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Ind. Eng. Chem. Res. 2005, 44, 8968-8971
KINETICS, CATALYSIS, AND REACTION ENGINEERING A Porous Reactor for Supercritical Water Oxidation: Experimental Results on Salty Compounds and Corrosive Solvents Oxidation E. Fauvel, C. Joussot-Dubien,* V. Tanneur, and S. Moussie` re CEA DEN/VRH/DTCD/SPDE/LFSM, BP 111, 26702 Pierrelatte Cedex, France
P. Guichardon, G. Charbit, and F. Charbit Laboratoire de Proce´ de´ s Propres & Environnement UMR 6181 CNRS, Europole de l'Arbois, BP 80, 13345 Aix en Provence Cedex, France
Supercritical water oxidation (SCWO) development is limited by corrosion and salt precipitation. Transpiring wall reactors are emerging to cope with these problems. A reactor has been developed in which the inner porous shell is composed of pure R-alumina, to handle organic effluents generated by nuclear activities. The reactor was not proven to be efficient enough to oxidize salty effluents. However, experimental results concerning the oxidation of a mixture of dodecane and tributyl phosphate, which is used as a model effluent, confirmed the ability of the reactor to treat corrosive wastes. High destruction rates were actually achieved (>98%). Phosphorus was totally recovered in the aqueous effluent as phosphoric acid. No corrosion was noticed in the reactor, except upstream from the waste injector. As expected, the inner alumina tube shielded the pressure vessel from corrosion. The assumed sensibility of alumina to thermal gradients was not a limiting factor of the reactor operation. Introduction To process hazardous wastes such as toxic compounds or radioactive organic compounds produced by nuclear industries, new technologies that are harmless to human beings and their environment are emerging. For example, supercritical water oxidation (SCWO), which involves the destruction of organic compounds in supercritical water (at a pressure of P > 22.1 MPa and temperature of T > 647 K), seems to be a good alternative technique to dispose of those dangerous wastes. Its main advantages are well-known and linked to the special physical properties of water under supercritical conditions (water is a nonpolar solvent in which organics and oxygen are completely miscible). High conversions are achieved in relatively short reaction times. However, the development of SCWO applications is limited by two main problems: material corrosion and salt deposition. Many studies have reported tests on several metallic materials to investigate their ability to withstand such an aggressive environment.1-3 The resistance of ceramic materials is also studied. Among them, pure alumina has been revealed to be quite effective in regard to withstanding corrosion attack in the presence of mineral acids and for a large range of temperature.4,5 A new generation of reactors is emerging to minimize salt precipitation and corrosion: the transpiring wall reactor. It consists of a double-wall reactor,6-8 where the inner porous wall is composed of stainless steel, high-porous sintered Alloy 625, or titanium. These sophisticated reactors might be well-suited to treat very hazardous wastes from nuclear activities. That is the reason the Nuclear Energy Division of the CEA is studying this type of reactor. Alumina was chosen as
the material for the inner wall, because of its excellent corrosion resistance. This paper describes the performance of the reactor, which has been developed for the treatment of salts and corrosive compounds. Mixtures of sodium sulfate and ethanol were chosen as model mixtures to simulate the behavior of salts in the reactor. Sodium sulfate is wellstudied in the literature,9 and it is not the root of corrosion, unlike sodium chloride. Ethanol was also introduced, together with sodium sulfate, to simulate the behavior of organic salty compounds in the SCWO reactor. For the concern of corrosive species, a synthetic solution of the dodecane-tributyl phosphate mixture was studied. The mixture is used in the nuclear reprocessing with a composition ratio of 70/30 (vol %). Its oxidation in a SCWO reactor yields phosphoric acid, which is corrosive. Experimental Section Reactor Design. The reactor design and the experimental setup were already published in previous papers.10,11 Therefore, only a very short description is given below. The reactor is composed of two main parts. The upper component is the reacting zone, where the double concentric wall is located. In this part, water is under supercritical conditions (generally ∼25 MPa and 723 K). The presence of a pure R-alumina tube delimits two parts in the upper supercritical area: an annular space between both walls and a tubular one. As in a doublewall reactor, the inner tube confines the reacting medium inside the tubular space. Thus, the stainless steel vessel has no contact with the aggressive solutions
10.1021/ie0505108 CCC: $30.25 © 2005 American Chemical Society Published on Web 10/26/2005
Ind. Eng. Chem. Res., Vol. 44, No. 24, 2005 8969
usually treated by SCWO. The annular space is filled with supercritical water, to supply a pressure balance between both sides of the inner wall and avoid mechanical stress on it. Our preliminary works concerning the supercritical water flow across the thickness of the porous tube confirmed that supercritical water in the annular space really entered the tubular space, creating a radial flow.11 Our hydrodynamic study, which was performed via residence time distribution, also showed that the annular fluid entirely flowed through the porous barrier. Reagents and Analytical Materials. Deionized water with a resistivity of ∼18.2 MΩ cm was produced by a water purification system from Millipore. Ethanol (99%), hydrogen peroxide (50 vol %), dodecane (35% n-dodecane among C12 molecules), tributyl phosphate (99%), and anhydrous sodium sulfate (99%) were supplied by Prolabo. In regard to the liquid effluent, it was analyzed with a total organic carbon (TOC) analyzer (Shimadzu, model TOC 5000A). The concentration of sulfate ion was determined by ion chromatography (IC) (Waters, model 432). The composition of the gaseous effluent was analyzed online via gas chromatography (GC) (Varian, model Star 3600 CX). Results Sodium Sulfate-Ethanol Mixture. The aqueous model waste was prepared with the composition of sodium sulfate noted as %salt (given in wt %) and the concentration of ethanol noted as [C2H5OH]w (given in mol/L). It was injected with a rather high flow rate (compared to the case of an introduction of pure organics) in the range of 5-10 mL/min, to avoid the inlet dip pipe plugging. The reactor performance concerning salty solutions was characterized by a salt mass balance criterion, noted as RS:
RS )
[SO42-](qaxial + qradial + qw) [SO42-]wqw
(1)
where [SO42-] is the molar concentration of sulfate measured via IC in the liquid output effluent (in units of mol/L) and [SO42-]w is the molar concentration of sulfate in the waste aqueous solution (in units of mol/ L). The parameters qaxial, qradial, and qw are, respectively, the axial, radial, and waste mass flow rates (in units of g/min). If RS equals zero, it means that all the sulfate sodium is accumulated in the reactor. The destruction of ethanol was expressed in term of conversion, noted as X:
X)1-
TOC TOC0
(2)
where TOC is the total organic carbon measured in the liquid effluent and TOC0 is the total organic carbon that should be measured if oxidation has not occurred. Every run with a sodium sulfate composition of >5 wt % led to inlet dip pipe plugging. This phenomenon obviously could have been avoided with a higher waste flow rate. However, it would have been difficult to reach supercritical temperatures in the reactor with such a huge amount of aqueous solution delivered at ambient temperature. That is the reason our study only involves sodium sulfate compositions of
