The present invention relates to a gas distributor arrangement in a three phase reactor or slurry bubble column reactor (SBCR).

SBCRs are employed to conduct many chemical reactions, particularly reactions in which the reactants are gaseous, the products include liquids, and a solid catalyst is required. In such a reaction, the gaseous reactants are introduced into a slurry of finely divided catalyst in a liquid medium which may contain a liquid reaction product. The gas introduction is achieved using a gas distributor.

A gas distributor for a slurry bubble column should satisfy some important requirements, which include:

• the gas should be rapidly mixed into the slurry;

• the catalyst particles should be well dispersed over a wide range of flow to the reactor;

• stagnant zones, especially at the bottom of the reactor, must be avoided; • penetration of catalyst particles into the gas distributor must be minimised;

• erosion of reactor walls and internals should be avoided;

• the distributor should not induce unacceptable attrition of the catalyst; and

• the pressure drop should be low, for economic reasons.

These requirements are addressed in WO 2005.094979.

It is an object of the present invention to provide a gas distributor arrangement that is simpler in design and function and simpler to operate over a wide range of flow and pressures. According to the invention, there is provided a slurry bubble column reactor comprising a reactor vessel having a gas outlet and a liquid outlet, and a gas distributor arrangement in the region of the bottom of the vessel when in its orientation for use, the gas distributor arrangement comprising an upper gas distributor, a lower gas distributor spaced from the upper distributor to a position nearer the bottom of the vessel and a tube open at both ends and with its longitudinal axis extending generally vertically when the vessel is in its orientation for use, the tube being arranged so that the upper gas distributor is located close to or in the vicinity of the upper open end of the tube, and the lower gas distributor is located at a level close to the lower end of the tube, whereby in use gas from the lower sparger rings rises into the tube, reduces the density of the slurry, and causes the slurry in the tube to rise.

Thus, in use, a difference in density is established between the slurry outside the tube and the slurry inside the tube by means of the gas from the lower distributor entering the tube, and this in turn causes slurry outside the tube to flow downwards thereby sweeping the inside wall of the vessel, and then upwards through the tube as the gas from the lower distributor is entrained.

In this way, any catalyst particles with a tendency to settle at the bottom of the reactor will be kept in circulation by movement of the slurry. In addition, stagnant zones at the bottom of the reactor vessel are avoided, thereby reducing any tendency for catalyst de-activation, due to inadequate supply of reactant gases to the liquid phase.

The system avoids difficulties associated with control of the gas nozzle pressures brought about by having a series of gas manifolds with their respective nozzles at difference heights at the bottom of the reactor, which will then have to overcome different hydrostatic pressures. Independent control of the pressure to the two gas distributors means that gas can be injected through the upper gas distributor at a lower pressure than through the lower gas distributor, if desired, or even through the lower gas distributor alone.

Preferably, the upper gas distributor comprises a manifold arrangement with a plurality of upper nozzles. The manifold arrangement may comprise one or more pipes each having a plurality of upper nozzles. Preferably, the manifold arrangement comprises a plurality of generally circular or part-circular concentric pipes each having a plurality of upper nozzles arranged along its length. Preferably, the concentric pipes are tubular toroids or toroid sections.

Preferably, the upper nozzles all lie in a common generally horizontal plane, with height variations typically < 100mm. Preferably, the diameter of the openings of the upper gas nozzles is equal to or greater than 5 mm. Preferably, the openings of the upper nozzles are arranged so that the principal axis of the jets of gas produced has an inclination of 90° or less below horizontal. Preferably, the upper gas distributor is connected to an upper gas supply arrangement, which gives a supply pressure to each of the nozzles with a variation between nozzle supply pressure of less than 100% of the average nozzle pressure drop.

Preferably, the lower gas distributor comprises a manifold arrangement with a plurality of lower nozzles. Preferably, the manifold arrangement comprises one or more pipes, each having a plurality of lower nozzles. Preferably the manifold arrangement comprises a plurality of generally circular or part- circular concentric pipes each having a plurality of lower nozzles arranged along its length. Preferably, the concentric pipes are tubular toroids or toroid sections. Preferably, the lower nozzles all lie in a common generally horizontal plane, with variations typically < 100mm. Preferably the diameter of the openings of the lower nozzle is equal to or greater than 5mm. Preferably the lower gas distributor is connected to a lower gas supply arrangement, which gives a supply pressure to each of the nozzles with a variation between nozzle supply pressure of less than 100% of the average nozzle pressure drop.

Preferably, the upper and lower gas distributors together cover from 30 to 100% of the cross-sectional area of the reactor , preferably from 40 to 80%. Preferably, the lower gas distributor represents from 2 to 12% of the cross- sectional area of the reactor, preferably from 5 to 8%. Preferably in normal operational use, the lower gas distributor is arranged to supply 4 to 20% of the gas flow to the reactor, preferably from 8 to 15%.

Preferably the tube is cylindrical and arranged generally in the reactor with its longitudinal axis substantially vertical. Preferably, the upper gas distributor is at a position between Im above the top of the tube and 50% of the tube length below the top of the tube. Preferably, the lower gas distributor is between 0.3m beneath and 0.3m above the bottom of the tube. Preferably there is an open area between the reactor bottom and the tube lower end to allow slurry flow from the outside of the tube into the tube. Preferably, the cross-sectional area of this is sufficient so that the average slurry velocity through this gap is slower than 5 m/s to avoid erosion of reactor bottom and attrition of catalyst particles. The velocity should however be >0.5 m/s to avoid settling of catalyst at bottom.

Preferably, the reactor vessel is generally cylindrical but at the bottom, there is a curved portion extending from the main cylindrical wall, a part-conical portion extending from the curved position, and a curved base. Preferably the bottom of the tube is at substantially the position where the part-conical portion joins the curved base.

The invention also extends to a method of conducting a chemical reaction involving gaseous reactants which comprises supplying the gaseous reactants to a reactor as described above, by way of the upper and lower gas distributors, in which method, the reactor contains a volume of slurry comprising a liquid phase and solid catalyst particles, and the catalyst particles are maintained in suspension in the slurry by rising gas bubbles from the gas distributors.

Preferably, the gas flowing through each individual nozzle exerts a dynamic pressure of less than 15 000 kg/m s ² , more preferably less than 10,000 kg/ms ² , for example a pressure in the range 200 to 8,000 kg/ms ² . "The dynamic pressure of the gas" is defined as "The pressure of a fluid resulting from its motion, equal to one half the fluid density times the fluid velocity squared" in Handbook of Chemistry and Physics, i.e. q=0.5rv ² , these are exactly the same units as for kinetic energy, which is defined also in Handbook of Chemistry and Physics "The kinetic energy per unit volume of a fluid parcel is thus 0.5rv ² , where r is the density and v is the speed of the parcel". Based on the definition, kinetic energy will have the units (kg/m ³ )*(m ² /s ² ) = kg/m s ² .

The method may be applied to a Fischer-Tropsch synthesis reaction, for example, one in which the reaction temperature is in the range 150 to 300 ⁰ C, the reaction temperature is in the range 200 to 26O ⁰ C, and the reaction pressure is in the range 1 to 100 bar. Preferably, the temperature is in the range 210 to 25O ⁰ C and the pressure is preferably in the range 10 to 50 bar. The invention also extends to methods of operating the reactors according to the invention to carry out reactions, to the products of those reactions and to further methods comprising various post-processing operations and to the products of such further methods.

The invention may be carried into practice in various ways, and one embodiment will now be described by way of example, with reference to the accompanying drawings in which:

Figure 1 is a schematic section of the bottom of a reactor vessel according to the invention;

Figure 2 is a top plan view of a lower gas distributor;

Figure 3 is an amplified section through a part of the lower gas distributor;

Figure 4 is a top plan view of part of the upper gas distributor;

Figure 5 is a section through a simple nozzle; and

Figure 6 is a section through an alternative form of nozzle.

Referring to figure 1, a slurry bubble column reactor comprises a vessel 11, an upper gas distributor or sparger 12, a lower gas distributor or sparger 13 and a tube 14.

The vessel 11 is generally cylindrical and has a gas outlet at the top (not shown) and a liquid product outlet (not shown). At the bottom, the vessel 11 has a curved portion 15, a part conical section 16 and a curved base 17. The tube 14 is a cylinder, open at the top 18 and bottom 19. The upper sparger 12 will be described in greater detail below with reference to Figure 4 but includes a series of concentric tubular gas distribution rings 21 which surround the tube 14 and are located at a level just below and close to the open top 18 and just at the position where the curved portion 15 of the vessel meets the part-conical section 16. The lower sparger 13 will be described in greater detail below with reference to figures 2 and 3, but includes a series of concentric tubular gas distribution rings 22 which are located within the circumference of the tube 14 and at a level close to but beneath the open bottom 19.

Turning now to Figures 2 and 3, the lower sparger 13 includes a main gas feed pipe 23, three equispaced radially extending manifolds 24 connected to the feed pipe 23 and beneath the manifolds 24, the rings 22. The rings 22 are connected to the manifolds 24 by means of respective gas supply tubes 25. It will be appreciated that while three rings 22 are shown, there could be fewer or more, depending on various factors, such as the dimensions of the reactor and the rings themselves.

The upper sparger 12 is shown in Figure 4. It consists of a main gas feed ring outside the reactor (not shown), three radically extending manifolds 26 connected to the main gas feed ring through the vessel wall 11, and beneath the manifolds 26, the rings 21. The rings 21 are connected to the manifolds 26 by means of respective gas supply tubes (not shown) in a similar fashion to the lower sparger 13. It will be appreciated that while seven rings 21 are shown in Figure 4, there could be fewer or more, depending on various factors, such as the dimensions of the reactor and the rings 21 themselves. It will also be appreciated that a gas feed ring arrangement similar to that used for the upper sparger 12 could be used for the lower sparger 13, in place of the main gas feed pipe 23.

Each of the rings 21, 22 is formed with a series of nozzles which are generally equispaced along the ring 21, 22, and are also similarly spaced from the nozzles on adjacent rings. The nozzles are shown in more detail in Figures 5 and 6.

Figure 5 shows a nozzle in the form of a simple hole 27 in a ring 21. Figure 6 shows an alternative nozzle in the form of a hole 28 in a ring 21 covered by a cowl 29 which has an aperture 31. In each case, the hole 27 or the aperture 31 faces downwards, 90° or less below the horizontal. The L/D-ratio for the nozzle should be >1, where L is the length of the nozzle and D is the diameter of the nozzle. Where the nozzle is a simple hole as in Figure 5, the length L is the depth of the hole, i.e. the thickness of the ring wall.

In use, the reactant gas is conveyed to the upper and lower feed pipes and enters the upper manifolds 26 and the lower manifolds 24. It then enters the respective rings 21, 22 via the respective tubes, and is injected into the slurry in the reactor vessel 11 through the nozzles. In this way, each nozzle produces a downward gas jet into the slurry. The jets near the vessel bottom create motion of the slurry near the surface of the reactor vessel 11 to prevent the catalyst from settling and keep the catalyst in motion in that area.

The flushing effect of the rings 21 of the upper sparger 12 which are further away from the vessel wall will tend to diminish. However this is addressed by the lower sparger 13. Gas from the nozzles in the lower sparger rings 22 rises into the tube 14. The density of the slurry in the tube 14 is therefore reduced. This tends to cause the slurry in the tube 14 to rise, which in turn draws slurry from the zone 32 down and into the tube 14. As this slurry descends, it flushes the wall of the vessel 11. This effect is assisted by the fact that in the region of the zone 32 the wall of the vessel 1 1 is the part-conical section 16.

There are several benefits to this design. Firstly, the two spargers can be designed without the need for a very narrow window for the distance between the reactor wall and the nozzles. There will be an improved turn down ratio of gas while still keeping the catalyst distributed in the liquid by using separate flow control to the 2 levels of sparger, the gas distribution can be controlled over a large feed flow range. Even with feeding gas only in the inner part of the reactor, a sufficient slurry circulation can be maintained to avoid sedimentation of catalyst outside the tube. The pressure drop can be kept low over the spargers due to operation at low gas velocity in sparger nozzles, but still maintain even distribution of gas, thus reducing attrition of the catalyst.

fable 1 below shows some examples for a reactor of 10 m ID. The slurry residence time below the top sparger outside tube should be as short as possible.

Table 1 shows calculated cases for some variations of tube diameter, gap area, fraction of feed gas to the bottom sparger and bottom and top sparger coverage. The calculations are based on Bernoulli's equations for calculating the driving force for the circulation.