Pervaporation separation model

C.K. Yeom, R.Y.M. Huang, Modelling of the pervaporation separation of ethanol—water mixtures through crosslinked poly(vinyl alcohol) membrane, J. Membr. Sci. 67 (1992) 39—55. 

Figure 9.3 The pervaporation process shown in Figure 9.1 can be described by the thermodynamically equivalent process illustrated here. In this model the total pervaporation separation /9pervap is made up of an evaporation step followed by a membrane permeation step [18]...

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. 

J. Lipzinski, G. Tragardh, 2001, Modelling of Pervaporation, Models to analyze and prediet the mass transfer transport in Pervaporation, Separation and Purifieation Methods, vol. 30(1), 49-25. 

Huang, J. Li, J. Jhan, X. Chen, C., A Modified Solution Diffusion Model and Its Application in the Pervaporization Separation of Alkane/Thiophenes Mixtures with PDMS Membrane. J. Appl. Polym. Sci. 2008,110, 3140-3148. 

Krupiczka, R., Koszorz, Z. (1999). Activity-based model of the hybrid process of an esterification reaction coupled with pervaporation. Separation and Purification Technology, 16, 55—59. 

Based on experimental results and a model describing the kinetics of the system, it has been found that the temperature has the strongest influence on the performance of the system as it affects both the kinetics of esterification and of pervaporation. The rate of reaction increases with temperature according to Arrhenius law, whereas an increased temperature accelerates the pervaporation process also. Consequently, the water content decreases much faster at a higher temperature. The second important parameter is the initial molar ratio of the reactants involved. It has to be noted, however, that a deviation in the initial molar ratio from the stoichiometric value requires a rather expensive separation step to recover the unreacted component afterwards. The third factor is the ratio of membrane area to reaction volume, at least in the case of a batch reactor. For continuous opera-... 

Reverse osmosis, pervaporation and polymeric gas separation membranes have a dense polymer layer with no visible pores, in which the separation occurs. These membranes show different transport rates for molecules as small as 2-5 A in diameter. The fluxes of permeants through these membranes are also much lower than through the microporous membranes. Transport is best described by the solution-diffusion model. The spaces between the polymer chains in these membranes are less than 5 A in diameter and so are within the normal range of thermal motion of the polymer chains that make up the membrane matrix. Molecules permeate the membrane through free volume elements between the polymer chains that are transient on the timescale of the diffusion processes occurring. 

Concentration polarization can dominate the transmembrane flux in UF, and this can be described by boundary-layer models. Because the fluxes through nonporous barriers are lower than in UF, polarization effects are less important in reverse osmosis (RO), nanofiltration (NF), pervaporation (PV), electrodialysis (ED) or carrier-mediated separation. Interactions between substances in the feed and the membrane surface (adsorption, fouling) may also significantly influence the separation performance fouling is especially strong with aqueous feeds. 

Krishna and Paschek [91] employed the Maxwell-Stefan description for mass transport of alkanes through silicalite membranes, but did not consider more complex (e.g., unsaturated or branched) hydrocarbons. Kapteijn et al. [92] and Bakker et al. [93] applied the Maxwell-Stefan model for hydrocarbon permeation through silicalite membranes. Flanders et al. [94] studied separation of C6 isomers by pervaporation through ZSM-5 membranes and found that separation was due to shape selectivity. 

Selective separation of hquids by pervaporation is a result of selective sorption and diffusion of a component through the membrane. PV process differs from other membrane processes in the fact that there is a phase change of the permeating molecules on the downstream face of the membrane. PV mechanism can be described by the solution-diffusion mechanism proposed by Binning et al. [3]. According to this model, selective sorption of the component of a hquid mixture takes place at the upstream face of the membrane followed by diffusion through the membrane and desorption on the permeate side. 

Generally, alcohols showed higher separation factors when present in model multicomponent solutions than in binary systems with water. On the other hand, aldehydes showed an opposite trend. The acmal tea aroma mixmre showed a rather different behavior from the model aroma mixmre, probably because of the presence of very large numbers of unknown compounds. Overall, the PDMS membrane with vinyl end groups used by Kanani et al. [20] showed higher separation factors and fluxes for most of the aroma compounds. Pervaporation was found to be an attractive technology. However, as mentioned above the varying selectivities for the different aroma compounds alter the sensory prohle and therefore application of PV for recovery of such mixmres needs careful consideration on a case-by-case basis. 

Cao B and Henson MA. Modeling of spiral wound pervaporation modules with application to the separation of styrene/ethylbenzene mixtures. J Membr Sci 2002 197 117-146. 

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. 

The hollow-fiber module is often used when the feed stream is relatively clean, such as in gas separation and pervaporation. It has also been used in the case of seawater desalination, but pretreatment is needed. The module construction given in Fig. 15 A is a typical RO module, where a central pipe is used to uniformly distribute the feed solution throughout the module. This is to avoid the problem of channelling in outside-in model, which means the feed has a tendency to flow along a fixed path, thus reducing the effective membrane surface area. In gas separation, as shown in Fig. 15B, the outside-in model is used to avoid high pressure losses inside the fiber and to attain a high membrane area (13). 

With the advent of process simulation packages, modeling of pervaporation and vapor permeation processes in a user added subroutine allows these unit processes to be included in overall separation schemes right from the conceptual stage. This enables many different combinations of pervaporation, for example, distillation to be studied and the optimum operating parameters for the preferred configuration to be selected very quickly. Such parameters include membrane feed temperature, which strongly influences the flux rate and, therefore. 

The most widely accepted model for describing the transport mechanism through the pervaporation membrane is a modified solution-difiusion model, where a permeating component is first dissolved or adsorbed in the membrane and then transported through it by a diffusion process. In this process, separation of the mixture is achieved by differences in the solubility and diffusibility of the individual components. 

Solution-diffusion model In the solution-diffusion model, permeates dissolve in the membrane material and then diffuse through the membrane down a concentration gradient. Separation is achieved between different permeates because of differences in the amount of material that dissolves in the membrane and the rate at which the material diffuses through the membrane. The solution-diffusion model is the most widely accepted transport mechanism for many membrane processes [209,210]. Selectivity and permeability of a pervaporation membrane mainly depend on the first two steps, that is, the solubility and diffusivity of the components in the membrane. According to this model, mass transport can be divided into the three steps the mechanism is shown in Fig. 3.11 ... 

Hasanoglu, A., Dinger, S. (2011). Modelling of a pervaporation membrane reactor during esterification reaction coupled with separation to produce ethyl acetate. Desalination and Water Treatment, 35, 286—294. 

Pervaporation is used to separate the liquid mixture. A phase transition occurs at the phase boundary on the permeate side, allowing desorption by vaporization. According to the solution-diffusion model, selectivity is primarily achieved because not all components in the mixture of substances can be dissolved equally well in the membrane material. Pervaporation involves a second selectivity step as a result of the required vaporization of the permeating components. For this, the partial pressure on the permeate side of the components must be lower than the saturated steam pressure. If only some of the components dissolved by the membrane boil at the operating point, the remainder of the components are not desorbed. 

The thermodynamics of irreversible processes are very useful for understanding and quantifying coupling phenomena. However, structure-related membrane models are more useful than the irreversible thermodynamic approach for developing specific membranes. A number of such transport models have been developed, partly based on the principles of the thermodynamics of irreversible processes, both for porous and nonporous membranes. Again, two types of structure will be considered here porous membranes, as found in microfUtration/uItrafiltration, and nonporous membranes of the type used in pervaporation/gas separation. 

To smdy separation in this flow vs. force configuration, it is useful to adopt a process model of pervaporation somewhat different from those employed in formulating equations (3.4.67a-h). Consider Figures 6.3.30(a) and (b). In Figure 6.3.30(a), the volatile liquid feed mixture is imposed on the feed membrane the permeation process may be described by equations in Section 3.4.2.I.I. On the other hand. Figure 6.3.30(b) illustrates a thermodynamically equivalent configuration that is somewhat different (Wijmans and Baker, 1993) the liquid feed is in equilibrium with a vapor phase this vapor phase is in contact with the feed side of the membrane while there... 

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