Energy-optimized process for dehydrating alcohols into olefins

Abstract

The present invention relates to a process for dehydrating an alcohol feedstock comprising: Vaporizing said alcohol feedstock mixed with a gas stream; Introducing said alcohol feedstock, vaporized and mixed with said gas stream, into a set of at least two adiabatic reactors; Separating the effluent from the last adiabatic reactor of step b) into an effluent comprising the olefin(s) corresponding to the dehydration of the feedstock and an aqueous effluent; Compressing the effluent comprising the olefin(s) from step c) in at least one compressor; Dividing the compressed effluent from step d) into two fractions; and then Recycling, in part or in whole, a first fraction of the compressed effluent upstream of step a) to constitute said gas stream. Figure 2 to be published

Description

Title of the invention: Process for dehydrating alcohols into olefins with energy optimization technical field

[0001] The present invention relates to a process for dehydrating alcohols into olefins, and in particular an alcohol having two carbon atoms, an alcohol having three carbon atoms, or mixtures thereof, into the corresponding olefin(s), exhibiting a satisfactory yield of the corresponding olefin(s) while advantageously limiting energy consumption. Prior art

[0002] Olefins are traditionally produced by catalytic cracking or steam cracking of hydrocarbons. To address the challenges related to dwindling petroleum resources and environmental concerns, it is necessary to develop non-petroleum-based methods for producing olefins such as ethylene and propylene. Such olefins are useful raw materials for numerous applications, including petrochemical applications, particularly for the production of polymers such as polyethylene, polypropylene, and other plastics, as well as applications related to fuel production where olefins are oligomerized into fuel fractions.

[0003] One of the most studied routes currently for the production of olefins is that of the catalytic dehydration of alcohols.

[0004] The dehydration reaction of ethanol to ethylene has been known and described in detail since the late 19th century. It is known that this reaction is highly endothermic, equilibrium-shifted towards ethylene at high temperatures. The temperature drop corresponding to the complete conversion of pure ethanol is 380°C. The article by H. Knôzinger and R. Kôhne, “The Dehydration of Alcohols over Alumina: The reaction scheme”, Journal of Catalysis (1966), 5, 264-270, considered the reference publication on the dehydration of alcohols, including ethanol, specifies that the catalyst often used to catalyze the dehydration reaction of ethanol is a monofunctional acidic catalyst, for example, gamma alumina.

[0005] Zeolites are also used for this application, and in particular ZSM5 since the 1980s, as for example in the article SN Chaudhuri et al., "Reactions of ethanol over ZSM-5", Journal of Molecular Catalysis 62:289-295 (1990).

[0006] In many alcohol dehydration processes, a diluent, in particular water, is used to reduce the Tendotherm of the dehydration reaction.

[0007] French patent FR 2998568 B1 relates to a process for dehydrating a charge The process converts ethanol to ethylene, including a step of vaporizing the ethanol feedstock, mixed with at least a portion of a treated water stream, which then acts as a thermal reaction diluent. This implementation requires the use of a compressor to vaporize the diluted feedstock.

[0008] Patent TW202308968 A describes a process for manufacturing propylene, by dehydration of propanol, in the presence of a large quantity of water.

[0009] French patent FR 3013707Blse relates to a process for producing a mixture of ethylene and propylene by dehydrating a mixture containing water, ethanol, and isopropanol, wherein the water content is between 30 and 75% by weight relative to the total weight of the mixture. French patent FR 3 013 708 B1 describes this same process, in which isopropanol is replaced by n-propanol.

[0010] These large quantities of water, used as a thermal fluid, make it possible to reduce the endotherm of the dehydration reactions of alcohols.

[0011] However, the use of water requires a vaporization step, usually coupled with the use of a compressor, which makes dehydration processes expensive, particularly in terms of investment and energy consumption.

[0012] To reduce investment costs and energy consumption associated with alcohol dehydration processes, an alternative was proposed consisting of replacing water with a gas, while retaining the effect on the endotherm.

[0013] US patent 4,232,179 A describes a process for dehydrating ethanol to ethylene in which the heat loss due to the dehydration reaction is at least partially compensated by introducing a heat transfer fluid into the reactor, mixed with the ethanol feed. The heat transfer fluid is either steam from an external source, an external flow from the process, or the recycling of a portion of the effluent from the dehydration reactor, i.e., including the ethylene produced, the generated water, and possibly by-products. The introduction of the feed mixture with said heat transfer fluid provides the heat necessary to maintain the temperature of the catalytic bed at a level compatible with the desired conversion.In cases where the heat transfer fluid is the effluent from the dehydration reactor, the introduction of additional equipment, a recycle compressor, is necessary for effluent recycling. Thus, this process does not improve the feed vaporization step but only reduces the endotherm of the dehydration reaction.

[0014] Application WO 2009 / 098268 A1 relates to a process for dehydrating ethanol to ethylene in which the endotherm is at least partially compensated by the introduction into the reactor of an inert heat transfer fluid of the hydrocarbon or CO2 type mixed with the feed. This heat transfer fluid provides a beneficial effect on The catalyst's performance is enhanced while improving the reactor's thermal balance through a dilution effect. Furthermore, the reactor pressure must be sufficient to allow the heat transfer fluid to be recycled via a pump, avoiding the need for a recycle compressor, thus minimizing costs.

[0015] Application WO 2007 / 063280 A1 describes a process for producing propylene by dehydrating propanol alone or in a mixture with ethanol, involving the use, prior to dehydration, of a column to isolate the propanol from the ethanol. The catalyst used for dehydration may be a heteropolyacid. The feedstock may include water, but due to the associated costs, particularly related to the size of the reactor used and water vaporization, the amount of water should preferably be less than 10% by weight relative to the total weight of the feedstock. Furthermore, the use of a mixture of ether and alcohol allows for higher yield and selectivity of olefin. A portion of the ether is then preferably recycled back into the dehydration reactors.

[0016] Although some alcohol dehydration processes have been able to do without the use of additional equipment such as compressors, the search for ever more efficient and cost-effective processes continues.

[0017] US patent application 2023 / 065667 Al relates to the conversion of alcohols comprising 1 to 5 carbon atoms (C1-C5) into olefin mixtures comprising 2 to 5 carbon atoms (C2-C5), by dehydration of the alcohol and oligomerization of the primary olefins generated in the presence of a single catalytic system, in a single reactor. These C2-C5 olefins can be readily oligomerized into basic products used in fuel production. A portion of the olefins recovered from the reactor outlet is recycled upstream of the reactor and combined with the C1-C5 alcohol feed. The process described in US patent application 2023 / 065667 Al results in a yield of ethylene, propylene, and butene exceeding 80% by weight.

[0018] Reducing energy consumption remains, however, one of the objectives in the field of alcohol-to-olefin conversion.

[0019] French patent FR2978145 B1 proposes a process for dehydrating an ethanol feedstock into ethylene, comprising in particular vaporizing said feedstock mixed with at least a portion of a purified, recycled water stream and compressing the vaporized mixture in a compressor before its introduction into at least one reactor where the dehydration reaction takes place. This process thus includes recycling a portion of the generated water as a heat transfer fluid. The process achieves high ethylene selectivity. While this technical solution significantly reduces energy consumption, it is nonetheless costly because it requires the use of a compressor to vaporize the diluted feedstock. Furthermore, this compressor operates under specific conditions, particularly in a to have a high temperature at the compressor outlet, and with the risk of being in contact with products that are not totally inert, for example compounds containing nitrogen, sulfur, oxygenated compounds.

[0020] US patent application 2024 / 0301296 A1 describes a process for converting oxygenated compounds into olefins comprising introducing said oxygenated compounds into a multi-stage adiabatic reactor comprising catalytic beds. The reactor thus comprises at least two inlets for the oxygenated compound feedstocks, such that the first inlet feedstock in contact with the first bed produces a reaction mixture which is mixed with a second inlet feedstock to produce a first effluent. This conversion process involves the combination of endothermic dehydration of the oxygenated compounds followed by exothermic oligomerization of the olefins. The multi-stage reactor thus makes it possible to balance the endothermic dehydration of the oxygenated compounds and the exothermic oligomerization of the olefins. The outlet stream of the multi-stage reactor comprises olefins containing between 2 and 5 carbon atoms (C2 to C5).If the endothermic nature of the dehydration reaction is compensated by the exothermic nature of oligomerization, improved selectivity in olefin mixtures (C2 to C5) is also desired.

[0021] The objective of the present invention is to propose a process for dehydrating a feed containing an alcohol having two carbon atoms, an alcohol having three carbon atoms or mixtures thereof, into the corresponding olefin(s), exhibiting reduced energy consumption and reduced investment and operating costs, said process also exhibiting excellent performance in terms of selectivity and conversion. Summary of the invention

[0022] The invention describes a process for dehydrating an alcohol feed containing an alcohol having two carbon atoms, an alcohol having three carbon atoms, or mixtures thereof, into the corresponding olefin(s) comprising: a. The vaporization of said alcohol charge mixed with a gas stream, preferably in a heat exchanger, and preferably by heat exchange with a hot stream, b. The introduction of said alcohol feedstock, vaporized and mixed with said gas stream, into a set of at least two adiabatic reactors in series, each of said at least two adiabatic reactors of said set containing at least one dehydration catalyst and in which the dehydration reaction takes place, at an inlet temperature into said set of between 300 and 550 °C and at an inlet pressure into said set between 0.3 and 1.8 MPa, the effluent from the last adiabatic reactor constituting said hot stream, c. Separation of the effluent from the last adiabatic reactor from step b) into an effluent comprising the olefin(s) corresponding to the dewatering of the feedstock, at a pressure below 1.6 MPa, and an aqueous effluent, d. Compression of the effluent comprising the olefin(s), from step c), in at least one compressor, e. The division into two fractions of the compressed effluent from step d) and comprising the olefin(s), then f. the recycling, in part or in whole, of a first fraction of the compressed effluent, upstream of step a) to constitute said gaseous stream, g. possibly, the purification of the second fraction of the compressed effluent.

[0023] The alcohol filler contains an alcohol having two carbon atoms, an alcohol having three carbon atoms, or mixtures thereof. The alcohol having two carbon atoms advantageously corresponds to a mono-alcohol comprising two carbon atoms, i.e., ethanol. The alcohol having three carbon atoms advantageously corresponds to a mono-alcohol comprising three carbon atoms, i.e., propanol, and more particularly to n-propanol or isopropanol (i-propanol), or a mixture of n-propanol and i-propanol.

[0024] In an embodiment where the feed contains ethanol, the process according to the invention is a process for dehydrating ethanol from an alcohol feed, to ethylene comprising: a. The vaporization of said alcohol charge mixed with a gas stream, preferably in a heat exchanger, and preferably by heat exchange with a hot stream, b. The introduction of said alcohol feedstock, vaporized and mixed with said gas stream, into a set of at least two adiabatic reactors in series, each of said at least two adiabatic reactors of said set containing at least one dehydration catalyst and in which the dehydration reaction takes place, at an inlet temperature into said set of between 300 and 550 °C and at an inlet pressure into said set of between 0.3 and 1.8 MPa, the effluent from the last adiabatic reactor constituting said hot stream, c. Separation of the effluent from the last adiabatic reactor of step b) into an effluent comprising ethylene, at a pressure below 1.6 MPa, and an aqueous effluent, d. Compression of the effluent containing ethylene, from step c), in at least one compressor, e. The division into two fractions of the compressed effluent from step d), then f. the recycling, in part or in whole, of a first fraction of the effluent compressed, upstream of step a) to constitute said gaseous flow, g. The purification of the second fraction of the compressed effluent.

[0025] In an embodiment where the feed contains isopropanol (i-propanol) or n-propanol, preferably i-propanol, the process according to the invention is a process for dehydrating i-propanol or n-propanol, preferably i-propanol, from an alcohol feed, in propylene comprising: a. The vaporization of said alcohol charge mixed with a gas stream, preferably in a heat exchanger, and preferably by heat exchange with a hot stream, b. The introduction of said alcohol feedstock, vaporized and mixed with said gas stream, into a set of at least two adiabatic reactors in series, each of said at least two adiabatic reactors of said set containing at least one dehydration catalyst and in which the dehydration reaction takes place, at an inlet temperature into said set of between 300 and 550 °C and at an inlet pressure into said set of between 0.3 and 1.8 MPa, the effluent from the last adiabatic reactor constituting said hot stream, c. Separation of the effluent from the last adiabatic reactor of step b) into an effluent comprising propylene, at a pressure below 1.6 MPa, and an aqueous effluent, d. Compression of the effluent containing propylene, from step c), in at least one compressor, e. The division into two fractions of the compressed effluent from step d), then f. the recycling, in part or in whole, of a first fraction of the effluent compressed, upstream of step a) to constitute said gaseous flow, g. Optionally, the purification of the second fraction of the compressed effluent.

[0026] In an embodiment where the feed contains a mixture comprising ethanol and isopropanol (i-propanol) or a mixture comprising ethanol and n-propanol, or a mixture comprising ethanol, n-propanol and i-propanol, preferably a mixture comprising ethanol and i-propanol, the process according to the invention is a process for dehydrating a mixture of ethanol and isopropanol (i-propanol) or a mixture of ethanol and n-propanol or a mixture of ethanol, n-propanol and i-propanol, preferably a mixture of ethanol and i- propanol, contained in an alcohol filler, in a mixture of ethylene and propylene comprising: a. The vaporization of said alcohol charge, mixed with a gaseous stream, preferably in a heat exchanger, and preferably by heat exchange with a hot stream, b. The introduction of said alcohol feedstock, vaporized and mixed with said gas stream, into a set of at least two adiabatic reactors in series, each of said at least two adiabatic reactors of said set containing at least one dehydration catalyst and in which the dehydration reaction takes place, at an inlet temperature into said set of between 300 and 550 °C and at an inlet pressure into said set of between 0.3 and 1.8 MPa, the effluent from the last adiabatic reactor constituting said hot stream, c. Separation of the effluent from the last adiabatic reactor of step b) into an effluent comprising a mixture of ethylene and propylene, at a pressure below 1.6 MPa, and an aqueous effluent, d. Compression of the effluent comprising a mixture of ethylene and propylene, obtained from step c), in at least one compressor, e. The division into two fractions of the compressed effluent from step d), then f. the recycling, in part or in whole, of a first fraction of the effluent compressed, upstream of step a) to constitute said gaseous flow, g. Optionally, the purification of the second fraction of the compressed effluent, comprising the mixture of ethylene and propylene, from step d).

[0027] The process according to the invention, by recycling a portion of the olefins upstream of step a) of feed vaporization, has the advantage over prior art processes of exhibiting excellent properties in terms of selectivity towards olefins and conversion, and of preserving the effect on the endotherm of the dehydration reactions, while limiting energy consumption. It also simplifies the process, notably by eliminating a feed compression step, and reduces the operational and investment costs of said process.

[0028] To reduce investment costs and in particular to avoid the use of additional equipment such as compressors, especially used to compress the alcohol feed mixed with a thermal diluent, in particular water, before its introduction into the reaction section, but also to reduce the energy consumption of the process, the inventors have replaced, at least in part, the water used as a thermal diluent, by a gas or a mixture of gases which is inert with respect to the dehydration reaction under the operating conditions from the dehydration step and which remains in a gaseous state at room temperature. This diluent includes the olefin(s) produced by the dehydration reaction.

[0029] In addition, thanks to judicious recycling, it is possible to reinject part of the olefin(s) produced with the liquid feed, upstream of step a) of feed vaporization and thus optimize the energy recovery of the process. List of figures [Fig 1]

[0030] Fig. 1 schematically represents the process of dehydrating a feed containing an alcohol having two carbon atoms, an alcohol having three carbon atoms or mixtures thereof, mixed with purified water, wherein a portion of the purified water stream and a portion of an unconverted alcohol stream is recycled upstream of step a) of vaporization.

[0031] The alcohol feed (1), optionally pretreated by any technique known to those skilled in the art, is mixed with a portion of a recycled purified water stream (6), and optionally with unconverted alcohol (not shown). The alcohol feed, optionally pretreated, mixed with a portion of the recycled purified water stream and a portion of the unconverted alcohol stream, is introduced into a heat exchanger in which the mixture undergoes heat exchange with the effluent (4) from the last adiabatic reactor. The enthalpy of condensation of the effluent from the last adiabatic reactor is used to vaporize the alcohol feed mixed with the recycled purified water stream and the unconverted alcohol stream, without the need for external heat input.

[0032] The alcohol charge mixed with the recycled purified water stream and the unconverted alcohol stream, vaporized, is then sent into a compressor.

[0033] The vaporized and compressed mixture of the feed and the two streams is then sent to a second single-phase gas heat exchanger, in which the mixture is heated by heat exchange with the effluent (4) from the last adiabatic reactor, which is introduced into the second heat exchanger. In the second single-phase gas heat exchanger, the vaporized and compressed feed is superheated, and the effluent from the last adiabatic reactor, in gaseous form, is desuperheated without being condensed.

[0034] The mixture of the feed and the two streams, vaporized, compressed, and heated in the single-phase gas heat exchanger, is then introduced into a furnace before being introduced into a first adiabatic reactor via line (2), so as to bring it to an inlet temperature in said first adiabatic reactor compatible with the temperature of the dehydration reaction. The effluent from the first reactor is sent to a second furnace before being introduced into a second adiabatic reactor via line (3).

[0035] The effluent from the second reactor then undergoes the two successive exchanges described above in the two exchangers.

[0036] The effluent from the second adiabatic reactor is sent to a gas / liquid separation column where it is separated into an effluent containing the olefin resulting from the dehydration of the alcohol (5) and an effluent containing water. The effluent containing the olefin (5) then undergoes compression before its final purification. A portion of the effluent containing water is recycled after cooling back into said gas / liquid separation column.

[0037] The portion of the effluent containing water not recycled in said gas / liquid separation column is sent to a second column. At least one stream of purified water (6) and (7) and at least one stream of unconverted alcohol (not shown) are then separated by distillation. A stream containing the light gases is also separated.

[0038] A portion of said unreacted alcohol stream (6b) (not shown) is thus recycled and mixed with at least a portion of the recycled purified water stream (6). The mixture of these two streams is recycled upstream of the first exchanger, to the optionally pretreated alcohol feed (1). The stream containing the light gases is returned to said gas / liquid separation column. [Fig 2]

[0039] Fig. 2 schematically represents the process of dehydrating a feed containing an alcohol having two carbon atoms, an alcohol having three carbon atoms or mixtures thereof, mixed with a gaseous effluent, in which part of the compressed effluent is recycled upstream of step a) of vaporization.

[0040] The alcohol feed (1), optionally pretreated by any technique known to those skilled in the art, is mixed with a portion of a recycled olefin stream (18). The alcohol feed, optionally pretreated, mixed with a portion of the recycled olefin stream, is introduced into a heat exchanger in which the mixture undergoes heat exchange with the effluent from the last adiabatic reactor. The enthalpy of condensation of the effluent from the last adiabatic reactor is used to partially vaporize the alcohol feed mixed with the recycled olefin stream, without the need for external heat input.

[0041] The alcohol feed mixed with the olefins stream and partially vaporized is separated into a gas stream (3) and a liquid stream (4). The liquid stream (4) enters a gas / liquid separation column along with a portion of the unconverted alcohol stream (14) so ​​as to recover the ethanol at the top of the column (5) and the aqueous phase at the bottom via the purge line (6).

[0042] The gas stream (5) is mixed with the stream (3) and then sent to a second single-phase gas heat exchanger, in which the mixture is heated by means of a heat exchange with the effluent (11) from the last adiabatic reactor which is introduced into said second exchanger. In said second single-phase gas exchanger, said vaporized charge is superheated and the effluent (11) from the last adiabatic reactor in gaseous state is "desuperheated" without being condensed into a stream (12).

[0043] The mixture of the vaporized and heated feed in the single-phase gas heat exchanger is then introduced into a furnace via line (7), before being introduced into a first adiabatic reactor via line (8), so as to bring it to an inlet temperature in said first adiabatic reactor compatible with the temperature of the dehydration reaction. The effluent from the first reactor is sent to a second furnace before being introduced into a second adiabatic reactor via line (10).

[0044] The effluent from the second reactor then undergoes the two successive exchanges described above in the two exchangers.

[0045] The effluent from the second adiabatic reactor (13) is sent to a gas / liquid separation column where it is separated into an effluent comprising the olefin resulting from the dehydration of the alcohol (16) and an effluent comprising water and possibly unconverted alcohol. A portion of the effluent comprising water and possibly some of the unconverted alcohol is sent via the line (14) to the gas / liquid separation column in order to recycle the unconverted alcohol, as described previously. The effluent containing the olefin, recovered at the top of the gas / liquid separation column (16), then undergoes compression.

[0046] The compressed effluent comprising the olefin (17) is divided into two parts: - A first fraction (18) which is recycled in part or in whole upstream of the mixing stage with the feed and - A second fraction (19) which is purified. Description of the implementation methods

[0047] According to the present invention, the expression "between ... and ..." means that the limit values ​​of the interval are included within the described range of values. If this were not the case and the limit values ​​were not included within the described range, this clarification will be provided by the present invention.

[0048] In the sense of the present invention, the different parameter ranges for a given step, such as pressure ranges and temperature ranges, can be used alone or in combination. For example, in the sense of the present invention, a preferred range of pressure values ​​can be combined with a more preferred range of temperature values.

[0049] In the context of the present invention, the expression "upstream of..." means being before the step of the process considered or before the device considered and the expression " "downstream of..." means to be after the process step considered or after the device considered.

[0050] In the following, particular embodiments of the invention may be described. They may be implemented separately or in combination with each other, without limitation of combinations where technically feasible.

[0051] The invention relates to a process for dehydrating a feed containing a two-carbon alcohol, a three-carbon alcohol, or mixtures thereof, into the corresponding olefin(s), offering a significant reduction in energy consumption compared to prior art processes, while maintaining high selectivity and conversion comparable to prior art processes that use water as a diluent. Furthermore, by recycling a portion of the olefins produced during the dehydration reaction upstream of step a) vaporizing said feed, the process according to the invention avoids the use of additional equipment, such as a compressor in a step compressing the feed mixture with the diluent, thereby considerably reducing the investment costs associated with said process. The charge

[0052] According to the invention, the alcohol feed contains an alcohol having two carbon atoms, an alcohol having three carbon atoms or mixtures thereof, said alcohol feed being mixed with a portion of a recycled olefin stream.

[0053] According to the invention, alcohol having two carbon atoms and alcohol having three carbon atoms are mono-alcohols, that is to say alcohols comprising only one hydroxyl function.

[0054] Thus, according to the invention, the alcohol having two carbon atoms corresponds to ethanol. The alcohol having three carbon atoms corresponds to propanol, and more particularly to n-propanol or isopropanol (i-propanol) or a mixture of n-propanol and i-propanol, preferably the alcohol having three carbon atoms corresponds to i-propanol.

[0055] The charge may thus contain ethanol, n-propanol, isopropanol (i-propanol), or mixtures thereof. The charge may therefore contain ethanol, n-propanol, i-propanol, a mixture comprising ethanol and n-propanol, a mixture comprising ethanol and i-propanol, a mixture comprising n-propanol and i-propanol, or a mixture comprising ethanol, i-propanol, and n-propanol. Preferably, the charge contains ethanol, i-propanol, or a mixture comprising ethanol and i-propanol.

[0056] Preferably, the charge contains at least 25% by weight, preferably at least 50% by weight, preferably at least 80% by weight of an alcohol having two atoms of carbon (ethanol), of an alcohol having three carbon atoms or mixtures thereof, the percentages being given here in weight relative to the total weight of the charge.

[0057] In particular, the charge comprises between 25% and 99.9% by weight, preferably between 50% and 99% by weight of an alcohol having two carbon atoms (ethanol), of an alcohol having three carbon atoms or mixtures thereof, the percentages being given here in weight relative to the total weight of the charge.

[0058] When the charge is a mixture of an alcohol having two carbon atoms and an alcohol having three carbon atoms, the charge preferably contains between 10% and 90% by weight of alcohol having two carbon atoms and between 90% and 10% by weight of alcohol having three carbon atoms, preferably between 20% and 80% by weight of alcohol having two carbon atoms and between 80% and 20% by weight of alcohol having three carbon atoms and preferably between 30% and 70% by weight of alcohol having two carbon atoms and between 70% and 30% by weight of alcohol having three carbon atoms, relative to the total weight of the charge.

[0059] When the charge is a mixture of ethanol, n-propanol and isopropanol, the charge preferably contains between 10% and 90% by weight of ethanol, between 45% and 5% by weight of n-propanol and between 45% and 5% by weight of isopropanol, preferably between 20% and 80% by weight of ethanol, between 40% by weight and 10% by weight of n-propanol and between 40% and 10% by weight of isopropanol, preferably between 30% and 70% by weight of ethanol, between 35% and 15% by weight of n-propanol and between 35% and 15% by weight of isopropanol, relative to the total weight of the charge.

[0060] When the alcohol in the charge contains two carbon atoms, it contains at least 80%, preferably at least 90%, preferably at least 95% by weight of ethanol, the percentages being given here in weight relative to the total weight of the alcohols contained in the charge.

[0061] In particular, the charge comprises between 80% and 99.9%, preferably between 90% and 99% by weight of ethanol, the percentages being given here in weight relative to the total weight of the alcohols contained in the charge.

[0062] When the alcohol of the charge contains three carbon atoms, it contains at least 70%, preferably at least 80%, preferably at least 90% by weight of n-propanol and / or i-propanol, the percentages being given here in weight relative to the total weight of the alcohols contained in the charge.

[0063] In particular, the alcohol charge comprises between 70% and 99.9%, preferably between 80% and 99% by weight of n-propanol and / or i-propanol, the percentages being given here in weight relative to the total weight of the alcohols contained in the charge.

[0064] When the alcohol charge contains a mixture of an alcohol having two carbon atoms and an alcohol having three carbon atoms, this mixture contains between 10% and 90% by weight of an alcohol having two carbon atoms and between 90% and 10% by weight of alcohol having three carbon atoms, preferably between 20% and 80% by weight of alcohol having two carbon atoms and between 80% and 20% by weight of alcohol having three carbon atoms and preferably between 30% and 70% by weight of alcohol having two carbon atoms and between 70% and 30% by weight of alcohol having three carbon atoms relative to the total weight of alcohols contained in the charge.

[0065] When the feed is a mixture of ethanol, n-propanol and isopropanol, the feed preferably contains between 10% and 90% by weight of ethanol, between 45% and 5% by weight of n-propanol and between 45% and 5% by weight of isopropanol, preferably between 20% and 80% by weight of ethanol, between 40% by weight and 10% by weight of n-propanol and between 40% and 10% by weight of isopropanol, preferably between 30% and 70% by weight of ethanol, between 35% and 15% by weight of n-propanol and between 35% and 15% by weight of isopropanol, relative to the total weight of the alcohols contained in the feed.

[0066] The alcohol filler containing an alcohol having two carbon atoms, an alcohol having three carbon atoms, or mixtures thereof may also include other alcohols, such as butanol and / or pentanol. According to the invention, the expressions "filler containing an alcohol having two carbon atoms" and "ethanol filler" are interchangeable.

[0067] According to the invention, the expressions "charge containing an alcohol having three carbon atoms" and "propanol charge" are interchangeable.

[0068] According to the invention, the expression "the charge contains a mixture of ethanol and n-propanol" means that the charge contains a mixture comprising predominantly ethanol and n-propanol relative to the other alcohols that may be included in the charge, the term "predominantly" meaning at least 70%, preferably at least 80%, and preferably at least 90% of a mixture of ethanol and n-propanol. The expression "the charge contains a mixture of ethanol and i-propanol" means that the charge contains a mixture comprising predominantly ethanol and i-propanol relative to the other alcohols that may be included in the charge, the term "predominantly" meaning at least 70%, preferably at least 80%, and preferably at least 90% of a mixture of ethanol and i-propanol.

[0069] According to the invention, the expression "the charge contains a mixture of ethanol, n-propanol and i-propanol" means that the charge contains a mixture comprising predominantly ethanol, n-propanol and i-propanol relative to the other alcohols that would be included in the charge, the term "predominantly" meaning at least 70%, preferably at least 80%, preferably at least 90% of a mixture of ethanol, n-propanol and i-propanol,

[0070] The alcohol charge may include other compounds, different from the alcohols defined above, such as water, for example, at a content of between 0.1 and 30% by weight relative to the total weight of the alcohol charge; oxygenated compounds other than alcohols, preferably at a content less than or equal to 1% by weight in relation to the total weight of the charge, for example ethers, acids, ketones, aldehydes and / or esters, and impurities in particular nitrogenous and / or sulfurous compounds, organic and mineral, preferably at a nitrogen and sulfur content less than or equal to 0.5% by weight in relation to the total weight of the alcohol charge, the percentages by weight being expressed in relation to the total weight of said alcohol charge.

[0071] Advantageously, said alcohol charge, containing an alcohol having two carbon atoms, an alcohol having three carbon atoms, or mixtures thereof, may be derived from non-fossil resources. Preferably, it is produced from renewable resources derived from biomass, preferably by fermentation of sugars from, for example, sugar crops such as sugar cane (sucrose, glucose, fructose, and sucrose), beets, or starchy plants (starch), or from lignocellulosic biomass or hydrolyzed cellulose (predominantly glucose and xylose, galactose), containing varying amounts of water.

[0072] Said alcohol charge containing an alcohol having two carbon atoms, an alcohol having three carbon atoms or mixtures thereof, can also be obtained from synthesis gas.

[0073] It can possibly be obtained by a process of synthesizing alcohol from fossil resources such as, for example, from coal, natural gas or carbonaceous waste.

[0074] Said charge containing an alcohol having two carbon atoms, an alcohol having three carbon atoms or mixtures thereof, can advantageously be obtained by hydrogenation of the corresponding acids or esters.

[0075] Optionally, said alcohol feed containing a two-carbon alcohol, a three-carbon alcohol, or mixtures thereof, is pretreated on an acidic solid and / or by partial evaporation and gas-liquid separation. This pretreatment step removes impurities contained in said feed, in particular nitrogen-containing compounds, sulfur-containing compounds, and optionally oxygenated compounds.

[0076] According to the present invention, the selectivity of the process is defined as the ratio between the quantity of target olefins produced and all the products resulting from the dehydration of the feedstock, excluding water and unconverted feedstock. According to the present invention, the expression "target olefins" means the olefins obtained from the dehydration of the alcohol feedstock.

[0077] According to the present invention, the feed conversion is defined as the ratio between the quantity of feed converted by the process and the quantity of feed entering the dehydration step.

[0078] The quantity of the converted charge is defined as the difference between the quantity of the charge entering the dehydration step and the quantity of the charge exiting the dehydration step.

[0079] According to the present invention, the process yield is defined as the ratio between the quantity of target olefins produced and the quantity of feedstock entering the process. According to the present invention, the expression "target olefins" means the olefins obtained from the dehydration of the alcohol feedstock. According to the present invention, the expression "the corresponding olefin(s)" means the olefin(s) obtained after the dehydration of the feedstock, the olefins having the same number of carbon atoms as said feedstock. Thus, the dehydration of ethanol leads to the production of ethylene, the dehydration of n-propanol or isopropanol leads to the production of propylene, and the dehydration of a mixture comprising ethanol and n-propanol and / or isopropanol leads to the production of ethylene and propylene.

[0080] The alcohol feed containing a two-carbon alcohol, a three-carbon alcohol, or mixtures thereof may optionally undergo a pretreatment step prior to step a) vaporization of said feed. This pretreatment step removes impurities contained in said feed, in particular nitrogen-containing compounds and sulfur-containing compounds, and possibly oxygenated compounds, so as to limit the deactivation of the downstream dehydration catalyst(s).

[0081] This pretreatment step is advantageously carried out by means known to those skilled in the art, such as those described in applications WO 2014 / 083261 A1 and WO 2020 / 126374 A1. Among these pretreatment techniques are, for example, the use of at least one advantageously ion-exchange resin; the adsorption of impurities onto solids, preferably at a temperature between 20 and 60°C; a sequence comprising a first hydrogenolysis step operating at a temperature between 20 and 80°C, followed by a capture step on an acidic solid at a temperature between 20 and 80°C; distillation; and / or partial evaporation and gas-liquid separation. In the case of the use of at least one resin, said resin is preferably acidic and is used at a high temperature between 70 and 200°C. The said acidic resin may optionally be preceded by a basic resin.

[0082] In the case where the pretreatment step is carried out by adsorption of impurities onto solids, said solids are advantageously chosen from molecular sieves, activated carbon, alumina and zeolites.

[0083] Said pretreatment step of the feed containing an alcohol having two carbon atoms, an alcohol having three carbon atoms or mixtures thereof makes it possible to produce a purified fraction of said feed in which the organic impurities have been eliminated, in order to obtain a purified alcohol feed that meets the level of impurities compatible with the dehydration catalyst. Step a)

[0084] According to the invention, the dehydration process includes a step a) of vaporizing said alcohol feed, optionally pre-treated, in a mixture with a gas stream, preferably in a heat exchanger, and preferably by heat exchange with a hot stream which is preferably the effluent from the last adiabatic reactor implemented in step b).

[0085] The enthalpy of condensation of said hot stream, which is preferably the effluent from the last adiabatic reactor implemented in step b), is used to partially vaporize the alcohol charge mixed with the gas stream, without external heat input.

[0086] At the end of step a), a mixture of said charge is obtained with a gaseous stream, which is vaporized at least partially, that is to say at least partially in the gaseous state.

[0087] Advantageously, step a) comprises a first phase of mixing the alcohol feed with the gas stream and a second phase of vaporizing said mixture. In particular, during said mixing phase, the alcohol feed, optionally pretreated, is mixed with a gas stream. Said gas stream most advantageously corresponds to at least a portion of one of the two fractions of the compressed effluent from step d). The mass ratio between the gas stream and the feed is between 0.5 and 2.0, preferably between 0.6 and 1.5. The mixture comprising the alcohol feed and the gas stream is then advantageously vaporized during the vaporization phase by heating, at a temperature enabling the vaporization of at least 50% of the stream, and preferably at least 60% of the stream, and most preferably at least 70% of the stream, and at a pressure between 0.3 and 1.8 MPa.

[0088] The applied pressure is sufficient to achieve the desired pressure for the dehydration reaction.

[0089] Said vaporization step is partial and allows recovery of a liquid stream comprising an unvaporized portion of the alcohols and a gaseous stream rich in alcohols which is sent to step b).

[0090] Advantageously, step a) includes a gas / liquid separation step by distillation of the liquid stream from step a).

[0091] The liquid stream is thus sent into a distillation column allowing a gaseous phase, comprising all the alcohols, to be recovered at the top of the column and an aqueous phase, which can be purged, to be recovered at the bottom.

[0092] The gas stream from the column head is preferably mixed with said alcohol-rich stream, in gaseous form, comprising said feed mixed with said gas stream.

[0093] In this very particular embodiment, the vaporized mixture, from step a) is in a gaseous state and advantageously has a temperature between 80 and 170°C and a pressure between 0.3 and 1.8 MPa.

[0094] In a preferred embodiment of the invention, the vaporized mixture from step a) is superheated, preferably in a heat exchanger, and preferably by heat exchange with the effluent from the last adiabatic reactor of step b). In this embodiment, said effluent from the last adiabatic reactor of step b), in the gaseous state, is then desuperheated without being condensed. The vaporized mixture from step a) is advantageously superheated to a temperature between 250 and 450°C and preferably between 280 and 420°C. In this embodiment, this step takes place in a single-phase gas heat exchanger. Step b)

[0095] According to the invention, the dehydration process includes a step b) during which the alcohol having two carbon atoms, and / or the alcohol having 3 carbon atoms of the alcohol charge undergoes a dehydration reaction.

[0096] According to the invention, during step b), said charge mixed with said vaporized gas stream is introduced into a set of at least two adiabatic reactors in series, each of said at least two adiabatic reactors of said set containing at least one dehydration catalyst and in which the dehydration reaction takes place, at an inlet temperature into said set of between 300 and 550°C, preferably at a temperature of between 350 and 500°C and at an inlet pressure into said set of between 0.2 and 1.8 MPa, preferably between 0.3 and 1.2 MPa, the effluent from the last adiabatic reactor constituting said hot stream of step a).

[0097] Preferably, said charge mixed with said vaporized gas stream, before entering the first adiabatic reactor in a set of at least two adiabatic reactors in series, is heated, for example in a furnace or heat exchanger, so as to bring it to an inlet temperature between 300 and 550 °C, compatible with the temperature of the dehydration reaction.

[0098] Preferably, the effluent from the earlier adiabatic reactor, advantageously located upstream of the last adiabatic reactor in the series, is heated, for example in a furnace or heat exchanger, before feeding the subsequent adiabatic reactor. The effluent from the last adiabatic reactor advantageously has a temperature of between 250 and 450°C at the outlet of the last adiabatic reactor, preferably between 280 and 400°C.

[0099] The effluent from the last adiabatic reactor advantageously has a pressure at the outlet of the last adiabatic reactor between 0.2 and 1.8 MPa, preferably between 0.2 and 1.6 MPa.

[0100] Advantageously, step b) in which the dehydration reaction takes place, employs at least two adiabatic reactors in series, and preferably at most ten, preferably at most five adiabatic reactors in series.

[0101] According to a particular embodiment of the invention, step b) employs two or three adiabatic reactors in series. In this embodiment, the mixture of said alcohol feed with said vaporized gas stream is advantageously introduced into the first reactor at an inlet temperature of between 300 and 550°C and preferably at a temperature of between 350 and 500°C, and at a pressure of between 0.2 and 1.8 MPa, and preferably between 0.3 and 1.2 MPa.

[0102] In this embodiment, in which step b) uses two adiabatic reactors in series, the effluent from the first adiabatic reactor advantageously exits said first reactor at a temperature between 250 and 400°C and at a pressure between 0.2 and 1.7 MPa. Said effluent is then advantageously introduced into a furnace such that the inlet temperature of said effluent into the second adiabatic reactor is between 300 and 550°C, and preferably between 350 and 500°C. Said effluent has an inlet pressure into said second reactor advantageously between 0.1 and 1.6 MPa, and preferably between 0.3 and 1.1 MPa.

[0103] The effluent from the second adiabatic reactor exits said second adiabatic reactor at a temperature advantageously between 250 and 450°C. The outlet pressure of said effluent from the second adiabatic reactor is advantageously between 0.2 and 1.8 MPa, preferably between 0.2 and 1.6 MPa.

[0104] According to another particular embodiment of the invention, step b) employs three adiabatic reactors in series. In this embodiment, the mixture of said alcohol feed with at least a part of said vaporized gas stream is advantageously introduced into the first reactor at an inlet temperature of between 300 and 550°C and preferably at a temperature of between 350 and 500°C, and at a pressure of between 0.2 and 1.8 MPa, and preferably between 0.3 and 1.2 MPa.

[0105] In this embodiment, in which step b) uses three adiabatic reactors in series, the effluent from the first adiabatic reactor advantageously exits said first reactor at a temperature between 250 and 450°C and at a pressure between 0.2 and 1.7 MPa. Said effluent is then advantageously introduced into a furnace such that the inlet temperature said effluent in the second adiabatic reactor shall be between 300 and 550°C and preferably between 350 and 500°C. Said effluent shall have an inlet pressure in said second reactor advantageously between 0.2 and 1.6 MPa and preferably between 0.3 and 1.2 MPa.

[0106] The effluent from the second adiabatic reactor exits said second adiabatic reactor at a temperature advantageously between 250 and 400°C. The outlet pressure of said effluent from the second adiabatic reactor is advantageously between 0.2 and 1.6 MPa. Said effluent is then advantageously introduced into a furnace such that the inlet temperature of said effluent into the third adiabatic reactor is between 300 and 550°C and preferably between 350 and 500°C. Said effluent has an inlet pressure into said third reactor advantageously between 0.2 and 1.6 MPa and preferably between 0.3 and 1.2 MPa.

[0107] The effluent from the third adiabatic reactor exits said third adiabatic reactor at a temperature advantageously between 250 and 450°C. The outlet pressure of said effluent from the third adiabatic reactor is advantageously between 0.2 and 1.1 MPa.

[0108] The dehydration reaction which takes place in a set of at least two adiabatic reactors of step b) of the process according to the invention advantageously operates at a weight-hourly rate of between 0.1 and 20 h1, and preferably between 0.5 and 15 h1. The weight-hourly rate is defined as the ratio of the mass flow rate of the feed containing an alcohol having two carbon atoms, an alcohol having three carbon atoms or mixtures thereof (the gas flow not being taken into account in defining the weight-hourly rate), to the mass of catalyst.

[0109] The dehydration catalyst used in each of the adiabatic reactors of said assembly in step b) is a catalyst known to those skilled in the art as a dehydration catalyst. Said catalyst is preferably an amorphous acid catalyst or a zeolitic acid catalyst.

[0110] Preferably, the zeolitic acid catalyst then comprises at least one zeolite selected from among zeolites having at least pore openings containing 8, 10, or 12 oxygen atoms (8 MR, 10 MR, or 12 MR). It is known to define the pore size of zeolites by the number of oxygen atoms forming the annular cross-section of the zeolite channels, called "member ring" or MR. Preferably, said zeolitic dehydration catalyst comprises at least one zeolite having a structural type selected from among the structural types MFI, FAU, MOR, FER, SAPO, TON, CHA, EUO, and BEA. Preferably, said catalyst of zeolitic dehydration includes a structural type MFI zeolite and preferably a ZSM-5 zeolite.

[0111] The zeolite used in the dehydration catalyst of each of the adiabatic reactors of said assembly, used in step b) of the process according to the invention, can advantageously be modified by desalumination or desilication according to any desalumination or desilication method known to a person skilled in the art.

[0112] The zeolite used in the dehydration catalyst of each of the adiabatic reactors of said assembly, used in step b) of the process according to the invention, can advantageously be modified by an agent capable of reducing its total acidity and improving its hydrothermal resistance properties. Preferably, said zeolite or said catalyst advantageously comprises phosphorus, preferably added in the form of H3PO4 followed by steam treatment after neutralization of the excess acid by a basic precursor such as, for example, sodium (Na) or calcium (Ca). Preferably, said zeolite comprises a phosphorus content of between 2.5 and 4.5 wt% relative to the total mass of the catalyst.

[0113] Preferably, the dehydration catalyst used in each of the adiabatic reactors of said assembly, in step b) of the process according to the invention, is the catalyst described in patent applications WO / 2009 / 098262, WO / 2009 / 098267, WO / 2009 / 098268 or WO / 2009 / 098269.

[0114] Preferably, the amorphous acid catalyst comprises at least one porous refractory oxide selected from alumina, alumina activated by a mineral acid deposition and silica, preferably alumina.

[0115] If the alcohol in said feedstock is ethanol, the dehydration catalyst used in step b) may be a zeolitic acid catalyst. If the alcohol in said feedstock is a three-carbon alcohol or a mixture of a three-carbon alcohol and ethanol, the dehydration catalyst used in step b) may be an amorphous acid catalyst.

[0116] Said amorphous or zeolitic dehydration catalyst used in each of the adiabatic reactors of said assembly in step b) of the process according to the invention may advantageously also comprise at least one matrix (also called a binder), of the oxide type, also called a binder. A matrix according to the invention is understood to be an amorphous or poorly crystallized matrix. Said matrix is ​​advantageously selected from the elements of the group formed by clays (such as, for example, natural clays such as kaolin or bentonite), magnesia, aluminas, silicas, silica-aluminas, aluminates, titanium oxide, boron oxide, zirconia, aluminum phosphates, titanium phosphates, zirconium phosphates, and the coal. Preferably said matrix is ​​chosen from the elements of the group formed by aluminas, silicas and clays.

[0117] The dehydration catalyst used in each of the adiabatic reactors of said assembly in step b) of the process according to the invention is advantageously shaped into grains of various shapes and sizes. It is advantageously used in the form of cylindrical or multilobed extrudates such as bilobed, trilobed, or multilobed, with a straight or twisted shape, but may optionally be manufactured and used in the form of crushed powder, tablets, rings, balls, wheels, or spheres. Preferably, the catalyst is in extruded form.

[0118] Said dehydration catalyst used in step b) of the process according to the invention is advantageously implemented in at least two reactors, in fixed bed or moving bed.

[0119] A preferred reactor is a radial reactor operating in up or down mode.

[0120] During step b) of the process according to the invention, the transformation of the feedstock is generally accompanied by the deactivation of the dehydration catalysts, in particular by coking and / or by adsorption of inhibitory compounds. The dehydration catalysts must therefore periodically undergo a regeneration step. Preferably, the reactors are used in an alternating regeneration mode, also called a swing reactor, in order to alternate the reaction and regeneration phases of said dehydration catalysts. The objective of this regeneration treatment is to burn off the organic deposits as well as the nitrogen- and sulfur-containing species contained on the surface and within said dehydration catalysts.

[0121] The regeneration of the dehydration catalysts used in said step b) is advantageously carried out by oxidation of the coke and inhibitor compounds under an air flow or under an air / nitrogen mixture, for example by using combustion air recirculation with or without water to dilute the oxygen and control the exothermicity of regeneration. In this case, the oxygen content at the reactor inlet can advantageously be adjusted by adding air. Regeneration takes place at a pressure between atmospheric pressure (0 bar relative) and the reaction pressure. The regeneration temperature is advantageously chosen to be between 400 and 600°C; it can advantageously vary during regeneration. The end of regeneration is detected when there is no more oxygen consumption, indicating complete combustion of the coke.

[0122] In step b) of the process according to the invention, the conversion of the alcohol having two carbon atoms contained in the alcohol feed is advantageously greater than 75%, preferably greater than 80% and preferably greater than 85%.

[0123] Advantageously, according to the invention, the conversion of ethanol in a dehydration process of a feed containing ethanol is greater than 85%, preferably greater than 90%, and preferably greater than 95%.

[0124] Advantageously, according to the invention, the conversion of ethanol in a process for dehydrating a feed containing a mixture of isopropanol and ethanol is greater than 75%, preferably greater than 80%, and preferably greater than 85%.

[0125] In step b) of the process according to the invention, the conversion of the alcohol having three carbon atoms contained in the alcohol feed is advantageously greater than 90%, preferably greater than 95% and preferably greater than 99%.

[0126] Advantageously, according to the invention, the conversion of isopropanol in a process for dehydrating a feed containing isopropanol is greater than 90%, preferably greater than 95%, and preferably greater than 99%.

[0127] Advantageously, according to the invention, the conversion of isopropanol in a process for dehydrating a feed containing a mixture of isopropanol and ethanol is greater than 90%, preferably greater than 95%, and preferably greater than 99%.

[0128] The effluent from the last adiabatic reactor of step b) is advantageously sent to a separation step c).

[0129] The effluent from the last adiabatic reactor of step b) can, prior to step c), be used as a hot stream in step a), in at least one heat exchanger, to heat, vaporize and possibly superheat the alcohol feed mixed with the gas stream.

[0130] According to a preferred embodiment of the invention, the effluent from the last adiabatic reactor of step b) is sent to a first heat exchanger, preferably a single-phase gas exchanger, in which it is desuperheated without being condensed by heat exchange with the alcohol feed mixed with the gas stream, vaporized, said alcohol feed being superheated. The desuperheated effluent from the last adiabatic reactor of step b) is then advantageously sent to a second heat exchanger, preferably a gas / liquid exchanger, in which it is partially condensed by heat exchange, so as to vaporize the alcohol feed mixed with the gas stream.In this embodiment, the effluent from the last adiabatic reactor of step b) which has successively passed through two heat exchangers to vaporize and superheat the alcohol feed mixed with the gas stream advantageously has a temperature between 60 and 150°C, at the end of step a) (i.e. at the outlet of the second heat exchanger). . Step c)

[0131] According to the invention, the dehydration process comprises a step c) of separating the effluent from the last adiabatic reactor of step b), and optionally from the outlet of at least one heat exchanger of step a), into an effluent comprising the olefin(s) corresponding to the alcohol or alcohol mixture which has undergone dehydration, said effluent comprising the olefin(s) being at a pressure of less than 1.6 MPa, and an aqueous effluent.

[0132] Step c) of separating said effluent from the last adiabatic reactor from step b) can be carried out by any method known to the person skilled in the art, such as a gas / liquid separation, and preferably a gas / liquid separation column.

[0133] Advantageously, the separation step c) is carried out in a quench tower in which the effluent from the last adiabatic reactor from step b) is both cooled and washed with an aqueous solvent so as to separate the water generated from the dehydration of the alcohol(s), the unconverted alcohol(s) and possibly impurities, in particular oxygenated impurities.

[0134] The aqueous effluent includes in particular water and unconverted alcohol.

[0135] Advantageously, at least a portion of the aqueous effluent from step c) is recycled upstream of the separation step c). This portion of the aqueous effluent is then advantageously cooled, in particular in a heat exchanger using an external or process-derived cold fluid, and is preferably purified according to known purification methods described below.

[0136] Advantageously, according to a particular embodiment, at least a portion of the aqueous effluent is recycled in step a) of the distillation step. It is then mixed with said liquid stream, comprising an unvaporized portion of the alcohols, from the vaporization step a) of the feed. This mixture is then sent to said distillation column during the distillation step, allowing the recovery at the top of the column of a gaseous phase, comprising all the alcohols, and at the bottom of an aqueous phase. This aqueous phase from the bottom of the distillation column comprises heavy products from said alcohol feed and / or from secondary reactions during the dehydration of said alcohol feed.

[0137] In a particular embodiment, at least part of the effluent comprising the olefin(s) in liquid form as well as heavier olefins is recycled in step c). Step d)

[0138] According to the invention, the dehydration process includes a step d) of compressing the effluent comprising the olefin(s), from step c), in at least one compressor, to obtain a compressed effluent, comprising the olefin(s).

[0139] Indeed, the effluent containing the olefin(s) from step c) is typically at a pressure between 0.1 and 1.6 MPa. Since this pressure is not compatible with the subsequent purification step(s) to obtain a purified olefin(s) stream, said effluent containing the olefin(s) from step c) must be compressed prior to said purification step(s).

[0140] The compression step (d) is advantageously implemented in any type of compressor known to those skilled in the art. In particular, the compression step (d) is advantageously implemented in at least one radial compressor with integrated gearbox or in at least one compressor comprising one or more blowers with a radial wheel connected in series with intercooling or in a positive displacement compressor with or without lubrication.

[0141] Said compression step may comprise one or more compression stage(s), i.e. may employ one or more compressor(s). Preferably, compression step d) employs 2 or 3 compressors.

[0142] Advantageously, the pressure of the compressed effluent, at the end of the compression step d), is between 2 and 4 MPa, preferably between 1.5 and 3.5 MPa.

[0143] According to the process of the invention, the compressor used has a cost and energy consumption much lower than those of the compressor used in prior art processes and in particular than those of the compressor used during an additional compression step which takes place upstream of the dehydration reaction when the diluent is water.

[0144] Indeed, when water is used as a diluent, this requires the use of a powerful compressor with significant energy consumption for the compression step upstream of the dehydration reaction. In the process of the invention, this compressor is not present, and the associated energy consumption is thus saved.

[0145] Advantageously, during the compression step (d), a portion of the compressed effluent condenses. This liquid portion may include, in particular, water and impurities and may then be sent to step (c). Step e)

[0146] According to the invention, the dehydration process comprises a step e) of dividing the compressed effluent from step d into two fractions, said compressed effluent advantageously comprising the olefin(s). Said first fraction of the compressed effluent comprising the olefin(s) represents at most 85% by weight of the total compressed effluent from step d). Preferably, said first fraction of the compressed effluent comprising the olefin(s) represents between 30% and 85% by weight of the total compressed effluent from step d). The second fraction of the effluent The tablet containing the olefin(s) then advantageously represents at least 15% by weight of the total compressed effluent from step d). Preferably, said second fraction of the compressed effluent containing the olefin(s) represents between 15% and 70% by weight of the total compressed effluent from step d).

[0147] In the case where the compression stage comprises several compression stages, preferably between 2 and 3, the first fraction of the compressed effluent can be separated after the first or second compression stage, and preferably from the first compression stage, when the pressure is between 0.5 and 1.8 MPa. The second fraction of the compressed effluent is recovered at the outlet of the last compressor, and its pressure is then greater than or equal to that of the first fraction, preferably between 2 and 4.5 MPa. Step f)

[0148] According to the invention, the process includes a step f) of recycling, in part or in whole, said first fraction of the compressed effluent, upstream of step a) to constitute said gaseous stream and which is advantageously mixed with the alcohol feed.

[0149] The weight ratio between the first fraction of the compressed and recycled effluent and the alcohol load is between 0.5 and 2.0, and preferably between 0.6 and 1.5. Step g) optional

[0150] The dehydration process may include a step g) of purification of the second fraction of the compressed effluent.

[0151] The purification step (g) can advantageously be carried out by any purification method known to those skilled in the art. By way of example, the purification step (g) can advantageously be carried out by the use of ion exchange resins, molecular sieves, by adding chemical agents to adjust the pH, such as, for example, sodium hydroxide or amines, and by adding chemical agents to stabilize the products, such as, for example, polymerization inhibitors selected from bisulfites and surfactants.

[0152] The process according to the invention makes it possible to dehydrate an alcohol feed containing a two-carbon alcohol, a three-carbon alcohol, or mixtures thereof, into the corresponding olefin(s). By recycling a portion of the olefins upstream of step a) vaporization of the feed, the process possesses excellent properties in terms of selectivity and yield, while limiting energy consumption and the associated costs of the process.

[0153] Advantageously, according to the invention, the selectivity of the process of dehydrating a feed containing ethanol, optionally in a mixture with propanol, into ethylene is greater than 85%, preferably greater than 90%, and preferably greater than 95%.

[0154] Advantageously, according to the invention, the selectivity of the process of dehydrating a feed containing isopropanol and / or n-propanol into propylene is greater than 85%, preferably greater than 90%, and preferably greater than 95%.

[0155] The efficiency of the dehydration process of a feed comprising two carbon atoms, according to the invention, is advantageously greater than 90%, preferably greater than 95%.

[0156] The yield of the dehydration process of a feed comprising three carbon atoms, according to the invention, is advantageously greater than 90%, preferably greater than 95%.

[0157] According to the invention, the process makes it possible to reduce operating costs by at least 10%, preferably by at least 25%, and preferably by at least 30%, compared to a process using water as a diluent.

[0158] Advantageously, according to the invention, the process of dehydrating a feed comprising ethanol makes it possible to reduce operating costs by at least 40%, preferably by at least 45%, preferably by at least 50%, compared to a process using water as a diluent.

[0159] Advantageously, according to the invention, the process of dehydrating a feed comprising propanol, in particular isopropanol, makes it possible to reduce operating costs by at least 35%, preferably by at least 40%, preferably by at least 45%, compared to a process using water as a diluent.

[0160] Advantageously, according to the invention, the process of dehydrating a feed comprising a mixture of isopropanol and ethanol makes it possible to reduce operating costs by at least 20%, preferably by at least 25%, preferably by at least 30%, compared to a process using water as a diluent.

[0161] According to the invention, the process also makes it possible to reduce investment costs by at least 10%, preferably by at least 20%, and preferably by at least 30%, compared to a process using water as a diluent.

[0162] Advantageously, according to the invention, the process of dehydrating a feed comprising ethanol makes it possible to reduce investment costs by at least 10%, preferably by at least 15%, preferably by at least 20%, compared to a process using water as a diluent.

[0163] Advantageously, according to the invention, the process of dehydrating a feed comprising isopropanol makes it possible to reduce investment costs by at least 10%, preferably by at least 15%, preferably by at least 20%, compared to a process using water as a diluent.

[0164] Advantageously, according to the invention, the process of dehydrating a feed comprising a mixture of isopropanol and ethanol makes it possible to reduce costs an investment of at least 10%, preferably at least 15%, preferably at least 20%, compared to a process using water as a diluent. Examples Example 1: not in accordance with the invention

[0165] Example 1 illustrates a process for dehydrating a feed comprising 93% ethanol by weight of the total feed, in which part of the purified water stream and part of the unconverted ethanol stream is recycled upstream of the feed vaporization step.

[0166] The flow numbers are those shown in [Fig. 1].

[0167] The ethanol charge considered is produced by fermentation of wheat, without extraction of glutens, by a dry milling type process according to the Anglo-Saxon term.

[0168] Step a)

[0169] Said ethanol feed is introduced, at a flow rate of 45,340 kg / h, mixed with 100,673 kg / h of recycled purified water from step c) and with 327 kg / h of unconverted ethanol from step c), into an exchanger at a pressure of 0.35 MPa.

[0170] The purified water stream from step c) acts as a thermal reaction diluent. The dilution of said ethanol feed by adding a portion of the purified water stream from step c) is carried out so as to achieve a diluent-to-feed mass ratio of 2.04 at the reactor inlet (66.5% water and 32.5% ethanol).

[0171] Table 1: Characteristics of the ethanol charge

[0172] [Tables] Charge Unit Ethanol 93.0% wt H2O 6.7% wt Other oxygenated gases 0.3% wt

[0173] In step a), the mixture of the feed with water and unconverted ethanol is heated in a first exchanger, by heat exchange with the reaction effluent from the last adiabatic reactor of step b).

[0174] Thus, 60.2 MW are exchanged between the mixture and the reaction effluent.

[0175] At the vaporization inlet (i.e., at the inlet of the heat exchanger in step a), said feed is at a temperature of 122 °C and a pressure of 0.35 MPa. At the heat exchanger outlet, the mixture is partially vaporized.

[0176] The liquid stream including an unvaporized portion of the alcohols is sent into a distillation column allowing recovery at the top of the column of a gaseous phase, including all the alcohols and at the bottom the aqueous phase recycled upstream of step a).

[0177] The gas stream from the column head is mixed with the alcohol-rich stream, in gaseous form, comprising said charge mixed with purified water.

[0178] Step a')

[0179] The mixture of feedstock, water, and unconverted ethanol from the heat exchanger in step a) is then compressed in a radial compressor with an integrated multiplier so that the pressure of said mixture after compression is equal to 0.81 MPa. The vaporized and compressed mixture is then heated in a second heat exchanger by means of heat exchange with the effluent from the last adiabatic reactor of step b). In said second heat exchanger, said vaporized and compressed mixture is superheated to a temperature of 373°C.

[0180] Step b), implementation in two adiabatic reactors

[0181] Said mixture, vaporized, compressed and superheated, is then introduced into a furnace so as to bring it to a temperature of 450°C. The mixture, vaporized, compressed and heated, is then introduced into the first adiabatic reactor at an inlet temperature of 450°C and an inlet pressure of 0.69 MPa.

[0182] The effluent from the first adiabatic reactor exits said first reactor at a temperature of 364°C and a pressure of 0.66 MPa; it is then introduced into a furnace so that the inlet temperature of said effluent into the second adiabatic reactor is 460°C. Said effluent has an inlet pressure of 0.63 MPa into said second reactor. The effluent from the second adiabatic reactor exits said second adiabatic reactor at a temperature of 390°C and a pressure of 0.6 MPa.

[0183] The two adiabatic reactors each contain a fixed bed of dehydration catalyst comprising 80% by weight of ZSM-5 zeolite treated with H3PO4 so that the P2O5 content is 3.5% by weight.

[0184] Step c)

[0185] The effluent from the second adiabatic reactor of step b) is sent to a gas / liquid separation column in the presence of an aqueous stream. An effluent containing ethylene at a pressure of 0.51 MPa is separated from an effluent containing water. This separation is carried out using a gas / liquid separation column, with recycling of all or part of the water produced at the bottom of the column to the top of the column.

[0186] Step d)

[0187] The effluent containing ethylene, recovered at the top of the gas / liquid separation column of step c), then undergoes compression.

[0188] In this example, the compression of the effluent containing ethylene is carried out in a multi-stage compressor comprising 3 compression stages. The pressure The final effluent containing ethylene is 3.31 MPa before its final purification.

[0189] Step e)

[0190] The effluent including water, recovered at the bottom of the gas / liquid separation column, is then separated into a purified water stream containing unconverted ethanol and a stream containing light gases by distillation.

[0191] Step f)

[0192] Part of the purified water stream containing the unconverted ethanol is recycled upstream of step a) of vaporization in the proportions described in step a). The stream containing the light gases is returned to step c).

[0193] The selectivity of the process in ethylene is 97.9%.

[0194] It is calculated as follows:

[0195] [Math 1]

[0196] (Ethylene contained in the effluent comprising ethylene) / (0.61 * quantity of ethanol converted)

[0197] where the quantity of ethanol converted is the ethanol contained in the ethanol feed minus the ethanol contained in the purged water streams and in the effluent containing ethylene. 0.61 g is the maximum quantity of ethylene obtained by dehydrating 1 g of pure ethanol.

[0198] The conversion of the ethanol feed in the dehydration step is 99.2%.

[0199] The conversion of the ethanol charge is defined, as a percentage, by the formula next:

[0200] [Math 2]

[0201] [1 - (hourly mass of ethanol output / hourly mass of ethanol input)] xlOO.

[0202] The energy balance of the scheme according to example 1 is given in table 2.

[0203] Table 2: Energy balance

[0204] [Tables2] Energy exchanged within the system Energy supplied to the system by an external input Quantity of heat exchanged in the 1st exchanger Quantity of heat exchanged in the 2nd exchanger Quantity of heat exchanged to vaporize the feed Step a) Quantity of heat exchanged in the furnaces Step b) Electricity required for compression Step a') Electricity required for compression Step d) Quantity of heat required at step e) Quantity of heat extracted on the gas / liquid separation column of Step e) 60.2 MW 9.6 MW 1.9 MW 13.1 MW 6.8 MW 1.9 MW 7.0 MW 15.1 MW

[0205] The scheme according to example 1 has a net energy consumption of +30.8 MW with a net amount of energy extracted of 15.1 MW and a heat exchange within the system of 69.8 MW. Example 2: in accordance with the invention

[0206] Example 2 illustrates a process for dehydrating a feed comprising 93% ethanol relative to the total weight of the feed, namely the same as that of the process illustrated in Example 1, in which 69.7% of the compressed effluent, comprising 96% ethylene relative to the total weight of the feed, is recycled upstream of step a) of vaporizing the feed.

[0207] The flow numbers are those shown in [Fig.2].

[0208] Step a)

[0209] The same ethanol feed as that used in Example 1, at a flow rate of 45,340 kg / h, is mixed with a gaseous ethylene stream from recycling step f), which is at a flow rate of 62,000 kg / h.

[0210] The dilution of said ethanol feed by adding part of the recycled gaseous ethylene stream from step f) is carried out in a mass ratio of diluent to feed equal to 1.4 (i.e. a mixture of 39.4% ethanol, 3.2% water, 55.6% ethylene and the remainder in impurities).

[0211] The mixture is then introduced into an exchanger at a pressure of 0.87 MPa, to be heated and partially vaporized by heat exchange with the effluent from the last adiabatic reactor of step b). Thus, 13.6 MW are exchanged between the mixture of the feed and the ethylene gas stream and the reaction effluent.

[0212] The temperature of the mixture at the inlet of the exchanger is equal to 48 °C (at 0.87 MPa).

[0213] The liquid stream including an unvaporized portion of the alcohols is mixed with the liquid stream containing unconverted ethanol from the bottom of the column of the gas / liquid separation of step c). The aqueous liquid mixture is sent to a distillation column allowing recovery at the top of the column of a gaseous phase, including all the alcohols and at the bottom the aqueous phase.

[0214] The gas stream from the column head is mixed with the alcohol-rich stream from vaporization, in gaseous form, comprising said charge mixed with recycled ethylene.

[0215] The mixture of the feed, the gas stream from the distillation column head and the vaporized ethylene gas stream is then heated in a heat exchanger, by means of a heat exchange with the reaction effluent from the second adiabatic reactor of step b). In said single-phase gas heat exchanger, said vaporized mixture is superheated to a temperature of 390 °C.

[0216] Step b), implementation in two adiabatic reactors

[0217] Said mixture of the charge and the vaporized and heated gas stream is then introduced in a furnace to bring it to a temperature of 460°C. The mixture of the feed and the vaporized and superheated ethylene gas stream is then introduced into the first adiabatic reactor at an inlet pressure of 0.68 MPa.

[0218] The effluent from the first adiabatic reactor exits said first reactor at a temperature of 380°C and a pressure of 0.65 MPa; it is then introduced into a furnace so that the inlet temperature of said effluent into the second adiabatic reactor is 460°C. Said effluent has an inlet pressure of 0.63 MPa into said second reactor. The effluent from the second adiabatic reactor exits said second adiabatic reactor at a temperature of 380°C and a pressure of 0.59 MPa.

[0219] The two adiabatic reactors each contain a fixed bed of dehydration catalyst, said catalyst being identical in both reactors and identical to that used in Example 1.

[0220] Step c)

[0221] The effluent from the second adiabatic reactor of step b) is sent to a gas / liquid separation column in the presence of an aqueous stream. An effluent containing ethylene at a pressure of 0.48 MPa is separated from an effluent containing water. This separation is achieved using a gas / liquid separation column, with the water produced at the bottom of the column being recycled to the top of the column.

[0222] Step d)

[0223] The effluent containing ethylene, recovered at the top of the gas / liquid separation column of step c), then undergoes compression.

[0224] In this example, the compression of the effluent containing ethylene is carried out in a multi-stage compressor comprising 3 compression stages. The final pressure of the effluent containing ethylene is 3.3 MPa before its final purification. The intermediate pressures reached in the first and second stages are 1.0 MPa and 1.9 MPa, respectively.

[0225] Step e)

[0226] The compressed effluent from the first compression stage of step d) and comprising ethylene is divided into two fractions:

[0227] - a first fraction representing 70% weight / volume of the compressed effluent derived from d) and corresponding to the recycled gaseous ethylene stream mixed with the ethanol feed in step a);

[0228] - a second fraction representing 30% weight / volume of the compressed effluent from from the first compression stage which is sent to the second and third compression stages of stage d) then is sent to a purification stage g).

[0229] Step f)

[0230] Said first fraction of the compressed effluent and comprising ethylene is recycled in full upstream of step a).

[0231] The different flows, in kg / h, are given in tables 3 and 4.

[0232] Tables 3 and 4: Composition of the main flows

[0233] [Tables3] Flow Description Unit Ethanol Charge Inflow into RI Outflow from R2 Effluent containing ethylene Flux No. 1 8 11 16 Total mass flow rate kg / h 45 339.8 110 060.6 1100 060.8 88 107.3 Ethylene kg / h 0 59 733.0 84 853.9 84 849.0 Ethane kg / h 0 34.8 49.3 49.3 C3 kg / h 0 212.7 (211.4 of C3 olefins and 1.29 of C3 olefins) 305.1 305.1 C4 kg / h 0 376.9 540.7 540.6 Oxygenated compounds (other than 'ethanol) kg / h 131.5 1128.2 1328.3 1072.8 Ethanol kg / h 42166.0 42477.0 352.4 117.3 h2o kg / h 3033.2 5728.7 22101.3 642.5

[0234] [Tables4] Description of the end X Unit Effluent including recycled ethylene Recycled unconverted ethanol Purge water Flow No. 18 14 6 Initial mass flow rate kg / h 62,000.0 22,172.7 19,451.6 Ethylene kg / h 59,728.6 4.4 0 Ethane kg / h 34.8 0 0 C3 kg / h 212.6 0.02 0 C4 kg / h 376.0 0.02 0 Oxygenated compounds (other than ethanol) kg / h 747.9 275.13 35.4 Ethanol kg / h 81.7 248.8 19.4 H2O kg / h 447.0 21,644.0 19,396.7

[0235] The selectivity of the process in ethylene, measured at the outlet of the reactors is 97.8%.

[0236] The conversion of the ethanol feed in the dehydration step is 99.2%.

[0237] The process selectivity and the load conversion are calculated as in Example 1.

[0238] The energy balance of the scheme according to example 2 is given in table 5.

[0239] Table 5: Energy balance

[0240] [Tables5] Energy exchanged within the system Energy supplied to the system by an external input Quantity of heat exchanged in the 1st exchanger Quantity of heat exchanged in the 2nd exchanger Quantity of heat exchanged to vaporize the feed - step a) Quantity of heat exchanged in the furnaces - step b) Electricity required for compression - step d) Quantity of heat extracted on the gas / liquid separation column - step c) 13.6 MW 18.3 MW 5.4 MW 12.9 MW 3.5 MW 5.1 MW

[0241] The process according to Example 2 has a net energy consumption of 21.8, which corresponds to a decrease in energy consumption of 29.2% ( 100x(30.8-21.8) / 30.8) compared to the energy consumption of the process in example 1 which uses water as a thermal diluent.

[0242] Furthermore, in Example 2, compared to Example 1, the compressor in step a') is not present, resulting in savings in terms of investment and operating costs. Moreover, the energy to be exchanged within the system in Example 2 is reduced by 54% (100*(69.8-31.9) / 69.8) compared to Example 1 because the water to be vaporized is present in a very small quantity in the feed in the process of Example 2, compared to the feed-diluent-ethanol mixture in the process of Example 1, thus reducing the requirements in terms of exchange capacity and representing a decrease in investment cost.

[0243] Finally, the amount of heat to be extracted during step e) is reduced by 66% in example 2 compared to example 1 ( 100*( 15.1 -5.1) / 15.1 ), because the water to be vaporized is in very small quantity compared to example 1.

[0244] Thus, the process for dehydrating a feed containing ethanol, in which a portion of the compressed effluent, ethylene, is recycled upstream of step a) vaporization of the feed, exhibits reduced energy consumption and operating costs compared to a dehydration process for the same feed in which water acts as a diluent. The process according to Example 2 also demonstrates excellent performance in terms of selectivity and conversion. Example 3: not in accordance with the invention

[0245] Example 3 illustrates a process for dehydrating a feed comprising 99.9% isopropanol by weight of the total feed, the remainder being water, in which a portion of the purified water stream and a portion of the unconverted isopropanol stream is recycled upstream of the feed vaporization step.

[0246] The flow numbers are those shown in [Fig. 1].

[0247] The isopropanol charge considered is produced by fermentation of wheat, without extraction of glutens, by a dry milling type process according to the Anglo-Saxon term.

[0248] Step a)

[0249] The isopropanol feedstock is introduced, at a flow rate of 5,375 kg / h, mixed with 6,981 kg / h of recycled purified water from step c) and with 10 kg / h of unconverted isopropanol from step c), into a heat exchanger at a pressure of 0.45 MPa. The purified water stream from step c) acts as a thermal reaction diluent. The dilution of the isopropanol feedstock by adding a portion of the purified water stream from step c) is carried out so as to achieve a diluent-to-feedstock mass ratio of 1.0 at the reactor inlet (50% water and 50% isopropanol).

[0250] In step a), the mixture of the feed with water and unconverted ethanol is heated in a first exchanger, by heat exchange with the reaction effluent from the last adiabatic reactor of step b). Thus, 4.6 MW are exchanged between the mixture and the reaction effluent. At the vaporization inlet (i.e., at the inlet of the heat exchanger in step a), the feed is at a temperature of 97 °C and a pressure of 0.45 MPa. At the heat exchanger outlet, the mixture is partially vaporized.

[0251] The liquid stream including an unvaporized portion of the alcohols is sent into a distillation column allowing recovery at the top of the column of a gaseous phase, including all the alcohols and at the bottom the aqueous phase recycled upstream of step a).

[0252] The gas stream from the column head is mixed with the alcohol-rich stream, in gaseous form, comprising said charge mixed with purified water.

[0253] Step a')

[0254] The mixture of feedstock, water, and unconverted isopropanol from the heat exchanger in step a) is then compressed in a radial compressor with an integrated multiplier so that the pressure of said mixture after compression is equal to 0.815 MPa. The vaporized and compressed mixture is then heated in a second heat exchanger by means of heat exchange with the effluent from the last adiabatic reactor of step b). In said second heat exchanger, said vaporized and compressed mixture is superheated to a temperature of 326 °C.

[0255] Step b), implementation in two adiabatic reactors

[0256] Said mixture, vaporized, compressed and superheated, is then introduced into a furnace so as to bring it to a temperature of 410°C. The mixture, vaporized, compressed and heated, is then introduced into the first adiabatic reactor at an inlet temperature of 410°C and an inlet pressure of 0.7 MPa.

[0257] The effluent from the first adiabatic reactor exits said first reactor at a temperature of 308°C and a pressure of 0.67 MPa; it is then introduced into a furnace so that the inlet temperature of said effluent into the second adiabatic reactor is 410°C. Said effluent has an inlet pressure of 0.64 MPa into said second reactor. The effluent from the second adiabatic reactor exits said second adiabatic reactor at a temperature of 342°C and a pressure of 0.61 MPa.

[0258] The two adiabatic reactors each contain a fixed bed of dehydration catalyst, said catalyst being an alumina consisting of 99% gamma alumina and having a sodium content of less than 50 ppm by weight and a sulfur content of less than 40 ppm by weight.

[0259] Step c)

[0260] The effluent from the second adiabatic reactor of step b) is sent to a gas / liquid separation column in the presence of an aqueous stream. An effluent containing propylene at a pressure of 0.485 MPa is separated from an effluent containing water. This separation is achieved using a gas / liquid separation column, with recycling of all or part of the water produced at the bottom of the column to the top of the column.

[0261] Step d)

[0262] The effluent containing propylene, recovered at the top of the gas / liquid separation column of step c), then undergoes compression.

[0263] In this example, the compression of the effluent containing propylene is carried out in a multi-stage compressor comprising 2 compression stages. The pressure of the effluent containing propylene is 1.8 MPa before its final purification.

[0264] Step e)

[0265] The effluent including water, recovered at the bottom of the gas / liquid separation column of step c), is then separated into a purified water stream containing unconverted isopropanol and a stream containing light gases by distillation.

[0266] Step f)

[0267] A portion of the purified water stream containing unconverted isopropanol is recycled upstream of step a) of vaporization in the proportions described in step a). The stream containing the light gases is returned to step c).

[0268] The selectivity of the process in propylene is 98.8%.

[0269] It is calculated as follows:

[0270] [Math 1]

[0271] (Propylene contained in effluent including propylene) / (0.70 * quantity of isopropanol converted)

[0272] where the quantity of isopropanol converted is the ethanol contained in the isopropanol feed subtracted from the isopropanol contained in the purged water streams and in the effluent including propylene 0.70 g is the maximum quantity of propylene obtained by dehydrating 1 g of pure isopropanol.

[0273] The conversion of the isopropanol load in the dehydration step is 98.6%.

[0274] The conversion of the isopropanol charge is defined, as a percentage, by the formula next:

[0275] [Math 2]

[0276] [1 - (hourly mass of isopropanol output / hourly mass of isopropanol input)] xlOO.

[0277] The energy balance of the scheme according to example 3 is given in table 2.

[0278] Table 7: Energy balance

[0279] [Tables7] Energy exchanged within the system Energy supplied to the system by an external input Quantity of heat exchanged in the 1st exchanger Quantity of heat exchanged in the 2nd exchanger Quantity of heat exchanged to vaporize the feed Step a) Quantity of heat exchanged in the furnaces Step b) Electricity required for compression Step a') Electricity required for compression Step d) Quantity of heat required at step e) Quantity of heat extracted on the gas / liquid separation column of Step e) 4.6 MW 0.5 MW 0.1 MW 1.3 MW 0.7 MW 0.10 MW 0.2 MW 0.9 MW

[0280] The scheme according to example 3 has a net energy consumption of +2.4 MW, a net amount of energy extracted of 0.9 MW and a heat exchange within the system of 5.1 MW. Example 4: in accordance with the invention

[0281] Example 4 illustrates a process for dehydrating a feed comprising 99.9% isopropanol by total feed weight, wherein at least 50% of the compressed effluent, comprising 98% propylene by total feed weight, is recycled upstream of step a) of evaporating the feed.

[0282] The flow numbers are those shown in [Fig.2].

[0283] Step a)

[0284] The same isopropanol feed as used in Example 3, at a flow rate of 5375 kg / h, is mixed with a gaseous propylene stream from recycling step f), which is at a flow rate of 4200 kg / h.

[0285] The dilution of said isopropanol feed by adding part of the recycled gaseous propylene stream from step f) is carried out in a mass ratio of diluent to feed equal to 0.78 (i.e. a mixture of 56% isopropanol, 0.3% water and 43% propylene and the remainder in impurity.

[0286] The mixture is then introduced into an exchanger at a pressure of 0.54 MPa, to be heated and partially vaporized by heat exchange with the effluent from the last adiabatic reactor of step b). Thus, 1.3 MW are exchanged between the mixture of the feed and the gaseous propylene stream and the reaction effluent. The temperature of the mixture at the inlet of the exchanger is equal to 49 °C (at 0.54 MPa).

[0287] The liquid stream comprising an unvaporized portion of the alcohols is mixed with the liquid stream containing unconverted isopropanol from the bottom of the column of the gas / liquid separation of step c). The aqueous liquid mixture is sent into a distillation column allowing the recovery at the top of the column of a gaseous phase, comprising all the alcohols and at the bottom the aqueous phase.

[0288] The gas stream from the column head is mixed with the alcohol-rich stream from vaporization, in gaseous form, comprising said charge mixed with recycled propylene.

[0289] The mixture of the feed, the gas stream from the distillation column head and the vaporized propylene gas stream is then heated in a heat exchanger, by means of a heat exchange with the reaction effluent from the second adiabatic reactor of step b). In said single-phase gas heat exchanger, said vaporized mixture is superheated to a temperature of 313 °C.

[0290] Step b), implementation in two adiabatic reactors

[0291] Said mixture of the charge and the vaporized and heated gas stream is then introduced in a furnace to bring it to a temperature of 410°C. The mixture of the feed and the vaporized and superheated gaseous propylene stream is then introduced into the first adiabatic reactor at an inlet pressure of 0.38 MPa.

[0292] The effluent from the first adiabatic reactor exits said first reactor at a temperature of 308°C and a pressure of 0.35 MPa; it is then introduced into a furnace so that the inlet temperature of said effluent into the second adiabatic reactor is 410°C. Said effluent has an inlet pressure of 0.32 MPa into said second reactor. The effluent from the second adiabatic reactor exits said second adiabatic reactor at a temperature of 343°C and a pressure of 0.29 MPa.

[0293] The two adiabatic reactors each contain a fixed bed of dehydration catalyst, said catalyst being identical in both reactors and identical to that used in Example 3.

[0294] Step c)

[0295] The effluent from the second adiabatic reactor of step b) is sent to a gas / liquid separation column in the presence of an aqueous stream. An effluent containing propylene at a pressure of 0.14 MPa is separated from an effluent containing water. This separation is achieved using a gas / liquid separation column, with the water produced at the bottom of the column being recycled to the top of the column.

[0296] Step d)

[0297] The effluent containing propylene, recovered at the top of the gas / liquid separation column, then undergoes compression.

[0298] In this example, the compression of the effluent containing propylene is carried out in a multi-stage compressor comprising 2 compression stages. The pressure of The effluent containing propylene has a pressure of 1.8 MPa. The intermediate pressure at the end of the first compression stage is 0.7 MPa.

[0299] Step e)

[0300] The compressed effluent from the first compression stage of step d) and comprising propylene is divided into two fractions:

[0301] - a first fraction representing 51% weight / volume of the compressed effluent from of d) and corresponding to the recycled gaseous propylene stream mixed with the isopropanol feed in step a);

[0302] - a second fraction representing 49% weight / volume of the compressed effluent from the first compression stage which is sent to the second compression stage of step d) and then is sent to a purification step g).

[0303] Step f)

[0304] Said first fraction of the compressed effluent and including propylene is recycled in part or in whole, upstream of step a). The different flows, in kg / h, are given in tables 8 and 9.

[0305] Tables 8 and 9: Composition of the main flows

[0306] [Tables8 Flow Description Unit Charge Isopropanol Flow entering RI Flow leaving R2 Effluent containing propylene Flow No. 1 8 11 16 Total mass flow rate kg / h 5,375.0 9,834.0 9,834.0 7,988.6 Propylene kg / h 5,375.0 4,122.0 7,840.0 7,839.0 Propane kg / h 0.4 0.77 0.8 Oxygenated compounds (other than isopropanol) kg / h 74.7 106.4 78.1 Isopropanol kg / h 5,385.5 12.4 5.5 H2O kg / h 251.7 1,874.4 65

[0307] [Tables9] Flow Description Unit Effluent including recycled propylene Recycled unconverted isopropanol Purge water Flow No. 18 14 6 Total mass flow rate kg / h 4,200.0 1,873.8 1,614.5 Propylene kg / h 4,121.0 1,873.8 Propane kg / h 0.4 0 Oxygenated compounds (other than isopropanol) kg / h 42.2 32.45 Isopropanol kg / h 2.9 8.0 0.4 H₂O kg / h 33.6 1,832.0 1,614.0

[0308] The selectivity of the process for propylene, measured at the outlet of the reactors, is 98.5%.

[0309] The conversion of the isopropanol feedstock in the dehydration step is 99.7%.

[0310] The process selectivity and the feed conversion are calculated as in Example 3.

[0311] The energy balance of the scheme according to example 4 is given in table 10.

[0312] Table 10: Energy balance

[0313] [TableauxlO] Energy exchanged within the system Energy supplied to the system by an external input Quantity of heat exchanged in the 1st exchanger Quantity of heat exchanged in the 2nd exchanger Quantity of heat exchanged to vaporize the load - step a) Quantity of heat exchanged in the furnaces - step b) Electricity required for compression - step d) Quantity of heat extracted on the gas / liquid separation column - step c) 1.3 MW 1.2 MW 0.4 MW 1.4 MW 0.3 MW 0.3 MW

[0314] The process according to Example 4 has a net energy consumption of 21 MW, which corresponds to a reduction in energy consumption of 12.5% ​​(100x(2.4-2.0) / 2.4) compared to the energy consumption of the process of Example 3 which uses water as a thermal diluent.

[0315] Furthermore, in example 4, compared with example 3, the compressor in step a') is not present, resulting in cost savings investment and operating costs. Furthermore, the energy to be exchanged within the system, in example 4 is reduced by 51% (100*(5.1-2.5) / 5.1) compared to example 3 because the water to be vaporized is in very small quantity in the feed in the process of example 4, compared to the feed-diluent-isopropanol mixture in the process of example 3, thus reducing the requirements in terms of exchange capacity and representing a reduction in investment cost.

[0316] Finally, the amount of heat to be extracted during step e) is reduced by 66% in example 4 compared to example 3 (100*(0.9-0.3) / 0.9), again because the water to be vaporized is in very small quantity compared to example 3.

[0317] Thus, the process for dehydrating a feed containing isopropanol, in which a portion of the compressed effluent, propylene, is recycled upstream of step a) of feed vaporization, exhibits reduced energy consumption and operating costs compared to a dehydration process for the same feed in which water acts as a diluent. The process according to Example 4 also demonstrates excellent performance in terms of selectivity and conversion. Example 5: not in accordance with the invention

[0318] Example 5 illustrates a process for dehydrating a feed comprising 45% ethanol and 45% isopropanol relative to the total weight of the feed, the remainder being water, in which a portion of the purified water stream and a portion of the unconverted ethanol and isopropanol stream is recycled upstream of the feed vaporization step.

[0319] The flow numbers are those shown in [Fig.1].

[0320] The charge comprising isopropanol and ethanol considered is produced by fermentation of wheat, without extraction of glutens, by a dry milling type process according to the Anglo-Saxon term.

[0321] Step a)

[0322] Said charge comprising isopropanol and ethanol is introduced, at a flow rate of 12,000 kg / h, mixed with 14,020 kg / h of recycled purified water from step c) and with 990 kg / h of an unconverted isopropanol and ethanol mixture from step c), into an exchanger at a pressure of 0.17 MPa.

[0323] The purified water stream from step c) acts as a thermal reaction diluent. The dilution of said feed comprising isopropanol and ethanol by adding a portion of the purified water stream from step c) is carried out so as to achieve a diluent-to-feed mass ratio of 1.0 at the reactor inlet (50% water and 50% of the isopropanol and ethanol mixture).

[0324] In step a), the mixture of the feed with water and the mixture of unconverted ethanol and isopropanol is heated in a first exchanger, by heat exchange with the reaction effluent from the last adiabatic reactor of step b).

[0325] Thus, 8.4 MW are exchanged between the mixture and the reaction effluent. At the vaporization inlet (i.e., at the inlet of the heat exchanger in step a), the feed is at a temperature of 99 °C and a pressure of 0.17 MPa. At the heat exchanger outlet, the mixture is partially vaporized.

[0326] The liquid stream including an unvaporized portion of the alcohols is sent into a distillation column allowing recovery at the top of the column of a gaseous phase, including all the alcohols and at the bottom the aqueous phase recycled upstream of step a).

[0327] The gas stream from the column head is mixed with the alcohol-rich stream, in gaseous form, comprising said charge mixed with purified water.

[0328] Step a')

[0329] The mixture of feedstock, water, and the unconverted ethanol-isopropanol mixture from the heat exchanger in step a) is then compressed in a radial compressor with an integrated multiplier such that the pressure of said mixture after compression is equal to 0.82 MPa. The vaporized and compressed mixture is then heated in a second heat exchanger by means of heat exchange with the effluent from the last adiabatic reactor of step b). In said second heat exchanger, said vaporized and compressed mixture is superheated to a temperature of 356 °C.

[0330] Step b), implementation in two adiabatic reactors

[0331] Said mixture, vaporized, compressed and superheated, is then introduced into a furnace so as to bring it to a temperature of 450°C. The mixture, vaporized, compressed and heated, is then introduced into the first adiabatic reactor at an inlet temperature of 450°C and an inlet pressure of 0.7 MPa.

[0332] The effluent from the first adiabatic reactor exits said first reactor at a temperature of 355°C and a pressure of 0.67 MPa; it is then introduced into a furnace so that the inlet temperature of said effluent into the second adiabatic reactor is 450°C. Said effluent has an inlet pressure of 0.64 MPa into said second reactor. The effluent from the second adiabatic reactor exits said second adiabatic reactor at a temperature of 372°C and a pressure of 0.61 MPa.

[0333] The two adiabatic reactors each contain a fixed bed of dehydration catalyst, said catalyst being an alumina consisting of 99% gamma alumina and having a sodium content of less than 50 ppm by weight and a sulfur content of less than 40 ppm by weight, as well as some iron and titanium impurities.

[0334] Step c)

[0335] The effluent from the second adiabatic reactor of step b) is sent to a gas / liquid separation column in the presence of an aqueous stream. An effluent A mixture containing ethylene and propylene at a pressure of 0.49 MPa is separated, along with an effluent containing water. This separation is achieved using a gas / liquid separation column, with the water produced at the bottom of the column being recycled to the top.

[0336] Step d)

[0337] The effluent comprising ethylene and propylene, recovered at the top of the gas / liquid separation column, then undergoes compression.

[0338] In this example, the compression of the effluent containing ethylene and propylene is carried out in a multi-stage compressor comprising 3 compression stages. The pressure of the effluent containing ethylene and propylene is 3.1 MPa before its final purification.

[0339] Step e)

[0340] The effluent including water, recovered at the top of the gas / liquid separation column, is then separated into a purified water stream and a stream of an unconverted ethanol and propanol mixture and a stream containing the light gases by distillation.

[0341] Step f)

[0342] Part of the purified water stream and part of the unconverted isopropanol and ethanol mixture stream are recycled upstream of step a) of vaporization in the proportions described in step a). The stream containing the light gases is returned to step c).

[0343] The selectivity of the process in propylene is 99.6% and the selectivity in ethylene is also 99.6%.

[0344] The conversion of the feed including isopropanol in the dehydration step is 99.9% and the conversion of the feed including ethanol is 84.2%.

[0345] The process selectivities and the charge conversions are calculated as in examples 1 and 3.

[0346] The energy balance of the scheme according to example 5 is given in table 12.

[0347] Table 12: Energy balance

[0348] [Tables 12] Energy exchanged within the system Energy supplied to the system by an external input Quantity of heat exchanged in the 1st exchanger Quantity of heat exchanged in the 2nd exchanger Quantity of heat exchanged to vaporize the feed Step a) Quantity of heat exchanged in the furnaces Step b) Electricity required for compression Step a') Electricity required for compression Step d) Quantity of heat required at step e) Quantity of heat extracted on the gas / liquid separation column of Step e) 8.4 MW 1.0 MW 1.7 MW 2.9 MW 2.0 MW 0.32 MW 1.0 MW 4.4 MW

[0349] The scheme according to example 5 has a net energy consumption of +7.92 MW, a net amount of energy extracted of 4.4 MW and a heat exchange within the system of 9.4 MW. Example 6: in accordance with the invention

[0350] Example 6 illustrates a process for dehydrating a feed comprising 45% isopropanol and 45% ethanol by weight of the total feed, namely the same as that illustrated in Example 5, in which at least 50% of the compressed effluent, comprising 51% propylene and 46% ethylene by weight of the total feed, is recycled upstream of step a) of vaporizing the feed.

[0351] Step a)

[0352] The same charge comprising a mixture of isopropanol and ethanol as that used in Example 5, at a flow rate of 12,000 kg / h, is mixed with a stream of the propylene and ethylene gaseous mixture from compression step f), which is at a flow rate of 9,000 kg / h.

[0353] The dilution of said ethanol feed by adding part of the recycled propylene and ethylene gaseous mixture stream from step f) is carried out in a diluent to feed mass ratio of 0.75 (i.e. a mixture of 52% of the isopropanol and ethanol mixture, 42% of the propylene and ethylene mixture, 6% water).

[0354] The mixture is then introduced into a heat exchanger at a pressure of 0.67 MPa, where it is heated and vaporized by heat exchange with the effluent from the last adiabatic reactor of step b). Thus, 2.66 MW are exchanged between the mixture of the feed and the propylene and ethylene gas mixture and the reaction effluent. The temperature of the mixture at the inlet of the heat exchanger is 42 °C (at 0.67 MPa).

[0355] The liquid stream including an unvaporized portion of the alcohols is mixed with the liquid stream containing isopropanol and unconverted ethanol from the bottom of the column of the gas / liquid separation of step c). The aqueous liquid mixture is sent to a distillation column allowing recovery at the top of the column of a gaseous phase, including all the alcohols and at the bottom the aqueous phase.

[0356] The gas stream from the column head is mixed with the alcohol-rich stream from vaporization, in gaseous form, comprising said charge mixed with recycled ethylene and propylene.

[0357] The mixture of the feed, the gas stream from the distillation column head and the vaporized propylene and ethylene mixture stream is then heated in a heat exchanger, by means of a heat exchange with the reaction effluent from the second adiabatic reactor of step b). In said single-phase gas heat exchanger, said vaporized mixture is superheated to a temperature of 380 °C.

[0358] Step b), implementation in two adiabatic reactors

[0359] Said mixture of the charge and the vaporized and heated gas stream is then introduced in a furnace to bring it to a temperature of 450°C. The mixture of the feed and the gas stream, vaporized and heated, is introduced into the first adiabatic reactor at an inlet pressure of 0.49 MPa.

[0360] The effluent from the first adiabatic reactor exits said first reactor at a temperature of 340°C and a pressure of 0.46 MPa; it is then introduced into a furnace so that the inlet temperature of said effluent into the second adiabatic reactor is 450°C. Said effluent has an inlet pressure of 0.43 MPa into said second reactor. The effluent from the second adiabatic reactor exits said second adiabatic reactor at a temperature of 394°C and a pressure of 0.42 MPa.

[0361] The two adiabatic reactors each contain a fixed bed of dehydration catalyst, said catalyst being identical in both reactors and identical to that used in Example 5.

[0362] Step c)

[0363] The effluent from the second adiabatic reactor of step b) is sent to a gas / liquid separation column in the presence of an aqueous stream. An effluent comprising ethylene and propylene at a pressure of 0.28 MPa is separated from an effluent comprising water. This separation is carried out using a gas / liquid separation column, with the water produced at the bottom of the column being recycled to the top of the column.

[0364] Step d)

[0365] The effluent comprising ethylene and propylene, recovered at the top of the gas / liquid separation column, then undergoes compression.

[0366] In this example, the compression of the effluent containing ethylene and propylene is carried out in a multi-stage compressor comprising three compression stages. The pressure of the effluent containing ethylene and propylene is 3.1 MPa before its final purification. The intermediate pressures reached in the first and second stages are 0.67 MPa and 1.6 MPa, respectively.

[0367] Step e)

[0368] The compressed effluent from step d) and comprising ethylene and propylene is divided into two fractions:

[0369] - a first fraction representing 55% weight / volume of the compressed effluent from of d) and corresponding to the recycled gaseous propylene and ethylene mixture flow mixed with the ethanol and isopropanol feed in step a);

[0370] - a second fraction representing 45% weight / volume of the compressed effluent from from the first compression stage which is sent to the second and third compression stage of step d) then is sent to a purification step g).

[0371] Step f)

[0372] Said first fraction of the compressed effluent comprising the mixture of propylene and ethylene is recycled in part or in whole upstream of step a).

[0373] The different flows, in kg / h, are given in tables 13 and 14.

[0374] Tables 13 and 14: Composition of the main flows

[0375] [Tables 13] Flow Description Unit Load isopropanol and ethanol Flow into RI Flow out of R2 Effluent containing propylene and ethylene Flow No. 1 8 11 16 Total mass flow rate kg / h 12,000 22,005 22,005 16 175.3 Propylene 4,587 8,361 8,368.9 Ethylene 4,171 7,362 7,362.8 Oxygenated compounds (other than isopropanol and ethanol) 70 109 93 Isopropanol 5,400 5,405 5 1.2 Ethanol 5,400 6,357 1,004 180.7 H2O 1,200 1,380 5,104 110.5

[0376] [Tables 14] Flow Description Unit Effluent including recycled propylene and ethylene mixture (QT) Recycles unconverted isopropanol and ethanol Purge water Flow No. 18 14 6 Total mass flow rate kg / h 9000 5914.7 4729.5 Propylene kg / h 4587 0 Ethylene kg / h 4171 Oxygenated compounds (other than isopropanol and ethanol) kg / h 51 19.3 Isopropanol kg / h 0.6 4.5 Ethanol kg / h 98.6 859 H2O kg / h 58.5 5032 4729.5

[0377] The selectivity of the process in propylene, measured at the outlet of the reactors is 99.3% and the selectivity in ethylene, measured at the outlet of the reactors is also 99.7%.

[0378] The conversion of the isopropanol feed in the dehydration step is 99.9% and the conversion of the ethanol feed is 86.3%.

[0379] The process selectivity and the load conversion are calculated as in Example 5.

[0380] The energy balance of the scheme according to example 4 is given in table 15.

[0381] Table 15: Energy balance

[0382] [Tables 15] Energy exchanged within the system; Energy supplied to the system by an external input; Quantity of heat exchanged in the 1st exchanger; Quantity of heat exchanged in the 2nd exchanger; Quantity of heat exchanged to vaporize the charge (step ea); Quantity of heat exchanged in the furnaces (step b); Electricity required for compression (step d); Quantity of heat extracted on the gas / liquid separation column (step c); 2.66 MW; 3.5 MW; 1.6 MW; 2.9 MW; 0.6 MW; 1.9 MW

[0383] The process according to Example 6 has a net energy consumption of 5.1 MW, which corresponds to a reduction in energy consumption of 35% (100x(7.9-5.1) / 7.9) compared to the energy consumption of the process of Example 5 which uses water as a thermal diluent.

[0384] In Example 6, compared with Example 5, the compressor in step a') is not present, resulting in savings in terms of investment and operating costs. Furthermore, the energy to be exchanged within the system in Example 6 is reduced by 34% (100*(9.4-6.16) / 9.4) compared to Example 5 because the water to be vaporized is present in a very small quantity in the feed in the process of Example 6, compared to the feed-diluent-ethanol-isopropanol mixture in the process of Example 5, thus reducing the requirements in terms of exchange capacity and representing a decrease in investment cost.

[0385] Finally, the amount of heat to be extracted during step e) is reduced by 56% in example 6 compared to example 5 (100*(4.4-1.9) / 4.4), again because the water to be vaporized is in very small quantity compared to example 5. The process according to the invention (examples 2, 4 and 6) thus makes it possible to maintain very good performance in terms of olefin selectivity and feed conversion, while reducing the operational costs and energy consumption related to the dehydration process of said feed.

[0386] Thus, the process for dehydrating a feed comprising isopropanol and ethanol, in which a portion of the compressed effluent, the propylene and ethylene mixture, is recycled upstream of step a) of feed vaporization, exhibits reduced energy consumption and operating costs compared to a dehydration process for the same feed in which water acts as a diluent. The process according to Example 4 also demonstrates excellent performance in terms of selectivity and conversion.

Claims

1.

2.

3. Demands A process for dehydrating an alcohol feedstock containing an alcohol having two carbon atoms, an alcohol having three carbon atoms, or mixtures thereof, into the corresponding olefin(s) comprising: a. the vaporization of said alcohol charge mixed with a gaseous stream, b. the introduction of said alcohol feedstock, vaporized and mixed with said gas stream, into a set of at least two adiabatic reactors in series, each of said at least two adiabatic reactors of said set containing at least one dehydration catalyst and in which the dehydration reaction takes place, at an inlet temperature into said set of between 300 and 550 °C and at an inlet pressure into said set of between 0.3 and 1.8 MPa, c. the separation of the effluent from the last adiabatic reactor from step b) into an effluent comprising the olefin(s) corresponding to the dewatering of the feedstock, at a pressure below 1.6 MPa, and an aqueous effluent, d. the compression of the effluent comprising the olefin(s), from step c), in at least one compressor, e. the division into two fractions of the compressed effluent from step d) and comprising the olefin(s), then f. the recycling, in part or in whole, of a first fraction of the compressed effluent, upstream of step a) to constitute said gaseous stream. A method according to claim 1, wherein in step a), said alcohol charge mixed with a gaseous stream is vaporized in a heat exchanger. A process according to claim 1 or claim 2, wherein in step a), said alcohol feed mixed with a gaseous stream is vaporized in a heat exchanger, by heat exchange with a hot stream, the effluent from the last adiabatic reactor constituting said hot stream.

4. A method according to any one of the preceding claims, wherein in step a), the mass ratio between the gas flow and the charge is between 0.5 and 2.

0.

5. A process according to any one of the preceding claims, wherein said first fraction of the compressed effluent comprising the olefin(s) represents at most 85% by weight of the total compressed effluent from step d).

6. A process according to the preceding claim, wherein said catalyst used in step b) is alumina.

7. A process according to any one of the preceding claims, wherein said alcohol feed containing an alcohol having two carbon atoms, an alcohol having three carbon atoms or mixtures thereof undergoes a pretreatment step prior to step a) of vaporizing said feed.

8. A process according to any one of the preceding claims, wherein the vaporized mixture from step a) is superheated with the effluent from the last adiabatic reactor of step b).

9. A process according to any one of the preceding claims, wherein the effluent from the last adiabatic reactor of step b) has, at the outlet of the last adiabatic reactor of step b) a temperature between 250 and 450°C and a pressure between 0.2 and 1.8 MPa.

10. A method according to any one of the preceding claims, wherein step b) employs two or three adiabatic reactors in series.

11. A method according to any one of the preceding claims, wherein the pressure of the compressed effluent, from the compression step d), is between 2 and 4 MPa.

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