Two microfiltration units were designed and constructed during this research.One was a dead-end strirred cell unit and the other was a tubular membrane crossflowunit, both with integrated backwash ability. Initial experiments on cross-flowmicrofiltration, in a pressure range of 2.5 to 30 psi, are conducted using tubularceramic membranes of both 1.4 and 0.8 μm nominal pore size and whey proteinconcentrate solution. Although 1.4 μm membranes do not retain any protein, apossible pore constriction mechanism is identified, as well as cake formation duringfiltration with 0.8 μm membranes. A potential “optimum” pressure difference is foundto be in effect during cross-flow microfiltration and is further investigated in afollowing set of customized experiments.Permeate flux decline in dead-end microfiltration of whey protein isolatesolutions is studied, using disc-type ceramic membranes of nominal pore size 0.8 μm.The tests involve five successive filtration cycles, under fixed filtration pressure, withintermediate backwashing. Flux decline analysis and membrane resistance areemployed to determine fouling mechanisms, with respect to applied pressure, in therange 2.5–10 psi. Data interpretation, suggests that more rapid fouling, as well assignificant surface-layer “compaction” effects may occur at the higher pressuresemployed. There is evidence that irreversible fouling effectively develops during thefirst cycle of the tests (of 30 min duration) and apparently does not significantlyincrease later on; however, reversible fouling occurs through the entire filtrationseries, being more intense during the first minutes of each cycle. The effect of proteinaggregates is investigated with the aid of DLS measurements and filtration tests withprefiltered solutions; it appears that whey protein aggregates present in the solution,are responsible (almost entirely) for the observed membrane fouling.Microfiltration of whey protein solutions by tubular ceramic membranes,under constant cross-flow and trans-membrane pressure, with periodic backwashing,is investigated using a fully instrumented pilot unit. Relatively large nominalmembrane pore size (0.8 μm) insures very high protein transmission, which isdesirable in applications such as microbial load reduction. In the first of a sequence ofthree filtration-backwashing cycles, irreversible and reversible fouling are identified,over the tested pressure range of 5 to 17.5 psi. Permeate and retentate concentrationdata complement the permeation rate measurements. Both irreversible and reversiblefouling is identified for the backwashing mode employed. Early in the first cycle,especially at the higher pressures, a pore constriction / blocking mechanism appears tobe responsible for the irreversible fouling. In the other two cycles only the reversiblefouling is significant, possibly due to some kind of protein layer formation on themembrane surface. The permeate flux level tends to increase by increasing transmembranepressure up to a near-optimum value of 10 psi, beyond which pressure hasa negative effect. This interesting trend is attributed to the interplay of cross-flowvelocity, which tends to reduce fouling by promoting re-suspension and breakage ofcolloidal protein agglomerates, with the trans-membrane pressure (and related flux)which leads to protein layer formation on the membrane and may impose compressivestresses, thereby increasing its resistance to permeation.An effective membrane cleaning procedure is proposed, combining bothtypical chemical treatment and ultrasounds. Evidence is also presented on the effect ofisotropic nature of the membranes, especially during the first few minutes of filtration.
Εθνικό Κέντρο Τεκμηρίωσης και Ηλεκτρονικού Περιεχομένου (ΕΚΤ)
