Refrigeration – Processes – Reducing pressure on compressed gas
Reexamination Certificate
2001-07-12
2003-09-23
Doerrler, William C. (Department: 3744)
Refrigeration
Processes
Reducing pressure on compressed gas
C062S050200, C062S118000, C062S309000
Reexamination Certificate
active
06622496
ABSTRACT:
FIELD OF THE INVENTION
This invention is related to a process and system for cooling a heat transfer fluid. In particular, this invention is directed to an external loop nonfreezing heat exchanger for cooling a heat transfer fluid with cryogenic fluid.
BACKGROUND OF THE INVENTION
Cryogenic fluids, such as liquid nitrogen, have been used successfully in a number of low-temperature freezing operations such as food or biological materials freezing. In theory, it was recognized that a number of chemical and pharmaceutical processes also could benefit from cryogenic cooling due to the low temperature and high driving force afforded by cryogenic liquids. However, although certain cryogenic fluids can provide very high heat transfer driving force, it has limited use to cool process liquid if freezing is undesirable. Many process liquids have freezing point far above that of liquid nitrogen, which boils at −195° C. This limits the use of liquid nitrogen in cooling process fluid in low temperature chemical process because the process fluid can potentially be frozen. Freezing the process fluid in chemical operation is undesirable and can be hazardous especially if the refrigeration is used to control exothermic reactions.
Properly designed direct contact cooling can reduce the potential freezing problem. This is carried out by injecting liquid nitrogen directly into process or heat transfer fluid. Unfortunately, it is not always acceptable to customers due to various reasons. Although the emission level is very low at this type of operating conditions, some manufacturer's site may not be able to accept any additional vapor into the solvent recovery system. For very large potential cryogenic cooling process (chilling the heat transfer fluid for freeze-drying can use up to 40 TPD of liquid nitrogen), manufacturer would prefer reusing the spent nitrogen. Therefore, indirect contact cooling in a heat exchanger is a preferred choice of operation. However, the freezing potential must be eliminated.
A conventional approach to solve the above problem is to design an over sized shell and tube heat exchanger. A heat transfer fluid or reactant is pumped into the tube side under high velocity. Liquid nitrogen is either sprayed or flooded into the shell side of the heat exchanger. One problem encountered by this approach is that the heat transfer fluid may cause problems as the liquid nitrogen downloads its latent heat of vaporization on the metal surface. When ice starts to grow and propagate, the heat transfer surface will lose its thermal conductivity. The result is either a heat exchanger losing its heat transfer capacity rapidly or having the content frozen totally solid. The unit must be defrosted before it can be put back to service. For reactions or applications that require very short batch time, an over-sized heat exchanger may still function for a limited time before losing its capability.
Another approach is to mix the liquid nitrogen with room temperature nitrogen gas to reduce the refrigerant driving force and provide a cryogenic gas with a warmer temperature than the boiling point of −320° F. However, all the latent heat of vaporization is lost in the mixing process. Although this approach may avoid freezing, the heat transfer fluid can be warmed as high as one desired, the nitrogen consumption rate is normally too high to be economically acceptable. Furthermore, the cold gas mixture will lose its sensible heat very rapidly due to the cryogenic fluid's low heat capacity, making it unacceptable for a number of applications.
Yet another approach is to use a heat transfer fluid with lower freezing points to receive the refrigeration from the liquid nitrogen. The lower freezing point heat transfer fluid is then used to cool another heat transfer or process fluid to the final desired temperatures. Such a stopgap measure may prolong the batch time before total freezing occurs. It also adds substantial complexity and cost to the process.
The prior arts have also proposed a complicated scheme by cycling inlet and outlet of cryogenic flow to avoid freezing. However, freezing may still occur eventually, even with this complicated cycling operation by a sequence of valves. Subsequently, these prior arts also require recycling part of the spent nitrogen to mix with the fresh liquid nitrogen. The liquid nitrogen and the spent nitrogen gas form a cryogenic cold gas mixture as refrigerant.
A cycling flow control mechanism then force these cold gas mixture to enter the heat exchanger in the front and then reverses flow to enter from the back. Such a complicated mechanism not only add significant capital and operating cost to the process, but it also deteriorate the recirculating pattern of the spent nitrogen gas. Such complicated cycling procedures are believed to be unnecessary and counter-productive to the mixing requirements of the spent nitrogen and fresh liquid nitrogen.
U.S. Pat. No. 5,456,084 discloses the above complex cryogenic cooling system for freeze dryers at which a sequence of valves cycle the flow of cryogen between the heat exchanger inlet and outlet. Part of the spent nitrogen is recycled alternatively between the inlet and outlet to vaporize and mix with the fresh cryogen liquid. There is no prior art that teaches or suggests the amount of recycle is needed to make the system workable. Furthermore, an eductor is generally not the right type of device to recirculate the cryogenic nitrogen.
U.S. Pat. No. 5,937,655 discloses a heat exchanger that contains a series of baffles and vaporizers inside a single heat exchanger where the liquid nitrogen is vaporized directly inside a series of vaporizer tube. As the vaporized nitrogen warms up by contacting the heat transfer fluid surface, it is re-directed by the baffles to be chilled by the vaporizing liquid nitrogen. Very high thermal efficiency can be achieved without any mechanical means. A draw back of such as system is the complexity of the internal devices in that it requires the system to be custom designed and fabricated individually. The heat exchanger must be custom built.
It is, therefore, desirable to have an effective means to convert all the latent heat of vaporization of the cryogenic liquid into sensible heat. It is also the objective of this invention to develop a process at which a conventional heat exchanger can be used while having the benefits of cooling without freezing.
It was found from this invention that the alternative cyclic operation of cryogen inlet and out let is not necessary to make the heat transfer from the cryogen without freezing the process fluid. It was also found from this invention that the amount of spent nitrogen needed to recycle must be at higher than the weight of the fresh cryogenic liquid. An amount less than that will have a domino-effect in that the spent nitrogen will not be sufficient to vaporize the liquid nitrogen, which in turn will not be able to entrain sufficient spent nitrogen and so on. The complete loop must allow for high gas flow at low-pressure drop without the complicated valve switching system blocking its way.
There is a general misconception that the freezing condition in heat exchangers occur because of the cold temperature of the liquid nitrogen. Most freezing occur because the liquid nitrogen can boil and transfer its latent heat of vaporization rapidly when come in contact with a warmer surface. The latent heat of vaporization is generally more than half of all the refrigeration available from the liquid nitrogen. Therefore, a very small section can become extremely cold during the initial contact. As a result, the heat transfer coefficient of the liquid nitrogen is significantly bigger than a cryogenic cold gas at the equivalent temperatures.
It is therefore desirable to provide a system in which the direct contact design does not cause the process fluid to freeze.
SUMMARY OF THE INVENTION
This invention is directed to a process for cooling a process fluid which comprises flowing a cool mixed refrigerant in a continuous unidirectional loop compri
Cheng Alan Tat Yan
DeVack Donald Leonard
Doerrler William C.
Follett Robert J.
Praxair Technology Inc.
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