GB2191213A - Integrated process for the production of liquid hydrocarbons from methane - Google Patents

Abstract

Liquid hydrocarbons are produced from methane by a process comprising the following steps:- (I) feeding methane to a pyrolysis zone maintained under such conditions that methane is pyrolysed to form a product comprising hydrogen, unsaturated hydrocarbons and aromatic hydrocarbons in a yield exceeding 30% wt, (II) partially quenching the pyrolysis product from step (I) with water to a lower temperature such that carbon formation is essentially prevented, (III) further quenching the product from step (II) or the product from (II) which has been subjected to oligomerisation and/or alkylation, with a liquid hydrocarbon to a temperature low enough to condense condensible components of the pyrolysis product, thereby achieving gas/liquid product separation. The liquid product from III may be hydrogenated.

Description

SPECIFICATION Integrated process for the production of liquid hydrocarbons from methane The present invention relates in general to the pyrolysis of methane and in particular to an improved integrated process for the production of liquid hydrocarbons by the pyrolysis of methane or methane-containing gaseous alkane mixtures, for example natural gas.

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, both in the presence and absence of pyrolysis catalysts, has been extensively studied since the 1920s. The reaction can be represented by the following overall equation: CH4 100000 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, the heat required under technical conditions (including preheat from 800"C to 1000"C) typically amounts to approximately 625 Kcal per kg total reactor feed.

Despite the vast research input over the past fifty years the process remains commercially unattractive for a variety of reasons.

Amongst these is the low methane conversion generally achieved at pressures required for conventional gas/liquid product separation, typically about 30 bar. Conversion to liquids at this pressure is limited to 20% at operating temperatures achievable with current technology (ca. 1100 C). This low conversion requires substantial recycle of unconverted feed to the pyrolysis reactor which in turn requires removal of at least part of the hydrogen byproduct since the presence of substantial amounts of hydrogen reduces the conversion in pyrolysis. While operation of a pyrolysis process at pressures of about 30 bar may be advantageous in terms of product separation, the above factors lead to low thermal efficiency and high capital costs.

Operation of the pyrolysis reactor at low pressure, to maximise conversion, requires compression of the product stream to pressures of approximately 30 bar to facilitate separation by conventional means. However, the pyrolysis product stream at high conversion contains a substantial amount of hydrogen requiring special compressors and high capital costs.

The present invention seeks to alleviate or eliminate the above problems.

Accordingly, the present invention provides a process for the production of liquid hydrocarbons from methane which process comprises the following steps.

(I) feeding methane to a pyrolysis zone maintained under such conditions that methane is pyrolysed to form a product comprising hydrogen, unsaturated hydrocarbons and aromatic hydrocarbons in a yield exceeding 30% wt, (II) partially quenching the pyrolysis product from step (I) with water to a lower temperature such that carbon formation is essentially prevented, (III) further quenching the product from Step (II) with a liquid hydrocarbon to a temperature low enough to condense condensible components of the pyrolysis product, thereby achieving gas/liquid product separation.

In a preferred additional step [Step (IIA)], the partially quenched pyrolysis product from step (II) may be reacted in an oligomerisation/alkyiation zone maintained under conditions whereby unsaturated hydrocarbons are oligomerised and/or utilised to alkylate aromatic hydrocarbons. Thereafter, the product from step (IIA) is further quenched in step (III).

In a further additional step [Step (is1, the liquid product from step (III) may be hydrogenated using the hydrogen-rich process off-gas separated in step (III).

As a consequence of the combined liquid quench/separation of Step III, the whole process may be operated at relatively low pressure, maximising conversion. Control of the temperature at the exit of the pyrolysis reactor, Step II, and the residence time in the optional gas phase oligomerisation/alkylation Step IIA increases the fraction of liquid products and the optional subsequent hydrogenation Step IV maximises the overall make and value of the liquid products.

Methane may be fed to the pyrolysis zone in substantially pure form or in the form of a mixture with other gaseous alkanes and/or CO2 in which methane constitutes the major component. A preferred mixture is natural gas.

Preferably natural gas is purified in conventional manner before use in the process of the invention. Also steam and/or CO2 may be cofed with the gaseous hydrocarbon.

The pyrolysis of methane may be accomplished in a variety of ways, including both catalysed and uncatalysed reactions. In a suitable uncatalysed reaction, the pyrolysis zone to which methane is fed may suitably take the form of a plurality of tubes which may be heated to the desired pyroiysis temperature by both convective and radiative heating means.

In order to withstand the elevated temperatures involved the pyrolysis tubes may be formed in a suitable ceramic material. In one arrangement of the pyrolysis zone the plurality of pyrolysis tubes are located within an outer shell, within which fuel gas and air at atmospheric pressure are burned, thereby generating heat which is transferred to methane within the pyrolysis tubes by both radiation and convection. This type of arrangement will be recognisable to those skilled in the art as being similar to that conventionally employed for steam reforming. A difference between methane pyrolysis and steam reforming is that temperatures employed in the former reaction are typically 1000"C and higher whereas temperatures employed in the latter reaction are generally at least 150"C lower.The use of a specific type of ceramic material in the construction of the pyrolysis tubes facilitates operation at these higher temperatures. Elevated pressures up to 10 bar may suitably be employed in the pyrolysis zone. Above this pressure, conversions may become too low to be economically feasible.

In a suitable catalysed pyrolysis reaction, the pyrolysis zone may suitably take the form of a fluid bed reactor employing autogenerated coke as catalyst by for example a process comprising the steps of: (A) feeding methane to a pyrolysis zone comprised of a reactor containing fluidisable particulate catalyst and a regenerator; the methane 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 (C5-C10+), unreacted methane 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 methane feed a major part of the heat necessary for its pyrolysis, (B) separating the pyrolysis product into a liquid portion comprising higher molecular weight hydrocarbons (C5-C10+) and a gaseous portion comprising hydrogen, C2-C4 hydrocarbons and unreacted methane, (C) separating the gaseous portion of the pyrolysis product recovered in step (B) into a first portion comprising hydrogen and a second portion comprising methane 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 (C) to produce a hydrogenated liquid hydrocarbon product.

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 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 catalysts 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 5 bar.Preferably the feed is preheated before entering the reactor, suitably to a temperature which substantially avoids methane decomposition and coke formation, for example from about 550 to 650"C.

Generally, the pyrolysis reaction may be operated at temperatures of 1000"C or greater and pressures up to 50 bar.

A suitable catalyst for use in the pyrolysis reaction is a metal(s) doped carbon catalyst, the metals being selected from Groups I to VIII of the Periodic Table. 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.

Suitably the carbon may be in the form of charcoal, graphite or a higher surface area graphitised carbon, for example that described in GB-A-2136704. The metal(s) may suitably comprise up to 15%, preferably up to 10%, by weight of the catalyst.

The aforesaid catalysts may be prepared by any of the methods conventionally employed in catalyst preparation. Generally, an impregnation technique will be found suitable for the preparation of catalysts active in the process of the invention. A convenient impregnation method comprises impregnating the carbon with a water soluble compound of the metal(s), for example the nitrate or if the nitrate is unavailable a double salt of ammonium and the metal. It is preferred to include in the impregnation solution a lower alkanol, for example methanol, in order to facilitate wetting of the carbon. Thereafter, it is preferred to heat the mixture, suitably by boiling under reflux. The solid catalyst is thereafter evaporated to dryness and further dried.Before use in the process of the invention the catalyst so-obtained is preferably calcined, suitably at a temperature in the range from 550 to 600"C in contact with an inert gas, for example nitrogen. The calcination is preferably effected 'in situ' immediately prior to contact with the methane-containing gaseous paraffinic hydrocarbon feedstock.

Some at least of the aforesaid catalysts, for example a tungsten doped charcoal catalyst, are regenerable. Regeneration of the catalyst may suitably be accomplished by treatment with an oxygen-containing gas, for example air, at elevated pressure, suitably up to 50 bar, for example about 30 bar, and at an elevated temperature suitably greater than 800"C.

It is preferred to co-feed steam to the regeneration reaction.

Alternatively, the pyrolysis reactor may suitably take the form of a silicon impregnated silicon carbide tube or a plurality thereof treated by contact with nitrogen, for example, at elevated temperature, suitably in the range from 800 to 1400, preferably from 1000 to 1400"C for a time sufficient to achieve a reduction in coke-forming activity. The material may be purchased in tubular form from the Carborundum Company (Trade name-Hexoloy) or Sigri Electrographit GmbH (Trade name-Silit Sk). Alternatively other forms of reactor and materials of construction may be employed.

The pyrolysis product exiting the pyrolysis zone may be used as a heat exchange medium to pre-heat the feed to the pyrolysis zone thereby cooling the pyrolysis product.

Preferably the pyrolysis product is partially quenched with water immediately after the pyrolysis reactor to a lower temperature, suitably in the range 500-800"C, such that further pyrolysis reactions, particularly of the product, to form carbon are minimised while oligomerisation/alkylation reactions proceed to increase the fraction of product that is liquid at normal temperature and pressure. The reduction in temperature by a primary water quench also allows the use of a hydrocarbon oil in a subsequent quench/product separation step. Use of a hydrocarbon oil quench directly at the pyrolysis reactor exit would lead to cracking of the quench oil and production of light gas, thereby reducing the efficiency of the process.

After water quenching the pyrolysis product to a temperature in the range 500-800"C, the gas stream will generally comprise hydrogen, acetylene, olefins and diolefins in the C2 to C5 range and aromatics in the C6+ range. The pyrolysis product is optionally fed depending on the amount of C2 to C5 unsaturated hydrocarbons in the pyrolysis product to an oligomerisation/alkylation zone maintained under oligomerisation/alkylation conditions wherein unsaturated hydrocarbons, particularly acetylene and lower olefins, for example ethylene, are oligomerised and/or utilised in the alkylation of aromatic hydrocarbons. Suitable oligomerisation/alkylation conditions will depend on the process pressure and the severity of the pyrolysis reaction, which determine the gas composition, and the temperature to which the gas is quenched.Typical conditions will be in the range 1-10 bar, 500-800"C and residence time in the range 1-30 seconds. Inclusion of this step can increase the yield of hydrocarbon liquid products from the process.

The gas exiting the oligomerisation/alkylation zone typically contains hydrogen, aromatics and C5+ hydrocarbons at 600-800"C. In Step (III) of the process of the invention the gas stream from Step (II) or optional Step (IIA) is further quenched with hydrocarbon oil to a temperature sufficiently low to condense the aromatics and C5+ olefinic hydrocarbon products and achieve separation of the normally liquid products from the gaseous components, namely hydrogen and light hydrocarbons, for example C1 to Cs hydrocarbons, thereby reducing the cost associated with conventional gas/liquid separation systems.

A preferred embodiment of the present invention will now be described in greater detail with reference to the accompanying Figure which is a simplified flow diagram. With reference to the Figure, 11 is the reactor furnace containing the pre-heat section, 12 is the water quench which may in practice be directly at the exit of the pyrolysis reactor, 13 is the oligomerisation/alkylation zone which may in practice be a transfer line, 14 is the liquid quench/separation unit, and 15 is an optional hydrogenation reactor.

Natural gas is fed through line 1 to the feed pre-heat/pyrolysis reactor furnace 11 which consists of a firebox enclosing a multi-tubular pyrolysis reactor with the preheat in the conventional bank of the furnace. The feed is preheated to the highest temperature that avoids the onset of pyrolysis and coke formation at the process pressure, typically 600-800"C depending on the amount of higher hydrocarbons present in the natural gas feed. The preheated gas then passes through the multi-tubular pyrolysis reactor. The tubes, fabricated in a ceramic material, are directly fired by combustion of gas with air at atmospheric pressure, the heat generated being transferred by radiation and convection to the natural gas within the tubes.The natural gas temperature is thereby raised to 1000-1300"C depending on the process pressures, typically 1-10 bar.

Generally the higher the pressure the higher the temperature required for economic conversion. The optimium residence time of the gas within the pyrolysis reactor tubes depends on the process pressure and temperatures but is typically 0.5-10 seconds. Pyrolysis of the gas produces a product comprising hydrogen, acetylene, C2-C5 olefins and diolefins and C6+ aromatics.

The hot product gas stream exits the pyrolysis zone via line 2 and is immediately quenched with water fed via line 3 to the quench zone 12 to a temperature low enough to prevent the pyrolysis products reacting further to form unwanted tars and carbon. This depends on process pressure and degree of conversion in the pyrolysis reactor but is typically 500-800"C. The product stream is passed via line 4 to the oligmerisation/alkylation reactor, 13, where it is allowed to react in a controlled manner, at the lower temperature, such that the acetylene and C2-C5 olefins oligomerise and/or alkylate the aromatic fraction of the products. The residence time in this region depends on temperature and pressure but is typically 1-10 seconds.The hot product from this stage is passed via line 5 to the final quench zone 14 where it is cooled to 150"C by a hydrocarbon liquid introduced via line 6. The final temperature to which the product stream is quenched depends on its composition but is typically 25-100"C to ensure that the liquid products of the pyrolysis and oligomerisation/alkylation reactions condense and mix with or dissolve in the quench oil.

The quench oil containing the liquid product is optionally drawn off via line 7. Process offgas, mainly hydrogen and unconverted methane, is withdrawn through line 8. Part of this stream may be recycled, the amount being such that the hydrogen content of the feed to the pre-heater does not exceed 20% vol. The remainder is used as feed for the pyrolysis reactor furnace or other applications such as enhanced oil recovery. Depending on the nature of the quench oil the products may be further enhanced in value by hydrogenation.

For conversion of gas associated with crude oil or where crude oil is available, this may be advantageously used as the quench oil. In this case the temperature of the product stream from the oligomerisation/alkylation reactor should be suitably adjusted by the severity of the water quench, 12, to minimise cracking of the crude oil which would otherwise produce unwanted light gas and reduce the overall liquid yield of the process. Where a crude oil quench is thus used the products dissolved in the crude oil may be hydrogenated by passing the crude oil containing the aromatic and olefinic pyrolysis products through line 9 together with a part of the off-gas containing a large amount of hydrogen, through a hydrogenation reactor, 15. The liquid product being withdrawn through line 10 and off-gas via line 11.

Where no crude oil is available as quench oil, the aromatic product of the process is a particularly suitable quench oil, being resistant to cracking during the quench and allowing light temperature duty which in turn reduces the primary water quench duty and allows higher temperatures and reactor rates in the oligomerisation/alkylation reactor. However, in this case hydrogenation of the whole product stream is not advantageous since the aromatic oil is required for recycle to the quench. In this mode of operation a proportion of the liquid drawn off at line 7 is recycled to the quench while the remainder is passed to the hydrogenation reactor.

Claims (8)

1. A process for the production of liquid hydrocarbons from methane which process comprises the following steps: (I) feeding methane to a pyrolysis zone maintained under such conditions that methane is pyrolysed to form a product comprising hydrogen, unsaturated hydrocarbons and aromatic hydrocarbons in a yield exceeding 30% wt, (11) partially quenching the pyrolysis product from step (I) with water to a lower temperature such that carbon formation is essentially prevented, (III) further quenching the product from Step (II) with a liquid hydrocarbon to a temperature low enough to condense condensible components of the pyrolysis product, thereby achieving gas/liquid product separation.

2. A process according to claim 1 wherein in an additional step [Step (IIA)] the partially quenched pyrolysis product from step (II) is reacted in an oligomerisation/alkylation zone maintained under conditions whereby unsaturated hydrocarbons are oligomerised and/or utilised to alkylate aromatic hydrocarbons.

3. A process according to either claim 1 or claim 2 wherein in an additional step [step (IV)], the liquid product from step (III) is hydrogenated using the hydrogen-rich process offgas separated in step (III).

4. A process according to any one of the preceding claims wherein the pyrolysis product from step (I) is used as a heat exchange medium to pre-heat the feed to the pyrolysis zone thereby cooling the pyrolysis product.

5. A process according to any one of the preceding claims wherein the pyrolysis zone takes the form of a silicon impregnated silicon carbide tube or a plurality thereof treated by contact with nitrogen at elevated temperature for a time sufficient to achieve a reduction in coke-forming activity.

6. A process according to any one of the preceding claims wherein a catalyst is employed in the pyrolysis zone, the catalyst being one or more of the metals molybdenum, tungsten lanthanum, cerium and ytterbium supported on a carbon.

7. A process according to any one of the preceding claims wherein methane is fed to the pyrolysis zone in the form of natural gas.

8. A process according to any one of the preceding claims wherein the pyrolysis product is partially quenched with water immediately after the pyrolysis reactor to a lower temperature in the range 500 to 800"C, such that further pyrolysis reactions to form carbon are minimised while oligomerisation/alkylation reactions proceed to increase the fraction of product that is liquid at normal temperature and pressure.