EP1720646A1

EP1720646A1 - Downcomers for slurry bubble column reactors

Info

Publication number: EP1720646A1
Application number: EP05708383A
Authority: EP
Inventors: Dag Schanke; Trond Myrstad; Pal Soraker; Olav Juliussen; Per Oscar Wiig
Original assignee: Statoil ASA; Petrosa; Petro SA
Current assignee: PETROSA; Equinor ASA
Priority date: 2004-02-24
Filing date: 2005-02-17
Publication date: 2006-11-15
Also published as: GB0404069D0; WO2005082513A1; GB2411365B; JP2007526114A; KR20070019995A; GB2411365A

Abstract

A downcomer (61) for use in a slurry bubble column reactor (60) comprises: an upper part (62) including a slurry inlet (66) through which slurry can enter the downcomer (61), gas separation means (65) arranged to separate gas bubbles from the slurry, and a gas outlet through which the separated gas can escape; an elongate, tabular-shaped intermediate section (63) which extends vertically in use and through which slurry can pass downwards; and a lower portion (64) which has a slurry outlet (71). The slurry outlet (71) is defined by a wall (69) which is inclined relative to the longitudinal axis of the intermediate section (63).

Description

Downcomers for Slurry Bubble Column Reactors

The present invention relates to downcomers for slurry bubble column reactors.

Downcomers are used in slurry bubble columns reactors (SBCRs) in order to achieve a more suitable and uniform distribution of catalyst particles in the vertical direction of the slurry phase, and a more uniform temperature within the SBCR. In general, downcomers are tubular structures positioned vertically in the SBCRs, and are provided with gas disengaging means and means for restricting the entry of rising gas bubbles into the lower end of the downcomer structures.

WO 98/50494 shows a downcomer which is provided with a gas disengaging zone positioned in the upper part of the downcomer. A cone shaped baffle below the bottom of the downcomer prevents upwardly moving gas bubbles from entering into the downcomer.

In EP 0674610, a downcomer is shown which is open at both ends and fully submerged in the slurry phase. The bottom end of the downcomer is shielded by a baffle arrangement which diverts gas bubbles from the bottom of the SBCR away from entering into the bottom end of the downcomer. The baffle arrangement may have the shape of a cone similar to that of WO 98/50494.

It is an object of the invention to achieve a more uniform vertical distribution of catalyst particles in a SBCR.

According to the invention, there is provided a downcomer for use in a slurry bubble column reactor comprising: an upper part including a slurry inlet through which slurry can enter the downcomer, and a gas outlet through which the separated gas can escape; an elongate, tubular-shaped intermediate section which extends vertically in use a d through which slurry can pass downwards; and a lower portion which has a slurry, outlet, the slurry outlet being defined at least in part by a wall which is inclined relative to the longitudinal axis of the intermediate section and in which the slurry outlet is an orifice which defines an aperture to the intermediate section, the aperture being the area of the opening from the orifice to the intermediate section in a plane perpendicular to the axis of the intermediate section, the aperture representing less than 2/3 of the transverse cross-sectional area of the intermediate section, whereby, in use, entry of upwardly moving gas bubbles into the downcomer via the slurry outlet is restricted.

Preferably, the upper part includes gas separation means arranged to separate gas bubbles from the slurry.

The aperture can be thought of as the area of a surface beneath the downcomer that would be illuminated by light travelling down the inside of the downcomer in a direction parallel to the axis of the intermediate section, the surface being perpendicular to the axis of the intermediate section.

Preferably, the inclined portion has a smooth, curved geometry and may be part of a tubular bend.

The outlet may be formed by a frusto-conical wall section, whereby the outlet has a smaller area than the transverse cross-sectional area of the intermediate section. The perimeter of the outlet may be inclined at an oblique angle to the axis of the intermediate section, or may be parallel to it. The outlet may be offset from the intermediate section when viewed in the direction of the axis of the intermediate section.

Preferably, any substantial part of the walls of the downcomer has an inclination that is steeper than the angle of repose of the catalyst particles under the prevailing conditions thereby preventing build-up of catalyst at such locations.

Preferably, the intermediate section includes means for restricting the passage of gas upwards inside the intermediate section when in use. Preferably, the means for restricting the passage of gas comprises a disc arranged horizontally inside the intermediate section, the disc covering at least 20% of the transverse cross-sectional area of the intermediate section.

Preferably, the intermediate section includes means for collecting gas bubbles in the slurry when the slurry bubble column reactor is in use. Preferably, the means for collecting gas bubbles comprises a tube with a flared skirt at its lower end. Preferably, the tube extends to the top of the downcomer.

The invention extends to a reactor, such as a slurry bubble column reactor, including one or more downcomers as described above. Preferably, the vertical length of the downcomer is in the range of 30-120% of the intended depth of the slurry in the slurry bubble column when in use. Preferably, there is a gas phase above the slurry, the gas outlet from the upper part being in communication with the gas phase.

The invention extends to the use of such a reactor to carry out a Fischer-Tropsch synthesis reaction.

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

Figures 1 to 5 show five alternative embodiments of the lower end of a downcomer in accordance with the invention;

Figure 6 is a view of the embodiment of Figure 1 to an enlarged scale;

Figure 7 is a section on the line AA in Figure 6;

Figure 8 is a vertical section through a reactor including a downcomer in accordance with the invention;

Figure 9 is a section on the line BB in Figure 8; Figure 10 is a graph showing the effect of gas velocity on downcomer operation;

Figures 11, 12 and 13 show alternative geometries for the downcomer outlet;

Figure 14 is a graph showing the effects of the geometries of the alternatives in Figures 11, 12 and 13;

Figure 15 is a graph showing the effect of the communication of the downcomer with the gas space above the slurry; and

Figure 16 is a graph similar to Figure 10, but with particles present in the liquid; and

Figures 17, 18 and 19 are graphs showing catalyst concentration profiles at different gas velocities.

The embodiment shown in Figure 1 represents one particular experimental set up which illustrates the principles and operation of the invention. It is to be understood that this embodiment may not correspond to a full scale industrial application.

The lower portion of the downcomer 11 is formed from a 7.62 cm (3 inches) tube which has a bend 12 at the bottom, defining an orifice 13 which is set at an angle of 45° to the axis of the tube. In practice, this design functioned well.

In Figure 2, the tube forming the downcomer 21 has a diameter of 127 mm (5 inches). It has an elbow joint 22 defining an orifice 23 which is parallel to the axis of the tube. This design functioned well in practice, though the flow rate through the downcomer was low.

The design shown in Figure 3 is similar to the design in Figure 2, but the tube additionally includes a disc 34 inside the downcomer 31, about 50 cm above the orifice 33. The disc 34 occupies about half the cross-sectional area of the downcomer 31. Again, this design worked well, however, when the disc 34 was replaced by a smaller disc occupying about a quarter of the cross-sectional area, and located 150 cm above the orifice 33, the performance of the downcomer was unsatisfactory.

The downcomer 41 shown in Figure 4 is also similar to that of Figure 2, but includes an inverted funnel 44 positioned above the orifice 43 and connected to a tube 45 which extends upwards to open above the top of the slurry phase.

The downcomer 51 in Figure 5 is again similar to that of Figure 2, but in this case, the lower part terminates in a truncated cone 54 which defines a smaller orifice 53 parallel to the axis of the tube.

Both the embodiments shown in Figures 4 and 5 functioned well.

Referring now to Figures 6 and 7, the concept of the aperture defined by the orifice will be amplified. As described in relation to Figure 1, the downcomer 11 teπninates in a bend 12 which defines the orifice 13. The outline of the tube is shown as the circle 14 in Figure 7, while the elliptical shape 15 represents the outline of the orifice 13. The area 16 bounded by the two sectors of the circle 14 and elliptical shape 15 is the aperture. It will be appreciated that in the arrangements shown in Figures 2, 3, 4, 5, 8, 9, 12 and 13, the aperture will have a zero value.

Figures 8 and 9 show a downcomer 61 extending vertically in a slurry bubble column reactor 60. It will be understood that in practice, there would probably be several downcomers in the reactor.

The downcomer 61 comprises an upper part 62, an intermediate section 63 and a lower portion 64.

The upper part 62 is about 260 cm in height, has an open top, and has an internal diameter of 11.2 cm. The bottom 78 cm of the upper part 62 has an increased internal diameter of 21 cm and constitutes a degassing zone 65. The degassing zone 65 includes a series of slurry inlets 66 to the interior of the downcomers which are located about 50 cm above the bottom of the degassing zone 65. The area of the slurry inlets 66 is 1.9 times the cross sectional area of the degassing section 65, and 6.7 times the cross sectional area of the intermediate section 63 of the downcomer 61.

The intermediate section 63 is a tube, 1200 cm in height and 11.2 cm in diameter.

The lower portion 64 comprises a tube 68 with a diameter of 11.2 cm and a bend 69, terminating in an orifice 71 which is parallel to the axis of the downcomer 61. In total, the lower portion has a height of about 97 cm.

The reactor 60 has a height of about 1600 cm and an internal diameter of 50 cm, and contains slurry liquid 72 to a height 73 of about 1550 cm, which is below the level of the open tope of the downcomer 61. This results in a gas space 74 above the liquid 72. There is a gas outlet 75 at the top of the reactor 60.

Near the bottom of the reactor 60, there is a gas inlet 76 and a gas distributor 77. The relative positions of the downcomer 61 and the gas inlet are shown in Figure 9. The reactor 60 also has a slurry outlet (not shown).

The invention, will now be further illustrated in the following Examples. The apparatus used was that described with reference to Figures 8 and 9. It is to be understood that in a commercial installation, a downcomer could have other dimensions than the downcomers referred to in the following Examples.

Example 1

Effect of gas velocity

The column with the downcomer shown in Figures 8 and 9 above was filled with water, air was injected near the bottom of the reactor, and the superficial gas velocity, u_g, was varied. The expanded liquid level above the top of the slurry inlet area, Δh_t0p, was kept approximately constant. In the table below, the corresponding liquid velocities, ui, in the downcomer are reported. The pressure difference inside and outside the downcomer, ΔP, is also included to show the relationship between this parameter and the liquid velocity. The results are shown in Table 1 and are also presented graphically in Figure 10.

Table 1

U_g (m/s) Δh_top (m) ui (m/s) ΔP (mmH2θ)

0.107 1.25 1.25 392

0.217 1.25 1.18 331

0.302 1.25 1.03 211

The results show that the efficiency of the downcomer decreases with increasing gas velocity. It should however be borne in mind that the function of the downcomer is to decrease the axial catalyst concentration profile. This profile decreases also with increasing gas velocities. It is therefore most important that the downcomer has a good efficiency at low gas velocities.

Example 2

Other possible downcomer outlet geometries

The downcomer outlet section helps prevent gas bubbles entering into the downcomer. Several other geometries than the 90° bend shown in Figures 8 and 9 are possible. A shorter version of the column shown in Figures 8 and 9 (6.6m height) was used with a 12.7 cm (5 inch) diameter downcomer, and three other possible geometries were investigated. These are:

- no bend at the slurry outlet, but the outlet diameter is reduced, such that the outlet cross sectional area is reduced by 50%; 135° bend, the upper half of the slurry outlet area is blocked; 90° bend, and with a gas collector inside in the form of a funnel covering half of the cross sectional area. These are shown as Figures 11, 12 and 13.

The results can be seen in Figure 14 and show that there is practically no difference in downcomer performance for the different options.

Other options for the downcomer outlet, which have given poor downcomer performance, have also been tested. In these tests, ΔP measurements were used to indicate performance rather than a flow meter. The designs tested included no bend at the slurry outlet, no reduction of the outlet diameter; 45°C bend, with a larger aperture area than Figure 16.

Results from experiments with the last option are shown in Table 2 below (the ΔP- values close to zero indicates no (or a very low) liquid flow inside the downcomer).

Table 2

Δh_top (m) U_g (m/s) ΔP (mmhfeO) 0.9-1.2 0.25-0.3 1.18

1.2-1.4 0.1-0.2 1.36

1.2-1.4 0.2-0.25 1.33

Example 3 Effect of the Upper Part of the Degassing Section

In EP 0674610 a downcomer fully submerged in the slurry phase is used. In this example a downcomer with such a slurry inlet is compared with a downcomer which is not submerged, and the slurry inlet is at the side of the degassing section. The column with the downcomer shown in Figures 8 and 9 was used, except that only the downcomer section below the slurry inlet was in use. The expanded liquid level above the top of the slurry inlet area, Δh_t0p, was kept approximately constant. In Table 3 below, the corresponding liquid velocities, ui, in the downcomer are shown. The results shown in Table 3 are compared with results obtained with a downcomer which is not submerged, but with the slurry inlet at the side of the degassing section, (ie. the present invention) in Figure 15. Table 3

U_g (m/s) Δh_top (m) uι (m/s) 0.106 1 0.98

0.108 1.05 0.98

0.208 1.25 0.92

0.312 1.2 0.847

Figure 15 shows that the present invention gives higher liquid velocities in the downcomer than a downcomer fully submerged in the slurry.

Example 4

Effect of Catalyst Concentration Profile The column with the downcomer shown in Figures 8 and 9 was used also for this example. In addition to water, the column was now filled with particles. The particles used were made of SiC, and had an average particle size of approximately 75 μm. The concentration of particles was approximately 200 g/1. The expanded liquid level above the top of the slurry inlet area, Δh_t0p, was kept approximately constant. The corresponding liquid velocities, ui, in the downcomer are shown in the Table 4 and Figure 16.

Table 4

Δhtop (m) U_g (mis) ui (m/s)

1.1 0.101 1.4

1.2 0.199 1.32

1.1 0.281 1.18

At each of the gas velocities, samples of slurry were withdrawn from the column, and the catalyst concentration was determined. Then the downcomer inlet was sealed, and the experiments were repeated for all three gas velocities. The catalyst concentration profiles in the experiments with and without the downcomer in use are shown for the different gas velocities in Figures 17, 18 and 19. These Figures clearly show the effect of the downcomers. When the downcomers was in use, the catalyst concentration was more uniform through the column than without the downcomer in use. (Note: The numbers on the x-axis on the Figures 17, 18 and 19 refer to the distance above the gas distributor. Negative numbers mean that samples have been withdrawn below this level).

Claims

1. A downcomer for use in a slurry bubble column reactor comprising: an upper part including a slurry inlet through which slurry can enter the downcomer and a gas outlet through which the separated gas can escape; an elongate, tubular-shaped intermediate section beneath the upper part which extends vertically in use and through which slurry can pass downwards; and a lower portion beneath the intermediate section which has a slurry outlet section, the slurry outlet section being defined at least in part by a wall which is inclined relative to the longitudinal axis of the intermediate section and in which the slurry outlet section terminates in an orifice which defines an aperture to the intermediate section, the aperture being die area bf the opening from the orifice to the intermediate section in a plane perpendicular to the axis of the intermediate section, the aperture representing less than 2/3 of the transverse cross-sectional area of the intermediate section, whereby, in use, entry of upwardly moving gas bubbles into the downcomer via the slurry outlet is restricted.

2. A downcomer as claimed in Claim 1, in which the upper part includes gas separation means arranged to separate gas bubbles from the slurry.

3. A downcomer as claimed in Claim 1 or Claim 2, in which the aperture represents less than 50% of the transverse cross-sectional area of the intermediate section.

4. A downcomer as claimed in Claim 3 in which the aperture represents less than 25% of the transverse cross-sectional area of the intermediate section.

5. A downcomer as claimed in Claim 4, in which the aperture represents 0% of the transverse cross-sectional area of the intermediate section.

6. A downcomer as claimed in any preceding Claim, in which a geometric projection of the lower portion on to a plane perpendicular to the axis of the intermediate section represents less than 110% of the transverse cross-sectional area of the intermediate section.

7. A downcomer as claimed in any preceding Claim, in which the outlet is formed by a frusto-conical wall section, whereby the outlet has a smaller area than the transverse cross-sectional area of the intermediate section.

8. A downcomer as claimed in any preceding Claim, in. which perimeter of the outlet is inclined at an oblique angle to the axis of the intermediate section.

9. A downcomer as claimed in any of Claims 1 to 7, in which the perimeter of the outlet is parallel to the axis of the intermediate section.

10. A downcomer as claimed in any preceding Claim in which the outlet is offset from the intermediate section when viewed in the direction of the axis of the intermediate section.

11. A downcomer as claimed in any preceding Claim, in which any substantial part of the walls of the downcomer has an inclination that is steeper than the angle of repose of the catalyst particles under the prevailing conditions thereby preventing buildup of catalyst at such locations.

12. A downcomer as claimed in any preceding Claim, in which the intermediate section includes means for restricting the passage of gas upwards inside the intermediate section when in use.

13. A downcomer as claimed in Claim 12, in which the means for restricting the passage of gas comprises a disc arranged horizontally inside the intermediate section, the disc covering at least 20% of the transverse cross-sectional area of the intermediate section.

14. A downcomer as claimed in any preceding Claim, in which the intermediate section includes means for collecting gas bubbles in the slurry when the slurry bubble column reactor is in use.

15. A downcomer as claimed in Claim 14, in which the means for collecting gas bubbles comprises a tube with a flared skirt at its lower end.

16. A downcomer as claimed in Claim 15 in which the tube extends to the top of the downcomer.

17. A slurry bubble column reactor including a downcomer as claimed in any preceding Claim secured within the reactor.

18. A reactor as claimed in Claim 17, in which the vertical length of the downcomer is in the range of 30-120% of the intended depth of the slurry in the slurry bubble column when in use.

19. A reactor as claimed in Claim 17 or Claim 18, including a gas phase above the slurry, the gas outlet from the upper part being in communication with the gas phase.

20. A reactor as claimed in any of Claims 17 to 19, in which the downcomer extends from the region of the top of the slurry to the region of the bottom of the reactor.

21. A reactor as claimed in any of Claims 17 to 20, including a plurality of downcomers.

22. The use of a slurry bubble column reactor as claimed in any of Claims 17 to 21 to carry out a Fischer-Tropsch synthesis reaction.

23. A method of carrying out a Fischer-Tropsch synthesis reaction which comprises supplying hydrogen gas and carbon monoxide gas to a slurry including a Fischer-Tropsch catalyst in a reactor as claimed in any of Claims 17 to 21.