WO1996039242A1

WO1996039242A1 - Treatment of hydrogen sulphide-containing gas streams

Info

Publication number: WO1996039242A1
Application number: PCT/GB1996/001337
Authority: WO
Inventors: Robert De Haan
Original assignee: Sasol Technology (Proprietary) Limited; Sasol Chemicals Europe Limited
Priority date: 1995-06-06
Filing date: 1996-06-05
Publication date: 1996-12-12
Also published as: ZA961746B; TW324667B; AU5906596A

Abstract

A process for treating a hydrogen sulphide-containing gas stream is provided. The process comprises bringing, in a reaction zone, the hydrogen sulphide-containing gas stream into contact with a precipitated iron oxide catalyst. Hydrogen sulphide in the gas stream is thereby converted to elemental sulphur.

Description

Treatment of Hydrogen-Sulphide Containing Gas Streams

THIS INVENTION relates to the treatment of hydrogen sulphide-containing gas streams. It relates in particular to a process for treating a hydrogen sulphide-containing gas stream.

According to the invention, there is provided a process for treating a hydrogen sulphide-containing gas stream, which process comprises bringing, in a reaction zone, the hydrogen sulphide-containing gas stream into contact with a precipitated iron oxide catalyst, thereby to convert hydrogen sulphide in the gas stream to elemental sulphur.

The Applicant has, surprisingly, found that such a precipitated iron oxide catalyst has a high activity for selectively oxidizing hydrogen sulphide in gas streams, such as gas streams containing hydrogen sulphide as well as high proportions of carbon dioxide and unsaturated hydrocarbons, to elemental sulphur. Furthermore, such a catalyst is cheaper to manufacture than known catalysts, such as impregnated carried based catalysts. The Applicant has further unexpectedly found that conversions in excess of 99% and sulphur yields between 85% and 90%, or higher, can be achieved; the balance of the converted hydrogen sulphide goes to S0_2; with substantially no sulphur trioxide being present in the product gas . The following reactions thus occur: nH₂S + 7₂0₂ → 7₈S₈ + nH₂0 (1)

S₈ + 80, 8S0, (3)

/₂S₈ + 8H,0 8H₂S 4S0, (4)

Because of the high activity of the catalyst, the process can advantageously be operated at a sufficiently low reaction temperature so that undesired overoxidation to sulphur dioxide in accordance with reactions (2) and (3) is largely avoided. Furthermore, as a result of the high catalyst activity, it was also unexpectedly found that a minimum average catalyst pore radius, eg of at least 150A, is not essential, and the catalyst pores can thus be much smaller, if desired.

Precipitated iron oxide catalysts having a broad range of compositions and prepared by any suitable process can, in principle, be used. However, in one embodiment of the invention, the catalyst prepared by the method of Frohning et al (CD Frohning, W Rottig and F Schnur in J Falbe (ed) , Chemierohstoffe aus Kohle, George Thieme, Stuttgart, 1977, p 234) can be used. This publication is hence incorporated herein by reference. However, in another embodiment of the invention, the catalyst may, in addition to the iron oxide, also contain other metal compounds, provided these compounds have no Claus activity. To stabilize the surface area and pore structure the catalyst may also contain suitable structural promoters, again providing that these have no Claus activity. Silica, alumina, silica alumina, titania, chromia and magnesia have been found to be suitable promoters. The promoter, when present, may be added to the precipitated iron oxide before drying thereof. Conveniently it may be added as an alkali waterglass solution or as a silica sol (when silica is used as the structural promoter) or in any suitable manner known to those skilled in the art. To further strengthen the catalyst it may be sintered at a suitable temperature before used. In other words, the precipitated iron oxide catalyst is then a sintered precipitated iron oxide catalyst. Sintering may be effected at between 500°C and 900°C, preferably at about 700°C. However, it has been found that sintering reduces the activity per volume of catalyst by reducing the surface area and therefore requires a higher reaction or operating temperature.

The precipitated iron catalyst may have a surface area of 2-300m²/g, typically 30-200m²/g. The catalyst may have a pore volume of 0, 9-0, 05cm³/g, preferably 0,6-0, 05cm³/g, and

*t-.y—p.i.^*c—a—l-Il^*],y, n0,4 Λ-e0t,*l*lcnnm,³³/ I rgv.

The catalyst will then have an average pore diameter determined according to the following formula (5) : 40000 x PV average pore diameter (A) = SA (5) where PV = pore volume in cm³/g

SA = BET surface area in m²/g

Thus, the average pore diameter may, at least in principle, range widely between about 7A when the pore volume is 0,05cm³/g and the surface area is 300m²/g, and about 18000A when the pore volume is 0,9cm³/g and the surface area is 2m²/g. However, the average pore diameter will normally be between about 10A and about 250A, for example from about 10A to less than 150A, typically 10A to 80A.

The process may include maintaining the reaction zone, ie may include effecting the reaction of the hydrogen sulphide to elemental sulphur, at substantially atmospheric pressure, and preferably at a temperature between 150°C and 350°C. At temperatures below 150°C, the conversion becomes unacceptably low, while above 350°C the selectivity to sulphur dioxide becomes unacceptably high. The catalyst can, however, be used at pressures greater than atmospheric pressure, if desired.

The process may include utilizing a space velocity of up to 8000 volumes gas per volume catalyst per hour in the reaction zone. With increasing space velocity the temperature needs to be increased to obtain the same or similar conversion. The preferred space velocity is, however, 1000-3000 volumes gas per volume catalyst per hour.

The process may also include feeding any suitable oxygen-containing gas, eg air, or even pure oxygen, into the reaction zone to provide oxygen for oxidation of the hydrogen sulphide. Sufficient oxygen-containing gas may then be used so that the oxygen to hydrogen sulphide molar ratio is at least 0,5:1, typically between 0,5:1 and 3,5:1. Higher ratios can be used, if desired, but it has been found that this will not increase sulphur yield. It has also been found that higher ratios do not affect the S0₂ formation. For a particular catalyst, the S0₂ formation was found to increase with increasing temperature, but to be independent of the 0₂ to H₂S ratio.

The gas stream may comprise, by volume, less than 20% hydrogen sulphide, eg 0,5-1,5% hydrogen sulphide and carbon dioxide. When the gas stream contains more than about 1,5% hydrogen sulphide, it will normally be necessary to cool down the reaction zone. Typically, such cooling may be effected by means of cooling units or by recycling quenched product gas from the reaction zone, to the reaction zone. Additionally, the gas stream may comprise light saturated and/or unsaturated hydrocarbons. When such hydrocarbons are present, they may typically be present in a proportion of up to 20% by volume, typically 1,5-2,0% by volume. The gas stream may, in particular, be the off or tail gas stream from the so-called Rectisol (trade mark) process.

The catalyst is preferably used in the form of a fixed bed, and catalyst particles having a size of 2mm to 5mm can be used.

The invention will now be described by way of the following non-limiting examples:

EXAMPLE 1

A catalyst was prepared according to the method described by Frohning et al (referred to above) . Thus, in accordance with this method, iron (Fe) and copper (Cu) are separately dissolved in nitric acid at elevated temperature. The clarified solutions are adjusted to lOOg Fe/1 and 40g Cu/1. A small excess of nitric acid is used to prevent precipitation caused by hydrolysis and the solutions are stored separately.

Precipitation is carried out by addition of a boiling iron- copper nitrate mixed solution (40g Fe and 2g Cu/1) to a boiling sodium carbonate solution. The addition is carried out within 2 to 4 minutes to a final pH of 7 to 8 with vigorous stirring and extraction of the released C0₂.

The resulting suspension is filtered and the filtercake is washed with steam condensate until free of alkali (sodium ions) . The washed filtercake is re-slurried with steam condensate and the slurry is mixed with an amount of potassium waterglass solution so that the catalyst contains ~25g Si0₂ per lOOg Fe. As technical potassium waterglass usually contains Si0₂ and K₂0 in a 2,5:1 mass ratio, the excess K₂0 has to be removed. An accurately determined amount of nitric acid is therefore added to the slurry. After another filtration step the filtercake is again washed with steam condensate.

The resultant filtercake has the composition by mass 100 Fe : 25 Si0₂ : 5 K₂0 : 5 Cu. The cake is partially dried, extruded and finally dried to a residual moisture content of 3 mass %. After crushing to a particle size of 2mm to 5mm and separation of over- and undersize particles the catalyst is obtained.

The catalyst had a BET surface area of 292 m²/g, a pore volume of 0,57 cm³/g and average pore diameter of 78A. The average pore diameter is based on the pores being cylindrical, and using formula (5) :

40000 x PV average pore diameter (A) = SA (5) where PV = pore volume in cm³/g

SA = BET surface area in m /g

In all the examples, a Micromeritics model Gemini 2375 (trade mark) surface area analyzer was used to determine the surface area and pore volume of the catalyst by nitrogen adsorption.

The catalyst was crushed and sieved to a particle size of 850-500 microns. Ten cm³ catalyst was loaded into a stainless steel tube inside an electric furnace. The temperature was measured by means of a thermocouple in the catalyst bed. The gas to be treated and air were fed through separate mass flow controllers and mixed before entering the reactor. The catalyst was heated to 220°C under air and the H₂S containing gas, comprising (by volume) 96% C0₂ and 1,5-2% light, unsaturated hydrocarbons was introduced in stages to avoid formation of FeS.

The results are shown below:

Tempera¬ Hours % H₂S in GHSV¹¹ 0₂:H₂S % % ture on¬ the feed molar Conver¬ Sulphur line (vol ratio sion yield basis)

208°C 417 0,68 2017 1,53 99,1 88,3

i) GHSV = gas hourly space velocity = volume gas per volume catalyst per hour

EXAMPLE 2

The catalyst of Example 1 was calcined in air at 700°C for 24 hours to decrease the surface area and increase the average pore size. The calcined catalyst had a BET surface area of 10,2 m²/g, a pore volume of 0,06 cm³/g and average pore diameter of 235A. The catalyst was also crushed and screened and 10 cm were loaded into the same reactor as in Example 1 above.

This catalyst gave the following results:

Tempera¬ Hours % H₂S in GHSV 0₂:H₂S % % ture on¬ the feed molar Conver¬ Sulphur line (vol ratio sion yield ' basis)

262°C 565 0,68 2036 3,15 99,4 89,7

EXAMPLE 3

A catalyst was prepared according to the following procedure:

A solution of 1kg sodium carbonate in 101 distilled water was heated to 80°C. To this was added a solution of 40g/l iron as ferric nitrate, also heated to 80°C, over a period of about 10 minutes to a final pH of 6,9. The resulting slurry was filtered and washed with distilled water to remove the sodium ions. The washed filter cake was re-slurried with distilled water and sufficient sodium waterglass solution was added to give 26 parts by mass of silica per 100 parts of iron. The slurry was again filtered and the filter cake dried at 80°C. The dried cake was broken up and screened to obtain 1,0-1,4mm particles.

The surface area of the catalyst was 269m²/g and the pore volume 0,49cm³/g. This gives an average pore diameter of 72A, calculated according to formula (5) . EXAMPLE 4

Five cm³ catalyst, prepared according to Example 3 was loaded in a 2cm diameter glass reactor tube inside an electric furnace. A 5cm layer of 2mm diameter glass beads was loaded on top of the catalyst. The temperature was measured by means of a thermocouple placed centrally in the catalyst bed. The gas to be treated and air were fed through separate mass flow controllers and mixed before entering the reactor. The gas to be treated comprised (by volume) 96% C0₂, 1,5-2% light saturated and unsaturated hydrocarbons and lOOOOppm H₂S. The results are shown below:

Tempera¬ Hours ppm H₂S GHSV¹' 0₂:H₂S % % ture on¬ in feed molar Conver¬ Sulphur line (vol basis) ratio sion yield

195°C 91 10000 2044 2,21 98,9 90,9

i) GHSV = gas hourly space velocity = volume gas per volume catalyst per hour

EXAMPLE 5

A catalyst prepared as in Example 3 was calcined in air at 700°C for 13 hours. The resulting calcined catalyst had a BET surface area of 97m²/g, a pore volume of 0,36cm³/g and average pore diameter of 148A. It was tested as in Example 4. The results are shown below:

230°C 184 11500 2036 2,34 99,0 90,2 These results show that a precipitated iron catalyst is highly active for the selective oxidation of low concentrations of H₂S in gas streams. The catalyst is not affected by unsaturated hydrocarbons and no deactivation could be detected after several hundred hours on stream. The examples show that a high surface area is desirable to give the highest possible activity and high sulphur yields.

EXAMPLE 6

A catalyst, prepared as in Example 3, was extruded to give, after drying, extrudates of 2,3mm diameter. The catalyst was calcined in air in a muffle furnace at 700°C for 13 hours. 351 of the calcined catalyst were loaded into a fixed bed adiabatic pilot plant reactor to give a bed depth of 500mm. A gas containing about 96% C0₂, 1,5-2% light saturated and unsaturated hydrocarbons and 11050ppm H₂0 was passed through the bed at a flow of 62,3Nm³/h together with 8,5Nm³/h air at an inlet temperature of 173°C and atmospheric pressure. The results are shown below:

Hours Inlet Outlet Pressure 0₂:H₂S % % % on temp. temp. drop molar Conver¬ Sulphur Sulphur line ratio sion selectivity yield

166 173°C 233°C 1, 9kPa 2,3 97,6 95,1 92,6

These results demonstrate the high activity of the catalyst, and because of the low operating temperature a high sulphur selectivity is obtained. Furthermore, because relatively large extrudates could be used, the pressure drop is very low. GC analysis of the feed and product streams show that only H₂S and parts per million levels of mercaptans, also present in the gas, are converted.

It is to be noted that, when reference is made to unsaturated hydrocarbons, in gas streams containing hydrogen sulphide and high proportions of carbon dioxide in the specification, examples of such unsaturated hydrocarbons are ethylene, propylene and butylene,- the reference to no Claus activity in the specification, means that the compounds do not promote the Claus back-reaction between water and sulphur, again to form S0₂ and H₂S; and the reference to BET surface areas is to surface areas, determined by N₂ adsorption according to the Brunauer Emmett Teller method.

In the process of the present invention, the catalyst, which can contain up to 70% by mass iron, remains in the iron oxide form and is not converted to iron sulphates; and the catalyst needs no conditioning to resist the formation of unwanted iron sulphides, as the catalyst appears to be relatively insensitive to the formation of such sulphides. Finally, it is to be noted that the process of the present invention can in principle be operated at a space velocity of up to 15000 hour^"1 or more.

Claims

CLAIMS :

1- A process for treating a hydrogen sulphide-containing gas stream, which process comprises bringing, in a reaction zone, the hydrogen sulphide-containing gas stream into contact with a precipitated iron oxide catalyst, thereby to convert hydrogen sulphide in the gas stream to elemental sulphur.

2. A process according to claim 1, wherein the gas stream comprises, by volume, less than 20% hydrogen sulphide, and carbon dioxide.

3. A process according to Claim 2, wherein the gas stream also includes light saturated and/or unsaturated hydrocarbons.

4. A process according to any one of Claims 1 to 3 inclusive, wherein the precipitated iron oxide catalyst has a surface area of 2-300m²/g and a pore volume of 0,9-0,05cm³/g.

5. A process according to any one of Claims 1 to 4 inclusive, which includes maintaining the reaction zone at substantially atmospheric pressure and at a temperature between 150°C and 350°C.

6. A process according to any one of Claims 1 to 5 inclusive, which includes utilizing a space velocity of up to 8000 volumes gas per volume catalyst per hour in the reaction zone.

7. A process according to any one of Claims l to 6 inclusive, which includes feeding an oxygen-containing gas into the reaction zone to provide oxygen for oxidation of the hydrogen sulphide, with sufficient oxygen-containing gas being used so that the oxygen to hydrogen sulphide molar ratio is at least 0,5:1,0.

8. A process according to any one of Claims 1 to 7 inclusive, wherein the precipitated iron oxide catalyst includes a structural promoter exhibiting little or no Claus activity.

9. A process according to any one of Claims 1 to 8 inclusive, wherein the precipitated iron oxide catalyst is a sintered precipitated iron oxide catalyst.

10. A novel process for treating a hydrogen sulphide- containing gas stream, substantially as described and illustrated herein.