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.

Description:
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. § 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. 
    
    
     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° C. to 260° 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° 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° 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.