A New Design of a Micro-Channel Cross-Flow Ultrafiltration Module with a Staggered Arrangement of Membranes

Cross-flow ultrafiltration of macromolecular solutions in a module with micro-channels is expected to have the advantages of fast diffusion from the membrane surface and a high ratio of membrane surface area to feed liquid volume. In addition, high shear stress will reduce the degree of fouling of the membrane. Though such kinds of ultrafiltration modules are expected to be used for separation and refining and as membrane reactors in micro-chemical processes, there have been few papers on their performance. In the previous studies of the authors [1], [2], micro-channel filtration modules were fabricated and the experimental investigation on the relationships between permeate fluxes and operational conditions of transmembrane pressure and feed flow rate was made. The modules consisted of feed liquid side and permeate liquid side channels. Membrane was put between two channels and experiments of cross flow ultrafiltration were carried out. PolyVinylPyrrolidone (PVP) aqueous solution was used as a model solute of macromolecules such as enzymes. Changes in permeate flux with time were obtained under various conditions. Based on the results, the relation between the permeate fluxes and the operational conditions including the length of the channel, were discussed. On the basis of the experimental results, a model on the transport phenomena considering the effect of the concentration polarization was proposed. By means of the model, the permeate fluxes under various conditions were estimated. From the model, the effects of the axial convection and the osmotic pressure difference were expected to be important factors in the permeation process. As increase of the concentration of the feed liquid in the axial direction, permeate flux decreased in the downstream region of the channel because of the effect of the concentration polarization and high osmotic pressure. Average permeate fluxes over the whole module were, therefore, not in proportion to the length of the channel. Considering the results, it is indispensable to reduce the concentration at the surface of the membrane in order to make the average permeate flux higher in the downstream. Though it might be effective to put a mixing device e.g. a static mixer, in the micro-channel, pressure loss through the device would become large. An alternative idea is to reverse the direction of the permeation. In the previous module, the membrane was put on the bottom side of the feed liquid channel. As a result, the feed liquid close to the membrane was concentrated extremely in the downstream of the micro-channel. In this study, a new module with a staggered arrangement of the membrane on the top and the bottom sides of the feed liquid channel was fabricated. The permeation direction changes at the axial position in the channel, where the side of the membrane changes from the bottom to the top, and vice versa. As results of the experiment, the permeate flux of the new module was larger than that of the previous one with the membrane set only on the bottom side. It appears that the staggered arrangement is effective to cancel the concentration polarization.