DE69514957T2 - Process and plant for the selective hydrogenation of catalytic cracked petrol - Google Patents

Description

The production of reformed petrol, which is a response to the new environmental standards, requires in particular a reduction in its concentration of olefins, aromatics (particularly benzene) and sulphur.

The reduction in the concentration of olefins is often obtained by etherification of 2-methylbutene to t-amyl methyl ether, even sometimes of 2-methylpentene and 3-methylpentene-2 to t-hexyl methyl ether.

Etherification is also an elegant method to reduce the content of olefins while converting them into high-octane ethers for research and engines.

However, the presence of diolefins in the catalytic cracking gasoline results in the deactivation of the etherification catalyst by the formation of lubricating oil residues, which also cause a deterioration in the quality of the ether product. Various processes have been proposed to solve this problem (EP-A-0 564 329, EP-A-0 554 151, FR-A-2 482 953, GB-A-1 346 778, GB-A-1 565 754).

A process has therefore been developed which makes it possible to selectively hydrogenate the diolefins to the corresponding olefins and, at the same time as the hydrogenation of the diolefins, to carry out the isomerization of at least some of the primary olefins and/or secondary olefins into tertiary olefins, for example the isomerization of 3-methylbutene-1, which is not etherifiable, into 2-methylbutene-2, which is etherifiable.

It has also been found that it is possible to sweeten at least part of the catalytic cracking gasoline by transforming the mercaptans into other sulphur-containing products, thanks to the special conditions according to the invention and a special device of the catalytic reactor according to the invention. Sweetening is understood to mean an addition reaction of the olefin to a mercaptan.

More specifically, the invention relates to a process for the selective hydrotreatment of catalytic cracked gasoline, in which the C5-210°C fraction is brought into contact with a catalyst containing 0.1 to 1% of palladium deposited on a support, under a pressure of 4-25 bar, a temperature of 80-200°C, with a liquid hourly space velocity (LHSV) of 1 to 10 h-1, in such a way as to reduce the mercaptan content of the gasoline.

The catalyst must contain palladium (0.1 to 1 wt.%, preferably 0.2-0.5 wt.%) deposited on a support containing at least 50%, preferably at least 90% alumina, advantageously the support is pure alumina.

Another metal may be added to create a bimetallic catalyst, in particular gold (Au/Pd ratio, expressed in parts by weight, above or equal to 0.1 and below 1, preferably in a range between 0.2 and 0.8).

The catalyst mainly contains palladium or palladium and gold.

The choice of operating conditions is particularly important. In the most general case, the operation is carried out under pressure in the presence of a quantity of hydrogen slightly higher than the stoichiometric value required to hydrogenate the diolefins. The hydrogen and the feed to be treated are blown in an ascending or descending stream into a preferred fixed-bed catalyst reactor. The temperature is generally between 80 and 200°C, in particular between 130 and 200°C, preferably between 150 and 170°C.

The pressure is sufficient to keep the greater part of the gasoline to be treated in a liquid phase in the reactor, namely, in the most general case, between 4 and 25 bar and preferably above 10 bar. A preferred pressure range is between 10 and 20 bar, preferably between 12 and 16 bar.

The hourly space velocity under these conditions is between 1-10 h⁻¹, preferably below 10 h⁻¹ and more advantageously between 4 and less than 10 h⁻¹.

In the same way, traces of oxygen can be injected with the petrol in very low concentrations, namely in the order of 100 to 500 ppm, based on moles.

The invention relates exclusively to catalytic cracked gasoline.

In contrast to pyrolysis gasoline, which contains about 50% diolefins, the fraction of catalytic cracking gasoline generally contains from 15 to 40% olefins (olefins, diolefins and cycloolefins) with a content of conjugated lower diolefins of up to 5%, generally more than 4%. After hydrogenation, the diene content is reduced to less than 1000 ppm. The diene content in the C5 cut and C6 cut after selective hydrogenation is generally reduced to less than 250 ppm.

The cyclopentadiene content in catalytic cracking gasoline is below 1 wt.%, generally below 0.5 wt.%, whereas in a steam cracking gasoline it can be as high as 20 wt.%.

The mercaptan content is between 1 and 300 ppm, generally below 200 ppm.

These special conditions allow the production of catalytic cracked gasoline directly downstream of the debutanization device. without the need for additional preheating or feed pumping, and preferably with a single reactor.

The invention also has as its subject an installation for treating by selective hydrogenation catalytic cracking gasoline comprising a catalytic cracking unit, followed by a fractionation unit to separate the C3-210°C fraction, a debutanization device, then a separation unit selected from a depentanization device and a dehexanization device, said installation including a selective hydrogenation unit located between the debutanization unit and the separation unit.

Fig. 1 shows a schematic representation of the method and the device according to the invention.

The catalytic cracking unit 1, for example a fluid catalytic cracking unit, provides an effluent which is fractionated in a fractionation unit 2 into a gas oil cut (LCO), a high boiling fraction (HCO) and a fraction containing C₃ hydrocarbons up to 210°C, preferably C₃ hydrocarbons up to 180°C and advantageously C₃ hydrocarbons up to 160°C, especially most preferably C₃ hydrocarbons up to less than 160°C (i.e. fraction end point of 160°C).

According to Fig. 1, a C3-180°C fraction is fed to the debutanizer 3 to separate the C3-C4 fraction.

The C₅-180°C fraction obtained (or the C₅-160°C or C₅-210°C fraction according to the fractionation used), called the C₅+ fraction of the catalytic cracked gasoline, is fed into a zone (or unit) 4 of the selective hydrogenation according to the invention, the hydrogen being introduced in the same way, through line 5 for example.

The hydrogenated fraction obtained is sent to the separation unit 6, which is a depentanization device (separation of C5) or a dehexanization device (separation of C5-C6). A C5 or a C5-C6 fraction is thus obtained, which is advantageously sent to the etherification unit, and a C7+ fraction which is sent to the gasoline tank.

According to one embodiment, the catalytic hydrogenation reactor 10 comprises a catalytic reaction zone through which all of the feed and the necessary amount of hydrogen flows to effect the desired reactions.

According to a preferred embodiment of the invention, the catalytic hydrogenation reactor 10 is designed in a particular way, as indicated in Fig. 2, namely with two catalytic zones 11 and 12, the first 11 of which is flowed through by the liquid feed (and a quantity of hydrogen supplied via pipe 5 which is less than the stoichiometrically necessary to convert all the diolefins into monoolefins) which flows in via pipe 15; in this first zone the sweetening also takes place; the second zone 12 receives the liquid feed flowing from the first zone (as well as the rest of the hydrogen, i.e. an amount of hydrogen sufficient to convert the remaining diolefins into monoolefins and to isomerize at least a portion of the primary and secondary olefins into tertiary olefins), for example injected via a lateral line 13 and distributed by means of a suitable diffusion device 14.

The proportion of the first zone (by volume) is a maximum of 50% of the sum of the two zones, preferably 15 to 30%.

Since the hydrogenation unit can also operate at pressures lower than those present in the debutanization device and the depentanization device (or the dehexanization device) does not require a pressure of 13 to 15 bar, the circulation of the feed through the installation is obtained by a slight reduction in pressure at the outlet of the hydrogenation unit. This allows it to function without additional pumps.

The high temperature at the bottom of the debutanizer and the high activity of the catalysts used according to the invention allow the unit for selective hydrogenation of the C5+ fraction of catalytic cracking to operate without a preheating furnace for the feed and to be very compact. This results in a unit that has low investment costs and great flexibility, which gives it unique advantages.

Compared to the classic hydrogenation units, the new process according to the invention for the selective hydrogenation of catalytic cracked gasoline does not require feed pumps or recycle pumps, nor a preheating furnace. Compared to a selective hydrogenation unit in a distillation column, the new process provides much greater flexibility by isolating the catalyst from the distillation column, which allows the catalyst to be replaced without having to stop the downstream units. It also allows the catalyst to provide greater activity thanks to temperatures and pressures higher than those achieved in the depentanization device.

Furthermore, the new process allows the C5, C6 and C7 210°C fractions to be treated simultaneously. This then allows pretreatment before etherification of the C5 and C6 fractions thanks to a flexible and inexpensive selective hydrogenation. The process allows the maximum of the C5 and C6 ether precursors to be obtained and also reduces the amount of antioxidants used in the converted gasoline.

According to the invention, not only the selective hydrogenation of the diolefins is realized, but also the isomerization of the primary and secondary olefins to tertiary olefins (for example 3-methylbutene-1 into 2-methylbutene-2 and 2-methylbutene-1).

The process therefore makes it possible to increase the quantity of etherifiable products and, consequently, the production output of ethers such as t-amyl ether. It also improves the quality of the petrol produced, which has improved stability to oxidation as well as an improved octane number.

The following example illustrates the invention.

100 cm3 of the LD265 catalyst from Procatalyse-GmbH with 0.3 wt.% palladium on an alumina support is placed in a 1.9 cm diameter stainless steel tube. This catalyst is constantly used for the hydrogenation of the C3 and C4 fractions of FCC and for steam cracking.

The catalyst is reduced by reduction under hydrogen at a flow rate of 30 l/h for 5 hours at 200ºC. The installation is cooled to 150ºC under nitrogen before injecting FCC gasoline with the characteristics shown in Table 1 below. The reactor is then pressurized to 14 bar and the gasoline is injected at the bottom of the reactor (100 cm³/h, i.e. a VVH of 1 h⁻¹). A quantity of hydrogen corresponding to a H₂/diolefins molar ratio of 1.6 is injected. The feed/hydrogen mixture flows through the catalytic bed in an ascending stream. The results obtained according to the invention are shown in Table 2.

The effect of space velocity is determined by increasing the value of VVH from 5 h⁻¹ (Example 2) to 10 h⁻¹ (Example 3) while maintaining the molar ratio of H₂/diolefins at 1.6.

The effect of reducing the molar ratio of H₂/diolefins is shown in Example 4, where this ratio is 1.4 and the value for liquid VVH is 10 h⁻¹.

Another series of catalytic tests was carried out to illustrate the invention. The catalytic zone is divided into two separate beds, the first containing 25 cm3 and the second 75 cm3 of LD265. The procedure is as described above in Example 4, except that the amount of hydrogen injected into the reactor together with the feed represents a molar ratio of 0.9. An injection device between the two beds allows the addition of a supplementary amount of hydrogen corresponding to a molar ratio of 0.5 with respect to the amount of diolefins initially present in the crude FCC gasoline, Example 5.

It can be seen that the staged addition of hydrogen under the conditions established in the invention not only improves the conversion of diolefins (as shown by an increase in the induction period), but also increases the amount of TAME precursors (i.e. 2-methylbutene-1 and 2-methylbutene-2) and - at least partially - sweetens the FCC gasoline.

Table 1. Fraction of FCC naphtha

Starting point 20ºC

End point 166ºC

S (total) 224 ppm

S (mercaptan) 72 ppm

Bromine index 67

MAV20

Paraffins 29.9 wt.%

Olefins, diolefins, cycloolefins 38.4% by weight

Naphthenes 9.1 wt.%

Aromatics 22.6 wt.%

C�5 fraction, total 29.5 wt.%

C�5 fraction, unsaturated 15.5 wt.%

C�5 fraction, etherifiable products 6.2 wt.%

C₆ fraction, total 22.3 wt.%

C₆ fraction, unsaturated 9.8 wt.%

C₆ fraction, etherifiable products 3.6 wt.% Table 2. Examples according to the invention

Claims (14)

1. Process for treating catalytic cracked gasoline by selective hydrogenation, which has a diolefin content of less than 5%, a cyclopentadiene content of less than 1% and a mercaptan content of between 1 and 300 ppm, characterized in that the gasoline fraction C5-210°C is contacted under a pressure of 4-25 bar at a temperature of 80-200°C at an LHSV (liquid hourly space velocity) of 1-10 h⁻¹ with a catalyst comprising 0.1 to 1% by weight of palladium deposited on a support containing at least 50% of alumina, such that the mercaptan content of the gasoline is reduced. 2. Process according to claim 1, characterized in that the catalyst contains 0.2-0.5 wt.% palladium. 3. Process according to one of the preceding claims, characterized in that the carrier contains at least 90% aluminum oxide. 4. Process according to one of the preceding claims, characterized in that the carrier is pure aluminum oxide. 5. Process according to one of the preceding claims, characterized in that the catalyst essentially comprises palladium. 6. Process according to one of the preceding claims, characterized in that the temperature is between 130 and 200°C. 7. Process according to one of the preceding claims, characterized in that the temperature is between 150 and 170°C. 8. Method according to one of the preceding claims, characterized in that the pressure is between 10 and 20 bar. 9. Method according to one of the preceding claims, characterized in that the pressure is between 12 and 16 bar. 10. Process according to one of the preceding claims, characterized in that the LHSV is between 4 and 10 h⊃min;¹. 11. Method according to one of the preceding claims, characterized in that the end point of the cut is at 180°C. 12. Method according to one of the preceding claims, characterized in that the end point of the cut is at 160°C. 13. Installation for treating a catalytic cracked gasoline by its selective hydrogenation, comprising a catalytic cracking unit followed by a fractionation unit for separating a C3-210°C cut, with a debutanizer, then a separation unit selected from a depentanizer and a dehexanizer, characterized in that a selective hydrogenation unit is arranged between the debutanizer and the separation unit. 14. Installation according to claim 13, characterized in that the selective hydrogenation unit is formed by a reactor comprising two catalytic zones, the first being penetrated by the liquid charge and a quantity of hydrogen less than the stoichiometry, and the second receives the liquid charge coming from the first zone and the rest of the hydrogen injected through a lateral pipeline.