GB2075125A - Method of driving a rotary machine - Google Patents

Abstract

Air in excess of the combustion air required by a furnace, e.g. a reformer furnace 1, is compressed by at least one compressor means 32 and is then heated with hot gaseous combustion products from the combustion section of the furnace 1 so as form first and second mass flows 38, 72 of heated compressed air. The first mass air flow 38, which is at a higher temperature than the second air flow 72, provides the combustion air via line 6 for the furnace 1 and is expanded through first turbine means 44 coupled to the or a first one of the compressor means 32. The second mass air flow 72 is superheated in chamber 73 by burning supplementary fuel in it, before being expanded through a second turbine means 68, 65 coupled to a machine to be driven, e.g. a synthesis gas compressor 51, 55, 62, and to the 32 or a second one of the compressor means 67. <IMAGE>

Description

SPECIFICATION Method and apparatus for driving a rotary machine This invention relates to a method and apparatus for driving a rotary machine, for example an electricity generator or a synthesis gas compressor.

Fossil fuel fired furnaces are used for numerous purposes in industry, for example for raising steam for power generation or for providing heat to chemical processes. In such furnaces the hot gaseous combustion products are used to heat, for example, boiler tubes or catalyst-filled tubes so as to heat the contents thereof by indirect heat exchange. In order to maximise efficiency of fuel utilisation care is taken to recover as much heat as possible from the hot gaseous combustion products and numerous proposals have been described with a view to recovering residual waste heat from the furnace exhaust gases. Commonly adopted expedients include use of such waste heat to pre-heat boiler feed water or incoming reactants and the like.In many chemical processes waste heat is used to ger,era3e steam which can be used to power turbines for generation of electricity or compression of gases or to provide heating in other parts of the plant. Typical of chemical plants which incorporate a furnace are synthesis gas plants employing steam reforming of a hydrocarbon feedstock.

Synthesis gas (i.e. a gas contairing a mixture of carbon oxide(s) and r,ydsogen in variable proportions} is produced in large quantities commercially as a feedstock for subsequent synthetic reactions. Methanol is synthesised catalytically in large quantities, for example, from an approximately 2:1 hydrogen: carbon oxide(s) synthesis gas mixture. An ap proximately 1:1 hydrogen:carbon monoxide synthesis gas mixture is used in hydroformylation. Synthesis gas is also a starting material for producing the 3:1 hydrogen:nitrogen mixture used in ammonia synthesis. Synthesis gas is also used, for example, in the production of synthetic petrols from coal.

One main route to synthesis gas 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 lirnits. This preheating takes place in one or more heat exchangers mounted in a convection section of the reformer furnace.Normally a steamdriven 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 stream feed for the reforming process. Such high pressure turbines and high pressure steam-raising plant are expensive to instal and maintain.

As with boiler furnaces 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 Gjy improving the recovery of waste heat from the furnace exhaust gases.

The present invention seeks to provide an improved method of driving a rotary machine.

It also seeks to provide an improved apparatus for drivi',g a rotary machine. The invention also seeks to provide an improved design of synthesis gas plant. It also seeks to provide an improved method of producing a synthesis gas. It further seeks to provide a synthesis gas 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 synthesis gas which requires less fuel input than a conventional synthesis gas plant.Yet again, the invention seeks to provide a synthesis gas plant that can be erected at a lower capital cost and on a smaller site than a conventional synthesis gas plant.

According to one aspect of the present invention there is provided a method of driving a rotary machine comprising burning a fuel in combustion air in a combustion section of a furnace, compressing air in at least one compressor means in excess of the combustion air required by the furnace, heating resulting compressed air by heat exchange with hot gaseous combustion products from the combustion section of the furnace so as to form first and second mass flows of heated compressed air, the first mass flow being at a first elevated temperature and comprising substantially the mass air flow required as combustion air, and the second mass flow being at a second elevated temperature less than the first elevated temperature and comprising substantially the mass air flow in excess of the mass air flow required as combustion air, expanding the first mass flow of heated compressed air through first turbine means coupled to the or to a first one of the compressor means, feeding expanded air from the first turbine means as combustion air to the furnace, burning supplementary fuel in the second mass flow of heated compressed air thereby to superheat it, and expanding resulting superheated mass flow through a second turbine means coupled to the machine to be driven and to the or to a second one of the compressor means.

In accordance with another aspect of the invention apparatus for driving a rotary machine comprises a furnace having a combustion section to which a fuel may be fed, and a combustion air supply conduit connected to the combustion section for supply of the air required for combustion of the fuel, at least one compressor means for compressing air in excess of the combustion air required by the furnace, heat exchanger means in the path of hot gaseous combustion products from the combustion-section of the furnace through which compressed air from said at least one compressor means may be passed, said heat exchanger means being arranged to deliver first and second mass flows of heated compressed air, the first mass flow being at a first elevated temperature and comprising substantially the mass air flow required as combustion air by the furnace, and the second mass flow being at a second elevated temperature less than the first elevated temperature and comprising substantially the mass air flow in excess of the mass air flow required as combustion air, first turbine means coupled to the or to a first one of the compressor means, the inlet of the first turbine means being arranged for receipt of the first mass flow of heated compressed air and the outlet of the first turbine means being connected to the combustion air supply conduit, supplementary burner means for burning supplementary fuel in the second mass flow thereby to superheat it, second turbine means coupled to the machine to be driven and to the or to a second one of the compressor means, and conduit means for supplying compressed air which has been superheated by the supplementary burner means to the inlet of the second turbine means.

The invention can thus provide for recovery of waste heat from the hot gaseous combustion products of a furnace by indirect heat exchange with compressed air passing through one or more heat exchanger means, a first mass flow of the resulting hot compressed air being used to power first turbine means coupled to air compressor means for generating the compressed air and the pass out air from this first turbine means being used as combustion air for the furnace, whilst a second mass flow of hot compressed air from the heat exchanger means is superheated by firing supplementary fuel therein and is used to power second turbine means coupled to the synthesis gas compressor, alternator or other rotary machine to be driven.

Moreover this second turbine means can also be coupled to the same air compressor means as the first turbine means or to a separate air compressor means. Because the inlet temperatures to the turbines means can be high, for example in excess of about 1 500 F (i.e. over about 81 5 C), low pressure gas turbine sets can be used operating, for example, at inlet pressures of about 60 p.s.i.a. (i.e. about 4.22 kg/cm2) and outlet pressures of about 1 5 p.s.i.a. (i.e. about 1.05 kg/cm2). The hot pass out air from the first turbine means by being used as combustion air reduces the fuel requirement of the furnace since it may be at a temperature of at least about 1000"F (i.e. at least about 538"C), for example.The still hot pass out gas from the second turbine means which may be at a similarly high temperature to that from the first turbine means can be returned to the waste heat recovery section of the furnace and used, for example, for preheating boiler feed water and/or process feedstock.

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 first and/or second compressor 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 compressor stages in series or a plurality of such multi-stage machines in parallel.

In the practice of the invention conventional gas turbine sets can be used. Thus it is preferred to use two such sets, each with its own compressor and turbine, but with the two compressors operating in parallel, and to pass the combined air flows from the compressors of the two sets through the heat exchanger means. Alternatively it is possible to use a single gas turbine set having a single compressor coupled to both the first and second turbines.

The heat exchanger means may comprise a single heat exchanger positioned in a waste heat recovery section of the furnace; in this case the second mass flow may be taken from the heat exchanger in the form of a bleed stream from a point intermediate its ends.

Alternatively the heat exchanger means may comprise two or more heat exchangers in series, with the second mass flow being taken as a bleed stream, for example, from a point intermediate successive heat exchangers. Although it will usually be convenient to withdraw the second mass flow from the heat exchanger means as a single bleed stream, it is of course possible to withdraw it as two or more bleed streams from different points.

Alternatively the heat exchanger means may comprise two heat exchangers, or two sets of heat exchangers, in parallel, one for heating the first mass flow and one for heating the second mass flow respectively.

The division of the compressed air into the first and second mass flows may thus take place after at least some heating of the compressed air by heat exchange with the hot gaseous combustion products has taken place or before heating. Hence a preferred method of driving a rotary machine in accordance with the invention includes the steps of supplying a mass flow of compressed air from the at least one compressor means to an inlet end of heat exchanger means positioned in the path of hot gaseous combustion products from the combustion section of the furnace, withdrawing the first mass flow of heated compressed air from an outlet end of the heat exchanger means, and withdrawing the second mass flow of heated compressed air intermediate the inlet and outlet ends of the heat exchanger means.

An alternative preferred method includes the steps of heating compressed air from the at least one compressor means in separate mass flows by heat exchange with the hot gaseous combustion products to provide the first and second mass flows of heated compressed air.

Although in many cases it will be convenient to heat the first and second mass flows by passage as a single stream or streams through one or more heat exchangers in series, it may in some cases be preferred to heat the mass flows as two or more parallel streams by passage through a plurality of heat exchangers or sets of heat exchangers in parallei.

In a plant employing two gas turbine sets with their compressors arranged in parallel the first turbine may be required to yield little or no external power; for example it may be required merely to drive the exhaust draught fan for drawing the waste gases from the furnace to the furnace stack. This means that the power developed by the first turbine is considerably in excess of that which would normally be required by its connected compressor. This surplus power can be used to flow more air through its associated compressor than is required by the first turbine. This surplus air flow can thus be used to supplement the output air flow from the compressor associated with the second turbine, the sum of this surplus air flow and the output air flow from the second turbine providing the second mass flow which is superheated and passed through the second turbine.In this way there may be provided a condition in which the compressor of the second gas turbine set demands less power from its connected turbine thereby making more power available for driving the synthesis gas compressor, alternator or other rotary machine.

In a steam reforming plant for the production of a synthesis gas, there can be used as hydrocarbon feedstock any feedstock that can hydrocarbon feedstock any feedstock that can be subjected to steam reforming. Such feedstocks include for example natural gas, ethane, propane, butane, liquefied petroleum gases, light naphtas, 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 feedstock the reactions are: CH4 + H20oCO + 3H2 (1) CO + H20CO2 + H2 (2) 2CO oCO2 + C (3) 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 temererature 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/hydrocarbon feedstock ratio is preferably chosen so as to minimise carbon deposition in the reformer tubes. Typically the steam/hydrncarbon feedstock is such as to represent a steam/carbon atom ratio of at least about 2:1 up to about 5:1 or higher, e.g. about 4:1.

The catalyst may be any conventional steam reforming catalyst. The conditions selected in the reformer tubes are those conven.ionally adopted for steam reforming. Typically the temperature ranges from about 700 C to about 1000"C, preferably about 750"C to 900'C, 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 1 5 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. Since the temperature of the reformed gases exiting the reformer tubes generally greatly exceeds the temperature required for subsequent processing (except when secondary reforming is to be carried out), 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 may be at a lower pressure than is required for subsequent processing, e.g.

methanol synthesis, it may thereafter be necessary to compress it.

According to a preferred aspect of the invention there is provided a method of producing a synthesis gas at elevated pressure greater than atmospheric pressure by steam reforming of a hydrocarbon feedstock which comprises burning a fue! in combustion air in a combustion section of a reformer furnace to heat a plurality of reformer tubes through which a steam/hydrocarbon feedstock mixture is passed at a steam reforming pressure greater than atmospheric pressure but less than the elevated pressure, compressing air in excess of the combustion air required by the furnace in at least one compressor means, heating resulting compressed air by heat exchange with the hot gaseous combustion products from the cornbustion section of the reformer furnace so as to form first and second mass flows of heated compressed air, the first mass flow being at a first elevated temperature and comprising substantially the mass air flow required as combustion air, and the second mass flow being at a second elevated temperature less than the first elevated temperature and comprising substantially the mass air flow in excess of the mass air flow required as combustion air, expanding the first mass flow of heated compressed air through first turbine means coupled to the or to a first one of the compressor means, feeding expanded heated compressed air from the first turbine means as combustion air to the combustion section of the furnace, burning supplementary fuel in the second mass flow of heated compressed air thereby to superheat it, and expanding resulting superheated mass flow through a second turbine means coupled to the or to a second one of the compressor means and to a synthesis gas compressor means thereby to compress synthesis gas from a pressure intermediate atmospheric pressure and the elevated pressure to the elevated pressure.

According to a further preferred aspect of the invention a synthesis gas plant for proddc- ing synthesis gas at an elevated pressure greater than atmospheric pressure by steam reforming of a hydrocarbon feedstock comprises a reformer furnace having a combustion section to which a fuel may be fed, a combustion air supply conduit connected to the com- bustion 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 but less than the elevated pressure, and a convection section for recovery of waste heat from the combustion gases from the combustion section, at least one compressor means for compressing air in excess of the combustion air required by the furnace, heat exchanger means in the convection section in the path of combustion gases from the combustion section of the furnace through which air from said at least one compressor means may be passed, said heat exchanger means being arranged to deliver first and second mass flows of heated compressed air, the first mass flow being at a first elevated temperature and comprising substantially the mass air flow required as combustion air by the furnace, and the second mass flow being at a second elevated temperature less than the first elevated temperature and comprising substantially the mass air flow in excess of the mass air flow required as combustion air, first turbine means coupled to the or to a first one of the compressor means, the inlet of the first turbine means being arranged for receipt of the first mass flow of heated compressed air and the outlet of the first turbine means being connected to the combustion air supply conduit, supplementary burner means for burning supplementary fuel in the second mass flow thereby to superheat it, synthesis gas compressor means for compressing synthesis gas from a pressure intermediate atmospheric pressure and the elevated pressure to the elevated pressure, second turbine means coupled to the synthesis gas compressor means and to the or to a second one of the compressor means, and conduit means for supplying compressed air which has been superheated by the supplementary burner means to the inlet of the second turbine means.

In one form of plant the second turbine means is coupled also to a second air compressor means, compressed air from which is also passed through the heat exchanger means. In this case there may be two gas turbine sets.

In another form of plant the first and second turbine means are coupled to a common air compressor means and form a single gas turbine set therewith.

In yet another embodiment the second turbine means comprises two gas turbines in parallel and the superheated mass flow from the supplementary burner means is divided, one part being used to drive a gas turbine coupled to the synthesis gas compressor means whilst the other part is used to drive a gas turbine coupled to an air compressor, the compressed air from which is fed to the heat exchanger means.

The mass flow through the second turbine means may be augmented, if desired, by injecting steam into the second mass flow prior to, simultaneously with, or subsequent to the superheating thereof. Such steam can be raised by indirect heat exchange with the combustion gases of the furnace.

The synthesis gas compressor means may comprise a single compressor stage or two or more compressor stages, possibly with intercooling between compressor stages.

The intermediate pressure of the synthesis gas corresponds approximately to the pressure prevailing in the reformer tubes, assuming that there is no negligible pressure drop downstream therefrom. Hence it preferably ranges from about 5 kg/cm2 up to about 30 kg/cm2. The elevated pressure depends on the end use of the synthesis gas. When, for example, the synthesis gas is intended for methanol synthesis, the elevated pressure may be in the range of from about 300 kg/cm2 up to about 400 kg/cm2 or more.

The composition of the synthesis gas may depend on the intended use thereof. A synthesis gas for hydroformylation contains an approximately 1:1 H2:CO mixture (e.g. 1.05:1 H2:CO), whereas that for methanol synthesis is typically about 2:1 H2:CO (e.g. 2.25:1 H2:CO). The exact composition of the synthesis gas can be varied in known manner by varying the conditions in the reformer tubes.

In order that the invention may be clearly understood and readily carried into effect a preferred form of synthesis gas 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 synthesis gas plant constructed in accordance with the invention; and Figures 2 to 9 are details of the flow sheets of modified forms of the plant of Fig. 1.

It will be appreciated by those skilled in the art that the drawings are diagrammatic and that various items of equipment wnich would be included in practice in an operational plant, such as valves, temperature measurement devices, pressure control devices, and the like, have been omitted for the sake of clarity.

Such standard items of equipment would be provided in accordance with conventional chemical engineering practice and form no part of the present invention.

Referring to Fig. 1 of the drawings, a synthesis gas plant for a methanol synthesis plant comprises a reformer furnace 1 having a reforming section 2 and a convection section 3. A plurality of reformer tubes 4, of which two only are shown in Fig. 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 desulphurizing agent. The desulphurized feedstock passes on via line 11 and is mixed with steam supplied via line 1 2. Typically the resulting steam/hydrocarbon feedstock mixture has a steam:carbon atom ratio of approximately 4:1. This mixture passes via line 1 3 to a heat exchanger 14 in which it is heated to inlet temperature (typically about 850"C) by the reformed gases exiting reformer tubes 4.From heat exchanger 14 line 1 5 conducts the heated mixture 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.

The hot reformed gases are led by way of line 1 6 to heat exchanger 14, in which they heat the incoming steam/hydrocarbon feedstock mixture, and pass on through line 1 7 to heat exchangers 18, 1 9 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 1 8 the hot reformed gases raise steam by heat exchange with water in line 26 connected to steam drum 27, the resulting steam being fed back to steam drum 27 by way of line 28. In heat exchanger 1 9 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 circulating hot water in line 30. Cooler condenser 21 is supplied with cooling water by means of line 31.

As can be seen from Fig. 1 steam for line 1 2 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 lines 35 and 36 to a heat exchanger 37 mounted in the convection section 3 of reformer furnace 1. Hot compressed air is withdrawn from heat exchanger 37 through line 38 and passes on to the inlet end of turbine 44 of gas turbine set 33. The outlet end of turbine 44 is connected to combustion air supply conduit 6.

As can be seen from Fig. 1 compressor 32 and turbine 44 are mounted on a common shaft 45, together with an induced draught fan 46 which serves to impel exhaust gases via lines 47 and 48 from convection section 3 of reformer furnace 1 to a stack 49.

Reference numeral 50 indicates a starter motor for gas turbine set 33.

Synthesis gas in line 24 is compressed in primary stage compressor 51 to an intermediate pressure and is fed through line 52 to a cooling stage 53 and thence via line 54 to the inlet end of secondary compressor 55 in which it is further compressed to methanol synthesis pressure. The gas from the exit end of compressor 55 passes on through line 56 to a further cooling stage 57 and thence via line 58 to form the make up gas for methanol synthesis, being mixed with the recirculating gas in line 59 from methanol synthesis plant 60 (which is of conventional design and includes suitable gas purification and methanol synthesis and recovery stages). The mixed gases are fed through line 61 to recycle compressor 62 and then to plant 60 via line 63.

Primary and secondary compressors 51 and 55 and recycle compressor 62 are mounted on a common shaft 64 and are driven by a second turbine stage 65 of a second gas turbine set 66. This is a two-shaft gas turbine set and comprises a compressor 67 and a first turbine stage 68, both mounted on a shaft 69, in addition to second turbine stage 65.

Compressor 67 draws in air, as indicated by arrows 70, into its inlet. Compressed air passes from its outlet via line 71 and 36 to heat exchanger 37. A stream of hot air is withdrawn from heat exchanger 37 via line 72 and is passed to a combustion chamber 73 to which fuel is also fed through line 74.

The resulting hot combustion gases are fed by way of line 75 to the inlet end of first turbine stage 68, which drives compressor 67, and thence directly to second turbine stage 65.

Since first turbine stage 68 and second turbine stage 65 are on different shafts these can operate at different speeds. From the outlet end of second turbine stage 65 the expanded gases flow through line 76 to convection section 3 of reformer furnace 1 in which such gases heat the incoming hydrocarbon feedstock in heat exchanger 8.

Reference numeral 77 indicates an alternative exhaust gas line from convection section 3.

At start up motor 50 is used to run up gas turbine set 33 and hence to commence forcing air through heat exchanger 37. At the same time fuel is supplied via lines 5 and 74 and ignited by suitable ignition means (not shown). As the temperature rises in convection section 3 the air temperature in line 38 will correspondingly rise until motor 50 can be cut out. An auxiliary heater (not shown) is used to raise steam in steam drum 27 and once the reformer tubes 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 Fig. 1 differs from a conventionally designed plant in that the forced draught fan for the reformer furnace combustion air and its associated driver, the induced draught fan for the exhaust gases from the convection section of the reformer furnace and its associated driver, and the air heater used for heat ing the combustion air, 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 37. 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 ex haust gases from the convection section of the furnace, the plant of Fig. 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 replaced by a second gas turbine set, again with a simplification of the plant since no steam condensers or condensate pumps are needed for this. Moreover since there are no high pressure steam turbines, there is no need to generate high pressure steam, e.g. at pres sures in the region of 100 kg/cm2; instead there is only a need for steam at process pressure for the steam reformer, e.g. a pres sure in the region of 30 kg/cm2, 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 conven tional 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 Fig. 1 will be reduced compared with those of a conventional plant, due to the simplification of the plant and to the reduction in steam pres sure. An additional benefit of the use of gas turbine 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 plants using high pressure steam turbines.

The plant of Fig. 2 is identical to that of Fig. Fig. 1 except that line 1 2 from steam drum 27 leads to a superheater 78 in convection section 3.

In the plant of Fig. 3 additional steam is raised in boiler 80 which is connected to steam drum 27 by lines 81 and 82. The steam in excess of that required in the steam reforming reaction is passed through line 83 to join line 72. The combustion gases from combustion chamber 73 are supplemented by this excess steam and increase the throughput of gas through turbine stages 68, 65, hence providing additional power, beyond that re quired to drive the synthesis gas compressors 51, 55 and 62. This additional power is used to drive alternator 84 and hence to generate electricity for export beyond battery limits or for use within the plant.

The plant of Fig. 4 has a similar arrange ment to that of Fig. 3 for raising steam. In this case, however, the excess steam is passed via line 85 to a steam turbine 86 which drives an alternator 87. Steam exiting turbine 86 is condensed in condenser 88 which is fed with cooling water via line 89.

In the plant of Fig. 5 the two gas turbine sets of the embodiments of Figs. 1 to 4 are replaced by a single gas turbine set. This has an air compressor 32 whose capacity is the sum of the capacities of the two air compressors 32 and 67 of the plants of Figs. 1 to 4.

There is thus a single feed line 35, 36 to heat exchanger 37. Turbine 44 and turbine stages 68 and 65 form part of the single gas turbine set which is of the two shaft type, turbine 44 and first turbine stage 68 being mounted on the same shaft as compressor 32 and on a different shaft from second turbine stage 65.

Fig. 6 shows yet another modification of the plant of Fig. 1. In this embodiment the flow of exhaust gases from combustion chamber 73 in line 75 is split into two, these two flows passing along lines 90 and 91 respectively to turbines 92 and 93 (which replace turbine stages 68 and 65 of the plant of Fig.

1). The expanded gases from turbines 92 and 93 pass along lines 94 and 95 respectively which lead to line 76.

In the embodiment of Fig. 6 not all of the gas flow passes through the synthesis gas compressor turbine. Both turbines 92 and 93 are supplied with hot gas at the same inlet temperature and pressure and can be relatively small machines. Thus the gas turbine set 96 (consisting of compressor 67 and turbine 92) is smaller than gas turbine set 66 of Fig. 1, for example.

If desired, the plant of Fig. 6 can be further modified to incorporate also the modification of Fig. 4. In this case steam is supplied through line 83 (shown in dotted lines in Fig.

6) and an alternator 97 (also shown in dotted lines in Fig. 6) can be driven also by turbine 92 to provide electricity for export beyond battery limits or for use within the plant.

In another modification (not iliustrated) of the plant of Fig. 1 high pressure steam is generated in steam drum 27 by heat exchange with the hot reformed gas and this steam, which may be for example, at a pressure in the region of 100 kg/cm2 or more, is passed through a noncondensing steam turbine to expand the steam and reduce its pressure to process pressure (e.g. about 30 kg/cm2). This non-condensing steam turbine can be used, for example, to drive an electrical alternator whose output is exported beyond battery limits or partially used within the plant.

In the plant of Fig. 7 compressor 32 feeds compressed air via line 35. The air required for combustion flows on via line 101 to heat exchanger 102 positioned in the convection section 3 in the path of the hot gaseous combustion products, whilst air in excess of the combustion air requirements of the furnace flow through line 103 to another heat exchanger 104 positioned in the convection section 3 downstream from heat exchanger 102 in the path of hot gaseous combustion products. The exit end of heat exchanger 102 is connected to line 38, whilst the exit end of heat exchanger is connected to line 105. The air in line 38 is at a higher temperature than the air in line 105.

Air is also compressed in compressor 67 of the second gas turbine set 66; the major part of the compressed air flows on through line 71 to a third heat exchanger 106 positioned in the convection section 3 of the furnace downstream from heat exchanger 102 in the path of gaseous combustion products, whilst a minor part flows along a bypass duct 107 to the turbine 108 of the second gas turbine set 66. (As shown in Fig. 7, turbine 108 is a single stage turbine; however, it can have two stages like stages 65 and 68 of Fig. 1). The exit end of heat exchanger 106 is connected to line 109, and the air flows in lines 105 and 109 are combined to flow on via line 72 to the combustion chamber 73.

In the plant of Fig. 8 the stream of compressed air in line 35 can be split between lines 11 O and 111, the proportions flowing through the two lines being controlled by means of valve 11 2. A major part of the compressed air from line 35 flows via line 110, in normal operation of the plant, through heat exchangers 11 3 and 114 arranged in series in the convection section 3 of the furnace. The exit end of heat exchanger 114 is connected to line 38. A minor part of the compressed air flow from line 35 is diverted via line 111 and is combined with the air flow in line 71 to flow on via line 11 5 through heat exchanger 11 6 and thence to line 72.

As in the plant of Fig. 1, the stream of hydrocarbon feedstock in line 7 is preheated in heat exchanger 8 and desulphurized in desulphurization vessel 10, prior to mixing with steam in appropriate ratio supplied via line 1 2. However, instead of being further heated by indirect heat exchange with the hot reformed gases, as occurs in the plant of Fig.

1, the steam/hydrocarbon feedstock mixture is heated to inlet temperature by heat exchange with the expanded gases in line 76 in heat exchanger 11 7. The turbine exhaust gases are exhausted to the stack 49 by way of line 118.

At start up of the plant of Fig. 8 valve 11 2 may be closed whilst the first gas turbine set 33 is run up to operating speed. Second gas turbine set 66 can be run up to operating speed independently and valve 11 2 adjusted to provide the desired ratio of compressed air flows in lines 110 and 111.

In the plant of Fig. 9 gas turbine set 33 is replaced by two gas turbine sets (not shown), the compressed air streams from their com pressors being combined and fed via iine 35 and the pass out air from their turbine being combined and fed via line 6. The outlet stream from heat exchanger 11 3 is divided and passed to two heat exchangers 119, 1 20 arranged in parallel, the outlet streams from which are passed to the inlets of the two gas turbine sets (i.e. the two sets that replace set 33) via lines 121, 122 respectively. Other- wise the plant of Fig. 9 is the same as that of Fig. 8.

Tne invention is further illustrated in the following Example.

Example A plant of the type illustrated in Fig. 1 is constructed, the output of the methanol synthesis plant 60 amounting to 1 500 short tons per day (1360 tonnes per day). The combustion air required of the reformer furnace is 500,000 Ibs per hour (226795 kg per hour) and is delivered at 1 5 psia (1.05 kg/cm2 absolute) and 1090 F (588 C;.

The design of heat exchanger 37 is such that there is a pressure drop across it of 1 2.5 psi (0.88 kg/cm2). Air compressor 32 has a compression ratio of 5:1, thus delivering air at an outlet pressure of 72.5 psia (5.10 kg/cm2 absolute) and an outlet temperature of 435"F (223.9"C) when the ambient air pressure is 14.5 psia (1.02kg/cm2 absolute) and the ambient air temperature is 60 F (15.6to). The inlet pressure to the turbine 44 is 60 psia (4.22 kg/cm2 absolute) and the outlet pressure 1 5 psia (1.05 kg/cm2 absolute).In passing through heat exchanger 37 the compressed air is heated to 1600"F (871"C). The power output of turbine 44 is thus 2711 2 HP (20225 kW). Approximately 1000 HP (746 kW) are required to drive fan 46 so that 26112 HP (19479 kW) are available to drive compressor 32. At an ambient air temperature of 60"F (15.6"C) this means that compressor 32 handles 731,400 Ibs per hour (51417 kg per hour) of air, i.e. an excess of 231,400 Ibs per hour (16267 kg per hour) over the combustion air requirement for the reformer furnace 1.

Air compressor 67 takes in air at 60"F (15.6"C) and a pressure of 14.5 psia (1.01 kg/cm2 absolute) at a rate of 252245 Ibs per hour (1 7733 kg per hour) and delivers this via line 71 at 72.5 psia (5.1 kg/cm2 absolute) and 435"F (223.9'C). 483654 Ibs per hour (34000 kg per hour) of air at 67.5 psia (4.75 kg/cm2 absolute) and 1000"F (537.8'C) are withdrawn from heat exchanger 37 via line 72 and are passed to combustion chamber 73.Sufficient fuel is supplied through line 74 to produce 78 x 106 BTU/hr (22680 kW) of, combustion heat which superheats the gas in line 75 to 1600"F (871 C). This superheated gas stream is expanded in passing through turbine stages 68 and 65 to 1 5 psia (1.05 kg/cm2 abso lute), its temperature dropping to 1057"F (569.4 C) in so doing. The combined power output of turbine stages 68 and 65 is 28021 HP (20904 kW), of which 9005 HP (6718 kW) are required to power air compressor 67.

This leaves 19016 HP (14186 kW) for driving the synthesis gas compressors 51 and 55 and the recycle gas compressor 62, a power output that is adequate for this capacity of methanol plant 60.

Compared with a conventional design of synthesis gas plant, the total fuel requirement for the reformer furnace 1 (as represented by the fuel supplied by lines 5 and 74) is reduced for the plant of Fig. 1.

Although the illustrated forms of plant are all designed to supply synthesis gas to a methanol synthesis plant, it will be apparent to the skilled reader that the teachings of the invention are applicable to other forms of synthesis gas plant relying on steam reforming of a hydrocarbon feedstock, for example plants for supplying synthesis gas to hydroformlyation plants as well as plants for supplying synthesis gas to ammonia plants. In these latter types of plant item 60 will be a hydroformylation plant or an ammonia synthesis plant, as the case may be and further process and/or purification stages are interposed as necessary between knock out pot 22 and compressor 51. Such further process stages may include, for example in the case of an ammonia synthesis plant, secondary reforming, water gas shift, CO2 removal and methanation stages. In addition it will be apparent that the teachings of the invention are applicable to other situations such as power stations.

In this case, for example, the furnace is used primarily to raise steam in boiler tubes rather than to heat reformer tubes and an electrical generation takes the place of the illustrated synthesis gas compressors.

Claims (23)

1. A method of driving a rotary machine comprising burning a fuel in combustion air in a combustion section of a furnace, compressing air in at least one compressor means in excess of the combustion air required by the furnace, heating resulting compressed air by heat exchange with hot gaseous combustion products from the combustion section of the furnace so as to form first and second mass flows of heated compressed air, the first mass flow being at a first elevated temperature and comprising substantially the mass air flow required as combustion air, and the second mass flow being at a second elevated temperature less than the first elevated temperature and comprising substantially the mass air flow in excess of the mass air flow required as combustion air, expanding the first mass flow of heated compressed air through first turbine means coupled to the or to a first one of the compressor means, feeding expanded air from the first turbine means as combustion air to the furnace, burning supplementary fuel in the second mass flow of heated compressed air thereby to superheat it, and expanding resulting superheated mass flow through a second turbine means coupled to the machine to be driven and to the or to a second one of the compressor means.

2. A method according to claim 1, in which the at least one compressor means comprises first and second compressor means coupled to the first turbine means and to the second turbine means respectively.

3. A method according to claim 1, in which the at least one compressor means comprises a single compressor means coupled both to the first turbine means and to the second turbine means.

4. A method according to any one of claims 1 to 3 which includes the steps of supplying a mass flow of compressed air from the at least one compressor means to an inlet end of heat exchanger means positioned in the path of hot gaseous combustion products from the combustion section of the furnace, withdrawing the first mass flow of heated compressed air from an outlet end of the heat exchanger means, and withdrawing the second mass flow of heated compressed air intermediate the inlet and outlet ends of the heat exchanger means.

5. A method according to any one of claims 1 to 3 which inciudes the steps of heating compressed air from the at least one compressor means in separate mass flows by heat exchange with the hot gaseous combustion products to provide the first and second mass flows of heated compressed air.

6. A method according to any one of claims 1 to 5 in which the mass flow through the second turbine means is augmented by injecting steam into the second mass flow prior to, simultaneously with, or subsequent to the superheating thereof.

7. A method of producing a synthesis gas at elevated pressure greater than atmospheric pressure by 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 through which a steam/hydrocarbon feedstock mixture is passed at a steam reforming pressure greater than atmospheric pressure but less than the elevated pressure, compressing air in excess of the combustion air required by the furnace in at least one compressor means, heating resulting compressed air by heat exchange with the hot gaseous combustion products from the combustion section of the reformer furnace so as to form first and second mass flows of heated compressed air, the first mass flow being at a first elevated temperature and comprising substantially the mass air flow required as combustion air, and the second mass flow being at a second elevated temperature less than the first elevated temperature and comprising substantially the mass air flow in excess of the mass air flow required as combustion air, expanding the first mass flow of heated compressed air through first turbine means coupled to the or to a first one of the compressor means, feeding expanded heated compressed air from the first turbine means as combustion air to the combustion section of the furnace, burning supplementary fuel in the second mass flow of heated compressed air thereby to superheat it, and expanding resulting superheated mass flow through a second turbine means coupled to the or to a second one of the compressor means and to a synthesis gas compressor means thereby to compress synthesis gas from a pressure intermediate atmospheric pressure and the elevated pressure to the elevated pressure.

8. Apparatus for driving a rotary machine comprising a furnace having a combustion section to which a fuel may be fed, and a combustion air supply conduit connected to the conbusion section for supply of the air required for combustion of the fuel, at least one compressor means for compressing air in excess of the combusion air required by the furnace, heat exchanger means in the path of hot gaseous combustrion products from the combustion section of the furnace through which compressed air from said at least one compressor means may be passed, said heat exchanger means being arranged to deliver first and second mass flows of heated compressed air, the first mass flow being at a first elevated temperature and comprising substantially the mass air flow required as combustion air by the furnace, and the second mass flow being at a second elevated temperature less than the first elevated temperature and comprising substantially the mass air flow in excess of the mass air flow required as combustion air, first turbine means coupled to the or to a first one of the compressor means, the inlet of the first turbine means being arranged for receipt of the first mass flow of heated compressed air and the outlet of the first turbine means being connected to the combustion air supply conduit, supplementary burner means for burning supplementary fuel in the second mass flow thereby to superheat it, second turbine means coupled to the machine to be driven and to the or to a second one of the compressor means, and conduit means for supplying compressed air which has been superheated by the supplementary burner means to the inlet of the second turbine means.

9. Apparatus according to claim 8, in which the at least one compressor means comprises first and second compressor means coupled to the first turbine means and to the second turbine means respectively.

10. Apparatus according- to claim 8, in which the at least one compressor means compresses a single compressor means coupled both to the first turbine means and be the second turbine means.

11. Apparatus according to any one of chaims 8 to 10, in which the heat exchanger means comprises a single heat exchanger positioned in a waste hear recovery section of the furnace, the seconc mass air flow being taken in the form of a bieed stream from the heat exchanger from a point intermediate its ends.

12. Apparatus according to any one of claims 8 to 10, in which the heat exchanger means comprises W or more heat exchange ers in series and in which the second mass flow is taken as a bleed stream fr/3m a point intermediate successive heat exchangers.

1 3. Apparatus according to any one of claims 8 to 10, in which the heat exchanger mass comprises two heat exchangers, or two sets of heat exchangers. in parailel. one for seating the firs mass ffflo-JY and one for heating the second mass rio-v respectively.

14. A synthesis gas plant for producing synthesis gas at an elevated pressure greater than atmospheric pressure by steam reforming of a hydrocarbon feedstock which comprises a reformer Furnace having a combustion section to which a fuel may be fed, a combustion air supply conduit connected so 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 refornied at 2 steam reforming pressure greater than atmospheric pressure but less than the elevated pressure, and a convection section for recovery of waste heat from the combustion gases from the combustion section, at least one compressor means for compressing air ::; excess of the combustion air required by the furnace, heat ex changer means in tile convection section in the path of combustion gases from the combustion section of the furnace through which air from said at least one compressor means may be passed, said heat exchanger means being arranged to deliver first and second mass flows of heated compressed air, the first mass flow being at a first elevated temperature and compnsing substantially the mass air flow required as combustion air by the furnace, and the second mass flow being at a second elevated temperature less than the first elevated temperature and compflsincj substantially the mass air flow in excess of the mass air flow required as combustion air, first turbine means coupled to the or to a first one of the compressor means, the inlet of the first turbine means being arranged for receipt of the first mass flow of heated compressed air and the outlet of the first turbine means being connected to the combustion air supply conduit, supplementary burner means for burning supplementary fuel in the second mass flow thereby to superheat it, synthesis gas compressor means for compressing synthesis gas from a pressure intermediate atmospheric pressure and the elevated pressure to the elevated pressure, second turbine means coupled to the synthesis gas compressor means and to the or to a second one of the compressor means, and conduit means for supplying compressed air which has been superheated by the supplementary burner means to the inlet of the second turbine means.

15. A synthesis gas plant according to claim 1 4, in which the second turbine means is coupled also to a second air compressor means, compressed air from which is also passed through the heat exchange means.

16. A synthesis gas plant according to claim 14, in which the first and second turbine means are coupled to a common air compressor means and form a single gas turbine set therewith.

17. A synthesis gas plant according to claim 14, in which the second turbine means comprises two gas turbines in parallel and the superheated mass flow from the supplementary burner means is divided, one part being used to drive a gas turbine coupled to the synthesis gas compressor means whilst the other par is used to drive a gas turbine coupled to an air compressor, the compressed air from which is fed to the heat exchanger means.

18. A method of driving a rotary machine conducted substantially as herein described with particular reference to any one of Figs. 1 to 6 of the accompanying drawings.

19. A method of driving a rotary machine conducted substantially as herein described with particular reference to any one of Figs. 7 to 9 of the accompanying drawings.

20. Apparatus for driving a rotary machine conducted and arranged substantially as herein described with particular reference to any cne cf Figs. 1 to 6 of the accompanying drawings.

21. Apparatus for driving a rotary machine constructed and arranged substantially as herein described with reference to any one of Figs. 7 to 9 of the accompanying drawings.

22. A synthesis gas plant constructed and arranged substantially as herein described with particular reference to any one of Figs. 1 to 6 of the accompanying drawings.

23. A synthesis gas plant constructed and arranged substantially as herein described with reference to any one of Figs. 7 to 9 of the accompanying drawings.