GB2191214A - Production of higher molecular weight hydrocarbons from methane - Google Patents

Abstract

Higher molecular weight hydrocarbons are produced from a methane-containing gas by: (A) pyrolysing the gas in fluidised catalytic bed reactor (2) at at least 800 DEG C to form a product comprising hydrogen, C2-C4 hydrocarbons, higher hydrocarbons (C5-C10+), unreacted gaseous paraffinic hydrocarbons and carbon which is laid down on the catalyst; (B) regenerating the catalyst with oxygen at elevated temperature in fluidised bed regenerator (3); (C) returning the hot regenerated catalyst via (9) to the reactor (2) and thereby supplying to the incoming gaseous feed a major part of the heat necessary for its pyrolysis, (D) separating the pyrolysis product into a liquid portion comprising higher molecular weight hydrocarbon (C5-C10+) and a gaseous portion comprising hydrogen, C2-C4 hydrocarbons and unreacted gaseous paraffinic hydrocarbon, (E) separating at (6) the gaseous portion recovered in step (D) into a first portion (12, 21) comprising hydrogen and a second portion (19) comprising unreacted gaseous paraffinic hydrocarbon and C2 to C4 hydrocarbons, (F) recycling the second portion (19) separated in step (E) in whole or in part to the reactor, and (G) passing (12) a part of the hydrogen separated as the first portion in step (E) as fuel to the regenerator. <IMAGE>

Description

SPECIFICATION The catalysed pyrolysis of methane-containing gaseous paraffinic hydrocarbons The invention relates in general to a process for converting methane-containing gaseous paraffinic hydrocarbons to higher molecular weight hydrocarbons. More specifically, the present invention relates to the catalysed pyrolysis of methane-containing gaseous paraffinic hydrocarbon feedstocks to a product comprising liquid C5+ hydrocarbons including aromatic hydrocarbons.

Huge reserves of natural gas exist in many remote areas. This gas can not be economically delivered to traditional markets and this surplus .availability results in the gas having an economic value well below its calorific equivalence with petroleum products. A direct conversion of natural gas into useful liquid hydrocarbons would allow the use of conventional tankers or possibly existing pipelines for transport of the liquid products.

The pyrolysis of methane and natural gases principally comprising methane to higher molecular weight hydrocarbons, including aromatics, has been known for a long time (see for example 'The production of gaseous, liquid and solid hydrocarbons from methane.

Part I-The thermal decomposition of methane', J.Soc. Chem. Ind., 11.1.29, pp 1-8, by H.M. Staniey and A.W. Nash; 'The heat-treatment of hydrocarbons with special reference to the gaseous hydrocarbons', J. Soc. Chem.

Ind., 21.08.31, pp 313-318, by A.E. Dunstan, E.N. Hague and R.V. Wheeler and 'Semiindustrial production of aromatic hydrocarbons from natural gas in Persia' Ind. Eng. Chem., March, 1934, pp 315-320, by W.H. Cadman).

The reaction can be represented by the following overall equation: CH4 , hydrocarbons + H2+coke This dehydrogenation involves a variety of free radical intermediates forming higher hydrocarbons in stepwise sequence. The final products can include acetylene, olefins and diolefins in the C2 to C5 range and aromatics in the C6+ range. By-products are hydrogen and coke.

The overall reaction is highly endothermic.

We have now devised an integrated process for the production of higher molecular weight hydrocarbons from a methane-containing gaseous paraffinic hydrocarbon feedstock.

Accordingly the present invention provides a process for the production of higher molecular weight hydrocarbons from a methane-containing gaseous paraffinic hydrocarbon feedstock which process comprises the steps of: (A) feeding the gaseous paraffinic hydrocarbon to a pyrolysis zone comprised of a reactor containing fluidisable particulate catalyst and a regenerator; the gaseous paraffinic hydrocarbon being fed to the reactor at a rate sufficient to cause fluidisation of the catalyst and being heated to a temperature of 800"C or greater such that it is pyrolysed to form a product comprising hydrogen, C2-C4 hydrocarbons, higher hydrocarbons (Cs-C,0+), unreacted gaseous paraffinic hydrocarbons and carbon which is laid down on the catalyst; the reactor and the regenerator intercommunicating to provide a passage for carbonised catalyst to pass under the influence of driving means from the reactor to the regenerator wherein the catalyst in fluidised mode is contacted with an oxygen-containing gas at a sufficiently elevated temperature to burn off at least a portion of the carbon thereon, thereby forming a flue gas comprising carbon oxides and further increasing the temperature of the catalyst; returning the hot regenerated catalyst via a device for controlling the flow of hot solids to the reactor and thereby supplying to the incoming gaseous paraffinic hydrocarbon feed a major part of the heat necessary for its pyrolysis, (B) separating the pyrolysis product into a liquid portion comprising higher molecular weight hydrocarbon (C5-Cao+) and a gaseous portion comprising hydrogen, C2-C4 hydrocarbons and unreacted gaseous paraffinic hydrocarbon, (C) separating the gaseous portion of the pyrolysis product recovered in step (B) into a first portion comprising hydrogen and a second portion comprising unreacted gaseous paraffinic hydrocarbon and C2 to C4 hydrocarbons, (D) recycling the second portion separated in step (C) in whole or in part to the reactor, (E) passing a part of the hydrogen separated as the first portion in step (C) as fuel to the regenerator, (F) optionally, passing a second part of the hydrogen separated as the first portion in step (C) to the reactor, and (G) optionally, hydrogenating the liquid portion of the pyrolysis product separated in step (B) using a third part of the hydrogen separated in step (C) to produce a hydrogenated liquid hydrocarbon product.

The methane-containing gaseous paraffinic hydrocarbon feedstock may be substantially pure methane or may be in the form of a mixture with other gaseous alkanes, for example ethane, propane or butane. Additionally, the feedstock may contain one or more of carbon monoxide, hydrogen, carbon dioxide and steam. The preferred feedstock is natural gas. The precise composition of natural gas will depend upon its origin but generally it will principally comprise methane with lesser amounts of ethane, propane and butane.

It is preferred to purify the natural gas by removing acid components, for example CO2 and hydrogen sulphide, in conventional manner prior to feeding to the reactor. The methane containing gaseous paraffinic hydrocarbon feedstock is supplemented by recycling (step (D)) unreacted gaseous paraffinic hydrocarbon and C2-C4 hydrocarbons separated in step (C).

Preferably it is further supplemented by the addition of hydrogen (step (F)) also separated in step (C). The addition of hydrogen can have substantial benefits in reducing carbon formation and increasing the selectivity to the desired product hydrocarbons in the pyrolysis reaction.

As regards the catalyst for the pyrolysis reaction, this may suitably be carbon in one of its forms, for example charcoal, graphite or a high surface area graphitised carbon such as that described in GB-A-2136704. Alternatively, the carbon may be doped with a metal selected from Groups I to VIII of the Periodic Table as described in our copending UK application No 8613672 (BP Case No. 6390). Suitable metals include iron, cobalt, manganese, chromium, molybdenum, tungsten, rhodium, rhenium, lanthanum, cerium, ytterbium, erbium, neodymium, gadolinium, terbium, holmium and praesodymium, and mixtures of two or more thereof. Preferred metals include molybdenum, tungsten, lanthanum, cerium and ytterbium.

The metal(s) may suitably comprise up to 15%, preferably up to 10%, by weight of the catalyst. In a further alternative a metal(s) doped silica or alumina catalyst, for example tungsten supported on silica or tungsten supported on alumina, may be employed. The catalyst must be of a particle size suitable for fluidisation, for example from about 0.4 mm to 2.0 mm.

The reactor may suitably take the form of a series of communicating fluidised beds, typically stacked one atop another so that catalyst may overflow and fall under gravity from a higher bed to a lower bed and eventually exit from the lowermost reactor below the methane-containing paraffinic hydrocarbon feed inlet. The feed may be subjected to progressively increasing temperatures as it passes up the successive fluidised beds comprising the reactor by arranging the recycle of hot catalyst from the regenerator to the individual fluidised beds accordingly. Typically for a reactor comprised of two fluidised beds, the uppermost bed may be at a temperature in the range from 950 to 1100 C and the lowermost bed at a temperature of from 650 to 950"C.

The pressure in the reactor is preferably elevated, for example in the range from 1 to 50 bar. Preferably the feed is pre-heated before entering the reactor, suitably to a temperature which substantially avoids methane decomposition and coke formation, for example from about 550 to 6500C The carbonised catalyst, having been cooled by heating the incoming feed gas, is passed from the reactor to the regenerator. The driving means for the passage of the catalyst to the regenerator may suitably be provided by the introduction of a transport gas, which is preferably air. The transport gas is preferably pre-heated to a temperature at least equal to that of the carbonised catalyst.

In the regenerator carbon laid down on the catalyst is burned off. As those skilled in the art will appreciate, this must be done in a controlled manner, particularly using carbon and metal(s) doped carbon catalysts, in order to prevent damage to or possible destruction of the catalyst. Burning off of the excess carbon is accomplished by contacting the carbonised catalyst with an oxygen-containing gas.

The oxygen-containing gas may be oxygen or air, preferably the latter. It is preferred to feed steam to the regenerator for ease of temperature control; this is a well-established technique whereby the steam reacts by the 'water gas' reaction: C+H20- ,H2+CO to moderate the exothermic combustion.

The major proportion of the large quantity of heat required in the reactor is provided by the recirculated hot catalyst. The remaining part of the heat required is provided by the feed pre-heat. The burning off of laid down carbon from the catalyst provides insufficient heat to provide all the endothermic heat of the pyrolysis reaction in the reactor. Consequently, hydrogen separated in step (C) is fed as fuel to the regenerator (step (E)), thereby raising still further the temperature of the solid catalyst, typically to about 1400"C. The air and hydrogen fed to the regenerator is preferably pre-heated, suitably by heat exchange with the flue gas exiting from the regenerator.

The hot regenerated catalyst is then returned via a device for controlling the flow of hot solids means to the reactor wherein it heats the feed gases to pyrolysis temperatures. The device may suitably be mechanical or pneumatical in nature. As mentioned hereinbefore the hot catalyst may be divided into separate streams, the streams being directed to the individual fluidised beds comprising a preferred form of the reactor in proportions appropriate to produce a temperature gradient within the reactor. The flue gas comprising carbon oxides formed by burning the carbon may be used to preheat the feeds to the regenerator.

The pyrolysis product comprising hydrogen, C2 to C4 hydrocarbons, higher hydrocarbons (C5-C10+) and unreacted gaseous paraffinic hydrocarbons exiting from the reactor is separated into a liquid portion comprising higher molecular weight hydrocarbons (C5-Cao+) and a gaseous portion comprising hydrogen, C2-C4 hydrocarbons and unreacted gaseous paraffinic hydrocarbon. The separation may suitably be achieved by in a first step cooling the product, suitably by (i) raising steam in a waste heat boiler, (ii) preheating the feed gas, and (iii) cooling water, in a second step flashing the cooled product, typically at about 40"C and 30 bar pressure and finally passing the flashed product to an absorber/stripper system.The heavier liquid products are condensed out in the cooling and the ligher ones are separated in the absorber/stripper system.

In step (C) the gaseous portion of the pyrolysis product recovered in step (B) is separated into a first portion comprising hydrogen and a second portion comprising unreacted gaseous paraffinic hydrocarbon and C2 to C4 hydrocarbons. The second portion is recycled to the reactor in step (D). A part of the separated hydrogen is passed as fuel to the regenerator in step (E). A second part of the separated hydrogen is preferably passed to the reactor. The reactor feed may contain up to 25 mole percent, typically about 5 mole percent hydrogen.

The separation in step (C) may suitably be effected by cryogenic separation, catalytic separation, pressure-swing adsorption or membrane separation. On the grounds of lower capital cost and simplicity of operation while giving relatively high hydrogen recoveries at adequate (about 90%) hydrogen purity, which is desirable if the optional step (G) is employed, membrane separation is a preferred alternative. Of the available membrane separation processes, that generally referred to as the Monsanto Prism Process is preferred. Further details of this process may be found, for example, in Oil and Gas Journal, February 6th, 1984 by S.l. Wang, D.M. Nicholas and S.P.

DiMartino in an article entitled "Analysis dictates H2 purification process" and in the proceedings of a Symposium by Monsanto at the Royal Garden Hotel, London, 17.11.83 entitied "Prism Separators". A second preferred alternative is a cryogenic separation which is a well-known technique requiring no further elaboration to those skilled in the art. An advantage of the cryogenic method over the membrane method is the ability to remove nitrogen with the hydrogen.

Preferably the process of the present invention includes a further step (G) in which the liquid portion of the pyrolysis product separated in step (B) is hydrogenated using a third part of the hydrogen separated in step (C) to produce a hydrogenated liquid hydrocarbon product. The desirability, or otherwise, of product hydrogenation will depend on commerical considerations regarding the potential outlets for the products and the location of the plant.

Thus, if the only requirement is for a "transportable liquid" then hydrogenation would not be essential since any necessary product refining could be carried out at the end-use destination of the transportable liquid. Typi caily, in the performance of step (G) the C5-Cg components are separated from the C10-C,1+ components in a splitter; the C5-Cg fraction is hydrogenated to produce a gasoline component, the benzene and C7-Cg aromatics thereby being saturated; the C10-C11+ fraction is hydrogenated to produce a diesel component, the naphthalene, C10Ar (aromatics) and C11+ thereby also being saturated. The technology by which unsaturated hydrocarbons are hydrogenated is well-known and requires no further elaboration.

An embodiment of the process of the present invention will now be further described with reference to the accompanying Drawings.

In the Drawings: Figure 1 is a process block diagram, and Figure 2 is a diagrammatic representation of the pyrolysis zone which includes the reactor and regenerator of Fig. 1.

With reference to Fig. 1, 1 is a pre-heat unit, 2 is a reactor, 3 is a regenerator, 4 is a compressor, 5 is a waste heat recovery and separation zone, 6 is a cryogenic separation unit and 7 to 21 are transfer lines.

With reference to Fig. 2, 2, 3 and 8 to 15 have the same significance as in Fig. 1, 22 and 23 are fluidised catalyst beds, 24 and 25 are overflow standpipes, 28 is a fluid bed stripper, 31 and 32 are devices for controlling the flow of hot solids and 26, 27, 29 and 30 are transfer lines.

Natural gas having the composition: Mole % wt % C1 88 74.8 C2 6 9.5 C3 2 4.7 C4 3 9.5 N2 1 1.5 is purified by acid gas removal/ZnO desulphurisers (not shown) and fed via line 7 to the pre-heater 1 wherein its temperature is raised to about 6500C by heat exchange with the hot reactor product (not shown). The pre-heated, purified natural gas is then fed through line 8 to the reactor 2 wherein it is further heated progressively to a temperature of about 1 000"C by contact with hot particulate catalyst, at which temperatures and under short residence time conditions the natural gas pyrolyses to form a product comprising for example hydrogen, ethylene, acetylene, propylene, propyne, benzene, styrene, naphthalene, 1 ,2,4,5-tetramethylbenzene, pentamethylbenzene, unreacted methane and carbon, which is laid down on the catalyst.

The reactor 2, fabricated in steel with a refractory liner, takes the form of two fluidised beds one on top of the other (22 and 23), the catalyst levels in the fluidised beds being maintained mechanically by means of the overflow standpipes 24 and 25. Product gas leaves the reactor by line 15. Carbonised catalyst leaves the reactor through line 26 at a temperature of about 700"C and is transported to the regenerator 3 through line 14 under the driving force of preheated air (about 300"C) introduced through line 27. Also fed to the regenerator 3 through lines 10 and 11 is preheated air (about 800"C), which is pressurised in compressor 4, and steam (about 500"C) and through line 12 hydrogen as fuel preheated to a temperature of about 650"C.

The regenerator is also fabricated in steel with a refractory liner. In the regenerator a controlled burning off of the carbon of the fluidised catalyst is effected and the catalyst temperature is raised to about 1400"C.

The regenerated catalyst is taken from the regenerator through line 9 and fed to the fluid bed stripper 28. From the fluid bed stripper the catalyst passes through the two lines 29 and 30 through the devices for controlling the flow of hot solids 31 and 32 to the fluidised beds 22 and 23, the amounts of catalyst delivered to the two fluidised beds depending upon the temperatures desired in the two beds. This may be adjusted automatically in response to temperature signals from the individual fluid beds.Typically, a part of the recycled catalyst is fed through line 29 to fluid bed 22 where the catalyst gives up heat from 1400"C to 1000"C before overflowing through the overflow standpipe 24 to fluid bed 23 wherein it gives up further heat from 1000"C to 960"C. The remaining part of the recycled catalyst is fed through line 30 to fluid bed 23 where the catalyst gives up heat from 1400"C to 960"C before overflowing together with overflowing catalyst from bed 22 through the overflow standpipe 25 and past the feed inlet point where it gives up further heat (to the incoming feed) from 960"C to about 700"C.

The flue gas leaving the generator through line 13 typically has the composition CO (0.5%), H2 (1.0%), H20 (28.0%), N2 (66.3%) and CO2 (4.2%), all percentages being by volume. This gas, having a temperature typically about 1400"C is used to generate steam in a waste heat boiler and to preheat air and fuel feeds to the regenerator.

The product gas exiting from the reactor through line 15 typically has a temperature of about 1000"C. It is cooled in stages by (i) generating steam in a waste heat boiler, (ii) preheating the reactor feed and finally (iii) the use of cooling water. The cooled product is passed to a flash vessel, typically maintained at 40"C and 30 bar pressure, wherein gaseous components, for example methane, hydrogen, nitrogen and C2 to C4 hydrocarbons are separated from the heavier products. The flashed gaseous product is then passed to an absorber column. Gaseous product from the absorber is passed through line 18 to the cryogenic separation unit 6.Bottoms product from the absorber is fed together with the liquid residue from the flash to a stripper column wherein it is separated into a C5 to C9 fraction for gasoline utilisation and a C10 to C11+ fraction for diesel utilisation. The cooling, flash, absorber and stripper is shown as a single operation (5) in Fig. 1.

In the cryogenic separation unit 6 the gaseous product is separated into a hydrogen-lean component, comprising gaseous hydrocarbons, including methane and C2 to C4 hydrocarbons, nitrogen and some hydrogen and a hydrogenrich component principally comprising hydrogen but also containing some nitrogen and methane. The hydrogen-lean component or at least that part remaining after a purge is taken off through line 20 is recycled through the preheat 1 to the reactor 2. A part of the hydrogen-rich portion is passed to the regenerator through line 12. A second part may be passed to the reactor (not shown). Another part may be used to hydrogenate the C5-Cg fraction and C10-C1,+ funcation obtained from the stripper column and the remainder may be used to generate steam.

Claims (6)

1. A process for the production of higher molecular weight hydrocarbons from a methane-containing gaseous paraffinic hydrocarbon which process comprises the steps of: (A) feeding the gaseous paraffinic hydrocarbon to a pyrolysis zone comprised of a reactor containing fluidisable particulate catalyst and a regenerator; the gaseous paraffinic hydrocarbon being fed to the reactor at a rate sufficient to cause fluidisation of the catalyst and being heated to a temperature of 800"C or greater such that it is pyrolysed to form a product comprising hydrogen, C2-C4 hydrocarbons, higher hydrocarbons (C,-C,,,), unreacted gaseous paraffinic hydrocarbons and carbon which is laid down on the catalyst; the reactor and the regenerator intercommunicating to provide a passage for carbonised catalyst to pass under the influence of driving means from the reactor to the regenerator wherein the catalyst in fluidised mode is contacted with an oxygen-containing gas at a sufficiently elevated temperature to burn off at least a portion of the carbon thereon, thereby forming a flue gas comprising carbon oxides and further increasing the temperature of the catalyst; returning the hot regenerated catalyst via a device for controlling the flow of hot solids to the reactor and thereby supplying to the incoming gaseous paraffinic hydrocarbon feed a major part of the heat necessary for its pyrolysis, (B) separating the pyrolysis product into a liquid portion comprising higher molecular weight hydrocarbon (C5-C10+) and a gaseous portion comprising hydrogen, C2-C4 hydrocarbons and unreacted gaseous paraffinic hydrocarbon, (C) separating the gaseous portion of the pyrolysis product recovered in step (B) into a first portion comprising hydrogen and a second portion comprising unreacted gaseous paraffinic hydrocarbon and C2 to C4 hydrocarbons, (D) recycling the second portion separated in step (C) in whole or in part to the reactor, (E) passing a part of the hydrogen separated as the first portion in step (C) as fuel to the regenerator, (F) optionally, passing a second part of the hydrogen separated as the first portion in step (C) to the reactor, and (G) optionally, hydrogenating the liquid portion of the pyrolysis product separated in step (B) using a third part of the hydrogen separated in step (C) to produce a hydrogenated liquid hydrocarbon product.

2. A process according to claim 1 wherein the methane-containing gaseous paraffinic hydrocarbon is natural gas.

3. A process according to either claim 1 or claim 2 wherein the feedstock is supplemented by recycling unreacted gaseous paraffinic hydrocarbon and C2-C4 hydrocarbons separated in step (C).

4. A process according to any one of the preceding claims wherein the feedstock is supplemented by the addition of hydrogen separated in step (C).

5. A process according to any one of the preceding claims including a further step (G) in which the liquid portion of the pyrolysis product separated in step (B) is hydrogenated to produce a hydrogenated liquid hydrocarbon product.

6. A process according to claim 1 substantially as hereinbefore described with reference to and as illustrated in Figs. 1 and 2 of the Drawings.