EP1217060B1 - Process for treatment of butadiene and acetylenes containing C4-hydrocarbons comprising the steps of distillation and selective hydrogenation - Google Patents

Description

The invention relates to a process for treating a feedstock comprising hydrocarbons having at least 4 carbon atoms per molecule, which are highly unsaturated. It applies in particular to the purification of an olefinic cut containing butadiene for the most part, in particular butadiene 1,3, highly recoverable product as raw material elastomers.

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

This section also contains acetylenic impurities, vinylacetylene (VAC), ethylacetylene (ETAC), generally up to 1 to 2% which create dysfunctions in the polymerization processes due to the presence of gums that these compounds tend to to create and therefore need to be eliminated.

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

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

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

According to another variant of the prior art, the charge to be treated can be introduced into a debutanizer. The C4 and acetylenic compounds are recovered at the top of the debutanizer and then hydrogenated in a reactor. The hydrogenation effluent is distilled again 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. Admittedly, acetylenic compounds were concentrated in the debutanizer since the C5s were separated in the bottom but the overall process requires an additional distillation column, which increases the investment and operating costs. In addition, the acetylenic ratio on butadienes is unchanged from the previous variant.

Furthermore, US Pat. No. 5,866,734 describes a process for the hydrogenation of an olefinic C4 cut predominantly containing butadiene and acetylenic compounds so as to substantially completely hydrogenate the compounds with multiple double bonds and triple bonds without loss 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 overcome the disadvantages of the prior art.

Another object is to selectively hydrogenate the acetylenic compounds contained in a C4 cut in the presence of hydrogen without causing excessive losses of butadienes and at the lowest possible cost. Another object is to purify a very rich butadiene cut while minimizing the maximum butadiene losses associated with the distillation and hydrogenation of said cut.

Specifically, the invention relates to a process for treating a feedstock comprising hydrocarbons having at least 4 carbon atoms per molecule, as described in the wording of claim 1, said feed comprising diene compounds and mainly butadiene. as well as acetylenic compounds in a minor proportion, said process comprising a step of distillation of the feed introduced into a distillation zone which comprises a rectification zone and a depletion zone and at least one step of hydrogenation of the acetylenic compounds in a at least one hydrogenation zone to at least one catalytic bed under appropriate hydrogenation conditions in the presence of a hydrogen-containing gas, the process being characterized in that part of the liquid phase is withdrawn laterally in the liquid phase. charge circulating in the distillation zone enriched with acetylenic compounds, at the level of a level of racking In the distillation zone and in the depletion zone, the hydrogenation step is carried out in the hydrogenation zone which is external to the distillation zone. A hydrogenation effluent which is depleted of compounds is produced. acetylenic and enriched in oligomers and recycle said hydrogenation effluent in the rectification zone, the method being further characterized in that is recovered at the head of the distillation zone a C4 cut comprising substantially all the butadiene and depleted in acetylenic compounds and at the bottom of the distillation zone a C5 cut enriched in oligomers.

Other optional features of the invention are described in the claims of dependent claims 2 to 10.

By grinding zone is meant an area above the feed level of the distillation column.

By depletion zone is meant an area below the feed level of the distillation column.

The feedstock may be a steam cracking effluent which contains predominantly hydrocarbons of 4 to 5 carbon atoms per molecule and preferably a major portion of hydrocarbons having 4 carbon atoms.

This filler according to the invention may contain at least 20% by weight of butadienes and preferably at least 50% by weight in the C4 cut alone. Moreover, it generally contains at most 20% by weight of acetylenic compounds, advantageously at most 5% and preferably at most 2.5% by weight.

According to one characteristic of the invention, the withdrawal rate may be at most equal to twice that of the feedstock introduced into the column, advantageously at most equal to 1.5 times that of the column. It is by withdrawing from the depletion zone a flow of liquid fluid substantially equal to that of the feed introduced into the column that the best results are obtained.

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

Appropriate chromatographic measurements can be used to determine the ratio of concentrations of acetylenic compounds to butadiene and to extract Laterally the fluid when this ratio, on the relevant plateau of the column, is substantially higher and advantageously greater than that of the load.

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 with respect to the level of withdrawal in order to create an internal reflux in the column and thus to increase its power. of seperation.

The temperature increase related to the hydrogenation is generally low because the amount of hydrogenated products is very low. Nevertheless, it may 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 zone of the column, especially since it is preferable to reintroduce the fluid at the top of the column at a temperature substantially equal to that of the reintroduction plate, so as not to disturb the distillation column.

• 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 plates 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

As a general rule, the hydrogenation reactor is 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
• Space velocity of 3-10 h ^- 1, preferably 4 to 8 hours ^-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 catalyst volume divided by the fresh charge liquid volume measured at 15 ° C.

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

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

A hydrocarbon feed 1 comprising a C4 cut from a steam cracker and containing about 50% butadienes and 1 to 2% relative to the C4 cut alone of acetylenic compounds is introduced into a distillation column 2 called debutanizer. This column comprising about 40 theoretical plates, the load is introduced at the 20 ^th plateau. At the top of the column, the C4 fraction containing butadiene and approximately 1000 ppm of acetylenic compounds are recovered via line 3. Part of this fraction is reintroduced after condensation and separation 21, in the form of reflux at the top of the column while the other part is recovered by a line 6 for subsequent 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 product of the reaction, for example equal to 0.027 mol / mol.

This fluid is introduced into at least one hydrogenation reactor 8 fed with hydrogen via a line 9 under partial pressure conditions which substantially correspond to the stoichiometry of the hydrogenation of the acetylenic compounds. This reactor contains a fixed bed downflow (introduction from the top of the reactor of the liquid feedstock) of hydrogenation catalyst which may be palladium stabilized with 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 feedstock, butadienes which have not substantially been hydrogenated as well. oligomers produced in the hydrogenation zone. These oligomeric compounds which are heavy products are collected at the bottom of the column by a line 13 as well as the hydrocarbons containing 5 carbon atoms per molecule of the feedstock. Part is used to be introduced into a reboiler 14 and recycled down the column by a line 15.

The following examples illustrate the invention.

Example 1

A steam cracking feed C4 + C5, the composition of which is described in Table 1, is introduced into the device described in the figure at its bubble temperature. TABLE 1 Charge C4 C5 Head cup 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 feedstock in the column, the side drawdown level and the recycling level of the hydrogenated effluent are usually selected to obtain 1000 ppm by weight of acetylenic in the C4 cut while minimizing the loss of butadiene 1,3. This charge is introduced at the 20 ^th theoretical plateau of the column.

The purpose of the column is to separate the C4 cut from 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 plates: 40 (50 to 60 real trays).
• Operating pressure: 5 bar abs at the top, 5,3 bar abs at the bottom.
• Operating temperature: 45 ° C at the top, 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 specifications of the column are as follows:

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

Bubble point supply (56 ° C), pressure = 6 bar abs, flow rate = 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 inlet
• Space velocity: 4 h ^-1
• Hydrogen flow rate: 30 kg / h
• Catalyst = Palladium 2000 ppm
• LD 277® Procatalyse = 800 ppm gold

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 largely recover at the bottom of the column.

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

Conversion rate: The conversion rate of a product represents the amount of product (mass) that has disappeared.
Ex: Conversion rate of VAC = 0.95: $$1-{\left(\mathrm{Mass\; of}\mathrm{}\mathrm{VAC}\right)}_{\mathrm{exit\; at\; the\; top\; of\; the\; column}}/{\left(\mathrm{Mass\; of}\mathrm{}\mathrm{VAC}\right)}_{\mathrm{Entrance}}=0,95$$

This is the conversion rate observed.
Losses in butadiene : butadiene losses represent the amount of butadiene which is not recovered at the top of the column, ie butadiene which is hydrogenated to butene at the reactor plus butadiene which is lost in the bottom debutanizer in the C5 section.
The 1,3-butadiene losses, the isomer of commercial interest, are 2.7% while the content of acetylenic compounds in the top cut is 1000 ppm, which corresponds to a conversion rate of the vinylacetylene compound of 0.95.

Example 2 (comparative)

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

Example 3 (comparative)

Example 1 is repeated under identical conditions, but instead of partially withdrawing an effluent from the column from the depletion zone to the rectification zone, it is withdrawn from the depletion zone (35 ^th plateau) 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 negligible.
On the other hand, the content of acetylenic compounds at the top of the column is very high (greater than 8000 ppm). All acetylenic compounds within the column at the feed level ^(e tray 20) and extending in the head region are not hydrogenated by such a device.

Claims (10)

1. Process for treatment of a feedstock that comprises hydrocarbons with at least four carbon atoms per molecule, whereby said feedstock comprises diene compounds and primarily butadiene as well as acetylene compounds in a minor proportion, whereby said process comprises a distillation stage of the feedstock introduced in a distillation zone that comprises a rectification zone and a drainage zone and at least one stage for hydrogenation of acetylene compounds in at least one hydrogenation zone with at least one catalytic bed under suitable hydrogenation conditions in the presence of a gas that contains hydrogen, whereby the process is such that a portion of the feedstock that circulates in the distillation zone that is enriched with acetylene compounds is drawn off laterally in liquid phase at a suitable draw-off level in the distillation zone; the hydrogenation stage is carried out in the hydrogenation zone that is outside the distillation zone; a hydrogenation effluent that is low in acetylene compounds is produced; and said hydrogenation effluent is recycled in the rectification zone, whereby the process is characterized in that said suitable draw-off level is located in the drainage zone and also in that the hydrogenation effluent is oligomer-enriched and in that a C4 fraction that comprises essentially all of the butadiene and that is low in acetylene compounds is recovered at the top of the distillation zone, and an oligomer-enriched C5 fraction is recovered at the bottom of the distillation zone.
2. Process according to claim 1, wherein the feedstock is a steam-cracking effluent that contains for the most part hydrocarbons with four to five carbon atoms per molecule and preferably a majority of hydrocarbons with four carbon atoms.
3. Process according to one of claims 1 and 2, wherein the butadiene content in the feedstock is at least equal to 20% by weight and preferably equal to 50% by weight.
4. Process according to one of claims 1 to 3, wherein the feedstock contains at most 20% by weight of acetylene compounds, advantageously at most 5% and preferably at most 2.5%.
5. Process according to one of claims 1 to 4, wherein the draw-off flow rate is at most equal to twice the one of the feedstock, advantageously at most equal to one and one-half times the flow rate of the feedstock and preferably approximately equal to the one of the feedstock that is introduced in the distillation zone.
6. Process according to one of claims 1 to 5, wherein the feedstock is introduced at a level that essentially corresponds to the center of the distillation column; the lateral draw-off level is located below said center of the column at a height that corresponds to fewer than five theoretical plates; and the hydrogenation effluent is recycled above the center of the column at a level that corresponds to at most the height of the first five theoretical plates.
7. Process according to one of claims 1 to 6, wherein the ratio of the acetylene compounds/butadienes concentrations at the level of the lateral draw-off is essentially the highest.
8. Process according to one of claims 1 to 7, wherein the operating conditions of the distillation zone are as follows:

   Number of theoretical plates: 40, preferably 35-45

   Absolute pressure: 4-10 bar, preferably 5 bar

   Top temperature: 45°C, preferably 30°C to 50°C

   Bottom temperature: 95°C, preferably 90°C to 150°C
9. Process according to one of claims 1 to 8, wherein 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

   Volumetric flow rate 3 to 10 h^-1, preferably 4 to 8 h^-1

   Ratio of H₂/acetylene compounds (mol/mol) = 0.5 to 3, preferably 1.0 to 1.1

   Noble metal catalyst of group VIII, preferably palladium; 0.01 to 1% by weight stabilized by at least one metal of the group formed by Au, Ag, Sn.
10. Process according to one of claims 1 to 4, wherein the temperature of the hydrogenation effluent is controlled upstream from the recycling level in the rectification zone of the distillation column.

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