Kuo-Jen Hwang

Effect of morphology of polymeric membrane on the performance of cross-flow microfiltration

Effects of membrane morphology and operating conditions on the performance of cross-flow microfiltration are studied. Three kinds of membranes with the same mean pore size of 0.1 μm are selected for filtration experiments. They are MF-Millipore, Durapore and Isopore membrane. The variations of cake and membrane resistance during filtration using these membranes are measured and used for blocking analysis. Although complete pore blocking occurs, Isopore membrane results in the highest filtration rate due to less cake formation. The filtration rate of Durapore membrane is lower than that of Isopore membrane. It is because, more particles deposit on the surface of Durapore membrane to form filter cake. MF-Millipore membrane results in the lowest filtration rate due to the most cake formation and a serious pore blocking. The pore structures of each membrane are modeled in order to understand the mechanisms of pore blocking. At the initial stage of filtration, a standard blocking occurs in MF-Millipore membrane, an intermediate blocking in Durapore membrane, and a complete blocking in Isopore membrane. These blocking models are transferred to cake filtration after 10 min for all kinds of membranes. The pore blocking in each membrane is also observed and demonstrated by scanning electronic microscopy. Furthermore, filtration rate increases with either increasing cross-flow velocity or increasing filtration pressure.

FLUID FLOW THROUGH BASIC WEAVES OF MONOFILAMENT FILTER CLOTH

The effects of woven structures on fluid flow through basic weaves of monofilament filter cloths are studied numerically using the fluid-flow software FLUENT. The flow pattern and resistance to flow in the interstices are obtained as results of the numerical solution. The results show that the construction of the fabric pores has a significant influence on flow pattern in the interstices and downstream. Plain weave gives the highest fluid flow resistance, while satin weave has the lowest with the same thread count.

Dynamic analysis of cake properties in microfiltration of soft colloids

Abstract The properties of cakes formed using soft colloids in “dead-end” microfiltration were studied. The empirical power functions were used to relate the local cake properties. However, the cake compressibility factors are functions of time due to the retardation effect of cake compression. A dynamic analysis method is proposed to estimate the cake properties during soft colloid microfiltration. The results show that the entire filtration course can be divided into three stages. At the beginning of filtration, the deposition and rearrangement of the colloidal particles on the membrane surface cause the overall filtration resistance to increase. The average porosity and average specific filtration resistance of the cake vary slightly during this period of filtration. In the second stage, a rapid increase in filtration resistance and a decrease in cake porosity due to cake compression and colloid deformation can be found. A compact skin layer begins to be formed next to the filter membrane in this stage. The skin thickness is about 10–20% of the entire cake but this layer exhibits about 90% of the overall filtration resistance. The average cake porosity increases gradually in the third stage due to the loose packing in the newly formed cake. This trend can be reflected in the d t /d v versus v filtration curves and is demonstrated by the experimental data. The structure of the cake formed in each stage was observed using scanning electronic microscopy (SEM).

Compression of deformable gel particles

Abstract In order to study the deformation behavior of deformable particles under hydraulic drag and mechanical load such as in filtration, a number of in situ strengths of single calcium-alginate gel particle and inter-particle contact modes among gel particles were explored. A linear viscoelastic contact model based upon the Hertz theory was proposed to describe the deformation behavior of deformable gel particles. The effect of particle deformation due to frictional drag and mass of particles on the reduction of porosity was studied to examine how this variation led to the increase in contact area between particles. Both theoretical and experimental results show that the modified Hertz theory combined with the linear viscoelastic model can be used to predict the mechanical behaviors of gel particle under mechanical compression well. Experimental results also demonstrate that a given mechanical load would result in a higher degree of deformation of particles than under the same amount of hydraulic drag. Based upon the theoretical derivation and the verification of experimental results, a correlated relationship between specific surface area ratio and bed porosity, e, with a validated range of 0.05

Local properties of cake in cross-flow microfiltration of submicron particles

Abstract The local properties of filter cakes, such as porosity and specific filtration resistance, in cross-flow microfiltration of submicron particles are studied based on an analysis of force. The packing of particles in a filter cake can be divided into two modes. When the solid compressive pressure is smaller than the critical value, there exists an equilibrium distance between neighbouring particles due to the electrostatic repulsive force, and the local cake porosity can be estimated by using the cell model proposed in this study. When the solid compressive pressure is greater than the critical value, the compressive force can overcome the repulsive barrier, the particles then come into contact with neighbours, and the power-type empirical relationship between cake porosity and solid compressive pressure can be employed to estimate the local cake porosity. It can be found that the half of the cake near the filter membrane has a compact structure, and a high filtration resistance within the operating conditions of this study. On the other hand, the portion of cake near the cake surface has a high porosity due to the separation of particles. By using this model, the effect of electrolyte concentration on cake properties can be analyzed, and the estimated values of average porosity and average specific filtration resistance under various electrolyte concentrations, cross-flow velocities, and filtration pressures agree fairly well with the experimental data.

CONSTANT PRESSURE FILTRATION OF MONO-DISPERSED DEFORMABLE PARTICLE SLURRY

Constant pressure filtration experiments of Ca-alginate gel particle, Saccharomyces cerevisiae, and polymethyl methacrylate (PMMA) were conducted to study the local properties of cake layer formation by deformable particles. Effects of particle deformation due to frictional drag and cake mass on cake compression and contact area among particles were examined. The factors that lead to the increase in filtration resistance were discussed. The dynamic analysis proposed by Lu and Hwang in 1993 was modified to analyze formation and compression of cake during cake filtration of deformable particle slurry. A thin skin layer of low porosity and high resistance was formed next to the filter medium due to severe deformation, caused by frictional drag, of the first layer. The results of this study clearly indicate that neglecting the area-contact effect among particles will lead to an overestimate of cake porosity, and neglecting the transient effect of cake compression during gel layer formation will result in an underestimate of cake porosity. The characteristic values of filter cake obtained from dynamic analysis can be used to predict the performance of the filtration of slurries containing deformable particles.

The Role of Dynamic Membrane in Cross-Flow Microfiltration of Macromolecules

The effects of dynamic membrane formed by fine particles on the filtration rate and the rejection of macromolecules in cross-flow microfiltration are studied. Experiments were carried out using the binary suspensions prepared by polymethyl methacrylate (PMMA) particles and the Dextran macromolecules. The filtration rates, the rejections of Dextran and the cake properties under various operating conditions, such as the cross-flow velocity and the filtration pressure, are measured and discussed. The filtration rate increases with increases of the cross-flow velocity and the filtration pressure but decreases when the molecular weight of Dextran is increased. On the other hand, the rejection of Dextran increases with the increase of filtration pressure but decreases with the increase of cross-flow velocity. The standard capture equation for deep-bed filtration is adopted to relate the rejection of Dextran and the properties of dynamic membrane. The results show that the mass of the dynamic membrane is the most important factor on the rejection of Dextran. Using the experimental data obtained under various cross-flow velocities, the rejection of Dextran can be regressed as a single function of cake mass. It can be concluded that the rejection of macromolecules increases due to the presence of fine particles, and that the magnitude of the rejection increases with increasing the mass of cake.

Stress distribution in a confined wet cake in the compression—permeability cell and its application

Abstract Stress distribution in a confined wet cake is analysed by solving the equation of equilibrium of force acting on the system. An integrated equation is derived to estimate the stress distribution within the cake providing the boundary values around the cell are known. The results are applied to estimate the mean stress acting on the confined cake and also to correct the effect of side-wall friction of the compression — permeability (C—P) cell tests. These corrected data are then used to predict the performance of constant-pressure filtrations. From the results of these predictions, it is indicated that the simulated results based on the corrected C—P cell test data may give an average deviation around 5%, while the simulated results based on the CP cell test data without correction give a deviation higher than 10%.

Cross-flow microfiltration of submicron microbial suspension

The mechanism of cross-flow microfiltration of submicron microbes is studied. Experiments are carried out using pseudomonas suspensions. The effects of operating conditions on the filtration performance are discussed. The results show that the filtration rates increase with increasing the cross-flow velocity, while decrease with increasing the filtration pressure. The Brownian dynamic simulation method is adopted for simulating the trajectories of microbes in cross-flow microfiltration. The deposition flux of microbes on the membrane surface can then be estimated from the simulated trajectories. Based on the continuity equation of cake compression and Kozeny equation, a dynamic analysis method is developed for simulating the distributions of local cake properties, such as porosity and specific filtration resistance in the filter cake. The deformation of microbes, the area contact between microbes and the creeping effect of cake compression are taken into consideration. The simulated values of average cake properties agree with the experimental data. It can be found that a skin layer controlled the filtration rate will form next to the membrane surface. Although the thickness of the skin layer is only about 10% of the whole cake, it plays the major role on the filtration resistance. Increasing the filtration pressure results in more compact skin layer; therefore, a lower filtration rate is obtained. Moreover, a higher cross-flow velocity will results in a thinner but more compact cake, a higher filtration rate can be given.

Cross-flow microfiltration of dual-sized submicron particles

The effects of particle size distribution on the cake properties and the performance of cross-flow microfiltration of dual-sized particles are studied. An equation based on the force analysis at the critical condition of particle deposition is derived to relate the filtration rate and the cake properties. The packing porosities of dual-sized particles under various mixing fractions are predicted theoretically in accordance with two limiting conditions, cavern and displacement effects, and are compared to the simulated results and experimental data. The results show that either the theoretical predictions or simulation results agree with the experimental data except in the region near the lowest packing porosity. There has been an overestimation on cake porosity using simulation method and an overestimation using theoretical prediction about 30% near the lowest packing porosity. The average specific filtration resistance of cake can be estimated accurately by substituting the average particle diameter based on the surface area and the Kozeny constant calculated from the cell model into the Kozeny equation. The increase in the mixing fraction of large particles results in a decrease in specific filtration resistance of cake but in an increase in the cake mass. Therefore, the pseudo-steady filtration rate increases with an increase in the mixing fraction of large particles. Once the values of porosity and specific filtration resistance of cakes formed by mono-sized particles are known, the cake properties and the pseudo-steady filtration rates for various mixing ratios of dual-sized particles can be estimated using the proposed theory. The agreements between the calculated results and the experimental data demonstrate the reliability of the proposed method.