Abstract:
Apparatus and methods are described for transporting and cooling silicon-coated granules produced in a fluidized bed reactor. The described system allows consistent silicon-coated granule production with fewer impurities than traditional silicon granule coolers. Granules flow from the reactor into a cooling vessel and subsequently are transported to a post production treatment system below the cooler. The cooling vessel is constructed as a single standpipe, vertical or near vertical, with a pipe diameter that allows granules to flow freely while providing adequate residence time for cooling. The standpipe is cooled by flowing a cooling medium through a passageway that extends along an external surface of the standpipe. The passageway can be provided by a pipe jacket or conduit.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This claims the benefit of U.S. Provisional Application No. 61/495,744, filed Jun. 10, 2011, which is incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present disclosure relates to a system and method for transporting and cooling silicon-coated granules produced in a fluidized bed reactor. 
       BACKGROUND 
       [0003]    Pure or high grade polycrystalline silicon (polysilicon) is a critical raw material for both the semiconductor (SC) and photovoltaic (PV) industries. While there are alternatives for specific photovoltaic applications, polysilicon will remain the preferred raw material in the near and foreseeable future. Hence, improving the availability of and economics for producing polysilicon will increase the growth opportunities for both industries. 
         [0004]    The majority of polysilicon currently is produced by the commonly called Siemens hot-wire method wherein silicon is deposited by the decomposition of a silicon-bearing gas, typically silane or trichlorosilane (TCS). The silicon-bearing gas, usually mixed with other inert or reaction gases, is pyrolytically decomposed and deposited onto a heated silicon filament. 
         [0005]    Another method that has gained recent interest is the pyrolytic decomposition of silicon-bearing gas in a fluidized bed of silicon granules. The silicon-bearing gas, usually mixed with other inert or reaction gases, is pyrolytically decomposed and deposited onto the granules that have been heated by heaters surrounding the fluidized bed. This method is an attractive alternative to produce polysilicon for the photovoltaic and semiconductor industries due to significantly lower energy consumption and the possibility for continuous production. These benefits are the result of excellent mass and heat transfer, a substantially increased deposition surface and continuous production. Compared with the Siemens-type reactor, a fluidized bed reactor offers considerably higher production rates at a fraction of the energy consumption. The fluidized bed reactor also can operate continuously and be highly automated to significantly reduce labor costs. 
         [0006]    The fluidized bed reactor produces silicon in a granular form. In traditional designs of the silicon fluidized bed reactor, the produced granules are emptied into a granule handling system below the fluidized bed reactor. The granules usually are cooled before they enter the handling system to minimize the risk of high temperature, diffusion-related contamination and the need for high temperature equipment and instrumentation. Compact units with high cooling surface area, such as tube and shell coolers as described in  Chemical Engineer&#39;s Handbook , Perry and Chilton, 5 th  Edition, “Section 11—Heat Transfer Equipment,” traditionally are used for the cooling devices in such applications. These types of devices are prone to contaminate the granular silicon product because they have complex geometric surfaces that are difficult to coat with a non-contaminating material. They are also subject to process upsets due to cooling medium leaks from inherent mechanical and thermal stress issues. 
       SUMMARY 
       [0007]    Described herein are apparatuses and methods for transporting and cooling silicon-coated granules produced in a fluidized bed reactor. The described systems allow consistent silicon-coated granule production with fewer impurities than traditional silicon granule coolers. Granules flow from the reactor into a cooling vessel and subsequently are transported to a post production treatment system below the cooler. The cooling vessel is constructed as a single standpipe, vertical or near vertical, with a pipe diameter that allows granules to flow freely while providing adequate residence time for cooling. The standpipe primarily is cooled externally either by a jacketed pipe or with a cooling medium path extending in proximity to the external surface. The post treatment can include, but is not limited to, degassing hydrogen and traces of silane so granules can be handled under nitrogen or ambient atmosphere. 
         [0008]    These arrangements allow cooling with minimum risk of contamination from cooling medium leakage because leaks will be contained outside the standpipe. Leak reduction is enhanced with the cooling medium path extending around the standpipe&#39;s external surface. And the pipe shape, with only a peripheral contact surface, is inherently less prone to contamination than a system with tube bundles in a shell. The disclosed systems are more robust and provide safer production than conventional systems by preventing the cooling medium from contacting the silicon-coated granules and minimizing the risk for areas of reduced cooling medium flow. Such areas of reduced flow can lead to overheating and evaporation, and result in overpressure that will upset production. 
         [0009]    The standpipe can be lined or coated with non-contaminating material to produce higher quality material than traditional coolers. Additionally, the smoother flow path eliminates holdup in coolers after shutdown and thus increases overall production yields. It also facilitates maintenance cleanup during turnaround of a reactor. 
         [0010]    The cooled silicon-coated granules are delivered from the standpipe to a post-production treatment system below the reactor. The post-production treatment can include, but is not limited to, degassing hydrogen and traces of silane so granules can be handled under nitrogen or ambient atmosphere. 
         [0011]    A further refinement of the standpipe cooler provides improved granule quality through dedusting, silicon coating and dehydrogenation. Very fine silicon powder particles entrained within the product can be an explosion hazard under atmospheric conditions. Silicon powder particles can be entrained by countercurrent flow of gas through the pipe. Such entrainment will be much more efficient in a single tubular pipe design than a traditional cooler where multiple and rigorous flow paths make entrainment difficult. To further reduce the powder adhered to the granule surface, traces of silane can be introduced with the countercurrent gas to cause slow silicon deposition onto the granules. This deposition will create a chemically bonded layer of newly deposited silicon and result in a smoother granule surface. Adjusting the temperature profile and granule holdup through the standpipe cooler can improve dehydrogenation by allowing time for chemisorbed hydrogen to diffuse from the granules. 
         [0012]    Features and advantages will become apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    In the drawings: 
           [0014]      FIG. 1  is a schematic diagram of a first fluidized bed reactor and standpipe cooler system. 
           [0015]      FIG. 2  is a schematic diagram of a standpipe cooler having a cooling jacket. 
           [0016]      FIG. 3  is a schematic diagram of a standpipe cooler having an external helical cooling conduit. 
           [0017]      FIG. 4  is a schematic diagram of a standpipe cooler having an internal cooling conduit. 
           [0018]      FIG. 5  is a schematic diagram of a standpipe cooler having multiple injection points. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  shows a fluidized bed reactor and cooling system  100 . The system comprises a fluidized bed reactor vessel  102  having a bottom-mounted outlet, a cooling vessel  104 , and a post-production treatment system  106 . The illustrated cooling vessel  104  is a substantially vertical standpipe granule cooler. Silicon-coated granules  108  are produced in the fluidized bed reactor  102  through the chemical vapor deposition of silicon onto starter granules in the reactor. A silicon-bearing gas enters the reactor  102  through an inlet (not shown) and decomposes pyrolytically in the reactor vessel, which is maintained at a sufficiently elevated temperature. 
         [0020]    Starter granules may have any desired composition that is suitable for coating with silicon. Suitable compositions are those that do not melt or vaporize, and do not decompose or undergo a chemical reaction under the conditions present in the reactor chamber. Examples of suitable starter granule compositions include, but are not limited to, silicon, silica, graphite, and quartz. Starter granules may have any desired morphology. For example, the starter granules may be spheres, elongated particles (e.g., rods, fibers), plates, prisms, or any other desired shape. Starter granules also may have an irregular morphology. Typically starter granules have a diameter in the largest dimension of 0.1-0.8 mm, such as 0.2-0.7 mm or 0.2-0.4 mm. 
         [0021]    Examples of silicon-bearing gases include, but are not limited to, silane and trichlorosilane. For simplicity, the use of silane is discussed in the examples herein, but it should be understood that similar operation would be possible with other silicon-bearing gasses of the type used for the production of polysilicon. 
         [0022]    After growth to a sufficient size, silicon-coated granules  108  flow through an outlet nozzle  110  positioned at the bottom of the fluidized bed reactor  102  and then into a withdrawal pipe  112 , which provides a passageway between the reactor and the cooling vessel  104 . The granules  108  fall by gravity from the withdrawal pipe  112  through a standpipe inlet nozzle  114  into the standpipe main vessel  104  where the granules  108  form a moving packed bed  116 . The packed granule bed  116  moves slowly down through the pipe  104  and out through the standpipe outlet  118 . 
         [0023]    As the packed granule bed  116  moves down through the standpipe vessel  104 , the granules  108  are gradually cooled. Initial granule temperatures may be more than 1000° C. The main cooling is achieved by transferring heat to the cooled walls  120  of the pipe  104 . The standpipe  104  may be surrounded by a cooling device  122 . 
         [0024]    Additional gas can be injected through separate injector nozzles  124  into the withdrawal pipe  112 , into the standpipe  104 , or into the standpipe outlet  118 . This gas is referred to as withdrawal gas and can be any inert gas, appropriate silicon-bearing gas, or mixture thereof. A gas that is already present in the fluidized bed reactor  102  is preferred. 
         [0025]    The withdrawal gas has multiple purposes. Additional cooling can be achieved by the injection of cold withdrawal gas into the standpipe  104 . In some embodiments, the cold withdrawal gas flows co-currently with the granular flow. In other embodiments, the withdrawal gas typically flows countercurrently to the granular flow and creates a gas backflow into the reactor  102 , minimizing the risk of reactor gas diffusing into the withdrawal pipe  112  and standpipe  104  where it could cause wall deposition and granule agglomeration. The withdrawal gas also entrains powder and small particles, thereby separating powder and small particles from the product granules  108  and moving the powder and small particles back up into the reactor  102 , which minimizes escape of free-flowing powder and small particles with the product granules  108 . 
         [0026]    To further reduce the presence of powder adhered to the surfaces of product granules, traces of silicon-bearing gas can be introduced with the withdrawal gas and contacted with the granules  108  within the standpipe  104  at a temperature sufficient to cause slow silicon deposition onto the granules. This deposition creates a chemically bonded layer of newly deposited silicon and result in a smoother surface. The deposition reduces product dustiness by binding powder to the granules and also adds to the production yield. The concentration of silicon-bearing gas in the withdrawal gas and the gas flow rate can be balanced to minimize powder production and the potential for entrained silane in gases leaving with the product granules. 
         [0027]    A high enough withdrawal gas flow can entrain practically all granules  108 , thus limiting the flow of granules from the reactor  102  into the standpipe  104 . Further, the gas cools the granules leaving the reactor  102  while also becoming preheated. This preheated withdrawal gas enters the reactor  102  carrying heat that can be used in the reactor  102 , lowering the heating duty required of the bed heaters. 
         [0028]    The cooling rate of granules  108  within the standpipe cooler  104  is a function of temperature differential, heat transfer efficiency, cooling area and cooling time. The granular flow rate is typically dictated by the fluidized bed reactor production rate to avoid accumulation. The temperature gradient is modified by the cooling medium temperature and possible multistage design of the cooling device  122  to maintain maximum cooling. Heat transfer efficiency is generally a function of granule size and reactor wall cleanliness. There is little variation in heat transfer efficiency during operation. 
         [0029]    The size of the cooling area is a function of the packed bed level because most cooling occurs in the packed bed  116 . The cooling time is a function of granular hold-up time in the standpipe  104 . Granular hold-up time depends upon granular flow rates in and out of the standpipe  104 . Granular in-flow is partially controlled by modifying the withdrawal gas flow, but typically varies with conditions of the fluidized bed reactor  102 . Thus the primary control is the granule flow control device  126 . Under steady state operation, the packed bed  116  level will be constant since flow in and flow out are equal. If granules  108  are removed from the standpipe  104  at a faster rate than granules  108  are entering the standpipe  104  from the fluidized bed reactor  102 , the level of the packed bed  116  will decrease. Conversely, if granules  108  are removed from the standpipe  104  more slowly than granules  108  enter from the fluidized bed reactor  102 , the level of the packed bed  116  will increase. A lower level results in a smaller cooling area and less cooling time for a given granular flow rate. 
         [0030]    Adjusting the temperature profile and granule holdup time through the standpipe cooler  104  can improve dehydrogenation of the silicon-coated granules  108  by allowing time for chemisorbed hydrogen to diffuse from the granules  108 . Within these controls, the operation of the standpipe cooler  104  can be continuous or batch as desired. 
         [0031]    Cooled granular product exits through the bottom standpipe nozzle  118  and passes through a granule flow control device  126  into the post-production treatment system  106 . The granule flow control device  126  functions as a valve that controls the granular flow rate out of the standpipe  104  and can completely stop the granule flow if required. The valve can be any valve capable of operating with granule flow. Typical valves include ball valves, slide gate valves and pinch valves, among others. The granule flow control device  126  typically is not gas-tight, so gas isolation valves  128  are used to isolate the standpipe cooler  104  and fluidized bed reactor  102  from the post production treatment system  106 . 
         [0032]    The primary purpose of the post-production treatment system  106  is to further eliminate free hydrogen gas and powder from the product. More advanced treatments, such as vacuum dehydrogenation, high temperature or extended hold time purging, and non-hydrogen gas purges, also may be applied if desired. 
         [0033]    The granules in the packed bed are primarily cooled by the cold walls of the standpipe.  FIGS. 2 and 3  illustrate two types of wall-cooling. One skilled in the art will understand that other wall cooling arrangements are possible. 
         [0034]    In  FIG. 2 , a cooling jacket  200  surrounds the length of the standpipe  202 . The illustrated cooling jacket  200  is adjacent and concentric to the outer wall  204  of the standpipe  202 . Cooling medium  206  flows through a space between the outer wall  208  of the jacket  200  and the outer wall  204  of the standpipe  202 , thus cooling the outer wall  204  of the standpipe  202 . The cooling medium  204  is any free flowing medium, such as, but not limited to, cooling water, process gases or heated oil. Cooling medium  204  flows into a bottom opening  210  and out of a top opening  212  of the jacket  200 . 
         [0035]      FIG. 3  shows an arrangement where the cooling medium flows through a helical conduit or pipe  300  wound around the external wall  302  of the standpipe vessel. Cooling medium enters at the bottom opening  304  of the conduit  300  and exits at the top opening  306  of the conduit  300 . A conduit is preferred over the cooling jacket from a quality and safety standpoint because the conduit eliminates any risk of cooling medium contacting hot silicon-coated granules in case of a leak. Hence there is no risk of sudden gas production from boiling cooling medium in the process and also no risk of granules being contaminated by cooling medium. In the case of a cooling jacketed standpipe, this is a concern. Furthermore, the continuous flow in a conduit is preferred. In a jacketed standpipe, dead zones with no flow can lead to stagnant areas where cooling medium can overheat and start to boil. 
         [0036]    Cooling can be accomplished with a single one-through loop heat exchanger, as shown in  FIG. 3 , wherein a cooling tube is a continuous winding around the stand pipe cooler. Or cooling can be accomplished in multiple stages along various sections of the standpipe to create and control a temperature profile. Different cooling mediums and heat exchange configurations can be used at the various stages to optimize heat recovery. 
         [0037]      FIG. 4  illustrates an alternate embodiment of a standpipe cooler. An inner concentric wall  400  defines a substantially central channel  402  and an annular space  404  between the inner wall  400  and the outer wall  406  of the standpipe cooler. Cooling medium flows through the central channel. In some embodiments, cooling medium enters the central channel  402  through a bottom opening  408  and flows out of the central channel  402  through a top opening  410 . A packed bed of granules moves downward in the annular space  404  between the inner wall  400  and the outer wall  406  and is cooled by the countercurrent flow of cooling medium. In other embodiments, the cooling medium may enter through the top opening  410  and flow out through the bottom opening  408 , thus producing a co-current flow. 
         [0038]      FIG. 5  illustrates one system wherein multiple cooling loops  500   a - d  are staged so that the cooling temperature can be varied at different elevations within the standpipe to optimize, for example, gas preheating. For further control, multiple injection points  502   a ,  502   b  are provided so that gases can be injected in stages. 
         [0039]    The standpipe&#39;s inner surface may be coated with any material that reduces contamination of the granules. Examples of suitable coating materials include, but are not limited to, silicon carbide, pure silicon, quartz, and combinations thereof. Coatings can be added during standpipe manufacture. The geometry of a straight-through pipe allows coating materials to be applied by any suitable method, such as spray coating, chemical coating or slip-lining. 
         [0040]    In an alternate arrangement, the standpipe may be constructed of a non-contaminating material such as ceramic, silicon carbide, or polysilicon tiles. Another approach is to prepare the standpipe prior to each operation by applying a chemical pretreatment that adds a non-contaminating, or less contaminating, layer to the inner standpipe wall. 
       EXAMPLES 
     Example 1 
     Batch Production 
       [0041]    In batch production, the packed bed level increases over time as granules flow into the standpipe cooler. At certain time intervals or at pre-determined packed bed levels, a batch of cooled granules is released into the post production treatment section. In one example, the standpipe is rapidly filled with granules and the standpipe fills completely. The granules remain in the standpipe and cool for a certain period of time. During this time period, the level of granules in the fluidized bed reactor increases since the standpipe is full and granules cannot flow into the standpipe. After the granules in the standpipe have cooled, they are released into the post-production treatment section. As the cooled granules flow out of the standpipe, hot granules from the fluidized bed reactor flow into the standpipe. The release of cooled granules is stopped as soon as the temperature of granules flowing out of the standpipe starts to increase. As the standpipe refills, the bed level in the fluidized bed reactor decreases. 
         [0042]    In a typical example, granules flow into the standpipe at a temperature of about 700° C. The granule temperature drops over time while the granules cool in the standpipe. Once the temperature is acceptable for the downstream system, the cooled granules are released. Typical temperatures are shown in Table I. 
         [0000]    
       
         
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
             
             
               
                   
                 Initial bed temperature = 700° C. 
               
               
                   
                 Temperature after 15 minutes = 400° C. 
               
               
                   
                 Temperature after 30 minutes = 200° C. 
               
               
                   
                 Temperature after 60 minutes = 40° C. 
               
               
                   
                 Outlet temperature at release = 40° C. 
               
               
                   
                 Outlet temperature after 3 min = 45° C. 
               
               
                   
                 Outlet temperature after 5 min = 60° C. 
               
               
                   
                 Solids valve closed after 5.5 min at 100° C. 
               
               
                   
                   
               
             
          
         
       
     
       Example 2 
     Continuous Production 
       [0043]    In continuous operation, the solids outflow from the standpipe is adjusted such that the rates of granules entering and exiting the standpipe are equal and the level of the packed bed within the standpipe remains constant. During continuous operation, there will be a temperature profile, or gradient, through the packed bed. Typically the temperature is about 700° C. at the top of the packed bed where hot granules enter. The temperature decreases to about 40° C. at the bottom of the packed bed as granules flow out from the standpipe. 
         [0044]    In view of the many possible embodiments to which the principles of the disclosed invention may be applied, it should be recognized that the illustrated embodiments are only preferred examples and should not be taken as limiting the scope of the invention. Rather, the scope of the invention is defined by the following claims.