Thermal regulation process for a solid in a heat exchanger using cylindrical tube surfaces

The invention concerns a process for thermal regulation in a continuous fluidized bed treatment process for a powdered solid, wherein the solid is treated in a fluidized bed treatment zone, at least a portion of the solid is extracted from said zone and transported to an external heat exchanger (21) containing at least one array (22) of thermal exchange tubes in which vaporizable cooling fluid circulates, fluidized or mobile bed thermal regulation by indirect heat exchange with the fluid is carried out and the portion of regulated solid is extracted for recycling into the treatment zone or to another treatment zone (1). More precisely, said portion of solid is circulated in descending mode by means of an inert or non inert fluidization fluid across the array of tubes (22) which are wound such that the current of solid intersects said tubes and that the cooling fluid is circulated in one direction in the array of tubes.

FIELD OF THE INVENTION 
The invention concerns the use of an array of specially wound tubes in a 
fluidised or mobile bed heat exchanger. It generally concerns a process 
for thermal regulation or control in a continuous fluidised or mobile bed 
treatment process for powdered solid. The invention particularly concerns 
a process for regeneration of a used catalyst by heat exchange in a 
fluidised bed. The process is particularly applicable to the regeneration 
of catalysts which are particularly heavily loaded with hydrocarbon 
residues and coke following reaction with a hydrocarbon feedstock. It can 
apply to catalysts for hydrotreatment, hydrocracking or catalytic 
cracking, reforming catalysts or to any contact mass used, for example, in 
thermal cracking processes. 
By way of illustration, we shall describe the use of an array of tubes in a 
fluidised bed process for regeneration of a used catalyst from a catalytic 
cracking process using heavy feedstocks having a high Conradson carbon 
residue such as atmospheric pressure residue, vacuum residue, or 
deasphalted residue, these residues being capable of hydrotreatment. 
The process is particularly applicable to temperature control. 
BACKGROUND OF THE INVENTION 
Catalytic cracking processes convert hydrocarbon feedstocks into lighter 
products such as petrol. The feedstocks were initially fairly light, for 
example gas oils, and in order to achieve maximum conversion efficiency 
using highly active zeolite catalysts, it was necessary to remove the 
maximum amount of coke which deposited on these catalysts and reduced 
their activity. This was carried out in a regeneration step at a 
temperature of between 520.degree. C. and 800.degree. C. 
Increasing demand for fuels led refiners to use heavier and heavier 
feedstocks, containing high boiling point hydrocarbons, with boiling 
points of more than 550.degree. C., for example, and a high Conradson 
carbon residue or high concentration of metals. Large quantities of coke 
and heavy hydrocarbons can become deposited on the catalyst during the 
catalytic cracking phase. Combustion regeneration releases a large amount 
of heat which can cause the apparatus to deteriorate and deactivate the 
catalyst, particularly when exposed to temperatures of greater than 
800.degree. C. for long periods. It is therefore essential to control 
catalyst regeneration. This problem is particularly prominent when 
existing technology essentially designed for the treatment of conventional 
feedstocks is employed in a process using much heavier feedstocks. 
The following patent documents illustrate the state of the art: EP-A-0 092 
065, U.S. Pat. No. 2,963,422, EP-A-0 197 486, EP-A-0 153 214, EP-A-192 906 
and EP-A-0 093 063. In addition, European patent EP-A-0 403 381 describes 
a double chamber heat exchanger connected to a catalyst regenerator by a 
single entrance and exit opening, which limits the cooling efficiency of 
the exchanger. 
The prior art is further illustrated by U.S. Pat. No. 4,434,245 which 
describes thermal exchange of a catalyst which has been removed from a 
release zone located above a combustion zone and comprising vertical 
bayonet tubes. The drawback here is that a limiting layer of defluidised 
catalyst located at the tube walls flows along the tube and reduces 
thermal exchange. 
Complex and cumbersome technology is usually required to overcome this 
problem, thereby reducing the reliability of the system. 
A horizontal tubular plate positioned in the thermal exchanger and 
supporting the tubes will maintain the mechanical integrity of the 
assembly. However, widely different temperature and pressure conditions in 
different parts of the plate, for example 730.degree. C., 3 bars on the 
catalyst side and 275.degree. C., 60 bars on the coolant water side, 
create intense stresses which affect the mechanical integrity of the 
assembly. 
SUMMARY OF THE INVENTION 
One object of the invention is to overcome the problems associated with the 
prior art and obtain maximal thermal exchange in the most homogeneous 
fashion. 
The invention thus concerns a process for thermal regulation or control in 
a continuous fluidised or mobile bed treatment process for a powdered 
solid, wherein the solid is treated in a fluidised or mobile bed treatment 
zone, at least a portion of the solid is extracted from said zone and 
transported to an external thermal regulation or control zone which is 
advantageously elongate and has an axis of symmetry and which contains at 
least one array of thermal exchange tubes in which vaporisable cooling 
fluid circulates, fluidised or mobile bed thermal regulation by indirect 
heat exchange with the fluid is carried out and the regulated portion of 
solid is extracted for recycling into said treatment zone or to another 
treatment zone. More precisely, said portion of solid is circulated in 
descending mode by means of an inert or non inert fluidisation fluid 
across the array of tubes which are wound or disposed such that the 
current of solid intersects said tubes, preferably across substantially 
the entire cross section of the regulation zone, and such that the cooling 
fluid is circulated in one direction in the array of tubes. 
In a first preferred embodiment of the process, the thermal exchange array 
comprises tubes wound into a helix with a plurality of diameters with 
respect to a winding axis which is substantially parallel to the axis of 
symmetry of the regulation zone, such that tubes having the same winding 
diameter form a cylindrical layer or surface and that the different layers 
or surfaces thus formed are disposed one inside the other, preferably 
substantially concentrically. 
This configuration is described in French patent FR-A-2 124 043 as applied 
to steam generators which are reheated with liquid sodium. Within the 
context of the invention, this configuration produces maximum exchange in 
the fluidised bed since substantially the entire volume of the cylindrical 
thermal exchanger containing the dense phase solid is cooled by the array 
of tubes. 
In a second embodiment, the thermal regulation zone comprises a plurality 
of tubular arrays comprising a first series of substantially parallel tube 
sections whose axes are located in a first plane and a second series of 
substantially parallel tube sections whose axes are located in a second 
plane which is substantially parallel to the first plane, the sections in 
one plane being at a substantially equal and opposite inclination to that 
of the sections in the other plane with respect to the longitudinal plane 
containing said axis of symmetry, the tube sections with adjacent ends on 
one side being connected together by bent sections, the assembly of 
tubular arrays being positioned substantially parallel to the plane 
containing the axis of symmetry of the thermal regulation zone. 
Such a configuration is described in French patent FR-A 2 015 263. 
In a third embodiment, the thermal regulation zone comprises a plurality of 
tube arrays wherein each tube comprises tube sections which are 
substantially parallel to each other and located in the same plane, the 
ends adjacent to one side of two successive sections being connected 
together by bent sections, the assembly of tube arrays being positioned 
substantially parallel to the plane containing the axis of symmetry of the 
thermal regulation zone. 
These tube arrays may advantageously be used in a process for continuous 
fluidised bed regeneration of a used catalyst by combustion of the coke 
deposited on the catalyst during the course of a hydrocarbon conversion 
reaction in a reaction zone. The process thus comprises at least one 
regeneration zone into which said catalyst is introduced from said 
reaction zone, the catalyst is normally regenerated in a dense fluidised 
bed zone in the presence of a gas containing oxygen under regeneration 
conditions, at least a portion of the catalyst is extracted from the dense 
fluidised bed and transported to the thermal regulation or exchange zone, 
said portion of catalyst is cooled by indirect heat exchange with the 
cooling fluid, and the cooled portion of catalyst is reintroduced into the 
dense fluidised bed in the regeneration zone. 
In accordance with one mode of operation, the cooled catalyst in the lower 
portion of the thermal exchange zone is recycled to the dense bed of the 
generator from which it was extracted by means of co-current injection of 
fluidisation gas containing oxygen. The cooled catalyst is thus circulated 
to the regenerator in broadly ascending mode using recycling means 
comprising a catalyst evacuation conduit controlled by a valve connected 
to a Y or J junction, for example, which is itself connected to a catalyst 
gas lift which has fluidisation air injected at its base. 
A further mode of operation employing an array is used in a continuous 
fluidised bed regeneration process for a used catalyst by combustion of 
coke deposited thereon, comprising two regeneration zones. A first 
catalyst regeneration step is carried out in a first regeneration zone, 
the at least partially regenerated catalyst is transported to a second 
regeneration zone located above the first zone, a second regeneration step 
is carried out and at least a portion of the catalyst from the second 
regeneration zone is cooled under the conditions described above and the 
cooled catalyst is extracted for transport to the first regeneration zone 
or return to the second regeneration zone. 
If the catalyst is recycled to the first regeneration zone, it can be 
gravity fed (descending mode). 
When the catalyst is recycled to the first regeneration zone, it can be 
recycled in generally ascending mode using the recycling means described 
above comprising the gas lift. 
When recycling to the second regeneration zone, the catalyst can be 
recycled using the recycling means described above comprising the gas 
lift. 
In a further mode of operating an apparatus comprising two regeneration 
zones, the first catalyst regeneration step is carried out in the first 
regeneration zone, at least a portion of the catalyst from the first 
regeneration zone is cooled in a thermal exchanger in accordance with the 
invention and the cooled catalyst is recycled to the first regeneration 
zone using the recycling means described above comprising the gas lift. 
The catalyst is then transported from the first regeneration zone to the 
second which is located above the first zone. 
Whatever the winding of the tube array described above, the interaxial 
distance between the tubes defining the helical pitch is between 1.5 and 
10 times their diameter, preferably between 2 and 3 times their diameter. 
This distance can be that measured between tubes located in the same 
cylindrical surface or between two neighbouring cylindrical surfaces (in 
the case of the first embodiment). This distance may also be that measured 
for tubes positioned in accordance with the second and third embodiments. 
Good thermal exchange is obtained with a fluidisation gas flow rate of 
generally 0.01 m/s to 0.75 m/s, preferably 0.05 m/s to 0.3 m/s in the 
thermal exchange zone using a tube array wherein the cooling fluid and the 
vapour generated preferably circulates from the bottom to the top at a 
flow rate normally between 0.5 and 2.5 m/s, for example, preferably 1 to 2 
m/s, ie counter-current to the flow of catalyst in the thermal exchange 
zone. 
The catalyst current intersecting the tubes is permanently renewed at the 
tube surface within its descending current. 
High compactness (for example 13 to 16 m.sup.2 /m.sup.3 of exchanger) and 
ease of installation or removal are particular advantages. 
Further, the tube array can advantageously absorb radial and axial 
expansions. 
Finally, because of the high exchange efficiency, the system requires an 
installed surface area per unit volume which is less than that of the 
prior art. This has the advantage of leaving a greater volume for the 
circulating catalyst, thus retaining good fluidisation without the need 
for auxiliary means. 
The cooling fluid circulating in the exchanger may be air, water, water 
vapour or mixtures of these fluids. 
The catalyst which has been regenerated in accordance with the invention is 
also of conventional type, such as zeolite or amorphous type 
aluminosilicates, advantageously with a granulometry of 30 to 100 
micrometres.

DETAILED DESCRIPTION 
A first regenerator 1 in a catalytic cracking unit receives catalyst on 
which coke has been deposited during the course of the catalytic cracking 
reaction from a stripper (not shown) via line 2. This line opens into the 
catalytic bed at an appropriate point, preferably into the diluted phase 
located above dense fluidised bed 3. Regeneration gas containing oxygen is 
fed via line 4 to fluidisation means 5, for example a screen, ring or 
distribution line, situated at the base of the regenerator to fluidise the 
dense catalyst bed and effect continuous counter-current combustion of 
about 50% to 90% of the coke. Regeneration fumes and entrained catalyst 
are separated in cyclones 6 and the regeneration fumes containing the 
major combustion products carbon monoxide, carbon dioxide and water vapour 
are evacuated via line 7 to an incinerator. 
The temperature of fluidised bed 3 is measured by means of sensor 8. When 
this temperature falls below a set value T.sub.1, due to introduction of 
the relatively cold catalyst via lines 34 as will be described below, the 
flow of oxidising fluid (fluidisation fluid) to fluidisation means 5, 
regulated by control valve 33 on line 4, is increased until the 
temperature measured at 8 returns to the preset value. 
The partially regenerated catalyst particles are then transferred to a 
second regenerator 9 located above the first regenerator 1 via conduit 10 
which is supplied with air via line 11. Diffuser 12, supplied with air via 
line 13, is located at the base of the second regenerator. Combustion of 
the partially regenerated catalyst is carried out in dense bed 19 whose 
upper portion defines level 19a at a height which depends on the level of 
aeration. 
A portion of the regenerated catalyst is laterally evacuated into buffer 
chamber 14. Particle fluidisation in this chamber is normally controlled 
by annular diffuser 15 which is fed with fluidisation gas such as air or 
an inert gas via line 16. Regenerated catalyst particles are recycled from 
chamber 14 via conduit 35 fed by a riser (not shown) in a quantity which 
is determined by the opening or closing of a valve. The combustion gases 
in the upper portion of the second regenerator are separated from the 
catalyst particles by external cyclones 17 and evacuated via line 18 which 
is separate from fume evacuation line 7 from the first regeneration step. 
A portion of the hot catalyst and a portion of the fumes at a temperature 
of 600.degree. to 850.degree. C. are removed from dense bed 19 of the 
second regenerator at a point located above air injection means 12 and 
gravity fed via downwardly inclined conduit 20, for example at an angle of 
30 to 60 degrees with respect to the axis of the exchanger, into heat 
exchanger 21 adapted for indirect heat exchange. The exchanger is 
vertical, of elongate cylindrical form, and contains a thermal exchange 
array which lines the space containing substantially all the dense 
catalyst bed, within an envelope. The array is a tubular array composed of 
a plurality of tubes 22a, 22b wound in a helix with a plurality of 
diameters with respect to the vertical axis of the exchanger envelope. 
Layers with the same winding diameter form a cylindrical layer or surface 
and the various coaxial cylinders thus defined are located one within the 
other. The chamber which is delimited by the envelope contains the 
catalyst which is maintained as a dense bed through the tube array by 
means of fluidisation means 24 (ring or screen) into which a 
counter-current of air is introduced via line 25. The catalyst circulates 
from top to bottom through the array, intersecting the tubes across 
substantially the entire cross section of the exchanger and giving up its 
heat to an appropriate fluid such as pressurised water which is fed via 
line 23a. This line feeds the upper extremity of a central cylindrical 
conduit 40 which is thermally insulated and which acts as the winding axis 
for the helically disposed tubes and also acts as a stiffener and 
therefore as a support system. This axial cylindrical conduit, where 
practically no thermal exchange occurs, feeds the various cylindrical 
tubular surfaces at different distances from its lower end such that the 
water-vapour mixture rises counter-current to the catalyst. Thus the 
helixes with the greatest winding diameter are fed by the ends of bent 
tubes connected to the lowest points of the axial conduit while those with 
a smaller diameter are connected higher up. 
The upper part of the tubes of the array have ends which are bent 
vertically to connect to the tube surfaces of the steam chests or toric 
collectors (not shown), resulting in very high mechanical strength. The 
water-vapour mixture is evacuated via line 23b connected to the 
collectors. 
Conduit 20 carrying hot catalyst opens into the exchanger at a junction 
point located below level 19a of the dense bed of the second regenerator, 
for example at a point located at a distance of a quarter to a third of 
the height of the exchanger from upper extremity 26. The catalyst is in a 
dense fluidised bed due to fluidisation means 24 (ring) right across the 
tube array to a level 19b above the junction point and substantially 
identical to level 19a of the dense bed in regenerator 29. Level 19b is in 
general a function of the respective fluidisation flow rates in the second 
regenerator and in the thermal exchanger and thus of the respective 
densities. There may thus be a small difference between the catalyst 
levels in the regenerator and in the exchanger. 
The height of the exchange array and the parameters which determine its 
compactness are generally calculated so that the array occupies 
substantially the entire volume of the dense bed in the exchanger. 
Preferably, the height is equal to the highest level which the dense bed 
in the exchanger can attain, taking into account the differences in 
pressure which may exist in the exchange zone and in the regenerator. 
The height of the exchanger is selected so that, with respect to the level 
in the regenerator, a 1 to 2.5 m high free zone termed the release zone 27 
is created in the exchanger above the dense bed to allow the fluidisation 
gas and any regeneration fumes to be separated from the catalyst. A 
degassing line 28 evacuates the fumes and gases from the dilute phase at 
the upper extremity of the exchanger and carries them to the diluted 
fluidised phase 29 above the dense fluidised bed of the second 
regenerator. The diameter is selected so that the ratio of the diameter of 
the degassing line to that of the catalyst inlet conduit 20 is between 3 
and 6. The gas exit rate is generally between 3 and 15 m/s. 
Extraction and recycling means 34 comprises a conduit in which the catalyst 
flows under gravity and which is the first regenerator. The catalyst is 
transported by a lift recycled into the dense phase of the first 
regenerator, preferably above fluidisation means 5. 
Valve 30, for example a slide gate, is located at the exit to exchanger 21 
below the lower extremity of the first regenerator and upstream of the 
lift. This controls the rate of transfer of catalyst between the 
regenerators once the temperature of the regenerated catalyst exceeds the 
required preset value. 
The FIGURE is shown with a descending catalyst flow at the exit of the 
exchanger and with an ascending flow only in the first regenerator. In a 
further unillustrated embodiment, the catalyst is introduced directly into 
the dense phase of the first regenerator by descending flow. 
The rate of catalyst flow through the thermal exchanger is adjusted to 
maintain the temperature in the second regenerator and thus maintain the 
entry temperature into the reaction zone (riser) at a set temperature 
suitable for the feedstock to be cracked in the unit. 
Thermal control of the regeneration process is effected by a combination of 
the following means: 
Automatic control means 31 is connected to valve 30 located on catalyst 
evacuation conduit 17 from the exchanger. This means is also connected to 
temperature sensor 32 which monitors the local temperature in the dense 
bed of second regenerator 9. When the signal transmitted by the sensor 
indicates a value which exceeds a preselected value which depends on the 
regeneration parameters and is stored in the automatic control means, a 
signal is transmitted by the latter to valve 30 which increases the 
catalyst evacuation flow rate and thus increases the catalyst flow rate 
into the exchanger. This increase in flow rate helps to reduce the 
temperature of the first regeneration step as recorded by temperature 
sensor 8. Means 31 compensates by increasing the oxygen feed by adjusting 
valve 33 on line 4 which feeds the fluidisation means of the first 
regenerator. A larger amount of coke can thus be burned off. 
If, however, the signal transmitted by sensor 32 indicates a value which is 
lower than the set value, valve 30 is partially closed to reduce thermal 
exchange. At the same time, the oxygen consumption in the first 
regenerator is decreased and thus less coke is burned off, boosting the 
catalyst temperature in the second regenerator. The temperature is thus 
maintained at a substantially constant value across a range of desired 
values.