Process for the steam cracking of ethane

Abstract

The invention relates to a process for the steam cracking of a feedstock composed of at least 80% by weight, in particular of at least 90% by weight, of ethane, the process comprising a steam cracking of the feedstock in a furnace (2), then a quenching of the pyrolysis products, then a compression operation, then a series of successive operations on the products resulting from the quenching, the said series of operations comprising a washing operation, followed by a drying operation and at least one compression operation, and finally a fractionation by cryogenic distillation. A selective hydrogenation operation, followed by a catalytic conversion operation, will be inserted into the said process, after the drying operation and before the fractionation, in order to partially convert the ethylene predominantly into propylene.

Claims

1. A process for steam cracking of a feedstock comprising at least 80% by weight of ethane, the process comprising a steam cracking of the feedstock in a furnace (2), then a quenching of pyrolysis products, then a compression operation, then a series of successive operations on the compressed pyrolysis products resulting from the quenching and compression, wherein the series of operations comprise a washing operation, followed by a drying operation and at least one compression operation, and finally a fractionation by cryogenic distillation, wherein a selective hydrogenation operation, followed by a catalytic conversion operation with a recycle loop, is inserted into the process, after the drying operation and before the fractionation, on the products obtained after drying, in order to partially convert the ethylene predominantly into propylene.

2. The process according to claim 1, wherein the ethylene is partially converted into propylene and into other components comprising butene and aromatics.

3. The process according to claim 1, wherein the catalytic conversion operation is followed by at least one compression operation.

4. The process according to claim 1, wherein the catalytic conversion operation is carried out in one reactor (213) or in several reactors in series (213, 217).

5. The process according to claim 1, wherein the catalytic conversion operation is carried out in several reactors in series (213, 217) and is followed by a cooling operation after at least one or each of the said reactors.

6. The process according to claim 1, wherein the catalytic conversion operation is carried out in a reactor (213), wherein a portion of the products obtained by the catalytic conversion is recycled at the outlet of the reactor by reintroducing them, after a compression operation, into the reactor.

7. The process according to claim 1, wherein the selective hydrogenation and the partial catalytic conversion of the ethylene are carried out at pressures intermediate between that of the steam cracking operation and that of the fractionation.

8. The process according to claim 7, wherein the intermediate pressure during the selective hydrogenation operation is between 510.sup.5 and 210.sup.6 Pa.

9. The process according to claim 7, wherein the intermediate pressure during the operation of the catalytic conversion is less than or equal to 10.sup.6 Pa.

10. The process according to claim 1, wherein the feedstock comprises at least 90% by weight of ethane.

11. The process according to claim 1, wherein the catalytic conversion operation is followed by at least two compression operations.

12. The process according to claim 1, further comprising bypassing part of the products obtained after drying or after the selective hydrogenation to said at least one compression operation.

13. The process according to claim 1, wherein the partially converting the ethylene predominantly into propylene leads to more propylene than ethylene being present.

14. A plant for the steam cracking of a feedstock comprising at least 80% by weight of ethane, which plant is for the implementation of the process according to claim 1, the plant comprising, in series: a steam cracking furnace (2) to produce an effluent, a quenching unit (3) which quenches the effluent and conducts a first fractionation, a first compressor (5), then a series of units comprising a washing unit (7), wherein the effluent is contacted with sodium hydroxide, a dryer (10), at least one second compressor (9), and a unit for fractionation by cryogenic distillation (11) and comprising, between the drying unit (10) and the fractionation unit (11): a selective hydrogenation unit (20), followed by a catalytic conversion unit (21) in order to partially convert the ethylene into hydrocarbons comprising predominantly propylene, wherein the catalytic conversion unit (21) comprises a reactor (213) and a recycling pipe (23) at the outlet of the reactor for reintroduction of a portion of the product into the reactor or reactors after compression.

15. The plant according to claim 14, wherein the catalytic conversion unit (21) comprises a compressor (22, 24).

16. The plant according to claim 14, wherein the catalytic conversion unit (21) comprises one reactor or several reactors in series (213, 217).

17. The plant according to claim 16, wherein a cooler is positioned at the outlet of the reactor (213) or at the outlet of each of the reactors (213, 217).

18. The plant according to claim 14, wherein the catalytic conversion unit (21) comprises two-stage compressors.

19. The plant according to claim 14, further comprising a bypass line 28, which connects the outlet of the drying unit 10 or the outlet of the selective hydrogenation section 20 to the inlet of the at least one second compressor 9.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be described in more detail below using nonlimiting examples of the process according to the invention, illustrated by the following figures:

(2) FIG. 1: a block diagram representation of a steam cracking plant provided for treating naphtha in a conventional way,

(3) FIG. 2: the plant of FIG. 1 modified according to the invention in order to be suitable for a feedstock of ethane type by incorporating an ethylene conversion unit,

(4) FIG. 3: a detailed representation of a first alternative form of the ethylene conversion unit represented in FIG. 2,

(5) FIG. 4: a detailed representation of a second alternative form of the ethylene conversion unit represented in FIG. 2.

(6) The same references correspond to the same components in all the figures. Generally, throughout the present text, when it is mentioned that a section/operation is followed by another, or when the terms of upstream or downstream type are mentioned, reference is made to the general direction of progress of the feedstock from its entry into the steam cracking furnace up to its departure from the unit for fractionation by distillation.

(7) Description of FIG. 1

(8) The feedstock arrives via the pipe 1 as a mixture with steam towards the steam cracking furnace 2. At the outlet of the furnace, the effluent is conveyed to a quenching section 3 producing a rapid quenching of the effluent and a first fractionation, which will remove the heaviest fractions and the condensed water. The gaseous fraction exiting from this section is conveyed via the pipe 4 to a compression section 5, generally comprising three compression stages and intermediate condensations. On departing from this section 5, the gaseous fraction is conveyed via the pipe 6 to a section for washing with sodium hydroxide solution 7. The gaseous outflow from this washing operation is conveyed via the pipe 8 to a further compression section 9. At the outlet of the compression 9, the compressed gas is conveyed to a drying section 10 and then to a section 11 for cryogenic distillation and for selective hydrogenations. This section makes it possible to fractionate the gas: there is a hydrogen outlet 12, a combustible gas outlet 13, an ethylene outlet 14 and a C.sub.3+ outlet 15 (C.sub.3+ are the hydrocarbons comprising at least three carbons: propylene and heavier than propylene). The ethane outlet 16 makes it possible to recycle the unconverted ethane to the steam cracking furnace 2.

(9) Description of FIG. 2

(10) The feedstock arrives via the pipe 1 as a mixture with steam towards the steam cracking furnace 2. At the outlet of the furnace, the effluent is conveyed to a quenching section 3 producing a rapid quenching of the effluent and a first fractionation, which will remove the heaviest fractions and the condensed water, and the gaseous fraction exiting from this section is conveyed via the pipe 4 to a compression section 5, generally comprising three compression stages and intermediate condensations. At the outlet of this section 5, the gaseous fraction is conveyed via the pipe 6 to a section for washing with sodium hydroxide solution 7. The gaseous outflow from this washing operation is conveyed via the pipe 8 to a drying section 10.

(11) At the outlet of the drying 10, the gas is subjected to a selective hydrogenation section 20 which removes all the diolefins which, at high temperature, might foul the items of equipment of the conversion section 21 which is positioned following the selective hydrogenation section 20. In this section 21, approximately 30% of the ethylene is converted per pass to produce mainly propylene but also butenes and aromatics. This section 21 is at a fairly low pressure, of at most 20 bars; it is thus necessary to recompress the stream at the outlet. This recompression takes place in two stages; a first stage with the compression section 22 makes it possible to achieve a pressure sufficient to recycle a portion 23 of the effluent towards the conversion section 21, so as to increase the ethylene conversion; the remainder is recompressed by the compression section 24. It is then directed into the final existing compression section 9 (already present in the conventional steam cracking line of FIG. 1) and then towards the fractionation section 11. The items of equipment for the fractionation of the C.sub.3+ compounds already exist in a steam cracking plant originally designed for naphtha: they thus make possible the departure from this section, apart from the hydrogen 12, the combustible gas 13 and the ethylene 14, of propylene 25, butenes 26 and petrol 27. The ethane outlet 16 makes it possible to recycle the unconverted ethane to the steam cracking furnace 2. A bypass line 28 connecting the outlet of 10 or 20 to the inlet of the compression section 9 makes it possible to operate with a variable and flexible feedstock in the section according to the invention (it is optional).

(12) Description of FIG. 3

(13) This figure describes in detail a first alternative form of the ethylene conversion unit 21 of FIG. 2, an alternative form comprising two (or more) reactors with intermediate cooling. The other items of equipment of the plant of FIG. 2, in particular those described as initiation (that is to say, the steam cracking furnace 2), and those described as regeneration (compressor, exchangers, separator, and the like), are not represented in FIG. 3 (or in FIG. 4).

(14) The gas coming from the selective hydrogenation section 20 arrives via the pipe 210 at the feedstock/effluent exchanger 211, where it is preheated up to the operating temperature and conveyed via the pipe 212 to the first reactor 213. In this reactor, a zeolite-based catalyst makes it possible to convert the ethylene into propylene and other products which are enhanceable in value. As the reaction is very exothermic, it is necessary to cool the stream at the outlet of this first reactor by generation of steam 215 in order to cool the effluent 214 down to the operating temperature. The cooled stream is conveyed via the pipe 216 to the second reactor 217.

(15) Two reactors are represented. In some cases, it may be necessary to install more of them if the concentration of ethylene at the inlet, and thus the exothermicity, is very high.

(16) At the outlet of the reactor 217, the converted effluent is conveyed via the pipe 218 to the feedstock/effluent exchanger 211, where it is cooled by indirect exchange with the feedstock (in this instance, the products entering the exchanger in question), and then to a refrigerating means 220, where the cooling is terminated using cooling water. It is also possible to use air to provide the cooling at the refrigerating means 220. It is thus possible to use, in order to carry out this cooling, either a water exchanger or an air exchanger.

(17) At the outlet of the heat exchanger 220, the effluent is conveyed via the pipe 221 to a demisting vessel 222, where possible traces of liquid are separated from the gas phase. The gas phase is taken up into the pipe 223, compressed by the compressor 22, cooled by the water refrigerating means 225 and then conveyed towards another compression section 24 via the pipe 226.

(18) According to the desired degree of conversion of ethylene, a portion of the gas at the outlet of the refrigerating means/heat exchanger 225 can be recycled to the feedstock/effluent exchanger 211. This is because the conversion of the ethylene per pass is approximately 30%; it may be necessary to have a higher conversion in order to use yet again the steam cracking furnaces.

(19) Description of FIG. 4

(20) In this figure, according to another alternative form, the ethylene conversion unit only has a single reactor and resorts to high recycling.

(21) The gas coming from the selective hydrogenation section 20 arrives via the pipe 210, as a mixture with the recycling gas arriving via the pipe 23, at the feedstock/effluent exchanger 211, where it is preheated up to the operating temperature and conveyed via the pipe 212 to the reactor 213. The reaction is very exothermic but, with the dilution due to high recycling, it is possible to have a moderate increase in temperature (between 30 C. and 50 C.) and thus to operate with just one reactor.

(22) At the outlet of the reactor 213, the converted effluent is conveyed via the pipe 218 to the feedstock/effluent exchanger 211, where it is cooled by indirect exchange with the feedstock, and then to the refrigerating means 220, which is a water exchanger, where the cooling is terminated using cooling water. It is also possible to use an air exchanger. At the outlet of this exchanger, the effluent is conveyed via the pipe 221 to a demisting vessel 222, where possible traces of liquid are separated from the gas phase.

(23) The gas phase is taken up into the pipe 223, compressed by the compressor 22, cooled by the water refrigerating means 225 and then conveyed towards another compression section 24 via the pipe 226. A portion of the compressed gas is returned, via the pipe 23, to the feedstock/effluent exchanger 211. High recycling (between 100% and 250% by weight, with respect to the feedstock) makes it possible to have just one reactor and a high conversion of the ethylene to give propylene and other advantageous compounds.

(24) Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

(25) In the foregoing and in the examples, all temperatures are set forth uncorrected in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.

(26) The entire disclosures of all applications, patents and publications, cited herein and of corresponding French application No. 18/50.558, filed Jan. 24, 2018, are incorporated by reference herein.

EXAMPLES

(27) These examples result from simulation studies. For the sake of simplification, they were worked out while regarding the feedstock as 100% ethane, whereas, under real conditions, the feedstock also contains a few percent of other compounds (less than 10% and generally at most 5% of minor compounds).

Comparative Example 1

(28) In a first step, a base case is simulated with a naphtha steam cracker operating with an ethane feedstock, with a plant in accordance with FIG. 1.

(29) The initial ethane feedstock comes from a series of distillations incorporating a demethanizer, which separates the methane at the top, and then a de-ethanizer, which separates the C.sub.3+ compounds at the bottom. The purity is greater or lesser depending on the strictness of these distillations: in this instance, the composition of the feedstock is 96% by weight ethane, the remainder consisting of methane and propane.

(30) The steam cracking furnace 2 treats 100 t/h of ethane, including 38 t/h of recycling and 62 t/h of fresh feedstock.

(31) The flow rate is limited by the refrigeration compressors of the fractionation section 11.

(32) 48.85 tonne/h of ethylene and 2.6 t/h of pure hydrogen are produced at the outlet of the fractionation section 11, but very few other economically enhanceable compounds.

(33) The refrigeration compressors have a power of 2.05 MW for the ethylene compressor and of 28.9 MW for the propylene compressor.

Example 2 According to the Invention

(34) The introduction of an ethylene conversion unit into the steam cracking installation, as represented in FIGS. 2 and 4, that is to say in the alternative form of the invention where just one catalytic conversion reactor is used, is simulated. The feedstock flow rate is increased, while keeping the power of the refrigeration compressors of the fractionation section lower than that of the base case according to Comparative Example 1.

(35) It is then possible to treat 135.5 t/h of ethane in the steam cracking furnace, including 83 t/h of fresh feedstock and 52.5 t/h of recycling.

(36) At the inlet of the conversion section 210, diolefins have already been hydrogenated, the pressure is 5.7 bars and the temperature is 35 C., the flow rate is 135.5 t/h, the average molar mass of the gaseous effluent is 20.1 g/mol and the molar composition is as follows (in %):

(37) TABLE-US-00001 Hydrogen 32.1 Methane 4.6 Ethane 25.5 Ethylene 35.7 Propane 0.3 Propylene 0.6 Butenes 0.6 C.sub.5+ 0.2 CO 0.4

(38) A high recycling 23, of 240 t/h, is applied and, at the outlet 226 of the conversion section, after the cooler 225, the situation has returned to 5.7 bars and 35 C., with a flow rate still of 135.5 t/h. The average molar mass is increased to 21.9 g/mol and the molar composition is then (in %):

(39) TABLE-US-00002 Hydrogen 35.0 Methane 5.0 Ethane 28.2 Ethylene 13.3 Propane 0.8 Propylene 15.4 Butenes 1.6 C.sub.5+ 0.4 CO 0.4

(40) At the outlet of the cryogenic fractionation section 11, 22.7 t/h of ethylene, 40.3 t/h of propylene, 3.6 t/h of pure hydrogen and 7.8 t/h of C.sub.4+ are produced.

(41) C.sub.4+ and C.sub.5+ represent, with the same convention as for C.sub.3+, hydrocarbons respectively exhibiting at least 4 and at least 5 carbons.

(42) The regeneration compressors have a power of 1.15 MW for the ethylene compressor and of 26.8 MW for the propylene compressor.

(43) It is seen that, with the same refrigeration compressors, it is possible to increase the flow rate of fresh feedstock by more than 30% and to produce an amount of olefins increased by approximately 30%, while taking into account only the ethylene and the propylene. There are also other economically enhanceable products, such as butenes and aromatics.

(44) In this scenario, there is much more propylene than ethylene: the ratio by weight is approximately 1.8.

Example 3 According to the Invention

(45) The introduction of an ethylene conversion into the steam cracking installation, represented in FIGS. 2 and 3, that is to say in the alternative form of the invention where two catalytic conversion reactors are used in series, is simulated.

(46) The same base case is used as in Comparative Example 1, with a naphtha steam cracker operating on ethane.

(47) The steam cracking furnaces treat 100 t/h of ethane, including 38 t/h of recycling and 62 t/h of fresh feedstock. The flow rate is limited by the refrigeration compressors of the fractionation section. 48.85 tonne/h of ethylene and 2.6 t/h of pure hydrogen are produced, but very few other economically enhanceable compounds. The refrigeration compressors have a power of 2.05 MW for the ethylene compressor and of 28.9 MW for the propylene compressor.

(48) The introduction of an ethylene conversion into the steam cracker, as represented in FIGS. 2 and 3, is simulated and the flow rate of feedstock is increased, while keeping the power of the refrigeration compressors of the fractionation section lower than that of the base case.

(49) It is then possible to treat 128.1 t/h of ethane in the steam cracking furnaces, including 78.9 t/h of fresh feedstock and 49.2 t/h of recycling.

(50) At the inlet of the conversion section 210, diolefins have already been hydrogenated, the pressure is 5.2 bars and the temperature is 35 C., the flow rate is 128.1 t/h, the average molar mass is 20.1 g/mol and the molar composition is as follows (%) (same composition as in Example 1):

(51) TABLE-US-00003 Hydrogen 32.1 Methane 4.6 Ethane 25.5 Ethylene 35.7 Propane 0.3 Propylene 0.6 Butenes 0.6 C.sub.5+ 0.2 CO 0.4

(52) The pressure is slightly lower than in Example 1 as, without recycling, the pressure at the inlet of the first conversion reactor is lower (it is adjusted in order to have 1.5 bars of olefin partial pressure).

(53) At the outlet 226 of the conversion unit 21, after the compressor 22 and the cooler 225, the situation has returned to a pressure of 5.2 bars and to a temperature of 35 C., the flow rate is 128.1 t/h, the average molar mass has increased to 21.2 g/mol and the molar composition is then:

(54) TABLE-US-00004 Hydrogen 34.0 Methane 4.8 Ethane 27.2 Ethylene 22.2 Propane 0.4 Propylene 8.7 Butenes 2.1 C.sub.5+ 0.2 CO 0.4

(55) At the fractionation outlet, 36.9 t/h of ethylene, 22.2 t/h of propylene, 3.4 t/h of pure hydrogen and 8.6 t/h of C.sub.4+ are produced.

(56) The regeneration compressors have a power of 1.52 MW for the ethylene compressor and of 28.9 MW for the propylene compressor.

(57) It is seen that, with the same refrigeration compressors as those used in the plant provided for treating naphtha, it is possible, in this case and without recycling, to increase the flow rate of fresh feedstock by 27% and to produce an amount of olefins increased by 21%, while taking into account only the ethylene and the propylene, as there are also other economically enhanceable products, such as butenes and aromatics.

(58) In this case, there is less propylene than ethylene and the ratio by weight is approximately 0.6, and an even lower ratio is possible by conveying only a fraction from the outlet of the selective hydrogenation unit 20 to the conversion unit 21, which makes it possible to have the ethylene conversion below a certain threshold, for example below 30%.

(59) The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

(60) From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.