This invention relates to novel processes for the production of power which utilise/combine the operation of a turbine with a separate process which as a part of the feed to the turbine generates a pressurised gas stream which is not in itself capable of supporting combustion. There are a wide variety of processes which produce such gas streams but this invention finds particular application when that process is a nitric acid producing facility or a fluid catalytic cracking process. The process of this invention may utilise novel combinations of apparatus which may be incorporated into the design of new facilities incorporating both the turbine and separate processing facilities. Alternatively the process may be carried out by retro-fitting new components into existing plants.

Summary of the prior art

The production of power by burning a fuel and passing the hot gaseous products produced by that combustion to a turbine is an established technology. There have been proposals to integrate these power producing processes with other processes.

USP 3,798,898 describes a gas turbine prime mover which incorporates a chemical oxidation reactor. A portion of the compressed air which is required for the conventional operation of the turbine is used in the oxidation reaction and the gases produced by that oxidation are injected into the turbine.

British Patent Application 2097476 describes a procedure in which the operation of a turbine is combined with the disposal of a gaseous residue, e.g. from a gas well. The gaseous residue is used to cool and dilute the hot combustion gases before they enter the turbine. British Patent 1146292 describes a process for the production of nitric acid by the oxidation of ammonia In which the tail gases emerging from the absorber are heated by heat exchange with the hot gaseous product of the oxidation stage. International Patent Publication 89/01091 describes

processes in which the excess high temperature heat generated in an exothermic chemical reaction process is used to heat a pressurised gas stream comprising a quantity of gas which is additional to that required or produced by the process and the heated gas stream is passed to a turbine. In one embodiment of that disclosure it is stated that where the gas stream contains oxygen, a fuel may be burnt in the stream in order to increase its temperature. Summary of the invention The processes of the present invention integrate power generation technologies which utilise gas turbines with processes such as nitric acid producing processes which produce significant volumes of gas normally as an end product, which is incapable of supporting combustion. We have discovered that the combination of such a pressurised gas stream with a combustion supporting gas and a fuel and the combustion of the fuel or with the gaseous products produced by the combustion of the fuel results in the production of a hot pressurised gas stream which can be used to generate power efficiently. This combination can be conveniently carried out by mixing the pressurised gas stream with the fuel and the combustion supporting gas and burning the fuel in a gas turbine. The combustion enables the temperature of the pressurised gas stream to be raised in effect to a level which permits the efficient generation of power.

The invention may potentially be usefully applied to any process which produces a pressurised gas stream which is not capable of supporting combustion. Preferably the gas stream is at a pressure of at least 15psig and more preferably at least 25 psig. Gas streams which are at a higher pressure may. be employed. Processes which generate gas streams at pressures greater than say 25 psig often incorporate facilities for the recovery of energy usually by passing the stream through an expander. The pressure of the gas emerging from the expander is generally in the range 0 to 10 psig. In general the processes of

this invention utilise a gas stream which is at a pressure within this range. In the preferred embodiments the processes of this invention utilise a gas stream which is a product of the existing process and can thereby be designed to operate in conjunction with an existing process without affecting the operation of that process.

The temperature of the gas stream utilised in the processes of this invention may also vary. In general existing processes may incorporate facilities for the recovery of heat energy from a gas stream having a temperature greater than 50°C. Whilst hotter gas streams may be utilised in the processes of this invention, it is preferred to use cooler ones having temperatures in the range 0 to 900°C.

The composition of the gas stream may vary widely provided that 1t is one which does not support combustion. Most commonly those streams will be air streams 1n which the oxygen content has been reduced and will thereby consist primarily of nitrogen.

The process from which the gas stream is generated should be one which can incorporate the combustion step without significant loss of efficiency or introduction of safety hazards. In those processes where the pressurised gas stream is normally subject to further processing the process must be one which can tolerate the presence of the combustion products and unburnt oxygen in the gas stream which leaves the turbine. A pressurised gas stream which is deficient in oxygen is one which either contains insufficient oxygen to combust the quantity of the fuel needed to raise the temperature of the gas stream to the predetermined level and/or one which contains a concentration of oxygen which is insufficient to support that combustion. Whether or not a particular gas stream is deficient in oxygen. ay be determined by routine experiment. The present invention finds application wherever the concentration of oxygen is insufficient to enable the temperature of the gas at the inlet to reach the predetermined level. The pressurised gas stream is preferably one which is substantially free from oxygen.

The fuel may be combusted in a separate vessel into which the oxygen containing gas and optionally a portion of the pressurised gas stream are introduced and the gaseous products of that combustion mixed with the pressurised gas stream or the remaining portion of that gas stream. Preferably the fuel, the pressurised gas stream and the oxygen containing gas are introduced into a burner and the fuel combusted. The gaseous output of the burner may then be. passed to the turbine. The output of the turbine may be used to provide any form of shaft work, including but not limited to internal compression requirements, external compression requirements or electrical power generation.

Any fuel may be utilised although those which can be completely combusted and do not leave a solid residue are preferred. The fuel may conveniently be methane, ethane, propane, butane, natural gas or town gas or a liquid hydrocarbon.

The amount of fuel burnt will preferably be sufficient to raise the temperature of the gas stream at the turbine inlet to a level at which that turbine generates power most efficiently.

Typically existing turbine equipment operates most efficiently when the inlet temperature is in the range 500 ^β C to 1500°C more usually 900°C to 1200 ^C C. Inlet temperatures below these limits or below the ideal for any particular turbine may be useful where for example the amount of power generated is greater than required at that time or where the cost of the fuel detracts from the economic advantage of operating the turbine at maximum efficiency.

The quantity of fuel burnt will preferably be sufficient to raise the temperature of the pressurised gas stream by at least 50°C and more preferably by at least 250°C. The exhaust gases which exit the turbine may well be at a temperature which renders them useful in a heat recovery system. Processes in which heat is recovered in this manner represent a preferred aspect of the present invention. The exhaust gases from the turbine or from the heat recovery system may be vented to the atmosphere. Alternatively they may be compressed and

recycled to the pressurised gas stream or to a part of the process from which that pressurised gas stream is produced. A second alternative is that the exhaust gases may be useful either as the whole or a part of a product gas stream which finds utility downstream of the turbine. In these alternatives the gaseous products produced by the combustion of the fuel should be compatible with the system to which they are passed.

The processes of this invention find a first particular application where the pressurised gas stream comprises the tail gas stream which Is produced from the absorption stage of a nitric acid production process. Such processes involve the oxidation of ammonia with air. The reaction of this oxidation stage can be represented by the equation

4NH ₃ + 50 ₂ ^■ > 4NO + 6H ₂ 0 The nitric oxide produced is further oxidised to nitrogen peroxide which itself reacts with water to produce nitric ac d. The overall reaction at this stage can be represented by the equation

2H ₂ 0 + 4N0 + 30 ₂ + 4HN0 ₃ Commercial nitric acid production plants can operate either at constant pressure or at dual pressures where the pressure in the oxidation stage is less than that at the absorption stage.

The use of lower pressures at the oxidation stage is advantageous in that it facilitates the condensation of the water produced by the oxidation reaction. The use of higher pressures at the absorption stage Is advantageous in that it reduces the size of the apparatus required and enables a more concentrated acid to be produced. Both the constant pressure and the dual pressure processes are used on a large scale. In all those commercial operations the production facility comprises some means whereby the energy generated by the highly exothermic oxidation reaction can be recovered and used, e.g. to generate power. The existing proposals to recover this energy, e.g. in British Patent 1146292 and in International Patent Publication 89/01091 are all directed to systems which are integrated with the production plant in some

way with the result that the amount of energy which can be recovered is relatively inflexible. The processes of the present invention offer a greater degree of flexibility particularly in terms of regulating the amount of power produced so as to meet the requirements of the site of operation at any particular time. The processes of this invention may also be operated in conjunction with any of the known processes for energy recovery. In particular they may be operated in conjunction with the processes described in International Patent Application 89/01091. In this prior application an amount of pressurised gas which is additional to that required or produced by the nitric acid process is heated with a portion of the excess high temperature heat produced by the oxidation reaction and expanded to generate work. In practice such procedures require that the size of the air compressor will be increased (as compared to a conventional process) as will the size of the turbine/expander. Also additional heat exchanger capacity must be installed in order to permit the heating of the supplementary gas. The additional amount of heated gas which can be passed to the expander is limited by the quantity of heat available to heat 1t. This earlier patent application also describes the idea that when the supplementary gas which is heated is one which is capable of supporting combustion, its temperature can be increased still further by burning a suitable fuel in it. The amount of power which could be generated by expanding this heated gas stream is, however, still limited by the amount of supplementary gas which is available. The use of the processes of this invention in conjunction with the processes which are described in International Patent Application 89/01091 (the relevant portions of which are hereby incorporated by reference) removes this constraint and introduces a greater flexibility into the operation.of the nitric acid facility and the power producing turbine.

Brief description of the drawings relating to nitric acid production

Figure 1 is a diagrammatic sketch illustrating the operation of one type of conventional process for the production of nitric acid.

Figure 2 Is a diagrammatic sketch of the process illustrated in Figure 1 but modified so as to operate in accordance with the present invention.

Detailed description of the invention In Figure 1 the principal features of the apparatus are a reactor 1, an absorber 2, a bleaching tower 3, an air compressor 4 and an effluent gas expander 5. The apparatus further comprises heat exchanger units 6 to 12. In use ammonia is fed through line 13 and is heated in exchangers 6 and 7 before entering line 14. Air is compressed in compressor 4 and passed through line 15 and (in part) line 16 to exchanger 10 where it is heated and passed to line 14 where it mixes with the ammonia. The gas mixture in line 14 is fed to reactor 1. A product gas stream leaves the reactor through line 17 and passes through exchangers 8, 9, 10 and 11 before being fed to the absorber 2. A part of the compressed air is passed through line 18 to the bleaching tower 3. Water is fed to the absorber through line 19. A product nitric acid stream is collected through line 20. An exit gas stream leaves the absorber 2 through line 21 and passes through exchangers 12 and 9 to expander 5. A tail gas stream leaves expander 5 through line 22.

In Figure 2 the principal features which are identical with those of Figure 1 are identified with the same number with the addition of the suffix A. Thus 1A represents the reactor. The apparatus further comprises a burner 23, a compressor 24 _. an expander 25 and a heat exchanger 26. In operation the exit gas stream in line 21A passes through exchanger 12A and 9A and enters burner 23. Compressed air from compressor 24 is fed into the burner through line 27 and a fuel is fed into the burner through line 28. The fuel is burnt and the gaseous products of the

combustion are mixed with the exit gas stream to produce a combined gas stream which passes through line 29 to expander 25. The depressurised gas leaves the expander through line 30 and is passed to heat exchanger 26. Water is fed through line 31 to generate steam in line 32. A tail gas stream leaves exchanger 26 through line 33.

In a typical operation according to the process represented in Figure 1 the gas stream entering the expander 5 may be at a temperature of 315 ^β C and a pressure of 10 bar g. The expander might generate about 2.6 MW of power. The tail gas in line 22 will be at a temperature of 77°C. For a typical plant producing 250 short tonnes per day of nitric acid the overall energy requirements would be a power input of 3 MW; a steam export of 23000 lb/hour and a fuel import of zero. In a typical process according to this invention and as illustrated in Figure 2 the temperature of the gas stream in line 29 will be raised to about 1100°C, compared with a temperature of only about 320 ^C C for the gas stream from the heat exchanger 9A, by combustion in the burner 23 of the fuel supplied through the line 8 and the oxygen of the air supplied through the line 27 from the compressor 24. This additional air has been designed at approximately 41% of the gas stream in the line 21A from the absorber 2A to the burner 23. The expander 25, receiving at its inlet the combined gas streams from the burner 23, via the line 29, will be designed to accept the raised temperature of 1100°C as inlet temperature, and could be of a size so as to generate up to 9.7 MW of power, compared with only 2.6 MW of power recovered by the expander 5 1n the plant, not Incorporating the invention, shown in Figure 1. The temperature of the gas in line 30 may be reduced to about 600°C. The heat exchanger 26 could generate up to 28MMBTU/hour of high pressure steam and the gas stream in line 33 will then have a temperature of 132°C. The overall energy balance of this process represents a fuel import of about 50.3 MMBTU/hour, a power export of 2.16 MW (instead of a power import of 3.07 MW) and a steam export of 45000 Ib/hr.

The energy performance of this embodiment of the invention, compared to the typical prior art, may be summarised in the following table:

TABLE 1

Air Compression Other Power Consumers Tail Gas Expansion Power Export Steam Export

Fuel Consumption

The process of this invention finds a second particular application where the pressurised gas stream is the hot flue gas produced from the regeneration of the catalyst in the Fluid Catalytic Cracking (FCC) process.

FCC is used to convert gas oil fractions including atmospheric and vacuum distillates, coker gas oil and deasphalted oils into light olefinic hydrocarbons, LPG, high octane gasoline, middle distillates and petrochemical feedstocks.

The process consists of four main components: a reactor, a stripper, a regenerator and a fractionator. Fresh feed and recycle feed from the fractionator are mixed with regenerated catalyst. Heat from the hot catalyst vaporizes the oil and raises it to the required reaction temperature. As the oil and catalyst rise through the reactor, the cracking reaction takes place. Vapours from the reactor pass overhead through cyclones to the fractionator where the reactor products are separated.

Spent catalyst from the reactor is stripped with stearø to remove cracked products trapped in the catalyst. The stripped catalyst then falls into the regenerator bed. Here, coke deposited on the catalyst during the reaction is burnt off tsslng air. The coke may either be completely burnt to form carbon dioxide or partially burnt to form carbon monoxide. This completes the catalyst cycle.

Hot flue gases from the regenerator are used for energy recovery before being vented to the atmosphere. In many FCC units, regenerator flue gas is expanded in a turbine before passing through a waste heat boiler to recover power and high pressure steam.

The processes of this invention introduce additional air and fuel into the regenerator flue gas stream. This air is used to combust the -fuel which results in higher temperatures and larger gas flows to the regenerator tail gas expander. The additional air may be supplied by an independent air compressor or an enlarged main air blower.

The processes of this invention may be utilised in FCC processes which completely or partially burn the coke in the regeneration step. In the processes of this invention any carbon monoxide will be oxidised to form carbon dioxide. Brief description of the drawings relating to the FCC processes

Figure 3 is a simplified flow diagram of the FCC process. Figure 4 is a diagrammatic representation of an arrangement usual in FCC units to recover power and steam.

Figure 5 is a diagrammatic representation of the arrangement of Figure 4 modified to operate in accordance with the invention. Detailed Description of the invention

In Figure 3 the principal features of the apparatus are a reactor 1, a stripper 2 and a regenerator 3. Gases produced in reactor 1 pass through line 4 .to a fractionator 5 where the products are separated. A portion of untreated feedstock is recycled through line 6 and mixed with fresh feedstock in line

7. The resulting feedstock passes through line 8 and is contacted with hot catalyst in reactor 1. Steam is introduced through line 9 to strip products trapped in the catalyst. The stripped catalyst falls into the regenerator 3. Air is introduced through line 10 and burns off coke deposited on the catalyst. Hot flue gases from this step are recovered through line 11.

Figure 4 shows the regenerator 3 and the lines 10 and 11. A compressor 12 is used to provide the air in line 10. The hot gases in line 11 are passed to expander 13. The exhaust gases from expander 13 pass through line 14 to a boiler 15, through which they pass at essentially atmospheric pressure. High pressure steam is generated in line 16. Boiler feed water is fed through line 17, and cooled flue gas leaves the boiler through line 18. The steam can be passed to line 19 to drive steam turbine 20. Low pressure steam exits through line 21. A motor/generator 22 is mounted on the shaft.

In Figure 5 the features which are identical with those in Figures 3 and 4 are identified by the same numeral with the addition of the suffix A. Thus 3A represents the regenerator. The apparatus further comprises line 23 which connects burner 24 with compressor 12A. Compressor 12A is enlarged compared to 12 so as to be capable of providing the requirement of line 23. Line 25 is an inlet through which fuel is introduced into burner 24. The fuel is combusted and the hot gases pass through line 26 to expander 13A. 27 represents a generator mounted upon the shaft.

A typical 25,000 Barrels per Day FCC unit operates at a compressor exhaust pressure of 37psig and an expander inlet pressure of 25psig. A comparison between such a process as illustrated in Figure 4 and operated according to the invention as in Figure 5 is presented in the following table:

power equivalent 5,678 hp

Net Fuel Import 0 MMBtu/h

Net Power Export 10,172 hp 28,908 hp 18,736 hp

Incremental Efficiency 43.51

Additional comparisons between the process illustrated in Figure 4, and modified in accordance with the invention as in Figure 5, have been made when operating at expander inlet pressures of 85 psig and 185 psig. The results are presented in the following tables 3 and 4 respectively:

TABLE 3

Net Power Export 14,661 hp 25,038 hp 10,377 hp

Incremental Efficiency 79.0%