Membrane separators: flow patterns

Figure 10.11 Idealized flow patterns in membrane separation.

Gas and liquid flow up along the membrane, then turn to the inlet of a narrow channel at the top of the chamber. This flow pattern enhances continuous replacement of electrolyte over the whole membrane surface. It is especially effective in eliminating gas stagnation at the top zone of the electrolysis area. The DAM-type system ensures that the fine-bubble flow is constant through its narrow channel and that smooth gas separation occurs at the outlet of the channel. Gas and liquid flow separately through an upper duct, an outlet nozzle and an outlet hose, then to a... 

The effect of concentration polarization on specific membrane processes is discussed in the individual application chapters. However, a brief comparison of the magnitude of concentration polarization is given in Table 4.1 for processes involving liquid feed solutions. The key simplifying assumption is that the boundary layer thickness is 20 p.m for all processes. This boundary layer thickness is typical of values calculated for separation of solutions with spiral-wound modules in reverse osmosis, pervaporation, and ultrafiltration. Tubular, plate-and-ffame, and bore-side feed hollow fiber modules, because of their better flow velocities, generally have lower calculated boundary layer thicknesses. Hollow fiber modules with shell-side feed generally have larger calculated boundary layer thicknesses because of their poor fluid flow patterns. 

For a defect-free ideal membrane, the selectivity is independent of thickness, and either permeability ratios or permeance ratios can be used for comparison of selectivi-ties of different materials. Nonideal module flow patterns, defective separating layers, impurities in feeds, and other factors can lower the actual selectivity of a membrane compared to tabulated values based on ideal conditions (Koros and Pinnau, 1994). 

In a separate parametric study, Mohan and Govind(l)(9) analyzed the effect of design parameters, operating variables, physical properties and flow patterns on membrane reactor. They showed that for a membrane which is permeable to both products and reactants, the maximum equilibrium shift possible is limited by the loss of reactants from the reaction zone. For the case of dehydrogenation reaction with a membrane that only permeates hydrogen, conversions comparable to those achieved with lesser permselective membranes can be attained at a substantially lower feed temperature. 

None of the above studies, however, deals with the detailed hydrodynamics in a membrane reactor. It can be appreciated that detailed information on the hydrodynamics in a membrane enhances the understanding and prediction of the separation as well as reaction performances in a membrane reactor. All the reactor models presented in Chapter 10 assume very simple flow patterns in both the tube and annular regions. In almost all cases either plug flow or perfect mixing is used to represent the hydrodynamics in each reactor zone. No studies have yet been published linking detailed hydrodynamics inside a membrane reactor to reactor models. With the advent of CFD, this more complete rigorous description of a membrane reactor should become feasible in the near future. 

An application of microfluidic reactors is the development of a membraneless fuel cell. Two streams, one containing a fuel such as methanol, the other an oxygen-saturated acid or alkaline stream, are merged without mixing. The laminar flow pattern in the narrow channel helps to maintain separate streams without the use of membrane separators. Opposite walls function as the electrodes and are doped with catalyst. Ion exchange, protons for the add system, takes place through the liquid-liquid interface. This is an example of a solid-liquid-liquid-solid multiphase reactor. ... 

Figure 13.3-4. Idea flow patterns in a membrane separator for gases (a) complete mixing, (h) cross-flow, (c) countercurrent flow, (d) cocurrent How.

The performance of a membrane separator may be predicted on the basis of permeation fluxes and material balances. The formulation of these relationships for a particular module depends on the flow patterns of the fluid on both sides of the membrane. 

In this model the flow on the residue side is parallel to the membrane and on the permeate side it is normal to the membrane and away from it as shown in Figure 18.4. This flow pattern is typical in membrane separators where the velocity is high on the high-pressure residue side. On the permeate side the pressure is low and the permeate is forced to flow away from the membrane. 

The field of membrane separations is radically different from processes based on vapor-liquid or fluid-solid operations. This separation process is based on differences in mass transfer and permeation rates, rather than phase equilibrium conditions. Nevertheless, membrane separations share the same goal as the more traditional separation processes the separation and purification of products. The principles of multi-component membrane separation are discussed for membrane modules in various flow patterns. Several applications are considered, including purification, dialysis, and reverse osmosis. 

FIGURE 20.6-2 Idealized flow patterns in a membrane gas separator. 

Countercurrent and Cocurrent Plug Flows. The model equations for these flow patterns cannot be solved analytically. Oishi and coworkers first derived the general model eqnstions for a binary-component system with porous media.19 Walawenderand Stem,16 Blaisdell and Kammermeyer,1 and Pan and Habgood17 later reported solutions for similar membrane separators. The cocurrent-counteicurrent combiner inu flow pattern also lies been studied by Pen ned Habgood.17... 

FLOW PATTERNS IN MEMBRANE SEPARATORS. There are several ways of arranging the surface area in a gas separator, and some of these are illustrated in Fig. 26.5 for hollow-fiber membranes with an external skin. Only a few fibers are shown, and their si2S is greatly exaggerated for clarity. A commercial separator has up to a million fibers in a shell several inches in diameter. The fibers are sealed... 

In Example 9.2, perfect mixing was assumed on both sides of the membrane. Three other idealized flow patterns, common to other mass-transfer processes, have been studied countercurrent flow, cocurrent flow, and crossflow. For a given cut, the flow pattern can significantly affect the degree of separation achieved and the membrane area required. For a given membrane module geometry, it is not always obvious which idealized flow pattern to assume. Hollow-fiber modules are the most versatile since they may be designed to approximate any of the three flow patterns mentioned above. 

Calculation of the degree of separation of a binary mixture in a membrane module for cocurrent or countercurrent flow patterns involves the numerical solution of a system of two nonlinear, coupled, ordinary differential equations (Walawender and Stem, 1972). For a given cut, the best separation is achieved with countercurrent flow, followed by crossflow, cocurrent flow, and perfect mixing, in that order. The crossflow case is considered to be a good, conservative estimate of module membrane performance (Seader and Henley, 2006). 

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. 

In most of this chapter we will make the assumption that both sides of the membrane are perfectly mixed. This assumption greatly simplifies the mathematics. The resulting design will be conservative in that the actual apparatus will result in the same or better separation than predicted. The effect of other flow patterns will be explored in section 17.7. 

Explain and analyze the effects of flow patterns on the separation achieved in membrane systems... 

As emphasized in the preceding sections, the successful application of membranes for separation purposes depends primarily on discovery of economically competitive membranes with high peimselectivities and permeabilities. However, the ensuing considerations, such as membrane configurations and flow patterns of the feed and the permeant streams, are also very important in determining the performance of Ae final separator system. 

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