Characterization Of Fluid Dynamics And Mass Transfer In An Electrochemical Oxidation Reactor For Organic Pollutants

Electrochemical oxidation means conversion or destruction of organic substrates in wastewater under the influence of the electric field. It can take place through two different oxidation mechanisms:(1) direct anodic oxidation, where the pollutants are destroyed at the anode surface;(2) indirect oxidation where a mediator is electrochemically generated to carry out the oxidation. Electrochemical oxidation method is a perspective technology for organic wastewater treatment because its advantages such as free secondary pollution, environmental compatibility, mild reaction conditions, easy automation, etc. However, the practical application of this technology still faces some urgent problems such as low current efficiency and weak mass transfer performance. The purpose of this thesis is accelerating the industrialization of the "green" electrochemical wastewater treatment technology. Aiming to the above existing problems, we make a series of fundamental engineering researches on the electrochemical reaction kinetics for organic pollutant degradation, flow characteristic and mass transport process in the electrochemical reactor.(1) Application of electrochemical oxidation technology for practical chemical produce wastewater treatment Good effect was achieved by the treatment of reverse osmosis concentrated wastewater from refinery and saponification wastewater from propylene oxide production using two-dimension or three-dimension electrochemical reactor. Type of electrochemical reactor, current density, electrolysis time, electrolyte flow rate, electrode materials and their opposite area were important factors that effects the organic pollutant degradation results. However, some problem such as high energy consumption, block of three-dimension electrode, mechanism and kinetic of organic degradation was appeared. The solution of these problems needs our theoretical study about electrochemical engineering.(2) Kinetic of organic electrochemical oxidationThe whole degradation process of oxalic acid in a cylindrical electrochemical reactor with active Ti/IrO2-Ta2O5 anode was studied. The relation between instantaneous current efficiency and organic concentration was established base on the two paths for direct electrochemical oxidation, and the kinetic model was verified by experiments. The experimental results are consistent with the established kinetic model. Furthermore, the competition between the two paths was explored with model parameter analysis.The indirect electrochemical oxidation process of oxalic acid under the condition of chloride ion existence was studies. Organic degradation in this system is a typical series electrochemical-chemical process. When the amount of chloride ion is too little, the current efficiency is very low, that is because the reaction system occurs mainly direct electrochemical oxidation or oxygen generation. Only the indirect electro-oxidation reflects significant degradation effect when organic wastewater containing some amount of chloride ions. The variety of pH showed that, electrolyte maintains strong acid until the removal percentage of oxalic acid larger than 80%.The indirect electrochemical degradation of oxalic acid is agreed with the role of first order reaction kinetics. The value of the Hatta number, which approximated to 0.1 indicates that, most of the reactions between the organic and the active chlorine take place both in the bulk solution and in the reaction zone near the anode.(3) Flow characteristics and mass transfer performance of electrochemical reactorThe residence time distribution in a laboratory scale filter-press type electrochemical reactor was determined using common impulse-response technique. The phenomenon of tailing indicated the existence of "dead zone" in the reactor. This phenomenon reduces and even disappears at high flow rate. The two peaks for RTD curves can be well expressed by the composite flow model about two paths. And path-1 is the flow moves through the holes of the electrode; path-2 is the fluid flows along the internal walls of the filter-press type electrochemical reactor. The effect of flow rate and electrode configuration on the flow model parameters was investigated. Because of the enhancement of convection and the decrease of boundary layer thickness, the parameters for path-2 are significantly influenced by flow rate. And the model parameters of path-1 reflected the effect of electrode configuration on the total flow characteristics.Determination of mass transfer coefficient for vertical tubular electrochemical reactor with parallel electrodes has shown that mass transfer coefficient increased with the decrease of expansion ratio, but it is little related to distribution and amount of circular arranged holes in baffles. The CFD simulation result showed the appearance of jet flow downstream the sudden expansion entrance. In effect, the high rates of shear induce the rotation of the solution and small eddies are formed, the level of electrolyte turbulence enhances and plays a role in improving mass transfer performance.The study of mass transfer enhancement for mesh electrode in a tubular electrochemical reactor indicated that, the insert of mesh electrode has changed the flow pattern from laminar to turbulence, and the thickness of boundary layer gets smaller. This leads to the improvement of force convection and enhancement of mass transfer performance.The comparison of CFD simulation and experimental results showed that the assumption related to Nernst diffusion layer is correct. The single-phase CFD simulation method is a useful tool to describe the flow characteristic and predict the local mass transfer performance in such an electrochemical reactor. In this thesis, a kinetic model described the whole degradation process of organic pollutants was established based on the mechanism of electrochemical oxidation. And this model overcomes the shortage of former literatures discussed by control steps. Furthermore, according to the special requirements for electrochemical oxidation reactor, we have raised a novel method investigating the electrolyte layer near the electrode. These studies have provided some useful information on the selection, design and optimization of electrochemical oxidation reactor for organic pollutants.