Microfiltration membrane performance

SPEC was essentially able to market their Zr02-based ultrafiltration membranes to an already existing market in the sense that these membranes replaced polymeric UF membranes in a number of applications. They also developed a certain number of new applications. For Ceraver, the situation was different. When the Membralox membranes were first developed, microfiltration was performed exclusively with dead-end polymeric cartridge filters. In parallel to the development of inorganic MF membranes, Ceraver initiated the development of cross-flow MF with backflushing as a new industrial process. 

Inorganic membranes have also been studied. Thus, AFM has been used to probe the surface morphology and pore structure of micro- and ultrafiltration membranes, both in contact and noncontact mode, the latter being very suitable for soft and delicate materials. One of the first reports concerned alumina microfiltration membranes (Anapore) [45] and the authors performed statistical analysis to obtain the pore size distribution from the AFM... 

Chen ZA, Deng MC, Yong C, He GH, Ming W, and Wang JD. Preparation and performance of cellulose acetate/polyethyleneimine blend microfiltration membranes and their applications. J. Membr. Sci. 2004 235 73-86. 

Heidenreich S and Scheibner B. Hot gas filtration with ceramic filters Experiences and new developments. Filtr. Sep. 2002 May 22-25. Heidenreich S and Wolters C. Hot gas filter contributes to IGCC power plant s reliable operation. Filtr. Sep. 2004 June 22-25. Larbot A, Bertrand M, Marre S, and Prouzet E. Performances of ceramic filters for air purification. Sep. Purif. Technol. 2003 32 81-85. DeFriend KA and Barron AR. A simple approach to hierarchical ceramic ultrafiltration membranes. J. Membr. Sci. 2003 212 29-38. Endo Y, Chen D-R, and Pui DYH. Collection efficiency of sintered ceramic filters made of submicron spheres. Filtr. Sep. 2002 March 43-47. Sakol D and Konieczny K. Application of coagulation and conventional filtration in raw water pre-treatment before microfiltration membranes. Desalination 2004 162 61-73. 

Vemhet A, Pellerin MP, Belleville J, Planque J, and Moutounet M. Relative impact of major wine polysaccharides on the performances of an organic microfiltration membrane. Am. J. Enol. Vitic. 1999 50(l) 51-56. 

The effects of pH and ionic strength on the performance of an a-alumina microfiltration membrane from U.S. Filter was evaluated by Nazzal and Wies-ner [46]. Concerning pH effect on flux, results obtained in this work perceptibly differ from the previous one. Here the membrane operated at a significantly higher permeation rate at a pH well below the isoelectric point of the membrane. This variance can be explained considering the isoelectric point of the membrane was found at pH = 8.3 in this case while it was at pH = 3.5 in the... 

Elzo, D. et al.. Charge effects on inorganic membrane performance in a cross-flow microfiltration process. Colloids Surf. A, 138, 145, 1998. 

Hua FL, Tsang YF, Wang YJ, Chan SY, Chua H, and Sin SN. Performance study of ceramic microfiltration membrane for oily wastewater treatment. Chem. Eng. J. 2007 128 169-175. 

Abadi SRH, Sebzari MR, Hemati M, Rekabdar F, and Mohammad T. Ceramic membrane performance in microfiltration of oily wastewater. Desalination 2011 265 222-228. 

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. 

In order to achieve a high membrane performance, both selectivity and permeability should be high. However, the function of a membrane depends also on its structural integrity and stability, and hence limitations exist for reducing membrane thickness to reduce membrane resistance and increase permeability. This is not a critical problem in microfiltration where the pores enable a size-based selection for... 

Pore-filling MIP composite membranes had been first prepared by Dzgoev and Haupt [100]. They casted the reaction mixture into the pores of a symmetric microfiltration membrane from polypropylene (cutoff pore size 0.2 pm) and performed a cross -linking copolymerization of a functional polyacrylate for imprinting protected tyrosine. Hattori et al. [101] had used a commercial cellulosic dialysis membrane (Cuprophan) as matrix and applied a two-step grafting procedure by, (i) activation of the cellulose by reaction with 3-methacryloxypropyl trimethoxysilane from toluene in order to introduce polymerizable groups into the outer surface layer, (ii) UV-initiation of an in situ copolymerization of a typical reaction mixture (MAA/EDMA, AIBN) for imprinting theophylline. 

Finally, in Chap. 8, attempts are made to correlate the AFM parameters, such as nodule and pore sizes, to the membrane performance data. Membranes used for a variety of membrane processes, including reverse osmosis, nanofiltration, ultrafiltration, microfiltration, gas and vapor separation, pervaporation, and other membrane separation processes, are covered in this chapter. AFM parameters are also correlated to membrane biofouhng. This chapter also includes appUcations of AFM to characterize biomedical materials, including artificial organs cind drug release. 

This implies that microfiltration membranes are porous media containing macropores and ultrafiltration membranes are also porous with mesopores in the top layer. Hence, the definition porous covers both the macropores and mesopores. With membranes of these type it is not the membrane (material) which is characterised but the pores in the membrane. Here the pore size (and pore size distribution) mainly determines which particles or molecules are retained and which will pass through the membrane. Hence, the material is of little importance in determining the separation performance. On the other hand, with dense pervaporation/gas separation membranes, no fixed pores are present and now the material itself mainly determines the performance. 

In summary, the bubble-point method is a very simple technique for characterising the largest pores in microfiltration membranes. Active pores are determined with this technique. A disadvantage is that different results are obtained when different liquids are used for charaaerisatiorL In addition, the rate of pressure increase and the pore length may influence the result. Pore size distributions can be obtained by performing this technique by a stepwise increase of the pressure. 

All these examples clearly indicate that not only is the membrane performance important in microfiltration but particularly the chemical and thermal resistance of the materials used. Furthermore, the control of fouling is extremely important as well and this is discussed further in chapter VIL... 

Cellulose acetate (CA) can also be used as a hydrophilic additive to enhance the performance of PVDF microfiltration membranes cast by the phase inversion process... 

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