Title: Production of liquid and, optionally, gaseous products from gaseous reactants
Abstract: A process for producing liquid and, optionally, gaseous products from gaseous reactants includes feeding, at a low level, gaseous reactants into a slurry bed, allowing the gaseous reactants to react as they pass upwardly through the slurry bed, withdrawing any gaseous product and unreacted gaseous reactants from a head space above the slurry bed and withdrawing liquid product and/or slurry bed to maintain the slurry bed at a desired level. The process further includes passing boiler water, as a first heat transfer fluid, in indirect heat exchange relationship through the slurry bed to remove heat from the slurry bed, allowing the heated boiler water to flash and separate to form pressurised steam, controlling the pressure of the steam to be substantially constant, and passing a second heat transfer fluid in indirect heat exchange relationship through the slurry bed to remove heat from the slurry bed.
Patent Number: 6,864,293 Issued on 03/08/2005 to Steynberg
| Inventors:
|
Steynberg; Andre Peter (Vanderbijlpark, ZA)
|
| Assignee:
|
Sasol Technology (Proprietary) Limited (Johannesburg, ZA)
|
| Appl. No.:
|
322835 |
| Filed:
|
December 18, 2002 |
Foreign Application Priority Data
| Dec 20, 2001[ZA] | 2001/10471 |
| Current U.S. Class: |
518/719 |
| Intern'l Class: |
C07C 027//00 |
| Field of Search: |
518/712
|
References Cited [Referenced By]
U.S. Patent Documents
| 4258006 | Mar., 1981 | Flockenhaus et al. | 422/146.
|
| 4407974 | Oct., 1983 | Flockenhaus et al. | 518/711.
|
| 4539016 | Sep., 1985 | Flockenhaus et al.
| |
| 5409960 | Apr., 1995 | Stark | 518/700.
|
| 5527473 | Jun., 1996 | Ackerman | 210/767.
|
| Foreign Patent Documents |
| 2807422 | Aug., 1979 | DE | .
|
| 0099690 | Jul., 1983 | EP | .
|
| 2193444 | Feb., 1988 | GB | .
|
| WO 0045948 | Aug., 2000 | WO | .
|
| WO 0136066 | May., 2001 | WO | .
|
Other References
Great Britain Search Report corresponding to GB 0229203.5 completed May 21,
2003.
|
Primary Examiner: Parsa; J.
Attorney, Agent or Firm: Myers Bigel Sibley & Sajovec, P.A.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority from U.S. Provisional Application No.
60/342,668, filed Dec. 20, 2001 and under 35 U.S.C. .sctn. 371 from South
African Patent Application No. 2001/10471, filed in English on Dec. 20,
2001, the disclosures of which are incorporated by reference herein in
their entirety.
Claims
What is claimed is:
1. A process for producing liquid and, optionally gaseous products from
gaseous reactants, which process includes
feeding, at a low level, gaseous reactants into a slurry bed of solid
particles suspended in a suspension liquid;
allowing the gaseous reactants to react as they pass upwardly through the
slurry bed, thereby to form liquid and, optionally, gaseous products;
withdrawing any gaseous product and unreacted gaseous reactants from a head
space above the slurry bed;
withdrawing liquid product and/or slurry from the slurry bed to maintain
the slurry bed at a desired level;
passing boiler water, as a first heat transfer fluid, in indirect heat
exchange relationship through the slurry bed to remove heat from the
slurry bed;
allowing the heated boiler water to flash and separate to form pressurised
steam;
controlling the pressure of the steam to be substantially constant; and
passing a second heat transfer fluid in indirect heat exchange relationship
through the slurry bed to remove heat from the slurry bed, wherein the
average temperature of the second heat transfer fluid in indirect heat
exchange relationship with the slurry bed being lower than the average
temperature of the boiler water in indirect heat exchange relationship
with the slurry bed.
2. A process as claimed in claim 1, in which the first heat transfer fluid,
which is boiler water, removes at least 50% of the total heat removed from
the slurry bed by the first and second heat transfer fluids.
3. A process as claimed in claim 1, which includes cooling the second heat
transfer fluid and returning it for heat exchange duty to the slurry bed,
the second heat transfer fluid thus being cycled continuously through the
slurry bed, in a substantially closed system.
4. A process as claimed in claim 1, which includes controlling the
temperature of the slurry bed by controlling an operating temperature of
the second heat transfer fluid passing in indirect heat exchange
relationship through the slurry bed.
5. A process as claimed in claim 1, in which the second heat transfer fluid
is water, the process including pumping the water to a pressure sufficient
substantially to prevent evaporation of the water to form steam at the
operating temperature and pressure of the water.
6. A process as claimed in claim 1, in which the second heat transfer fluid
is water, the process including allowing steam to be formed by the water.
7. A process as claimed in claim 1, which includes selectively increasing a
heat transfer surface area between the second heat transfer fluid and the
slurry bed, and decreasing a heat transfer surface area between the first
heat transfer fluid and the slurry bed, in order to increase the total
heat removal rate achieved by the first and second heat transfer fluids,
and/or selectively decreasing a heat transfer surface area between the
second heat transfer fluid and the slurry bed, and increasing a heat
transfer surface area between the first heat transfer fluid and the slurry
bed in order to decrease the total heat removal rate achieved by the first
and second heat transfer fluids.
8. A process as claimed in claim 7, which includes switching heat transfer
surface area in contact with the first heat transfer fluid and the slurry
bed to be in contact with the second heat transfer fluid and the slurry
bed, and/or vice versa.
Description
BACKGROUND OF THE INVENTION
THIS INVENTION relates to the production of liquid and, optionally, gaseous
products from gaseous reactants. In particular, it relates to a process
for producing liquid and, optionally, gaseous products from gaseous
reactants, and to an installation for producing liquid and, optionally,
gaseous products from gaseous reactants.
Many reactions, such as the Fischer-Tropsch synthesis reaction are highly
exothermic and the effective design of a heat removal system is essential
to control the reaction for industrial applications. This is also the case
for the Fischer-Tropsch slurry phase reaction. Typically, heat removal is
effected by passing boiler water through cooling pipes submerged in a
slurry bed within which the Fischer-Tropsch synthesis reaction takes
place. The boiling water is pumped from a steam drum through the cooling
pipes and the heated water is then returned to the steam drum where it
flashes to form steam. The steam passes out of the steam drum via a
pressure control valve to a steam header. Often, the amount of steam
generated is in excess of total requirements, but not enough high pressure
steam is produced.
In the prior art of which the applicant is aware, the heat removal rate is
matched with the heat generation rate of the Fischer-Tropsch synthesis
reaction by varying the pressure in the steam drum. As will be appreciated
by those skilled in the art, pressure changes in the steam drum changes
the boiling temperature of the water in the cooling system and hence it
changes the temperature of the water and steam in the cooling pipes in
contact with the slurry bed, and the heat removal rate.
A disadvantage of the prior art heat removal system is that a sudden
increase in heat generation in the slurry bed may cause operating
problems, since a sudden increase in heat generation may cause a sudden
drop in pressure in the steam drum, which may result in cavitation of the
pumps that deliver the boiler water to the cooling pipes. This may result
in a failure of the cooling system, leading to overheating of the slurry
bed and thus damaging the catalyst in the slurry bed.
It is an object of this invention to provide a process and installation for
producing liquid and, optionally, gaseous products from gaseous reactants,
in which the temperature control of the slurry bed is improved and which
can provide more optimum steam production.
According to one aspect of the invention, there is provided a process for
producing liquid and, optionally, gaseous products from gaseous reactants,
which process includes
feeding, at a low level, gaseous reactants into a slurry bed of solid
particles suspended in a suspension liquid;
allowing the gaseous reactants to react as they pass upwardly through the
slurry bed, thereby to form liquid and, optionally, gaseous products;
withdrawing any gaseous product and unreacted gaseous reactants from a head
space above the slurry bed;
withdrawing liquid product and/or slurry from the slurry bed to maintain
the slurry bed at a desired level;
passing boiler water, as a first heat transfer fluid, in indirect heat
exchange relationship through the slurry bed to remove heat from the
slurry bed;
allowing the heated boiler water to flash and separate to form pressurised
steam;
controlling the pressure of the steam to be substantially constant; and
passing a second heat transfer fluid in indirect heat exchange relationship
through the slurry bed to remove heat from the slurry bed.
The first heat transfer fluid, which is boiler water, may remove at least
50%, preferably at least 75%, of the total heat removed from the slurry
bed by the first and second heat transfer fluids.
The average temperature of the second heat transfer fluid in indirect heat
exchange relationship with the slurry bed may be lower than the average
temperature of the boiler water in indirect heat exchange relationship
with the slurry bed.
The pressure of the steam may be controlled at at least 14 bar(g),
preferably at least 16 bar(g).
The process may include cooling the second heat transfer fluid and
returning it for heat exchange duty to the slurry bed. In other words, the
second heat transfer fluid may be cycled continuously through the slurry
bed, in a substantially closed system.
The cooling of the second heat transfer fluid may be effected by means of
indirect heat exchange with a cooling fluid, e.g. air.
The process may include controlling the temperature of the slurry bed by
controlling an operating temperature of the second heat transfer fluid
passing in indirect heat exchange relationship through the slurry bed.
The second heat transfer fluid may be water. The process may include
pumping the water to a pressure sufficient substantially to prevent
evaporation of the water to form steam at the operating temperature and
pressure of the water. Thus, the water may be pumped to a pressure of at
least 28 bar(g), preferably at least 34 bar(g), e.g. about 40 bar(g).
Instead, the process may include allowing steam to be formed by the second
heat transfer fluid. In this case, the water may be pumped to a pressure
of between about 2 bar(g) and about 12 bar(g), preferably between about 4
bar(g) and about 10 bar(g).
The process may include selectively increasing a heat transfer surface area
between the second heat transfer fluid and the slurry bed, and decreasing
a heat transfer surface area between the first heat transfer fluid and the
slurry bed, in order to increase the total heat removal rate achieved by
the first and second heat transfer fluids. Instead, or in addition, the
process may include selectively decreasing a heat transfer surface area
between the second heat transfer fluid and the slurry bed, and increasing
a heat transfer surface area between the first heat transfer fluid and the
slurry bed in order to decrease the total heat removal rate achieved by
the first and second heat transfer fluids. This may be effected by
switching heat transfer surface area in contact with the first heat
transfer fluid and the slurry bed to be in contact with the second heat
transfer fluid and the slurry bed, and/or vice versa.
The solid particles may be catalyst particles for catalysing the reaction
of the gaseous reactants into the liquid product, and, when applicable,
the gaseous product. The suspension liquid may be the liquid product, with
the slurry bed being contained in a reaction zone of a slurry reactor or
bubble column using a three-phase system comprising solid catalyst
particles, liquid product, and gaseous reactants and, optionally, product.
The gaseous reactants may be capable of reacting catalytically in the
slurry bed to form liquid hydrocarbon product and gaseous hydrocarbon
product by means of Fischer-Tropsch synthesis, with the gaseous reactants
being in the form of a synthesis gas stream comprising mainly carbon
monoxide and hydrogen.
The catalyst may be an iron based Fischer-Tropsch catalyst or a cobalt
based Fischer-Tropsch catalyst. Typically, the catalyst particles have a
particle size range such that no catalyst particles are greater than 300
microns and less than 5% by mass of the catalyst particles are smaller
than 22 microns.
The process may include allowing slurry to pass downwardly from a high
level in the slurry bed to a lower level thereof, through at least one
downcomer located in a first downcomer region of the slurry bed, as well
as through at least one further downcomer located in a second downcomer
region of the slurry bed, with the second downcomer region being spaced
vertically with respect to the first downcomer region, thereby to
redistribute solid particles within the slurry bed, as disclosed in
International Application No. WO 99/03574, the specification of which is
incorporated herein by reference.
According to another aspect of the invention, there is provided an
installation for producing liquid and, optionally, gaseous products from
gaseous reactants, the installation including
a reactor vessel having a slurry bed zone which, in use, will contain a
slurry bed of solid particles suspended in a suspension liquid;
a gas inlet in the vessel at a low elevation within the slurry bed zone,
for introducing gaseous reactants into the vessel;
a gas outlet in the vessel above the slurry bed zone, for withdrawing
unreacted gaseous reactants and, when present, gaseous product from the
vessel;
a liquid outlet in the vessel within the slurry bed zone, for withdrawing
liquid product from the vessel;
a first, steam-producing, cooling arrangement for bringing boiler water in
indirect heat exchange relationship with the slurry bed zone, the first
cooling arrangement including pressure control means for providing steam
from the first cooling arrangement at a substantially constant pressure;
and
a second cooling arrangement for bringing a heat transfer fluid in indirect
heat exchange relationship with the slurry bed zone.
The first cooling arrangement may include a steam drum and a steam header.
The pressure control means may be configured or configurable to control
the pressure in the steam header at a preselected set point.
The second cooling arrangement may be a steam producing cooling arrangement
for producing steam at a lower pressure than the first cooling
arrangement. The second cooling arrangement may thus include a steam drum.
The second cooling arrangement may be a closed cooling circuit which
comprises an indirect heat exchanger for cooling the heat transfer fluid
by means of exchange of heat with a cooling medium. The indirect heat
exchanger may be an air cooler for cooling the heat transfer fluid with
air. When the second cooling arrangement is a steam producing cooling
arrangement and is a closed cooling circuit, it may include a condensate
collecting drum in flow communication with the indirect heat exchanger for
collecting condensate from the indirect heat exchanger.
The installation may include temperature control means for controlling the
temperature of the slurry bed, in use. The temperature control means may
be configured to control the slurry bed temperature by controlling an
operating temperature of the heat transfer fluid in the second cooling
arrangement.
The first cooling arrangement and the second cooling arrangement may be in
selective flow communication with each other, to allow at least a portion
of the first cooling arrangement selectively to carry heat transfer fluid
from the second cooling arrangement, in indirect heat exchange
relationship with the slurry bed zone, and/or vice versa.
The first cooling arrangement may have a pressure rating high enough to
require the use of schedule 40 piping and 300 lb flanges.
When the second cooling arrangement is not a steam producing cooling
arrangement, it may have a pressure rating high enough to require the use
of schedule 80 piping and 600 lb flanges.
SUMMARY OF THE INVENTION
When the second cooling arrangement is a steam producing cooling
arrangement, it may have a pressure rating compatible with the use of
piping with a schedule less than 40 and with 150 lb flanges.
The installation may include at least one downcomer located in a first
downcomer region in the slurry bed zone and through which, in use, slurry
can pass downwardly and at least one further downcomer located in a second
downcomer region in the slurry bed zone, with the second downcomer region
being spaced vertically relative to the first downcomer region, the
slurry, in use, also passing downwardly through this downcomer, as
disclosed in WO 99/03574.
BRIEF DESCRIPTION OF THE DRAWING
The invention will now be described, by way of example, with reference to
the accompanying diagrammatic drawings in which
FIG. 1 shows schematically one embodiment of an installation in accordance
with the invention for producing liquid and, optionally, gaseous products
from gaseous reactants; and
FIG. 2 shows schematically another embodiment of an installation in
accordance with the invention for producing liquid and, optionally,
gaseous products from gaseous reactants.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1 of the drawings, reference numeral 10 generally
indicates an installation according to the invention for producing liquid
and, optionally, gaseous products from gaseous reactants.
The installation 10 includes an upright cylindrical slurry reactor or
bubble column 12, with a gas inlet 14 leading into a gas distributor (not
shown) inside the reactor and a gas outlet 16 leading from the top of the
reactor. Liquid product outlets 18, only one of which is shown, lead from
the reactor 12 at any convenient level. The installation 10 includes a
first, steam-producing cooling arrangement generally indicated by
reference numeral 20. The first cooling arrangement 20 includes a steam
drum 22, a steam header 24, a pressure control valve 26 and a boiler water
circulation pump 28. A boiler water line 30 leads from the steam drum 22
through the boiler water circulation pump 28 into a manifold 32, which may
be inside and/or outside the reactor 12. A plurality of cooling pipes 34
leads from the manifold 32 into and through the reactor 12 and into a
manifold 36, which may be inside and/or outside the reactor 12. A return
line 40 leads from the manifold 36 back to the steam drum 22.
The pressure control valve 26 is located in a steam line 42 which connects
the steam drum 22 to the steam header 24. One or more steam lines 44 lead
from the steam header 24 to steam users (not shown).
A second cooling arrangement is generally indicated by reference numeral
50. The second cooling arrangement 50 includes an air cooler 52 and a pump
54. A water line 56 extends between the air cooler 52 and the pump 54. A
cooling water line 58 leads from the pump 54 into a manifold 60 which may
be inside and/or outside the reactor 12. A plurality of cooling pipes 62
is connected to the manifold 60 and a manifold 64 and extends through the
reactor 12. A return line 66 leads from the manifold 64 to the air cooler
52. The manifold 64 may be inside and/or outside the reactor 12.
On an inlet side of the cooling pipes 62 and the cooling pipes 34, a
connecting line 68 connects one of the cooling pipes 62 and one or more of
the cooling pipes 34. Similarly, on an outlet side of the cooling pipes 62
and the cooling pipes 34, a connecting line 70 also connects the cooling
pipe 62 and the cooling pipe 34 connected by the connecting line 68.
Valves 72, 74 are provided between the connecting line 68 and the
manifolds 60, 32 and valves 76, 78 are provided between the connecting
line 70 and the manifolds 64, 36. Furthermore, a valve 80 is located in
the connecting line 68 and a valve 82 is located in the connecting line
70. It is to be appreciated that more cooling pipes or groups of cooling
pipes 34, 62 may be interconnected in this fashion and that the valves
will typically be located outside the reactor 12.
In use, synthesis gas comprising mainly carbon monoxide and hydrogen as
gaseous reactants, is fed into the bottom of the reactor 12 through the
gas inlet 14, the gas typically being uniformly distributed through a grid
plate or sparger system (not shown) inside the reactor. The gaseous
reactants pass upwardly through a slurry bed 84 comprising Fischer-Tropsch
catalyst particles, typically an iron or cobalt based catalyst, suspended
in liquid product. The slurry bed is operated to have a normal level 86
above the cooling coils 62, with a head space 88 being provided above the
slurry bed 84. As the synthesis gas bubbles through the slurry bed 84, the
gaseous reactants therein react catalytically to form liquid product,
which thus forms part of the slurry bed 84. From time to time, or
continuously, liquid phase comprising liquid product is withdrawn through
the outlet 18, with catalyst particles having been separated from the
liquid product in a suitable internal filtration system (not shown).
Alternatively, the filtration system may be located externally to the
reactor 12, with an additional system (not shown) to return the separated
catalyst particles to the reactor 12 then being provided.
Typically, the reactor 12 includes downcomers (not shown) to achieve
uniform redistribution of catalyst particles within the slurry bed 84, and
also to ensure uniform heat distribution throughout the slurry bed 84, as
described in the specification of WO 99/03574.
The Fischer-Tropsch reactions taking place in the slurry bed 84 are highly
exothermic and the slurry bed 84 is thus operated at a desired temperature
in the range of 210.degree. C. to 260.degree. C. In order to control the
temperature of the slurry bed 84 at the desired temperature, heat is
removed from the slurry bed 84 by means of the first cooling arrangement
20 and the second cooling arrangement 50.
In the first cooling arrangement 20, boiler water is continuously
circulated through the slurry bed 84 by means of the boiler water
circulation pump 28 and the cooling pipes 34. In the slurry bed 84, the
water inside the cooling pipes 34 is heated by indirect heat exchange and
a mixture of water and steam is formed. The water and steam mixture is
returned through the return line 40 to the steam drum 22, where the water
and steam separate, with the steam passing through the pressure control
valve 26 to the steam header 24. Fresh boiler water is added to the first
cooling arrangement 20 through a feed line 23.
The pressure control valve 26 is configured to control the pressure in the
steam drum 22 in abnormal or transient operating conditions. During normal
operation this valve is open so that the steam drum 22 is at substantially
the same pressure as the steam header 24, which pressure is typically
controlled using conventional means (not shown) at a pressure typically
about 16 bar(g). Thus, as will be appreciated, the pressure control valve
26 and the conventional means used to control the pressure in the steam
header 24 during normal operation are not used to control the temperature
of the slurry bed 84.
In the second cooling arrangement 50, boiler quality water is circulated
through the slurry bed 84 in indirect heat exchange by means of the pump
54 and the cooling pipes 62. The operating pressure of the water in the
second cooling arrangement 50 is about 40 bar(g), ensuring that steam
formation inside the cooling pipes 62 is substantially prevented. The
inlet temperature of the water into the cooling pipes 62, i.e. in the
manifold 60, is typically at least 100.degree. C.
The water in the cooling pipes 62 is returned through the manifold 64 and
the return line 66 to the air cooler 52, at a temperature of typically at
most 200.degree. C. In the air cooler 52, the water is cooled by indirect
heat exchange with ambient air before the water is returned to the cooling
pipes 62.
In order to control the temperature of the slurry bed 84, the temperature
of the cooling water inside the cooling pipes 62 is controlled. This may
be achieved, for example, by manipulating the operation of the air cooler
52 or by providing a bypass line around the air cooler 52.
If the combined heat duty of the cooling arrangements 20 and 50 becomes
particularly large due to a sudden release of heat in the slurry bed 84,
boiler water from the second cooling arrangement 50 can be used to replace
some of the boiler water in the first cooling arrangement 20 which is in
indirect heat exchange relationship with the slurry bed 84. This is
achieved by opening the valves 80 and 82 and closing the valves 74 and 78.
The valves 72 and 76 will normally be open. Water from the cooling
arrangement 50 then passes through one of the cooling pipes 34 of the
cooling arrangement 20 before it is returned to the air cooler 52. As will
be appreciated, since the operating temperature of the boiler water in the
second cooling arrangement 50 is lower than the operating temperature of
the boiler water in the first cooling arrangement 20, the combined heat
removal capacity of the first and second cooling arrangements 20, 50 is
thereby increased.
A large decrease in cooling duty can in similar fashion be catered for by
closing the valves 72 and 76, which will normally be open, and opening the
valves 80 and 82. The valves 74 and 78 are normally open. In this fashion,
boiler water from the first cooling arrangement 20 is circulated through
one of the cooling pipes 62 before being returned to the steam drum 22. As
the boiler water temperature of the first cooling arrangement 20 is higher
than the temperature of the boiler quality water in the second cooling
arrangement. 50, such an arrangement will reduce the combined heat removal
capacity of the first and second cooling arrangements 20, 50.
Referring to FIG. 2 of the drawings, reference numeral 100 generally
indicates another embodiment of an installation in accordance with the
invention for producing liquid and gaseous products from gaseous
reactants. Parts or features of the installation 100 which are the same as
or similar to those of the installation 10 of FIG. 1, are indicated with
the same reference numerals.
The installation 100 is very similar to the installation 10, but a major
difference is the fact that the second cooling arrangement 50 of the
installation 100 is a steam producing cooling arrangement. Thus, the
closed cooling arrangement 50 is operated at a pressure such that, in the
cooling pipes 62, the water is evaporated to form a mixture of steam and
water, which is fed to a steam drum 102 where the mixture separates into
steam and water. The steam is then transferred by means of a steam line
104 to the air cooler 52, whereas the water is removed by means of a flow
line 106 to a condensate tank 108. In the air cooler 52, the steam is
condensed and the condensate is removed to the condensate tank 108 by
means of a flow line 110.
The operating pressure of the second cooling arrangement 50 of the
installation 100 is substantially lower than the operating pressure of the
first cooling arrangement 20 of the installation 100. This ensures that
the temperature of the boiler quality water entering the cooling pipes 62
is also lower than the temperature of the boiler water entering the
cooling pipes 34, as is the case with the installation 10.
By allowing steam to be formed in the second cooling arrangement 50, the
piping can be designed with a much lower pressure rating than in the case
of the installation 10, where steam is not allowed to form. However, as
shown in FIG. 2, the second cooling arrangement 50 then requires a steam
drum 102 and a condensate tank 108.
The installation 10, 100, as illustrated, decreases the cost of heat
removal equipment, compared to the prior art and improves reactor
temperature control. Higher pressure steam can be produced, at a more
constant pressure. As a result of the pressure of the steam being higher,
the cost of using the steam for process heating and the driving of steam
turbines is reduced.
The quantity of steam produced is less than for the conventional
installation of which the applicant is aware. However, the steam that is
produced is of a higher pressure, and the decrease in steam production is
often advantageous because excess steam must often be condensed because
there are not sufficient users for the steam that is generated.
*
