Gas-Separation Membranes

The acid gas removal and upgrading the heating value of the natural gases can be divided into three broad categories  

bulk removal of carbon dioxide, hydrogen sulfide, and water 

The most important step in the membrane separation process is the selection of the membrane material, its properties, and its selectivity to the different components of the gas stream. The membrane area and operational power consumption are the major factors to be considered in the design of a membrane gas-separation system once the suitable membrane is selected. Capital investment is affected by the membrane area, while the operating cost could be altered by the power requirements. However, the individual (specific to the separation problem) design parameters (e.g., pressure ratio between feedstock and permeate stream, reflux fraction for a recycle permeator, and relative areas for a cascade system) must be considered during the design process. 

Suitability of a membrane for a particular process is decided based on the following factors  

adaptability of the membrane to changes in the flow rates of feedstock 

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Status of Membrane Gas Separation Technology," Nitrogen 173, 25—29 (May—June 1988). 

Hydrogen Hydrogen recovery was the first large commercial membrane gas separation. Polysulfone fiber membranes became available in 1980 at a time when H9 needs were rising, and these novel membranes qiiickly came to dominate the market. Applications include recovery of H9 from ammonia purge gas, and extraction of H9 from petroleum crackiug streams. Hydrogen once diverted to low-quahty fuel use is now recovered to become ammonia, or is used to desulfurize fuel, etc. H9 is the fast gas. 

In gas separation with membranes, a gas mixture at an elevated pressure is passed across the surface of a membrane that is selectively permeable to one component of the mixture. The basic process is illustrated in Figure 16.4. Major current applications of gas separation membranes include the separation of hydrogen from nitrogen, argon and methane in ammonia plants the production of nitrogen from ah and the separation of carbon dioxide from methane in natural gas operations. Membrane gas separation is an area of considerable research interest and the number of applications is expanding rapidly. 

Fig. 16.4. Schematic diagram of the basic membrane gas separation process.

In the present study, we fabricated hollow fiber membrane modules and performed experiments at several conditions. The energy consumption of this process is compared to those of conventional gas absorption processes and membrane gas separation processes. 

Membrane gas-separation systems have found their first applications in the recovery of organics from process vents and effluent air [5]. More than a hundred systems have been installed in the past few years. The technique itself therefore has a solid commercial background. Membranes are assembled typically in spiral-wound modules, as shown in Fig. 7.3. Sheets of membrane interlayered with spacers are wound around a perforated central pipe. The gas mixture to be processed is fed into the annulus between the module housing and the pipe, which becomes a collector for the permeate. The spacers serve to create channels for the gas flow. The membranes separate the feed side from the permeate side. 

Gas separation processes with membranes have undergone a major evolution since the introduchon of the first membrane-based industrial hydrogen separation process about two decades ago. The development of high selectivity mixed-matrix membranes will further advance the technology of membrane gas separation processes within the next decade. 

Figure 19.6. Gas permeation equipment and performance, (a) Cutaway of a Monsanto Prism hollow fiber module for gas separation by permeation, (b) Flowsketch of a continuous column membrane gas separator, (c) Composition profiles of a mixture of C02 and Oz in a column 5 m long operated at total reflux [Thorman and Hwang in ( Turbak, Ed.), Synthetic Membranes II, American Chemical Society, Washington DC, 1981, pp. 259-279],...

The sorption and diffusion behaviour of gas mixtures is of particular interest from the point of view of membrane gas separation, which is steadily gaining in importance by virtue of its low energy requirements. On the basis of the dual mode sorption model, one may reasonably expect that sorption of a binary gas mixture A, B in the polymer matrix will exhibit little gas-gas interaction and hence will tend to occur essentially additively. In the Langmuir-like mode of sorption, on the other hand, there will be competition between A and B for the limited number of available sites. These considerations led 67) to the following reformulation of Eqs. (8) and (9)... 

The general conclusion to be drawn from the above discussion is that the dual gas sorption and mobility model has very useful predictive potential in the field of membrane gas separation. At the present stage, no comparable predictive capability is discernible in the alternative treatments so far published 33 >34 63). 

Membrane Gas Separations for Chemical Processes and Energy Applications... 

Another important factor in membrane gas separation is the pressure differential, P1 - P11 the greater this difference the less membrane area required. The membrane area is exactly proportional to the inverse of the pressure differential, i.e.,... 

There are many commercial applications for membrane gas separations, some of which are currently being marketed, and others being tested. 

The development of high-flux anisotropic membranes and large-surface-area membrane modules for reverse osmosis applications in the late 1960s and early 1970s provided the basis for modem membrane gas separation technology. The first company to establish a commercial presence was Monsanto, which... 

The three factors that determine the performance of a membrane gas separation system are illustrated in Figure 8.12. The role of membrane selectivity is obvious not so obvious are the importance of the ratio of feed pressure (p ) to permeate pressure (pt) across the membrane, usually called the pressure ratio, [Pg.317]

Figure 8.12 Parameters affecting the performance of membrane gas separation systems...

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