Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
  • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
  • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B33/00—Silicon; Compounds thereof
  • C01B33/02—Silicon
  • C01B33/021—Preparation
  • C01B33/027—Preparation by decomposition or reduction of gaseous or vaporised silicon compounds other than silica or silica-containing material
  • C01B33/035—Preparation by decomposition or reduction of gaseous or vaporised silicon compounds other than silica or silica-containing material by decomposition or reduction of gaseous or vaporised silicon compounds in the presence of heated filaments of silicon, carbon or a refractory metal, e.g. tantalum or tungsten, or in the presence of heated silicon rods on which the formed silicon is deposited, a silicon rod being obtained, e.g. Siemens process
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02K—DYNAMO-ELECTRIC MACHINES
  • H02K21/00—Synchronous motors having permanent magnets; Synchronous generators having permanent magnets
  • H02K21/38—Synchronous motors having permanent magnets; Synchronous generators having permanent magnets with rotating flux distributors, and armatures and magnets both stationary

Definitions

• the present system and method relates to cryogenic cooling of intermediate process streams during the polysilicon production processes, and more particularly to methods and systems for the refrigeration recovery during purification of silane in a fluidized bed polysilicon production process.
• Silicon produced from this and similar processes are called polycrystalline silicon. Because of the high resistivity of the silicon seed rods, the Siemens process requires two power supplies—one for preheating the rods into a conductive state, and the second for superheating the rods by conduction. Most of the energy from the hot silicon rods is radiated into water-cooled bell jars covering the Siemens reactor.
• the fluidized bed process of manufacturing polysilicon offers some significant economic advantages compared to the Siemens process for the production of polysilicon.
• the energy losses and hence the energy consumption are considerably reduced in the fluidized bed process because the decomposition operates at a lower temperature, and cooling the bell jar is not required.
• Another advantage in the fluidized bed process is that very large reactors may be constructed and operated continuously, reducing further the capital and operating costs.
• the end products in the fluidized bed process for manufacture of polysilicon are small granules of polysilicon that may have some commercial advantages, such as when continuous feeding polysilicon into the customer's process is required.
• silicon fluoride is distilled into a gaseous feed of silane in hydrogen. After distillation of the silicon fluoride, the influent gaseous silane feed is purified/separated in a purification unit and thermally decomposed in a fluidized bed to produce polysilicon. Silicon seed particles are introduced into the fluidized bed sustained by a stream of silane and hydrogen. The silicon from the decomposed silane attaches to the seed particles in the fluidized bed reactor, which grow to granule sized pellets during their free fall to the bottom of the reactor.
• the influent gaseous feed comprising silane in hydrogen is purified/separated through a series of heat exchangers and economizers that use liquid and gaseous nitrogen to separate these intermediate process streams, namely into a hydrogen stream and a silane stream.
• the incoming gaseous feed (12) of silane and hydrogen at a flow rate of about 975 kg per hour generally comprises about 2% silane (S1H 4 ) and 98% hydrogen (H 2 ) at a temperature of about 25°C and a pressure of about 0.66 MPa.
• This incoming gaseous feed or process stream (12) is cooled in a series of heat exchangers and economizers to a prescribed final temperature where the silane and hydrogen are separated in a phase separator.
• This multi-step sequence of cooling the influent process stream (12) includes an economizer (13) which pre-chills the silane and hydrogen gas stream (12) to a temperature of about -80 °C using cold hydrogen gas (22).
• This pre-chilled silane and hydrogen stream (14) is then directed to a second, relatively small economizer (15) that further cools the pre-chilled silane and hydrogen stream (14) to an intermediate temperature of about -144 °C using gaseous nitrogen (32) at about - 164 °C.
• the resulting cooled silane and hydrogen stream (16) is directed to a cryogenic heat exchanger (17) where it is cooled with liquid nitrogen at about -179°C to the final prescribed process temperature of about -165°C.
• the fully cooled silane and hydrogen stream (18) is directed to a phase separator (19) where the silane is condensed to a liquid product (20) to be directed to the fluidized bed and the resulting cold hydrogen gas (22) at about -160°C is directed back to the economizer (13) to pre-chill the influent process stream (12).
• the used hydrogen stream (24) is vented or used elsewhere in the plant.
• the cooling medium for cryogenic heat exchanger (17) and economizer (15) is liquid and/or gaseous nitrogen flowing through the cooling circuit.
• the nitrogen used in the polysilicon purification process originates from a source of liquid nitrogen (not shown).
• the liquid nitrogen is at about -179°C and a pressure of 0.5MPa is supplied to the cryogenic heat exchanger (17) at a flow rate of between about 1 150 to 1500 kg per hour where it cools the silane and hydrogen process stream (16) to the final prescribed temperature of about -160°C.
• the nitrogen stream (32) at about - 164°C exiting the cryogenic heat exchanger (17) is routed to the economizer (15) where it pre-chills the silane and hydrogen process stream (14) and exits the economizer (15) in a gas stream (34) at about -130°C.
• the 1500 kg per hour nitrogen gas stream (34) is eventually directed to another gas to air heat exchanger (35) where the cold nitrogen gas (34) is warmed to an exhaust or vent temperature of about 10°C at a pressure of about 0.3MPa using incoming warm air (38) which exits the heat exchanger (35) as cold air (40).
• the present invention may be characterized as a method_for cryogenic cooling of a silane in hydrogen process stream in the production of polysilicon, the method comprising the steps of: (a) pre-chilling a process stream of silane in hydrogen using using a cooling stream and one or more economizers; (b) cooling the pre-chilled process stream with liquid nitrogen in a cryogenic heat exchanger to a prescribed final temperature; (c) separating the cooled process stream at the prescribed final temperature into a product of liquid silane and a cold hydrogen stream; (d) recycling the cold hydrogen stream to form part of the cooling stream in the one or more economizers to pre-chill the process stream; (e) forcibly directing a portion of the used hydrogen stream from one or more of the economizers to an auxiliary heat exchanger; (f) directing the nitrogen stream from the cryogenic heat exchanger to the auxiliary heat exchanger to re-cool the used hydrogen stream; and (g) directing the re- cooled, used hydrogen stream to
• the present invention may also be characterized as a cryogenic cooling system comprising: (i) a process stream of silane in hydrogen; (ii) a source of liquid nitrogen; (iii) a cryogenic heat exchanger for cooling the process stream using the liquid nitrogen; (iv) a phase separator disposed downstream of the cryogenic heat exchanger, the phase separator adapted for separating the cooled process stream into a product of liquid silane and a cold hydrogen stream; (v) one or more economizers for pre-chilling the process stream with the cold hydrogen stream, the one or more economizers disposed upstream of the cryogenic heat exchanger; (vi) a first recycle conduit coupling the outlet of the phase separator to the one or more economizers to direct the cold hydrogen stream from the phase separator to at least one economizer to pre-chill the process stream; (vii) a second heat exchanger coupled to the cryogenic heat exchanger and adapted for using nitrogen exiting from the cryogenic heat exchanger to cool a used
• the excess refrigeration capacity of the nitrogen stream exiting from the cryogenic heat exchanger is transferred first to the used hydrogen stream flowing through the second heat exchanger and subsequently to the process stream flowing through one or more economizers and the excess refrigeration capacity of the cold hydrogen stream exiting the phase separator is directly transferred to the process stream flowing through one or more of the economizers.
• FIG. 1 is a schematic illustration of the fluidized bed process for production of polysilicon
• FIG. 2 is a schematic illustration of a prior art cryogenic cooling and separation system used in the fluidized bed process for production of polysilicon;
• FIG. 3 is a schematic illustration of a preferred embodiment of the cryogenic cooling and separation system in accordance with the present invention.
• FIG. 4 is a schematic illustration of an alternate embodiment of the cryogenic cooling and separation system
• FIG. 5 is an illustration of a three stream heat exchanger that can be used to achieve the refrigeration recovery associated with the present invention.
• Fig. 6 is an illustration of an alternate concept for a heat exchanger that can be used to achieve the refrigeration recovery associated with the present invention.
• the incoming gaseous feed (52) of silane and hydrogen at a prescribed flow rate of about 975 kg per hour generally comprises about 2% silane (S1H 4 ) and 98% hydrogen (H 2 ) at a temperature of about 25°C and a pressure of about 0.66 MPa.
• this incoming gaseous feed or process stream (52) is cooled in a series of economizers and heat exchangers to a prescribed final temperature where the silane and hydrogen are separated in a phase separator (59).
• This preferred process of cooling the influent process stream includes first pre-chilling the silane and hydrogen influent stream (52) in an economizer (53) to a temperature of about -80°C using a cooling stream including the cold hydrogen stream.
• This pre-chilled silane and hydrogen stream (54) is then directed to a second economizer (55) that further cools the pre-chilled silane and hydrogen stream (54) to an intermediate temperature of -167°C also using a cooling stream (64) that includes the cold hydrogen stream (62).
• the resulting cooled silane and hydrogen stream (56) is directed to a cryogenic heat exchanger (57) where it is further cooled with liquid nitrogen at about -179°C to a colder final prescribed process temperature of about -173°C.
• This fully cooled silane and hydrogen stream (58) is then directed to a phase separator (59) where the silane is condensed to a liquid product (60) to be directed to the fluidized bed and the resulting cold hydrogen stream (62) at about -172°C is used in pre- chilling and cooling the influent and intermediate silane and hydrogen process streams (52,54) in the economizers (53,55) as described in more detail below.
• the cryogen used in the cryogenic heat exchanger (57) is preferably liquid nitrogen (80) from a source of liquid nitrogen.
• the liquid nitrogen (80) is supplied to the cryogenic heat exchanger (57) at a flow rate of only 554 kg per hour, a temperature of about -179°C, and a pressure of 0.4MPa where it cools the silane and hydrogen process stream (56) to a colder final prescribed temperature of about - 173°C.
• the nitrogen stream (82) exiting the cryogenic heat exchanger (57) at about - 164°C is routed to an auxiliary heat exchanger (75) where it is used to provide re- cooling of the used hydrogen gas (76).
• Nitrogen gas (84) exiting the auxiliary heat exchanger (75) is then directed to another gas to air heat exchanger (85) where the nitrogen gas (84) at about 14°C is warmed to an exhaust or vent temperature of about 25°C at a pressure of about 0.3MPa using incoming warm air (88) which exits the heat exchanger (85) as cool air (90).
• the above-described used hydrogen gas (76) represents a portion of the warm hydrogen gas (72) exiting the economizer (53).
• the warm, used hydrogen gas (72) exiting the economizer (53) is preferably divided into two streams.
• One portion of the warm, used hydrogen gas (74) is vented or directed elsewhere in the plant whereas the second portion of the warm, used hydrogen gas (76) is recycled to the second or auxiliary heat exchanger (75) using a blower (73).
• This second portion of the warm, used hydrogen gas (76) is re-cooled in the auxiliary heat exchanger (75) using the nitrogen stream (82) exiting the cryogenic heat exchanger (57).
• the re- cooled, used hydrogen stream (78) is then combined with the cold hydrogen stream (62) from the phase separator (59).
• the combined hydrogen cooling stream (64) is directed to first to the economizer (55) to cool the intermediate silane and hydrogen stream (54) and then to the economizer (53) to pre-chill the warm, influent silane and hydrogen stream (52).
• the influent process stream is cooled to a lower temperature. This, in turn, reduces the amount of nitrogen needed in the cryogenic heat exchanger to obtain the desired or prescribed final temperature for separation. The reduction in nitrogen
• cryogenic cooling system and method for purification of silane in the fluidized bed polysilicon production process disclosed herein against the prior art cryogenic cooling system and method, it is apparent that significant operating cost savings in terms of reduced cryogen consumption and lower operating pressures can be achieved.
• the reduction in cryogen consumption alone should allow the plant to realize between about 20% and 50% improvement without sacrificing or curtailing the purification of silane or the production of polysilicon.
• Fig. 4 there is shown a schematic illustration of an alternate, more generic embodiment of the present cryogenic cooling and separation system.
• the influent or feed process stream (152) is a gaseous stream of silane in hydrogen which is cooled in a multi-step process and fully cooled process stream (158) at about -173°C is subsequently separated into liquid silane stream (160) and a hydrogen stream (162).
• the cooling and separation of the influent or feed process stream (152) is
• the cryogen source used in the cryogenic heat exchanger (157) is preferably liquid nitrogen (180) delivered at approximately -179°C and 0.4 MPa to cool the pre- chilled process stream prior to its separation.
• the nitrogen stream (182) exiting the cryogenic heat exchanger (157) at a temperature of about -164°C is then directed to a second or auxiliary heat exchanger (175) where it cools the warm hydrogen stream (176) (i.e., hydrogen gas).
• the spent gaseous nitrogen (186) at about 14°C is subsequently vented to the atmosphere or released for other uses within the plant. This multi-step use of the cryogenic nitrogen recovers and utilizes a significant portion of the available refrigeration capacity of the cryogen.
• the separation of the fully cooled process stream (158) of silane and hydrogen in the separator (159) produces a liquid silane stream (160) at -173°C and cold hydrogen stream (162) at about -172°C.
• the cold hydrogen stream (162) is then recirculated to the economizer (153) for cooling the 25°C influent or feed process stream (152) to an intermediate pre-chilled process stream (154).
• the used hydrogen gas (172) exiting the economizer (153) at a temperature of about 1 1 °C is divided into two streams.
• One portion of the used hydrogen stream (174) is vented or used elsewhere in the plant whereas the second portion of the used hydrogen stream (176) is forcibly recycled to the second or auxiliary heat exchanger (175) using a blower (173).
• This portion of the used hydrogen stream (176) is re-cooled in the second or auxiliary heat exchanger (175) to a temperature of about -147°C using the cold nitrogen stream (182) exiting to cryogenic heat exchanger (157).
• the re-cooled, used hydrogen stream (178) is then combined with the cold hydrogen stream (162) from the phase separator (159).
• the combined hydrogen stream (164) is directed to the economizer (153) to directly cool the influent or feed process stream (152).
• the embodiment schematically illustrated in Fig. 4 provides improved refrigeration capacity recovery of both the cold hydrogen stream and the cryogenic stream. Specifically, the excess refrigeration capacity of the nitrogen stream exiting from the cryogenic heat exchanger is transferred indirectly to the influent process stream whereas the excess refrigeration capacity of the cold hydrogen stream exiting the phase separator is transferred directly to the influent process stream. This improved refrigeration recovery applied to the influent process stream of silane and hydrogen reduces the amount of nitrogen needed in the cryogenic heat exchanger to obtain the desired or prescribed final temperature for separation.
• Fig. 5 is an illustration of a three stream integrated heat exchanger (200) that can be used to achieve the refrigeration recovery associated with the present invention.
• one of intakes to the heat exchanger (200) is the influent process stream of silane in hydrogen (252) with the corresponding outlet stream being the fully cooled process stream (258) which is directed to the phase separator (259).
• the phase separator (259) produces a liquid silane stream (260) at -173°C and cold hydrogen stream (262) at about -172°C.
• the second intake stream to the three stream heat exchanger (200) is liquid nitrogen (280) at a temperature of about -179°C and the corresponding outlet is the nitrogen gas (284) at a temperature of about 14°C.
• the third stream is the cold hydrogen gas (262, 270) and the corresponding outlet is the used hydrogen gas (272) at about 1 1 °C which can be vented to the atmosphere (274) or otherwise used within the plant.
• Fig. 6 is an illustration of an alternate concept for an integrated heat exchanger (200) that, similar to the heat exchanger of Fig. 5, can also be configured to achieve the refrigeration recovery associated with the present invention. This embodiment is similar to that of Fig. 5, but further illustrates the recycling of the used hydrogen gas (276) via the blower (273) through the heat exchanger (200) to be combined with the cold hydrogen gas (262) to form a combined hydrogen stream (264).

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• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
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• Inorganic Chemistry (AREA)
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• Physics & Mathematics (AREA)
• Mechanical Engineering (AREA)
• Thermal Sciences (AREA)
• General Engineering & Computer Science (AREA)
• Separation By Low-Temperature Treatments (AREA)
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• 2011
  • 2011-11-08 EP EP11842423.3A patent/EP2641044A1/de not_active Withdrawn
  • 2011-11-08 WO PCT/US2011/059711 patent/WO2012067892A1/en active Application Filing
  • 2011-11-08 CN CN201180065193.3A patent/CN103429979B/zh not_active Expired - Fee Related
  • 2011-11-08 US US13/884,647 patent/US20140007615A1/en not_active Abandoned

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