Mass membrane separation process

Figure 19.1 gives an overview of some of the most common membrane separation techniques, their application range and their denotation. It should be pointed out that the terminology for membrane separation processes is partly traditional. The kind of membrane-solute interactions and the respective mass-transport phenomena can therefore not necessarily be derived from the designation of the membrane separation, and should always be evaluated for the individual application envisaged. 

Membrane separation processes are discussed in Chapter 8. Liquid-phase mass transfer rates at the surface of membranes - either flat or tubular - can be predicted by the correlations given in this chapter. 

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. 

In analogous manner, residue curve maps of the reactive membrane separation process can be predicted. First, a diagonal [/e]-matrix is considered with xcc = 5 and xbb = 1 - that is, the undesired byproduct C permeates preferentially through the membrane, while A and B are assumed to have the same mass transfer coefficients. Figure 4.28(a) illustrates the effect of the membrane at nonreactive conditions. The trajectories move from pure C to pure A, while in nonreactive distillation (Fig. 4.27(a)) they move from pure B to pure A. Thus, by application of a C-selective membrane, the C vertex becomes an unstable node, while the B vertex becomes a saddle point This is due to the fact that the membrane changes the effective volatilities (i.e., the products xn a/a) of the reaction system such that xcc a. ca > xbbO-ba-... 

The clogging effect can be considered as a reduction in the value of the surface filtration constant for practical purposes. Indeed, when clogging takes place, the surface filtration constant can be given by its initial value ko multiplied by a decreasing time function. This assumption is frequently used when the function is obtained from experiments [3.19, 3.20]. In our example, if we do not consider the friction (and heat transfer) we can note that only a concrete mass transfer problem can be associated with the membrane separation process. The first step before starting to build the general mathematical model, concerns the division of the system into different elementary sections. Indeed, we have a model for the filtration device (i.e. the membrane and its envelope), for the pump (P) and for the reservoir of concentrated suspension (RZ) (Fig. 3.7). 

Further details of the mass transfer enhancement techniques in membrane separation processes are reported by Belfort and Al-Bastaki and Abbas. In this entry, the focus is on the flow instabilities produced by Dean vortices in curved and coiled tubes because of their advantages over the other techniques, viz., lower axial dispersion, better radial mixing, residence time distribution closer to plug flow, higher mass... 

Several research groups experimentally and theoretically reported the enhancement in mass transfer in membrane separation processes using Dean vortices, and their findings are summarized in Tables 1 and 2. 

Several novel techniques for mass transfer enhancement reported in the literature have been discussed in detail. The present study is focused on the mass transfer enhancement in the rate-controlled separation processes using flow instabilities. There are a large number of examples about the success of flow instabilities produced by Dean vortices in improving the performance by increasing flux and reducing fouling in membrane separation processes. Several curved modules... 

Mass transfer in fiber bundles is a problem of great practical importance for membrane separation processes. Such processes commonly utilize a bundle of randomly packed hollow fibers enclosed in a case to contact two process streams. Ports on the case permit one to introduce and remove streams from the space inside the fibers, the lumen, and the space outside the fibers, the shell. Figure 2.6 illustrates the construction of a typical hollow-fiber membrane module (Bao et al., 1999). 

The technical aspects outlined above illustrate that for a process optimization all process parameters must be considered, since they strongly affect each other. It should also be pointed out that although vapor permeation and pervaporahon are membrane separation process, their selectivity and effectiveness may be significantly determined by mass and heat transport phenomena occurring beyond the membrane surfaces. 

In many cases of practical interest, the membrane s mass transfer resistance is significant, i.e., the wall Sherwood number is small, leading to relatively low mass transfer rates of the solute. The diffusive flux of the permeate through the membrane can be increased by introducing a carrier species into the membrane. The augmentation of the flux of a solute by a mobile carrier species, which reacts reversibly with the solute, is known as carrier-facilitated transport (25). The use of carrier-facilitated transport in industrial membrane separation processes is of considerable interest because of the increased mass transfer rates for the solute of interest and the improved selectivity over other solutes (26). 

Traditionally, it was believed that RCMs were only suitable for equilibrium-based separations and could not be used for the representation of kinetically based processes [15]. However, the differential equations which describe a residue curve are simply a combination of mass balance equations. Because of this, the inherent nature of RCMs is such that they can be used for equilibrium- as well as non-equilibrium-based processes. This now allows one to consider kinetically based processes, such as reactive distillation (see Chapter 8) as well as membrane separation processes. 

However, in spite of the known advantages and applications of liquid membrane separation processes in hollow-fiber contactors, there are scarce examples of industrial application. The industrial application of a new technology requires a reliable mathematical model and parameters that serve for design, cost estimation, and optimization purposes allowing to accurate process scale-up. " The mathematical modeling of liquid membrane separation processes in HFC is divided into two steps (1) the description of the diffusive mass transport rate and (2) the development of the solute mass balances to the flowing phases. 

In conclusion, the mathematical description of the overall diffusive flux in liquid-membrane separation processes needs the calculation of the corresponding mass transport parameters (kt, k 11.. AC kL A) and equilibrium param-... 

Mass transport through a membrane has been described by several semiempirical mathematical models including Pick s, Hagen-PoisseuiUe s and Ohm s laws. For pressure-driven membrane-separation processes (RO, NF, UF, MF, GS), the transport relationship is given by ... 

MD is one of the emerging nonisothermal membrane separation processes, known for abont 50 years but still reqniring development for its industrial implementation. MD refers to the thermally driven transport of vapor through porous hydrophobic membranes, the driving force being the vapor pressure difference between the two sides of the membrane pores. Simnltaneous heat and mass transfer occurs in this process, and different MD contignrations include direct contact MD, sweeping gas MD, vacuum MD, and air gap MD. 

Membrane separation processes such as gas permeation, pervaporation, reverse osmosis (RO), and ultrafiltration (UF) are not operated as equilibrium-staged processes. Instead, these separations are based on the rate at which solutes transfer though a semipermeable membrane. The key to understanding these membrane processes is the rate of mass transfer not equilibrium. Yet, despite this difference we will see many similarities in the solution methods for different flow patterns with the solution methods developed for equilibrium-staged separations. Because the analyses of these processes are often analogous to the methods used for equilibrium processes, we can use our understanding of equilibrium processes to help understand membrane separators. These membrane processes are usually either conplementary or conpetitive with distillation, absorption, and extraction. 

This chapter presents an introduction to the four membrane separation methods most commonly used in industry gas permeation, RO, UF, and pervaporation. At the level of this introduction the mathematical sophistication needed to understand the membrane processes is approximately the same as that needed for the equilibrium-staged processes. A background in mass transfer (Chapter 15) will be helpful but is not essential. Detailed descriptions of these membrane separation processes are found in Baker et al. (1990), Eykanp (199Z), Geankoplis (2Q03), Kucera (2010). Noble and Stern (1985), Mohr et al. (1988), Mulder (1996), Osada and Nakagawa (1992), Hagg (1998), Ho and Sirkar (1992). and Wankat (1990). 

Facilitated mass transfer Similarly to LLE, the selectivity and efficiency of the liquid membrane separation process can be considerably improved if a suitable extractant with a high selectivity for the analyte of interest is used. This extractant, often referred to as the carrier, facilitates the mass transfer of the analyte between the feed and receiver solutions. The liquid membrane phase in this case usually consists of a suitable liquid extractant or an extractant dissolved in an organic solvent (diluent). The extractant facilitates the transport of the analyte from the feed phase to the liquid membrane phase by chemically interacting with it. This interaction leads to the selective extraction of the analyte into the... 

Merdaw, A. A., A. O. Sharif, G. A. W. Derwish, Mass Transfer in Pressure-Driven Membrane Separation Processes, Part II. Chemical Engineering Journal, 2011, 168(1), 229-240. 

During the last decades interest in membrane separation processes and in solutions for packaging problems has given a substantial boost to research on sorption and mass transport in polymeric materials. Several interpretative models for the solubility and diffusivity in rubbery polymers are now available which, at least in principle, allow for the prediction of the permeability of low molecular weight species in polymeric films above their glass transition temperature. 

The oxygen-hemoglobin system is but one of many examples of adsorption processes that occur in the human body. These are often irreversible in nature (i.e., involve chemisorption) and do not properly fall into the category of phase equilibria. All such events, however, do involve the transport of mass in one form or another, with both membranes and fluid barriers defining the mass transfer resistance. We will return to this topic in Section 8.2 dealing with membrane separation processes. 

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