System and method for cryogenic cooling of a process stream with enhanced recovery of refrigeration

Abstract

A system and method for improved cryogenic cooling of process gases is provided. The disclosed system and method provides for the cryogenic cooling of a silane and hydrogen gas 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 gas 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 gas process stream.

What is claimed is: 
     
         1 . A method for cryogenic cooling of a process stream comprising the steps of:
 pre-chilling an influent of warm process gas in an economizer;   cooling the pre-chilled process gas with a cryogen in a cryogenic heat exchanger to a prescribed final temperature;   separating the cooled process gas at the prescribed final temperature into a condensable product and a cold spent process gas;   recycling the cold spent process gas to the economizer to pre-chill the influent of warm process gas;   forcibly directing a portion of the used process gas recycled to the economizer to an auxiliary heat exchanger; and   directing the spent cryogen from the cryogenic heat exchanger to the auxiliary heat exchanger to re-cool the used process gas;   wherein the excess refrigeration capacity of the cold spent process gas is directly transferred to the influent warm process gas flowing through the economizer and the excess refrigeration capacity of the spent cryogen is indirectly transferred to the influent warm process gas flowing through the economizer; and   wherein the amount of cryogen needed to cool the pre-chilled process gas in the cryogenic heat exchanger to a prescribed temperature is minimized.   
     
     
         2 . The method of  claim 1  wherein the cryogenic cooling is applied to a polysilicon manufacturing process and wherein the warm process gas is a gaseous stream of silane in hydrogen, the cryogen is nitrogen in both liquid and gaseous form, the condensable product is liquid silane at temperatures of lower than about −173° C. and the cold spent process gas is hydrogen gas at temperatures lower than about −172° C. 
     
     
         3 . A cryogenic cooling system comprising:
 an influent process stream;   a source of cryogen;   a cryogenic heat exchanger for cooling the process stream using the cryogen;   a phase separator disposed downstream of the cryogenic heat exchanger, the phase separator adapted for separating the cooled process stream into a condensable product and a cold spent process gas;   an economizer for pre-chilling the influent process stream with the cold spent process gas, the economizer disposed upstream of the cryogenic heat exchanger;   a first recycle conduit coupling the outlet of the phase separator to the economizer to direct the cold spent process gas from the phase separator to the economizer to pre-chill the influent process stream;   a second heat exchanger coupled to the cryogenic heat exchanger and adapted for using spent cryogen from the cryogenic heat exchanger to cool a stream of used process gas recycled to the economizer;   a second recycle conduit coupling the outlet of the economizer through the second heat exchanger and to either the first recycle conduit or the inlet of the economizer to pre-chill the influent process stream;   a blower disposed in operative association with the second recycle conduit to forcibly drive the used process gas from the outlet of the economizer through the second heat exchanger to either the first recycle conduit or the inlet of the economizer:   wherein excess refrigeration capacity of the spent cryogen from the first heat exchanger is transferred first to the used process gas flowing through the second heat exchanger and subsequently to the influent process stream flowing through the economizer; and   wherein excess refrigeration capacity of the cold spent process gas exiting the phase separator is transferred to the influent process stream flowing through the economizer.   
     
     
         4 . The cryogenic cooling system of  claim 3  wherein the cryogenic cooling system is integrated in a polysilicon manufacturing process and wherein the process gas is a gaseous stream of silane in hydrogen, the cryogen is nitrogen in both liquid and gaseous form, the condensable product from the phase separator is liquid silane and the cold spent process gas from the phase separator is hydrogen gas. 
     
     
         5 . An improvement to a cryogenic cooling system comprising a cryogenic heat exchanger for cooling a process stream; a phase separator downstream of the cryogenic heat exchanger for separating the cooled process stream into a condensable product and a cold spent process gas, and an economizer for pre-chilling the process stream upstream of the cryogenic eat exchanger, the improvement further comprising:
 a second heat exchanger coupled to the cryogenic heat exchanger;   a first recycle conduit coupling the outlet of the phase separator to the economizer to direct the cold spent process gas from the phase separator to the economizer;   a second recycle conduit coupling the outlet of the economizer through the second heat exchanger and to either the first recycle conduit or the inlet of the economizer;   a blower disposed in operative association with the second recycle conduit to forcibly drive used process gas from the outlet of the economizer through the second heat exchanger and to either the first recycle conduit or the inlet of the economizer;   wherein excess refrigeration capacity of the spent cryogen exiting the first heat exchanger is transferred to the used process gas flowing through the second heat exchanger subsequently to the influent process stream flowing through the economizer; and   wherein the excess refrigeration capacity of the cold spent process gas exiting the phase separator is transferred directly to the influent process stream flowing through the economizer.   
     
     
         6 . The improvement of  claim 5  wherein the cryogenic cooling system is integrated in a polysilicon manufacturing process and wherein the process gas is a gaseous stream of silane in hydrogen, the cryogen is nitrogen, the condensable product from the phase separator is liquid silane and the cold spent process gas from the phase separator is hydrogen gas.