Abstract:
A method and apparatus for converting gaseous reactants to liquid products catalyzed by stable catalysts. The method comprises providing a rotatable catalyst bed comprising Gas to Liquid (GTL) catalyst, feeding a gaseous stream of reactants into the catalyst bed and providing a pressure drop across the catalyst bed such that the gaseous stream flows through the catalyst bed so as to produce a gas output and a liquid product, and rotating the catalyst bed so as to enhance passage of said liquid product from the catalyst bed. The preferred apparatus comprises a rotatable fixed bed catalyst system including a catalyst active for converting syngas to hydrocarbons, a feed gas line for providing syngas to the catalyst bed, a liquid output line for receiving liquid output from the catalyst bed, and a gas output line for receiving gas output from the catalyst bed.

Description:
RELATED APPLICATIONS 
     The present application claims benefit of priority from U.S. application Ser. No. 60/255,243, filed Dec. 13, 2000, and entitled “Rotating Annular Catalytic Reactor.” 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a method of operating a catalyst bed for the preparation of More particularly, this invention relates to a rotatable catalyst bed comprising a gas to liquid (GTL) catalyst. Still more particularly, the present invention relates to a rotatable bed multiphase catalytic reactor that allows removal of liquid products from the catalyst bed as they form and thus reduces the formation of waxes and other high molecular weight products. 
     BACKGROUND 
     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 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 entails the catalytic hydrogenation of carbon monoxide to produce any of a variety of products ranging from methane to higher alkanes and aliphatic alcohols. The reaction is carried out by contacting the hydrogen and carbon monoxide with a catalyst. The reaction gives off a large amount of heat. When the Fischer-Tropsch reaction is carried out in fixed-bed reactors, this high heat of reaction results in an increase in the temperature of the catalyst bed above that of the surrounding environment. Excessive temperature rises can lead to inferior product distribution, and can damage the catalyst if not controlled. 
     When the Fischer-Tropsch process is carried out in a fixed bed reactor, synthesis gas is fed via an inlet into direct contact with the catalyst while heat is removed from the catalyst bed via heat exchange catalyst and a heat exchange medium, e.g., water. The heat exchange medium is typically contained in one or more tubular conduits passing through the catalyst bed. The optimum temperature gradient between the catalyst and the heat exchange medium must be one wherein the catalyst produces a product having the desired spectrum of hydrocarbons, while the catalyst bed remains thermally stable. 
     Slurry reactors avoid the problem of paraffinic buildup by suspending the catalyst in the liquid reaction products. On the other hand, the mass transfer rate, which is determined by the ability of the feed gases to reach a catalyst surface, is greatly reduced as compared to fixed-bed reactors. This is because the liquid products of the catalyzed reaction coat the surfaces of the catalyst bed and thus reduce the contact between the feed gases and the catalyst. In addition, catalysis continues while the liquid products remain in contact with a catalyst surface. As the reactions proceed, some of the hydrocarbons grow large enough that they undergo a phase transformation from liquid to solid at reactor conditions. Solids formed in the catalyst bed in this manner are highly undesirable, as they obstruct the surface of the catalyst are relatively difficult to dislodge. 
     The foregoing issues arise in gas-to-liquid systems, including Fischer-Tropsch systems. Hence, a need exists for a gas-to-liquid system that avoids the inefficiency and operating difficulty caused by the buildup of liquid and solid reaction products in the catalyst bed. It is further desired to provide a system that allows control over the residence time or contact time for the liquid products in the catalyst bed. In addition, the desired system would provide the advantageous mass transfer rate of fixed bed reactors, and yet avoid the need for period removal of solids from the bed. 
     SUMMARY OF THE INVENTION 
     The present system and apparatus avoids the inefficiency and operating difficulty caused by the buildup of liquid and solid reaction products in the catalyst bed. The present system enhances separation of the products and allows control over the residence time or contact time for the liquid products in the catalyst bed. In addition, the present system provides the advantageous mass transfer rate of fixed bed reactors, and yet avoids the need for periodic removal of solids from the bed. The desired process allows the removal of liquid products from the catalyst bed at a desired rate, which means that the residence time of the liquids in the reactor, and thus the product distribution, can be controlled. 
     The present invention is applicable to any GTL reactions that are catalyzed by stable solid catalyst, such as Fischer-Tropsch reactions or methanol synthesis reactions. 
    
    
     BRIEF DESCRIPTION OF THE FIGURE 
     For a more detailed understanding of the present invention, reference will be made to the accompanying FIGURE, which is a schematic cross-section of a rotating gas to liquids reactor constructed in accordance with a preferred embodiment. 
    
    
     DETAILED DESCRIPTION 
     While the present invention is described below in the context of a Fischer-Tropsch system, it will be understood that the devices and principles disclosed herein are equally applicable to any gas-to-liquids operation that uses a solid catalyst. Reactor 
     Referring now to the FIGURE, a preferred embodiment of the present system comprises a reactor  10  that includes reactor housing  12  and an annular catalyst bed  20 . Annular bed  20  is defined by inner and outer catalyst retainers  22 ,  24 , respectively, which preferably comprise concentric tubular members having a common vertical axis  28 . The inside of inner catalyst retainer  22  defines a central chamber  23 , an annulus  25  is defined between catalyst retainers  22  and  24 , and an annular outer chamber  27  is defined between outer catalyst retainer  24  and the reactor housing  12 . 
     In one preferred embodiment, the radius of retainer  22  is between 2 and 20 cm and the radius of retainer  24  is between 4 and 40 cm, but it is understood that retainers  22  and  24  can have any desired radius, so long as annulus  25  is wide enough to contain a desired amount of catalyst. Similarly, bed  20  is preferably but not necessarily between 5 and 50 cm tall. In an alternative embodiment, central chamber  23  can be eliminated, or replaced with any alternative gas distribution device that allows gas to flow radially outward through the full height of the catalyst bed at substantially uniform pressure. 
     Central chamber  25  preferably includes a lower end  30  and an upper end  32 , with upper end  32  being closed by an end wall  34 . Annulus  23  is preferably packed with a suitable Fischer-Tropsch catalyst system, which may comprise supported or unsupported Fischer-Tropsch catalyst provided in a form having a relatively high surface area, such as saddles, rings, stacked layers of mesh, sponge, porous particles, or the like, such as are known in the art. Inner and outer walls  22 ,  24  are preferably perforated or comprise mesh or the like, so as to allow the passage therethrough of gas and liquid while still containing the catalyst packing. 
     Reactor  10  further includes an feed gas inlet  14 , a gas outlet  16  and a liquid outlet  18 . Feed gas inlet  14  opens into lower end  30  of chamber  23 . A rotating seal  31  is preferably provided between the stationary gas inlet  14  and the rotating bed  20 . Gas outlet  16  and liquid outlet  18  both preferably communicate with outer chamber  27 . 
     According to a preferred embodiment of the invention, annular catalyst bed  20  is mounted on a bearing (not shown) so as to be rotatable around axis  28 . The system includes a motor  40  engaging bed  20  for driving rotation of bed  20 . While motor  40  is shown mounted at the upper end of the bed  20 , it will be understood that the position of motor  40  is not important to operation of the present system. Likewise, it is not important that the bed rotate relative to the rest of the reactor  10 ; the entire reactor  10  can rotate in the manner described herein. In an alternative embodiment, catalyst bed is rotatable about a non-vertical axis, such as a horizontal axis, although such an embodiment is not preferred. 
     Reactor  10  preferably includes a cooling system  50 , which, in one preferred embodiment, comprises at least one cooling tube  52  in thermal contact with outer chamber  27 . Cooling tube  52  can comprise a coiled tube that spirals around the circumference of chamber  27 , as shown, or can be any other suitable configuration that is suitable for effective heat exchange between the cooling system and the contents of chamber  27 . A cooling medium, such as water, enters cooling tube  52  at its inlet end  54  and exits at its exit end  56  after absorbing heat from chamber  27 . The heated medium can be used as a heat source in another system, or simply cooled and recycled. It will be understood that cooling system  50  can take other forms, including any suitable heat-removal device capable of removing heat from the system without interfering with the catalytic reaction, including but not limited to multiple tubes, cooling fins, heat sinks, etc. 
     Operation 
     In operation, the inside of reactor  20 , and in particular catalyst bed  20 , is maintained at desired Fischer-Tropsch reaction-promoting conditions, such as are known in the art. Catalyst bed  20  is rotated at a predetermined rotation rate, which is set as discussed below. A feed gas stream comprising a mixture of CO and hydrogen (syngas) enters inner chamber  23  via inlet  14 . Because inlet  14  is at a slightly higher pressure than outlet  18 , the gas flows radially outward through the perforated catalyst retainers and the catalyst of catalyst bed  20 . As the gas contacts the catalyst in the catalyst bed, it reacts to form hydrocarbons, according to the Fischer-Tropsch mechanism. The annular configuration and substantially radial gas flow produces a substantially uniform residence time for the gas in the catalyst bed. 
     The hydrocarbons produced in the Fischer-Tropsch process range from single-carbon methane gas, up to C11+ and higher. Some of the produced hydrocarbons are liquids at the Fischer-Tropsch reactor conditions. In a conventional fixed-bed reactor, these liquids would tend to accumulate in the interstices of the catalyst bed, thereby reducing the effectiveness of the catalyst for gas-to-liquid conversion. In the present reactor, however, the rotation of bed  20  produces sufficient centrifugal force to cause the liquid products to migrate radially outward, “falling” toward outer catalyst retainer  24 . Once at the outer surface of retainer  24 , the produced liquids flow under the force of gravity to the floor of the reactor, and then out through outlet  18 . If the rotation rate of the catalyst bed is high enough, droplets of liquid may be flung outward from the surface of retainer  24  and may or may not reach housing  12  before dropping to the bottom of outer chamber  27 . In any event, the rotation of the bed tends to facilitate removal of produced liquids from the catalyst surfaces and thereby increases operating efficiency. 
     It is preferred that bed  20  rotate with sufficient angular velocity to generate a centrifugal force at least as great as the force of gravity and, more preferably, at least about two times the force of gravity. Because the radial acceleration resulting from rotation of the reactor can be controlled, the residence time of the produced liquids in the reactor can be controlled to some extent. Because outer retainer  24  is perforated, the thickness of the catalyst bed places an upper limit on the residence time, since liquids reaching retainer  24  exit the bed and are no longer in contact with the catalyst. At the same time, the residence time cannot be longer than the time that it would take the liquids to fall downward through the height of the bed under the force of gravity alone. 
     An example of a preferred technique for setting the centrifugal force generated by rotating the bed  20  is as follows. For a bed  20  of radius R rotating at an angular velocity of w(rev/sec), the radial acceleration at the outer surface of the bed is given by v2/R, where v=2πRw. Thus, for an annular bed  20  having an outer radius equal to 10 cm, the angular velocity w required to generate a force at the outer radius equal to the acceleration of gravity (980 cm/sec2) is only 1.57 rev/sec. The angular velocity w required to generate a force at the outer radius equal to twice the gravitational force is only 2.23 rev/sec. 
     Hence, it is relatively easy to generate within the particle bed a radial acceleration that is greater than the acceleration of gravity and thus remove liquid products from the catalyst bed more efficiently than by using gravity alone to remove the liquids. While the radial acceleration can be increased by increasing the rotation rate, it may be preferred to allow the liquid products to remain in contact with the catalyst for some amount of time. In addition, the type of catalyst system, i.e. its permeability of the catalyst bed, will affect how quickly liquids pass through the bed. Hence, selection of the preferred rotation rate will depend on the reactor dimensions, the type of catalyst bed, and the desired residence time. 
     Various modifications to the embodiments described above can be made. For example, outer retainer  24  can be constructed to include grooves or channels that direct liquids leaving the bed  20  to a desired point. Likewise, the rotation of bed  20  need not be continuous, but can be pulsed or intermittent, with the rate of rotation varying between a predetermined upper value and a predetermined lower value, with the lower value including zero rotation. If desired, rotation of bed  20  can be controlled by feedback from the reactor itself. For example, the mounting of bed  20  can include a weight sensor. Upon the accumulation of liquids in bed  20 , the weight sensor produces a signal that in turn causes bed  20  to be rotated. Bed  20  can be rotated for a predetermined amount of time or until the sensor signal indicates that the liquid level in bed  20  has returned to a desired level. Alternatively, intermittent rotation of bed  20  can be controlled by a timer, with the length of the rotational and non-rotational periods being independently predetermined. 
     Gases produced in reactor  20  exit via gas outlet  16 , while liquids produced in reactor  20  exit via outlet  18 . The gases can be burned, exported from the system, recycled through the Fischer-Tropsch process via recycle line  17 , or otherwise disposed of as desired. Similarly, the liquid hydrocarbons exiting via outlet  18  can be burned, exported from the system, or otherwise disposed of as desired. 
     Catalyst 
     The present methods can be used in conjunction with any gas to liquid catalysis system, including any suitable Fischer-Tropsch catalyst system, including supported and unsupported catalysts. Since the reactor uses a fixed bed, the catalyst system is not subjected to the mechanical erosion that increases catalyst attrition in slurry reactors. Hence, catalyst systems that are not robust enough for slurry bed reactors can be used in the present system. The catalytically active materials can include but are not limited to iron, nickel, cobalt, ruthenium, and combinations thereof, with and without one or more promoters such as manganese, vanadium, platinum, palladium and other elements, such as are known in the art. These catalysts can be supported on suitable catalyst supports, or can be provided in an unsupported form, so long as sufficient catalytic area and gas flow area are provided. 
     Feed Gases 
     During conversion, the Fischer-Tropsch reactor is charged with feed gases comprising hydrogen or a hydrogen source and carbon monoxide. H2/CO (syngas) mixtures suitable as a feedstock for conversion to hydrocarbons according to the process of this invention can be obtained from light hydrocarbons such as methane by means of steam reforming or partial oxidation. The hydrogen is preferably provided by free hydrogen, although some Fischer-Tropsch catalysts have sufficient water gas shift Fischer-Tropsch activity to convert some water to hydrogen for use in the Fischer-Tropsch process. It is preferred that the mole ratio of hydrogen to carbon monoxide in the feed be greater than 0.5:1 (e.g., from about 0.67:1 to 2.5:1). The feed gas may also contain carbon dioxide or other compounds that are inert under Fischer-Tropsch reaction conditions, including but not limited to nitrogen, argon, or light hydrocarbons. The feed gas stream should contain a low concentration of compounds or elements that have a deleterious effect on the catalyst. The feed gas may need to be treated to ensure low concentrations of sulfur or nitrogen compounds such as hydrogen sulfide, ammonia and carbonyl sulfides. 
     During conversion, the gas hourly space velocity through the reaction zone may range from about 100 volumes/hour/volume catalyst (v/hr/v) to about 10,000 v/hr/v. The reaction zone temperature is typically in the range from about 160° C. to about 300° C. Preferably, the reaction zone is operated at conversion promoting conditions at temperatures from about 190° C. to about 260° C. The reaction zone pressure is typically in the range of about 80 psig (653 kPa) to about 1000 psig (6994 kPa). 
     While the preferred embodiments of the invention have be disclosed herein, it will be understood that various modifications can be made to the system described herein without departing from the scope of the invention. For example, the various inlet, outlet and cooling lines and the catalyst bed itself can be reconfigured, the mechanism used to provide the rotational force tot he catalyst bed can be varied, and the placement and type of feed gas inlet can be altered. Without further elaboration, it is believed that one skilled in the art can, using the description herein, utilize the present invention to its fullest extent.