Critical stability conditions in skimmed milk crossflow microfiltration: impact on operating modes

There exists a critical ratio (convection towards the membrane/erosion) in crossflow microfiltration, MF, below which there is no marked fouling by co lloidal particles and above which performance are altered: sharp increase o f fouling, reduced operating time, large decrease in permeability and solut e transmission. This paper outlines the impact of the critical ratio on ski mmed milk MF processing (separation of casein micelles from the soluble pro teins) and gives objective elements (taking into account the critical ratio ) for the selection of start-up procedures and stationary modes of operatio n: controlled transmembrane pressure or permeation flux; "static" or "dynam ic" counterpressure, the latter achieved by circulating the permeate co-cur rent to the retentate to maintain an equal transmembrane pressure profile a long the filtering path. Results showed that the controlled transmembrane p ressure mode of operation was appropriate for conducting skimmed milk filtr ation, since it prevented from sharp increase of the hydraulic resistance o f fouling and maintained nearly constant performances (permeability, select ivity) over the course of the time. In order to determine the more appropri ate counter pressure mode and to predict skimmed milk performances, a four step method was proposed : i) assessment of the critical ratio; ii) selecti on of the critical flux at the required wall shear stress; iii) determinati on of the corresponding critical transmembrane pressure in "dynamic" counte r-pressure; iv) calculation of the MF performance in "static" mode taking i nto account the gradient of transmembrane pressure induced by the retentate pressure drop.