GB2137606A - Process for the purifying Claus tail gas - Google Patents

Abstract

The invention relates to a process for purifying Claus tail gas by catalytic reduction of the said gas with formation of hydrogen sulphide, cooling of the gas, condensing and separating water from the exhaust gas, adjusting the H2S/SO2 ratio necessary for the further Claus reaction by combining the tail gas with an SO2- containing gas produced by combustion and conducting the combined gases through a first Claus catalyst stage below the sulphur dew point. The process is characterised in that the SO2- containing gas produced by combustion is cooled to a regeneration temperature or cooling temperature and conducted through the second catalyst stage charged with sulphur or respectively freed from sulphur and thereby regenerates or cools the said stage, the sulphur taken up in the regeneration phase is condensed out and separated from the SO2-containing gas by cooling and the SO2-containing gas, after being combined with the tail gas, is conducted through the first catalyst stage for the formation and separation of sulphur.

Description

SPECIFICATION Process for the purifying Claus tail gas The invention relates to a process for purifying Claus tali gas, with simultaneous recovery of sulphur, by catalytic reduction of the tail gas at elevated temperature with formation of hydrogen sulphide, cooling the gas, condensation and separation of water from the tail gas, adjustment of the H2S/SO2 molar ratio necessary for a further Claus reaction by combining the tail gas with an SO2-containing gas produced by combustion of an H2S-containing gas and conducting the combined gases through a first Claus catalyst stage below the sulphur dew point, while a second Claus catalyst stage is regenerated by passing a hot gas therethrough.

A method of purifying Claus tail is known from German Auslegeschrift 21 01 112, in which water is extracted from the gases after their hydrogenation before they are further reacted in an additional Claus stage. It is true that the formation of sulphur after the Claus reaction is promoted by the extraction of water. Since the Claus stage, however, operates above the sulphur dew point, the sulphur yield is not optimal, because of the less favourable equilibrium position of the reaction. It is indicated as being 98 to 99.5%. A substantially similar method is known from German Offenlegungsschrift 21 34 742, in which the hydrogenated Claus tail gas, largely freed from water, is mixed, prior to the additional Claus stage, with an SO2-containing gas produced by burning an H2S-containing gas in the H2SlSO2 molar ratio which is necessary for the Claus reaction.In this case also, the additional Claus stage is operated above the sulphur dew point, so that the said disadvantage is also produced in this case.

A method of removing hydrogen sulphide and sulphur dioxide from Claus tail gases is known from German Patent Specification 29 23 895, in which the tail gases are separated into two component flows, of which the first is conducted at temperatures above the sulphur dew point through a catalyst bed charged with sulphur and the bed is regenerated by the vaporised sulphur being separated from this stream by cooling and condensation, and this component stream is united with the second component stream and the united streams are conducted at temperatures below the sulphur dew point through a second catalyst bed. Although the yield of sulphur is raised to about 98.5 by this additional cold bed stage, the pollution of the atmosphere and the sulphur loss are still considerable, so that the method needs to be improved.

Finally, a method of purifying Claus tail gas is known from European Patent Application 0 059 548, in which the sulphur components contained in the tail gas are reduced catalytically to hydrogen sulphide, water is condensed out of the said gas and then, after adding a quantity of an SO2-containing gas, as is necessary for the further Claus reaction, the further reaction is carried out in a cold bed below the sulphur dew point. In this case, a hot regeneration gas is to be produced for the regeneration of the charged cold bed.

The present invention seeks to improve the separation of sulphur from the tail gas of Claus installations with the assistance of a Claus catalyst stage connected on the outlet side. It is more especially intended to provide a Claus tail gas purification with low-temperature Claus stages, with which the regeneration gas for the Claus stages is so available from the process that its temperature, corresponding to the regeneration and cooling phases, can be controlled in a heat exchanger.

According to the invention, the purification method for Claus tail gas as initially referred to is characterised by the fact that the SO2-containing gas produced by combustion is cooled to a regeneration temperature or cooling temperature and is conducted through the second catalyst stage charged with or freed from sulphur and thereby regenerates or cools this stage, the sulphur taken up in the regeneration phase is condensed out of the SO2containing gas by cooling and separated and the S02-containting gas, after being united with the tail gas, is then conducted through the first catalyst stage for the sulphur formation and separation.

Using the method according to the invention, we have found that the yield, which in any case is good with Claus low-temperature catalyst stages, is not only improved by the previous water separation, but also essential operational advantages are achieved: since the S02-containing combustion gas is available hot, it is no longer necessary to heat a cold gas to the regeneration temperature. A flexible temperature control according to the regeneration and cooling phases is readily possible with the naturally hot S02-containing combustion gas. Finally, the Claus reaction in the cold bed offers the advantage that the cold waste or exhaust gas from the water condensation and separation no longer has to be heated to high temperature. The sulphur yield amounts to about 99 to 99.8%.

With the preferred embodiment of the method according to the invention, the SO2-containing combustion gas is cooled during the regeneration phase to a temperature in the range from 160 to 3600C and, during the cooling phase, to a temperature in the range from 100 to 160 C. The S02-containing gas is available, from the combustion, at for example 400 to 4500C. The cooling is preferably effected by the hot SO2-containing gas, before entering the second catalyst stage, being conducted through a heat exchanger with a regulatable by-pass for the purpose of adjusting the regeneration or cooling temperature.Depending on the setting of the by-pass valve, the cooling action of the heat exchanger being traversed by the gas and thus the gas temperature at the outlet of the exchanger and inlet of the second catalyst stage (for regenerating or cooling) can be varied and adapted to the particular phase.

According to the preferred form of the method according to the invention, the S02-containing gas is produced by burning a part of the Claus tail gas after reduction and water separation has taken place and possibly a part of the acid gas fed to the Claus installation. The burning of dehydrated Claus tail gas supplies an S02-containing gas with a correspondingly low water content. The use of this gas, after being combined with the remaining part of the H2S-containing Claus tail gas in the cold bed Claus stage guarantees that also the combined gases have a low water content and consequently the equilibrium of the Claus reaction is favourable.

The flow of the SO2-containing gas through the Claus catalyst stages is reversed when the second stage, after regeneration has taken place, is cooled and the first stage is charged with sulphur. The H2S-containing waste gas is in each case connected to the S02-containing gas stream or flow, which is flowing to the catalyst stage which is in the reaction phase. The reaction temperature of the united gases is expediently determined by the heating-up temperature of the H2S-containing waste gas.

The SO2-containing combustion gas is advantageously produced by partial combustion of the H2S-containing gas. Due to the sub-stoichiometric combustion, there is still H2S remaining in the combustion gas, as a result of which this gas is particularly suitable for the regeneration.

The invention is hereinafter more fully described by reference to the drawing, in which is represented the flow diagram of an installation for carrying into effect one embodiment of the method.

The tail gas of a Claus installation delivered at 1400C through a pipe 1 is supplied to an in-line burner 2, in which its temperature is raised to 350 C.

In the burner 2, heating or fuel gas is burnt with air, which is supplied through the pipes 3 and 4. At the same time, hydrogen is supplied as reduction gas through a pipe 5. Through a pipe 6, a mixture of H2S, SO2, COS, CS2, S, H2 and N2 passes to the hydrogenation reactor 7, in which SO2 and also the carbon disulphide compounds and elementary sulphur are hydrogenated to H2S. The hydrogenation reactor is provided with a cobalt/molybdenum catalyst. The reduction takes place at about 300 C. The exhaust flow of the hydrogenation reactor is first of all cooled in the heat exchanger 8 with the cold dehydrated tail gas and then in the tubular boiler 9 with boiler feed water supplied through a pipe 10, with generation of steam, to 1 600C.At this temperature, the hydrogenated gas passes into a cooling tower 11, in which it is further cooled to about 40 C by direct cooling with water. In the cooling tower 11, the major part of the water present in the hydrogenated Claus tail gas is condensed. The water is constantly circulated by pumping and in this way, by cooling in the condenser 12, is kept as far as possible close to ambient temperature. The quantity of condensed water is drawn off at 13. The H2S dissolved in the water is driven off in a stripper (not shown) and returned into the Claus installation.

The dehydrated tail gas leaves the cooling tower at 40 C and is heated in the heat exchanger 8 to 140 C.

About one third of this gas passes by way of a pipe 14to a burner 15, in which heating or fuel gas supplied through a pipe 16 is burnt with air supplied through a pipe 17. At the same time, acid gas from the Claus installation can be supplied through a pipe 18. The SO2-containing gas of combustion which leaves the burner 15 at a temperature of 400 to 450 C passes into a heat exchanger 19 which is provided with a hot by-pass 26 and in which it can be cooled to variable temperatures in the range from 140 to 3600C with generation of steam.The SOrcontaining gas stream, cooled for example to 3500C, then serves above all for the regeneration of the Claus catalyst stage 22 charged with elementary sulphur via a pipe 25, by which this gas stream is conducted through the stage and there takes up the vaporising sulphur.

The SOrcontaining gas stream charged with sulphut then passes into the sulphur condenser 21, in which it is cooled to about 1400C with generation of steam and sulphur condensation and separation.

The gas stream is then combined with the remaining two thirds of the dehydrated Claus tail gas from pipe 20 and supplied at 1 40"C to the Claus catalyst stage 23. Taking place here is an extensive conversion to sulphur of the H2S and SO2 contained in the gas mixture, which sulphur is deposited on the catalyst of this stage. The tail gas thus freed from sulphur thereafter leaves the installation by way of a pipe 24.

After the catalyst in the stage 22 is regenerated by sulphur vaporisation, the temperature of the SO2containing gas is lowered by opening the by-pass pipe on the exchanger 19, so that the catalyst of the stage 22 is cooled to below the sulphur dew point.

When the catalyst in the stage 22 is cooled and the catalyst of the stage 23 is charged with sulphur, the temperature of the SO2-containing gas in pipe 25 is raised again by closing the valve in the by-pass pipe 26 of the exchanger 19. The SO2-containing gas flow is then switched to the Claus catalyst stage 23, in order to regenerate this stage. The Claus reaction and sulphur separation then take place in the stage 22. The liquid sulphur separated in the sulphur condenser 21 is drawn off at 21 a or 21 b, depending on the direction of flow of the SO2-containing#gas.

Apart from the composition of the gas flows, the manner in which the cold bed Claus stage 21-25 operates corresponds to the installation which is described in German Patent Specification 29 23 895.

Example 37,000 Nm3/h of tail gas from a Claus installation with 0.8% by volume of H2S, 0A% by volume of SO2, 0.2% by volume of COS + CS2, 1.4% by volume of H2 and 1.5 g/Nm3 of elementary sulphur, which are available at a temperature of 130 C under a pressure of 1.3 bar (a), are heated by means of combustion of 420 Nm3/h of heating or fuel gas in an in-line burner to 3000C. Before the heating operation, 135 Nm3/h of H2 rich gas is admixed with the tail gas. The mixture reacts over a CoMo catalyst on an aluminium oxide base at 3200C. Effected in this way is the hydrogenation of SO2 and the hydrolysis of the carbon disulphide compounds to hydrogen sulphide. The tail gas contains 1.36% by volume of H2S and also traces of COS. The gas is cooled to 40 C, first of all indirectly and thereafter by direct heat exchange with circulating water. A condensation of 7.62 t/h of water is hereby obtained, which is separated from the water circulation. The gas coming from the direct condenser is heated with the gas coming from the hydrogenation stage to 1400C in a gas/gas heat exchanger.

Of the 30,900 Nm3/h of the hydrogenated, dehydrated gases, which contain 551 Nm3/h of H2S, about 40% are separated out. The separated gas stream is heated to about 41 00C in an in-line burner, in which are burnt 460 Nm3/h of acid gas (60% by volume of H2S) and in this way the necessary quantity of SO2 is produced. 14,655 Nm3/h of combustion gas with 1.87% by volume of SO2 and 1.50% by volume of H2S are formed.

This gas is cooled in a heat exchanger with by-pass to 3500C and directly conducted into a Claus catalyst stage charged with sulphur. The sulphur vaporises from the catalyst into the hot SO2containing gas stream, which is thereafter cooled in a sulphur condenser to 1400C,the sulphur being again condensed and separated from the gas stream. The SO2-containing gas is thereafter combined with the remaining 60% of the H2S-containing gas, the temperature of 140 C, which is suitable for the low-temperature Claus reaction, being adjusted in the mixture. The mixture is thereafter conducted through another Claus catalyst stage, in which formation and separation of sulphur take place on the catalyst.The gas stream of 33,195 Nm3/h, which is conducted into this additional Claus catalyst stage, contains 1.66% by volume H2S, 0.83% by volume SO2 and about 5.3% by volume of H20. By the conversion to sulphur, which takes place in this catalyst stage, the content of the sulphur compounds in the tail gas are lowered to 600 ppmv of SO2 and 1200 ppmv of Hips.

After the separated sulphur is completeiy volatilised from the first Claus catalyst stage, the temperature of the SO2-containing gas flowing into this stage is lowered to 1400C and as a result the first catalyst stage is cooled to about 140 C. After cooling the first catalyst stage and completely charging the second catalyst stage, the temperature of the SO2containing gas is again raised to 350 C, and the SO2-containing gas is switched to the second Claus catalyst stage, in order to regenerate this latter, while at the same time the Claus reaction and sulphur charging are effected in the first catalyst stage.

Claims (7)

1. Process for purifying Claus tail gas, with simultaneous recovery of sulphur, by catalytic reduction of the tail gas at elevated temperature with formation of hydrogen sulphide, cooling the gas, condensing out and separating the water from the tail gas, adjustment of the H2S/SO2 molar ratio for the further Claus reaction by combining the tail gas with an SO2-containing gas generated by combustion of an H2S-containing gas and conducting the combined gases through a first Claus catalyst stage below the sulphur dew point, while a second Claus catalyst stage is regenerated by passing a hot regeneration gas therethrough, wherein the SO2- containing gas produced by combustion is cooled to a regeneration temperature or cooling temperature, respectively, and is conducted through the second catalyst stage, which is charged with sulphur and freed from sulphur, respectively, and thereby respectively regenerates or cools this latter stage, the sulphur absorbed in the regeneration phase is condensed out and separated from the SO2containing gas by cooling and then the S02containing gas, after being combined with the tail gas, is conducted through the first catalyst stage for the formation and separation of sulphur.

2. Process according to claim 1, wherein the SO2-containing combustion gas is cooled during the regeneration phase to a temperature in the range from 160 to 3600C and during the cooling phase to a temperature in the range from 100 to 1 600C.

3. Process according to claim 1 or 2, wherein the SO2-containing combustion gas, prior to entering the second catalyst stage, is conducted through a heat exchanger with adjustable by-pass for the purpose of adjusting the regeneration temperature and cooling temperature, respectively.

4. Process according to any one of claims 1 to 3, wherein the SO2-containing gas is produced by burning a part of the Claus tail gas after water separation has taken place and possibly a part of the acid gas supplied to the Claus installation.

5. Process according to any one of claims 1 to 4, wherein the flow of the SO2-containing gas through the Claus catalyst stages is reversed when the second stage is cooled and the first stage is charged.

6. Process according to any one of claims 1 to 5, wherein the SO2-containing combustion gas is produced by partial combustion of a gas which contains Hips.

7. Process for purifying Claus tail gas, substantially as described herein with reference to the accompanying drawing or with reference to the Example.