Patent Number: 043138454
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT This invention provides an improved system for performing the process described in U.S. Pat. No. 3,957,676. In accordance with this invention a high-rate acid digester is provided which reacts the combustible waste with sulfuric acid in a well mixed reactor where the acid and waste are intimately mixed through the whole volume of the chemical reaction vessel without establishing critical concentrations of radioactive material. This contrasts with previous systems employed to carry out the patented process wherein the waste was either batch wise or incrementally added to a stagnant pool of acid. Furthermore, the system of this invention has the capability of providing a continuous throughout. The apparatus of this invention basically includes a deep annular vessel 10, for example approximately 39 inches (1 meter) deep, having an outside diameter 14 of approximately 30 inches (0.76 meters) and an inside diameter 12 of approximately 24 inches (0.61 meters). The vessel includes a number of airlift circulators and gas bubblers 16 which extend from the top cover of the vessel 18 into and substantially through the annular cavity 17 to a depth well below the surface level of sulfuric acid, which substantially fills the cavity. An inlet conduit 20 is provided for permitting the introduction of solid waste material. The waste to be digested is funneled through inlet port 22 and is transported by a ram 24 to the inlet conduit 20 where it is distributed into the annular cavity of concentrated sulfuric acid. The waste enters the top of the annular digester where the recirculators spray the acid solution over the waste at high flow rates. The action of the gas bubblers and the recirculators are designed to cause the waste to be swept under the surface of the hot sulfuric acid. Reaction of the waste with the acid produces a carbon slurry residue and an off gas mixture. The gas bubblers supply the air used to oxidize the off gases. Nitric acid or nitrogen dioxide is added to the reaction to oxidize the carbon slurry residue. The nitric acid or nitrogen dioxide can be introduced into the reaction through the recirculators or through a separate inlet 28 and can be added either incrementally or continuously at the rate required to fully oxidize the carbon slurry residue. The rate of addition can be established in advance of the reaction from the nature and volume of waste to be digested. The intimate contact of the sulfuric acid with the reaction products facilitates a more complete and efficient reaction. It has been observed that significantly less energy input is needed to drive the waste/acid reaction of this invention than had previously been required by the prior art process. Desirably, the reaction vessel is surrounded by a heating jacket 26 which includes auxiliary heating coils to maintain the reaction temperature within the permissible range of between 220.degree. to 330.degree. C. The rate of the reaction drops off significantly below 230.degree. C., and much below 220.degree. there is a possibility of the formation of nitrated compounds, which is undesirable. 200.degree. C. therefore has proved to be a practical lower limit for carrying out the process. The upper limit of 330.degree. C. is set to maintain the process below the boiling point of sulfuric acid. Preferably, the temperature is maintained at a value up to 260.degree. C. The heating jacket, which functions in part as an insulator, retains the exothermic heat produced during the reaction to reduce the amount of energy that must be added to the process. During the process the off gases are routed through a deentrainment unit 30 to recover any captured acid that might have been entrained, which can then be returned to the reaction cavity. Also, while the process is taking place, the product slurry 32 is drained on a regular basis so that the reaction may be carried on continuously. The slurry is routed to a recovery or residue ash disposal system. Thus, the improved system of this invention increase the efficiency of the acid digestion process and provides a continuous through-put capability.