Microfiltration process limitations

Many models have been published to calculate the microfiltration process. One important factor is the concentration polarization, which represents the most important limiting physical obstacle. At high particle concentration and with time, a layer is formed on the membrane. This layer is responsible for the flux reduction. A comprehensive overview on this technique is given by Ripperger52 and Staude.53 Often similar or identical module types are used in microfiltration and ultrafiltration. 

A range of membrane processes are used to separate fine particles and colloids, macromolecules such as proteins, low-molecular-weight organics, and dissolved salts. These processes include the pressure-driven liquid-phase processes, microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), and the thermal processes, pervaporation (PV) and membrane distillation (MD), all of which operate with solvent (usually water) transmission. Processes that are solute transport are electrodialysis (ED) and dialysis (D), as well as applications of PV where the trace species is transmitted. In all of these applications, the conditions in the liquid boundary layer have a strong influence on membrane performance. For example, for the pressure-driven processes, the separation of solutes takes place at the membrane surface where the solvent passes through the membrane and the retained solutes cause the local concentration to increase. Membrane performance is usually compromised by concentration polarization and fouling. This section discusses the process limitations caused by the concentration polarization and the strategies available to limit their impact. 

The solid-liquid separation of shinies containing particles below 10 pm is difficult by conventional filtration techniques. A conventional approach would be to use a slurry thickener in which the formation of a filter cake is restricted and the product is discharged continuously as concentrated slurry. Such filters use filter cloths as the filtration medium and are limited to concentrating particles above 5 xm in size. Dead end membrane microfiltration, in which the particle-containing fluid is pumped directly through a polymeric membrane, is used for the industrial clarification and sterilisation of liquids. Such process allows the removal of particles down to 0.1 xm or less, but is only suitable for feeds containing very low concentrations of particles as otherwise the membrane becomes too rapidly clogged.2,4,8... 

The early stages of cross-flow microfiltration often follow such a pattern. However, the growth of the cake is limited by the cross-flow of the process liquid. There are several ways of accounting for the control of cake growth. A useful method is to rewrite (16.6.1) as ... 

A limitation to the more widespread use of membrane separation processes is membrane fouling, as would be expected in the industrial application of such finely porous materials. Fouling results in a continuous decline in membrane penneation rate, an increased rejection of low molecular weight solutes and eventually blocking of flow channels. On start-up of a process, a reduction in membrane permeation rate to 30-10% of the pure water permeation rate after a few minutes of operation is common for ultrafiltration. Such a rapid decrease may be even more extreme for microfiltration. This is often followed by a more gradual... 

S-layer ultrafiltration membranes (SUMs) are isoporous structures with very sharp molecular exclusion limits (see Section III.B). SUMs were manufactured by depositing S-layer-carrying cell wall fragments of B. sphaericus CCM 2120 on commercial microfiltration membranes with a pore size up to 1 pm in a pressure-dependent process [73]. Mechanical and chemical resistance of these composite structures could be improved by introducing inter- and intramolecular covalent linkages between the individual S-layer subunits. The uni-... 

Size ranges for membrane processing by reverse osmosis, ultrafiltration and microfiltration are shown in Figure 19.1. Reverse osmosis is effective in removing solvents away from dissolved molecules. Because of limitations in crushing strengths of membranes, pressures are limited to maxima of about 1000 psi... 

Mass-transport limitations are common to all processes involving mass transfer at interfaces, and membranes are not an exception. This problem can be extremely important both for situations where the transport of solvent through the membrane is faster and preferential when compared with the transport of solute(s) - which happens with membrane filtration processes such as microfiltration and ultrafiltration - as well as with processes where the flux of solute(s) is preferential, as happens in organophilic pervaporation. In the first case, the concentration of solute builds up near the membrane interface, while in the second case a depletion of solute occurs. In both situations the performance of the system is affected negatively (1) solute accumulation leads, ultimately, to a loss of selectivity for solute rejection, promotes conditions for membrane fouling and local increase of osmotic pressure difference, which impacts on solvent flux (2) solute depletion at the membrane surface diminishes the driving force for solute transport, which impacts on solute flux and, ultimately, on the overall process selectivity towards the transport of that specific solute. 

Only a few reports are available on the preparation of MFGM from commercially available sources and the opportunities to exploit fully the utilization of MFGM as a functional material are so far limited by the lack of available products and commercially feasible preparation methods. The development of methods for the extraction of MFGM from buttermilk through microfiltration may increase the opportunity to produce this ingredient on a commercial scale. On the other hand, before the economics of such processes can be appreciated, the unique functionality of MFGM isolates needs to be understood better. 

Microfiltration is a unit operation for the separation of small particles. The separation limits are between 0.02 and 10 (jum particle dimensions. Microfiltration can be carried out in a dead-end mode and a cross-flow mode. In downstream processing, the cross-flow filtration is carried out continuously or discontinuously. The most important parameters that determine the productivity of cross-flow microfiltration are transmembrane pressure, velocity, particle size and surface, viscosity of the liquid and additives such as surfactants, and changing the surface and surface tension. 

Today the majority of polymeric porous flat membranes used in microfiltration, ultrafiltration, and dialysis are prepared from a homogenous polymer solution by the wet-phase inversion method [59-66]. This method involves casting of a polymer solution onto an inert support followed by immersion of the support with the cast film into a bath filled with a non-solvent for the polymer. The contact between the solvent and the non-solvent causes the solution to be phase separated. This process involves the use of organic solvents that must be expensively removed from the membrane with posttreatments, since residual solvents can cause potential problems for use in biomedical apphcations (i.e., dialysis). Moreover, long formation times and a limited versatihty (reduced possibUity to modulate cell size and membrane stmcture) characterize this process. 

Membrane processes, in general, are very attractive for their simplicity and flexibility. They are capable of achieving separations at a molecular level. Membrane modules are often compact and easily scaleable. For clarification and concentration, microfiltration, ultrahltration, and reverse osmosis are the current methods of choice. RO has been widely used in the food industries as an attractive alternative to classical evaporahon the only hmitahon being its dependence on osmotic pressure, which practically limits concentration of fluid streams to 25°Bx-30°Bx. Hence, currently it is used more as a preconcentration step. In recent years, membrane processes, notably pervaporahon, membrane dishUahon and osmotic membrane distillation (OMD) [21], have been used either by themselves or in combinahon with other membrane processes to overcome the problems associated with thermal processes. 

Some complementary techniques can be used, such as membrane processes, for example. Urban wastewater treatment by membrane (e.g. microfiltration) can be envisaged up to a virtual disinfection, but the industrial development of these processes is still limited by the rather low value of the permeate fluxes and by membrane fouling that is... 

In summary, we have described an anaerobic process for singlecell protein from crude carbohydrates. The inhibitory by-products are simultaneously converted into methane. Mass transfer limitations can be avoided by using microfiltration rather than dialysis. Further study of the kinetics and improvements in yield will be necessary in order to make an economic comparison with other processes for single-cell protein. 

The limiting oil-concentration for the first stage, microfiltration, is determined by the tolerable concentration of oil in the recycled product. Since the oil retention capacity of the ultrafiltration stage is independent of feed concentration, the final concentration of the retentate is solely determined by the phase inversion "oil/water -> water/oil". At this concentration, the overall water recovery of the process is above 97.5%. However, the retentate of the process still contains too much water if incineration or refining is considered. Therefore, an evaporation step must be included in the process. Almost certainly evaporation will not be economical for the small capacities indicated in Figure 6.34 and a central evaporation station for the tentates from several production lines should be considered. 

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