Crossflow module

In the crossflow module illustrated in Figure 8.5(a), the pooled permeate stream has a water concentration of 1.88%. The counterflow module illustrated in Figure 8.5(b) performs substantially better, providing a pooled permeate stream with a concentration of 3.49%. Not only does the counterflow module perform the separation twice as well, it also requires only about half the membrane area. This improvement is achieved because the gas permeating the membrane at the residue end of the module contains much less water than the gas permeating the membrane at the feed end of the module. Permeate counterflow dilutes the permeate gas at the feed end of the module with low-concentration permeate gas from the residue end of the module. This increases the water concentration driving force across the membrane and so increases the water flux. 

Counterflow modules are always more efficient than crossflow modules, but the advantage is most noticeable when the membrane selectivity is much higher than the pressure ratio across the membrane and a significant fraction of the most permeable component is being removed from the feed gas. This is the case for air-dehydration membrane modules, so counterflow capillary modules are almost always used. With most other gas-separation applications, the advantage offered by counterflow designs does not offset the extra cost of making the counterflow type of module, so they are not widely used. 

Norton Co., 1984, Asymmetric ceramic microfiltcrs - Testing tubular crossflow modules. Product Brochure. 

Ceramic membrane systems are achieving widespread appheation in the place of centrifuges. These systems do not require filter aid media or a separate follow-on clarification step. HoUow-fiber crossflow modules are preferred for the separation of mammalian cells, which require particularly gentle handling. They can also be used as disposable filters for perfusion and... 

The present contribution describes a novel low pressure, high flux system which utilizes an "in situ" dynamically formed silica membrane particularly suited for the ultrafiltration of emulsions. The support for this selective layer of silica was a pleated, thin channel crossflow module il (tradename "Acro-flux", Gelman Sciences, Inc.) containing 0.1 m of 0.2 urn pore size acrylonitrile copolymer membrane. 

Membrane Formation. In earlier work. 2.) it was found that fumed silica particles could be dispersed in aqueous suspension with the aid of ultrasonic sound. Observations under the electron microscope showed that the dispersion contained disc-like particles, approximately 150-200 1 in diameter and 70-80 1 in height. Filtration experiments carried out in the "dead-end" mode (i.e., zero crossflow velocity) on 0.2 urn membrane support showed typical Class II cake formation kinetics, i.e., the permeation rate decreased according to equation (12). However, as may be seen from Figure 7, the decrease in the permeation rate observed during formation in the crossflow module is only t 1, considerably slower than the t 5 dependence predicted and observed earlier. This difference may be expected due to the presence of lift forces created by turbulence in the crossflow device, and models for the hydrodynamics in such cases have been proposed. 

Figure 10. Dependence of the permeation rate on the crossflow pumping rate in the UF of a 3% oil emulsion by a dynamically formed 5iO membrane in a pleated crossflow module 3, Trial 2 X, Trial 3 A, Trial 4 , Trial 6 O, Trial 7 A, Trials.

Figure 12. Long-term UF performance of a dynamically formed SiOt membrane in a pleated crossflow module P = 1.5 atm 1.67-gpm crossflow initial feed concentration = 3.6% T = 32°C.

Stirred cell systems were selected for the experimental work for a number of reasons (1) volumes are small which is required for the use of IHSS reference material, (ii) membrane samples are small which allows the use of a new membrane for each experiment, (iii) the solution chemistry can be precisely controlled, (iv) experiments are relatively short and thus the investigation of a great number of parameters is possible, and (v) the concentration in the cell represents the concentration in a crossflow module (recovery about 70%). A comparison of mass transfer values was demonstrated in the case of NF in Chapter 7. Drawings of the filtration equipment are shown in Appendix 2, A hydrodynamic analysis is also shown in Appendix 2. 

A PIV application was used to observe particle movements/distributions in a crossflow module during filtration of yeast cells (without spacer) [85]. A particle velocity map was also defined to describe flow distribution along the rectangular crossflow module (without membrane) with spacers [86]. In another experiment, the flow velocity distribution within the fiber bundle (nine fibers in a 3 x 3 array) was observed to identify the presence of dead zones (indicated by particle movements) during bubble injections [84]. It was found that if small bubbles were introduced from the center of the bundle, a dead zone was formed and caused fiber blockage. 

Equations (20-66) and (20-67) present single-pass formulas relating retentate solute concentration, retentate crossflow, permeate flow, and membrane area. For relevant low-feed-concentration applications, polarization is minimal and the flux is mainly a function of pressure. Spiral or hollow fiber modules with low feed channel and permeate pressure drops are preferred. 

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. 

By resorting to the so-called membrane recycle bioreactors (MBR) (Bubbico et al., 1997 Enzminger and Asenjo, 1986), continuous recycling of the culture broth through crossflow MF modules allows removal of the inhibiting metabolites, this helping to maximize cell density in the bioreactor, as well as bioproduct formation rate. Further ED treatment of MF permeates gives rise to two streams, a diluted one to be recycled back into the bioreactor, and a concentrated one to be supplementary refined. 

Figure 8.5 Comparison of (a) crossflow, (b) counterflow and (c) counterflow sweep module performance for the separation of water vapor from air. Membrane water/air selectivity = 100, water permeance = 1000 gpu.

Figure 11.3 Partial fluxes of isoamyl alcohol, ethyl acetate, isoamyl acetate and ethyl hexanoate as a function of their feed crossflow velocity (bottom axis) and Reynolds number (top axis) in a singlechannel module, using a POMS-PEI composite membrane. Notice that external mass-transfer limitations are not fully overcome when soluteswith a high affinity towardsthe membrane are processed (Adapted from Ref. 32.)...

A comprehensive presentation of all membrane types, modules and geometries is beyond the scope of this chapter, reference available membrane books for details [12,17, 55, 60, 71, 77,90]. The examples in Figure 16.2 are an illustration of a typical membrane module and installation. The most widespread FS membrane system is mounted as a spiral-wound (SW) unit. In the SW example the actual membrane module is shown together with how they are mounted inside a pressure vessel. A typical installation is shown where several pressure vessels are subsequently mounted in a stack. Pressurized HF units are typically operated as a crossflow system. In the example shown the HF modules are mounted vertically and arranged in a skid. Several variations of the theme can be found depending on the type of module and the manufacturer, where Figure 16.2 is not specific to a particular item. 

On the contrary, no general expression is available for calculating the mass-transfer coefficient at the shell side. In the literature, in fact, different equations are proposed, depending on the type of module and on the type of flow (parallel or crossflow). Probably, this is due to the fact that the fluidodynamics of the stream sent outside the fibers is strongly affected by the phenomena of channeling or bypassing and it is not well defined as for the stream, which is sent into the fibers. Hereinafter some of the different expressions proposed are reported. 

The membrane emulsification can be considered as a case of microdevice emulsification process [17, 18] in which the porous membrane is used as microdevices. Membrane emulsification carried out in quiescent conditions is also referred to as static membrane emulsification, while membrane emulsification carried our in moving conditions (either the membrane, i.e., rotating module, or the phase, i.e., crossflow) is also referred to as dynamic membrane emulsification (Figure 21.2(b)). 

Figure 21.12 Marketed equipments for membrane emulsification, (a) Plant for crossflow membrane emulsification produced by SPG Technologies Co. Ltd (http //www.spg-techno. co.jp/) (b) spiral-wound metallic membrane module produced by Micropore Technologies (http //www.micropore.co.uk/).

Contactors with flat-sheet and cylindrical walls are used but only hollow fiber (HF) contactors in cylindrical modules in several sizes are available commercially [31]. Flat-sheet contactors are widely used in analytical chemistry [32-34]. There are two main types of H F contactors, those with parallel flow of phases in fiber lumen and in shell or crossflow of phases. A FIF contactor with crossflow of phases is shown in Figure 23.2. More details on their construction and sizes available are presented in the producer s web site [31]. 

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