Abstract:
The present invention relates to a method and system for water removal and optionally liquid product separation in slurry reactors operating at Fischer-Tropsch conditions. More particularly, the present invention includes a water stripping system that allows the reaction water to the stripped in an external vessel, with a relatively high rate of catalyst and wax circulation. In a preferred embodiment of the present invention, a method for removing water from a slurry reactor containing a water-rich slurry includes removing a portion of water-rich slurry from the slurry reactor, stripping water from the water-rich slurry using a dry gas to form a water-reduced slurry and a water-rich gas stream, and returning the water-reduced slurry back to the reactor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to concurrently filed, commonly assigned, co-pending U.S. Provisional applications Ser. No. 10/320,311, entitled “Water Removal in Fischer-Tropsch Processes,” and Ser. No. 10/315,371, entitled “Method For Reducing Water Concentration In A Multi-Phase Column Reactor,” both hereby incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for the preparation of hydrocarbons from synthesis gas, i.e., a mixture of carbon monoxide and hydrogen, typically labeled the Fischer-Tropsch process. Particularly, this invention relates to a method and apparatus for in situ water removal in multi-phase column reactors operating at Fischer-Tropsch conditions. 
     BACKGROUND OF THE INVENTION 
     Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive. To improve the economics of natural gas use, much research has focused on the use of methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids, which are more easily transported and thus more economical. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is converted into a mixture of carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted into useful hydrocarbons. 
     This second step, the preparation of hydrocarbons from synthesis gas, is well known in the art and is usually referred to as Fischer-Tropsch synthesis, the Fischer-Tropsch process, or Fischer-Tropsch reaction(s). Fischer-Tropsch synthesis generally entails contacting a stream of synthesis gas with a catalyst under temperature and pressure conditions that allow the synthesis gas to react and form hydrocarbons. 
     More specifically, the Fischer-Tropsch reaction is the catalytic hydrogenation of carbon monoxide to produce any of a variety of hydrocarbon products ranging from methane to higher alkanes. Research continues on the development of more efficient Fischer-Tropsch catalyst systems and reaction systems that increase the selectivity for high-value hydrocarbons in the Fischer-Tropsch product stream. 
     There are continuing efforts to design reactors that are more effective at producing these desired products. Product distribution, product selectivity, and reactor productivity depend heavily on the type and structure of the catalyst and on the reactor type and operating conditions. Catalysts for use in such synthesis usually contain a catalytically active metal of Groups 8, 9, or 10 (in the New notation of the periodic table of the elements, which is followed throughout). In particular, iron, cobalt, nickel, and ruthenium have been abundantly used as the catalytically active metals. 
     Originally, the Fischer-Tropsch synthesis was operated in packed bed reactors. These reactors have several drawbacks, such as temperature control, that can be overcome by gas-agitated slurry reactors or slurry bubble column reactors. Gas-agitated reactors, sometimes called “slurry reactors” or “slurry bubble columns,” operate by suspending catalytic particles in liquid and feeding gas reactants into the bottom of the reactor through a gas distributor, which produces small gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are converted to gaseous and/or liquid products. The gaseous products enter the gas bubbles and are collected at the top of the reactor. Liquid products are recovered from the suspending liquid using different methods, for example, by passing the slurry through a filter that separates the liquid from the catalyst solids, and then separating the liquids. 
     A known problem in slurry reactors, however, is that water is continuously formed during Fisher-Tropsch synthesis in the reactors. This is known to limit conversion and prematurely deactivate catalyst systems such as iron and cobalt-based Fisher-Tropsch catalysts through an oxidation mechanism. As is well known in the art, a high water partial pressure correlates to a high deactivation rate. This is detrimental to the overall system performance, since two requirements for a successful commercial application of cobalt-based Fischer-Tropsch catalysts are a stable performance (low deactivation rate) and, for middle distillates production, a high wax selectivity (or a high alpha value). 
     For any given cobalt-based catalyst, along with the H2/CO ratio and the reaction temperature, the total pressure is a parameter that has a direct influence on the wax selectivity, in that a higher pressure will result in a higher wax selectivity. However, a higher total pressure (at any given degree of per-pass conversion) also correlates to a higher water partial pressure and therefore a higher deactivation rate. Therefore, if reactors are operated at conditions conducive to higher alpha values (higher pressures), per-pass conversion will necessarily have to be low to avoid premature catalyst deactivation due to water. A low per-pass conversion is undesirable, however, because it results in higher capital investment and operating costs. Additionally, for iron-based catalysts, the water not only has a negative effect on the catalyst deactivation rate, but it also inhibits the rate of reaction. 
     The water partial pressure is therefore a constraint that will not allow the realization of the kinetic and/or wax selectivity potential of iron and cobalt-based Fisher-Tropsch catalysts. Therefore, in order to improve the efficiency of slurry reactors using iron and cobalt-based catalyst systems, there exists a need for a method to remove water formed during Fisher-Tropsch synthesis. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a system and method for water removal and optional liquid product separation in slurry reactor systems operating at Fischer-Tropsch conditions. More particularly, the present invention includes a water stripping system that allows the reaction water to be stripped and heavy liquid products to be removed in an external vessel. The term “heavy liquid” products is herein defined as hydrocarbons in the wax range, that is hydrocarbons heavier than Carbon 19. Generally, in stripping, a liquid containing a dissolved liquid or gas, such as water, flows down a column while a stripping gas flows up the column at conditions such that the dissolved liquid or gas comes out of solution and is carried off with the stripping gas. In the present invention this system can remove water dissolved in the wax, and potentially water contained in the very small bubbles in the wax, therefore allowing a higher per-pass conversion at pressures conducive to high alpha values while protecting the catalyst from excessive oxidation. By allowing a higher pass per conversion, fewer stages may be necessary to achieve a suitable overall conversion. 
     In a preferred embodiment of the present invention, a method for removing water from a slurry reactor containing a water-rich slurry includes removing a portion of water-rich slurry from the slurry reactor, stripping water from the water-rich slurry using an dry gas to form a water-reduced slurry and a water-rich gas stream, and returning the water-reduced slurry back to the reactor. Generally, the slurry includes gaseous reactants and products, liquid hydrocarbon products ranging from light liquid products to heavy liquid products, and catalyst. In some embodiments, a fraction of the heavy liquid products may be separated from the remaining slurry prior to returning the slurry back to the reactor. 
     In another preferred embodiment of the present invention, a method for producing hydrocarbons includes contacting a synthesis gas with a hydrocarbon synthesis catalyst in a slurry body comprising the catalyst and gas bubbles in a hydrocarbon slurry liquid having light and heavy components, under reaction conditions effective to form hydrocarbons and unavoidable secondary products, such as water, from the synthesis gas. A portion of the slurry from the slurry body then passes into a gas-disengaging zone to separate gas bubbles from the slurry and form gas-reduced slurry. Next, the gas-reduced slurry passes into a stripping zone, wherein the gas-reduced slurry is contacted with a dry stripping gas, which at least partially removes water therein to form water-reduced slurry. Lastly, the water reduced slurry passes back into the slurry body. In some embodiments, a fraction of the liquid component of the hydrocarbon slurry is separated from the remaining slurry prior to passing the slurry back into the slurry body. 
     The present invention allows higher per-pass conversions of syngas and/or use of higher total pressures at any given degree of conversion, while protecting the Fischer-Tropsch catalyst from an excessive oxidation rate. By returning the water-reduced slurry back into the reactor and optionally removing a fraction of the heavy liquid products, the catalyst inventory in the reactor is kept approximately constant. In some instances, circulation of the liquid back to the reactor may improve the temperature profile in the reactor. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     A better understanding of the present invention can be obtained when the following detailed description of the preferred embodiment is considered in conjunction with the following drawings: 
     FIG. 1 is a schematic diagram of a Fischer-Tropsch reactor system including a gas-stripping unit in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is a schematic diagram of a first gas stripping system in accordance with one embodiment of the present invention; 
     FIG. 3 is a schematic diagram of a gas stripping and settling system in accordance with a preferred embodiment of the present invention; and 
     FIG. 4 is a schematic diagram of an alternative gas stripping and settling system in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Where natural gas of suitable quantity and quality is available, gas stripping may be employed to extract undesirable components, such as water, from a Fischer-Tropsch system. In a preferred embodiment of the present invention, a stream of dry gas is introduced in countercurrent flow to the water/wax/hydrocarbon slurry. In some embodiments, hydrogen is the preferred stripping gas. Introduction of the stripping gas to the slurry reduces the water partial pressure in the mixture, creating a drive force for mass transfer from the liquid to the gas phase. 
     Process 
     Referring now to FIG. 1, a system  100  in accordance with a preferred embodiment of the present invention includes slurry reactor  120 , a de-gasser  130 , and a water stripping system  140 . Reactor  120  includes a tank  126 , a catalyst system (not shown), inlets  112  and  157 , and outlets  121 , and  123 . De-gasser  130  preferably includes a tank  136 , optional baffle plates (not shown), an inlet  129 , and outlets  131  and  133 . Water stripping system  140  includes a vessel  142 , inlets  139  and  145 , and outlets  141  and  147 . If included, the baffle plates are preferably not heated. The interrelationship and separation of these components are discussed in detail below. 
     Slurry Reactor 
     As described earlier, slurry reactors operate by suspending catalytic particles in liquid by feeding gas reactants in line  110  into the bottom of reactor  120  through inlet  112 , which produces gas bubbles (not shown). As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are converted to gaseous and liquid products. The gaseous products enter the gas bubbles and exit at the top of reactor  120  through outlet  121  into line  122 . Liquid products  124  leave reactor  120  as a water-rich slurry via outlet  123  and enter de-gasser  130  at inlet  129 . A valve  125  in line  124  regulates the flow of slurry to de-gasser  130 . Outlet  123  is preferably positioned near the top of the slurry bed. It is known to operate slurry bed Fischer Tropsch reactors in a variety of ways, including but not limited to: plug flow of gas through the catalyst bed and well-mixed or back-mixed gas flow. 
     De-Gasser 
     De-gasser  130  may include any suitable de-gassing equipment. For example, when liquid droplets are entrained in a gas, separation is enhanced by allowing the liquid drops to hit and adhere to a solid surface, such as a baffle plate. Similarly, when a gas is dissolved in a liquid, separation may be enhanced by inducing the gaseous constituent to assume the vapor phase. Inducing the gaseous constituent to assume the vapor phase requires disturbing the equilibrium between the gas and the liquid. This may be done by heating the liquid, thus lowering the solubility of the gas, by passing a second gas through the liquid so as to sweep out the dissolved gas, or by lowering the pressure above the liquid. Various other de-gassing techniques are known in the art and can be used in the present system. In one preferred embodiment of the present invention, de-gasser  130  includes simply a baffle plate. 
     As the slurry enters de-gasser  130 , it flows downward and is optionally guided by baffle plates (not shown). A portion of the gas dissolved in the slurry flows upward, forming a gas stream, which exits the top of de-gasser  130  into line  132  and a degassed water-rich slurry, which exits the bottom of de-gasser  130  into line  134 . The gas stream  132  exits de-gasser  130  through outlet  131  at the top of de-gasser  130 . The gas stream  132  may optionally be combined with gaseous products stream  122 , as shown. Degassed water-rich slurry line  134  exits de-gasser  130  via outlet  133  at the bottom of de-gasser  130  and enters water-stripping system  140  at inlet  139 . The stream leaving de-gasser  130  via line  134  is preferably essentially free of gas bubbles and contains essentially all of the liquid and solids leaving reactor  120 . 
     Stripping and Separation System 
     The slurry containing the liquids and solids flow from de-gasser  130  into striping system  140 , wherein it is stripped of water. A first embodiment of a stripping system  140  is illustrated in detail in FIG.  2 . System  140  preferably includes a cylindrical column, or tower,  142  equipped with a gas inlet  145  and a distribution chamber  166  at the bottom; a liquid inlet  139  and an optional distributor  160  at the top; and liquid and gas outlets  147  and  141  at the bottom and top, respectively. The inlet liquid in line  134 , which contains the water-rich slurry, is distributed into vessel  142  by distributor  160 . A dry stripping gas, such as hydrogen, methane, nitrogen, carbon or any combination of them, enters distribution chamber  166  at the bottom of vessel  142  and flows upward, countercurrent to the flow of the liquid. The dry stripping gas does not have to be 100% pure and it may contain small amounts of other gases, for instance, carbon monoxide, carbon dioxide, light hydrocarbons, etc. In some embodiments, the gas is sparged into vessel  142 , increasing the area of contact between the liquid and gas, and encouraging intimate contact between the phases. Contact between phases can also be improved by placing packing elements  143 , or metal tubes, rods, or screens (not shown) inside vessel  142  so as to slow the flow of the gas bubbles upward through the slurry. If packing elements  143  are used, it is preferred to provide a supporting grid (shown in phantom) beneath packing elements  143 , so as to prevent them from settling on the bottom of vessel  142 . 
     The water in the slurry is stripped by the dry gas entering the vessel, and water-rich gas leaves the top of the tower through outlet  141  into line  146 . The water content in the slurry decreases as the slurry flows downward in vessel  142 , so that the slurry leaving the bottom of vessel  142  through liquid outlet  147  is essentially water-free. The water-free slurry in line  148  can be exported via line  156 . 
     Referring again to FIG. 1, the de-watered slurry mixture  148  is more preferably recycled into reactor  120  at inlet  157  so that the catalyst is conserved. A valve  155  on line  148  regulates slurry flow to slurry reactor  120 . In some embodiments, a portion  156  of slurry mixture  148  may be removed for other uses such as sampling for quality control purposes, etc. 
     Still referring to FIG. 1, the water-rich vapor phase stream leaving vessel  142  via line  146  comprises the gaseous stripping agent, water, and various amounts of other vaporized products such as unreacted stock from line  110 , and part of the gaseous products formed in reactor  120 . In some embodiments, system  140  may include a wet gas purifier  150 , wherein wet gas stream  146  is separated into components including dry gas and water/light hydrocarbons mixture. A valve  149  may be used to send all or a portion of gaseous stream  146  to wet gas purifier  150  or to be mixed with streams  122  and/or  132 . Dry gas from wet gas purifier  150  may then be recycled via line  172  back into vessel  142  via feed line  144 . Optionally, either wet gas stream  146  or dry gas stream  172  may be combined with outlet streams  122  and/or  132 . Also optionally, part or all of the dry gas in stream  172  may be sent to a further purification section (not shown) via line  174  or may be purged from the system via line  176 . 
     In wet gas purifier  150 , at least a portion of the gas stream in line  146  is condensed so that two phases are formed, namely a stripping agent rich phase and a water-rich phase. The stripping agent rich phase is preferably returned to stripper  140 . Subsequent processing of the water-rich phase may be performed by processes known in the art to recover the material and render the water suitable for disposal. 
     In some embodiments, in addition to stripping out the water, it may be desirable to separate a fraction of the liquid products from the catalyst prior to recycling the de-watered slurry. A stripping and settling system  240  that is suitable for this dual purpose is illustrated in FIG.  3 . System  240  is an alternative to system  140 , inasmuch as it includes a stripping vessel  242  equipped with a gas inlet  245 , a slurry inlet  239 , slurry and gas outlets  247  and  241  at the bottom and top, respectively. System  240  further includes a liquid outlet  253  intermediate between the top and bottom of vessel  242 . System  240  preferably also includes at least one internal baffle plate  250  and may optionally include a slurry distributor (not shown) and a gas distribution chamber (not shown). Baffle plate  250  defines a sparging zone  261  on one side thereof and a quiescent zone  251  on the other side thereof. 
     The water-rich slurry containing catalyst particles, hydrocarbon liquids and water enters the top of vessel  242  via line  134 . As in the embodiment of FIG. 2, a dry stripping gas enters the bottom of vessel  242  from line  144  and flows upward through the slurry. The dry stripping gas does not have to be 100% pure and it may contain small amounts of other gases, for instance, carbon monoxide, carbon dioxide, light hydrocarbons, etc. Preferably, the gas is sparged into vessel  242 , forming bubbles  243 . The water in the liquid is removed by the stripping gas, and water-rich stripping gas leaves the top of the vessel through outlet  241 . 
     Because the catalyst particles are much denser than the liquids in the slurry, they begin settling as soon as the slurry enters the vessel. In sparging zone  261 , however, rising gas bubbles  243  tend to prevent complete settling of the particles. Hence, in this embodiment, baffle plate  250  is preferably provided so as to define a quiescent zone  251  that is essentially free of rising gas bubbles and in which the hydrocarbon liquids  252  can be separated from the catalyst particles  254  using the density difference between the catalyst particles  254  and the liquid product  252 . In a preferred operation, relatively or essentially water-free slurry flows under baffle plate  250  into quiescent zone  251 . Because the catalyst particles  254  are denser than the liquid product  252 , they tend to settle to the bottom of vessel  242 . 
     In a preferred embodiment, vessel  242  is sized such that the residence time of the slurry therein is sufficient to allow most or essentially all of the catalyst particles to settle out of an upper portion of the hydrocarbon liquid. In a further preferred embodiment, the floor of vessel  242  includes a collection area  257  in which further settling of particles  243  can occur. Collection area  257  is optionally positioned under quiescent zone  251 . In some instances, it may be preferred to position outlet  247  as far away from sparging zone  261  as possible. In an alternative embodiment shown in FIG. 4, collection area  257  need not be positioned asymmetrically and can be conical or sloped so as to enhance settling and separation of particles  243 . The settled catalyst and another portion of the liquid product exit the bottom of vessel  242  via outlet  247  and follow the preferably gravity-driven circulation loop  148  back to reactor  120  (FIG.  1 ). Sufficient liquid should be removed through outlet  257  to ensure that the catalyst-containing slurry in line  148  is flowable and, if necessary, pumpable. 
     A second portion of the liquid product, typically comprising catalyst-free or substantially catalyst-free liquid product, can be removed from vessel  242  via outlet  256 . Outlet  256  is positioned preferably in quiescent zone  251  and at a sufficient height above the floor of vessel  242  to minimize the possibility that stray particles  254  will pass through it. If desired, a screen  258  may optionally be included at outlet  256 , to ensure that no catalyst enters line  253 . The amount of liquid withdrawn through line  253  is preferably set such that substantially no solid particles are withdrawn. 
     Alternative Operation 
     In some instances, it may be desirable to remove the heavy liquid products from the product prior to recycling the slurry without stripping. In such cases, gas inlet  245  is closed and baffle plate  250  may be optionally removed. 
     Computer Simulation 
     The operating conditions (i.e. pressure, temperature) of the stripping system  140 ,  240  are preferably similar to conditions of the slurry reactor  120  because a circulating loop is established between the two components. Using typical conditions (e.g. T=425° F., P=400 psia), for a nominal liquid flow rate of 10,000 kg/h in the slurry circulation loop (reactor to stripper to reactor) the following stripping streams were simulated using a commercial process simulator. The results are listed in Table 1 below. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Stripping 
                 % Water removal 
               
               
                   
                 Gas 
                 from slurry 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Hydrogen 
                 82 
               
               
                   
                 Methane 
                 80 
               
               
                   
                 Nitrogen 
                 79 
               
               
                   
                 Light Hydrocarbons (&lt;C 10 ) 
                 &lt;20 
               
               
                   
                   
               
             
          
         
       
     
     In each simulation, stripping gases were injected at a rate of 8 kg-mol/h per 10,000 kg/h of slurry flow in the stripping loop. As can be seen, the water stripping performance is better for hydrogen than for methane and nitrogen. However, the effect of the stripping gas downstream also plays a role in determining the most suitable stripping gas, as discussed below. 
     For a reactor configuration of multiple reactor stages in series, the overhead gas stream of the water stripper may be mixed with the overhead of the reactor, and after cooling and condensing steps, the gas mix is sent to the next reaction stage. In this configuration, wet hydrogen, produced in the water stripper, tends to react in the next stage reactor, while methane and nitrogen act as inert components. This higher inert content in the reactant mixture is detrimental to the economics of the process because of under-utilization of reactor volume and a possible decrease in the Fischer-Tropsch reaction rate. For these reasons, it is preferred to use a hydrogen-rich stream as the stripping gas in the present water stripping system, although other gases may also be used. 
     While the present invention has been disclosed and described in terms of a preferred embodiment, the invention is not limited to the preferred embodiment. For example, while the present invention has been described with a vessel with internals to favor contact between slurry and the stripping gas, it should be understood that other vessel designs may be used. In addition, various modifications to the operating conditions and stripping gases, among others, can be made without departing from the scope of the invention. In the claims that follow, any recitation of steps is not intended as a requirement that the steps be performed sequentially, or that one step be completed before another step is begun, unless explicitly so stated.