WO2001091893A1

WO2001091893A1 - Method for optimising a production increase operation

Info

Publication number: WO2001091893A1
Application number: PCT/FR2001/001448
Authority: WO
Inventors: Pierre Avrillier; Fabien Illy
Original assignee: L'air Liquide, Societe Anonyme A Directoire Et Conseil De Surveillance Pour L'etude Et L'exploitation Des Procedes Georges Claude
Priority date: 2000-05-26
Filing date: 2001-05-14
Publication date: 2001-12-06
Also published as: FR2809332B1; CA2407725A1; EP1289640A1; FR2809332A1; AU2001260416A1

Abstract

The invention concerns a method for optimising a production increase operation. Said method is designed for production increase in a chemical process comprising: at least a step (B) which consists in compressing a gas fluid containing at least part of the reagents of the chemical process or their reaction products; at least part carried out under pressure with at least a step (C) which consists in drawing a product of the chemical process and recuperating a gaseous residue; at least a step which consists in recuperating (D) at least part of the thermal power (W) of at least part of the gaseous residue, the chemical process comprising at least a step (F) which consists in recuperating energy, the production increase process being associated with the production of excess energy in a least a step (F) and a reduction of the ratio between the thermal power and total flux of drawn products, and consists in using at least part of the recuperated excess energy in at least a step (F) for increasing the thermal power of at least part of the fluid residue.

Description

Method for optimizing a debottlenecking operation.

The present invention relates to a method for optimizing a stripping operation. The term "stripping" of a chemical production unit is understood to mean any modification of this unit intended to increase its production. The invention applies to chemical processes comprising:

at least one step, hereinafter designated by B, of compression of a gaseous fluid containing at least part of the reagents entering into said critical process or of products resulting from a step of reaction of said reagent,

at least one part produced under pressure, hereinafter designated by E, comprising at least one step hereinafter designated by C during which a product derived from said chemical process is drawn off and a gaseous residue is recovered,

- At least one step, hereinafter designated by D, for recovering at least a portion of the thermal power (hereinafter designated by W) from at least a portion of said gaseous residue, in which it is sought to modify the flow rate of the reagents used so as to increase the production capacity of the installation even though the capacity of said installation to circulate gaseous products under the required operating conditions (pressure, temperature, etc.) is limited due to the fact inherent limitations of the compression and / or energy recovery devices used. It is not uncommon in the industry that, in such processes, an attempt is made to increase the production capacity of the installation by carrying out what is traditionally called "debottlenecking" and that this debottlenecking is accompanied the production of excess energy, either directly in a step implementing the process flows, or even in an additional step, for example a step of producing heat at a point in the process.

This additional energy production is, in some cases, felt rather like a hindrance at the level of the process insofar as it is not always reusable directly in a useful way, at the place where the energy is released. In addition, it is very common for debottlenecking to lead, in particular because it occurs with an increase in the quantity of reaction which are withdrawn, to a decrease in the energy flow of the residual gas flow recovered after the withdrawal of said product.

It is to this type of process, in which the stripping leads to a real energy imbalance, that the present invention proposes to provide a solution consisting precisely in rebalancing, at least partially, the energy balance of the whole process and in optimize the use of all new sources of energy linked to the stripping operation.

Thus, the present invention proposes, in particular, to provide an original solution to the problem of injecting a gas intended to boost a production unit even when the latter unit can no longer circulate a higher flow rate. Such a problem is posed, in particular, in all the oxygen doping operations of a spouted unit when it operates in air because of the presence of a compressor and which has at least one recovery turbine.

As is apparent in particular from the examples, the invention proposes, very particularly, a solution in the case of the manufacture of nitric acid from ammonia and proposes, very particularly, an improvement to the process as it is described in patent EP 0 834466. The invention also provides a solution in the case of the synthesis of ammonia from methane and air and proposes, very particularly, an improvement to the process as described in international patent application WO 98/45211.

The invention also provides an improved solution to the processes for the manufacture of acetic acid by oxidation of n-butene, in the presence of a catalyst, in which the catalytic oxidation is' doped with oxygen and leads to the formation of excess energy. In general, the invention provides a particularly original solution consisting in recovering the excess energy produced - at a point of a chemical unit, during the debottlenecking, by using this energy to increase the energy value of a residual flow. obtained after drawing off at least one of the products from the process. Thus, according to one of its essential characteristics, the invention relates to an improvement to the debottlenecking processes intended to increase the production capacity of a unit for producing a chemical in a chemical process comprising:

at least one step (B) for compressing a gaseous fluid containing at least part of the reagents entering into said chemical process or products resulting from a step of reacting said reagents, at least one part produced under pressure with at least one step (C) during which a product derived from said chemical process is drawn off and a gaseous residue is recovered,

- At least one recovery step (D) of at least part of the thermal power (W) of at least part of said gaseous residue, the chemical process further comprising at least one step (F) during which energy is recovered, said debottlenecking being accompanied by the production of excess energy in at least one step (F) and by a reduction in the ratio between the thermal power (W) and the total flow of products withdrawn during of said chemical process, said improvement consisting in using at least part of the excess energy recovered during at least one step (F) to increase the thermal power (W) of at least part of said gaseous residue. The present invention, in its different variants, proposes the recovery of an excess of energy linked to the debottlenecking of a chemical process. In fact, the increase in process capacity leads, in many cases, to an increase in the energy released by a reaction, which compared to normal walking will generate an excess of energy. It will be understood that the excess energy can come, in particular, from an exothermic reaction using the reagents used in the context of the process. It can be included in a step of the process which is generally less exothermic than the reaction itself or even athermal. In this case, it is less the increase in the reaction rate, or in the temperature resulting from the reaction, than the increase in the flow rates of the reactants under similar conditions which will lead to an increase in the energy flow of the products of the reaction and therefore create the excess energy that the invention proposes to use.

Likewise, it will be understood that the exothermic reaction may not take place directly in the process flow but also indirectly. Thus, in the case of a process which requires an external supply of heat, the debottlenecking of the process may require an increase in the quantity of heat supplied indirectly. It is therefore necessary to distinguish the reactants of the exothermic reaction which supply their heat externally from the process fluids which will react. In general, the heat of reaction of the exothermic reaction can be recovered in different ways during step F. We can either consider recovering it directly by, for example, a boiler, on the process fluids, during the reaction or on the hot products of this reaction, or even on the fluids associated with the means for heating the process fluids, which would have served to supply energy to the process. We can also recover it indirectly by using an intermediate flow to transport energy.

The excess energy recovered in this step will be used according to the invention, directly or indirectly, to increase the energy value of the ^'flow of said waste gas prior to said energy recovering step.

The invention is intended for all processes where the stripping is accompanied by a reduction in the ratio between the thermal power W and the total flow of product withdrawn during the chemical process. 'The effect of debottlenecking on the thermal power W of the residual flow can be felt in different ways. In fact, as is particularly the case when the latter comprises oxygen doping of an oxidation reaction carried out in the presence of an inert gas, the flushing is often done with a reduction in the flow rate of the residue. gaseous. However, since the stripping can also be accompanied by an increase in the temperature of the gaseous residue, it is not excluded that the thermal power of the gaseous residue does not vary appreciably during the stripping.

The invention is particularly intended, in the event that the decrease in the ratio between the heat flow W and the total flow of the withdrawn products decreases, due to an increase in the flow of the withdrawn products.

According to an advantageous variant of the invention, at least part of the excess energy is used to produce steam, the production of this steam can be carried out for example by a boiler located on the process gases resulting from a reaction exothermic or a boiler located on smoke used to supply energy to the fluids of the chemical process.

This vapor can in particular be produced under pressure and then sent to a device intended to recover its energy, for example to a turbine or a heat exchanger.

The use of this vapor in step D of recovering the energy of the compressed gaseous residue can be done indirectly, for example through an exchanger with the part of the compressed gaseous residue, but preferably at through an injection of the vapor produced in the part of the compressed gaseous residue.

Step (C) of recovery of a product resulting from the chemical process can also take place in different ways. It may, in particular, be recovery by distillation, condensation of a liquid or by crystallization of a solid or by absorption of the liquid / gas type or by adsorption for example of the gas-solid type or even by permeation .

The excess energy used in the energy recovery stage of the compressed gaseous residue can also correspond to different schemes. Indeed, for example the compressed gaseous residue can be separated into several streams and / or undergo different chemical reactions before its recovery stage over all or part of its flow rate.

When the excess energy used in the recovery stage is produced by mixing with steam (for example by injecting steam into the gaseous residue), it is more advantageous that the gaseous residue no longer has a function essential in the process (combustion fumes, residual gases or purges before or after treatment). It is thus possible to limit the impact on the rest of the process of mixing the gaseous residue and the vapor.

This energy recovery step of the compressed gaseous residue product is advantageously carried out by expansion of this gaseous product in a turbine and / or by heat exchange.

Other characteristics and advantages of the invention will appear more clearly in the light of the description which will follow given, in particular, with reference to FIGS. 1 to 14 appended hereto and which respectively present: - FIG. 1: a diagram representing a sequence particular steps of the process according to the prior art;

- Figure 1A: a diagram representing a particular sequence of steps of the method according to the prior art, the compression step being at the start of the sequence; - Figure 1B: a diagram representing a particular sequence of the steps of the process according to the prior art, the heat recovery step being located on the products of the exothermic reaction;

- Figure 2: the sequence shown in Figure 1 incorporating the improvement of the invention; - Figure 3: a particular example of the process of the prior art corresponding to an oxidation reaction in which the oxidation is carried out by air;

- Figure 4: the same example as in Figure 3 but with the use of oxygen-enriched air so as to be able to increase the production capacity of the installation;

- Figure 5: an example of improvement according to the invention of the method shown in Figure 4;

- Figure 6: a schematic representation of a process according to the prior art corresponding in particular to a process for preparing nitric acid by oxidation of ammonia in the presence of air;

- Figure 7: the same process as that shown in Figure 6 but using oxygen enriched air so as to be able to increase the production capacity of nitric acid of the installation; - Figure 8: the improvement of the invention applied to the method shown in Figure 7;

- Figure 9: a particular example of pressurized process of the prior art corresponding to an oxidation reaction in which the oxidation is carried out with air; - Figure 10: a schematic representation of a process according to the prior art corresponding in particular to a process for preparing ammonia by oxidation and reforming of methane in the presence of air and steam;

- Figure 11: the improvement of the invention applied to the same process as that shown in Figure 10 but using oxygen-enriched air in the secondary reformer, so as to be able to increase the ammonia production capacity of the 'installation;

- Figure 12: the improvement of the invention applied to the same process as that shown in Figure 10 but using air enriched in oxygen in the secondary reformer, and air also enriched in oxygen for the combustion carried out in view of heating the reactants in the primary reformer, so as to be able to increase the ammonia production capacity of the installation;

- Figure 13: the simplified block diagram of a process for the production of acetic acid by direct oxidation of n-butene according to the prior art; - Figure 14: the refinement of the invention applied to the same process as that shown in Figure 13, to recover energy excess swells during the oxidation step to increase the energy flow of the gaseous residue obtained after drawing off the acetic acid.

The diagram shown in FIG. 1 corresponds to an example of a succession of steps that can occur in the prior art. It only represents the steps useful for understanding the present invention, other steps could be inserted while remaining within the scope of the invention.

1

More specifically, in FIG. 1, step A represents an exothermic reaction carried out in the presence of a gas I and of various reactants II during which energy III is therefore produced (step F). The product from step A is subjected to a compression step B, then to a step C carried out under pressure (also step E) during which an IN product is recovered.

The pressurized gaseous residue from step C is then subjected to a step D for recovering its energy N. The gas flow from this step is denoted NI. FIG. 2 represents the improvement proposed by the invention in the context of a succession of steps which is the same as that shown in FIG. 1.

In this improvement, the fluxes II ′ and I ′ of the reactants introduced in stage A are increased, which leads to an increase in the energy supplied during the exothermic reaction A. The energy then becomes IIP and part of this energy III ″ is injected into the gaseous residue before the latter is subjected to step D of recovery of its energy V. The gaseous residue resulting from this step is denoted VF.

Those skilled in the art will readily understand, in particular in the light of the description which follows, that the sequence of steps as shown in the diagrams of FIGS. 1 and 2 is not the only one that can be envisaged in the context of invention since in particular the same improvement can be envisaged with a process in which the exothermic reaction is carried out under pressure and where, consequently, stage B of compression of the gas could be located upstream from stage A of exothermic reaction as shown for example in Figure 1A. In addition, step A described in FIG. 1 can be non-simultaneous with the step of recovering energy F as suggested in FIG. 1B.

Thus, according to one of its variants, the invention applies very particularly to a process for recovering the excess energy supplied in the context of a stripping operation intended to increase the capacity of production of an installation in which a chemical process is implemented comprising:

- at least one exothermic reaction (A) carried out in the presence of a process gas, at least part of which is inert with respect to the exothermic reaction,

- at least one step (F) of recovering at least part of

1 the energy of said reaction

- at least one step (E) carried out under pressure,

- at least one step (B) for compressing a gas, - at least one step for recovering at least one product from said chemical process,

- at least one step of recovery of a gaseous residue, and

- At least one step of recovering the energy of at least part of said gaseous residue during said compression step, said stripping being carried out with an increase in the flow rate of the reactants used in said exothermic reaction and accompanied by an increase in the energy supplied by said exothermic reaction, according to which at least part of the excess energy supplied by the exothermic reaction is used to increase the energy value of the flow of said gas before said step of recovering its energy.

The invention applies particularly to cases of processes comprising at least one exothermic reaction and more particularly to cases where the exothermic reaction is an oxidation reaction and in particular an oxidation reaction carried out with an oxygen-containing gas . It is particularly applicable to oxidation reactions with an oxygen-enriched gas.

The invention applies very particularly to cases where the exothermic reaction uses at least part of the reactants entering the chemical process. In such methods, the invention provides an improvement, in particular in cases where the debottlenecking is carried out with an additional injection of oxygen, and more particularly in cases where this additional oxygen injection is carried out in the context of doping. to oxygen from an oxidation reaction carried out on the process fluids. The invention is particularly applicable to cases where this additional injection of oxygen is accompanied by a reduction in the inert gases circulating in the unit for producing the chemical.

According to one of its variants, the invention relates to an improvement according to which the chemical process comprises an exothermic reaction step carried out in the presence of an inert gas with respect to said exothermic reaction, at least one step of compressing the gaseous product resulting from said reaction followed by at least one step during which at least one product derived from said chemical process is separated from said gaseous product, the gaseous residue then being subjected to a step of recovering its energy and according to which said stripping is carried out with a reduction in the proportion of the inert gas with respect to said exothermic reaction in said gaseous residue and an increase in the flow of reactants used in said exothermic reaction, characterized in that part of said excess energy provided by the exothermic reaction is used to increase the energy of the gaseous residue before it is subjected to said tape recovery of its energy.

In fact, when the flow rate of the reagents used in the exothermic reaction is increased, it is very often necessary to reduce the proportion of gases inert with respect to the exothermic reaction in the gas in the presence of which said said is carried out. exothermic reaction to limit the increase in total flow and the appearance of bottlenecks that could result. This has the consequence of reducing the energy flow of the gaseous residue before the step of recovering its energy, the gaseous residue being composed mainly of inert gases. It will therefore be understood that the invention applies very particularly to these cases by making it possible to increase, thanks to the contribution of a part of the excess energy supplied by the exothermic reaction, the energy value of the flow of the gas for which seeks to recover energy.

A detailed description of such a process is given with reference to FIGS. 3 to 5. FIG. 3 symbolically represents a process in which an exothermic oxidation is carried out in a reactor 1 by introducing air and reactants respectively into 2 and 3 . The energy released by the exothermic reaction is recovered (its recovery which takes place simultaneously) and symbolized by arrow III. The flow leaving the oxidation device 1 is introduced into a compressor 5, then subjected to one or more process steps carried out under pressure E. During one of the steps of this process (C), an IN product is recovered. The gaseous residue from the pressurized process then undergoes a step D of recovering its energy by introduction into a turbine 6. The recovered energy is materialized by the arrow N, the gas flow at a lower pressure is denoted NI.

When a production unit of the type represented in FIG. 3 is found to be gulled and it is desired to increase the product by using oxygen, a conventionally used solution consists in lowering the air flow rate of the unit to circulate reagents and additional products (including oxygen) through the unit. This solution is represented in FIG. 4 where the introduction of air I has been replaced by an introduction of oxygen-enriched air (P and I ") into the oxidation device 1. The oxygen enrichment of the air introduced to carry out the oxidation makes it possible to process a larger quantity of IP reagents and to recover, consequently, a quantity of IIP energy and of IV products greater than what was achieved in the diagram of FIG. 3. As the capacity of said installation to circulate gaseous products under the required operating conditions (pressure, temperature, etc.) is limited due to the imitations inherent in the compression and / or energy recovery devices used, it should reduce the inertia flow (Ν2 in the case of Pair) and in doing so the air flow (P) to circulate the excess of reagents and products in particular at the compression step B. To exceed the system of government nominal, it is necessary, firstly, to provide, by means of an injection of oxygen from the air, the molecular oxygen of the air which has been removed by reducing the air flow. We then understand that the first cubic meters of oxygen only compensate for the oxygen in the missing air. The oxygen utilization efficiency is therefore degraded.

In addition, any mechanical increase in the capacity of compression step B is unnecessary because if it could circulate more air under normal conditions, the flow should be reduced by as much to pass the necessary reagents and oxygen in the conditions of running down. Mechanical upgrading does not improve the efficiency of oxygen use.

In addition, when energy III is recovered in the form of steam and used in a steam turbine, any increase in the flow of energy III in IIP, for example due to an increase in temperature or flow rate, may not be accepted. by the steam turbine. We may be forced to limit the increase in capacity or to no longer use the excess energy (IIP -III) in these conditions. A decrease in the energy efficiency of the unit can then be observed.

The replacement of the nitrogen in the air with oxygen and the other process reagents also leads to a reduction in the flow of gases passing through the turbine 6 for recovering energy V when all or part of the products from the chemical process are withdrawn from the main stream or when the reactions implemented in the process reduce the amount of gaseous material and therefore decrease the energy recovered (V).

The solution proposed by the invention in the case of the diagram shown in FIG. 4 is illustrated by FIG. 5 and consists in using part III "of the excess energy (IH'-III) produced during the oxidation, of the oxygen doping, to increase the energy value of the gas flow before it enters the turbine 6.

Thus, while in the prior art this production of excess energy, generally not used, was considered as rather undesirable or limiting, the invention on the contrary provides a means of recovering this energy by increasing the amount of energy recovered at the outlet of the turbine 6.

Indeed, the gases entering the turbine 6 are mainly generally ballast gases, that is to say inert gases mixed, where appropriate, with by-products from the process or other products not separated during from step C.

The additional energy supplied in the context of the present invention to the turbine 6 can then be valued in different ways. In particular, consideration could be given to transmitting it to the compression step B (here represented by the compressor 5) in order to compress all or part of the reagents and additional oxygen with a lesser reduction in the air flow rate. The part of the injected oxygen which is used to replace the oxygen in the air can thus be draώed for a final capacity of the identical unit.

Thus, it is very clear that the invention makes it possible to improve the yield of the unit after doping with oxygen.

As explained above, the process of the invention applies to chemical processes according to which part E carried out under pressure comprises several stages.

This is the case in particular which is illustrated in FIGS. 6 to 8 where the pressure process comprises two steps E1 and E2. More specifically, in the diagram shown in FIGS. 6 to 8, part E of the chemical process carried out under pressure comprises:

a first step E1 in which part or all of the product resulting from the exothermic reaction is separated from the inert or unreacted gas in this exothermic reaction,

- A second step E2 during which this product is brought to react with a new reagent to form the product whose manufacture is targeted in the process (product noted IN in the diagram of Figures 1 and 2).

More specifically, in the diagram shown in FIG. 6, the reactants used in the exothermic reaction A are introduced into the reactor 9 as well as the gas, part of which is inert with respect to the exothermic reaction (introduction respectively in 7 and 8).

The product flow resulting from this exothermic reaction A is compressed in stage B, then subjected to stage Ei of partial condensation in a condenser 13 making it possible to concentrate the product of the exothermic reaction in the liquid phase. The gaseous and Hquid products thus separated are then subjected to a second step also under pressure E ₂ . They are more precisely introduced via the lines 14 and 15 into a device 17 where the product of the exothermic reaction reacts with a new reagent introduced by the canahsation 18 to produce the product IN withdrawn during step C by the canahsation 20.

Furthermore, the energy III released by the exothermic reaction is recovered in the form of vapor in a boiler 10, then expanded in the turbine 12 after having possibly been subjected to a stage intended to "desuperheat" it, if necessary (this stage is materialized by arrow 23).

In the case of FIG. 7 where the production of the exothermic reaction A has been increased by increasing the flow rate of the incoming reactants and decreasing the proportion of inert gases in the gas introduced, if the total gas flow introduced must be generally constant, a additional introduction of reactive gas (in 7 'and 8') means a decrease in the reactive gas flow in 7.

This increase in the flow of reagents introduced in reaction step A leads to the production of additional energy. It is this additional energy (IIP-III) supplied which is valued in the diagram shown in FIG. 8. An industrial example of a process implementing the process illustrated in FIG. 8 is given in detail in Example 1 It illustrates how it is possible to rebalance the energy within the framework of the manufacture of nitric acid in a process implementing:

- a step of oxidizing ammonia with oxygen-enriched air to form nitrogen oxides, at the end of which energy is recovered, - a step of oxidizing nitrogen monoxide in nitrogen dioxide and / or its dimer,

a step of reacting nitrogen dioxide and / or its dimer with water to form nitric acid, at the end of which the nitric acid is drawn off and a gaseous residue is recovered. In the process illustrated in this example, the debottlenecking is done with a doping of the air with oxygen leading to the recovery of excess energy at the end of the step of oxidation of rammoniac, excess energy that l 'is used to increase the energy flow of the gaseous residue after withdrawal of nitric acid. Another industrial example shown diagrammatically in FIGS. 9 to 12 is given in Example 2 and illustrates the application of the method of the invention in the context of a stripping operation carried out on a method implementing a sequence of steps. as depicted in Figure 1 A.

This process is schematically represented in FIGS. 9 to 12. It is a process for the synthesis of rammoniac using:

- a primary reforming step (A ' ₂ and F' ₂ ),

- a secondary or autothermal reforming step (A'i and E 'leading to the formation of a synthesis gas,

- a synthesis gas cooling step (F'i), - a synthesis gas treatment step (E ' ₂ ) followed by:

- An ammonia synthesis loop comprising a step (C) of withdrawing the ammonia and rejection of a gaseous residue under pressure.

In such a process, the debottlenecking comprises an oxygen doping of the reaction carried out in the secondary reforming step and a nitrogen doping of the reaction carried out before or in the ammonia synthesis loop, leading to the production of excess energy at the end of the secondary reforming step. According to the invention, said excess energy is used to increase the energy flow of the gaseous residue from said ammonia synthesis loop. As is also apparent from Example 2, it is also possible to carry out doping with combustion oxygen during the reforming step. primary, which leads to the supply of excess energy which, too, is used to increase the energy flow of the residual gas from the ammonia synthesis loop.

Such a process is an example of a process where excess energy is recovered at several points.

Another application illustrated by example 3 shows that it is possible to advantageously distribute the excess energy at several points of the flow.

This example shows how it is possible, according to the invention, to exploit the excess energy supplied during the synthesis of acetic acid in a process using a step of catalytic oxidation of n-butene doped with oxygen.

More specifically, according to this example, the excess energy recovered is recovered by the oxidation reaction of n-butene to increase the energy of the gaseous residue obtained after drawing off the acetic acid. Example 3 is an example that it is advantageous to reuse excess energy to increase the residual flux at several points.

EXAMPLES

Example 1 Application to the case of oxygen doping of a process for the manufacture of nitric acid

The industrial process for manufacturing nitric acid by oxidation of ammonia is conventionally carried out in an installation as described in a simplified manner in FIG. 6. FIG. 7 will be described with reference to the improvement described in patent EP 0 834 466 consisting of doping the air used for the ammonia oxidation reaction with oxygen. FIG. 8 shows the improvement proposed by the invention applied to the method shown schematically in FIG. 7.

In fact, nitric acid is generally prepared on an industrial scale using a process using catalytic oxidation of ammonia. Such a process firstly comprises the reaction of ammonia and oxygen (generally supplied by air) in the presence of a catalyst which is generally a platinum cloth in a reactor in order to selectively obtain nitric oxide and water according to the reaction: 4NH ₃ + 5O ₂ -> 4NO + 6H ₂ O In general, a large excess of air is introduced relative to the stoichiometric quantities necessary for this reaction to control the flammability of the reaction mixture and the external temperature of the reactor and to supply the oxygen used in the oxidation reactions later.

The gaseous effluent from the reactor is then cooled in a series of heat exchangers to oxidize the nitrogen monoxide with oxygen and transform it into nitrogen dioxide and its dimer according to the reaction:

2NO + O ₂ = 2NO ₂

The NO, NO ₂ mixture is then cooled and then condensed to form a liquid phase and a gas phase. In the liquid phase, the nitrogen dioxide is in equilibrium with its dimer which can react with water to form nitric acid. 3NO ₂ = 3/2 N ₂ O ₄ + H ₂ O → 2HNO ₃ + NO.

A liquid phase is obtained containing dilute nitric acid and nitrogen dioxide and its dimer and a gas phase containing a gas depleted in nitrous vapor. The Hquide and gas phases are introduced at different levels into an absorption column into which water is also introduced for the complete absorption of NO ₂ according to the same reaction

3NO ₂ - 3/2 N ₂ O ₄ + H ₂ O → 2HNO ₃ + NO

The NO resulting from this latter reaction is oxidized in its turn in the absorption column.

The diagram shown in Figure 6 corresponds to the implementation of such a method. Specifically, in the diagram of FIG. 6, air is introduced through the canahsation 7 and ammonia through the canahsation 8 into the oxidation reactor 9 where the reaction A of ammonia oxidation takes place . The energy (III) supplied by this reaction is recovered via the boiler 10 and sent to a turbine 12. The reaction flow from step A (mainly a mixture of N _2,

O ₂ , NO, NO ₂ and N ₂ O) is subjected to a compression step B in the compressor 11, then is separated by condensation in the condenser 13 where a liquid phase is formed containing inter alia dilute nitric acid and a gas phase with a lower content of nitrous vapor. These two phases are introduced separately (respectively at 14 and 15) into the oxidation-absorption column 17 where the remaining nitrogen oxides are transformed into nitric acid by the introduction of water through the pipe 18. This nitric acid is extracted ( product IN) via line 20. The unreacted gas, mainly composed of nitrogen and oxygen with a low nitrogen oxide content, is extracted from the column by line 19, then sent to turbine 22 for recovering its energy (N).

In the process represented in FIG. 7, in order to increase the quantity of nitrogen oxide supplied at the end of reaction A, the air is doped with oxygen which makes it possible to increase the quantity of ammonia introduced and leads to an increase in energy recovered at the end of the ammonia oxidation step (IIP).

FIG. 8 illustrates the recovery of this excess energy (III ") to increase the energy of the flow leaving the oxidation adsorption column.

Thus, the process of the invention can be applied particularly effectively to a chemical process with catalytic oxidation and withdrawal of products in the case of the stripping of a nitric acid unit of the bi-pressure type.

Such a process is almost self-producing energy with a turbine on the steam circuit produced during oxidation and a recovery turbine on the gases leaving the redox adsorption column. The more detailed description of such a method is then made with reference to FIGS. 6 to 8.

FIG. 6 illustrates an air-operated unit which operates at maximum capacity, that is to say that the compressor 11 cannot circulate more fluid under the same pressure conditions. The unit includes an oxidation zone A, the energy of which is recovered in the form of vapor in the boiler 10, then expanded in the turbine 12. A condenser 13 sends a gaseous flow and a fluid flow to the column of oxides. absorption 17 where the nitric acid extracted in 20 is produced.

The gas recovered at the top of the column at 19 is expanded in the turbine 22 so as to recover the compression energy. The normalized flow (for example normal pressure and temperature conditions) of the gases recovered in 19 is lower than the standardized flow rate of gases from the compression step, due to the production of acid. This column gas essentially consists of the nitrogen ballast of the air. The compressor 11 must circulate the reaction products resulting from the reaction of the oxygen in the air introduced at 7 with the ammonia introduced at 8. This gas undergoes a cooling step. It is generally composed of approximately 80 to 85% nitrogen, 10% oxygen and 5 to 10% of a NO-NO ₂ mixture. An injection of water or steam symbolically represented by arrow 23 makes it possible to regulate the temperature of the superheated steam which enters the turbine 12. In the case of doping with oxygen illustrated by FIG. 7, and corresponding to the process described in patent EP 0 834466, oxygen and water are injected via line 7 'in admixture with the air introduced by line 7 before the oxidation step A. The gas flow rate is found to be increased due to the presence of oxygen because, as in all doping, the flow rate of reagent in 8 "must also be increased. The resulting flow rate would therefore be greater than the flow rate obtained in the case of FIG. 6. The compressor being spouted , it cannot circulate such a gas flow rate. It is therefore necessary, in this case, to reduce the entry of air introduced in 7. In doing so, the total quantity of molecular oxygen which enters the process, so increase the oxygen flow not introduced in 7 'compared to the quantity strictly necessary for doping. The oxygen yield is thus degraded.

The reduction in the flow rate of the air at the inlet has the effect of reducing the flow rate of the nitrogen ballast present in the unit and, in particular, in the gas leaving in 19 from the oxidation-absorption column and entering the recovery turbine 22. It therefore appears that the reduction in air intake is useful for circulating more products in the unit but that it unbalances the energy balance of the unit. It is this problem that the invention particularly aims to solve.

The doping of NH ₃ oxidation increases the thermal load to be evacuated by the steam circulating in the boiler. Both the temperature and the flow rate of the steam produced in the boiler 10 can be increased. The steam turbine 12 is designed to operate at a certain flow rate and a certain temperature. If the temperature of the superheated steam is too high, an injection of water or additional steam at 23 must be used to "desuperheat" it. This further increases the total steam flow arriving in the turbine 12. Furthermore, "desuperheating" has a certain cost which is passed on to the doping. The specific cost of the oxygen yield is degraded. In addition, once the turbine is ducted, the excess vapor III "is expanded in the low pressure network. Thus, the system is again ducted without external energy supply. The oxygen yield has therefore been degraded on the one hand by replacing the molecular oxygen in the air, on the other hand by the additional cost of overheating and possibly by the discharge of excess steam on the low pressure network.

The improvement proposed by the invention shown in FIG. 8 makes it possible to optimize the energy resources of the doped unit, and therefore not to degrade the oxygen yield. Indeed, the added energy created in the boiler 10 is no longer perceived as a hindrance to debottlenecking but as the appearance of an energy imbalance on the unit due to doping and which is rebalanced thanks to the present invention .

In fact, thanks to the improvement of the invention, the surplus produced by the steam produced in the boiler 10 is drawn off, which the turbine 12 cannot accept even after desuperheating at 23. This surplus is discharged into the gas coming from the column of oxidation absorption 17 before its introduction into the turbine 22 to increase or restore the load of this turbine. Indeed, the turbine 22 is under-loaded before this introduction because the air flow has been reduced and therefore the gaseous residue from the column mainly composed of nitrogen ballast from the air is reduced thereby. The expansion turbine is then able to supply an excess of power to the compressor 11. It is thus possible to continue to boost the oxygen unit until the turbine 22 is in turn choked or until the compressor It can no longer accept a higher speed of rotation. In the same way, one can use the energy thus available at the level of the compressor while respecting its maximum speed of rotation, to increase the flow of air inlet introduced in 7 by lowering the flow of oxygen introduced in 7 'of way to improve performance.

Taking into account the mechanical characteristics of the three rotating machines 11, 22 and 12, there is thus an optimum making it possible to minimize the quantity of oxygen used according to the desired debottlenecking rate.

It is therefore possible to increase the yield of the oxygen used for the debottlenecking while optimizing the use of the additional energy. Example 2: Application in the case of an industrial ammonia manufacturing process

Another example of application of the invention is shown in Figures 1 A and 9 to 12 where the entire process is carried out under pressure. In the process of this example, the compression step B is at the head of the line and more precisely, the diagram shown in FIG. 9 corresponds to a unit using air with an exothermic oxidation reaction carried out under pressure. De Pair is introduced into I, then once compressed in stage B, it reacts with a gaseous mixture II containing inter alia a mixture of CH4, H2 and CO itself already under pressure. The set of reactions between I and II comprises an exothermic reaction carried out in a step A. The gaseous products of these reactions are cooled in step F where the energy III is recovered in the form of vapor. The cooled gaseous product is then treated and purified in a step E2 before entering a chemical synthesis loop comprising a step C during which the IN product is drawn off and a gaseous residue is recovered under pressure which is then recovered in the turbine D before further treatment or lower pressure incineration (NI).

The industrial process for manufacturing ammonia is conventionally carried out in an installation as described in a simplified manner in FIG. 9, it is a little more detailed in FIG. 10.

The exothermic reaction step A ′ ₂ comprises the combustion of a mixture of natural gas and air introduced at 110 and generates at 111 hot fumes. The energy of these fumes is recovered in step F ′ ₂ to preheat and react a reagent introduced in 109, for example natural gas and steam under pressure and leads to a mixture of synthesis gas by reforming in 103 .

The fumes from step A ' ₂ , after cooling in step F' ₂ are introduced at 112 into the exchanger F ' ₃ where their energy is recovered.

Air is introduced at 101 into the compressor 102 (step B). This air reacts in step A with the mixture of natural gas partially steam reformed in step F ' ₂ . The reforming reaction continues in step E. The gas from 104 of the so-called primary reforming step consisting of steps A ' ₂ and F' ₂ and secondary reforming consisting of steps A ', - and E' is then subjected to a step Ε of recovering its energy under form of vapor at 105. The synthesis gas is then subjected to a step E ' ₂ of purification and treatment then it enters a synthesis loop 122 comprising a step C during which ammonia is drawn off in 106 and a residual gas is recovered under pressure composed mainly of nitrogen and hydrogen in 107. The energy of this gas can be recovered in a turbine before being treated or incinerated at lower pressure. It will be noted that all of the steps A'i- and E '] constitute the secondary reformer also called an autothermal reformer denoted 121 and that all of the steps A' ₂ and F ' ₂ constitute the primary reformer denoted 120.

FIG. 11 illustrates the use of the improvement described in patent WO 98/45211 consisting in doping with oxygen the air used for the oxidation reaction carried out in the secondary reformer 121 in the case of the improvement proposed by the present invention.

The increase in capacity for an ammonia production installation is more problematic for the upstream part of the unit (which corresponds to the formation of synthesis gas before step E ′ ₂ ) than for the downstream part (treatment of the synthesis gas and NH ₃ synthesis loop). Indeed the compressor 102 is often at

of its possibilities and cannot circulate more air under the temperature and pressure conditions required by the process.

If the increase in the thermal load cannot be envisaged in the primary reformer 120 and more particularly in steps F ′ ₂ and F ′ ₃ , it is necessary to supply energy through the secondary reformer. This increase in process capacity can be done by increasing the flow rate of the gas introduced at 109 and by injecting oxygen at 113 into the secondary reformer 121 in step A and nitrogen at 114 before or in the loop. synthesis 122. The oxygen supplied will react with the excess of unreacted natural gas at the outlet of F ′ ₂ because the increase in the flow rate of gas introduced in 109 could not be compensated by the increase in the thermal load in F ' ₂ . The energy III 'recovered in F is greater than III because the conditions of temperature and pressure in the gaseous effluent at 104 are identical to normal operation but the gas flow rates are greater. This additional energy (IIP-III) is converted into additional steam by the boiler of stage F This steam III "can be injected into the purge gases before entering the turbine D.

In another application of the invention shown in FIG. 12, it can be seen that if all of the exchangers F ' ₂ and F' ₃ can withstand an increase in its thermal load, it is possible to also inject oxygen at 115 to increase the heat of the combustion reaction A ' ₂ . This in doing so, the excess energy due to the increased capacity can be distributed. The excess energy III'-III in vapor form III "is then recovered simultaneously in steps Fj _. And F ' _3. This example also makes it possible to illustrate the application of the invention for the recovery of energy from exothermic reactions which are located on gases which have given up their heat to process fluids and not on the process gases themselves.

Example 3: Application in the case of an industrial process for the manufacture of acetic acid Another example of application of the present invention may be the direct oxidation of n-butene in air to acetic acid as described in "Petrochemical Processes 2 "by Chauvel et al. The whole process is under pressure and it is described in a simplified manner in FIG. 13 and the principle is represented in FIG. 1 A. The global equation can be summarized by:

nC ₄ H ₈ + 2 O ₂ = 2 CH ₃ -COOH

The reaction is exothermic and takes place in the gas phase in the presence of a large excess of oxygen (and consequently of air) under 15-30 bar. The oxidation reaction A '* _! between air (I) introduced in 201 and n-butene (II) introduced in 203 a er, in the presence of a catalyst, for example in the presence of vanadium oxide, in a reactor 204. This reactor can be a reactor at fixed ht or at fluidized ht and in this case, the catalyst is introduced in 209. The gaseous product of the reaction is cooled (step F ") in an exchanger 205, and at the same time generates for example steam at 220, then the cooled product is washed in a scrubber-type washing device 206 to extract (step Ci) the organic compounds in a water phase at 207.

The residual gas issuing in 211 from step Ci is then preheated in an exchanger 218 and burned (step A " ₂ ) in a finishing oven 212 to remove any trace of hydrocarbon before discharge. The combustion fumes in 219, always under pressure, are cooled in step F "ι against the current in P exchanger 218 and then expanded in a recovery turbine 214 (step D) to generate mechanical power retransmitted to the air compressor 202. The exchanger 218 ensures good thermal efficiency of the overall process and facilitates the combustion of unreacted hydrocarbons contained in the gaseous product at 211.

From the liquid phase recovered in 207, it is sought to extract the final product, acetic acid in 208, by a succession of distihation steps with or without solvent and flashes, all of these steps being denoted C ₂ . The catalyst

1 present in this liquid, containing non-separated acetic acid, can also be recovered by a centrifugation device 210 (step E "ι) carried out on the liquid products recovered in 216 throughout these separation steps. catalyst recovered in 209 will then be reinjected into reactor 204.

FIG. 14 which illustrates the improvement of the invention, is then described with reference to an increase in capacity of the unit shown in FIG. 13. This increase in capacity consists in doping oxygen in 222, the air used for the oxidation reaction carried out in step A "ι. This figure illustrates the improvement proposed by the invention applied to the oxidation process of n-butene. When an increase in capacity is desired and the compressor 202 can no longer circulate more air, it is advisable to inject oxygen (I ") under pressure directly into the reactor 204. It is then necessary to maintain a pressure high enough to force the gaseous product through the exchanger 205 and the washer 206. As mentioned above, it may then be necessary to reduce the air flow rate passing through the compressor 202 in order to be able to circulate, under normal conditions of pressure and temperature, the excess t of gaseous product created by the increase in capacity. After drawing off the liquid product in 207 in the washer 206, the gaseous effluent sees its flow decrease with respect to the flow generated by the reactor 204 compared to normal operation. The turbine 214 then sees its output decrease due to the dύninution of the nitrogen ballast in the reaction step A "ι. The application of the invention therefore consists in recovering the excess energy given up by the hot gases from the reactor 204 in the exchanger 205 in order to use it to increase the energy value of the gas phase leaving the washer 206.

It can be clearly seen in this example that the use of this excess energy III ″ may not be optimal if it is used just before stage D of recovery of the compression energy of part of said compressed gas at B by example by transferring it at 221a directly into the gaseous product at 213. It may be wise to increase the temperature of the flow at 217 by using this energy which is introduced in 221b, however if energy III is recovered in the form of steam in 220, it may prove contraindicated to inject steam in 221 so as not to compromise the smooth running of the reactions involved in the oven 212. We can then consider injecting the excess of vapor in 221c in the hot fumes in 219. Thus the energy III "will be given up at two points. Thus, according to this example, another apphcation of the invention is illustrated which consists in making the best use of the excess heat by not not necessarily limited to a point of use of said energy.

Claims

1. Improvement in debottlenecking processes intended to increase the production capacity of a unit producing a chemical in a chemical process comprising:

at least one step (B) for compressing a gaseous fluid containing at least part of the reagents entering into said chemical process or products resulting from a step of reacting said reagents,

at least one part produced under pressure with at least one step (C) during which a product derived from said chemical process is drawn off and a gaseous residue is recovered,

- At least one recovery step (D) of at least part of the thermal power (W) of at least part of said gaseous residue, the chemical process further comprising at least one step (F) during which energy is recovered, said debottlenecking being accompanied by the production of excess energy in at least one step (F) and by a reduction in the ratio between the thermal power (W) and the total flow of products withdrawn during of said chemical process, said improvement consisting in using at least part of the excess energy recovered during at least one step (F) to increase the thermal power (W) of at least part of said fluid residue.

2. Improvement according to claim 1, ^' characterized in that said decrease in said ratio between the heat flow (W) and the total flow of products withdrawn during said chemical process is due to an increase in the flow of products withdrawn.

3. Improvement according to one of claims 1 or 2, characterized in that the energy recovered in at least one of the steps (F) is recovered in the form of vapor under pressure.

4. Improvement according to one of claims 1 to 3, characterized in that said excess energy is recovered in the form of vapor and injected into said gaseous residue.

5. Improvement according to one of claims 1 to 4, characterized in that said energy recovered in one of the steps (F) is used at several points of said gaseous residue.

6. Improvement according to one of claims 1 to 5, characterized in that the flow subjected to said compression step comprises gases inert with respect to said chemical process.

7. Improvement according to one of claims 1 to 6, characterized in that said energy recovered in at least one of the steps (F) comes from at least one exothermic reaction (A).

8. Improvement according to claim 7, characterized in that said exothermic reaction is an oxidation.

9. Improvement according to one of claims 7 or 8, characterized

₄ r in that said exothermic reaction implements at least part of the reactants entering into said chemical process.

10. Improvement according to one of claims 1 to 9, characterized in that said debottlenecking is carried out with an additional injection of oxygen.

11. Improvement according to claim 10, characterized in that said additional oxygen injection is accompanied by a reduction in the inert gases circulating in said unit.

12. Improvement according to one of claims 1 to 11, according to which said chemical process comprises a step of exothermic reaction carried out in the presence of an inert gas with respect to said exothermic reaction, at least one step of compression of the product. gaseous from said reaction followed by at least one step during which at least one product from said chemical process is separated from said gaseous product, the gaseous residue then being subjected to a step of recovering its energy and according to which said stripping is carried out with a decrease in the proportion of inert gas with respect to said exothermic reaction in said gaseous residue and an increase in the flow of reactants used in said exothermic reaction, characterized in that part of said excess energy supplied by the exothermic reaction is used to increase the energy of the gaseous residue before it is subjected to said step of energy recovery.

13. Use of the improvement according to one of claims 1 to 12 in the case of the manufacture of nitric acid in a process using:

a step for the oxidation of rammoniac by oxygen-enriched air to form nitrogen oxides, at the end of which energy is recovered, a step of oxidizing nitrogen monoxide to nitrogen dioxide and / or its dimer,

a step of reacting nitrogen dioxide and / or its dimer with water to form nitric acid, at the end of which the nitric acid is drawn off and a gaseous residue is recovered.

14. Use according to claim 13 according to which said debottlenecking is done with a doping of the air with oxygen leading to the recovery of excess energy at the end of the ammonia oxidation step, characterized in that said excess energy is used to increase the energy flow of said gaseous residue after the withdrawal of nitric acid.

15. Use of the process according to one of claims 1 to 12 in the case of the synthesis of ammonia in a process using:

- a primary reforming step (A ' ₂ and F' ₂ ),

- a secondary or autόthermal reforming step (A'i and E'i) leading to the formation of a synthesis gas,

- a stage of treatment of synthesis gas (E ' ₂ ) followed by:

- a synthesis gas cooling step (F'i),

a loop for synthesizing the ammonia comprising a step of withdrawing the ammonia and rejecting a gaseous residue under pressure,

16. Use according to claim 15, according to which said debottlenecking comprises an oxygen doping of the reaction carried out before or in the secondary reforming step and a nitrogen doping of the reaction carried out before or in the ammonia synthesis loop. , leading to the production of excess energy at the end of the secondary reforming steps, characterized in that said excess energy is used to increase the energy flow of the gaseous residue from said rammoniac synthesis loop.

17. Use according to claim 15 or 16, characterized in that one further carries out a doping with combustion oxygen during the primary reforming step, leading to the supply of excess energy than the it is used to increase the energy flow of said residual gas from said rammoniac synthesis loop.

18. Use of the process according to one of claims 1 to 12 in the case of the synthesis of acetic acid in a process using a catalytic oxidation step of n-butene doped with oxygen.

19. Use according to claim 18, characterized in that the excess energy supplied by the oxidation reaction of n-butene is recovered to increase the energy of the gaseous residue obtained after drawing off the acetic acid.