EP1742901A1

EP1742901A1 - System and process for synthesis of methanol

Info

Publication number: EP1742901A1
Application number: EP05743070A
Authority: EP
Inventors: Gregor August Jenzer; Thian Hoey Tio; Pieter Lammert Zuideveld
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 2004-05-07
Filing date: 2005-05-04
Publication date: 2007-01-17
Also published as: JP2007536347A; RU2006143195A; CN1950316A; RU2386611C2; AU2005240368A1; WO2005108336A1; AU2005240368B2

Abstract

System and process for synthesising methanol out of a source gas containing light hydrocarbons. The system comprises - a hydrocarbon gas supply (3) for providing the source gas;- an oxygen supply (1) for providing an oxygen containing fluid;- a syngas reactor (2) in the form of a gasification reactor, arranged to receive the source gas from the hydrocarbon gas supply (3) and the oxygen containing fluid from the oxygen supply (1), the syngas reactor (2) being arranged to produce an intermediate reaction product from the source gas and the oxygen containing fluid, and to discharge the intermediate reaction product, the intermediate reaction product comprising syngas;- a methanol reactor (5), arranged to receive at least a fraction of the intermediate reaction product from the syngas reactor, and arranged to let the intermediate reaction product react into at least methanol via a methanol forming reaction, the methanol reactor being provided with a methanol outlet (11) for discharging the formed methanol and an offgas outlet (12) for discharging an offgas.

Description

SYSTEM AND PROCESS FOR SYNTHESIS OF METHANOL

Field of the invention The present invention relates to a system and process for synthesising methanol out of a source gas containing light hydrocarbons. Background of the invention For the purpose of this specification, "light hydrocarbons" is a term used to embrace hydrocarbon molecules containing one, two, or three carbon atoms. Examples of light hydrocarbons are methane, ethane, and propane. More in particular, the invention relates to a system and process for synthesising methanol out of natural gas. Syngas, which is a mixture containing CO, C0 , and H₂, can be reacted into methanol via a known catalytic process. There are several processes to produce syngas from light hydrocarbons for methanol synthesis. In WO 98/28248 steam reforming of methane is disclosed. The syngas produced by steam reforming contains more H₂ relative to CO and C0₂ than required for the catalytic methanol reaction. The synthesis gas mixture is separated into a hydrogen-rich stream and a hydrogen-depleted stream containing carbon oxides and methane. The H₂ in the hydrogen-rich stream is burned to provide energy to the steam reforming reaction, and the hydrogen-depleted stream is recycled into the steam reforming zone. Another known process to form syngas for methanol production is autothermal reforming. The syngas from this process contains more CO and C0₂ compared to H₂ than required for the catalytic methanol reaction. The known processes produce syngas at pressures of around 25 to 40 bar. The subsequent methanol reaction, however, requires a higher pressure in order to reach a reasonable efficiency. US-A-5496859 describes a a process to prepare methanol from syngas . The syngas is prepared by a combination of a non-catalyzed partial oxidation of natural gas and a gas heated steam reforming of natural gas. By using this combination the required molar ratio between hydrogen and carbon monoxide as specified in the description is achieved to carry out the methanol synthesis reaction. EP-A-111376 describes a process to prepare methanol from syngas. The syngas mixture is prepared by a non- catalyzed partial oxidation of methane, ethane or propane or a mixture thereof. From the effluent of the methanol synthesis step methanol is isolated. From the remaining mixture carbon dioxide is partly removed by washing at the methanol synthesis pressure. The gas mixture depleted of carbon dioxide is recycled to the methanol synthesis step. In this manner the required molar ratio between hydrogen and carbon monoxide as specified in the description is achieved to carry out the methanol synthesis reaction. According to the description the process is advantageous because no steam reformer is required. An advantage of the process of EP-A-111376 is that no steam reformer is required as in the process of US-A- 5496859. Combining a gasification reactor and a steam reformer reactor as in US-A-5496859 is not simple. A disadvantage of the process of EP-A-111376 is that a relatively complex separation process is applied downstream the methanol synthesis. That process is complicated by the use of a washer with recycle. The object of the present invention is to provide a simple process to prepare methanol wherein the disadvantages of the process of EP-A-111376 are overcome. Summary of the invention The present invention provides a system for synthesising methanol out of a source gas containing light hydrocarbons, the system comprising

- a hydrocarbon gas supply for providing the source gas; - an oxygen supply for providing an oxygen containing fluid;

- a syngas reactor, arranged to receive the source gas from the hydrocarbon gas supply and the oxygen containing fluid from the oxygen supply, the syngas reactor being arranged to produce an intermediate reaction product from the source gas and the oxygen containing fluid, and to discharge the intermediate reaction product, the intermediate reaction product comprising syngas;

- a methanol reactor, arranged to receive at least a fraction of the intermediate reaction product from the syngas reactor, and arranged to let the intermediate reaction product react into at least methanol via a methanol forming reaction, the methanol reactor being provided with a methanol outlet for discharging the formed methanol and an offgas outlet for discharging an offgas; whereby the syngas reactor comprises a gasification reactor, and wherein a gas flow path extends between the hydrocarbon gas supply and the methanol outlet, which gas flow path, in flow direction, passes through at least the hydrocarbon gas supply, the syngas reactor, the methanol reactor and the methanol outlet, and wherein the system further comprises an H₂ supply system for injecting an H₂ containing gas into the gas flow path through an H₂ inlet located upstream the methanol reactor and downstream the syngas reactor; - a pressure swing absorber as H₂ separator arranged to receive at least a fraction of the offgas discharged through the offgas outlet and to separate H₂ from the received offgas and to discharge separated H₂ into the H₂ supply system. In another aspect, the present invention provides a process of synthesising methanol out of a source gas containing light hydrocarbons, the process comprising steps of: extracting the source gas from a hydrocarbon gas supply;

- reacting the light hydrocarbons from the source gas with oxygen to produce a syngas-containing intermediate reaction product via a syngas forming reaction; reacting at least a fraction of the intermediate reaction product into methanol via a methanol forming reaction whereby also an H-containing offgas is produced; discharging the methanol and offgas through a methanol outlet and an offgas outlet downstream the methanol forming reaction; whereby the syngas forming reaction comprises gasification; . separating H₂ from at least part of the offgas; injecting separated H₂ into a gas flow path extending between the hydrocarbon gas supply and the methanol outlet. Gasification is a non-catalytic partial oxidation process that advantageously requires essentially no steam, although steam may be added to amounts of typically less than 10 % of the source gas. A gasification reactor, or gasifier, is a reactor wherein gasification is performed. An advantage of gasification to produce the syngas as the intermediate reaction product for methanol synthesis is that it can discharge the intermediate reaction product at a pressure of higher than 60 bar, preferably around 80 bar. Consequently, only a light compressor or even no compressor is required between the syngas reactor and methanol reactor to bring the syngas to the desired pressure for the methanol reactor. Also, gasification produces relatively low amounts of inert components such as methane, nitrogen, and argon, which enhances the methanol reaction efficiency. Moreover, the ratio CO/C0₂ from the gasifier is very favourable in CO, because only very limited amounts of C0₂ form in the water gas shift equilibrium since no or only little steam is added to the process. A high CO/C0₂ ratio is advantageous, because the formation of methanol from CO is more efficient than that from C0₂. Brief description of the Figures The invention will be described hereinafter in more detail and by way of example, with reference to the accompanying drawings in which: Fig. 1 schematically shows . the system according to an embodiment of the invention; Fig. 2 schematically shows the system according to anther embodiment of the invention; Fig. 3 schematically shows a flow diagram of the system and process of the invention. Detailed description of the invention In order to further improve the system, the system can optionally further comprise an H₂ supply system for injecting an H₂ containing gas into a gas flow path through an H₂ inlet located upstream the methanol outlet; an H₂ separator arranged to receive at least a fraction of the offgas discharged through the offgas outlet and to separate H₂ from the received offgas and to discharge separated H₂ into the H₂ supply system; whereby said gas flow path extends between the hydrocarbon gas supply and the methanol outlet and, in flow direction, passes through at least the hydrocarbon gas supply, the syngas reactor, the methanol reactor and the methanol outlet. Herewith the composition of the syngas is optimised to meet the ideal stoichiometric ratio for the methanol forming reaction. It is an advantage that the H₂ separated from the offgas can be discharged at approximately the same pressure as desired for the methanol reaction in the methanol reactor. The H₂ inlet is best located downstream the syngas reactor, so that it does not interfere with the reaction in the syngas reactor. Nor does it have to be heated to the reaction temperature. It has been found feasible by reinjecting H₂ from the offgas into the syngas to bring the stoichiometric number to between 2.0 and 2.1. In an embodiment of the invention, the methanol reactor is a single pass reactor. This means that no internal recycle in the methanol synthesis process is employed. An advantage hereof is that accumulation of inert components in the methanol reactor can be minimised. Detailed description of the Figures In the Figures like reference numerals relate to like components. Referring to Figs. 1 and 2 there is schematically shown a system for the synthesis of methanol. An oxygen supply is provided in the form of an air separation unit 1. A syngas reactor is provided in the form of gasifier 2, which is arranged to receive the oxygen containing output stream of the air separation unit 1. The 0 content of the output stream can typically be higher than 99 %. The gasifier 2 also receives a source gas containing light hydrocarbons. The source gas can be provided in the form of a stream 3 of natural gas. The gasifier can be a refractory lined vessel. It is equipped with a co-annular burner designed to ensure proper mixing of the fuel with the oxygen. A source gas preheater (not shown) can optionally be provided to increase the gasification efficiency. For details on gasification reference is made to a book "Gasification" by Ch. Higman and M. van der Burgt, Elsevier Science, 2003 (ISBN 0-7506-7707-4) pages 128-140. Still referring to Figs. 1 and 2, the syngas from the gasifier 2 is discharged as an intermediate reaction product via syngas cooler 17 through line 4 and fed, via an optional gas cleaning unit 6, to a methanol reactor 5 for producing methanol. The optional gas cleaning unit 6 can comprise, amongst others, a scrubber, a wet scrubber, a filter or combinations thereof. The methanol reactor 5 can be of any known type, such as either an isothermal type or an adiabatic type. A suitable methanol reactor is available from Johnson Matthey (formerly Synetix) or Lurgi. Note that a compressor does not have to be provided in line 4 for the syngas. The syngas discharged from the syngas reactor comprises at least H₂, CO, and C0₂. The suitability of the syngas composition for the methanol forming reaction is expressed as the stoichiometric number SN of the syngas, whereby expressed in the molar contents [H₂] , [CO], and [C0₂], SN = ( [H₂] - [C0₂] ) / ( [CO] + [C0₂] ) . It has been found that the stoichiometric number of the syngas produced by gasification of the source gas is lower than 2, and typically lies between 1.5 and 1.8. This is lower than an ideal stoichiometric ratio of about 2.05 for forming methanol in the methanol forming reaction. As will be discussed below, a recycle into the synthesis loop of hydrogen separated from methanol synthesis offgas can be provided to increase the SN. The following Table I gives a comparison of syngas produced by the steam reforming method (SMR) , the autothermal reforming method (ATR) , and the gasification method (SGP) . The numbers are obtained by equilibrium calculations and optimization of oxygen addition. The parameters of the ATR and SMR such temperature and pressure are typical, and reference is made to a conference paper "Syngas Technologies for Mega Methanol plants" presented by Wolfgang Hilsebein at the CMAI World Methanol Conference 2003 in Phoenix AZ.

Table 1: Typical syngas properties for a sample natural gas

In the table, the steam/carbon ratio is defined as the molar ratio of steam over carbon atoms in the hydrocarbons. Several advantageous properties are apparent from the Table I, including a high CO/C0₂ ratio, SN not far below 2, pressure of greater than 60 bar, low level of inerts including methane, nitrogen and argon. The advantageous properties of the syngas produced by gasification of light hydrocarbons are advantageous for the production of methanol using only limited recycling. Thus a process is provided which can prepares syngas at or near the required pressure for methanol synthesis and which process does not involve a steam reforming process step. The methanol reactor 5 can optionally be provided with an internal recycle loop 7 (as depicted in Fig. 1) comprising a methanol recycle compressor 8 to compensate for the pressure drop across the methanol reactor 5. However, a single pass reactor as depicted in Fig. 2 has the advantage that less inert components will accumulate in the methanol reactor. A single pass reactor or methanol process is preferably defined as a process wherein the content of recycled carbon as carbon monoxide and carbon dioxide calculated on the total of carbon as present in the feed to the methanol synthesis step or methanol reactor is smaller than 15 mol%, more preferably smaller than 2 mol%. The down stream end of the methanol reactor is connected to a gas-liquid separator 10 via line 9. The gas-liquid separator has a methanol outlet 11 for discharging the methanol that has been formed in the methanol reactor 5, and an offgas outlet 12 for discharging an offgas that emerges from the methanol reactor 5. The offgas outlet 11 is connected to an H separator 14 arranged to receive at least a fraction of the offgas discharged through the offgas outlet and to separate H₂ from the received offgas. Line 15 carries the hydrogen-rich stream from the H₂ separator, and is connected to line 4 via an H₂ inlet. This way, a hydrogen containing gas is injected into the syngas upstream the methanol reactor. A pressure swing absorber (PSA) forms a suitable H₂ separator. PSA type processes are well known and are for example described in the above referred to textbook "Gasification" by Ch. Hig an and M. van der Burgt,

Elsevier Science, 2003 (ISBN 0-7506-7707-4) pages 310- 311. An advantage of the PSA is that a pure stream of H₂ can be obtained whereas all the other gases including inert gases are removed from the cycle. This gives a high tolerance on the quality of source gas that can be converted into methanol using the system and process of the invention. Moreover, the drop of hydrogen pressure across the PSA is small, so that the H₂ can be fed back into the synthesis loop with only moderate recompression for which compressor 16 is optionally provided. The purity of the hydrogen as separated from at least part of the offgas of the methanol forming reaction is preferably greater than 90 mol%, more preferably greater than 95 mol and even more preferably greater than 99 mol%. Separating H from at least part of the offgas is preferably performed by means of a pressure swing absorber process. The pressure at which the hydrogen is obtained in the PSA is preferably above 60 bar. It has been found that the PSA is preferably operated at a higher pressure than typically applied in PSA operations. The somewhat lower yield and purity of the hydrogen then obtained is then compensated by the fact that an additional recompression can be omitted or that a smaller recompression step can be used for the hydrogen to the methanol synthesis pressure. A membrane-separator or a cold box separator can also be used, but that would require more recompression of the separated H₂ before injecting into the synthesis loop. Depending on the quality of the source gas, a shift reactor may be required upstream of the H₂ separator to further increase the H₂ content. The system is optionally provided with means for further processing the discharged methanol, such as one or more distillation columns 13 for purifying the discharged methanol. Gasification in gasifier 2 can be performed at a pressure between 60 and 80 bar, preferably close to 80 bar, and a temperature of around 1300-1400 °C. The hot reactor effluent is then cooled down to about 350 °C in the syngas cooler 17. In the syngas cooler, saturated steam is produced at pressures up to about 100 bar. The saturated steam is used for preheating of the oxygen and, optionally, for preheating of the source gas. The methanol forming reaction can be performed at a pressure of between 30 and 150 bar, for example 75 bar and a temperature of between 200 and 320 °C, for example 230 °C. The hydrogen recovery in the PSA can be performed at for example 60 bar. Fig. 3 shows a schematic process flow chart. A corresponding Table II is also presented, that shows calculated compositions and flow rates of through the lines that are identified in Fig. 3 by letters a to i . The calculation is made for a low quality and low cost natural gas as the source gas, having a particularly high concentration of N₂. Comparing columns d and e, it can be seen that SN is enhanced from 1.1 to 2.01 by injection of the hydrogen recycle stream i. It can also be seen that the inerts in stream g are effectively removed from the synthesis loop via stream h since the methanol synthesis process is here taken as a once-through process. In conclusion, surprisingly, a very attractive methanol production process is obtained by employing partial oxidation by gasification, for the conditions as described above. Neither a syngas compressor nor a shift are required, because the methanol synthesis can be a once through process, and because in the preferred embodiment this system a SN of > 2 can be reached just by combining the fresh syngas with recycled hydrogen. It is surprising that this relatively simple system, which is to the knowledge of the inventors unique in gasification, gives exactly the conditions that are needed for methanol synthesis out of a source gas containing light hydrocarbons . The gasification can run at high temperature and pressure and without addition of steam. Addition of a little steam can be beneficial for prevention of possible coking during the heat up phase and soot production, but it also has an adverse effect on the C0/C0₂ ratio in the syngas and on the oxygen consumption of the process since more fuel must be burned to bring the steam up to the gasification temperature.

Claims

C L A I M S

1. System for synthesising methanol out of a source gas containing light hydrocarbons, the system comprising a hydrocarbon gas supply for providing the source gas; - an oxygen supply for providing an oxygen containing fluid; a syngas reactor, arranged to receive the source gas from the hydrocarbon gas supply and the oxygen containing fluid from the oxygen supply, the syngas reactor being arranged to produce an intermediate reaction product from the source gas and the oxygen containing fluid, and to discharge the intermediate reaction product, the intermediate reaction product comprising syngas; - a methanol reactor, arranged to receive at least a fraction of the intermediate reaction product from the syngas reactor, and arranged to let the intermediate reaction product react into at least methanol via a methanol forming reaction, the methanol reactor being provided with a methanol outlet for discharging the formed methanol and an offgas outlet for discharging an offgas; whereby the syngas reactor comprises a gasification reactor, and wherein a gas flow path extends between the hydrocarbon gas supply and the methanol outlet, which gas flow path, in flow direction, passes through at least the hydrocarbon gas supply, the syngas reactor, the methanol reactor and the methanol outlet, and wherein the system further comprises - an H₂ supply system for injecting an H₂ containing gas into the gas flow path through an H₂ inlet located upstream the methanol reactor and downstream the syngas reactor; a pressure swing absorber as H₂ separator arranged to receive at least a fraction of the offgas discharged through the offgas outlet and to separate H₂ from the received offgas and to discharge separated H₂ into the H₂ supply system.

2. The system of any claim 1, wherein the syngas discharged from syngas reactor comprises at least H₂, CO, and C0₂ whereby their molar contents [H₂] , [CO] , and [C0₂] satisfy a relation ( [H₂] - [C0₂] ) / ( [CO] + [C0₂] ) < a stoichiometric ratio for forming methanol in the methanol forming reaction.

3. The system of any one of the previous claims, wherein the oxygen containing fluid comprises gaseous 0₂.

4. The system of any one of the previous claims, wherein the methanol reactor is a single pass reactor.

5. Process of synthesising methanol out of a source gas containing light hydrocarbons, the process comprising steps of: extracting the source gas from a hydrocarbon gas supply; reacting the light hydrocarbons from the source gas with oxygen to produce a syngas-containing intermediate reaction product via a syngas forming reaction; reacting at least a fraction of the intermediate reaction product into methanol via a methanol forming reaction whereby also an H₂-containing offgas is produced; - discharging the methanol and offgas through a methanol outlet and an offgas outlet downstream the methanol forming reaction; whereby the syngas forming reaction comprises gasification; . separating H₂ from at least part of the offgas; injecting separated H₂ into a gas flow path extending between the hydrocarbon gas supply and the methanol outlet.

6. Process according to claim 5, wherein the separated hydrogen has a purity of greater than 90 mol%.

7. Process according to any one of claims 5 and 6, wherein separated H₂ is injected into the syngas- containing intermediate reaction product and before performing the methanol forming reaction.

8. Process according to any one of claims 5-7, wherein separating H₂ from at least part of the offgas is performed by means of a pressure swing absorber process.