PERVAPORATION

PERVAPORATION

331 SEPARATION OF LIQUID MIXTURES/PERVAPORATION Applications of Pervaporation Per~,aporation is unique among membrane separations, involving a change...

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SEPARATION OF LIQUID MIXTURES/PERVAPORATION Applications of Pervaporation Per~,aporation is unique among membrane separations, involving a change of phase to achieve separation. It offers a means of separating miscible liquids of similar molar mass and is an alternative method to distillation. However the relative cost of membrane units in comparison to distillation equipment means that in most separations, where distillation performs efficiently, pervaporation is not a viable alternative. In addition pervaporation will not generally be economically viable as a multi-stage separation process. Where pervaporation can prove to be a useful method is in the separation, or removal, of small amounts of one liquid from a liquid mixture. Pervaporation also then becomes attractive in the separation of liquid mixtures which form azeotropes, ie where vapour and liquid have the same composition in equilibrium, as standard distillation cannot achieve this separation. There are many mixtures of organic solvents and organic solvents with water which exhibit azeotropic compositions. The composition frequently has a low percentage of one component, for example of water, in azeotropes of alcohols (see Table 1). Included in Table 1 are typical permeation rates and selecfivites achieved in pervaporation with a grafted (poly vinyl pyrrolidine onto PTFE) membrane. There are potentially a large number of separations amenable to pervaporation which can be classified as either mixtures with water (aqueous) or non-aqueous mixtures (see Table 2). This range of mixtures occurs predominately in the chemical industry, although other applications are in the food and pharmaceutical industries to process heat-sensitive produc~s, in waste water applications to remove trace quantities of volatile organic (chlorinated hydrocarbons) and in analytical procedure to concentrate components for detection. Pervaporation is used to overcome the difficult stage of a separation, ie to overcome azeotrope limitations in distillations. It would rarely be used as an isolated method rather it would be part of a hybrid separation with for example distillation or even reverse

332

HANDBOOK OF INDUSTRIAL MEMBRANES

T A B L E 1 - Liquid mixtures with azeotropic composition. Typical positive azeotropic mixtures (characterised by a minimum boiling temperature) which can be separated by pervaporation through a permselective membrane obtained by grafting polyvinylpyrolidone onto a thin polytetrafluoethylene film [ 1] ,

A-B Azeotropes (A=Fast component)

Tb (~

,

Azeotrope characteristics

,,,

,

i

Selectivity

,

To(~

c(%)

I

i Permeate flux ' (kg/h m 2) I

A: Chloroform B:n-Hexane

61.2 69.0

60.0

72.0 28.0

3.9

1.25

2.65

A: Ethanol B" Cyclohexane

78.5 81.4

64.9

30.5 69.5

16.8

2.89

1.10

A: Butanol-1 B" Cyclohexane

117.4 81.4

78.0

10.0 90.0

23.5

7.23

0.30

A: Water B: Ethanol

100.0

78.2

4.4 95.6

2.9

2.68

2.20

A: Water B" t-Buthanol

100.0 82.8

79.9

11.8 88.2

41.0

7.17

0.35

A: Water B: Tetrahydrofurane

1O0 65.5

63.8

5.7 94.3

19.1

9.24

0.94

A: Water B: Dioxane

100.0 101.3

87.8

18.4 81.6

18.1

4.36

1.33

A: Ethanol B" Ethylacetate

78.5 77.2

71.8

31.0 69.0

2.4

1.67

0.95

A: Methanol B: Acetone

64.7 56.2

55.7

12.0 88.0

2.9

2.36

78.5 80.1

67.8

32.4 67.6

1.3

1.18

82.8

A: Ethanol B : Benzone

i Ul

II

. . . . . . .

i

0.65 2.90

|

The selectivities e~ and 13are defined by the following ratios, respectively: a-

(c'A/c'a)_

c'(1-c)

(CA/CB)

C(1--C') '

b = - C' C

Where c and c' are the weight concentrations of the faster permeant (A) in the feed (c) and the permeate (c'), respectively. Tb denotes the normal boiling point of an organic compound or of an azeotropic mixture (at 1 atm). Pervaporation temperature: T = 25~

SEPARATION OF LIQUID MIXTURES/PERVAPORATION

333

T A B L E 1 (continued) - Liquid mixtures with azeotropic composition. Fractionation of negative azeotrpic mixtures (characterised by maximum boiling temperature) by pervaporation through a membrane obtained by grafting polyvinylpyrolidone onto a thin polytetrafluoroethylene film [ 1]

A-B Azeotropes (A=Fast component)

Tb (~

A: Chloroform B" Acetone A: Chloroform B" M.E.K.

Azeotrope characteristics

Tb(~

c(%)

61.2 56.2

64.7

80.0 20.00

61.2 79.6

79.9

Permeate flux (kg/h m 2)

l

Selectivity

. i

1.8

1.10 I

17.0 83.0

i

0.85 1.50

1.0

1.0

1.4

1.09

1.25

1.0

1.0

2.74

77.0 23.0

2.7

1.17

0.27

J

A: Butanol- 1 B: Pyridine

117.7 115.3

118.7

71.0 29.0

A: Water B: Formic acid

100.0 100.7

107.1

22.5 77.5

A: Acetic acid B" Dioxane

118.1 101.3

A: Acetic acid B" D.M.F.

118.1 153.0

159.0

26.0 74.0

1.2

1.14

0.04

A: Formic acid B" Pyridine

100.7 115.3

150.0

53.5 36.5

2.8

1.31

0.22

A: Acetic acid B: Pyridine

118.1 115.3

139.7

35.0 65.0

1.0

1.0

! !

A: Propionic acid B: Pyridine

119.5

140.9 115.3

74.0 26.0

0.09 I

I

[__

A = Faster permeant; T = 25~ Parameters c~, 13, c and c' and Tb are the same as in Table 5.1 M.E.K. = Methylethylketone; D.M.F. = N, N-dimethylformamide.

TABLE 2 - Classification of mixtures for pervaporation.

Aqueous

Dehydration Trace organics

Non Aqueous

Polar/non polar

water/ethanol (ethanol/water, aromatics/water)

.,

Aromatics/aliphatics S aturatd/unsaturated Isomers ..

alcohols/aromatics (methanol/toluene) alcohols/aliphatics (ethanol/hexane) (cyclohexane/benzene) (butane/butene) (C8 isomers)

HANDBOOK OF INDUSTRIAL MEMBRANES

334

F I G U R E

1 -

A process flow diagram of pervaporation.

osmosis. Thus for example with an azeotrope with a high water content (water/pyridine) pervaporation would only be used once to produce the two non-azeotrope fractions for further processing. The most important applications of pervaporation are removal of water from organics removal of organics from water(and gases) separation of organic mixtures concentration of aqueous solutions

Isopropyl alcohol dehydration plant.

SEPARATION OF LIQUID MIXTURES/PERVAPORATION

335

TABLE 3 - Applications of pervaporation.

MAIN APPLICATIONS Dehydration o f organic solvents and mixtures

9 9 9 9 9 9

alcohols : ethanol, propanol, butanol, etc., ketones : acetone, MEK, MIBK, etc., esters : ethyl, butyl, propyl, acetates, etc., others : THF, dioxane, MTBE, glycol ether, acetonitrile, etc., acetic acid, organic amines, pyridine.

The PV process can easily dehydrate solvent mixtures. The binary or ternary azeotropes can be dehydrated without the use of entrainers. In some cases, where distillation is not possible, prevaporation is the only alternative to costly incineration of waste solvent streams. 9 Debottlenecking of existing entrainer distillation plants is also an effective application. Removal o f organics f r o m aqueous streams

Several applications are covered by PERVAP| organophilic membranes. 9 Waste Waterpurification

A wide range of organic solvents can be extracted: 9 hydrocarbons, chlorinated hydrocarbons, 9 esters, ketones, ethers, 9 alcohols. The PERVAP| systems can reduce COD requirement of water water streams going to biological treatment units, or preconcentrate organic wastes to incinerators. 9 Wine and beer dealcoholisation

The main advantage of PERVAP| systems in this case is a far better recovery of aromas than with any other process. 9 Aroma recovery and concentration

PERVAP| membranes allow recovery and concentration of many compounds in the food industry. The main industrial applications are in the dehydration of organic solvent mixtures and the removal of organics from aqueous streams (see Table 3). Dehydration

The dehydration of organic/water mixtures has rapidly become one of the main areas of application of PV. The splitting ofwatefforganic azeotropes by pervaporation is common, either used as the sole dehydration step or used in conjunction with distillation for final dehydration. There are many organic streams in industry which become contaminated with, or contain low concentrations, of water (<10%). Methods to achieve dehydration include distillation and adsorption. Distillation will become very expensive when used to remove small amounts of water. Adsorption with desiccants (alumina, zeolities), although having relatively low capital outlay, requires regeneration. This regeneration requires high

HANDBOOK OF INDUSTRIAL MEMBRANES

336

energy costs. Adsorbent replacement costs, disposal and generation of hazardous gaseous effluents are also other factors against adsorption. Table 4 gives a list of organics currently dehydrated by pervaporation. Pervaporation is generally economic with water contents of approximately 10% and less, with final product water content of hundreds ofppm to 10 ppm attainable. To go much below these water contents requires significantly greater installed membrane area and possibly a greater reduced pressure on the permeate side. TABLE 4 - Organic solvents dehydrated by pervaporation.

Solvent

1-Butanol n-ButanoL t-Butanol THF Xylene Methanol Methanol/IPA Caprolactam Ethanol/IPA

Water Content Feed Product (wt.%) (ppm)

8.4 5.4 10.4 0.4 0.1 7.1 0.21 10.3 0.6

135 800 581 220 140 1650 300 671 610

Solvent

Water Content Feed Product (wt.%) (ppm)

Ethanol/MeOH Ethanol/benzene Allyalcohol Trichlene MEK Methylene chloride Ethylene dichloride Chlorothene

2.9 14.1 4.85 0.01 3.8 0.20 0.22 0.0617

780 320 620 8 220 140 10 12

For water contents above 10%, extraction and other methods are generally more economic, while below 0.1% content of feed, adsorption is generally preferable. Many of the applications cited in Table 4 are for the small scale recycling of solvents (acetone, isopropyl alcohol) in pharmaceutical and speciality chemical companies where water contamination arises from a reaction or from other separations. Other applications are for bulk chemical processing (ethylene glycol, methyl ethyl ketone) where the objective is generally debottling and improvement in economics of existing distillations and perhaps adsorption. Anhydrous Alcohol Production

Ethanol production is either based on fermentation or on synthesis methods such as the sulphuric acid process and direct catalytic hydration (of ethene). Fermented ethanol product is typically 8% to 12% by volume, which after several stages of distillation to rectify and purify is produced as a near azeotropic mixture. Anhydrous ethanol for chemical and fuel use is obtained typically by azeotropic distillation with benzene trichoroethene, etc. The direct hydration route uses extractive distillation, with water, to free the ethanol of impurities and is thus purified by distillation in a similar manner to the fermented product. Azeotropic distillation is a relatively expensive procedure and in addition there is some concern on environmental and health grounds over the use of some of the dehydrating agents. Pervaporation is considered to be an appropriate and competitive replacement for azeotropic distillation in the production of anhydrous ethanol (see Fig 2). A product of 99.5% ethanol is produced and a permeate, containing a relatively high percentage of

SEPARATION OF LIQUID MIXTURES/PERVAPORATION

337

Azeotropic__ Mixture,~~'~..._ ..... r L ~

Feed. . Solution 9

I ~ team L'~ Water

"~'

Distillation Tower

F..0 Solution 9! -V

Steam~

Azeotropic Distillation Tower

"i

Vacuum Pumo

,~ ~Water Alcohol Distillation Tower

Water Steam r~Alcohol

Water

Pervaporation Separation Unit

~ . . _ . ~ Anhydrous ethanol (99.3vot% EtOH)

[ Azeotropic mixture (96.4vo1% EtOH)

L,~-J

]CApervaporationmembrane

Permeate fraction 88.5vo1% EtOH 11.5vo1% H20

§

recycled as feed (to distill tower)

FIGURE 2 - Dehydration of alcohol by pervaporation and distillation.

HANDBOOK OF INDUSTRIAL MEMBRANES

338

ethanol, which is recycled back to distillation. It is particularly acceptable method of ethanol production for medical use. The installation of pervaporation plants has slowly increased over the last ten years (see Table 5). These units are of varying capacity, with the largest designed to produce 150,000 dm 3 per clay of anhydrous ethanol (0.2%) water from a pre-distilled feed of 93 % ethanol. The membrane area required is 2,200 m 2, which is the form ofthe fiat sheets of a composite of a PVA active layer/PAN backing layer/PET supporting layer in several plate and frame modules. T A B L E 5 - P e r v a p o r a t i o n unit installations.

Prevaporation operation Ethanol dehydration B6th6niville sugar refinery, France (150,000 1 d-~) Provins sugar refinery, France (30,000 1 d-~) Smaller plants (1,000-12,000 1 d-~)

No. of plants 1 1

11

Isopropanol dehydration Production capacity ranging from 5,000 to 15,000 1 d-t Dehydration of ethylacetate (1,000-6,000 1 d-~) Dehydration of ethers (tetrahydrofurane, dimethoxyethane) Production capacity ranging from 2,000 to 6,000 1 d-~ Dehydration of ketones (6,000 1 d-~) Dehydration of other organic solvents Production capacity ranging from 750 to 15,000 1 d-l Multipurpose plants (integrated systems) Total number of operational units + 25 pilot plants (4m2 surface area membrane each) installed to test the applicability of the technique to potential fractionation problems t |

6

3 33

SEPARATION OF LIQUID MIXTURES/PERVAPORATION

Plate and frame module. Solvent Concentration Inlet Outlet Flow rate Utility requirements Steam Power Membrane area

Height Length Width

339

Spiral wound module.

Ethanol

Ethanol

Ethanol

Ethanol

85.7% 99.8%

93.9% 99.8%

85.7% 99.95%

93.9% 99.95%

1,195 kg/h

1,500kg/h

840 kg/h

970 kg/h

195 kg/h 85 kW

110 kg/h 85 kW

145 kg/h 85 kW

83 kg/h 85 kW

480 m2 3,000 mm 7,500 mm 2,000 mm Ethanol dehydration plant.

Plate and frame pervaporation module.

HANDBOOK OF INDUSTRIAL MEMBRANES

340

Solvent Recycling Electronics and pharmaceutical industries require good quality solvents for drying and cleaning purposes. During use many solvents become contaminated with water and thus need to be refined before recycling. Azeotropic distillation is commonly used in these applications. However pervaporation can economically remove this water and reduce the concentration to a few tens of ppm (see Table 6).

TABLE 6 - Relative economics of pervaporation and azeotropic distillation for isopropyl alcohol refining.

Utility consumption Steam (kg/h IPA) Electricity (kwh/h - IPA) Cooling water (t/h - IPA) Running cost (Yen/kg- IPA) Steam Electricity Cooling water Entrainer (Benzene) Membrane Total

Prevaporation

Azeotropic distillation

0.3 0.03 0.055

1.6 0.01 0.01

0.9 0.6 0.55

4.8 0.2 1.0 0.03

1.9 6.03

3.95 , ,

where: Steam Electricity Cooling water Membrane renewal

: : : :

3,000 yen/t 20 yen/kw - h 10 yen/t once/3 years

Solvent vapours such as chlorinated hydrocarbons are generated in several industries, eg electronic, pharmaceutical, and for environmental reasons are recovered and not released to atmosphere. Adsorption on activated carbon is effective, but this on regeneration with steam, produces a mixture of condensed water and solvent. After phase separation of this mixture a solvent phase, containing a low percentage of water, is produced. Dehydration of this solvent by pervaporation is effective.

Examples In many cases where water contaminated solvents occur the concentration of the water in the permeate will give a mixture which will readily phase separate on cooling. This phase separation allows much greater recovery of the organics. A typical example is the dehydration of dichloroethene (shown in Fig 3.) or ethlene dichloride (EDC). In this plant saturated EDC (0.2% wt H20) form a condenser is preheated before entering the PV module equipped with PVA membranes. The pervaporation produces a purified EDC (< 10 ppm water) in one pass, with a permeate containing approximately 50% water. This

SEPARATION OF LIQUID MIXTURES/PERVAPORATION

341

0.15 wt.% H 20 90oc

700

tuba1

Saturated EDC ~ (0.2 wt.% H20 t)9.8wt.% EDC) Preheat

Steam 6 bar

[

PV

45-55% H 20

= Membranes

1

Purified EDC ( < 10 ppm H 20)

~1Condenserl

99.19% H20 0.81% EDC

To Steam Stripper

or Other Recovery

FIGURE 3 - Pervaporation of dichlorethene for solvent purification.

permeate is condensed and phase separation occurs to produce an EDC rich layer (0.15 wt % H20), which is recycled and a water rich layer containing 0.81% EDC. Other commercial examples of this technique include the processing ofjet engine fuels and halogenated refrigerants. Another reported application is for dewatering isopropyl alcohol (IPA). This is a debottle-necking application in which the IPA at 85% is taken to 95% by PV prior to introduction into an extractive distillation column. Dehydration of cleaning agent

Pervaporation systems are installed for use at an LCD manufacturing'plants. Traditionally, microchip washing has been performed by chlorinated hydrocarbons but with the steady demise of CFCs, isopropanol is becoming a preferred cleaning agent. The alcohol picks up water during the process and eventually is either discarded or regenerated. This expensive prospect can be avoided by the use of a compact pervaporation unit, which allows the straightforward dehydration of isopropanol and similar solvents, whilst also offering the convenience of on-site recycling. The Pervaporation system typically dehydrates 6 m 3 of isopropanol per day, which is used as a cleaning agent.The IPA final concentration is typically 98.9%. Biotechnology

Pervaporation has several features which are attractive for separations in biotechnology; i) low temperature, ii) low pressures, iii) high cross-flow velocities are not needed and iv) additional chemicals are not required. There are four suggested classes of bioseparation for pervaporation, which require membranes with different characteristics. Direct bioproduct recovery Volatile by-product removal Concentration of sensitive bioproducts Dehydration of low molecular organics The selective removal of organic byproducts is not a particularly well developed area,

342

HANDBOOK OF INDUSTRIAL MEMBRANES

requiring highly selective membranes. The concentration of sensitive bioproducts, requires primarily high water flux membranes, as the products are usually high molecular species, eg amino acids, enzymes, which are non-permeable to pervaporation membranes. Dehydration of low molecular bioproducts by pervaporation will only be in the last stage of downstream processing.

Direct Product Recovery Fermentation can produce many organic components, from a range of feedstocks, notably ethanol but also butanol, isopropanol, acetone, 2-3 butanediol, glycol and acetic acid. These products inhibit the fermentation process as they increase in concentration, eventually resulting in termination of fermentation. The volatile nature of the products makes it attractive to continuously remove these species during fermentation and maintain long-term fermentation at an optimum rate. The separation must retain the fermentation media, cells, nutrients etc, whilst removing fermentation product and by-product solvents. Practised procedures for extraction of ethanol from fermentations include i) Vacuum evaporation- ethanol is removed at a temperature of 30-35~ ii) Stripping- ethanol is removed in a stripping gas iii) Extraction - liquid extraction with suitable water miscible organic solvents. The PV membranes for this application must exhibit high flux and good selectivity for the organic solvent products. Economics of the process will likely require hollow fibre membranes, e.g. composite hollow fibre, consisting of a polysulphone porous support with a thin inner coating or organophilic polydimethly siloxane. However an economically competitive membrane for this application is still awaited.

Removal of Organics from Water The applications of organic pervaporation from aqueous solution are generally targeted at pollution control, solvent recovery, organic concentration for disposal and speciality processes. One suggested application is in the treatment of wash waters used to remove organics from solvent laden airstreams, ieair scrubbing. The dilute aqueous solution is treated by pervaporation to remove the solvent in the permeate and to produce water, with a minor amount of solvent, to recycle to the air scrubbing unit. The contamination of water by small amounts of organics requires appropriate means of pollution control. There are many possible alternative methods for this problem, which include air stripping, carbon adsorption, biological treatment, steam stripping and incineration. If the aqueous stream contains a few percent of organic, then recovery of the solvent adds favourably to the overall process economics. Pervaporation is an economically viable alternative over a wide range of organic concentrations; approximately 100 ppm to 60,000 ppm (see Fig 4). The process of pervaporation can quite selectively recover the organic species. The membranes in these applications are rubbery polymers, such as silicone rubber, polybutadiene, natural rubber, polyether copolymers etc. The selectivity of the separation depends significantly on the type of organic compound (see Fig 5. For example silicon rubber selectivity decreases as the hydrophobicity of the

SEPARATION OF LIQUID MIXTURES/PERVAPORATION

343

50

40

20

10

o

o

0.2

0.4

0.6

Feed concentration

FIGURE 4 -

0.8

1 .o

(wl~)

Competitive range of pervaporation for organic in water separation.

organic decreases. Thus high selectivities are achieved separating benzene, 1, 1, 2 trichloroethene and toluene, good selectivities with ethylacetate and acetone, but only modest values with ethanol and acetic acid. Furthermore different elastonomers (Table 7) give significantly different values of selectivity for individual compounds. Ethene-propene terpolymer is much more selective for toluene and trichloretlylene than silicone rubber, exhibiting a separation factor is excess of 30,000.

FIGURE

5 - Separation of organic compounds from water by pervaporation using a silicone rubber composite membrane.

344

HANDBOOK OF INDUSTRIAL MEMBRANES

TABLE 7 - Elastomeric membranes for removal of organics from water by pervaporation. Flurocarbon elastomer-FKM Poly-acrylate rubber-ACM Polyuerethane-AU

r-l Toluene

1 Trichloroethylene

..r.._

Epichlorohydrinterpolymer-ETERA Nitrile butadiene rubber-NBR(38) Nitrile butadienerubber-NBR(33) NitrUe butadiene rubber-NBR(28)_~

Polydimethylsiloxane-PDMS Polymorbornene-PNR Nitrile butadiene rubber-NBR(18)J ~

Polychloroprene-CR NitrUe butadiene rubber-NBR(0)

_2 Polyoctenamer-OT_ Ethene-propeneterpolymer-EPDM

I

I

I

20,000 40,000 60,000 80,000 Separation factor

The process of pervaporation with rubbery membranes is also suitable for recycling dilute solutions (1% tO 2%) of organic solvents such as ethyl acetate. A process has been developed to treat approximately 90 m3/day of a 2% ethyl acetate solution using silicone rubber membranes. 90% of the ethyl acetate is recovered as a 96.7% product. The process is similar to that depicted in Fig6 ... The separation of polar solvents such as ethanol, acetic acid and formic acid from water can not generally be achieved with a separation factor much greater than 5 to 10, with most rubber membranes. At the moment this has prevented commercial applications in solvent recoveries and fermentations. Progress however is being made in this area with modified rubbery membranes, where for example silicone rubber containing dispersed zeolite has shown selectivity greater than 40 for ethanol/water. Pollution Control

An example of pollution control is the treatment of groundswater contaminated with 0.1% of 1, 1, 2-trichloroethane. As shown in Fig 6, the process, with a membrane unit having a separation factor of 200, removes 99% of the organic compound and produces a permeate of 4.1% organic. The permeate rapidly phase separates into an organic rich (>99%) layer and aqueous stream containing 0.4% organic, which is recycled to the incoming groundwater for re-treatment. O r g a n i c / O r g a n i c Separations

The dominant separation method for organic mixtures in the petroleum and chemical processing industries is distillation. It is an energy intensive process estimated that around 40% of the total energy consumed by the chemical processing industries is in distillation.

345

SEPARATION OF LIQUID MIXTURES/PERVAPORATION

When the organic components have similar boiling points and relative volatilties, the separation is difficult and the energy cost is particularly high. Separation of azeotropes is not possible by simple distillation and thus entrainers are added to increase relative volatility of the major components and thus break the azeotrope. Several important separations in petroleum and chemical industries rely on the use of entrainers (see Table 8). These processes require additional separations to recover the entrainer. TABLE 8 - Binary hydrocarbon azeotrope mixtures and possible entrainers. ,,,

Hydrocarbon system 2,2-Dimethylbutane/cyclopentane Cyclohexane/2,4-dimethylpentane Methyl cyclohexane/ 2,4-dimethylpentane 3-Methylpentane/1 -hexane 3-Methylpentane/2-ethyl, 1-butene 2-Ethyl, 1-butene/n-hexane 2,2,4-Trimethylpentane/ 2-2,4-trimethylpentene- 1 n-Heptane/2,2,4-trimethylpentene- 1 Ethyl benzene/p-xylene p-Xylene/m-xylene m-Xylene/o-xylene

Relative volatility

Best entrainer

Relative volatility

1.006 1.006

n-Propylamine Acetone

0.987 1.025

1.046 1.009 1.037 1.056

Ethanol Methylene chloride Ethyl formate Chloroform

1.085 1.159 1.156 1.094

1.040 1.045 1.035 1.020 1.105

Isopropyl acetate Isopropyl acetate 2-Methyl butanol 2-Methyl butanol Methyl isobutanol carbinol

1.129 1.129 1.079 1.029 1.150

In certain cases a suitable entrainer is not available and then alternative separation methods must be considered. For a good overall separation factor for pervaporation, ideally both the membrane separation factor and the evaporation separation factor (relative volatility) should be large. With mixtures of low relative volatility, good separation is reliant upon a reasonable separation factor for the pervaporation membrane. For example with a benzene/cyclohexane mixture which has an azeotrope composition of approximately 50% benzene, pervaporation with a 20 ktm thick crosslinked membrane (polymeric/alloy of polyphosphonate and acetyl cellulose) will produce a permeate with more than 90% benzene (see Fig 7). Further purification of this benzene permeate, and the cyclohexane rich residue, by pervaporation would not be economic. Rather standard distillation of both these streams would be used to produce the respective pure components. Both distillations would realise azeotropic (or near azeotropic) mixtures as second products, which would then be recycled for further purification. A critical factor in the development of organic/organic separations is the availability of membranes, to withstand the continuous long-term exposure to the organic compounds; frequently at elevated temperatures. An interesting example ofpervaporation applied to organic/organic separations is in the production of methyltertiary butyl ether from methanol and isobutene (C4) (Fig 8). This

346

HANDBOOK OF INDUSTRIAL MEMBRANES

100

I 80

60

.y

I

liquid

in product

(wW.) 40

l, 20

_

1[/SSSSS ~Z~ I

1

_

I

0 0

20

40

60

80

100

Benzene in feed (wt%)

FIGURE 7 - Vapour product vs feed composition for pervaporation of a benzene/cyclohexane mixture.

process produces a reactor product mixture of all three components of which both the methanol and ether and methanol and C4 form azeotropes. A processhas been developed in which pervaporation is integrated in the system to separate out the methanol and recycle it backto the reactor. The membrane used is made from cellulose acetate. Cellulose acetate membrane has a separation factor for methanol from MTBE of over 1000, because the material is hydrophilic and methanol is more polar than MTBE or the isobutene. R e c o v e r y of Volatile Bioproducts

The considerable variety of low molecular weight bioproducts can be put into the distinguishable groups of biosynthetic chemicals, aroma compounds and essential oils and "concrete' s". TABLE 9 - Oxychemicals produced from fermentation.

Low Boilers

High (or Non) Boilers

Ethanol - Ethylene - Butadiene - Octane Enhancer - Industrial

Ethylene Glycol Adipic Acid Acetic Acid Acrylic Acid Glycerol

Isopropanol Acetone Methylethylketone

Propylene Glycol n-Butanol Citric Acid Sorbitol Propionic Acid Fumaric Acid

1,4-Butanediol

SEPARATION OF LIQUID MIXTURES/PERVAPORATION

347

MTBE Reaction Chemistry

CH3

CH3

I

I

CH3OH + C. = CH2 ~ / CH3 Methanol I$obutene

CH3OH --C --CH 3

I CH 3

Methyl Tert-butyl ether

Pervaporation Process before Debutanlzer

. C4 raffinate

C4 Methanol

Pervaporation unit

MTBE

Methanol

Pervaporation on Debutanizer Sidedraw

C4

un,,

Methanol

r'~176176 ,

MTBE

FIGURE 8 - Integration of pervaporation in the MTBE production process.

Biosynthestics are oxychemicals which can be produced by fermentation of renewable biomass on a large scale (see Table 9). Several of these have been produced in commercial quantifies (ethanol, acetic acid, isopropanol, acetone, glycol and n-butanol) and are presently produced from petrochemcial feedstocks. Fermentation products are mainly constituents of the aqueous solutions and pervaporation is required to selectivity transfer the product or byproducts, through the polymeric membrane. Thus organophilic membranes such as silicone rubber (polydimethylsiloxcane, PDMS) in various configurations are used to selectively remove the organic solute. Much of the research on pervaporation membranes has focused on impeding transport of water. The enrichment of organic solute achieved by pervaporation is determined by the "pervapouration coefficient" which is essentially the product of activity coefficient and the ratio of pure component vapour pressure to permeate pressure. Thus although pervaporation of low boilers such as ethanol

HANDBOOK OF INDUSTRIAL MEMBRANES

348

is relatively easy, the fact that, under pervaporation conditions, activity coefficients of organic solutes are high and increase with dilution enables high boiling solutes to be enriched. Oxy Chemicals Fermentation of biomass by specific yeast or bacterium typically produces several metabolites with the primary bioproduct. For example in the case of fermentation of glucose to ethanol by Sacc-horomyces cerevisiae (yeast) (see Table 10). These metabolites inhibit microbial activity as their concentrations increase and thus removal of these species as well as the main product during fermentation is of interest. A typical study is in the fermentation of ethanol by S cerevisiae using a PDMS (1 m coating) on polysulphone membrane (see Table 10). TABLE 10 -- Pervaporation of bioproducts form S cerevisiae using PDMS on PSU membrane.

Component Ethanol Acetaldehyde Ethylacetate Methanol Isobutanol Methyl butanol Acetic acid

Fermenter (w-%)

Permeate (w-%)

4.8 0.026 0 0 0.01 0.006 0.007

26.1 0.25 0.05 0.02 0.11 0.05 0

Organic Flux (g/m2h) 63.9 0.61 0.1 0.06 0.28 " 0.13 0

A problem with the use of pervaporation in this mode is that certain inhibiting metabolites, such as acetic acid, do not pervaporate. Acetone, Butanol-Ethanol

Fermentation of sugars by microganisms of the clostridium group yield mainly n-butanol, isopropanol, ethanol and acetone. The production of acetone, butanol and ethanol (ABE fermentation) using C. acetobutylicum Weizmann has seen commercial operation, although it is not in current use today. The fermentation is inhibited by butanol at concentrations of 1% and byproducts, acetic and butyric acids and thus accumulation of solvents in the fermentation broth is low. Recovery of n-butanol (B.pt 118C) by distillation from water at low concentrations (1%) is not economic. Thus processes based on membranes are alternatives for product recovery. These include, pervaporation, pertraction, membrane distillation and reverse osmosis. Pervaporation with PDMS membranes (Hollow fibre) of butanol gives enrichment factors of 20 and greater. Overall recovery of solvent products will be by an integrated procedure involving other separation such as stripping and three phase distillation. AROMA Compounds

Aroma compounds (odours and fragrances) are volatile metabolites released from cultures

SEPARATION OF LIQUID MIXTURES/PERVAPORATION

349

of micro-organisms and fungi. These compounds are alcohols, aldehydes, aliphatic esters, lactones and terpenes and are correlated with the micro-organism and odours (see Table 11). The organic solute species are typically high boiling compounds and are in dilute aqueous solutions and thus pervaporation with organophilic membranes is ideally suited to product recovery, at least to achieve an immediate reduction in volume of feedstock. T A B L E 11 - A r o m a c o m p o u n d s .

Microorganism

Odour

Constituents

Bacillus subtilis Ceratocystis moniliformis

Soybean Fruity, banana, peach, pear, rose

Ceratocystis variospora

Fragrant, geranium

Ceratocystis virescens

Rose, fruity

Corynebacterium glutamicum Daedalea quercina Inocybe corydalina Kluyveromyces lactis Lentinus cochleatus Lenzites sepiaria Mycoacia uda

Soybean

Tetramethylpyrazine 3-Methylbutyl acetate, ~5-and ~,-decalactone, geraniol, citronellol, nerol, linalool, c~-terpineol Citronellol, citronellyl acetate, geranial, neral, geraniol, linolool, geranyl acetate 6-Methyl-5-hepten-2-ol acetate, citronellol, linalool, geraniol, geranyl acetate Tetramethylpyrazine

Penicillium decumbens

Apples Fruity, jasmine Fruity, rose Anisaldehyde Slightly spicy Fruity, grassy, almond Pine, rose, apple, mushroom

Pholiota adiposa Polyporus croceus Polyporus obtusus Poria xantha Pseudomonas perolens Pseudomonas taetrolens Stereum murrayi Stereum rugosum Streptomyces odorifer

Earthy Narcissus Jasmine Lemon Musty, potato Musty, Potato Vanilla Fruity, banana Earthy, camphor

Trametes odorata

Honey, rose, fruity, anise Anisaldehyde Coconut

Trametes suaveolens Trichoderma viride

Cinnamic acid methyl ester Citronellol, linalool~ geraniol p-Methylacetophenone, p-tolyl1-ethanol, p-tolylaldehyde Thujopsene, 3-octanone, 1-octen-3-ol, nerolidol, I]-Phenylethyl alcohol

2-Methoxy-3-isopropylpyrazine 2-Methoxy-3-isopropylpyrazine

trans- 1,10-Dimethyl-trans-9decalol, 2-exo-hydroxy-2-

methylbornane Methyl phenylacetate, geraniol, nerol, citronellol 6-Pentyl-2-pyrone

350

HANDBOOK OF INDUSTRIAL MEMBRANES

Pervaporation of several aroma compounds has been achieved with silicone rubber (including chemically modified) and polyether-polyamide block copolymer apple (and fruit)juice aroma compounds -

gamma - decalactone (peach) produced by fermentation of castor oil

-

6-pentyl-2-pyrone, a natural aroma compound of coconut fragrance, from trichoderma viride culture medium

As a rule pervaporation enrichment is higher with esters, lower with aldehydes and lower again with alcohols.

Essential Oils Essential oils are natural products of various parts of plants which can be recovered by steam distillation or solvent extraction. The characteristic and important fragrance is due to a combination of the many constituents (several hundred in many cases) and thus isolation of these constituents is rarely required and could detract from essential product quality. Pervaporation of essential oils, which are normally volatile with steam, is thus possible and has been demonstrated in the recovery of juniper oil.

Enhanced esterification by pervaporation Esterification reactions generate water as part of the production of the ester. Esterification reactions are typically reversible processes and the degree of conversion is limited by equilibrium conditions. When water formed as a byproduct in a reaction is continuously removed from the reaction mixture, the formation of the wanted product can be shifted beyond the thermodynamic equilibrium and full conversion of the reactants can be obtained. The main advantages observed are: reaction (batch) times are reduced; space-time yield of a given reactor can be increases by a factor of 2 to 3; downstream separation and purification of the wanted product is facilities or even eliminated; energy costs are reduced; and -

when one of the reactants is used at a slight excess, 100% yield of the wanted product is possible.

9 Pervaporation has been applied to the continuous removal of water from esterification reaction mixtures and can extract water either directly or from a side loop of the reactor, as the membranes used are fully stable against the reactants and the acidic catalyst. When at least one of the reactants has a volatility sufficiently higher than that of the product, an evaporated stream can be treated in the vapour phase and returned to the reactor, thus water can be removed at the reaction temperature or at any freely chosen temperature.

SEPARATION

OF LIQUID MIXTURES/PERVAPORATION

351

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