| Along with the rapid development of agriculture and industry in China, all kinds of high-concentrated hard-degradation liquid waste containing toxic and harmful organic matters have correspondingly increased, which has do great harm to the environment. As an emerging green technology of water treatment, the technology of supercritical water oxidation (SCWO) can completely oxidize the hard-degraded organic matters in the liquid waste, and the post-treated gas-liquid product has little pollution, which is regarded as the most promising liquid waste treatment technology. However, the industrialization extension has been influenced severely by the corrosion and plugging in the SCWO reactor. Due to its benign anti-corrosion and anti-salt deposit performance, the transpiring wall reactor has become the primary type of reactor in the SCWO system. Currently, the study on the transpiring wall reactor mainly focuses on the treatment effect of the organic matters and the stability of the system in the long-term operation. It is crucial that whether the water film layer at sub-critical temperature exists at the internal face of the porous tube in the transpiring wall reactor, which determines the anti-corrosion and anti-deposit performance of transpiring wall reactor. Currently, there are few studies on the factors that influences the formation and thickness of water film layer with sub-critical temperature on the internal face with porous tube, and no precise operation guidance has been shaped to such a reactor.SCWO reaction is an exothermic reaction. When the organic concentration is higher than a certain value, the system will realize the heat auto-compensation, and the excess heat can be recycled. Therefore, it is necessary to analyze the energy utilization in the SCWO reaction system, discuss the operation cost of the liquid waste treatment and seek for the approaches which improve the economic of the system so as to boost the development of industrialization of this technology.Pointing to the above situation, the SCWO reaction system majoring in transpiring wall reactor has been firstly established, meanwhile, the energy recycling in the system has been taken into consideration. The definition of transpiring water layer has been introudced into the design and operation of the transpiring wall reactor, the top transpiring water has been injected into the reactor at sub-critical temperature and the water film layer forms at sub-critical temperature in the internal face of porous tube, which can ensure the oxidation effect in the reaction area. The bottom transpiring water enters the reactor for refrigeration with normal temperature, which is to ensure the outlet temperature of the reactor is lower than the sub-critical temperature of water, that is, the sub-critical soluble salt area forms at the bottom of the reactor. In order to recycle and utilize the heat generated by the reaction, the reaction products with high temperature and pressure in the outlet of reactors will primarily be split-flow to preheat the organic liquid waste and transpiration water respectively through two heat exchangers and then they mix and heat up the heating hot water through the third heat exchanger.The systematic experiment has been performed with the way that simulates the organic liquid waste with methanol water solution and the oxidant with oxygen, which has researched on the items as follows, that is, the consequences of all the operation parameters for the strainaway rate of the organic matter, the element of gas phase product, the temperature distribution of the axial in the reactor, the effective remaining length and time of the supplies. (The parameters mainly include the liquid waste flow, the temperature of liquid waste in the reaction inlets, the organic matter concentration in the liquid waste, the water flow of the evaporation, and transpiration water temperature in different layers). From the perspective of treatment effect and capability of the system, the stable operation and energy conservation, the proper operation parameters have been determined. The experiment study result has proved that the TOC strainaway rate relies on the highest temperature and the effective residence time in the reactors, in order to acquire the TOC strainaway rate as high as 99%, the effective residence time should keep more than 15s. The methanol water solution in the reactor outlet is the key issue that influences whether the SCWO has reacted in the reactors. SCWO reaction will not work until the temperature of the methanol water solution in the reactor inlet is higher than 369℃, the degradation effectiveness of TOC will be over 99%. The concentration of the methanol water solution has a linear function relationship with the highest temperature of reaction, the higher the concentration is, the higher the reactive temperature and the strainaway rate of TOC is. When the concentration is less than or equal to 2%, the highest temperature of the reaction is less than 500℃, and the residence time is less than 15s, and the methanol cannot degrade completely. However, if the concentration increased to 8%, the highest temperature of reaction will increase rapidly to over 700℃. If the concentration is over-high, the overheat phenomenon existing in the top of the reactors will not do good to the long-term operation, finally, the proper concentration of the methanol water solution will be selected within 4%～6%. If the flow of the methanol water solution raises, the effective residence time of the materials will drop evidently, and the organic matters will not be degraded completely. If the flow of liquid waste in the experiment system is less than 14kg/h, the degraded effectiveness of the TOC will reach 99%, the evaporativity and the transpiration water temperature in the inlet of reactor will have little influence on the axial temperature distribution in the reactors and the degraded effectiveness of TOC. The evaporativity in the experiment system is within 0.04-0.08, and even if the temperature of the middle-layer evaporation water in the inlet of the reactor drops to 105℃, the strainaway rate of TOC will reach to more than 99%. Along with the rising temperature of the transpiration water in the top layer, the effective residence time of materials will increase, more than 99% of the TOC degrading efficiency will be gained only when the temperature of transpiration water in the top layer is higher than 285℃.The precise measurement of the radial temperature in the reactor will be hard to achieve in the process of experiment, and the online measurement of the water film thickness cannot be achieved, therefore, the proper physical model and mathmatical model has been determined in accordance with the material flow and reaction characteristics in the transpiring wall reactor; the temperature field distribution and water film thickness distribution within the transpiring wall reactor have been calculated and studied with the help of FLUENT 6.2 commercial software.The RNG k-εmodel has been applied into the flow model in simulation, and the oxidizing reaction with single step has been taken; the Arrhenius rate and mixed rate are calculated by the finite rate/vortex dissipative model with thin layer, and the smaller one of them has been applied, in addition, the Darcy law should be obeyed within the porous area. In order to gain the astringency of the calculation, the physical parameter of the water near the sub-critical points will be calculated with the peak linearization. The structure size of the physical model of transpiring wall reactor is completely in accordance with the actual reactor size in the experiment system. The transpiring wall reactor has a structure with axial symmetry; in order to reduce the calculated amount, the half of the model has been taken for calculation and analysis in the paper. Finally, the results of the axial temperature distribution in the reactors acquired from the simulation calculation and experiments have been compared, and the alteration trends is almost the same, and the error is less than 8%.The influence law of all the operation parameters on the temperature filed distribution and water film thickness in the reactor has been determined by the numerical value simulation of the water film reactors, and the parameter selection in the process of calculation is determined by the reference to the experiment results. The calculation result has proved that the influence law of all the operation parameters on the axial temperature distribution within the reactors have completed identical to the experiment conclusions, the water film thickness of the reactors will go through the process that low firstly, even low to the minus value, then rise gradually, the areas that existing minus value is only in the transpiration water area in the top layer. The minus value existing in the water film thickness has proved that the temperature within the porous tube has surpassed the sub-critical temperature of the water. The temperature of the organic solution in the inlet of the reactor has little influence on the changes of water film thickness, as long as it can guarantee the starting performance of SCWO reaction. The organic solution’s concentration, flow and the temperature of evaporation water in the middle layer has little influence on the size of the water film thickness in minus value areas; within the area of water film layer at sub-critical temperature, along with the lower of the concentration and flow as well as the drop of the transpiration temperature in the middle layer, the thickness of water film will increase correspondingly. And the specific selection boundary will be in accordance with the experiment results and the strainaway rate of TOC should be also taken into consideration. Along with the uprising of the transpiration water flow in the top layer, the areas of water film thickness with minus value have gradually decreased, which has little influence on the water film thickness at the bottom of the reactor. The transpiration water temperature has the most powerful influence on the water film thickness in the operation parameters; along with the drop of the transpiration water temperature in the top layer, the areas of water film thickness with minus value have decreased evidently, and the water film thickness at the bottom of reactors has rise evidently. When the transpiration intensity is 0.06 and the transpiration water temperature in the top layer drop to about 250℃, the thickness of water film is positive value, which suggests that the water flow of the porous wall of the whole reactor is sub-critical temperature. However, according to the experiment results, when the temperature is over-low, it will impact on the strainaway effectiveness of the TOC. After having determined the temperature, flow and organic concentration in the liquid waste inlet, and the intensity and temperature of the transpiration water in the middle and bottom layer, and when the water film thickness emerges the sub-critical value, a linear relationship will present between the transpiration water temperature and the transpiration intensity in the top layer.After the influence law of all the operation parameters on the water film thickness has been determined, what should be done is to establish the relevance equation between all the parameters or between the reaction structure and the water film thickness, determine the conditions to form the specific sub-critical water film layers to guide the design and correct parameters selection of the transpiring wall reactor system. According to the basic principle of material heat exchange within the transpiring wall reactor, the basic equation for heat exchange between materials has been deduced. Through simplifying and combining the numerical value calculation, neglecting the parameters that has little influences on the water film thickness, then the mathematical relations between all the parameters is established and the conditions that form the sub-critical water film layers is determined. The change of sub-critical water film layer in the internal face of porous tube is caused by the exchange between the transpiration water and the mainstream materials, on the one hand, the transpiration heat exchange is caused by the partial pressure between mainstream materials and transpiration water; on the other hand, the touching heat exchange is caused by the temperature difference between mainstream materials and transpiration water.Whether the temperature in the outlet of transpiring wall reactor is lower than 374℃of the critical temperature of water, which depends on the transpiration water with normal temperature in the low layer; the transpiring wall reactor has been selected as the controlling volume in the paper, according to the quality balance and energy balance, and the mathematical relevance between the transpiration water and other parameters in the systems have been determined, that is, the conditions that form the sub-critical salt-solutions areas within the reactors, so as to guide the selection of the transpiration water in the bottom layers.In order to discuss the application outlook of the SCWO energy recycling system based on the transpiring wall reactor, the availability of the energy and treatment cost in the system has been calculated and analyzed, which has suggested t hat the availability of the system can reach 54.06% with the optimized parameter in the experiment and simulation research, and the energy loss caused by the depressurization of back pressure valve reaches about 20%. In order to improve the economy of the system, the further research should be done on the available approaches to using pressure energy of the system. The economic analysis on the liquid waste treatment with 300m3/d, the COD in liquid waste with 40000mg/L has proved that, the treatment cost of system is 33.05￥/t, in which the cost of oxygen has covered 71.8% of the total cost. In order to improve the economy of the system, it is necessary to seek for new oxidant application method to reduce the operation cost of the system.Having kept the high TOC strainaway rate in the system, according to the determined conditions that form the sub-critical water film layers, after the best operation parameters have been selected for the conditions that form the sub-critical salt solutions areas, transpiring wall reactors can effectively relieve the corrosion and salt deposits of the SCWO system; along with the development of the application manner of system pressure energy and the optimization of the application manner of oxidant, SCWO technology has widely application for dealing with the organic waste with high concentration and tough degradation. |
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