Gas-permeation module

An internally-staged, gas-permeation module is used for the oxygen enrichment of air, using the flow arrangement shown in Fig. 5.206. Enrichment depends on differing membrane permeabilities for the oxygen and nitrogen to be separated. The permeation rates are proportional to the differences in component partial pressures. 

A perfectly mixed gas permeation module is separating carbon dioxide from nitrogen using a poly (2,6 - dimethylphenylene oxide) membrane. The feed is 20.0 mol% carbon dioxide and is at 25 °C. The module has 50.0 m of membrane. The module is operated with a retentate pressure of 5.5 atm and a permeate pressure of 1.01 atm. We desire a permeate that is 40.0 mol% carbon dioxide. The... 

Historically, the basis of gas permeation module design was first proposed by Weller and Steiner in 1950. Nowadays, modem computation techniques provide numerical solutions to the problems thanks to dedicated routines. Orthogonal collocation methods or perturbation methods " are reported to be particularly attractive when a minimum resolution time and computational efforts are required. Several of these routines have been implemented in commercial process simulation software, where advantage can be taken of thermodynamics or unit operation design packages in order to simulate hybrid or multi-stage operations with gas separation membranes. Nevertheless, much effort has been devoted to... 

Figure 5.11 The five different flow patterns applied in gas permeation modules.

For high natural gas flows with low carbon dioxide concentrations absorption processes prove to be more profitable than gas permeation modules. 

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],...

Hollow fine fiber membranes are extremely fine polymeric tubes 50-200 micrometers in diameter. The selective layer is on the outside surface of the fibers, facing the high-pressure gas. A hollow-fiber membrane module will normally contain tens of thousands of parallel fibers potted at both ends in epoxy tube sheets. Depending on the module design, both tube sheets can be open, or as shown in Figure 8.1, one fiber end can be blocked and one open. The high-pressure feed gas flows past the membrane surface. A portion of the feed gas permeates the membrane and enters the bore of the fiber and is removed from the open end of the tube sheet. Fiber diameters are small because the fibers must support very large pressure differences feed-to-permeate (shell-to-bore). 

In the crossflow module illustrated in Figure 8.5(a), the pooled permeate stream has a water concentration of 1.88%. The counterflow module illustrated in Figure 8.5(b) performs substantially better, providing a pooled permeate stream with a concentration of 3.49%. Not only does the counterflow module perform the separation twice as well, it also requires only about half the membrane area. This improvement is achieved because the gas permeating the membrane at the residue end of the module contains much less water than the gas permeating the membrane at the feed end of the module. Permeate counterflow dilutes the permeate gas at the feed end of the module with low-concentration permeate gas from the residue end of the module. This increases the water concentration driving force across the membrane and so increases the water flux. 

The hollow fiber membranes are the optimum choice for gas separation modules due to their very high packing density (up to 30,000 m /m may be attained [1]). Figure 4.21 shows alternative configurations for such modules [108]. Modifications of this configuration exist, where possibility for introduction of sweep gas on permeate side is included, or fibers may be arranged transversal to the flow in order to minimize concentration polarization [109,110]. The hollow fiber membranes are usually asymmetric polymers, but composites also exist. Carbon molecular sieve membranes may easily be prepared as hollow fibers by pyrolysis. 

Some more module configurations are reported for use in MD. Lawson and Lloyd [77] have designed a laboratory-scale MD module as shown in Figure 19.13, where the membrane was sandwiched between the two half-cells, and several hose clamps held the module together. The total area available is 9.7 cm and the smooth transitions at the module entrance as well as exit allow achievement of relatively high Reynolds numbers, whereby conventional boundary layer equations are applicable. The module does not require a support in low pressure-drop applications such as DCMD. Wider permeate channels would require a support for VMD experiments. A porous sintered stainless steel material has been used for the gas permeation experiments. 

R. Rautenbach and W. Dahm, Simplified calculation of gas-permeation hollow fiber modules for the separation of binary mixtures. /. Membr. Science, 26 (1986) 319. 

Common practical applications of permeation separation phenomena include gas permeation, dialysis, and reverse osmosis. A variety of equipment modules for carrying out these processes are in use. The membrane type and the way it is incorporated in the device define the different separator assemblies that are commonly available. 

Single membrane units can be evaluated based on their geometry and operation conditions. Zolandz and Fleming [4] provide a good description for gas permeation systems and models for design purposes. Sender [5] discusses the use of cascades (or staging) for various series and/or parallel sets of membrane modules. 

In the case of reverse osmosis, the relative flow configuration does not affect the performance to any large extent. As already indicated, the situation is quite different for gas permeation. While countercurrent flow improves the separation efficiency of hollow fiber modules in reverse osmosis only slightly, as far... 

There are four specific tasks (1) HDPE treatment and development of alternative tank liner materials for hydrogen gas permeation reduction (2) enhancing epoxy resin toughness to improve the tank carbon liber overwrap performance (3) materials and process evaluation for manufacturing the ISS protective shell and (4) engineering an optimized unitary gas control module. An additional task (5) is the formulation of a roadmap for protot5q)ing and... 

Apply the perfect-mixing and crossflow models to solve gas permeation problems in membrane modules. 

Commercial membrane separation processes include reverse osmosis, gas permeation, dialysis, electrodialysis, pervaporation, ultrafiltration, and microfiltration. Membranes are mainly synthetic or natural polymers in the form of sheets that are spiral wound or hollow fibers that are bundled together. Reverse osmosis, operating at a feed pressure of 1,000 psia, produces water of 99.95% purity from seawater (3.5 wt% dissolved salts) at a 45% recovery, or with a feed pressure of 250 psia from brackish water (less than 0.5 wt% dissolved salts). Bare-module costs of reverse osmosis plants based on purified water rate in gallons per day are included in Table 16.32. Other membrane separation costs in Table 16.32 are f.o.b. purchase costs. 

Gas permeation is used to separate gas mixtures, for example, hydrogen fixjm methane. High pressures on the order of 500 psia are used to force the molecules through a dense polymer membrane, which is packaged in pressure-vessel modules, each containing up to 4,000 ft of membrane surface area. Membrane modules cost approximately 35/ft of membrane surface area. Multiple modules are arranged in parallel to achieve the desired total membrane area. 

Gas permeation Membrane surface area. A, ft — Cp = 35A Membrane module... 

Gas permeation systems typically use hollow-fiber or spiral-wound membranes, although hollow-fiber systems are more common tBaker. 2004k Cellulose acetate membranes are used for carbon dioxide recovery, polysulfone coated with silicone rubber is used for hydrogen purification, and conposite membranes are used for air separation. The feed gas is forced into the membrane module under pressure. Retentate, which does not go through the membrane, will become concentrated in the less permeable gas. Retentate exits at a pressure that will be close to the input pressure. The more permeable species will be concentrated in permeate. Permeate, which has passed through the membrane, exits at low pressure. The operating cost for a gas permeator is the cost of conpression of the feed gas and the irreversible pressure difference that occurs for the gas that permeates the membrane. A typical hollow-fiber unit will contain 5000 m membrane area per m at a cost of approximately 200/m. ... 

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