Transmembrane pressure difference

Feed High pressure feed side AP = Transmembrane pressure difference An = Osmotic pressure difference Membrane Concentrate... 

Solution—Diffusion Model. In the solution—diffusion model, it is assumed that (/) the RO membrane has a homogeneous, nonporous surface layer (2) both the solute and solvent dissolve in this layer and then each diffuses across it (J) solute and solvent diffusion is uncoupled and each is the result of the particular material s chemical potential gradient across the membrane and (4) the gradients are the result of concentration and pressure differences across the membrane (26,30). The driving force for water transport is primarily a result of the net transmembrane pressure difference and can be represented by equation 5 ... 

Microfiltration (MF) and ultrafiltration (UF) involve contacting the upstream face ofa porous membrane with a feed stream containing particles or macromolecules (B) suspended in a low molecular weight fluid (A). The pores are simply larger in MF membranes than for UF membranes. In either case, a transmembrane pressure difference motivates the suspending fluid (usually water) to pass through physically observable permanent pores in the membrane. The fluid flow drags suspended particles and macrosolutes to the surface of the membrane where they are rejected due to their excessive size relative to the membrane pores. This simple process... 

Successful performance of inorganic membranes depend on three types of variables and their interactions. The first type is related to the characteristics of the feed stream such as the molecular or particulate size and/or chemical nature of the species to be separated and concentration of the feed to be processed, etc. The second type is membrane dependent Those factors are the chemical nature and pore size of the membrane material and how the membrane and its accessory processing components are constructed and assembled. The third type is processing conditions such as pressure, transmembrane pressure difference, temperature, crossflow velocity and the way in which the membrane flux is maintained or restored as discussed earlier in this chapter. 

Transmembrane pressure. The driving force for membrane permeation is the transmembrane pressure or transmembrane pressure difference... 

As in many other membrane separation applications, the clean water flux is essentially proportional to the transmembrane pressure difference (TMP). When solutes, macromolecules or particulates are to be separated from the solvent (e.g., water), the permeate flux is first a linear function of the TMP and is in the pressure controlled regime. Although similar to the behavior of water flux, the permeate flux is nevertheless lower. Beyond a "threshold pressure," the permeate flux is insensitive to TMP due to concentration and gel polarization near the membrane surface. This behavior is so-called mass transfer controlled. It appears that the larger pore membrane, 50 nm in pore diameter, reaches the threshold pressure sooner than the finer pore membrane, 4 nm in pore diameter. There is a significant advantage of operating the membranes at a higher... 

Besides some measures of separation efficiency such as the separation factor and extent of separation defined above, some quantity indicative of the throughput rate of a membrane system is needed to compliment the permselectivity of the membrane. It is quite common and practical in the membrane technology to use a phenomenological expression to relate the permeate flux (Ja in the unit of cm (STP)/s-cm7) of a given gas (A) through the membrane to the driving force, the transmembrane pressure difference (Ap) as follows ... 

Figure 10.16b Mole fraction profile of a Claus reaction in a catalytic nonpermselective membrane at 300X in the absence of a transmembrane pressure difference [Sloot et al., 19901...

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. 

Recycling some portion of the permeate or retentate stream and introducing feed at intermediate locations are effective methods for improving the reaction conversion. The relative permeabilities of the reactant(s) and product(s) and whether the permeate or retentate is recycled all affect the effectiveness of these measures for conversion enhancement. To compensate for the variations in the transmembrane pressure difference and consequently in the permeation rate, the concept of a location-dependent membrane ]x rmeability has been proposed. The effects of this approach and the average permeation rate arc discussed. 

Dynamic viscosity of the permeate (Pa s) Ion conductivity (mS cm ) Transmembrane pressure difference (kPa) Effective capillary length (m)... 

Efforts to stabilize BLMs by the use of polymerizable lipids have been successful, but the electrochemical properties of these membranes were greatly compromised and ion channel phenomena could not be observed [21]. Microfiltration and polycarbonate filters, polyimide mesh, and hydrated gels have been used successfully as stabilizing supports for the formation of black lipid films [22-25] and these systems were observed to retain their electrical and permeability characteristics [24]. Poly(octadec-l-ene-maleic anhydride) (PA-18) was found to be an excellent intermediate layer for interfacing phospholipids onto solid substrates, and is sufficiently hydrophilic to retain water for unimpeded ion transfer at the electrode-PA-18 interface [26]. Hydrostatic stabilization of solventless BLMs has been achieved by the transfer of two lipid monolayers onto the aperture of a closed cell compartment however, the use of a system for automatic digital control of the transmembrane pressure difference was necessary [27]. 

Decrease of the transmembrane pressure-difference as a consequence of the drop in pressure due to friction (- decrease of permeate flux). 

Since flux and selectivity increase with increasing transmembrane pressure difference and the modules can tolerate pressure differences of about 100 bars, the reader might question why the first unit is operated with a transmembrane pressure difference of only 60 bars. The reason is that the H2 recovery system has been added to an existing plant and that the first stage of the synthesis compressor would not accept the permeate flux of both units. 

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