LOW-ENERGY CONSUMPTION METHOD FOR DEHYDRATING ETHANOL INTO ETHYLENE

Abstract

A process for dehydrating an ethanol feedstock to give ethylene, includes:

a) a vaporization stage;

b) a heating stage;

c) a dehydration stage in a multitubular reactor comprising tubes having a length of between 2 and 4 m, said tubes comprising a, preferably zeolitic, dehydration catalyst, the feedstock having an inlet temperature of greater than 400° C. and less than 550° C. and an inlet pressure of between 0.8 and 1.8 MPa, the heat transfer fluid having an inlet temperature of greater than 430° C. and less than 550° C. and a mass flow rate such that the ratio of the mass flow rates of the heat transfer fluid relative to the feedstock is greater than or equal to 10;

d) separation into an effluent comprising ethylene and an aqueous effluent;

e) purification of the aqueous effluent and separation of a stream of purified water and a stream of unconverted ethanol.

Claims

1. A process for dehydrating an ethanol feedstock to give ethylene, comprising: a) a stage for vaporizing a vaporization feedstock comprising said ethanol feedstock in an exchanger by means of a heat exchange with a dehydration effluent resulting from stage c), so as to produce a vaporized feedstock; b) a stage for heating said vaporized feedstock in an exchanger by means of a heat exchange with a thermal fluid, so as to produce a superheated feedstock having a temperature of greater than 400° C. and less than 550° C.; c) a stage for dehydrating said superheated feedstock so as to produce a dehydration effluent, wherein said stage for dehydrating comprises a reaction section comprising at least one multitubular reactor in which the dehydration reaction takes place, said multitubular reactor comprising a plurality of tubes having a length of between 2 and 4 m and a shell, said tubes each comprising at least one fixed bed comprising at least one dehydration catalyst, said superheated feedstock being introduced into said tubes at an inlet temperature of greater than 400° C. and less than 550° C. and at an inlet pressure of between 0.8 and 1.8 MPa, a heat transfer fluid circulating inside said shell at a mass flow rate such that the ratio of the mass flow rate of said heat transfer fluid in the shell relative to the mass flow rate of said superheated feedstock introduced into said tubes is greater than or equal to 10, said heat transfer fluid having a temperature at the inlet into the shell of said multitubular reactor of greater than 430° C. and less than 550° C.; d) a stage for separating the dehydration effluent resulting from stage c) into an effluent comprising ethylene at a pressure of less than 1 MPa and an effluent comprising water; e) a stage for purifiying at least a portion of the effluent comprising water resulting from stage d) and the separation of at least one stream of purified water and at least one stream of unconverted ethanol.

2. The process as claimed in claim 1, wherein said dehydration catalyst used in stage c) is a zeolitic catalyst comprising at least one zeolite selected from zeolites having at least pore apertures containing 8, 10 or 12 oxygen atoms (8 MR, 10 MR or 12 MR), preferably of MFI structural type.

3. The process as claimed in claim 1, wherein said ethanol feedstock additionally undergoes a stage of pretreatment prior to vaporization stage a).

4. The process as claimed in claim 1, wherein at least a portion, preferably all, of the stream of unconverted ethanol resulting from stage e) is recycled and introduced upstream of the exchanger of vaporization stage a).

5. The process as claimed in claim 1, wherein the pressure of said vaporization feedstock at the inlet of said vaporization stage a) is between 0.1 and 2.0 MPa, preferably between 0.1 and 1.4 MPa, preferentially between 0.8 and 1.3 MPa and very preferably between 1.0 and 1.2 MPa.

6. The process as claimed in claim 1, wherein said thermal fluid of said stage b) is selected from the group of molten salts, in particular from: NaNO.sub.3—KNO.sub.3 mixtures, NaNO.sub.3—NaNO.sub.2—KNO.sub.3 eutectic mixtures and mixtures of the fluoride salts NaF and NaBF.sub.4, preferably NaNO.sub.3—NaNO.sub.2—KNO.sub.3 eutectic mixtures.

7. The process as claimed in claim 1, wherein the length of the tubes of said multitubular reactor of dehydration stage c) is between 2.5 and 3.5 m.

8. The process as claimed in claim 1, wherein the temperature of said superheated feedstock at the inlet into the multitubular reactor is greater than or equal to 410° C., very preferably greater than or equal to 420° C.

9. The process as claimed in claim 1, wherein the temperature of said superheated feedstock at the inlet into the multitubular reactor is less than or equal to 500° C., preferentially less than or equal to 480° C. and very preferably less than or equal to 450° C.

10. The process as claimed in claim 1, wherein the pressure of said superheated feedstock at the inlet into the multitubular reactor is between 0.8 and 1.1 MPa, preferentially 0.85 and 1.0 MPa and very preferably between 0.90 and 0.95 MPa.

11. The process as claimed in claim 1, wherein the heat transfer fluid of said stage c) is selected from the group consisting of: molten salts and oils of the high-performance lubricant type, preferably molten salts.

12. The process as claimed in claim 11, wherein said heat transfer fluid is selected from the group of molten salts, and in particular from: NaNO.sub.3—KNO.sub.3 mixtures, NaNO.sub.3—NaNO.sub.2—KNO.sub.3 eutectic mixtures and mixtures of the fluoride salts NaF and NaBF.sub.4, preferably NaNO.sub.3—NaNO.sub.2—KNO.sub.3 eutectic mixtures.

13. The process as claimed in claim 1, wherein said thermal fluid used in heating stage b) is the same as the heat transfer fluid used in the multitubular reactor of dehydration stage c).

14. The process as claimed in claim 1, wherein the temperature of said heat transfer fluid at the inlet into the shell of said multitubular reactor is greater than or equal to 450° C., very preferably greater than or equal to 470° C.

15. The process as claimed in claim 1, wherein the temperature of said heat transfer fluid at the inlet into the shell of said multitubular reactor is less than or equal to 500° C., preferentially less than or equal to 495° C.

16. The process as claimed in claim 1, wherein the ratio of the mass flow rate of said heat transfer fluid in the shell relative to the mass flow rate of said superheated feedstock introduced into said tubes is between 11 and 15, preferably between 12 and 14.

17. The process as claimed in claim 1, wherein said ethanol feedstock is an ethanol feedstock produced from a renewable source derived from biomass.

Description

DESCRIPTION OF THE FIGURES

[0109] FIG. 1 schematically represents the dehydration process according to the invention in the case of the dehydration of a concentrated ethanol feedstock with a stage of pretreatment of the ethanol feedstock, a recycling of at least a portion of the stream of unconverted ethanol upstream of vaporization stage a) and a complete thermal integration of the streams resulting from the purification. In this embodiment, the thermal fluid of the heating stage and the heat transfer fluid of the dehydration stage are the same fluid.

[0110] The ethanol feedstock is introduced into a pretreatment zone (2) via the pipe (1). The pretreated ethanol feedstock (3) is then mixed in the pipe (5) with a portion of the stream of unconverted ethanol (4) resulting from the purification zone (15) and recycled via the pipe (4). The pretreated ethanol feedstock as a mixture with a recycled portion of the stream of unconverted ethanol is introduced via the pipe (5) into an exchanger El in which said mixture undergoes an exchange of heat with the stream of unconverted ethanol (16) resulting from the purification zone (15). Said mixture is then introduced via the pipe (6) into a second exchanger E2 in which it undergoes an exchange of heat with the stream of purified water (17) resulting from the purification zone (15).

[0111] Said mixture comprising the pretreated ethanol feedstock and a recycled portion of the stream of unconverted ethanol, preheated in the exchangers E1 and E2, is then sent via the pipe (7) into an exchanger E3 in which it undergoes an exchange of heat with the dehydration effluent resulting from the multitubular reactor R1. Said dehydration effluent is introduced into the exchanger E3 via the pipe (10) and leaves said exchanger via the pipe (11). The latent heat or enthalpy of condensation of the dehydration effluent resulting from the multitubular reactor R1 is used to vaporize the ethanol feedstock mixed with the recycled stream of unconverted ethanol, without the supply of external heat. A vaporized feedstock (8) is obtained at the outlet of the exchanger E3.

[0112] The vaporized feedstock is sent via the pipe (8) to a gas/liquid exchanger E4 in which said vaporized feedstock undergoes an exchange of heat with a heat transfer fluid (32), for example molten salts. A superheated feedstock (9), in gas form, at a temperature compatible with the temperature of the dehydration reaction is obtained at the outlet of exchanger E4.

[0113] The superheated feedstock is then introduced via the pipe (9) into the multitubular reactor Rl. The heat transfer fluid circulates in the shell of the reactor R1 into which it is introduced via the pipe (33) and exits the reactor R1 via the pipe (35). The heat transfer fluid at the inlet of the exchanger E4 and of the reactor R1, that is to say in the pipes (31), (32) and (33), is at a temperature greater than that of the temperature of the feedstock at the inlet into the multitubular reactor Rl. At the outlet of the exchanger E4 and of the reactor R1, the heat transfer fluid is then sent via the pipe (36) to an oven (30), for example a tubular oven, where it will be reheated. The reheated heat transfer fluid (31) is then returned to the exchanger E4 and the multitubular reactor R1 via the respective pipes (32) and (33).

[0114] The dehydration effluent resulting from the reactor R1 then undergoes an exchange of heat described above in the exchanger E3 into which it is introduced via the pipe (10). At the outlet of the exchanger E3, the dehydration effluent is sent via the pipe (11) to a gas/liquid separation column (12) where it is separated into an effluent comprising ethylene (13) and an effluent comprising water (14). A portion of the effluent comprising water is recycled after cooling into the column (12) via the pipe (20). The portion of the effluent comprising water which is not recycled into the column (12) is sent via the pipe (14) to a purification and separation stage (15). At the outlet of the purification and separation stage (15), at least one stream of purified water (17) and at least one stream of unconverted ethanol (16) are then obtained. A stream containing the light gases (19) is also separated and recycled to the gas/liquid separation column (12). After heat exchange in the exchangers E2 and El respectively, the stream of purified water (18) is recovered.

[0115] The following examples illustrate the invention without limiting the scope thereof.

EXAMPLES

Example 1

In Accordance with the Invention

[0116] Example 1 illustrates a process according to the invention.

[0117] The ethanol feedstock under consideration is produced by the fermentation of wheat, without extraction of gluten, by a process of dry milling type.

[0118] The composition of the ethanol feedstock is given in table 1, column 1.

[0119] The ethanol feedstock is pretreated on a TA801 type pretreatment resin at a temperature of 120° C. and a pressure of 1.15 MPa. At the end of this pretreatment, the amount of nitrogen-based compounds is reduced (See table 1).

[0120] Vaporization stage a)

[0121] Said ethanol feedstock is mixed with the recycled stream of unconverted ethanol resulting from purification stage e), the composition of which is given in table 1, column 3. The vaporization feedstock is then obtained: its composition and its mass flow rate are given in table 1, column 4.

TABLE-US-00001 TABLE 1 Mass compositions and mass flow rates of the different streams of the process recycled Effluent pretreated stream of at Effluent composition ethanol ethanol unconverted Vaporization reactor comprising Purified (% by mass) feedstock feedstock EtOH feedstock outlet ethylene water Ethanol 93.0  87.6  8.8 87.0  0.7 0.2 0.0 Water 6.7 7.7 88.7  12.5  46.1  0.9 99.8  ethylene 0.0 0.0 0.0 0.0 51.5  96.1  0.0 aliphatic 0.0 0.0 0.0 0.0 0.1 0.3 0.0 compounds oxygen-based 0.3 4.5 2.5 0.5 0.9 1.1 0.2 compounds other than ethanol Olefins other 0.0 0.0 0.0 0.0 0.7 1.5 0.0 than ethylene Nitrogen-based  0.005  0.000  0.000  0.000  0.000  0.000  0.000 compounds Total flow rate 46 870     46 870     3594    50 465     50 458     27 007     19 857     (kg/h)

[0122] The vaporization feedstock is introduced into an exchanger at a pressure of 1.03 MPa and is vaporized by exchange of heat with the dehydration effluent resulting from the multitubular reactor. At the exchanger outlet, a vaporized feedstock is obtained in gas form.

[0123] Heating stage b)

[0124] The vaporized feedstock is introduced into a gas/liquid exchanger. The heat transfer fluid used is Dynalene MS-1 (NaNO.sub.3—NaNO.sub.2—KNO.sub.3 eutectic mixture) sold by Dynalene, at a temperature of 470° C. At the exchanger outlet, the superheated feedstock is at 420° C.

[0125] Dehydration stage c)

[0126] The superheated feedstock is then introduced into a multitubular reactor comprising 2283 tubes and a shell in which the Dynalene MS-1 used in the previous stage circulates. The characteristics of the multitubular reactor are described in table 2.

[0127] The multitubular reactor comprises a dehydration catalyst placed in the tubes of the multitubular reactor, said catalyst comprising 80% by weight of ZSM-5 zeolite treated with H.sub.3PO.sub.4 such that the content of P.sub.2O.sub.5 is 3.5% by weight.

TABLE-US-00002 TABLE 2 Characteristics of the multitubular reactor and of the dehydration stage Parameters Number of tubes 2283 Height of the tubes (m)   3 Outside diameter of the tubes (mm)  50.8 Inside diameter of the tubes (mm)  45.3 Inlet temperature of the feedstock (° C.)  420 Inlet pressure of the feedstock (MPa)   0.91 Inlet temperature of the molten salts (° C.)  470 Flow rate of the feedstock in the reactor (kg/h)  50 465 Flow rate of the molten salts in the reactor (t/h)  639.1 Ratio of the molten salt/feedstock mass flow rates About  12.7 WWH* (h.sup.−1)   7 Temperature of the effluent at the reactor outlet (° C.)  408 Pressure of the effluent at the reactor outlet (MPa)   0.71 *WWH (weight per weight per hour), being the weight hourly space velocity, is defined as being the ratio of the mass flow rate of the feedstock entering the reactor, i.e. in this case (mass flow rate of the ethanol feedstock + mass flow rate of the recycled stream of unconverted ethanol), to the mass of dehydration catalyst present in the multitubular reactor.

[0128] The effluent obtained at the reactor outlet, or dehydration effluent, is analyzed by gas chromatography. Its composition is given in table 1, column 5.

[0129] The rate of conversion of the ethanol at the reactor outlet is very satisfaying: it is 99.1%. It is calculated as follows:

[1(hourly mass of EtOH at the reactor outlet/hourly mass of EtOH at the reactor inlet)]100.

[0130] The selectivity of the process for ethylene is approximately 98%. It is calculated as follows: (amount of ethylene contained in the effluent comprising ethylene)/(0.61×amount of converted ethanol),

[0131] wherein the amount of converted ethanol is the amount of ethanol contained in the vaporization feedstock subtracted from the amount of ethanol contained in the unconverted ethanol effluent ; 0.61 g is the maximum amount of ethylene obtained by dehydrating 1 g of pure ethanol.

[0132] Separation stage d)

[0133] The effluent resulting from the multitubular reactor of stage c) then undergoes a heat exchange with the vaporization feedstock, as described above, and is sent to a gas/liquid separation column. An effluent comprising ethylene at a pressure equal to 0.60 MPa is separated, as is an effluent comprising water. This separation is carried out by the use of a gas/liquid separation column, with recycling of the water produced at the bottom of the column to the top of the column and after cooling and injection of a neutralizing agent.

[0134] The effluent comprising ethylene then undergoes compression to increase its pressure to 2.78 MPa before its final purification.

[0135] Purification stage e)

[0136] A stream of purified water and a stream of unconverted ethanol as well as a stream containing the light gases are then separated by conventional low-pressure distillation of the effluent comprising water resulting from separation stage d).

[0137] The separated stream of unconverted ethanol is reintroduced in its entirety as a mixture with the ethanol feedstock upstream of vaporization stage a).

[0138] The equivalent primary energy consumption, or specific consumption of the process, is 6 GJ equivalent per ton of ethylene produced.

Example 2

In Accordance with the Invention

[0139] Example 2 illustrates a process according to the invention. In example 2, the same dehydration catalyst as that of example 1 (80% by weight of ZSM-5 zeolite treated with H.sub.3PO.sub.4 such that the content of P.sub.2O.sub.5 is 3.5% by weight) is used, but it is at the end of the cycle.

[0140] The same ethanol feedstock as in example 1 is used. It is pretreated as described in example 1 (on TA801 resin at 120° C. and 1.15 MPa).

[0141] The characteristics of the multitubular reactor are identical to those of the reactor of example 1.

[0142] The same heat transfer fluid as in example 1 is used (Dynalene MS-1).

[0143] Only the physical characteristics of the feedstock and of the molten salts at the inlet of the multitubular reactor and of the dehydration effluent vary compared to those of example 1: [0144] inlet temperature of the feedstock: 430° C.; [0145] inlet pressure of the feedstock: 0.92 MPa; [0146] flow rate of the feedstock: 50 465 kg/h; [0147] inlet temperature of the molten salts: 495° C.; [0148] flow rate of the molten salts: 639.1 t/h; [0149] temperature of the effluent at the reactor outlet: 432° C.; [0150] pressure of the effluent at the reactor outlet: 0.7 MPa.

[0151] The rate of conversion of the ethanol at the reactor outlet, calculated in the same way as in example 1, is 99.8%.

[0152] The selectivity of the process for ethylene, calculated in the same way as in example 1, is 99.9%.

[0153] The equivalent primary energy consumption, or specific consumption of the process, is 5.72 GJ equivalent per ton of ethylene produced.

EXAMPLE 3

Not In Accordance with the Invention

[0154] Example 3 illustrates a process for converting ethanol into ethylene, for example described in patent application WO 2013/011208.

[0155] The ethanol feedstock under consideration is the same as that of example 1. It is pretreated as described in example 1.

[0156] The process illustrated in example 3 comprises:

[0157] i) a stage of pretreatment of the ethanol feedstock on TA801 resin at 120° C., at 1.15 MPa.

[0158] ii) a stage for mixing the pretreated ethanol feedstock with a recycled portion of the stream of purified water and with the stream of unconverted ethanol which result from purification stage v), to obtain a vaporization feedstock comprising 65% by weight of water and 35% by weight of ethanol;

[0159] iii) a stage for vaporizing the obtained vaporization feedstock, by heat exchange with the dehydration effluent resulting from stage iii);

[0160] iv) a stage of compression of the vaporized feedstock;

[0161] v) a stage for dehydring the compressed vaporized feedstock, implemented in a succession of two adiabatic reactors, each comprising a fixed bed comprising a dehydration catalyst (the same catalyst as that of example 1), each of the adiabatic reactors being preceded by an oven for heating the reaction medium to temperatures compatible with the dehydration reaction: the feedstock is heated to a temperature at the inlet into the first adiabatic reactor of 460-480° C.; at the outlet of the first adiabatic reactor, the exiting effluent has lost 107° C. and is reheated in a second oven to 430-450° C. before entering the second adiabatic reactor;

[0162] vi) a stage of separation in a “quench” column of an effluent comprising ethylene and of an effluent comprising water;

[0163] vii) a stage of purification of the effluent comprising water and separation of at least one stream of purified water and one stream of unconverted ethanol.

[0164] The overall rate of ethanol conversion, at the outlet of the second adiabatic reactor, is equal to 99.2%. The selectivity for ethylene in the process of example 3 is 97.8%. The overall rate of conversion and selectivity for ethylene are calculated in the same way as in example 1.

[0165] The energy index of the process of example 3, not in accordance with the invention, is high: the primary energy consumption of the process of example 3 is at least 7.3 GJ per ton of ethylene.