GB2150550A - Process for the removal of hydrogen cyanide - Google Patents

Abstract

Process for the removal of hydrogen cyanide from a gaseous stream containing hydrogen cyanide, comprising the following steps: (a) contacting said gaseous stream with a solution containing ammonium polysulphide to convert hydrogen cyanide into ammonium thiocyanate, thereby producing a solution containing ammonium polysulphide and ammonium thiocyanate, and a gas stream having reduced hydrogen cyanide content; (b) removing at least a portion of the solution containing ammonium polysulphide and ammonium thiocyanate from the contact zone; (c) decomposing ammonium polysulphide and precipitating sulphur in the solution removed in step (b), thereby producing hydrogen sulphide, sulphur and a remaining ammonium thiocyanate-containing solution; (d) removing sulphur from said remaining solution and hydrolyzing the ammonium thiocyanate in said remaining solution, producing ammonia, hydrogen sulphide and carbon dioxide. a

Description

SPECIFICATION Process for the removal of hydrogen cyanide Tne invention relates to a process for the removal of hydrogen cyanide from a gaseous stream containing hydrogen cyanide.

The presence of hydrogen cyanide (HCN) in various gaseous streams complicates removal of additional impurities, e.g., removal of H2S or CO2, and poses problems insofar as product quality and pollution control requirements are concerned. In particular, gas streams derived from the gasification of solid carbonaceous fuel, such as coal, generally have significant minor quantities of HCN which must be dealt with before the gas is utilized.

Accordingly, a practical and efficient procedure for removing impurity HCN might have great economic importance. The invention is such a process.

The invention, therefore, relates to a process for the removal of hydrogen cyanide from a gaseous stream containing hydrogen cyanide, comprising the following steps: (a) contacting said gaseous stream in a contact zone with a solution containing ammonium polysulphide to convert hydrogen cyanide into ammonium thiocyanate, thereby producing a solution containing ammonium polysulphide and ammonium thiocyanate, and a gas stream having reduced hydrogen cyanide content; (b) removing at least a portion of the solution containing ammonium polysulphide and ammonium thiocyanate from the contact zone; (c) decomposing ammonium polysulphide and precipitating sulphur in the solution removed in step (b), thereby producing hydrogen sulphide, sulphur and a remaining ammonium thiocyanate-containing solution;; (d) removing sulphur from said remaining solution and hydrolyzing the ammonium thiocyanate in said remaining solution, producing ammonia, hydrogen sulphide and carbon dioxide.

The hydrolysis of the ammonium thiocyanate is carried out under appropriate conditions of temperature and pressure, and ammonia, carbon dioxide, and hydrogen sulphide are produced. These gases may be recycled and/or recovered, if desired, by known techniques. The process is preferably operated continuously.

The reactions for the process may be shown, as follows:

The particular gas streams treated according to the invention are not critical, as will be evident to those skilled in the art. Any gaseous stream containing HCN and from which it is desired to remove the HCN, and which itself does not react with the ammonium polysulphide or interfere substantially therewith may be treated according to the invention. Gaseous streams or effluents particularly suited to the invention include fuel gases produced by gasification procedures, e.g., fuel or effluent gases derived from or produced by the gasification of coal, petroleum, shale, tar sands, etc., wherein a significant quantity of HCN is present.In such gasification processes, the gaseous effluents are often quenched with water or gaseous liquids, and gaseous streams derived from stripping the liquids may contain HCN and may also be treated by the invention. The HCN content of such streams may vary, but the invention will preferably be employed with streams having an HCN content of from 0.002 percent to 0.1 percent by volume. Higher how contents even above 1% vol are also possible. As indicated, the process of the invention is preferably continuous, i.e., make-up ammonium sulphide or polysulphide is continuously supplied to the contact zone, and a portion or "bleed" of ammonium thiocyanate solution is continuously removed from the contact zone. The volumes of make-up and bleed will depend, inter alia, on the concentration of HCN in the gaseous stream, and thus cannot be given with precision.Those skilled in the art may suitably adjust solution flows.

Suitable conditions, i.e., appropriate temperature and pressure, sufficient contact time, proper pH, and appropriate concentrations of ammonium polysulphide and water are employed to achieve the HCN conversion. Temperatures in the contact zone of from about 2000 to about 8000 may be employed, with temperatures of from 2500 to 6000 being preferred. The pH of the ammonium polysulphide solutions will range from about 8 to 10, preferably 8.5 to 9.5, and concentrations of ammonium polysulphide will preferably range from 0.01 to 1, preferably 0.1 to 0.5 moles per litre. The most important variable controlling HCN removal and conversion is the amount of elemental sulphur available to maintain the polysulphide concentration.In general, the polysulphide solution will have at least a stoichiometric amount of the polysulphide sulphur with respect to the HCN, and preferably up to 3 or 4 times the stoichiometric amount.

Elemental sulphur may be supplied in the contact zone to maintain this concentration. H2S and NH3 in the feed gas do not interfere with HCN removal or conversion, and NH3 may actually help rejuvenate the solution. The ammonium polysulphide solution may be supplied on a continuous basis to the contact zone as make-up, or steps can be taken, in some cases, to generate the ammonium polysulphide to some extent in situ. Contact times may range from 1 to 8 minutes, preferably 3 to 5 minutes. Those skilled in the art may select suitable contact or scrubbing devices to carry out the contacting or scrubbing.

As the HCN is removed from the gaseous stream by reaction with the ammonium polysulphide solution, at least a portion of the solution, now containing ammonium thiocyanate, is removed. In the portion of the solution ammonium polysulphide is decomposed and sulphur precipitated. Preferably, the ammonium polysulphide is decomposed by lowering the pH of the solution removed in step (b). Suitably, the solution removed is contacted or mixed with a sufficient amount of a suitable pH-lowering composition to lower the pH, decompose ammonium polysulphide and precipitate sulphur, and produce H2S. The H2S released may be treated or recovered, as desired. Generally, lowering of the pH to a range of 7.5 to 8 will be sufficient to precipitate the sulphur. Any suitable composition which will lower the pH the required amount may be employed.Suitably acids, such as H2S04, HCI, HNO3 and acetic acid solution, may be employed. Suitable acidic gases, which will be taken up or react in the solution, such as HCI or H2S, may be added to the solution.

Organic acids may be used, as well as other hydrogen ion-supplying compositions. Moreover, the hydrogen ion-supplying material or composition need not be pure; even dilute solutions and those containing extraneous matter may be employed, so long as the extra components in the solution do not interfere with the ammonium polysulphide decomposition or react with the ammonium thiocyanate. As indicated, the hydrogen ion-supplying composition will be supplied in an amount sufficient to decompose the ammonium polysulphide and precipitate sulphur. This amount will depend on a number of factors, and may readily be determined by those skilled in the art in the particular case.

In another preferred embodiment, the ammonium polysulphide in the solution removed in step (b) is decomposed by stripping this solution. Preferably, the solution is stripped by heating it, passing a non-reactive gas through it or by a combination of heating and non-reactive gas passing.

The stripping decomposes ammonium polysulphide, producing H2S and NH3 and precipitating sulphur.

While a separate stripping zone may be provided, it is an advantage of the invention that the portion or stream may be supplied to a suitable stripping zone in a given process. For example, the portion may be supplied to a sour water stripping zone. As used herein, the term "sour water" refers to water containing H2S and NH3, such a composition or streams thereof being commonly available in refinery, gasification process, or other industrial operations. In such a case, the H2S and ammonia from the decomposition of the ammonium polysulphide may be recovered or treated with the stripped H2S and ammonia from the sour water. Sour water streams may also contain extraneous matter, such as fines or solids, if the sour water stream is derived from washing operations.In general, such streams will contain from about 0.005 percent by weight to about 1.3 percent by weight H2S, and about 0.03 percent by weight to about 0.6 percent by weight of NH3. If present, solids or fines may be present from infinitesimal amounts to amounts of from about 2 percent by weight to about 5 percent by weight, and their presence or absence may determine final treatment or disposal of sulphur precipitated in the stripping zone.

It is therefore preferred to mix the solution removed in step (b) with a H25 and NH3-containing aqueous mixture and to strip the solution together with the aqueous mixture.

Whatever the case, as indicate, the solution orsolution-sourwater mixture may be stripped by heating or use of flow of a non-reactive gas (or both). If heat alone is applied to the solution or mixture, sufficient heat will be supplied to decompose the ammonium polysulphide. Again, if heating is employed, it may be necessary to cool the stripped gases before further treatment. Suitable devices for this approach include, for example, a conventional packed or tray column with a re-boiler. Generally, temperatures on the order of about 80 C to about 120 C, preferably about 90 C to about 110 C, will be sufficient to decompose the ammonium polysulphide and precipitate sulphur.

If a non-reactive gas is employed, it will be supplied at a suitable pressure, e.g., 4 bar to 15 bar, to strip H2S and NH3 from the ammonium polysulphide containing solution. Any suitable stripping device may be used, such as a packed column or a tray column. Different devices may be used (whether stripping is accomplished by heat, gas flow, or a combination thereof) where plugging by solids may be a problem. In any event, any suitable non-reactive gas may be employed. As used herein, the term "non-reactive" implies that the gas, (or reactant products thereof with components of the sourwater-solution mixture) does not convert the ammonium thiocyanate in the removed solution to an undesired species, such as back to HCN, to any substantial extent.In general, gases non-reactive to ammonium thiocyanate, i.e., those that do not react with the ammonium thiocyanate in the portion to any substantial extent under the conditions employed, may be used. Suitable gases, under the conditions in the stripping zone, include air, steam, carbon dioxide, oxygen, nitrogen, and inert gases. Steam is much preferred, since it can provide heatforthe stripping and may be condensed easily, leaving a relatively concentrated H2S-NH3 stream. Those skilled in the art may adjust volumes and velocities of the stripping gas to appropriate levels. As indicated, heat may be supplied in the case of a stripping gas to assist the stripping.

The stripped gas or gases may be recovered or treated, as desired. Thus, the H2S and NH3 stripped may be returned to make-up for ammonium polysulphide, or may be sent to conventional gas clean-up steps.

Alternately, the NH3 and H2S may be separated, such as by use of a two section stripping zone with alternate acid addition and base addition in the zones to free the respective gases.

Precipitated sulphur is preferably removed from the remaining mixture or solution prior to hydrolysis. This may be accomplished by filtration or other appropriate technique. For example, since the temperature of the mixture or solution is ultimately to be raised to hydrolysis conditions, the temperature of the mixture may be raised to a point sufficient to melt the sulphur but not sufficient to cause hydrolysis of the ammonium thiocyanate. The molten sulphur may then be easily removed.

If the solution removed in step (b) is mixed with sour water, the volume of solids in the sour water is high, and the amount of sulphur is low, sulphur recovery may be uneconomic, and the recovered "sludge" may simply be sent to waste. Finally, the sulphur may simply be melted in the hydrolysis zone and recovered, recycled, or sent to waste.

Assuming prior sulphur removal, the ammonium thiocyanate-containing remaining mixture or solution is forwarded to a hydrolysis zone where it is subjected to conditions to hydrolyze the ammonium thiocyanate.

Water may be added, if necessary. The NH3, H2S, and COP may be recovered or sent to conventional gas treatment steps. Temperatures in the hydrolysis zone are important, and will range from about 20000 to about 300 C. In general, pressures will range from about 20 to about 100 bar.

If sulphur recovery is made before the hydrolysis step, quite minor amounts of sulphur still may remain in the solution to be hydrolyzed. In that event, suitable provision may be made for recovery or removal to waste of this minor quantity. The residual stream, after the ammonium thiocyanate hydrolysis, may be treated further, or may be used in other plant operations.

In order to demonstrate the removal of HCN from a gaseous stream, the following experiments were conducted.

Procedure A stream of nitrogen containing 1 percent by volume HCN and 0.5 percent by volume H2S was passed at atmospheric pressure at a rate of 2 volumes of gas per volume of solution per minute into a flask containing a 0.3 M solution of ammonium sulphide having 1.56 M sulphur suspended therein. The pH of the solution was 8.9, and the volume of gas treated was about 210 volumes of gas per volume of solution. Temperature of the system was maintained at about 8000. Greater than 99.8 percent of the HCN was removed, and conversion to thiocyanate approached 100 percent.

In a similar manner, a series of runs was made, and the conditions and results are, as set out below: Solution: 0.30 M (NH4)2S Gas Composition: 1% HCN in N2; H2S and NH3 content as indicated below Gas Flow Rate: 290-330 cl/min Pressure: 1 bar Volume of HCN/Volume of Solution ~ 2.1 cl/cl T Elemental Initial NH3 in H2S in HCN HCN f C) sulphur in Solution Feed Feed Removed Converted Solution pH ("/ov) (%v) (%) 1%1 (M) 25 1.6 8.8 0 0.55 > 99.8 99 50 1.6 9 0 0.45 > 99.8 97 80 1.6 8.9 0 0.55 > 99.8 100 50 1.6 7 0 0.55 > 99.4 98 50 0.3 9 0 0.55 > 99.8 97 50 0.06 9 0 0.55 > 98 76 50 0.5 9 1 0.55 > 99.8 100 50 0.5 7 1 0.55 > 99.4 96

Claims (12)

1. Process for the removal of hydrogen cyanide from a gaseous stream containing hydrogen cyanide, comprising the following steps: (a) contacting said gaseous stream in a contact zone with a solution containing ammonium polysulphide to convert hydrogen cyanide into ammonium thiocyanate, thereby producing a solution containing ammonium polysulphide and ammonium thiocyanate, and a gas stream having reduced hydrogen cyanide content; (b) removing at least a portion of the solution containing ammonium polysulphide and ammonium thiocyanate from the contact zone:: (c) decomposing ammonium polysulphide and precipitating sulphur in the solution removed in step (b), thereby producing hydrogen sulphide, sulphur and a remaining ammonium thiocyanate-containing solution; (d) removing sulphur from said remaining solution and hydrolyzing the ammonium thiocyanate in said remaining solution, producing ammonia, hydrogen sulphide and carbon dioxide.

2. Process according to claim 1, in which ammonium polysulphide is decomposed by lowering the pH of the solution removed in step (b).

3. Process according to claim 2, in which the pH is lowered to a value ranging from 7.5 to 8.

4. Process according to claim 2 or 3, in which the pH is lowered by addition of a H2SO4-, HCI-, HNO3- or CH3COOH-solution or H2S- or HCI-gas to the solution removed in step (b).

5. Process according to claim 1, in which ammonium polysulphide is decomposed by stripping the solution removed in step (b).

6. Process according to claim 5, in which the solution is stripped by heating it, by passing a non-reactive gas through it or by a combination of heating and non-reactive gas passing.

7. Process according to claim 6, in which the solution is heated to a temperature of 80-120 C.

8. Process according to claim 6 or7, in which the non-reactive gas is selected from air, steam, carbon dioxide, nitrogen and the inert gases.

9. Process according to any one of claims 5-8 in which the solution removed in step (b) is mixed with a H2S- and NH3-containing aqueous mixture, and the solution is stripped together with the aqueous mixture.

10. Process according to any one of claims 1-9, in which the amount of ammonium polysulphide solution supplied in step (a) contains at least a stoichiometric amount of polysulphide with respect to the hydrogen cyanide.

11. Process according to any one of claims 1-10, in which the gaseous stream comprises a stream derived from the gasification of solid carbonaceous fuel.

12. Process for the removal of hydrogen cyanidefrom a gaseous stream containing hydrogen cyanide according to claim 1 substantially as hereinbefore described with reference to the experiments.