GB2512710A

GB2512710A - Process for treating gas mixtures

Info

Publication number: GB2512710A
Application number: GB1401982.2A
Authority: GB
Inventors: Peter Edward James Abbott; Suzanne Rose Ellis; Martin Fowles; David Wails
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2013-03-01
Filing date: 2014-02-05
Publication date: 2014-10-08
Anticipated expiration: 2034-02-05
Also published as: GB2512710B; WO2014132028A1; GB201401982D0; GB201303723D0

Abstract

A process is described for treating a gas mixture comprising hydrogen, carbon monoxide and one or more olefins, comprising the steps of (i) passing the gas mixture and steam over a first catalyst comprising one or more precious metals selected from Pd and Pt supported on a catalyst support, to form a first gas mixture depleted in olefin and carbon monoxide and enriched in hydrogen, (ii) passing the first gas mixture over a second catalyst comprising a metal selected from Ni, Rh and Ru, to form a treated gas mixture and (iii) recovering the treated gas mixture. The gas mixture may comprise a tail-gas from a Fischer-Tropsch process.

Description

I

Process fortreating gas mixtures This invention relates to a process for treating gas mixtures comprising hydrogen, carbon oxides and olefins.

Gas mixtures comprising hydrogen, carbon oxides and olefins may be generated as a tail gas from the Fischer-Tropsch synthesis of hydrocarbons. The tail gas represents a valuable source of carbon and hydrogen and therefore it is desirable to recycle the tail gas.

US2008/0234397 discloses a method of treating off-gas from a Fischer-Tropsch reaction comprising the steps of: (i) recovering the off-gas from a Fischer-Tropsch reaction, and (ii) hydrogenating a portion of the olefins present in said off-gas. Carbon monoxide in the off-gas was also converted by the water-gas shift reaction to carbon dioxide or or methanated to form methane. The catalyst arrangements for performing the hydrogenation and water-gas shift or methanation stages included in one embodiment a Ni-containing pre-reforming catalyst to promote hydrogenation of unsaturated molecules in the off-gas as well as convert carbon monoxide by the water-gas shift reaction.

However we have found that the use of Ni catalysts to treat gases comprising hydrogen, carbon monoxide and olefins requires a narrow operating window to prevent carbon formation, which reduces the efficiency of the process. The present invention overcomes the problems of the previous process and allows operation over a wider range of temperatures, pressures and feed gas compositions.

Accordingly, the invention provides a process for treating a gas mixture comprising hydrogen, carbon monoxide and one or more olefins, comprising the steps of (i) passing the gas mixture and steam over a first catalyst comprising one or more precious metals selected from Pd and Pt supported on a catalyst support, to form a first gas mixture depleted in olefin and carbon monoxide and enriched in hydrogen, (U) passing the first gas mixture over a second catalyst comprising a metal selected from Ni, Rh and Ru, to form a treated gas mixture and (iii) recovering the treated gas mixture.

The gas mixture may be derived from various sources, but suitably comprises a tail-gas from a Fischer-Tropsch process. Such gases typically comprise hydrogen, carbon monoxide, carbon dioxide, methane and other light hydrocarbons, inerts such as nitrogen and argon, and one or more olefins such as ethylene, propene and butene. Typical Fischer-Tropsch processes are described for example in the aforesaid US200B/0234397. The amounts of the various components of the gas mixture will depend, amongst other things, on the type and operation of the FT process from which the tail gas is derived and whether any upstream treatment of the tail gas has taken place. Hence in one embodiment, the hydrogen content of the tail gas may be in the range 35-45 mole %, the carbon monoxide content in the range 23-33 mole %, the carbon dioxide content in the range 15-25 mole%, the methane content up to 10 mole %, the C2-C4 olefins content in the range 0.5-1.5 mole % and the inerts (nitrogen) content in the range 3-10 mole%. In another embodiment, the hydrogen and carbon monoxide content together may be in the range 15-25% vol, the carbon dioxide content in the range 35-40% vol, the methane content 30-35% vol, the olefins content 1-3% vol and the inerts content 5-7% vol. Other gas compositions may also be treated using the present invention. We have found that lower CO contents in the gas mixture improve the olefin hydrogenation conversion.

The composition of the gas mixture may be adjusted prior to passing it to the first catalyst by addition of one or more additional gases. The additional gases that may be added include one or more of hydrogen, carbon monoxide, carbon dioxide and methane. In one embodiment the gas mixture comprises a mixture of a tail-gas from a Fischer-Tropsch process and natural gas.

Preferably the natural gas, which includes associated gas and coal-bed methane, comprises >95% vol methane.

Steam is combined with the gas mixture priol to feeding it to the first catalyst. The amount of steam should be sufficient to allow the water-gas shift reaction to proceed. Typically the amount of steam, expressed as steam to carbon ratio, may be in the range 1-4. By the term "steam to carbon ratio" we mean the ratio of the number of moles of steam to the number of gram atoms of carbon in the feed, including carbon monoxide and carbon dioxide.

The first catalyst comprises a precious metal selected from Pt and Pd. These metals provide good hydrogenation and water-gas shift activity but low activity for methanation. Other precious metals such as Ru or Rh are not suitable because they promote the exothermic methanation reaction The Pt and/or Pd may be supported on an oxidic catalyst support comprising an oxide selected from the group consisting of alumina, silica, titania, zirconia, magnesia, calcia, ceria and mixtures thereof Preferred rst catalysts comprise Pt and/or Pd supported on a support comprising alumina, zirconia, ceria-zirconia or ceria-zirconia-alumina.

Alumina supports may include the higher surface area transition aluminas such as gamma, delta and theta alumina, but alpha alumina may also be used. Pt catalysts on a low acidity support are especially preferred. The total amount of Ft and/or Pd in the first catalyst is preferably in the range 0.2-1.5% by weight, preferably 0.25-1% by weight.

Various promoters may be included in the first catalyst, such a zinc oxide, which may reduce the poisoning effect of CO on the precious metal and may trap any sulphur species in the gas mixture by formation of ZnS. Alkali metal oxide and alkaline metal oxide promoters may also be included in alumina supports.

The first catalyst may be prepared by impregnation of shaped support materials, such as pellets or extrudates of the support material, with suitable Pd and/or Pt compounds, such as the nitrates, followed by drying and calcination. Alternatively the first catalyst may be prepared using a powdered support and the resulting catalyst powder wash-coated onto shaped supports such as pellets, extrudates, or monoliths, such as ceramic or metal honeycombs.

A preferred first catalyst comprises 0.25-1% wt Pt on alumina pellets.

The second catalyst comprises one or more of Ni, Rh and Ru. These catalysts may also be prepared by impregnation on catalyst supports according to the methods described above for the first catalyst.

In a preferred embodiment, the second catalyst is a Ni-containing pre-reforming catalyst. Such catalysts typically comprise »= 40% wt Ni (expressed as NiO) and therefore preparation is preferably by co-precipitation of a Ni-containing material with alumina and promoter compounds such as silica and magnesia. The precipitation may be performed by adding a suitable alkaline precipitant to a solution of nickel nitrate in the presence of alumina or an alumina precursor compound, as well as promoters or promoter metal precursor compounds.

Suitable pie-reforming catalysts preferably comprise Ni (expressed as NiO) in the range 45- 85% wt, more preferably 45-65%, most preferably 45-55% wt, and alumina.

The second catalyst is preferably in the form of cylindrical pellets or extrudates, which may be lobed or fluted and comprise one or more through holes. The catalyst width or diameter is preferably in the range 2-10mm with an aspect ratio, i.e. length/(width or diameter) in the range 0.5-5. 4-hole cloverleaf pellets with a diameter in the range 3-10 mm and a length 3-6 mm are especially preferred as they provide a reduced pressure drop and are belier able to deal with any carbon deposition that may occur in service. Pre-reforming catalysts are available commercially from Johnson Matthey PLC as the CRG F and CRG LH series of products.

The catalysts are disposed as fixed beds and the gas mixture passed through the first catalyst and then the second catalyst in sequence. In one embodiment, the first and second catalysts may be placed in separate reaction vessels, preferably with temperature adjustment of the first gas mixture between vessels. Thus the temperature of the first gas mixture depleted in olefin and carbon monoxide and enriched in hydrogen may be adjusted before passing it to the second catalyst. In another embodiment, the first and second catalysts are placed in the same reaction vessel with the first catalyst disposed upstream of the second catalyst such that the gas mixture entering the vessel contacts first with the first catalyst and then contacts with the second catalyst and then the treated gas mixture is recovered from the vessel. Preferred relative amounts of pelleted first and second catalyst in the process are preferably 20-60% by volume first catalyst and 80-40% by volume second catalyst.

The first and/or second catalyst bed may be subjected to cooling by means of a coolant passing through heat exchange tubes or plates disposed within the catalyst. However, preferably the first and/or second catalysts are operated adiabatically, i.e. without applied cooling.

Typically the mixture of feed gas and steam may be heated to a suitable inlet temperature to promote the desired reactions and fed to the first catalyst over which the exothermic hydrogenation and water gas shift reactions occurs to produce the first gas mixture depleted in olefins and carbon monoxide and enriched in hydrogen. The resulting first gas, with or without intermediate cooling, passes to the second catalyst over which endothermic steam reforming reactions as well as exothermic water gas shift and methanation reactions occur.

In order to promote the desired reactions, the inlet temperature to the first catalyst is preferably in the range 350-550°C, preferably 350-450°C.

The process of the present invention is particularly useful for operation at relatively low pressures. Thus the process may be operated at a pressure in the range 1-30 bar abs, preferably 1-10 barabs, more preferably 1-7 bar abs.

The treated gas may be used for a variety of purposes and is better suited for downstream processing than untreated gas. In one embodiment the treated gas mixture is subjected to a step of steam reforming to generate a reformed gas mixture comprising hydrogen and carbon oxides. Before steam reforming, the treated gas may be mixed with other suitable gas mixtures such as natural gas, refinery gas streams, hydrogen or carbon dioxide. The steam reforming may be performed in a conventional steam reformer using conventional steam reforming catalysts. A description of this process may be found for example in the Catalysts Handbook, (MV. Twigg ed.), Manson Publishing, 1989.Chapters.

The reformed gas mixture may be fed to processes for the synthesis of methanol and/or ammonia, but is preferably fed to a Fischer-Tropsch process for the synthesis of hydrocarbons.

This process may generate the gas mixture to be treated according to the present invention.

Alternatively, or in addition the reformed gas mixture may be used as a source of hydrogen for the hydrogenation or hydroconversion of Fischer-Tropsch products.

The invention is further illustrated by the following examples.

Example I

Pt alpha alumina catalysts were prepared by impregnation of alumina to produce pellets containing 0.5% wt and 0.75% wt Pt.

These pellets were used to treat a FT tail gas mixture comprising hydrogen, methane, carbon monoxide, carbon dioxide, ethane and containing ethene at 0.5%, in the presence of steam (at 48% of total gas flow) at 1 bar abs pressure at an inlet temperature range of 240-440°C. The results show for both catalysts a reduction in the ethene observed in the outlet gas from about 0.8% on a dry basis at 240°C to about 0.1% at 350°C, and at a level at or below 0.1% up to 440°C.

It was observed surprisingly that lower CO contents in the tail gas improved the olefin hydrogenation conversion.

In all cases no carbon formation was observed and no evidence of any pressure drop increase.

The tests also indicated that no significant methanation or reforming of methane occurred under these conditions.

Further tests with the 0.75% Pt on alumina catalyst on a tail gas comprising hydrogen, methane, carbon dioxide, carbon monoxide, ethane, propane, isobutene, nitrogen and containing propene/isobutene at 0.4 mole % in total at an inlet temperature of 350°C at 6 bar abs and a steam:carbon ratio of about 1.4 (including CO & C02). The results again showed no carbon formation or pressure drop or reforming of the methane or C2+ hydrocarbons and significant conversion of the propylene (>95% over 150 hrs) and isobutylene (>70% over 150 hrs). The order of ease of hydrogenation is ethylene> propylene> isobutylene. The results therefore demonstrate the high effectiveness of the first catalyst in reducing the amount of olefins in the tail gases.

Example 2

Tests were performed with a bed of pelleted catalyst containing 0.75% wt Pt on alumina as a first catalyst and a bed of CRH LHR, a 45-50% Ni (expressed as NiO) pre-reforming catalyst available from Johnson Matthey PLC as the second catalyst. The CRG LF-IR catalysts was in the form of 3.4 mm diameter cylindrical pellets of length 3.5 mm. The bed depth was 330 cm and the bed diameter 10.2 mm. The first catalyst represented about 45% of the total bed volume.

Two tail gas compositions were tested, each comprising different amounts of hydrogen, methane, carbon dioxide, carbon monoxide, ethane, propane, propylene, iso-butane, iso-butylene and nitrogen. In the first gas mixture the total propylenelisobutylene content was 0.99 mole % and in the second it was 0.44 mole %.

The tests were performed at Ca. 6 bar abs at an inlet temperature of 350°C, with a steam:carbon ratio about 1.4 (including CO & C02).

The results showed both satisfactory conversion of the olefins and reforming of the C2+ hydrocarbons. Over 180 hrs of test, there was no apparent increase in pressure drop due to carbon formation.

In comparison, testing of the CRG LE-IR catalyst without the first Pt/alumina catalyst upstream under the same conditions resulted in a noticeable increase in pressure drop and carbon formation on the catalyst.

Claims

Claims.1. A process for treating a gas mixture comprising hydrogen, carbon monoxide and one or more olefins, comprising the steps of (i) passing the gas mixture and steam over a first catalyst comprising one or more precious metals selected from Pd and Pt supported on a catalyst support, to form a first gas mixture depleted in olefin and carbon monoxide and enriched in hydrogen, (U) passing the first gas mixture over a second catalyst comprising a metal selected from Ni, Rh and Ru to form a treated gas mixture and (iii) recovering the treated gas mixture.
2. A process according to claim 1 wherein the gas mixture comprises a tail-gas from a Fischer-Tropsch process.
3. A process according to claim 1 or claim 2 wherein the gas mixture comprises a mixture of a tail-gas from a Fischer-Tropsch process and natural gas.
4. A process according to any one of claims 1 to 3 wherein the first catalyst comprises Pt and/or Pd on an oxidic catalyst support comprising an oxide selected from the group consisting of alumina, silica, titania, zirconia, magnesia, calcia, ceria and mixtures thereof.
5. A process according to any one of claims 1 to 4 wherein the first catalyst comprises Pt and/or Pd supported on a support comprising alumina, zirconia, ceria-zirconia or ceria-zirconia-alumina.
6. A process according to any one of claims 1 to 5 wherein the first catalyst comprises 0.25-1% wt Pt on alumina pellets.
7. A process according to any one of claims 1 to 6 wherein the second catalyst is a Ni-containing pre-reforming catalyst.
8. A process according to claim 7 wherein the pre-reforming catalyst comprises Ni (expressed as NiO) in the range 45-85% wt, more preferably 45-65%, most preferably 45-55% wt, and alumina.
9. A process according to any one of claims 1 to 8 wherein the first and second catalysts are placed in the same reaction vessel with the first catalyst disposed upstream of the second catalyst such that the gas mixture entering the vessel contacts first with the first catalyst and then contacts with the second catalyst and then the treated gas mixture is recovered from the vessel.
10. A process according to any one of claims 1 to 8 wherein the temperature of the first gas mixture depleted in olefin and carbon monoxide and enriched in hydrogen is adjusted before passing it to the second catalyst.
11. A process according to any one of claims 1 to 10 wherein the first and/or second catalysts are operated adiabatically.
12. A process according to any one of claims ito 11 wherein the process is operated at an inlet temperature to the first catalyst in the range 350-550°C, preferably 350-450°C.
13. A process according to any one of claims 1 to 12 wherein the process is operated at a pressure in the range 1-30 bar abs, preferably 1-10 bar abs, more preferably 1-7 bar abs.
14. A process according to any one of claims 1 to 13, wherein the treated gas mixture is subjected to a step of steam reforming to generate a reformed gas mixture comprising hydrogen and carbon oxides.
15. A process according to claim 14 wherein the reformed gas mixture is fed to a Fischer-Tropsch process for the synthesis of hydrocarbons.
16. A process according to claim 14 or claim 15 wherein the reformed gas mixture is used as a source of hydrogen for the hydrogenation orhydroconversion of Fischer-Tropsch products.