GB2546867A

GB2546867A - Methanol process

Info

Publication number: GB2546867A
Application number: GB1620655.9A
Authority: GB
Inventors: Edward James Abbott Peter; James George Ithell Thomas
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2016-01-11
Filing date: 2016-12-05
Publication date: 2017-08-02
Anticipated expiration: 2036-12-05
Also published as: GB201600475D0; WO2017121978A1; GB201620655D0; GB2546867B

Abstract

A process for the synthesis of methanol comprising the steps of: (i) passing a synthesis gas mixture 24 comprising a make-up gas 20 through a reactor (e.g. a tube cooled converter 34) containing a methanol synthesis catalyst 36 to form a product gas stream 38, (ii) cooling and recovering methanol 48 from the product gas stream thereby forming a methanol-depleted gas mixture 50, (iii) dividing the methanol-depleted gas mixture into a purge gas stream 54 and a loop gas stream 22 by means of a purge valve 56 and (iv) combining the loop gas stream with the make-up gas to form the synthesis gas mixture, wherein the purge valve is maintained at a fixed position so that the pressure of the loop gas increases or decreases in accordance to increases or decreases in the pressure of the make-up gas. The process allows for a fluctuating stream of hydrogen 14, e.g. from water electrolysis powered by wind or solar energy. The synthesis may be performed at 10-120 bar and 130-350 ºC. Hydrogen may be recovered from the purge gas stream for further use, and the methanol may be used in a fuel cell for electrical power generation.

Description

Methanol process

This invention relates to a process for synthesising methanol.

Methanol synthesis is generally performed by passing a synthesis gas comprising hydrogen, carbon oxides and any inert gases at an elevated temperature and pressure through one or more beds of a methanol synthesis catalyst, which is often a copper-containing composition. Methanol is generally recovered by cooling the product gas stream to below the dew point of the methanol and separating off the product as a liquid. The crude methanol is typically purified by distillation. The process is often operated in a loop: thus the remaining unreacted gas stream is usually recycled to the synthesis reactor as part of the synthesis gas via a circulator. Fresh synthesis gas, termed make-up gas, is added to the recycled unreacted gas to form the synthesis gas stream. A purge stream is often taken from the circulating gas stream to avoid the build-up of inert gases. The process is typically operated at a target operating pressure and the purge valve is opened and closed to maintain this pressure.

It has been proposed to use energy sources to generate hydrogen via electrolysis of water and combine it at the correct stoichiometric ratio with captured carbon dioxide to form a synthesis gas for methanol production. Such a design may be used for storage of surplus renewable energy, such as wind power and solar power, when power generation exceeds the capacity of the local electricity grid. However, due to the intermittent nature of such renewable energy sources, the surplus power available for hydrogen production will vary. For example, if the electrolyser is close-coupled to a wind turbine or group of wind turbines it could vary on an hourly or even minute basis. This in turn means that the flow of hydrogen gas, and the carbon dioxide, could fluctuate quickly. Thus one problem that needs to be solved is how to operate a methanol synthesis process with rapidly fluctuating synthesis gas pressures. Normally methanol synthesis loops are not capable of operating with large, rapid changes in the feed. This can cause rapid changes in the catalyst operating temperature, which at best could cause the efficiency of methanol production to change rapidly, but at worst could cause the catalyst to go ‘off strike’ and cease making methanol completely due to temperature in the catalyst falling too low. Another problem, which needs to be addressed therefore, is how to maintain a methanol process if the flow of hydrogen is very small, or ceases completely.

One method of overcoming fast changes in feed flows is to build-in gas storage into the feed gas lines. These gas buffer vessels can hold changing volumes of gas, while the hydrogen and carbon dioxide flow rates to the downstream methanol plant ramp up and down only gradually. However, this incurs extra cost in provision of pressurised gas storage. US2003039600 (A1) discloses a control scheme for conversion of variable composition synthesis gas to liquid fuels such as methanol in a three-phase or slurry bubble column reactor (SBCR). The control scheme allows operation to achieve constant or optimum liquid fuel production and constant or limited purge gas flow with highly variable synthesis gas feed condition. This is accomplished by adjusting one or more of the recycle ratio, water addition, and bypass flow. However, SBCR’s are complex and they are not generally used for methanol synthesis.

The invention described herein overcomes the problems of the previous processes by utilizing the innate gas storage already in the methanol loop.

Accordingly the invention provides a process for the synthesis of methanol comprising the steps of: (i) passing a synthesis gas mixture comprising a make-up gas through a reactor containing a bed of particulate methanol synthesis catalyst to form a product gas stream, (ii) cooling and recovering methanol from the product gas stream thereby forming a methanol-depleted gas mixture, (iii) dividing the methanol-depleted gas mixture into a purge gas stream and a loop gas stream by means of a purge valve and (iv) combining the loop gas stream with the make-up gas to form the synthesis gas mixture, wherein the purge valve is maintained at a fixed position so that the pressure of the loop gas increases or decreases in accordance to increases or decreases in the flow of the make-up gas.

By adopting this operating mode, the volume of the methanol loop itself is being utilised to modulate the methanol reaction rate. When the hydrogen feed flow is high, the loop will continue to pressurise, until the methanol production rate balances out the rate of hydrogen and carbon dioxide addition and vice versa.

The invention allows a methanol synthesis loop to operate down to a 5 % turndown of make-up gas and be responsive to rapid load changes.

The make-up gas typically comprises hydrogen and carbon dioxide. Carbon monoxide may also be present. The make-up gas may be generated by the steam reforming of methane or naphtha using established steam reforming processes, including pre-reforming. However, the present invention is of particular effectiveness in utilising make-up gases generated by combining electrolytic hydrogen with a carbon dioxide containing gas stream. By the term “electrolytic hydrogen” we mean a hydrogen-containing gas stream formed by the electrolysis of water/steam. The carbon dioxide-containing gas stream may be a captured carbon dioxide gas stream recovered from a power plant that combusts carbon-containing fuels, such as coal or biomass, or separated from a gas produced by anaerobic digestion of biomass. Alternatively, the carbon dioxide could be a pure carbon dioxide stream, such as that extracted from an ammonia plant, or that produced by a fermentation process, or from a geothermal source. Alternatively, off-gases from refineries or other chemical processes comprising principally hydrogen and carbon oxides may also be used.

The make-up gas is compressed using conventional compression equipment, combined with the loop gas and passed to the methanol synthesis reactor.

The hydrogen and carbon content of the synthesis gas mixture fed to the methanol synthesis catalyst in the reactor preferably should be adjusted so that the desired stoichiometry for the methanol synthesis reactions is achieved. The reactions may be depicted as follows; CO + 2H2 ^CHsOH CO2 + 3H2 ^ CH3OH + H20

The composition of synthesis gas at the reactor inlet is preferably as follows; 15-45 mol% carbon dioxide, 55-85 mol% hydrogen and the balance one or more inert gases.

In the present invention, at least part of the methanol-depleted gas mixture is used as the loop gas stream. Thus the methanol-depleted gas may be divided into a loop gas stream, which is combined with make-up gas, and optionally other gas streams to form the synthesis gas mixture. A purge stream is recovered from the methanol-depleted gas mixture.

The reactor may be an un-cooled adiabatic reactor. Alternatively, a cooled reactor may be used in which heat exchange with a coolant within the reactor may be used to minimise or control the temperature. A number of cooled reactor types exist that may be used. In one configuration, a fixed bed of particulate catalyst is cooled by tubes or plates through which a coolant heat exchange medium passes. In another configuration, the catalyst is disposed in tubes around which the coolant heat exchange medium passes. The reactor may be a quench reactor, or a reactor selected from a tube-cooled converter or a gas-cooled converter, wherein the catalyst bed is cooled in heat exchange with the synthesis gas. Alternatively, the reactor may be cooled by boiling water under pressure, such as an axial flow steam-raising converter, or a radial flow steam-raising converter. In each case, the reactors contain fixed beds of methanol synthesis catalyst through which the synthesis gas is passed.

In an adiabatic reactor the synthesis gas may pass axially, radially or axially and radially through a bed of particulate methanol synthesis catalyst. The exothermic methanol synthesis reactions occur resulting in an increase in the temperature of the reacting gases. The inlet temperature to the bed therefore is desirably cooler than in cooled reactor systems to avoid over-heating of the catalyst which can be detrimental to selectivity and catalyst life.

Alternatively, a cooled reactor may be used in which heat exchange with a coolant within the reactor may be used to minimise or control the temperature rise. Where the inlet temperature, the temperature within the catalyst bed, and the outlet temperature from the reactor are similar the reactor may be described as isothermal. A number of cooled reactor types exist that may be used. The reactor may be an axial steam raising converter, a radial-flow steam raising converter, a gas-cooled converter or a tube cooled converter. In each of these, a bed of particulate catalyst is cooled by tubes or plates through which a coolant heat exchange medium passes. Alternatively, the synthesis reactor may be a quench reactor in which one or more beds of particulate catalyst are cooled by a synthesis gas mixture injected into the reactor within or between the beds.

In an axial-flow, steam-raising converter (aSRC), the synthesis gas typically passes axially through vertical, catalyst-containing tubes that are cooled in heat exchange with boiling water under pressure. The catalyst may be provided in pelleted form directly in the tubes or may be provided in one or more cylindrical containers that direct the flow of synthesis gas both radially and axially to enhance heat transfer. Such contained catalysts and their use in methanol synthesis are described in WO2012146904 (A1). Steam raising converters in which the catalyst is present in tubes cooled by boiling water under pressure offer a useful means to remove heat from the catalyst. However, while the aSRC offers the highest cooling factor, it makes poorer use of the reactor volume so the reactor shell is relatively large for the quantity of catalyst that it holds. Furthermore, aSRCs can suffer from a high pressure drop.

In a radial-flow steam raising converter (rSRC) the synthesis gas typically passes radially (inwards or outwards) through a bed of particulate catalyst which is cooled by a plurality of tubes or plates through boiling water under pressure is fed as coolant. Such reactors are known and are described for example in US4321234. A rSRC has poorer heat transfer than an aSRC but has very low pressure drop, hence it favours operation with high recycle ratio.

In a tube-cooled converter (TCC), the catalyst bed is cooled by feed synthesis gas passing through open ended tubes disposed within the bed that discharge the heated gas to the catalyst. TCC’s therefore can provide sufficient cooling area for a range of synthesis gas compositions and is able to be used under a wide range of conditions. As an alternative to a TCC, a gas cooled converter (GCC), may be used to cool the catalyst bed by passing the synthesis gas though tubes in a heat exchanger-type arrangement. A GCC is described for example in US 5827901. The use of a TCC is preferred over the GCC in that it is simpler and cheaper to fabricate due to the use of open topped tubes and the elimination of the upper header and all of the differential expansion problems that the gas cooled converter raises. A TCC therefore has the advantage of low equipment cost and lower outlet temperature, which favours the synthesis reaction equilibrium, but it has a lower heat transfer than aSRC and higher pressure drop than rSRC.

In a quench reactor, the one or more beds of particulate catalyst are cooled by a synthesis gas mixture injected into the reactor within or between the beds. Such reactors are described, for example, in US3458289, US3475136 and US4411877.

The reactor is preferably a tube cooled converter (TCC) containing a bed of methanol synthesis catalyst in which a plurality of tubes are disposed, though which the synthesis gas passes before being fed to the methanol synthesis catalyst.

In the present process, the flow of the make-up gas may decrease or increase rapidly, for example, because of a fall or rise in the amount of power to produce electrolytic hydrogen. The flow of synthesis gas through the reactor and its operating pressure will therefore decrease or increase and it is necessary that the methanol synthesis catalyst is able to function through these and other fluctuations.

Even with the pressure varying according to the invention, it may still be challenging to achieve a smoothly-changing production rate if the catalyst bed cannot cool down or heat up fast enough to cope with the feed flow rate changes. This could be a problem with the usual method of reactor temperature control, which is to keep the catalyst inlet temperature substantially constant across a range of operating rates, which results in a higher weighted average temperature of the catalyst bed (WABT) at high rates of production, and vice versa. This mode of control could mean, for instance, that when there is a rapid increase of feed rate, the catalyst bed temperature cannot heat up fast enough and the cooler circulating gas will quench the reaction, making the catalyst go ‘off strike’.

One way of managing this is to control the temperature of the catalyst bed such that it is maintained within a desired range. This may be achieved by adjusting the temperature of the synthesis gas at the inlet of the reactor such that the weight average bed temperature of the methanol synthesis catalyst is maintained in response to a reduction in make-up gas flow, and vice versa, to sustain the methanol synthesis. Therefore, the catalyst bed does not need any substantial extra heat or cooling, when a rapid change of feed rate occurs.

This can be achieved, when necessary, by using one or more heat exchangers external to the reactor. In a preferred arrangement, the synthesis gas mixture fed to the reactor is heated in a gas-gas heat exchanger using the product gas stream from the reactor, with the heating being supplemented by a small external heat supply, for example an electric heater, when the rate of methanol production is too low to provide enough heat to preheat the feed itself.

The synthesis gas may also be heated by passing it through tubes or plates disposed within the methanol synthesis catalyst in a tube-cooled converter (TCC) or gas cooled converter (GCC). In a preferred arrangement, the synthesis gas is passed through at least a gas-gas heat exchanger where it is heated by the product gas and then passed through tubes or plates within the reactor where it is heated by the reacting gases passing through the methanol synthesis catalyst. Other temperature adjustment of the feed gas may be performed using conventional heat exchange apparatus.

The methanol synthesis catalysts are preferably copper-containing methanol synthesis catalysts, in particular the methanol synthesis catalyst in the synthesis reactor is a particulate copper/zinc oxide/alumina catalyst. Particularly suitable catalysts are Mg-doped copper/zinc oxide/alumina catalysts as described in US4788175.

Methanol synthesis may be effected in the reactor at pressures in the range 10 to 120 bar abs, and temperatures in the range 130°C to 350°C. The pressure of the synthesis gas at the reactor inlet is preferably 50-100 bar abs, more preferably 70-90 bar abs at maximum operating rate, but may fall to 10-20 bar abs. The temperature of the synthesis gas at the synthesis reactor inlet is preferably 200-250°C and at the outlet preferably 230-280°C. A purge gas stream is recovered from the loop. The purge stream is controlled by means of a purge valve. Such purge valves are known but are conventionally opened or closed in response to the build-up of gaseous inventory in the loop and to maintain an optimum fixed operating pressure. In contrast, in the present invention the purge valve is maintained at a fixed position so that the pressure of the loop gas increases or decreases in accordance to increases or decreases in the flow of the make-up gas. Accordingly, the operating strategy in the present invention is to maintain a fixed loop purge valve position and allow the loop to pressurise/depressurise as the make-up gas rate changes. Thus for an instantaneous decrease in make-up gas flow rate from steady state conditions there is a net loss of substance from the loop and the loop begins to depressurise. Depressurisation will continue until either steady state where the pressure reaches a point where the flow through the purge valve balances the flows in and out and moles lost to reaction, or the make-up rate changes again where the loop would continue its dynamic response. In this way, changes in the make-up gas rate are compensated for by changing pressure in the loop due to mass flow imbalance, and complete turndown of the loop can be achieved. Also circulation mass flow drops at low pressure, so in the case of a tube cooled converter or gas cooled converter, there is less flow and less cooling of the converter at low rate.

The recycle ratio of loop gas to make-up gas will vary in the process in accordance with the variation in pressure. By the term “recycle ratio”, we mean the molar flow ratio of the recycled loop gas to the make-up gas that form the synthesis gas mixture fed to the reactor. The recycle ratio to form the synthesis gas mixture may be in the range 0.01:1 to 25:1.

The purge gas typically comprises hydrogen and carbon oxides and may be used for hydrogen recovery, for example by pressure-swing absorption or by using suitable membranes, or may be subjected to one or more further processing stages including autothermal reforming, water-gas shift and methanol synthesis.

The product gas comprising unreacted hydrogen and carbon dioxide, along with methanol vapour is cooled to below the dew point to condense liquid methanol. The cooling may be performed using conventional heat exchange apparatus. Thus the product gas stream from the reactor may be cooled in one or more stages of heat exchange, e.g. with water or air cooling, to condense methanol therefrom, which may suitably be recovered using gas-liquid separators. The cooling may be performed to fully or partially condense the methanol. Preferably substantially all the methanol is condensed from the product gas stream. The recovered liquid methanol stream may be further processed, for example by one or more, preferably two or three, stages of distillation to produce a purified methanol product. Alternatively, the crude methanol may be fed recovered and stored.

The methanol product, with or without purification, may be subjected to further processing, for example to produce dimethyl ether or formaldehyde, but in one embodiment is stored for use in future electrical power generation. Thus the methanol may be fed to a direct methanol fuel cell to generate electrical power, or may be subjected to reforming in a methanol reformer containing a methanol reforming catalyst to generate a hydrogen gas stream, which may be fed to a conventional fuel cell for electrical power generation. Alternatively, the methanol may be used as a fuel.

The invention will be further described by reference to the figure in which;

Figure 1 depicts a process according to one embodiment of the present invention, and Figure 2 is a graph of inlet and turn temperatures in a tube-cooled converter in a process according to Figure 1 for constant weight average bed temperature at varying hydrogen availability.

It will be understood by those skilled in the art that the drawings are diagrammatic and that further items of equipment such as feedstock drums, pumps, vacuum pumps, compressors, gas recycling compressors, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, level controllers, holding tanks, storage tanks and the like may be required in a commercial plant. Provision of such ancillary equipment forms no part of the present invention and is in accordance with conventional chemical engineering practice.

In Figure 1, a carbon dioxide stream 10 is compressed in a compressor 12 and combined with a stream of pressurised electrolytic hydrogen 14. The combined make-up gas stream is fed by line 16 to a compressor 18 which further raises its pressure. The pressurised make up gas 20 is combined with a loop gas stream 22 and the resulting synthesis gas fed via line 24 to a gas-gas heat exchanger 26 were it is heated in exchange with a product gas stream. The temperature of the heated synthesis gas in line 28 is adjusted if necessary by means of a heat exchanger 30 and the temperature-adjusted synthesis gas fed to a plurality of tubes 32 disposed within a tube-cooled converter 34 containing a bed 36 of a particulate copper-based methanol synthesis catalyst. The synthesis gas is heated as it passes through the tubes and is discharged into a space above the catalyst bed 36. The synthesis gas then passes through the bed 36 where the methanol synthesis takes place to form a product gas comprising unreacted hydrogen and carbon dioxide, steam and methanol vapour. The product gas is fed from the reactor 34 to the gas-gas heat exchanger 26 where it is partially cooled in heat exchange with the synthesis gas. The partially cooled gas 40 is fed to one or more further heat exchanges 42 where it is cooled to below the dew point to condense methanol and steam. The cooled stream is fed by line 44 to a gas-liquid separator 46 where a crude liquid methanol stream is separated from the gases. The crude methanol stream is recovered from the separator 46 via line 48. The methanol-depleted gas stream recovered from the separator 46 is fed via line 50 to a compressor 52 where the pressure is increased to form the loop gas 22, which is combined with the pressurised make-up gas 20. The methanol-depleted gas stream 50 is divided before the compressor and a portion 54 removed as a purge gas stream. A purge valve 56 is set at a fixed position in the purge gas stream 54.

The Invention is further illustrated by reference to the following Example.

Example 1 A flow sheet was modelled according to the process shown in Figure 1. The base case, for 100 % electrolytic hydrogen availability, is taken as a 20 MTPD methanol plant, using hydrogen and carbon dioxide feeds. Carbon dioxide was compressed to the pressure of the hydrogen from electrolysis. This make up feed of carbon dioxide and hydrogen was compressed to the loop pressure and introduced to the synthesis loop. The synthesis loop used a tube cooled converter. The inlet streams were as follows:

Table 1

The maximum hydrogen flow rate was set to achieve 20 MTPD of methanol. The carbon dioxide flow rate was set to achieve a stoichiometry of R = 2.05 throughout.

The model shows that the energy requirements for steady state operation ranged from 10.7 MW producing 20 MTPD to 0.54 MW producing 0.86 MTPD. The varying make-up rate was compensated for by varying the loop pressure, and this varied between 80 barg at the 20 MTPD case and 13 barg at the 0.86 MTPD case. The carbon efficiency of the process was less at lower rates; 93 % for the 20 MTPD case, and 80 % at the 0.86 MTPD case. In comparison, a methanol loop running with a fixed pressure of 80 barg would only be capable of operating at energy requirements between 10.7 and 7.5 MW.

The time to pressurise the loop to 80 barg (100 % availability) can be as short as 2 minutes if there is an instantaneous change in make-up rate. Additional heating was provided to ensure that the reactor did not lose temperature and stop methanol production. This was achieved by maintaining the weight average bed temperature across the range of pressures by raising the converter inlet temperature at lower make up rates. This required additional power for heating at lower make up rates. The model outputs for the extremes of operation were as follows;

Table 2

Figure 2 shows how the converter inlet temperature and tube turn temperature in the TCC vary with hydrogen availability if the weight average bed temperature is maintained. This shows how the temperature rises are reasonable. The additional power requirement is also shown for each case. Also shown in Figure 2 is the required input power to maintain bed temperature at low synthesis gas flowrates. Above 40 % availability it is assumed that the interchanger will have sufficient capacity to provide the extra heat to the feed gas. A by-pass around the gas-gas interchanger 26 may be provided to improve temperature control, so that for > 40 % hydrogen availability there is no need for external heat to provide the temperature rise as more heat can be recovered from the exit gas by closing the bypass valve.

The process of Figure 1 was modelled at 100% hydrogen availability and 30% hydrogen availability for a 20 MTPD methanol plant with a WABT of 253.1 °C to illustrate the mass balance.

Table 3

Table 4

Claims

Claims.

1. A process for the synthesis of methanol comprising the steps of: (i) passing a synthesis gas mixture comprising a make-up gas through a reactor containing a bed of particulate methanol synthesis catalyst to form a product gas stream, (ii) cooling and recovering methanol from the product gas stream thereby forming a methanol-depleted gas mixture, (iii) dividing the methanol-depleted gas mixture into a purge gas stream and a loop gas stream by means of a purge valve and (iv) combining the loop gas stream with the make-up gas to form the synthesis gas mixture, wherein the purge valve is maintained at a fixed position so that the pressure of the loop gas increases or decreases in accordance to increases or decreases in the flow-rate of the make-up gas.

2. A process according to claim 1 wherein the make-up gas comprises electrolytic hydrogen.

3. A process according to claim 1 or claim 2 wherein the reactor is selected from an uncooled adiabatic reactor, or a cooled reactor selected from a tube cooled converter, an axial flow steam-raising converter, a radial flow steam-raising converter, a gas-cooled converter or a quench reactor.

4. A process according to claim 3 wherein the reactor is a tube-cooled converter containing a bed of methanol synthesis catalyst in which a plurality of tubes are disposed though which the synthesis gas passes before being fed to the methanol synthesis catalyst.

5. A process according to any one of claims 1 to 4 wherein a weight average bed temperature of the methanol synthesis catalyst is maintained constant in response to an increase or decrease in make-up gas, to sustain the methanol synthesis by controlling the reactor inlet temperature by means of one or more heat exchangers external to the reactor.

6. A process according to any one of claims 1 to 5 wherein the synthesis gas mixture fed to the reactor is heated in a gas-gas heat exchanger using the product gas stream from the reactor.

7. A process according to any one of claims 1 to 6 wherein methanol synthesis in the reactor is performed at pressures in the range 10 to 120 bar abs.

8. A process according to any one of claims 1 to 7 wherein methanol synthesis in the reactor is performed at a temperature in the range 130°C to 350°C.

9. A process according to any one of claims 1 to 8 wherein the purge gas stream recovered from the methanol depleted gas mixture is used for hydrogen recovery, or is subjected to one or more further processing stages including autothermal reforming, water-gas shift and methanol synthesis.

10. A process according to any one of claims 1 to 9 wherein the product gas stream from the reactor is cooled in one or more stages of heat exchange to condense methanol therefrom.

11. A process according to any one of claims 1 to 10 wherein the methanol product, with or without purification, is used for the generation of electrical power.