Seminar Report

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1. INTRODUCTION[9]

In chemical engineering, a separation process is used to transform mixture of substances into

two or more distinct products. The separated products could be different in the chemical

properties or some physical property such as size, or crystal modification or other separation into

different components.

Barring a few exceptions, almost every compound/element is found naturally in an impure state

such as a mixture of two or more substances. Many times need to separate it into its individual

components arises and separation applications in the field are very important. A good example is

that of crude oil which is a mixture of various hydrocarbons and is valuable in this natural form.

Demand is greater for the purified various hydrocarbons such as natural gases, gasoline, diesel,

jet fuel, lubricating oils, asphalt, etc.

Separation processes can be termed as mass transfer processes. The classification can be based

on means of separation, mechanical or chemical. The choice of separation depends on pros and

cons of each. The mechanical separations are usually favored if possible due to the lower cost of

the operations as compared to chemical separations but for the systems that cannot be separated

by purely mechanical means (e.g. crude oil), only chemical separation is the remaining solution.

The mixture could exist as a combination of any two or more states: solid-solid, solid-liquid,

solid-gas, liquid-liquid, liquid-gas, gas-gas, solid-liquid-gas mixture, etc.

Depending on raw mixture, various processes can be employed to separate mixtures. Many times

two or more of these processes have to be used in combination to obtain the desired separation

and in addition to chemical processes, mechanical processes can also be applied where possible.

In separation of crude oil, one upstream distillation operation will feed its two or more product

streams into multiple downstream distillation operations to further separate the crude, and so on

until the final products are purified.

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2. LIST OF DIFFERENT SEPARATION PROCESSES[9]

Separation Operation Separating Agent Industrial Application

Distillation Mass transfer Purification of styrene

Absorption Liquid absorbent Separation of CO2 from

combustion products by

ethanolamine

Liquid-liquid extraction Liquid solvent Recovery of aromatics

Evaporation Heat transfer Evaporation of water from a

solution of urea and water

Leaching Liquid solvent Extraction of sucrose from

sugar beets with hot water

Dialysis Porous membrane with

pressure gradient

Recovery of caustic from

hemicelluloses

Reverse osmosis Nonporous membrane with

pressure gradient

Desalination of water

Microfiltration Microporous membrane with

pressure gradient

Removal of bacteria from

drinking water

Ultrafiltration Microporous membrane with

pressure gradient

Separation of whey from

cheese

Pervaporation Nonporous membrane with

pressure gradient

Separation of azeotropic

mixtures

Adsorption Solid adsorbent Purification of water

Chromatography Solid adsorbent or liquid

adsorbent on a solid support

Separation of xylene isomers

and ethyl benzene

Electrolysis Electric force field Concentration of heavy water

Electrodialysis Electric force field and

membrane

Desalination of water

Table 1

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3. MEMBRANE SEPARATION PROCESSES

The separation of liquids and gases are commonly accomplished using membrane separation

methods. This includes dialysis, reverse osmosis, and ultra filtration. Hybrid and more exotic

membrane methods that have also proven effective are electro dialysis, helium separation

through glass, the hydrogen separation through Palladium and alloy membranes, immobilized

solvent and liquid-surfactant membranes.

The permeation of liquids and gases through polymeric membranes occurs where a constituent

passes through the membrane by diffusion and sorption by fluid on other side of the membrane.

The driving force is achieved either by the pressure or concentration difference across

membrane.

In general, the membrane separation techniques are especially useful in separating:

1. Mixtures of the similar chemical compounds,

2. Mixtures of the thermally unstable components since no heating is needed, and

3. In conjunction with the conventional separation methods such as using membranes to

break the azeotropic mixtures before feeding them to a distillation column.

In membrane separation, spent metal removal fluids are pumped from a process tank at a

moderate pressure and rapid flow permeable membrane to the series of membranes. This flow is

referred to as the feed rate. The large molecules and virtually all petroleum products are blocked

at membrane surface and the compounds that do not pass through the membrane are referred to

as reject.

The water-like solutions that pass through membrane are referred to as the "permeate" and rate at

which permeate flows through the membrane is called the flux rate.

3.1 OSMOSIS[3]

Osmosis is the movement of water molecules through a selectively-permeable membrane down a

water potential gradient. More specifically, it is the movement of water across a selectively

permeable membrane from an area of high water potential (low solute concentration) to an area

of low water potential (high solute concentration). It may also be used to describe a physical

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process in which any solvent moves, without input of energy, across a semipermeable membrane

(permeable to the solvent, but not the solute) separating two solutions of different concentrations.

Osmosis releases energy, and can be made to do work, but is a passive process, like diffusion.

Net movement of solvent is from the less-concentrated (hypotonic) to the more-concentrated

(hypertonic) solution, which tends to reduce the difference in concentrations. This effect can be

countered by increasing the pressure of the hypertonic solution, with respect to the hypotonic.

The osmotic pressure is defined to be the pressure required to maintain an equilibrium, with no

net movement of solvent. Osmotic pressure is a colligative property, meaning that the osmotic

pressure depends on the molar concentration of the solute but not on its identity.

Osmosis is important in biological systems, as many biological membranes are semipermeable.

In general, these membranes are impermeable to organic solutes with large molecules, such as

polysaccharides, while permeable to water and small, uncharged solutes. Permeability may

depend on solubility properties, charge, or chemistry, as well as solute size. Water molecules

travel through the plasma cell wall, tonoplast (vacuole) or protoplast in two ways, either by

diffusing across the phospholipid bilayer directly, or via aquaporins (small transmembrane

proteins similar to those in facilitated diffusion and in creating ion channels). Osmosis provides

the primary means by which water is transported into and out of cells. The turgor pressure of a

cell is largely maintained by osmosis, across the cell membrane, between the cell interior and its

relatively hypotonic environment.

3.2 VARIATION

3.2.1 Reverse osmosis

Reverse osmosis is a separation process that uses pressure to force a solvent through a

semipermeable membrane that retains the solute on one side and allows the pure solvent to pass

to the other side. More formally, it is the process of forcing a solvent from a region of high solute

concentration through a membrane to a region of low solute concentration by applying a pressure

in excess of the osmotic pressure.

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3.2.2 Forward osmosis

Osmosis may be used directly to achieve separation of water from a "feed" solution containing

unwanted solutes. A "draw" solution of higher osmotic pressure than the feed solution is used to

induce a net flow of water through a semipermeable membrane, such that the feed solution

becomes concentrated as the draw solution becomes dilute. The diluted draw solution may then

be used directly (as with an ingestible solute like glucose), or sent to a secondary separation

process for the removal of the draw solute. This secondary separation can be more efficient than

a reverse osmosis process would be alone, depending on the draw solute used and the feedwater

treated. Forward osmosis is an area of ongoing research, focusing on applications in desalination,

water purification, water treatment, food processing, etc.

3.3 REVERSE OSMOSIS

Reverse osmosis (RO) is a filtration method that removes many types of large molecules and

ions from solutions by applying pressure to the solution when it is on one side of a selective

membrane. The result is that the solute is retained on the pressurized side of the membrane and

the pure solvent is allowed to pass to the other side. To be "selective," this membrane should not

allow large molecules or ions through the pores (holes), but should allow smaller components of

the solution (such as the solvent) to pass freely.

In the normal osmosis process the solvent naturally moves from an area of low solute

concentration, through a membrane, to an area of high solute concentration. The movement of a

pure solvent to equalize solute concentrations on each side of a membrane generates a pressure

and this is the "osmotic pressure." Applying an external pressure to reverse the natural flow of

pure solvent, thus, is reverse osmosis. The process is similar to membrane filtration. However,

there are key differences between reverse osmosis and filtration. The predominant removal

mechanism in membrane filtration is straining, or size exclusion, so the process can theoretically

achieve perfect exclusion of particles regardless of operational parameters such as influent

pressure and concentration. Reverse osmosis, however, involves a diffusive mechanism so that

separation efficiency is dependent on solute concentration, pressure, and water flux rate. Reverse

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osmosis is most commonly known for its use in drinking water purification from seawater,

removing the salt and other substances from the water molecules.

3.3.1 APPLICATIONS

Drinking water purification

Around the world, household drinking water purification systems, including a reverse osmosis

step, are commonly used for improving water for drinking and cooking.

Such systems typically include a number of steps:

a sediment filter to trap particles, including rust and calcium carbonate

optionally, a second sediment filter with smaller pores

an activated carbon filter to trap organic chemicals and chlorine, which will attack and

degrade TFC reverse osmosis membranes

a reverse osmosis (RO) filter, which is a thin film composite membrane (TFM or TFC)

optionally, a second carbon filter to capture those chemicals not removed by the RO

membrane

optionally an ultra-violet lamp for disinfecting any microbes that may escape filtering by

the reverse osmosis membrane

3.4 DESALINATION[1][2]

Areas that have either no or limited surface water or groundwater may choose to desalinate

seawater or brackish water to obtain drinking water. Reverse osmosis is the most common

method of desalination, although 85 percent of desalinated water is produced in multistage flash

plants.

Large reverse osmosis and multistage flash desalination plants are used in the Middle East,

especially Saudi Arabia. The energy requirements of the plants are large, but electricity can be

produced relatively cheaply with the abundant oil reserves in the region. The desalination plants

are often located adjacent to the power plants, which reduces energy losses in transmission and

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allows waste heat to be used in the desalination process of multistage flash plants, reducing the

amount of energy needed to desalinate the water and providing cooling for the power plant.

Sea Water Reverse Osmosis (SWRO) is a reverse osmosis desalination membrane process that

has been commercially used since the early 1970s. Its first practical use was demonstrated by

Sidney Loeb and Srinivasa Sourirajan from UCLA in Coalinga, California. Because no heating

or phase changes are needed, energy requirements are low in comparison to other processes of

desalination, but are still much higher than those required for other forms of water supply

(including reverse osmosis treatment of wastewater).

The Ashkelon seawater reverse osmosis (SWRO) desalination plant in Israel is the largest in the

world. The project was developed as a BOT (Build-Operate-Transfer) by a consortium of three

international companies: Veolia water, IDE Technologies and Elran.

The typical single-pass SWRO system consists of the following components:

Intake

Pretreatment

High pressure pump

Membrane assembly

Remineralisation and pH adjustment

Disinfection

Alarm/control panel

3.4.1 Pretreatment

Pretreatment is important when working with RO and nanofiltration (NF) membranes due to the

nature of their spiral wound design. The material is engineered in such a fashion as to allow only

one-way flow through the system. As such, the spiral wound design does not allow for

backpulsing with water or air agitation to scour its surface and remove solids. Since accumulated

material cannot be removed from the membrane surface systems, they are highly susceptible to

fouling (loss of production capacity). Therefore, pretreatment is a necessity for any RO or NF

system. Pretreatment in SWRO systems has four major components:

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Screening of solids: Solids within the water must be removed and the water treated to

prevent fouling of the membranes by fine particle or biological growth, and reduce the

risk of damage to high-pressure pump components.

Cartridge filtration: Generally, string-wound polypropylene filters are used to remove

particles between 1 – 5 micrometres.

Dosing: Oxidizing biocides, such as chlorine, are added to kill bacteria, followed by

bisulfite dosing to deactivate the chlorine, which can destroy a thin-film composite

membrane. There are also biofouling inhibitors, which do not kill bacteria, but simply

prevent them from growing slime on the membrane surface and plant walls.

Prefiltration pH adjustment: If the pH, hardness and the alkalinity in the feedwater result

in a scaling tendency when they are concentrated in the reject stream, acid is dosed to

maintain carbonates in their soluble carbonic acid form.

CO3−2

+ H3O+ = HCO3

- + H2O

HCO3- + H3O

+ = H2CO3 + H2O

Carbonic acid cannot combine with calcium to form calcium carbonate scale. Calcium

carbonate scaling tendency is estimated using the Langelier saturation index. Adding too

much sulfuric acid to control carbonate scales may result in calcium sulfate, barium

sulfate or strontium sulfate scale formation on the RO membrane.

Prefiltration antiscalants: Scale inhibitors (also known as antiscalants) prevent formation

of all scales compared to acid, which can only prevent formation of calcium carbonate

and calcium phosphate scales. In addition to inhibiting carbonate and phosphate scales,

antiscalants inhibit sulfate and fluoride scales, disperse colloids and metal oxides, and

specialty products can be to inhibit silica formation.

High pressure pump

The pump supplies the pressure needed to push water through the membrane, even as the

membrane rejects the passage of salt through it. Typical pressures for brackish water range from

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225 to 375 psi (15.5 to 26 bar, or 1.6 to 2.6 MPa). In the case of seawater, they range from 800 to

1,180 psi (55 to 81.5 bar or 6 to 8 MPa). This requires a large amount of energy.

3.4.2 Membrane assembly

Fig. 1 The layers of a membrane.

The membrane assembly consists of a pressure vessel with a membrane that allows feedwater to

be pressed against it. The membrane must be strong enough to withstand whatever pressure is

applied against it. RO membranes are made in a variety of configurations, with the two most

common configurations being spiral-wound and hollow-fiber.

3.4.3 Remineralisation and pH adjustment

The desalinated water is very corrosive and is "stabilized" to protect downstream pipelines and

storages, usually by adding lime or caustic to prevent corrosion of concrete lined surfaces.

Liming material is used to adjust pH between 6.8 and 8.1 to meet the potable water

specifications, primarily for effective disinfection and for corrosion control.

3.4.4 Disadvantages

Household reverse osmosis units use a lot of water because they have low back pressure. As a

result, they recover only 5 to 15 percent of the water entering the system. The remainder is

discharged as waste water. Because waste water carries with it the rejected contaminants,

methods to recover this water are not practical for household systems. Wastewater is typically

connected to the house drains and will add to the load on the household septic system. An RO

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unit delivering 5 gallons of treated water per day may discharge 40 to 90 gallons of wastewater

per day to the septic system.

Large-scale industrial/municipal systems have a production efficiency closer to 48%, because

they can generate the high pressure needed for more efficient RO filtration.

3.5 NANOFILTRATION [9]

Nanofiltration is a relatively recent membrane filtration process used most often with low total

dissolved solids water such as surface water and fresh groundwater, with the purpose of

softening (polyvalent cation removal) and removal of disinfection by-product precursors such as

natural organic matter and synthetic organic matter.

Nanofiltration is also becoming more widely used in food processing applications such as dairy,

for simultaneous concentration and partial (monovalent ion) demineralisation.

Fig. 2 Nanofiltration operation

3.5.1 Principle

Nanofiltration (NF) is a cross-flow filtration technology which ranges somewhere between

ultrafiltration (UF) and reverse osmosis (RO). The nominal pore size of the membrane is

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typically about 1 nanometer. Nanofilter membranes are typically rated by molecular weight cut-

off (MWCO) rather than nominal pore size. The MWCO is typically less than 1000 atomic mass

units (Daltons). The pressure drop across the membrane required is lower (up to 3 MPa) than the

one used for RO, reducing the operating cost significantly. However, NF membranes are still

subject to scaling and fouling and often modifiers such as anti-scalants are required for use.

3.5.2 Water purification applications

In much of the developing world, clean drinking water is hard to come by, and nanotechnology

provides one solution. While nanofiltration is used for the removal of contaminants from a water

source, it is also commonly used for desalination. As seen in a recent study in South Africa, tests

were run using polymeric nanofiltration in conjunction with a reverse osmosis process to treat

brackish groundwater. These tests produced potable water, but as the researchers expected, the

reverse osmosis removed a large majority of solutes. This left the water void of any essential

nutrients (calcium, magnesium ions, etc.), placing the nutrient levels below that of the required

World Health Organization standards. This process was probably a little too much for the

production of potable water, as researchers had to go back and add nutrients to bring solute

levels to the standard levels for drinking water consumption.

Providing nanofiltration methods to developing countries, to increase their supply of clean

water, is a very inexpensive method compared to conventional treatment systems. However,

there remain issues as to how these developing countries will be able to incorporate this new

technology into their economy without creating a dependency on foreign assistance.

3.5.3 Nanofiltration membranes broaden the use of membrane separation technology

Most reverse osmosis (RO) research has concentrated on the development of single-pass

seawater membranes. The success of these high rejection membranes has created interest in other

applications requiring less demanding salt rejection, or desiring the elimination of salt from a

feed stream (diafiltration), or having severe chemical resistance requirements. All would prefer

to operate at lower net driving pressures than demanded by the high rejection membranes. These

membranes have been designated as ―quo; nanofiltration‖quo; membranes to distinguish them

from the ―quo; hyperfiltration‖quo; seawater membranes. The first is XP45, a polyamide

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membrane with a low sodium chloride rejection that makes it an excellent candidate for

applications such as the processing of salty cheese wheys and pharmaceutical preparations. The

second is NF70, another polyamide, a low pressure membrane with rejections suited for

converting mildly brackish water and organic-laden raw water to potable water that meets WHO

standards. The third is XP20, a new developmental membrane for the maintenance of electroless

copper plating baths.

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4. DISTILLATION[9]

Distillation is a method of separating mixtures based on differences in their volatilities in a

boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a

chemical reaction.

Commercially, distillation has a number of applications. It is used to separate crude oil into more

fractions for specific uses such as transport, power generation and heating. Water is distilled to

remove impurities, such as salt from seawater. Air is distilled to separate its components—

notably oxygen, nitrogen, and argon—for industrial use. Distillation of fermented solutions has

been used since ancient times to produce distilled beverages with a higher alcohol content. The

premises where distillation is carried out, especially distillation of alcohol, are known as a

distillery.

4.1 Types of Distillation

1. Laboratory scale distillation

Simple distillation

Fractional distillation

Steam distillation

Vacuum distillation

Air-sensitive vacuum distillation

Other Types

o The process of reactive distillation involves using the reaction vessel as the still.

In this process, the product is usually significantly lower-boiling than its

reactants. As the product is formed from the reactants, it is vaporized and

removed from the reaction mixture. This technique is an example of a continuous

vs. a batch process; advantages include less downtime to charge the reaction

vessel with starting material, and less workup.

o Pervaporation is a method for the separation of mixtures of liquids by partial

vaporization through a non-porous membrane.

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o Extractive distillation is defined as distillation in the presence of a miscible, high

boiling, relatively non-volatile component, the solvent that forms no azeotrope

with the other components in the mixture.

2. Azeotropic distillation

Breaking an azeotrope with unidirectional pressure manipulation

Pressure-swing distillation

3. Industrial distillation

Multi-effect distillation

4.2 Separation of azeotropic mixtures [15]

A separation through the azeotropic point in one column can not be done. There is a need for

different unit operation for such kind of problems. For a main classification of azeotropic

distillation operation we can distinguish between unit operation with use of an entrainer

(extractive distillation and azeotropic distillation) and without an entrainer (Vacuum distillation

and pressure swing distillation).

There exist three types of azeotropic mixtures, the heterogeneous and the low boiling and the

high boiling homogeneous azeotropic mixtures. Homogeneous azeotrops have one liquid phase,

heterogeneous azeotrops separate into two liquid phases at the azeotropic point. These mixtures

have a miscibility gap. For low-boiling (e.g. acetonitrile/water) azeotrops the azeotropic mixture

is separated from the top of the column and the pure product from the bottom of the column. For

high-boiling azeotrops it is the other way around. The product is at the top, the azeotropic

mixture at the bottom of the column (e.g. water/nitric acid). The ten most produced basic

products in Germany (methanol, benzene, toluene, xylene, acetic acid, ...) generate over 120

homogeneous azeotropic mixtures [VCI 2006, Ponton 2007], so there is a big industrial

relevance for the separation of homogeneous azeotropic mixtures. In this work I will concentrate

on low boiling azeotrops because most azeotrops - especially those encountered in solvent

recycling applications - fall in this category [Frank 1997].

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Fig. 3 Y vs X Diagram

4.2.1 Extractive distillation

For the separation of homogeneous close boiling or azeotropic mixtures, extractive

distillationcould be used. A low volatile liquid is added to the mixture as an entrainer to increase

the volatility over the whole concentration region by decreasing the partial pressure or the

volatility of one component. The main problem of the process is the choice of the right entrainer.

The entrainer has to fulfil many different properties. The boiling point of the entrainer must be

much higher than the boiling points of the other components, it has to be thermal stable, cheap

and non toxic, to mention only the main characteristics [Düssel & Warter 1998]. In general, it is

difficult and expensive to use an entrainer because of the additional recycling process. This

means additional investment and operation costs and a more complex automation.

The newest type of extractive distillation uses ionic liquids as an entrainer. The main advantage

of ionic liquids is the absence of its own vapor pressure, so it is easy to separate them from

vaporizable liquids. Because of their saline character, they have a big influence on the phase

equilibrium. It is much easier to shift azeotropic points or create miscibility gaps [Beste et al.

2005, Jork et al. 2004, Seiler et al. 2004].

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4.2.2 Azeotropic distillation

In contrast to the extractive distillation the azeotropic distillation uses an entrainer to create a

heterogeneous low boiling azeotrope with one of the original components [Knapp & Doherty

1992, Lei et al. 2005]. In this case the phase separation of the condensed vapor is used. For this a

decanter on top of the column is necessary. Both liquid phases have different concentrations of

entrainer. For example the light phase has more entrainer with more low boiling liquid and in the

other phase has more high boiling liquid inside. Each phase is separated in a different column to

get pure products and recycle of the entrainer at the same time. So in this constellation the

process structure sketched in Fig.3 will be used.

The main disadvantage of the azeotropic distillation against the extractive distillation is the

higher energy demand because of the vaporization of the entrainer [Hoffmann 1964, Onken

1975, Doherty & Caldarola 1985, Lei et al. 2005].

4.2.3 Vacuum distillation

If it is possible to shift the azeotropic point with temperature change induced from a pressure

change, a pressure reduction in the column can be used. The azeotropic point shifts to higher

concentrations of the low boiling component and it is also possible to erase the azeotrope. The

disadvantages of the vacuum distillation are mainly the costs of the process and the complexity

of the process because of the vacuum, so it is not often used [Grassmann et al. 1997].

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4.2.4 PRESSURE SWING DISTILLATION

The pressure swing distillation (PSD) is a process for the separation of homogeneous azeotropic

mixtures.

The PSD process uses the pressure sensitivity of the binary azeotropic point [Sattler & Feindt

1995, Lei et al. 2005]. If the pressure is increased, the azeotropic point shifts to lower

concentrations of the low boiling component. So a separation of the azeotropic mixture at

different pressures is possible (Fig. 4). In this work the mixture acetonitrile/water is used as an

example for low-boiling homogeneous azeotropic mixtures.

Depending on the feed composition based on the component acetonitrile, the feed concentration

could be lower or higher than the azeotropic point. The effect is that it is possible to get two

different high-boiling products. If the feed concentration is lower than the azeotropic point, the

bottom product is water and above the bottom product is acetonitrile.

For the process structure this means that in the continuous case two columns operating at two

different pressures are needed or in the discontinuous case one column operating at two different

pressures in at least two loops.

XAcetonit

ririle

Fig. 4 Y vs X diagram of the mixture acetonitrile –

water at different pressure

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The main advantage of the PSD process is the process intensification which means an abdication

of an entrainer and therefore a reduction of columns and stages for the recycling of the entrainer.

Furthermore there is a possibility of heat integration for the continuous process. In this case the

heat of the condenser of the high pressure column (HP) is used for heating up the low pressure

column (LP). The disadvantages of the process are a higher complexity of the process and a

more complex automation, therefore the development of applicable process control strategies are

much more difficult. There is also a gap of experimental data in the literature and industrial

applications are seldom published. An overview about industrial applications and PSD-suitable

azeotropic mixtures is given in table 1. There is a big relevance for industry using this process.

One possible reason why process designers do not consider PSD is that azeotropic data

frequently are not available at non-atmospheric pressures and the generating of such data is

expensive [Frank 1997]. To solve the problem of missing azeotropic data see the work of

[Wasylkiewicz et al. 2003]. Wasylkiewicz and his co-author developed an algorithm that applies

bifurcation theory together with an arc length continuation and a rigorous stability analysis. This

method is a robust scheme for finding all homogeneous as well as heterogeneous azeotrops

predicted by a thermodynamic model at a specified pressure. Also a lot of research is done to

expand the thermodynamical properties data bases for pure components and mixtures [Gmehling

et al. 1981, Ponton 2007, Gmehling 2004].

4.2.4.1 Continuous pressure swing distillation

Two columns are in operation for the continuous pressure swing distillation system at two

different pressures (Fig. 4, Fig. 5-A). Feed streams with different concentrations have to be put

into the suitable column, depending on the concentration under or above the azeotropic point.

For concentrations under the azeotropic point, the feed is put into the low pressure column. For

concentrations above the azeotropic point the feed has to be put into the high pressure column. In

both columns pure product is withdrawn from the bottom, acetonitrile from the bottom of the

high pressure column and pure water from the bottom of the low pressure column. At the top of

the columns there are azeotropic mixtures with concentrations depending on the pressure in the

column. Each distillate stream is recycled into the other column, so there is a mass integration

between the columns. The respective distillation region of low and high pressure operation are

overlapping.

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4.2.4.2 Batch pressure swing distillation

The batch process is one of the best known distillation processes. It is mostly used in fine

chemistry, for seasonal products, in the pharmaceutical, and in food industry, despite the

competition of the continuous process [S0rensen 1994, S0rensen & Skogestad 1996, Mutjaba

2004]. Mainly the energy demand is much higher than for the continuous processes [Hasebe et

al. 1999]. But if the whole producing costs are considered there could be an advantage of the

discontinuous process compared to the continuous process [Oppenheimer & S0rensen 1997]. But

one main advantage is that the process structure (one column) is much simpler than for a

continuous operation and or flexible in the scope of product changes and also product amount

changes.

The discontinuous process uses one column which is operated in two loops at different operation

pressures (Fig. 5-C1/C2). In the first loop (e.g. atmospheric pressure) the mixture is added to the

column and the high boiling component (component 1, high boiling) is drained at the bottom and

the azeotropic mixture at the top. The process ends if the bottom purity runs out of specification

and then the process stops. After that the pressure will be changed (e.g. high pressure). The

pressure change leads to a shift of the azeotropic point and therefore of the azeotropic

concentration at the top of the column. Now the other component (component 2, high boiling)

will be drained from the bottom because the column operate in the other distillation region (Fig.

4). The azeotropic mixture (at a different pressure, means a different composition) will drained

from the top of the column. The process ends, if the specification runs out of the set points.

Examples of PSD binary azeotropes

Tetrahydofuran (THF) Water

Acetonitrile Water

Methanol Methyl Ethyl Ketone

Acetone Methanol

Ethanol Ethyl Acetate

Benzene Isopropanol

Phenol Butyl Acetate

Propanol Toluene

Acetic Acid Toluene

Table 2

20

Feed Feed

End of process: ErxJ of process;

Fig.5

Pressure swing distillation;

A: continuous,

B: semi continuous,

C1: discontinuous (inverted),

C2: discontinuous (regular).

21

5. PERVAPORATION

Pervaporation is a method for the separation of mixtures of liquids by partial vaporization

through a non-porous or porous membrane.

5.1 Applications

Pervaporation is effective for diluting solutions containing trace or minor amounts of the

component to be removed. Based on this, hydrophilic membranes are used for dehydration of

alcohols containing small amounts of water and hydrophobic membranes are used for

removal/recovery of trace amounts of organics from aqueous solutions.

Pervaporation is a very mild process and hence very effective for separation of those mixtures

which cannot survive the harsh conditions of distillation.

Solvent Dehydration: dehydrating the ethanol/water and isopropanol/water azeotropes

Continuous water removal from condensation reactions such as esterifications to

enhance conversion and rate of the reaction.

Removing organic solvents from industrial waste waters.

Combination of distillation and pervaporation/vapour permeation

Concentration of hydrophobic flavour compounds in aqueous solutions (using

hydrophobic membranes)

Recently, a number of organophilic Pervaporation membranes have been introduced to the

market. Organophilic Pervaporation membranes can be used for the separation of organic-

organic mixtures, e.g.:

Reduction of the aromatics content in refinery streams

Breaking of azeotropes

Purification of product stream after extraction

Purification of organic solvents

22

5.2 Recent advances in sulfur removal from gasoline by Pervaporation

Pervaporation (PV) is today considered as a promising unit operation for separation of

organic–organic liquid mixtures and is being investigated extensively in chemical and

petrochemical industries. Recently, PV applications in environment cleanup operations,

especially in the removal of sulfur compounds from gasoline have attracted increasing attention

worldwide. Gasoline desulphurization by PV is a newly emerged technology in which sulfur

components can be preferentially removed from the gasoline feed due to its higher affinity

with, and/or quicker diffusivity in the membrane. A considerable amount of background

information, current state and trends of the new PV application in gasoline desulphurization are

dealt with. The article focuses on the PV membranes development, interactions between

gasoline components and membranes, the improvement in process engineering,

techoeconomical analysis and the technology scale up. Finally, some suggestions for further

research were presented with the aim of reducing the cost in introducing the PV process into

refineries for desulphurization.

5.3 Recent advances in cellulosic membranes for gas separation and Pervaporation

Cellulose acetate membranes have been used commercially for many gas separation

applications in recent years. Advances have been made in understanding their behavior in the

presence of various vapors and under severe operating conditions, for example at very high

carbon dioxide and hydrogen sulphide partial pressures. In addition, a new membrane module

design has been developed for use in high recovery systems and at high gas flow rates.

Extension of cellulose acetate gas separation membrane technology into the Pervaporation field

has resulted in a new application related to the production of methyl t-butyl ether (MtBE). In

this case the membrane is used to remove methanol from MtBE and hydrocarbons to increase

the reaction yield.

23

5.4 Novel hybrid separation processes based on Pervaporation for THF recovery

Design of novel hybrid processes is performed for the industrial recovery of tetrahydrofuran

(THF) from two different highly non-ideal mixtures. The novel hybrid processes combine the

advantages of batch and/or continuous distillations, extractive distillation, and pervaporation

for efficient separation of highly non-ideal mixtures. Pervaporation, used for the final

dewatering of the THF, is the last step in the separation technologies and carried out with

available industrial technology (Sulzer Chemtech GmbH, Batch pervaporation BP models,

PERVAP® 2210 membrane type). It is necessary, however, to design a proper pre-separation

process of the mixture to be pervaporated and coordinate its operation with the pervaporation

unit. Since methanol is also present in one of the mixtures, its behavior during pervaporation is

also investigated. It shows that since the methanol is able to permeate the membrane, it should

be separated in the pre-separation process. The vapor–liquid equilibrium data indicate that with

extractive distillation it becomes possible to break the THF—methanol azeotrope and the

appropriate application of pervaporation makes the further reduction of the recovery costs

possible. The total annual costs of the novel hybrid separation processes range between 10.3

and 54% of that of the old technology based on chemical dewatering. The THF loss decreases

to the 7.5 and 17% of that of the old technology and shows that pervaporation is also a

powerful tool for the application of the principles of the sustainable development and

consumption.

24

6. Reactive Distillation [11]

Reactive distillation is an advanced technique of reaction process operation.

The (energetic) advantages of reactive distillation over a process comprising a reactor and a

distillation unit including a recycle stream for unconverted reactant. The reactor + distillation

column was optimized with respect to reboiler energy consumption via the recycle and reflux

ratio.

Reactive distillation is a process where the chemical reactor is also the still. Separation of the

product from the reaction mixture does not need a separate distillation step, which saves energy

(for heating) and materials.

This technique is especially useful for equilibrium-limited reactions such as esterification and

ester hydrolysis reactions. Conversion can be increased far beyond what is expected by the

equilibrium due to the continuous removal of reaction products from the reactive zone. This

helps reduce capital and investment costs and may be important for sustainable development

due to a lower consumption of resources.

Being a relatively new field, research on various aspects such as modeling and simulation,

process synthesis, column hardware design, non-linear dynamics and control is in progress.

The suitability of RD for a particular reaction depends on various factors such as volatilities of

reactants and products along with the feasible reaction and distillation temperature. Hence, the

use of RD for every reaction may not be feasible. Exploring the candidate reactions for RD,

itself is an area that needs considerable attention to expand the domain of RD processes.

WHY REACTIVE DISTILLATION (RD)?

Benefits

Increased speed

Lower costs – reduced equipment use, energy use and handling

Less waste and fewer byproducts

25

Improved product quality– reducing opportunity for degradation because of less heat

Consider a reversible reaction scheme: A + B ⇌ C + D where the boiling points of the

components follow the sequence A, C, D and B. The traditional flow-sheet for this process

consists of a reactor followed by a sequence of distillation columns; see Fig.6. The mixture of

A and B is fed to the reactor, where the reaction takes place in the presence of a catalyst and

reaches equilibrium. A distillation train is required to produce pure products C and D. The

unreacted components, A and B, are recycled back to the reactor. In practice the distillation

train could be much more complex than the one portrayed in Fig. 6(a) if one or more

azeotropes are formed in the mixture. The alternative RD configuration is shown in Fig. 6(b).

The RD column consists of a reactive section in the middle with non-reactive rectifying and

stripping sections at the top and bottom. The task of the rectifying section is to recover reactant

B from the product stream C. In the stripping section, the reactant A is stripped from the

product stream D. In the reactive section the products are separated in situ, driving the

equilibrium to the right and preventing any undesired side reactions between the reactants A

(or B) with the product C (or D). For a properly designed RD column, virtually 100%

conversion can be achieved.

Fig.6

The most spectacular example of the beneits of RD is in the production of methyl acetate.

26

Fig. 7

(a) Reactive distillation concept for synthesis of MTBE from the acid-catalysed

reaction between MeOH and isobutene. The butane feed is a mixture of reactive

iso-butene and non-reactive n-butene. (b) Reactive distillation concept for the

hydration of ethylene oxide to ethylene glycol (c) Reactive distillation concept for

reaction between benzene and propene to form cumene. (d) Reactive distillation

concept for reaction production of propylene oxide from propylene chlorohydrins and

lime. The reactive sections are indicated by grid lines.

For the acid catalysed reaction between iso-butene and methanol to form methyl tert-butyl

ether: iso-butene + MeOH ⇌ MTBE, the traditional reactor-fol-lowed-by-distillation concept is

particularly complex for this case because the reaction mixture leaving the reactor forms three

minimum boiling azeotropes. The RD implementation requires only one column to which the

butenes feed (consisting of a mixture of n-butene, which is non-reactive, and iso-butene which

is reactive) and meth-anol are fed near the bottom of the reactive section. The RD concept

shown in Fig. 7(a) is capable of achieving close to 100% conversion of iso-butene and

methanol, along with suppression of the formation of the unwanted dimethyl ether

(Sundmacher, 1995). Also, some of the azeotropes in the mixture are "reacted away" (Doherty

& Buzad, 1992).

27

For the hydration of ethylene oxide to mono-ethylene glycol: EO + H2O⟶ EG, the RD

concept, shown in Fig. 7(b) is advantageous for two reasons (Ciric & Gu, 1994). Firstly, the

side reaction EO + EG → DEG is suppressed because the concentration of EO in the liquid-

phase is kept low because of its high volatility. Secondly, the high heat of reaction is utilised to

vaporise the liquid-phase mixtures on the trays. To achieve the same selectivity to EG in a

conventional liquid-phase plug-low reactor would require the use of 60% excess water (Ciric &

Gu, 1994). Similar beneits are also realised for the hydration of iso-butene to tert-butanol

(Velo, Puig-janer & Recasens, 1988) and hydration of 2-methyl-2-butene to tert-amyl alcohol

(Gonzalez & Fair, 1997).

Several alkylation reactions, aromatic + olefin ⇌ al-kyl aromatic, are best carried out using the

RD concept not only because of the shift in the reaction equilibrium due to in situ separation

but also due to the fact that the undesirable side reaction, alkyl aromatic + olefin ⇌ di-alkyl

aromatic, is suppressed. The reaction of propene with benzene to form cumene, benzene +

propene ⇌ Cumene (Shoemaker & Jones, 1987; see Fig. 7(c)), is advantageously carried out in

a RD column because not only is the formation of the undesirable di-isopropylben-zene

suppressed, but also the problems posed by high exothermicity of the reaction for operation in

a conventional packed-bed reactor are avoided. Hot spots and runaway problems are alleviated

in the RD concept where liquid vaporisation acts as a thermal lywheel. The alkylation of iso-

butane to iso-octane, iso-butane + n-butene ⇌ iso-octane, is another reaction that benefits from

a RD implementation because in situ separation of the product prevents further alkylation: iso-

octane + n-butene ⇌ C12 H24 (Doherty & Buzad, 1992).

The reaction between propylene chlorohydrin (PCH) and Ca(OH)2 to produce propylene oxide

(PO) is best implemented in an RD column, see Fig. 7(d). Here the desired product PO is

stripped from the liquid-phase by use of live steam, suppressing hydrolysis to propylene glycol

(Bezzo, Bertucco, Forlin & Barolo, 1999).

28

7. Advanced purification of petroleum refinery wastewater by catalytic vacuum

distillation.

In our work, a new process, catalytic vacuum distillation (CVD) was utilized for purification of

petroleum refinery wastewater that was characteristic of high chemical oxygen demand (COD)

and salinity. Moreover, various common promoters, like FeCl(3), kaolin, H(2)SO(4) and

NaOH were investigated to improve the purification efficiency of CVD. Here, the purification

efficiency was estimated by COD testing, electrolytic conductivity, UV-vis spectrum, gas

chromatography-mass spectrometry (GC-MS) and pH value. The results showed that NaOH

promoted CVD displayed higher efficiency in purification of refinery wastewater than other

systems, where the pellucid effluents with low salinity and high COD removal efficiency

(99%) were obtained after treatment, and the corresponding pH values of effluents varied from

7 to 9. Furthermore, environment estimation was also tested and the results showed that the

effluent had no influence on plant growth. Thus, based on satisfied removal efficiency of COD

and salinity achieved simultaneously, NaOH promoted CVD process is an effective approach

to purify petroleum refinery wastewater.

29

8. Recent Advances in Catalytic Distillation

Catalytic distillation (CD) is a novel green reactor technology that combines a heterogeneous

catalytic reaction and separation via distillation in a single distillation column. It is an excellent

example of process intensification. There are many possible advantages in carrying out a

chemical process using CD. They include enhanced product yield and selectivity, reduction of

capital and operating costs, enhanced catalyst lifetime, reduction of waste treatment streams,

and higher energy efficiency. Owing to the current concern of the impact of greenhouse gases

such as carbon dioxide on the environment, a major benefit of CD is related to the utilization of

the reaction heat for distillation, which reduces the energy consumption and hence reduces the

production of greenhouse gases.

30

9. Hybrid separation processes—Combination of reactive distillation

with membrane separation [14]

Over the years, the focus of the chemical and process industry has shifted towards the

development and application of integrated processes combining the mechanism of reaction and

separation in one single unit. This trend is motivated by benefits such as a reduction in

equipment and plant size and improvement of process efficiency and safety, and hence a better

process economy. Reactive distillation is an important example of a reactive separation

process. Especially for equilibrium reactions like esterifications, ester hydrolysis and

etherifications, the combination of reaction and separation within one zone of a reactive

distillation column is a well-known alternative to conventional processes with sequential

reaction and separation steps (Hiwale et al., 2004; Kaibel et al., 2005). In several cases, non-

ideal aqueous-organic mixtures are formed which tend to form azeotropes. They can be

overcome using membrane separations like pervaporation and vapour permeation since they

are very selective and not limited by vapour-liquid equilibrium (Rautenbach, 1997).

Consequently, a hybrid process consisting of membrane-assisted reactive distillation

contributes to sustainable process improvement due to arising synergy effects and allows for

reduction of investment and operational costs.

A review of hybrid processes combining pervaporation with one or more other separation

technologies can be found in (Lipnitzki et al., 1999). The analysis of hybrid separation

processes combining membrane separation with conventional distillation is described in (Kreis

and Gorak, 2006). An example for the investigation of a reactive hybrid process concept is the

transesterification of methyl acetate and butanol to butyl acetate and methanol by the

combination of reactive distillation and pervaporation, as examined by (Steinigeweg and

Gmehling, 2004). The industrially operated hybrid process for the continuous production of

fatty acid esters by reactive distillation and pervaporation is presented by (von Scala et al.,

2005). In this work, the heterogeneously catalysed esterification of propionic acid (ProAc) with

1-propanol (POH) to n-propyl propionate (ProPro) and water (H2O) is investigated:

C3H8O + C3H6O2 ⇌ C6Hl2O2 + H2O (1)

The esterification reaction is reversible; the equilibrium constant is a weak function of

temperature. As catalyst, the strongly acidic ion exchange resin Amberlyst 46TM

from Rohm &

31

Haas is used. Amberlyst 46TM

has acidic active sites (sulfonic-acid groups) only at the surface

of the styrene-co-divinylbenzene matrix (Lundquist, 1995). This catalyst shows thermal

stability up to 120°C and is tailor-made for esterifications because the competing side product

formations, e.g. etherification and dehydration of the alcohol, are suppressed (Blagov et al.,

2006).

9.1 Process description

One possible process alternative for n-propyl propionate synthesis in one apparatus is the

removal of the desired product (ProPro) at the bottom of the reactive distillation column while

at the top, an almost azeotropic aqueous-organic mixture (POH/H2O) is obtained. A

hydrophilic membrane unit is located in the distillate stream to remove water out of the

process. The water depleted retentate is recycled back to the column. The coupling of the

reactive distillation column with a membrane module results in a hybrid process (Figure 8).

Fig. 8 Reactive distillation column with a membrane separation located in the distillate stream

32

10. Production of Pure Ethanol from Azeotropic Solution by Pressure Swing

Adsorption [12]

Ethanol and water mixture forms an azeotrope at a temperature of 351K and a mixture

concentration of 95% (w/w) ethanol. With the rising demand of pure ethanol for various

applications, there is a need of getting nearly pure ethanol considering the energy and

efficiency aspects.

Conventionally, azeotropic distillation has been employed in production of fuel-ethanol. In

Azeotropic distillation, dehydration is carried out in presence of entrainer like benzene or

cyclohexane. Although benzene has been banned in several countries for its carcinogenic

effect, cyclohexane is still being employed. Moreover, this distillation method is very energy

intensive.

To bring down energy consumption and to ensure high level of dryness in final ethanol

product, Zeolite has proved to be ideal. There have been several researches on adsorption of

water from ethanol/water mixture and it is suggested that dehydration by adsorption on 3A

zeolite has the advantage that the micropores are too small to be penetrated by alcohol

molecules so that water is adsorbed without competition in the liquid phase.

It requires little energy input and operates on cycles of short duration. Therefore, it has high

adsorbent productivity and is often capable of producing very pure product. Despite many

literatures which studied on adsorption of water on 3A zeolite through simulations and

experimental works, there has been no real effort on the investigation of its productivity and

performance on actual PSA system. This research aims to study the actual effects of different

operating parameters on the efficiency of PSA system mainly in terms of product recovery and

enrichment.

33

The process flow diagram for Pressure swing adsorption is shown in the Fig.9.The parameters

which affect the adsorption rate, recovery and enrichment of ethanol are feed rate, feed

concentration, adsorption pressure and cycle time.

Fig. 9 Process flow diagram of the PSA pilot plant

34

Table 3

It can be seen in from the experimental results that increasing flow rate and cycle time could

significantly increase the percentage of ethanol recovery. After every given cycle time the

adsorber needs to be regenerated. Certain amount of ethanol that is left in the voidage and dried

ethanol that is used as purge stream are taken out during regeneration. As a result, the higher

the amount of ethanol being fed during adsorption process, the higher the percentage of the

ethanol recovery of the PSA system. Likewise, the shorter the cycle time, the more amount of

dried ethanol as purge stream is needed during regeneration. However, the effect of the cycle

time on the product concentration cannot be clearly seen since the amount of zeolite packed in

the adsorber was in abundant and the breakthrough had not yet occurred.

It can be suggested that increasing adsorption pressure to increase the water partial pressure

and hence increasing the adsorption capacity can improve the quality of the product or the

ethanol concentration. Furthermore, it was shown that as the flow rate is increased, the ethanol

product concentration was also higher.

35

11. Hybrid PSA-Membrane Gas Separation Process [13]

Advances in non-cryogenic gas separation process applications over the past 20 years have

been driven by the need to improve efficiency and reduce cost, via alternatives to several

traditional, energy-intensive gas separation processes (distillation, chemical absorption). High-

purity hydrogen, which is foreseen as the fuel for the future, is commercially produced by

pressure swing adsorption (PSA), a typically low product recovery process. Previous studies

(Sircar et al., 1999; Sircar & Golden, 2000) identified that integrating a membrane module into

PSA can improve the overall recovery of the separation process. Membrane gas separation

processes are also shown to be cost-effective in separating greenhouse gases from gaseous

mixtures at high purity (CO2 capture and sequestration). Numerous studies (Bhide et al., 1998;

Naheiri et al., 1997, Zolandz & Fleming, 1992) show that combinations of a membrane module

and another separation process offer lower cost and better separation performance than an all-

membrane separation system. The first combination of a membrane and an adsorption

separation process is attributed to Mercea and Hwang (1994); a PSA unit was used to improve

the O2 enrichment performance of a Continuous Membrane Column (CMC), and the

combination featured superior economics and separation performance over both PSA and CMC

processes. Feng et al. (1998) proposed an integrated process in which gas permeation is

included in the sequential steps of PSA, hence considering permeation occurring in a cyclic

fashion. Hydrogen purification from a gaseous mixture has also been studied: results show that

a hybrid PSA-membrane achieves higher purity compared with a standalone PSA process.

Other PSA-membrane combinations are shown to improve the performance of either of the two

units (Sircar et al., 1999; Esteves & Mota, 2002), yet none presents a detailed mathematical

model and numerical solution procedure for simulation and optimisation. The main goal of this

paper is thus to study the potential of a PSA-membrane HSS by developing a rigorous

mathematical model for its dynamic simulation and optimisation, and by using it to obtain

relevant results and design conclusions. Air separation is the exemplary case study for the

hybrid gas separation process.

36

11.1 Process Description: Hybrid Separation Systems (HSS)

All hybrid PSA-membrane processes are classified into two categories in the literature: (a)

Membrane followed by PSA (Class I), (b) PSA followed by membrane (Class II). Rigorous

mathematical models combine all equations describing the dynamic behaviour of the

membrane separation module into the cyclic operating steps of the PSA process; such models

are sets of Integral Partial Differential and Algebraic equations (IPDAEs) and their

implementation for dynamic simulation and optimization is often challenging and

cumbersome.

HSS I: In a Class-I HSS flowsheet, the membrane comes before the PSA (Figure 10a). The

first processing step is feeding fresh compressed gas into a hollow fibre module: the permeate

is obtained at the shell side (atmospheric pressure), while the residue stream (assumed to be at

feed pressure) is obtained at the tube side of the fibre module. Depending on PSA selectivity,

the membrane residue or permeate is used as PSA feed: in N2 production (HSS with same

selectivity) the N2-rich (residue) stream is fed to PSA; in O2 production (HSS with opposite

selectivity) the O2-rich (permeate) stream is used. Either the residue or the recompressed

permeate is fed in the first step (pressurisation), yet the high-pressure residue stream is the only

fed in the second step (adsorption).

HSS II: In a Class-II HSS flowsheet, the membrane comes after the PSA (Figure 10b); Sircar

et al. (1999) considered such a HSS to improve the recovery of a H2 PSA process. The cyclic

steps of this HSS start with fresh feed introduction into the PSA unit; then, the purge gas from

each PSA bed passes through the membrane to increase recovery. The membrane residue

stream obtained can be recycled as fresh feed to the PSA bed or (in the case of multiple beds),

the permeate stream can be used for purging other beds. Generally, feed conditions for the PSA

unit depend on the membrane module (HSS I); feed conditions for the membrane unit depend

on the PSA beds effluent (HSS II).

The present study is based on separation selectivity towards the target species. A binary gas

mixture Hybrid Separation System (HSS) in which the gas more adsorbed in the PSA is more

permeable through the membrane is a HSS with same selectivity; when the same gas is the

least permeable, then we have a HSS with opposite selectivity. Polymeric membranes are

usually only selective to O2 (O2 being obtained as permeate), but for PSA, either O2 or N2 can

be more adsorbed (depending on the adsorbent used). The combined HSS mathematical model

37

of this paper thus considers (Akinlabi, 2006): (a) A dual-bed PSA unit (producing N2 on

carbon molecular sieve and O2 on zeolite 5A), and (b) A steady-state, isothermal, cross-flow

permeation hollow fibre membrane module.

Fig. 10 The two Hybrid Separation System (HSS) flow sheets considered:

(a) HSS I,

(b) HSS II.

38

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39