HYBRID DISTILLATION-PERVAPORATION SYSTEMS

The combination of distillation and membrane separation has been explored in the literature for many years. One of the applications is to separate azeotropic mixtures. A distillation column is used to make a preliminary separation with one of the product streams being one of the pure components. The other product stream has a composition fairly close to the azeotropic composition. This stream is fed to a membrane separation unit that achieves the separation not on the basis of VLE, but based on differences in diffusion rates through a solid membrane. The fluid passing through the membrane is called the permeate. The fluid that does not pass through the membrane is called the retentate. Composition differences are the driving forces for mass transfer. 

Design and Control of Distillation Systems for Separating Azeotropes. By William L. Luyben and I-Lung Chien Copyright 2010 John Wiley Sons, Ihc. 

Pervaporation has been the subject of many papers for several decades. The literature is quite extensive with many papers and several books on the subject. The 1991 book edited by Huang gives 262 citations in the first chapter. The book by Rautenbach and Albrect, written in 1989, has a chapter on pervaporation. There are many papers that discuss the fundamentals of diffusion, adsorption, evaporation, and membrane characteristics. Some papers develop detailed mass-transfer models. A number of papers discuss design considerations and present comparative economic studies of pervaporation vctsus alternatives such as distillation. 

Industrial applications began in the 1970s. This was made possible by the development of highly selective polyvinyl alcohol composite membranes. The process is clearly discussed by Sander and Soukup. They provide engineering performance curves for removing water from an ethanol-water mixture. The curves show how the temperature and ethanol concentration of the feed affect the flux of water through the membrane and the composition of the permeate stream. These performance curves are used in this chapter to provide a simple pervaporation model that captures the behavior of the process with sufficient fidelity for use in control studies. 

Many papers point out the economic advantages of pervaporation over conventional processes. A quantitative economic comparison between azeotropic distillation and hybrid column-pervaporation is presented by Guerreri, whose results show higher capital cost but much lower energy cost for the ethanol-water separation. A review of industrial applications of pervaporation, coupled with either distillation columns or chemical reactors, is 

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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. 

Daviou M. C., P. M. Hoch, and A. M. Eliceche, Design of membrane modules used in hybrid distillation/pervaporation systems, Ind. Engng. Chem. Res., 43, 3403-3412 (2004). 

Figure 17-14. Hybrid pervaporation system coupled with distillation (A) general two-column system, (B) sinqilified one-column system shown for dehydration of ethanol...

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... 

Van HV, Vanden AL, Buekenhoudt A, Dotremont C, and Ley sen R. Economic comparison between azeotropic distillation and different hybrid systems combining distillation with pervaporation for the dehydration of isopropanol. Sep. Purific. Tech. 2004 37(l) 33-49. 

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. 

Thus, theoretically, membrane technology may be the best candidate to couple reaction and separation. In the hterature, it is possible to find some hybrid units coupling the reliability of the reactive distillation system with the pervaporative technology when an esterification reaction is mn (Riemenschneider Bolt, 2000 Buchaly et al., 2012 Bida, Gongping, Xueliang, Wang, Wanqin, 2012). This allows for a reduction of energy consumption and an enhancement of process quality compared with stand-alone reactive distillation. 

The main problem in membrane processes, especially for UF and MF separations, is the decrease of permeate flux caused by concentration polarization and fouling, whereas other membrane processes such as gas separation and pervaporation are less affected. Different approaches have been studied to reduce fouling. Hybrid systems using different types of membrane operations (e.g., distillation, dialysis, NF, pervaporation, and osmosis) prevent microbial fouling, offering a strong potential for the use of new types of thin-film composite membranes. 

Membranes can also be used to purify a mixmre and attain composition beyond the azeotropic composition. The pervaporation process features a liquid feed, a liquid retentate, and a vapor permeate. While gas-phase membrane processes are essentially isothermal, the phase change in the pervaporation process produces a temperature decrease as the retentate flows through the unit. Since flux rates decrease with decreasing temperature, the conventional pervaporation unit consists of several membrane modules in series with interstage heating. The vapor permeate must be condensed for recovery and recycle, and refrigeration is usually required. Hybrid systems of distillation columns and pervaporation units are frequently used in situations where distillation alone is impossible or very expensive. An important application is the removal of water from the ethanol-water azeotrope. Chapter 14 will discuss the details of design and control of such processes. 

It is quite common in the chemical process industry to use various kinds of salts to dehydrate complex mixtures of solvents to avoid azeotropic distillation. These salts can cause many serious problems in the whole process. They can also cause a msting problem in the whole piping system. The disposal of such toxic wastewater is a colossal issue for the chemical industry. The AZEO SEP pervaporation hybrid system can eliminate the use of salt, and it can breakdown the azeotropes for further process or use. 

Small- to medium-size footprint ethanol plants are fading away from the industry due to the high operafronal eost and the high capital cost. AZEO SEP pervaporation hybrid systems offer altemafrves for conventional four-column distillation. This includes only one column hybrid with AZEO SEP. This can reduce the operational cost by as much as 30%. (See Figs. 17.1 and 17.2.)... 

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