IE903596A1

IE903596A1 - Catalysts for treating gaseous effluent and a method of¹treating the effluent

Info

Publication number: IE903596A1
Application number: IE359690A
Authority: IE
Original assignee: Rhone Poulenc Chimie
Priority date: 1989-10-09
Filing date: 1990-10-08
Publication date: 1991-05-08
Also published as: PT95535A; CA2027015A1; FI904949A0; BR9005001A; NO904329L; EP0422999A1; FR2652759A1; CN1054015A; RU1837957C; JPH03193140A; KR910007578A; NO904329D0; FR2652759B1

Abstract

Catalysts for the treatment of gas containing sulphur compounds. These catalysts, which contain, as an essential compound, an element which is catalytically active for the reaction of oxidation of hydrogen sulphide to sulphur, are monolithic in form. This form makes it possible to improve the activity and the selectivity for sulphur in the reaction of direct oxidation of hydrogen sulphide.

Description

CATALYSTS FOR TREATING GASEOUS EFFLUENT AND A METHOD OF TREATING THE EFFLUENT RHONE-POULENC CHIMIE, a French Body Corporate of 25 Quai Paul Doumer, 92408 Courbevoie, France. -1IE 903596 The invention concerns catalysts for treating gases, particularly gaseous industrial effluent, containing sulphur conpounds, with a view to catalytic conversion of the sulphur conpounds into conpounds which can easily be eliminated.

More particularly, it concerns catalysts for oxidation of sulphurous hydrogen or of carbon conpounds of sulphur, directly into sulphur.

Some industrial effluents, and particularly effluent from Claus units, also described as tail gas, contain sulphur and/or polluting oxidisable sulphur conpounds. These have to be treated to convert them, by oxidation, to conpounds which can easily be eliminated, such as sulphur dioxide and/or sulphur trioxide.

The sulphur conpounds present in the effluent are chiefly hydrogen sulphide and organic sulphur conpounds such as carbon sulphide, carbon oxysulphide and/or mercaptans.

The effluent may equally be treated to convert the sulphur conpounds to elemental sulphur by direct oxidation. The elemental sulphur may easily be eliminated or recovered, e.g. by condensation.

Several methods of oxidising these conpounds are already known. The sinplest consists of burning the effluent at high tanperature. iThen the hydrogen sulphide content is too low, however, it is difficult to maintain a hicfl-i enough flame tenperature to obtain stable combustion of the sulphur conpounds.

Catalytic methods of oxidising the sulphur conpounds to S, SO2 or SO3 have been proposed as a means of treating the gases with a low hydrogen sulphide concentration.

Of the catalysts proposed those based on titanium oxide appear to havo a high performance. European Patents 115449, 60742 and 78690 for exanple, describe various catalysts based on titanium oxide which are used in oxidising hydrogen sulphide.

Other catalysts have also been proposed. Tiius Patent US 4092404 describes an oxidation catalyst based on vanadium, while European Patent No. 39266 describes a catalyst based on iron. The catalysts are generally used in the form of a bed made up of a quantity of particles arranged in a colunru the stream of gas to be treated passes through the bed.

Ibe catalyst beds h&ui ·· fa» been made up of particles of catalyst in the form of cylindrical or spherical granules shaped by moulding or extrusion. However, such catalysts do not have high initial activity if the contact time is short, so it is necessary to work with contact times of about 3 seconds or more. Hence there is a drop in the selectivity of the reaction of direct oxidation of hydrogen sulphide to sulphur (reaction 1) due to the reverse reaction (reaction 2,, which will be described as the reverse Claus reaction: (1) HjS + 1 02-----> 1 S„ + H20 n (2) 2 sn + ^2°-----> 2HJS + S02 n <----The invention aims chiefly to remedy this disadvantage by proposing the use of a catalyst in monolithic form for the reaction of direct oxidation of hydrogen sulphide to 6ulphur.

Applicants have found surprisingly that the catalyst in monolithic form gave better catalyst selectivity for the reaction of direct oxidation of hydrogen sulphide than for the reverse Claus reaction.

To this end the invention proposes a catalyst for treating gases containing sulphur conpounds, with a view to oxidation of those conpounds to sulphur conpounds which can easily be eliminated and particularly to elemental sulphur, the essential constituent being an element which catalyses the oxidation reaction of the sulphur conpounds, characterised in that the catalyst is in monolithic form.

Monolithic form refers to the fact that the catalyst has a structure of the alveolar or honeycomb type, conprising a number of hexagons, squares, triangles or other polygons or circles or a sinusoidal wave shape or a spider’s web or spiral shape. A catalyst of the honeycomb type contains cavities or pores with about 50 to about 70% porosity.

The catalyst generally has a cylindrical or parallelepipedal shape, containing orifices parallel with its generatrix over its whole length and thus allowing the gases to pass into the channels or ducts.

A first embodiment of the invention is a so-called mass catalyst, that is to say, the catalytically active phase is extruded in monolithic form.

Another embodiment of the invention is a supported catalyst, that is to say, the catalytically active phase is deposited on a monolithic carrier, which is thus a rigid substrate with a honeycomb type structure.

The essential constituent of the catalyst according to the invention is a catalytically active element selected from the group conprising aluminium oxide, titanium oxide, cerium oxide, zirconium oxide, iron oxide, zinc oxide, chromium oxide, molybdenum oxide, cobalt oxide, nickel oxide, vanadium oxide or a mixture of these.

For exanple, titanium oxide may be used alone or mixed with a plurality of oxides such as alumina, silica, zirconium oxide, oerium oxide, tin oxide, trivalent rare earth oxides, molybdenum oxide, cobalt oxide, nickel oxide, iron oxide, chromium oxide, zinc oxide, vanadium oxide or the like. The same applies to cerium oxide, zirconium oxide, alumina and the other oxides mentioned above as catalytically active elements.

The oxides of catalytically active elements described above, which are suitable for the invention, rrey be any oxides of these elements, whatever their method of preparation or their source.

The proportion of catalytically active element relative to the total weight of the finished catalyst nay range from 0.5 to 100% and preferably from about 60 to 99%.

The catalyst of the invention nay also contain additives to facilitate shaping and additives to inprove the final mechanical properties.

Additives which are conventionally used in shaping techniques may be included when preparing the catalyst of the invention. These give the oough obtained by kneading the rheological properties adapted to the shaping process. Some exanples of shaping additives are: cellulose, carboxy methyl cellulose, carboxy ethyl cellulose, tail oil, xanthan gums, surfactants, flocculating agents such as polyacrylamides, carbon black, starches, stearic acid, polyacrylic alcohol, polyvinyl alcohol, biopolymers, glucose, polyethylene glycol and the like.

Finally, the quantity of such additives may range from 0.1 to 15% by weight relative to the finished catalyst.

It is also possible to use conplementary constituents which can inprove the mechanical properties of the formulations. These nay be selected from the group conprising clays, silicates, alkaline earth netal sulphates, ceramic fibres, asbestos or silica.

The catalyst advantageously conprises an alkaline earth metal sulphate, preferably calcium sulphate, or an ammonium sulphate.

These constituents may be used in quantities of up to 99.5%, particularly up to 60% and more specifically up to 30% by weight of the finished catalyst.

The so-called mass catalyst of the invention nay be prepared by any appropriate known process to obtain a mass monolith.

The monolith may be obtained by shaping a mixture containing the constituents of the catalyst.

A mixture is kneaded, based on water, powder containing at least one catalytically active element and the possible additives of the type mentioned above.

The mixture thus obtained is then put into monolithic form by conventional methods of preparation by extrusion, rolling, solidification of elements in sheet form etc.

Tne resultant monolith may be subjected to a drying operation at a tenperature ranging, for exanple, from 100 to 150°C, for a very variable period of 10 to 20 hours.

It may possibly be followed by a calcining operation at a tenperature ranging from about 350 to about 500°C, for a period usually of from 1 to 8 hours.

The so-called mass catalysts of monolithic form generally have a specific surface area ranging from 5 to 300 and preferably from 50 to 120 m /g. The specific surface area expressed is a B.E.T surface area, that is to say, it is determined by adsorption of nitrogen in accordance with ASTM standard D 3663-76, based on the BRUNAUER-ΕΜΜΕΊΤ-TELLER method - The Journal of American Society, 60, 309 (1938).

In another embodiment of the invention a catalytically active element is deposited on a monolithic carrier.

Ihe monolithic carrier may be a carrier with non-basic properties, such as alumina, silica, cerium oxide, zirconium oxide, titanium oxide or the like.

It may be prepared and shaped as described above, from a powder of oxide or hydrated oxide and possibly of shaping additives and additional constituents to inprove the mechanical properties as described above.

Monolithic carriers of known types are also suitable, such as refractory monolithic carriers, e.g. monoliths made of metal or ceramic material.

The metallic monoliths used are in particular those obtained from alloys of chromium, aluminium and cobalt, such as those known under the trade mark KANTHAL, or those obtained fron alloys of iron, chromium, aluminium and yttrium, and known under the trade nark FECRALLGY. The metal nay also be carbon steel or ordinary cast iron.

The ceramic monoliths used are in particular those in which the nain material is: cordierite, alumina, mullite, zirconium, zirconnullite, barium titanate, porcelain, thorium oxide, magnesium oxide, steatite, and boron or silicon carbides.

The various monolithic carriers such as those mentioned above nay then be inpregnated to deposit the catalytically active element.

Impregnation is carried out in known manner by putting the carrier into contact with a solution, sol or gel containing at least one catalytically active element in the form of an oxide or oxide precursor.

The operation is generally effected by steeping the carrier of monolithic structure in a specific volume of solution of at least one precursor of a catalytically reactive element.

A solution of a precursor of a catalytically active element is understood as being a solution of a salt or compound of the element, or of at least one of the elements, forming the catalytic phase, the salts and conpounds being decomposable by heat.

The concentration of salt in the solution is chosen according to the quantity of active phase to be deposited on the monolithic carrier.

The area impregnated with active phase is determined by the volume of solution adsorbed. Thus according to one feature of the invention, the volume of catalytic phase adsorbed is equal to the total pore volume of the monolithic carrier to be impregnated. The pore volume may be determined by the known method with a mercury porosimeter, or the quantity of water absorbed by a sample may be measured. It is also possible to impregnate the carrier by steeping it in the solution of the precursor(s) of catalytically active element(s) and to drain away the excess solution.

The monolithic carrier may then be subjected to a drying operation and possibly to calcination under the conditions mentioned above.

These operations may be repeated with the same carrier when the carrier has been dried and calcined, and a plurality of elements may be deposited successively on the carrier and on specific areas, which may vary.

The monolithic carrier is thus covered with a layer of a catalytically active phase, the thickness of which may range from 2 to 500 microns. This layer represents from 0.5 to 95% of the weight of the monolithic carrier and provides a specific surface area generally of from 50 to 120 rn^/g.

According to the invention, the method of oxidising hydrogen sulphide and sulphur oonpounds conprises putting the gaseous effluent into contact with a gas containing oxygen, in the presence of the catalyst in monolithic form.

The gaseous effluent used in the method of the invention may come from various sources. In particular it may be a natural gas, a gas derived from gasification of coal or heavy oils, or a gas obtained by hydrogenating sulphur conpounds in the effluent from a sulphur plant.

The gaseous effluent containing the sulphur conpounds has a conposition which may vary both in the nature of its constituents and in their proportions. Thus it contains hydrogen sulphide but possibly other organic sulphur conpounds, such as carbon sulphide (CS2) and/or carbon oxysulphide (COS) and/or me reaptans.

The gaseous effluent to be treated may vary widely in conposition. It generally contains less than 15% by volume of hydrogen sulphide and preferably from 0.1 to 10% by volume. It may also contain CS2 and/or COS in a nexinum total concentration of 3% by volume and preferably 1%.

As for the gas used to oxidise the hydrogen sulphide, this is generally air, possibly air enriched with oxygen, or pure oxygen or mixtures of oxygen and an inert gas such as nitrogen in varying proportions.

The amount of gas is such that the quantity of oxygen is at least equal to, and preferably greater than, the stoichimetric quantity required to oxidise all the sulphur conpounds to sulphur. The amount of oxygen present in the gas advantageously represents an excess of about 15 to 100% over the stoichimetric quantity.

The desulphurising process according to the invention is carried out at tenperatures over 150°C and preferably from 200 to 550°C.

The time for which the gas stream is in contact with the catalyst of the invention is arranged so that the desired conversion rate is obtained.

In the method of the invention the contact time is short, preferably from 0.5 to 2.5 seconds, corresponding to a high space velocity. The WH, representing the volume of gas treated per volume of catalyst per hour, is preferably over 1500 h_1b and more preferably from 2000 to 10,000 h"\ The gaseous effluent containing the sulphur conpounds and the gas containing free oxygen may be put into contact with the catalyst separately. However, with a view to obtaining a very homogeneous gaseous reaction medium, it is preferable first to mix the effluent with the oxygen containing gas and to put the resultant mixture into contact with the catalyst of the invention.

The resultant mixture, hereinafter described as the gas stream, may vary greatly in conposition. The proportions of the various constituents in a preferred embodiment of the invention will be specified by way of exanple: - hydrogen sulphide =* 0.1 to 3% = 0.05 to 2% - oxygen - water - nitrogen = 3 to 30% = balance to 100% A preferred embodiment of the method of the invention conprises taking a gaseous effluent with a potential H2S content of less than 5% (free H2S + H2S from CS2 and COS) and a gas containing oxygen - the molar ratio being from 1.0 to 2.3:1 and preferably from 1.3 to 1.8:1 - and putting them into contact with a catalyst in monolithic form. It is preferable to use a so-called mass catalyst of titanium oxide, extruded in monolithic form.

The gas stream from the catalytic stage may be treated to condense and separate the sulphur in known manner.

The method of the invention gives a stream containing very little S02, preferably less than 50 ppm.

Ihe catalyst in monolithic form thus gives better selectivity for the reaction of direct oxidation of hydrogen sulphide than for the reverse CIAUS reaction leading to the formation of S02· An exanple of the invention (Exanple 2) and a comparative exanple (Exanple 1) will now be given.

Conparative Exanple 1 Ihe starting material is a suspension of titanium oxide obtained after hydrolysis and filtration in the conventional method of reacting ilmenite with sulphur. A suspension of lime is added to neutralise all the sulphates. Ihe suspension is dried at 150°C for one hour. The pcwder obtained is kneaded in the presence of water and nitric acid in the following preportions: - TiO2 pcwder - hho3 - h2o 58% 2% 40% The resultant dough is extruded through a cylindrical die 4 ram in diameter to give extrusions of cylindrical form.

After being dried for 15 hours at 120°C and calcined for 2 hours at 450°C, the extrusions have the following properties: - diameter of extrusions - specific surface area - total pore volume 3. 5 nm 120 m2/g 0.35 cm^/g Exanple 2 The dough obtained in Example 1 is extruded through a die of square section with sides of 20 nm, containing 16S small squares with sides of 1.4 nm.

The monoliths are then dried at 120°C for 15 hours then calcined for 2 hours at 450°C.

The properties of the catalyst obtained are as follows: - monoliths of square section with sides of 20 nm - specific surface area : 110 m /g - total pore volume : 0.30 m2/g Catalytic Tests The purpose of the catalytic test is to conpare the activity of the catalysts in Examples 1 and 2 in direct oxidation of hydrogen sulphide to sulphur and SO2· *E 903596 A gas of the following conposition (by volume) is fed into the reactor. h2s : 1%c2 : 0.5% h2o 7% bn2 : 91.5% With isothernal operation, at a tenperature of 200°C and for an identical volume of reactor filled with catalyst, the volume velocity of the gases is 7,200 h"\ calculated at normal tenperature and pressure.

The contact time of the gases is 0.5 second.

The gases discharged are analysed by chromatography in gas phase.

The activity of the catalysts is conpared by measuring the rate of converting hydrogen sulphide, and the percentage of SC2 formed is determined.

The results obtained are set out in Table I below: Table I Type of catalyst Conparative Exanple 1 Ti02 extrusions Exanple Ti02 monoliths Conversion rate Formation of SO2 27% 50C ppm 57% < 50 ppm The results in Table I clearly show the superiority of the catalyst according to the invention to the prior art catalyst, not only in eliminating the gas, but also in its activity and its selectivity for sulphur.

Claims

1. A catalyst for treating gases containing sulphur conpounds with a view to oxidising those conpounds to sulphur conpounds which can easily be eliminated, particularly to elemental sulphur, its essential constituent being an element which catalyses the oxidation reaction of the sulphur conpounds, characterised in that said catalyst is in nonolithic form.

2. Ihe catalyst of Claim 1, characterised in that it has a structure of an alveolar or honeycomb type, enabling the gases to pass into the channels or ducts.

3. The catalyst of Claim 1 or 2, characterised in that it contains cavities or pores with a porosity of about 50 to 7C%.

4. The catalyst of any of Claims 1 to 3, characterised in that it is in mass form: the catalytically active phase being extruded in nonolithic form.

5. The catalyst of any of Claims 1 to 3, characterised in that it is in supported form: the catalytically active phase being deposited on a carrier in monolithic form.

6. The catalyst of Claim 5, characterised in that the carrier in monolithic form is selected from the group conprising alumina, silica, cerium oxide, zirconium oxide and titanium oxide.

7. The catalyst of Claim 5, characterised in that the carrier in monolithic form is a metallic monolith or a monolith made of ceramics.

8. The catalyst of any of Claims 1 to 7, characterised in that the catalytically active element is an oxide of elements selected from the group conprising titanium, cerium, zirconium, aluminium, iron, zinc, chromium, molybdenum, cobalt, nickel and vanadium.

9. Ihe catalyst of any of Claims 1 to 8, characterised in that it contains from 0.5 to 100% by weight of catalytically active element (expressed in oxide), and preferably from 60 to 99%.

10. The catalyst of any of Claims 1 to 9, characterised in that it conprises an alkaline earth metal sulphate or an ammonium sulphate.

11. The catalyst of any of Claims 1 to 10, characterised in that at least one oxide of a netal selected from the group conprising cerium, zirconium, molybdenum, cobalt, silicon, trivalent rare earths, nickel, iron, tin, aluminium and titanium, chromium, zinc and vanadium.

12. The catalyst of Claim 1, characterised in that it is formed by a monolith of titanium oxide.

13. A method of oxidising hydrogen sulphide and sulphur conpounds to sulphur, characterised in that it conprises putting a gaseous effluent containing the sulphur conpounds into contact with a gas containing oxygen, in the presenoe of the catalyst of any of Claims 1 to 12.

14. The method of Claim 13, characterised in that the gaseous effluent conprises from 0.1 to 10% by volume of hydrogen sulphide, and preferably from 0.1 to 3%.

15. Ihe method of Claim 13 or 14, characterised in that the reaction tenperature is from 200 to 55CPC.

16. Ihe method of any of Gains 13 to 15, characterised in that the contact time is from 0.5 to 2.5 seconds.

17. The method of any of Claire 13 to 16, characterised in that the space velocity is over 1500 h”^.

18. The method of any of Gains 13 to 17, characterised in that the space velocity is from 2000 to 10,000 h _1 . •Ε 903596

19. A catalyst according to Claim 1, substantially as hereinbefore described and exemplified.

20. A method according to Claim 13 of oxidising hydrogen sulphide and sulphur compounds to sulphur, substantially as hereinbefore described and exemplified.

21. Sulphur whenever obtained by a method claimed in a preceding claim.