Cryogenic Air Distillation With Companded Nitrogen Refrigeration

Process and apparatus are disclosed for production of gaseous oxygen plus optional byproducts (argon and/or nitrogen) and optionally including a minor amount of liquid product for oxygen purities from below 90% to above 99.5% and for high O.sub.2 recoveries. The invention results in improved recovery of product and/or byproduct because more work and resulting refrigeration is obtained from the nitrogen routed to the refrigeration expander, while no additional power import is required and no substantial increase in capital expenditure is required. This leaves more liquid N.sub.2 reflux available for increasing recovery than would be possible in the absence of the invention. The invention is appropriate for large-scale tonnage oxygen-producing plants such as those found in industrial applications.

In the classical dual pressure process for air distillation, cooled air is supplied to a high pressure rectifier. The HP rectifier overhead reboils the bottom of a lower pressure nitrogen removal column, and also evaporates the oxygen product. The rectifier liquid products are routed to the LP column--kettle liquid as feed, and LN.sub.2 as overhead reflux (directly injected). The required process refrigeration may be obtained by either of two approximately equivalent methods: work expansion of part of the supply air to an appropriate height of the LP column, or work expansion of part of the HP rectifier overhead N.sub.2 to exhaust pressure (approximately the LP column overhead pressure).

The recovery of O.sub.2 is determined by the ability of the LN.sub.2 reflux at the LP column overhead to rectify O.sub.2 out of the N.sub.2 exhaust. For the classical process described above, there is more than adequate LN.sub.2 reflux available even after satisfying refrigeration requirements to achieve very high O.sub.2 recoveries, on the order of 98% or more. However, whenever any process enhancement is introduced in order to achieve increased byproduct recovery, increased purity, or reduced energy consumption, there is usually also a corresponding reduction in the LN.sub.2 available for reflux, and hence a decrease in O.sub.2 recovery.

For example, LN.sub.2 coproduct or pressurized GN.sub.2 coproduct obviously cause a mole-for-mole reduction in LN.sub.2 reflux availability. Increasing the O.sub.2 delivery pressure by LOXBOIL results in less air supplied to the HP rectifier, and hence less LN.sub.2 available as reflux. LOX coproduct decreases the HP rectifier reflux, which is LN.sub.2. Argon byproduct requires an auxiliary argon rectifying section of the LP column, which communicates at its bottom with the vapor and liquid streams of the LP column between the argon stripping section and nitrogen stripping section of the LP column. The argon rectifier is conventionally reboiled by exchanging latent heat with part of the kettle liquid. This raises the quality of the feed to the LP column, and hence increases the need for LN.sub.2 reflux. As a final example, low-energy triple-pressure processes such as disclosed in U.S. Pat. Nos. 3,688,513, 4,507,134, and 4,578,095 also supply one part of the air to the HP rectifier, and gasify part of the kettle liquid, thereby decreasing the LN.sub.2 reflux availability on two counts.

In summary many different hypothetical improvements to air separation end up being less advantageous than first supposed, or even disadvantageous, due to the reduction in LN.sub.2 reflux availability and the resulting decrease in O.sub.2 recovery. What is needed is a method or apparatus which makes more LN.sub.2 reflux available than in conventional practice, without additional power requirement or excessive increase in equipment cost. That is one primary objective of the disclosed invention.

It is known to expand part of the supply air to produce refrigeration, and drive a warm end compressor with the expansion work. U.S. Pat. No. 2,666,303 discloses such an apparatus in which overhead N.sub.2 from the LP nitrogen removal column is compressed, thereby lowering LP column pressure below atmospheric pressure. West German patent application No. 2854508 by Rohde, published 6/19/80, discloses such an apparatus wherein the warm compressor compresses the expansion air before cold expansion.

Other prior art references involving similar companders include U.S. Pat. Nos. 3,261,168, 4,133,662, and Russian Patent No. 756150.

It is known to warm HP rectifier overhead N.sub.2 to near ambient pressure, compress it with an externally powered compressor, re-cool it, and expand it back to HP rectifier overhead pressure, thereby providing the relatively large amount of refrigeration necessary in liquefaction cycles. One example is U.S. Pat. No. 3,319,427. It is also known to use a warm-end compressor driven by the expander to pick up part of the external compression duty, as disclosed in U.S. Pat. No. 4,099,945.

It is known to boil liquid nitrogen in a latent heat exchanger at the overhead of the argon rectifying section of a dual pressure column (U.S. Pat. No. 3,729,943) and of a triple pressure column (U.S. Pat. No. 4,578,095).

None of the above prior art references addresses or solves the problem of providing increased LN.sub.2 reflux and hence increased O.sub.2 recovery in a flowsheet wherein a discharge stream of pressurized N.sub.2 gas must be expanded for refrigeration.

In the disclosed solution to the above problem the pressurized nitrogen stream to be expanded and exhausted is caused to produce more refrigeration work by being warmed and then further compressed in a compressor powered by the N.sub.2 refrigeration expander. After companding (both expansion and compression) the N.sub.2 is discharged at approximately atmospheric pressure; this is necessary because otherwise, if the N.sub.2 remained pressurized after companding, no net refrigeration would be accomplished unless an externally powered compressor were also incorporated. The present disclosure makes it possible to develop all required process refrigeration without the addition of an externally powered N.sub.2 compressor, although such an addition is not excluded from this invention.

The disclosed invention, consisting of cold expansion and warm compression of an initially pressurized N.sub.2 stream plus final exhaust at atmospheric pressure, results in about 20 to 30% more refrigeration being developed per mole of N.sub.2 processed through the compander. The increase in specific refrigeration can be used to either decrease the total compander flow, making more LN.sub.2 reflux available, or to make more refrigeration, thus allowing withdrawal of more liquid coproduct; or any combination.

It is possible to conduct either expansion or compression first, and to recycle all, part, or none of the N.sub.2 back into the cold box after compression. These three embodiments plus the various tradeoffs applying to each are described below. In general any of the three embodiments may be used with any flowsheet using discharge of pressurized nitrogen to produce refrigeration, since none of the embodiment recycle N.sub.2 to any part of the actual separation process such as the distillation columns.

FIGS. 1a, 1b, and 1c illustrate the compander embodiments which extract additional work from a pressurized nitrogen stream being discharged to ambient pressure. In FIG. 1a, cold expansion to below atmospheric pressure is followed by warm compression to atmospheric pressure, and no N.sub.2 is recycled. In FIG. 1b cold expansion to approximately atmospheric pressure is followed by warm compression of only a minor fraction to expander inlet pressure, and only that fraction is recycled. In FIG. 1c warm compression occurs first, all the N.sub.2 is recycled into the cold box, and then cold expanded to approximately atmospheric pressure.

FIG. 2 illustrates the FIG. 1a embodiment as applied to a dual pressure high purity O.sub.2 flowsheet incorporating increased argon recovery enhancements, and FIG. 3 illustrates the FIG. 1b embodiment as applied to a triple pressure low energy high purity O.sub.2 flowsheet.

Referring to FIG. 1a, cold pressurized is initially superheated in sensible heat exchanger 3 at least sufficiently to preclude condensation during work-expansion, and then is work-expanded in cold expander 1, thereby depressurizing to below atmospheric pressure and also cooling the N.sub.2. It is then heated to near ambient temperature by heat exchange with gases entering the cold box, and routed to the suction of warm compressor 2, which raises the pressure sufficiently for discharge to ambient. Warm compressor 2 is directly powered by cold expander 1. Heat exchanger 3 may be a single component or multiple cores, split streams, etc.

In FIG. 1b, the differences from FIG. 1a are that the N.sub.2 is only depressurized to slightly above atmospheric pressure and only a minor fraction of the warm N.sub.2 is compressed, but all the way up to expander inlet pressure, and the compressed fraction is cooled and combined with the cold pressurized N.sub.2 to form the expander supply stream.

In FIG. 1c, the entire cold pressurized N.sub.2 stream is warmed in 3 to near ambient, compressed in 2, cooled back to near-cold-box temperature, expanded in 1 to about atmospheric pressure, and discharged via 3. Whereas in many traditional flowsheets there is little difference between expanding air or nitrogen for refrigeration, in some advanced flowsheets for making high purity oxygen plus coproduct argon there is a significant difference. FIGS. 2 and 3 are examples. Traditional air expansion or HP rectifier N.sub.2 expansion results in less reboil through the oxygen-argon stripper and also the argon rectifier, thus necessitating more stages and pressure drop to achieve high O.sub.2 purity, and also decreasing the potential for argon recovery. If instead the expansion vapor is generated in the argon rectifier above the bottom, those problems are solved. Boiling liquid nitrogen is a preferred method of generating such vapor. The offsetting problem is that since the pressure of the N.sub.2 vapor sent to expansion is appreciably lower than the HP rectifier overhead pressure, more N.sub.2 is required, thus reducing LN.sub.2 reflux. However, that problem is minimized in either or both of two ways: first, by companded expansion of the pressurized N.sub.2 as disclosed herein; and second, by evaporating the N.sub.2 at as high a pressure as possible, e.g., at an intermediate height of the argon rectifier.

FIG. 2 illustrates both of the above techniques, as applied to a dual pressure high purity plant. Supply air is processed conventionally through main exchanger 3, HP rectifier 5, LP N.sub.2 removal column 6, including argon rectifier 7. LOX is gasified by total condensation of part of the feed air in 8, and the liquid air is split to intermediate reflux both rectifier 5 and column 6 via valves 9 and 10 respectively. Kettle liquid is split via valves 11 and 12, and part is evaporated in 13 to reflux the overhead of 7. HP rectifier overhead LN.sub.2 is split by valves 14 and 15, part supplying indirect latent heat exchange reflux to column 7 via latent heat exchanger 16. The cold pressurized N.sub.2 from 16 is the feed to the companded expansion device, expander 1 and compressor 2. Although the FIG. 1a embodiment is illustrated, it could equally well be the FIG. 1b or 1c embodiment. Also, exchanger 16 could alternatively be located at the top of rectifier 7 vice intermediate, with only small penalty. Other features illustrated in FIG. 2 are explained more fully in copending application Ser. No. 853,461 filed 04/18/86 by Donald C. Erickson, which is incorporated by reference.

Referring to FIG. 3, a low energy triple flowsheet similar to those in U.S. Pat. No. 4,578,095 is illustrated, except that in order to provide positive assurance against argon freezeup at the top of argon removal column 17, liquid N.sub.2 is evaporated in latent heat exchanger 18 at a pressure sufficient to ensure temperatures above the argon freezing temperature, i.e., a pressure of about 31 psia or higher. The evaporated fraction is then routed to the companded N.sub.2 expansion apparatus (expander 1 and compressor 2). Kettle liquid refluxes an intermediate height of 17 by latent heat exchanger 19; alternatively the column 17 intermediate liquid can directly exchange latent heat with N.sub.2 removal column 20 intermediate vapor. FIG. 3 also illustrates total condensation LOXBOIL in 21 and liquid air split intermediate refluxing via valves 22 and 23. The companded expander is the partial recycle or FIG. 1b embodiment and hence may include cooler 4, although either other embodiment could also be used. Also, if other means are available to positively preclude argon freezeup, it would be desirable to interchange the locations of exchangers 18 and 19, so as to produce the pressurized N.sub.2 at higher pressure.

Argon removal from 17 may be as vapor or liquid. A preferred technique is as liquid, using a barometric leg to increase the pressure well above atmospheric pressure before evaporation.

The basic disclosure of FIG. 1 obviously applies to the work-expansion of other cold pressurized gases besides N.sub.2.