GB2040995A

GB2040995A - Process for treating a C4 hydrocarbon cut to recover butadiene

Info

Publication number: GB2040995A
Application number: GB7938941A
Authority: GB
Original assignee: IFP Energies Nouvelles IFPEN
Current assignee: IFP Energies Nouvelles IFPEN
Priority date: 1978-11-10
Filing date: 1979-11-09
Publication date: 1980-09-03
Also published as: GB2040995B; IT7927214A0; JPS5569521A; IT1127217B; FR2440984B1; NL187850C; NL7908195A; NL187850B; DE2945075A1; BE879802A; FR2440984A1; DE2945075C2; JPS6215047B2

Abstract

A C4 hydrocarbon cut containing 1 ,3-butadiene, acetylenic hydrocarbons and saturated or olefinic hydrocarbons is treated to recover butadiene by admixing the fresh C4 cut with a recycle stream, hydrogenating the resultant mixture, fractionating the product, feeding back an acetylenic stream and recovering butadiene.

Description

SPECIFICATION Process for treating a C4 hydrocarbon cut to recover butadiene The invention concerns an improved process for recovering 1 3-butadiene from mixtures of C4 hydrocarbons.

An old technique consists of selectively hydrogenating the acetylenic hydrocarbons which are admixed with butadiene, but high butadiene losses are a serious drawback of this process (see, for example, US Patent 3,075,917).

Extraction techniques have then been experimented to avoid the hydrogenation of the charge.

These techniques can be used to fractionate a C4 hydrocarbons mixture containing 1 ,3-butadiene to a raffinate mainly containing saturated and monoolefinic C4 hydrocarbons, a butadiene cut and a cut of relatively high acetylenic hydrocarbon content.

This can be effected with two successive extractions, either both of the liquid/liquid type, or both of the extractive distillation type, or one of each type. An equivalent technique comprises an extraction of one of these types, followed with a distillation to separate butadiene from the acetylenics. All these possibilities wili be referred to below as "extraction".

The first step (extraction) leads to a raffinate (mainly saturated and monoolefinic hydrocarbons) and an extract containing a mixture of butadiene with acetylenic hydrocarbons dissolved in the extraction solvent. After withdrawal of the extraction solvent, the above mixture is fractionated, by extraction or distillation, to butadiene and a cut of high acetylenic hydrocarbons content.

The latter cut is often rejected, which represents a substantial loss of butadiene, eventually up to 4 to 8%, since, in order to avoid the dangerous handling of a cut of too high actylenic content, it is preferred to have a portion of butadiene, as diluent discharged with the acetylenics.

It has however been recently proposed to selectively hydrogenate the so-recovered C4 acetylenic cut, so as to recycle it to the extraction zone kand recover butadiene therefrom (see, for example, US Patent 4,049,742).

However, the hydrogenation of a cut having a too high content of acetylenics cannot be effected easily, owing to explosion hazards, excessive heat release and excessive gum formation. It is then recommended to dilute the cut of high acetylenic content with a part of the raffinate obtained in the process (a fraction having a high content of saturated and monoolefinic hydrocarbons). This results in a repeated treatment of the same components, thus in a higher cost of the process, which may be so high that all the advantages of the improved butadiene extraction yield are lost. Furthermore, in that known process, 1,2-butadiene accumulates and must be separated.

The invention largely avoids the above drawbacks.

According to a general embodiment, the present process comprises the following steps: a) admixing the fresh C4 hydrocarbon charge with a recycle stream as hereinafter defined, b) selectively hydrogenating the acetylenic hydrocarbons of the so-constituted mixture, while limiting the conversion rate of vinylacetylene to a value from 20 to 95%, preferably 50 to 90%, c) separating the hydrogenation product by extraction, in known manner, to (i) a raffinate mainly comprising saturated and monoolefinic hydrocarbons, (ii) a cut of high 1,3-butadiene content and (iii) a cut of high acetylenic hydrocarbon content, d) feeding back at least one part of said cut of high acetylenic hydrocarbon content to step (a), as recycle stream, for admixture with the fresh C4 hydrocarbon charge, e) recovering the cut of high 1,3-butadiene content, obtained in step (c), which constitutes the product of the process.

The conditions of the reaction of selective hydrogenation are well known. The temperature is usually 0--1000C, preferably 10--500C, and the pressure is sufficient to maintain the hydrocarbons at least partly in the liquid state.

Partial vaporization is however possible, to absorb reaction heat. Hydrogen is in sufficient amount to satisfy the well-known stoichiometric ratio of the reaction. The feed rate of the liquid charge is usually 1 to 50 volumes per volume of catalyst per hour.

The operation may be isothermal or preferably adiabatic.

The hydrogenation can proceed in downflow or upflow stream.

The catalyst is one commonly used in that type of reaction, for example a metal of the group Veil, such as nickel, cobalt, platinum or preferably palladium. A carrier may be used, for example alumina or silica. The metal content is, for example, 0,005 to 3%, preferably 0.1 to 1% by weight. Additional metals may be present, as well-known. In certain cases, several catalyst beds with different metals are used.

Hydrogen may be used either pure or diluted in an inert gas.

As shown above, the vinylacetylene conversion rate is limited. According to a preferred embodiment, a vinylacetylene amount of 300 to 5000 parts per million, preferably 500 to 3000 ppm (by mole) is left in the hydrogenation product. The conversion rate of ethylacetylene is preferably 10 to 60%.

The vinylacetylene conversion can be easily controlled by selection of the operating variables. it increases, for example, in otherwise unchanged conditions, when the temperature or the hydrogen partial pressure increases or when the feed rate decreases.

This technique of limited selective hydrogenation thus departs from the technique described, for example, in US 3,075,917 where the conversion of the mixture of vinylacetylene with ethylacetylene is far higher, about 97%, corresponding to vinylacetylene conversions higher than 99%, vinylacetylene being the most reactive compound.

The extraction (liquid/liquid extraction or extractive distillation) may be effected with a conventional selective solvent for example, dimethylacetamide, dimethylformamide, fu rfu ral, N-methyi pyrrolidone, formylmorpholine, acetonitrile, etc. . .

The volume of the cut of high acetylenics content, recycled to step (a), according to the preferred embodiment of the invention, represents usually 1 to 1 5%, preferably 2 to 1 0%, by volume of the fresh charge (C4 cut), these values having no limitative significance.

The mixtures of C4 hydrocarbons to be treated according to the invention usually comprise at least one monooiefin, for example 1-butene, 2-butene and/orisobutene, 1,3-butadiene and at least one acetylenic hydrocarbon, for example, vinylacetylene and/or ethylacetylene. Saturated hydrocarbons, butane and/or isobutane, may be present, possibly with small amounts of 1 ,2-butadiene and/or lighter or heavier hydrocarbons. These mixtures may be obtained for example, by steam-cracking or dehydrogenation of hydrocarbon stocks.

The acetylenic hydrocarbon content of the fresh charge is usually from 0.1 to 5% by weight or more and the 1 ,3-butadiene content is 1070% by weight or more. The vinylacetylene content is often higher than 0.6% by mole.

The recycle stream to be admixed with the fresh charge may comprise, for example, 10 to 85% by weight (preferably 50 to 80%) of 1 3-butadiene and 5 to 80% by weight of C4 acetylenic hydrocarbons, optionally also C4 monoolefinic hydrocarbons and traces of C4 saturated hydrocarbons.

The accompanying figures illustrate, as basic schemes, two embodiments of the invention, which mainly include: - the operation of selective hydrogenation, - the operation of extraction effected in two successive steps of the same type or different types, - as well as the recycling of the cut of high acetylenics content obtained as concentrate at the term of the second extraction step.

According to figure 1, the fresh C4 hydrocarbon cut to which hydrogen has been added is fed through line 1 to the hydrogenation reactor 2 which also receives the recycle stream 3. The effluent (line 4) is fed to the separation drum 5 hydrogen is discharged through line 6. The hydrogenated C4 fraction is fed through duct 7 to the extractor 8 also fed with solvent through line 9. A raffinate is discharged from the top through line 10 (mainly saturated and monoolefinic hydrocarbons). This raffinate is a gas or a liquid, depending on the type of extraction: extractive distillation or liquid/liquid extraction.

The bottom product is fed to column 11 through line 12: the distillate, which comprises 1,3 butadiene and acetylenic hydrocarbons, is fed through line 13 to the distillation columns 1 4a and 1 4b.

1 ,3-butadiene (duct 1 5) is separated from the top of column 1 4bf the bottom product of column 1 4b is a cut which comprises relatively heavy acetylenic hydrocarbons; they are recycled through line 3, in admixture with the relatively light acetylenic hydrocarbons of line 1 6a; a portion may be discharged, if desired, through line 1 6c.

The description of Figure 2 is identical up to column 11 inclusively. The effluent (line 13) is supplied to extractor 17, fed with extraction solvent through line 1 8. Gaseous or liquid butadiene is collected at the top (line 19), depending on the type of extraction :extractive distillation or liquid/liquid extraction. The extract is supplied through line 20 to the distillation column 21. A top fraction containing acetylenic hydrocarbons is recovered; it is recycled through line 3. A portion thereof may be discharged through line 22.

The following examples illustrate the invention, without limiting the scope thereof. Examples 2 and 4, in conformity with the invention, are conducted according to the embodiments iilustrated respectively in Figures 2 and 1. Examples 1 and 3, which are conducted without recycling, are given by way of comparison.

EXAMPLE 1 (Comparison) A steam-cracking cut having the % molar composition given in Table I (second column) is treated.

TABLE I

Inlet of the Hydrogenation Outlet of the Hydrogenation Reactor Reactor Composition % by mole % by mole Saturated hydrocarbons 1.33 1.60 Monoolefins 46.55 47.77 1,2-butadiene 0.13 0,11 1,3-butadiene 50.54 50.33 Ethylacetylene O.t8 0.09 Vinylacetylene 1.27 0.10 The charge is passed in a hydrogehation reactor containing a 0.2% palladium-on-alumina, catalyst, at400C, 8 bars and a VVH of 15.

The hydrogen feed rate is so adjusted as to convert 92% of vinylacetylene, which conversion was previously determined as giving a maximum recovery of butadiene at the outlet from the extraction stage.

The composition, at the outlet of the hydrogenation reactor, is given in Table I (in % by mole).

The hydrogenated C4 cut is fed to an evaporator (gum-remover): the vapor is fed to a unit for extractive distillation in 2 successive stages, fed with N-methyl pyrrolidone. There is thus recovered: - an olefinic raffinate of high butene content, containing about 0.3% by mole of butadiene, - very pure 1 ,3-butadiene (15 ppm of acetylenic hydrocarbons) with a molar yield of 97.9%, - a concentrate of acetylenics and allenics diluted with butadiene (about 35% by mole of acetylenics and allenics and 65% by mole of butadiene). This concentrate represents a loss of 1 .4viz of the butadiene of the initial C4 cut.

EXAMPLE 2 The procedure is the same as in example 1; however, it is not the fresh charge alone which is subjected to hydrogenation, but a mixture of the fresh charge with a recycle stream constituted by the concentrate of acetylenics and allenics recovered by extraction.

The composition of the fresh charge is the same as in example 1.

The recycle rate is 4.6% by weight, based on the fresh charge.

The molar composition of the resultant mixture, at the inlet and the outlet of the hydrogenation reactor, is given in Table II. The vinylacetylene conversion was 81.8% and the ethylacetylene conversion 37.8% in the course of the hydrogenation.

TABLE II

Inlet of the Hydrogenation Outlet of the Hydrogenation Reactor Reactor Composition % by mole % by mole Saturated hydrocarbons 1.22 1.41 Monoolefins 44.69 45.97 1,2-butadiene 0.93 0.80 1,3-butadiene 51.03 51.27 Ethylacetylene 0.45 0.28 Vinylacetylene 1.48 0.27 The hydrogenation product has been fed to the same 2-stage extraction unit as described in example 1. The yield of 1 ,3-butadiene of the same purity as in example 1 was 99.94% by weight, which shows a substantial improvement as compared to the process without recycling.

Yields above 100% have been obtained (since butadiene forms from vinylacetylene); for example, a yield, of 100. 7% is obtained with a recycle rate of 10%.

EXAMPLE 3 (comparison) Example 1 is repeated, except that the 2-stage extractive distillation is replaced by a one-stage extractive distillation with N-methylpyrrolidone, followed with a superfractionation. The feed charge is the same as in example 1.

The 1 3-butadiene yield, of the same purity as in example 1, is 98% by mole.

The concentrate from the extraction stage is formed of 20% acetylenics and allenics, 45% 1,3 butadiene and 35% 2-butene.

EXAMPLE 4 The procedure is as in example 3; however the fresh charge subjected to hydrogenation is admixed with a recycle stream constituted by the concentrate of acetylenics and allenics recovered from the extraction stage (extractive distillation + superfractionation).

The composition of the fresh charge is the same as in example 1.

The recycle rate is 4.6% by weight of the initial charge.

The hydrogenation product has been fed to the unit of extractive distillation + superfractionation as in example 3. The yield of butadiene of the same purity was 99.96% by weight, which shows a substantial improvement as compared to the same process without recycling.

Many modifications may be brought to the above examples. For example, a single reactor or several reactors in series may be used, the hydrogen supply being effected at one time or fractionwise at the inlet of each reactor, or progressively at various points of the reactor(s).

Claims

1. A process for treating a C4 hydrocarbon cut containing 1 3-butadiene, at least one acetylenic hydrocarbon and at least one saturated and/or monoolefinic hydrocarbon, in order to recover 1,3butadiene therefrom, which comprises the following steps of: a) admixing the fresh C4 hydrocarbon cut with a recycle stream as defined below, b) passing the resultant mixture with hydrogen in contact with a hydrogenation catalyst, under conditions suitable for the selective hydrogenation of the acetylenic hydrocarbons, and limiting the conversion of vinylacetylene to a value from 20 to 95%.

c) fractionating the hydrogenation product, by extraction, in a known manner, to (i) a raffinate mainly containing saturated and/or monoolefinic hydrocarbons, (ii) a cut of high 1 ,3-butadiene content and (iii) a cut of high acetylenic hydrocarbons content, d) feeding back at least a part of said cut of high acetylenic hydrocarbons content to step (a), as recycle stream, and admixing it with the fresh C4 hydrocarbon cut, and e) collecting the cut of high 1 3-butadiene content, obtained in step (c), which constitutes the main product of the process.

2. A process according to claim 1, wherein the selective hydrogenation is effected with a vinylacetylene conversion rate of 50 to 90%.

3. A process according to claim 1, wherein the fresh C4 hydrocarbon cut contains 0.1 to 5% by weight of acetylenic hydrocarbons and 10 to 70% by weight of 1 ,3-butadiene.

4. A process according to claim 3, wherein the vinylacetylene content of the fresh charge is at least 0.6% by mole.

5. A process according to claim 1, wherein the fractionation comprises two successive extractions with a solvent.

6. A process according to claim 1, wherein the fractionation comprises an extraction with a solvent followed with a distillation.

7. A process according to claim 1, wherein the volume of the recycle stream represents 1 to 1 5% by volume of the fresh C4 hydrocarbon charge.

8. A process according to claim 1, wherein the recycle stream comprises 10 to 85% by weight of 1 ,3-butadiene and 5 to 80% by weight of C4 acetylenic hydrocarbons.

9. A process according to claim 1, wherein the C4 hydrocarbon cut is a product obtained by steamcracking of hydrocarbons.

10. A process according to claim 1 , wherein the selective hydrogenation is effected at least partially in liquid phase at 0--1000C, at a hourly feed rate of 1 to 50 (liquid) volumes per volume of catalyst, with a group VIII metal catalyst.

1 A process according to claim 1, wherein 300 to 5000 ppm by mole of vinylacetylene is left in the product of the selective hydrogenation before extraction.

12. A process according to claim 1, substantially as hereinbefore described in Example 2 or 4.

13. A process according to claim 1 , carried out in apparatus diagrammatically illustrated in either figure of the accompanying drawings.

14. 1 ,3-Butadiene obtained by a process according to any one of claims 1 to 13.