Zhanfeng Cui

The use of gas bubbling to enhance membrane processes

The introduction of gas/liquid two-phase flow has been shown to significantly enhance the performance of some membrane process applications. This paper reviews the state-of-the-art of this technique. In particular, the review focuses on the use of gas bubbles and slugs in microfiltration and ultrafiltration with flow inside tubes and fibres, across flat sheets and outside fibres. Examples discussed are applications in biotechnology, bioseparations and water and wastewater treatment. Some practical issues and future trends are addressed.

Adipose-derived stem cell: a better stem cell than BMSC

To further study the proliferation and multi‐differentiation potentials of adipose‐derived stem cells (ADSCs), the cells were isolated with improved methods and their growth curves were achieved with cck‐8. Surface protein expression was analyzed by flow cytometry to characterize the cell phenotype. The multi‐lineage potential of ADSCs was testified by differentiating cells with adipogenic, chondrogenic, osteogenic, and myogenic inducers. The results showed that about 5 × 105 stem cells could be obtained from 400 to 600 mg adipose tissue. The ADSCs can be continuously cultured in vitro for up to 1 month without passage and they have several logarithmic growth phases during the culture period. Also, the flow cytometry analysis showed that ADSCs expressed high levels of stem cell‐related antigens (CD13, CD29, CD44, CD105, and CD166), while did not express hematopoiesis‐related antigens CD34 and CD45, and human leukocyte antigen HLA‐DR was also negative. Moreover, stem cell‐related transcription factors, Nanog, Oct‐4, Sox‐2, and Rex‐1 were positively expressed in ADSCs. The expression of alkaline phosphatase (ALP) was detected in the early osteogenic induction and the calcified nodules were observed by von Kossa staining. Intracellular lipid droplets could be observed by Oil Red staining. Differentiated cardiomyocytes were observed by connexin43 fluorescent staining. In order to obtain more stem cells, we can subculture ADSCs every 14 days instead of the normal 5 days. ADSCs still keep strong proliferation ability, maintain their phenotypes, and have stronger multi‐differentiation potential after 25 passages. Copyright

Flux enhancements with gas sparging in downwards crossflow ultrafiltration: performance and mechanism

Abstract In this study gas-liquid two-phase crossflow ultrafiltration was studied in downwards flow condition. A commercially available tubular membrane module was installed vertically and the feed solution and the injected gas bubbles flow downwards inside the membrane tubes. The permeate flux achieved in such an operation was compared to conventional single phase ultrafiltration and also to gas sparged upwards crossflow operation. The effect of operating parameters, including liquid and gas flow rates, transmembrane pressure (TMP) and feed concentrations, was examined with dextran solutions. Flux increases of up to 320% were achieved with gas sparging in the experimental study compared to single liquid phase crossflow ultrafiltration. This enhancement is much more profound when the liquid phase is in laminar flow, particularly when the concentration polarisation is more severe, for example, at a high TMP, a higher feed concentration, and a low liquid crossflow velocity. The enhancing effect is not very significant when liquid flow itself is turbulent. The flow pattern proved to be an important parameter and the operation could be optimised to achieve maximum enhancement with minimum gas injection.

GAS-LIQUID TWO-PHASE CROSS-FLOW ULTRAFILTRATION OF BSA AND DEXTRAN SOLUTIONS

Abstract A method is proposed for reducing concentration polarisation and membrane fouling by injection air into the feed stream, creating a gas—liquid two-phase flow across the membrane surface. The injected air promotes turbulence, increasing the superficial cross-flow velocity of the process fluid, suppressing the polarisation layer and enhancing the ultrafiltration process. Experiments were carried out using a tubular membrane (PCI, 100 kDa MWCO), mounted vertically and horizontally, with BSA, dextran and dyed dextran solutions. A range of transmembrane pressures and liquid and gas flow rates were tested, with the liquid in pulsatile and steady flow driven by peristaltic and centrifugal pumps. On the addition of air to the liquid stream, permeate flux was observed to increase by up to 60% for dextran, 113% for dyed dextran and 91% for BSA. The rejection ratios were also improved with an increase of between 5 and 10%. Significant enhancements could be achieved at low gas flow rates, with the degree of enhancement increasing further at higher gas flow rates. Vertically mounted membranes showed a 10 to 20% higher permeate flux when compared with horizontally installed membranes under the same two-phase flow operation.

Gas sparging to enhance permeate flux in ultrafiltration using hollow fibre membranes

Abstract This study focuses on the use of gas-liquid two-phase crossflow to overcome concentration polarisation in the ultrafiltration of macromolecular solutions as applied to hollow fibre membrane systems. The experimental work was conducted on a purpose built pilot-plant scale rig with albumin and dextran as the test media. The effect of gas injection on the permeate flux and membrane sieving coefficient was examined experimentally at different transmembrane pressures, feed concentrations and gas to liquid flow ratios. The results were encouraging, with flux enhancements of 20–50% obtained for dextrans and 10–60% for albumin, when air was injected into the system over the range of process variables examined. The sieving coefficient of albumin was considerably reduced when gas-liquid two-phase cross-flow was used. These results were compared to those obtained with tubular membrane systems, and an additional mechanism, based on physical displacement of the concentration polarisation boundary layer is proposed. The operational difficulty related to protein foaming is also discussed.

Microfibrils, elastin fibres and collagen fibres in the human intervertebral disc and bovine tail disc

The distribution of microfibrils was studied immunohistochemically in intervertebral discs taken from young normal human surgical cases and from the bovine tail. Co‐localization of microfibrils and elastin fibres was examined by dual immunostaining of fibrillin‐1 and elastin. Collagen fibre network orientation was studied by using polarized filters. A similar microfibrillar network was seen in both bovine and human discs with network organization being completely different from region to region. In the outer annulus fibrosus (OAF), abundant microfibrils organized in bundles were mainly distributed in the interterritorial matrix. In addition, the microfibril bundles were orientated parallel to each other and co‐localized highly with elastin fibres. Within each lamella, co‐localized microfibrils and elastin fibres were aligned in the same direction as the collagen fibres. In the interlamellar space, a dense co‐localized network, staining for both microfibrils and elastin fibres, was apparent; immunostaining for both molecules was noticeably stronger than within lamellae. In the inner annulus fibrosus, the microfibrils were predominantly visible as a filamentous mesh network, both in the interterritorial matrix and also around the cells. The microfibrils in this region co‐localized with elastin fibres far less than in the OAF. In nucleus pulposus, filamentous microfibrils were organized mainly around the cells where elastin fibres were hardly detected. By contrast, sparse elastin fibres, with a few of microfibrils, were visible in the interterritorial matrix. The results of this study suggest the microfibrillar network of the annulus may play a mechanical role while that around the cells of the nucleus may be more involved in regulating cell function.

Fractionation of BSA and lysozyme using ultrafiltration: effect of pH and membrane pretreatment

Selective transmission of a solute through membranes proves to be a challenge in ultrafiltration processes. This is because the transport of a solute through an ultrafiltration membrane does not depend on size alone, but on several other factors such as solute-solute and solute-membrane interactions. By manipulating physicochemical parameters and process variables (eg. pH, ionic strength, concentration of solute, etc.) and by membrane modification, it is possible to enhance the transmission of a particular solute and thus enhance fractionation of solutes. In this paper, the effect of pH on fractionation of BSA and lysozyme by ultrafiltration through 50 kDa MWCO (molecular weight cut off) polysulfone membrane has been examined. It was found that the selectivity of solute separation for dilute mixtures of BSA and lysozyme was very much pH dependent and varied from 3.3 at pH 5.2 to 220.0 at pH 8.8. However, at a higher feed concentration, the transmission of lysozyme through polysulfone membrane decreases quite dramatically resulting in lower throughput of product. An attempt has been made to enhance the transmission of lysozyme through the polysulfone ultrafiltration membrane by pretreating the surface of the membrane by adsorption of another protein, myoglobin. An increase in lysozyme transmission of up to 63% with respect to native membrane was observed. The stability of this pretreatment and its effect on permeate flux have been examined. The pretreated membrane was used to fractionate BSA/lysozyme mixtures. Even at higher feed concentration, enhanced fractionation with respect to native membrane was observed due to highly enhanced transmission of lysozyme through the pretreated membrane.

CFD modelling of gas-sparged ultrafiltration in tubular membranes

Abstract In ultrafiltration processes, injecting gas to create a gas–liquid two-phase crossflow operation can significantly increase permeate flux and, moreover, can improve the membrane rejection characteristics. It has been shown that controlled pulse injection to generate slug flow is more advantageous than uncontrolled gas sparging, especially when the gas flow rate is low. The slug size and frequency affect the performance of ultrafiltration, and there exits an optimal slug size and frequency to achieve high permeate flux. Previous studies have been based on the analysis of the experimental data and mass-transfer correlations. In this work, an attempt is made to model the slug flow ultrafiltration process using the volume of fluid (VOF) method with the aim of understanding and quantifying the details of the permeate flux enhancement resulting from gas sparging. For this numerical study, the commercial CFD package, FLUENT, is used. The first part of the model uses the VOF method to calculate the shape and velocity of the slug, the velocity distribution and the distribution of local wall shear stress in the membrane tube (neglecting the wall permeation effect). The second part links the local wall shear stress to the local mass-transfer coefficient that is then used to predict the permeate flux. In order to validate the model, experimental data reported in the literature over a wide range of gas and liquid velocities, slug frequencies, and transmembrane pressures are compared with the CFD predictions. Good agreement is obtained between theory and experiment.

Effect of Extracellular pH on Matrix Synthesis by Chondrocytes in 3D Agarose Gel

In cartilage tissue engineering, the determination of the most appropriate cell/tissue culture conditions to maximize extracellular matrix synthesis is of major importance. The extracellular pH plays an important role in affecting energy metabolism and matrix synthesis by chondrocytes. In this study, chondrocytes were isolated from bovine articular cartilage, embedded in agarose gel, and cultured at varied pH levels (7.3–6.6). Rate of lactate production, total glycosaminoglycan (GAG) and collagen synthesis, as well as total cell numbers and cell viability were evaluated after culturing for up to 7 days. The results showed the rate of lactic acid production over the 7‐day culture was significantly affected by extracellular pH; acidic pH markedly inhibited the production of lactate. Also, a biphasic response to extracellular pH in regard to total GAG synthesis was observed; the maximum synthesis was seen at pH 7.2. However, the collagen synthesis was not pH‐dependent within the pH range explored. In addition, within the conditions studied, total cell numbers and cell viability were not significantly affected by extracellular pH. In conclusion, even minor changes in extracellular pH could markedly affect the metabolic activities and biosynthetic ability of chondrocytes. Consequently, the control of extracellular pH condition is crucially important for successful cartilage tissue engineering and for the study of chondrocyte physiology and functions.

Effect of bubble size and frequency on the permeate flux of gas sparged ultrafiltration with tubular membranes

Abstract Gas sparged ultrafiltration experiments are performed using a tubular membrane module with solutions of dextran and human serum albumin (HSA) as the test media. Air is injected, in a controlled manner with the ability to adjust bubble size and frequency independently, into the membrane module to create a gas–liquid two-phase crossflow operation. The effects of bubble size and frequency on the permeate flux of the sparged ultrafiltration are studied experimentally. It is found that the permeate flux increases with the bubbling frequency in the examined range. The effect of bubble size on flux can be divided into two regions, an increasing region for smaller bubbles and a plateau region for larger slugs. The results are discussed on the basis of bubble wake hydrodynamics.