Side-stream membrane modules

Phattaranawik J, Fane AG, Pasquier ACS. Studies of air slug distributions and preliminary membrane fouling by optical monitoring in a side-stream membrane module. Sep. Sci. Technol. 2009 44 3793-3813. 

Cabral and coworkers [253] have investigated the batch mode synthesis of a dipeptide acetyl phenylalanine leucinamide (AcPhe-Leu-NH2) catalyzed by a-chymotrypsin in a ceramic ultrafiltration membrane reactor using a TTAB/oc-tanol/heptane reverse micellar system. Separation of the dipeptide was achieved by selective precipitation. Later on the same group successfully synthesized the same dipeptide in the same reactor system in a continuous mode [254] with high yields (70-80%) and recovery (75-90%). The volumetric production was as high as 4.3 mmol peptide/l/day with a purity of 92%. The reactor was operated for seven days continuously without any loss of enzyme activity. Hakoda et al. [255] proposed an electro-ultrafiltration bioreactor for separation of RMs containing enzyme from the product stream. A ceramic membrane module was used to separate AOT-RMs containing lipase from isooctane. Application of an electric field enhanced the ultrafiltration efficiency (flux) and it further improved when the anode and cathode were placed in the permeate and the reten-tate side respectively. 

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]. 

In the case of the counterflow/sweep membrane module illustrated in Figure 8.5 (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... 

Reaction, Reaction side Membrane module stream on the reaction volume referred to... 

Sweep Membrane module inlet stream on permeate side referred to... 

Complete membrane systems can be operated in a variety of modes with e.g. CO- or counter flow of feed (high pressure side) and permeate (low pressure side) streams and with membrane modules coupled in different ways. Permeation and separation in these complex engineering systems will not be treated in this chapter. Heat and mass transfer limitations on the gas-membrane surfaces or interfaces can be important with high fluxes and/or strongly adsorbing gases as well as in membrane reactors. These effects will not be treated explicitly but are introduced in experimental results, e.g., by variation of sweep rates of permeated gases. 

Mass transfer in the feed and strip solutions is limited by the extent of concentration polarization. On the feed side of the membrane, concentration polarization refers to an increase in the concentration of solutes at and near the feed-membrane interface because of evaporation of water into the membrane pores (Fig. 1). The resulting solute concentration gradient between the membrane-feed interface, where the concentration is greatest, and the bulk solution induces diffusive transport of rejected solutes back through the concentration polarization boundary layer into the bulk stream. Bulk solution is simultaneously transported to the membrane wall by convection. When equilibrium has been established under a given set of operating conditions (stream flow rate, temperature, fluid dynamics imposed by membrane module design), the rate of back diffusion is equal to the rate at which the solutes are carried to the membrane surface by convective flow. ... 

Regardless of the membrane module design, effective performance is dependent on rapid mass transfer at the feed side of the membrane. There are several comprehensive references on mass transfer [10], so it will not be addressed in detail here. However, one should consider mass transfer because it affects two design issues the manifolding of the feed stream to each membrane, and the feed channel design and dimensions. The detailed analysis of these issues differs somewhat when one considers stacked planar modules and tubular modules, but the fundamental objectives are quite similar. 

The goal of the model for membrane unit for gas separation is to predict the flow rate and composition of retentate and permeate streams, for a given feed stream containing n components, membrane type and area, and permeate pressure. Here, the process boundary and variables are limited to one of the membrane modules shown in Figure 4.5. In this section, the solution-diffusion mechanism is used to predict the separation behavior of the membrane. In the development of a membrane model, it is assumed that the process is at steady state, pressure is constant on feed side, and permeability of a component through the... 

For gas separation applications, the feed stream is usually fed into the shell side of the module. This implies that the dense selective layer should be located on the outside of the polyaniline hollow fiber membrane. To achieve this desired morphology, the polyaniline hollow fiber was spun using an air gap between the spinneret and the coagulation bath. The residence time in the air gap influences the amount of solvent evaporation, which in turn governs the thickness of the dense separating layer on the outer surface of the hollow fiber. By adjusting the residence time from a few seconds to 30 s, the thickness of the dense separating layer on the outer surface of the hollow was successfully varied between 0.5 and 5 xm. 

Typical layout of a PRO-based power plant is represented in Figure 9.5. Seawater from the offshore intake is first moved to a sedimentation basin. Then, an electric pump supplies a proper head to overcome the pressure drop of the filtration system and avoid cavitation in the main pump. A complex filtration system is required both on seawater and freshwater to prevent membrane fouling that negatively affects its permeation. After filtration, pressure of the seawater stream is increased to some bar (stream S2) before it is addressed to the permeate side of the osmotic membrane module. 

Provided that the pressure difference between the two sides of the membrane is set to a value lower than the difference in their osmotic pressure, a water flux establishes across the membrane from the low- to the high-pressure side (i.e. from the low to the high salt concentration side). Therefore, the water flow rate on the high-pressure side of the membrane increases. The seawater stream diluted by pure water in the membrane module (brackish stream B1) is then sent to a hydrauhc turbine connected to the same shaft of the main pump. Since the turbine head equals the pump head (apart from the pressure losses along the circuit and the small head provided by the EPl pump) and the turbine flow rate is higher than the pump flow rate because of the water permeated across the membrane, a net mechanical power output is available at the shaft. A generator connected to the shaft finally converts mechanical to electric power. 

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