Improvement of filtration kinetics by pressure electrofiltration

摘要

The requirement to dewater finely dispersed products is rising world-wide. Because the flow resistance increases with decreasing particle size, the mechanical dewatering of fine particle suspensions is time-consuming. The electrofiltration potential increases with the particle surface area. Therefore the combination of mechanical and electrical filtration seems to be an effective method to enhancethe filtration kinetics. Experiments at the Institute of Mechanical Process Engineering and Mechanics (MVM) at the University of Karlsruhe (TH) shwed that this combination results in an acceleration of the filtration kinetics Thus the filtration time was less than half compared to traditional pressure filtration. If an electric field is applied in addition to pressure dewatering, four additional effects occur. On the hand, the electrokinetic effects of electroosmosis and electrophoresis benefit filtration. The electrophoresis decreases the migration velocity of the particles. The electroosmosis moves the surrounding diffusive layer of the particles and promotes the filtration flow. On the other hand, electrolysis and a decrrease in thermal viscosity occur. As a result of the applied electric field, the fluid is electrolytically decomposed and electrolytic gas is generated. This gas displaces water but also increase the electric esistance between the electrodes. The electric resistance of the bulk converts electrical energy into thermal energy, resulting in a temperature rise of the suspension or filter cake. An increasing temperature causes a lower viscosity and easier dewatering. These single effects and their influence on the process have to be known in order to succeed in industrial implementation of the process. The dewatering effect of the generated electrolytic gas can be calculated by a continuity balance. The change in viscosity can be eliminated by knowledge of the temperature. Based on the root equation of cake building filtration, Yukawa developed an equation for pressure electrofiltration. The electrokinetic effects can be separatedusing this equation. The electroosmotic pressure and the electrophoretic coefficient can then be calculated if the filter media and bulk resistance are known. Experiments on a specially constructed pressure/electro filter showed that the acceleration of the filtration kinetics is mainly caused by electrophoresis. Electroosmosis only has a little effect. The bulk resistance, which mainly influences the filtration kinetics, will be abated by the electrophoretically reduced particle velocity.