Carbon Dioxide Capture for the Oxidative Coupling of Methane Process - A Case Study in Mini-plant Scale - Repke-Stunkel Paper Stuenkel Repke Mini-plant

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Chemical Engineering Research and Design

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arbon dioxide capture for the oxidative coupling ofethane process – A case study in mini-plant scale

. Stünkela,∗, A. Dreschera, J. Windb, T. Brinkmannb, J.-U. Repkea, G. Woznya

Berlin Centre of Technology, Department of Process Engineering, Straße des 17. Juni 135, 10623 Berlin, GermanyHelmholtz-Zentrum Geesthacht, Centre for Materials and Coastal Research, Max-Planck-Straße 1, 21502 Geesthacht, Germany

a b s t r a c t

The oxidative coupling of methane (OCM) to ethylene is a promising alternative for the oil based industry. In this

process, beside the valuable product ethylene, unwanted by-products like CO2 are produced. Hence, the gas stream

has to be refined further. The process is not applied in the industry yet, because of high separation costs. This article

focuses particular on the CO2 purification of the OCM product stream. Therefore a case study was done for a design

task of 90% CO2 capture from 25 vol% in the OCM product gas with an operation pressure of 32 × 105 Pa. Within

the article is shown, how to resolve the lack of high separation cost for the purification and the development of an

integrated, energy efficient CO2 capture process for the OCM refinery is described. Therefore a state of the art chemical

absorption process using monoethanolamine (MEA) was developed and optimized for the base case. Therefore Aspen

Plus® with the build-in rate based model for the mass transfer with an electrolyte NRTL – approach and chemical

equilibrium reactions for the water–MEA–CO2 system as well as kinetic reactions based on the MEA-REA package was

applied. In order to improve the energetic process performance, gas permeation with dense membranes was studied

as process alternative. For this purpose a membrane unit was developed in Aspen Custom Modeler® (ACM). The

solution-diffusion model with the free-volume-theory for gas permeation including Joule–Thomson effect as well as

concentration polarization (Stünkel et al., 2009) was applied successfully. Furthermore several selective materials for a

composite membrane with experimentally determined parameters were studied by this model and it was found, that

a matrimide membrane provides the best selectivity performance for the OCM CO2 capture. Based on this material a

membrane module was installed to form a hybrid separation process in combination with the amine based absorption

process. The comparison of the state of the art process with the novel hybrid separation process shows an energy

saving of more than 40% for the OCM CO2 capture. In the experimental study the stand alone performance of each

unit, as well as the performance of the hybrid process were studied and the results are presented in this article.

© 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Keywords: Carbon dioxide capture; Membrane; Absorption; Oxidative coupling of methane; Hybrid process

including the down streaming simultaneously in a mini-plant

. Introduction

hile the fossil fuel becomes shorter and the price for crudeil rises since the last decades provides the oxidative couplingf methane a new route for the petrochemical industry (Behrnd Kleyensteiber, 2010). The process based on a hot catalyzedas phase reaction at temperatures up to 750 ◦C to convertsethane to ethylene (Jaso et al., 2010). Thus, the OCM opens

p natural- or biogas as a new feedstock for a wide range

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f chemical products (Hall, 2005). Beside the desired productthylene, unwanted by products like carbon dioxide are pro-

∗ Corresponding author. Tel.: +49 303 147 9817; fax: +49 303 142 6915.E-mail address: [email protected] (S. Stünkel).Received 31 October 2010; Received in revised form 14 January 2011; A

263-8762/$ – see front matter © 2011 The Institution of Chemical Engioi:10.1016/j.cherd.2011.02.024

duced and the reaction product gas has to refine further. Dueto high separation cost is this process not applied in indus-trial yet, although several process alternatives for the OCMare purposed in the literature (Salerno et al., 2010). So far,all purposed processes associated with high energy demandand cost-intensive downstream procedures for separation andgas recycling. To overcome the limitation, a novel approach ofconcurrent engineering is applied to study the whole process,

ure for the oxidative coupling of methane process – A case study in

ccepted 11 February 2011

scale (Deibele and Dohm, 2006). In order to investigate thewhole OCM process from reaction over purification to sep-

neers. Published by Elsevier B.V. All rights reserved.

ARTICLE IN PRESSCHERD-729; No. of Pages 10

2 chemical engineering research and d

Nomenclature

˛ selectivity [–]� Lenard Jones diameter [Å]ı membrane thickness [m]c concentration [mol/m3]D diffusion coefficient [m2/s]f fugacity [Pa]He Henry coefficient [Pa]L permeance [mol/Pa m2 s]n molar flux [mol/s m2]p pressure [Pa]P permeability [mol/Pa m s]S Solubility coefficient [mol/m3 Pa]T Temperature [K]x liquid molar fraction [–]y vapour molar fraction [–]

aration in continuous operation, an integrated downstreamconcept was developed (Stünkel et al., 2009). Due to the yieldlimitation of 30% for conventional OCM reactors (Jaso et al.,2010), the goal of the downstream process synthesis is a fur-ther energetic and economic improved process performanceto overcome the lack.

Hence, the whole process was divided into three units,shown in Fig. 1: the reaction unit, the purification unit andthe separation unit; and all of them are investigated concur-rently in a mini-plant scale. Furthermore, design cases weredefined for the process synthesis of each unit under consid-eration of their interactions. Based on the general frameworkof Fig. 1, process conditions and the design task for the CO2

separation unit was developed, which are given in Table 1. Theremoval of acid gas components is a key step in the gas refin-ery to reach the product purity. Hence, for the CO2 removal areseveral process options available, but for a selective removalis the chemical absorption process a state of the art process(Kohl and Nielsen, 1997; IPCC, 2005). Novel approaches likemembrane separation reaches more interests and studied anddiscussed as well in this article.

2. Base case design: chemical absorptionprocess

A chemical absorption process using monoethanolamine as asolvent was chosen to design a base case, which is presentedin Fig. 2. Such a chemical absorption process is favoured forspecies, which contains acid-based functional groups, like car-

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bon dioxide. Another aspect is the fast kinetic of the MEA–CO2

reaction, which affects the height of the absorption column,in order to provide a sufficient gas–liquid contact time. Those

Fig. 1 – Flow diagram o

esign x x x ( 2 0 1 1 ) xxx–xxx

amine processes are well established and commonly used forindustrial CO2 separation (Hochgesand, 2002). The advantagesof amine processes are the high selectivity regarding CO2,reusing the solvent and recovering the heat by recuperation.Otherwise, they also cause high thermal energy demand forsolvent regeneration in the desorption unit, that leads to sev-eral process concepts with improved energy performance. Theconcentration range of MEA in industrial plants is between 15and 35 wt% (Kohl and Nielsen, 1997) and it was assessed forthe base case to 30 wt%. Operating the column in an indus-trial relevant regime the F-Factor has to be in the range of0.15–1.5 Pa0.5. The process was designed as simple as possibleto handle a wide range of operation conditions and to build abenchmark to compare process alternatives.

2.1. Simulation model

The base case process was investigated theoretically and asimulation model was implemented by the commercial toolAspen Plus®. Hence a rigorous process model was imple-mented using the provided build-in packages. This modelincludes the build-in electrolyte NRTL package ELECNRTL,with chemical equilibrium reactions for the liquid phase(Austgen et al., 1989) and the Redlich–Kwong equation of statefor the gas phase. Furthermore the MEA-REA package wasapplied successfully in the packed column that considers theliquid phase reaction kinetics of the MEA with the CO2. Thekinetic reactions are essential for the description of the rigor-ous model for the absorption (Kucka et al., 2003). The build-inRateSep approach was used to describe the mass transfer inthe packed column. The discretization height per simulationsegment of the absorption and desorption column was foundwith 0.1 m by a sensitivity study. Further increase shows noeffect on the results of the CO2 concentration profile, Fig. 3.

2.2. Parameter study and process design

The effect of solvent flow rates on the regeneration energydemand was studied for the constant carbon capture of thedesign task iteratively. Hence, the reboiler heat duty was keptconstant and the solvent flow rate was varied in a closed loopconfiguration (Fig. 2), to reach the design task of 90% CO2 cap-ture. Afterwards was the reboiler heat duty decreased in orderto change the solvent regeneration and a new optimal solventflow rate was found for the design case. The thermal regen-eration energy demand was calculated per kg captured CO2

from the raw gas. The aim of this procedure is to find the opti-mum between regeneration energy and solvent flow rate. Theresults for a particular gas load of 0.7 Pa0.5 are presented in

ure for the oxidative coupling of methane process – A case study in

Fig. 4 and Table 2.Furthermore the effect of the column height on the cap-

tured CO2 was studied in standalone simulation of the

f the OCM-Process.

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Table 1 – Process condition and design task of the CO2 capture.

Gas temperature Gas pressure CO2 C2H4 CO2 removal

40 ◦C 32 × 105 Pa 25 vol% 18 vol% 90%

Fig. 2 – Flow sheet of the amine based absorption process and photo of the mini-plant process.

Table 2 – Base case design of a conventional CO2 capture process.

System pressure Solvent flow Gas load MEAconcentration

Solventregeneration

Absorptionpacked height

Thermal energydemand

32 × 105 Pa 150 kg/h 0.7 Pa0.5 30 wt%

0

2

4

6

8

10

12

14

16

0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5

CO2gasph

aseconcen

tra�

on[vol%]

Absorp�oncolumn height [m]

Discre�za�oneffect on the CO2 concentra�on profile

0.5m discre�za�on height per segment0.2m discre�za�on height per segment0.1m discre�za�on height per segment0.05m discre�zation height per segment

Fig. 3 – Effect of absorption column discretization onc

ahaicfcw

column kept constant. While for different desorption pressure

oncentration profile.

bsorption column for each solvent flow. A minimal columneight is necessary to realize the contact time of the liquidnd the gas phase. Therefore the height of the column wasncreased for each flow rate till no effect on the CO2 gas phaseoncentration was obtained. The column height was variedrom 0.5 to 5 m and structured packing was chosen for the

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olumn internals. However, based on this simulation studyas the process designed and developed for the experimental

Table 3 – Process design and experimental setup.

Absorptionoperationpressure

Absorptioncolumn

diameter

Absorptionpacked height

Specificpackingsurfacea

32 × 105 Pa 0.04 m 5 m 450 m2/m3

a Rombopak 12 M for the absorption column.b Rombopak 9 M for the desorption column.

47% 5 m 2.79 MJ/kgCO2

study (Table 3). While state of the art design procedures forprocess equipment are followed (Perry and Green, 2008) andthe key parameters of the process design are given in Table 3.

2.3. Experimental model validation

To validate the simulation results and to study continuousoperation, the process was build in a mini-plant scale, likethe flow sheet and a photo shows (Fig. 2), while the techni-cal details presented in Table 3. A Sick-Maihak infrared onlinegas analyzer was used for hydrocarbon and CO2 detectionwith an accuracy of ±1%. Furthermore was a titration methoddeveloped for the liquid phase, to determine the CO2 loadingand the amine concentration with an accuracy of ±6%; ±10%respectively. In a first preliminary experimental study for dif-ferent absorption pressure the desorption pressure effect onthe regeneration was investigated and the simulation resultswas proofed experimentally. In Table 4 are the experimentaloperation conditions presented for this preliminary study. Inthis study was the gas load, the raw gas feed pressure, the CO2

raw gas concentration and the reboiler duty of the desorption

ure for the oxidative coupling of methane process – A case study in

the flow rate was adapted, to reach the design task of 90% CO2

capture. The simulation results are compared with the exper-

Desorptionoperationpressure

Desorptioncolumn

diameter

Desorptionpacked height

Specificpackingsurfaceb

2.5 × 105 Pa 0.1 m 4 m 350 m2/m3

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Table 4 – Preliminary experimental study conditions.

Absorptionpressure

Gas load Liquid load CO2 raw gasconcentration

Reboilerduty

Desorptionpressure

Solventflow rate

10 × 105 Pa 0.45 Pa0.5 15–16 m3/m2 h 15 vol

Fig. 4 – Simulation results-thermal energy demand fordifferent solvent flows with constant carbon capture.

imental data in Fig. 5. They are obtained for desorption toppressure of 2 × 105 Pa with a solvent flow rate of 20 kg/h.

2.4. Results and discussion – preliminaryexperimental study

The results of desorption top pressure variation are presentedin Table 5 for a standard absorption process configurationwithout any heat integration or process optimization. It wasobserved, that the minimum specific energy demand for thisprocess conditions was achieved by 2 × 105 Pa desorption toppressure. In this experimental study a solvent regenerationof 70% was achieved. Thereby was the CO2 loading of therich solvent flow determined with 0.48 molCO2 /molMEA; whilethe lean solvent flow was loaded with 0.13 molCO2 /molMEA.Furthermore shows the comparison of the experimental con-centration profile for the absorption column a good agreementwith the simulation results in Fig. 5, left side. While the pre-diction of the temperature profile by the simulation exhibit alarge gap to the obtained experimental profile. Beside others,

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could the heat loss of none insulate absorption column a pos-sible explanation for the discrepancy. Thus, the heat loss hasto be taken in to account by the simulation model. Neverthe-

Absorption CO2 concentration profiles

0

1

2

3

4

5

6

50 10 15 20

CO2 gas concentration [vol%]

Ab

sorp

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n c

olu

mn

hei

gh

t [m

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Aspen Plus results

Experimental results

Fig. 5 – Left side: comparison simulation results for CO2 concentrexperimental absorption temperature profile with simulation res

% 3.41 kW 1.5/2/2.5 × 105 Pa 15–20 kg/h

less seems the effect of temperature on the reaction kineticor on the CO2 solubility negligible to reproduce the concen-tration profile by the simulation with a good agreement to theexperiments, as Fig. 5 shows.

2.5. Base case – experimental study results

Consequently, the design case for an ordinary reactor gasstream was reproduced in the mini-plant, based on the resultsof the preliminary experimental study and they are pre-sented in Table 6. In this study could the design task of 90%CO2 removal achieved by the operation conditions presentedin Table 7. The benchmark for a further process improve-ment was found with 5 MJ/kgCO2

with an ethylene loss of 6%.Remarkable on this result is the solvent regeneration of 15%,in comparison to 70% found in the preliminary experimentalstudy of Section 2.4. Thus the solvent flow rate was increasedby nearly 70% to achieve 90% CO2 removal in the base casecompared to the flow rates of the preliminary experimentalstudy.

3. Process alternatives

3.1. Process synthesis – alternative processes

The design task of the purification unit is to deplete 90%of the carbon dioxide from reaction product stream. For adetailed process synthesis, particular substance property dataare required and listed in Table 8.

Several strategies are known for process synthesis (Kohland Nielsen, 1997; Douglas, 1995; Barnicki and Siirola, 2004).The one, followed in this approach is a knowledge-based sep-aration system synthesis (Barnicki and Fair, 1992). Followingthis procedure, the general separation tasks can be classifiedinto: (1) enrichment, (2) sharp separation and (3) purification.The enrichment is the concentration increase of a species in

ure for the oxidative coupling of methane process – A case study in

one of the product streams (Barnicki and Fair, 1992), whereastwo high-purity product streams of two species resulting bysharp separation. While for the first separation class no high

Absorption temperature profile

0

1

2

3

4

5

6

200 40 60 80 100 120

Temperature [° C]

Ab

sorp

tio

n c

olu

mn

hei

gh

t [m

]

Aspen Plus results

Experimental results

ation profile with the experiments, right side: comparison ofult.

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Table 5 – Results of desorption top pressure study.

Desorption top pressure CO2 pure gasconcentration (vol%)

Captured CO2 (kg/h) Specific energyinput [MJ/kgCO2]

1.5 × 105 Pa 1.83 1.71 7.342 × 105 Pa 1.21 1.77 7.092.5 × 105 Pa 1.45 1.73 7.25

Table 6 – Process condition and design task of the CO2 capture.

Gas temperature Gas pressure Gas load CO2 C2H4 CH4 N2

pabburc(tsctaesT

3Cahobvgsae

3Tfo

Fr

40 ◦C 32 × 105 Pa 0.33 Pa0.5

urity or high recovery is achievable, but with the second sep-ration class two high purity or high recovery streams cane achieved. The classification of a sharp separation task cane proofed by the ration of the key components in the prod-ct streams, which has to be higher than 9 or less than 0.1espectively. The key components for the CO2 removal are thearbon dioxide (CO2), that has to remove and the ethyleneC2H4) as the product. Purification, in this context representshe removal of one low concentration component, in this casetudy: the removal of CO2 from 15 vol%. For the OCM designase, the sharp separation has to be considered to fulfill theask as a one step separation. However, it can be considereds a two step separation task as well, which consists of annrichment followed by a purification separation. For all threeeparation tasks the applicable processes are presented inable 9, and discussed in detail in the next following sections.

.1.1. Cryogenic distillationryogenic distillation should apply only for high throughputsnd is economical only for a volatility of the key componentsigher than 2 (Barnicki and Fair, 1992). For a system pressuref 32 × 105 Pa results the relatively volatility to ˛CO2/C2H2

≈ 1,ut for a system pressure of 1 × 105 Pa becomes the relativelyolatility to ˛CO2/C2H4

≈ 3, according to Fig. 6. Thus, the cryo-enic distillation at 32 × 105 Pa should not applied, but, for aystem pressure of 1 × 105 Pa this process has to consider as anlternative. Hence, the cryogenic distillation was not consid-red as a process alternative for this design case of 32 × 105 Pa.

.1.2. Physical absorption

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o consider the physical absorption as a applicable processor this separation task a selectivity for the key componentsf SCO2/C2H4

> 4 is recommended (Barnicki and Fair, 1992). The

ig. 6 – Vapour liquid equilibrium for CO2/C2H4 to determine theight side.

25 vol% 18 vol% 17 vol% 40 vol%

selectivity for two common physical absorbents: methanoland water was observed by using Eq. (2) in combination withthe Henry approach, Eqs. (2) and (3). Therefore the selectiv-ity for methanol results to SCO2/C2H4

= 1.11 and for water asabsorbent to SCO2/C2H4

= 1.03 respectively. The results show,that physical absorption should not considered as an alterna-tive for the CO2 removal of the OCM process.

SabsCO2/C2H4

= xCO2

xC2H4

(1)

xCO2 HeCO2 = yCO2 p (2)

xC2H4 HeC2H4 = yC2H4 p (3)

3.1.3. Chemical absorptionChemical absorption is favoured for species, which con-tains acid-based functional groups, like the carbon dioxideto remove them with high selectivity concerning valuableproduct. Several chemical absorbents are purposed in the lit-erature like caustic potash, caustic soda or alkanolamines(Kohl and Nielsen, 1997). As state of the art absorbent inCO2 separation alkanolamines are favoured with low molec-ular weight, low partial pressure, low corrosivity or no toxicbehavior (Thiele, 2007). Considering the physical and chem-ical properties of alkanolamines, they are best suitable forCO2 gas purification. Moreover exist several alkanolamineslike monoethanolamine (MEA), diethanolamine (DEA) ormethyldiethanolamine (MDEA) and each of them with a par-ticular field of application like low CO2 partial pressure orsulphuric acid components (Kriebel and Schlichting, 2002).

ure for the oxidative coupling of methane process – A case study in

However, monoethanolamine is the widespread used alka-nolamine in CO2 capture when a high selectivity is required.Therefore, monoethanolamine with a concentration of 30 wt%

relatively volatility at 32 × 105 Pa, left side and 1 × 105 Pa,

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Table 7 – Experimental base case results for 90% CO2 capture from OCM reaction gas stream.

Liquid load(m3/m2 h)

Solvent flow(kg/h)

˛loaded(molCO2 /molMEA)

˛unloaded(molCO2 /molMEA)

Desorptionpressure (105 Pa)

Reboiler duty(kW)

Ethylene loss(%)

Thermal energydemand (MJ/kgCO2

)

42.3 55 0.45 0.39 2 4.7 6 5

Table 8 – Substances properties data (Bird et al., 2007).

Component State 25 ◦C, 1 × 105 Pa Molar mass [g/mol] Boiling point 1 atm [◦C] Melting point 1 atm [◦C] Kinetic diameter � [A] Critical properties

Tc [K] pc [Pa]

Methane Vapour 16 −162 −182 3.8 191.1 45.8 × 105

Ethylene Vapour 28 −103.72 −169.18 4.228 282.4 50 × 105

Ethane Vapour 30 −89 −183 4.388 305.4 48.2 × 105

Nitrogen Vapour 28 −195 −210 3.667 126.2 33.5 × 105

Carbon dioxide Vapour 44 Over critical gas 3.996 304.2 72.8 × 105

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Table 9 – Process alternatives and indicators forseparation processes (Barnicki and Fair, 1992).

Process alternative Indicator

Cryogenic distillation Relative volatilityPhysical absorption Separation factor for gas solubility

using Henry’s approachMolecular sieve

adsorptionDifference in shape size and kineticdiameter

Equilibrium limitedadsorption

Ratio of the equilibrium loading forthe key components

Membrane separation Separation factor, ration of thecomponent permeability

Chemical absorption Chemical family of the componentsCondensation Difference in normal boiling point

wa

3TtdacltoCasEcciS

52a

3Ttlstt

3Csst

Catalytically conversion Product worth

as chosen for the chemical absorption process used in thisrticle.

.1.4. Molecular sieve and equilibrium adsorptiono decide if molecular sieve adsorption is a proper alterna-ive, the species have to differ in shape size and their kineticiameters. The physical size properties of the commercialvailable adsorbents are listed in Table 10. The componentsan be sized by their kinetic diameters based on Table 8 as fol-owed: �C2H6 > �C2H4 > �CO2 > �CH4 > �N2 . Remarkable is, thathe kinetic diameter of the carbon dioxide is in the middlef the components and all diameters are very close together.ause of this reason is the separation by size not efficientpplicable and the molecular sieve adsorption was not con-idered as an alternative for the carbon dioxide removal.quilibrium based adsorption is only suitable for species con-entration less than 10 vol% and for a selectivity of the keyomponents larger than 2. It was found, that the selectiv-ty for equilibrium loading of the key components results to

CO2/C2H4= 1.74, based on experimental measurements for a

A molecular sieve found in the literature (Pakseresht et al.,002). Thus, the equilibrium adsorption was not taken intoccount as a process alternative.

.1.5. Condensation and catalytically conversionhe separation by condensation should be considered when

he difference in normal boiling point of the components isarger than 40 K. The separation by catalytic conversion is onlyuitable for impurities. Cause of those reasons the condensa-ion and catalytically conversion of the carbon dioxide was notook into consideration as process alternatives.

.1.6. Membrane processesonsidering membrane separation as an economical feasibleeparation technique, the selectivity of the key components

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hould be larger than 15 (Barnicki and Fair, 1992). The selec-ivity for the key components can be obtained by the ratio of

Table 10 – Commercial available adsorbents form(Barnicki and Fair, 1992).

Category Nominal aperturesize [Å]

Zeolite type

5 3 3A Linde 3ADavison4 4 4A Linde, 4A Davison3 5 5A Linde, 5A Davison2 8 10× Linde1 10 13× Linde, 13× Davison

esign x x x ( 2 0 1 1 ) xxx–xxx 7

the permeability with Eq. (4).

˛CO2/C2H4= PCO2

PC2H4

= DCO2 SCO2

DC2H4 SC2H4

(4)

Permeability was determined by the pressure increasemethod for single gas components by the GKSSb. Based ontheir results the components selectivity was calculated forpromising selective membrane materials and the results arepresented in Table 11. The most promising material for CO2

removal from the OCM reaction product gas is a polyimidecomposite membrane, which consists of matrimide for theselective layer.

3.1.7. Results for the alternative separation processsynthesisThe above discussion of recommended unit operation fromTable 9, shows that a membrane process can be consideredas an alternative process concept for the CO2 separation fromthe OCM reaction product. The presence of hydrocarbons inthe gas stream and the high feed pressure are the key factorsthat have to consider by the process synthesis for alterna-tives. Only tailor made separation processes can be appliedto the OCM process for achieving high purity in combinationwith low product loss by low energy demand. Therefore eithera one step separation process for a sharp separation can beapplied or a two step separation process, consisting of enrich-ment and purification can be adopted. In the following sectionboth cases are considered for the membrane process.

3.2. Membrane processes as alternative CO2

separation

Gas permeation processes with dense membranes wereapplied as an efficient method to improve the energy demandof CO2 capture (Baker and Richard, 2002). Therefore, they wereinvestigated as an alternative in comparison to the base case.While industrial applications for gas permeation are rare, thepotential of membrane in separation technique are vast andthe applications rises (Brinkmann, 2006). Several materialslike cellulose or polymer based materials can be used as aselective layer for the separation of CO2 from hydrocarbons(Lin and Freeman, 2005). In this work were cellulose acetate(CA), polyethylene oxide (PEO), polydimethylesiloxane (PDMS)and matrimide (PI) investigated as selective layer with poly-dimethylsiloxane for the support layer in a GKSSb compositeflat sheet membrane configuration. Experiments for single gaspermeation behavior were carried out by the GKSSb, based onthe pressure increase method.

3.2.1. Gas permeation membrane modelFirst valuation of the separation efficiency was achievedby process simulation using the solution-diffusion model(Brinkmann, 2006), Eq. (5) and was improved by applying thefree-volume theory for temperature dependent gas perme-ation.

ni = S.D

ı· �fi,M = P

ı· �fi,M (5)

ni = Li(T, ci) · �fi,M (6)

ure for the oxidative coupling of methane process – A case study in

For technical membranes the quotient of permeability Pand membrane thickness ı are combined to the permeance L,which has to be obtained experimentally. The permeance can

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Table 11 – Selectivity for a membrane area of 0.5 m2.

CA PEO POMS PI

SCO2/C2H4 12.24 3.5 1.26 16.5SCO /C H 14.25 2.75 1.04 54.55

14.1

SCO2/CH4 17.6

be specified with the free-volume theory including temper-ature, pressure and concentration dependency. Furthermoreare free-volume parameters obtained experimentally andparameters were adapted by the GKSSb. The permeation fluxdescribing equation using the permeance is given in Eq. (6)and forms the base of the developed model in Aspen CustomModeler®. Beside the temperature and concentration depen-dency of the permeation behavior, the mass transfer andnon-ideal effects are taken into account. Those effects arethe concentration polarization of the enriched componentalong the membrane and the Joule–Thomson effect of cool-ing by decompression of a real gas. Furthermore two differentmodule structures were investigated in this model. The firststructure tread the membrane as a one flat membrane with anoverall length of 7.14 m and a wide of 0.07 mm, discretized with100 steps over the whole length. While in the second structurethe real module structure was taken into account as describedbelow. The base of this structure is formed by one membranesheet with the size parameter of Table 12. The flow in a com-partment is the flow between two baffles and was separateduniform to each membrane sheet of this compartment andthe permeation behavior was calculated only for one sheet.After the calculation of one sheet of the compartment, theflows were combined accordingly to the numbers of sheets inthe compartment. In this way, the retentate of the compart-ment is the feed for the next compartment. The flow patternis closer to the reality. The module structure is presented inTable 12.

3.2.2. One and two stage membrane processTwo different kinds of membrane processes were investigatedin this study: a one stage membrane process and a two stagemembrane process, presented in Fig. 7. The raw gas conditionsfor both processes were the same as for the base case and theyare given in Tables 1 and 6. However, first screenings of thedifferent materials were done, for a constant membrane area.Anticipate the results of the study, it was observed, that thedesign task of 90% CO2 capture could be reached only with anethylene losses of more than 40% by Eq. (7) with a one stageprocess. This is uneconomically for the OCM process. Hencea two step separation process was considered that consistsof a membrane process followed by amine based absorption.The design tasks in particular are the reduction of the CO2

concentration down to 15 vol% with the membrane and therest has to remove by the following absorption process.

( )

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Ethyene loss : 1 − nC2H4–Puregas

nC2H4–Rawgas× 100% (7)

Table 12 – Membrane sheet parameter.

Length 0.08 mWide 0.07 mNumber of sheets 44Numbers of compartments 7Structure (sheets in the compartment) 7–7–6–6–6–6–6–6

3.28 39.94

3.2.3. Experimental investigationIn the next step the simulation results were proofed byan experimental study with a GKSS flat sheet envelopetype membrane module of 0.5 m2 composite membrane withmatrimide as the selective layer. In order to understand themembrane behavior, preliminary experiments were carriedout for the stand alone membrane module, with which theeffect of feed pressure, CO2 concentration and velocity wasinvestigated. In Table 13 are the varied parameter and theirrange presented. The structure of the module with their par-ticular compartments and sheets are given in Table 12.

3.2.4. ResultsThe simulation results for the CO2 and the C2H4 concentra-tion in the retentate side are compared with the experimentalresults and presented in Fig. 8. While the simulation results forthe CO2 concentration in the retentate side with the detailedmodule structure represents the experiments with an aver-age error of 10%. In comparison to this, the model with theone sheet structure represents the experiments with 20% errorfor the CO2 concentration in the retentate side. Whereas thesimulation results for the detailed module structure for theC2H4 concentration represents the experiments with an aver-age error of 8.4%. While, in comparison to this, the one sheetstructured model represent the experiments with 6.8% errorfor C2H4 in the retentate side. Consequently is the agreementof simulation with experiments for the C2H4 better than for theCO2 concentration. Nevertheless is the prediction for experi-ments of more than 13 vol% CO2 feed concentration with anaverage error of 2.4% much better than the prediction of exper-iments with lower CO2 feed concentration with an error of45%. Generally can be resumed that simulation model of thedetailed membrane structure represents the real behavior wellwith a maximum error of 10% and the model can be used tooptimize the process.

4. Hybrid process: combinedmembrane-amine process

To fulfill the design task, a membrane process was combinedwith an absorption process and results in a hybrid process(Fig. 9). While the membrane unit removes 50% of the CO2

down to 15 vol%, the absorption process removes the rest ofthe CO2 down to 1 vol%. This process fulfills the design taskof more than 90% CO2 capture with an experimentally deter-mined thermal energy demand of only 2.78 MJ/kgCO2

. Thus anenergy saving of more than 40% was achieved by using the

ure for the oxidative coupling of methane process – A case study in

hybrid process in comparison with the base case. Whereasthe hybrid process product losses is with 13.3% acceptable,but higher than those of the base case. The detailed experi-

Table 13 – Preliminary experiments for the PI membranemodule.

Pressure [Pa] 5 × 105, 10 × 105 and 32 × 105

CO2 concentration [vol%] 15 and 25Velocity [m/s] 0.25–0.8

ARTICLE IN PRESSCHERD-729; No. of Pages 10

chemical engineering research and design x x x ( 2 0 1 1 ) xxx–xxx 9

Fig. 7 – Left side: one stage membrane process-right sight: two stage membrane process.

Parity plot - retentate CO2 concentration ACM model and experiment

0,00

5,00

10,00

15,00

20,00

25,00

30,00

0,00 5,00 10,00 15,00 20,00 25,00 30,00

Experimental retentate CO2 concentration [vol%]

Sim

ula

tio

n r

eten

tate

CO

2

c

on

cen

trat

ion

[vo

l%] Simulation 1 plate

Simulation of module structure

Parity plot of retentate C2H4 concentration for ACM model and experiment

0,00

5,00

10,00

15,00

20,00

0,00 5,00 10,00 15,00 20,00

Experimental retentate C2H4 concentration [vol%]

Sim

ula

tio

n r

eten

tate

C H 2

4

c

on

cen

trat

ion

[vo

l%] Simulation 1 plate

Simulation of module structure

Fig. 8 – Parity plot between experimental and simulation retentate results for CO2 concentration (left side) and C2H4

concentration (right side).

Fig. 9 – Left side: flow diagram of the hybrid CO2 capture process: combined membrane and absorption process; right side:mini-plant design for experimental investigations.

Table 14 – Design and results of the membrane unit in the hybrid process.

Raw gas pressure Membrane area Membrane material CO2 feed CO2 removal Ethylene loss

de

mihtt

32 × 105 Pa 0.5 m2 Matrimi

ental results of the hybrid separation process are presentedn Tables 14–16. The performance of the membrane unit in theybrid separation unit is listed in Table 14, those of the absorp-

Please cite this article in press as: Stünkel, S., et al., Carbon dioxide captmini-plant scale. Chem Eng Res Des (2011), doi:10.1016/j.cherd.2011.02.024

ion unit are listed in Table 15 and the overall performance ofhe hybrid process is listed in Table 16.

Table 15 – Design and results of the absorption unit in the hybr

Absorption column height Liquid flow Absorption solv

2.5 m 55 kg/h 30 wt% MEA

25 vol% 50% 13.75%

Beside the energetic aspect, the process flexibility is anadvantage of the hybrid process, to handle a wide rangeof CO2 concentration by using the membrane and absorp-

ure for the oxidative coupling of methane process – A case study in

tion process, separated or together. Nevertheless improvesthe hybrid process the overall economics, achieves 40% of

id process.

ent CO2 raw gas CO2 removal Ethylene loss

15 vol% 40% 10.79

ARTICLE IN PRESSCHERD-729; No. of Pages 10

10 chemical engineering research and d

Table 16 – Overall performance of the hybrid process.

Thermal energy demand CO2 removal Ethylene loss

2.75 MJ/kgCO290% 13.3

energy saving and reduces investment cost due to smallerequipment size for components like the column, reboiler,condenser and pumps. In contrast, only additional invest-ment costs have to be spent for the membrane module ofthe hybrid process. However, the developed models werevalidated successfully and can be used for the processoptimization.

5. Conclusions

Within this article was the design and development of analternative separation process presented. This process wascompared to a state of the art process in order to improvethe energetic performance for constant carbon capture. Asa side condition the product loss was chosen and was eval-uated. Nevertheless a case study was presented for theCO2 capture of the OCM Process. Therefore was a singlemembrane unit, a two stage membrane unit, a stand aloneabsorption process and the combination of a membranewith an absorption process to a hybrid separation processstudied and discussed. As a result a hybrid separation pro-cess was developed, based on rigorous simulation that savesmore than 40% energy in comparison to the base case withacceptable product losses. Furthermore was the rigoroussimulation model validated experimentally in the installedmini-plant and the hybrid separation process was reproducedexperimentally.

Acknowledgements

The authors acknowledge the support from the Cluster ofExcellence “Unifying Concepts in Catalysis”, coordinated bythe Berlin Institute of Technology and funded by the GermanResearch Foundation (DFG).

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