Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2001-03-02
2002-03-26
Doerrler, William C. (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S050200, C062S903000, C165S166000
Reexamination Certificate
active
06360561
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present Application relates to the heating of pumped liquid oxygen to safely provide high pressure gaseous oxygen without use of a gas compressor by use of a heat exchanger having specific geometry requirements for the oxygen flow channels and their associated walls and has particular, but not exclusive, application to the cryogenic separation of air to provide a high pressure gaseous oxygen product. It provides both a heat exchanger for heating high pressure liquid oxygen and a method of providing high pressure gaseous oxygen by indirect heat exchange against a heat exchange fluid such as air, nitrogen and the like.
BACKGROUND OF THE INVENTION
Some chemical processes such as partial oxidation of hydrocarbon fuels require large quantities of high pressure oxygen because it is often more economic to carry out the process at high pressure. Cryogenic air separation is the technology of choice for the supply of such oxygen and the oxygen obtained from such separation can be pressurized in two ways. Gaseous oxygen (“GOX”) from the air separation unit (“ASU”) can be compressed to the required pressure or a pumped liquid oxygen cycle can be employed in which liquid oxygen (“LOX”) is pumped to the required pressure and heated to ambient temperature against a condensing boosted air or nitrogen stream. Sometimes the LOX is pumped to an intermediate pressure, vaporized against the boosted stream and then compressed to the required pressure.
There are several disadvantages associated with use of a high pressure gaseous oxygen compressor. Such compressors are expensive compared to air or nitrogen compressors and also tend to have lower aerodynamic efficiencies, as the machine clearances tend to be larger in order to minimize the possibility of a machine ‘rub’ and consequent fire caused by reaction of the compressor material with the oxygen. There is always a safety concern associated with the use of gaseous oxygen compressors, especially high pressure ones, due to the possibility of a compressor fire.
The above disadvantages make it preferable to use a pumped LOX cycle. There is a large body of patents and published literature concerning many aspects of pumped LOX cycles. Usually, the ASU heat exchangers are separated into two units; one using aluminum plate fin heat exchanger cores at low to medium pressure for the medium pressure air feed and returning nitrogen streams and a second aluminum high pressure plate fin heat exchanger for oxygen heating. However, it is known to combine all the duties in one aluminum high pressure plate fin heat exchanger.
An important consideration in the choice of aluminum plate fin heat exchangers is that, although reaction between LOX and aluminum can be explosive, it does require initiation by a primary energy release similar to the need for a booster explosion to detonate TNT. The reaction is much easier to initiate the higher the oxygen pressure and accordingly the pressure in aluminum heat exchangers is limited. However, the risk of an explosion if a primary energy release took place is not eliminated. Accordingly, when high pressure gaseous oxygen is required, it is current practice to limit the pressure of oxygen which is vaporized in an aluminum plate fin heat exchanger and to add an oxygen compressor to boost the resultant GOX to the required pressure. This adds equipment capital cost and compressing oxygen to high pressure also has safety implications in that oxygen compressor fires can occur.
It has been proposed to provide high pressure GOX by heating pumped LOX in a coil heat exchanger comprising copper, or copper based alloy, tube wound onto a central mandrel. Copper and copper based alloys such as cupro-nickel are ideal for this purpose because, in general, combustion cannot be initiated for copper below its melting point. However, the disadvantage of such copper wound coil heat exchangers is that they are very expensive and very large, as compared to a compact plate fin type heat exchanger.
A pumped LOX wound coil heat exchanger could be fabricated using stainless steel (“SS”) or other cryogenically suitable ferrous alloy. It is known that SS will not explode when reacting with either liquid or gaseous pure oxygen, but instead simply burns. Thus a heat exchanger used in pumped LOX heating would be much safer when fabricated from SS rather than from aluminum, especially as the relatively thick walls of tubing provides thermal inventory to quench an energy release if one were to start. The article “Flammability Limits of Stainless Steel Alloys 304, 308, and 316” by Barry L. Werley and James G. Hansel (ASTM STP 1319; 1997) reports that thicker tube walls inhibit reaction between oxygen and SS. However, wound coil heat exchangers fabricated from SS are very expensive and very large, as compared to compact plate fin heat exchangers.
It is known that plate fin heat exchangers can be fabricated from SS. Such a heat exchanger could be used for high pressure pumped LOX heat exchanger service and would be safer than an aluminum heat exchanger. However, in current practice, a SS plate fin heat exchanger contains many very thin SS fins, usually having a thickness of less than about 10% of channel hydraulic mean diameter (the hydraulic mean diameter of a channel is calculated by dividing 4 times its cross-sectional area by its wetted perimeter), and the ratio of heat transfer surface area to SS weight is very high. Thus, in the event of a local reaction between oxygen and a thin SS fin, there would be little local metal thermal inventory to help quench the reaction and, accordingly, there would be more safety concerns related to the use of such heat exchangers for high pressure oxygen service than for the thicker walled SS wound coil heat exchangers.
Printed Circuit Heat Exchangers (PCHE) are a well known compact type of heat exchanger for use primarily in the hydrocarbon and chemical processing industries and have been commercially available since at least 1985. They are constructed from flat metal plates into which fluid flow channels are chemically etched or otherwise formed in a configuration suitable for the temperature and pressure-drop requirements of the relevant heat exchange duty. Conventionally, the metal is SS such as, for example, SS 316L; Duplex alloy such as, for example, Duplex alloy 2205 (UNS S31803); or commercially pure titanium. The channeled plates are stacked so that a plurality of spaced layers of passages are formed by closure of the channels in each plate by the base of a respective adjacent plate; the stacked plates are diffusion or otherwise bonded together to form heat exchange cores; and fluid headers or other fluid connections are welded or otherwise connected to the core in order to direct fluids to respective layers of the passages. In diffusion bonding, grain growth between metal parts is caused by pressing surfaces metal surfaces together at temperatures approaching the melting point to effect a solid-state type of weld. A fluid to be heated is passed through channels of some layers (“heating layers”) and heated by indirect heat exchange against a warmer heat exchange fluid passing through channels of one or more intermediate layers (“cooling layers”). Usually, the plates from which the heating and cooling layers are formed have different channel designs.
Existing PCHE applications in hydrocarbon processing include, for example, hydrocarbon gas processing; PCHE applications in power and energy include, for example, feedwater heating and chemical heat pumps; and PCHE applications in refrigeration include chillers and condensers; cascade condensers and absorption cycles. It is reported that PCHEs can operate at temperatures from about −273° C. to about 800° C.
It is the primary object of this invention to provide a competitive method of supplying high pressure gaseous oxygen from an ASU without the use of an oxygen compressor and without incurring the risk of a reaction between oxygen and the heat exchanger material used in the oxygen heating process.
SUMMARY OF THE INVENTION
It h
Allam Rodney J.
O'Connor Declan P.
Air Products and Chemicals Inc.
Doerrler William C.
Jones, II Williard
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