GB2082574A - Methanol synthesis - Google Patents

Abstract

A method and plant are disclosed for synthesising methanol from synthesis gas in which crude product methanol is vaporised and superheated and the resulting superheated crude methanol vapour is then expanded through a turbine which can be used to drive one of the synthesis gas compressors. Typically such synthesis gas is obtained by steam reforming of a hydrocarbon feedstock in a reformer furnace, waste heat generated in the reforming process being used to vaporise and/or superheat the crude methanol vapour. A turbine-compressor set may advantageously be used to compress air for combustion in the reformer furnace, the pass out air from the compressor being heated by heat exchange with hot combustion products from the combustion section of the reformer furnace and expanded through turbine prior to delivery to the combustion air duct. Additional power is available from the combustion air turbine for driving one of the synthesis gas compressor stages.

Description

SPECIFICATION Methanol synthesis This invention relates to a method and plant for methanol synthesis.

Methanol is synthesised catalytically in large quantities, for example, from an approximately 2:1 hydrogen:carbon oxide(s) synthesis gas mixture.

One main route to synthesis gas for methanol synthesis utilises steam reforming of a hydrocarbon feedstock, e.g. methane, as the primary reaction step. In this step a mixture of steam and hydrocarbon is passed through catalyst filled reformer tubes positioned in a reforming section of a reformer furnace. The exhaust gases from the reforming section of the furnace are used to preheat the steam/hydrocarbon mixture, the hydrocarbon feedstock and/or the steam feed, as well as to raise steam for use in the plant and for export beyond battery limits. This preheating takes place in one or more exchangers mounted in a convection section of the reformer furnace. Normally a steam-driven fan is used to feed air to the furnace chamber, whilst a steam driven compressor is used to compress the synthesis gas up to the reaction pressure required for subsequent process steps, e.g. methanol synthesis.According to current practice the synthesis gas compressors are commonly driven by high pressure back pressure steam turbines, the medium pressure pass out steam from which is used as steam feedforthe reforming process. Such high pressure turbines and high pressure steam-raising plant are expensive to instal and maintain.

In a conventional methanol synthesis plant the thermal efficiency of the reformer furnace is less than 100% because waste heat is lost in the furnace exhaust gases and considerable effort has been made, and still continues to be made, to improve the overall efficiency of a synthesis gas plant by improving the recovery of waste heat from the furnace exhaust gases.

The present invention seeks to provide an improved design of methanol synthesis plant. It also seeks to provide an improved method of producing methanol. It further seeks to provide a methanol synthesis plant which obviates the need to use high pressure back pressure steam turbines and hence obviates the requirement to generate high pressure steam. Additionally, the invention seeks to provide a method of producing methanol which requires less fuel input than a conventional methanol synthesis plant. Yet again, the invention seeks to provide a methanol synthesis plant that can be erected at a lower capital cost and on a smaller site than a conventional methanol synthesis plant.

According to one aspect of the present invention there is provided a method of producing methanol which comprises contacting synthesis gas under methanol synthesis conditions with a methanol synthesis catalyst, vaporising resulting crude methanol at elevated pressure, superheating resulting crude methanol vapour, and expanding resulting superheated crude methanol vapour through a turbine means arranged to drive a rotary machine.

In a particularly preferred method methanol is produced from a synthesis gas resulting from steam reforming of a hydrocarbon feedstock by steps which comprise burning a fuel in combustion air in a combustion section of a reformer furnace to heat a plurality of reformer tubes mounted therein, passing a steam/hydrocarbon feedstock mixture to be reformed through the reformer tubes at a steam reforming pressure greater than atmospheric pressure, providing a methanol synthesis section for synthesising methanol at a methanol synthesis pressure greater than the steam reforming pressure from synthesis gas resulting from steam reforming in the reformer tubes of the reformer furnace, vaporising crude methanol from the methanol synthesis section at elevated pressure, superheating resulting crude methanol vapour, expanding resulting superheated crude methanol vapour through a first turbine means, passing expanded crude methanol vapour from the first turbine means to a methanol purification section, compressing air in air compressor means, passing resulting compressed air through heat exchanger means positioned in a convection section of the reformer furnace thereby to heat the compressed air by indirect heat exchange with the products of combustion from the combustion section of the reformer furnace, expanding heated compressed air from the heat exchanger means through second turbine means coupled to the air compressor means, feeding expanded heated compressed air from the second turbine means as combustion air to the combustion section of the reformer furnace, and compressing synthesis gas resulting from the steam reforming step to the methanol synthesis pressure in a synthesis gas compressor means in at least two stages, each stage being coupled to a corresponding one of the first and second turbine means.

According to another aspect of the present invention a methanol synthesis plant for producing methanol from a synthesis gas resulting from steam reforming of a hydrocarbon feedstock comprises means for producing synthesis gas, a methanol synthesis section for synthesising methanol at an elevated methanol synthesis pressure, means for vaporising crude methanol from the methanol synthesis section at elevated pressure, superheater means for superheating resulting crude methanol vapour, and a turbine means coupled to a rotary machine and arranged to be driven by resulting superheated crude methanol vapour.

A particularly preferred form of methanol synthesis plant comprises a reformer furnace having a combustion section to which a fuel may be fed, a combustion air supply conduit connected to the combustion section for supply of the air required for combustion of the fuel, a plurality of reformer tubes positioned in the combustion section for passage of a steam/hydrocarbon feedstock mixture to be reformed at a steam reforming pressure greater than atmospheric pressure, and a convection section for recovery of waste heat from the combustion gases from the combustion section, a methanol synthesis section for synthesising methanol at a methanol synthesis pressure greater than the steam reforming pressure from synthesis gas resulting from steam reforming in the reformer tubes of the reformer furnace, means for vaporising crude methanol from the methanol synthesis section at elevated pressure, superheater means for superheating resulting crude methanol vapour, a first turbine means arranged to be driven by the expansion of resulting superheated crude methanol vapour, a methanol purification section for purifying crude methanol discharged from the firstturbine means, air compressor means for compressing air, heat exchanger means in the convection section in the path of products of combustion from the combustion section of the reformer furnace through which air from the air compressor means may be passed, second turbine means coupled to the air compressor means and arranged to be driven by heated compressed air from the heat exchanger means and to discharge resulting expanded air into the combustion air supply conduit, and synthesis gas compressor means for compressing synthesis gas resulting from the steam reforming of the hydrocarbon feedstockto the methanol synthesis pressure in at least two stages, each stage being coupled to a corresponding one of the first and second turbine means.

The invention can thus provide for recovery of waste heat from the combustion products of the reformer furnace by vaporising crude methanol (still under pressure from the methanol synthesis section) and then superheating this to provide a superheated high pressure crude methanol vapour the expansion of which is used to drive a first turbine coupled to one of the synthesis gas compressor stages, whilst further waste heat is recovered from the combustion products by indirect heat exchange with compressed air passing through a heat exchanger, the expansion of the resulting hot compressed air being used to drive a second turbine coupled to another of the synthesis gas compressor stages and to an air compressor for generating the compressed air and the exhaust stream from this second turbine being used as combustion air from the furnace.Because the inlet temperatures to the turbines can be high, for example in excess of about 1500"F (i.e. over about 815"C), low pressure turbine/compressor sets can be used operating, for example, at inlet pressures of about 60 p.s.i.a. (i.e. about4.22 kg/cm2 absolute) and outlet pressures of about 15 p.s.i.a. (i.e. about 1.05 kg/cm2 absolute). The hot pass out air from the second turbine by being used as combustion air reduces the fuel requirement of the reformer furnace since it may be at a temperature of at least about 1000"F (i.e. about 538"C), for example.The crude methanol vapour which passes out from the first turbine may be art a similarly high temperature to the exit temperature of the air from the second turbine and can be used, for example, for heating crude methanol vapour, or for pre-heating boiler feed water and/or process feedstock. Since there may be additional power available from the first and/or second turbines beyond that required to drive the compressor or compressors associated therewith, it is possible to couple an alternator or other rotary machine to be driven to the first and/or second turbine. In an alternative arrangement at least some of the heat for vaporising and/or superheating the crude methanol vapour may be provided by the hot reformed gases exiting the reformer tubes.Steam may be raised by heat exchange with the hot reformed gases and/or the combustion gases in the heat recovery section of the reformer furnace both for use in the process and for export beyond battery limits or for driving an alternator.

The first and/or second turbine means may each comprise a single stage machine or a plurality of single stage machines in parallel or a multi-stage machine comprising two or more turbine stages in series or a plurality of such multi-stage machines in parallel.

The air compressor means may comprise a single stage machine or a plurality of single stage machines in parallel or a multi-stage machine comprising two or more compressor stages in series or a plurality of such multi-stage machines in parallel.

In the practice of the invention conventional turbine/compressor sets can be used.

The power developed by the first turbine and/or by the second turbine may be considerably in excess of that which would normally be required by its respective connected compressor This surplus power from either turbine can be used to drive the exhaust draught fan for drawing the waste gases from the furnace to the furnace stack and/or an alternator for producing electrical power for use within the plant or for export beyond battery limits.

The power output of the first turbine can be boosted by mixing steam with the crude methanol vapour so as to increase the mass flow through the first turbine. This steam can be raised by heat exchange with the hot reformed gases and/or with the hot combustion gases in the heat recovery section of the reformer furnace. The steam in the vapour mixture exiting the first turbine can then be condensed in the methanol purification section of the plant.

The hydrocarbon feedstock used in the steam reforming step can be any feedstock that can be subjected to steam reforming. Such feedstocks include for example natural gas, ethane, propane, butane, liquefied petroleum gases, light naphthas, medium naphthas and the like.

In the reformer tubes various gas phase reactions occur in the presence of the catalyst. Taking, for example, methane as an example of a suitable feedstockthe reactions are:

The first reaction is known as the reforming reaction, the second is the shift reaction and the third is the carbon reaction. The first two reactions are beneficial but the third reaction, the carbon reaction, is an undesirable side reaction. To minimise deposition of carbon it is accordingly expedient to choose a relatively low pressure, a relatively high temperature and a relatively high steam to hydrocarbon ratio.

Usually it will be preferred to utilise a relatively low molecular weight feedstock to reduce the relative amount of carbon oxides in the gas.

The steam/h'ydroca rbon feedstock ratio is preferably chosen so as to minimise carbon deposition in the reformer tubes. Typically the steam/hydrocarbon feedstock is such as to represent a steam/carbon atom ratio of at least about 2:1 upto about 5:1 or higher, e.g. about4:1.

For methanol synthesis the synthesis gas is typically required to be an approximately 2:1 H2:CO mixture (e.g. 2.25:1 H2 : CO). If the hydrocarbon feeds took does not yield a synthesis gas of the required ratio (e.g. the theoretical H2:CO ratio for methane is 3:1), then CO2 may be injected into the steam/hydrocarbon feedstock mixture to be reformed. Such CO2 can, if necessary, be obtained by scrubbing the combustion gases exiting the reformer furnace with ethanolamine solution.

The catalyst may be any conventional steam reforming catalyst. The conditions selected in the reformer tubes are those conventionally adopted for steam reforming. Typically the temperature ranges from about 700"C, preferably about 750"C to 9000C, e.g. about 850"C, whilst the pressure may range from atmospheric pressure up to about 30 kg/cm2 absolute. Usually, however, it will be preferred to operate at a pressure of about 5 to about 15 kg/cm2 absolute in the reformer tubes.

The reformed gases exiting the reformer tubes are at a high temperature e.g. in the region of 850"C, and contain usually a high proportion (e.g. about 70% by volume or more) of steam, the balance comprising an essentially 2:1 Hop : CO mixture. Since the temperature of the reformed gases exiting the reformer tubes usually greatly exceeds the temperature required for methanol synthesis, it is generally practical to recover much of the sensible heat in the gases by indirect heat exchange with incoming steam/hydrocarbon feedstock mixture, by raising steam and/or by heating boiler feed water and the like. It will usually be preferred to cool the reformed gases sufficiently to condense the unreacted steam.

Since the resulting synthesis gas will be at a pressure (e.g. from about 5 kg/cm2 up to about 30 kg/cm2) that is lowerthan is required for subsequent proces- sing, i.e. methanol synthesis (e.g. from about 300 kg/cm2 up to about 400 kg/cm2 or more), it is thereafter necessary to compress it. This is done by the synthesis gas compressor means which includes two or more stages each driven by a corresponding one of the first and second turbine means. Provision may be made for intercooling between the stages of the synthesis gas compressor means.In order that the invention may be clearly understood and readily carried into effect a preferred form of methanol synthesis plant and several modifications thereof, together with the method of operation thereof, will now be described by way of example only with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a flow sheet of a methanol synthesis plant constructed in accordance with the invention; and Figures 2 to 4 are details of the flow sheets of modified forms of the plant of Figure 1.

Referring to Figure 1 of the drawings, a methanol synthesis plant includes a reformer furnace 1 having a reforming section 2 and a convection section (or heat recovery section) 3. A plurality of reformer tubes 4, of which two only are shown in Figure 1, are positioned in reforming section 2 for the passage therethrough of a steam/hydrocarbon feedstock mixture to be reformed. Fuel can be supplied via line 5 to a burner nozzle or nozzles (not shown) whilst combustion air is supplied to reforming section 2 through line 6.

The hydrocarbon feedstock, e.g. methane, is supplied via line 7 and is preheated in heat exchanger 8 in convection section 3, typically to a temperature in the range of from about 400"C to about 420 C. The preheated feedstock is passed on through line 9 to a desulphurization vessel 10, which contains a charge of a suitable desulphurization agent. The desulphurized feedstock passes on via line 11 and is mixed with steam supplied via line 12. Typically the resulting steam/hydrocarbon feedstock mixture has a steam:carbon atom ratio of approximately 4:1.This mixture, now at inlet temperature (typically about 850"C) and at a pressure of, for example 20 kg/cm2 absolute, passes via line 13 to the inlet end of reformer tubes 4, which are packed with a suitable catalyst, such as nickel on a refractory support consisting of calcium and aluminium oxides. Carbon dioxide can be admitted into the steam/feedstock mixture via line 14 in order to adjust the H2:CO ratio in the reformed gases exiting the reformer tubes 4 to the desired approximately 2:1 ratio.

The hot reformed gases are led by way of lines 15 and 16 to heat exchanger 17 and pass on through line 18 to heat exchangers 19 and 20 and then to cooler/condenser 21 and condensate drum 22. Condensate is removed from drum 22 via line 23 whilst the resulting synthesis gas passes on via line 24.

Reference numeral 25 indicates a demister in drum 22.

In heat exchanger 17 the hot reformed gases raise steam by heat exchange with water in line 26 connected to steam drum 27, the resulting steam water-mixture being fed back to steam drum 27 by way of line 28. In heat exchanger 19 the reformed gases are cooled further by heat exchange with boiler feed water in line 29. Heat exchanger 20 provides for further cooling of the reformed gases against circuiating hot water in line 30. Cooler/condenser 21 is supplied with cooling water by means of line 31.

As can be seen from Figure 1 steam for line 12 is supplied from steam drum 27.

Air for combustion of the fuel supplied via line 5 to reforming section 2 is drawn into the inlet end of compressor 32 of a gas turbine set 33 as indicated by arrows 34. The resulting compressed air is fed from the outlet end of the compressor 32 via line 35 to a heat exchanger 36 mounted in the convection section 3 of reformer furnace 1. Hot compressed air is withdrawn from heat exchanger 36 through line 37 and passes on to a combustion chamber 38, which can be supplied with fuel via line 39 and thence via line 40 to the inlet end of turbine 41 of gas turbine set 33. The outlet end of turbine 41 is connected to combustion air supply conduit 6.As can be seen from Figure 1 compressor 32 and turbine 41 are mounted on a common shaft 42, together with the primary stage 43 of the synthesis gas compressor train (and possibly also an induced draught fan (not shown) which serves to impel exhaust gases exiting convection section3 of reformerfurnace 1 via line44to a suitable stack (also not shown)).

Reference numeral 45 indicates a starter motor for gas turbine set 33 which is also mounted on shaft 42.

Motor 45 can be an electric motor or a gas expansion motor.

Synthesis gas in line 24 is compressed in primary compressor stage 43 to an intermediate pressure and is fed through line 46 to a cooling stage 47 and thence via line 48 to the inlet end of the secondary stage 49 of the synthetic gas compressor train in which it is further compressed to methanol synthesis pressure. The gas from the exit end of second compressor stage 49 passes on through line 50 to a further cooling stage 51 and thence via line 52 to form the make up gas for methanol synthesis, being mixed with the recirculating gas in line 53 from methanol synthesis plant 54. The mixed gases are fed through line 55 to recycle compressor 56 and then to plant 54 via line 57.

Secondary compressor stage 49 and recycle compressor 56 are mounted on a common shaft 58 and are driven by a second turbine 59.

The methanol synthesis plant 54 is of conventional design and includes suitable gas purification and methanol synthesis stages. The methanol synthesis reactor or reactors can be of conventional design and contain(s) a suitable methanol synthesis catalyst. The gases exiting the methanol synthesis stage are cooled to condense product methanol and the resulting mixture of unreacted gas and crude methanol is passed via line 60 to separator 61. Crude methanol, still at methanol synthesis pressure, is withdrawn through line 62 whilst unreacted gases are recirculated in line 53. A purge stream may be withdrawn in known manner from methanol plant 54 in order to prevent excessive build up of inert gases, such as nitrogen in the recirculating gas.

The crude methanol in line 62 consists predominantly of methanol but contains also some water and minor amounts of by-products, including dimethyl ether and higher alcohols. This crude methanol is pre-heated in pre-heater63 by indirect heat exchange with the combustion gases of the reformer furnace in convection section 3 and is further heated in heat exchanger 64. The resulting crude methanol vapour (mainly a methanol vapour/steam mixture) still at methanol synthesis pressure, passes on through line 65 to crude methanol drum 66. From crude methanol drum 66 the crude methanol vapour passes on by way of line 67 to a superheater 68 which is also mounted in convection section 3 of the reformerfurnace 1 and is heated by the combustion gases from the combustion section 2.The resulting compressed, superheated crude methanol vapour passes at high pressure on through line 69 to the inlet end of turbine 59. The outlet end of turbine 59 is connected to line 70 which leads to heat exchanger 64 and thence to the topping column 71 of the methanol purification section of the plant. Hot crude methanol vapour exits turbine 59 at low pressure (e.g. 60 p.s.i.a. (4.21 kg/cm2 absolute), gives up heat to the incoming crude methanol in heat exchanger 64, and is supplied still in vapour form to topping column 71. A pressure reducing valve (not shown) t may be fitted in line 70 to enable topping column to operate at atmospheric pressure, if desired.

Light ends containing any dimethyl ether present are removed overhead from topping column 71 through line 72 and are condensed against cooling water in condenser 73. Part of the light ends condensate is returned to topping column 71 by means of line 74, whilst the remainder is removed via line 75.

The bottoms product flows on through line 76 to the product tower 77. Methanol product is recovered overhead through line 78 and is condensed against cooling water in condenser 79. A proportion of the condensate is returned to product tower 77 through line 80 whilst the remainder is recovered via line 81 for storage or for further purification, e.g. by redistillation. Higher alcohols are removed via line 82 and are condensed by condenser 83, whilst a bottoms stream consisting essentially of water is removed via line 84.

Reference numeral 85 indicates a boilerforthe crude methanol; this is supplied from crude methanol drum 66 by means of line 86. The resulting crude methanol/vapour is returned to methanol drum 66 by way of line 87.

The output of turbine 59 will usually exceed the power required to drive second synthesis gas compressor stage 49 and the recycle compressor 56.

Hence an alternator 89 can be coupled to shaft 58.

If desired high pressure steam can be supplied through line 90 and mixed with the crude methanol vapour in line 67 prior to passage through the superheater 68. In this way the mass flow through turbine 59 can be increased which results in a corresponding increase in the power output of turbine 59 and in the power generated by alternator 89. The condensate from the high pressure steam added by means of line 90 appears in line 84. Hence the methanol distillation section of the plant becomes the condenser for this additional steam flow.

In the event of the output of steam from steam drum 27 exceeding the process requirements any excess can be exported from the plant by way of line 91.

At start up, motor45 is used to run up gas turbine set 33 and hence to commence forcing air through heat exchanger 36. At the same time fuel is supplied via lines 5 and 39 and ignited by suitable ignition means (not shown). As the temperature rises in convection section 3 the airtemperature in line 40 will correspondingly rise until motor 45 can be cut out and the supply of fuel through line 39 can also be cut off. An auxiliary heater (not shown) is used to raise steam in steam drum 27 and once the reformertubes 4 are up to the correct temperature feedstock is allowed to pass along line 7. Alternatively an inert gas such as nitrogen, can be passed through reformer tubes 4 until steam has been raised in steam drum 27, whereupon feedstock can be supplied along line 7.

As will be appreciated by the skilled reader, the plant of Figure 1 differs from a conventionaliy designed plant in that the forced draught fan for the reformer furnace combustion air and its associated driver, the air heater used for heating the combustion air, and possibly also the induced draught fan for the exhaust gases from the convection section of the reformer furnace and its associated driver, all of which are used in a conventional plant, are all replaced by a single gas turbine set and a heat exchanger, i.e. gas turbine set 33 and heat exchanger 36.In addition, since conventional designs usually utilise steam turbines to drive the forced draught fan for the furnace combustion air and the induced draught fan for the exhaust gases from the convection section of the furnace, the plant of Figure 1 obviates the need to provide condensers and condensate pumps for such steam turbines.

Furthermore, the high pressure steam turbine conventionally used to drive the synthesis gas compressor is also replaced by a turbine/compressor set, again with a simplification of the plant since no steam condensers or condensate pumps are needed forthis. Moreover since there are no high pressure steam turbines, there is no need to generate high pressure steam, e.g. at pressures in the region of 100 kg/cm2 absolute; instead there is only a need for steam at process pressure for the steam reformer, e.g. a pressure in the region of 30 kg/cm' absolute, and then only for the amount of steam required in the steam reforming process itself.This represents a considerable saving in capital since the steam drum can be smaller than in a conventional plant and can be of lighter construction, as can also be the associated pipework and steam control valves. An added advantage is that maintenance costs of the plant of Figure 1 will be reduced compared with those of a conventional plant, due to the simplification of the plant and to the reduction in steam pressure. An additional benefit of the use of gas turbine and turbine/compressor sets is that these are compact units and hence the ground area required for the synthesis gas plant can be reduced compared with conventional plants. Significant savings in fuel are to be expected compared with conventional piants using high pressure steam turbines.

The plant of Figure 2 is identical to that of Figure 1 exceptthat line 12 from steam drum 27 leads to a superheater 92 in convection section 3.

In the plant of Figure 3 steam in line 12 is superheated by passing through heat exchanger 93 in which it undergoes indirect heat exchange with reformed gas in line 16.

Figure 4 shows another modification of the plant of Figure 1. High pressure steam is raised in conventional manner by heat exchange with hot reformed gases in heat exchangers 17, 19 (not shown in Figure 4) and is led from steam drum 27 to a superheater 94 via line 95. The resulting superheated high pressure steam (e.g. at about 100 kg/cm2 absolute or more) is passed via line 96 to a back pressure steam turbine 97 which is coupled to an alternator 98 by means of shaft 99. The exhaust steam flows on through line 100 to a superheater 101 in which it is resuperheated by hot reformed gas in line 16 and passed on via line 12 to the steam reforming step.

Alternatively the steam exhausted from turbine 97 can be condensed in condenser 102.

In an alternative variation, also indicated in Figure 4, steam drum 27 is operated at medium pressure (e.g. at about 50 kg/cm2 absolute) and part of the superheated steam is passed via lines 103 and 100 to superheater 101 for passage to line 12.

The invention is further illustrated in the following Example.

Example A methanol synthesis plant of the type illustrated in Figure 1 whose output amounts to 1500 short tons per day (1360 metric tons per day) has a combustion air requirement for the reformer furnace of 563,388 Ibs per hour (255553 kg per hour). This is delivered at 15 p.s.i.a. (1.05kg/cm2 absolute) and 1032 F (556C).

The design of heat exchanger 36 is such that there is a pressure drop across it of about 10 psi (0.70 kg/cm2). Air compressor 32 has a compression ratio of about 5:1, thus delivering air at an outlet pressure of about 72.5 p.s.i.a. (5.10 kg/cm2 absolute) and an outlet temperature of 435"F (223.9"C) when the ambient air pressure is 14.5 p.s.i.a. (1.02 kg/cm' absolute) and the ambient air temperature is 60"F (15.6"C). The inlet pressure to the turbine 41 is 62.5 p.s.i.a. (4.39 kg/cm2 absolute) and the outlet pressure 15 p.s.i.a. (1.05 kgicm2 absolute). in passing through heat exchanger. 36 the compressed air is heated to 1 500oF (816 C). The power output of turbine 41 is thus 29733 HP (22189 kW), of which 20111 HP (15008 kW) are needed to drive compressor 32, the remaining 9622 HP (7181 kW) being available to drive first compressor stage 43 of the synthesis gas compres sortrain. The crude methanol flow in line 62 amounts to 158,842 Ibs per hour (72050 kg per hour).

After passage through superheater 68 the superheated methanol vapourlsteam mixture is at a temperature of 1300 F (704"C) and a pressure of 1360 p.s.i.a.

(95.6 kgicm2 absolute). In expanding through turbine 59 the temperature of this mixture falls to 121 8T (659 C) and its pressure to 60 p.s.i.a. (4.21 kgicm2 absolute). The power output of turbine 59 is 13583 HP (10137 kW).

The total power required to drive the two synthesis gas compressor stages 43 and 49 and the recycle compressor 56 is 21086 HP (15736 kW). As stated above, 9622 HP (7181 kW) are available from turbine 41 to drive first synthesis gas compressor stage 43.

Hence 11464 HP (8555 kW) are required to drive second synthesis gas compressor stage 49 and recycle compressor 56. Since the power output of turbine 59 is 13583 HP (10137 kW) this means that there is 2119 HP (1581 kW) available to drive alternator 89 andíor to drive the induced draught fan for impelling exhaust gases in line 44 to the stack.

Claims (23)

1. A method of producing methanol which comprises contacting snythesis gas under methanol synthesis conditions with a methanol synthesis catalyst, vaporising resulting crude methanol at elevated pressure, superheating resulting crude methanol vapour, and expanding resulting superheated crude methanol vapour through a turbine means arranged to drive a rotary machine.

2. A method of producing methanol from a synthesis gas resulting from steam reforming of a hydrocarbon feedstock, which comprises burning a fuel in combustion air in a combustion section of a reformer furnace to heat a plurality of reformer tubes mounted therein, passing a steam/hydrocarbon feedstock mixture to be reformed through the reformer tubes at a steam reforming pressure greaterthan atmospheric pressure, providing a methanol synthesis section for synthesising methanol at a methanol synthesis pressure greater than the steam reforming pressure from synthesis gas resulting from steam reforming in the reformer tubes of the reformer furnace, vaporising crude methanol from the methanol synthesis section at elevated pressure, superheating resulting crude methanol vapour, expanding resulting superheated crude methanol vapourthrough a first turbine means, passing expanded crude methanol vapour from the first turbine means to a methanol purification section, compressing air in air compressor means, passing resulting compressed airthrough heat exchanger means positioned in a convection section of the reformer furnace thereby to heat the compressed air by indirect heat exchange with the products of combustion from the combustion section of the reformer furnace, expanding heated compressed air from the heat exchanger means through second turbine means coupled to the air compressor means, feeding expanded heated compressed air from the second turbine means as combustion air to the combustion section of the reformer furnace, and compressing synthesis gas resulting from the steam reforming step to the methanol synthesis pressure in a synthesis gas compressor means in at least two stages, each stage being coupled to a corresponding one of the first and second turbine means.

3. A method according to Claim 2, in which heat is recovered from the crude methanol vapour passing out from the first turbine means by indirect heat exchange with at least one of crude methanol vapour, boiler feed water and process feedstock.

4. A method according to Claim 2 or Claim 3, in which the power output from at least one of the first and second turbine means exceeds the power required to drive the compressor means associated therewith and in which the resulting excess power is used to drive an alternator or similar rotary machine.

5. A method according to any one of Claims 2 to 4, in which at least some of the heat required for vaporising and/or superheating crude methanol is provided by hot reformed gases exiting the reformer tubes.

6. A method according to any one of Claims 2 to 5, which includes the further step of raising steam by heat exchange with hot gases selected from the hot reformed gases and the combustion gases in the heat recovery section of the reformer furnace.

7. A method according to any one of Claims 2 to 6, in which steam is mixed with the crude methanol vapour prior to passage through the first turbine means.

8. A method according to Claim 7, in which steam for admixture with the crude methanol vapour is raised by heat exchange with hot gases selected from hot reformed gases and hot combustion gases in the heat recovery section of the reformer furnace.

9. A method according to Claim 7 or Claim 8, in which steam in the vapour mixture exiting the first turbine means is condensed in the methanol purification section.

10. A methanol synthesis plant for producing methanol from a synthesis gas resulting from steam reforming of a hydrocarbon feedstock comprising means for producing synthesis gas, a methanol synthesis section for synthesising methanol at an elevated methanol synthesis pressure, means for vaporising crude methanol from the methanol synthesis section at elevated pressure, superheater means for superheating resulting crude methanol vapour, and a turbine means coupled to a rotary machine and arranged to be driven by resulting superheated crude methanol vapour.

11. A methanol synthesis plant comprising a reformer furnace having a combustion section to which a fuel may be fed, a combustion air supply conduit connected to the combustion section for supply of the air required for combustion of the fuel, a plurality of reformer tubes positioned in the combustion section for passage of a steam/hydrocarbon feedstock mixture to be reformed at a steam reforming pressure greater than atmospheric pressure, and a convection section for recovery of waste heat from the combustion gases from the combustion section, a methanol synthesis section for synthesising methanol at a methanol synthesis pressure greater than the steam reforming pressure from synthesis gas resulting from steam reforming in the reformer tubes of the reformer furnace, means for vaporising crude methanol from the methanol synthesis section at elevated pressure, superheater means for superheating resulting crude methanol vapour, a first turbine means arranged to be driven by the expansion of resulting superheated crude methanol vapour, a methanol purification section for purifying crude methanol discharged from the first turbine means, air compressor means for compressing air, heat exchanger means in the convection section in the path of products of combustion from the combustion section of the reformer furnace through which air from the air compressor means may be passed, second turbine means coupled to the air compressor means and arranged to be driven by heated compressed air from the heat exchanger means and to discharge resulting expanded air into the combustion air supply conduit, and synthesis gas compressor means for compressing synthesis gas resulting from the steam reforming of the hyd-' rocarbon feedstock to the methanol synthesis pressure in at least two stages, each stage being coupled to a corresponding one of the first and second turbine means.

12. A plant according to Claim 11,furtherinclud- ing means for recovering heat from the crude methanol vapour passing out from the first turbine means by indirect heat exchange with at least one of crude methanol vapour, boiler feed water and process feedstock.

13. A plant according to Claim 11 or Claim 12, in which the power output from at least one of the first and second turbine means exceeds the power required to drive the compressor means associated therewith and in which the resulting excess power is used to drive an alternator or similar rotary machine.

14. A plant according to any one of Claims 11 to 13, in which at least one of the means for vaporising crude methanol and the superheater means com prises a heat exchanger means arranged to impart heat to the crude methanol by indirect heat exchange with hot reformed gases exiting the reformer tubes.

15. A plant according to any one of Claims 11 to 14, further including a heat exchanger means for raising steam by heat exchange with hot gases selected from the hot reformed gases and the combustion gases in the heat recovery section of the reformer furnace.

16. A plant according to any one of Claims 11 to 15, including means for mixing steam with the crude methanol vapour prior to passage through the first turbine means.

17. A plant according to Claim 16, further including steam raising means for raising steam for admixture with the crude methanol vapour by heat exchange with hot gases selected from hot reformed gases and hot combustion gases in the heat recovery section of the reformer furnace.

18. A plant according to Claim 16 or Claim 17, in which means are provided for recovering from the methanol purification section aqueous condensate resulting from condensation in the methanol purification section of the admixed steam.

19. A method of producing methanol conducted substantially as herein described with particular reference to Figure 1 orto Figure 1 as modified by any one of Figures 2 to 4.

20. A method of producing methanol conducted substantially as herein described and exemplified.

21. A methanol synthesis plant constructed and arranged substantially as herein described with particular reference to Figure 1 orto Figure 1 as modified by any one of Figures 2 to 4.

22. A methanol synthesis plant constructed and arranged substantially as herein described and exemplified.

23. Methanol whenever prepared by a method according to any one of Claims 1 to 9, 19 and 20 or in a plant according to any one of Claims 10 to 18, 21 and 22.