Membrane separation pressure drop

Spiral-wound modules are much more commonly used in low pressure or vacuum gas separation appHcations, such as the production of oxygen-enriched air, or the separation of organic vapors from air. In these appHcations, the feed gas is at close to ambient pressure, and a vacuum is drawn on the permeate side of the membrane. Parasitic pressure drops on the permeate side of the membrane and the difficulty in making high performance hollow-fine fiber membranes from the mbbery polymers used to make these membranes both work against hollow-fine fiber modules for this appHcation. 

Pervaporation operates under constraints similar to low pressure gas-separation. Pressure drops on the permeate side of the membrane must be small, and many prevaporation membrane materials are mbbery. For this reason, spiral-wound modules and plate-and-frame systems ate both in use. 

The Pe number is an important parameter which has an influence on the performance of the membrane process. Permeation and surface area cire coupled via the Pe number. In the equation Qh2 is the permeation of the fastest permeating component (usually H2 in this study). In membrane gas separation processes Pe is usually between 0.1 and 1.0. For new applications Pe = 0.5 can be taken as a first guess. The actual performance of the systems depends on many more parameters than the Pe number only, i.e. membrane selectivity, pressure drop, sweep gas flow to feed gas flow ratio, composition of the feed. 

Pressure drop due to capillary phenomena Pressure drop in the outlets of the separator Pressure drop in the straight channel of separator Pressure drop across the membrane... 

The factors to consider in the selection of crossflow filtration include the flow configuration, tangential linear velocity, transmembrane pressure drop (driving force), separation characteristics of the membrane (permeability and pore size), size of particulates relative to the membrane pore dimensions, low protein-binding ability, and hydrodynamic conditions within the flow module. Again, since particle-particle and particle-membrane interactions are key, broth conditioning (ionic strength, pH, etc.) may be necessary to optimize performance. 

In ultrafiltration, the effluent is passed across a semiper-meable membrane (see Chapter 10). Water passes through the membrane, while submicron particles and large molecules are rejected from the membrane and concentrated. The membrane is supported on a porous medium for strength, as discussed in Chapter 10. Ultrafiltration is used to separate very fine particles (typically in the range 0.001 to 0.02 xm), microorganisms and organic components with molar mass down to 1000 kg kmol. Pressure drops are usually in the range 1.5 to 10 bar. 

The objective of the present study is to develop a cross-flow filtration module operated under low transmembrane pressure drop that can result in high permeate flux, and also to demonstrate the efficient use of such a module to continuously separate wax from ultrafine iron catalyst particles from simulated FTS catalyst/ wax slurry products from an SBCR pilot plant unit. An important goal of this research was to monitor and record cross-flow flux measurements over a longterm time-on-stream (TOS) period (500+ h). Two types (active and passive) of permeate flux maintenance procedures were developed and tested during this study. Depending on the efficiency of different flux maintenance or filter media cleaning procedures employed over the long-term test to stabilize the flux over time, the most efficient procedure can be selected for further development and cost optimization. The effect of mono-olefins and aliphatic alcohols on permeate flux and on the efficiency of the filter membrane for catalyst/wax separation was also studied. 

Novel Processing Schemes Various separators have been proposed to separate the hydrogen-rich fuel in the reformate for cell use or to remove harmful species. At present, the separators are expensive, brittle, require large pressure differential, and are attacked by some hydrocarbons. There is a need to develop thinner, lower pressure drop, low cost membranes that can withstand separation from their support structure under changing thermal loads. Plasma reactors offer independence of reaction chemistry and optimum operating conditions that can be maintained over a wide range of feed rates and H2 composition. These processors have no catalyst and are compact. However, they are preliminary and have only been tested at a laboratory scale. 

Porous ceramic membrane layers are formed on top of macroporous supports, for enhanced mechanical resistance. The flow through the support may consist of contributions due to both Knudsen-diffusion and convective nonseparative flow. Supports with large pores are preferred due to their low resistance to the flow. Supports with high resistance to the flow decrease the effective pressure drop over the membrane separation layer, thus diminishing the separation efficiency of the membrane (van Vuren et al. 1987). For this reason in a membrane reactor it is more effective to place the reaction (catalytic) zone at the top layer side of the membrane while purging at the support side of the membrane. 

Parallel-plate hemodialyzers using flat membranes, with several compartments in parallel, separated by plastic plates, are now only available from Hospal Co (Crystal and Hemospal models). Blood circulates between two membranes and the dialysate between the other side of membrane and the plastic plate. These parallel-plate dialyzers have a smaller blood-pressure drop than hollow-fiber ones and require less anticoagulants as flat channels are less exposed to thrombus formation than fibers, but they are heavier and bulkier and thus less popular. A recent survey of the state-of-the-art in hemodialyzers is given in [13]. 

Microdialysis was achieved in a fused silica chip with in situ photopattemed porous membrane, as shown in Figure 5.13. Phase-separation polymerization of the membrane (7-50 pm thick) was formed between posts. The posts maximize the mechanical strength of the membrane so that it can withstand a pressure drop of 1 bar. Low MW cutoff (MWCO) membrane, which can be formed by using less organic solvent, 2-methoxyethanol, appears to be more transparent (see Figure 5.13). This low MWCO membrane can be used to dialyze away low MW molecules, such as rhodamine 560, but not fluorescently labeled proteins (insulin, BSA, anti-biotin, and lactalbumin). Fligh MWCO membrane, which was formed by more organic solvent, allows diffusion of lactalbumin [347]. 

Dialysis continues to meet certain specialized applications, particularly those in biotechnology and the life sciences. Delicate substances can be separated without damage because dialysis is typically performed under mild conditions ambient temperature, no appreciable transmembrane pressure drop, and low-shear flow. While slow compared with pressure-driven processes, dialysis discriminates small molecules from large ones reliably because the absence of a pressure gradient across the membrane prevents convective flow through defects in the membrane. This advantage is significant for two... 

Microporous membranes with nominal pore sizes in the 0.1 — 10 pm range are usually employed in animal cell separation. Such membranes are able to retain the cells, but unable to retain solutes of low or high molar mass (Figure 11.12). Microfiltration uses a pressure drop, usually in the 0.5-2 bar range, as the driving force to promote separation (Nobrega et al., 2005). 

Early in this chapter we defined viscosity and discussed its role in opposing flow. By resisting flow, viscosity often limits the flowrate to less than optimal values. Higher pressure drops can be used to offset viscous drag but the pressure drop necessary to force fluid at the desired rate through narrow pores is often unattainable. Thus the rate of flow is often viscosity limited. For example, the rate of fluid transfer through both membranes and chromatographic columns is often limited by viscous resistance this in turn limits the speed of separation. 

In gas permeation, a gas species is separated based mainly on its permeability in hollow fiber and spiral wound membranes. The hollow fiber systems can have an inside diameter up to 200 xm and hence very large surface-to-volume ratios, but high pressure drops inside the tubes. The basic flow equation for a species / is... 

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