THE TREATMENT OF GASEOUS SUBSTANCES

Abstract: A method for the treatment of a gaseous acidic substance which includes delivering the said substance to a packed tower (3) or scrubber in which the gaseous acidic substance is intimately contacted with a supply of a dilute aqueous extractant solution, the tower being in an extractant solution recirculation loop in which extractant solution is delivered to the tower and is removed from the tower and the concentration of acid in solution in the loop is maintained in an acid concentration range by addition of dilute aqueous solution to the loop and the extraction of aqueous solution of greater acidic concentration from the loop.

Description: THE TREATMENT OF GASEOUS SUBSTANCES The present invention relates to the treatment of gaseous substances, in particular acidic fumes formed by chemical reactions, eg in chemical processing plants. A feature of many organic and inorganic chemical processes is that acidic products which are either primary products or by-products are formed. Familiar examples of such processes include halogenation of aromatic compounds, eg chlorination of benzene, acid regeneration, eg breakdown of metallic chlorides or sulphates in order to recover hydrochloric acid or sulphur dioxide respectively, and hydrolysis of uranium hexafluoride giving hydrofluoric acid. Presently, such acidic gaseous compounds are recovered either in condensers followed by treatment of the tail gases to reduce the acidic content to an acceptable environmental level or in some instances without condensation. A variety of methods are used to achieve the above objective and include absorbtion in sparged towers, stirred reactors, packed towers (scrubbers) and spray towers. A feature of many of these processes is that the acidic component is neutralised with alkali, generally sodium or potassium hydroxide or an amine which is subsequently regenerated. In packed towers where acid absorbtion and neutralisation take place simultaneously difficulties arise owing to the high heat of reaction which can be generated. Such heat may deform plastics material employed as packing material or in construction of the packed tower. Further, control of the reaction is complicated particularly if the feed composition varies. It is an object of the present invention to provide an improved method for the treatment of a gaseous acidic substance which does not exhibit the aforementioned disadvantage. According to the present invention there is provided a method for the treatment of a gaseous acidic substance which includes delivering the said substance to a packed tower or scrubber in which the gaseous acidic substance is intimately contacted with a supply of a dilute aqueous extractant solution, the tower being in a extractant solution recirculation loop in which extractant solution is delivered to the tower and is removed from the tower and the concentration of acid in solution in the loop is maintained in an acid concentration range by addition of dilute aqueous solution to the loop and the extraction of aqueous solution of greater acidic concentration from the loop. In a preferred form of the method according to the invention, * che said tower has an outlet for any of the remaining gaseous substance which has not been extracted in the said tower and any remaining gaseous substance is delivered to a second packed tower or scrubber in which the gaseous acidic substance is intimately contacted with a supply of dilute aqueous extractant solution, the second packed tower being in a second extractant solution recirculation loop in which extractant solution is delivered to the second tower and removed from the second tower and the concentration of acid in solution in the second loop is maintained in a second acid concentration range by control of the addition of dilute aqueous solution to the second loop and of the extraction of aqueous solution containing any acidic substance introduced into the tower from the second loop. In this way the first tower may be used to buffer variations in concentration of the incoming gas stream and provide the second tower with a gas stream of essentially constant acidic gas content. The dilute aqueous solution introduced into each recirculation loop may be water or a solution containing less than 0.5 per cent by weight of inorganic and/or organic additive or additives, eg heptyl alcohol. The packed tower or towers or scrubbers may be constructed in a known manner. The packing material may be any known inert solid material of high surface area. Commercially available synthetic products or natural materials, eg gravel or pebbles, may be used. Each recirculation loop may comprise a reservoir for the recirculating solution. The reservoir may be provided by a tank located below the tower. The solution may be removed from the tower to the tank by suitable drainage means. Fresh dilute aqueous solution may be added to each loop at the reservoir of that loop, eg at or near to the top of a tank. Aqueous solution containing the said acidic substance may be removed from each loop after the reservoir. Desirably, the acid concentration of the solution in each said reservoir is controlled so as to be maintained in a given concentration range (or substantially constant) by adjustment of the rate at which fresh aqueous solution is introduced into the recirculation loop and acidic aqueous solution is extracted from the loop. The addition and extraction may be controlled independently by suitable known fluid control devices, eg valves, optionally together with associated flow rate meters. For example, the acid concentration in the extractant solution in the reservoir of the first extractant solution recirculation loop may be maintained in the range 2 to 20 per cent, eg 5 to 12 per cent, by weight. The acid concentration in the extractant solution in the reservoir of the second extractant solution recirculation loop may be maintained in the range 0 to 2 per cent, eg 0 to 0.5 per cent by weight. A pump may be employed in each said recirculation loop to convey extractant solution from the said reservoir in that loop to the tower in that loop, eg at or near the upper end of the tower. A supply of the inlet gaseous substance may be introduced into each tower at or near the lower end thereof whereby the supply is able to rise through the packing of the tower to contact the extractant liquid falling, ie percolating, through the packing of the tower. The recirculation of extractant solution in each recirculation loop may be carried out continuously. After the process has been started and the desired acid concentration has been reached in each loop (this may be detected by the provision of suitable pH detectors located in the said reservoir) extractant solution may also be continuously added to and removed from the loop to maintain the concentration in the required range, desirably at a substantially constant level. The method according to the present invention may conveniently be one in which the acidic gaseous substance to be extracted is a by-product associated with one or more non-acidic gases to be treated or recovered separately. For example, the supply of gaseous acidic substance delivered to the first mentioned tower may include the said one or more non-acidic gases. The said non-acidic gas may, for example, comprise hydrogen used in the hydrolysis and reduction of uranium hexafluoride. The hydrolysis produces hydrogen fluoride as a gaseous acidic by-product. The gases may optionally be carried in a stream of an inert carrier gas such as nitrogen. Beneficially and unexpectedly the present invention shows the following advantages compared with the prior art. (a) There are no substantial heats of reaction generated which might cause degradation of material employed in the packed tower construction or packing. (b) Heats of dilution are controllable by adjusting the reservoir volume and extractant solution removal and fill rates in each recirculation loop. (c) As the extractant solution acid concentration is known packed towers having suitable dimensions and contacting properties can be accurately designed. (d) Because the extractant solution acid concentration is maintained in a given range if the acid concentration of the supplied gas stream varies there is no consequent effect on extraction performance. (e) As the acid concentration of the stream of extractant solution removed from the recirculation loop is known (or is controlled within a given range and may be measured) further treatment of the removal stream, eg blending and storage or neutralisation, are facilitated. Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a block schematic diagram of an arrangement for removing an acidic gaseous substances from a gas flow by an embodiment of the present invention. The arrangement shown in Figure 1 may for example be used to remove HF from a gas stream comprising HF and hydrogen in nitrogen carrier gas, eg the HF having been produced by the hydrolysis of uranium hexafluoride and hydrogen which is used to reduce the uranyl fluoride which is a product of the said hydrolysis reaction. As shown in Figure 1, the gaseous input to the arrangement is delivered via a pipeline 1 to the lower end 3a of a first packed tower 3. Extractant solution is delivered via a pipeline 4 by a pump 5 from a base tank 7 to the upper end 3b of the tower 3. Gas from the pipeline 1 rises through the packed tower 3 and contacts the extractant liquid percolating downward through the tower 3. Acidic constituents in the gas flow are thereby dissolved in the extractant solution. The extractant solution collects at the lower end 3a of the tower 3 and is allowed to drain via a pipeline 6 to the base tank 7 which acts as a reservoir for the solution. The tank 7 is in a recirculation loop comprising the tower 3, pipeline 6, tank 7 and pipeline 4. Fresh extractant solution, eg clean water, is introduced into the tank 7 at the upper end thereof from a pipeline 9 incorporating a control valve 11. Extractant solution collected in the base tank 7 and pumped by the pump 5 may be bled from the pipeline 4 via a pipeline 13 incorporating a control valve 15. The level of extractant solution collecting in the base tank 7 is monitored via a suitable level monitor 17 and the pH of the solution in the tank 7 is monitored by a pH monitor 19. In use of the arrangement shown in Figure 1, the acid concentration of the solution in the base tank 7 is allowed to build up to a concentration in the range 5 to 12 per cent by weight. On achieving this concentration the rate at which the tank 7 is allowed to fill and the rate at which solution is bled from the pipeline 13 are set by adjustment of the valves 11, 15 thereby fixing the level and concentration of extractant solution in the tank 7. Exit gas from the tower 3 will usually contain a low concentration of remaining acidic gas and this is removed by a further packed tower 21 acting in a manner similar to the packed tower 3. The exit gas is delivered from the upper end 3b of the tower 3 to the lower end 21a of the further tower 21 by a pipeline 20. Extractant solution is delivered via a pipeline 24 by a pump 25 from a base tank 27 to the upper end 21b of the tower 21. Gas from the pipeline 20 rises through the packed tower 21 and contacts the extractant liquid percolating downward through the tower 21. Acidic constituents in the gas flow are thereby dissolved in the extractant solution. The extractant solution collects at the lower end 21a of the tower 21 and is allowed to drain via a pipeline 26 to the base tank 27 which acts as a reservoir for the solution. The tank 27 is in a recirculation loop comprising the tower 21, pipeline 26, tank 27 and pipeline 24. Fresh extractant solution, eg clean water, is introduced into the tank 27 at the upper end thereof from a pipeline 29 incorporating a control valve 31. Extractant solution collected in the base tank 27 and pumped by the pump 25 may be bled from the pipeline 24 via a pipeline 33 incorporating a control valve 35. The level of extractant solution collecting in the base tank 27 is monitored via a suitable level monitor 37 and the pH of the solution in the tank 27 is monitored by a pH monitor 39. In use of the arrangement shown in Figure 1, the acid concentration of the solution in the base tank 27 is allowed to build up to a maximum level in the range 0 to 0.5 per cent by weight. The valves 31, 35 are thereafter adjusted to set the level of extractant solution in the base tank 27 and maintain the acid concentration at the desired level. Clean non-acidic gas is extracted from the upper end of the tower 21 via an exit pipeline 41 incorporating a pump 43. The bleeds extracted via the pipelines 13 and 33 may be fed to storage tanks and thereafter neutralised with an alkaline solution in a conventional manner.

Claims: Claims 1. A method for the treatment of a gaseous acidic substance which includes delivering the said substance to a packed tower or scrubber in which the gaseous acidic substance is intimately contacted with a supply of a dilute aqueous extractant solution, the tower being in a extractant solution recirculation loop in which extractant solution is delivered to the tower and is removed from the tower and the concentration of acid in solution in the loop is maintained in an acid concentration range by addition of dilute aqueous solution to the loop and the extraction of aqueous solution of greater acidic concentration from the loop. 2. A method as in Claim 1 and wherein the said tower has an outlet for any of the remaining gaseous substance which has not been extracted in the said tower and any remaining gaseous substance is delivered to a second packed tower or scrubber in which the gaseous acidic substance is intimately contacted with a supply of dilute aqueous extractant solution, the second packed tower being in a second extractant solution recirculation loop in which extractant solution is delivered to the second tower and removed from the second tower and the concentration of acid in solution in the second loop is maintained in a second acid concentration range by control of the addition of dilute aqueous solution to the second loop and of the extraction of aqueous solution containing any acidic substance introduced into the tower from the second loop. 3. A method as in Claim 1 or Claim 2 and wherein the dilute aqueous solution introduced into the or each recirculation loop is water or a solution containing less than 0.5 per cent by weight of inorganic and/or organic additive or additives. 4. A method as in Claim 1, 2 or 3 and wherein the or each recirculation loop comprises a reservoir for the recirculating solution, the reservoir being provided by a tank located below the tower and the solution being removed from the tower to the tank by drainage means. 5. A method as in Claim 4 and wherein fresh dilute aqueous solution is added to the or each loop at the reservoir of that loop at or near to the top of a tank and aqueous solution containing the said acidic substance is removed from each loop after the reservoir. 6. A method as in any one of Claims 4, 5 or 6 and wherein the acid concentration of the solution in each said reservoir is controlled so as to be maintained in a given concentration range or substantially constant by adjustment of the rate at which fresh aqueous solution is introduced into the recirculation loop and acidic aqueous solution is extracted from the loop. 7. A method as in Claim 6 which includes a first packed tower and a second packed tower and wherein the acid concentration in the extractant solution in the reservoir of a first extractant solution recirculation loop including the first tower is maintained in the range 2 to 20 per cent and the acid concentration in an extractant solution in the reservoir of the second extractant solution recirculation loop including the second tower is maintained in the range 0 to 2 per cent. 8. A method as in any one of the preceding claims and wherein the recirculation of extractant solution in the or each recirculation loop is carried out continuously. 9. A method as in any one of the preceding claims and wherein the acidic gaseous substance to be extracted is a by-product associated with one or more non-acidic gases to be treated or recovered separately. 10. A method as in Claim 9 and wherein the said non-acidic gas comprises hydrogen and the acidic substance comprises hydrogen fluoride. 11. A method as in Claim 9 or Claim 10 and wherein the gases are carried in a stream of an inert carrier gas. 12. A method as in Claim 1 and substantially as hereinbefore with reference to the accompanying drawings.

Inventors: FILSON ARTHUR COLE, ROBINSON PETER JOHN, FILSON, ARTHUR, COLE, ROBINSON, PETER, JOHN
Applicants: BRITISH NUCLEAR FUELS PLC, FILSON ARTHUR COLE, ROBINSON PETER JOHN, FILSON, ARTHUR, COLE, ROBINSON, PETER, JOHN
Application year: 1995
Main classification: B01D53
IPC codes: B01D53