In the process of crossflow microfiltration, a deposit of cake layer tends to form on the membrane, which usually controls the performance of filtration. The previous studies about cake formation focused on the deposition of monodisperse suspension and cake formation although most real fields are at the mercy of polydisperse condition.
Cake formation at polydisperse condition has been studied experimentally and compared with that at monodisperse condition reported in previous researches. Kaolin and two kinds of $CaCO_3$ are used as particles and polysulfon hollow fiber microfilter is used as membrane for the experiment. This study evaluates the effects of particle size, shear rate, and feed concentration which affect cake formation in crossflow microfiltration. As the results of the evaluation, this study identifies the factor more critical to cake formation and suggests the strategy to reduce cake layer.
In order to study cake formation at polydisperse condition, the study starts from the following hypothesis. "Polydisperse characteristics in cake formation are generated from monodisperse characteristics."
The mass transfers of particles are different according to there sizes at polydisperse condition, but there is no representative particle size that represents the characteristic of cake formation well at polydisperse condition. EDSD(Effective Diameter for Shear induced Diffusion) is introduced as a representative particle size to estimate the size distribution of particles in terms of shear induced diffusion. Statistical analyses show that it can describe the effect of particle size on cake formation at polydisperse condition well.
At polydisperse condition, the effect of shear rate and feed concentration on cake formation are similar to those at monodisperse condition. Cake formation increases with the decrease of shear rate and the increase of feed concentration. But polydispersity makes the quantitative differences of the effects between monodisperse and polydisperse condition. Especially the effect of shear rate is less sensitive than that at monodisperse condition because of polydisperse effect. Polydisperse effect here means that the polydisperse particles whose diffusion coefficients are over a certain critical value have same diffusivity.
Comparing all the effects of particle size(EDSD), shear rate, and feed concentration on cake formation, this study indicates that EDSD is the most influential factor for cake formation at polydisperse condition and suggests that increasing EDSD is the most efficient way to decrease cake formation. So the focus of this study moves to the reduction of cake layer by coagulation which is a way of increasing EDSD.
Cake layer in crossflow microfiltration can be reduced by coagulation of particles. This is because enlarging particle size by coagulation increases shear induced diffusivity and the back transport of rejected particles.
Flocs made of several particles may be broken when they are passing through a pump. This study is to look at the floc breaking phenomenon in relation with cake layer reduction experimentally. Kaolin is used as the particle and aluminum sulfate as the coagulant for the experiment.
The reduction of cake resistance by coagulation is observed in the range of 17% to 53% at various coagulation conditions. The results of particle size analyses show that the aggregated particles in feed are completely disaggregated by the pump(this result is valid in our case; the degree of floc breaking increases with the increase of the size of particle which makes floc.) but re-aggregated in the membrane module. This suggests that the re-aggregation of particles is critical to cake reduction and flux enhancement.
Re-aggregation is proved to occur at the movable cake layer in the vicinity of membrane. Charge neutralization of particles is better for re-aggregation at the movable cake layer than sweep flocculation although it has some drawbacks in operation.
The results of this study show that rapid mixing and charge neutralization of particles are important to reduce cake layer more efficiently. Inline mixer system is thus expected as a good system for coagulation-crossflow microfiltration and the effectiveness of inline mixer system is partially proved in our experimental condition.