Process fluid cooling method and apparatus

Abstract: A method of cooling a process liquid in which a process liquid stream is withdrawn by pump 16 from a process liquid, and in a container 10 is frozen in a chamber 18 into a conveyable particulate form. The conveyable particulate form is introduced back into the process liquid within the container 10.

Description: The present invention relates to a method of cooling a liquid process fluid in which a stream of the process fluid is extracted and frozen into a conveyable particulate form and the conveyable particulate form is introduced back into the liquid process fluid. More particularly, the present invention relates to such a method and apparatus in which the process fluid is frozen by direct heat exchange with a liquid cryogen coolant. There are industrial cooling applications that arise with respect to process fluids, namely, products and intermediate products of industrial processes or intermediate chemical reactions used in forming such products. Typical methods for cooling such process fluids while they are contained within reaction vessels include both indirect and direct means. A typical indirect method would be to provide the reaction vessel with an external jacket through which a cooling fluid can be circulated. This cooling fluid in turn is circulated through an external heat exchanger in which the coolant is cooled through indirect heat exchange with a cryogenic liquid, for instance, liquid nitrogen. A common direct method involves the addition of a refrigerant into the process fluid. This may be water ice produced from a nearby ice plant, which is unceremoniously dumped into the reaction vessel and melts to provide cooling to the process fluid. Alternatively it may be a cryogenic coolant, such as dry ice or liquid nitrogen, which vaporises in the process fluid to provide the cooling. Existing methods based on the above have limitations and problems. The indirect methods are complex and constrained by the materials of construction of the reaction vessel and the available heat transfer surface of the reaction vessel. The current direct methods in the case of water ice are difficult to control, limited by the temperature of the ice, and may not be preferred due to the dilution effects as the ice melts. The direct methods using vaporising cryogenic fluids may increase the size of the reaction vessel required due to gas hold-up in the process fluid and can be rejected due to problems with foaming of the process fluid or entrainment in the gas exhaust. As will be discussed, the present invention solves the prior art cooling problems, mentioned above, by a method and apparatus that involves the freezing of the process fluid itself and therefore, can be conducted by methodology and apparatus that are far simpler than methods and apparatus used in the prior art. According to the present invention there is provided a method of cooling a liquid process fluid comprising: forming a liquid process stream from said liquid process fluid; freezing said process fluid contained within said liquid process stream into a conveyable particulate form; and introducing said conveyable particulate form back into said liquid process fluid. The method according to the invention can be conducted in a batch form in which the liquid process stream is removed from a container containing a batch of the liquid process fluid and the resultant conveyable particulate form of the process fluid is then introduced back into the container. Additionally, the present invention also has application to a flowing liquid process fluid. In such case, a liquid process stream is removed from a conduit or pipe and the liquid process stream is frozen into a particulate form. The particulate form is then introduced back into the conduit by gravity, positive pressure or perhaps a constrictive venturi-like section of a pipe to create a reduced pressure within the flow. The invention also provides an apparatus for cooling a liquid process fluid, comprising a container for the liquid process fluid; conduit means for withdrawing a liquid process stream from said container; freezing means communicating with said container via said conduit means for freezing the liquid process stream into a conveyable particulate form; and means associated with said freezing means for reintroducing said conveyable particulate form back into the process liquid within said container. As will be discussed, the freezing of the process fluid can be accomplished by direct heat exchange with a cryogen in an apparatus of simpler design than cryogenic cooling circuits of the prior art. In practice, the direct heat exchange can be conducted countercurrently to conserve cryogen. Additionally, the frozen particulate form can be sub-cooled to well below its freezing point. This will reduce the mass flow rate of the liquid process fluid required to be removed from the container in a typical application of the present invention. The present invention is advantageous if the temperature of the process liquid is to be maintained near its freezing point because particles of ice or snow produced by the present invention allow for such temperature maintenance without the complications that might arise in prior art methods, such as indirect heat exchangers or direct cryogen injection, where bulk freezing might occur. The present invention also eliminates foaming problems that might occur when cryogenic coolants such as nitrogen are introduced directly in process fluids. A yet further advantage of the present invention is that it can be conducted without dilution of the process fluid. For example, the process fluid can comprise components that react with one another in the main body of the process fluid. Since it is only a frozen particulate form of the process fluid that is being reintroduced into the main body of the process fluid, changes in pH of the process fluid can be avoided. The term "particulate form" as used herein means a frozen form of the process fluid (having the appearance of snow or ice particles). Also, the term "cryogen" as used herein means a liquefied gas such as nitrogen, oxygen, argon, carbon dioxide. The invention will now be described by way of example with reference to the accompanying drawing which is a schematic diagram of an apparatus for carrying out a method in accordance with the present invention. With reference to the drawing an apparatus 1 in accordance with the present invention is illustrated. Apparatus 1 is provided with a container 10 for containing a process fluid 12. Process fluid 12 may be a fluid that is undergoing chemical reaction and is thereby liberating heat. Process fluid 12 may be introduced into and removed from container 10 by pumping process fluid 12 or its precursors with a pump and a suitable conduit positioned within container 10. A separate inlet and outlet to container 10 could be provided for this purpose. A liquid process stream is conducted in a pipe 14 by provision of a circulation pump 16. The liquid process stream is introduced into a freezing chamber 18 having a top vent 20 by one or more nozzles 22. Nozzle 22 can be an atomising nozzle or a spray nozzle that effects a break up and that directs the process fluid downwardly so that the process fluid descends in the freezing chamber. Two coolant nozzles 24 and 26 are located below process nozzle 22 to inject the liquid coolant into freezing chamber 18. Embodiments of the present invention may be constructed with one coolant nozzle or with three or more. The coolant utilised is one that is selected to freeze the process fluid into a conveyable particulate form. In the illustrated embodiment, the coolant is liquid nitrogen. In an appropriate case, the process fluid could be frozen into the conveyable particulate form through the use of a higher temperature coolant, for instance water. The resultant direct heat exchange causes the liquid nitrogen to vaporise within freezing chamber 18 and ascend to top vent 20. Thus, a countercurrent flow of coolant versus process fluid is set up to more efficiently utilise the cryogen. The subject invention could be effected, when appropriate, by injecting the process fluid immediately into a liquid cryogen region. Such liquid cryogen region can be formed in the freezing chamber by one or more nozzles or a ring-like manifold immediately beneath nozzle 22. Furthermore, although direct heat exchange is preferred for the sake of simplicity, a freezer utilising indirect cooling could be used to form the conveyable particulate form of the process fluid. The conveyable particulate form 28 of the process fluid falls by action of gravity and collects in the bottom of freezing chamber. The bottom of the freezing chamber is provided with an opening 30 in communication with container 12. A rotary valve 32 having vanes 34 is located within bottom opening 30 of freezing chamber 18. Vanes 34 prevent conveyable particular form 30 from falling directly into container 10 and therefore process fluid 12, as well as preventing the cold cryogenic gas from escaping freezing chamber 18. Although not illustrated, a motor or other actuating means is connected to rotary valve 32 to rotate vanes 34 and thereby introduce the conveyable particulate form back into container 10 and therefore process fluid 12. Preferably, the speed of the motor is controllable so that the rate of rotation of rotary valve 32 can in turn be controlled. Such control allows there to be a degree of control exerted over the amount of cooling provided by conveyable particulate form 28. For instance, if the speed of the motor were increased, there would be an increase in the rate of cooling provided to process fluid 12. Alternatively, rotary valve 32 could be replaced with a valve or damper which could be opened periodically or partly to allow the conveyable form 30 to enter container 10. Although not illustrated, an embodiment of the present invention could be constructed in which the freezing chamber 18 is maintained at an elevated pressure. Top vent 20 would then be equipped with an appropriate back pressure regulating device, not shown. In this case, conveyable form 30 could be introduced into a pressurised receiver, such as liquid process fluid flowing through a conduit. A pressurised freezing chamber would also allow more flexibility in locating the freezing chamber because the conveyable form would not need to rely on gravity for re-introduction into the process fluid.

Claims: A method of cooling a liquid process fluid (12) comprising: forming a liquid process stream from said liquid process fluid; freezing said process fluid contained within said liquid process stream into a conveyable particulate form (28); and introducing said conveyable particulate form back into said liquid process fluid. A method according to Claim 1, wherein said liquid process fluid is frozen by direct heat exchange with coolant. A method according to Claim 2, wherein said coolant is a cryogen. A method according to Claim 2, wherein said coolant and liquid process stream are introduced into a freezing chamber (18) so that said liquid process fluid freezes within said freezing chamber by direct heat exchange with said coolant. A method according to Claim 4, wherein said coolant is introduced into said freezing chamber having a vent (20) so that it vaporises within said freezing chamber, rises, and is vented from said vent. A method according to Claim 4 or Claim 5, wherein said process fluid is sprayed into the freezing chamber. A method according to any one of Claims 4 to 6, wherein said coolant is sprayed into the freezing chamber downstream of the introduction of said process fluid into the freezing chamber and so that sprays of the coolant and process fluid contact one another. A method according to any one of Claims 4 to 7, wherein said process stream is introduced into said freezing chamber above said coolant so that said process stream descends in said freezing chamber, said direct heat exchange is conducted countercurrently and said frozen particulate form descends in said freezing chamber under gravitational influence. A method according to any one of the preceding Claims, wherein: said liquid process fluid is located within a container (10); said liquid process stream is removed from said container; and said conveyable particulate form is introduced back into the container. A method according to any one of the preceding Claims, further comprising controlling rate of introduction of said conveyable particulate form so as in turn to control the cooling of said liquid process fluid. Apparatus for cooling a liquid process fluid (12) comprising: a container (10) for the liquid process fluid; conduit means (14) for withdrawing a liquid process stream from said container; freezing means communicating with said container via said conduit means for freezing the liquid process stream into a conveyable particulate form (28); and means (32) associated with said freezing means for reintroducing said conveyable particulate form back into the process liquid within said container. Apparatus according to Claim 11, wherein: said freezing means comprises: a freezing chamber (18) having a vent (20); at least one process fluid nozzle means (22) located below said vent for introducing said process stream into said freezing chamber so that the process fluid contained within said process stream descends in said freezing chamber; and at least one coolant nozzle means (24, 26) located below said process nozzle means for introducing a cryogen into said freezing chamber so that it vaporises, rises within said freezing chamber and is vented from said vent and thereby countercurrently exchanges heat with said process stream. Apparatus according to Claim 12, wherein: said freezing chamber (18) is located on the top of said container and has a bottom opening (30) in communication with said container; and said conduit has a pump (16) for pumping said liquid process stream to said freezing chamber.

Inventors: CLEMENTS STEPHEN, LEE RON C, CLEMENTS, STEPHEN, LEE, RON C.
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1996
Main classification: F25C1
IPC codes: F25D3, B01J2, F25C1