Permeated-side flow

The inlet flow on the permeate side wouldbe that of an inert gas, for instance, which continuouslysweepsthelowersideofthemembraneandclearsthepermeate.Ifthe flowis... 

Backflushing is another way of cleaning heavily fouled membranes. During back flushing a slight overpressure is applied to the permeate side of the membrane forcing solution from the permeate side to the feed side of the membrane. The flow of solution lifts deposited materials from the surface. Typical back flushing pressures are 5-15 psi [48]. 

We have been studying the novel process for CO2 separation named membrane/absorption hybrid method. The advantages of this process are that high gas permeance and selectivity were obtained. The concept of this process is shown in Fig. 1. Both feed gas and absorbent solution are supplied to the inside of hollow fibers. While Ae liquid flows upward inside the hollow fibers, absorbent solution absorbs CO2 selectively and it becomes a rich solution. Most of rich solution permeates the membrane to the permeate side maintained at reduced pressure, where it liberated CO2 to become a lean solution. Compared to a conventional gas absorption... 

The schematic diagram of the experimental setup is shown in Fig. 2 and the experimental conditions are shown in Table 2. Each gas was controlled its flow rate by a mass flow controller and supplied to the module at a pressure sli tly higher than the atmospheric pressure. Absorbent solution was suppUed to the module by a circulation pump. A small amount of absorbent solution, which did not permeate the membrane, overflowed and then it was introduced to the upper part of the permeate side. Permeation and returning liquid fell down to the reservoir and it was recycled to the feed side. The dry gas through condenser was discharged from the vacuum pump, and its flow rate was measured by a digital soap-film flow meter. The gas composition was determined by a gas chromatograph (Yanaco, GC-2800, column Porapak Q for CO2 and (N2+O2) analysis, and molecular sieve 5A for N2 and O2 analysis). The performance of the module was calculated by the same procedure reported in our previous paper [1]. 

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. 

Membranes can also be used as a reactor where catalysts are used frequently. The membrane may physically segregate the catalyst in the reactor, or have the catalyst immobilized in the porous/microporous structure or on the membrane surface. The membrane having the catalyst immobilized in/on it acts almost in the same way as a catalyst particle in a reactor does, except that separation of the product(s) takes place, in addition, through the membrane to the permeate side. All such configurations involve the bulk flow of the reaction mixture along the reactor length while diffusion of the reactants/products takes place generally in a perpendicular direction to/from the porous/microporous catalyst. 

Figure 63. Six flow models for multilayer diffusion and capillaiy condensation (1) multilayer difliision, (2) capillary condensation at feed side. (3) entire pore filled with condensate, (4) bulk condensate at feed side, (5) bulk condensate at feed side and capillary condensate at permeate side and (6) total bulk condensate data taken from Lee and Hwang (1986).

Figure 4.1 also shows the formation of concentration polarization gradients on both sides of the membrane. However, in most membrane processes there is a bulk flow of liquid or gas through the membrane, and the permeate-side composition... 

Figure 4.1 shows the concentration gradients that form on either side of a dialysis membrane. However, dialysis differs from most membrane processes in that the volume flow across the membrane is usually small. In processes such as reverse osmosis, ultrafiltration, and gas separation, the volume flow through the membrane from the feed to the permeate side is significant. As a result the permeate concentration is typically determined by the ratio of the fluxes of the components that permeate the membrane. In these processes concentration polarization gradients form only on the feed side of the membrane, as shown in Figure 4.3. This simplifies the description of the phenomenon. The few membrane processes in which a fluid is used to sweep the permeate side of the membrane,...

In the discussion of concentration polarization to this point, the assumption is made that the volume flux through the membrane is large, so the concentration on the permeate side of the membrane is determined by the ratio of the component fluxes. This assumption is almost always true for liquid separation processes, such as ultrafiltration or reverse osmosis, but must be modified in a few gas separation and pervaporation processes. In these processes, a lateral flow of gas is sometimes used to change the composition of the gas on the permeate side of the membrane. Figure 4.14 illustrates a laboratory gas permeation experiment using this effect. As the pressurized feed gas mixture is passed over the membrane surface, certain components permeate the membrane. On the permeate side of the membrane, a lateral flow of helium or other inert gas sweeps the permeate from the membrane surface. In the absence of the sweep gas, the composition of the gas mixture on the permeate side of the membrane is determined by the flow of components from the feed. If a large flow of sweep gas is used, the partial... 

Figure 4.14 (a) Flow schematic of permeation using a permeate-side sweep gas sometimes used in laboratory gas separation and pervaporation experiments, (b) The concentration gradients that form on the permeate side of the membrane depend on the volume of sweep gas used. In laboratory experiments a large sweep-gas-to-permeate-gas flow ratio is used, so the concentration of permeate at the membrane surface is very low... 

Figure 4.16 An illustration of a counter-flow module for the separation of nitrogen from air. Directing the permeate to flow counter to the feed sweeps the permeate side of the membrane with a flow of oxygen-depleted gas. This increases the oxygen flux and decreases the nitrogen flux through the membrane...

The importance of the pressure ratio in separating gas mixtures can be illustrated by considering the separation of a gas mixture with component concentrations (mol%) nio and njo at a feed pressure of p0. A flow of component across the membrane can only occur if the partial pressure of component i on the feed side of the membrane, is greater than the partial pressure of component i on the permeate side of the membrane, nkpi. That is... 

In the case of the counter-flow/sweep membrane module illustrated in Figure 4.18(c) a portion of the dried residue gas stream is expanded across a valve and used as the permeate-side sweep gas. The separation obtained depends on how much gas is used as a sweep. In the calculation illustrated, 5 % of the residue gas is used as a sweep even so the result is dramatic. The concentration of water vapor in the permeate gas is 13 000 ppm, almost the same as the perfect counter-flow module shown in Figure 4.18(b), but the membrane area required to perform the separation is one-third of the counter-flow case. Mixing separated residue gas with the permeate gas improves the separation The cause of this paradoxical result is illustrated in Figure 4.19 and discussed in a number of papers by Cussler et al. [16]. 

Figure 4.19(a) shows the concentration of water vapor on the feed and permeate sides of the membrane module in the case of a simple counter-flow module. On the high-pressure side of the module, the water vapor concentration in the feed gas drops from 1000 ppm to about 310 ppm halfway through the module and to 100 ppm at the residue end. The graph directly below the module drawing shows the theoretical maximum concentration of water vapor on the permeate side of the membrane. This maximum is determined by the feed-to-permeate pressure ratio of 20 as described in the footnote to page 186. The actual calculated permeate-side concentration is also shown. The difference between these two lines is a measure of the driving force for water vapor transport across the membrane. At the feed end of the module, this difference is about 1000 ppm, but at the permeate end the difference is only about 100 ppm. 

Figure 4.19 The effect of a small permeate-side, counter-flow sweep on the water vapor concentration on the permeate side of a membrane. In this example calculation, the sweep flow reduces the membrane area by two-thirds...

A third possibility, illustrated in Figure 9.7(c), is to sweep the permeate side of the membrane with a counter-current flow of carrier gas. In the example shown, the carrier gas is cooled to condense and recover the permeate vapor, and the gas is recirculated. This mode of operation has little to offer compared to temperature-gradient-driven pervaporation, because both require cooling water for the condenser. However, if the permeate has no value and can be discarded without condensation (for example, in the pervaporative dehydration of an organic solvent with an extremely water-selective membrane), this is the preferred mode of operation. In this case, the permeate would contain only water plus a trace of organic solvent and could be discharged or incinerated at low cost. No permeate refrigeration is required [36],... 

Methane and air were fed to the reactor with an O/C ratio of 1.0 and additional steam at different S/C ratios. The reaction was performed under atmospheric pressure and a nitrogen flow was maintained on the permeate side. The results obtained with the membrane were compared with others generated with the same reactor type without a catalytic membrane in the temperature range 400-575 °C, which is low for methane reforming [51]. 

Since selectivity is a function of the local permeate flux, and thus the local transmembrane pressure, selectivity can be further improved by maintaining a nearly uniform and low transmembrane pressure throughout the ultrafiltration module. Following the early work from Sandblom, HPTFF technology uses, when required, a cocurrent permeate flow that is accomplished by the addition of a coflow loop and pump on the permeate side. 

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