DE69415084T2 - Process for improving a hydrocarbon feed - Google Patents

Description

The present invention relates to a process for upgrading a hydrocarbon feedstock boiling substantially in the gasoline range to provide a gasoline blend pool having an improved octane content and a reduced aromatics content.

One of the main objectives of oil refining today is to produce gasolines that meet increasing environmental requirements, product quality and have a high octane number.

EP-A-0 553 931 describes such a process, which has already been proposed to obtain a gasoline with an improved octane number.

For gasoline, this means that the octane specification must now be met without lead-containing additives, with fewer aromatics, especially benzene, fewer olefins and lower gasoline vapor pressure.

The aim of the present invention is to create a process for producing gasolines that meet both the increasing environmental demands on product quality and the high octane requirement.

It has now been found that gasolines with a high octane number and a considerably reduced aromatics content, in particular benzene, can be produced if an upgrading process is used which comprises a special sequence of process steps.

Accordingly, the present invention relates to a process for creating a gasoline blend pool with an improved octane content and a reduced aromatics content, which process comprises:

a) subjecting a hydrocarbon feedstock boiling substantially in the gasoline range to separation and recovering therefrom a first hydrocarbon feedstream comprising C6 and smaller hydrocarbons, and a second hydrocarbon feedstream comprising C6 and larger hydrocarbons;

(b) subjecting at least a portion of the second hydrocarbon feed stream to a separation in which normal paraffins and optionally monoisoparaffins are separated from diisoparaffins;

c) recovering therefrom a first separation effluent comprising normal paraffins and optionally monoisoparaffins, and, a second separation effluent comprising diisoparaffins;

d) subjecting at least a portion of the first separation effluent to a reformate step to produce a reformate;

e) carrying out a hydrogenation step on at least part of the reformate obtained; and

f) Transferring the second separation effluent from stage c) into a gasoline mixture pool.

In this way, a direct octane improvement of the gasoline blend pool formed is achieved, while a significant reduction in the aromatics content, especially benzene, is achieved. In refineries with limited gasoline production due to octane and/or capacity limitations, this octane improvement can enable increased gasoline production.

The two hydrocarbon feed streams originating from the feedstock in stage a) may conveniently be obtained by distillation. Conveniently, the two hydrocarbon feed streams are adjacent fractions obtained by distillation. Depending on the sharpness of the intersections of the selected fractions in the distillation, there may of course be some overlap between the adjacent fractions.

Preferably, the separation in step a) is carried out in such a way that the first hydrocarbon feed stream comprises essentially C₅ and smaller hydrocarbons. When the first hydrocarbon feed stream comprises essentially C₅ and smaller hydrocarbons, this feed stream need not be subjected to an isomerization process but can advantageously be introduced directly into the gasoline blending pool. Conveniently, at least a portion of the feedstock to be upgraded can be subjected to hydrogenation in step e) before being subjected to separation in step a).

The gasoline range boiling hydrocarbon feedstock may conveniently be obtained by distillation of crude oil or by catalytic cracking, although it may be obtained by other cracking processes such as thermal cracking, delayed coking, visbreaking and flexicoking. Such gasoline feedstocks typically contain unacceptable amounts of sulfur and nitrogen and benefit from hydrotreating before being subjected to the process of the present invention.

The process according to the present invention is expediently carried out in such a way that in step b) both the normal paraffins and the monoisoparaffins (singly branched paraffins) are separated from the diisoparaffins (doubly branched paraffins).

This is conveniently accomplished by passing at least a portion of the second hydrocarbon feed stream into a separation zone containing a shape-selective molecular separation sieve having a pore size between 5.5 x 5.5 and 4.5 x 4.5 Å, but excluding 4.5 x 4.5 Å, which pore size is sufficient to allow the entry of normal paraffins and monoisoparaffins, but prevents the entry of diisoparaffins, other multiply branched paraffins, cyclic paraffins and aromatic hydrocarbons.

In this way, the normal paraffins and the monoisoparaffins can be separated selectively from the diisoparaffins.

Subsequently, the first separation effluent, which comprises both normal paraffins and monoisoparaffins, and the second separation effluent, which comprises diisoparaffins, can be obtained.

Subsequently, at least a portion of this first separation effluent is subjected to the reforming step. Preferably, substantially all of the first separation effluent is subjected to the reforming step, although a portion of it may be used as a preferred chemical feed, for example as a feed for a highly selective (dehydro)cyclization process.

Preferably, the normal paraffins are first separated from the monoisoparaffins and diisoparaffins, while the monoisoparaffins are subsequently separated from the diisoparaffins. For this purpose, a multi-selective molecular sieve adsorption system can be used, which has special separation properties.

Preferably, the multiple separation sieve system to be used comprises a first molecular sieve having a pore size of 4.5 x 4.5 Å or smaller, designed to allow the adsorption of normal paraffins in a selective manner over monoisoparaffins, diisoparaffins, other multi-branched paraffins, cyclic paraffins and aromatic hydrocarbons, and a second molecular sieve having a pore size between 5.5 x 5.5 and 4.5 x 4.5 Å, but excluding 4.5 x 4.5 Å, to allow the adsorption of monoisoparaffins (and any residual normal paraffins) over diisoparaffins, other multi-branched paraffins, cyclic paraffins and aromatic hydrocarbons which can be passed directly into a refinery gasoline blend pool.

In operation, at least a portion of the second hydrocarbon feed stream is first contacted with the first shape selective molecular separation sieve as defined above to produce a first separation effluent comprising the normal paraffins and a second separation effluent comprising both mono- and diisoparaffins The latter separation effluent is subsequently contacted with the second shape-selective molecular separation sieve as described above.

Subsequently, a third separation effluent containing monoisoparaffins and a fourth separation effluent containing diisoparaffins can be obtained. At least a portion of the first and third separation effluents can be subjected to the reforming stage.

Preferably, substantially all of the first and third separation effluents are subjected to the reforming step. In a further embodiment of the present invention, at least a portion of the first and third separation effluents may be conveniently used as a preferred chemical feedstock as mentioned above.

The multiple selective molecular sieve adsorption system as described above comprises at least two molecular sieves. These may be arranged in separate vessels or they may be arranged in a stacked flow pattern within a vessel.

This first molecular sieve can be a calcium 5 Å zeolite or any other sieve with similar pore dimensions. It is not necessary to dimension the first sieve so that all normal paraffins are adsorbed, but this is preferred so that the second molecular sieve does not have to function as a normal paraffin adsorption sieve.

The second molecular sieve in this process step is illustrated by a molecular sieve with eight and ten ring members and with pore diameters between 5.5 x 5.5 and 4.5 x 4.5 Å, but exclusively 4.5 x 4.5 Å.

The preferred second molecular sieve of the present invention is exemplified by a ferrierite molecular sieve. It is preferred that the ferrierite sieve be in a hydrogen form, but it may optionally be exchanged with a cation of an alkali metal or alkaline earth metal or with a transition metal cation.

The second molecular sieves of the present invention include ferrierite and other analogous shape-selective materials with pore openings between the dimensions of calcium 5 Å zeolite and ZSM-5. Other examples of crystalline sieves include aluminum phosphates, silicoaluminum phosphates, and borosilicates.

The aluminum phosphate, silicoaluminum phosphate and borosilicate molecular sieves, which can be used as a second molecular sieve, have a pore size between 5.5 x 5.5 and 4.5 x 4.5 Å, but exclusively 4.5 x 4.5 Å.

It is possible that the second molecular sieve comprises a large pore zeolite that has been ion exchanged with cations to return the effective pore size of the sieve to the previously mentioned dimensional range.

When using multi-selective molecular sieve adsorption systems, the order of the sieves, whether in separate vessels or in a stacked version, is very important. If the sieves are interchanged, the process loses effectiveness because the larger sieve quickly fills with normal paraffins, preventing the efficient adsorption of monoisoparaffins.

The respective sieves employed in a multiple selective molecular sieve adsorption system should be arranged in a process sequence such that sufficient adsorption of the normal paraffin hydrocarbons occurs first and then the adsorption of the monoisoparaffins. Each of these respective sieves can be provided with a common desorbent stream or each sieve can have its own desorbent stream. The desorbent is preferably a gaseous material such as a hydrogen gas stream.

Conveniently, at least part of the reformate obtained in step d) is subjected to a separation from which a light fraction comprising C6 and smaller hydrocarbons and a heavy fraction comprising C6 and larger hydrocarbons are obtained.

Subsequently, at least part of the light fraction and optionally at least part of the heavy fraction are subjected to hydrogenation in stage e). Conveniently, C5 and smaller hydrocarbons are separated from the light fraction before they are subjected to hydrogenation in stage e).

Any conventional hydrogenation catalyst can be used in the hydrogenation step. An example of such a catalyst is a catalyst comprising at least one component of a Group VIII and/or Group VIb metal on a silica-alumina-containing support. Preferably, a platinum component on an amorphous silica-alumina support is used. The hydrogenation step can conveniently be carried out under conventional hydrogenation conditions. Typically, the hydrogenation is carried out at a temperature in the range of 150 to 30,000 and a hydrogen partial pressure of 10 to 30 bar.

Conveniently, at least a portion of the light and heavy fractions are recovered. In another attractive embodiment of the process according to the present invention, at least a portion of the light and heavy fractions is subjected to a separation in which normal paraffins and optionally monoisoparaffins are separated from diisoparaffins, thereby obtaining a first hydrocarbon product stream comprising normal paraffins and optionally monoisoparaffins and a second hydrocarbon product stream comprising diisoparaffins. At least a portion of the first hydrocarbon product stream can be used as a preferred chemical feedstock as mentioned above.

The separation is expediently carried out in such a way that both the normal paraffins and the monoisoparaffins are separated from the diisoparaffins. This is expediently achieved by passing at least a portion of the light and heavy fractions into a separation zone which has a shape-selective molecular separation sieve with a pore size between 5.5 x 5.5 and 4.5 x 4.5 Å, but excluding 4.5 x 4.5 Å, the pore size being sufficient to prevent the entry of To allow normal paraffins and monoisoparaffins, but to prevent the entry of diisoparaffins.

In this way, normal paraffins and monoisoparaffins can be separated selectively from diisoparaffins, other multi-branched paraffins, cyclic paraffins and aromatic hydrocarbons.

As a result, a first hydrocarbon product stream containing both normal paraffins and monoisoparaffins and a second hydrocarbon product stream containing diisoparaffins can be obtained.

Conveniently, the separation is carried out in such a way that first the normal paraffins are separated from the monoisoparaffins and diisoparaffins, while the monoisoparaffins are subsequently separated from the diisoparaffins. For this purpose, a multi-selective molecular sieve adsorption system can be used, as described above.

When such a multi-selective molecular sieve adsorption system is used upstream of the reforming stage, at least a portion of the light and heavy fractions is fed to the first molecular sieve.

When a multi-selective molecular sieve adsorption system is used both upstream and downstream of the reforming stage, the normal and monoisoparaffins originally present are first separated from the diisoparaffins, while the normal paraffins and monoisoparaffins formed in the reforming stage are subsequently separated from the isoparaffins.

The application of a multi-selective molecular sieve adsorption system both upstream and downstream of the reforming stage is very attractive as it offers product flexibility coupled with product quality.

Therefore, in a preferred embodiment of the present invention, a multi-selective molecular sieve adsorption system both upstream and downstream of the reforming stage.

The separations upstream and downstream of the reforming stage, in which the normal paraffins and, if applicable, the monoisoparaffins are separated from the diisoparaffins, are preferably carried out in the same separation zone.

Conveniently, the light fraction containing C6 and smaller hydrocarbons and the heavy fraction containing C6 and larger hydrocarbons were obtained from the reformate by distillation.

Conveniently, the light and heavy fractions are adjacent fractions obtained by distillation. However, depending on the sharpness of the intersections of the selected fractions in the distillation, there may be some overlap between the adjacent fractions.

In a further embodiment of the present invention, the reformate obtained in step d) is first subjected to a separation in which a gaseous fraction is separated from a liquid fraction, after which the liquid fraction is separated into the light fraction containing C₆ and smaller hydrocarbons and the heavy fraction containing C₆ and larger hydrocarbons.

Any conventional reforming catalyst can be used in the reforming stage. Preferably, a catalyst with a substantially (dehydro)cyclization selectivity is used in the reforming stage. An example of such a catalyst is a platinum-containing catalyst, in which the platinum is present, for example, in the range of 0.005 wt.% to 10.0 wt.%.

The catalytic metals provided with the reforming function are preferably noble metals of group VIII of the periodic table of elements, such as platinum and palladium. The reforming catalyst can be present as such or it can be mixed with a binding material.

It is generally recognized that the application of noble metal-containing reforming catalysts usually requires a pretreatment in the form of a catalytic hydrotreatment of the feedstock to be upgraded. In this way, nitrogen compounds and sulfur compounds can be separated from the feedstock, which compounds would otherwise significantly reduce the performance of the reforming catalyst.

The reforming stage can conveniently be carried out under conventional reforming conditions. Typically, the process is carried out at a temperature of 450 to 550ºC and a pressure of 3 to 20 bar. The reaction section in which the reforming stage is to be carried out can conveniently be divided into several stages or reactors.

The present invention will now be explained by way of example.

Example

A process according to the present invention is carried out in accordance with the flow diagram as schematically shown in Fig. 1.

A hydrocarbon feedstock boiling substantially in the gasoline range and having the properties set forth in Table 1 is introduced via line 1 and line 1a into a distillation column 2 in which the feedstock is separated into two hydrocarbon feedstreams. A first hydrocarbon feedstream comprising C5 and smaller hydrocarbons is withdrawn via line 3 and passed to a gasoline blend pool 4. A second hydrocarbon feedstream comprising C5 and larger hydrocarbons is withdrawn via line 5 and passed to a separation zone 6 which contains two molecular sieves 7 and 8. The molecular sieve No. 1 (7) is a commercially available zeolite having a pore size of 4.5 by 4.5 Å or smaller. Molecular sieve 8, referred to as molecular sieve No. 2, has a pore size of 5.5 x 5.5 to 4.5 x 4.5 Å, but excludes 4.5 x 4.5 Å. The first molecular sieve 7 selectively adsorbs normal paraffins over monoisoparaffins. en, diisoparaffins, other multi-branched paraffins, cyclic paraffins and aromatic hydrocarbons. A fraction containing normal paraffins is taken off via a line 9 and introduced into a reforming reactor 10. The separation effluent, which has essentially been freed from normal paraffins, is discharged via a line 11 and brought into contact with molecular sieve No. 2 (8). The monoisoparaffins are adsorbed in this molecular sieve, while diisoparaffins and other multi-branched paraffins and cyclic paraffins are passed through the sieve without being adsorbed. A fraction containing monoisoparaffins is taken off via a line 12 and transferred to the reforming reactor 10. The remaining separation effluent (diisoparaffin fraction), which has now essentially been freed from normal paraffins and monoisoparaffins, is discharged via a line 13 and transferred to the gasoline mixture pool 4. In the reforming step, a commercially available highly selective (dehydro)cyclization catalyst is used under typical semi-regenerative reforming conditions. The resulting reformate is then withdrawn via a line 14 and introduced into a distillation column 15. In the distillation column 15, the reformate is separated into a gaseous fraction and a liquid fraction. The gaseous fraction is withdrawn via a line 16, the liquid fraction is removed via a line 17. The liquid fraction is then transferred to a distillation column 18. In the distillation column 18, the liquid fraction is separated into a first fraction containing C₅ and smaller hydrocarbons, a second fraction containing C₆ and C₇ hydrocarbons, and a third fraction containing C₅ and larger hydrocarbons. The first fraction is removed from the distillation column 18 via a line 19 and transferred to the gasoline mixture pool 4. The second fraction is introduced into a hydrogenation unit 20 via a line 21. A hydrogen stream is introduced into the hydrogenation unit 20 via a line 22. The hydrogenated product obtained from the hydrogenation unit 20 is then processed together with the feedstock to be improved via lines 23 and 1a. The third fraction is removed from the distillation column 18 via a Line 24 is disconnected and transferred to the gasoline mixture pool 4.

100 parts by weight (GT) of feedstock in line 1 yield the various product fractions in the following quantities:

11.7 GT first hydrocarbon feed stream (line 3)

107.3 GT second hydrocarbon feed stream (line 5)

21.6 GT normal paraffin fraction (line 9)

85.7 GT first separation partial effluent (line 11)

23.3 GT monoisoparaffin fraction (line 12)

62.4 GT diisoparaffin fraction (line 13)

44.9 GT reformate fraction (line 14)

5.8 GT gaseous fraction (line 16)

39.1 GT liquid fraction (line 17)

1.4 GT first fraction (line 19)

18.1 GT second fraction (line 21)

0.9 GT hydrogen stream (line 22)

19.0 GT hydrogenated product stream (line 23)

19.6 GT third fraction (line 24)

In gasoline blend pool 4, 5.3 GT of butane, 17.5 GT of MTBE are added to the gasoline obtained via line 25. In this way, 117.9 GT of a total gasoline are obtained that has the maximum allowed RVP specification. The total gasoline obtained in gasoline blend pool 4 has the properties given in Table 2.

From Table 2 it is clear that a very attractive gasoline in terms of octane number and aromatics content, especially benzene, can be obtained by applying the present invention. In conventional upgrading processes, gasolines with a significantly higher content of aromatics, especially benzene, are obtained.

Table 1

C (wt%): 85.2

H (wt%): 14.8

S (ppm): < 1

d (15/4): 0.731

Initial boiling point: 56

10 wt.% rec.: 64

30 wt.% rec.: 92

50 wt.% rec.: 106

70 wt.% rec.: 127

90 wt.% rec.: 149

Final boiling point: 197

RON: 55.7

Naphthenes: 27.2

Aromatics: 10.3

Table 2 Gasoline properties:

RON: 95.0

Total aromatics (vol%): 23.2

Benzene (vol%): 1.1

Naphthene (vol%): 37.5

RVP (kpa): 60

Claims (7)

1. A process for producing a gasoline blend pool with an improved octane content and a reduced aromatics content, which process comprises: a) subjecting a hydrocarbon feedstock boiling substantially in the gasoline range to separation and recovering therefrom a first hydrocarbon feedstream comprising C6 and smaller hydrocarbons and a second hydrocarbon feedstream comprising C6 and larger hydrocarbons; (b) subjecting at least a portion of the second hydrocarbon feed stream to a separation in which normal paraffins and optionally monoisoparaffins are separated from diisoparaffins; c) recovering therefrom a first separation effluent comprising normal paraffins and optionally monoisoparaffins, and a second separation effluent comprising diisoparaffins; d) subjecting at least a portion of the first separation effluent to a reforming step to produce a reformate; e) carrying out a hydrogenation step on at least a part of the reformate obtained; and f) Transferring the second separation effluent from stage c) into a gasoline mixture pool. 2. Process according to claim 1, wherein in step b) both the normal paraffins and the monoisoparaffins are separated from the diisoparaffins and at least a portion of the normal paraffins and monoisoparaffins thus obtained are subjected to the reforming step. 3. Process according to claim 2, wherein first the normal paraffins are separated from the isoparaffins and subsequently the monoisoparaffins are separated from the diisoparaffins. 4. A process according to any one of claims 1-3, wherein at least a portion of the reformate stream obtained in step d) is separated into a light fraction containing C6 and smaller hydrocarbons and a heavy fraction containing C6 and larger hydrocarbons, at least a portion of the light fraction and at least a portion of the heavy fraction being subjected to hydrogenation in step e). 5. A process according to claim 4, wherein at least a portion of the reformate is first separated into a gaseous fraction and a liquid fraction, after which the liquid fraction is separated into the light fraction containing C₆ and smaller hydrocarbons and the heavy fraction containing C₆ and larger hydrocarbons. 6. Process according to claim 4 or 5, wherein at least a portion of the light fraction and at least a portion of the heavy fraction are subjected to a separation, in which normal paraffins and optionally monoisoparaffins are separated from diisoparaffins, and a first hydrocarbon product stream containing normal paraffins and optionally monoisoparaffins and a second hydrocarbon product stream containing diisoparaffins are obtained, which second hydrocarbon product stream is transferred to the gasoline blend pool. 7. A process according to claim 6, wherein the separation is carried out in such a way that both the normal paraffins and the monoisoparaffins are separated from the diisoparaffins.