Oligomerization of C4 streams with very low 1 butene content

Abstract

An OCTOL process is disclosed which by limitation of the conversion in its individual oligomerization steps is set up particularly for the productive utilization of C.sub.4 feedstock mixtures with a low 1-butene content and which nevertheless yields a C.sub.8 product mixture having an Iso index suitable for the preparation of plasticizer alcohols.

Claims

1. A process for preparing a C.sub.8 olefin or a C.sub.12 olefin by oligomerization of butenes, comprising: a) oligomerizing a portion of butenes present in a hydrocarbon mixture comprising a 2-butene and a further linear butene other than the 2-butene, to give one or more C.sub.8 olefins and to give one or more C.sub.12 olefins and optionally to give one or more C.sub.12+ olefins, by contacting of the hydrocarbon mixture with an oligomerization catalyst arranged in a reaction zone, at a reaction temperature prevailing in the reaction zone, to give an oligomerizate comprising the prepared oligomers and one or more unreacted butenes; b) removing the unreacted butenes from the oligomerizate; and c) optionally, recycling a part of the removed unreacted butenes to the oligomerizing a); wherein: the sequence of a), b) and c) is carried out at least once with inclusion of the recycling c); during the first time that the oligomerizing a) is carried out, a concentration of 1-butene in the hydrocarbon mixture, based on the concentration of linear butenes, is less than or equal to an equilibrium concentration of 1-butene, resulting from the reaction temperature of the sequence carried out for the first time, within the fraction of the linear butenes present in the hydrocarbon mixture during the first time; the C.sub.8 olefins are obtained as a C.sub.8 product mixture whose Iso index is less than 1.2; during the sequence carried out for the first time, the conversion of butenes, assessed directly over a first reaction zone, is limited to a first limit value of between 26.1 and 40 wt %; and the limiting of the conversion in the oligomerization is accomplished by limiting the reaction temperature of the oligomerization to a maximum temperature of between 40 C. and 140 C.

2. The process of claim 1, wherein: the sequence of a), b) and c) is carried out at least two times one after another, and in the second sequence carried out, the conversion of butenes, assessed directly at a second reaction zone, is limited to a second limiting value of between 5 and 50 wt %.

3. The process of claim 2, wherein: the sequence of a), b) and c) is carried out at least three times one after another, and in the third sequence carried out, the conversion of butenes, assessed directly at a third reaction zone, is limited to a third limiting value of between 5 and 65 wt %.

4. The process of claim 3, wherein: the sequence of a), b) and c) is carried out at least four times one after another, and in the fourth sequence carried out, the conversion of butenes, assessed directly at a fourth reaction zone, is limited to a fourth limiting value of between 5 and 80 wt %.

5. The process of claim 4, wherein: the sequence of a), b) and c) is carried out at least five times one after another, and in the fifth sequence carried out, the conversion of butenes, assessed directly at a fifth reaction zone, is limited to a fifth limiting value of between 5 and 95 wt %.

6. The process of claim 1, wherein an overall conversion of butenes achieved after all of the sequences of a), b) and c) have been carried out is between 5 and 100 wt %.

7. The process of claim 1, wherein the limiting of the conversion in the oligomerization is accomplished by limiting the reaction temperature of the oligomerization to a maximum temperature of between 45 C. and 120 C.

8. The process of claim 1, wherein more than 80 wt % of the oligomers prepared are C.sub.8 olefins.

9. The process of claim 1, wherein a composition of the C.sub.8 olefins obtained is as follows, adding up to 100 wt %: n-octenes: 10 to 25 wt %; methylheptenes: 50 to 80 wt %; dimethylhexenes: 10 to 30 wt %.

10. The process of claim 1, wherein the C.sub.8 product mixture has an Iso index of less than 1.1.

11. The process of claim 1, wherein the hydrocarbon mixture in the sequence carried out for the first time is a reactant mixture having the following composition, which adds up to 100 wt %: 1-butene: less than 10 wt %; 2-butenes: 20 to 90 wt %; isobutene: less than 5 wt %; n-butane: less than 80 wt %; isobutane: less than 80 wt %; and others: less than 5 wt %.

12. The process of claim 1, carried out in a plant within which exactly one reaction zone is assigned to each sequence carried out.

13. The process of claim 1, wherein in each sequence apart from the last sequence, a non-recycled part of the removed unreacted butenes is provided as a hydrocarbon mixture for the subsequent step sequence.

14. The process of claim 13, further comprising: performing at least one selected from the group consisting of: (a) totally hydrogenating the non-recycled part of the removed unreacted butenes in the last sequence, to obtain a butane mixture; (b) oxidatively or non-oxidatively dehydrogenating the non-recycled part of the removed unreacted butenes in the last sequence, to obtain butadiene; (c) hydroformylating the non-recycled part of the removed unreacted butenes in the last sequence, to obtain pentanals; (d) oxidizing the non-recycled part of the removed unreacted butenes in the last sequence, to obtain maleic anhydride; (e) metathesizing the non-recycled part of the removed unreacted butenes in the last sequence; (f) hydrating the non-recycled part of the removed unreacted butenes in the last sequence, to obtain butanols; (g) alkylating the non-recycled part of the removed unreacted butenes in the last sequence; (h) isomerizing the non-recycled part of the removed unreacted butenes in the last sequence; (i) carbonylating the non-recycled part of the removed unreacted butenes in the last sequence; (j) cracking the non-recycled part of the removed unreacted butenes in the last sequence in a steamcracker or in a fluid-catalytic cracker, to obtain hydrocarbons having less than four carbon atoms; and (k) combusting the non-recycled part of the removed unreacted butenes in the last sequence to give thermal energy.

15. The process of claim 1, wherein oligomers obtained in individual sequences are combined and then separated into a C.sub.8 product mixture, into a C.sub.12 product mixture and into a C.sub.12+ product mixture.

Description

(1) Further aspects of the present invention will become apparent from the description that now follows of a number of embodiments. For this purpose, the following figures offer the following schematic representations:

(2) FIG. 1: one-stage process;

(3) FIG. 2: two-stage process;

(4) FIG. 3: three-stage process;

(5) FIG. 4: three-stage process with product combination before the last separating column.

(6) FIG. 1 shows a simplified flow diagram of an oligomerization process of the invention. Starting material is a reactant mixture 1, which comes, for example, from a fluid-catalytic petroleum cracker, may have been subjected to preliminary purification, and is provided as a continuous stream of material. The reactant mixture 1 comprises a mixture of hydrocarbons having four carbon atoms, including the C.sub.4 olefins, 1-butene, cis-2-butene, trans-2-butene and isobutene, and also the C.sub.4 alkanes n-butane and isobutane. No attention is paid here to other organic or inorganic constituents which typically occur within C.sub.4 cuts. The particular nature of the reactant mixture 1 provided is that its 1-butene content is unusually low. The composition of reactant mixture 1 is as follows: 1-butene: less than 5 wt % 2-butenes: 20 to 90 wt % isobutene: less than 1 wt % n-butane: less than 80 wt % isobutane: less than 80 wt % others: less than 2 wt %

(7) Reactant mixture 1 is passed into a reaction zone 2. At the reaction temperature prevailing there, it comes into contact with an oligomerization catalyst arranged in the reaction zone 2, and so some of the butenes present in the reactant mixture 1 react with one another to give oligomers, and are taken off from the reaction zone 2 in an oligomerizate 3. The oligomerizate is a mixture of the oligomers formed, of the unreacted butenes, and of those constituents of the reactant mixture that behave inertly in the reaction, such as the butanes.

(8) The oligomers include C.sub.8 olefins such as n-octenes, methylheptenes and dimethylhexenes which are formed by the oligomerization of two C.sub.4 olefins. Where three butenes or one butene and one previously formed octene oligomerize with one another, the products are C.sub.12 olefins (dodecenes). Four butenes oligomerizing with one another, or two butenes and one previously formed octene, or two previously formed octenes or one butene and one previously formed dodecene, lead to C.sub.16 olefins.

(9) The oligomerization carried out in reaction zone 2 forms predominantly C.sub.8 olefins; C.sub.12 olefins are the greatest by-product. The olefins with more than twelve carbon atoms are formed only in comparatively small fractions and are referred to collectively as C.sub.12+ olefins.

(10) The composition of the oligomerizate, adding up to 100 wt %, is typically as follows: butanes less than 80 wt % 1-butene 1 to 5 wt % 2-butenes 10 to 80 wt % n-octenes 1 to 10 wt % methylheptenes 5 to 40 wt % dimethylhexenes 1 to 15 wt % C.sub.12 olefins 1 to 10 Wt % C.sub.12+ olefins 0.1 to 2 wt %

(11) The oligomerization catalyst, which is not shown in the figures, is a heterogeneous, nickel-containing catalyst. Employed with preference is a supported catalyst comprising a support material such as silicon dioxide or aluminium oxide or mixtures thereof, or aluminosilicates or zeolites, for example. The supports may comprise sulphur in the form of sulphate, sulphide or other types of compound. Suitable oligomerization catalysts are known in the technical literature and are described for example in DE 4339713 A1 or in WO 2001/37989 A2 or in WO 2011/000697 A1.

(12) For the preparation of the supported nickel catalysts used there are a variety of ways. For example, such catalysts may be prepared by joint precipitation of nickel compounds and support material (i.e. aluminium compounds and/or silicon compounds), filtration and subsequent heat treatment. Another option is to apply nickel compounds to a suitable support material, by impregnation or sprayed application, for example, with subsequent calcining. To prepare the catalysts by the impregnating method, nickel compounds such as nickel nitrate, nickel chloride or amine complexes, for example, may be used. Support materials used are preferably commercially available catalyst supports such as, for example, amorphous mixed silicon aluminium oxides carrying the designation Grace DAVICAT, available from Grace, or zeolites (e.g. MCM41) from Mobil Oil.

(13) Especially preferred is the use of titanium free supports and supported catalysts, consisting substantially of nickel oxide, aluminium oxide and silicon oxide. These catalysts contain preferably 5 to 50 mass % nickel, more particularly 10 to 30 mass % nickel. The aluminium contents are in the range from 5 to 30 mass %, more particularly in the range from 7 to 20 mass %. The fractions of silicon are in the range from 10 to 40 mass %, with the range from 20 to 30 mass % being particularly preferred. The stated mass fractions are based on the total metal content. As further components, these catalysts may contain 0.1 to 2 mass % of alkali metal oxide, alkaline earth metal oxide, lanthanum oxide or oxides of the rare earths, and optionally shaping auxiliaries.

(14) In macroscopic terms, the nickel catalyst used in accordance with the invention is employed in a form in which it presents a low resistance to flow. The oligomerization catalyst is preferably in the form of shaped bodies such as granules, pellets, tablets, cylinders, beads, strand extrudates or rings.

(15) In terms of apparatus, the reaction zone 2 is implemented preferably as a shell-and-tube reactor or as a serial or parallel connection of a plurality of reactors. Even when the oligomerization is carried out in a plurality of serially connected reactors, the oligomerization step here is a single step, since in the terminology of the present invention, an oligomerization step always concludes with a removal step. More later on this in reference to FIG. 2.

(16) The shell-and-tube reactor preferably employed comprises a multiplicity of flow-traversed tubes with a catalyst filling. The reactant mixture 1 flows in at the start of the tubes, optionally in a mixture with recycle stream 7; at the end of the tubes, the oligomerizate 3 is taken off. The heat of reaction that forms in the course of the exothermic oligomerization reaction is taken off preferably not via the outflowing oligomerizate 3, but instead via an external cooling medium (not shown). The cooling medium flows through a jacket surrounding the tube bundle, allowing heat exchange without exchange of matter to take place between the reaction mixture and the cooling medium. The cooling medium does not participate in the reaction; accordingly, the shell-and-tube reactor also fulfils the function of a heat exchanger. At its most simple, the cooling medium is suitably water or an organic heat-transfer fluid such as Marlotherm from Sasol Germany GmbH, for example.

(17) Setting the reaction temperature within the oligomerization by means of the cooling medium is of particular interest since in accordance with the invention, the limitation of the reaction temperature represents an important measure for limiting the conversion. Very preferably, therefore, the reaction temperature is to be limited to a comparatively low figure of between 50 and 100 C., this being made possible by the use of the external cooling medium.

(18) The pressure within the reaction zone is selected such that the C.sub.4 hydrocarbons present are in liquid phase. The pressure is set accordingly to between 0.1 to 70 MPa, preferably from 0.1 to 10 and very preferably from 0.5 to 4 MPa.

(19) The specific catalyst space velocity (WHSV) is between 0.1 and 5 min.sup.1, preferably 0.2 and 3 min.sup.1.

(20) The oligomerizate 3 taken off from the reaction zone 2 is then introduced into a separating device in the form of a distillation column 4, in which it is separated conventionally by distillation into a top stream 5, containing the inert butanes and the butenes not reacted in the oligomerization, and into a liquid phase stream 6, containing the oligomers prepared. The distillation takes place preferably under a pressure of 0.1 to 1 MPa, preferably under 0.2 to 0.5 MPa. Because of the considerable difference in the molecular weight and the resultant distinct difference in the boiling points between the C.sub.4 hydrocarbons taken off at the top and the oligomers with eight or more hydrocarbons in the liquid phase, separation within the distillation column 4 is achieved with comparatively little technical complexity, and so more detailed comments are unnecessary. Further information on the design of the distillative purification of oligomerizates is found in EP1029839A1.

(21) The top stream 5 containing the unreacted butenes is divided into a recycle stream 7 and a transfer stream 8. The recycle stream 7 is mixed with the reactant stream 1 originally provided, and supplied again to the oligomerization 2. The transfer stream 8 is passed on for further production utilization of the butenes and butanes it contains (not shown). The proportion of the top product recycled, in other words the division ratio of the streams 7 to 5 and 8 to 5, is a further parameter, alongside the reaction temperature, for limiting the conversion within the reaction zone 2. In accordance with the invention, indeed, the conversion within the reaction zone 2 is limited to a first limit value of between 5 and 40 wt %. The stated conversion is assessed immediately at the reaction zone 2, in order words within the assessment boundary 9 drawn with dashed lines. What is meant is therefore the conversion per pass based on the reactor feed, which is composed additively of the fresh feed 1 and the (optional) recycle stream 7.

(22) The conversion within the assessment boundary 9 is limited on the one hand by the restriction on the reaction temperature through appropriate cooling of the reaction zone 2 via the cooling medium, and also via the size of the recycle stream 7.

(23) In the liquid phase of the distillation column 4, the liquid phase stream 6 is taken off, containing the oligomers prepared. These oligomers will also be separated in accordance with their molecular weight (not shown in FIG. 1).

(24) The oligomerization process represented in FIG. 1 constitutes the simplest embodiment of the invention, in which the step sequence of providing, oligomerizing, removing and optionally recycling is run through only a single time. On account of the inventive limitation on the conversion within the assessment boundary 9, drawn in with dashed lines, to not more than 40%, it is possible, without recycle stream 7, to achieve only an overall conversion of 40% assessed over the entire process. In order to increase the overall conversion, the stated step sequence is performed preferably with recycle stream 7 and/or a number of times after one another, for example twice as shown in FIG. 2.

(25) In the two-stage process of FIG. 2, the step sequence represented in FIG. 1 is run through twice one after another, and so the overall process 10 is subdivided into a first step sequence 101 and a second step sequence 102. Since the overall process 10 is operated continuously, the apparatus required in each step sequence is present twice, accordingly, and is connected serially. This is referred to as a reactor cascade.

(26) Within the step sequence 101 run through for the first time, a C.sub.4 hydrocarbon mixture is provided for the first time as reactant mixture 11, and is then oligomerized in a first reaction zone 12, and the first oligomerizate 13 obtained is separated in a first distillation column 14 into a first top stream 15 and a first liquid phase stream 16. One part of the first top stream 15 is returned to the preceding oligomerization 12, for conversion of butenes not reacted there so far, while the other part is transferred as a first transfer stream 18 into the second step sequence 102. In this second sequence it serves as provided hydrocarbon mixture 21 for the second stage of the oligomerization, which takes place in a second reaction zone 22. The second oligomerizate 23 obtained therein is again separated, in a second distillation column 24, into a second top stream 25 and a second liquid phase stream 26. The top stream 25 of the second distillation column 24 is divided into a second recycle stream 27 and a second transfer stream 28.

(27) The liquid phase stream 26 of the second distillation column 24 is combined with the first liquid phase stream 16, and supplied for joint fractionation 29 of the oligomers present therein. Possible embodiments of the fractionation 29 are elucidated with reference to FIGS. 3 and 4.

(28) The part of the removed unreacted butenes not recycled to the second and hence last step sequence 102 is not oligomerized any more, and is passed on with the second transfer stream 28 for productive utilization 30. Utilization 30 consists at its most simple of the combustion of the non-recycled top product of the last column 24. If a fluid-catalytic cracker or steamcracker is available in the plant vicinity, it is appropriate to pass stream 28 back into the cracker and to separate it there into hydrocarbons having less than four carbon atoms. Where no such cracker is available, the butenes present in the non-recycled second transfer stream 28 may be subjected to total hydrogenation, with the consequence that a butane mixture is obtained in the utilization 30, and is suitable as propellant gas or fuel gas for private use. There are also other utilization possibilities for the unreacted butenes per se, their profitability being dependent on the selling situation and on the composition of the stream of material 28 leaving the oligomerization.

(29) FIG. 3 shows a further embodiment of the invention, in which the C.sub.4 hydrocarbon mixture 11 provided the first time is oligomerized by triple runthrough of the step sequence in a total of three reaction zones 12, 22, 32. Each reaction zone 12, 22, 32 is assigned its own distillation column 14, 24, 34, in which the oligomers prepared in each preceding oligomerization run are removed. For this purpose, the liquid phase streams of the distillation columns 14, 24, 34 are combined and supplied to a joint fractionation.

(30) For the fractionation of the oligomers prepared, the combined liquid phase streams 36 are first of all passed into a C.sub.8 column 37. In this column, the actual target product of the process, a C.sub.8 product mixture, is removed at the top by distillation. The C.sub.8 product mixture consists almost exclusively of C.sub.8 olefins, with the following composition, which adds up to 100 wt %: n-octenes: 10 to 25 wt % methylheptenes: 50 to 80 wt % dimethylhexenes: 10 to 30 wt %.

(31) The Iso index of the C.sub.8 product mixture 38 obtained therein is less than 1.1.

(32) About 80% of all the butene oligomers formed are di-butenes and are within the C.sub.8 product mixture, making the selectivity of the process very high in terms of the desired target product (C.sub.8 olefins).

(33) The liquid phase product 39 of the C.sub.8 column 37, containing the oligomers prepared and having twelve and more than twelve carbon atoms, is supplied to a C.sub.12 column 40, where it is separated into a C.sub.12 product mixture 41, which is taken off at the top, and into a C.sub.12+ product mixture 42 in the liquid phase of the C.sub.12 column 40.

(34) About 7% to 17% of the oligomers formed are the C.sub.12 olefins present in the C.sub.12 product mixture 41. The C.sub.12 product mixture, which is still formed to a significant extent, can be used for the production of detergent alcohols.

(35) The olefins having more than twelve carbon atoms which are present in the C.sub.12+ product mixture can be hydrogenated and admixed to light heating oil or to diesel fuel.

(36) FIG. 4 shows another inventive variant of a three-stage process. In the case of the embodiment shown in FIG. 4, the optional step of recycling is omitted within the step sequence carried out the first time. Accordingly, the entire top stream 15 of the first distillation column 14 is transferred as a first transfer stream 18 into the second step sequence, to provide the hydrocarbon mixture 21 needed for the second stage.

(37) Within the context of the invention, it will also be possible to omit the recycling in a stage other than the first stage, or it is possible even to carry out a number of step sequences without the recycling step. In at least one step sequence, however, recycling should be provided. In the case of a one-stage process, the recycling is carried out in the first and only step sequence, as a logical necessity.

(38) Relative to the embodiment of a three-stage process shown in FIG. 3, the fractionation of the oligomers prepared is performed differently in this case: accordingly, the liquid phase streams 16 and 26 of the first and second distillation columns 14, 24 are combined with the oligomerizate 33 of the third stage and then supplied to the third distillation column 34. The third distillation column 34, accordingly, is given a greater size than in the embodiment shown in FIG. 3. The liquid phase stream 36 of the third distillation column 34 then corresponds to the combined liquid phase streams 36 of the embodiment shown in FIG. 3. The fractionation of the oligomers from the combined liquid phase streams 36 corresponds to the embodiment shown in FIG. 3.

EXAMPLE 1 (NOT INVENTIVE)

(39) The non-inventive example 1 was conducted in accordance with WO 99/25668 A1 in a largely adiabatically operated tube reactor with the following dimensions: length 2.0 m, internal diameter 32.8 mm. The reaction was carried out under an absolute pressure of 3 MPa in the liquid phase.

(40) The feedstock used was a hydrocarbon mixture containing the following components, adding up to 100 wt %: 1-butene 25% 2-butene 51% isobutene less than 1% isobutane less than 2% n-butane more than 21%

(41) Contrary to the teaching of the present invention, the mixture therefore contained an amount of 1-butene which is above the concentration of 1-butene that comes about in the thermodynamic equilibrium of the n-butenes at reaction temperature (in this case, reactor entry temperature 60 C.). At a temperature of 60 C., this value is about 4.1% in the overall mixture, or 5.4% within the n-butene fraction.

(42) A part of the stream of the unreacted butenes was returned to the reactor (recycle); the recycled quantities were selected, in accordance with the teaching of WO 99/25668 A1, such that the oligomer concentration does not exceed 25% at any point in the reactor and does not fall below 10% in the reactor effluent. The individual concentrations of the oligomers can be found in Table 1.

(43) The catalyst used was a material prepared in accordance with Example 1 of WO 2011/00697 A1 and aftertreated in accordance with Example 4 of the same publication.

(44) The product stream was analysed for its composition by means of gas chromatography (GC). To identify the octene skeleton isomers, a hydrogenating GC analysis method was used, in which the oligomeric olefins are first hydrogenated to alkanes. The resultant alkanes are then separated chromatographically and detected. It is possible to differentiate between three relevant C.sub.8 isomers: n-octane (formed from n-octenes), methylheptane (formed from methylheptenes) and dimethylhexane (formed from dimethylhexenes). The composition of the hydrogenated C.sub.8 mixture is compiled in Table 1.

EXAMPLES 2 TO 5 (NOT INVENTIVE)

(45) The examples were carried out in accordance with WO 99/25668 A1 in a largely adiabatically operated tube reactor with the following dimensions: length 2.0 m, internal diameter 32.8 mm. The reaction was carried out under an absolute pressure of 3 MPa in the liquid phase. Feedstocks used were two hydrocarbon mixtures with different 1-butene/2-butene ratios but with a constant total amount of n-butenes. The concentrations of the n-butenes are given in Table 1. In addition, the mixtures contained the following components, which add up to 100 wt %: isobutane less than 2% n-Butane more than 21% Isobutene less than 1%

(46) An of the mixtures therefore contained 1-butene amounts below the 1-butene concentration which comes about in the thermodynamic equilibrium of the n-butenes at the reaction temperature, which is set here at 60 C. and measured at the reactor entry.

(47) A part of the stream of the unreacted butenes was returned to the reactor (recycle), as described in Example 1.

(48) The catalyst used was the same material as in Example 1.

(49) TABLE-US-00001 TABLE 1 Analysis of Examples 1 to 5 Example No. 1 2 3 4 5 Fresh feed [g/h] 850 850 850 850 850 1-Butene concentration 25.0 0.5 0.5 4.0 4.0 in fresh feed [wt %] 2-Butene concentration 51.0 75.5 75.5 72.0 72.0 in fresh feed [wt %] Recycle amount [g/h] 1500 4200 1500 4200 1500 Entry temperature [ C.] 60.0 60.0 60.0 60.0 60.0 Exit temperature [ C.] 120.4 96.3 119.3 96.6 119.0 Per pass conversion [wt %] 50.7 22.8 45.9 23.4 46.4 Concentration of oligomers 23.7 10.3 22.9 10.4 23.0 in reactor effluent [wt %] Total conversion [%] 86.5 80.3 83.3 81.1 83.7 C.sub.8 selectivity [%] 80.6 86.9 81.9 86.7 81.7 Mass fractions in hydro- genated C.sub.8 mixture [wt %] n-Octane 17.2 11.7 12.6 12.2 13.1 Methylheptane 62.8 68.0 61.6 68.0 62.4 Dimethylhexane 18.9 19.2 24.7 18.8 23.4 Iso index 1.017 1.075 1.123 1.067 1.105

EXAMPLES 6 TO 12 (INVENTIVE)

(50) Examples 6 to 12 were carried out in a three-stage reactor cascade of largely isothermally operated tube reactors with the following dimensions: length 2.0 m, internal diameter 32.8 mm. The oligomerization was carried out in each case under an absolute pressure of 3 MPa in the liquid phase. Feedstocks used were two hydrocarbon mixtures with different 1-butene/2-butene ratios but the same total amount of n-butenes. The concentrations of the n-butenes are given in Table 2. In addition, the mixtures contain the following components, which add up to 100 wt %: isobutane less than 2% n-butane more than 21% isobutene less than 1%

(51) All of the mixtures therefore contained 1-butene quantities below the 1-butene concentration which comes about in the thermodynamic equilibrium of the n-butenes at reaction temperature (in this case, reactor entry temperature 60 C.), or were free from 1-butene within the bounds of analytical detectability (Example 12).

(52) The catalyst used was the same heterogeneous nickel catalyst as in Examples 1 to 5.

(53) Downstream of each reaction stage, the oligomers were removed from the butanes and unreacted butenes and were analysed for their composition as described in Example 1. A part of the stream containing butanes and unreacted butenes was returned to the preceding reactor (for amounts see Table 2). The part of this mixture not recycled was used as fresh feed for the subsequent reaction stage (where present).

(54) TABLE-US-00002 TABLE 2 Analysis of Examples 6 to 11 Example No. 6 7 8 9 10 11 12 Fresh feed [g/h] 850 850 850 850 850 850 850 1-Butene concentration 0.5 0.5 0.5 4.0 4.0 4.0 0.0 in fresh feed [wt %] 2-Butene concentration 75.5 75.5 75.5 72.0 72.0 72.0 76.0 in fresh feed [wt %] Recycle amount [g/h] 1st stage 100 100 100 100 100 100 100 2nd stage 100 100 100 100 100 3rd stage 100 100 100 Entry temperature [ C.] 1st stage 60.0 60.0 60.0 60.0 60.0 60.0 60.0 2nd stage 60.0 60.0 60.0 60.0 60.0 3rd stage 70.0 70.0 70.0 Exit temperature [ C.] 1st stage 60.9 60.9 60.9 60.9 60.9 60.9 60.9 2nd stage 60.6 60.6 60.6 60.6 60.6 3rd stage 70.4 70.4 70.4 Per pass conversion [wt %] 1st stage 35.1 35.1 35.1 34.7 34.7 34.7 35.1 2nd stage 42.5 42.5 42.5 42.5 42.5 3rd stage 59.2 59.2 59.2 Concentration of oligomers in reactor effluent [wt %] 1st stage 26.4 26.4 26.4 26.1 26.1 26.1 26.4 2nd stage 27.2 27.2 27.2 27.2 27.2 3rd stage 26.9 27.0 26.9 Overall conversion [%] 38.7 68.2 89.4 38.4 67.9 89.3 89.4 C.sub.8 selectivity [%] 84.7 83.9 82.6 84.7 83.9 82.7 82.6 Mass fractions in the hydrogenated C.sub.8 mixture [wt %] n-Octane 14.4 14.7 14.5 15.2 15.1 14.8 14.5 Methylheptane 66.6 66.8 66.3 67.8 67.5 66.9 66.2 Dimethylhexane 18.9 18.5 19.1 17.0 17.4 18.3 19.3 Iso index 1.045 1.039 1.047 1.019 1.024 1.035 1.049

CONCLUSION

(55) The comparison of the non-inventive Examples 1 and 2 shows that the 1-butene content has a perceptible influence on the Iso index of the resulting C.sub.8 olefin mixture. Examples 2 to 5 show that the oligomerization process known from the prior art is unsuitable for producing di-butenes that can be used for plasticizer production from C.sub.4 streams with a low 1-butene content.

(56) But the comparison of the non-inventive Examples 2 to 5 with the inventive Examples 6 to 12 demonstrates that in accordance with the process of the invention, C.sub.8 mixtures with a low Iso index of below 1.05 can be prepared even when the reactant stream employed contains extremely small fractions of 1-butene, or none. The three-stage processes in this case achieve an overall conversion of approximately 90%. The C.sub.8 selectivity is slightly above that of the conventional process.

(57) The inventively modified OCTOL process has therefore been set up, by limitation of the conversion in its individual oligomerization steps, in a particular way for the productive utilization of C.sub.4 feedstock mixtures having a low 1-butene content, but nevertheless yields a C.sub.8 product mixture having an Iso index suitable for the production of plasticizer alcohols.

LIST OF REFERENCE NUMERALS

(58) 1 reactant mixture 2 reaction zone 3 oligomerizate 4 distillation column 5 top stream 6 liquid phase stream 7 recycle stream 8 transfer stream 9 assessment boundary 10 overall process (two-stage) 101 first step sequence 102 second step sequence 11 reactant mixture 12 first reaction zone 13 first oligomerizate 14 first distillation column 15 first top stream 16 first liquid phase stream 17 first recycle stream 18 first transfer stream 21 hydrocarbon mixture provided for the second stage 22 second reaction zone 23 second oligomerizate 24 second distillation column 25 second top stream 26 second liquid phase stream 27 second recycle stream 28 second transfer stream 29 fractionation 30 productive utilization 32 third reaction zone 33 third oligomerizate 34 third distillation column 36 combined liquid phase streams/third liquid phase stream 37 C.sub.8 column 38 C.sub.8 product mixture 39 liquid phase product of the C.sub.8 column 40 C.sub.12 column 41 C.sub.12 product mixture 42 C.sub.12+ product mixture