The effect of linear velocity and flux on performance of ceramic graded permeability membranes when processing skim milk at 50°C

Abstract

Raw milk (about 500 kg) was cold (4°C) separated and then the skim milk was pasteurized at 72°C and a holding time of 16s. The milk was cooled to 4°C and stored at ≤ 4°C until processing. The skim milk was microfiltered using a pilot-scale ceramic graded permeability (GP) microfilter system equipped with 0.1-µm nominal pore diameter ceramic Membralox membranes. First, about 155 kg of pasteurized skim milk was flushed through the system to push the water out of the system. Then, additional pasteurized skim milk (about 320 kg) was microfiltered (stage 1) in a continuous feed-and-bleed 3× process using the same membranes. The retentate from stage 1 was diluted with pasteurized reverse osmosis water in a 1:2 ratio and microfiltered (stage 2) with a GP system. This was repeated 3 times, with total of 3 stages in the process (stage 1 = microfiltration; stages 2 and 3 = diafiltration). The results from first 3 stages of the experiment were compared with previous data when processing skim milk at 50°C using ceramic uniform transmembrane pressure (UTP) membranes. Microfiltration of skim milk using ceramic UTP and GP membranes resulted in similar final retentate in terms of serum proteins (SP) removed. The SP removal rate (expressed by kilogram of SP removed per meter-squared of membrane area) was higher for GP membranes for each stage compared with UTP membranes. A higher passage of SP and SP removal rate for GP than UTP membranes was achieved by using a higher cross-flow velocity when processing skim milk. Increasing flux in subsequent stages did not affect membrane permeability and fouling. We operated under conditions that produced partial membrane fouling, due to using a flux that was less than limiting flux but higher than critical flux. Because the critical flux is a function of the cross-flow velocity, the difference in critical flux between UTP and GP membranes resulted only from operating under different cross-flow velocities (6.6 vs 7.12 for UTP and GP membranes, respectively). Conditions that allow microfiltration operation at higher flux will reduce the membrane surface area required to process the same amount of milk in the same length of time. Less membrane surface area reduces investment costs and uses less energy, water, and chemicals to clean the microfiltration system.

A process efficiency assessment of serum protein removal from milk using ceramic graded permeability microfiltration membrane

“…These differences are due likely to variation in the TS contents of the batches of milk used and the VCF actually achieved at the end of filtration, as VCF is more difficult to estimate in batch mode. The fluxes were nevertheless slightly higher than those obtained in similar studies-72.5 kg/h per m 2 by Zulewska and Barbano (2014) and 71.8 kg/h per m 2 by Zulewska et al (2009)-whereas the overall pressures maintained (Rpi, Rpo, and Ppo) were lower. The same tendency was noted during DF, the flux reaching, on average, 115.4 kg/h per m 2 after DF 1 and 139.3 kg/h per m 2 after DF 2, compared with 84.5 and 92.7 kg/h per m 2 in a similar experiment conducted by Zulewska and Barbano (2014).…”

“…This is due to NPN and other solids (mostly lactose) consisting of molecules too small to be rejected by the membrane. The casein as a percent of TP value was expected to increase between DF 1 and 2, as reported in the literature (Zulewska and Barbano, 2014), but remained essentially unchanged ( Table 3). The clean water flux was typically 365 kg/h per m 2 and the fouled water flux was 272 kg/h per m 2 .…”

“…A final VCF of 3× was chosen because this a common value sought in MF processing of skim milk using GP membranes (Zulewska et al, 2009;Zulewska and Barbano, 2014). The batch process used in our study allowed accurate determination of flux and composition during skim milk concentration and DF at an average TMP of 152 kPa.…”

“…Spiral-wound polymeric membrane (0.3 μm pore size) and Isoflux (TAMI, Nyons, France) ceramic membrane have been found less effective than UTP or GP systems (Zulewska et al, 2009;Adams and Barbano, 2013). Previous work by Zulewska and Barbano (2014) evaluated the effect of diafiltration (DF) on the efficiency of SP removal during MF of skim milk. Most studies of the performance of GP membranes have been conducted under feed-and-bleed conditions, which allow continuous production of retentate with a volumetric concentration factor (VCF) of 3× by adjusting permeate and retentate removal rates during the process.…”

“…Raising the TMP above the critical flux increases the thickness of the fouling layer and decreases SP transmission (Gésan-Guiziou et al, 1999). Zulewska and Barbano (2014) determined that the critical flux of skim milk in a continuous feedand-bleed process with a VCF of 3× using a 0.1-μm GP membrane was 64 L/m 2 per hour. Under the same conditions, the maximal flux attainable (limiting flux) was estimated at 96 L/m 2 per hour.…”