GB2155453A - Treatment of "Stretford" redox solutions - Google Patents

Abstract

The growth and activity of microbiological organisms in an aqueous redox solution comprising at least one salt of an anthraquinone disulphonic acid and at least one water soluble vanadium compound and containing at least one inorganic thio compound is controlled by including at least one water soluble thiocyanate, and introducing oleum or sulphuric acid having a concentration of from 10 to 98% by weight into the bottom of a mass of said solution whereby the molar ratio of sulphuric acid to thiocyanate is at least 1:1 in excess of that required for reaction with any other thio compound, maintaining the reaction mass of said redox solution, thio compounds and acid at a temperature of from 70 DEG C to boiling temperature at ambient pressure, recovering insoluble salts from the reaction mass and returning the redox liquor containing thiocyanate for use in a redox reaction, and thereby to maintain the thiocyanate at a desired level, e.g. about 50gm/litre.

Description

SPECIFICATION Treatment of effluents This invention relates to gas purification processes and, more particularly, to the removal of acid gases from hydrocarbon gas streams.

Acid gases such as hydrogen sulphide can be removed from gas streams by both chemical and physical absorption methods. One well known process is the "Stretford Process" originally described in UK Patent Specification No. 948270.

One problem associated with the Stretford Process has been the build up of oxidised sulphur compounds such as thiosulphate, sulphate and, in some applications, thiocyanate. The working liquor cannot be discharged directly to waste since not only will valuable chemical reagents, e.g. vanadium and anthraquinone compounds, be lost but also such discharge is environmentally unacceptable because of the pollutant effect of the sulphoxy compounds.

The problem in regard to these sulphoxy compounds has been largely overcome by the application of a desalting process described in our US Patent Specification No. 4360508.

A second problem has been that the presence of bacteria and other micro-organisms in Stretford Process solutions can cause operational difficulties with regard to alkali consumption, sulphur flotation and separation. It may be desirable in some cases to use a biocide in solution to control the level of microbial contamination and several proprietary biocides have been identified which are compatible with the process operating conditions. It has, however, been found that these biocides are not totally stable to the process chemistry and it is necessary to add biocide to the solution to maintain its concentration of an effective level because of its degradation. This continual dosing of biocide adds to the daily operating cost of the plant and is generally considered to be uneconomic.

It has been found that suitable biocidal agents are dissolved thiocyanates, e.g. sodium thiocyanate, which are formed in Stretford solutions when the plant is treating gas containing hydrogen cyanide.

It is proposed, therefore, to add thiocyanate to process solutions where it has not been formed due to the presence of HCN in the gas being treated, as a biocidal treatment. Although this addition is acceptable in principal, in practice however, the presence of an added salt in solution will reduce the allowable concentrations of sodium sulphate and thiosulphate which are formed as byproducts in solution and will be lost at an unacceptable rate in the solution purge necessary to control these byproduct salts. The presence of sodium thiocyanate in the effluent would also be environmentally unacceptable in many cases.

This problem can be unexpectedly avoided if the byproduct salts sodium thiosulphate and sulphate were to be treated in accordance with the process prescribed in US 4360508 since this process has limited effect on sodium thiocyanate. Therefore the considerations of purge disposal would be overcome and the lower sulphate and thiosulphate concentrations required because of the presence of thiocyanate would be less critical.

In accordance with the present invention there is provided a process for controlling the growth and activity of micro-organisms in an aqueous redox solution comprising at least one salt of an anthraquinone disulphonic acid and at least one water soluble vanadium compound and containing at least one inorganic thio compound including at least one water soluble thiocyanate, which process comprises introducing oleum or sulphuric acid having a concentration of from 10 to 98% by weight in to the bottom of a mass of said solution whereby the molar ratio of sulphuric acid to thiocyanate is at least 1::1 in excess of that required for reaction with any other thio compound, maintaining the reaction mass of said redox solution, thio compounds and acid at a temperature of from 70 C to boiling temperature at ambient pressure, recovering insoluble salts from the reaction mass and returning the redox liquor containing thiocyanate for use in a redox reaction.

Preferably, the redox liquor after removal of insoluble sulphur compounds, such as sulphates, will contain about 50gm/litre of dissolved thiocyanate.

In order to remove otherthio compounds such as thiosulphates in accordance with the process described in US Patent No. 4360508, the molar ratio of acid to thiosulphate is from 1:2 to 1:3. Under the maximum loading conditions for optimum thiosulphate removal the destruction rate of thiocyanate is about 10%.

However, we have found that the rate of thiocyanate destruction is a function of the excess acid required for thiosulphate destruction. For example, in a thiosulphate removal process operated at 100or in which the acidification stage duration is three hours, the rate of thiocyanate destruction for varying thiocyanates to acid ratios is given in the following table: CNS/SO4 (molar) CNS Conversion % 1:1 0 1:2 48 1:3 93 1:4 98 Thus, in order to maintain a level of thiocyanate in the liquor at, say, 50% gm/litre. The excess acid required calculated once the initial concentration of thiocyanates and otherthio compounds is known.

The invention will be illustrated by the following example.

Three hundred and seventy three (373) litres of a redox liquor having the composition: Feed Composition-gil V 0.57 ADA 1.03 Na2 CO3 10.5 NO HOB 14.5 Na2 S2 O3 186.6 Na CNS 64.8 Na2 SO4 28.8 was reacted with 17 litres of 94% sulphuric acid over a period of 2.75 hours at 97 C.

The composition of the liquor after neutralisation was as foliows: Composition after reaction and neutraffsation - gil V 0.55 ADA 0.99 Na2 S2 O3 8.5 Na CNS 57.9 Na2 SO4 228.3 Thiosulphate Conversion % 95 Thiocyanate Conversion % 10.6 The process of the invention is applicable for control of microbiological growth and activity in acid gas removal processes.

We therefore propose a process whereby gases or non-polar liquids containing hydrogen sulphide may be purified, whereby sodium sulphate of high quality may be recovered and whereby other sulphoxy compounds which are produced may be destroyed.

This process may comprise contacting said gas or non-polar liquid with an aqueous alkaline wash liquor comprising a salt of an anthraquinone disulphuric acid or derivative thereof, a compound of vanadium, and at least one water soluble thiocyanate, contacting said liquor, after contact with said gas or liquid, with an oxygen containing gas, taking a first portion of said oxygenated liquor, acidifying said portion as described aforesaid, returning said acidified liquor to the remainder of the oxygenated liquor, taking a second portion of said mixed oxygenated and acidified liquor, removing elemental sulphurfrom said second portion and subjecting said elemental sulphur fee second portion to a solubility lowering technique to induce crystallisation of sodium sulphate, removing said sodium solphate and returning said second portion, now free of elemental sulphur and sodium sulphate, to the remainder of said oxygenated liquor.

As described aforesaid, the process of the present invention may be employed to increase the overall efficiency of the Stretford Process. Thus, the main purification process may be carried out as described in our UK Patent Specification No. 948270 as well as according to our earlier UK Patent Specification No's 971233 and 878251 and US Patent No's 2,997,439 and 3,035,889, all of which are incorporated herein by reference.

The aqueous alkaline solution may contain approximately 0.5% by weight of any of the isomeric anthraquinone disulphonic acids (which of course will be present in the form of their salts), and may be initially made alkaline by adding ammonia or an alkali metal carbonate or bicarbonate or other base. It has a pH above 7, the preferred value being from 8.5 to 9.5.

The compound of a metal having at least two valency states may be an ortho-, meta-, or pyrovanadate of ammonia vanadate or sodium orthovanadate. Which ever salt is initially added, it would appear that a meta-vanadate is formed in a solution having a pH of about 9. It is preferably added in such quantity as to give a solution of concentration M/1000 to M/20, although concentrations outside this range may be used.

Other metal compounds which may be used in addition to the vanadates are salts or iron, copper manganese, chromium, nickel and cobalt, for example, ferrous sulphate or ferric chloride. Such salts may be used in concentrations of M/1 000 to Mid 00.

Good results are obtainable also by using vanadates together with salts of iron.

A chelating or sequestering agent for the vanadate may be added. Examples of such agents include soluble tartrate such as sodium potassium tartrate or tartaric acid or ethylene diamine tetracetic acid (referred to hereinafter as EDTA), or citric acid or soluble citrates, present in sufficient quantity to complex at least a portion of the vanadate, in order to maintain the solubility of the vanadate in the presence of hydrosulphide.

In carrying out the process of removal of hydrogen sulphide it appears that absorption of the hydrogen sulphide is an alkaline solution occurs with formation of hydrosulphide which is then oxidised. This procedure results in the accelaration of the oxidation process as shown by the following figures relating to the time taken to oxidise 50% of the hydrosulphide in a solution initially containing 340 parts per million of hydrosulphide, using different concentrations of vanadate.

Anthraquinone Time for 50% Disuiphonic Acid Vanadate Conversion M/100 Nil 60 minutes M/100 M/1000 18 minutes M/100 M/500 8 minutes M/100 M/200 2 minutes M/100 M/100 1 minute For best results the higher vanadate and/or metal salt concentrations should be used where the hydrosulphide concentration in the alkaline solutions due to the absorption of hydrogen sulphide is higher.

The precipitated sulphur may be removed, e.g. by filtration, either before or after the regeneration of the solution.

After acidification of the portion of oxidised liquor, the portion is returned to the main liquor stream. The portion will contain all the original components of the Stretford liquor, elemental sulphur, sulphuric acid, sulphate and thiocyanates. Although the pH of the portion is acid, when admixed back into the main stream, the overall pH will not drop.

A further portion of this main stream is now taken and subjected to a sulphur removal step. The sulphur present in the stream is solid elemental sulphur suspended in the liquid phase. This sulphur may be removed either by physical removal technique such as filtration or by chemical techniques to solubilise the sulphur in the aqueous phase of the liquor. The sulphur may be solubilised for example by heating the oxide stream to between 850C and boiling. Preferably the solubilising reaction is effected in a delay tank and the residence time may be from 2 to 5 hours. At the end ofthesolubilising step, all the liquor components are in the aqueous phase. This aqueous phase may then be subjected to a solubility lowering technique. for example as described above. The precipitated sulphate may then be removed for example by filtration and the mother liquor returned to the main process stream. The thiocyanates present in the liquor may be destroyed by contacting the liquor with hydrogen sulphide, i.e. when the liquor is reused for the hydrogen sulphide purification step.

By this process, coal and other fuel gases, effluent air streams, liquid hydrocarbons and other materials can be purified so as to be free from hydrogen sulphide.

Claims (3)

1. A process for controlling the activity and growth of microbiological organisms in an aqueous redox solution comprising at least one salt of an anthraquinone disulphonic acid and at least one water soluble vanadium compound and containing at least one inorganic thio compound including at least one water soluble thiocyanate, which process comprises introducing oleum or sulphuric acid having a concentration of from 10 to 98% by weight into the bottom of a mass of said solution whereby the molar ratio of acid to thiocyanate is at least 1: :1 in excess of that required for reaction with any other thio compound, maintaining the reaction mass of said redox solution, thio compounds and acid at a temperature of from 70 C to boiling temperature at ambient pressure, recovering insoluble salts from the reaction mass and returning the redox liquor containing thiocyanate for use in a redox reaction.

2. A process as claimed in Claim 1 in which the level of dissolved thiocyanate is of about 50gm/litre.

3. A process for the control of growth and activity of microbiological oganisms substantially as hereinbefore described.