Wei-Ming Lu

Selective Particle Deposition in Crossflow Filtration

Abstract To study the mechanism of particle deposition in crossflow filtration, hydrodynamic forces exerted on a spherical particle touching the surface of filter medium are analyzed to derive the critical selective cut-diameter of the deposited particles under various crossflow velocities and filtration rates in a crossflow filtration system. Experimental data of turbulent crossflow filtration of dilute light calcium carbonate suspension agree with the prediction of this theory within 30% error under the crossflow velocity of from 0.57 to 1.14 m/s. Equations to estimate the characteristics of crossflow filtration, such as steady-state filtration rate and average specific resistance of cake, are also presented.

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.

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.

Effects of baffle design on the liquid mixing in an aerated stirred tank with standard Rushton turbine impellers

Abstract The effects of width and number of baffles in mechanically agitated vessels with standard Rushton turbine impellers are examined for systems with and without aeration. The insertion of the appropriate number of baffles clearly improves the extent of liquid mixing. However, excessive baffling (i.e. n b > 8or B/T> 0.2 ) and sparging gas through the impeller would interrupt liquid mixing and lengthen the mixing time. This study found that sparging gas through the impeller leads to an increase in the mixing time of more than 20% because it reduces the liquid pumping capacity of the impeller. A numerical technique was applied to examine the same effect on several extreme baffle conditions and the cases of higher gas flow rates and rotational speeds. To generalize our results obtained, the numerical technique was applied to simulate for a system with triple impellers. The trends of the mixing time were found to be very similar to the single impeller system. By correlating the mixing time with n b , B/T, Q g and N , the following correlation is obtained for the system with single Rushton turbine impeller under non-gassed and aerated systems.Nt M = 55.7(n b ) −0.30 ( B/T ) −0.1535 ( Q g /ND 3 ) 0.0296 and the similar correlation for the triple impeller system can be given asNt M = 46.5(n b ) −0.295 ( B/T ) −0.327 ( Q g /ND 3 ) 0.010

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.

Studies on the mechanism of cross-flow filtration

Abstract A hydrodynamic mechanism of slurry particles depositing onto a filter septum in a turbulent cross-flow filtration is proposed. The distribution of fluid velocity and the particle concentration within the cross-flow filtration system are numerically analyzed to study the rate of particle deposition, the growth of the cake, and the attenuation of filtration rate during operation. The effects of the operating parameters on the characteristics of the cross-flow filtration are discussed.

Methods to determine the relationship between cake properties and solid compressive pressure

This paper summarized the methods used to determine the relationship between cake properties and solid compressive pressure. The dynamic simulation method proposed by Lu and Hwang (Mechanism of cake formation in constant pressure filtration, Separation Technol. 3 (1993) 122–132) is modified by considering the variation of the pressure drop across the septum and specific surface area of particles. The importance of the accuracy of the value of ϵi is also examined, and the effects of the operating condition, such as pressure and slurry concentration, are also discussed. The results of the obtained relationships between ϵ vs. Ps and α vs. Ps for different compactness materials through the proposed dynamic simulation method give the results very close to the corrected CP Cell test data. To compare the reliability of the obtained local cake properties vs. Ps by various methods, the results obtained for different compactness materials are used to predict the performances of the constant pressure filtrations. The results of the dynamic simulation can give the best prediction with an average deviation around 4%, while the corrected CP Cell test data may give an average deviation around 5%, and the uncorrected CP Cell data gives an average deviation around 10%.