FLUIDIZED-BED CATALYTIC CRACKING PROCESS The present invention relates to a fluidized-bed catalytic cracking process for converting a hydrocarbon feed into lighter products. This is a known process, which comprises the steps of (a) contacting the hydrocarbon feed in a reactor riser with hot regenerated catalyst particles passing through the reactor riser as a fluidized bed to obtain a gaseous effluent stream with spent catalyst particles dispersed therein; (b) separating the spent catalyst particles from the gaseous effluent stream; (c) passing the spent catalyst particles to a catalyst bed in a vertical stripping zone, in which the spent catalyst particles are stripped by contacting the catalyst particles with upwardly flowing stripping gas; (d) discharging the stripped spent catalyst particles to a regenerator, in which coke deposited on the catalyst particles is combusted to obtain hot regenerated catalyst particles and returning the hot regenerated catalyst particles to the reactor riser for use in step (a).

The present invention relates more in particular to improving the efficiency of stripping the spent catalyst particles to better remove adsorbed hydrocarbons from the particles.

USA patent specification No. 5 284 575 discloses a fluidized-bed catalytic cracking process, wherein spent catalyst particles are stripped in a vertical fast fluidized bed zone, in which the superficial stripping-

gas velocity is so high that fast fluidized bed conditions are ensured. To do so the superficial gas velocity is greater than about 1 m/s and suitably between about 1.5 and 3 m/s. As a consequence the catalyst particles will flow upwards, concurrently with the upwardly flowing stripping gas. To allow discharging stripped catalyst particles, the particles pass over the top of the stripping zone into a catalyst transport zone located around the stripping zone.

Stripping can as well be carried out at lower superficial steam velocities. An example of such stripping is given in USA patent specification No. 5 380 426. This publication discloses a fluidized- bed catalytic cracking process, wherein spent catalyst particles are stripped in a vertical stripping zone/In the stripping zone the catalyst particles flow downwardly opposite to stripping steam. Stripped catalyst particles are removed from lower end of the stripping zone. Part of the removed catalyst particles is recycled to the upper end of the vertical stripping zone through a slender transport riser, and the remainder is supplied to a regenerator. The superficial steam velocity in the vertical stripping zone is in the range of about 0.03 to 1 m/s.

Applicant has now found that stripping at superficial steam velocities in the range of from 0.3 to 1 m/s can be improved by changing the design of the vertical stripping zone so that the height of the vertical stripping zone is sufficiently large compared to the diameter of the stripping zone. In this way, stripping can be done so efficient that recycling is not required.

To this end in the fluidized-bed catalytic cracking process according to the present invention the super- ficial stripping-gas velocity in the vertical stripping zone is between 0.3 and 1.0 m/s, and the ratio of the height of the catalyst bed in the vertical stripping zone to the diameter of the vertical stripping zone is greater than 3.

Wherein the superficial stripping-gas velocity is determined under the conditions prevailing in the vertical stripping zone.

In the specification, the term aspect ratio'will be used to refer to the ratio of the height of the catalyst bed in the vertical stripping zone to the diameter of the vertical stripping zone.

The combination of a relatively large aspect ratio (which gives good staging) and a superficial gas velocity in the range of from 0.3 to 1.0 m/s (which gives a good mass transfer) provides an improved stripping efficiency.

Suitably the conditions are so selected that the superficial steam velocity in the vertical stripping zone is between 0.30 and 0.95 m/s, and more suitably between 0.35 and 0.60 m/s, and suitably the aspect ratio is less than 25 and more suitable less than 10. A very suitable combination is a superficial steam velocity of between 0.35 and 0.60 m/s and an aspect ratio of between 4 and 8.

The invention will now be described by way of example in more detail with reference to the accompanying drawings, wherein Figure 1 shows schematically a cross-section of the lower end of a reactor vessel including a vertical stripping zone according to the present invention;

Figure 2 shows a diagram with curves of constant coke to regenerator in the aspect ratio-superficial steam velocity plane; and Figure 3 shows a diagram with curves of constant coke to regenerator in the steam to catalyst ratio- superficial steam velocity plane.

Reference is now made to Figure 1 which shows a cross section of the lower end of a reactor vessel 1 which is used in the process of the present invention.

A reactor riser 2 passes through the lower end of the reactor vessel 1. The reactor riser 2 extends to into the vapour space (not shown) of the reactor vessel 1, in which vapour space separators (not shown) are arranged for separating during normal operation catalyst particles from reaction effluent. The separated catalyst particles are passed to the lower end of the reactor vessel 1 through a dipleg 3 of a separator (not shown).

To remove catalyst particles from the reactor vessel 1 there is provided a catalyst discharge con- duit 4. To strip the catalyst particles, the lower end of the reactor vessel is provided with a stripping-gas distributor 5. The annular space between the inner surface of the side wall of the lower end of the reactor vessel 1 and the outer surface of the reactor riser 2 defines a catalyst bed zone 8.

During normal operation, a hydrocarbon feed is contacted in the reactor riser 2 with hot regenerated catalyst particles passing through the reactor riser 2 as a fluidized bed to obtain a gaseous effluent stream with spent catalyst particles dispersed therein. Spent catalyst particles contain coke deposited on them and hydrocarbons still adhered to the particles. In the

separator (s), the spent catalyst particles are separated from the gaseous effluent stream, and through dipleg 3, the spent catalyst particles are passed to a catalyst bed in the catalyst bed zone 8, in which the spent catalyst particles are stripped by contacting the catalyst particles with upwardly flowing striping-gas in the form of steam supplied through the stripping-gas distributor 5.

After being stripped, that is after adhered hydrocarbons have been removed, the stripped spent catalyst particles is discharged via the catalyst discharge conduit 4 to a regenerator (not shown), in which coke on the catalyst particles is combusted to obtain hot regenerated catalyst particles, which are returned to the reactor riser 2 so that they are used again.

Attention is now drawn more closely to the catalyst bed zone 8. The catalyst bed present in this zone 8 extends from the bottom 9 of the reactor vessel 1 to an upper level which is indicated by means of a dashed line referred to with reference numeral 10. In the catalyst-bed zone, three zones can be defined from bottom to top, (a) a compacting zone 12, extending from the bottom 9 to the stripping-gas distributor 5; (b) a vertical stripping zone 13, extending from the stripping-gas distributor 5 to the beginning of the widening section 14 of the reactor vessel 1 ; and (c) a deceleration zone 15, extending from the beginning of the widening section 14 to the upper level 10 of the catalyst bed. The compacting zone 12 should be so designed as to reduce the steam entrainment to the regenerator (not shown), and the presence of the deceleration zone 15 reduces the catalyst entrainment

with the upwardly flowing stripping gas. In the absence of a widening section, the vertical stripping zone extends to the upper level of the catalyst bed.

The aspect ratio is the height of the catalyst bed in the vertical stripping zone 13 divided by its diameter, wherein the diameter is defined as 4 (A*4/z), A being the cross-sectional area of the vertical stripping zone 13. The latter definition is chosen to take into account the situation as shown in the drawing wherein the vertical stripping zone 13 has an annular cross section. The height of the catalyst bed in the vertical stripping zone 13 equals in most cases to the height of the vertical stripping zone 13.

Applicant has found that when the conditions are so selected, that the superficial steam velocity in the vertical stripping zone 13 is between 0.3 and 1.0 m/s, and that the ratio of the height of the catalyst bed in the vertical stripping zone 13 to the diameter of the vertical stripping zone 13 is greater than 3, the catalyst is stripped in a counter-current way and stripping is done more efficiently. Suitably the conditions are so selected that the superficial steam velocity in the vertical stripping zone is between 0.30 and 0.95 m/s, and more suitably between 0.35 and 0.60 m/s, and suitably the aspect ratio is less than 25 and more suitable less than 10. A very suitable combination is a superficial steam velocity of between 0.35 and 0.60 m/s and an aspect ratio of between 4 and 8.

To predict the performance of stripping according to the invention, model calculations have been performed, and Table 1 gives the obtained results.

In Table 1 the area is the cross-sectional area of the vertical stripping zone, the height is the height of the vertical stripping zone, the aspect ratio is height over diameter of the vertical stripping zone, the superficial steam velocity is the steam velocity at conditions prevailing in the vertical stripping zone, the solids mass flow rate is the superficial downward mass flow rate of the catalyst particles in the stripping zone, the mass transfer rate is the product of mass transfer coefficient (in m/s) and transfer area (in m2) divided by the bed volume (in m3), the number of axial mixing stages is used to define the hydrodynamic regime of the stripping zone (one stage corresponds to an ideal mixer and an infinite number of stages corresponds to plug flow), and the coke to regenerator is the change in amount of coke on the catalyst particles after stripping (in % mass of feedstock, m. o. f.) of the process of the invention as compared to the base case.

Table 1. The results of model calculations. Base case Invention Area (in m2) 20.91 10.46 Height (in m) 20 23.2 Aspect ratio 3.9 6.4 Superficial steam velocity 0.25 0.5 (inm/s) Solids mass flow rate 33 66 (inkg/m2/s) Bed inventory (in 1 000 kg) base base-140 Catalyst residence time (in s) 400 200 Mass transfer rate base 1.7*base Number of axial mixing stages base 1. 9*base Bed pressure build-up base 1.0*base coke to regenerator base base-0.94 (in % m. o. f.)

From Table 1 it can be concluded that with the stripping of the present invention not only the catalyst inventory in the stripping zone is significantly reduced, but in addition the stripping efficiency is improved as well. These results are obtained for a similar pressure build-up.

Further model calculations were carried out to investigate the effects of changes in the aspect ratio on the superficial steam velocity and the amount of steam required to maintain the coke to regenerator at a constant level. The model calculations were made for a vertical stripping zone wherein, during normal

operations the bed pressure build-up was maintained at a level equal to that of the base case. To this end the model calculations were made for a vertical stripping zone having a cross-sectional area in the range of from 3.1 to 20.9 m2 and a solids mass flow rate in the range of from 33 to 222 kg/m2/s. Figure 2 shows a diagram of superficial steam velocity (Vg in m/s) as a function of aspect ratio (R). Line 25 represents the superficial steam velocity Vg required to maintain the coke to regenerator at base-0.9, and line 27 represents the superficial steam velocity Vg required to maintain the coke to regenerator at base-0.6. Figure 2 clearly shows that for an aspect ratio in between 4 and 8 the required superficial steam velocity is substantially constant.

A further advantage of stripping in a stripping zone with an aspect ratio in that range is that stripping is done more efficiently, that is to say, less steam is required. This is demonstrated with in Figure 3, which shows a diagram of steam to catalyst mass ratio (SCR) as a function of aspect ratio (R).

Line 30 represents the steam to catalyst mass ratio SCR required to maintain the coke to regenerator at base-0.9, and line 31 represents the steam to catalyst mass ratio SCR required to maintain the coke to regenerator at base-0.6. Figure 3 clearly shows that for an aspect ratio in between 4 and 8 the required steam to catalyst mass ratio SCR is decreasing substantially.

Suitably a gas distributor in the form of a fluffing ring 18 is provided in the compacting zone 12 to give a modest amount of fluidizing steam in order to improve passing the catalyst particles into the

catalyst discharge conduit 4. Moreover the upper end of the vertical stripping zone 13 can contain a gas distributor 19 for pre-stripping.

To reduce entrainment, the superficial gas velocity at the upper level 10 of the catalyst bed is about half the superficial gas velocity in the stripping zone.