Fractionation process for a process for oligomerising light olefins

Abstract

The invention relates to a process for oligomerising light olefins in which the effluent from the oligomerisation section is passed to a prefractionator that leads to at least one head fraction containing a mixture of liquefied petroleum gas and light gasoline and a bottom fraction containing a mixture of heavy gasoline and middle distillate, the said head fraction being passed to a debutaniser that leads to at least one liquefied petroleum gas cut and a light gasoline cut, the said bottom fraction and at least part of the said light gasoline cut being passed to a separator enabling at least a gaseous fraction, a gasoline fraction and a gasoil fraction to be obtained.

Claims

1. Process for the oligomerisation of light olefins in which the effluent from the oligomerisation section is passed to a prefractionator, which leads to at least one head fraction containing a mixture of liquefied petroleum gas and light gasoline and a bottom fraction containing a mixture of heavy gasoline and middle distillate, the said head fraction being passed to a debutaniser that leads to at least one liquefied petroleum gas cut and a light gasoline cut, the said bottom fraction and at least part of the said light gasoline cut being passed to a separator enabling at least a gaseous fraction, a gasoline fraction and a gasoil fraction to be obtained.

2. Process according to claim 1, in which the said head fraction contains a liquefied petroleum gas (LPG)/light gasoline mixture, containing less than 15% by weight with respect to the hydrocarbon feedstock containing at least 8 carbon atoms, and the said bottom fraction containing a heavy gasoline/middle distillate mixture containing less than 15% by weight with respect to the hydrocarbon feedstock containing at most 5 carbon atoms.

3. Process according to claim 1, in which the said prefractionator contains between 5 and 20 theoretical trays, operates at a pressure between 0.1 and 2 MPa, the feedstock being supplied to a tray situated at the head of the column, the head vapours being withdrawn at a temperature between 100 and 150 C. and the bottom liquid being withdrawn at a temperature between 170 C. and 220 C.

4. Process according to claim 1, in which the said liquefied petroleum gas cut contains hydrocarbons containing 4 or fewer carbon atoms and the said light gasoline cut contains hydrocarbons containing at least 5 carbon atoms.

5. Process according to claim 1, in which the said debutaniser contains between 15 and 35 theoretical trays and operates at the same pressure as the prefractionator, the feedstock being fed between the theoretical trays 8 and 25, the trays being numbered starting from the head of the column, the head vapours are removed, condensed and subcooled to a temperature between 35 and 55 C. in an air-cooled condenser, and the bottom liquid is removed at a temperature between 100 C. and 130 C.

6. Process according to claim 1, in which the said separator contains between 20 and 40 theoretical trays and operates at a lower pressure than the pressure of the said debutaniser, namely between 0.01 MPa and 0.6 MPa.

7. Process according to claim 1, in which the said bottom fraction obtained from the prefractionator is fed between the trays 5 and 15 of the said separator, the said bottom fraction obtained from the debutaniser is fed between the theoretical trays 2 and 12 of the said separator, the trays being numbered starting from the head of the column, the head vapours being removed, condensed and subcooled to a temperature between 40 C. and 80 C. in an air-cooled condenser and the bottom liquid being removed at a temperature between 200 C. and 240 C.

8. Process according to claim 1, in which the said bottom fraction and the said bottom cut supply the said separator at two different levels.

9. Process according to claim 1, in which the said bottom cut is in part recycled to the prefractionation column.

10. Process according to claim 9, in which the said bottom cut is preferably introduced to several trays above the reboiler.

11. Process according to claim 9, in which the degree of recycling of the said bottom cut is between 5% by weight and 20% by weight.

12. Process according to claim 1, in which the bottom effluent from the said separator is used for a heat exchange with the reboiler of the said debutaniser.

13. Process according to claim 1, in which the effluent obtained from the last reactor is cooled before its pressure is released and it is introduced to the prefractionator, which enables heat to be recovered and the reboiler of the said debutaniser to be reheated.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0043] FIG. 1 shows a conventional fractionation process scheme according to the prior art for obtaining a LPG fraction, a gasoline fraction and a middle distillate fraction. The oligomerate (1.1) is passed to a debutaniser (1.A), which separates at least two fractions, namely a column head fraction (1.2) and a column bottom fraction (1.3). The said column bottom fraction (1.3) is then passed to a separator (1.B) that separates in turn two fractions (1.4 and 1.5).

[0044] FIG. 2 shows an embodiment of the process according to the invention in which a prefractionation column (2.C) is installed upstream of the process shown in FIG. 1. The head fraction containing a mixture of liquefied petroleum gas and light gasoline (2.6) from the prefractionator (2.C) is passed to the debutaniser (2.A), while the bottom fraction containing a mixture of heavy gasoline and the middle distillate (2.7) is passed directly to the separator (2.B).

[0045] FIG. 3 shows an embodiment similar to that shown in FIG. 2 of the process according to the invention in which, in addition, a part of the bottom fraction containing a mixture of heavy gasoline and middle distillate (3.7) from the debutaniser (3.A) is recycled (3.8) to the said prefractionator (3.C).

[0046] Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

[0047] In the foregoing and in the examples, all temperatures are set forth uncorrected in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.

[0048] The entire disclosures of all applications, patents and publications, cited herein and of corresponding application No. FR 1563036, filed Dec. 22, 2015 are incorporated by reference herein.

EXAMPLES

[0049] All the examples 1 to 3, 21 t/hr. of oligomerate obtained from an oligomerisation section consisted of 28% by weight of hydrocarbons containing 4 or fewer carbon atoms, 14% by weight of hydrocarbons containing 5 carbon atoms, 10% by weight of hydrocarbons containing 6 carbon atoms, 2% by weight of hydrocarbons containing 7 carbon atoms, 12% by weight of hydrocarbons containing 8 carbon atoms and 34% by weight of hydrocarbons containing at least 9 carbon atoms.

[0050] The operating conditions are adjusted so as to fractionate the said oligomerate into 3 products, the amounts of which are given in the table below:

TABLE-US-00001 Product Flow rate (t/hr.) Liquefied petroleum gas 5.8 Gasoline 7.8 Middle distillate 7.4

[0051] The liquefied petroleum gas contains 2% by weight of hydrocarbons containing 5 or more carbon atoms.

[0052] The gasoline is characterised by an initial boiling point D 86 of 13 C. and a final boiling point D 86 of 140 C., and a content of hydrocarbons containing 4 or fewer carbon atoms of only 1.9% by weight.

[0053] The middle distillate is characterised by an initial boiling point D 86 of 164 C. and a flash point determined by the Nelson method of 43.9 C.

Example 1 (Comparative)

[0054] The process of Example 1 is shown in FIG. 1. The oligomerate (1.1) is passed to a debutaniser (1.A) which separates it into at least two fractions, namely a column head fraction (1.2) and a column bottom fraction (1.3). The said column bottom fraction (1.3) is then passed to a separator (1.B) that separates in turn two fractions (1.4 and 1.5). The bottom product from the debutaniser (1.4) exchanges its heat with the effluent feeding it (1.3). The debutaniser and the separator operate with 23 theoretical trays, their feed positions being optimised so as to minimise the heat dispensed to the reboilers. Accordingly the feed of the debutaniser (1.A) is located at tray 13 and the feed of the separator (1.B) is located at tray 8, the trays being numbered starting from the head of the column. The debutaniser (1.A) operates at 0.65 MPa and the separator (1.B) operates at 0.2 MPa.

Example 2 (According to the Invention)

[0055] The process of Example 2 is shown in FIG. 2. In Example 2, a prefractionation column (2.C) is installed upstream of the process of Example 1. The head fraction from the prefractionator (2.6) is passed to the debutaniser (2.A) while the bottom fraction (2.7) is passed directly to the separator (2.B).

[0056] The prefractionator (2.C) comprises 14 theoretical trays and operates at 0.65 MPa. The bottom effluent from the separator (2.5) is used for a heat exchange with the reboiler of the debutaniser (2.A) before it is cooled and stored. The bottom cut of the debutaniser (2.3) and the bottom fraction (2.7) obtained from the prefractionation are passed in whole to the separator (2.B), respectively to tray 4 and to tray 10, the trays being numbered starting from the head of the column.

Example 3 (According to the Invention)

[0057] The process of Example 3 is shown in FIG. 3. In Example 3 the same process as in Example 2 (FIG. 2) is implemented, with in addition the recycling (3.8) of 0.5 t/hour of the said bottom cut (3.3) from the debutaniser (3.A) to the prefractionator (3.c) at the level of tray 12, the trays being numbered starting from the head of the column and the column containing 14 trays.

TABLE-US-00002 Example Example Example 1 2 3 Prefractionator Flow rate (tons/hour) 11.9 11.9 Reboiler service (MW) 0.63 0.58 Reboiler temperature ( C.) 197.6 187.1 First column Reflux rate (tons/hour) 13.9 12.8 12.8 Side reboiler service (MW) Side reboiler temperature ( C.) Reboiler service (MW) 1.0 0.53 0.52 Reboiler temperature ( C.) 145.0 116.7 116.4 Condenser service (MW) 1.9 1.75 1.75 Condenser temperature 45.0 45.0 45.0 ( C.) Second column Reflux rate (tons/hour) 5.4 4.2 4.1 Reboiler service (MW) 1.20 1.08 1.13 Reboiler temperature ( C.) 224 224 224 Condenser service (MW) 1.7 1.4 1.4 Condenser temperature 45.0 61.0 61.0 ( C.) Total reboiler services 2.2 2.24 2.23 Total hot utilities (MW) 2.2 1.71 1.71 Not in In In accordance accordance accordance

[0058] The process according to the invention enables, for the same amounts and specifications of obtained products, the consumption of hot utilities to be reduced in the fractionation stage of the oligomerate by 22% compared to a simple fractionation carried out by means of two successive distillations without a prefractionation section.

[0059] Furthermore, by a simple partial recycling of the light gasoline cut depleted in middle distillate to the prefractionation column, it is possible to adjust the proportion of the heavy and light gasoline cuts, so as to regulate the temperature of the reboiler of the prefractionation column.

[0060] The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

[0061] From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.