Membrane filtration systems are used in a variety of processing industries where their performance meet and exceed the requirements in cost and quality. However, it is a challenge to design a small pore-size membrane system that treats very concentrated, large-volume streams within a reasonable time period. In the processing industries, several membrane technologies are used to separate various fluid streams where the concentrate or filtrate contains high-value products. Nevertheless, pore blocking is one of the major factors determining the applicability, efficiency and performance of the membrane filtration and separation system. Inside and outside membrane pore blockages lead to concentration polarization and cake buildup that reduces the flux rate and increases losses in system efficiency. There are four pore blocking mechanisms identified and modeled (complete, standard, intermediate and cake). Several experimental and theoretical works exist that describe the pore flow and blocking process. Depending on the processing fluid and membrane characteristics, all or some of the blocking mechanisms will be exhibited during the filtration process. Understanding the fluid and membrane size and characteristics in addition to pore blocking mechanisms is very important to designing effective membrane filtration systems that overcome the drawbacks associated with membrane performance. Furthermore, developing a membrane filtration system with a target cleaning process that controls membrane performance declines and maintains a reasonable flux for an extended period of time requires understanding and identifying the cause of membrane blocking. In this study, the membrane blocking during the filtration process was investigated experimentally. The experiment was designed to simulate the characteristics of a fluid stream encountered in food processing. The higher concentration was selected to manage the experiment time as well as to address worst-case scenarios, while the lower concentrations were selected to manage the filter area reduction. Dead-end filtration of two yeast solution concentrations were filtered through two different filter areas. In addition, the dynamic tests were conducted with shear generated using an impeller operated at various rotational speeds. Several tests were performed and the filtrate volume, time, pressure and agitation rate were recorded. The volume was measured with a graduated cylinder and the time measured in seconds. The results show the membrane blocking process is significantly affected by the membrane and fluid characteristics. The plots of pore blocking models and the experimental membrane filtrate data show the dominant pore blocking observed for both filters and flow process is cake filtration. The side-by-side comparison also indicates that the dominant pore blocking mechanisms depend on time. Thus, the initial and final pore blocking may not be attributed to the same pore blocking mechanism. Although it cannot be clearly shown from the current study, some part of the experimental flux profile may also be shaped by the combined pore blocking effects.
Effects of Particles Diffusion on Membrane Filters Performance
