Membrane transmembrane pressure drop

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. 

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. 

Neglecting convection effects, the solution-diffusion model gives the following expressions for water (1) and salt (2) molar fluxes through a membrane with a selective layer thickness of L and a transmembrane pressure drop Ap (Merten, 1966) ... 

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... 

Dynamic filtration modules present a relative movement between the membrane and the module, or between the membrane and a rotor. Thus, it is possible to adjust the shear stress independently of the feed flow rate and of the transmembrane pressure drop. 

It should be obvious from the above discussion that though the ability of the high temperature membrane reactor to increase the reactor yield is important equally important is the membrane s ability to efficiently separate hydrogen at high temperatures and to sustain a transmembrane pressure drop. 

Hydrophilic MF membranes can be made by the dry-wet phase inversion technique. The latter technique is also applicable in making PVDF membranes. On the other hand, other hydrophobic MF membranes are made by the TIPS technique. In particular, semicrystalline PE, PP, and PTFE are stretched parallel to the direction of film extrusion so that the crystalline regions are aligned to the direction of stretch, while the noncrystalline region is ruptured, forming long and narrow pores. Hydrophobic membranes do not allow penetration of water into the pore until the transmembrane pressure drop reaches a threshold pressure called liquid entry pressure of water. These membranes can therefore be used for membrane distillation. Tracketching method is applied to make MF membranes from PC. 

The DMF module (PaU Corp., New York) which consists of several disks mounted on the same shaft [2, 135]. The reported studies of the apphcation of the rotating disk dynamic membrane indicate that high shear-induced filtration is much less sensitive to the solids concentration. Advantage of the rotational system is that it permits operation at both very low transmembrane pressure-drop, and low upstream mass velocity, without loss in depolarization efficiency. It is thus possible with this system to achieve cleaner separation of solute components, than is achievable with conventional systems. Equipment for this process is, unfortunately, significandy more costly, and maintenance costs much higher also, than those for conventional membrane systems. 

If the gel layer is not the limiting resistance to flow, the membrane must be, and the flux should be proportional to the transmembrane pressure drop. Experimental data deny this-showing threshold pressure (above which flux is independent of pressure) (see Figure 3.48). 

Fortunately, hollow fibers may be cleaned by back-washing which tends to compensate for their propensity to foul. Manufacturers of tubes, plate and frame units, and spiral wound modules do not recommend back-washing due to problems with membrane delamination and glue line seal rupture. Because hollow fibers are self-supporting and hold up well under the compression force of a reverse transmembrane pressure drop, they can easily withstand back-wash pressures of 15 to 20 psi. However, the back-wash fluid should be filtered to remove any particles which would tend to lodge in the porous wall of the fiber. 

Porous membranes used for reactant feed are typically mesoporous or macro-porous. Two problems arise control of the rate of addition and distribution of reactant B, and back-diffusion of reactant A. To control the uniformity of the distribution of B, we want the membrane to present enough resistance to equalize pressure on the reactant B side, giving a constant transmembrane pressure drop along the tube. This may mean modification of thin y-alumina mesoporous membranes or use of Vycor glass tubes.A higher pressure drop also helps reduce the driving force for back-diffusion, which was exploited in the pneumatic... 

In this reaction scheme, P is the desired product and S is the undesired byproduct. In the case the reaction rates are proportional to the partial pressure of reactant B (ri = kiPB and t 2 = p y respectively) the kinetics are favourable if nlslow reaction 2 more than reaction 1, inducing an increased selectivity for the desired product P. For this purpose, mainly porous membranes are used. To control the uniformity of the distribution of B, the membrane should have sufficient resistance to equalize the pressure on the reactant side, i.e. a constant transmembrane pressure drop along the tube [15, 16]. Another problem to tackle in this type of systems is back diffusion of reactant A and product(s) P and S. Here also, an increased pressure drop across the membrane will be advantageous, although it also decreases the permeation rate of B, which potentially leads to problems balancing the feed rate to the reaction rate 1. 

While aU the above mentioned membrane separation processes utiHze the transmembrane pressure drop as the driving force, there are other membrane separation processes based on different driving forces. 

UF and MF have been successfully employed in analytical chemistry. The principal factors that should be considered for analytical and technological use of membranes with aqueous media are the pore size and pore size distribution, solution flow, and degree of hydrophilicity. The solution flow (flux) through an MF or UF membrane is given by the equation J = P/ R, where / is the solution flux, R is a phenomenological resistance coefficient, and P is the transmembrane pressure drop. A pump or a gas (e.g., nitrogen) bottle can be used as a pressure source (50-500 kPa). The other main features of the filtration system are a membrane filtration unit and reservoirs. 

Figure 4.10.75 Dehydrogenation of ethane in a membrane reactor for a transmembrane pressure drop of about 0.7 bar and a tube side pressure of 1.7 bar (detaiis in Champagnie, Tsotsis, and Minet, 1990).

Where (P//) is permeance, Q. is permeation rate of gas at standard temperature and pressure (STP), Ap is transmembrane pressure drop, A is area of membrane and is ideal separation factor or selectivity. 

Van Reis et al. (1999) found that high separation factors for solute species with size differences less than an order of magnitude were obtainable using high-performance tangential-flow filtration. Their results indicate that maintaining a constant transmembrane pressure drop across the entire length of the membrane leads to a much finer fractionation of proteins. 

Ultrafiltration (UF) and diafiltration (DF) are very popular methods for conditioning the clarified culture broth for chromatography. As general rule, dilute animal cell culture supernatants are concentrated up to a factor of 20 and even more prior to fractionation. Ultrafiltration is a process in which a solution containing macromolecules is passed across a functional membrane, while macromolecules are retained by the membrane. The process is driven by the pressure drop across the membrane also called transmembrane pressure (PT) expressed as... 

If a transmembrane pressure difference is imposed at a given constant temperature, the reaction zone will be shifted toward the lower pressure side. The mole fraction of the reactant entering the lower pressure side of the membrane surface drops to a level lower than that in the absence of a pressure difference. It has been shown [Sloot et al., 1990] that the molar fluxes of, say, hydrogen sulfide increases as the pressure on its side increases, thus potentially reducing the membrane area required. A serious drawback with this mode of operation, however, is the amount of inert gas introduced. 

The top product recycle mode in Figure 11.12 brings part of the permeate stream at a lower pressure to join the feed suream at a higher pressure. Thus, additional energy external to the membrane reactor will be required to recompress the recycled permeate. On the contrary, in the bottom product recycle, also shown in Figure 11.12, only the transmembrane pressure difference and the longitudinal pressure drop need to be overcome between the recycled portion of the bottom product (or retentate) and the feed. Therefore, the required pressure recompression is expected to be small compared to the top product recycle mode. 

On examining the effect of membrane compaction on the membrane permeability, Lawson et al. [100] concluded that the transmembrane flux in MD could be increased significantly up to 11 % with relatively small pressure drops <70 kPa. 

However, although the permeate flux can be increased by the increase in tangential velocity, the pressure drop in the supply channel can become very high and result in large reduction in transmembrane pressure, causing the process failure. It is necessary to optimize the speed and configuration of the membrane to obtain higher filtration efficiency [1]. 

FIGURE 16.39 Effect of the content of oil drops on transmembrane flux at two different transmembrane pressures (single pass through the membrane). (From Vladisavljevic, G.T., Shimizu, M., and Nakashima, T., J. Membr. Set, 244 (1-2), 97-106, 2004. With permission.)... 

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