Catalytic cracking process for the treatment of a fraction having a low conradson carbon residue

Abstract

Process for the fluidized-bed catalytic cracking of a weakly coking feedstock having a Conradson carbon residue equal to or less than 0.1% by weight and a hydrogen content equal to or greater than 12.7% by weight, comprising at least a step of cracking the feedstock, a step of separating/stripping the effluents from the coked catalyst particles and a step of regenerating said particles, the process being characterized in that at least one coking, carbonaceous and/or hydrocarbonaceous effluent having a content of aromatic compounds of greater than 50% by weight, comprising more than 20% by weight of polyaromatic compounds, is recycled to homogeneously distributed and weakly coked catalyst, before regeneration, in order to adjust the delta coke of the process.

Claims

1. Plant for implementing a process for fluidized bed catalytic cracking, comprising at least a main reactor and optionally at least a secondary reactor, at least a disengager and a stripper, and a single-stage or multistage regenerator, characterized in that the stripper contains, level with a dense catalyst bed, at least one zone equipped with at least one structured packing element positioned upstream of a device for dispersing a coking fraction with respect to circulation of a stream of catalyst particles, wherein said structured packing element(s) are formed by interlacing plates, strips or fins constituting a screen, said screen occupying less than 10% of the area of flow cross section in a vessel in which it is placed, but covering, in projection on said section, the entire area thereof.

2. Plant according to claim 1, characterized in that the stripper contains at least two zones equipped with at least one structured packing element that are associated with two fluid-dispersing devices, one for dispersing coking fractions, the other for dispersing the stripping fluid, these devices being located downstream of said structured packing elements relative to the stream of catalyst particles.

3. Plant according to claim 2, characterized in that the coking fractions and stripping fluid dispersion devices are chosen from spraying, rod-type injectors, venturi pressurized atomizing injectors, fluidization rings and sparger tubes.

4. Plant according to claim 1, characterized in that the stripper is located in one and the same vessel as the disengager.

5. Plant according to claim 1, characterized in that the stripper is located in a different vessel downstream of the disengager, but still positioned upstream of the regenerator.

6. Plant according to claim 5, characterized in that the disengager and/or the stripper comprises, respectively, at the outlet of said disengager and/or inlet of the catalyst particles into said stripper, at least one structured packing element followed by a device for dispersing stripping fluids for the pre-stripping of the catalyst particles.

7. Plant according to claim 1, characterized in that when the stripper comprises a plurality of structured packing elements for the intercalated recycle and stripping, each structured packing element being associated with a fluid-dispersing device, the volumes occupied for the recycle and the stripping are respectively from 25 to 65% and from 35 to 75% of the volume of stripping zone or vessel.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The invention will now be described with reference to the appended non-limiting drawings in which:

(2) FIG. 1 is a section through a disengager-stripper in a single vessel;

(3) FIG. 2 is a diagram representing a catalytic cracking unit with two cracking reactors, a primary reactor and a secondary reactor, for which the stripping vessel is independent of the vessel for disengaging the coked catalyst/cracking effluents mixture.

(4) FIG. 3 represents, in cross section, two methods of filling stripping vessels with structured packings, each associated with a dispersion device downstream.

(5) FIG. 1 represents one embodiment of the reactor/stripper part of the plant for implementing the invention: it comprises a riser reactor (R1) equipped at its upper end with the single disengager/stripper (1) comprising the disengager part (1a) and the stripper part (1b) in the same vessel. This stripper part is equipped with three packings composed of several structured elements (I.sub.1, I.sub.2 and I.sub.3). The stream of catalyst particles circulating from top to bottom, downstream of each packing have either steam dispersion rings (D.sub.1 and D.sub.2) for stripping the catalyst particles or a hydrocarbonaceous compound recycle injector (2). The pipe (8) connects the disengager/stripper (1) to a regenerator (not shown).

(6) FIG. 2 represents the whole of an FCC unit implementing the process of the invention according to one particular mode. The unit as represented comprises two reactors (R.sub.1, main reactor, and R.sub.2) both fed with a feedstock (C.sub.1, C.sub.2, C.sub.1 being a feedstock according to the present invention). The effluents and the catalysts coked by the feedstocks in the two riser reactors are brought together in one and the same disengager (1). The unit also comprises a separate and independent stripper (5), connected to the disengager (1) via the pipe (7), and to the two-stage regenerator (3 and 4) via the pipe (8). The stripper (5) is equipped with three packings composed of several structured elements (I.sub.1, I.sub.2 and I.sub.3). Within the stripper (5), with the stream of catalyst particles circulating from top to bottom, downstream of each packing either steam dispersion rings for stripping the catalyst particles (D.sub.1 and D.sub.2) or a hydrocarbonaceous compound recycle injector (2) are introduced.

(7) FIG. 3 represents, according to sections A-A and B-B, two methods of filling the stripper with packings composed of a non-identical number of structured elements. In these two sections, the stream of catalyst particles circulates from top to bottom. According to the section A-A, two successive packings (I.sub.2) and (I.sub.3) with injection of a recycle of hydrocarbons (10) and (11) via an injector, and a packing I.sub.1 for stripping the particles by steam dispersion via the dispersion ring D.sub.1. According to the section B-B, the stripper is equipped with three packings composed of several structured elements, two stripping steps corresponding to packings (I.sub.2) and (I.sub.3) and to the dispersion rings (D.sub.1) and (D.sub.2) frame a coking step comprising the recycle of hydrocarbon via the injector (10) and the packing (I.sub.2).

(8) The examples, like the figures described above, aim to describe the invention without limiting the scope thereof.

EXAMPLE 1

(9) The present example shows the advantages of the present invention by comparing the efficiency in terms of product yield when weakly coking feedstocks are cracked in an FCC unit with or without recycle of coking fractions.

(10) A base case may be distinguished in which there is no recycle with a fluid catalytic cracking (FCC) unit having a single riser reactor with a capacity of 40 000 barrels per day, i.e. 240 tonnes per hour, and treating a corresponding hydrotreated VGO feedstock.

(11) The main properties of the feedstock are given in Table 1 below.

(12) TABLE-US-00001 TABLE 1 Feedstock Hydrotreated VGO Density g/cm.sup.3 0.8610 H.sub.2 content wt % 13.5 Sulphur content ppm by weight 330 Nitrogen content ppm by weight 550 CCR (Conradson <0.1 carbon residue) Ni content ppm by weight <2 V content ppm by weight <2

(13) This unit with no recycle of coking fraction into the stripper is carried out under the conditions presented in Table 2.

(14) TABLE-US-00002 TABLE 2 C/O 8.6 Riser outlet temperature, C. 525 delta coke wt % 0.60 Regenerator temperature, C. 671

(15) The regeneration temperature is too low, which may cause afterburning or post-combustion reactions of the coke which is only incompletely burnt off. Indeed, additional combustions may take place in the dilute phase of the bed fluidized in the regenerator, by combustion of the entrained particulate coke following the incomplete combustion in the dense phase thereof. In order to obtain a complete combustion, the optimal regeneration temperature required to prevent such phenomena is generally equal to or greater than 690 C.

(16) The associated yield structure, that is to say the amounts of products obtained by cracking the feedstock, is provided in Table 3.

(17) TABLE-US-00003 TABLE 3 Yield relative to the feedstock wt % Dry gases 1.98 LPG C3/C4 22.81 Petrol C5-220 C. 56.50 LCO (220-360 C.) 9.53 >360 C. 3.98 Coke 5.22

(18) In the second case, a slurry fraction resulting from the unit itself is recycled, as described in FIG. 1, to the dense phase of the stripper (1b), via 4 dispersion devices (2) positioned equidistantly downstream of a first packing (I.sub.2) comprising several structured elements that make it possible to homogenize the stream of descending catalyst particles and to obtain good contact between the latter and the recycled slurry, and thus a thoroughly homogeneous deposit of additional coke on the catalyst. The positioning of the dispersion devices in the stripper downstream of the packing with which they are associated is chosen so that the overall contact time between the slurry and the catalyst particles is 70 seconds for a descent rate of the catalyst particles of 65 kg/m.sup.2/s.

(19) A second packing (I.sub.3) is located in the lower part of the dense phase of the stripper (1a) associated with a device for dispersing a stripping fluid (D.sub.1), here, steam: the dispersion of steam makes it possible to strip the light products loaded with hydrogen atoms resulting from the cracking of the coking fraction. These light hydrocarbons will be recovered and mixed with the effluents from the reactor (R.sub.1) in order to then be distilled and finally upgraded in the refinery. In this way, the coke (Q.sub.r) resulting from the coking of the polycondensed or polyaromatic heavy hydrocarbons that are not very rich in hydrogen, is added to the coke (Q.sub.i) resulting from the cracking of the feedstock in the reactor (R.sub.1) in order to constitute the amount of coke (Qt) needed for the heat balance of the unit, before being sent to the regenerator. As this additional coke is free of an excess of hydrogen due to the stripping after the cracking reaction, the risks of hot spots appearing that are damaging to the catalyst, linked to the combustion of the hydrogen and also an excessive production of steam in the regenerator, will be avoided.

(20) A third packing (I.sub.1) associated with a device for dispersing stripping fluid (D.sub.2), mainly steam, is positioned upstream of the first packing (I.sub.2) in the dense phase of the stripper (1a) in order to carry out a pre-stripping of the catalyst particles before they encounter said coking fraction and thus help to restore a considerable portion of the catalytic activity and therefore of the coking power of said catalyst particles. The positioning of the devices for dispersing steam corresponds to that of the devices for dispersing the coking fraction: the targeted overall contact time between the stripping fluid and the catalyst particles is 70 seconds for a descent rate of the catalyst particles of 65 kg/m.sup.2/s.

(21) Collated in Table 4 below are the yields obtained for the recycling of a slurry to a dense phase of catalyst particles in the stripper when there is: State 1; neither prior stripping (or pre-stripping I.sub.3+D.sub.2), nor packing upstream of the recycle, but a terminal stripping (I.sub.1+D.sub.1) State 2; no pre-stripping (I.sub.3+D.sub.2), a packing (I.sub.2) upstream of the dispersion device (2) for the recycle of slurry and finally a terminal stripping (I.sub.1+D.sub.1). State 3; a pre-stripping (I.sub.3+D.sub.2), followed by the recycle of slurry (I.sub.2+2) and finally a terminal stripping (I.sub.1+D.sub.1).

(22) TABLE-US-00004 TABLE 4 Yield relative to the feedstock State 1 State 2 State 3 Dry gases (wt %) 1.94 2.32 2.44 LPG C3/C4 (wt %) 2.44 3.18 3.66 Petrol C5-220 C. (wt %) 11.87 13.89 14.93 LCO (220-360 C.) (wt %) 29.00 28.62 28.09 Slurry >360 C. (wt %) 39.54 33.13 31.40 Coke (wt %) 15.09 18.86 19.81

(23) In this table it is observed that the introduction of a packing comprising structured elements upstream of the recycle of slurry makes it possible to increase the amount of coke that will be deposited on the catalyst, and that the addition of a step of pre-stripping the catalyst before bringing it into contact with the coking fraction makes it possible to still further increase the cracking and the coking effect of this fraction.

(24) In order to illustrate the contribution of the invention, Table 5 shows, for the unit in question, the gains as regards the amount of coke deposited on the catalyst (Q.sub.t), or else delta coke, and also the corresponding increase in the temperature of the dense phase in the regenerator for a throughput of the coking fraction recycled to the stripper of 6 t/h.

(25) TABLE-US-00005 TABLE 5 State 1 State 2 State 3 Regenerator temperature ( C.) 680 686 691 Delta coke (wt %) 0.63 0.66 0.68 C/O (weight/weight) 8.2 7.9 7.7

(26) Thus, depending on the configuration envisaged for the recycle of the coking fraction to the stripper and therefore on the resulting amount of coke (see Table 4) deposited on the catalyst, the temperature within the regenerator increases from 671 C. for the configuration with no recycle to 691 C. for the State 3 configuration and thus limits the afterburning phenomena linked to temperatures of the dense phase that are too low, typically below 690 C.

EXAMPLE 2

(27) The present example shows the advantage of the present invention for making it possible to equilibrate the heat balance of a catalytic cracking unit with a deficit of coke in the regenerator operating in combustion mode by cracking of a weakly coking feedstock.

(28) In this example, the catalytic cracking unit has a capacity of 340 t/h and treats a highly paraffinic feedstock originating from a hydrocracker. This feedstock has a density of 0.86, a Conradson carbon residue, determined by the ASTM D 482 standard, of less than 0.1% by weight and a content of metals (nickel+vanadium) of less than 0.1 ppm.

(29) In Table 6 below, the first column collates the characteristics of this unit treating said feedstock with no recycle of heavy hydrocarbons into the stripper. By calculating the heat balance of the unit, a very small amount of coke on the catalyst, of 0.4% by weight, is obtained, which results in a very low temperature for the dense phase of the fluidized bed in the regenerator, barely above 640 C. Increasing the injection of air beyond the amount mentioned does not make it possible to increase this temperature beyond this threshold.

(30) To raise the temperature of the catalyst, a heavy hydrocarbon, in this case slurry (350+), the density of which is 1.083 and the Conradson carbon residue of which is greater than 10% by weight, originating from the bottom of the primary fractionating column of the catalytic cracking unit is recycled. This recycling consists in injecting said heavy hydrocarbon into the stripper at the inserts dividing the grains of catalyst coked by the feedstock. The results of the heat balance are given in the second column of Table 6.

(31) It is observed that by recycling 20 t/h of slurry to the stripper, the amount of coke deposited on the catalyst via cracking increases significantly, which then makes it possible to obtain a dense phase temperature which is perfectly satisfactory for ensuring the combustion of the coke on the catalyst via injection of a reasonable amount of air.

(32) It is observed in this case that the hydrogen content of the coke increases slightly due to the adsorption of heavy molecules on the catalyst in the stripper, the H/C (hydrogen/carbon) molecular ratio of which is greater than that of the coke initially deposited on the catalyst following the cracking of the feedstock in the reactor. This increase of hydrogen in the coke is desired here because the combustion of this additional hydrogen helps to increase the temperature of the dense phase of the fluidized bed in the regenerator.

(33) TABLE-US-00006 TABLE 6 Without recycle With Recycle Feedstock throughput t/h 340 340 Heavy HC recycle t/h 0 20 throughput Preheat Temperature C. 250 250 Reaction Temperature C. 518 518 C/O wt % 9.5 6.8 Delta coke (Qt) 0.51 0.72 H in coke wt % 6.94 7.35 Dense Phase C. 642 694 Temperature Injected air throughput t/h 229 247

(34) From Table 6 it is observed that the amount of coke increases on the catalyst (Qt varying from 0.51 to 0.72) and that the excessively low temperature of the unit with no recycle is increased to more than 690 C., this ensuring the re-equilibration of the heat balance of the unit.