GB2191212A - 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 pyrolysis product comprising hydrogen, unsaturated hydrocarbons and aromatic hydrocarbons, (II) reacting the pyrolysis product from step (I) in an oligomerisation/alkylation zone maintained under conditions whereby unsaturated hydrocarbons are oligomerised and/or are utilised to alkylate aromatic hydrocarbons, (III) reacting the oligomerisation/alkylation product with at least part of the hydrogen remaining in the product thereby to hydrogenate hydrogenatable unsaturated hydrocarbons in the oligomerisation/alkylation product, and (IV) recovering liquid hydrocarbons.

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 1000"C hydrocarbons+H2+coke This dehydrogenation involves a variety of free radical intermediates forming higher hydrocarbons in stepwise sequence. The final products 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 about 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 may be mentioned the low methane conversions generally achieved, necessitating high recycle ratios and overall gas flows; the high heat supply necessary for the highly endothermic pyrolysis reaction and the necessity to separate by-product hydrogen from the product gas giving rise to a large capital investment in hydrogen separation plant. The present invention seeks to alleviate to some extent the problems associated with low methane conversions and simultaneously eliminate the costly hydrogen separation step.

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 pyrolysis product comprising hydrogen, unsaturated hydrocarbons and aromatic hydrocarbons, (II) reacting the pyrolysis product from step (I) in an oligomerisation/alkylation zone maintained under conditions whereby unsaturated hydrocarbons are oligomerised and/or are utilised to alkylate aromatic hydrocarbons, (III) reacting the oligomerisation/alkylation product with at least part of the hydrogen remaining in the product thereby to hydrogenate hydrogenatable unsaturated hydrocarbons in the oligomerisation/alkylation product, and (IV) recovering liquid hydrocarbons from the hydrogenated oligomerisation/alkylation product.

As a consequence of the oligomerisation/alkylation and hydrogenation reactions, the unsaturated C2 hydrocarbons which would otherwise be lost to the liquid hydrocarbon product, are converted to useful liquid products, thereby increasing the overall make and quality of liquid products without any change in the conversion. The overall product distribution is also shifted in the direction of more useful products. Moreover, a part of the hydrogen produced in the pyrolysis reaction is removed, thereby reducing the size of the separation.

In a preferred embodiment of the present invention a part of the remaining hydrogen, ie that hydrogen not used in the hydrogenation of unsaturated hydrocarbons, is recycled together with unconverted methane to the pyrolysis zone. In this embodiment the hydrogen separation plant is eliminated completely.

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. Steam and/or carbon dioxide may also be added to the feed.

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 pyrolysis temperature by both radiative and convective 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 50 bar may suitably be employed in the pyrolysis zone.

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-C,O+), 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-C1o+) 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 pre-heated 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 suitable 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 the 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' immediateiy 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 cofeed 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.

Preferably the pyrolysis product exiting from the pyrolysis zone is used as a heat exchange medium to preheat the feed to the pyrolysis zone, thereby cooling the pyrolysis product to a temperature suitable for the oligomerisation/alkylation step. Alternatively, or in addition, following pyrolysis, the pyrolysis product may be subjected to a rapid quench to reduce its temperature to a value at which coke formation is substantially eliminated.

Whichever pyrolysis step is employed the pyrolysis product will generally comprise in addition to hydrogen, acetylene, olefins and diolefins in the C2 to C5 range and aromatics in the C6+ range. The pyrolysis product is fed 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.Suitably oligomerisation/alkylation may be performed in the absence of a catalyst if the temperature, pressure and reaction time are arranged accordingly, for example employing temperatures between 600 and 900"C and pressures between 1 and 30 bar the gas phase oligomerisation/alkylation reactions can be accomplished depending upon the composition of the pyrolysis product. Alernatively, oligomerisation/alkylation can be performed using suitable catalysts at lower temperatures than those required for homogeneous gas phase oligomerisation/ alkylation. Suitable oligomerisation catalysts and oligomerisation/alkylation conditions will be wellknown to those skilled in the art and require no further elaboration.

In step (III) the oligomerisation/alkylation product is reacted with at least part of the hydrogen remaining in the product thereby to hydrogenate hydrogenatabie unsaturated hydrocarbons present in the oligomerisation/alkylation product. Suitable hydrogenation catalysts and hydrogenation conditions will be well-known to those skilled in the art and require no further elaboration.

In a particularly preferred embodiment of the present invention the oligomerisation/alkylation step [step (II)] and the hydrogenation step [step (III)] are incorporated into a single oligomerisation/alkylation/hydrogenation step. Catalysts suitable for effecting a combined step of this type include for example noble metals of Group VIII of the Periodic Table, or other metals, for example zinc, in the form of the elemental metal, the oxide or sulphide in combination with an acidic support, such as silica/alumina, chromia zeolites and clays. The catalyst may be used in the form of a fixed bed, a fluidised bed or as a slurry in a suitable medium. Depending upon the temperature employed for this step, different product distributions may be obtained.

In step (IV) of the process of the invention, liquid hydrocarbons are separated from gaseous components comprising unreacted methane and hydrogen and the liquid hydrocarbons recovered.

This may suitably be accomplished by cooling and feeding to a conventional flash and oil absorber/stripper system. Preferably a part, for example up to 50% by volume, of the separated gaseous component is recycled as feed to step (I) of the process.

A preferred embodiment of the present invention will now be described in greater detail with reference to the accompanying Figure which takes the form of a simplified flow diagram.

With reference to the Figure, 16 is a feed purification unit (which unit is optional and can in general be omitted, particularly for homogeneous gas phase operation), 17 is a feed preheat unit, 18 is a reactor furnace, 19 is a flue gas extractor, 20 is a flue gas cooler assembly, 21 is an air blower, 22 is an oligomerisation/hydrogenation reactor, 23 is a product separation unit and 24 is a recycle gas compressor.

Natural gas is fed through line 1 to the feed purification unit 16 (optional) equipped with conventional acid gas removal and desulphurisation facilities. The purified natural gas is passed through line 2 to the feed preheat unit consisting of a heat exchanger which raises the temperature of the gas to the highest value which avoids the onset of pyrolysis and coke formation at the pressure employed, for example a temperature of about 800"C. The preheated gas is passed through line 3 to the reactor furnace 18 which consists of a firebox enclosing a multi-tubular reactor through which the natural gas passes. 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 tubular reactor.The natural gas temperature is thereby raised to at least 1000"C at process pressures up to 50 bar. The optimum residence time of the gas within the pyrolysis reactor tubes will depend on the temperature. For processes operating at about 1000"C residence times in the range from 0.1 to 10 seconds are suitable. Higher temperatures may be employed with correspondingly shorter residence times. Pyrolysis of the gas produces a pyrolysis product comprising hydrogen, acetylene, C2 to C5 olefins and diolefins and C6+ aromatics. Flue gases are withdrawn from the reactor furnace 18 through line 14, e.g. by a flue gas extractor 19, and are passed to the flue gas cooling assembly 20 incorporating steam generation facilities. Additionally, the thermal energy of the flue gas may be used for furnace air preheat.An air blower 21 blows air through the flue gas cooler 20 and via line 13 to the reactor furnace 18. Flue gas is withdrawn from the flue gas cooler 20 through line 15, optionally containing a quench.

Pyrolysis product passes from the multi-tube pyrolysis reactor through line 4 to the heat exchanger portion of the feed preheat 17 wherein it imparts a major portion of its heat content to the feed and in doing so is cooled to a temperature suitable for the oligomerisation/hydrogenation reaction. From the heat exchanger the product gases pass through line 5 to the oligomerisation/hydrogenation reactor 22 wherein unsaturated hydrocarbons and in particular unsaturated C2 hydrocarbons are oligomerised and alkylate aromatics. The products then pass through line 6 to the product separation unit 23 comprising cooling, flash and oil absorber/stripper facilities wherein liquid products are seprated from gaseous components principally comprising unreacted methane and hydrogen. The separated liquid product is recovered through line 7.

Gaseous components are withdrawn through line 8. Up to 50% by volume is recycled through line 9 via the recycle gas compressor 24 to the feed preheat unit 17. Part of the gaseous product may be passed through line 11 as fuel for the reactor furnace 18. The excess gas is recovered through line 10.

Pyrolysis [Step (I)] CATALYST PREPARATION A Merck charcoal support was impregnated with a water soluble compound of the metal.

Whenever available the nitrate salt of the metal was used; in all other cases the double salt of ammonium and the metal was employed, specifically for Cr, Mo and W.

The impregnation was carried out in a rotary evaporator. In order to improve the wetting of the charcoal, 10% of methanol was added to the solution. The amount of the impregnating agent was calculated to give the pre-determined amount of the metal on the basis of the pure metal. The mixture was boiied for 12 hours under reflux. Thereafter, the mixture of charcoal and the solution was evaporated to dryness. The dry catalyst was further dried for 24 hours at 12Q C in a laboratory drier.

The catalyst was charged to the reactor in which methane pyrolysis was to be carried out and was calcined in situ at 550 to 600"C in a stream of nitrogen for 4 hours.

The following catalysts were prepared in the aforesaid manner: 2% metal on Merck charcoal:- Fe, Co, Mn, Cr, Mo, W and Co; 1% each metal on Merck charcoal:- Co+Mo; 0.5% metal on Merck charcoal:- La, Ce, Yb, Eu, Nd and Gd.

CATALYST TESTING Examples 1 to 17 Following calcination, the gas was changed from nitrogen to methane and the temperature was raised to 1000"C; the pressure was up to 10 bar and the GHSV was between 2000 and 4000 h--1.

The reaction conditions and the results obtained are reported in the following Table.

Comparison Test A The procedure of Examples 1 to 17 was repeated using charcoal alone in place of the metal doped charcoal catalyst used in the Examples.

Comparison Test B The procedure of Examples 1 to 1 7 was repeated using silicon carbide in place of the metal doped charcoal catalyst used in the Examples.

The reaction conditions and the results obtained are reported in the following Table 1.

CATALYST REGENERATION Example 18 A 2% tungsten doped charcoal was tested as catalyst under the following conditions: Feed: 95% vol. methane; 5% vol. ethane Temperature: 1000"C Pressure: 1 bar GHSV: 1000h-' Before regeneration the conversion was 18.0% wt. and the C2+ yield was 11.6% wt.

The catalyst was then regenerated for 30 minutes in air. After regeneration the conversion was 16.4% wt. and the C2+ yield was 9.2% wt.

This Example demonstrates the regenerability of the tungsten doped charcoal catalyst.

TABLE 1

Example Catalyst GHSV Press CH4 C2+ Yield No (h-l) (BarAbs) Conversion % wt 1 1200 1 9.7 5.7 2 2% Charcoal 1000 1 16.2 8.8 3 600 1 26.0 10.9 4 2X Mo/Charcoal 600 1 14.6 11.8 5 2X Fe/Charcoal 600 1 11.4 7.9 6 2% Cr/Charcoal 600 1 6.8 3.8 7 2Z Co/Charcoal 600 1 7.3 4.7 8 1X Co/1% Mo/Charcoal L 600 1 6.0 4.0 9 0.5% La/Charcoal 600 1 15.5 10.9 10 0.5X Yb/Charcoal 600 1 11.8 8.2 11 400 1 20.9 12.8 12 833 7 22.9 20.6 13 0.5% Ce/Charcoal 500 2 14.4 11.6 14 600 1 11.5 8.0 15 0.5% Nd/Charcoal 600 1 6.5 5.8 16 0.5X Pr/Charcoal 600 1 7.9 4.6 17 0.5X Gd/Charcoal 600 1 6.7 4.3 Comparative Charcoal 600 1 5.8 3.7 Test A Comparative Silicon Carbide 600 1 1.41 1.2 Test B Oligomerisation/Alkylation (Step II) Examples 19 to 22 (i) Catalyst Treatment The zeolite catalyst used was an ammonium ion exchanged MFI type zeolite. Analysis of this catalyst provided the following data: Na content (atomic absorption) = 0.07% wt.

Si/AI ratio (bulk by XRF) =15.9 SINAI ratio (framework by 29Si NMR) = 19.2+2.0 The zeolite was pelletised with a 26 mm diameter die by applying a pressure of 0.13 GPa.

The pellets were granulated and sieved to between 420 and 600 um. Pretreatment of the zeolite catalyst (3.5 ml) was effected in a stream of nitrogen (100 ml min-) by increasing the temperature from room temperature up to 400 C at 20 min-, maintaining the final temperature for 3h, and then cooling in nitrogen back to room temperature.

(ii) Catalytic Measurements The apparatus used was a continuous flow system constructed mainly from stainless steel.

A feed gas simulating a pyrolysis product obtainable from step (I) and having the composition (by vol):65.3% methane, 27.4% hydrogen, 3.9% acetylene, 1.8% ethylene, 0.1% ethane and 1.5% benzene was passed over the activated catalyst under ambient conditions and the temperature increased at a rate of 20C min-' up to the value as shown in the Table. Gas samples, taken at regular intervals, were analysed by on-line chromatography. Liquid samples were collected over one hour periods, at ice temperature, and analysed using a capillary PLOT fused silica column.

The reaction conditions and the results obtained after 2 hours on stream are given in Table 2.

The maximum yield of liquid products was obtained at 650K (Example 3). Under these conditions 91% mol conversion of the unsaturated C2 feed with 91% C-mol selectivity to C6+ products was observed.

TABLE 2

Example 1 2 3 4 GHSV (1) 1030 1024 1002 990 P/atm 1 1 1 1 T/K 550 600 650 700 Conversion/% mol (2) Acetylene 12 48 97 100 Ethylene - 9 77 72 Benzene - - 12 - H2 4 - - - Selectivity/% C-mol CH4 3 6 - - 02H4 1 - - - C2H6 - 2 1 - C3 - - 6 5 C4 - - 2 2 C5-C1 1 ~ ~ ~ ~ Benzene 88 50 - 14 Toluene - - 6 6 Ethyl benzene 2 18 40 18 Other C2-a 2 2 7 6 C3-a - 2 5 3 C4+ - a 3 12 16 13 C10+ (naphthalene +) 3 8 17 33

Claims (11)

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 pyrolysis product comprising hydrogen, unsaturated hydrocarbons and aromatic hydrocarbons, (II) reacting the pyrolysis product from step (l) in an oligomerisation/alkylation zone maintained under conditions whereby unsaturated hydrocarbons are oligomerised and/or are utilised to alkylate aromatic hydrocarbons, (Ill) reacting the oligomerisation/alkylation product with at least part of the hydrogen remaining in the product thereby to hydrogenate hydrogenatable unsaturated hydrocarbons in the oligomerisation/alkylation product, and (IV) recovering liquid hydrocarbons from the hydrogenated oligomerisation/alkylation product.

2. A process according to claim 1 wherein hydrogen not used in the hydrogenation of unsaturated hydrocarbons in step (III) is recycled together with unconverted methane.

3. A process according to either claim 1 or claim 2 wherein the oligomerisation/alkylation step [step (II)] and the hydrogenation step [step (III)] are incorporated into a single oligomerisation/alkylation/hydrogenation step.

4. A process according to any one of the preceding claims wherein the pyrolysis product exiting from the pyrolysis zone is used as a heat exchange medium to preheat the feed to the pyrolysis zone, thereby cooling the pyrolysis product to a temperature suitable for the oligomerisation/alkylation step.

5. A process according to any one of the preceding claims wherein the pyrolysis product is subjected to a rapid quench to reduce its temperature to a value at which coke formation is substantially eliminated.

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

7. A process according to any one of the preceding claims wherein the pyrolysis zone takes the form of a fluid bed reactor employing autogenerated coke as catalyst.

8. A process according to any one of claims 1 to 6 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.

9. 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.

10. A process according to any one of the preceding claims wherein oligomerisation/alkylation is carried out in the presence of a catalyst.

11. A process according to any one of the preceding calims wherein in step (IV) the liquid hydrocarbons are recovered by cooling and feeding to a flash and oil absorber/stripper system.