GB2246141A

GB2246141A - Two stage process for conversion of hydrocarbons

Info

Publication number: GB2246141A
Application number: GB9111123A
Authority: GB
Inventors: Antony Harold Patrick Hall; Alistair William Winstanley
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1990-07-19
Filing date: 1991-05-23
Publication date: 1992-01-22
Also published as: GB9015864D0; GB9111123D0

Abstract

A two-stage process for the upgrading of a feedstock comprising C2 to C8 hydrocarbons comprises a first dehydrogenation step wherein the feedstock is contacted with a dehydrogenation catalyst at elevated temperature to produce a first product, said dehydrogenation catalyst comprising a platinum group metal on a silicalite support; and a second step wherein at least part of the first product is passed over a zeolite catalyst at elevated temperature to produce a second product rich in tertiary olefins and/or aromatics.

Description

TWO STAGE PROCESS FOR CONVERSION OF HYDROCARBONS The present invention relates to a two-stage process for the conversion of aliphatic hydrocarbons.

The conversion of aliphatic hydrocarbons such as ethane, propane and butane into aromatics is well known. The process requires a catalyst and typically, conventional aluminosilicates including zeolites have been employed. In such processes, the aliphatic hydrocarbon feedstock is passed over a catalyst at elevated temperatures in the vapour phase. Indeed, our UK patent No 1561590 discloses a process for the preparation of aromatics from open chain hydrocarbons using a zeolite catalyst containing gallium.

Additionally, prior art includes disclosures on a two-stage process for the conversion of aliphatic hydrocarbons. In particular, US Patent No 4554393 discloses a two-stage process comprising a first step of passing the reactant over a potassium or cerium promoted chromia/alumina catalyst and a second reaction step using a crystalline aluminosilicate. Furthermore, US Patent No 4788364 discloses a two stage process using a fluidised bed while US Patent No 4746763 discloses a process comprising a zeolite catalyst and a non-zeolite type catalyst. Aromatic hydrocarbons are produced in the above disclosed processes but we have now found a two-stage process in which selectivity to aromatic hydrocarbons can be fineiy controlled. Additionally, the products obtained comprise very low levels of benzene and in the light of current legislation, this is most favourable.

Accordingly, the present invention provides a two-stage process for the upgrading of a feedstock comprising C2 to C8 hydrocarbons comprising a first dehydrogenation step wherein the feedstock is contacted with a dehydrogenation catalyst at elevated temperature to produce a first product, said dehydrogenation catalyst comprising a platinum group metal on a silicalite support; and a second step wherein at least part of the first product is passed over a zeolite catalyst at elevated temperature to produce a second product rich in tertiary olefins and/or aromatics.

The second product as made is suitable for use in gasoline blending. Equally advantageous, etherification of the tertiary olefins contained within the product can result in high octane oxygenates.

It is preferred that the first stage of the process is operated as a high temperature dehydrogenation stage and the second stage is operated as a lower temperature stage. Stage two initially produces an aliphatic liquid product. This product may be aromatised in situ to varying extents if so desired.

The present invention provides a process for the upgrading of aliphatic hydrocarbon compounds wherein the selectivity to aromatic compounds can be adjusted by control of reaction conditions.

Moreover, the selectivity to methane and ethane from the hydrocracking of the reaction products is reduced. Selectivity to liquids in the gasoline range is greatly enhanced and the end-product is a liquid product suitable for use in gasoline blending.

In addition, the process allows the product stream from stage 1 to be passed directly into stage 2. By-products such as hydrogen, methane and ethane may be removed at the end of stage 2 and surprisingly it is found that in the present process, the presence of hydrogen in the second step prolongs the life of the catalyst used in the second stage.

The first reaction stage is essentially dehydrogenation of the saturated aliphatic hydrocarbons. The feedstock is suitably C2 C8 hydrocarbons, preferably C3 - C4 hydrocarbons. The hydrocarbons include paraffinic hydrocarbons and may suitably include ethane, propane, n-butane, isobutane, n-pentane, isopentane, n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, n-heptane. The feedstock may of course include materials other than paraffins, for example olefins or cycloparaffins.

The feedstock is passed over the dehydrogenation catalyst at elevated temperatures. Suitably, a temperature in the range of from 550-650 C is employed, preferably 570-600"C. A pressure of suitably 50-600 KPa (0.5-6 bar) absolute, preferably 100-300 KPa (1-3 bar) absolute may be used in the first stage of the process.

It is preferred to use a relatively high space velocity in the first stage of the process. Space velocity is defined as the weight of feed per hour divided by weight of catalyst. Suitably, space velocities ranging from 3 to 10 hourly, preferably from 4 to 6 hour -l may be used.

The catalyst used in the first stage of the process is a silicalite support and a platinum group metal. The preferred structure is disclosed in EP-A-212850 and is a framework comprising essentially silicon and oxygen atoms. The platinum group metal is suitably supported by the silicalite framework.

The platinum group metal may suitably be at least one of platinum, ruthenium, iridium, rhodium or palladium. The preferred metal is platinum. The metal is preferably present, at least in part, in the elemental form. The catalyst may suitably contain up to 10% by weight platinum group metal, preferably up to 5% by weight and most preferably 0.01 to 2% by weight of the platinum group metal.

Silicalite can exist in a number of different structural forms depending upon the route by which it is prepared. Thus one form (silicalite I) is described in US Patent No 4061724 which relates to a silica polymorph consisting of crystalline silica which after calcination in air at 600"C for one hour has a characteristic X-Ray powder diffraction pattern similar to that of ZSM-5. Another form (silicalite II) is described in a publication in Nature, 280, 664-665 (1979) by D M Bibby, N B Milestone and L P Aldridge.

Structurally, silicalite II has the same relationship to ZSM-11 as silicalite I has to ZSM-5. It has been proposed that silicalite I, for example, merely represents an extreme end member of the ZSM-5 type of aluminosilicate zeolite. The material silicalite I is designated to have the MFI structure.

The catalyst may also optionally comprise a metal promotor.

The metal promotor may suitably be selected from tin or zinc. The preferred catalysts comprising the metal promotors are disclosed in our European applications EP-A-0351066 and EP-A-0351067 respectively. The promotor may be present entirely or in part in the framework of the silicalite support. Alternatively some or all of the metal promotor may be deposited on the surface and in the pores of the support. It is preferred that at least some of the metal promotor is present in the elemental form. The amount of promotor present in the catalyst may vary widely. Suitably, the catalyst contains 0.05 to 20% by weight, preferably 0.1 to 15% by weight of promotor.

The catalyst is preferably activated, suitably by thermal treatment, for the purposes of decomposing thermally decomposable compounds. The thermal treatment may suitably be carried out in the presence of air or an inert gas such as nitrogen. Alternatively, or in addition to, the catalyst may be reductively activated by heating in the presence of a reducing gas such as hydrogen. It is possible to combine the thermal treatment and the reductive treatment into a single operation.

The catalyst may suitably be prepared by any of the known techniques for preparing catalysts. A suitable method for example comprises impregnating a silicalite with a soluble thermally decomposable compound of the platinum group metal. Optionally, a soluble thermally decomposable compound of the metal promotor may be added. A mineral acid, for example nitric acid, may be added to the impregnation solution or solutions in order to facilitate better the dispersion of the metallic component. The platinum group metal and the metal promotor may be introduced together by impregnation with a single solution or separately. If they are introduced separately, a preferred process comprises impregnating with a metal promotor containing solution; calcining the resultant material; impregnating with a platinum group metal-containing solution; and re-calcining.

The product obtained from the first stage of the reaction is essentially alkenes. Selectivity to the alkenes may be 70-80% with by-products comprising methane and ethane. The product from the first stage of the process is preferably passed directly into the second stage.

The first stage product is passed over a zeolite catalyst, for example ZSM-5. The catalyst may suitably contain a metal promotor such as gallium or zinc. The metal promotor may suitably be present in the catalyst at a concentration of 0.1 - 10 wt g, preferably 0.5 - 2 wtX.

The second stage of the process is preferably operated at a temperature which is lower than that of the first stage. A temperature of suitably 300-450"C, preferably 320-400 C may be used. The amount of aromatic compounds produced at the end of stage 2 may be varied by varying process conditions such as temperature and pressure, particularly applicable to stage 2. A low concentration of aromatics can be produced if so desired, by operating the present process stage 2 at the lower end of the temperature range. Equally, a greater concentration of aromatics can be obtained by operating the second stage at the upper end of the temperature range. A pressure of suitably 50-600 KPa (0.5-6 bar) absolute, preferably 100-300 KPa (1-3 bar) absolute may be applied during stage 2 of the present process.It is preferred that stage 1 and stage 2 are operated at equivalent pressure.

It is envisaged that the selectivity to aromatics may also be varied by varying the space velocity. The space velocity of the catalyst bed of stage 2 may range from 2 to 10 hourly. A low concentration of aromatics can be obtained if so desired by operating the process at the upper end of the range. Equally, operation at the low end provides a product rich in aromatics.

The product from the process may comprise aromatic hydrocarbons, unconverted olefins, converted olefins and hydrogen.

Equally possible, is the presence of small amounts of paraffinic hydrocarbons eg methane and ethane. Unconverted material, particularly C3-C4 paraffins containing small quantities of olefins are preferably recycled to the process.

It is preferred to recover the hydrogen from the product. The hydrogen may be utilised elsewhere or recycled to the first stage of the process and utilised as a diluent. The presence of hydrogen in the second stage of the process does not appear to adversely affect the catalyst activity. Indeed, no deactivation is apparent after prolonged use of the catalyst eg 80 hours on stream at constant reaction conditions.

The converted olefins contain substantial amounts of C4 -C7 tertiary olefins suitable for etherification. The aromatic hydrocarbons are substantially C6 -C10 hydrocarbons. By suitable choice of reaction conditions, the production of benzene can be minimised.

The process of the present invention is now described with reference to the following examples: Example 1. Preparation of 0.5 wt % Pt/3 wt % Zn/Silicalite.

- Stage 1 catalyst 600g of an aqueous solution containing 20% by weight tetrapropylammonium hydroxide (TPAOH) were added with stirring to 2000 grams of Ludox AS40 (Trade Mark, ex Dupont) containing 40% by weight silica (ammonia stabilised). The resultant hydrogen had the molar composition of 4.4 TPAOH:1.4 NH3:100Si02:700 H2O. The hydrogel was heated at 175"C for 72 hours in a pressure vessel under autogenous pressure. The vessel was then cooled and the product filtered, washed with distilled water and dried at 100"C.

The product was calcined at 600 C in air for 48 hours prior to stirring in 20% by weight nitric acid (silicalite/solution = 0.25 by weight) for 1 hour at room temperature, filtered, washed in distilled water, dried and calcined again at 6000C for 16 hours.

27g of the treated silicalite were then mixed with 350g of an aqueous solution containing 2.7g of Zn(C2H302)2.2H20 and the mixture was in a rotary evaporator under vacuum. The resulting solid was placed in an air oven at 80"C for 2 hours. The solid was then calcined at 500 C in air for 16 hours. The zinc impregnated solid was mixed with 350g of aqueous solution containing 0.24g of Pt(NH3)4C12.H20. The mixture was dried in a rotary evaporator under vacuum.

Example 2. Preparation of 0.8% Ga/ZSM-5/Aluminium Phosphate/Alumina - Stage 2 catalyst.

This catalyst was prepared according to US Patent 4636483 wherein phosphoric acid was added to an aqueous solution of hexamethylenetetramine (HMT) in an amount to yield a phosphorus content of the finished catalyst equal to about 11 wt%. A second solution was prepared by adding a ZSM-5 type zeolite to enough alumina sol, prepared by digesting metallic aluminium in hydrochloric acid to yield a zeolite content in the finished catalyst equal to about 67 wt%. These two solutions were commingled to achieve a homogenous admixture of HMT, phosphorus, alumina sol and zeolite. This admixture was dispersed as droplets into an oil bath maintained at about 200"F. The droplets remained in the oil jath until they set and formed hydrogel spheres.These spheres were removed from the oil bath, water washed, air dried and calcined at a temperature of about 900 F. A solution of gallium nitrate was used to impregnate the spheres to achieve a gallium content on the finished catalyst equal to about 1 wt%. After impregnation, the spheres were calcined a second time, in the presence of steam at a temperature of about 1200 F.

Example 3 A 3.5 ml (1.87g) sample of the product of Example 1 was placed in a first stainless steel reactor tube. A 20 ml (11.52g) sample of the product of Example 2 was placed in a second, stainless steel reactor tube, both tubes being connected in series. The two tubes were subsequently sealed and the temperature of the first tube raised to 400"C. A flow of air was introduced into the first tube and conditions maintained for 16 hours. The a-ir flow was then stopped and replaced by a nitrogen purge. The temperature of the first tube was raised to 530"C and the nitrogen purge maintained for a further 8 hours. The nitrogen flow entering the first tube was then replaced by a hydrogen flow, again at 530"C for 16 hours.

The second tube was treated at 535"C with a nitrogen purge for 2 hours.

The hydrogen inlet was then closed, both reactors purged with nitrogen in series and the temperature increased to 570ec. Both tubes were then pressurised to 200 KPa (2 bar) absolute. Nitrogen was replaced by propane and introduced into the first reactor tube at a rate of 4.85 gas hourly space velocity. An ethane cofeed was also introduced at a rate of 0.56 gas hourly space velocity. The product of the reaction in the first tube was passed directly into the second reaction tube via trace wire heated piping.

The reaction products obtained at the end of stage 2 were separated by a condenser system into liquid and gas phases and analysed using gas and liquid chromatography.

Conversion of propane at 46 hours on stream was found to be 57.6% with selectivity to liquid of 58Z. The liquid product contained 100% aromatics of which 26% was attributable to benzene.

Example 4 A 3 ml sample of the product of Example 1 was placed in a first stainless steel reactor tube. A 20 ml sample of the activated product of Example 2 was placed in a second stainless steel reactor tube.

The contents of the two tubes were treated as outlined in Example 3. Following the hydrogen treatment, the temperature in the first tube was raised to 600C and pressurised to 2 bar. The temperature in the second tube was raised from room temperature to 370"C and also pressurised to 200 KPa (2 bar) absolute. Propane was passed into the first reactor tube at a rate of 4.85 weight hourly space velocity. The furnace controls in each stage were adjusted to maintain an average bed temperature of 600 C and 370"C. The product from the first tube was passed directly into the second tube. The reaction products obtained at the end of stage-2 were separated by a condenser system into liquid and gas phases and analysed using liquid and gas chromatography.

Conversion of propane at 47.5 hours on stream was found to be 31.7% with selectivity to liquid of 65.2%. The liquid product comprised 78.9% aromatics of which 10% was attributable to benzene.

Example 5 Two stainless steel reactor tubes were prepared with the respective catalysts components according to Example 4. The contents of the tubes were treated as outlined in Example 3.

Following the hydrogen treatment, the temperatures of the first and second tubes were increased to 6000C and 320 C respectively. The two tubes were pressurised to 200 KPa (2 bar) absolute and propane introduced into the first tube as outlined in Example 4.

Conversion of propane at 72 hours on stream was found to be 36.8% with 72.2% selectivity to liquids. Aromatic content in the liquid product was found to be 39.9 % with 2% attributable to benzene.

Example 6 Two stainless steel reactor tubes were prepared with the respective catalyst components according to Example 4. The contents of the tubes were created as outlined in Example 3. Following the hydrogen treatment, the temperatures of the first and second tubes were increased to 595"C and 320"C respectively The two tubes were pressurised to 200 KPa (2 bar) absolute and propane introduced into the first tube at a rate of 5.05 weight hourly space velocity.

Conversion of propane at 51 hours on stream was found to be 37.2% with selectivity to liquids of 73.9to. The aromatic content in the liquid product was found to be 9% with 1% attributable to benzene.

Comparative Example 1 A 20 ml (11.40g) sample of the catalyst prepared according to Example 2 was placed in a stainless steel reactor tube and the temperature raised to 5350C under flowing nitrogen at atmospheric pressure. When the reactor had come to temperature, the reactor was purged with nitrogen for 2 hours. The pressure was raised to (200 KPa (2 bar) absolute and propane passed through the reactor at a rate of 0.8 weight hourly space velocity, the furnace controls being adjusted to maintain an average bed temperature of 535"C. The reaction products were separated into gas and liquid phases in a condenser system and analysed by gas chromatography.

Conversion of propane at 46.8 hours on steam was found to be 60.8% with selectivity to liquids of 55.3%. The aromatic content of the liquid product was found to be 100% with 28.5% attributable to benzene.

Claims

1. A two-stage process for the upgrading of a feedstock comprising C2 to C8 hydrocarbons comprising a first dehydrogenation step wherein the feedstock is contacted with a dehydrogenation catalyst at elevated temperature to produce a first product, said dehydrogenation catalyst comprising a platinum group metal on a silicalite support; and a second step wherein at least part of the first product is passed over a zeolite catalyst at elevated temperature to produce a second proruct rich in tertiary olefins and/or aromatics.

2. A two-stage process according to claim 1 wherein the first stage is operated as a high temperature dehydrogenation stage and the second stage is operated as a lower temperature stage.

3. A two-stage process according to claim 2 wherein the first stage is operated in a temperature range of from 550-650 C and the second stage is operated in a temperature range of from 300-450"C.

4. A two-stage process according to any of the preceding claims wherein the first and second stages are operated in the pressure range of from 50-600 KPa absolute.

5. A two-stage process according to any of the preceding claims wherein the platinum group metal is selected from platinum, ruthenium, iridium, rhodium or palladium.

6. A two-stage process according to claim 5 wherein the platinum group metal is platinum.

7. A two-stage process according to any of the preceding claims wherein the dehydrogenation catalyst comprises a metal promotor selected from zinc or tin.

8. A two-stage process according to claim 7 wherein the metal promotor is zinc.

9. A two-stage process according to the preceding claims wherein the zeolite catalyst is ZSM-5.

10. A two-stage process according to claim 9 wherein the zeolite catalyst comprises a metal promotor selected from gallium or zinc.