Patent Number: 060305878
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Generally, designs of supercritical water oxidation systems found in the literature are composed of six basic components. The first is a means to pressurize the waste water; the second, an oxidant which is added and mixed with the waste water; the third, is a source of heat, the fourth, a reaction chamber which provides sufficient volume for the reaction to proceed and capable of withstanding the temperatures and pressures encountered during supercritical operation, the fifth, is a pressure letdown means; and the sixth, is a means of effluent cooling. In a water liquid/vapor phase diagram, shown in FIG. 1 one may see that there is a critical point of temperature (about 700.degree. F.) and a critical point of pressure (about 3,200 psia) over which there is only one single fluid phase, and which, although represents neither liquid nor vapor, behaves and seems to have more of a vapor character than of a liquid one. The single-phase condition occurring above the critical points is called supercritical condition. The use of very high pressures at elevated temperatures, however, presents serious challenges in the construction of a reactor capable of withstanding these adverse conditions. In order to meet this situation, excessively elongated reactors having accordingly small diameter have been largely utilized in the past. These conventional reactors, however, exhibit a number of disadvantages which include restrictions on waste feed materials (to preclude corrosive feeds and products of destruction), increased plugging potential due to small diameters, restrictions on feed rates, increased investment cost, and safety hazards. Furthermore, nearly all conventional approaches have been drawn to continuous flow systems. The preferred embodiment of the instant invention avoids these limitations by approaching the problem in an entirely different manner. Rather than attempting to provide a system based on a continuous flow reactor, a batch reactor has been designed. Furthermore, in order to avoid many of the problems associated with containing a pressurized, supercritical liquid a reactor has been provided which maintains supercritical conditions only within a limited zone near the center of the reactor. The operational concept of the present invention is as follows: waste is loaded into a reactor vessel and a solution of hydrogen peroxide and water is added. The solution is then heated at which point the hydrogen peroxide decomposes to oxygen and water. Air or oxygen could also serve as the oxidizer, but water is still required as the reaction medium. After the reactor is sealed, the fluid is heated by means of electrical resistance heaters located along the axis of the reactor. As the system heats up, the internal pressure increases until the temperature and pressure of the system reach supercritical conditions near the center of the reactor. Coincidentally, the fluid near the reactor wall is cooled by conduction through the wall to a heat transfer means attached to the exterior of the reactor. Above the critical pressure of 3204 psia, fluids can be heated and cooled between subcritical and supercritical temperatures with no discontinuity in density. Consequently, the reactor does not have separate liquid and gas phases; the density changes in a continuous gradient from liquid-like conditions in a lower temperature region near the wall to a gas-like supercritical condition at the center of the vessel. At the desired operating conditions, the system reaches an equilibrium in which the fluid continuously circulates through the vessel due to natural convection: the heat input at the center is balanced by the heat loss at the wall. At supercritical conditions, organic wastes and oxygen react spontaneously to form water, carbon dioxide, acids, and salts. As the fluid circulates through the supercritical region in the center of the reactor, all the waste is destroyed. The system is then cooled and the contents of the reactor analyzed to verify that the treated waste has been completely consumed. The reactor is then emptied and prepared for another batch process. It is worth noting that organic matter decomposes readily under supercritical conditions, and in the presence of oxygen carbonaceous compounds oxidize completely to carbon dioxide, sulfur, compounds mostly to SO.sub.3, and nitrogen compounds decompose mostly to molecular nitrogen. It is also worth noting that under supercritical water oxidation conditions, only small amounts of nitrogen oxides are produced. The invention can be explained most effectively by referring to FIG. 2A. The reactor of the present invention comprises a conventional pressure vessel 200 fabricated from high strength steel or other structural material member such as alloys of nickel, titanium, aluminum, and the like, and alloys thereof. The vessel 200 further comprises a cap 201 and shell 202. Both the cap 201 and shell 202 have a mating seal ring flange 203 and 204 located at, an running around, the outside edge of each. The seal ring flanges 203 and 204 are used to permit the to two parts of the reactor to be secured together. A series of clamps 205 are provided which are placed at equidistant locations around the perimeter cap/shell assembly ring flanges. The clamps 205 compress the two flanges against a metal compression gasket 206 provided between cap 201 and shell 202 in order to provide a hermetic seal within the closed pressure vessel 200. Clamps 205 are here a Graylok type "c" clamp but may be any convenient device or method. These might include, but not be limited to, nuts and bolts through a plurality of equidistant spaced through holes, hydraulic clamps, or a screw clamp. In all respects the vessel comprising the reactor is conventional, except that both the cap 201 and shell 202 are provided with hollow, cylindrical protuberances 207 and 208 lying along the central axis and extending deep into the interior of vessel 200. These protuberances are closed to the interior of vessel 200 but open to the surrounding exterior environment. High watt-density heating cartridges 209 are inserted into the protuberances such that the cartridge wall and the exterior wall of the protuberance mate and form a tight fitting seal. The heating cartridge may include a contact layer 210 for providing better conductive contact with the vessels' exterior wall. Shell 202 is provided with a cooling portion 211, such as a cooling jacket or the like, by circulating a fluid media, preferably water, through an envelop constructed directly on shell 202 and using the exterior wall of shell 202 as one wall of the jacket. Alternatively, cooling portion 211 may be established by attaching a separate manifold around the shell wall 202a and securing it in intimate contact with the wall. Cooling portion 211 is used to maintain shell exterior wall 202a at temperatures substantially below the supercritical temperatures setup in the reactor center zone. In practice this temperature would range between about 400.degree. F. and 575.degree. F. while the interior of the reactor fluid is expected to temperatures in excess of 1000.degree. F. This temperature difference will provide a large density gradient within the fluid medium and will insure that the fluid circulates throughout the reactor. This circulation promotes rapid oxidation in the hot center zone by constantly moving reactants into this zone while removing reaction products. It also has the added benefit of minimizing corrosion of the interior wall. Such circulation thus helps to maintain the efficiency of the oxidation process. In addition, an internal baffle 212 or series of baffles in the form of long cylinders spaced slightly away from the shell interior wall 202b may be incorporated in the reactor interior in order to further enhance circulation by nominally isolating the central supercritical zone from the cooler walls. Baffle 212 shields the interior wall from the effects of side circulation eddies and further helps to establish a discrete convective circulation between the wall and the vessel interior. Each baffle is supported by standoff supports to hold it off the interior wall and bottom of the reactor. These supports may be any convenient device or method such as, but not limited to angled supports welded at different heights around the circumference of the interior walls and around the bottom of the vessel. Alternatively the baffle itself may incorporate arms and legs or a series of openings in through it's surface at strategic locations. An alternate embodiment shown in FIG. 2B, includes a reactor shell 202 having central hollow tube 213 running the length of the reactor. This tube would be open to the outside environment, but closed to the interior of reaction vessel 200. The tube 213 would be sealed or secured to the cap/shell assembly in any of a number of ways. Any techniques well known to the art may be used for the hermetic connection or sealing including but are not limited to welding, pipe fitting/compression sealing, and the like. Most preferably, tube 213 would run up, and extend out of a hole fitted with a male nipple 214 welded, or otherwise secured, to the reactor cap 201. The tube 213 would be attached to shell 202 by welding it to a close fitting hole in the shell bottom end. Tube 213 would then be passed through threaded male nipple 214, the nipple having a close fitting inside diameter for passing the tube through, the tube secured by a ferret/gasket means 215. High watt-density heating cartridges 209 again are placed at or near the center of the tube for heating the central zone of the reactor. In operation, the reactor vessel would be loaded with a quantity of water, the oxidizer added, the waste materials, which are to be destroyed, added, and the vessel sealed. Circulating water would then be admitted through the cooling jacket and the heating elements slowly powered. The internal temperature and pressure of the vessel are closely monitored as the heating elements gradually bring the water inside the reactor achieves a supercritical state. By controlling the rate of heating a cooling, adjusting the current to the heaters and the volume and temperature of cooling water passed over the vessel walls, the desired internal circulation is established. This circulation constantly replenishes reactants into the supercritical region helping the reaction proceed. Once it has been determined that no further reaction is taking place, power is removed from the center heating elements and the vessel is allowed to cool. The processed and cooled brine is sampled and analyzed for composition and then further treated in order to precipitate and remove any metals in the effluent stream, to neutralize acids and to remove dissolved salts. It should be pointed out that in the different embodiments of the present invention, oxygen could be replaced by other oxidants or mixtures of oxidants, or mixtures of oxidants and other gases or liquids. It should also pointed out that the is reactors of the present invention are particularly useful in the case of supercritical water conditions, especially in the presence of corrosive compounds, such as for example halogens, and the like. The examples and the description of the above embodiments have been given for illustrating purposes only and they should not be construed as restricting the scope of the present invention. Features described in one embodiment of this invention may be combined with any appropriate features of other embodiments, or replace features of other embodiments for the purpose of exemplifying the scope of the invention.