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.

Description

FIELD OF THE INVENTION

The present system and method relates to improved cryogenic cooling of process gases, and more particularly to a system and method for the cryogenic cooling of a process stream with concurrent recovery of refrigeration from both the spent cryogen and the cold process stream.

BACKGROUND

Cryogens such as liquid nitrogen are often used to cool process gas streams to very low temperatures in many manufacturing processes. Example of using cryogenic cooling systems in manufacturing processes include: cooling reactors of exothermic reactions; chemical process that require low temperatures to improve product selectivity; recover condensable products from a gaseous stream; and solution crystallization to purify products from a mixture of dissolved solids.

The economics of using cryogenic liquid nitrogen for cooling in many manufacturing processes depends heavily on the ability to recover the refrigeration from the spent nitrogen and from the cooled process gas stream. Ideally, the temperature of the spent nitrogen and the process gas stream should be warmed to about room temperatures or the final desirable downstream operating temperatures without external heating or temperature adjustments. Conventionally, the spent nitrogen is a nitrogen gas stream that is directed to an economizer where it is used to cool against the incoming process stream.

FIG. 1 shows an example of a conventional vapor phase product recovery process. As seen therein, a warm process stream (12) is first pre-cooled in an economizer (13) cooled by a cold nitrogen gas stream (26). The partly cooled process stream (14) is then directed to a primary heat exchanger (15) where the liquid nitrogen (24) is vaporized and cools the process stream (14) to it final temperature. The resulting process stream (16) is then directed to a phase separator (17) while the vaporized cold nitrogen gas (26) is routed back to the economizer (13) to cool the incoming warm process stream (12). The final product (20) is condensed in the phase separator (17) and recovered as a liquid with the residual cold process gas stream (30), without the condensed product, is vented or otherwise discarded or recycled within the plant. Most, if not all of the refrigeration capacity in the cold process gas stream is lost.

In other prior art systems, the refrigeration capacity from the cold gas process stream is recovered but the refrigeration capacity from the cold nitrogen gas stream exiting the primary heat exchanger is lost. FIG. 2 is a schematic illustration of this alternate arrangement for recovering some of the refrigeration from a cryogen gas stream used to produce a condensable product. As seen therein, a warm process stream (12) is first pre-cooled in an economizer (13) cooled by the cold process gas stream (30) exiting the phase separator (17). The partly cooled or pre-chilled process stream (14) is then directed to the primary heat exchanger (15) where the liquid nitrogen (24) is vaporized and cools the process stream (14) to it final temperature. The resulting cooled process stream (16) is then directed to a phase separator (17) while the vaporized cold nitrogen gas (26) is vented. The desired product (20) is condensed in the phase separator (17) and recovered as a liquid with the residual cold process gas stream (30), without the condensed product, is recycled back to the economizer (13) to pre-chill the incoming warm process gas (12) after which it is vented or otherwise discarded.

Using these conventional cryogenic cooling processes, a substantial amount of refrigeration capacity is not recovered. If either of the spent nitrogen gas stream or the cold process stream, without the condensed product, is used to pre-chill the incoming warm process gas stream, there will be little thermodynamic driving force for the other cold stream to transfer additional refrigeration values to the pre-chilled process stream. Refrigeration capacity recovery is even more difficult if any phase transformations of the cryogen or the process stream are involved. In most industrial applications, it is estimated that about half of the refrigeration capacity cannot be recovered by the above-described conventional methods as the refrigeration capacity of the liquid nitrogen is dissipated in the form of latent heat.

What is needed, therefore, is a cryogenic cooling system that minimizes the use of cryogens and also maximizes the recovery of the available refrigeration capacity within the cooling and purification processes.

SUMMARY OF THE INVENTION

The present invention may be charterized as a method for cryogenic cooling of a process stream comprising the steps of: (a) pre-chilling an influent of warm process gas in an economizer; (b) cooling the pre-chilled process gas with a cryogen in a cryogenic heat exchanger to a prescribed final temperature; (c) separating the cooled process gas at the prescribed final temperature into a condensable product and a cold spent process gas; (d) recycling the cold spent process gas to the economizer to pre-chill the influent of warm process gas; (e) forcibly directing a portion of the used process gas recycled to the economizer to an auxiliary heat exchanger; and (f) directing the spent cryogen from the cryogenic heat exchanger to the auxiliary heat exchanger to re-cool the used process gas. Using this method, 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. This, in turn minimizes the amount of cryogen needed to cool the pre-chilled process gas in the cryogenic heat exchanger to the prescribed final temperature.

The present invention may also be characterized as a cryogenic cooling system comprising: (i) a process stream; (ii) a source of cryogen; (iii) a cryogenic heat exchanger for cooling the process stream using the cryogen; (iv) a phase separator adapted for separating the cooled process stream into a condensable product and a cold spent process gas; (v) an economizer for pre-chilling the process stream with the cold spent process gas; (vi) 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 process stream; (vii) 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; (viii) 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 process stream; (ix) 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 inlet of the economizer. The excess refrigeration capacity of the spent cryogen from the cryogenic 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. Similarly, 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.

Still further, the present invention may be characterized as an improved cryogenic cooling system comprising a cryogenic heat exchanger for cooling an influent 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 heat exchanger, the improvement further comprising: (i) a second heat exchanger coupled to the cryogenic heat exchanger; (ii) 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; (iii) 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; and (iv) a blower disposed in operative association with the second recycle conduit to forcibly drive 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. Using the above-described improvement, the excess refrigeration capacity of the spent cryogen from the cryogenic 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. Similarly, 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.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the present invention will be more apparent from the following, more detailed description thereof, presented in conjunction with the following drawings, wherein:

FIG. 1 is a schematic illustration of a cryogenic cooling system showing the conventional process of recovering refrigeration from a spent cryogen stream using an economizer;

FIG. 2 is a schematic illustration of a cryogenic cooling system showing the conventional process of recovering refrigeration from a cold process stream using an economizer; and

FIG. 3 is a schematic illustration of the present cryogenic cooling system depicting an improved system and process for concurrently recovering refrigeration from both a cold process stream and a spent cryogen stream.

DETAILED DESCRIPTION

Turning to FIG. 3, there is shown a schematic illustration of the present cryogenic cooling system depicting the improved system and method for concurrently recovering refrigeration from both a cold process stream and the spent cryogen using an economizer. As is well known in this art, eeconomizers are mechanical devices intended to reduce energy consumption during a process, or to perform useful functions like pre-chilling a process gas.

The present system and method uses both direct refrigeration recovery of the cold spent process gas to the influent warm process gas via the economizer as well as indirect refrigeration recovery of the spent cryogen to the recycled process gas in an auxiliary heat exchanger and subsequently to the influent warm process gas via the economizer. This dual refrigeration recovery approach improves the performance of the economizer which in turn reduces the amount of cryogen needed to achieve the desired or prescribed final temperature for product separation.

In the illustrated embodiment, the influent warm process stream (52) is cooled in a multi-step process using an upstream economizer (53) and a primary cryogenic heat exchanger (55). The influent warm process stream (52) is first cooled to a pre-chilled process stream (54) at a prescribed intermediate temperature and then to a fully cooled process stream (56) at the prescribed final temperature. The fully-cooled process stream (56) is then directed to a phase separator (57) where the process stream is separated into a liquid product (60) and a cold spent process gas (80). It should be noted however, that this cooling of the influent warm process stream (52) may also be accomplished in a series of cooling steps using a plurality of economizers or heat exchangers.

The cryogen source used in the cryogenic heat exchanger (55) is preferably liquid nitrogen (64) to cool the pre-chilled process gas prior to its separation. The amount and flow of liquid nitrogen required to attain the prescribed final temperature is dependent, in part on the process stream being cooled and the intermediate temperature attained by the economizer. The spent nitrogen (66), in gaseous form at about is then directed to a second or auxiliary heat exchanger (59) where it cools the recycled, used process gas (86). The gaseous nitrogen (68) exiting the auxiliary heat exchanger (59) is subsequently vented or released to the atmosphere. 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 (56) into a condensable product and a cold spent process gas occurs at the final prescribed temperature. The cold spent process gas (80) is then recirculated to the economizer (53) for cooling the influent warm process stream (52) to an intermediate process stream (54). The now warm, used process gas (84) is divided into two streams. One portion of the warm, used process gas (85) is vented or directed elsewhere in the plant whereas the second portion of the warm, used process gas (86) is recycled to the second or auxiliary heat exchanger (59) using an blower (87). This warm, used process gas (86) is re-cooled in the second heat exchanger (59) using the spent nitrogen gas (66). The re-cooled, used process gas (88) is then combined with the cold, spent process gas (80) from the separator (57). The combined stream (82) is directed to the economizer to cool the influent warm process stream (52).

The excess refrigeration capacity of the spent nitrogen (66) from the cryogenic heat exchanger (55) is transferred indirectly to the influent warm process stream (52) by first transferring the refrigeration capacity to the recycled, used process gas (86) flowing through the second heat exchanger (59) which, in turn, is subsequently used to cool the influent warm process stream (52) flowing through the economizer (53). In addition, the excess refrigeration capacity of the cold spent process gas (80) exiting the phase separator (57) is transferred directly to the influent warm process stream (52) flowing through the economizer (53).

By using both direct refrigeration recovery and indirect refrigeration recovery, the influent warm process gas is pre-chilled to a lower temperature. This, in-turn, reduces the amount of cryogen needed in the cryogenic heat exchanger to obtain the desired or prescribed final temperature for separation. The reduction in cryogen use lowers the operating costs associated with the cryogenic cooling system and process.

INDUSTRIAL APPLICABILITY

The present cryogenic cooling system and method is useful in many industrial applications, including for example in the manufacture of polysilicon using the Ethyl Corp developed fluidized bed process. In such polysilicon production application, the influent or feed process stream (52) is a gaseous stream of silane in hydrogen which is cooled in a multi-step process and the resulting cooled process stream (56) is subsequently separated into liquid silane (60) and hydrogen gas (80). The cooling and separation of the influent or feed process stream (52) is accomplished using a first economizer (53) followed by a cryogenic heat exchanger (55) and then the separator (57). It should be noted however, that this initial cooling of the influent or feed process stream can be accomplished in a series of steps using one or more economizers or heat exchangers.

The cryogen source used in the cryogenic heat exchanger (55) is preferably liquid nitrogen (64) at −179° C. to cool the process gas prior to its separation. The spent nitrogen (66), in gaseous form at about −164° C., is then directed to a second heat exchanger (59) where it cools the warm, spent process gas (86) (i.e. hydrogen gas). The gaseous nitrogen (68) at about 14° C. is subsequently vented or released to the atmosphere. 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 cooled process stream (56) of silane (SiH₄) and hydrogen gas (H₂) produces liquid silane at −173° C. and hydrogen gas at about −172° C. The cooled hydrogen gas (80) is then recirculated to the economizer (53) for cooling the 25° C. influent or feed process stream (52) to an intermediate pre-chilled process stream (54). The warm, spent hydrogen gas (84) at about 11° C. is divided into two streams. One portion of the warm, spent hydrogen stream (85) is vented or used elsewhere in the plant whereas the second portion of the warm, used hydrogen gas (86) is recycled to the second heat exchanger (59) using an auxiliary blower (87). This warm, used hydrogen gas (86) is re-cooled in the second heat exchanger (59) to a temperature of about −147° C. using the cold nitrogen gas (66). The re-cooled hydrogen gas (88) is then combined with the cold, spent hydrogen gas (80) from the separator (57). The combined hydrogen stream (82) is directed to the economizer (53) to cool the influent or feed process stream (52).

From the foregoing, it should be appreciated that the present invention thus provides an improved method and system for cryogenic cooling of a process stream. While the invention herein disclosed has been described by means of specific embodiments and processes associated therewith, numerous modifications and variations can be made thereto by those skilled in the art without departing from the scope of the invention as set forth herein or sacrificing all its material advantages.

Claims

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; anddirecting 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; andwherein 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; andwherein 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; andwherein 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.