Membrane separation processes characteristics

Although ED is more complex than other membrane separation processes, the characteristic performance of a cell is, in principle, possible to calculate from a knowledge of ED cell geometry and the electrochemical properties of the membranes and the electrolyte solution. 

PVA-PAcr.Ac. membranes have been tested also for ethanol separation from ethanol/toluene mixture, by using pervaporation technique. The reported data concerning the separation process characteristics are presented in table 8. 

The interest in ceramic membranes grew, together with the interest in membrane separation processes, due to their specific properties. They are chemically stable, can withstand high temperatures and are noncompressible. These characteristics made them the only materials available, which could withstand the harsh environment in the isotope separation. On the other hand, the brittleness of most materials is a problem and so is the selectivity. 

The most important characteristic of nonporous membranes is that they are hydrophobic and contain no pores in the polymeric structure. This means that these membranes not only selectively act as a barrier to particles and polar species, but they also provide unique selectivity and specificity for the permeation and transport of a specific group of compounds that can readily solubilize and diffuse in the membrane material. The analyte extraction rate (permeability) in a nonporous membrane separation process is governed by the solution-diffusion mechanism, as commented on earlier. 

The possible products of a reactive membrane separation process are influenced by the mass transfer characteristics of the applied membranes. In the following section it is shown how the concepts and tools, being developed for reactive vapor-liquid separation, can also be used to analyze the feasibility of membrane separators. 

It is essential to understand the basic characteristics of the membrane, such as membrane materials and their formation, so that proper selection of membrane separation processes can be made. 

NF is a pressure-driven membrane separation process with characteristics between reverse osmosis and UF. NF was introduced in the early 1980s and has since gained popularity due to improved selectivity for mono- and multivalent ions, low operating pressures, and relatively low capital and... 

The design and scale-up of liquid-membrane separation processes need separation and concentration mathematical models as reported in Section 29.2.1. When complex solutions such as wastewaters are treated, several simplifications according to the specific characteristics of the system are usually assumed in order to reduce the number of parameters and mathematical complexity of the EPT model. From a kinetic point of view, the transport through the membrane... 

Huang, R. Y. M., and J. W. Rhim, Separation Characteristics of Pervaporation Membrane Separation Processes, in R. Y. M. Huang (Ed.), Pervaporation Membrane Separation Processes, Elsevier, Amsterdam, 1991, pp. 111-180. 

Vane LM, Alvarez FR. Effect of membrane and process characteristics on cost and energy usage for separating alcohol-water mixmres using hybrid vapor stripping-vapor permeation process. J Chem Technol Biotechnol 2015 90 1380-90. 

Currently, virtually all industrial scale separations of hydrocarbons are performed by distillation (43,44). Distillation alone is inherently inefficient when the vapor/liquid equilibrium line is close to the operating lines in McCabe-Thiele diagrams. This occurs when components have similar volatilities, form azeotrope(s), or when high product purity is required. While it is unlikely that membrane separation processes will displace distillation, the use of membranes to eiihance the performance of distillation columns is a possibility, if membrane materials with the necessary performance characteristics can be developed. Membrane/distillation column hybrid processes offer significant advantages in a number of situations ... 

Second, other types of membrane separation processes can also be proposed for the CO2/N2 separation step, in order to play the same role as the absorption process. In that case, it is absolutely necessary to carefully estimate the operating costs (OPEX) and capital costs (CAPEX) of the operation, so that an overall CO2 capture cost (in euros or per ton of recovered CO2) can be precisely estimated. This data will obviously be of utmost importance in order to evaluate the chances of the membrane process to possibly compete with the standard capture process. A CO2/N2 separation efficiency is clearly needed in that case. Depending on the membrane material, this characteristic can be obtained based on a physical mechanism (solution-diffusion in a dense polymer, for instance, which corresponds to a classical membrane gas separation operation ), or a chemical reaction in a polymeric matrix based on fixed sites (so-called fixed site carrier membranes ) or with membranes making use of a mobile selective carrier (liquid membranes ). 

Reverse osmosis membrane separations are governed by the properties of the membrane used in the process. These properties depend on the chemical nature of the membrane material, which is almost always a polymer, as well as its physical stmcture. Properties for the ideal RO membrane include low cost, resistance to chemical and microbial attack, mechanical and stmctural stabiHty over long operating periods and wide temperature ranges, and the desired separation characteristics for each particular system. However, few membranes satisfy all these criteria and so compromises must be made to select the best RO membrane available for each appHcation. Excellent discussions of RO membrane materials, preparation methods, and stmctures are available (8,13,16-21). 

A very high separation factor has been obtained in phenol dehydration by using pervaporation process and PVA/PAA as membranes. The membrane composition and the process characteristics are presented in table 1. 

Table 1. Characteristics of the separation process by pervaporation function of the membrane composition and structure, composition of feed mixture and temperature [18]...

Adsorptive separation is a powerful technology in industrial separations. In many cases, adsorption is the only technology available to separate products from industrial process streams when other conventional separation tools fail, such as distillation, absorption, membrane, crystallization and extraction. Itis also demonstrated that zeolites are unique as an adsorbent in adsorptive separation processes. This is because zeolites are crystalline soUds that are composed of many framework structures. Zeolites also have uniform pore openings, ion exchange abiUty and a variety of chemical compositions and crystal particle sizes. With the features mentioned, the degree of zeoUte adsorption is almost infinite. It is also noted that because of the unique characteristics of zeoHtes, such as various pore openings, chemical compositions and structures, many adsorption mechanisms are in existence and are practiced commercially. 

Table 2.1 Overview of main polymer membrane characteristics and membrane-based processes for molecular separations in liquid phase.

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