WO2007009987A1

WO2007009987A1 - Hydrocarbon synthesis process

Info

Publication number: WO2007009987A1
Application number: PCT/EP2006/064364
Authority: WO
Inventors: Robert Martijn Van Hardeveld; Lip Piang Kueh; Thomas Joris Remans
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2005-07-20
Filing date: 2006-07-18
Publication date: 2007-01-25

Abstract

A process for the synthesis of hydrocarbon products from syngas in one or more syngas conversion reactors, the or each conversion reactor having: A fixed bed of hydrocarbon synthesis catalysts; a syngas entry stream system comprising one or more entry streams into the reactor; two or more reactor product exit streams, one of which is a liquid waxy hydrocarbon stream having a heavy tail fraction including oxygenates, another one being a gaseous product stream comprising unconverted syngas, gaseous hydrocarbons, steam and optionally inert gaseous compounds; and a system for recycling at least a portion of the waxy hydrocarbon stream back into the reactor, by contacting syngas with the hydrcarbon synthesis catalyst and converting syngas into liquid and gaseous hydrocarbons, wherein in that portion of the liquid waxy hydrocarbon stream being recycled, the oxygenates in the heavy tail fraction are removed before said stream is reintroduced as a recycle stream into the conversion reactor.

Description

HYDROCARBON SYNTHESIS PROCESS

The present invention relates to a hydrocarbon synthesis process, particularly but not exclusively to a Fischer-Tropsch process.

The Fischer-Tropsch process can be used for the conversion of hydrocarbonaceous feed stocks into liquid and/or solid hydrocarbons. The feed stock (e.g. natural gas, associated gas and/or coal-bed methane, residual (crude) oil fractions, tar sand extracts, biomass or coal) is converted in a gasifier, optionally in combination with a reforming unit, into a mixture of hydrogen and carbon monoxide (this mixture is often referred to as synthesis gas or syngas).

The synthesis gas is then fed into a Fischer-Tropsch reactor where it is converted in one or more steps over a suitable catalyst at elevated temperature and pressure into paraffinic compounds ranging from methane to high molecular weight modules comprising up to 200 carbon atoms, or, under particular circumstances, even more. The hydrocarbons formed in the Fischer-Tropsch reactor can then proceed to a hydrogenation unit, optionally a hydroisomerisation/hydrocracking unit, and thereafter to a distillation unit.

For a general overview for the Fischer-Tropsch process reference is made to Fischer-Tropsch Technology, Studies in Surface Science and Catalysis, Vol. 152, Steynberg and Dry (ed.) Elsevier, 2004, Amsterdam, 0-444-51354-X. Reference is further made to review articles in Kirk Othmer, Encyclopedia of Chem. Techn. and Ullmann's Encyclopedia of Ind. Chem., Vol. 6, 4^th edition, p. 584 ff. Numerous types of reactor systems have been developed for carrying out the Fischer-Tropsch reaction. For example, Fischer-Tropsch reactor systems include fixed bed reactors, especially multi-tubular fixed bed reactors, fluidised bed reactors, such as entrained fluidised bed reactors and fixed fluidised bed reactors, and slurry bed reactors such as three-phase slurry bubble columns and ebulated bed reactors.

Generally, two main products streams are recovered from a Fischer-Tropsch reaction: a waxy hydrocarbon stream, and a gaseous stream, which on cooling results in light hydrocarbons, impure water and off gas. The off gas comprises unconverted synthesis gas, C2-C4 olefins, C1-C4 hydrocarbons, CO2, and inert gases such as N2 and Ar. The waxy hydrocarbon stream is generally that fraction comprising the C15+, preferably C20+_? hydrocarbons. The light hydrocarbons mainly comprise C_$-\—C15 hydrocarbons, optionally mainly C5-C20~hydrocarbons .

It is known to recycle one or more of the product streams back into the Fischer-Tropsch reactor for various purposes, in particular to increase the percentage conversion of the reactants, and/or to increase the percentage yield of certain desired products. It is also known to recycle light hydrocarbons, as evaporation of the light hydrocarbons will reduce the duty of the cooling system.

For example, the use of liquid recycles as a means of improving the overall performance in a fixed-bed design are described in the art. Such a system is also called a "trickle flow" reactor (as part of a sub-set of fixed-bed reactor systems) in which both reactant gas and liquid are introduced (preferably in an up flow or down flow orientation with respect to the catalyst) simultaneously. The presence of the flowing reactant gas and the liquid improves heat removal and temperature control, thus enhancing the reactor performance with respect to CO conversion and product selectivity. Hitherto, there have been no reported problems in the art with the recycled products used. However, as the Fischer-Tropsch reaction is improved in the art, longer (and therefore heavier) hydrocarbons are being formed, meaning that the percentage of the heavy tail fractions formed by the reaction is increasing. Thus, recycle streams are increasingly involving greater amounts of heavy tail fractions, and/or the same percentage of a heavy tail fraction has a greater amount of heavier hydrocarbons . It has appeared, now, that the recycle of the heavy hydrocarbons results in the deactivation of the Fischer- Tropsch catalyst. In a large series of experiments it has surprisingly been discovered that the deactivation of the Fischer-Tropsch catalyst can be avoided by removing the heavy oxygenates from the heavy recycle stream. This can be done for instance by distillation or by hydrogenation . Hydrogenation includes mild hydrogenation in which no isomerisation occurs, as well as hydroisomerisation and/or hydrocracking. Thus, the present invention relates to a hydrocarbon synthesis reaction in a fixed bed reactor, especially a multitubular reactor, in which a heavy hydrocarbon recycle is used, in which the oxygenates are removed from the heavy hydrocarbon recycle. These oxygenates may be removed by physical methods, e.g. distillation or removal by membranes, or by chemical methods, especially reaction with hydrogen. - A -

It is an object of the present invention to reduce the amount of such substances in a hydrocarbon synthesis recycle stream which could cause catalyst deactivation.

Thus, the present invention provides a process for the synthesis of hydrocarbon products from syngas in one or more syngas conversion reactors, the or each conversion reactor having: a fixed bed of hydrocarbon synthesis catalyst; a syngas entry stream system comprising one or more entry streams into the reactor; two or more reactor product exit streams, one of which is a liquid waxy hydrocarbon stream having a heavy tail fraction including oxygenates, another one being a gaseous product stream comprising unconverted syngas, gaseous hydrocarbons, steam and optionally inert gaseous compounds; and a system for recycling at least a portion of the waxy hydrocarbon stream back into the reactor, by contacting syngas with the hydrocarbon synthesis catalyst and converting syngas into liquid and gaseous hydrocarbons, wherein in that portion of the liquid waxy hydrocarbon stream being recycled, the oxygenates in the heavy tail fraction are removed before said stream is reintroduced as a recycle stream into the conversion reactor.

Preferably, the process of the present invention also removes any olefins and/or water in the heavy tail fraction of the portion of the waxy hydrocarbon stream being recycled, and being reintroduced into the conversion reactor. The olefins may be removed by hydrotreatment, e.g. hydrogenation (without skeletal rearrangement), hydroisomerisation and/or hydrocracking, or by physical means, e.g. selective adsorption or membranes. It is observed that hydrotreatment reactions to remove the oxygenates usually will also remove any olefins present in the reaction mixture. Water may be removed by means of drying over e.g. mol sieves, silicagel, alumina etc. Water may also be removed during distillation.

The term "heavy tail fraction" as used herein relates to that portion of the waxy hydrocarbon stream from the conversion reactor having a boiling point generally above 540 ⁰C suitably above 500 ⁰C, preferably above 490 ⁰C. In the process of the present invention the oxygenates present in the heavy tail fraction need to be removed. Preferably also the oxygenates with a lower boiling are removed too.

The portion of waxy hydrocarbon stream being recycled could be in the range 20-75%, preferably 30-50%, of the waxy hydrocarbon products in the reactor exit stream. It is possible to remove the oxygenates and optionally any olefins and/or water in a number of ways. According to one embodiment of the present invention, the oxygenates are removed by separation of the heavy tail fraction from the remainder of the waxy hydrocarbon stream being reintroduced as a recycle stream into the conversion reactor. Preferably, the separated fraction is that fraction of the waxy hydrocarbon stream heavier than the middle distillates, more preferably being that fraction having a boiling point in the range of about 350-490 ⁰C. The separation of the heavy tail fraction could be carried out by one or more processes known in the art. These include distillation and/or fractionation, e.g. by cooling. If necessary or desired, at least a part, possibly all, of that portion of the waxy hydrocarbon streams being reintroduced as a recycle stream into the conversion reactor could also be hydrogenated after separation of the heavy tail fraction, so as to further ensure that any oxygenates and any olefins and/or water are not present in the recycle stream.

According to another embodiment of the present invention, the oxygenates and optionally any olefins and/or water can be removed by hydrogenation of the heavy tail fraction of that portion of the waxy stream being reintroduced into the conversion reactor as a recycle stream.

The hydrogenation of the desired portion of the waxy hydrocarbon stream can be carried out in a number of different ways.

In one method, the waxy hydrocarbon stream is passed to a hydrogenation unit, in which all product fractions are hydrogenated, and thus then useable in the recycle stream to the conversion reactor. Thus all oxygenates, including those in the heavy tail fraction, have been hydrogenated, and thus removed.

In another method, the waxy hydrocarbon stream is passed to a hydrocracking (HC) unit, wherein generally lighter hydrocarbon fractions are created. From the HC unit, the fractions known as the waxy raffinates therefrom can particularly be used as the recycle stream to the conversion reactor.

The waxy raffinates are generally that fraction of hydrocarbon products boiling between, at the lower end, in the range of about 300-390 ⁰C, to an upper end in the range of about 420-570 ⁰C. More preferably, the range is between 330-370 ⁰C at the lower end, to about 450-520 ⁰C at the upper end. Even more preferably, the waxy raffinates are that fraction of hydrocarbons having a boiling point between approximately 350 ⁰C and 490 ⁰C.

The hydrocracking could be carried out by one or more units, which includes known secondary units or processes such as secondary distillation of initial hydrocracked fractions .

In another embodiment of the present invention, any hydrogenated recycle stream could pass through a high vacuum unit (HVU) to help further reduce the level of catalyst contaminants in the stream.

In another embodiment of the present invention, the recycle stream could be added to one or more other conversion reactor feed and/or recycle streams. This includes, but is not limited to, a portion of the waxy hydrocarbon stream to be recycled but which does not undergo any removal step or process.

The hydrogen gas for the hydrogenation can be provided by any suitable source. Whilst one source is pure hydrogen, any source providing sufficient hydrogen to effect the invention is usable, such as reformed hydrogen prepared by a steam methane reforming process. Such a process could be carried out by a unit or apparatus integral with, or associated with, the hydrocarbon synthesis system. Hence, the hydrocarbon synthesis system may include a readily available source of sufficiently high quality hydrogen to provide the required hydrogen gas .

The temperature during hydrogenation in a hydrogenation unit will generally be in the range 200-450 ⁰C, more suitably 300-400 ⁰C, and more suitably approximately 360 ⁰C. The pressure during hydrogenation will generally be in the range 5-100 bar, more preferably 40-80 bar, more preferably 60-70 bar.

Generally, the hydrogenation unit will include one or more catalysts to assist the hydrogenation reaction or reactions. Many hydrogenation catalysts are known in the art, and such can include copper/zinc oxide, nickel, nickel/tungsten, cobalt/molybdenum and sulfided nickel/molybdenum. More preferably hydrogenation may be conducted using a first reactor preferably with a Cu/Zn0 catalyst. Preferably Mn is added to the Cu/Zn0 catalyst to promote hydrogenation. Typically the CuZnO is prepared by providing Cu0/Zn0 and reducing the CuO in the reactor prior to hydrogenating a portion of the olefins. Noble metal based catalysts such as palladium and platinum could also be used.

Preferably, the process for the production of hydrocarbon products from syngas is the Fischer-Tropsch process, followed by a hydrogenation process, a hydroisomerisation process and/or a hydrocracking process, followed by distillation of the reaction products into fractions, e.g. naphtha, kero, gasoil, waxy raffinate, base oil, wax, solvents, drilling fluids and LPG. Possible conditions, parameters and products for a Fischer-Tropsch process are known in the art.

Preferably, the conversion reactor includes one or more catalysts. More preferably, a Fischer-Tropsch catalyst is used which yields substantial quantities of paraffins, more preferably substantially unbranched paraffins. A part may boil above the boiling point range of the so-called middle distillates, to normally solid hydrocarbons. A most suitable catalyst for this purpose is a cobalt-containing Fischer-Tropsch catalyst. The term "middle distillates", is a reference to hydrocarbon mixtures of which the boiling point range corresponds substantially to that of kerosene and gas oil fractions obtained in a conventional atmospheric distillation of crude mineral oil. The boiling point range of middle distillates generally lies within the range of about 150 to about 350 ⁰C.

The present invention also provides apparatus for the production of hydrocarbon products from syngas comprising: one or more syngas conversion reactors, at least one reactor having a syngas entry stream system comprising one or more entry streams into the reactor, two or more reactor product exit streams, one of which is a waxy hydrocarbon stream having a heavy tail fraction including oxygenates, and a system for recycling a portion of the waxy hydrocarbon stream back into the reactor; and means for removing the oxygenates in the heavy tail fraction of the portion of the waxy hydrocarbon stream being recycled before said stream is reintroduced into the reactor. The present invention also provides hydrocarbons whenever prepared by a hydrocarbon synthesis process as hereinbefore described. Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawing, Figure 1, which shows a flow diagram for a number of embodiments of a Fischer-Tropsch process in accordance with the present invention. Figure 1 shows part of a Fischer-Tropsch system, which is usually integral with or associated with other units, apparatus and processes which assist in the Fischer-Tropsch process. The preparation of synthesis gas by one or more methods is well known in the art, such as the Shell Gasification Process. A comprehensive survey of this process can be found in the Oil and Gas Journal, September 6, 1971, pp 86-90.

The Fischer-Tropsch synthesis is also well known to those skilled in the art and involves synthesis of hydrocarbons from a gaseous mixture of hydrogen and carbon monoxide, by contacting that mixture at reaction conditions with a Fischer-Tropsch catalyst.

Products of the Fischer-Tropsch synthesis may range from methane to heavy paraffinic waxes. Preferably, the production of methane is minimised and a substantial portion of the hydrocarbons produced have a carbon chain length of a least 5 carbon atoms. Preferably, the amount of C5+ hydrocarbons is at least 60% by weight of the total product, more preferably, at least 70% by weight, even more preferably, at least 80% by weight, most preferably at least 85% by weight. Reaction products which are liquid phase under reaction conditions may be separated and removed, optionally using suitable means, such as one or more filters. Internal or external filters, or a combination of both, may be employed. Gas phase products such as light hydrocarbons and water may be removed using suitable means known to the person skilled in the art.

Fischer-Tropsch catalysts are known in the art, and typically include a Group VIII metal component, preferably cobalt, iron and/or ruthenium, more preferably cobalt. Typically, the catalysts comprise a catalyst carrier. The catalyst carrier is preferably porous, such as a porous inorganic refractory oxide, more preferably alumina, silica, titania, zirconia or mixtures thereof. The optimum amount of catalytically active metal present on the carrier depends inter alia on the specific catalytically active metal. Typically, the amount of cobalt present in the catalyst may range from 1 to 100 parts by weight per 100 parts by weight of carrier material, preferably from 10 to 50 parts by weight per 100 parts by weight of carrier material.

The catalytically active metal may be present in the catalyst together with one or more metal promoters or co- catalysts. The promoters may be present as metals or as the metal oxide, depending upon the particular promoter concerned. Suitable promoters include oxides of metals from Groups HA, IHB, IVB, VB, VIB and/or VIIB of the Periodic Table, oxides of the lanthanides and/or the actinides. Preferably, the catalyst comprises at least one of an element in Group IVB, VB and/or VIIB of the Periodic Table, in particular titanium, zirconium, manganese and/or vanadium. As an alternative or in addition to the metal oxide promoter, the catalyst may comprise a metal promoter selected from Groups VIIB and/or VIII of the Periodic Table. Preferred metal promoters include rhenium, platinum and palladium.

A most suitable catalyst comprises cobalt as the catalytically active metal and zirconium as a promoter. Another most suitable catalyst comprises cobalt as the catalytically active metal and manganese and/or vanadium as a promoter.

Catalysts as discussed above are susceptible to various substances, which include oxygenates, olefins and water. Such substances are found in many fractions of hydrocarbon products, but their presence in heavy tail fractions is of greater concern, which presence is increasing in recycle streams as the Fischer-Tropsch reaction is made more efficient in the art.

References to the Periodic Table and groups thereof used herein refer to the previous IUPAC version of the Periodic Table of Elements such as that described in the 68^th Edition of the Handbook of Chemistry and Physics (CPC Press) .

The promoter, if present in the catalyst, is typically present in an amount of from 0.1 to 60 parts by weight per 100 parts by weight of carrier material. It will however be appreciated that the optimum amount of promoter may vary for the respective elements which act as promoter.

The Fischer-Tropsch synthesis is preferably carried out at a temperature in the range from 125 to 350 ⁰C, more preferably 175 to 275 ⁰C, most preferably 200 to 260 ⁰C. The pressure preferably ranges from 5 to 150 bar abs . , more preferably from 5 to 80 bar abs.

The Fischer-Tropsch synthesis can be carried out in a slurry phase regime or an ebullating bed regime, wherein the catalyst particles are kept in suspension by an upward superficial gas and/or liquid velocity.

Hydrogen and carbon monoxide (synthesis gas) is typically fed to a three-phase slurry reactor at a molar ratio in the range from 0.4 to 2.5. Preferably, the hydrogen to carbon monoxide molar ratio is in the range from 1.0 to 2.5.

The gaseous hourly space velocity may very within wide ranges and is typically in the range from 500 to 10000 Nl/l/h, preferably in the range from 1000- 4000 Nl/l/h. It will be understood that the skilled person is capable to select the most appropriate conditions for a specific reactor configuration and reaction regime.

Typically, the superficial liquid velocity is kept in the range from 0.001 to 4.00 cm/sec, including liquid production. It will be appreciated that he preferred range may depend on the preferred mode of operation.

In Figure 1, syngas 8 is fed into a Fischer-Tropsch reactor labelled HS ( for the 'heavy synthesis') 10. The HS 10 may be one or more reactor units, acting either in parallel, series or both, and is represented in Figure 1 as a singular box.

The HS 10 has a syngas entry stream system comprising one or more entry streams into the reactor, the detail of which is not shown in Figure 1.

The HS 10 also has two or more product exit streams, full details of which are not shown in Figure 1.

Figure 1 shows an off gas product stream labelled "HOG" 40 from the reactor 10. Line 12 is a waxy hydrocarbon stream from the reactor 10.

The waxy hydrocarbon stream 12 will generally comprise the heavier fractions from the HS 10, and it is common that some of this fraction is recycled directly back into the HS 10 for reasons described above. However, the waxy hydrocarbon stream 12 still contains amounts of oxygenates, and possibly water and/or olefins, which are known to deactivate Fischer-Tropsch catalysts. Even if the level of such contaminants is not high, their continuing circulation through the HS 10 will still lead to gradual deactivation of the catalyst in the HS 10 over time. Reactivation of catalyst material is a significant process which it is preferred to avoid or delay where possible . The invention still encompasses the possible arrangement wherein a portion of the waxy hydrocarbon stream 12 is recycled directly back into the HS 10 without removal of the catalyst contaminants therein (e.g. by line 32) . However, by combining such a direct recycle stream with recycle stream as hereinafter described, the amount of catalyst contaminants in the combined recycle stream will still be less than using a recycle stream without any oxygenate removal action. In Figure 1, there are shown three options labelled

A, B and C for use of the waxy hydrocarbon stream 12. Each of these options can be used mutually exclusive to the other, or in combination with one or more of the other, and optionally in combination with one or more other oxygenate-removing processes known in the art. One or more other recycle streams may also be used, but are not shown for clarity in Figure 1.

Option A involves passing the waxy hydrocarbon feed 12 to a separation unit (SU) 14, which in this example could be a distillation unit . The distillation will create a number of product streams therefrom. For the present invention, that fraction which is generally boiling in the range 350-490⁰C can be taken from the separation unit 14 and passed along line 24 into a recycle stream 30 passing back into the HS 10.

The recycle fraction is generally that fraction from the separation unit 14 which is between the middle distillates having a boiling point in the range 150- 350 ⁰C, and the heavy tail fraction, generally having a boiling point above 500 ⁰C, which fraction is drawn off by line 38 in figure 1. The action of Option A serves to totally separate the heavy tail fraction (through line 38) from the intended recycle stream going through line 24. In the heavy tail fraction in line 38 will be the oxygenates associated with said fraction, and any olefins and/or water therewith and not desired to be recycled, which are hence removed from the recycle stream.

If necessary or desired, at least a portion of the fraction from the separation unit 14 that is going to be used as the recycle stream could subsequently be fed into a hydrogenation unit (HU) 18 as hereinafter described, to further ensure that any remaining oxygenates, and any possible remaining olefins and/or water still in that portion of the intended recycle stream, are hydrogenated.

In option B, the waxy hydrocarbon stream 12 follows line 16 into a hydrogenation unit (HU) 18. Hydrogenation units are well known in the art, and possible sources of hydrogen gas for the unit 18 have been described hereinbefore. Any or all of the products of the HU 18 can be taken through pipe 26, for recycle in line 30 back into the HS 10. By hydrogenating all the waxy hydrocarbon stream 12,

16, all the resultant product stream from the hydrogenation unit 18 can be used as the recycle stream in the present invention, as all relevant oxygenates and any olefins and/or water will have been hydrogenated. In this way, nothing from the waxy hydrocarbon stream 12, 16 need be separated or otherwise removed in the recycle system.

If desired or necessary, at least a portion of the product from the HU 18 could further be sent through line 36 to a hydrocracking unit 22 as hereinafter described. In option C, the waxy hydrocarbon stream 12 follows line 20 into a hydrocracking (HC) unit 22. The action of an HC unit 22 is also well known in the art, and again, a heavy fraction can be taken from the unit 22 through line 28, for use in recycle line 30 back into the HS 10.

The HC unit 22 may involve a first distillation wherein various products ready for use are provided, and from which the heavy tail fraction could be sent to a second distillation unit. Preferably, the product line to be reintroduced into the HS unit 10 through recycle line 28 and 30 is that fraction from the second distillation unit known in the art as the waxy raffinates, as hereinbefore described.

In a hydrocarbon synthesis system, there are other units, apparatus and processes which are integral or associated with the arrangement shown in Figure 1. Thus, each of options A, B and C may be separate or part of or integral with other units, apparatus and processes in the hydrocarbon synthesis system. Moreover, options A, B and C could be exclusive, or used with one or more of the other options, optionally simultaneously or at separate times or stages, such as discussed above. Thus, the actual material that is recycled into the HS could involve the combination of two or more streams of material from different locations, processes, units or apparatus in the recycle system and/or from other parts of the hydrocarbon synthesis system.

Claims

C L A I M S

1. A process for the synthesis of hydrocarbon products from syngas in one or more syngas conversion reactors, the or each conversion reactor having: a fixed bed of hydrocarbon synthesis catalysts; a syngas entry stream system comprising one or more entry streams into the reactor; two or more reactor product exit streams, one of which is a liquid waxy hydrocarbon stream having a heavy tail fraction including oxygenates, another one being a gaseous product stream comprising unconverted syngas, gaseous hydrocarbons, steam and optionally inert gaseous compounds; and a system for recycling at least a portion of the waxy hydrocarbon stream back into the reactor, by contacting syngas with the hydrocarbon synthesis catalyst and converting syngas into liquid and gaseous hydrocarbons , wherein in that portion of the liquid waxy hydrocarbon stream being recycled, the oxygenates in the heavy tail fraction are removed before said stream is reintroduced as a recycle stream into the conversion reactor.

2. A process as claimed in claim 1 wherein the portion of the waxy hydrocarbon stream being recycled, any olefins and/or water in the heavy tail fraction are also removed before said stream is reintroduced as a recycle stream into the conversion reactor.

3. A process as claimed in claim 1 or claim 2 wherein the portion of waxy hydrocarbon stream being recycled is in the range 20-75%, preferably 30-50%, of the waxy hydrocarbon products in the reactor exit stream.

4. A process as claimed in any one of claims 1 to 3 wherein the oxygenates are removed by separation of the heavy tail fraction from the remainder of the waxy hydrocarbon stream before said stream is reintroduced as a recycle stream into the conversion reactor.

5. A process as claimed in claim 4 wherein the portion of the waxy hydrocarbon stream being recycled to the conversion reactor is partly, substantially or wholly that fraction of the waxy hydrocarbon stream heavier than the middle distillates.

6. A process as claimed in claim 5 wherein said portion of the waxy hydrocarbon stream to be recycled has a boiling point in the range of about 350-490⁰C.

7. A process as claimed in any one of claims 4 to 6 wherein the separation is carried out by distillation and/or fractionation.

8. A process as claimed in any one of claims 1 to 3 wherein the oxygenates and optionally any olefins and/or water are removed by hydrogenation of that portion of the heavy tail fraction being reintroduced into the conversion reactor, or wherein the hydrogenation is carried out by a hydrogenation unit, or wherein the hydrogenation is carried out by a hydrocracking unit.

9. A process as claimed in claim 7 or 8 wherein the removal of oxygenates and optionally any olefins and/or water is carried out by a combination of a distillation unit and a hydrogenation unit, or a distillation unit and a hydrocracking unit, or a hydrogenation unit and hydrocracking unit, or a distillation unit, a hydrogenation unit and a hydrocracking unit, preferably wherein any hydrogenated recycle stream is passed through a high vacuum unit.

10. A process as claimed in any one of the preceding claims wherein the recycle stream is combined with one or more other recycle streams prior to being reintroduced into the conversion reactor.

11. A process as claimed in any one of the preceding claims wherein the hydrocarbon synthesis process is a Fischer-Tropsch process, the process being followed by a hydrogenation process, a hydroisomerisation process and/or a hydrocracking process, followed by distillation of the reaction products into fractions, e.g. naphtha, kero, gasoil, waxy raffinate, base oil, wax, solvents, drilling fluids and LPG.

12. Apparatus for the synthesis of hydrocarbon products from syngas comprising: one or more syngas conversion reactors, at least one reactor having a syngas entry stream system comprising one or more entry streams into the reactor, two or more reactor product exit streams, one of which is a waxy hydrocarbon stream having a heavy tail fraction including oxygenates, and a system for recycling a portion of the waxy hydrocarbon stream back into the reactor as a recycle stream; and means for removing the oxygenates, and optionally any olefins and/or water, in the heavy tail fraction of the portion of the waxy hydrocarbon stream before said stream is reintroduced into the conversion reactor.