Countercurrent membranes

Moreover, the membrane could be mounted as an interface between the apolar substrate and the polar oxidant in a membrane reactor, avoiding the use of any solvent. Dilution of the reagents by solvent and competition between solvent and reagents on the active sites can thus be avoided. In the countercurrent membrane reactor, the substrate and the oxidant are circulated at each side of the membrane and alkanes can be oxidized with peroxides without solvents. Of course, the system carries all of the other advantages of membrane reactors continuous operation and easy separation. 

Figure 9.11 depicts the profiles of the feed-side mole fractions of CO and C02 along the length of the countercurrent membrane reactor. The modeling results showed that this membrane reactor could convert CO via the WGS reaction and then decrease CO concentration from 1% to 9.82ppm along with the removal of almost all the C02 from the hydrogen product. In the membrane reactor, the removal of C02 enhanced the WGS reaction. 

Figure 2. Profiles of Carbon Monoxide Mole Fractions in the Hydrogen Products Along the Countercurrent Membrane Reactors for the Synthesis Gases With (1) 18.63% CO from Steam Reforming and (2) 5% CO from Autothermal Reforming...

Figure 4. Effect of Sweep-To-Feed Molar Flow Rate Ratio on Hydrogen Recovery for the Countercurrent Membrane Reactor With the 18.63% CO Syngas From Steam Reforming...

Countercurrent Membrane Reactors in Terms of the Profiles of Carbon Monoxide Mole Fractions in the Hydrogen Products along the Reactors... 

In Figure 25.22, the scheme of experimental setup used for membrane extraction experiments was presented. The setup consisted of membrane contactor with aqueous and organic circuits, two pumps, and the control equipment flowmeters, pressure gauges, and valves. Two phases aqueous and organic solutions circulated countercurrently. Membrane contactor X50 2.5 x 8 Liqui-Cel Extra-Flow, Celgard, was used in the system. The characteristics of the membrane were shown in Table 25.17. The small volume module houses 11,000 capillaries with 1.9 m inner surface area. The module possesses the central baffle, which enables uniform flow inside the shell. 

Countercurrent membrane systems can also use a sweep gas or retentate reflux on the permeate side to increase separation fBaker. 20021. The spreadsheet in Appendix 17.A.3 is easily modified for these systems. 

H3. A countercurrent membrane module with a poly(dimethylsiloxane) membrane operates at 35°C to separate air (assume air is 20.9% oxygen and 79.1% nitrogen). We want a permeate product that is 23.5% ojgrgen (above this limit for safety reasons, stainless steel has to be used in all later equipment). Fjjj = 100,000 cm (STP)/s, Pj. = 1.5 atm, Pp = 1.0 atm, and tjns = 0.00002 cm. 

In the following part of this section, we provide simple mathematical descriptions of a few common features of two-phase/two-region countercurrent devices, specifically some general considerations on equations of change, operating lines and multicomponent separation capability. Sections 8.1.2, 8.1.3, 8.1.4, 8.1.5 and 8.1.6 cover two-phase systems of gas-Uquid absorption, distillation, solvent extraction, melt crystallization and adsorption/SMB. Sections 8.1.7, 8.1.8 and 8.1.9 consider the countercurrent membrane processes of dialysis (and electrodialysis), liquid membrane separation and gas permeation. Tbe subsequent sections cover very briefly the processes in gas centrifuge and thermal diffusion. 

Consider equation (8.1.427) describing the variation of Xaz with membrane area in countercurrent membrane gas separation in a hollow fiber device with no longitudinal diffiision/dispersion and symmetrical hollow fiber membrane. The feed-gas composition varies from x/ f to Xa21 as per Figure 8.1.51(a). 

Hollow-fiber membranes may be run with shell-side or tube-side feed, cocurrent, countercurrent or in the case of shell-side feed and two end permeate collection, co- and countercurrent. Not shown is the scheme for feed inside the fiber, common practice in lower-pressure separations such as air. 

Air is commonly run with tube-side feed. The permeate is run countercurrent with the separating sldn in contact with the permeate. (The feed gas is in contact with the macroporous back side of the membrane.) This configuration has proven to be superior, since the permeate-side mass-transfer problem is reduced to a minimum, and the feed-side mass-transfer problem is not limiting. 

Partial Pressure Pinch An example of the hmitations of the partial pressure pinch is the dehumidification of air by membrane. While O9 is the fast gas in air separation, in this apphcation H9O is faster still. Special dehydration membranes exhibit a = 20,000. As gas passes down the membrane, the pai-dal pressure of H9O drops rapidly in the feed. Since the H9O in the permeate is diluted only by the O9 and N9 permeating simultaneously, p oo rises rapidly in the permeate. Soon there is no driving force. The commercial solution is to take some of the diy air product and introduce it into the permeate side as a countercurrent sweep gas, to dilute the permeate and lower the H9O partial pressure. It is in effect the introduction of a leak into the membrane, but it is a controlled leak and it is introduced at the optimum position. 

As described above, the application of classical liquid- liquid extractions often results in extreme flow ratios. To avoid this, a completely symmetrical system has been developed at Akzo Nobel in the early 1990s [64, 65]. In this system, a supported liquid-membrane separates two miscible chiral liquids containing opposite chiral selectors (Fig. 5-13). When the two liquids flow countercurrently, any desired degree of separation can be achieved. As a result of the system being symmetrical, the racemic mixture to be separated must be added in the middle. Due to the fact that enantioselectivity usually is more pronounced in a nonaqueous environment, organic liquids are used as the chiral liquids and the membrane liquid is aqueous. In this case the chiral selector molecules are lipophilic in order to avoid transport across the liquid membrane. 

Fig. 5-13. Schematic representation of the Akzo Nobel enantiomer separation process. Two liquids containing the opposing enantiomers of the chiral selector (FI and K) are flowing countercurrently through the column (4) and are kept separated by the liquid membrane (3). The racemic mixture to be separated is added to the middle of the system (1), and the separated enantiomers are recovered from the outflows of the column (2a and 2b) [64],...

Figure 9. H2 ( ) / n-butane ( ) separaticm with the ccxnposite zeolite-alumina membrane (fluxes in the permeate as a function of the tenq>erature). A mixture of hydrogen, n-btitane and nitrogen (12 14 74) was fed in the tube (Fig. 2) with a flow rate of 4.8 1/h. Sweep gas (N2), countercurrent mode, flow rate 4.3 1/h.

Removal of reaction products can shift the equilibrium, forcing the reaction to go to completion. This can be effected by evaporation of products from the reaction mixture (reactive distillations), extraction (including supercritical extraction) of products from the reaction mixture (reactive extractions), or membrane processes. Counter- and cocurrent operation also falls within this category. If the reaction is equilibrium-limited or inhibited by reaction products countercurrent operation outperforms cocurrent operation. 

Dynantics of Heat Exchangers, Simple Batch Extraction, Multi-Solute Batch Extraction, Multistage Countercurrent Ctiscade, Extraction Cascade with Backmixing, Countercurrent Extraction Cascade with Reaction, Absorption with Chemical Reaction, Membrane Transfer Processes... 

Primary metals manufacturing operations have experienced source reduction and recycle/reuse benefits similar to those available to metal finishing operations, including conserving waters through countercurrent rinsing techniques, and utilizing electrolytic recovery, customized resins, selective membranes, and adsorbents to separate metal impurities from acid/caustic dips and rinsewaters to thereby allow for recycle and reuse. 

---