New fields of application

11.3.1 Pervaporation

Description of the process

The term 'pervaporation' comprises two Latin words: 'per' (through) and 'vapor' (vapor, steam). Pervaporation is a separation process in which one

268 Handbook of waste management and co-product recovery Table 11.3 Some applications of reverse osmosis

Dairy industry

Concentrating of milk17

Concentrating and dematerializing of whey11,17

Other processes with animal proteins

Concentrating of egg white18

Beverage industry

Concentrating of fruit and vegetable juices such as:

• apple juice19

• orange juice20

• tomato juice20

Reduction of alcohol in beer and wine21 Concentrating of wine22 Concentrating of tea23

Sugar industry

Concentrating of low-viscosity juice24 Water production

Ultra-pure water production from river water25 Drinking water26,27 Wastewater treatment23,28

Fine chemicals production

Lactic acid29 l-Phenylalanine30

or more components (the permeate) are extracted selectively from a liquid mixture (the feed) by use of a membrane. The membrane is nearly impermeable to all the remaining components of the solution (retentate) (Fig. 11.8).

Membranes used for pervaporation are so-called dense (non-porous) composite membranes. Composite membranes consist of at least two materials and are built up in layers. There is a difference between selective layers and supporting (stabilizing) layers (Fig. 11.9). The selective layer is responsible for the separation effect. Supporting layers provide strength to the membrane. In most cases the selective layer is a polymer, e.g. poly-dimethylsiloxane (PDMS) (Fig. 11.10). The separation effect is caused by a stronger interaction of specific substances with the membrane. These substances are selectively absorbed by the membrane polymer and are transported through the membrane by diffusion.

As found in the process of reverse osmosis, the interactions are electrochemical and/or steric. Those chemicals that are enriched in the permeate 'dissolve' more easily in the membrane polymer than those that remain in

Feed (liquid)

Feed (liquid)

Permeate (gaseous)

Fig. 11.8 Test cell of a pervaporation unit, schematic.

Membrane

Permeate (gaseous)

Retentate (liquid)

Fig. 11.8 Test cell of a pervaporation unit, schematic.

Supporting I—I layer

Selective layer h

Acc.V Spot Magrt WU I-1 50 pm

Fig. 11.9 Scanning electron micrograph of a composite membrane.

Fig. 11.10 General chemical structure of silicones; for PDMS, R = CH3.

n the retentate. According to the solution-diffusion model,31 which describes this mechanism, the material transport through the membrane, the so-called 'permeation', is subdivided into five steps:

1 Diffusion of dissolved substance to the membrane.

2 Sorption of the component into the membrane.

3 Diffusion of the component through the membrane.

4 Desorption of the component out of the membrane.

5 Removal of the component into the gaseous phase by evaporation.

The driving force for the transport of molecules through the membrane is a gradient of chemical potential m This gradient is maintained in the first place by a partial pressure difference and also by a temperature difference. Because of a low pressure of p < 20 mbar on the permeate side, the absorbed components evaporate into the gaseous phase and can be transported away (Fig. 11.11).

In a vacuum, ideal gas behavior may be assumed for the permeate side without restricting the precision. Furthermore, the pressure dependence of the chemical potential on the feed side may be ignored since operating systems do not involve a high overpressure.

Hence, chemical potential can be described as:

xif pip

It follows that the driving force can be increased by:

• reducing the permeate pressure, pP;

• raising the temperature, T;

• increasing the concentration x, of the component in the feed.

Fig. 11.11 Gradient of chemical potential over the membrane.

In the process of pervaporation, the separation of the components is connected with a transition from liquid to gas. The pervaporation can also be interpreted as a distillation that is hindered by a membrane. However, in contrast to a distillation, pervaporation does not depend on vapour-liquid balances and can therefore be used to separate azeotropic mixtures.

A critical factor in the practical application of pervaporation is the selectivity of the process, which depends on the membrane used. The already mentioned PDMS membrane is a hydrophobic (organophilic) polymer for the separation of organics from water. There are also hydrophilic (organo-phobic) membranes for the separation of water from organics (e.g. poly-vinyl alcohol (PVA) membranes). Some examples for the application of pervaporation are given in Table 11.4.

The selectivity of the membrane towards specific components i in the solution is required for the separation activity of the membrane. These components permeate through the membrane and become less concentrated in front of the membrane, while less permeable substances j are enriched in front of the membrane. The weaker the turbulence of the flow and the weaker the convective remixing of the components, the stronger the concentration and de-concentration in the laminar boundary layer on the feed side. If the flow is laminar, a concentration gradient can only be compensated by diffusion (which is slow compared with convection). There is a transport resistance. This phenomenon is called 'concentration polarization' and is shown schematically in Fig. 11.12.32

Application of pervaporation

While hydrophilic pervaporation is state of the art for the dehydration of organic solvents, organophilic pervaporation is hardly applied on an indus-

Table 11.4 Examples of the application of pervaporation

Process

Application

Hydrophilic pervaporation

Dehydration of solvents

Increase in chemical reaction by removing the

reaction water

Removal of methanol and ethanol from

hydrocarbons

Hydrophobic (organophilic)

Dealcoholization of beer

pervaporation

Removal of ethanol, butanol, acetone, etc. from

fermentation broths

Treatment of waste water containing organic

material

Production of aromatizing agents

Treatment of laboratory waste water

Target organophilic

Separation of organic-organic azeotrop

pervaporation

Separation of isomers

Fig. 11.12 Concentration polarization, schematic.32 Jj and Jt are the flows of components j and i, respectively.

trial scale, even though there are many interesting application fields for organophilic pervaporation. One of these applications involves the removal of alcohol from wine and beer. Studies of this topic show that the problem with current techniques, such as distillation, is that many flavor compounds are removed in addition to the ethanol and water, leading to a negative impact on the taste. This problem can be solved by using an organophilic membrane with high selectivity or by using a hydrophilic membrane. Hydrophilic membranes can be used for the reduction of polar substances and can therefore separate short-chain alcohols as well. To reduce the water flow, the difference in partial pressure of water could be reduced by applying a water-vapor-rich carrier gas stream on the permeate side. With this technology the ethanol concentration of chardonnay wine could be decreased to 0.5% without any losses of taste and flavor.35

The depletion of flavor components from aqueous solutions exemplifies a further application of organophilic pervaporation in food technology on which there are many studies. In general there are two main areas: the separation of flavors in fruit juice production and the separation of flavors from fermentation broths. The driving force of the application of pervap-oration in fruit juice production is the high loss of flavors during juice concentration. This is especially the case when using vaporization. The main target of process optimization must be the minimization of these losses by separating flavor and product stream. After the concentration of the beverage, flavors can be added to the product (see Fig. 11.13).

Using pervaporation in a bioreactor hybrid process One example of an industrial possibility for the application of organophilic pervaporation is the combination of a fermentation process with a pervap-oration bioreactor hybrid process. In a fermentation process the target product is usually part of a very complex mixture consisting of metabolites, proteins, sugars and inorganic salts, as well as cells and cell fragments.

Loss stream flavor

Loss stream flavor

stream stream

Fig. 11.13 Separation of flavor and product stream in beverage processing.6

stream stream

Fig. 11.13 Separation of flavor and product stream in beverage processing.6

Another problem is the product inhibition caused by other metabolites. By using a coupling of fermentation and organophilic pervaporation the products can be separated and the concentration of inhibiting substances could be reduced, which enables a continuous fermentation. An increase of productivity of from 80 to 500% is possible compared with a non-integrated batch process.37 Table 11.5 shows an overview of possible products and microorganisms used.

The pervaporation bioreactor hybrid process can be realized by integrating a membrane in a reactor or by using an external process unit as shown in Fig. 11.14. The application of membrane processes in downstream processing of fermentation solutions is usually inhibited by biofouling. Hence, an integration of a microfiltration between the reactor and the pervaporation unit is necessary to avoid fouling on the pervaporation membrane. Additionally, by immobilization of the cells, the exploitation of suitable modules or a semi-continuous cleaning of the pervaporation module, fouling can be avoided. An additional processing of the permeate can be carried out by distillation or by other membrane processes.

11.3.2 Membrane extraction

Perstraction or membrane-supported extraction is an efficient method of separating process solutions and is an interesting alternative to established extraction processes. In this process, non-miscible phases are separated by a porous membrane. Hydrophilic and organophilic membranes can be employed. By using a hydrophilic membrane the pores are filled by the aqueous phase, by using an organophilic membrane they are filled by the organic phase. The wetting phase is floating within the membrane pores, so that on the surface of the pores an immobilized phase surface is generated. A breakthrough through the pores is avoided by applying a pressure

Table 11.5 Overview of pervaporation-bioreactor hybrid processes described in the literature

Application

Microorganisms

Separation of acetoin and butandiol

Bacillus subtilis

Separation of acetone-butanol-

Clostridium saccharoperbutylacetonicum

ethanol

C. beyerinckii

C. acetobutylicum

Separation of benzaldehyde

Bjerkandera adusta

Separation of butanol

C. saccharoperbutylacetonicum

C. acetobutylicum

Separation of butanol and isopropanol

C. beyerinckii

Saccharomyces cerevisiae

Candida pseudotropicalis

Separation of ethanol

Candida thermohydrosulfuricum

S. carlsbergensis

Internal process unit

External process unit

CO2

CO2

Fig. 11.14 Comparison of internal and external process unit in a pervaporation (PV) bioreactor hybrid process.

Fig. 11.14 Comparison of internal and external process unit in a pervaporation (PV) bioreactor hybrid process.

on the side of the wetting liquid. In this way a mass transfer without dispersion can be enabled. The principle of perstraction is depicted in Fig. 11.15.38'39

The main advantage of this process compared with an established extraction process is the enlarged surface and hence an enlarged exchange area. The exchange area using a hollow-fiber membrane contactor is 200 times larger compared with a packed column.38 Other advantages compared with a conventional separation process are shown in Table 11.6. The process is limited by the costs of the membrane. As in all membrane processes the limited stability and potential fouling can complicate the application. Due to agglomeration, the wetting properties of the membrane can change and can hence endanger the process. Compared with conventional extraction the additional resistance of the membrane must be taken into account; this can limit the efficiency of the process.

Pores

Pores

Membrane matrix

Concentration profile

Exchange area

Fig. 11.15 Principle of perstraction.

Concentration profile

Exchange area

Membrane matrix

Fig. 11.15 Principle of perstraction.

Table 11.6 Advantage of perstraction compared with conventional processes

Advantages compared with conventional extraction

Advantages compared with thermal separation processes

Immobilization of the phase surface, no mix of the phases No difference in density necessary for separation of phases High flow rate due to the fixing of the surfaces

Separation of aceotropes possible Low energy costs No temperature stress of feed and permeate

Of particular interest is perstraction in the food industry, especially for the separation of fermentation products. One example involves the continuous separation of flavors from fermentation broths. By immobilization, the mixing of the phases and hence the stress for the microorganisms can be reduced and a continuous extraction can be enabled. Additionally, temperature stress of microorganisms and substrate can be avoided. In the example below, the extraction of the flavors 1-octene-3-ol, 2-heptanone, isoamyl acetate and ethyl butyrate is examined. The extraction agent was hexane. The hollow fiber was Celgard® hollow fibers made of hydrophobic polypropylene (see Fig. 11.16). With this experimental set-up, high extraction rates can be achieved. After 18 min, 98% of the flavors could be extracted. Figure 11.17 shows the high efficiency of the process.41 It can be seen that extraction using hollow-fiber membrane contactors is a most

Idealized pore, filled with organic phase

Hexane as organic strip-solution

Hexane as organic strip-solution

Flavor solution as process stream

Membrane matrix

Flavor solution as process stream

Cross-section of a contactor

Hollow fibre inside

Hexane

Fig. 11.16 Extraction of an aqueous solution of flavors using a hollow-fiber membrane contactor (after Wickramasinghe et al.,36,40 and Laufenberg and Cussler41).

Hollow fibre inside

Central feed stream, outside

Hexane

Central feed stream, outside

System set up

• Hydrophobic membrane for extraction module

• Organic phase: hexane or vegetable oil

• Module geometry:

- hollow fibre arranged among a porous central tube

- flow along a cylindrical bundle shaped in slice and 'donut' geometry

- reduced membrane resistance, improved separation factor

• Loss of membrane area =10%, as a result of turbulent flow an enlarged efficiency of =400%

• Pore interface is a function of

- transmembrane pressure

- pore size and geometry

- interface tension

- wetting properties

Fig. 11.16 Extraction of an aqueous solution of flavors using a hollow-fiber membrane contactor (after Wickramasinghe et al.,36,40 and Laufenberg and Cussler41).

-Isoamyl acetate X 1-Octene-3-ol

*

■ Ethy

butyrate

*

S o

- y

A V

Fig. 11.17 Membrane extraction, -ln cilc0 versus time, all flavours in aqueous solution, feed flow 4.8 ml s-1; extraction agent, hexane.39

efficient method of extracting organic substances from aqueous organic solutions. The extraction agent, hexane, is of considerable concern in German food legislation, and an easy separation enables a quality product. An interesting alternative is seen in the application of vegetable oil as an extraction agent. Here no additional reprocessing is necessary, which reduces costs. The product could be flavored oil which can be applied in food without any further purification.

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