Kemerovo, Kemerovo, Russian Federation
The possibility of intensifying the ultrafiltration concentrating of food substance solutions by the separation of the near-membrane flow part that comprises the concentration boundary layer (or diffusion layer) enriched by a useful component has been investigated in this study. A mathematical model of the longitudinal development of polarization on a membrane with consideration of its selectivity (rejection coefficient) has been proposed. The efficiency of the separation of the near-membrane layer has theoretically been estimated on the basis of this model. Some constructions of membrane modules with the separation of the near-membrane layer have been proposed. Experiments have shown that the proposed method allows the concentrate to be enriched in the continuous-flow module by 9‒10%, which is much higher than for the traditional concentrating process. The calculated concentration coefficients are in good agreement with experimental values.
ultrafiltration, concentrating, intensification, concentration polarization, rejection coefficient, concentration coefficient.
INTRODUCTION
The development of simple and economical methods for the separation, purification, and concentrating of liquid media is one of the most important problem in the food industry and, especially, in the dairy industry. Membrane technologies, which have a number of advantages in comparison with traditional separation methods, are especially noteworthy [1, 2]. This explains a profound interest in membrane processes, to which a considerable number of theoretical and experimental studies have been devoted.
However, membrane methods have some disadvantages reducing the efficiency of the process. The most essential of them is the formation of the diffusion boundary layer with an increased concentration of rejected substances on the membrane surface (i.e. concentration polarization), which promotes the formation of a gel layer hindering the removal of a solvent.
The weakening of concentration polarization is a traditional way of increasing the efficiency of membrane equipment [3]. It is attained via the turbulization of a flow with mechanical turbulizers [4‒7] or gas sparging [9‒13] or via the physical effect on a flow with mechanical vibrations [14‒16], ultrasound [17‒20], or an imposed electrical field [21]. All these methods lead to additional expenditures, complicate the structure of an apparatus, degrade the quality of a processed product, and increase its cost.
In this work, we consider the possibility of the intensification of ultrafiltration concentrating by the separation of the near-membrane part of a solution flow as a resulting product. It comprises the diffusion boundary layer, the concentration in which is appreciably higher than in the major part of a flow (flow middle). This enables the obtaining of a highly concentrated product at lower energy consumption, since a solution flows in the laminar regime.
The objective of our work is to perform the theoretical and experimental analysis of the efficiency of the separation of the near-membrane layer, to estimate the effect of geometrical and regime parameters, and to describe the technical implementation of the proposed idea.
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