Luhui Ding

Simultaneous Convective and Diffusive Mass Transfers in a Hemodialyser

The mass transfer in a hemodialyser in the presence of combined dialysis and ultrafiltration has been calculated by integration of mass fluxes across the boundary layers in blood and dialysate phase taking into account the partial rejection of solute as well as changes in local blood flow and ultrafiltration flux along the membrane. Clearances of creatinin, vitamin B12, and myoglobin have been calculated as a function of blood and ultrafiltrate flow rate and were found to be in good agreement with in vitro measurements. The data suggest the following empirical correlation for the hemodiafiltration clearance.

Microfiltration and ultrafiltration of UHT skim milk with a vibrating membrane module

Abstract Dairy applications of a vibratory shear-enhanced (VSEPPUR) filtration system were investigated using a 500 cm 2 membrane area pilot. Commercial UHT skim milk was ultrafiltered with a 50 kDa PES membrane for concentrating total proteins. The effect of mean membrane shear rate γ w was investigated by lowering the vibration frequency. The permeate flux was found to vary as γ w 0.533 at initial concentration and as γ w 0.567 at a volume reduction ratio (VRR) of 1.8. The permeate flux decayed with the logarithm of concentration and the maximum VRR calculated by extrapolation to zero flux was 8.66. The critical flux for stable filtration was found to be 76 l h −1 m −2 at the maximum frequency of 60.75 Hz, corresponding to a mean shear stress on the membrane of 50 Pa. Microfiltration tests were carried out with a 0.1 μm pores Teflon membrane for separating casein micelles. The permeate flux increased with γ w , but with a lower exponent than in UF, especially at high frequency. The critical flux at 60.75 Hz and initial concentration was 64 l h −1 m −2 , thus lower than in UF. A comparison with critical fluxes reported in the literature for UF and MF of skim milk indicates a larger domain of filtration stability for the VSEPPUR than for cross-flow filtration with tubular membranes.

Milk protein concentration by ultrafiltration with rotating disk modules

Abstract Ultrafiltration of UHT skimmed milk has been carried out using a smooth disk and a disk equipped with 6-mm-high vanes rotating at high angular velocity ω near a circular 50 kD membrane. Permeate turbidity was less than 2 NTU confirming complete casein micelles retention. The addition of vanes to the disk increases the permeate flux by about 56% due to higher fluid core velocity resulting in higher shear rate at the membrane The permeate flux increases as ω0.9 and remains independent of concentration until a volume reduction ratio (VRR) of 3 at 2000 rpm. The maximum theoretical VRR corresponding to zero flux was 9.6 for a disk with vanes.

Dynamic filtration of blood: a new concept for enhancing plasma filtration.

We have shown previously that blood flow pulsations created by intermittent squeezing of the inlet blood line significantly increased the plasma filtration rate in membrane plasmapheresis. However, in order to avoid hemolysis, the filtration increase had to be limited to about 50%. We have now devised a more efficient pulsation generator. By properly matching the tubing compliance and the pulsation amplitude, it is possible to extract 50 ml/min of plasma from 90 ml/min of blood at 36% hematocrit with a 1000 cm2 polypropylene hollow fiber filter without hemolysis. Simultaneous recording of the time course of plasma filtration rate measured by an electromagnetic flow meter and transmembrane pressure showed that the increase in mean plasma flow rate was due to a dynamic filtration process which prevents the establishment of concentration polarization. The transmembrane pressure (Ptm) increases over a 0.5-second interval when the tube is squeezed. The membrane responds with an increase in filtration since the concentration polarization layer takes a few seconds to build up. The Ptm then drops when the tube is released before the polarization layer has time to build up appreciably and a sudden acceleration of the blood flow (velocity spike) helps clean the membrane, reducing the polarization. Tests with bovine blood show that the system is very efficient in reducing membrane plugging with small area filters.

Comparison of three different systems used for flux enhancement: application to crossflow filtration of yeast suspensions

The aim of this work is to characterize more accurately, through experimental or theoretical approaches, the hydrodynamics of three particular systems usually used for flux enhancement: shear-enhanced filtration with a vibrating membrane module, gas/liquid two-phase flows and Dean vortices. The correlation between local hydrodynamics and permeate flux obtained during crossflow filtration of a yeast suspension under different operating conditions confirmed the importance of the wall shear stress in these three systems.

Effect of ethanol on ultrafiltration of bovine albumin solutions with organic membranes

This paper investigates the ultrafiltration of albumin-ethanol solutions on polysulfone hollow fiber membranes with 30 kDa cut-off. The aim is to identify the mechanisms responsible for the observed permeate flux reduction in presence of ethanol. The variations of permeate flux with transmembrane pressure and wall shear rate fit the usual pattern of flux limitation by concentration polarization. Thus, although ethanol significantly increases the permeate viscosity, the data show that the flux decrease is not a direct consequence of the viscosity increase but rather due to reduced albumin diffusivity which decreases the back transport to the bulk solution. The specific resistance of the albumin layer on the membrane was found to be unaffected by the presence of ethanol. However the fouling potential of our solutions was found to be significantly increased by the addition of ethanol. Thus the observed flux reduction due to ethanol seems to be explained by a combination of a thicker polarization layer caused by reduced back transport and increased membrane fouling. A 10% increase in filtrated volume can be obtained by imposing periodic retrofiltrations which decrease fouling.

Kinetics of concentration-polarization formation in crossflow filtration of plasma from blood: experimental results

Abstract The formation and disappearance of the concentration-polarization layer in crossflow microfiltration of blood has been investigated by submitting a membrane to step changes in the transmembrane pressure, while recording the time variation of pressure and permeate flux. When the pressure is suddenly raised from an unpolarized regime to a level corresponding to complete concentration-polarization, the permeate flux reaches a peak in 0.4–0.6 sec, which surpasses the equilibrium level by 60 to 80% depending upon the pressure, returning to the concentration-polarization equilibrium level in 3–4 sec. This data shows that the membrane retains its original permeability only during the first 0.5 sec of the pressure change and that concentration-polarization takes about 3–4 sec to build-up. This formation time decreases with increasing pressure. When the pressure returns to its initial level, the concentration-polarization disappears instantaneously and the process can be repeated at a frequently up to 0.7 Hz. However, when a stepped pressure increase is applied in the concentration-polarization regime, with initial pressure above 150 mmHg, the permeate flux hardly changes, which confirms that once concentration-polarization is established, the system membrane-polarization layer ceases to behave as a porous medium even with increased resistance. These data can explain the permeate flux increase observed when pressure and flow pulsations at 1 Hz are superimposed on the retentate.

Concentration polarization formation in ultrafiltration of blood and plasma

This paper describes a technique for investigating the kinetics of concentration polarization formation in ultrafiltration of bovine blood, plasma and albumin (BSA) solutions using polysulfone hollow fiber membranes of 30000 Da cut-off. The mebrane is submitted to step pressure changes while monitoring the variation of instantaneous permeate flux. When the pressure is suddenly raised from an unpolarized regime to a high polarization one, the permeate flux reaches a peak in 4 ot 7 sec and decays back to its equilibrium level in 2 min, for blood and plasma, and in less than 1 min, for albumin solution. With blood, the apparent filtration resistance (neglecting osotic pressure) reaches its maximum of 6 × 1013 m−1 when 200 ml of blood has been filtered (on a 0.65 m2 membrane). The red cell contribution to this resistance is about 20%. The filtration resistance is higher with plasma than with albumin, at the same protein concentration. In the initial buildup of concentration polarization the resistance increases linearly with albumin mass carried towards the membrane, with a rate independent of concentratiion. Nevertheless, the final resistance at equilibrium increases with increasing albumin concentration. The time for complete reversal of concentration polarization is estimated by repeating pressure increments, while increasing the time interval between two increments, until the permeate peak following the pressure step reaches its maximum. This time is found to be of the order of 12 sec for BSA solutions.

Albumin recovery enhancement in membrane plasma fractionation using pulsatile flow.

In therapeutic plasmapheresis using cascade filtration, it is important to maximize albumin recovery while rejecting as many γ-globulins as possible. Several membrane fractionation techniques were investigated using fresh bovine and human plasma and cellulose acetate filters (PF 100, AKZO). In dead end mode the sieving coefficients were found to decrease as transmembrane pressure increased. This was due to membrane plugging during the course of filtration after about 20 minutes which lead to a rapid increase in transmembrane pressure. In single pass mode the albumin recovery factor generally remains around 40% since the permeate flux is much less than the inlet flow. When strong pulsations (4 to 6 Hz) were superposed on the inlet plasma flow in single pass mode, the albumin sieving coefficient remained at about 0.95 while the permeate flux was increased by 106%. As a result a recovery factor of more than 80% could be sustained for at least 90 minutes without membrane plugging. Therefore pulsatile flow plasma fractionation seems to be an interesting approach to combine continuous operation with high albumin recovery

Stepwise membrane fouling model for shear-enhanced filtration of alfalfa juice: experimental and modeling studies

Alfalfa juice is predominantly created from the production of fodder pellets for cattle. Due to its high nutritive value and abundant sources, an inappropriate treatment may cause important environmental problems. Membranes are used to separate and concentrate leaf protein from alfalfa juice. However, membrane fouling reduces process efficiency and hinders wide application. This study investigated the fouling of MF and UF membranes by foulants of alfalfa juice. Permeate flux gradually increased with TMP in a stepwise pattern. A stepwise multisite Darcys law model (SMDM) was proposed to simulate the stepwise multisite fouling process. Besides, the resistance coefficient and compressibility for different steps and sites were calculated to explain the complex fouling process. The effects of feed composition, membrane and hydraulic conditions on the stepwise fouling process were investigated. These various factors have a strong effect on the fouling process of industrial membranes. A series of long term tests were utilized to study flux decline and membrane fouling at various steps and sites of the fouling process. The results of this investigation can help us to understand the fouling process of alfalfa juice and facilitate membrane fouling control.