Feed Air Cooling Using Waste Nitrogen In Air Separation By Cryogenic Distillation
Publication Date: 2003-Sep-12
The IP.com Prior Art Database
Cryogenic distillation is an established technology for air separation. All air separation units (ASU) require a "front-end" pretreatment system to remove H2O, CO2, and other impurities that will otherwise freeze at cryogenic temperatures and clog the heat exchangers and distillation columns. Impurity removal in the front-end is commonly done by fixed bed adsorption technology, employing alumina for H2O removal, and alumina or zeolite molecular sieve 13X for CO2 removal. The spent adsorbents are regenerated by purging the beds at a pressure lower than the feed pressure (in pressure swing adsorption or PSA), and/or at a temperature higher than the feed temperature (in temperature swing adsorption or TSA). Besides the adsorbers, the front-end comprises means for cooling the compressed air to the feed temperature; for example, a direct contact aftercooler (DCAC). The dry purge gas is typically waste N2 (dry N2 or dry N2-enriched air) from the ASU, especially in O2 production plants. In general, a part of the waste N2 is used for regenerating the adsorbers in PSA or TSA, and also for cooling the beds to the feed temperature, in TSA. The remainder of the waste N2 is used for evaporative cooling of the spent cooling water from the DCAC. Even though the waste N2 is typically at the ambient temperature, it is an effective cooling medium by virtue of its dryness. That is, a small portion of the cooling water evaporates into the dry waste N2, the heat of evaporation serving to cool the remainder of the water. This evaporative cooling is typically done in a counter-current contact cooling tower. The cooling water leaving the tower is sent to the main air compressor (MAC) for interstage cooling, and/or to the DCAC for aftercooling the compressed air. The N2 stream leaving the tower, typically saturated with H2O, is vented, despite the fact that it is about as cold as the cooling water which leaves the tower. The basis of the present proposal is the recognition that the cooling power of the cooled waste N2 stream (abbreviated henceforth as CWN) can be put to use in the process. For example, as illustrated in Figure 1, CWN is used in gas-gas heat exchanger (exemplified by, but not restricted to, a plate-fin exchanger) to cool the compressed air.