EP1217060A1 - Process for treatment of butadiene and acetylenes containing C4-hydrocarbons comprising the steps of distillation and selective hydrogenation - Google Patents

Abstract

A process for treating a 4 carbon atom charge which contains diene compounds and a minor portion of acetylene compounds is described. Part of the fluid circulating in a distillation zone which is enriched with acetylenic compounds is withdrawn laterally, preferably in the exhaustion zone, a selective hydrogenation step is carried out in a hydrogenation zone external to the distillation zone. The hydrogenation effluent produced is recycled to the rectification zone. A C4 cut comprising the butadiene and depleted in acetylenic compounds is recovered at the head and a C5 cut enriched in oligomers at the bottom. <IMAGE>

Description

The invention relates to a method for treating a charge comprising hydrocarbons with at least 4 carbon atoms per molecule, strongly unsaturated. It applies in particular to the purification of an olefinic cut containing mostly butadiene, including 1,3 butadiene, produced highly recoverable as a raw material for elastomers.

The prior art is illustrated by patent applications WO-97 24413 and EP-A-0 273 900.

This cut also contains acetylenic impurities, vinyl acetylene (VAC), ethylacetylene (ETAC), generally 1 to 2% which create dysfunctions in the polymerization processes due to the presence gums that these compounds tend to create and which therefore must be eliminated.

To eliminate acetylenic compounds, it is known to hydrogenate them to presence of a catalyst containing a noble metal from group VIII of the table periodical of the elements, alone or with promoters, deposited on a support such than alumina or silica.

This is how in the main hydrogenation processes of compounds acetylenic, oligomers are formed (mainly dimers and trimers) in the hydrogenation reactor which must then be separated from the butadiene.

The hydrogenation reactor can be installed upstream of the debutanization. Under these conditions, the ratio of acetylenic compositions on butadienes is very weak in the charge and the selective hydrogenation of these compounds are difficult to perform resulting in relatively low butadiene losses high.

According to another variant of the prior art, the load to be treated can be introduced in an debutanizer. The C4 and acetylenic compounds are recovered at the top of the debutanizer, then hydrogenated in a reactor. The hydrogenation effluent is again distilled in another distillation column. The purified C4 cut is recovered at the top of the distillation column while the oligomers obtained at the bottom of said column are recovered. Certainly acetylenic compounds were concentrated in the debutanizer since the C5 were separated in the bottom but the overall process requires an additional distillation column, which which increases investment and operating costs. In addition, the acetylenic ratio on butadienes is unchanged compared to the variant previous.

Furthermore, US Pat. No. 5,866,734 describes a process for the hydrogenation of a olefinic C4 cut mainly containing butadiene and compounds acetylenic so as to substantially completely hydrogenate the lossless multiple and triple bond compounds of unsaturated hydrocarbons with a double bond, the reaction being able to take place in a catalytic distillation column.

One of the objects of the invention is to remedy the drawbacks of the prior art.

Another object is to selectively hydrogenate acetylenic compounds contained in a C4 cut in the presence of hydrogen without necessarily cause excessive butadiene losses at the lowest cost possible. Another object is to purify a cut very rich in butadiene while minimizing as much butadiene losses from distillation and the hydrogenation of said cut.

Specifically, the invention relates to a method of treating a charge containing hydrocarbons with at least 4 carbon atoms per molecule, said filler comprising diene compounds and mainly butadiene as well as acetylenic compounds in minor proportion, said process comprising a stage of distillation of the charge introduced into a zone of distillation which includes a rectification zone and a depletion zone and at minus a step of hydrogenation of acetylenic compounds in at least a hydrogenation zone with at least one catalytic bed under conditions suitable hydrogenation in the presence of a gas containing hydrogen, the process being characterized in that it is withdrawn laterally in the liquid phase part of the charge circulating in the distillation zone enriched in acetylenic compounds, up to an appropriate level of withdrawal in the distillation zone and preferably in the exhaustion zone, the stage is carried out of hydrogenation in the hydrogenation zone which is external to the zone of distillation, a hydrogenation effluent depleted in compounds is produced acetylenic and enriched in oligomers and said hydrogenation effluent is recycled in the rectification zone, the process being further characterized in that one recovers at the head of the distillation zone a C4 cut comprising substantially all butadiene and depleted in acetylenic compounds and at the bottom of the zone distillation a C5 cut enriched in oligomers.

By rectification area is meant an area above the level supply feed to the distillation column.

Depletion zone means an area below the level supply feed to the distillation column.

The feedstock can be a steam cracking effluent which mainly contains hydrocarbons of 4 to 5 carbon atoms per molecule and preferably one most of hydrocarbons with 4 carbon atoms.

This charge according to the invention can contain at least 20% by weight of butadienes and preferably at least 50% by weight in section C4 alone. Besides, she generally contains at most 20% by weight of acetylenic compounds, advantageously at most 5% and preferably at most 2.5% by weight.

According to a characteristic of the invention, the withdrawal rate can be at most equal to twice that of the charge introduced into the column, advantageously at most 1.5 times that of the column. It is by drawing from the area depletion a liquid fluid flow rate substantially equal to that of the load entered in the column which gives the best results.

According to a particularly advantageous characteristic of the process, the charge can be introduced at a level corresponding substantially to the middle of the distillation column, the lateral racking level is located below said middle of the column at a height corresponding to less than five trays theoretical and we recycle the hydrogenation effluent above the middle of the column, at a level corresponding at most to the height of the first five theoretical platforms.

It will be possible to determine by appropriate chromatographic measurements the ratio of the concentrations of acetylenic compounds on butadiene and withdraw laterally the fluid when this ratio, on the considered plate of the column, is substantially higher and advantageously higher than that of the charge.

At the level of the reinjection of the product of the hydrogenation reaction, it is advantageous to reinject this flow as high as possible in the column compared at the racking level in order to create an internal reflux in the column and therefore to increase its separation power.

The increase in temperature linked to hydrogenation is generally small because the quantity of hydrogenated products is very small. Nevertheless, it can be advantageous to control the exothermicity of the hydrogenation step and the temperature of the hydrogenation effluent upstream of the recycling level in the rectification area of the column, especially since it is preferable to reintroduce the fluid at the top of the column at a substantially equal temperature to that of the reintroduction tray, so as not to disturb the column of distillation.

• Nombre de plateaux théoriques 40, de préférence 35 - 45
• Pression absolue 4 - 10 bar (1 bar = 10⁵ Pa)
• Température de tête : 45 °C, de préférence 30 °C à 50 °C
• Température de fond : 95 °C, de préférence 90 °C à 150 °C

The operating conditions of the distillation column are usually as follows

• Number of theoretical platforms 40, preferably 35 - 45
• Absolute pressure 4 - 10 bar (1 bar = 10 ⁵ Pa)
• Head temperature: 45 ° C, preferably 30 ° C to 50 ° C
• Bottom temperature: 95 ° C, preferably 90 ° C to 150 ° C

• Pression absolue : 2 à 70 bar (1 bar = 10⁵ Pa), de préférence 5 à 15 bar
• Température : 30 à 60 °C, de préférence 35 à 45 °C
• Vitesse spatiale 3 à 10 h^-1, de préférence 4 à 8 h^-1

The hydrogenation reactor is generally operated under the following conditions:

• Absolute pressure: 2 to 70 bar (1 bar = 10 ⁵ Pa), preferably 5 to 15 bar
• Temperature: 30 to 60 ° C, preferably 35 to 45 ° C
• Spatial speed 3 to 10 h ^- 1, preferably 4 to 8 h ^-1

• Rapport H₂/composés acétyléniques (mole/mole) : 0,5-3, de préférence 1,0 à 1,1
• Catalyseur : soit le nickel ou une masse de captation contenant du cuivre, soit un métal noble du groupe VIII, de préférence le palladium, stabilisé par au moins un métal Au, Ag, par exemple 0,01 à 1 % poids de métal par rapport au poids total du catalyseur ledit rapport H₂/composés acétyléniques sera ajusté en fonction des spécifications requises pour l'effluent de tête de la colonne de distillation.

The space velocity represents the volume of catalyst divided by the liquid volume of fresh charge measured at 15 ° C.

• H ₂ / acetylenic compounds ratio (mole / mole): 0.5-3, preferably 1.0 to 1.1
• Catalyst: either nickel or a capture mass containing copper, or a noble metal from group VIII, preferably palladium, stabilized by at least one metal Au, Ag, for example 0.01 to 1% by weight of metal relative to the total weight of the catalyst, said H ₂ / acetylene compounds ratio will be adjusted according to the specifications required for the overhead effluent from the distillation column.

The invention will be better understood from the following figure, illustrating a mode of schematically preferred embodiment.

A hydrocarbon charge 1 comprising a cut. C4 resulting from a steam cracker and containing approximately 50% of butadienes and 1 to 2% compared to the C4 cut only of acetylenic compounds is introduced into a column 2 of distillation called debutanizer. This column comprising about 40 theoretical plates, the charge is introduced at the level of the 20 ^th plate. At the top of the column, the fraction C4 containing butadiene and approximately 1000 ppm of acetylene compounds is recovered via a line 3. Part of this fraction is reintroduced after condensation 20 and separation 21, in the form of reflux 5 at the top of the column while the other part is recovered by a line 6 for further treatment, extraction by solvent for example.

At the ^23rd tray of the column is laterally discharged through a line 7 a liquid fluid which the ratio of concentrations acetylenic compounds / butadiene is substantially the highest in the column taking into account dilution by a factor 2 by reaction product, for example 0.027 mol / mol.

This fluid is introduced into at least one hydrogenation reactor 8 supplied with hydrogen by a line 9 under partial pressure conditions which correspond substantially to the stoichiometry of the hydrogenation of the acetylene compounds. This reactor contains a fixed downflow bed (introduction from the top of the reactor of the liquid charge) of hydrogenation catalyst which can be palladium stabilized by gold on a gamma alumina support. The hydrogenation effluent is collected by a line 10, cooled in exchanger 11 and recycled in the ^4th theoretical plate, for example of the rectifying zone of the column, at a temperature substantially equal to that of the ^4th tray .

This effluent contains in particular the olefinic compounds initially present in the charge, the butadienes which have not been substantially hydrogenated thus than the oligomers produced in the hydrogenation zone. These compounds oligomers which are heavy products are collected at the bottom of the column by a line 13 as well as the hydrocarbons with 5 carbon atoms per molecule of load. A part is used to be introduced into a reboiler 14 and recycled at the bottom of the column by a line 15.

The following examples illustrate the invention.

Example 1

A charge C4 + C5 of steam cracking whose composition is described in Table 1 is introduced into the device described according to the figure, at its bubble temperature. Charge C4 C5 Head cut Bottom cut Phase Liquid Liquid Liquid Mass percentages PROPANE 0.01 0.01 0.00 pROPADIENE 0.01 0.01 0.00 propyne 0.02 0.02 0.00 IBUTANE 0.27 0.38 0.00 ISOBUTENE 13.66 19.22 0.00 1BUTENE 9.06 14.40 0.01 13BUTADIENE 32.52 44.51 0.04 NBUTANE 3.53 4.96 0.02 TRANS2BUTENE 4.26 6.66 0.06 CIS2BUTENE 2.00 3.11 0.10 VAC 0.73 0.03 0.05 1BUTYNE (ETAC) 0.20 0.07 0.06 12BUTADIENE 0.40 0.31 0.15 ISOPENTANE 1.67 0.44 4.68 2METHYL1BUTENE 3.21 0.73 9.27 Isoprene 9.52 1.86 28.23 PENTANE 3.33 0.62 9.96 TRANS2PENTENE 1.43 0.26 4.29 pentadiene 3.33 0.44 10.39 CYCLOPENTADIENE 9.76 1.81 29.18 cyclopentene 0.48 0.07 1.49 CYCLOPENTANE 0.60 0.07 1.89 15ETHYLCYCLOHEXADIENE - 0.00 0.10 123TRIMETHYLINDENE - 0.00 0.02

The feed level of the feed in the column, the lateral draw-off level and the level of recycling of the hydrogenated effluent are usually chosen in order to obtain 1000 ppm by weight of acetylenics in the C4 cut while minimizing the loss in butadiene 1.3. This charge is introduced at the level of the 20 ^th theoretical plate of the column.

The purpose of the column is to separate the C4 section from the other hydrocarbons.

Nombre de plateaux théoriques : 40 (50 à 60 plateaux réels).

Pression de service : 5 bar abs en tête, 5,3 bar abs en fond.

Température de service : 45 °C en tête, 95 °C en fond.

The operating conditions of the column were set as close as possible to those of an industrial column. That is to say :

Number of theoretical platforms: 40 (50 to 60 real platforms).

Operating pressure: 5 bar abs at the head, 5.3 bar abs at the bottom.

Serving temperature: 45 ° C at the head, 95 ° C at the bottom.

• Taux de reflux massique liquide/distillat = 1,2
• Teneur en C4 dans la coupe C5 : 0,5 %

Alimentation au point de bulle (56 °C), Pression = 6 bar abs, débit = 30 T/h.The column specifications are as follows:

• Liquid / distillate mass reflux rate = 1.2
• C4 content in C5 cut: 0.5%

Bubble point supply (56 ° C), Pressure = 6 bar abs, flow = 30 T / h.

Pression absolue : 5 bar

Température : 40 °C en entrée

Vitesse spatiale : 4 h^-1

Débit d'hydrogène : 30 kg/h

Catalyseur = Palladium 2000 ppm

LD 277® Procatalyse = Or 800 ppm

Is laterally withdrawn at the ^23rd a liquid effluent theoretical plate whose ratio acetylene compounds / butadienes is equal to 0.027 mol / mol The draw rate is equal to the feed flow rate.
This effluent is introduced into the catalytic hydrogenation reactor which is operated under the following conditions, in the presence of hydrogen:

Absolute pressure: 5 bar

Temperature: 40 ° C at input

Spatial speed: 4 h ^-1

Hydrogen flow: 30 kg / h

Catalyst = Palladium 2000 ppm

LD 277® Procatalyse = Gold 800 ppm

The hydrogenation effluent is recycled in the rectification zone of the column at the ^4th theoretical plate. The C5 hydrocarbons as well as the oligomers produced during the hydrogenation reaction will mainly recover at the bottom of the column.

At the top of the column, the C4 hydrocarbons are recovered, the composition of which is given in table 1 (head section) while at the bottom of the column, we recovers a C5 cut containing oligomers.

Conversion rate: the conversion rate of a product represents the quantity of product (by mass) that has disappeared.
Ex: VAC conversion rate = 0.95: 1- (VAC mass) outlet at the top of the column / (VAC mass) Entrance = 0.95

This is the conversion rate observed.
Butadiene losses: butadiene losses represent the quantity of butadiene which is not recovered at the top of the column, that is to say the butadiene which is hydrogenated into butene at the level of the reactor plus the butadiene which is lost at the bottom of the debutanizer in section C5.
Losses of 1,3 butadiene, the isomer with commercial interest, are 2.7% while the content of acetylenic compounds in the head section is 1000 ppm, which corresponds to a conversion rate of the vinylacetylene compound of 0.95.

Example 2: (comparative)

Example 1 is repeated under identical conditions, but instead of partially withdrawing an effluent from the column, from the exhaustion zone to the rectification zone, it is withdrawn from the rectification zone (10 ^th tray) and recycling is carried out. the effluent in the rectification zone (7 tray ^e). The withdrawal rate is fixed at 20 T / h, the VAC conversion rate at 0.9.
In this configuration, the content of acetylenic compounds in the C4 cut collected at the head is approximately 3000 ppm and the losses of butadiene are approximately 2%.

Example 3: (comparative)

Example 1 is repeated under identical conditions, but instead of partially withdrawing an effluent from the column, from the exhaustion zone to the rectification zone, it is withdrawn from the exhaustion zone (35 ^th tray) and the recycled after hydrogenation ^32nd tray in the stripping zone.

The butadiene content at the inlet of the hydrogenation reactor being very low (less than 1%), the losses are therefore negligible.
On the other hand, the content of acetylenic compounds at the top of the column is very high (greater than 8000 ppm). All the acetylenic compounds entering the column at the feed level (20 ^th tray) and leaving in the head zone are not hydrogenated by such a device.

Claims (10)

Process for treating a feedstock comprising hydrocarbons containing at least 4 carbon atoms per molecule, said feedstock comprising diene compounds and mainly butadiene as well as acetylenic compounds in minor proportion, said process comprising a step of distillation of the feedstock introduced in a distillation zone which comprises a rectification zone and a depletion zone and at least one step of hydrogenation of the acetylenic compounds in at least one hydrogenation zone with at least one catalytic bed under suitable hydrogenation conditions in presence of a hydrogen-containing gas, the process being characterized in that part of the charge flowing in the distillation zone enriched in acetylenic compounds is withdrawn laterally in the liquid phase, up to a level of appropriate racking in the distillation zone and preferably in the exhaustion zone, hydrogenation step in the hydrogenation zone which is external to the distillation zone, a hydrogenation effluent depleted in acetylenic compounds and enriched in oligomers is produced and said hydrogenation effluent is recycled in the rectification zone, a process being further characterized in that a C4 cut comprising substantially all of the butadiene and depleted in acetylenic compounds is recovered at the head of the distillation zone and at the bottom of the distillation zone a C5 cut enriched in oligomers. The method of claim 1 wherein the feed is an effluent from steam cracking which mainly contains hydrocarbons of 4 to 5 atoms of carbon per molecule and preferably a major part of 4-hydrocarbons carbon atoms. Method according to one of claims 1 and 2, wherein the content of butadienes in the load is at least equal to 20% by weight and preferably equal to 50% by weight. Method according to one of Claims 1 to 3, in which the charge contains at most 20% by weight of acetylenic compounds, advantageously at most 5% and preferably at most 2.5%. Method according to one of claims 1 to 4, in which the flow of withdrawal is at most equal to twice that of the load, advantageously at more equal to one and a half times the flow of the charge and preferably substantially equal to that of the feed introduced into the distillation zone. Method according to one of Claims 1 to 5, in which the charge is introduced at a level corresponding substantially to the middle of the column distillation, the lateral racking level is located below said middle of the column at a height corresponding to less than five theoretical plates and the hydrogenation effluent is recycled above the middle of the column, at a level corresponding at most to the height of the first five trays theoretical. Method according to one of claims 1 to 6 wherein the ratio of acetylenic / butadiene compound concentrations at the racking level lateral is substantially the highest. Process according to one of Claims 1 to 7, in which the operating conditions of the distillation zone are as follows: Number of theoretical platforms: 40, preferably 35 to 45 Absolute pressure: 4- 10 bar, preferably 5 bar Head temperature: 45 ° C, preferably 30 ° C to 50 ° C Bottom temperature: 95 ° C, preferably 90 ° C to 150 ° C Method according to one of Claims 1 to 8, in which the operating conditions in the hydrogenation zone are as follows: Absolute pressure: 2 to 70 bar, preferably 5 to 15 bar Temperature: 30 to 60 ° C, preferably 35 ° C to 45 ° C Space velocity 3 to 10 h ^-1 , preferably 4 to 8 h ^-1 H ₂ / acetylenic compounds ratio (mole / mole) = 0.5 to 3, preferably 1.0 to 1.1 Group VIII noble metal catalyst, preferably palladium; 0.01 to 1% by weight stabilized with at least one metal from the group formed by Au, Ag, Sn. Method according to one of Claims 1 to 4, in which the temperature of the hydrogenation effluent upstream of the recycling level in the rectification zone of the distillation column.

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