The present invention relates to a plate fin heat exchanger assembly and a system for use in a cryogenic air separation plant. The invention permits simplification of the design of a cross-flow subcooler for an air separation unit while maintaining global performance.
A cryogenic air separation unit includes a main heat exchanger for cooling feed air against return streams from a column system used to separate the air from the heat exchanger. The column system contains at least one column in which the air is separated at a cryogenic temperature by distillation.
The column system may comprise a single column only but frequently included a higher pressure distillation column and a lower pressure distillation column. Liquid streams such a bottom liquid richer in oxygen that the feed air and a sidestream liquid richer in nitrogen that the feed air are expanded in valves and sent from the higher pressure column to the lower pressure column or another part of the column system, for example the top condenser of an argon column.
In one example of a typical air separation unit, saturated bottom liquid and nitrogen enriched liquid(s) from the higher pressure distillation column are sub-cooled in a heat exchanger against a nitrogen stream from the lower pressure distillation column (lower pressure column) before the sub-cooled streams are sent to the lower pressure distillation column. Sub-cooling the bottom liquid and nitrogen enriched liquid stream(s) prior to introduction into to the lower pressure distillation column tends to minimize flashing of such liquid streams in the column, thereby maximizing liquid reflux in the lower pressure column which enhances the recovery of oxygen product and argon product.
In addition, sub-cooling of the bottom liquid and nitrogen enriched liquid streams aids in the recovery of refrigeration from the nitrogen streams, namely the nitrogen product stream and/or the waste nitrogen reducing the external refrigeration requirements for the air separation plant.
Sub-cooling the bottom liquid and nitrogen enriched liquid streams is preferably targeted at temperatures very close to the temperatures of nitrogen product stream and/or the waste nitrogen stream in order to recover most of the refrigeration and maximize refrigeration recovery from the nitrogen streams.
The liquid oxygen product is frequently subcooled in the heat exchanger before being sent to a storage tank.
Typical liquid subcoolers are described in “Cryogenic Engineering”, ed B. A. Hands, Academic Press, 1986, pp.213 and 215-216.
Usually, the exchange of heat between the nitrogen streams from the lower pressure column and the kettle liquid and shelf liquid streams from the higher pressure column is carried out using a Brazed Aluminum Heat Exchanger (BAHX), commonly referred to as a sub-cooler. This sub-cooler could be a separate, stand-alone heat exchanger or may be packaged within the primary heat exchanger shell and integrated therewith.
A sub-cooler typically would involve high capital costs as well as packaging challenges and may also result in high pressure drops of the cooling nitrogen streams. It is desirable to provide offers with more design flexibility in terms of selection of the quantity, dimensions, and number of layers, and flow direction for each stream traversing through the sub-cooler.
Classical cross-flow subcooler design orients the warm liquid streams according to temperature level with the streams requiring the coldest outlet temperature exiting the exchanger at the cold end. For example, in an air separation process using a double column with a lower pressure column whose base is thermally linked to the top of a higher pressure column, the designer may send two side streams from the higher pressure column to the lower pressure column: a liquid nitrogen stream and a lean liquid stream, richer in oxygen and poorer in nitrogen than the liquid nitrogen stream.
Lean liquid and liquid nitrogen are cooled first against waste nitrogen gas and low pressure nitrogen gas, containing more nitrogen than the waste nitrogen gas, both of which come from the low pressure column. This partially warms the gaseous nitrogen streams before they are used to cool the warmer fluids, such as the feed air in the main heat exchanger.
Liquid oxygen enters the subcooling exchanger at a temperature colder than the lean liquid and liquid nitrogen but exits at a temperature warmer than the outlet temperature of lean liquid and liquid nitrogen. This creates the need for different passage widths for different warm fluids at the same point in the exchanger.
Simulation of such a design with different passage widths at the same point in the exchanger is complex. When the liquid oxygen flow rate passing through the subcooler is very small compared to the other flow rates, an approximation can be made with negligible impact to the performance of the exchanger. However, the larger the liquid oxygen flow rate, such as that for a process producing large amounts of liquid oxygen product or a pumping process with liquid oxygen passing through the storage tank before being vaporized in the main exchanger, the more accurate the calculation required.
In this way heat is transferred from one layer to another. The stream D enters the top of the subcooler (crossed out arrow D) through an inlet, flows straight down through the subcooler and emerges in a warmed state from the warm end of the subcooler via an exit. Stream D is the cold stream, waste nitrogen, to which heat is transferred and comes from the low pressure column of an air separation unit. Streams A and B are liquid nitrogen and lean liquid from the higher pressure column, and stream C is liquid oxygen from the bottom of a low pressure column. Liquid oxygen C enters the subcooling exchanger at a temperature colder than the inlet temperature of lean liquid and exits the exchanger at a temperature warmer than the outlet temperature of lean liquid.
The liquid C crosses the subcooler in a direction orthogonal to the direction of flow of gas D, then reverses direction to return in the opposite direction, both inlet and outlet being at the same side of the subcooler.
Liquids A and B each have their respective inlet and outlet at opposite sides of the subcooler.
What is needed therefore is an improved sub-cooler heat transfer assembly and an improved heat transfer system for a cryogenic air separation plant that mitigates the above-identified problems.
Classical subcooler design involves cools the warm fluids requiring the coldest outlet temperatures in the coldest part of the exchanger. The main idea of the invention is to cool the liquid oxygen in the coldest part of the exchanger rather than the lean liquid and liquid nitrogen. This eliminates the need for multiple passage widths at the same temperature level in the exchanger, facilitating in-house design.
The LOX could also be cooled in a second subcooler arranged in parallel, in a process similar to that of US20060169000 but it would require an additional core and controlling the gaseous nitrogen flow rates to each core.
According to the invention, there is provided a plate fin heat exchanger assembly for a cryogenic air separation unit, comprising: a heat exchanger having at least two cryogenic liquid inlets, at least two cryogenic liquid outlets, at least one nitrogen-rich stream inlet at a first end of the heat exchanger and at least one nitrogen-rich stream outlet at a second end of the heat exchanger, the heat exchanger configured to receive a flow of at least one nitrogen-rich stream of the air separation unit at the at least one nitrogen-rich stream inlet and separate flows of at least two cryogenic liquids at the at least two cryogenic liquid inlets; the heat exchanger configured for receiving a first flow of at least one cryogenic liquid of an air separation unit and for channeling the first flow of the at least one cryogenic liquid in a cross flow orientation from a first of the cryogenic liquid inlets to a first of the cryogenic liquid outlets; the heat exchanger being configured for receiving a second flow of at least one cryogenic liquid of an air separation unit and for channeling the second flow of the at least one cryogenic liquid from a second of the cryogenic liquid inlets to a second of the cryogenic liquid outlets; the heat exchanger further configured for receiving a portion of the flow of the at least one nitrogen-rich stream and for channeling a portion of the flow of the at least one nitrogen-rich stream in a first direction within the first heat exchange segment from the at least one nitrogen-rich stream inlet to the at least one nitrogen-rich stream outlet to sub-cool both the first flow of the at least one cryogenic liquid and the second flow of the at least one cryogenic liquid and wherein the first direction is generally orthogonal to the first flow of the at least one cryogenic liquid and preferably to the second flow of the at least one cryogenic liquid wherein the inlet of the first of the cryogenic liquids is closer to the first end than the outlet of the second of the cryogenic liquids.
Other optional features of the invention include:
The invention will now be described in greater detail with reference to
The stream C in this case is cooled in two different layers, this being an optional feature.
In this way heat is transferred from one layer to another. The stream D enters the top of the subcooler (crossed out arrow D) through an inlet, flows straight down through the subcooler and emerges in a warmed state from the warm end of the subcooler via an exit. Stream D is the cold stream, waste nitrogen, to which heat is transferred and comes from the low pressure column of an air separation unit. Streams A and B are liquid nitrogen and lean liquid from the higher pressure column, and stream C is liquid oxygen from the bottom of a low pressure column.
In
The liquid oxygen stream is cooled to a temperature at most 15° C., preferably at most 10° C., above the temperature at which the at least one nitrogen rich stream enters the nitrogen-rich stream inlet
Lean liquid LL from the top of the higher pressure column is sent to the warm end of the subcooler in the same layer as liquid C and is removed in a cooled state from the middle of the subcooler.
In
Liquid nitrogen LIN from the top of the higher pressure column is sent to the warm end of the subcooler in the same layer as liquid C and is removed in a cooled state from the middle of the subcooler.
The LOX will not necessarily be colder than in the prior art process, but may be so if the first section of the exchanger performs better than expected. As such, a partial bypass of LOX is installed (not shown) in order to control the temperature with this configuration. The LIN and LL will be warmer than in the prior art, as the WN and GAN have been warmed by the LOX. However the impact the oxygen recovery is minimal. Either the LIN and LL temperatures are only slightly changed or the LR and AL temperatures may be colder than hi the prior art, which has a compensating effect, depending on the ratio of the different warm and cold streams in the exchanger.
The subcooler comprises three regions 1,2,3, the region 1 operating below a temperature T1, the region 3 operating at a temperature T2, greater than T1 and region 2 operating between T1 and T2.
The warming gases are waste nitrogen WN and low pressure gaseous nitrogen LPGAN, both from the lower pressure column of the air separation unit.
The cooling streams are liquid oxygen LOX from the lower pressure column, liquid nitrogen LIN from the top of the higher pressure column, lean liquid LL from the top of the higher pressure column, containing more oxygen than liquid LIN and liquefied air AL taken from the higher pressure column, a conduit or a turbine outlet.
Here once again it is the liquid oxygen which is cooled exclusively in the coldest part 1 of the subcooler. The liquid oxygen flows at substantially at right angles to the gaseous nitrogen flows and no liquid inlet is closer to the cold end of the subcooler and no liquid outlet is closer to the cold end of the subcooler. The liquid oxygen stream is cooled to a temperature at most 15° C., preferably at most 10° C., above the temperature at which the at least one nitrogen rich stream enters the nitrogen-rich stream inlet
Liquefied air AL is sent exclusively to the warm end of the subcooler and removed exclusively from a section 3 operating at the warmest temperatures. Rich liquid LR taken from the higher pressure column sump is sent exclusively to the warm end of the subcooler and removed exclusively from a section 3 operating at the warmest temperatures.
Lean liquid LL is sent to central region 2 of the subcooler and removed from that region operating between temperatures T1 and T2. Liquid LIN taken from the higher pressure column sump is sent exclusively to the central region 2 of the subcooler and removed exclusively from that region operating between temperatures T1 and T2.
Filing Document | Filing Date | Country | Kind |
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PCT/CN2018/112184 | 10/26/2018 | WO | 00 |