| Supercritical water oxidation is a new technology which is used for organic waste harmless treatment or producing multiple thermal fluids used for oilfield injection production. Transpiring wall reactor is the most effective solution to solve both the corrosion and plugging problems in the supercritical water oxidation process. However, corrosion and plugging still exist in the preheating section during the transition from subcritical to supercritical condition, and the reliability of the system is greatly reduced. The available experimental and industrial reactors are usually designed by the residence of the feed. However, other critical design parameters are neglected, such as:fuel type, degradation efficiency, the protection of the reactor wall by water film, the ignition and extinction characteristics of the feed, the dissolution and discharging of the inorganic salt, causing the irrational reactor structure. Thus, an inner preheating transpiring wall reactor for supercritical water oxidation was designed. The preheating of the feed can be transited from the exterior to the interior of the reactor. Therefore, the feed can be injected into the reactor at room temperature. Experimental study was conducted to investigate the ignition and extinction character of the hydrothermal flame inside the reactor, and the operating range of a stable operation were obtained. The effect of the operating parameters on the temperature profiles and the products of the reactor outlet were investigated to gain the optimized operating conditions. A computational fluid dynamic model of an inner preheating transpiring wall reactor for supercritical water oxidation was developed based on the real size of the reactor in the experiment. The effect of the operating parameters on the flow field of the reactor was analyzed, and the product characteristics of the reactor outlet at the critical condition were specially discussed. The influence of the reactor structure (reactor diameter and reactor length) on the temperature profile and product was also studied. A new method using the heat load parameters (the sectional heat load and the volume heat load) as criterion for the design of the transpiring wall reactor was proposed. And heat load data at certain conditions were obtained.An inner preheating transpiring wall reactor for supercritical water oxidation was firstly designed. A coaxial burner was introduced base on the structure of the transpiring wall reactor. The feed was preheated and ignited by the entertainment of the auxiliary heat source, and the preheating of the feed was transfered from the exterior to the interior of the reactor. Thus, the feed injected into the reactor at room temperature can be realized, and the problem of the corrosion and plugging in the preheating section can be solved.The prerequisite for the injection of the feed into the reactor at room temperature is the formation the hydrothermal flame inside the reactor. The ignition process of the hydrothermal flame accompanies by the sharp increase of the reaction temperature Trl and the concentration of CO2in the gaseous effluent, and the sharp decline of the concentration of TOC (Total Organic Carbon) in the aqueous effluent and CO in the gaseous effluent. It can be concluded that the most important prerequisite for ignition is the existence of a single-phase mixture at the outlet of the burner. The extinguishing process is on the contrast to the ignition process. The extinguishing process of the hydrothermal flame accompanies by the sharp decline of the reaction temperature Tr1and the concentration of CO2in the gaseous effluent, and the sharp increase of the concentration of TOC in the aqueous effluent and CO in the gaseous effluent. The stable hydrothermal flame can be kept even when the auxiliary heat source is lower than the critical temperature of the water. So it can be deduced that heat can be transfer from the downstream of the hydrothermal flame to the two-phase zone at the outlet of the burner by recirculation mixing and interdiffusion of species.The ignition temperature can be lowered when the increases of the feed concentration, the flux ration of the feed flow and the auxiliary heat source and the feed flow increase. The extinction temperature will also decrease when the feed concentration and the flux ration of the feed flow and the auxiliary heat source increase, while the extinction temperature increases when the feed flow increases. The ignition temperature of450-600℃and the extinction temperature of250-400℃are present at feed concentration of25-45wt.%, feed flow of2-5kg/h and the flux ration of the feed flow and the auxiliary heat source of0.2-0.55. The result based on the study of the hydrothermal flame characteristics shows that the feed injected into the reactor at room temperature can be realized by the inner preheating transpiring wall reactor, and at the same time, the temperature of the auxiliary heat source can be lower than the critical temperature to keep the hydrothermal flame stable, thus reducing the energy consumption of the system. The character of the fuel can also affect the ignition and extinction temperature of the feed. Generally, lower ignition temperatures and extinction temperatures will be present when the reaction heats per unit mass of the fuel are higher. The exit velocity of the burner was accelerated by changing the size of the burner. The increase of the exit velocity at the same operating parameters can enhance the mixing of the feed, thus lowering the ignition temperature. However, the extinction temperature will be increased when the exit velocity increases, which show that low velocity of the feed is favor for the stability of the hydrothermal flame.The effect of the operating parameters on the temperature profiles and the products of the reactor outlet were investigated based on the stable operation, energy saving, degradation efficiency, and the optimized operating conditions were obtained.The temperature of the whole reactor increases when the flux ration of the feed flow and the auxiliary heat source increases, and the reaction temperatures linearly increase. Besides, the supercritical length and the useful reaction time will increase when the flux ration of the feed flow and the auxiliary heat source increases, accompanying by higher degradation efficiency. The proper flux ration of the feed flow and the auxiliary heat source for the system is0.3-0.5. The decrease of the flow of the auxiliary heat source can also increase the flux ration of the feed flow and the auxiliary heat source. However, the reaction temperature can reach nearly900℃, which may exceed the temperature-resistance of the reactor material and is not conducive to the safety operation, when the flow of the auxiliary heat source is lowered to2.79kg/h. The auxiliary heat source flow for the experiment is selected at6-14kg/h. The supercritical length and the useful reaction time will be reduced when the temperature of the auxiliary heat source declines, causing lower degradation efficiency. So, the auxiliary heat source temperature is usually controlled at480-550℃.The temperature inside the reactor will also increase when the feed flow increases. The reaction temperature will increase firstly with the increase of the feed, but to certain feed flow, the reaction temperatures rise slowly or even no longer increase for the high velocity of the feed. The useful reaction time will decrease when the feed flow increases, thus are not conducive for the degradation of the fuel. The TOC removal efficiency of more than99%can be obtained at feed flow of less than4kg/h. The increase of the feed concentration will also increase the temperate of the whole reactor. The reaction temperature increases significantly with the increase of the feed concentration. Besides, the useful reaction time can be longer at high feed concentrations, and higher degradation efficiencies will be present.The transpiring flow relation has little influence on the reaction temperature, but the increase of the transpiring flow relation will lower the temperature of the reactor and the useful reaction time. The proper transpiring flow relation for the reactor is0.8-1.2. The increase of the transpiring water temperature, the temperature inside the reactor and the useful reaction time will increase. TOC removal efficiency of more than99%can be achieved when the transpiring water temperature exceeds250℃. The effect of the type of the fuel on the reactor was studied with the solution of glycerin, methanol, ethanol, acetone and water as artificial fuel. The temperature profile was greatly influenced by the reaction heat per unit mass of the fuel. Higher temperature profiles will present at higher reaction heats per unit mass. Besides, the length at supercritical temperature and the useful reaction time will increase at higher reaction heats per unit mass, which is conducive for the degradation of the feed.The quantitative description of the degradation law of the fuel by useful reaction time can be expressed as:the concentration of TOC in the aqueous effluent of the reactor can be lower than50ppm with the corresponding TOC removal efficiency higher than99%, and the concentration of CO in the gaseous effluent is less than0.1%, when the useful reaction time exceeds10.5s.The experimental measurement inside the reactor is difficult for the higher pressure and high temperature. A computational fluid dynamic model of an inner preheating transpiring wall reactor for supercritical water oxidation was developed based on the real size of the reactor in the experiment. The finite rate model was adopted in the simulation. And some assumption and simplification was made:the oxidation of fuel was divided into two steps with CO as the main intermediate product. The simulation results show a good agreement with the experimental temperature profiles and the aqueous and the gaseous products. An important characteristic of the flow field inside the reactor is the existence of the recirculation areas due to jet entrainment and natural convection in the upper part of the reactor. It is likely that a continuous stirred tank reactor (CSTR) is formed in the upper part of the reactor due to the existence of the eddy. The eddy is beneficial for the mixing of the feed, but it can destroy the water film in the upper section of the reactor and suppress the oxidation of fuel.The transpiring flow relation and the transpiring water temperature are two important factors which affect the formation of the subcritical water film. Compared with higher transpiration intensities, lower transpiring water temperatures are more conducive to the formation of water film, accompanied by a more significant decline of oxidation efficiency of the feed. The effect of the feed concentration, the flux ration of the feed flow and the auxiliary heat source and the feed flow on the flow fields well agree with the experimental result at the same operating conditions. But we are more interesting in the temperature profiles on the inner wall of the porous tube in the simulation which can’t be measured in the experiment. So the critical condition was defined which represents the existence of the ideal water film on the inner wall of the porous tube (keep the temperatures of the inner wall of the porous tube less or equal to374℃). The useful reaction time will decline when the feed concentration, the auxiliary heat source and the feed flow and the feed flow increase, resulting in the decreased efficiencies of fuel degradation, and the increased concentration of TOC and CO. Besides, the effect of the type of the fuel on the flow fields also agrees well with the experimental results at the same condition. However, the TOC and CO in the effluent will increase when the reaction heat per unit mass increase at critical conditions.Considering the irrational aspects in the design of the experimental reactor, different reactor diameters and lengths were test by numerical method for the optimization of the reactor structure. The increase of the reactor diameter.will increase the transpiration intensity, thus improving the formation of the water film on the inner porous wall. Besides, the increase of the reactor diameter can also prolong the useful reaction time, thus improving the degradation of the feed. The shortening of the reactor length can also increase the transpiration intensity, which is beneficial for the formation of the water film. However, the useful reaction time will be shortened at shorter reactor lengths. It can be concluded that a proper reactor diameter and length will be present for a certain operating condition.A new method using the heat load parameters (the sectional heat load and the volume heat load) as criterion for the design of the transpiring wall reactor was proposed. The heat load data at critical condition are obtained at certain criterions:a useful reaction time for fuel of11s; the formation of the subcritical water film; a dissolution time for inorganic salt of6s. The sectional heat load will increase when the feed concentration, the flux ration of the feed flow and the auxiliary heat source and the feed flow increase. Besides, the sectional heat load will also increase when the reaction heat per unit mass of the fuel increases.The fitting result of the initial chemical oxygen demand (COD) of the fuel and their sectional heat load shows that the sectional heat load is in an exponential growth relationship with the initial COD, and their relationship can be expressed by the formation:qF=4.57x103COD0.12. The volume heat load will increase when the feed flow and the flux ration of the feed flow and the auxiliary heat source increase, but decrease with the increase of the feed flow. Beside, the volume heat load will also increase when the reaction heat per unit mass of the fuel increases. The fitting result of the initial chemical oxygen demand (COD) of the fuel and the volume heat load shows that the e volume heat load is in an exponential downward relationship with the initial COD, and their relationship can be expressed by the formation:qv,r=2.50×104COD-0.09+Δqv,r and qv,a=1.44x104COD-0.08+Δqv,a. |
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