Pervaporation distillations

Reliable capital and operating cost comparisons between pervaporation and distillation are not available. Pervaporation is less capital and energy intensive than distillation or adsorption processes for small plants treating less than 5000 L/h of feed solution. However, because of the modular nature of the process, the costs of pervaporation are not as sensitive to economies of scale as are the costs of distillation and adsorption processes. Distillation costs, on the other hand, scale at a rate proportional to 0.6-0.7 times the power consumption. Thus, distillation remains the most economical process for large plants. The cross-over point at which distillation becomes preferable to pervaporation from an energy and economic point of view currently appears to be 5000 L/h processing capacity. Bergdorf has made an analysis of the comparative costs of pervaporation, distillation and other processes [43],... 

Figure 9.20 Potential configurations for pervaporation-distillation hybrid processes...

On the other hand, a pervaporation membrane can be coupled with a conventional distillation column, resulting in a hybrid membrane/distillation process (228,229). Some of the investigated applications of such hybrid pervaporation membrane/distillation systems are shown in Table 9. In hybrid pervaporation/ distillation systems, the membrane units can be installed on the overhead vapor of the distillation column, as shown in Figure 13a for the case of propylene/ propane splitting (234), or they can be installed on the feed to the distillation column,... 

TABLE 9 Possible Applications of Hybrid Pervaporation/Distillation Systems... 

Several authors have already developed methodologies for the simulation of hybrid distillation-pervaporation processes. Short-cut methods were developed by Moganti et al. [95] and Stephan et al. [96]. Due to simplifications such as the use of constant relative volatility, one-phase sidestreams, perfect mixing on feed and permeate sides of the membrane, and simple membrane transport models, the results obtained should only be considered qualitative in nature. Verhoef et al. [97] used a quantitative approach for simulation, based on simplified calculations in Aspen Plus/Excel VBA. Hommerich and Rautenbach [98] describe the design and optimization of combined pervaporation-distillation processes, incorporating a user-written routine for pervaporation into the Aspen Plus simulation software. This is an improvement over most approaches with respect to accuracy, although the membrane model itself is still quite... 

Fig. 4.21. Experimental data of the selectivity Si = i -xi in a pervaporation distillation process of the mixture iso-propanol (l)-water (2). Sintered metal plate, thickness L = 1 mm. Pore diameters between 100 and 1 mp [31].

Study of a novel Heat Integrated Hybrid Pervaporation Distillation Process Simulation and Experiments... 

In the present work a new developed heat integrated hybrid pervaporation distillation process is modeled and experimental studies are carried out to analyze the effect of the heat integration in the process. With the results of the experiments, the model is validated and a comparison between industrial scale non heat integrated and heat integrated processes is done. As a result, three main advantages are presented in the approach a) reduction of the necessary external energy supply into the process, b) improvement in the pervaporation separation performance and c) reduction in the necessary membrane surface. 

Figure 1 Hybrid pervaporation distillation process flow sheet. Left with external heat exchangers, right with heat integration.

For the study of the process, a set of partial differential model equations for a flat sheet pervaporation membrane with an integrated heat exchanger (see fig.2) has been developed. The temperature dependence of the permeability coefficient is defined like an Arrhenius function [S. Sommer, 2003] and our new developed model of the pervaporation process is based on the model proposed by [Wijmans and Baker, 1993] (see equation 1). With this model the effect of the heat integration can be studied under different operating conditions and module geometry and material using a turbulent flow in the feed. The model has been developed in gPROMS and coupled with the model of the distillation column described by [J.-U Repke, 2006], for the study of the whole hybrid system pervaporation distillation. 

For the model validation and the analysis of the heat integration in the hybrid pervaporation distillation process, a laboratory plant has been built at the TU -Berlin and prepared for the connection with the distillation column (see fig. 3). With this plant experiments with a flat PVA-based (Polyvinylalcohol from GKSS) hydrophilic membrane have been done. A heat exchanger has been built within the pervaporation module. The temperature in the heat exchanger has been necessary to avoid the temperature drop between feed and retentate streams in the pervaporation process. In the process a 2-Propanol/ Water mixture has been separated. The concentration of 2-Propanol in the feed is between 80 and 90 % in weight and the temperature range in the experiments was between 70 and 90°C. The feed flow is turbulent and the system fully insulated to avoid heat looses. The pressure in the permeate side has been kept at 30 mbar and the feed pressure at 1.5 bar. 

M.T. Del Pozo Gomez, A. Klein, J.-U. Repke, G. Wozny, 2007, Study and Design of a Heat-integrated Hybrid Pervaporation-Distillation Process, ECCE-6 1357 Distillation, Absorption Extraetion. 

Hommerich, U. and R. Rautenbach, Design and optimization of combined pervaporation/ distillation processes for the production of MTBE. Journal of Membrane Science, 1998, 146(1) 53 64. 

Pervaporation, as a non-integrated process, is typically utilized for dehydration and for the recovery or removal of organics from aqueous solutions and sometimes also for the separation of organic mixtures (Neel, 1995). Also many hybrid processes have been developed where PV is coupled with other processes, such as different membrane processes (e.g., reverse osmosis, or organophilic pervaporation coupled with hydrophilic pervaporation), distillation, reactive distillation and, of course, reaction. With these aspects in mind, PV appears particularly suitable to keep the concentration of a by-product low, or to continuously recover a product while it is formed. Note that these are the main objectives typically pursued in membrane reactors. 

Fig. 42. Integrated distillation/pervaporation plant for ethanol recovery from fermentors. The distillation columns concentrate the ethanol—water mixture from 5 to 80%. The pervaporation membrane produces a 99.5% ethanol product stream and a 40—50% ethanol stream that is sent back to the distillation...

Because a preconcentration step is probably needed to make the final sequence more economical, it is logical to start with the opportunistic separation. This separation produces one of the products, pure water, as the underflow and a concentrated distillate appropriate for feed into either strategic separation. Arbitrarily choosing pervaporation first, the retentate has a composition on the 2-propanol-rich side of the azeotrope, whereas the permeate is pure water. No further strategic separations are required. 

Fig. 4. Distillation of aqueous isopropyl alcohol (IPA), where A is the a2eotrope, combined with (a) pervaporation and (b) adsorption.

Of these five methods all but pressure-swing distillation can also be used to separate low volatiUty mixtures and all but reactive distillation are discussed herein. It is also possible to combine distillation and other separation techniques such as Hquid—Hquid extraction (see Extraction, liquid-liquid), adsorption (qv), melt crystallization (qv), or pervaporation to complete the separation of azeotropic mixtures. 

An important characteristic of pervaporation that distinguishes it from distillation is that it is a rate process, not an equilibrium process. The more permeable component may be the less-volatile component. Perv oration has its greatest iitihty in the resolution of azeotropes, as an acqiinct to distillation. Selecting a membrane permeable to the minor corTiponent is important, since the membrane area required is roughly proportional to the mass of permeate. Thus pervaporation devices for the purification of the ethanol-water azeotrope (95 percent ethanol) are always based on a hydrophihc membrane. 

Dehydration The growing use of isopropanol as a clean-rinse fluid in microelectronics produces significant quantities of an 8.5-90 percent isopropanol waste. Removing the water and trace contan ii-nants is required before the alcohol can be reused. Pervaporation produces a 99.99 percent alcohol product in one step. It is subsequently polished to remove metals and organics. In Europe, dehydration or ethanol is the largest pei vaporation application. For the very large ethanol plants typical of the United States, pei vaporation is not competitive with thermally integrated distillation. 

Alcohol is a clean energy source that can be produced by the fermentation of biomass. However, it needs to be highly concentrated. In general, aqueous alcohol solutions are concentrated by distillation, but an azeotrope (96.5 wt% ethanol) prevents further separated by distillation. Pervaporation, a membrane separation technique, can be used for separation of these azeotropes pervaporation is a promising membrane technique for the separation of organic liquid mixtures such as azeotropic mixtures [34] or close-boiling point mixtures. 

MTBE is a well known enhancer of the number of octanes in gasoline and as excellent oxygentated fuel additives that decrease carbon monoxide emissions. Therefore, MTBE has been one of the fastest growing chemicals of the past decade. MTBE is produced by reacting methanol with isobutylene from mixed-C4 stream liquid phase over a strong acid ion-exchange resin as catalyst. An excess of methanol is used in order to improve the reaction conversion. This excess has to be separated from the final product. The pervaporation technique, more energy efficient and with lower cost process, has been proposed as alternative to distillation [74],... 

The authors suggested that a combination of pervaporation with a conventional separation technique such as a hybrid distillation-pervaporation system could be useful economically to break the azeotropy. 

At present, there is one main commercial application of pervaporation, the production of high purity alcohol by a hybrid process which also incorporates distillation. Such separations use cellulose-acetate-based composite-membranes, with an active layer of polyvinyl alcohol, for example. Membrane fluxes are in the range 0.45-2.2 kg/m2 h. Pervaporation... 

Finally it should be noted that inorganic membranes suitable for separation by multilayer diffusion and capillary condensation are also appropriate for performing pervaporation (a technique in which the feed is liquid and the permeate is gas) and distillation at reduced pressure (where gases with overlapping condensation regions are separated). 

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