Refrigeration – Cryogenic treatment of gas or gas mixture – Liquefaction
Reexamination Certificate
1999-09-23
2001-05-15
Doerrler, William (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Liquefaction
Reexamination Certificate
active
06230518
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for the production of liquid air with an oxygen content of between 16 and 21 mol % in a low-temperature process, whereby atmospheric air is used as a feed gas. In a warm part of the process, H
2
O, carbon dioxide and contaminants entrained in the air are removed; cold values are produced by compression and engine expansion of process streams, and the liquid air is obtained in a cold part of the process by low-temperature rectification.
In addition, the invention relates to a liquefier for implementing the process comprising a purification station, at least one compressor for compressing process gas, at least one expansion machine for process gas and a rectification column, a head cooling unit and a bottoms heating unit.
BACKGROUND OF THE INVENTION
A process and a liquefier have been disclosed in the article in Process Engineering (March 1997) “The Air that I Breathe.” Air is liquefied in a low-temperature process, subjected to low-temperature rectification, and a liquid air product with an oxygen content of between 16.5 and 21% oxygen is produced. This air is produced by mixing an oxygen product and a nitrogen product (Synthetic Liquid Air, SLA). In this system, it is disadvantageous that there is a waste of energy to separate the air into liquid products of oxygen and nitrogen which are recombined to form SLA.
SUMMARY OF THE INVENTION
An object of the invention is, therefore, to provide a process and apparatus for the production of liquid air at a low cost. Upon further study of the specification and appended claims, other objects and advantages of the convention will become apparent.
To achieve the process aspect of the invention, there is provided a process for the production of liquid air with an oxygen content of between 16 and 21 mol % in a low-temperature process, comprising purifying atmospheric air to remove H
2
O, carbon dioxide and contaminants entrained in the air; producing cold values by compression and engine expansion of at least one process stream, and obtaining the liquid air in a cold part of the process by low-temperature rectification, wherein the improvement comprises conducting said low-temperature rectification in a rectification column having less than four theoretical plates, withdrawing a purified liquid air head product from the rectification column, withdrawing a liquid bottoms stream from the rectification column, vaporizing the liquid bottoms stream in indirect heat exchange with air to be cooled prior to being fed to the rectification column, and venting resultant warm residual gas into the atmosphere or passing said resultant warm residual gas at least partially to the purifying step.
Thus, a characteristic feature of the process according to the invention is that the liquid air is produced with use of less than four theoretical plates as a liquid head product in the rectification and that in addition a liquid bottoms product is obtained in the rectification, used exergetically and vented as a warm residual gas into the atmosphere or used at least partially in the purifying of the compressed gas. Whereas, previously it was necessary to employ theoretical plates an order of magnitude higher to produce an oxygen product and a nitrogen product, now only a small fraction of the separative work is conducted. In addition, obtaining the liquid air as a head product avoids the requirement for intermediate storage of liquid oxygen and liquid nitrogen. Also, contaminants, for example hydrocarbons, are discharged with the liquid bottoms product. The energy content of the bottoms product is largely used, and the residual gas that accumulates after use can be fed for still an additional use.
In an advantageous embodiment of the process according to the invention, the purifying can be carried out adsorptively, wherein the residual gas can be used as a regeneration gas and/or a purge gas. Since the liquid bottoms product is removed from the rectification column to avoid a concentration of hydrocarbons in the rectification column and in the air product, and since purge gas and regeneration gas are required for adsorptive purifying, the use of the residual gas for such purposes provides synergy insofar as it is unnecessary to prepare regeneration gas and purge gas extrinsically of the process.
In a more comprehensive embodiment of the invention, a heated gas stream from the cold part of the process can be admixed to the atmospheric air, and the resulting hot mixed feed can be compressed to a starting pressure for engine expansion and then purified. As an alternative, the atmospheric air can be precompressed, a heated gas stream from the cold part of the process can be admixed, and the resulting warm mixed feed can be compressed to a starting pressure for engine expansion and then purified.
In another embodiment of the process according to the invention, the atmospheric air is precompressed and then purified, a heated gas stream from the cold part of the process is admixed, and the resulting warm mixed feed is compressed to a starting pressure for engine expansion.
The most advantageous embodiment of the compression and purifying steps in each case is determined by optimizing the process and by the availability of commercial compressors.
It is preferable to precool one part of the purified warm mixed feed compressed to the starting pressure for engine expansion countercurrently against a cold gas and against at least one fluid, e.g. the liquid bottoms product, from the rectification column to a starting temperature for the engine expansion and to further cool the resultant gas by engine expansion. It is further preferred that another part of said warm mixed feed is both precooled and countercurrently cooled again, at least partially liquefied and then depressurized isenthalpically and fed to the rectification as a throttled feed.
A head gas from the rectification column can be admixed to the engine expanded mixed feed, and both together used as the cold gas for the countercurrent cooling. This embodiment of the process according to the invention is especially advantageous when both gas streams are present at the same pressure.
The liquid bottoms product can be evaporated and heated against the warm mixed feed which is cooled. In this case, the cold content of the bottoms product is used in an especially efficient manner.
In another embodiment of the process according to the invention, the liquid bottoms product is evaporated by indirect cooling of the warm mixed feed, heated to a starting temperature for passage through a gas turbine for residual gas, expanded by the residual gas turbine, cooled again as a result and is again used to cool the warm mixed feed. The engine expansion in the residual gas turbine has advantages if an adequately high pressure drop exists between the rectification pressure and either the atmosphere or the pressure during the purifying step.
Turning now to the apparatus aspect of the invention, a characteristic feature of the liquefier according to the invention is that the bottoms heating of the rectification column is designed as indirect heating with an electric heater or with a heating tube arrangement, whereby the heating tube arrangement carries a suitable warm fluid, preferably a warm process gas, and that the number of separating stages corresponds to less than four theoretical plates.
The electric heater is especially suitable for small units, in which a correspondingly higher power consumption is not important compared to the cost for the installation of a heating tube arrangement with related process gas lines. The low number of separating stages ensures a small overall pressure drop for the gas conversion in the rectification column. A head condenser is avoided since liquid from the throttled feed forms the column reflux. In the process according to the prior art, however, columns with condensers and a considerable number of separating stages are used. The liquid air produced according to the invention thus requires less investment and less energy because of the smal
Hahn Eric
Voit Jurgen
Doerrler William
Drake Malik N.
Linde Aktiengesellschaft
Millen White Zelano & Branigan P.C.
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