System and method for purification of silane using liquid nitrogen in a polysilicon production process

Abstract

A system and method for improved cryogenic cooling of process streams in polysilicon manufacturing is provided. The disclosed system and method provides for the cryogenic cooling of a silane and hydrogen process stream during the manufacture of polysilicon with concurrent recovery of refrigeration capacity from the vaporized nitrogen as well as the recovery of refrigeration capacity from the cold hydrogen stream. The improved cryogenic cooling system and method reduces the overall consumption of liquid nitrogen without sacrificing cooling performance of the cryogenic cooling of the silane and hydrogen process stream.

What is claimed is: 
     
         1 . A method for cryogenic cooling of a silane in hydrogen process stream in the production of polysilicon, the method comprising the steps of:
 pre-chilling a process stream of silane in hydrogen using a cooling stream and one or more economizers;   cooling the pre-chilled process stream with liquid nitrogen in a cryogenic heat exchanger to a prescribed final temperature;   separating the cooled process stream at the prescribed final temperature into a product of liquid silane and a cold hydrogen stream;   recycling the cold hydrogen stream to form part of the cooling stream in the one or more economizers to pre-chill the process stream;   forcibly directing some or all of the used hydrogen stream from the one or more economizers to an auxiliary heat exchanger; and   directing the nitrogen stream from the cryogenic heat exchanger to the auxiliary heat exchanger to re-cool the used hydrogen stream; and   directing the re-cooled, used hydrogen stream to form part of the cooling stream in the one or more economizers to pre-chill the process stream.   
     
     
         2 . The method of  claim 1  wherein the cooling stream is comprised of a mixture of the cold hydrogen stream and the used hydrogen stream and wherein the excess refrigeration capacity of the cold hydrogen stream is directly transferred to the process stream and the excess refrigeration capacity of the nitrogen stream is indirectly transferred to the process stream via the used hydrogen stream. 
     
     
         3 . The method of  claim 2  further comprising the step of adjusting the characteristics of the cooling stream by venting a portion of the used hydrogen stream prior to mixing with the cold hydrogen stream to alter the mixture of the cold hydrogen stream and the used hydrogen stream forming the cooling stream. 
     
     
         4 . The method of  claim 1  wherein the cryogenic cooling of the silane in hydrogen process stream is controlled by adjusting the flow of the incoming silane in hydrogen process stream; the flow of liquid nitrogen through the cryogenic heat exchanger; and the flow of the used hydrogen gas through the auxiliary heat exchanger. 
     
     
         5 . A cryogenic cooling system comprising:
 a process stream of silane in hydrogen;   a source of liquid nitrogen;   a cryogenic heat exchanger for cooling the process stream using the liquid nitrogen;   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;   one or more economizers for pre-chilling the process stream h the cold hydrogen stream, the one or more economizers disposed upstream of the cryogenic heat exchanger;   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 the economizer to pre-chill the process stream;   a second heat exchanger coupled to the cryogenic heat exchanger and adapted for using the nitrogen stream exiting from the cryogenic heat exchanger to cool a used hydrogen stream;   a second recycle conduit coupling the outlet of the one or more economizers through the second heat exchanger and to either the first recycle conduit or the inlet of the one or more economizers to pre-chill the process stream;   a blower disposed in operative association with the second recycle conduit to forcibly drive the used hydrogen stream from the outlet of the one or more economizers through the second heat exchanger and to either the first recycle conduit or the inlet of the one or more economizers;   wherein excess refrigeration capacity of the nitrogen 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 the one or more economizers; and   wherein excess refrigeration capacity of the Bold hydrogen stream exiting the phase separator is directly transferred to the process stream flowing through the one or more economizers.   
     
     
         6 . The system of  claim 5  wherein the cryogenic heat exchanger and the second heat exchanger are integrated into a three stream heat exchanger wherein the nitrogen stream cools the pre-chilled process stream and the used hydrogen stream. 
     
     
         7 . The system of  claim 5  wherein the cryogenic heat exchanger and the one or more economizers are integrated into a multi-stream heat exchanger wherein the cold hydrogen stream pre-chills the process stream and the nitrogen stream cools the pre-chilled process stream. 
     
     
         8 . The system of  claim 5  wherein the cryogenic heat exchanger, the auxilliary heat exchanger and the one or more economizers are integrated into a multi-stream heat exchanger wherein the cold hydrogen stream pre-chills the process stream and the nitrogen stream cools the pre-chilled process stream and the used hydrogen stream. 
     
     
         9 . The system of  claim 5  further comprising a third heat exchanger for transferring any remaining refrigeration capacity from the nitrogen stream to cool an air stream.