Aluminum plate-fin heat exchangers are widely used in air separation equipment, mainly including main heat exchangers for multi-stream heat exchange, subcoolers, etc., as well as condensers and evaporators for upper and lower towers, crude ammonia towers and pure hydrogen towers, and liquefiers in the hydrogen production system.
1. Main Heat Exchanger
The main heat exchanger realizes heat exchange between air as raw material gas, pressurized air, and return gases such as oxygen, nitrogen, and waste nitrogen. Air is cooled from room temperature to about 100K, pressurized air is cooled from room temperature to the temperature before the expander, and each return gas is reheated from low temperature to room temperature.
The flow channel arrangement of the main heat exchanger mainly adopts a stacked configuration, i.e., one hot flow layer corresponding to two cold flow layers. However, under special circumstances (such as high-pressure fluids or situations involving phase change), it is not strictly necessary to follow the 1:2 ratio of hot to cold channels. Some high-pressure hot flows or cold flows with phase change may adopt a single-layer arrangement, i.e., one hot flow layer corresponding to one cold flow layer. The flow channel arrangement should be determined according to specific conditions. When arranging channels, it should be considered that each fluid is distributed as uniformly and dispersedly as possible within the core. Under the premise of overall heat balance, local heat balance should also be taken into account to avoid uneven hot and cold distribution within a single core. It should be noted here that during the start-up of the air separation plant, the return oxygen and nitrogen have not yet been generated, and only the return waste nitrogen can provide cooling capacity. To accelerate the start-up speed and allow the pressurized air to cool down quickly before entering the turbo-expander, the waste nitrogen can theoretically be arranged adjacent to the pressurized air. However, the local heat balance should still be comprehensively considered, and such arrangement should be adopted more frequently under appropriate conditions.
2. Liquid Air and Liquid Nitrogen Subcooler
The liquid air and liquid nitrogen subcooler utilizes cold gaseous nitrogen and waste nitrogen to subcool liquid air and liquid nitrogen, thereby reducing the vaporization rate, increasing the reflux in the upper tower, and improving the distillation conditions. Generally, the liquid side uses serrated fins with a large heat transfer coefficient, low height, and wide spacing, while the gas side uses serrated fins with high height and dense spacing. Due to the increased scale of air separation equipment leading to larger plate-fin heat exchanger dimensions, other types of fins can also be selected to reduce the core cross-sectional dimensions without increasing resistance, thus facilitating manufacturing.
3. Condenser-Evaporator
The condenser-evaporator (hereinafter referred to as the main condenser) functions to condense the nitrogen at the top of the lower tower and evaporate the liquid oxygen at the bottom of the upper tower, thereby providing reflux for the lower tower and rising vapor for the upper tower. The main forms of condenser-evaporators primarily adopted by Hangyang are: the condenser-evaporator integrated with the upper and lower towers, where the heat exchange units generally adopt a star-shaped single-layer arrangement for convenient assembly with the distillation column. When a larger heat transfer area is required, a vertical double-layer configuration is adopted. As the scale of air separation equipment produced by Hangyang continues to increase, the dimensions of plate-fin heat exchangers have also grown, and even vertical single-layer or double-layer configurations can no longer meet the requirements. Considering transportation limitations, some extra-large air separation plants in recent years have begun to adopt horizontal configurations for their main condensers. In addition to the above forms, there are also high-heat-flux shell-and-tube main condensers and film-type main condensers, among others. Some literature has introduced and summarized the typical structures of main condensers in large-scale air separation equipment, with emphasis on the slit micro-film double-layer condenser-evaporator jointly developed by Hangyang and Xi'an Jiaotong University.
Both sides of the plate-fin main condenser involve phase-change heat transfer with relatively high heat transfer coefficients, so porous fins with relatively small fin heights are adopted. The liquid oxygen evaporation channels are open at both upper and lower ends, and the evaporated liquid oxygen forms the rising vapor for the upper tower. Nitrogen is drawn from the top of the lower tower and enters the core unit, and the condensed liquid nitrogen is collected in the header at the lower part of the core and led out through the outlet pipe. Considering the safety of liquid oxygen operation, the liquid oxygen side adopts a fully flooded configuration with a relatively large fin pitch.
4. Heat Exchangers in the Hydrogen Production System
Air separation plants equipped with hydrogen systems include not only the main heat exchanger, main condenser, and subcooler, but also heat exchangers such as the crude hydrogen condenser, crude hydrogen liquefier, pure hydrogen condenser, and pure hydrogen evaporator.
4-1. Crude Hydrogen Condenser
The crude hydrogen condenser is located at the top of the crude hydrogen tower, condensing the crude hydrogen gas with a hydrogen content of approximately 55% from the upper part of the crude hydrogen tower. Liquid air drawn from the bottom of the lower tower is throttled and enters the crude hydrogen condenser for evaporation. The plate-type core usually consists of one unit, where the liquid air evaporation channels are open at both upper and lower ends. The crude hydrogen gas enters the plate-type unit from the top of the crude hydrogen tower through a bridge pipe and the upper header, and the condensed liquid hydrogen flows back to the upper tray to participate in distillation. The crude hydrogen gas condensation channel and the liquid air evaporation channel are arranged in a cross-flow or counter-flow single-layer configuration.
4-2. Crude Hydrogen Liquefier
Depending on the process flow, a crude hydrogen liquefier may sometimes be required. A portion of the liquid nitrogen drawn from the pure hydrogen condenser enters the crude hydrogen liquefier for evaporation, and a portion of the non-condensable gas drawn from the crude hydrogen side of the crude hydrogen condenser enters the crude hydrogen liquefier for further condensation. The condensed crude liquid hydrogen then enters the pure hydrogen tower. The plate-type core usually consists of one unit, where the liquid nitrogen evaporation channel fin dimensions are 6.5 mm × 1.7 mm × 0.2 mm; the crude hydrogen gas condensation channel fin dimensions are 6.35 mm × 2.0 mm × 0.2 mm. The crude hydrogen gas condensation channel and the liquid nitrogen evaporation channel are arranged in a single-layer configuration.
4-3. Pure Hydrogen Condenser
The pure hydrogen condenser is located at the top of the pure hydrogen tower, condensing and liquefying the pure hydrogen gas from the upper part of the pure hydrogen tower. Liquid nitrogen from the main condenser evaporates. The plate-type core usually consists of one unit, where the liquid nitrogen evaporation channels are open at both upper and lower ends. The pure hydrogen gas enters the plate-type unit from the top of the pure hydrogen tower through a bridge pipe and the upper header, and the condensed liquid hydrogen flows back to the upper tray to participate in distillation. The pure hydrogen gas condensation channel and the liquid nitrogen evaporation channel are arranged in a counter-flow single-layer configuration.
4-4. Pure Hydrogen Evaporator
The pure hydrogen evaporator is located at the bottom of the pure hydrogen tower, evaporating the liquid hydrogen at the bottom of the pure hydrogen tower. Nitrogen from the lower tower is condensed and liquefied. The plate-type core usually consists of one unit, where the liquid hydrogen evaporation channels are open at both upper and lower ends. The pure hydrogen gas condensation channel and the liquid nitrogen evaporation channel are arranged in a counter-flow single-layer configuration.
