Pervaporation-Column System

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,... 

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

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.)... 

A flow scheme for an integrated distillation-pervaporation plant operating on a 5 % ethanol feed from a fermentation mash is shown in Figure 9.10. The distillation column produces an ethanol stream containing 80-90 % ethanol, which is fed to the pervaporation system. To maximize the vapor pressure difference and the pressure ratio across the membrane, the pervaporation module usually... 

Fig. 4.21. Different ways of improving pervaporation efficiency. (A) By halting the flow in the acceptor chamber without disrupting the overall dynamic system. (B) By on-line retention of transferred volatile species and elution in the opposite direction. (C) By use of a packed flow-cell to integrate reaction and detection. E eluent, lEC ion-exchange column, PR preconcentration, EL elution. (For other abbreviations, see previous figures.) (Reproduced with permission of Wiley Sons.)...

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. 

Existing entrainer distillation systems can also be effectively debottlenecked using pervaporation/vapor permeation (Fig. 15). Normally, the rectification column will be operating to give a product as close to the azeotrope as possible, running with a high reflux. 

To debottleneck the system, reflux in the rectification column is reduced, giving more overhead product, but with a higher water content. The pervaporation unit is sized to remove enough water that the subsequent entrainer column is also unloaded. Both columns can then realize a significant capacity increase. 

This feed is pressurized and heated to 85°C. It is then fed to a system similar to Figure 17-14, except the retentate can be reheated to 85°C and be sent to a second pervaporation stage. Additional stages can be added if needed. A 20-micron thick polyacrylonitrile film membrane is used (see Figure 17-161. Assume that each stage of pervaporation is perfectly mixed and operates at 25°C. The final retentate (the product) needs to be at 98.5 wt % ethanol or higher. The permeate streams are pressurized and recycled to the distillation column. If we desire to process 100 kg/h of the 90 wt % ethanol distillate, find ... 

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. 

The purpose of this chapter is to study the dynamie eontrol of a pervaporator system coupled with a distUlation column. The example system is the important ethanol-water separation. The commercial simulation tools of Aspen Technology are used in this study. 

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