Membrane processes, concept

Process Concepts. Hybrid systems involving gas-phase adsorption coupled with catalytic processes and with other separations processes (especially distillation and membrane systems) will be developed to take advantage of the unique features of each. The roles of adsorption systems will be to efficiently achieve very high degrees of purification to lower fouUng contaminant concentrations to very low levels in front of membrane and other separations processes or to provide unique separations of azeotropes, close-boiling isomers, and temperature-sensitive or reactive compounds. 

Process Concept The application of a direct elec tric field of appropriate polarity when filtering should cause a net charged-particle migration away from the filter medium. This electrophoretic migration will prevent filter-cake formation and the subsequent reduction of filter performance. An additional benefit derived from the imposed electric field is an electroosmotic flux. The presence of this flux in the membrane and in any particulate accumulation may further enhance the filtration rate. 

The concept of cross-flow microfiltration is shown in Figure 16.11, which represents a cross-section through a rectangular or tubular membrane module. The particle-containing fluid to be filtered is pumped at a velocity in the range 1-8 m/s parallel to the face of the membrane and with a pressure difference of 0.1-0.5 MN/m2 (MPa) across the membrane. The liquid penneates through the membrane and the feed emerges in a more concentrated form at the exit of the module.1617 All of the membrane processes are listed in Table 16.2. Membrane processes are operated with such a cross-flow of the process feed. 

B. Vigelund, K. Aasen, Development of a hydrogen membrane for the HMR process concept, Proceedings of the 8th International Conference on Greenhouse Gas Technologies (www.GHGT8.no), 20-23 June 2006, Trondheim, Norway. 

Researchers at Degussa AG focused on an alternative means towards commercial application of the Julia-Colonna epoxidation [41]. Successful development was based on design of a continuous process in a chemzyme membrane reactor (CMR reactor). In this the epoxide and unconverted chalcone and oxidation reagent pass through the membrane whereas the polymer-enlarged organocatalyst is retained in the reactor by means of a nanofiltration membrane. The equipment used for this type of continuous epoxidation reaction is shown in Scheme 14.5 [41]. The chemzyme membrane reactor is based on the same continuous process concept as the efficient enzyme membrane reactor, which is already used for enzymatic a-amino acid resolution on an industrial scale at a production level of hundreds of tons per year [42]. 

Membrane extraction encompasses a class of liquid-phase separations where the primary driving force for transport stems from the concentration difference between the feed and extractant liquids rather than a pressure gradient, as is the case with most of the other processes discussed above. A microporous membrane placed between the feed and the extractant liquids functions primarily as a phase separator. The degree of separation achievable is determined by the relative partition coefficients among individual solutes. This operationx is known as membrane solvent extraction. If a nonporous, permselective membrane is used instead, however, the selectivity of the membrane would be superimposed on the partitioning selectivity in this case the process may be referred to as perstraction. These process concepts are illustrated in Fig. 34. 

The reasoning behind developing different membrane reactor concepts is based on the realization of selective transport processes. Typically, certain components should be brought into - or removed selectively from - a reaction zone. Thus, an essential requirement for the successful operation of membrane reactors is to understand, and quantify, these transport processes correctly. 

The chemzyme membrane reactor is based on the same continuous process concept as the efficient enzyme membrane reactor, which has already been applied for enzymatic a-amino acid resolution on industrial scale at a production level of hundreds of tons per year (Drauz and Waldmann 2002 Wandrey and I iaschcl 1979 Wandrey et al. 1981 Groger and Drauz 2004). 

Membrane processes can be operated in two major modes according to the direction of the feed stream relative to the orientation of the membrane surface dead-end filtration and crossflow filtration (Figure 1.1). The majority of the membrane separation applications use the concept of crossflow where the feed flows parallel to and past the membrane surface while the permeate penetrates through the membrane overall in a... 

At these assumptions and simplifications the thermodynamic network analysis (TNA) [90] can be applied to analyze LM transport. Certainly in the case of a real specific system, the detailed mechanism of reaction-diffusion interfacial phenomena should be taken into account as far as possible. The above assumptions allow maintaining a concept of a homogeneous reaction. Any universal model does not exist, and in the description of a real membrane process the accessible knowledge concerning the specific interfacial processes should be taken into account. The model presented can be regarded as a simplified example only. 

Howell JA. Future of membranes and membrane reactors in green technologies and for water reuse. Desalination, 2004 162(10) 1-11. Noronha M, Britz T, Mavrov V, Janke HD, and Chmiel H. Treatment of spent process water from a fruit juice company for purposes of reuse Hybrid process concept and on-site test operation of a pilot plant. Desalination, 2002 143(2) 183-196. 

Other recent ideas on (i) simultaneous sorption-reaction process concepts using the principles of a novel PSA technology for the production of fuel cell-grade hydrogen and (ii) improving the H2 recovery from existing H2 PSA processes by integrating it with additional PSA units or nanoporous SSF adsorbent membrane systems are reviewed. 

On behalf of KTI an experimental programme on these reactor concepts has been started at the University of Southern California (USC). Some of the experimental results, concerning the use of Knudsen diffusion membranes are available in the literature [32,40]. These data have been used to calculate the economics of an isothermal propane dehydrogenation membrane reactor concept and are compared with the commercial Oleflex and Catofin processes, based on an adiabatic concept. The experimental circumstances of these lab-scale experiments, especially residence time, pressures and gas composition are not the same as in commercial, large-scale processes. However, we do not expect these differences to have a great influence on the results of the work presented here. 

Following the results of the adiabatic reactor concept it is expected that high selective membranes will further improve the economics. However, it should be recognised that the process conditions in an isothermal concept are more severe than in an adiabatic concept. In particular, decoking conditions can be a problem in using high selective membranes. Detailed calculations on the isothermal membrane reactor concept are being performed and will be reported in future. 

The CCRO concept has not been proven in practice thus, an objective of the present work was to demonstrate the process concept experimentally. Various RO membranes were characterized to determine if their use for ethanol enrichment by CCRO would be more energy-efficient than by distillation, and to identify membrane characteristics that affect the performance of the process. 

---