Research Collection · 29. On-Line Control of Batch Cooling Crystallization 29 30. Monitoring of Particle Size Distribution in Crystallization from Solution 30 31. Gas Antisolvent

Research Collection

Report

2001 Science Odysseythe research activities 2001 at the Institute of ProcessEngineering

Publication Date: 2002

Permanent Link: https://doi.org/10.3929/ethz-a-004261508

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Research Activities 2001: Contents

III

Contents

Preface..................................................................................................................................V Information for visitors .........................................................................................................VII

Particle Technology and Aerosol Engineering...............................................................Prof. Sotiris Pratsinis

1. Modeling of Aggregation and Fragmentation of Suspensions in Stirred Tanks 1 2. Theory on Diffusion of a Drying Polymer Film 2 3. Sol-gel Synthesis of Spray Granulation of Porous Titania 3 4. Model for Electrospray Deposition on a hot Plate 4 5. Flame Spray Pyrolysis for Synthesis of Nanoparticles 5 6. Flame Aerosol Microreactor for Synthesis of Non-Aggregated Nanoparticles 6 7. Large Scale Production of Silica-Carbon Nanoparticles in a Turbulent

Hydrogen-Air Flame Aerosol Reactor 7 8. Fourier Transform Infrared (FTIR) Monitoring of Electrically Assisted

Aerosol Flame Reactors 8 9. Flame Synthesis of Vanadia-Titania-Silica Mixed Oxide Catalysts 9 10. Monitoring the Formation and Growth of Titania Nanoparticles 10 11. CFD Simulation of a Premixed CH4/N2/O2 Flat Flame Used for Synthesis

of Titania Nanoparticles from TTIP 11 12. Nanoparticle Precursors for Cu(In,Ga)Se2 Solar Cells 12 13. Synthesis of Bismuth Nanoparticles 13 14. The Effects of Buoyancy on Thermophoretic Deposition of Aerosol Particles

in a Laminar, Vertical Flow 14 15. Automatic Detection of Spherical Primary Particles in TEM Images of Fractal

Aggregates 15 16. Determination of Functional Groups (OH-) on the Surface of Nanoparticles by

Thermogravimetric Analysis 16

Multiphase Transport Phenomena and Reactions ........................................................... Prof. Philipp Rudolf von Rohr

17. Wet Oxidation of Model Substances 17 18. Transpiring Wall Reactor for the Supercritical Water Oxidation 18 19. Operating Conditions for a Transpiring Wall Reactor for Supercritical Water

Oxidation 19 20. Rotating Biological Contactor for the Degradation of Organic Compounds from

Waste Air 20 21. Novel aspects of biological waste gas treatment systems 21

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Research Activities 2001: Contents

IV

22. Quantitative Visualization of Large-Scale Structures in Turbulent Convective Fields 22 23. Droplet Production and Transport from Disintegrating Bubbles above a

Gas-Liquid Interface 23 24. Tubular Reactor for Reactions in Liquid-Gas-Mixtures 24 25. Plasma Enhanced Coating of Powders 25 26. Novel Coating Process for Fine Powders 26 27. Plasma Enhanced Chemical Vapor Deposition using a Pulsed Microwave Plasma 27 28. Deposition of Silicon Oxide Thin Films on Paper using Plasma Enhanced Coating

Technologies 28

Advanced Separation Processes ............................................Prof. Marco Mazzotti

29. On-Line Control of Batch Cooling Crystallization 29 30. Monitoring of Particle Size Distribution in Crystallization from Solution 30 31. Gas Antisolvent Recrystallization of Specialty Chemicals 31 32. Modeling Gas Antisolvent recrystallization 32 33. Mass transfer effects in GAS recrystallization 33 34. Scale-up of precipitation processes 34 35. Experimental investigation of complex nonlinear dynamics in homogeneous azeo-

tropic distillation 35 36. Design and Operation of Simulated Moving Bed Processes for Fine Chemical and

Pharmaceutical Separations 36 37. Gradient mode operation of Simulated Moving Beds 37 38. Bio-Separations using Simulated Moving Beds 38 39. Enantioseparations through SMB and crystallization 39 40. Separation of Enantiomers through Continuous Supercritical Fluid Simulated

Moving Bed (SF-SMB) Chromatography 40 41. Thermodynamics of Supercritical Adsorption 41 42. Modeling the Adsorption Behavior of Supercritical Fluids 42 43. Application of the method of projection onto convex sets to inverse problems in

adsorption and chromatography 43 44. Enantioseparations through achiral chromatography 44 45. Gas Chromatographic SMB (GC-SMB) Technology for Chiral Separations 45 46. Analysis and Design of Simulated Moving Bed Reactors 46

Bio Process Technology..............................................................Prof. Fritz Widmer

47. Forming of Angiopolar Ceramic Cell Carriers by Dip Coating 47 48. Mass Transfer Characteristics of a Novel Gas-Liquid-Reactor 48 49. Prilling and Freezing of Aqueous Solutions in an Organic Phase with Following

Freeze Drying of the Particles 49

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Research Activities 2001: Preface

V

Preface

“2001: A Space Odyssey” was the title of the 1968 movie by Stanley Kubrick (1928-1999), where beside questions about men’s evolution and about the ultimate meaning of life, a future of space exploration and exploitation, of complex and controversial relationships between men and machines, of new life-science developments was imagined. At that time, the year 2001 represented the deep future, so remote in space and time that everything would be possible and achievable by an ever-progressing, far-reaching technology. For a whole generation the scientific and technological challenges that were implicit to that artistic outlook at the future were among the strongest motivations towards research and science.

Fast enough, the deep future has become the very present. It is the year 2001, and we recognise that Kubrick’s outlook was possibly biased in overestimating the progress in space technology due to the moon-race of the ‘60s, and in underestimating the much, much faster advances of life sciences and bio-technology. However, the total-control over humans by the super-computer HAL 9000 in the movie on the one hand and the nowadays widespread, all-permeating diffusion of the PC-based Internet on the other seem to differ in the modality but not in the significance of their impact on the life of mankind. Technology has advanced, and is advancing tremendously in all its aspects, and the scientists, researchers and students at the Institute of Process Engineering at ETH Zurich play an active, responsible, and concerned role in this development. That is why we have dared to pay homage to Kubrick as an artist and to its cult-movie, and to ambitiously title this Research Activities booklet “2001: A Science Odyssey”.

Process Engineering is concerned with the physical, chemical, and biological transformation of matter. Its objective is the industrial implementation of the corresponding techniques, which implies a clear understanding and control of the scale-up of a technology from the laboratory to the pilot to the production plant. The variety of topics and areas of engineering science involved in the whole of our projects represents the broad scope of Process Engineering itself.

The main goal of the research activity at the Institute of Process Engineering is the development of efficient, safe and sustainable processes. New techniques are being investigated for the development of new products to match the requirements of our society and of the new emerging technologies, such as life sciences and micro- and nano-systems. Examples of new techniques are chemical or biological processes for the down-stream treatment of waste gases or wastewater, separation processes based on chromatography or crystallisation, and processes where toxic solvents are replaced by the environmentally benign high-pressure carbon dioxide. Examples of product-oriented applications are new processes for cell encapsulation, plasma enhanced

Research Activities 2001: Preface

VI

coating, synthesis of drug microparticles and production of nanoparticles. We investigate processes not only in terms of their sequence of transformation but also address the design of special equipment and the issue of system analysis and integration.

All of the diverse projects presented in this report share a number of similar features. In addition to the common motivation mentioned above there are two aspects, which are often important. Firstly, most projects have a strong interdisciplinary character and call for an analysis at different scales, from the molecular to the particle level, from the plant equipment viewpoint to the overall system integration. Secondly, most projects are tackled by coupling the theoretical analysis and the experimental investigation.

The research activity at the Institute of Process Engineering has tight links with industry, in particular equipment and plant design, chemical, food and pharmaceutical industries. These connections are important for industry, which obtain highly qualified support in developing innovative projects, and for academia, since in the field of engineering and technology innovations are driven by applications. Moreover, industrial as well as federal support of our research activity allows us to undertake more basic and fundamental research, tackling problems that industry will face in one or two decades from now.

Despite the broad scope of Process Engineering the research activity at the Institute of Process Engineering is focused on four complementary areas:

♦ Particle Technology and Aerosol Engineering in the group of Prof. Sotiris Pratsinis;

♦ Multiphase Transport Phenomena and Reactions in the group of Prof. Philipp Rudolf von Rohr;

♦ Advanced Separation Processes in the group of Prof. Marco Mazzotti;

♦ Bio-Process Technology in the group of Prof. Fritz Widmer, who retired in October 2000, and in the group of the new Assistant Professor, who will be appointed during the present year.

The research projects within the above four groups are described in detail in the next pages.

Marco Mazzotti

Head of the Institute

Information for Visitors

VII

How to reach us:

With public transportation: Within five to ten minutes by tram 6 starting at main station (Bahnhofsstrasse) or by tram 10 starting at main station (Bahnhofsplatz) to ETH/Universitätsspital.

By car: It takes about five minutes from Central (near main station) to ETH Zentrum (at Universitätsspital). In the vicinity of ETH Zentrum it is almost impossible to find a parking place.

By foot: It takes about 10 minutes, starting at main station through Walche-Brücke first, then up the stairs to Haldenegg and finally walking along Leonhardstrasse and Tannenstrasse. Alternatively one can walk to Central and then up the stairs and leftward along Auf der Mauer, till Leonhardstrasse.

Institute ofProcess

Engineering

Particle Technology and Aerosol Engineering

1

1. Modeling of Aggregation and Fragmentation of Suspen-sions in Stirred Tanks G. BARTHELMES and S.E. PRATSINIS

Keywords: aggregation, population balances, stirred tank, compartmental model, CFD

In suspensions, particles often form aggregates or “flocs” due to attractive interactions. The structure of these flocs depends, among others, on the flow conditions of the suspension. Hydrodynamic forces can disrupt the flocs and lead to fragmentation. The superposition of aggregation and fragmentation can be modeled by population balances. Since the floc size distributions show self-preserving properties [1] they can be represented by a characteristic size. This allows to consider the aggregates to be monodisperse with a shear dependent size and to apply a very simple population balance.

In the past the models mostly considered homo-geneous flow conditions. When we examine floccu-lation in a stirred tank, as is often done experiment-ally, we get strong spatial variations of the flow (cf. Fig. 1), with high shear regions close to the stirrer and almost stagnant fluid on top. Therefore we can expect that the floc size within the tank is not homogeneous. This can be modeled by dividing the tank volume into a number of compartments and by solving the population balance equations in these compartments [2]. The flow itself is simulated by FLUENT, a commercial CFD package. Since we only consider dilute suspensions, the flow itself does not change by the presence of the aggregates.

References [1] SPICER P.T., PRATSINIS S.E., “Coagulation and Fragmentation: Universal Steady-State

Particle-Size Distribution”, AIChE J., 42, 1612-1620 (1996) [2] SCHÜTZ S., PIESCHE M., “Modellierung von Flockungsprozessen mit Hilfe von

Populationsbilanzen“, Chemie Ingenieur Technik 71, 1174-1178 (1999)

Fig. 1: Velocity VectorDistribution in a Stirred Tank with a Rushton Impeller (at 100 rpm)


2

2. Theory on Diffusion of a Drying Polymer Film S. VEITH and S.E. PRATSINIS

Keywords: diffusion models, polymers, drying

The investigation of diffusion in polymers and gels is crucial to predict for example drug release rates in polymer tablets or the impact of toxic compounds like plasticizers on human health.

Diffusion in polymers is a very complicated process, where diffusion coefficients can vary by more than a factor of 1010. Diffusion depends on the properties of the diffusant, the polymer network and the solvents. The obstruction by the polymer network, the hydrodynamic interactions in the system and the thermodynamic agitation should be all considered to understand the diffusion in polymer solutions, gels and even solids.

The main theoretical models are based on the different physical concepts such as obstruction and free volume effects and hydrodynamic interactions [1]. These can be regarded as molecular effects. Furthermore the application of the above theoretical concepts is restricted on the actual state of the polymer.

During a drying process the state of the polymer will change upon solidification thus influencing the mechanism of diffusion. Furthermore changes in the macroscopic morphology like the matrix porosity will be induced. For example the effective diffusivity in such a system is given by [2]:

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The scope of this research is to model the diffusion during a drying process taking above mentioned physical changes of the matrix system into account.

References [1] MASARO L., ZHU XX., “Physical models of diffusion for polymer solutions, gels and solids”,

Prog. Polym. Sci. 24, 731-775 (1999) [2] CUSSLER E.L., “Diffusion-Mass Transfer in Fluid Systems”, Second Edition, Cambridge

University Press (1999)


3

3. Sol-gel Synthesis of Spray Granulation of Porous Titania J. KIM, O. WILHELM and S.E. PRATSINIS

Keywords: sol-gel process, spray-drying, titania, zirconia, alumina

With large surface area, uniform pore size distribution, and better chemical stability, sol-gel made porous titania can be used as a ceramic membrane top layer, adsorbent, and catalyst support. For technical applications, however, titania powder in granule form is preferred. Therefore, porous titania granules were fabricated by spray-drying of sol-gel solutions. The properties of granules strongly depend on feed solutions. In this research, the effect of peptization and dopants was investigated.

The peptization of the feed solution considerably influenced phase composition, pore structure, and morphology of the product granules. The granules from unpeptized slurry consisted of hard aggregates which were loosely packed, showing bimodal pore size distribution[1]. In contrast, the granules from peptized sol were quite compact with smooth surfaces, resulting in monomodal pore size distribution[2]. Fig. 1 shows coarse and fine microstructure attributed to spray drying of unpeptized slurry and peptized sol, respectively.

When alumina or zirconia was added into titania sol, the product doped titania granules retarded the anatase-to-rutile phase transformation and showed a significantly improved textural stability compared to the pure titania granules. After heat treated at 600°C for 2 hours, the doped titania granules have specific surface area of 94-102 m2/g and are mostly of anatase phase, while pure titania granules processed at the same conditions have specific surface area of about 2 m2/g in anatase and rutile phases [2].

100nm

(b)

5µµµµm

(a)

5µµµµm100nm

Fig.1: SEM of spray dried titania granules from (a) unpeptized slurry, and (b) peptized sol.

References [1] SONG K.C., PRATSINIS S.E., “Synthesis of bimodally porous titania particles by hydrolysis of

titanium tetraisopropoxide“ J.Mater.Res., 15, 2322-2329 (2000) [2] KIM J., SONG K.C., WILHELM O., PATSINIS S.E., “Sol-gel synthesis and spray granulation of

porous titania powder“ Chem. Ing. Tech., submitted (2001)

5µµµµm


4

4. Model for Electrospray Deposition on a hot Plate O. WILHELM, L.J. GAUCKLER*, L. MÄDLER, D. PEREDNIS*, and S.E. PRATSINIS

Keywords: electrospray, thin films

The electro spray pyrolysis process provides the potential to produce thin (0.1 to 10 µm) and gastight ceramic coatings even on porous substrates. Deposition of e.g. thin zirconia films can be used as an electrolyte for solid oxide fuel cells and will enable to operate these cells at temperatures as low as 700°C [1]. In addition, large electrochemically active surface areas and low interface polarization is expected due to the special type of layer formation directly on the electrode surface. The control of the drying process is important to steer the droplet size and the precursor content having a great influence on the thin coatings and their mentioned properties and is thereby a target for modeling the drying process.

Electrohydrodynamic spraying or electrospraying provides a technique that is capable to produce a nearly monodisperse drop size distribution in the micrometer range connected with a high deposition efficiency. The produc-tion of particles and surfaces with special properties is a key issue of electrospraying. A number of functional ceramic thin films was deposited by van Zomeren et al. [2], and Kelder et al. [3]. Gañán-Calvo et al. [4], developed a numerical model to describe the droplet transport. Here a numerical model is developed that is describing the droplet transport and mass transfer of the spray by evaporation during deposition. Figure 1 shows results for different ethanol vapor concentrations of a calculation performed with an ethanol electrospray. This model reveals process conditions for formation of uniform or granulate deposits. References [1] BOHAC P., GAUCKLER L.,”Chemical spray deposition of YSZ and GCO solid electeolyte

films”, Solid State Ionics , 199, 317 (1999) [2] VAN ZOMEREN A.A., KELDER E.M., MARIJNISSEN J.C.M., SCHOONMAN J., “The production

of thin films of LiMn2O4 by electro spraying” J. Aerosol Sci., 25, 1229 (1994) [3] KELDER J.E.M., NIJS O.C.J., SCHOONMAN J.,”Low-temperature synthesis of thin-films of YSZ

and MACEO(3) using electrostatic spray-pyrolysis (ESP)”, Solid State Ionics, 68, 5 (1994) [4] GAÑÁN-CALVO A.M., LASHERAS J.C., DÁVILA J., BARRERO A., “The electrostatic spray

emited from an electrified conical meniscus”, J. Aerosol Sci.,25 (6), 1121-1142 (1994) *ETH-Zurich, Laboratory of Non-metallic Inorganic Materials

0

5

10

15

20

25

30

35

40

150 200 250 300 350 400substrate temperature [°C]

drop

let s

ize

[mm

]non-zero background concentration (Diffusion only)

zero background concentration

Fig. 1: Size distribution of ethanol droplets during deposition for different substrate temp-eratures and background vapor concentration


5

5. Flame Spray Pyrolysis for Synthesis of Nanoparticles L. MÄDLER, R. MUELLER, and S.E. PRATSINIS

Keywords: metal and mixed metal oxides, nanoparticles, spray flame

The growing demand of nanoparticles with very defined properties such as purity, stoichiometric ratio, and crystalline phase at low prices is targeted using flame spray pyrolysis (FSP) which has proven to be capable of producing nanograined materials for high technology applications from a broad spectrum of liquid precursors.

In FSP a fine and homogeneous spray of a solution containing the liquid precursor and fuel is introduced into a combustion zone using the oxidant as the dispersion gas in a pressurized atomizer applying a CH4/O2 supporting flame as ignition source (Figure 1). This method has been successfully applied to the preparation of SiO2 particles from a liquid mixture of HMDSO/fuel with a molar ratio of 0.1 resulting in a production rate of 9 g/h, where the specific surface area of the produced silica powder can be controlled from 400 to 60 m2/g corresponding to an equivalent particle diameter of 7 to 40 nm [1]. Control over powder properties was achieved by the type of oxidant, its flow rate forming the spray flame as well as the liquid fuel. It was further shown by TEM analysis that the corresponding morphology of particles formed in the gas phase [2].

Using the advantage to process liquid precursor/fuel system directly the easily adaptable system was further utilized to prepare e.g. TiO2, Al2O3 and other metal oxides and mixed metal oxides from metal organic and nitric precursors embedded in various fuel types.

This research enables us to fundamental understand the FSP for synthesis of tailor-made nanoparticles including spray combustion and particle dynamics. It may unravel a new tool for synthesis of a broad spectrum of functional nanoparticles with closely controlled size, purity, stoichiometry, and crystallinity. Direct applications of this research are expected in the field of catalysis, sensors, and specific electronics

References [1] MÄDLER L., MUELLER R., PRATSINIS S.E., “Controlled Synthesis of Nanostructured Particles

by Flame Spray Pyrolysis”, AIChE 2000 Annual Meeting, Los Angeles, CA. [2] PRATSINIS S.E., “Flame Aerosol Synthesis of Ceramic Powders”, Prog. Energy Combust. Sci.,

24, 197-219, (1998)

Fig 1: Long exposure time (1/60 s) image of silica producing spray flame.


6

6. Flame Aerosol Microreactor for Synthesis of Non-Aggregated Nanoparticles K. WEGNER and S.E. PRATSINIS

Keywords: aerosol flame synthesis, microreactor, nanoparticles, silica, titania

The aim of this project is the controlled synthesis of non-aggregated metal-oxide nanoparticles with diameters between 1 and 50 nm, which can be used in catalysis, thin film technology, protective metal and polymer coatings, or as inorganic/polymeric nanocomposites. Therefore, flame aerosol reactors are employed, which are widely used in industry for large-scale manufacture of oxide and carbon nanoparticles [1] and offer high versatility in control of product particle properties [2].

Here, the effect of the geometric dimension of a co-flow diffusion flame aerosol reactor on the product particle size and morphology is studied. The flame microreactor consists of five concentric stainless steel tubes, the outer tube having a diameter of 9 mm. Fumed silica and titania nanoparticles are synthesized by oxidation of hexamethyldisiloxane (HMDSO) and titania-tetra-isopropoxide (TTIP) as precursor with two different burner configurations. The influence of the oxidant flow rate and composition on the specific surface area of these powders is determined by nitrogen adsorption and transmission electron microscopy. Titania and silica nanoparticles with a specific surface area ranging from 30 to 250 m2/g and of spherical to aggregated morphology can be made with the aerosol flame microreactor by controlling the temperature history of the process.

An investigation of flow mixing in coaxial jets by computational fluid dynamics and tracer gas analysis as well as flame temperature measurements revealed that properties generally attributed to microreactors, such as an improved fluid mixing [3] can also be applied to the diffusion flame microreactor.

References [1] PRATSINIS S.E., “Flame Aerosol Synthesis of Ceramic Powders“, Prog. Energy Combust. Sci.

24, 197-219 (1998) [2] WEGNER K., PRATSINIS S.E., “Aerosol Flame Reactors for Synthesis of Nanoparticles“,

KONA 18, in press (2000) [3] EHRFELD W., HESSEL V., HAVERKAMP V., “Microreactors“, in Ullmann’s Encyclopedia of

Industrial Chemistry, 6th ed., Wiley-VCH, Weinheim (1999)

Fig. 1: Diffusion flame microreactor and a 10 cents coinof 1.8 mm diameter.


7

7. Large Scale Production of Silica-Carbon Nanoparticles in a Turbulent Hydrogen-Air Flame Aerosol Reactor H.K. KAMMLER, R. MUELLER and S.E. PRATSINIS

Keywords: fumed silica, carbon blacks, diffusion flame, high production rates

Flame aerosol technology is used for large scale manufacture of pigmentary titania, carbon black, fumed silica and alumina, lightguide preforms, to name a few [1]. The combination of carbon black and silica is more effective in reinforcing rubber compared to carbon black alone providing the capability for manufacture of so-called "green tires". As the silica surface is covered with silanol groups, adding an organosilane coupling agent, forms a silica network in the rubber. This filler-to-filler network enhances the tire reinforcement decreasing the rolling resistance by up to 24%, while wet traction and treadwear are similar to conventional tires [2], thus the fuel consumption is significantly decreased and therefore pollution of air and environment is reduced. The carbon is needed to ensure good dispersibility and static electricity dissipation [2].

Synthesis of flame-made nanostructured silica-carbon powders is studied at high production rates (up to 700 g/h) in a turbulent diffusion flame reactor (Figure 42), addressing also the required safety concerns. A commercial hydrogen-air burner is used for synthesis of pure silica and composite silica-carbon particles by oxidation of hexamethyl-disiloxane. The product powder is collected continuously in a baghouse filter unit (left hand side of Figure 42), cleaned periodically by air-pressure shocks. The effect of the fuel (hydrogen) flow rate, powder production rate, and total oxidant flow rate on product particle size, morphology, and composition is investigated. Nitrogen adsorption (BET), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA) coupled with a mass spectrometer (MS) are employed to characterize specific surface area and powder composition. Typically, aggregates of silica-carbon composite particles (0 to 1.5 wt% carbon) are made having specific surface areas of 75 to 250 to m²/g at production rates of 125 to 700 g/h corresponding to particle concentrations of 17 to 93 g/m3 (2 to 10 % solid fraction by weight). References [1] PRATSINIS S.E., "Flame aerosol synthesis of ceramic powders", Prog. Energy Combust. Sci.,

24, 197-219 (1998) [2] BYERS T.J., MCNEISH A.A., "Current advances in tire compounding technology for rolling

resistance" paper presented at the Carbon Black World 1997 Conference, San Antonio, TX

Fig. 42: Picture of the hydrogen/air-flame aerosol reactor making silica-carbon nano-particles at rates up to 700 g/h.


8

8. Fourier Transform Infrared (FTIR) Monitoring of Elec-trically Assisted Aerosol Flame Reactors H.K. KAMMLER and S.E. PRATSINIS

Keywords: FTIR, titania, nanoparticles, flame aerosol process, electric fields

External electric fields are intriguing in gas phase particle formation as they can be readily implemented into the set-up and are quite effective for precise particle size control. Electrical charges drastically reduce the average particle size [1]. Recently it was shown that electric fields can also control the primary particle size for production rates up to 87 g/h [2].

The most important parameters in flame aerosol processing, however, are the flame temperature and the residence time of the particles in the hot flame zone. As it is not possible to measure flame temperatures in the electric field with conventional thermo-couple injection methods, Fourier Trans-form Infrared (FTIR) spectroscopy is used in this set-up [3]. From this technique, detailed flame temperature profiles can be obtained even when using the external electric fields.

Figure 40 shows flame temperatures obtained with FTIR in a premixed titanium tetraisopropoxide (TTIP)/methane/oxygen flame in the absence and in presence of an electric field created by plate electrodes. The flame temperature is not affected by the electric field close to the burner, but is significantly lower with increasing distance. The higher the applied potential the more the flame temperature is lowered at the higher locations, thus the residence time of the particles in the hot flame region is shorter, resulting in smaller primary particles. When unipolarly charged ions are provided due to the electric field, however, the charged particles have the tendency to expand by electrostatic dispersion, thus resulting in a concentration decrease and shorter residence times of the particles in the flame. Both decrease also the primary particle size and facilitate the precise control of particle size. References [1] VEMURY S., PRATSINIS S.E., KIBBEY L., "Electrically controlled flame synthesis of nanophase

TiO2, SiO2, and SnO2 powders", J. Mater. Res. 12, 1031-1042 (1997) [2] KAMMLER H.K., PRATSINIS S.E., "Electrically-assisted flame aerosol synthesis of fumed silica

at high production rates," Chem. Eng. Process. 39, 219-227 (2000) [3] MORRISON P.W., RAGHAVAN R., TIMPONE A.J., ARTELT C.P., PRATSINIS S.E., "In situ

Fourier transform infrared characterization of the effect of electrical fields on the flame synthesis of TiO2 particles", Chem. Mater. 9, 2702-2708 (1997)

Fig. 40: Effect of an electric potential on flame temperature (obtained by FTIR) of a TiO2 producing premixed flame.

1500

1750

2000

2250

2500

0 20 40 60 80

Height above burner, mm

Ave

rage

flam

e te

mpe

ratu

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0 kV/cm1.0 kV/cm1.5 kV/cm2.0 kV/cm


9

9. Flame Synthesis of Vanadia-Titania-Silica Mixed Oxide Catalysts W. J. STARK, K. WEGNER, S.E. PRATSINIS and A. BAIKER*

Keywords: vanadia, catalysts, mixed oxide particles, aerosol flame reactors

NO removal from stationary and mobile combustion sources became a major environmental issue in the last years. Highly efficient catalysts for the selective catalytic reduction (SCR) of NO by ammonia or hydrocarbons either consist of dispersed noble metals or mixed oxides. One of the most active oxides is vanadia/titania. It removes NO down to ppm level at temperatures as low as 150°C. There is abundant knowledge on these catalysts and their preparation by classical methods. However, no approaches have been done for a high temperature synthesis.

We used flame aerosol synthesis to produce vandia/titania as small ceramic spheres with diameters around 20 to 50 nm. Particles can be produced with specific surface area between 20 and 120 m2/g and consist mainly of anatase. No formation of distinct vanadia particles is visible in HRTEM. Spectroscopic investigations using Laser Raman spectroscopy, X- ray photoelectron spectroscopy, UV-VIS and X-ray fluorescence revealed that vanadia mainly stays on the surface of these nanoparticles. Catalytic activity in the SCR of NO by ammonia was measured in a fixed bed microreactor in the group of A. Baiker. Catalytic activity is comparable and in some cases even superior to the best catalysts prepared by classical methods. Selectivity to nitrogen, a key issue in these systems is better than 99% up to 300 °C. This opens a broad application window for off gas treatment in challenging environments. The overall catalytic behavior is very similar to classically prepared catalysts and shows that flame aerosol synthesis can be successfully used in the production of heterogeneous catalysts.

References [1] STARK W. J., WEGNER K., PRATSINIS S. E., BAIKER A., "Flame Aerosol Synthesis of

Vanadia-Titania Nanoparticles. Structural and Catalytic Properties in SCR of NO by NH3", J. Catal., 197, 182-191, (2001)

[2] PRATSINIS S.E., “Flame Aerosol Synthesis of Ceramic Powders”, Prog. Energy Combust. Sci. 24, 197-219, (1998)

*ETH-Zurich, Laboratory of Technical Chemistry

Fig. 1: High Resolution TEM of flame-made vanadia- titania nanoparticles. Inserted: Diffraction pattern.


10

10. Monitoring the Formation and Growth of Titania Nanoparticles. S. TSANTILIS, H.K. KAMMLER and S.E. PRATSINIS

Keywords: surface growth, titania, nanoparticle

The objective of the present study is to explore the formation and growth of titania nanoparticles from titanium tetraisopropoxide (TTIP) in a premixed methane flame. Sectional and monodisperse aerosol dynamics models [1] for simultaneous gas phase chemical reaction, surface growth and coagulation are further modified to account for the effect of sintering. Simulation results are then compared with in-house experimental data for various inlet precursor concentrations.

Figure 1 shows how the predictions from monodisperse models compare to experimental data for three different TTIP reaction schemes, namely gas phase hydrolysis (dash-dot line), gas phase thermal decomposition (dash line) and surface

growth (solid line), for the same sintering mechanism. Although all the proposed reaction schemes predict rather similar average primary particle diameters far away from the burner tip ( x = 20 cm), model predictions show more significant deviations at the very early stages of particle growth. The smallest particle sizes are predicted by gas phase thermal decomposition for all the locations above the burner tip. Simulation results for hydrolysis are close to their experimental counterparts for x > 2 cm leading however to considerable overpredictions at earlier locations. This trend stems from the fact that the currently available in the open literature hydrolysis reaction is very fast as it is based on the assumption of

rather high water to TTIP ratios. Sectional calculations elucidate the effect of the above reaction schemes on the primary particle size distributions. The geometric standard deviations for gas phase thermal decomposition are higher than the corresponding experimental results at locations close to the burner tip. Finally, a model accounting for surface growth which is later on taken over by hydrolysis seems to be a more reasonable mechanism. References [1] PRATSINIS S. E., ARABI-KATBI O., MEGARIDIS C. M., MORRISON JR P. W., TSANTILIS S.,

KAMMLER H. K. "Flame Synthesis of Spherical Nanoparticles", Journal of Metastable and Nanocrystalline Materials, 8, 511-518 (2000)

Fig. 1: Model predictions and experimentaldata. The bars are one standard deviationbelow and above the average.


11

11. CFD Simulation of a Premixed CH4/N2/O2 Flat Flame Used for Synthesis of Titania Nanoparticles from TTIP. S. TSANTILIS, H.K. KAMMLER and S.E. PRATSINIS

Keywords: CFD, titania, premixed flames

Flame processes are by far the most widely used ones for manufacture of commercial quantities of nanoparticles (carbon blacks, fumed silica, SiO2 and pigmentary titania, TiO2). Typically, in flame reactors, nanostructured powders are produced virtually without control at high temperatures and extremely short process residence times (less than a second). This makes representative particle sampling, model development and process control very difficult.

The present work focuses on synthesis of titania nanoparticles in premixed flat flame reactors. More specifically, we simulate the flow dynamics of a premixed CH4/N2/O2 flat flame reactor [1] used for synthesis of titania powders by TTIP (titanium-tetraisopropoxide) oxidation/decomposition. Temperature and velocity profiles are derived using the commercial fluid dynamics software FLUENT 5 for a variety of process conditions (total gas flowrate, process temperature, inlet precursor

concentration, and burner geometry). This provides an alternative way of verifying temperature measurements done in the flame by other methods such as Fourier Transform Infrared Spectroscopy (FTIR). Figure-1 shows an example of temperature contours of a pure methane premixed flame under process conditions similar to those employed in synthesis of

titania nanoparticles [1]. Temperature and velocity profiles from such files are compared with their experimental counterparts. Finally, the CFD information is combined [2] with several in-house aerosol dynamics models for the prediction of various particle properties (such as particle diameter) along the flame. References [1] ARABI-KATBI O. I., PRATSINIS S. E., MORRISON JR. P. W., MEGARIDIS C. M., "Monitoring

Flame Synthesis of Titania by In Situ Fourier Transform Infrared (FTIR) Spectroscopy and Thermophoretic Sampling", Combust. Flame, in press (2001)

[2] JOHANNESSEN T., PRATSINIS S. E., LIVBJERG H., "Computational fluid-particle dynamics for the flame synthesis of alumina particles", Chem. Eng. Sci., 55, 177-191 (2000)


12

12. Nanoparticle Precursors for Cu(In,Ga)Se2 Solar Cells M. KAELIN, K. WEGNER, A.N.TIWARI* and S.E. PRATSINIS

Keywords: solar cells, copper, indium, gallium, jet aerosol flow condenser

As Cu(In,Ga)Se2 based thin-film solar cells have already proven their potential for high efficiency [1], efforts are now concentrating on the development of low cost and large area production. Layers of good structural and electronic properties are grown with high vacuum deposition methods at the price of high processing costs. Spray deposition, screen printing and doctor blade do not generally produce high quality thin-films but are low cost large area techniques. Layers of high quality can be deposited using nanoscale precursor materials because of their higher reactivity for film formation as the number of atoms or molecules on their surface is comparable to that inside the particles.

The aim of this project is to synthesize nanoparticle precursors of optimum composition, size and morphology to obtain ~2 µm thick compact Cu(In,Ga)Se2 layers of large grain size for high-efficiency cells. Jet aerosol flow condensers [2] are employed for the production of the nanoparticles by melt evaporation due to their high flexibility in control of product particle properties. Furthermore, it is possible to synthesize metal alloys by co-evaporation [3] and to carry out reactions like oxidation and selenization. Thus, single component particles such as In, Ga or Cu can be produced as well as metal alloys, selenides and oxides.

Fig. 1: Sintering of nanoparticle precursor films for synthesis of commercial solar cells.

The product nanoparticles are deposited on a substrate, forming a precursor layer which is then subjected to a sintering/selenization step (Fig.1). The resulting thin film of desired stoichiometry and microstructure can be used to process solar cells. References [1] CONTRERAS M.A., EGAAS B., RAMANATHAN K., HILTNER J., SWARTZLANDER A., HASOON

F., NOUFI R., “Progress Towards 20% Efficiency in Cu(In,Ga)Se2 Polycrystalline Thin-Film Solar Cell”, Progress in Photovoltaics: Research and Applications, 7, 311-316 (1999)

[2] HAAS V., BIRRINGER R., GLEITER H., PRATSINIS S.E., “Synthesis of Nanostructured Powders in an Aerosol Flow Condenser”, J. Aerosol Sci. 28, 1443-1453 (1997)

[3] KONRAD H., HAUBOLD T., BIRRINGER R., GLEITER H., “Nanostructured Cu-Bi Alloys Prepared by Co-evaporation in a Continuous Gas Flow”, Nanostructured Mater., 7, 605-610 (1996)

*Thin Film Physics Group, Laboratory for Solid State Physics, ETH Zürich

nanopowder absorber layerMo backcontact

Glas substrate

inert Se atmosphere500°C


13

13. Synthesis of Bismuth Nanoparticles K. WEGNER and S.E. PRATSINIS

Keywords: bismuth nanoparticles, jet aerosol flow condenser, aerosol dynamics, computational fluid dynamics

Nanostructured metal particles are used in a variety of applications such as catalysis, superalloys, and thin film coatings in the chemical and electronics industries. Here, a process is developed for continuous production of metal nanoparticles by melt evaporation [1] and is performance-tested by the example of bismuth, used in the production of dry phototools for printed circuit boards [2].

Bismuth vapor is produced in an externally heated tube flow condenser containing a crucible with the Bi-melt. Transport of the vapor in an argon carrier gas jet of furnace temperature is followed by quenching in a diluter, particle formation and product powder collection on a filter. This experimental design separates the evaporation from the particle formation (quenching) zone allowing to systematically study the influence of cooling rate and diluter configuration on particle properties. Specifically, the particle residence time distribution in the process is investigated for different quenching configurations as well as the effect of evaporation temperature, pressure, carrier and quenching gas flow rate. The characteristics of product particles are analyzed by nitrogen adsorption, transmission electron microscopy and X-ray diffraction.

Computational fluid dynamics (CFD) is applied for process design, elucidating the detailed temperature and velocity profiles in the complex geometry of the reactor. The data is verified by flow visualization experiments and temperature measurements and is interfaced with an aerosol dynamics model, accounting for nucleation, condensation and coagulation [3]. References [1] GRANQVIST C.G., BUHRMAN R.A., “Ultrafine Metal Particles“, J. Appl. Phys., 47, 2200-2219

(1976) [2] EICKMANS J., LEENDERS L., LAMOTTE J., DIERKSEN K., JACOBSEN W., “Mastertool: A New

Dry Phototool in the Production of Printed Circuit Boards“, Circuit World, 22, 26-32 (1996) [3] PANDA S., PRATSINIS S.E., “Modelling the Synthesis of Aluminum Particles by Evaporation-

Condensation in an Aerosol Flow Reactor“, Nanostructured Mater., 5, 755-767 (1995)

Fig.1: Aerosol jet flow condenser for synthesis of metal nanoparticles.


14

14. The Effects of Buoyancy on Thermophoretic Deposition of Aerosol Particles in a Laminar, Vertical Flow J.K. WALSH, S.R. DAHL*, C.M. HRENYA*, S.E. PRATSINIS, and A.W. WEIMER*

Keywords: thermophoresis, buoyancy, monodisperse aerosol dynamics

The flow configuration under consideration consists of laminar gas flow in a relatively cold, vertical tube. Such flow systems can be applied to a variety of applications such as the production of ceramic powders in high temperature aerosol flow reactors, production of optical fiber preforms, and dissociation of methane in a solar furnace [1,2]. Predicting the characteristics of the thermophoretic deposition are important for optimization of process efficiency.

Here the modeling of the thermophoretic deposition of aerosol particles consists of two steps. First, the temperature and velocity of the continuous phase are predicted at every point within the tube by solving the mass, momentum and energy balance for the fluid phase. Second, using these profiles as inputs, the concentration of the particles and the level of deposition are predicted via an aerosol population balance, which accounts for the effects of particle convection and thermophoresis. The particles are assumed to be monodisperse.

The effects of buoyancy have been found to slightly increase the overall deposition level of downward flow as compared to the case of upward flow. A more dramatic effect has been observed in the deposition as a function of axial distance. As seen in figure 1, more particles are deposited at a shorter distance in downward flow through a vertical pipe than in upward flow. This behavior is primarily due to the tendency of buoyancy to “flatten” the downward flow velocity profiles, which allows the particles more time to deposit on the walls of the reactor. It should be mentioned that buoyancy in downward flows also leads to reduced temperature gradients, which will lessen the amount of particle deposition. This latter effect, however, is not great enough to overcome the increased residence time of the particles. Hence, both the overall and local deposition levels are found to be greater for the downward flow case. This research is supported by the U.S. National Science Foundation.

References [1] PRATSINIS S. E., WANG G. Z., et al. “Aerosol Synthesis of AlN By Nitridation of Aluminum

Vapor and Clusters.” Journal of Materials Research 10 (3) 512-520 (1995) [2] PRATSINIS S. E., KIM K.S, “Particle Coagulation, Diffusion and Thermophoresis in Laminar

Tube Flows”, Journal of Aerosol Science 20 (1) 101-111. (1989) *University of Colorado-Boulder, Department of Chemical Engineering, Boulder, Co USA

00.05

0.10.15

0.20.25

0.30.35

0.40.45

0.5

0 0.1 0.2 0.3 0.4 0.5Axial distance (m)

Loca

l dep

ositi

on e

ffici

ency

Upward flow

Downward flow

Fig. 1: Local deposition efficiency for Twall=500 and Tinlet=1500


15

15. Automatic Detection of Spherical Primary Particles in TEM Images of Fractal Aggregates. G. SKILLAS, H.K. KAMMLER and S.E. PRATSINIS

Keywords: particle size distribution, microscopy, automatic detection

Dobbins and Megaridis [1] developed a technique allowing transmission electron microscope (TEM) images of flame made particles to be taken at different positions in the flame. That way the growth history of the particles as well as the processes which lead to their aggregation is made visible. The wealth of information contained in the images is usually analysed qualitatively. The quantitative analysis is difficult, involving manual counting of each particle on the image. The process is prone to human error and rather tedious. Several approaches to automatic detection have been tried [2]. The algorithm presented here recognises objects which resemble circles and to extract their attributes (size, location) from the image. For this, the original image (Fig. 1A) is thresholded (Fig. 1B) and differentiated (Fig. 1C). In Fig. 1D the detected discs can be seen.

To find the radii and centres of the discs in an image the correlation of circles of various sizes with the underlying differentiated image (Fig. 1C) is computed. This is equivalent to solving a two-dimensional wave propagation differential equation backwards in time, assuming a point source. At time zero the wave energy density will be maximal and focused in the origin of the wave. The origin of the wave corresponds to the centre of the disc. The time steps needed to back-propagate the solution provide a measure of the diameter of the disc and subsequently of the particle diameter. An array of checks are made to avoid detecting false-negative and false-positive particles. These checks, improve the reliability of the algorithm substantially. The accuracy of the programm is confirmed by comparisons to more than 10000 handcounted titania particles from a premixed flame aerosol reactor. References [1] DOBBINS R.A., MEGARIDIS C.M., “Morphology of flame-generated soot as determined by

thermophoretic sampling”, Langmuir 3, 254-259 (1987) [2] KRUIS F.E., VANDENDEREN J., BUURMAN H., SCARLETT B., “Characterisation of

agglomerated and aggregated aerosol particles using image analysis” Part. Part. Syst. Charact. 11, 426-435 (1994)

Fig. 1: Different images involved in the particle detection. (A) The original image. (B) After thresholding. (C) Taking the image derivative. (D) The detected circles.


16

16. Determination of Functional Groups (OH-) on the Surface of Nanoparticles by Thermogravimetric Analysis R. MUELLER, H.K. KAMMLER, K. WEGNER, and S.E. PRATSINIS

Keywords: TGA-MS analysis, nanoparticles, silica, titania, sol-gel, OH-groups

Thermogravimetric analysis (TGA) is applied as a simple and fast characterization method for nanosized fumed silica, silica-carbon and titania powders as well as for silica and titania sol-gel powders. It is demonstrated that it is possible to distinguish directly between physically adsorbed and chemically bound water and the carbon content with the thermogravimetric balance (TGA/SDTA851e, LF/1100°C, Mettler Toledo AG). The physically adsorbed water (Step 1) is attributed to the loss in weight up to 120°C [1], while the chemically bound water (Step 2) is attributed to the loss in weight from 120 to 800°C for silica powders [2] and from 120 to 500°C for titania powders [3] in N2 atmosphere. The carbon content (Step 3) of silica-carbon powders is attributed to the weight loss from 800 to 1000°C in O2 atmosphere (Figure 1). Further-more, it is shown that flame made powders have a high purity when coupling a mass spectrometer (Quadstar 422, Balzers Instruments) with the thermobalance [4]. Comparing the TGA plot of Aerosil 380 and various fumed silica powders, they show similar behavior in the first two steps, indicating similar powder composition (Figure 1). This allows calcul-ation of the OH-/nm2 for silica and titania powders. The composition of sol-gel powders depends strongly on the ratio water to precursor: at (5:1 & 20:1) by-products of the precursor, solvent and/or catalyst could be detected in the powder while at a ratio of 1000:1 no volatile compounds have been found, thus the reaction of the precursor allcoxide was fully completed. References [1] HOCKEY J.A., “The surface properties of silica powders”, Chem. Ind., 57-63 (1965) [2] ILER R. K., “The chemistry of silica”, John Wiley & Sons, New York (1979) [3] BOEHM H.P., HERMANN M., “Bestimmung des aktiven Wasserstoffs, thermische Entwäs-

serung und Rehydroxylierung”, Z. anorg. allg. Chem., 353, 156-167 (1967) [4] MUELLER R., KAMMLER H.K., WEGNER K., PRATSINIS S.E., “Thermogravimetrische Analyse

von nanoskaligen Partikeln”, Chem. Ing. Tech., 72, 1079-1080 (2000)

98

99

100

101

102

103

104

0 20 40 60 80Time, min

Norm

aliz

ed s

ampl

e w

eigh

t, w

t% .

0

200

400

600

800

1000

Tem

pera

ture

, °C

white powderSSA: 220 m2/g

OH/nm2: 3.4

gray powderSSA: 106 m2/g

OH/nm2: 2.8dark gray powder

SSA: 75 m2/gOH/nm2: 2.5

Aerosil 380OH/nm2: 2.6 (Degussa: 2-3)

Fig. 1: Comparison of various flame made silica powders with a commercial available Aerosil 380.

Step 1 Step 2 Step 3

N2 O2

Multiphase Transport Phenomena and Reactions

17

17. Wet Oxidation of Model Substances I. RAFFAINER and PH. RUDOLF VON ROHR

Keywords: wet oxidation, promoter, pollutants

Wet Oxidation (WO) of specific liquid wastes (and even of slurries/sludges) with oxy-gen under mild conditions has proven to be feasible and to work successfully. The combination of Wet Oxidation and biological treatment for wastewater purification is both ecologically and econo-mically advantageous since the Wet Oxidation converts the toxic substances into biodegradable products. The Promoted Wet Oxidation (PWO) uses an additional pro-moter (water-soluble organics and Fe-ions) and lower temperature as the known Wet (Air) Oxidation. The following three compounds were chosen as model-substances: Aniline, Orange II (azo dye) and Nitrobenzene. The experiments with the azo dye show good conversions with PWO and moderate conversions with WO (temperature range: 130…190°C). Nitrobenzene degrades slowly and Aniline does not degrade at the observed temperature range using

WO. The PWO shows good conversion with both model pollutants. The conversion rate depends on the promoter adding.

These examinations are showing the positive influence of the promoter in a normally unfavourable range of WO-parameters. Therefore, the proposed process has three parameters for optimisation of a given wastewater problem: Reaction temperature, oxygen pressure and promoter concentration. As showed by the PWO of nitrobenzene, the formation of toxic intermediates can be a problem and has to be controlled by enough reaction time or by promoter adding.

Supported by ETH Zurich

References [1] VOGEL F., HARF J., HUG A., RUDOLF VON ROHR PH., Env. Prog., 18 (1): 7-13 (1999)

WO-Step

ASP/Bio.-Step

Pretreat-ment

Off-gas Off-gas

Sludge

purifiedwater

2 4

3

1

Fig. 1: Industrial Wastewater treatment: Wastewater streams (1) areseparated in biodegradable (3) and critical streams (2) The toxic or non-biodegradable stream is converted by the WO-step to a biodegradable stream (4). The final mineralisation is processed by a bio-chemical treatment (e.g. Activated Sludge Process).

NO2

NO2

OH

NO2

OH

NO2

OH

NO2

NO2

COOH

COOH

COOH

HOOC

CO2 H2ONO3-

HOOC-COOH

HCOOH CH3COOH NH4+

Fig. 2: WO of Nitrobenzene: intermediates and products.


18

18. Transpiring Wall Reactor for the Supercritical Water Oxidation B. WELLIG and PH. RUDOLF VON ROHR

Keywords: transpiring wall reactor, supercritical water oxidation, transport phenomena, waste water treatment, reactor and process design

Supercritical Water Oxidation (SCWO) is a high-pressure process for the disposal of toxic and hazardous waste waters. The corrosion of load-bearing vessel walls and the plugging of reaction vessels due to precipitation of salts and solids have been considered the two most well known technical difficulties of SCWO. Within the framework of this project, a novel tubular reactor type is investigated. The characteris-tic feature of the Transpiring Wall Reactor (TWR) lies in the prevention of any wall contact of hot corrosive or particle containing fluid. Porous, non-load-bearing trans-piring wall elements are inserted to impose on the established flow field an inward-directed radial velocity component near the inner reactor wall. This velocity compo-nent provides convective mass transport to oppose the diffusion mechanisms that typically lead to the undesirable deposition of salt particles precipitated from super-critical water inside the reactor. The transpiration flow simultaneously protects the reactor walls from the hot corrosive reaction mixture. The hydrothermal flame within the flow allows that the reactants reach reaction temperature without any wall contact. The experiments in the TWR are carried out under a number of different flow condi-tions, varying parameters such as type and concentration of artificial waste water, temperature, pressure, salt type and concentration, and inner geometry of the pressure vessel as well as the liner design.

HWA

TI

TI

desalinated water & fuel

oxygen

waste water

transpiringwater

oxygen

Diameter: 34 / 72 mm

Inner length: 350 mm

Construction material:Alloy 625

Operating pressure: 250 - 420 bar

Burst pressure: 600 bar

Wall temperature: max. 600°C

Transpiring Wall: high porous sintered metall (average pore ∅ 3mm)

TIA

PIC

TWR

TACH TIC

TACH

TACH

TIC

TIC

TACH TIC

TICTACH

TICTACH

TACH TIC

TIFI

FI

FI

FI

FI

FI

FI

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PI PI PI PI

desalinated water

desalinated water &fuel (e.g. methanol)

oxygen

oxygen

artif

icia

l was

te w

ater

trans

pirin

g w

ater

mea

sure

men

t of a

xial

and

radi

alte

mpe

ratu

re a

nd v

eloc

ity p

rofil

es

gas(chemicalanalysis)

liquid(chemicalanalysis)

cooling water

Fig. 1: Design of the transpiring wall reactor. Fig. 2: Simplified PI-scheme.

Supported by ETH Zurich

References [1] WEBER M., WELLIG B., RUDOLF VON ROHR PH., “SCWO Apparatus Design - Towards

Industrial Availability”, Proc. of Corrosion 99, S.Antonio, Texas, April 25-30 (1999)


19

19. Operating Conditions for a Transpiring Wall Reactor for Supercritical Water Oxidation K. LIEBALL and PH. RUDOLF VON ROHR

Keywords: numerical simulation (CFD), turbulent reactive flow, parameter analysis, nonlinear physical properties, large temperature and density gradients, supercritical water oxidation (SCWO)

Supercritical fluids possess unique physicochemical properties depending on density that can be exploited to provide an adjustable reaction environment. In the last decades, a great number of processes were developed, one of them being the supercritical water oxidation (SCWO). SCWO is a high pressure process for the disposal of toxic and hazardous aqueous wastes. Water in its supercritical state (above 374 °C and 221 bar) is much less polar than in its liquid state and therefore becomes a good solute for organic compounds and gases, and a very poor solute for salts. The one-phased reaction mixture features no phase transfer resistance and fast reaction rates resulting in fast overall conversions. This allows short residence times and therefore small reactor volumes. The reaction temperature is low compared to gaseous combustion so that the effluent remains harmless. Yet SCWO reactors have to cope with two real challenges: the corrosion of the reactor walls and the plugging of reaction vessels due to the precipitation of salts. It is mainly due to these difficulties that SCWO has still not been applied to corrosive and salt containing wastes on an industrial scale. The elaboration of more sophisticated apparatus design and process techniques may overcome the aforementioned problems.

Using the concept of a transpiring wall seems to be promising. It is being explored by several research groups throughout the world (Sandia National Laboratory, CA; Forschungszentrum Karlsruhe, Germany; and ETH Zurich, Switzerland). Through the transpiring wall relatively cold pure water enters the reactor. This convective mass flow forms a protective film between the hot corrosive reaction mixture and the wall. If the temperature of the transpiring water is low enough (preferably subcritical) it is able to redissolve salt particles that might have reached and adhered to the wall.

The temperature is chosen as low as possible yet if it is set too low natural convection cells will prevail in the reactor and the wanted plug flow behavior vanishes. Also for least pure water consumption and cooling of the reaction mixture a minimum transpiration flux is needed. This flux rate is constrained by the performance of the protection of the wall. A parameter analysis using CFD simulations have been performed to determine the influences of bulk flow rate and temperature as well as transpiration flux and temperature on the protection of the wall.

Supported by the Swiss National Science Foundation (SNF). In collaboration with the Massachusetts Institute of Technology (MIT)


20

20. Rotating Biological Contactor for the Degradation of Organic Compounds from Waste Gas I. VINAGE and PH. RUDOLF VON ROHR

Keywords: biological waste gas treatment, biofilm, rotating biological contactor, transport phenomena

Operational problems of biological waste gas treatment facilities such as clogging of trickling filters due to unlimited biofilm growth are well known and have not been resolved yet. Introducing the Rotating Biological Contactor (RBC) system, which is widely spread in waste water treatment, to applications in waste gas purification may eventually overcome these drawbacks. In this reactor, rotating discs, which are partially immerged in water, are used as support for the biofilm. The advantage of this system is the imminent control of biofilm growth due to shear stress caused by disc rotation in the water phase. For the treatment of waste gas, the pollutant has to be absorbed from waste gas to water. Design considerations have addressed the optimization of mass transfer rates through intimate gas-liquid contacting and led to the development of a new, modified RBC system. The simulated waste gas is introduced to the reactor through a hollow shaft, ensuring a

uniform gas supply. The principle of the reactor is shown in Fig. 1. This system exhibits sufficient absorption rates for compounds having low solubility in water because of the constantly renewed thin water film on the discs and the concentration gradient in the water film due to the reaction in the biofilm.

The goal of this work is the experimental investigation and the theoretical description of fundamental aspects of VOC removal in this system combined

with the characterization of the influence of the main operating parameters such as rotational speed of the biofilm support, gas loading and gas flow. Toluene is used as model VOC compound. Degradation results over a long period are illustrated in Fig. 2.

Supported by Colasit AG, Spiez and Kommission für Technik und Innovation (KTI), Bern

volatile products (CO2)VOC, oxygen

biofilm rotating disc

rotating hollow shaft

liquid film

Fig. 1: Principle of the waste RBC

0.4

0.3

0.2

0.1

0.0

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(g/m

2 h)

806040200Time after start-up (days)

35

30

25

20

15

10

5

0

EC (g/m

3h)

Fig. 2: Toluene Elimination Capacity (EC) andSurface Elimination Capacity(SEC) after start-up, gas flow= 1m3/h, disc rotation=6 rpm


21

21. Novel Aspects of Biological Waste Gas Treatment Systems M. STUDER and PH. RUDOLF VON ROHR

Keywords: biofilm, waste gas, clogging

Waste gas streams, which are suited to be cleaned with the aid of biofilms, often have fluctuating concentrations, due to batch processes and shift production. These variations cause a major problem for most biological treatment systems for waste gas. The substrate level can be too low or even too high for an optimal activity of the biofilm, and can also inhibit its growth. The peaks in the load state another difficulty. Waste gas can leave the biological treatment unit, if the concentration exceeds the maximum degradation capacity of the system.

The strong growth of biofilms can cause problems of clogging. It is a consequence of an active biomass. This problem is particularly known for biotrickling filters (BTF). It was analyzed in a previous Ph.D. thesis. Ruediger [1] used computer tomography to examine the biofilm growth in a BTF. The clogged spots are well visible, as gray filled areas in figure 1. Most biological air treatment systems must use means to limit the thickness of the biofilm. A novel

innovative system to reach this task was developed. Vinage investigates a Rotating Biological Contactor (RBC) for waste air treatment (see previous abstract). The system has been proven to limit the growth of the biofilm, but still acts on the most active top layer.

The two main goals of this project are :

• A new internal buffering system will be developed. It will be able to ‘store’ the waste gas and to continuously release it to feed the biofilm, without the need of preceding or down-stream secondary units.

• A formation of the biofilm will be reached, which makes it possible to eliminate the old and for that reason less active or even dead biomass. This will result in limiting the thickness without influencing the top layer.

References [1] RUEDIGER, P.“Abluftreinigung in Biofilmreaktoren mit inerten Trägern,” Diss. Nr. 13229

ETH Zürich (1999)

Fig. 1: Computer tomographic cut through a packingof type Mellapak 250.Y Sulzer (CH) covered withbiofilm. The packing is black, the biofilm after 50 daysof growth gray. The white spots represent hollow spaces [1].


22

22. Quantitative Visualization of Large-Scale Structures in Turbulent Convective Fields

A. GÜNTHER and PH. RUDOLF VON ROHR

Keywords: waves, liquid crystal thermometry, PIV

Fundamentals of a technique for simultaneously measuring 2-D temperature and velocity fields have been developed. Thermochromic liquid crystal (TLC) particles are suspended in the transparent working fluid and change their color as a function of the local fluid temperature. The dependency of the temperature calibration on the fluids refractive index, the angle of illumination, and the properties of the light source have been thoroughly assessed. In combination with particle image velocimetry (PIV), the local velocity and temperature are simultaneously accessible in a plane. The technique is applicable to a wide range of experimental configurations and flow situations. It is first applied to turbulent Rayleigh-Benard convection [1]. From the transient temperature and velocity fields, large-scale flow structures, and local Nusselt numbers can be determined. The second reference situation is the turbulent, fully developed flow over a train of solid waves, see Fig. 1. The considered Reynolds numbers (150-6000 defined with the bulk velocity and the half-channel height) for the water flow between a flat top and a sinusoidal bottom wall are sufficiently low, which allows us to compare our data to results from direct numerical simulations. The wave amplitude, a, is twenty times smaller than the wavelength, Λ; causing partial flow separation behind the wave crests. The flow conditions are similar to ones of Hudson’s [2] single point (LDV) measurements. Whole field measurements are performed to experimentally document the existence of wave-induced longitudinal vortices – produced, e.g., by the Görtler, or the Craik-Leibovich type 2 (CL-2) [3] mechanisms – and their effect on the local transport of momentum and heat. We acknowledge financial support from ETH Zurich. Measurement technology is partially provided by TSI GmbH, Germany

References

[1] GÜNTHER, A., RUDOLF VON ROHR, PH. ASME-NHTC-34, Paper 12049, Pittsburgh (2000) [2] HUDSON J.D., Ph.D. Thesis, University of Illinois, Urbana (1993) [3] PHILIPPS W.R.C., WU Z., J. Fluid Mech. 272, 235-254 (1994)

Fig. 1: Flow over sinusoidal waves. Vectors represent instantaneous velocity fluctuations in a vertical, streamwise oriented plane.


23

23. Droplet Production and Transport from Disintegrating Bubbles above a Gas-Liquid Interface

A. GÜNTHER and PH. RUDOLF VON ROHR

Keywords: multiphase gas-liquid flow, entrainment, PDA

Droplet production from a single bubble that disintegrates at a gas-water interface was assessed in the original work of Blanchard [1]. Our focus is to experimentally determine droplet entrainment at the surface of a boiling pool or bubble column, involving the disintegration of multiple bubbles, and accounting for the role of droplet transport in the gas atmosphere (Fig. 1). A profound understanding of the involved mechanisms is relevant to applications in, e.g., nuclear reactor safety, resuspension in distillation columns, and oceanography. Gas-bubbles rise to the water surface, where they disintegrate and produce small film (F) and larger jet drops (J). Experimental conditions are given by the surface tension, the void fraction and bubble size distribution in the water, and the diameter and velocity distribution of the ejected droplets above the surface. Depending on their size and on the flow conditions in the gas atmosphere, produced droplets are either transported, or their presence is restricted to a sedimentation layer (thickness δ). Cosandey [2] performed integral measurements of the entrained liquid mass. We use phase Doppler anemometry (PDA) to separately address droplet production and transport. Two experimental situations with similar gas velocities and bubble sizes are considered: In a 0.10 m diameter column, air bubbles rise in de-ionized water at atmospheric conditions. The facility is advantageous for detailed PDA measurements in the vicinity of the water surface, i.e. for determining droplet production and δ. Fig. 1 shows a joint diameter-velocity distribution from PDA measurements, which allows to distinguish between regions of film and jet drop production. For most applications, the small film droplets are transported, whereas the gas velocities are not sufficient to transport the larger jet drops. Above a 0.6 m diameter boiling water pool that is located inside an insulated pressure vessel (volume 5m3), measurements with the system steam-water are performed at saturation conditions and elevated pressures. Financial support from PSEL is acknowledged. Measurement technology is partially provided by TSI GmbH, Germany.

References [1] BLANCHARD D.C., Progress in oceanography. 1, 71-202 (1963) [2] COSANDEY J.O., Diss. ETH No.13414, ETH Zurich, Zurich (1999)

Fig. 1: Production of film (F) and jet (J) drops from PDA measurements at a location 10 mm above the surface.


24

24. Tubular Reactor for Reactions in Liquid-Gas-Mixtures P. STAHL and PH. RUDOLF VON ROHR

Keywords: tubular reactor, multiphase flow, gamma-densitometry

Tubular plug-flow reactors with small diameter are recommendable in case of critical chemical reactions, as small reaction volumes and a narrow residence time distribution allow to gain a proper control and a reduction of the hazard potential.

Within this project a horizontal tubular reactor for gas releasing liquid-reactions is designed and modelled for turbulent flow. The physical processes within the reactor are described by a numerical calculation program, which takes into account both the fluid dynamic phenomena of two-phase flow and the reaction kinetics found in suitable laboratory experiments. Its results are verified in a pilot facility, designed as a glass double-tube of 5 m length and composed of modular elements. Measuring points for temperature, pressure and void fraction are provided between the modules. The tube jacket serves for temperature controlling by heat exchange, the reaction itself takes place in the inner reactor tube, which has a diameter of 7 mm.

Figure 1 shows the comparison between calculation and measurement for the model reaction 2 MnO4

- + 6 H+ + 5 H2O2 → 2 Mn2+ + 8 H2O + 5 O2. The experimental conditions can be taken from the caption.

Fig. 1: Pressure, Liquid void fraction and Gas mass flow along the reactor tube for reaction with ML0* = 0.086 kg/s liquid inflow, thereof KMnO4-fraction: 2,63 g/kg solution, H2O2-fraction: 3,79 g/kg solution and H2SO4-fraction: 11,05 g/kg solution. Inlet temperature: T0 = 16,3 °C.

The void fraction is measured by gamma-densitometry.

Supported by Emil Barell Foundation, Basel, CH

11.11.21.31.41.51.61.71.81.9

2

0 1000 2000 3000 4000 5000Length [mm]

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sure

[bar

]

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0.2

0.4

0.6

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1.2

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Fl.-H

old-

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as-M

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[g/s

]


25

25. Plasma Enhanced Coating of Powders M. KARCHES and PH. RUDOLF VON ROHR

Keywords: powder, plasma, coating, CFB

Thin film technologies are widely used to modify surface properties. Improvements may involve e.g. hardness, abrasion, adhesion, permeability, refractive index, catalytic activity or biocompatibility. Taking different film technologies into consideration, chemical vapour deposition (CVD) is particularly promising. By enhancing the deposition process with a low temperature, non-equilibrium plasma, the necessary reaction temperatures are much lower. Coating powder materials is a promising way to form novel and valuable products with special optical effects, controlled release of active substances, catalytic activity or high chemical resistance.

This project combines Plasma-CVD for uniform, high-rate and low-temperature coating with the fluidized bed technique for intense mass transfer. A Circulating Fluidized Bed with about 1 kg of powder is operated under vacuum and plasma is generated in the riser tube by coupling microwaves (Fig. 1). As a model application for mechanistic studies, NaCl crystals are coated with silica films, using HMDSO as precursor and O2 as oxidant [1]. Glass beads are coated with ultrathin titania films for photocatalytic applications. Abrasive particles are saved from high temperature decomposition by alumina barrier films.

At optimum operating conditions, reactor flow is in fast-fluidized regime with 1-3% solid loading. The required microwave power for stable plasma generation increases with the oxygen partial pressure. Greater than 80 % film yield is achieved for precursor-limited conditions and up to 1 g/min SiO2 has been deposited. Particle temperature can be controlled well below 80°C, but hot spots at the reactor wall can cause higher temperatures with wall-adhesive materials. The residence time distribution in the CFB is described and modeled to predict treatment uniformity.

The plasma reactor is also used to improve the wettability of hydrophobic polymer particles. Accurate dosage and dispersion for an efficient treatment of cohesive powders is achieved using a vacuum-screw feeder. A few seconds treatment in air plasma are sufficient to generate the required hydrophilic property. The obtained powder can be dispersed in water without additional surfactants. References [1] KARCHES M. et al., “A Circulating Fluidized Bed for PECVD on Powders at low

temperatures”, Surface & Coating Tech. 116-119, 879-885 (1998)

to vacuumunit

process gas

L-valve

cyclone

viewport

plasmasource

Fig. 1: Plasma-CFB


26

26. Novel Coating Process for Fine Powders B. BORER and PH. RUDOLF VON ROHR

Keywords: powder, plasma, coating, CFB, PECVD

In a previous project, powder materials could be successfully coated with various thin films in a Circulating Fluidized Bed (CFB) using a low-temperature, non-equilibrium plasma, which was generated in the riser by coupling microwaves. This unique combination of a CFB and the Plasma Enhanced Chemical Vapor Deposition (PECVD) leads to a uniform, high-rate and low-temperature coating process at film yields up to 80% [1].

However at certain process conditions the coupling of the microwaves was not ideal which resulted in unstable plasma. Furthermore fine, cohesive powders plugged up the reactor tubes, thus the coating-process stopped after a short period of time.

The goal of this project is to improve the given CFB facility for increased operational ranges (pressure, plasma power, particle diameter, etc). In a first step, a radio frequency (RF) source will be applied to the reactor. The advantages of a RF-source compared to a microwave-source are:

• higher operating flexibility,

• more homogeneous plasma prevents hot spots in the reaction zone,

• easier plasma analysis due to the open antenna geometry.

The optimization of the fluid-dynamic design and the use of adapted dispersing equipment will allow an operation even with fine, cohesive powders (dp < 20 µm).

In a second step of the project the PECVD process will be theoretically and experimentally investigated in a downer reactor. The uniform and steady state flow pattern and the narrow residence time distribution of this reactor type facilitate the characterization of the plasma process properties.

Another issue of the project is the application of a pulsed RF-source, which allows stable plasma at very low power densities. At this operating mode it is expected to improve the treatment of temperature sensitive materials. References [1] KARCHES M. et al., “A Circulating Fluidized Bed for PECVD on Powders at low

temperatures”, Surface & Coating Tech. 116-119, 879-885 (1998)

Fig. 1: Downer Reactor.

to vacuum unit

process gas

RF-

plas

ma

reac

tor

product

screw feeder

cyclone

powder storage


27

27. Plasma Enhanced Chemical Vapor Deposition using a Pulsed Microwave Plasma E. BAPIN and. PH. RUDOLF VON ROHR

Keywords: PECVD, pulsed plasma, plasma diagnostics, thin films

Plasma enhanced chemical vapor deposition (PECVD) is an established technique for the deposition of silicon dioxide films and has wide ranging applications in microelectronics, integrated circuit manufacturing and optics. Organosilicone/O2 plasmas tend to be used instead of SiH4/O2 or SiH4/N2O, especially on an industrial scale: they yield better step coverage and are safer and less toxic. Moreover, they offer the possibility of varying the carbon content of the film by changing the process conditions. Very low C and H-content SiO2 with outstanding dielectric properties can be deposited as well as plasma polymerized SiOxCyHz, which are known to be efficient membranes and vapor barriers.

The successful synthesis of plasma-generated thin films for specific end-uses requires mastering control of their morphology, chemical composition and of their deposition rate. This is achieved by a careful selection of the process parameters. Pulsing the plasma discharges also opens up promising perspectives in the control film growth kinetics. It may induce an alteration of the discharge chemistry which can lead to changes in the properties of the deposited films [1,2]. Still, few results are available in the literature about the deposition of SiO2 with pulsed microwave plasmas [3].

The microwave PECVD of SiO2 from oxygen/ TEOS and oxygen/HMDSO is addressed under continuous wave (CW) and pulsed modes. For this purpose, a new reactor equipped with optical emission spectroscopy and Langmuir probe has been built. The influence of the reactor geometry and of the main process parameters on the film characteristics has been investigated in the CW mode. The study focuses especially on the film deposition rate, chemical composition and on its morphology. Gas phase analysis and plasma diagnostics have been performed and reveal useful insights on the deposition process. Films, which are deposited using pulsed plasmas of equivalent power, are compared. Special attention is devoted to the role of the pulse frequency, and the duty cycle. Pulsed plasmas with an equivalent average power are found to improve the film morphology and, under given conditions, to increase the deposition rate. References [1] WATANABE Y. et al., Appl. Phys. Lett,. 53, 1263-1265 (1988) [2] SUGAI H. et al., J. Vac. Sci. Technol., A13, 897-893 (1995) [3] BROCKHAUS A. et al., Contrib. Plasma Phys., 39, 399-409 (1999)

pump outlet

substrate

emissionspectroscopy

massspectrometry

microwavering resonator

carriergas inlet

monomer gasinlet

Fig. 1: Scheme of the diagnostics mounted on the PECVD reactor


28

28. Deposition of Silicon Oxide Thin Films on Paper using Plasma Enhanced Coating Technologies A. GRÜNIGER and PH. RUDOLF VON ROHR

Keywords: plasma enhanced chemical vapour deposition (PECVD), silicon oxide, thin films, packaging material, paper, gas permeation

The demand for highly functional packaging materials, for instance in food or pharmaceutical industry, is increasing. However, additional to functionality, ecological aspects gain more and more importance. The aim of this project is to assess the scientific fundamentals to combine the ecological advantages of paper with the good gas barrier performance of silicon oxide thin films. These films offer several advantages: they are transparent, colorless, microwaveable and do not affect the paper recyclability. Moreover, they absorb UV radiation and can be sterilized without any major change in their properties.

Fig. 1: Layer structure of SiOx-coated paper (SEM-Graph)

Due to the possibility of operating in a low temperature range, Plasma Enhanced Chemical Vapor Deposition (PECVD) is a suitable technique for the coating of temperature sensitive materials such as paper or PET. The deposition of silicon oxide on PET has already been successfully carried out with a PECVD-Reactor built at our Institute.

Because of its complex morphology the deposition of a coherent film on paper is difficult. A proper pretreatment or even precoating by conventional methods is essential for a successful plasma coating. An important aspect in this application-oriented project is the investigation of paper and coating morphology and its influence on gas permeation through the layers. The main criterion is the improvement of the gas barrier performance, especially against oxygen and water vapour permeation.

Supported by Cham Paper Group, Cham and Kommision für Technik und Innovation KTI, Bern

10 µm 1 µm

SiOx-Layer

Pigmented Coating

Basis Paper

Advanced Separation Processes

29

29. On-Line Control of Batch Cooling Crystallization J. WORLITSCHEK and M. MAZZOTTI

Keywords: cooling crystallization, model predictive control, on-line monitoring, particle size distribution

Batch crystallizers are used extensively in chemical industry, often for small scale production of high value specialty chemicals. The control objectives of batch crystallization processes are defined in terms of product purity, crystal habit or morphology, average particle size, particle size distribution (PSD), bulk density, product filterability and dry solids flow properties. Most of previous control studies have dealt with finding the open-loop, supersaturation versus time trajectory that allows to obtain the desired PSD [1]. Uncertainties in parameter estimation, inaccuracies of the model and process disturbances have to be taken into account for optimal design and on-line control strategies of crystallizers. The main issues to be addressed are model parameter estimation, development of nonlinear control strategies and on-line monitoring of PSD and supersaturation.

The main objective of the project is the design of a nonlinear model predictive control system for batch cooling crystallizers and its experimental application to a model system. A one-liter batch cooling crystallizer is used for the experimental investigation. A deterministic population balance model accounting for solution thermodynamics, crystal growth, primary and secondary nucleation has been developed so far and solved

by means of the software package Parsival. Figure 2 shows the simulation of the PSD profile for a seeded batch cooling crystallization of paracetamol in ethanol. A linear cooling policy is applied and the estimated growth and nucleation parameters are used. Growth of the seeds, and secondary nucleation with formation of new particles can be observed.

References [1] RAWLINGS J. B., MILLER S. M., WITKOWSKI W.R., “Model Identification and Control of

Solution Crystallization Processes: A Review“, Ind. Eng. Chem. Res., 32, 1275-1296, (1993)

Fig. 1: Experimental Set-up

Fig. 2: Simulation of a batch cooling crystallization


30

30. Monitoring of Particle Size Distribution in Crystallization from Solution J. WORLITSCHEK and M. MAZZOTTI

Keywords: crystallization, particle size distribution, on-line monitoring, Focused Beam Reflectance Measurement FBRM, ultrasound spectroscopy

The most important limitation to optimization and control of crystallization processes is represented by the intrinsic difficulty of obtaining an accurate state estimation. On-line monitoring of particle size distribution (PSD), which is closely related to product quality and process productivity, has therefore become an important issue. In this project, two promising techniques for on-line and in-situ measurements of the PSD are considered, namely Focused Beam Reflectance Measurement (FBRM) and ultrasound spectroscopy.

The basic principle of the FBRM method is given in figure 1, where it is shown that it measures the chord length distribution (CLD) of a particle population. In this project first, a model has been introduced that allows to transform a known PSD into the corresponding CLD [1]. Then, a special technique to measure the CLD of single particles has also been developed, as a tool to gain a better understanding of FBRM features and potential (see Figure 2).

An alternative to FBRM is provided by ultrasound spectroscopy. This has been shown by estimating successfully both size and concentration of the growing crystals during a seeded cooling crystallization of potassium sulfate in aqueous solution using the measurement of the acoustic attenuation spectra [2]. References [1] RUF A., WORLITSCHEK J., MAZZOTTI M., “Modeling and Experimental Analysis of PSD

Measurements through the FBRM Method“, Part. Part. Syst. Charact. 17, 168 - 180 (2000). [2] HIPP A.K., WALKER B., MAZZOTTI M., MORBIDELLI M. “In Situ Monitoring of Batch

Crystallization by Ultrasound Spectroscopy“, Ind. Eng. Chem. Res. 39, 783-789 (2000)

1000Chord length s [µm]

Cou

nts

per s

ec

CPS

100100.8

crossinglaser beam vb

s = ∆ t vb

chord(a) (b)

(c)

Fig. 1: Basic principle of the FBRM method

0µm

x [ µµ µµ

m]

-500

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500

1000

(1)(2)

(3)

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y [µµµµm]0 500 1000 1500 2000 2500 3000 3500 4000

354µm 595µm

420µm

500µm297µm

250µm

Fig. 2: Contour plot of the FBRM measurement of a single ceramic sphere in water [1].


31

31. Gas Antisolvent Recrystallization of Specialty Chemicals G. MUHRER, C. LIN, W. DÖRFLER and M. MAZZOTTI

Keywords: micronization, recrystallization, compressed fluids, GAS process

Among the high pressure gas assisted particle formation technologies that are feasible to match the increasing demand for industrial manufacturing of micron or submicron size solid particles, antisolvent techniques are considered particularly promising for the micronization of specialty chemicals and pharmaceuticals, including proteins [1,2].

These techniques allow microparticles with controlled particle size distribution (PSD) and product quality to be produced under mild and inert conditions. The batch GAS process (see the high-pressure precipitator in Figure 1) exploits the low solubility of most pharmaceutical compounds in CO2, which is used as antisolvent for the solute initially solubilized in an organic solvent. Upon addition of high pressure CO2, the solution is expanded, its solvent power is reduced, and precipitation is triggered. Recently the effect of the carbon dioxide addition rate has been thoroughly investigated (cf. Figure 2 for an example of the product obtained) and a conceptual explanation of it has been presented [3].

In this project, we study the effect of antisolvent addition rate, precipitation temperature, initial solution concentration and volume, and choice of organic solvent on the particle size distribution and morphology of the final product. Current experimental work addresses the precipitation of phenanthrene and acetaminophen from different organic solvents using compressed CO2 as antisolvent. Future work will also include polymers and proteins.

Supported by ETH Zurich partly through grant TH-22./99-1

References [1] SUBRAMANIAM B., RAJEWSKI R.A., SNAVELY K., “Pharmaceutical processing with

supercritical carbon dioxide“, J. Pharm. Sci. 86, 885-890 (1997) [2] REVERCHON E., “Supercritical antisolvent precipitation of micro- and nanoparticles“, J.

Supercrit. Fluids 15, 1-21 (1999) [3] MÜLLER M., MEIER U., KESSLER A., MAZZOTTI M., “Precipitation of an organic using

compressed carbon dioxide as antisolvent“, Ind. Eng. Chem. Res. 39, 2260-2268 (2000)

Fig. 1: Laboratory scale high pressure GAS recrystallization vessel

Fig. 2: SEM micrograph of spherical particles precipitated from ethanol at 25°C and a CO2 addition rate of 220 ml/min [3]


32

32. Modeling Gas Antisolvent Recrystallization G. MUHRER and M. MAZZOTTI

Keywords: GAS process, modeling, nucleation, growth, population balances

In order to deepen the understanding of the interplay of the various physical phenomena governing Gas Antisolvent recrystallization (GAS), mathematical modeling is required besides ongoing experimental assessment of the influence of key operating parameters. A suitable model for the GAS process has to account for thermodynamics of near-critical metastable solutions, gas-liquid and liquid-solid mass transfer, micro- and macro-mixing, and crystallization kinetics. The latter involve primary and secondary nucleation, crystal growth, agglomeration, and breakage and are of key importance as all these phenomena influence each other and largely determine the final product properties, especially the particle size distribution [1]. A mathematical model for GAS recrystallization has been developed decoupling volume expansion upon CO2 addition and population balances governing the particle formation process. Volumetric expansion is described using the Peng-Robinson EOS, and assuming thermodynamic equilibrium and negligible mass transfer resistance. Thermodynamic data refer to the model system toluene/phenanthrene/CO2, and nucleation and growth rates are estimated using predictive models from crystallization theory [1].

As shown in Fig. 1., model results qualitatively confirm the experimental trend that increasing the CO2 addition rate yields smaller particles [2]. In the presence of significant secondary nucleation, bimodal particle size may be obtained with the model as well.

This project is supported by ETH Zurich partly through Grant TH-22./99-1

References [1] MERSMANN, A., BARTOSCH, K., BRAUN, B., EBLE, A., HEYER, C., “Möglichkeiten einer

vorhersagenden Abschätzung der Kristallisationskinetik”, Chem. Ing. Tech. 16, 17-30 (2000) [2] MÜLLER M., MEIER, U., KESSLER, A., MAZZOTTI, M., “Precipitation of an organic using

compressed carbon dioxide as antisolvent“, Ind. Eng. Chem. Res. 39, 2260-2268 (2000)

0.1

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T = 25°C

Fig 1: Effect of addition rate on product mean particle size in experiments (left) and simulations (right).


33

33. Mass Transfer Effects in GAS Recrystallization C. LIN, G. MUHRER and M. MAZZOTTI

Keywords: GAS process, modeling, mass transfer

In the Gas AntiSolvent recrystallization process the compressed gas anti-solvent addition rate is one of the key parameters in controlling the final particle size distribution. What is important is how fast the anti-solvent is added to the solution, which depends on both the anti-solvent feed flow rate and the mass transfer rate within the crystallization vessel. So far, mathematical modeling of GAS recrystallization reported in the literature has addressed phase equilibrium thermodynamics mostly [1]. In order to improve the understanding of the GAS process, the aim of this project is to include mass transfer effects into the GAS model that has been developed in our group.

Under the assumption of well-mixed bulk liquid and gas phases, a set of material balance equations is used to describe the CO2 mass flux between the two phases. Since both the mole fraction of the antisolvent in the liquid phase and the mass fluxes may be rather large, the two film theory for large fluxes has been used. A dimensionless correlation for the mass transfer coefficients on the liquid side has been developed, which accounts for either bubbling aeration or surface aeration depending on the CO2 feed position. Model parameters can be estimated from pressure relaxation experiments at different stirring rates (see Fig. 1). The model accounts also for the variation of system properties such as pressure, density, viscosity, and liquid volume typically observed during GAS operation. The effect of mass transfer limitations on the particle size distribution of the final product is currently explored through both simulations and experiments.

This project is supported by ETH Zurich partly through Grant TH-22./99-1

References [1] KIKIC I., LORA M., AND BERTUCCO A., “A thermodynamic analysis of three-phase equilibria

in binary and ternary systems for applications in RESS, PGSS, and SAS”, Ind. Eng. Chem. Res. 35, 5507-5515 (1997)

60

50

40

30

20

10

0

Pres

sure

(bar

)

2000150010005000Time (s)

N=500 rpm; M=10 g/mina) Simulated result; b) experimental data

60

50

40

30

20

10

0

Pres

sure

(bar

)

2000150010005000Time (s)

N=100 rpm; M=10 g/mina) Simulated result;b) experimental data

Fig.1: Pressure build-up in a closed vessel during carbon dioxide addition to a toluene solution at different stirring rates: comparison between experimental data and simulations.


34

34. Scale-up of Precipitation Processes L. VICUM and M. MAZZOTTI

Keywords: precipitation, mixing, scale-up, population balance, particle size distribution

Precipitation is the rapid crystallization of sparingly soluble materials, usually as a result of a chemical reaction or of physical changes in a solution. Although it is widely used in the chemical, fine chemical and pharmaceutical industry, general criteria for its scale-up do not exist.

This is mainly due to the difficulty in modeling precipitation, since several complex phenomena are involved. These include mixing, from the micro-, to the meso- and to the macroscale, chemical reaction, particle nucleation and growth, agglomeration and breakage. All these are coupled and have widely different time constants and length scales. In general, however, supersaturation is so large that nucleation is faster than mixing, which is then the controlling mechanism for the particle properties, i.e. particle size distribution and morphology. Under these circumstances all proposed models and design criteria have been unsuccessful when applied to precipitation scale-up [1,2,3].

The aim of this project is to enhance the understanding of precipitation, with particular focus on the process scale-up. Using laboratory-scale experiments, kinetic parameters for nucleation, growth and secondary effects such as agglomeration are determined. Based on these process parameters, population balances can be used to describe the particle formation. The effect of mixing on the course of precipitation is accounted for by coupling the population balance equations with the mixing model on the microscale, since this is where precipitation and mixing interact [4]. Using such a model will allow for the selection of the process operating parameters based on the information extracted from laboratory scale experiments and on information about the geometry and mixing characteristics of the final crystallizer. Following this approach, it will be possible to reduce the experimental effort for a successful scale-up of precipitation processes.

References [1] BALDYGA J., PODGORSKA W., et al. “Mixing-precipitation model with application to double

feed semibatch precipitation” Chem. Eng. Sci., 50 1281 (1995) [2] FRANKE J., MERSMANN A., “The influence of operational conditions on the precipitation

process”, Chem. Eng. Sci., 50 1737 (1995) [3] ZAUNER R., JONES A. G., “Scale-up of continuous and semibatch precipitation processes”,

Ind. Eng. Chem. Res., 39 2392 (2000) [4] BALDYGA J., BOURNE, J. R., “Turbulent mixing and chemical reations”, John Willey & Sons,

New York (1999)


35

35. Experimental Investigation of Complex Nonlinear Dynamics in Homogeneous Azeotropic Distillation M. MORARI* and M. MAZZOTTI

Keywords: azeotropic distillation, nonlinear dynamics, limit cycle

In the case of homogeneous azeotropic distillation, the occurrence of complex, nonlinear dynamic phenomena such as steady state multiplicity and sustained oscillations, has been recently demonstrated through extensive simulations and thorough theoretical investigation [1,2]. Steady state multiplicity has also been confirmed experimentally [3], and it has been shown that operation in the unstable steady state is possible through the action of a properly designed control system, with potential benefit in terms of separation performance [4]. The occurrence of the limit cycles shown in Fig. 1 in homogeneous azeotropic distillation lacks of experimental confirmation so far, and the goal of this project is to provide such an assessment, as well as an evaluation of its relevance from the viewpoint of industrial operation.

Fig.1: Calculated bifurcation diagram for the system acetone/benzene/n-heptane for a column configuration where the flow rates of the vapor from the top and of the distillate are controlled. The distillate flow rate is the bifurcation parameter. The system exhibits steady state multiplicity with an unstable intermediate steady state. The upper branch undergoes two supercritical Hopf bifurcations yielding limit cycle behavior in a rather large range of distillate flow rates. A homoclinic bifurcation can also be observed where the limit cycle collides with the intermediate s.s. branch [2].

This poses several scientific and technical challenges, which deserve to be tackled in three successive steps: (1) selection of a suitable experimental system and of feasible operating conditions; (2) design and build-up of an experimental set-up that allowsoscillations to be developed and to be observed; (3) experimental investigation and final assessment.

References [1] BEKIARIS N., MESKI G.A., RADU C.M., MORARI M., “Multiple steady states in

homogeneous azeotropic distillation“, Ind. Eng. Chem. Res. 32, 2023-2038 (1993) [2] LEE M., DORN C., MESKI G.A., MORARI M., “Limit cycles in homogeneous azeotropic

distillation“, Ind. Eng. Chem. Res. 38, 2012-2027 (1999) [3] GÜTTINGER T., DORN C., MORARI M., “Experimental study of multiple steady states in

homogeneous azeotropic distillation“, Ind. Eng. Chem. Res. 36, 794-802 (1997) [4] DORN C., GÜTTINGER T., WELLS J., MORARI M., KIENLE A., KLEIN E., GILLES E.D.,

“Stabilization of an azeotropic distillation column“, Ind. Eng. Chem. Res. 38, 506-515 (1998)

* Institut für Automatik, ETH Zürich


36

36. Design and Operation of Simulated Moving Bed Processes for Fine Chemical and Pharmaceutical Separations S. ABEL , M. MAZZOTTI, M. MORBIDELLI* and M. MORARI °

Keywords: chromatography, multi-component separations, Simulated Moving Bed, process control

Nowadays, the Simulated Moving Bed (SMB) technology is adopted in the food, agrochemical, pharmaceutical and biotechnology industries for difficult applications, such as the resolution of racemates, and it is considered attractive for complex separation tasks, such as bio-separations or separations of natural mixtures involving a number of difficult-to-characterize compounds [1]. Therefore, there are now a large number of potential small-scale applications of the SMB technology that call for a new SMB paradigm, which exploits the flexibility and versatility of the technology. Proper implementation of SMBs in production will require the application of robust control techniques [2]. The issue of process control under uncertainties, e.g. the competitive adsorption behavior of multi-component mixtures that can never be measured precisely, will also have to be addressed [3]. SMBs are constituted of several chromatographic columns with inlets and outlets, whose position within the column carousel switches periodically. Therefore SMBs reach only a cyclic steady state, where compositions change periodically and exhibit nonlinear dynamics with dead-times and lengthy analytic techniques for product quality assessment. These features pose fundamental questions and challenges on both SMB technology and control theory. The objective of this project will be achieved by pursuing the following specific goals: • synthesis of a new SMB paradigm and development of the related modeling tools,

e.g. an SMB virtual platform; • development of an SMB control concept and realization and assessment of the

control system; • realization of an experimental SMB set-up for testing and assessing the new

paradigm and the control strategy. Supported by ETH Zurich partly through grant TH-23’/00-1

References [1] JUZA M., MAZZOTTI M., MORBIDELLI M., ”Simulated moving-bed chromatography and its

application to chirotechnology”, Trends Biotechnol. 18, 108-118 (2000) [2] MORARI M. , ZAFIRION E., “Robust process control”, Prentice Hall, Englewood Cliffs (1989) [3] KLOPPENBURG E., GILLES E.D., “Automatic control of the simulated moving bed process for

C8 aromatics separation using asymptotically exact input /output-linearization”, J. Process Contr. 9, 41-50 (1998)

* Laboratorium für Technische Chemie, ETH Zürich; ° Institut für Automatik, ETH Zürich


37

37. Gradient Mode Operation of Simulated Moving Beds S. ABEL, M. MAZZOTTI and M. MORBIDELLI*

Keywords: solvent gradient, temperature gradient, SMB The SMB technology has already shown great potential for various separations, e.g. enantioseparations. It is possible to further improve SMB performance if each section is optimized independently. To this aim, an adsorption strength gradient along the unit would be needed. In section 1 of the SMB (see Fig. 1) a rather weak adsorption is preferable, because all species have to be desorbed; then the adsorption strength should increase up to section 4, where all species have to be adsorbed. The adsorption strength gradient can be realized in different ways. One option is to operate the sections at different temperatures (temperature gradient mode [1]); another one is to operate with a supercritical solvent and apply a pressure gradient [2]. A further possibility is to use different mobile phase compositions in the different sections (solvent gradient mode [3]). This is realized by feeding streams with a different solvent composition at the feed and desorbent inlet, respectively. In this case a step gradient between the two inlets would be generated. It is the aim of this project to develop a design method for these gradient mode operation of the SMB, and to compare the gradient and the isocratic operating mode, where temperature and mobile phase composition are constant, in terms of productivity and desorbent consumption. For design purposes a simplified model is used where mass transfer resistance and axial dispersion are neglected, i.e. the Equilibrium Theory model. This approach yields the so-called Triangle Theory, which – when properly extended to the gradient mode SMB - provides a rather accurate prediction of the optimal operating conditions for the desired separation and a qualitative and quantitative explanation of the main features of SMBs. Supported by ETH Zurich partly through grant TH-23’/00-1 References [1] MIGLIORINI C., WENDLINGER M., MAZZOTTI M., “Temperature gradient operation of a

simulated moving bed unit”, Ind. Eng. Chem., in press (2001) [2] MAZZOTTI M., STORTI G., MORBIDELLI M., “Supercritical fluid simulated moving bed

chromatography”, J. Chromatog. A 786, 309-320 (1997) [3] ABEL S., MAZZOTTI M., MORBIDELLI M., “Solvent gradient operation of Simulated Moving

Beds”, submitted to J. Chromatogr. A (2001) * Laboratorium für Technische Chemie, ETH Zürich

FeedA,B,D

DesorbendD

RaffinatB,D

ExtractA,D

Direction of fluidflow and port

switching

Section 1

Section 3

Section 4Section 2

Fig.1: Scheme of an SMB


38

Fig. 1: Desk-top SMB unit (modified ÄKTA Explorer)

38. Bio-Separations using Simulated Moving Beds S. ABEL, J. STADLER * and M. MAZZOTTI

Keywords: Bio-separations, SMB, nucleosides, plasmid DNA

Chromatography is a key step for clean, high purity bio-separation processes. The continuous simulated moving bed technology is a successful answer to scale-up needs, since it overcomes some of the limitations of single-column preparative chromatography [1]. Design and optimization tools developed for SMB enantioseparations are now available to support the application of SMBs to bio-separations.

In this context, the objective of the project is twofold. On the one hand we aim at proving the possibility of using a modified ÄKTA system (from AP Biotech AB, Uppsala, Sweden) as a desktop SMB unit. The latter will extend the capabilities of the former, and allow for fast and reliable development of new SMB bio-separations.

On the other hand, the new experimental set-up will be tested on two important applications. The first one is focussing on plasmid DNA purification by using size exclusion chromatography, which could be one out of at least two chromatographic steps. The preparation of large quantities of plasmids, free of chromosomal DNA, RNA and endotoxins is of interest for gene therapy [2]. The second model application is the four-component separation of nucleosides (deriving from DNA digests), i.e. the DNA building blocks, on a polystyrene divinylbenzene resin. Nucleosides are needed at extremely high purity for synthesizing DNA used to engineer new proteins, for instance. Design and simulation tools for SMB, as well as gradient mode and cleaning in place implementation of the SMB concept will be exploited [3]. References [1] JUZA M., MAZZOTTI M., MORBIDELLI M., “Simulated moving-bed chromatography and its

application to chirotechnology “,Trends Biotechnol. 18, 108-118 (2000) [2] OLLIVIER M., STADLER J., “Large scale purification of plasmid DNA for use in gene therapy“,

in Gene therapy of cancer, Walden et al. eds, Plenum Press, New York, (1998) [3] ABEL S., MAZZOTTI M., MORBIDELLI M., “Solvent gradient operation of Simulated Moving

Beds”, submitted to J. Chromatogr A (2001) *AP Biotech Europe, Freiburg, Germany


39

(+) enantiomer (-) enantiomer

solvent

M1

FRE

M2

Fig.1: Typical phase diagram for the ternary system constituted of the two enantiomers and the solvent.

39. Enantioseparations through SMB and Crystallization S. ABEL, J. WORLITSCHEK and M. MAZZOTTI

Keywords: Simulated Moving Bed, crystallization, enantiomers

The Simulated Moving Bed (SMB) technology is becoming an established technique for the high purity separation of enantiomers, using stable but expensive chiral stationary phases (CSPs) [1]. SMB productivity, i.e. the amount of separated racemate per unit mass of CSP, decreases sharply when increasing the purity requirement [2].

High enantiomeric purity can also be achieved in a single stage by crystallization, provided that the feed consists of a non-racemic mixture. Figure 1 illustrates a typical ternary phase diagram [3].

Combining a low-purity, high-productivity SMB step with successive crystallization (one stage for each enantiomer, as shown in Fig. 2) may drastically reduce the CSP demand, improve process economics, and achieve the high purity required by drug production [4]. Issues to be addressed within this project will be: system characterization, process optimization, and experimental verification of the concept. Histidine hydrocloride in water, using Crownpack CR(+) as CSP, is being used as model system.

References [1] JUZA M., MAZZOTTI M., MORBIDELLI M., “Simulated moving-bed chromatography and its

application to chirotechnology“, Trends Biotechnol. 18, 108-118 (2000) [2] BIRESSI G., LUDEMANN-HOMBOURGER O., MAZZOTTI M., NICOUD R.-M., MORBIDELLI M.,

“Design and optimization of a SMB unit: role of deviations from equilibrium theory“, J. Chromatogr. A 876, 3-15 (2000)

[3] COLLET A., “Separation and purification of enantiomers by crystallisation methods“, Enantiomer 4, 157-172 (1999)

[4] LORENZ H., SHEEHAN P., SEIDEL-MORGENSTERN A., “Coupling of simulated moving bed chromatography and fractional crystallisation for efficient enantioseparation“, J. Chromatogr. A 908, 201-214 (2001)

M1

crystals of (+)

desorbent

E

R

M2

SMB

crystals of (-)

F

Fig. 2: Combined SMB and crystallization process.


40

40. Separation of Enantiomers through Continuous Supercritical Fluid Simulated Moving Bed (SF-SMB) Chromatography A. RAJENDRAN, M. MAZZOTTI, R. M. NICOUD# and M. MORBIDELLI*

Keywords: supercritical fluids, enantiomers, simulated moving bed, carbon dioxide

There are two main advantages in replacing the traditionally used organic eluent with a supercritical fluid, in particular CO2: 1) the easy separation of products and solvent and full compatibility with any product bound to be used on humans; 2) the easy tunability of properties, such as density, viscosity and solubility.

The main performance improvements are due to the possibility of tuning the elution strength of the mobile phase. In particular a SF-SMB can be operated in two modes:

- isocratic mode, where pressure in the 4 sections of the unit is uniform;

- pressure gradient mode, where different pressure levels are kept in the four sections, by using back-pressure valves. Since the adsorption strength decreases with density, the decreasing pressure gradient yields also a decreasing elution strength gradient. The elution strength is maximum in section 1, where the more retained component must be eluted, and it is minimum in section 4, where also the weakest component must be retained by the stationary phase [1, 2].

The tetralol enantioseparation on Chiralcel OD (Daicel) using supercritical CO2 modified with ethanol as eluent has been carried out in a pilot scale SF-SMB unit at NOVASEP [3]. It is the first time that a chiral separation is carried out in a SMB that uses a supercritical fluid as eluent. The complete separation of the tetralol enantiomers has been achieved, operating both in isocratic and pressure gradient mode. The productivity was found to be three times higher for the case in which a pressure step of about 50 bar was imposed between sections 2 and 3, with respect to the case where the pressure was kept constant. These results are consistent with theoretical predictions, thus paving way for further process improvement and performance optimisation. Supported partly by the Swiss National Science Foundation through Grant SNF 21-55674.98

References [1] MAZZOTTI M., STORTI G., MORBIDELLI M., “Supercritical fluid simulated moving bed

chromatography.”, J. Chromatogr. A, 786, 309-320 (1997) [2] DI GIOVANNI O., MAZZOTTI M., MORBIDELLI M., DENET F., HAUCK W., NICOUD R. M.,

“Supercritical fluid simulated moving bed chromatography. II. Langmuir isotherm.”, submitted to J. Chromatogr. A (2000)

[3] DENET F., HAUCK W., NICOUD R. M., DI GIOVANNI O., MAZZOTTI M., JAUBERT J. N., MORBIDELLI M., “Enantioseparation through supercritical fluid Simulated Moving Bed (SF-SMB) chromatography.”, submitted to Ind. Eng. Chem. Res. (2000)

#NOVASEP, Vandoeuvre-les Nancy, France; *Laboratorium für Technische Chemie, ETH Zürich


41

41. Thermodynamics of Supercritical Adsorption A. RAJENDRAN, O. DI GIOVANNI, W. DÖRFLER, M. MAZZOTTI and M. MORBIDELLI*

Keywords: supercritical fluids, adsorption, critical phenomena, excess properties, carbon dioxide

Supercritical fluid chromatography represents a powerful technique to match the need of pharmaceutical, agrochemical and food industries for purer products and cleaner processes, which arises from stricter regulations and growing environmental concerns. Its rational design requires the characterisation of the adsorption equilibria of the components to be separated in the supercritical solvent, often in the presence of a polar modifier to enhance solubility. Experiments demonstrate that the adsorption isotherm of a solute in sc-CO2 depends on the total pressure. In particular the solute loading on the stationary phase decreases, when increasing the density of the mobile phase. This behavior can be explained by considering carbon dioxide competitive adsorption.

To this aim, we are carrying out sorption measurements (see Figure 1) of different supercritical solvents on mesoporous and microporous stationary phases, using a Rubotherm® magnetic suspension balance with provision for in-situ density measurements [1]. Peculiar effects observed in the measurements are being studied in the light of theories on critical adsorption and critical depletion. These experiments combined with pulse and frontal chromatographic experiments in a supercritical chromatograph will be used to characterize the behaviour of processes such as the SF-SMB.

Supported partly by the Swiss National Science Foundation through Grant SNF 21-55674.98

References [1] DI GIOVANNI O., DÖRFLER W., MAZZOTTI M., MORBIDELLI M., “Adsorption of Supercritical

Fluids on Mesoporous Adsorbents.”, submitted to Langmuir (2000) *Laboratorium für Technische Chemie, ETH Zürich

0.30

0.25

0.20

0.15

0.10

0.05

0.00

nex [g

/g]

8006004002000ρ [g/L]

312.0 K 320.3 K 335.8 K 349.9 K 367.6 K 386.0 K 415.3 K 465.9 K

Fig 1: CO2 excess adsorption isotherms on silicagel [1].


42

42. Modeling the Adsorption Behavior of Supercritical Fluids T. HOCKER and M. MAZZOTTI

Keywords: supercritical adsorption, lattice density functional theory

Despite the importance of supercritical adsorption phenomena in a number of new, promising separation processes, relatively little is known about the underlying thermodynamics compared to the bulk phase behavior of supercritical fluids. However, advances in lattice [1] as well as off-lattice [2] density functional theory allow new insights into this fascinating area.

This project investigates the influence of surface structure on the adsorption behavior of supercritical fluids using a lattice model that has proven to be an accurate approximation to the 3D Ising model [1]. Compared to classical nonrandom lattice models such as quasi-chemical theory, the used formalism has the advantage of being very flexible in that it allows one to model systems with complex boundaries (porous surfaces [3], energetically heterogeneous surfaces), molecules with directional bonds, as well as molecules of different sizes and shapes.

As an example of supercritical adsorption, Gibbs (or excess) isotherms as a function of the bulk density onto slit pores of different widths are shown in Fig.1. The temperature is held constant at T/Tcr = 1.0001, whereas the ratio of adsorption to fluid-fluid interaction energies, εs/ε, is unity. It is worth noting the significant correlation between the predicted adsorption behavior and the width of the slit pores. Adsorption data obtained by using a magnetic suspension balance will be exploited for model development and parameter estimation [4]. The comparison between experimental data and model predictions will be thoroughly analyzed. References [1] ARANOVICH G.L., DONOHUE M.D., “Analysis of Adsorption Isotherms: Lattice Theory

Predictions, Classification of Isotherms for Gas-Solid Equilibria, and Similarities in Gas and Liquid Adsorption Behavior“, J. Colloid Interface Sci., 200, 273-290 (1998)

[2] EVANS R., “Fluids Adsorbed in Narrow Pores: Phase Equilibria and Structure“, J. Phys. Condens. Mat., 2, 8989-9007 (1990)

[3] ARANOVICH G.L., DONOHUE M.D., “Adsorption Hysteresis in Porous Solids“, J. Colloid Interface Sci., 205, 121-130 (1998)

[4] DI GIOVANNI, O., DÖRFLER, W., MAZZOTTI, M., MORBIDELLI, M., “Adsorption of Supercritical Fluids on Mesoporous Adsorbents”, submitted to Langmuir (2000)

Fig. 1: Isotherms for supercritical adsorption onto slit pores of different widths. Shown are predictions from lattice density functional theory [1].


43

43. Application of the Method of Projection onto Convex Sets to Inverse Problems in Adsorption and Chromatography T. HOCKER and M. MAZZOTTI

Keywords: inverse problems, projection onto convex sets, ill-posed problems, adsorption

The “projection onto convex sets” (POCS) method, origi-nally developed in the ’60s, has been successfully applied for many years to estimation problems, mainly in the fields of image processing, signal recovery, and optics [1]. It allows one to incorporate into an iteration scheme available information about the experimental data and the measure-ment error as well as a priori constraints based on physical intuition (such as non-negativity). To our best knowledge, Ref. [2] provides the first application of the POCS method to calculate adsorption-energy distributions of heteroge-neous surfaces. Other examples of inverse problems in adsorption and chromatography include the calculation of pore-size distributions of heterogeneous surfaces, as well as the calculation of adsorption isotherms from pulse chromatograms. Note that all those examples can be transformed into a linear matrix equation of type b=A f + n, where b denotes the available experimental data, A is the design matrix, f is the unknown, and n is the measurement error. Despite its apparent simplicity, solving the above equation for f almost always represents a highly "ill-posed" problem for which it is hopeless to find an exact solution. Indeed, there usually is an infinite number of (physical and unphysical) solutions f* which all lead to values b* practically indistinguishable from the original data. It is important to note that the POCS method doesn't lead to a unique “optimum” solution. Rather, a feasible solution is found within a solution space that is consistent with all imposed constraints (see Figure 1).

The “size” of this solution space depends on how large the measurement errors are; it also depends on the accuracy of the error statistics, and the number and significance of a priori constraints used.

Fig. 2 illustrates results where the POCS method has been used to recover energy distributions from simu-lated adsorption data containing normally distributed errors [2]. The excellent recoveries obtained demonstrate its potential as a rigorous and robust tool for solving inverse problems. References [1] STARK H., YANG Y., Vector Space Projections, Wiley & Sons, New York (1998) [2] HOCKER T., DONOHUE M.D., “Adsorption-Energy Distribution of Heterogeneous Surfaces

Predicted from Projections onto Convex Sets“, submitted to J. Colloid Interface Sci. (2000)

Fig. 2: Recovery of an adsorption-energy distribution from simulated data [2].

Fig. 1: Three closed and convex sets (constraints) with a non-empty inter-section.


44

44. Enantioseparations through Achiral Chromatography R. BACIOCCHI*, G. ZENONI°, M. MORBIDELLI° and M. MAZZOTTI

Keywords: enantiomers, chromatography, modeling

Chromatography is a key technique for the analytical, preparative, and production scale separation of enantiomers, particularly in the pharmaceutical and fine chemicals industries. Although it is common belief, that this separation can be accomplished only using a chiral stationary phase, it has been recently shown that under certain circumstances a non-racemic mixture of specific chiral compounds can be separated in two fractions which differ in enantiomeric excess (e.e.) also on an achiral stationary phase.

This happens for instance for the enantiomers of bi-naphthol in chloroform, as shown in Fig. 1. In this case achiral chromatography on Licrosphere 100 NH2 furnishes two fractions constituted of the pure enantiomer present in excess and of the racemic mixture, respectively [1]. This is demonstrated by on-line monitoring the outlet concentration of both enantiomers during a pulse chromatogram by using a UV-detector and a polarimeter in series [2]. This is the first time that 100% e.e. is achieved through achiral chromatography.

Furthermore, we have provided experimental evidence of the presence of homo- and hetero-chiral dimers in solution through NMR experiments and develop a consistent physico-chemical model of the solution itself and of the competitive achiral adsorption equilibria. When combined with a standard rate model of the chromatographic column this has allowed also for the first time for a qualitative and quantitative description of all the experimentally observed phenomena. Among these, it is worth mentioning the effect of the enantiomeric excess and of the overall concentration of the injected pulse on the chromatographic behavior. References [1] BACIOCCHI R., ATTINÁ M., ZENONI G., VALENTINI M., MAZZOTTI M., MORBIDELLI M.,

“Achievement of 100% e.e. of binaphthol enantiomers through achiral chromatography“, in preparation (2001)

[2] ZENONI G., PEDEFERRI. M., MAZZOTTI M., MORBIDELLI M., “On-line monitoring of enantiomer concentration in chiral simulated moving bed chromatography“, J. Chromatogr. A, 888, 73-83 (2000)

* Università di Roma “Tor Vergata”, Italy; °Laboratorium für Technische Chemie, ETH Zürich

0.4

0.3

0.2

0.1

0.0706050403020100

time (min)

Con

cent

ratio

n (g

/l)

R

S

Fig. 1: On-line monitored experimental elution profiles of a 100 µL pulse of a 20/80 solution of S(-) and R(+)-1-1'-bi-2-naphthol at 30 g/L total concentration [1].


45

45. Gas Chromatographic SMB (GC-SMB) Technology for Chiral Separations A. RAJENDRAN, G. BIRESSI*, M. MAZZOTTI and M. MORBIDELLI*

Keywords: chiral separation, inhalation anesthetics, simulated moving bed, gas chromatography

.Preparative chromatography is an important tool in fine chemical and pharmaceutical separations. The SMB technology is obtaining more and more attention due to its advantages in terms of productivity and eluent consumption. This concept which was traditionally applied to separations in the liquid phase could be extended to the separation of volatile substances like inhalation anesthetics (e.g. Enflurane and Isoflurane) in the gas phase. A gas chromatographic SMB laboratory unit for the separation of volatile enantiomers has been designed and built and the continuous gas chromatographic separation of the enantiomers of enflurane [1,2] and those of isoflurane [3] have been carried out. Figure 1 shows a detail of the experimental GC-SMB unit. The complete separation regions were identified and the effects of key variables like switch time, flow rates, pressure levels, temperature and feed concentration were experimentally studied [2]. Nowadays, only the racemate anesthetic is used in clinical practice and it will be interesting to test the activity of the single enantiomers. The experiments performed in our facility will allow production of sufficient quantities of the enantiomers for clinical trials. Both experimental and modeling studies will be performed to better understand the differences and similarities between HPLC-SMB and GC-SMB systems

References [1] BIRESSI G., QUATTRINI F., JUZA M., MAZZOTTI M., SCHURIG V., MORBIDELLI M., “Gas

chromatographic simulated moving bed separation of the enantiomers of the inhalation anesthetic enflurane.”, Chem. Eng. Sci. 55, 4537-4547 (2000)

[2] BIRESSI G., MAZZOTTI M., MORBIDELLI M., “Experimental investigation of the behaviour of gas phase Simulated Moving beds.”, submitted to Chem. Eng. Sci. (2000)

[3] BIRESSI G., RAJENDRAN A., MAZZOTTI M., MORBIDELLI M., “The GC-SMB separation of the enantiomers of Isoflurane.”, submitted to Separat. Sci. & Tech. (2001)

*Laboratorium für Technische Chemie, ETH Zürich

Fig 1: The columns (U-shaped) and valves in the separation section of the GC-SMB unit.


46

46. Analysis and Design of Simulated Moving Bed Reactors F. LODE*, M. MAZZOTTI and M. MORBIDELLI*

Keywords: simulated moving bed, chromatographic reactor, heterogeneous catalysis, ion-exchange resins, esterification

Simulated Moving Bed Reactors (SMBRs) combine continuous simulated countercurrent chromatographic separation with chemical reaction. With two phases moving in continuous countercurrent flow, process performance is significantly improved both in terms of productivity and eluent consumption, and SMBR thus becomes a competitive alternative to traditional reactors with two separate units for reaction and separation. Especially for applications in which reaction conversion in conventional operations is limited by chemical equilibrium, SMBRs can simultaneously achieve the goals of total conversion of reactants and complete separation of the products within a single apparatus. Reactive SMB has been given more and more attention recently, particularly for reactions catalysed by acidic ion exchange resins, e.g. esterifications [1,2], partial oxidations, as well as some enzyme reactions [3], among others.

This project aims at deepening the understanding of the behavior of SMBRs through a better description of the elementary phenomena involved [4,5]. This allows a parametric analysis of the behavior of SMBRs to be carried out [6]. Based on that the operating conditions of the process are optimized. All these theoretical achievements are assessed by running experiments on a pilot unit for the synthesis of methylacetate using a commercial sulfonated ion-exchange resin as both catalyst and selective sorbent [7]. This project is partly supported by F. Hoffmann - La Roche AG, Basel, CH

References [1] MAZZOTTI M., KRUGLOV A., NERI B., GELOSA D., MORBIDELLI M., “A continuous

chromatographic reactor: SMBR ”, Chem. Eng. Sci., 51, 1827 (1996) [2] KAWASE M., SUZUKI T. B., INOUE K., YOSHIMOTOI K., HASHIMOTO K., “Increased

esterification conversion by application of the Simulated Moving Bed Reactor ”, Chem. Eng. Sci., 51, 2971 (1996)

[3] MENSAH P., GAINER J. L., CARTA G., “Adsorptive control of water in esterification with immobilized enzymes”, Biotechnol. Bioeng., 60 , 445 (1998)

[4] MAZZOTTI M., GELOSA D., NERI B., MORBIDELLI M., “Dynamics of a chromatographic reactor: esterification catalysed by acidic resins”, Ind. Eng. Chem. Res., 36 , 3163 (1997)

[5] MAZZOTTI M., KRUGLOV A., NERI B., GELOSA D., MORBIDELLI M., “Kinetics of the liquid-phase esterification catalysed by acidic resin”, Ind. Eng. Chem. Res., 36 , 3 (1997)

[6] MIGLIORINI C., FILLINGER M,. MAZZOTTI M., MORBIDELLI M., “Analysis of Simulated Moving Bed Reactors ”, Chem. Eng. Sci., 54, 2475 (1999)

[7] LODE F., HOUMARD M., MIGLIORINI C., MAZZOTTI M., MORBIDELLI M., “Continuous reactive chromatography”, Chem. Eng. Sci., in press (2001)

*Laboratorium für Technische Chemie, ETH Zürich

Bio Process Technology

47

47. Forming of Angiopolar Ceramic Cell Carriers by Dip Coating K. RULOFF and F. WIDMER

Keywords: dip coating, ceramic processing, cell carriers

Today a lot of people still die of liver disease although their illness could be cured by a liver transplantation (USA: ca. 30’000 per year). The reason for this is an acute shortage of donor organs. Thousands of lives could be saved by the development of an artificial liver. One possible design for such an artificial liver consists of hundreds of cell carriers filled with hepatocytes that are cultivated in vitro.

The cell carriers used for such an organ have to have a so-called “angiopolar” shape. That means they must be able to induce differentiated blood vessel when implanted in living tissue in order to imitate the complicated blood circulation found in the liver. Such a shape is characterised by a gradual transition from convex to concave and can be found for example on a hollow sphere with a smoothly edged opening.

Fig. 1: Process for the production of angiopolar cell carriers In this project a process (patent applied) to form such cell carriers with porous ceramics has been developed. It combines dip-coating of a spherical core of polystyrene with subsequent gelling of a component of the ceramic slurry used for the coating. The canula used to handle the core during dip-coating contains a repulsion mechanism and allows the generation of a defined opening in the shell of the cell carrier. The steps of the process are shown in Fig. 1. The investigation of the process is conducted with a laboratory-scale plant. In cooperation with the chair for biocompatible materials (BWB-ETHZ), supported by ETH Zurich


48

48. Mass Transfer Characteristics of a Novel Gas-Liquid-Reactor F. BAIER and F. WIDMER

Keywords: loop reactor, gas/liquid flow, mass transfer, bubble size distribution

Loop-reactors with external circulation and heat exchanger stand out for dispersing the gas without the use of any stirrers and their easily adjustable heat exchanger surface. One of the most recent developments is the Advanced Buss Loop Reactor.

The operating principle is shown in Fig. 1: The continuous circulation of the reaction solution through the specially designed reaction mixer causes the entrainment of the gas component and its intensive mixing with the liquid phase. Since the circulation pump is able to handle gas fractions up to 50 % (v/v) the disengagement of the two phases at the bottom of the autoclave is not necessary. Thus the effective reaction volume is optimally enlarged to the complete loop-reactor volume.

The main objective of this research is to deepen the basic knowledge of the gas-liquid mixing in this reactor. For this purpose a pilot scale Advanced BLR has been constructed which allows the variation of the reaction mixer geometry, the system pressure and circulation rate of the liquid phase. Furthermore the properties of the two phases are also varied. In dependence of these parameters the volumetric mass transfer coefficient kLa and the local bubble size distributions are measured.

Fig. 2 shows two local bubble size distributions in a 0.25 M Na2SO4-solution with air at ambient pressure. Special attention should be paid to the very small bubbles which are formed by reaction mixer. Supported by the Commission for Technology und Innovation (KTI), Bern, and Kvaerner Process Technology AG, Pratteln, CH

3

5

6

2

4

1

Fig. 1: Operating principle of the Advanced BLR: 1 reaction mixer, 2 reaction autoclave, 3 heat exchanger, 4 circulation pump, 5 gas supply, 6 gas recycle

0 300 600 900 1200 1500Bubble diameter (µµµµm)

0.000

0.002

0.004

0.006

0.008

Num

ber d

ensi

ty

0 300 600 900 1200 1500Bubble diameter (µµµµm)

0.000

0.001

0.002

0.003

Num

ber d

ensi

ty

Near reactor wall Reactor centre

Fig. 2: Local bubble size distributions


49

49. Prilling and Freezing of Aqueous Solutions in an Organic Phase with Following Freeze Drying of the Particles A. BERGER and F. WIDMER

Keywords: fluid prilling, droplet crystallization, lyophilisation, monodisperse particles, mannitol, polivinylpyrrolidone

Spherical microbeads having uniform size are widely used in the pharmaceutical industry, where well-known and reproducible operation conditions are important to characterize the process and product. The prilling method, which has been developed at the Institute of Process Engineering, allows the production of monodisperse particles.

The physical effects which are combined for this process are

• the break up of a laminar, aqueous jet in an non-miscible liquid (heptane),

• the freezing of the aqueous droplet in the cool heptane bath and

• the sublimation of the remaining water.

Fig. 1 shows the prilling apparatus: The aqueous solution is pumped by a syringe pump (5) through the nozzle (2). The prilling chamber (1) – filled with heptane – is cooled to minus 30°C by the cooling circuit (heat exchanger (11), gear wheel pump (4)). The frozen particles are collected in the vial (12). In a second step the frozen particle are freeze dried in the vial with a lyophilisator.

Monodisperse Mannitol- and Polyvinyl-pyrrolidone-particles are obtained in a wide range of diameters (see Fig. 2). The particles are porous and resolve easily in water.

The process is very gentle to the product, so even temperature sensitive substances can be dried. Although the mechanical stability of the beads is low, they are free flowing.

Fig. 2: Freeze dried particles (Mannitol) produced with the same nozzle (300 µm diameter)

M

1

2

3

5

6

8

8

9

1 prilling chamber2 nozzle3 vibrator4 gearwheel pump5 syringe pump6 double piston pump7 separation chamber8 damper9 particle outlet

TC

TITI

TI

TI

PI

PI

ColdNitrogen

7

4

10

11

12

10 Inlet reservoir11 heat exchanger12 vial

Fig. 1: Prilling apparatus

Research Collection · 29. On-Line Control of Batch Cooling Crystallization 29 30. Monitoring of Particle Size Distribution in Crystallization from Solution 30 31. Gas Antisolvent

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