RU2793786C2 - Propylene stream production method and related production equipment - Google Patents

Abstract

FIELD: process for production of a stream (11) of propylene.

SUBSTANCE: method includes the following steps: introducing a feed stream (26) enriched in C4 and/or C5 hydrocarbons and at least one stream (22) enriched in ethylene into the metathesis reactor (28); extracting the metathesis product (32) at the outlet (28) of the metathesis reactor; introducing the metathesis product (32) into the ethylene stripper (30); producing an overhead stream (70) at the head of the ethylene stripper (30) enriched in ethylene and a feed stream (78) at the bottom of the ethylene stripper (30); introducing the feed stream (78) into the propylene remover (34) and extracting from the bottom of the propylene remover (34) the bottom stream (88) containing C4+ hydrocarbons; extracting the propylene stream (11) from the overhead stream (80) of the propylene stripper (34); laterally extracting the recirculation stream (92) enriched in C4 and/or C5 hydrocarbons, and returning the recirculation stream (92) to the metathesis reactor (28). The method is characterized by the presence of the following stage: lateral extraction of the purge stream (90), enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene, in the propylene distillation column (34). The invention also relates to a device.

EFFECT: use of the present invention allows metathesis to be performed demonstrating improved yield and/or increased productivity without significant adverse effect on the structure of the equipment.

16 cl, 3 dwg, 2 tbl

Description

The present invention relates to a process for producing a propylene stream, comprising the following steps:

- introducing a feed stream enriched in C4 and/or C5 hydrocarbons and at least one stream enriched in ethylene into the metathesis reactor;

- extraction of the metathesis product at the outlet of the metathesis reactor;

- introduction of the metathesis product into the ethylene stripper;

- production at the head of the ethylene stripper of the head stream enriched in ethylene, and at the bottom of the ethylene stripper of the feed stream;

- introducing said feed stream into the propylene remover and recovering a bottom stream containing C4+ hydrocarbons in the bottom of the propylene remover;

- extracting the propylene stream from the head stream of the propylene stripper;

- lateral withdrawal of the recirculation stream enriched in C4 and/or C5 hydrocarbons and return of said recirculation stream to the metathesis reactor.

Such a process is carried out in particular within or in parallel with a hydrocarbon cracker, in particular a steam cracker and/or an oil refinery (Fluid Catalytic Cracker (FCC) or other unit).

Propylene is produced using a process for contacting a 2-butene stream and an ethylene stream in a metathesis reactor.

In one known process, the 2-butene stream and the metathesis ethylene stream come from a steam cracker. As another source of butene, C4 is optionally used from a petroleum refinery (catalytic cracker or otherwise). The C4 cut(s) are typically processed upstream of the metathesis to produce a feed that is butadiene-free, isobutene-lean, and has a 2-butene/1-butene ratio as oriented as possible in the 2- butene. 2-butene can also be obtained by dimerizing ethylene to form 1-butene followed by isomerization of 1-butene to form 2-butene.

In the case of a C4 steam cracked cut, said cut is typically treated in a butadiene selective hydrogenation unit prior to processing in a distillation column to remove isobutene, which also allows 1-butene to be hydroisomerized to form 2-butene. Alternatively, butadiene can be isolated using a dedicated device and/or isobutene can be isolated using a methyl tert-butyl ether (MTBE) type device. After that, the raffinate isolated from this device (devices) and enriched in 2-butene is sent to the metathesis reactor.

There is also a separation section downstream of the metathesis device to process the effluent. The first distillation column (hereinafter referred to as "ethylene stripper") separates unreacted ethylene from the C3+ distillate produced using the metathesis reactor.

The ethylene recovered in the top stream is separated into an ethylene purge stream and an ethylene recycle stream to the metathesis reactor.

The C3+ cut produced in the bottoms of the ethylene remover is sent to a second distillation column (hereinafter referred to as "propylene remover"). Propylene is produced from the top of this column. The shoulder strap enriched in C4 hydrocarbons is taken in the liquid phase between the feed tray and the bottom part of the column, after which it is sent for recycling to the metathesis reactor.

Such a process is not optimal in terms of overall propylene yield and/or reactor cycle time.

First of all, sending paraffinic compounds (isobutane, n-butane, ...) for recycling to the reactor is unnecessary, since these saturated compounds are inherently inert with respect to the metathesis reaction.

In addition, the isobutene metathesis reaction with ethylene does not produce new products, while the isobutene metathesis reaction with itself and/or other C4 olefinic hydrocarbons is limited in the presence of excess ethylene. Therefore, sending isobutene for recycling to the metathesis reactor is not advantageous.

When these compounds are present in the device in large quantities, they block the active sites of the catalyst, resulting in little or no yield, and they accumulate in the recycle stream enriched in C4 hydrocarbons. This will result in an unnecessary overhead and/or limitation on the device, unless it is required to provide higher capacity hardware.

In addition, the cut, enriched in C4 hydrocarbons and intended for recycling to the metathesis reactor, is separated from the propylene distillation column as a result of selection in the liquid phase from the bottom zone of the column. This arrangement means that some of the heavy components (particularly C6+ hydrocarbons) produced during the metathesis reaction and/or produced from side reactions are also recycled to the reactor. Such compounds cause fouling and premature coking of the catalyst, which can reduce the cycle time of the catalysts used for the metathesis reaction and, ultimately, their life.

Therefore, one object of the invention is to provide a process for the production of very high quality propylene from a metathesis reactor, showing improved yield and/or increased productivity without significant adverse effect on the structure of the equipment used to implement the process.

For these purposes, the invention relates to a method corresponding to the above type and characterized by the presence of the following further stage:

- side outlet in the propylene distillation column of the purge stream enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene.

The method according to the invention may include one or more of the following characteristics, taken individually or in any technically feasible combination:

said method includes sending a purge stream enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene for recycling to a steam cracker;

- the head stream from the propylene remover column is at least partially condensed to obtain a liquid fraction, while the specified liquid fraction is separated into a reverse reflux introduced at the N1 level of the propylene remover column, and a propylene stream, and the purge stream is enriched in C4 paraffinic hydrocarbons and / or enriched for isobutene, taken from the side of the propylene stripper at the N2 level, located below the N1 level;

- side withdrawal of the recirculation stream is carried out at the N4 level of the propylene distillation column, located below the N2 level of the side outlet of the purge stream enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene;

- side selection of the recirculation flow is carried out in the gas phase;

- a purge stream enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene is taken in the gas phase;

said method, corresponding to the above type, comprises the following steps:

* cooling and/or condensation of the recirculation stream,

* the introduction of the cooled and/or condensed recirculation stream l in the specified supplied shoulder strap; And

the feed stream is formed from a C4 stream from a steam cracker and/or a C4 stream from a petroleum refinery, advantageously from catalytic cracking.

The subject of the invention is also a process for producing a propylene stream, comprising the following steps:

- introducing a feed stream enriched in C4 and/or C5 hydrocarbons and at least one stream enriched in ethylene into the metathesis reactor;

- extraction of the metathesis product at the outlet of the metathesis reactor;

- introduction of the metathesis product into the ethylene stripper;

- production at the head of the ethylene stripper of the head stream enriched in ethylene, and at the bottom of the ethylene stripper of the feed stream;

- introducing said feed stream into the propylene remover and recovering a bottom stream containing C4+ hydrocarbons in the bottom of the propylene remover;

- extracting the propylene stream from the head stream of the propylene stripper;

- side selection of the recirculation stream enriched in C4 and/or C5 hydrocarbons, and the return of the specified recirculation stream to the metathesis reactor;

characterized in that the side selection of the recirculation stream is carried out in the gas phase.

The process according to the invention optionally includes a side draw in the propylene distillation column of a purge stream enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene. It may include one or more of the above characteristics taken in isolation or in any technically feasible combination.

The object of the invention is also a plant for the production of propylene, containing:

a metathesis reactor provided with at least one feed stream enriched in C4 and/or C5 hydrocarbons and at least one feed stream enriched in ethylene, wherein the metathesis reactor produces a metathesis product;

- an ethylene stripper fed with a metathesis product from the metathesis reactor, wherein the ethylene stripper produces an ethylene-rich overhead stream and produces a feed stream at the bottom;

- a propylene stripper receiving said feed stream and producing a bottom stream containing C4+ hydrocarbons and an overhead stream enriched in propylene at the bottom;

- a block for extracting the propylene stream from the head stream of the propylene stripper;

- block for lateral selection of the recirculation stream enriched in C4 and/or C5 hydrocarbons, and for returning the recirculation stream to the metathesis reactor;

characterized by the presence of:

- a block for side outlet of the purge stream enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene in the propylene stripper.

The installation according to the invention may contain one or more of the following characteristics, taken in isolation or in any technically possible combination:

said plant comprises at least one device for recirculating a purge stream enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene to the steam cracker;

said plant comprises a condenser suitable for at least partially condensing the head stream from the propylene remover to obtain a liquid fraction, and a separator suitable for fractionating said liquid fraction into a reflux introduced at the N1 level of the propylene remover and a propylene stream, wherein the block for sampling the purge stream enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene, located laterally at the N2 level located below the N1 level;

- the side take-off unit is suitable for taking the recirculation stream enriched in C4 hydrocarbons in the gas phase;

- the diversion unit is suitable for withdrawing a purge stream enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene in the gas phase;

- said installation contains:

* block for cooling and/or condensation of the recirculation flow,

* block for the introduction of the cooled and/or condensed recirculation stream in the supplied shoulder strap;

- the propylene stripper contains an internal dividing wall defining a first area located on one side of the dividing wall facing the inlet for the feed stream, and the opposite area located on the other side of the wall for separating from the inlet for the feed stream, while the block for the side the outlet of the purge stream enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene opens into said opposite region;

- propylene distillation column includes two fractionating columns connected in series;

- the plant comprises a condenser suitable for at least partial condensation of the overhead stream from the ethylene stripper to obtain a liquid fraction and a vapor purge stream, and a separator suitable for fractionating the liquid fraction into a reflux introduced into the ethylene stripper and a recirculating ethylene stream returned to metathesis reactor.

The object of the invention is also a plant for the production of propylene, containing:

a metathesis reactor provided with at least one feed stream enriched in C4 and/or C5 hydrocarbons and at least one feed stream enriched in ethylene, wherein the metathesis reactor produces a metathesis product;

- an ethylene stripper fed with a metathesis product from the metathesis reactor, wherein the ethylene stripper produces an ethylene-rich overhead stream and produces a feed stream at the bottom;

- a propylene stripper receiving said feed stream and producing a bottom stream containing C4+ hydrocarbons and an overhead stream enriched in propylene at the bottom;

- a block for extracting the propylene stream from the head stream of the propylene stripper;

- a block for lateral withdrawal of the recirculation stream enriched in C4 and/or C5 hydrocarbons and return of said recirculation stream to the metathesis reactor,

characterized in that the side extraction unit is configured to extract the recirculation flow in the gas phase.

The plant according to the invention optionally includes a unit for side-drawing a purge stream enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene in the propylene distiller.

It may include one or more of the above characteristics, taken individually or in any technically feasible combination.

The invention will become better understood after reading the following description of the invention, which is presented by way of example only and is given with reference to the accompanying drawings, including:

fig. 1 is a functional block diagram for a first production plant for carrying out the method according to the invention;

fig. 2 is a detailed view of the metathesis device from the setup of FIG. 1;

fig. 3 is a detailed view of a second production plant for carrying out the method according to the invention.

In the following, the same reference numeral designates a fluid circulating in a pipe and a pipe that conveys that fluid. Also, unless otherwise indicated, percentage levels will be molar percentage levels and pressures will be in the appropriate bar.

The first plant 10 for producing a propylene stream 11 is illustrated in FIG. 1 and 2. Plant 10 is integrated into a plant 12 for the production of hydrocarbon derivatives, in particular a plant for cracking and separating hydrocarbons, illustrated in part in FIG. 1.

The plant 12 for the production of hydrocarbon derivatives includes at least one steam cracker 13 and/or an oil refinery 13 of the catalytic cracker or other type, and distillation columns 13A suitable for successfully separating hydrocarbon cuts.

In particular, the installation 12 for the production of hydrocarbon derivatives contains equipment 14 for the production of raw cut 16 C4 hydrocarbons, equipment 17 for the selective hydrogenation of butadiene contained in the feed cut 16, which also provides hydroisomerization of 1-butene from cut 16 to form 2-butene, and a column 18 for isobutene separation.

Equipment 17 and 18, respectively, can be replaced with butadiene separation devices, followed by isomerization of 1-butene to form 2-butene (block 17) and an MTBE unit for methyl tert-butyl ether (block 18).

Installation 12 for the production of hydrocarbon derivatives, in addition, contains equipment 20 for the production of shoulder strap 22 enriched in ethylene.

As can be said with reference to FIG. 2, plant 10 comprises at least one piece of equipment 24 for purifying ethylene-rich overhead 22 and for purifying C4-rich olefin feed 26 coming from separation column 18. Plant 10 further comprises a metathesis reactor 28 , a first separation column (hereinafter referred to as "ethylene demultiplexer 30") for separating the metathesis product 32 produced in reactor 28, and a second separation column (hereinafter referred to as "propylene demultiplexer 34") for separating separating the effluent from the ethylene stripper 30.

Ethylene remover 30 and propylene remover 34 are each equipped with a condenser and reflux system 36 and 38, respectively, each including a condenser 40A, 40B, respectively, a separator 42A, 42B, respectively, and a reflux pump 44A, 44B, respectively. .

Each of the ethylene remover 30 and the propylene remover 34 is also equipped with a bottom reboiler 46 and 48, respectively.

The plant 10 further comprises a recirculation loop 50 for the C4 enriched stream, including a distillation unit 52 in the propylene distiller 34 and a heat exchanger 54 for cooling the C4 enriched stream.

Unit 10 further comprises, in this example, a block 58 for a side draw stream enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene in the propylene remover 34. The side draw block 58 includes at least one side drain in the propylene remover 34 .

The method for producing propylene in plant 10 will now be described.

Initially, a hydrocarbon feedstock 16 C4 is produced from a steam cracker 13 and/or an oil refinery unit 13 (a catalytic cracker or other unit) and from separation steps in equipment 14.

In the case when this shoulder strap 16 is produced only using a steam cracker and in the advantageous case when the latter operates under conditions of medium severity (for example, P / E \u003d 0.61), C4 shoulder strap contains trace amounts of acetylenic C4 compounds (for example, less than than 5 mol.%), from 40 mol.% to 50 mol.% butadiene (in particular, approximately 45 mol.%), from 15 mol.% to 25 mol.% 1-butene (in particular, approximately 19 mol. %), from 5 mol.% to 15 mol.% 2-butene (in particular, about 10 mol.%), from 15 mol.% to 25 mol.% isobutene (in particular, about 20 mol.%) and from 2 mol.% to 10 mol.% C4 paraffinic compounds (in particular, about 6 mol.% n-butane and isobutane).

The effluents from a petroleum refinery (FCC or other units) are similar except for the percentage of paraffinic compounds which is more significant, eg greater than 11 mole %.

Typically, cut 16 is first introduced into equipment 17 to remove butadiene by selective hydrogenation, which also hydroisomerizes 1-butene to form 2-butene. Thus, the cut 16 is contacted with a hydrogen stream 60 and a hydrogenated stream 62 of C4 hydrocarbons is produced, including 1-butene, 2-butene, isobutene, and C4 paraffins. Alternatively, cut 62 is produced from the isolation of butadiene.

Thereafter, the butadiene-free hydrogenated stream 62 can be processed in several ways to produce an isobutene-depleted stream, for example, using an isobutene separation column 18.

In this case, column 18 produces an overhead stream 64 which contains separated isobutene, isobutane from hydrogenated stream 62 and a portion of 1-butene, since 1-butene has a boiling point very close to that of isobutene (the difference at atmospheric pressure between the boiling point of isobutene and the boiling point of 1-butene is less than 1°C).

Column 18 produces in the cubic part of the flow constituting the supplied shoulder strap 26.

The feed stream 26 contains mainly (eg, more than 60 mol%) normal olefins, normal paraffins, and in small amounts (eg, less than 20 mol%) isoolefins and isoparaffins.

Alternatively, isobutene can also be removed using a variety of reactions that include reaction with methanol to form MTBE, reaction with water to form tert-butyl alcohol (TBA), or reaction with itself to form a C8 stream.

The feed stream 26, if produced from a C4 steam cracker stream (when operating advantageously at a typical P/E ratio of 0.61), treated by hydrogenation, after this fractionation to remove isobutene, contains a lot of 2-butene (for example , about 80 mol.% or more). Alternatively, in accordance with the other treatments explained above and/or other origin of the C4 cut, the presence of 2-butene in the composition is less.

In other embodiments, the C4 cut is mixed with (or even replaced by) the C5 hydrocarbon cut.

The feed stream 26 is enriched in C4 and/or C5 olefinic hydrocarbons, with the level of C4 and/or C5 olefinic hydrocarbons in the feed stream 26 generally being greater than 65 mol%.

The following description of the invention refers to a feed stream 26 formed from C4 hydrocarbons, but the same description can be extrapolated to a feed stream 26 formed from a mixture of C4 and C5 hydrocarbons, or even to a C4 and C5 hydrocarbon cut or feed stream 26 , consisting only of C5 hydrocarbons.

Parallel to this, an ethylene-enriched cut 22 is produced, for example, from the steam cracker 13 and from the separation stages in the equipment 20.

Ethylene-rich cut 22 when produced from a steam cracker typically has an ethylene molar content greater than 99.9%. However, this process does not require such a high degree of purity, and chemically pure ethylene (at lower levels, ie, 99 mole % or less) is also acceptable.

Thereafter, feed cut 26 and ethylene-rich cut 22 are processed in facility 24 to capture poisons for metathesis catalysts such as water, sulfur, alcohols, CO ₂ , nitrogen compounds, heavy metals, etc.

Depending on the source of the loads and their conditions of the boundary of the "red line" installation, the equipment 24A, 24B is a specialized or combined protective layers. Typically, they also process the ethylene recycle stream 76 and the C4 rich recycle stream 92, as will be described below.

In the example shown in FIG. 2, protective layers 24A, 24B are specialized. Feed cut 26 (and associated recycle stream 92) is processed on liquid phase layer 24A to produce processed feed stream 66, and ethylene-rich cut 22 (and associated recycle stream 76) is processed on vapor phase bed 24B to produce obtaining shoulder strap 67, enriched in recycled ethylene.

Thereafter, the processed feed stream 66 is mixed with the processed ethylene-rich cut stream 67. The mixture forms a feed stream 67A for the reactor 28, which is vaporized, heated and introduced into the metathesis reactor 28.

In one (not shown) embodiment, cut 26 and cut 22 (as well as respective streams 92 and 76) are mixed and introduced into the same protective layer 24A, functioning in the liquid phase, in order to obtain a cut 67A for feeding the reactor 28. Cut 67A sent to the metathesis reactor 28 after evaporation and reheating.

In a known manner, in the metathesis reactor 28, due to the presence of a catalyst, a metathesis reaction takes place between 2-butene and ethylene. This stimulates, as a main reaction, the exchange of fragments between 2-butene and ethylene molecules to form propylene molecules. The metathesis reaction can theoretically take place between any olefinic compounds, since it is a mutual permutation of alkyl groups relative to double bonds. Therefore, many side reactions take place in the reactor. All these reactions are governed by the laws of equilibrium.

In order to promote equilibrium with respect to the main reaction, the flow rate for the ethylene-rich cut 22 is optimized so that the operation is carried out in an excess of ethylene. Thus, the ratio between the molar flow rates for ethylene and butene (1-butene and 2-butene) in the 67A feed to the reactor advantageously ranges from 1.3 to 3.

The metathesis reaction takes place at a temperature generally in the range of 250° C. to 380° C. and at a pressure in the range of 20 bar to 35 bar.

The catalyst used generally comprises a combination of a metathesis catalyst (a transition metal optionally combined with an oxide, such as tungsten oxide supported on silica or rhenium oxide supported on an alumina support) and a catalyst. , intended for isomerization, (metal oxide from group II of the periodic table of elements, for example, magnesium oxide or calcium oxide).

The metathesis product 32 is produced at the outlet of the metathesis reactor 28 .

The metathesis product 32 contains ethylene, propylene, C4 olefinic hydrocarbons, including unreacted 1-butene and 2-butene, heavier olefinic compounds, and paraffinic hydrocarbons.

Thereafter, the metathesis product 32 is introduced into the ethylene stripper 30 at the first intermediate feed level L1.

The pressure prevailing in the ethylene stripper 30 is typically in the range of 18 bar to 30 bar. The ethylene stripper 30 is a cryogenic column. Condenser 40A typically uses C3 hydrocarbons as the refrigerant.

An overhead stream 70 is produced from the top of the ethylene stripper 30. The overhead stream 70 typically has an ethylene content of greater than 95 mole %. Top stream 70 is at least partially condensed in condenser 40A and then fed to separator 42A.

In separator 42A, the partially condensed overhead stream 70 is separated into a liquid stream 71 and a vapor purge stream 74.

Liquid stream 71 recovered in vessel 42 is typically pumped and separated into reflux 72 and ethylene recycle stream 76. Vapor purge stream 74 aims to reduce the concentration of light compounds that can accumulate in the ethylene recycle stream 76 (methane, ethane and the like).

The ethylene recovered in the recycle stream 76 typically corresponds to more than 95% of the ethylene contained in the metathesis product 32.

Reflux 72 is introduced at level L2 of ethylene stripper 30 with level L2 above feed level L1.

The ethylene recycle stream 76 is recycled to the metathesis reactor 28. In the example illustrated in FIG. 2, stream 76 is mixed with ethylene-rich cut 22 upstream of processing unit 24.

A feed stream 78 is produced at the bottom of the ethylene remover 30 to feed the propylene remover 34.

Feed stream 78 contains propylene, typically about 99% of the propylene contained in the metathesis product 32, as well as C4 hydrocarbons that have not been converted in the metathesis reactor 28, and heavier C5+ hydrocarbons resulting from secondary metathesis reactions and/or already present in the chase served 26.

The ethylene content is almost zero (eg, less than 20 ppmv, as it must be compatible with the ethylene specification for the propylene stream 11, which is typically very low (less than 40 ppmv). ppmv, in particular about 30 ppmv).

Feed stream 78 is introduced into depropylene column 34 at feed level N3.

The pressure prevailing in the depropylene column 34 is typically in the range of 15 bar to 25 bar.

The propylene stripper 34 produces an overhead stream 80 which is very high purity propylene. Advantageously, overhead stream 80 meets the specifications typical of polymer grade propylene and is greater than 99.6% v/v or even greater than 99.9% v/v.

The overhead stream 80 is at least partially condensed, typically using water as the cooling fluid, in condenser 40B and then introduced into separator 42B. A liquid fraction 82 is withdrawn from the bottom of separator 42B, which is then pumped into pump 44B before separation into reflux 84 and propylene stream 11. Reflux 84 is reintroduced into depropylene column 34 at the N1 level above the N3 level.

As mentioned above, the propylene stream 11 is of "polymer quality" at a propylene molar level of generally greater than 99.6% by volume, or even greater than 99.9% by volume, ethylene content, generally less than 50 ppm. by volume, typically on the order of 30 ppm. by volume, and the level of content of C4 hydrocarbons, in the General case, constituting less than 40 ppm. by volume, typically on the order of 20 ppm. by volume.

The propylene recovered in stream 11 typically corresponds to more than 99.5% of the propylene contained in feed stream 78.

The selection from the recirculation stream 92, enriched in C4 hydrocarbons and containing the main amount (in the advantageous case more than 70%) of n-butene compounds of the metathesis product 32, which have not been converted in the metathesis reactor 28, is carried out from the propylene distillation column 34 using a block 52 for sampling at level N4 below the supply level N3.

Typically, this recycle stream 92 also contains a portion of the C5 hydrocarbons produced in the metathesis reactor 28 and/or present in the feed stream 26.

The recycle stream 92 is recycled to the metathesis reactor 28. Thus, it is cooled in the heat exchanger 54 before being reintroduced into the feed stream 26 upstream of the purification equipment 24.

Advantageously, the amount of C5 olefinic hydrocarbons in the recycle stream 92 is optimized so as to limit or even block certain side reactions in the metathesis reactor 28.

Bottom stream 88 is withdrawn from the bottoms of propylene remover 34. Bottom stream 88 purges C6 and heavier hydrocarbons, as well as C4 and C5 hydrocarbons that cannot react by the metathesis mechanism.

The purge ratio is usually set to contain enough paraffinic C4 hydrocarbons to prevent their accumulation in the recycle stream 92. Alternatively, another (not shown) distillation column follows the propylene remover 34, receiving the bottom stream 88 in order to separate the C4 compounds and C5+.

In accordance with the invention, a purge stream 90 enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene is withdrawn from the propylene distiller 34 using a side draw unit 58 at an N2 drawoff level below the feed level N3 and above the N4 level for recirculation sampling. flow 92.

Purge stream 90 contains light non-reactive components such as C4 paraffinic hydrocarbons, in particular isobutane and n-butane. It also contains some light olefins which are unattractive to the process, in particular isobutene.

The amount of C4 paraffinic hydrocarbons and/or isobutene in the purge stream 90 is generally more than 50 mol%, in particular more than 60 mol%.

Since the boiling points of the C4 hydrocarbons are very close, the purge stream 90 also contains 1-butene and 2-butene. The position of this tap in depropylene column 34 and the flow rate recovered from depropylene column 34 is optimized to minimize carryover of these latter two compounds.

Therefore, the molar flow ratio between the purge stream 90 and the feed stream 78 is adjusted depending on the amount of n-butane, isobutane and isobutene present in the feed stream 26. Through the use of the purge stream 90, a purge is carried out with respect to about 90% of the C 4 paraffinic hydrocarbons present in the feed chase 26.

The purge stream 90 is advantageously returned to the steam cracker 13 and/or to the catalytic cracker 13 . Thus, the paraffinic hydrocarbons present in purge stream 90 are used to produce additional olefins that are separated upstream of unit 10.

The overall yield for the propylene production process by the metathesis mechanism is generally defined as:

The degree of conversion in the metathesis reactor 28 is determined as:

The theoretical maximum degree of conversion is very much dependent on loading as it is determined by the balance of all the metathesis reactions involved. In all cases, the expected values are on the order of 70% as the maximum, or even less for the case where cut 26 is enriched in 1-butene. Therefore, to obtain attractive overall yields (eg, of the order of magnitude in the range of 80% to 90%), it is essential to recycle the unreacted 1-butene and 2-butene to the metathesis reactor 28 . Thus, at a constant flow rate for recirculation stream 92 enriched in C4 hydrocarbons:

- the more this stream 92 is enriched in 1-butene and 2-butene, the better the overall yield will be; vice versa,

- the more a given stream 92 contains compounds inert to the metathesis reaction (typically paraffins) and/or compounds capable of restricting the main 2-butene to propylene reaction (typically isobutene), the less good the overall yield will be.

Therefore, it is also stimulated that, with a constant total efficiency:

- the richer the recirculation stream 92 would be in terms of 1-butene and 2-butene, the lower would be the required flow rate; vice versa,

- the more recycle stream 92 would contain compounds inert to the metathesis reaction (usually paraffins) and/or compounds capable of restricting the main reaction of converting 2-butene to propylene (usually isobutene), the greater would be the required flow rate .

In accordance with the invention, diverting purge stream 90 enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene reduces the flow rate for recycle stream 92. Thus, the size of the equipment (and lines) used for recirculation is greatly reduced. Similarly, the total flow through the purification equipment 24 (e.g. guard beds 24A, 24B), in the metathesis reactor 28 in the ethylene remover 30 and in the propylene remover 34 is reduced.

In addition, there is no need to impose strict restrictions on the level of isobutene in the feed stream 26 to the metathesis reactor 28, since the isobutene is removed using the purge stream 90. This results in energy savings and high performance, downsizing of the preprocessing equipment. in particular columns 18 for separating isobutenes.

In conclusion, the method according to the invention offers the possibility of eliminating bottlenecks ("bottlenecks") in the plant 10. In fact, as seen above, the addition of a purge stream 90 enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene provides allowing plant 10 to operate with limited C4 hydrocarbon recycle stream 92 and/or improve overall yield at the same stream 92 recycle rate. This advantage increases propylene production by increasing feed stream 26 without the need for significant modifications to existing equipment.

Recycle stream 92 is taken in liquid form. In one embodiment, the departure recycle stream 92 enriched in C4 hydrocarbons is advantageously taken in the gas phase. Thus, in comparison with the withdrawal of liquid in the recirculation stream 92, the amount of heavy components, in particular, the amount of C6+ hydrocarbons, is significantly reduced.

By way of illustrating the recovery in the propylene remover 34, the various compounds present in the recycle stream 92 obtained from the feed 26 corresponding to the description in Table 1 below are illustrated in Table 2 below. form:

(X) = flow rate for connection X

Table 2 compares a prior art plant devoid of paraffinic hydrocarbon purge stream 90 C4 with plant 10 according to the invention. Two hypotheses are presented for the withdrawal of the purge stream 90 according to the invention (liquid withdrawal or gas withdrawal).

The option in which the recycle stream 92 is gaseous is also shown in Table 2.

Supplied shoulder strap 26 is characterized by the following composition:

Table 1

The composition of the supplied shoulder strap 26 (mol.%) Propylene 0.6 Isobutene 2.1 1-butene 5.3 2-butene 59.6 n-butane 26.0 Isobutane 5.4 Pentenes 1.0

For all data for the various options in Table 2, the same mass flow rate for the recycle stream 92 C4 hydrocarbons is considered (generally, the mass flow rate for the recycle stream 92 is considered identical to the mass flow rate for the feed cut 26) and the same amount of excess ethylene in the reactor 28 metathesis.

table 2

Prior Art Column Recovery 34 (no purge stream 90) The degree of recovery in the propylene remover 34 in accordance with the present invention (with a purge stream of 90) Liquid purge stream 90 Gaseous purge stream 90 Extraction P of purge gas 90 + recycle gas 92 Isobutane 79% 74% 71% 53% Isobutene 76% 74% 72% 61% 1-butene 75% 73% 72% 63% n-butane 66% 69% 70% 72% 2-butene 65% 68% 69% 73% Pentenes 34% 36% 36% 40% C6+ 26% 25% 25% 9%

By implementing the purge stream 90, the amount of 2-butene recovered in the recycle stream 92 is significantly greater, allowing for better recycling of this compound and therefore a better overall propylene yield.

Conversely, the amount of light paraffinic compounds that are inert to the metathesis reaction (in particular isobutane) and/or light compounds capable of imposing restrictions on the main reaction of the conversion of 2-butene to propylene (in particular isobutene) is significantly reduced, which allows avoid unnecessary recycling of these compounds to the reactor.

In the example shown, the progress made is even more attractive when the purge stream 90 is taken in gaseous rather than liquid form and when the hydrocarbon recycle stream 92 C4 itself is taken in gaseous rather than liquid form.

In addition, the recovery of C6+ hydrocarbons in the depropylene column goes from a typical value of approximately 25% when withdrawing recycle stream 92 in liquid form to less than 10% when withdrawing in gaseous form. The sharp decrease in recovery of C6+ hydrocarbons from liquid to vapor recycle stream 92 is independent of the presence of purge stream 90 enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene.

By withdrawing vapor phase recycle stream 92, heavy compounds, which are sources of fouling and premature coking of the metathesis catalyst, are almost no longer recycled to reactor 28, extending catalyst cycle time. Since successful regenerations at very high temperature reduce the activity of the catalyst, the catalyst must be replaced after several cycles. Therefore, the extension of the cycle time increases the service life of the catalysts.

Yields expected from an embodiment of purge stream 90 enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene depend on the composition of the feed stream 26. In the example above (feed stream 26 corresponding to Table 1):

- at the same flow rate for the recirculation stream 92 in the form of a cut enriched in C4 hydrocarbons, the total yield increases by 1.5%,

- at the same production of propylene, the flow rate for the recycle stream 92 in the form of a shoulder strap enriched in C4 hydrocarbons is reduced by approximately 15%, which reduces the size of the purification equipment 24, the metathesis reactor 28, the ethylene remover 30 and the propylene remover 34 by an amount of the order of 6%.

The choice between purge stream 90 in gaseous or liquid form represents a compromise between increased production and capital investment costs (CAPEX), since the final destination of purge stream 90 must also be taken into account.

In one embodiment, illustrated in FIG. 3, the plant 10 according to the invention still includes a purge stream 90 comprising paraffinic hydrocarbons, C4 and/or enriched in isobutene. Recycle stream 92 enriched in C4 hydrocarbons is withdrawn from propylene distiller 34 in liquid form.

In addition, the propylene remover 34 includes a distillation column arranged in accordance with the concept of split streams using a vertical internal dividing wall 300, as seen in FIG. 3. The dividing wall 300 in the distillation column defines a first region 302 located on one side of the dividing wall 300 facing the inlet for the feed stream 78 (left in Fig. 3) and the opposite region 304 located on the other side of the dividing wall 300 from feed inlet 78, (right in FIG. 3).

Thus, the heaviest compounds (in particular, C6+ hydrocarbons) of the feed stream 78 are bypassed directly through the bottom of the distillation column, through the first region 302 (on the left of Fig. 3) and discharged into the bottom stream 88.

Light compounds are stripped off at the top of the same side of the dividing wall 300. Propylene is recovered from the top in stream 11, and the C4-C5 hydrocarbons continue their separation in the second region 304 defined by the dividing wall 300 (right of FIG. 3).

A purge stream 90 enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene is withdrawn (in gaseous or liquid form) in a second region 304 defined by dividing wall 300, and a recycle stream 92 enriched in C4 hydrocarbons is withdrawn below.

Stream 92 is advantageously withdrawn in the liquid phase while maintaining a low recovery of heavy compounds, since the latter are not present at that point in propylene remover column 34. The use of a split stream column as propylene remover column 34 extends the runtime for metathesis reactor 28 in accordance with the same principles as the corresponding principles presented above for the propylene distiller of FIG. 2 when recycle stream 92 is taken in gaseous form.

In one embodiment, the process is carried out with a side withdrawal of the recycle stream 92 enriched in C4 and/or C5 hydrocarbons, carried out in the gas phase, but in the absence of a purge stream 90 containing C4 paraffinic hydrocarbons and/or enriched in isobutene.

Claims (36)

1. A process for the production of a stream (11) of propylene, comprising the following steps: introducing a feed stream (26) enriched in C4 and/or C5 hydrocarbons and at least one stream (22) enriched in ethylene into the metathesis reactor (28); - extraction of the metathesis product (32) at the outlet (28) of the metathesis reactor; - introduction of the metathesis product (32) into the ethylene stripper (30); - production at the head of the ethylene stripper (30) of the head stream (70) enriched in ethylene and at the bottom of the ethylene stripper (30) of the feed stream (78); - introducing the feed stream (78) into the propylene distiller (34) and removing from the bottoms of the propylene remover (34) a bottom stream (88) containing C4+ hydrocarbons; - extracting the propylene stream (11) from the top stream (80) of the propylene stripper (34); - side selection of the recirculation stream (92), enriched in C4 and/or C5 hydrocarbons, and return of the recirculation stream (92) to the metathesis reactor (28); characterized by the presence of the following further stage: - side outlet of the purge stream (90), enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene, in the propylene distillation column (34). 2. The process of claim 1, comprising recycling the purge stream (90) enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene to the steam cracker. 3. Process according to claim 1 or 2, wherein the overhead stream (80) from the propylene remover (34) is at least partially condensed to produce a liquid fraction (82), wherein the liquid fraction (82) is separated into reflux (84) , introduced at the N1 level of the propylene remover column (34), and a propylene stream (11), wherein the purge stream (90), enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene, is taken from the propylene remover column (34) from the side at the N2 level located below level N1. 4. The method according to any one of paragraphs. 1-3, in which the side draw of the recycle stream (92) is carried out at the N4 level of the propylene distiller (34) located below the N2 level of the side draw of the purge stream (90) enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene. 5. The method according to any one of paragraphs. 1-4, in which the side selection of the recirculation stream (92) is carried out in the gas phase. 6. The method according to any one of paragraphs. 1-5, in which the purge stream (90) enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene is withdrawn in the gas phase. 7. The method according to any one of paragraphs. 1-6, including the following steps: - cooling and/or condensation of the recirculation stream (92), - introducing the cooled and/or condensed recycle stream (92) into the feed stream (26). 8. The method according to any one of paragraphs. 1-7, in which the feed stream (26) is formed from a C4 stream of a steam cracker and/or a C4 stream of an oil refinery. 9. Installation (10) for the production of propylene (11), containing: - a metathesis reactor (28) provided with at least one feed stream (26) enriched in C4 and/or C5 hydrocarbons and at least one stream (22) enriched in ethylene, wherein the metathesis reactor (28) produces a product (32) metathesis; - an ethylene stripper (30) fed with a metathesis product (32) from a metathesis reactor (28), wherein the ethylene stripper (30) produces an ethylene-enriched head stream (70) in the upper part and produces a feed stream (78) in the bottom part ); a propylene stripper (34) receiving feed stream (78) and producing bottom stream (88) containing C4+ hydrocarbons and head stream (80) rich in propylene at the top; - a block for extracting the propylene stream (11) from the head stream (80) of the propylene stripper (34); - block (52) for lateral selection of the recirculation stream (92), enriched in C4 and/or C5 hydrocarbons, and for returning the recirculation stream (92) to the metathesis reactor (28); characterized by the presence of: - a block (58) for lateral removal of the purge stream (90), enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene, in the propylene distillation column (34). 10. Plant (10) according to claim 9, comprising at least one device for recycling the purge stream (90) enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene to the steam cracker. 11. Plant (10) according to claim 9 or 10, comprising a condenser (40B) suitable for at least partial condensation of the overhead stream (80) from the propylene remover (34) to obtain a liquid fraction (82), and a separator (42B) suitable for fractionating the liquid fraction (82) into a reflux (84) introduced at the N1 level of the propylene distiller (34) and a propylene stream (11), while a block (58) for removing the purge stream (90) enriched in C4 paraffinic hydrocarbons and / or enriched in isobutene, located laterally at the N2 level, located below the N1 level. 12. Installation (10) according to any one of paragraphs. 9-11, in which the side extraction unit (52) is suitable for withdrawing the recirculation stream (92) enriched in C4 hydrocarbons in the gas phase. 13. Installation (10) according to any one of paragraphs. 9-12, wherein the diversion unit (58) is suitable for diverting the purge stream (90) enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene in the gas phase. 14. Installation (10) according to any one of paragraphs. 9-12 containing: - a block (54) for cooling and/or condensing the recirculation stream (92), - a unit for introducing the cooled and/or condensed recirculation stream (92) into the feed stream (26). 15. Installation (10) according to any one of paragraphs. 9-14, in which the propylene remover column (34) comprises an internal dividing wall (300) defining a first region (302) located on one side of the dividing wall (300) facing the inlet for the feed stream (78), and the opposite region (304) located on the other side of the dividing wall (300) from the inlet for the feed stream (78), while the block (58) for the side outlet of the purge stream (90) enriched in C4 paraffinic hydrocarbons and/or enriched in isobutene, opens into the opposite region (304). 16. Installation (10) according to any one of paragraphs. 9-15, in which the propylene remover (34) includes two fractionation columns connected in series.

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