Enhanced dairy membrane operations through control of deposit formation on membrane surfaces

Abstract Text:

The objective is to present recent developments and insights on novel ways to operate dairy membrane operations at higher levels of efficiency and predictability. Boundary layer phenomena at the surfaces of membranes, in particular adsorption, retention and deposit formation are still not fully understood. Hence, adverse effects reducing flux and unpredictable permeation of components cannot reliably be prevented. Deposits reduce flux and in many cases dominate the system´s retention characteristics. Deposit formation is dependent on the position along the membrane surface, which is a result of the pressure drop in crossflow situations. These aspects will be discussed with particular regard to whey concentration and milk protein fractionation by means of microfiltration as the possibly most challenging example. The related phenomena were investigated experimentally and theoretically in terms of assessing deposit properties (casein micelle multi-layers, in this case) such as thickness and porosity as a function of position along the membrane. For this purpose, special membrane prototypes were constructed enabling the measurement of flux and convective transport of material through the membrane pores in four sections along a tubular, industrially sized ceramic membrane. Similarly, the effect of membrane length has been studied for spiral wound modules (SWM), which are the dominating membrane type in dairy installations. Deposit amounts and structures were assessed by means of chemical analysis and synchroton based X-ray analysis, using the novel GISAXS technique (Grazing Incidence Small Angle X-Ray Scattering). This way, casein micelle deformation was found to occur as a result of elongational flow of the filtrate stream towards the membrane surface. Further to that, a method for improved whey ultrafiltration performance as a result of a pre-microfiltration step is presented. The removal of protein aggregates in whey increases flux levels considerably, and a reduction of the microbial load is achieved. Novel cascade-like combinations of UF and RO/NF are discussed as a means to achieve high levels of concentration for whey, and, for comparison, for milk. The UF step removes the protein, such that the RO step has only to cope with osmotic pressure and remains unaffected by deposit formation. Thus, higher concentration levels, an increased flux can be achieved and concentration in shorter times at reduced energy levels. UF was assessed in the form of various processing concepts, namely conventional polymeric spiral-wound and ceramic crossflow systems, as well as dynamic rotating ceramic membranes, which are able to produce and to cope with high protein concentration levels. 

Keywords: Membrane technology, Microfiltration, Milk protein fractionation, Milk protein concentration