GB2029386A - Removal of H2S from gases and liquid hydrocarbons - Google Patents

Abstract

The material containing hydrogen sulphide is washed with an aqueous alkaline solution containing one or more compounds having the general formula: <IMAGE> where A is a C1-C4 straight or branched chain alkylene radical, B is a -SO3M, -CO2M, -PO3HM or -PO3M2 grouping, where M is hydrogen, or a cation giving a water-soluble derivative, R is hydrogen, a cation giving a water soluble derivative, or C1-C4 straight or branched alkyl, R1 is hydrogen, methyl or -COOH and m is 0 or 1 the hydrogen sulphide being oxidised and sulphur liberated; and the reduced anthraquinone sulphonamide being oxidised by contact with free oxygen or a gas containing it. Optionally there is present in addition a compound of a metal having at least two valency states, and if necessary a chelating or sequestering agent for retaining such metal compounds in solution. <IMAGE>

Description

SPECIFICATION Gas purification process The present invention relates to a purification process especially to a process for removing hydrogen sulphide from gases or liquid hydrocarbons containing it as impurity.

The removal of hydrogen sulphide as sulphur from gases is described in British Patents 871,233 and 948,270 in which certain anthraquinone disulphonic acids are used. We have found surprisingly that certain anthraquinone sulphonamides as hereinafter defined have at least as good or superior activity to the anthraquinone disulphonic acids used in the removal of hydrogen sulphide as sulphur from gases by the methods described in the above patents.

According to the present invention there is provided a process for the absorption and subsequent removal as sulphur of hydrogen sulphide from gases or gas mixtures where the gas or gas mixture is washed with an aqueous alkaline solution of one or more anthraquinone sulphonamides having the formula::

where A is a C1-C4 straight or branched chain alkylene radical, B is a -S03M, -CO2M,-PO3HMor ~PO3M2 grouping, where M is hydrogen, or a cation giving a water-soluble derivative, R is hydrogen, or a cation giving a water-soluble derivative or C1-C4 straight or branched chain alkyl Ra is hydrogen, methyl or-COOH, and m is O or 1, whereby the hydrogen sulphide is oxidised and sulphur is liberated, and the reduced anthraquinone sulphonamide is oxidised by contact with free oxygen or a gas containing it.

The present invention further provides a process for the absorption and subsequent removal as sulphur of hydrogen sulphide from gases, gas mixtures and liquid hydrocarbons, in which the material containing hydrogen sulphide is washed with an aqueous alkaline solution containing one or more compounds of the above formula I, one or more compounds of a metal having at least two valency states, and if necessary one or more chelating or sequestering agents for retaining such metal compounds in solution.

When M or R is a cation giving a water-soluble derivative this is preferably an alkali metal, for example, sodium or potassium, or an unsubstituted or substituted ammonium cation for example ammonium or optionally substituted C1-C5 alkylammonium, examples of alkyl substituents being methyl, ethyl, propyl, butyl or mono-, di-, ortri-hydroxyethyl.

Preferably, B is a -SO3M grouping, R1 is hydrogen and M and R are hydrogen, sodium, potassium or ammonium.More preferably B is a -SO3M grouping, R1 is hydrogen and M and R are hydrogen, sodium, potassium or ammonium and A is a C1-C4 straight chain alkylene grouping. Especially preferred compounds are those in which R1 is hydrogen, M and R are hydrogen, sodium, potassium or ammonium and A is methylene or ethylene and m is 1.

The compounds of formula I and their methods of preparation are described in detail in our co pending British Patent Application No. 30844/78.

Examples of preferred compounds for use in the processes of the present invention are the di- and tetra-sodium, potassium and ammonium salts of N,N'-disulphomethylanthraquinone-2,6-disulphonamide, N,N'-disulphomethylanthraquinone-2,7-disulphonamide, N,N'-disulphomethylanthraquinone-1 ,5-disulphonamide, N,N'-'di#u Ipho-2'-ethylanthraquinone-2 ,6-disulphona mide, N,N'-dicarboxymethylanthraquinone-2,6-disulphonamide, and N,N-dicarboxymethylanthraquinone- 2,7-disulphonamide, and mixtures thereof.

In a recent development, our co-pending British Patent Application 30843/78 discloses the use, in the process of BP 948 270 of a sequestering agent comprising a compound containing at least one phosphonic acid grouping. Such sequestering agents may be used in this embodiment of the present invention.

The compound of a metal having at least two valency states may be an ortho-, meta-, or pyrovanadate of ammonia or of an alkali metal for example sodium ammonium vanadate or sodium orthovanadate. The two upper valency states of vanadium are preferably used.

When a vanadium compound is used as the compound of a metal having at least two valency states, an effective ratio of toe compound of formula I to the vanadium compound is conveniently in the range of 3:1 to 1 :2, preferably 2:1 to 1:1 and especially 2:1.5 by weight provided that the ratio of vanadium to hydrogen sulphide is 2:1.

It is desirable to use a compound of formula I which Is substantially free of halogen ions since the presence of halogen ions, particularly chloride ions causes corrosion problems in the plant. The compound of formula I should therefore preferably be prepared from the corresponding anthraquinone sulphonamide rather than the anthraquinone sulphonyl halide.

A particular advantage of a mixture of anthraquinone 2,6 and 2,7-disulphonamides of formula I is that they have improved solubility over the corresponding mixture of disulphonic acids.

The following Examples further illustrate the present invention.

EXAMPLES 1 to 8 Some laboratory tests were carried out to show the effectiveness of the compounds of the present invention as catalysts in the removal of H2S from gases, and to show that these compounds are superior to 2,7-anthraquinone disulphonic acid.

To carry out the test a synthetic simulation of a gas-absorbing solution is prepared to give: 25 gIl NaHCO3 5 9/l Na2CO3 lOg/I Na252O3 8 g/l NaCNS 3.8 gIl NaVO3 2 g/l Catalyst The test solution has a solution pH of 9.0-9.2.

The cell in which the tests were carried out is illustrated in the accompanying Figure 1 in which the cell 1 0 consists of a nominally 1 litre vessel made by Quickfit having body FV1 L and top MAF 3/52 containing an oxygen electrode 11, a temperature compensation probe 12, calomel electrode 13, platinum electrode 14, air inlet 15, and an aeration sintered disc 16.

The dissolved oxygen content is measured on an E.l.L. dissolved oxygen meter and the redox potential measured on a high impedance digital multimeter.

Procedure Prepare 1.5 litres of simulated gas-absorbing solution. Transfer 1 litre of the solution to the Cell, where the solution is oxygenated and de-oxygenated three times by alternately passing air and nitrogen at flow rates of 500 mls/min. The solution is finally left in a de-oxygenated state.

A minimum amount of the de-oxygenated solution is used to dissolve 3.75 g of Na2S which is then returned to the Cell. Although the process is for the removal of H2S the species formed when H2S dissolves in an alkaline absorbing solution is the HS- ion. Therefore, for ease of operation of the test it was decided to introduce the HS- ion using Na2S. The S-- ion from Na2S gives HS- at the solution pH of the simulated gas absorbing liquor. After the reduction with Na2S the redox potential and dissolved oxygen content are allowed to stabilise for 10 minutes while a low flow of nitrogen maintains agitation.

The solution is then re-oxidised by passing air at a flow rate of 500 mls/min. The dissolved oxygen content and the redox potential are monitored continuously. The oxidation is continued until the redox potential and dissolved oxygen concentration have stabilised. The solution is then de-aerated by passing nitrogen at a flow rate of 500 mis/min, and further reduced by a fresh addition of 3.75 g of Na2S. The procedure of reduction with further Na2S and subsequent re-oxidation by air blowing (with measurements of dissolved oxygen concentration and redox potential) is carried out three times and the precipitated sulphur is filtered off after each cycle.

Results The accompanying Figure 2 shows a schematic diagram of the results obtained from the Cell Test.

The parameters which indicate the efficiency of the catalyst system are: i) t20% - time for the solution to reach 20% of the saturation concentration of dissolved oxygen (ii) t,, -- time for the solution to reach 80% of the saturation concentration of dissolved oxygen.

(iii) the ratio of Es/Ef, where Es is the ledox potential of the system in the fully reduced state and Ef is a measure of the redox potential of the system in an oxidised state. For convenience in our test Ef is taken as the redox potential at t80%.

The results quoted in Table 1 are the average of the data from three cycles of reduction and reoxidation.

Table I shows a comparison of 2,7-anthraquinone disulphonic acid with the following compounds or mixtures of compounds of the present invention.

A. Tetrasodium N,N'-disulphomethylanthraquinone-2,6/2,7-disu Iphonamides.

B. Disodium N,N'-disu lphomethylanthraquinone-2,6-disulphonamide C. Disodium N,N'-disulpho-2-ethylanthraquinone-2,6-disulphonamide D. Tetrasodium N, N'-disu Ipho-2-ethylanth raqu inone-2,6/2,7-disulphonam ides.

E. Tetrasodium N,N'-dicarboxymethylanthraquinone-2,6/2,7/disulphonamides.

F. Tetrasodiu m salt of N,N'-disulphomethylanth raquinone-2,7-disulphonamide.

G. Tetrasodium salt of N,N'-disu Iphomethylanthraquinone-1 ,5-dissulphonamide H. Tetrasodium sa Its of N,N'-disu lphomethylanthraquinone- 1 ,6/1 .7-disulphonam ides.

TABLE 1 COMPARISON OF PRODUCTS OF THE PRESENT INVENTION WITH 2,7 ADA

Dissolved Oxygen Measurements Potential Measurements Example Catalyst t20% t80% Es Ef Ratio of mins mins mV mV Es/Ef Comparison 2,7 ADA 9 16 -430 -197 2.2 1 Compound A 9 18 411 -175 2.4 2 Compound B 7 10 -439 -180 2.4 3 Compound C 14 22 -420 -175 2.4 4 Compound D 7 24 -455 -157 2.9 5 Compound E 4 8 -477 -188 2.5 -16 Compound F 8 15 -404 -161 2.5 7 Compound G 7 9 -392 46 4.5 8 Compound H 7 9 -401 -148 2.7 Some of the reactions by which formerly-gaseous oxygen is fixed in solution are ionic in nature and are comparatively rapid. Generally, as long as there remain ionic compounds in a reduced state in the liquor the dissolved oxygen content remains at 5% or less of the saturation concentration of dissolved oxygen in the liquor.Therefore, whether these components of the solution are in reduced or oxidised state can be determined by measuring the dissolved oxygen concentration. The reoxidation times t20% and t,,. therefore give indications of the rate of system re-oxidation in the presence of the various catalysts.

The process, being an oxidative process, is dependent upon redox couples. The degree of oxidation of the solution determines the redox potential. Thus the degree of oxidation is measured instantly by measuring the redox potential.

The liquor consists of a mixture of at least three redox couples. The single electrode potential is related logarithmically tithe concentration of the oxidised and reduced species in solution: (Concentration of Oxidised forms) Potential = Standard Potential + constant x log10 (Concentration of Reduced forms) The ratio of Es/Ef has been taken by other workers to indicate the degree of re-oxidation that has occurred. The effectiveness of a catalyst can be decided from the combination of dissolved oxygen measurements and redox potential measurements. The results generally obtained with the Cell test appear to fall into three main types: i) those which have long times to t20% (i.e. 30 mins. or greater) but have a high ratio of Es/Ef (i.e. 2.2 or greater).

ii) those which have intermediate times to t20% (i.e. about 10 mins.) and a high ratio of Es/Ef (i.e. 2.2 or greater) iii) those which have fast times to t20% (i.e. 1-3 mins) but have a low ratio of Es/Ef (i.e. < 2.0) The interpretation of these results is that type (i) is a slow but effective catalyst, type (ii) is an effective catalyst, and type (iii) is an ineffective catalyst.

When the dissolved oxygen concentration in the solution has reached 20% of the saturation concentration the majority of the redox reaction has taken place. There is ample dissolved oxygen available so the time from t20% to t80% is mainly a function of the reactivity of the catalyst. Therefore, the smaller the time interval between t20% and t80% the more easily the catalyst is reoxidised and provided the ratio of Es/Ef is greater than 2.2, the more effective the catalyst would be in the system.

It can be seen that the compounds of formula I ail fall into the type (ii) category and have activity at least as good or superior to that of 2,7-anthraquinone disulphonic acid.

Claims (8)

1. A process for the absorption and subsequent removal as sulphur of hydrogen sulphide from gases or gas mixtures where the gas or gas mixture is washed with an aqueous alkaline solution of one or more anthraquinone sulphonamides of the general formula I

wherein A is a C1-C4 straight or branched chain alkylene radical, Bisa-SO3M, -CO2M, -PO3HM or-PO3M2 grouping where M is hydrogen or a cation giving a water-soluble derivative, R is hydrogen, a cation giving a water-soluble derivative or C1-C4 straight or branched chain alkyl, R1 is hydrogen, methyl or --COOH and m is O or 1 whereby the hydrogen sulphide is oxidised and sulphur is liberated, and the reduced anthraquinone sulphonamide is oxidised by contact with free oxygen or a gas containing it

2. -A process for the absorption and subsequent removal as sulphur of hydrogen sulphide from gases, gas mixtures and liquid hydrocarbons, in which the material containing hydrogen sulphide is washed with an aqueous alkaline solution containing one or more compounds of the formula I defined in claim 1, one or more compounds of a metal having at least two valency states, and if necessary, one or more chelating or sequestering agents for retaining such metal compounds in solution.

3. A process as claimed in claim 1 or claim 2 in which in the formula I, B is a -SO3M grouping, Rt is hydrogen, M and R are hydrogen, sodium, potassium or ammonium, A is methylene or ethylene and m is 1.

4. A process as claimed in claim 1 or claim 2 in which the compound of formula I is the di- or tetra-sodium, potassium or ammonium salt of: N,N'-disulphomethylanthrnquinone-2,6-disulphonamide N,N'-disulphomethylanthraquinone-2,7-disulphonamide N,N'-disulphomethylanthraquinone-1 ,5-disulphonamide N,N'-disulpho-2-ethylanthraquinone-2,6-disulphonamide N,N'-dicarboxymethylanthraquinone-2,6-disulphonamide or any mixtures thereof.

5. A process as claimed in any of claims 2 to 4 in which the metal having at least two valency states is vanadium.

6. A process as claimed in claim 5 in which the effective ratio of the compound of formula I to the vanadium compound is in the range of 3:1 to 1:2.

7. A process as claimed in claim 2 substantially as described in any of Examples 1 to

8.