Liquefaction

Liquid nitrogen and liquid oxygen are produced and stored in a back-up system to supply gases (after vaporization of the stored liquid) in the event of the cryogenic air separation plant shut-down. Liquid may also be supplied in tankers from a central plant location to an end-use site where the consumption of nitrogen and oxygen is not high and economically it is not justifiable to build a dedicated plant. Liquid nitrogen is also used as a source of refrigeration in such applications as food freezing.

In 1895, Carl von Linde built the first industrial-scale air liquefier. His liquefier used a Joule-Thompson (JT) valve to create refrigeration. His genius was the realization that, for the same pressure ratio across a JT valve, the amount of cooling (drop in temperature) increases rapidly as the absolute pressure of air is increased. Therefore, such a liquefier operated at about 125 atm while the pressure across the JT valve dropped to approximately 5 atm. In 1902,

Figure 10 A distillation scheme for UHP oxygen production.

Georges Claude demonstrated that it was possible to lubricate a piston expander with petroleum ether at cryogenic temperatures. He then built an air liquefier using his piston expander. Since this liquefier did not rely on a JT valve to supply all the refrigeration, it was much more efficient than the liquefier built seven years earlier by Linde. In 1935, Kapitza built a piston expansion engine with gas lubrication and in 1939 he built an air liquefier with an expansion turbine. Most modern liquefiers use expansion turbines.

Although the early 'masters' of cryogenics were interested in liquefying air, the focus of modern liquefiers is mainly to liquefy nitrogen. This is due to the dominant use of liquid nitrogen for refrigeration supply. Liquid oxygen is generally produced by supplying some liquid nitrogen as reflux to the low pressure column of a double-column process and by withdrawing an equivalent amount of liquid oxygen from the bottom of this column. The gaseous nitrogen needed for the nitrogen liquefier is provided by any of the suitable air distillation processes described earlier.

Liquefiers that are capable of producing in excess of 1000 tons per day of liquid nitrogen and oxygen are now in operation.

A two-expander nitrogen liquefier is shown in Figure 11.

Make-up nitrogen from an air distillation cold box is compressed to about 6 atm in a make-up compressor and is further compressed to a pressure in excess of 27 atm in a recycle compressor. The pressurized nitrogen leaving the recycle compressor is further boosted to a pressure in excess of 45 atm in compressor 1 and compressor 2, and then fed to a heat exchanger for cooling. A portion of the high pressure nitrogen stream is withdrawn near the warm end of the heat exchanger and expanded in a warm expander to provide a portion of the refrigeration needed for the liquefaction. A second portion of the high pressure nitrogen stream is withdrawn from an intermediate location of the heat exchanger and expanded in a cold expander to provide the refrigeration in the cold part of the heat exchanger. The remaining

Figure 11 A nitrogen liquefier.

portion of the high pressure nitrogen stream exits the cold end of the heat exchanger at a temperature below — 170°C and is sent to an optional dense fluid expander. The pressure drop across this dense fluid expander is maximized, subject to the constraint that very little vapour forms in the exhaust. The pressure of this stream is further reduced to about 6 atm in a JT value and the resulting two-phase stream is separated in separator I. The vapour from this separator and the exhaust streams from the cold and warm expanders are mixed at appropriate temperatures, warmed and returned to the recycle compressor. The liquid from separator I is further cooled and reduced to near atmospheric pressure through another JT valve. The resulting liquid is collected as liquid nitrogen from separator II and the vapour is recycled to the make-up compressor.

The liquefier shown in Figure 11 is quite efficient. The use of a dense fluid expander contributes to increased efficiency but its use is optional. The working pressure range of the modern brazed plate and fin aluminium heat exchangers now approaches 100 atm. For increased efficiency, the pressure of the high pressure nitrogen steam is increased to maximum feasible values. Recently, processes using more than two gaseous expanders have been suggested for incrementally higher efficiencies.

In Figure 11, if none of the expanders are used then the liquefaction process reduces to the one proposed by Carl von Linde. On the other hand, if the warm expander and the dense fluid expander are removed, then the resulting process is similar to the one used by Georges Claude.

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