JPH02233502A - Manufacture of sysnthetic methanol gas - Google Patents

Abstract

PURPOSE: To improve the formation of gaseous feedstock for methanol synthesis by dividing a gaseous mixture composed of a hydrocarbon raw material and steam to major and minor streams and joining these streams after reforming, separating the unreacted stream and further subjecting the stream to membrane sepn. thereby forming the synthetic gas contg. H₂ and carbon oxide.

CONSTITUTION: The hydrocarbon/steam mixture of 10 to 40 absolute bar is heated by a heat exchanger 4 and the major stream of 75 to 90% thereof is introduced to a major reformer 2 in a combustion heating type furnace 1 where the major stream is reformed by passing the stream through a catalyst 3. The stream is then passed on the outer side of an auxiliary reformer 6 at a gas temp. of 750 to 950°C and is sent to a reformed gas line 15. The minor stream is introduced to the auxiliary reformer 6 and is reformed by passing the stream through the catalyst 7 in an annular section 8 and is sent to the reformed gas line 15. The combined gas is subjected to condensation of the unreacted stream in a heat exchanger 19 and is separated by a catch pot 20. Part of the resulted moisture decreased gas is separated as a permeated stream in a membrane separator 23. The unpermeated tubular stream is joined with a circuit 24 stream component to form the synthetic gas of 1.8 to 2.5 in the molar ratio of H₂ and carbon oxides and the synthetic gas is then sent to a gas compressor 29.

COPYRIGHT: (C)1990,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to methanol and more particularly to the production of methanol synthesis gas, a gas containing hydrogen and carbonaceous compounds, by steam reforming raw hydrocarbons such as natural gas. Concerning building.

BACKGROUND OF THE INVENTION Steam reforming processes are well known and involve the use of a mixture of raw materials and steam. It is passed over the reforming catalyst, usually placed in an external heating tube. However, especially when the feedstock is primarily methane, for example when the feedstock is natural gas, the synthesis gas contains an excess of hydrogen over that required for methanol synthesis. In a synthesis gas containing stoichiometric proportions of hydrogen and carbon oxides for methanol synthesis, the ratio of the molar amount of hydrogen (minus the molar amount of carbon dioxide) to the total molar amount of carbon oxides (R ) is 2
Typically, synthesis gas produced by conventional stem reforming processes has a ratio (R) of the order of 2.5 or even more, such as about 3.

In US Pat. No. 4,337,170, it is proposed to include a combustion reformer and an auxiliary reformer. Feedstock Part 1: Bypassing the main reformer, the reformed gas from the auxiliary reformer tube is fed to the auxiliary reformer and reformed in a catalyst-containing tube disposed therein. The mixture of these reformed gases is then mixed along the surface of the auxiliary reformer tube in a countercurrent direction to the gas undergoing reforming in the auxiliary reformer tube. This development is currently It is useful for improving the value of plants.

For metallurgical and efficiency reasons, the pressure at which the reforming step is carried out is generally in the range 10-40 bar absolute, but in methanol synthesis processes even higher pressures, e.g. Since high pressures of absolute bars or more are also carried out, the synthesis gas usually has to be compressed after removal of unreacted steam but before use for methanol synthesis. 1 house in the rehoming process. Energy is unnecessarily expended in compressing such excess hydrogen, giving a gas containing more hydrogen than is required in the methanol syngas.

Furthermore, by providing an auxiliary reformer that increases the amount of raw material to be inspected during the reforming stage. The amount of gas to be compressed increases, which not only means that more power is required to perform the compression, but also if the existing plant is modified. This means that existing syngas compressors may be inadequate to handle that increased amount of syngas.

British Patent No. 21 40801 discloses that hydrocarbon W
i. by a process consisting of partial oxidation with fresh air, followed by a resulting gas flow t-shift reaction and then membrane separation to remove most of the nitrogen introduced by the use of air in the partial oxidation step. Methanol synthesis gas? It is proposed to manufacture.

However, such processes yield hydrogen-poor synthesis gas unless pre-heating is employed, combustion heaters are used, and an adiabatic stem reforming step is used. Moreover, such methods do not help to enhance the value of plants with conventional primary reforming stages.

We have found that most or all of the excess hydrogen can be removed from syngas produced by steam reforming prior to compression through the use of membrane separation methods.

In this way, the process using a combination of a combustion reformer and an auxiliary reformer can be modified so that the volume of syngas fed to the compressor is smaller than that of the synthesis gas that would be produced if the auxiliary reformer were not used. It will be possible to operate with a volume equal to or even smaller than that.

The invention therefore provides: ta) forming both main and sub-streams, each containing a hydrocarbon feedstock and steam; (b) directing the main stream over a steam reforming catalyst disposed in a tube heated by a combustion furnace; Passed at a pressure in the range of 10-40 bar absolute, thereby making the reformed main stream; tc+ also a side stream over the steam reforming catalyst placed in the tube of the auxiliary reformer at a pressure of 10-40 bar absolute. +d+ at said pressure within the range of said pressure, thereby creating a reformed substream; +d+ mixing the reformed substream with the reformed main stream, thereby forming a combined reformed gas stream; tel passing the reformed main stream over the outside of the auxiliary reformer tube, thereby providing heat to the auxiliary reformer tube; condensing the reaction steam in the form of water and separating the condensed water to obtain a moisture-reduced gas stream; subjecting the gas stream to a membrane separation operation effective to separate the gas stream from a non-permeable gas stream containing hydrogen and carbon oxides, the non-permeable gas stream together with the remainder of the moisture-reduced gas stream (if present); syngas fi'a-as done;
and the amount of hydrogen separated as said permeate stream is such that said synthesis gas stream is compressed to a pressure of at least 50 bar absolute; ) ratio R (as defined above).

In the process of the invention, the feedstock is preferably methane or natural gas containing a high proportion of methane, for example 90% (v/v). If the raw material contains a blood yellow compound,
Before feeding the feed to the reformer tubes, the feedstock (e.g. by passing over a hydrodesulphurization catalyst and then absorbing the hydrogen sulfide using a suitable absorbent, e.g. a zinc oxide bed).
Subjected to desulfurization treatment. It is usually desirable to introduce hydrogen-containing gas into the feedstock prior to hydrodesulfurization. This converts a small amount of the reformed gas, or the hydrogen-containing gas made therefrom (e.g., barge gas from the methanol loop to which the synthesis gas is fed, or part of the permeate gas stream), to a hydrodesulphurization catalyst. This can be done by recirculating the feedstock before passing it over. Before reforming, the stem is mixed with raw materials and this steam introduction
This can be carried out by direct injection of the stem and/or by saturation of the feedstock σ) (moisture) by contacting the feedstock with a heated water stream. The amount of steam introduced is 2 to 4 moles of steam (H2
It is preferable that the amount is such as to give 0). Some of the steam may be replaced by carbon dioxide if a carbon dioxide supply is available, thus obtaining a nearly stoichiometric synthesis gas. The feed/steam mixture is preferably combined, e.g. with a combined reformed gas stream and/or a flue from a combustion reformer, since this reduces the amount of hydrogen that has to be separated into two permeate streams. It is preheated by heat exchange with gas, and then a part of it is supplied as the main stream to the reforming tube of the combustion reformer. Both the main and secondary streams may be preheated separately (eg to different temperatures) and/or contain different proportions of steam and/or carbon dioxide. For example, the stem is
It may be introduced separately into both the main and secondary feed streams at °C. The main stream preferably contains 75-90% of the total amount of feedstock in both the main and secondary streams, preferably <1.

Combustion (heating) type reformer} process, preferably.The temperature of the reformed main stream leaving the catalyst of the combustion (heating) type reformer is in the range of 750 to 950°C, especially 850 to 90°C.
It is operated such that the temperature is within 0°C.

In a preferred embodiment of the invention, the auxiliary reformer tube is
a double tube type, i.e. an outer tube with each tube having a closed and chained end, an inner tube arranged concentrically within the outer tube, and an annular space between the inner tube and the outer tube; It consists of an inner tube that communicates with the outer tube at the closed end, and a stem reforming catalyst is disposed in the annular space of the inner tube. The reformed main flow is fed into the open end of the space, and at the same time the reformed main stream is fed across the outer surface of the outer tube, the reformed secondary stream is fed into the annular space from the end of the annular space near the closed end of the outer tube. left,
The flow then returns within the inner tube. One type of double tube reformer is described in EP-A-194067,
In this reformer, insulation is provided to minimize the amount of heat transferred through the inner tube firewall from the reformed side stream flowing within the inner tube. Heat transfer occurs from the reformed substream passing through the inner tube without such insulation, through the wall of the inner tube, to the substream undergoing reforming in the annular space containing the catalyst. It is preferable to cut it so that it occurs. This heat transfer has a dual effect: firstly, it supplies some of the heat required for reforming the sidestream, and secondly it causes cooling of the reformed sidestream. teeth,
The resulting reformed gas stream, consisting of a mixture of reformed main and side streams, is at a lower temperature and therefore contains less heat, thus reducing the heat recovery therefrom necessary for efficient operation. It has the advantage that it can be reduced.

The process gas stream (i.e., the reformed main stream, or a mixture thereof with a reformed side stream) is used to heat the auxiliary reformer tube. This offers the advantage of requiring thinner gauge material than is conventionally used in the industry.

This is because the pressure differential across the auxiliary reformer tube is relatively small, resulting primarily from the pressure drop experienced by the main stream as it passes through the combustion-heated reformer tube.

In the present invention, both the reformed primary and secondary streams are combined and the combined gas stream is used to heat the auxiliary reformer tubes as described in the aforementioned U.S. Pat. No. 43,371'70. However, it is advantageous to merge the reformed secondary gas stream with the reformed main gas stream after the reforming lance main gas stream has been used to heat the auxiliary reformer tube; Therefore, it is preferable.

The reformation of the feedstock that can be reformed in the auxiliary reformer depends on the amount of methane escape allowed in the moisture-reduced gas, and the desired temperature of the merging gas. The methane content of the stream [1 is the sum of the methane content of both the reformed main and secondary streams, for any given reformer feed, pressure and steam rate The amount depends on the temperature of the reformed main stream leaving the catalyst in the combustion-heated reformer, whereas the methane content of the reformed stream depends on the temperature of the reformed side stream leaving the catalyst in the auxiliary reformer. The temperature of the reformed stream leaving the auxiliary reformer catalyst is the temperature of the reformed main stream used to heat the auxiliary reformer, from the reformed side stream to the side stream undergoing reforming. depending on the heat (if present) and the relative proportions of both the main and secondary streams.Reformers are designed to maintain a moisture-reduced gas stream with an overall methane content of 2 to 10% by volume on a dry basis. It is preferable to operate within the range of .

After reforming, the combined reformed gas streams are
It is cooled below the dew point of the steam therein to condense unreacted steam in the form of water, which is then separated. This cooling is conventional, for example by indirect heat exchange with the reactants to be fed to the tubes of a combustion-heated reformer and/or an auxiliary reformer. By indirect heat exchange with water (generation of hot water and/or steam, which can be used as a process stem), and/or indirect heat exchange with steam to produce superheated steam (from which power can be recovered by a turbine) Alternatively, or in addition, at least a final portion of the cooling can be carried out by direct heat exchange with water, producing a stream of warm water that also contains condensed water; The hot water stream may be further heated and then used as a hot water stream for contacting the feedstock and effecting moisture saturation for introducing the process stem into the °C raw material.

After the water separation, at least a portion of the water-reduced stream is subjected to membrane separation treatment to separate the permeate stream containing hydrogen from the non-permeate stream containing hydrogen and carbon oxides, as is well known in the art. A wide variety of membrane materials can be used, examples of such membrane materials include, for example, polyimide and polyethersulfone. It is preferred to use a membrane having a relatively low permeation note for carbon oxides so that very little carbon oxides migrate into the permeate stream, and for this reason polyimide membranes are preferred.

It is essential that all of the moisture-reduced gas is subjected to membrane separation, so a portion of it may bypass the membrane separation step. By varying the rate of bypassing the membrane separation step,
Control over the composition of the synthesis gas can be provided. The amount of feedstock fed to the auxiliary reformer tubes and the amount of hydrogen removed as a retentate stream, the retentate stream and (if present) that portion of the moisture-reduced gas that bypasses the membrane separation stage. , such that the synthesis gas produced from , has an R ratio in the range of 1.8 to 2.5, and preferably the production volume of synthesis gas is less than the dry gas volume of the reformed main gas stream. Also, the size should not be larger than 1%. In particular, the amount of hydrogen removed as a permeate stream is preferably such that the volume of synthesis gas is not greater than the dry gas volume of the reformed main gas stream, so that no additional load is imposed on the synthesis gas compressor.

The permeate stream can be used as fuel for a combustion heated reformer or can be exported to a hydrogen user. A portion of the permeate stream can be used as a hydrogen-containing gas stream that is added to the feed prior to hydrodesulfurization of the feed, as described above.

It has been practiced to remove a purge stream from the methanol synthesis loop and use it as fuel for this purge combustion-heated reformer. Such purging avoids the accumulation of excess hydrogen resulting from the use of inerts in the loop, such as methane and often nitrogen (which may be present in small amounts in natural gas), and hydrogen-rich syngas. It was necessary for this purpose. With the present invention, it is possible to recirculate some or all of the methane-containing purge as feed to the reforming stage, since the membrane separator serves to remove some or all of the excess hydrogen. ，
If the membrane used is such that it separates the nitrogen into the permeate stream, it may be possible in some cases to recirculate the entire purge.

The portion of the purge that is not recycled, if such a portion exists, can be used as fuel for the combustion-heated reformer, along with the hydrogen-rich permeate stream from the membrane separator as described above. It can be done. The recirculating purge forms part of the feed to the reformer. ．． or <I,'Creduces the amount of fresh material required. Furthermore, since the recirculated purge contains hydrogen, it can be used as a hydrogen-containing gas added to the raw material prior to hydrodesulfurization of the fresh material.

Part or all of the recycled purge may also be subjected to a further gland separation step to separate the hydrogen as a hydrogen-containing permeate stream, which is then recycled to form part of the feedstock. , which has the advantage of reducing the amount of hydrogen recycled and reducing the load on the membrane separator processing the moisture-reduced gas. The hydrogen-containing permeate stream can be used as part of the fuel for a combustion-heated reformer.

If a portion of the purge needs to be recycled as hydrogen-containing gas (e.g. for hydrodesulphurization), the portion of the purge to be used for hydrodesulphurization should not be subjected to such a membrane separation step. is desirable.

In some cases, it may be desirable for both the main and secondary feed streams fed to the reformer to contain different proportions of recycle purge. For example, the main feed stream may include a small proportion of recycle purge (e.g., only the amount necessary to satisfactorily perform hydrodesulfurization and provide the amount of hydrogen required for 5), with a remainder 1; The raw material in the sub-feed stream is used in conjunction with the feedstock. actual,
In some cases, the entire feedstock of the side feed stream is passed through the recycle purge (preferably after it has been subjected to a methane separation step).
It may consist of

One form of the present invention will be explained with reference to the attached drawings (Fig. 1).
This attached figure is a schematic flow diagram in which a reformer with a single catalyst tube in each reformer is shown for clarity. In practice, of course, it is common for each reformer to be equipped with multiple tubes.

The figure shows a combustion-fired furnace 1 comprising a main reformer tube 2 in which a stem reforming catalyst 3 is arranged. A heat exchanger 4 is arranged in a flue gas duct 5 of the combustion furnace 1 . An auxiliary reformer 6 is provided in which each catalyst tube has a double tube structure in which a catalyst 7 is disposed within an annular portion 8 between an outer tube 9 and an inner tube 10. Although closed at the lower end, the upper end of the outer tube 9 opens into the granum chamber 11. Reformer
At the lower end of 6 a hot gas port 12 is provided and connected to the outlet of the tube 2 of the combustion-heated reformer. The reformer 6 is also provided with an outlet 13 for gas from the space outside the outer tube 9 and an outlet 14 communicating with the inner tube 1 port.

Outlets 13 and 14 communicate with reformed gas line 15, feed/steam supply line 16 with heat exchanger 4, and preheated reactant line 1 with heat exchanger 4.
4 communicates with the pipe 20 inlet. The auxiliary reformer supply line 18 is taken out from the heat exchanger 4 toward the plenum chamber 11 of the auxiliary reformer 6. The reformed gas line 15 passes through one or more heat exchangers 19 to a cavte pot 2 with a drain 21.
Contacting 0. A moisture reduction gas line 22 connects from the catchbot 20 to a membrane separator 23 with a bypass 24 having a flow control valve 25 . The membrane separator 26 has a permeate flow line 26 and a non-permeate flow line 27.The non-permeate flow line 27 is connected to a detour 2.
4 are connected to form a syngas delivery line 28, which supplies gas to a syngas compressor 29.

In a typical operation, a feed/steam mixture at a pressure of about 24 bar absolute is heated in a heat exchanger 4, and the main stream is then fed to the reformer tubes 2, while the side stream is fed to the line 1.
8 and then supplied to Glenafte Yamba-11. Mainstream 1
: It passes through the catalyst 3, is reformed by the heat supplied by the combustion furnace 1, becomes the reformed main stream, and then passes through the inlet 12 to the space outside the outer tube 9 of the double tube reformer 6. supplied and then via outlet 13,
It is sent to the reformed gas line 15. Annular section 8 between tubes 9 and 100 from sidestream hair eplenum champer 11
It is supplied to the catalyst 7 inside and reformed there. The reformed side stream exits the lower end of the annulus and passes upwardly within the inner tube 10 to the outlet 14 from where it enters the reformed gas line 15. The heat required for reforming the side stream is supplied from the reformed main stream passing outside the outer tube 9 and from the reformed side stream flowing upwardly within the inner side 10.

The combined reformed gas stream from reformed gas line 15 is cooled in heat exchanger 19 below the dew point of the stem therein to condense unreacted steam in the form of water. This condensed water is separated in two catch pots from which it is removed via a drain 21. The resulting moisture-reduced gas is fed via line 22 to a methane separator 26 where it passes through a permeate stream 26 and a non-permeate stream. It is separated into stream 27.

A portion of the moisture-reduced gas bypasses the trans-degree Celsius membrane separator via detour 24. The amount bypassing the membrane separator is controlled by valve 25.

Using the flow sheet described above with respect to the example drawings, and in a theoretical example using a hydrodesulfurized natural gas feedstock and a reforming pressure of 24 barre absolute, the gas composition flow rate at various stages of the reforming operation. and temperature are as shown in Table 1.

Table 1 *Exit, or bottom.

a,b higher than each represented by CH 2.
96 grade hydrocarbons at 6.5 and 2,0 kmol. h-'
In order to achieve the heat transfer through the outer wall of the outer tube 9 required to carry out the degree of reforming of the sub-feed stream shown in Table 1, 7. For a given number of tubes, tubes 9 are exposed to the reformed gas stream 12. It is calculated that it is necessary to have a length of 2 m.

If the inner tube 10 is omitted and the same number of outer tubes 9 of the same diameter are used, but with their lower ends open, the tubes 9
If the reformed side stream leaving the reformed main stream mixes with the reformed main stream and the resulting mixture is used to heat tube 9; the temperature and flow rate are calculated as shown in Table 2.

Table 2 * Higher than the outlet, i.e. bottom, a, b, respectively, represented by CH 2.
96 grade hydrocarbons 6.5 and 2, Q ktnol. h-
In this case, substantially the same degree of reforming of the subfeed stream is carried out at substantially the same inlet gas flow rate and temperature, and a product stream, i.e., stream 15, is obtained at substantially the same outlet temperature. To achieve the heat transfer through the outer wall of the outer tube 9 necessary for Since the side stream under reforming is not heated by the movement, the heat exchange surface area of the outer wall of tube 9 is
By approximately 14% (e.g. increasing the length of tube 9 exposed to both main and secondary streams, i.e. stream 12 plus the flow from the outlet of tube 9, from 2 m to 8.2 m) ) need to be increased.

Table 6 illustrates the production of near-stoichiometric synthesis gas from the combined reformed gas stream 15 of Table 1.

It is assumed here that there is no bypassing of the membrane separator, that the pressure of the gas fed to the membrane separator is approximately 22 bars absolute, and that the permeate stream has an intensification of approximately 2 bars absolute. , suppose. Furthermore, the membrane used has a hydrogen dicarbon dioxide permeation ratio of 69 and a hydrogen dicarbon dioxide permeation ratio of
5.8 and methane permeation t! l. It is also assumed that is an approximation to the carbon monoxide transmission note.

Table 6 In the specific example of Table 1, the proportion of raw material fed to the auxiliary reformer is about 23% of the total, so the system can be used to improve the capacity of the existing combustion-heated reformer by installing an auxiliary reformer. In the case of KJ engineering, lower reformed gas temperature, and an increase in the methane content (dry basis) of the syngas from 4.7% by volume (without auxiliary reformer or membrane separator) to 6.5% by volume. At the cost of this, production can be increased by about 25%.

A small proportion of carbon oxides, mainly carbon dioxide, is present in the permeate stream 26
The full benefit of increased reformer production cannot be realized in terms of the amount of methanol that can be produced; however, from Table 6 it can be seen that syngas stream 27 is about 17 % more carbon oxides, thus significantly increasing the amount of methanol that can be produced.

Furthermore, from Tables 1 and 6, the amount of synthesis gas produced (
Stream 27) is about 95% of the amount of dry gas in the SS IJ-homed main stream 12, thus significantly increasing the amount of methanol that can be produced, but also slightly reducing the amount of gas fed to the compressor. It can be seen that the compression power is reduced, thereby resulting in a saving in compression power.

[Brief explanation of the drawing]

Attached Figure 1 is a flow sheet of a preferred embodiment of the method of the present invention.

Claims (1)

Claims: 1. (a) forming primary and secondary streams, each containing a hydrocarbon feedstock and steam; (b) over a steam reforming catalyst disposed in a tube heated by a combustion furnace; passing the main stream at a pressure in the range of 10 to 40 bar absolute, thereby producing a reformed main stream; (c) passing a side stream over a steam reforming catalyst disposed in the tubes of the auxiliary reformer; (d) mixing the reformed substream with the reformed main stream, thereby creating a reformed substream; creating a combined reformed gas stream; (e) passing the reformed main stream over the outside of the auxiliary reformer tube, thereby providing heat to the auxiliary reformer tube; (f) combined reforming; a methanol synthesis gas stream comprising: cooling a decomposed gas stream to condense unreacted steam therein in the form of water, and separating the condensed water to obtain a moisture-reduced gas stream; (i) mixing the reformed substream with the reformed main stream before or after passing the reformed main stream beyond the outside of the auxiliary reformer tube to produce; (ii) a moisture-reduced gas stream; subjecting at least a portion of the hydrogen to a membrane separation operation effective to separate a permeate gas stream containing some of the hydrogen from a non-permeate gas stream containing hydrogen and carbon oxides; together with the remainder of the moisture-reduced gas stream (if present) to form a syngas stream, and the amount of hydrogen separated as said permeate stream is such that said syngas stream is in the range of 1.8 to 2.5 (iii) bring the synthesis gas stream to a pressure of at least 50 bar absolute; Compressing the synthesis gas. 2. The tubes to be heated by the combustion furnace are placed in a first shell, and the reformed main stream exits the first shell and enters the second shell with the auxiliary reformer tubes placed inside. and passing over the outer surfaces of the auxiliary reformer tubes to effect heating thereof.
Method described. 3. (i) Each of the auxiliary reformer tubes consists of an outer tube having a closed end and an inner tube disposed concentrically within the outer tube, with the inner tube extending between the outer tube and the outer tube. communicating with the annular space at the closed end of the outer tube, and a steam reforming catalyst is disposed in the annular space;
i) a sub-feed stream is fed into the open end of the annular catalyst-containing space between the outer and outer tubes; (iii) the reformed main stream is fed beyond the outer surface of the outer tube into said annular catalyst-containing space; and (iv) the reformed substream exits the annular space at the end of the annular space proximate the closed end of the outer tube and flows into the inner tube. 3. The reformed sub-stream passing through the inner tube and the sub-feed stream passing through the annular catalyst-containing space thus undergo a heat transfer. the method of. 4. A process according to any of claims 1 to 3, wherein the main feed stream comprises 75 to 90% of the total amount of hydrocarbons in both the main and secondary feed streams. 5. The reforming conditions and the proportions of both the main and secondary feed streams are such that the methane content of the combined reformed stream is within the range of 2 to 10% by volume on a dry basis. The method described in any of the above. 6. The amount of hydrogen separated in the permeate stream is such that, before compression, the volume of the methanol synthesis gas stream is not more than 10% greater than the dry gas volume of the reformed main stream. The method described in any of the above. 7. The separated hydrogen-containing permeate stream is used as a combustion furnace fuel to heat reformer tubes located within the combustion furnace, methanol synthesis is carried out within the synthesis loop, and methane-containing purge is carried out from the synthesis loop. 4. A purge stream is removed and at least a portion of this purge stream is recycled to form part of the total feedstock of both the main and secondary feed streams.
6. The method according to any one of 6. 8. At least a portion of the purge gas stream is subjected to a membrane separation step to separate a hydrogen-containing permeate stream and a methane-containing retentate stream that is used as part of the overall feed of both the main and secondary feed streams. Method described. 9. A process according to claim 7 or 8, wherein the hydrocarbons of the side feed stream consist entirely of methane from the recycle purge stream.