Supercritical water oxidation is an efficient process to treat hazardous organic compounds. The process is carried out at pressure P>22.1 MPa and temperature T>647K. The destruction rate of this process is as high as 99.9% without any production of nitrogen oxide (NOx) compounds. The reaction is not only rapid but also confining. Its performances are limited by salt precipitation and corrosion. So there was need to develop new reactor to solve these problems. It consists of a concentric double wall reactor in which the corrosive reactants are maintained inside an alumina porous tube where as pressure resistance is ensured by a stainless steel external vessel. A water flow through the porous pipe prevents sticky solid particles from depositing on the wall. The performances of this reactor were investigated at 723 K and 25 MPa with methanol as a model compound, and its destruction rate reached upto 99.9%. Due to high thermal gradients generated by the exothermic reaction, the pipe which plays an important role in the decrease of salt precipitation and corrosion can be broken. Thus, its behaviour must be controlled in-situ. The pressure drop measurement across the porous wall was used to check whether the inner pipe was still intact. The experiments confirm that the porous medium permeability is a characteristic constant of this medium. The permeability value remains equal whichever the fluid used, liquid water or supercritical water. In addition, the pressure drop measurement across the porous wall allows the control of the tube integrity.
INTRODUCTION
Wastes management is nowadays a topic of most interest. Hazardous wastes, such as toxic compounds or radioactive organic compounds produced by nuclear industries, cannot be destroyed by classical biological or thermal treatments. Thus, new concepts, which are harmless for human being and its environment, must be developed. Supercritical Water Oxidation (SCWO) is a high temperature and pressure technology that uses the properties of supercritical water for the destruction of organic compounds and toxic wastes. The process was developed by Modell and co-workers based on investigations undertaken in 1975. This process also involves the destruction of organic compounds in supercritical water (P>22.1 MPa and T>647 K), and seems to be a good alternative technique to dispose off the dangerous wastes. The SCWO process advantages are linked to the special physical properties of water under supercritical conditions. The low values of density, dielectric constant and ionic product actually induce a non-polar solvent behaviour of water. As a consequence, organics and oxygen are completely miscible with water and form a single phase.This monophase system and the high diffusivity of supercritical water makes it possible to carry out rapid reactions and achieve high destruction rates. Moreover, oxidation is confined which allows effluent control. Hydrocarbons are completely oxidised with formation of CO2 and H2O. Nitrogen present in the organic compound forms molecular nitrogen N2 and small amounts of N2O, so that no pollutants such as NOx are encountered in the gaseous effluent. The heteroatoms yield mineral acids, like HCl, H3PO4 and H2SO4. These acids with the oxidant create such aggressive environments that any material suffers from corrosion attack specially in presence of halogenated compounds. In order to prevent the reactor from corrosion, alkalis are usually added as neutralisation reagent. This leads to the formation of inorganic salts. Inorganic salts are highly soluble in water under ambient pressure and temperature at the same time they are insoluble under supercritical conditions because of the low density and the small dielectric constant of water. As a result, salts present in the waste or formed during the reaction, precipitate and can cause the reactor fouling. Corrosion and salt precipitation are the two main limitations of SCWO process. They have to be overcome in order to extend the SCWO applications. To cope with corrosion in the process a new generation of reactors called the double-wall reactors is emerging. These reactors separate corrosion stress from pressure stress. This reactor concept is based on a double-wall reactor in which the inner shell is porous and thus allows a water flow through it. A thin protecting water film is then formed on the inner surface which prevents sticky solids from depositing and reduces corrosion. Double-wall reactors are more sophisticated and are well suited to treat very hazardous wastes from nuclear activities as compared to tubular or vessel reactors.
This paper describes the design of a new reactor which belongs to the transpiring wall technology in which the common metallic porous inner shell is replaced by an α-alumina one. The inner ceramic tube plays an important role, so we must set an in-situ control method of the integrity of the pipe. The pressure drop measurement through the porous tube is easily accessible and can be a control of the inner tube behaviour. The definition of a flow model is necessary prior to investigate whether the pressure drop measurement is a reliable parameter to know the tube integrity.
EXPERIMENTAL SECTION
The vertical reactor is divided into two main parts. The upper one is the reacting zone locating the double concentric shell where the water is under supercritical conditions, generally around 25 MPa and 723K. The external vessel with an inner diameter of 24 mm is made of stainless steel 316. The inner porous barrier is composed of pure α-alumina as it is corrosion resistant in presence of chloride. The inner wall is a 0.5 m long tube with an overall diameter of 19 mm and an internal diameter of 15 mm. Two different tubes one with an average pore diameter given as of 0.8 μm and other as of 50 nm are available. The inner pipe confines the reacting medium inside the tubular space. The annular space is filled with supercritical water so as to supply pressure balance between both sides of the wall and avoid mechanical stress on it. A supercritical water radial flow through the porous barrier plays three important roles. First, it dilutes corrosive species at the vicinity of the wall and reduces their effect. Secondly, it cools the reactor medium where a highly exothermal reaction takes place. Finally, it also prevents sticky solids from depositing on the wall. Precipitate is driven downstream to the lower zone. The lower zone is a subcritical cooling one in which mineral salts are soluble. Cooling is supplied by an external jacket. Typical conditions in the output stream are usually 25 MPa and about 288 K. This cylindrical space is 0.3 m long with an internal diameter of 24 mm.Two chromato-graphic pumps deliver water inside the tubular zone with a flow rate ranging from 0 to 50 cm3 min-1. Pump 1 supplies the axial pure water flow. This induces a downward stream and avoids heat excess at the top of the reactor during oxidation. Pump 2 provides the radial flow across the porous medium. This radial stream is composed of a hydrogen peroxide solution, the oxidant of the process. Both flow rates are measured by a thermal mass flow meter. Waste is delivered by a third chromato- graphic pump called pump 3. Its flow rate ranged from 0 to10cm3 min-1. Waste is injected by the inlet dip pipe which supplies the feed inside the tubular reacting zone. Both water streams are heated at 653 K by two pre-heaters prior to their reactor input. Each pre-heater consists of a cast heater over a spiral shaped tube (OD=3.2 mm). Each cast heater delivers a 2 kW power. Reactor is heated at 723 K with three cast heaters (the power of each is 500 W) located at the outer side. A second cooling-jacket is placed after the stream output. It plays a preventive role in case of heat excess. The stream is then depressurized under ambient conditions by a back pressure regulator whose maximal pressure resistance is of 41.5 MPa. The resulting water–gas mixture is introduced into a gas–liquid separator. The liquid is collected at the bottom of the separator whereas the gaseous effluent is collected at the top. For the concern of studying the water flow through the porous wall, only one pump had to be in use. Hence pump 2, which delivers a radial flow, is the single operating pump. A pressure transmitter is placed between the pre-heater exit and the back-pressure regulator entrance and is calibrated in the pressure range from 0 to 0.15 MPa.
Reagents and Analytical Materials
Deionized water with a resistivity around 18.2 MΩcm is produced by a water purification system. Methanol (99%) and hydrogen peroxide (33 vol %) are used. The liquid effluent was analysed with a Total Organic Carbon Analyzer. The composition of the gaseous effluent is also analysed. Helium is used as the carrier gas. The apparatus is equipped with two detectors, a thermal conductivity detector (CO2, O2 and N2) and a flame ionization detector (CO2, CH4 and CO). Calibration of gases is completed by using standard gaseous mixtures.
RESULTS AND DISCUSSION
Methanol oxidation
The reactor performances were first tested using methanol as a model compound. Both the axial (pure water) and the radial (water and H2O2) flow rates were fixed at 10 g min-1. Pump 3 ensured pure methanol feed. The relative error made on the destruction rate of methanol is around 0.1%. which mean destruction rates remain steady at about 99.8% whatever the value of the inlet weight fractions of methanol. The oxidation was almost complete. In addition, a visual control of the inner tube revealed its perfect integrity after each run of experiment. Due to an oxidant feed with both axial and radial flows large heat is released at the top of the reactor during oxidation producing cracks on the tube. It confirmed that the inner ceramic tube is very sensitive to thermal gradients and it risks being broken during an oxidation. As a consequence, it is of most importance to control the exothermicity of the reaction and check the integrity of the inner barrier in-situ as well.
Water flow modelling in the inner porous tube
For a compressible fluid like supercritical water, the volume flow rate is unsteady along with the fact that a gas flow is sliding and not viscous. The velocity of gas molecules at wall contact is not equal to zero as it is the case of a liquid flow. The flow modelling is first studied using pressurized water (25 MPa) at ambient temperature. At a given radial mass flow rate, the pressure drop is measured with both tubes. With the inner tube offering an average pore diameter of 0.8 μm, the measured pressure drop is almost equal to the one without any tube. As a consequence, it is impossible to distinguish the presence or not of the tube. Whereas using a tube with a smaller average pore diameter of 50 nm not only enhanced the pressure drop but also constituted a validation of the pressure drop metering- device and allowed to know the permeability of the porous tube which is its constant characteristic. The flow rate variation is reduced in order to maintain the fluid temperature at 723 K. It is actually difficult for the heating system to heat water at 723 K when water flow rate is too low. At the operating conditions the supercritical water flow in the porous tube follows the classical Darcy’s law, regardless of the fact that supercritical water is a compressible fluid. It allows to model liquid and supercritical water flow through the inner porous tube.
Control mean of the tube integrity
During oxidation, the inner tube might be broken because of thermal gradients induced by SCWO process. A crack perpendicular to the axis can be noticed and the inner barrier is divided in two parts. To avoid the disassembly of the reactor for checking the integrity of the tube it is important to find an in-situ control method such as the pressure drop measurement in the porous medium. Considering supercritical water flow the values of the pressure drop in a broken tube are near the values obtained without tube. This is due to the low viscosity of supercritical water (3×10-5 Pa s). As a consequence, it is possible to control the tube integrity without opening the reactor, only by measuring the pressure drop with a supercritical water flow or a liquid water flow. When two water pumps are used to control the tube integrity during oxidation, we introduce the term of pressure difference instead of pressure drop. But the pressure differences measured with or without tube are so close that it is impossible to distinguish the presence or not of the tube. Thus, the integrity of the tube cannot be controlled during an oxidation. If one needs a continuous control of the tube integrity, it should be necessary to use an alumina having a lower pore diameter.
Conclusion
The double-wall reactor with an inner porous tube for supercritical water oxidation aims to the minimum salt deposition and corrosion. At 723 K and 25 MPa, the oxidation of methanol, used as a model compound, is achieved with high destruction rates (>99.5%). As the inner ceramic tube is sensitive to thermal gradients, the pressure drop measurement as a control method of its integrity can be used. Liquid and supercritical water flows are then modelled. They both follow the Darcy’s law determining the permeability of the porous tube. This characteristic property remains constant whatever the nature of fluid (liquid or supercritical). Even if the control is not possible during oxidation the pressure drop measurement, is an in-situ control method of the tube integrity. The pressure drop measurement caused by liquid (or supercritical) water flow in the porous tube allows checking if the tube is still intact avoiding the disassembly of the reactor.
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