Cryogenic air separation has been the major commercial source of air components for over half a century, and those components have become increasingly important industrial products. For example, steelmaking today is dependent upon pipe-line quantities of oxygen obtained from such separation.
The separation conventionally takes place in one or more cylindrical columns containing liquid-gas contacting structures wherein a liquid flows downwardly over the structures and gaseous vapor ascends through the structures in counter-current contact with the liquid. One type of such contacting structures is referred to as packing comprising a labyrinth of surfaces over which the liquid flows as a film and sufficient void spaces for the ascending vapor to pass through easily in contact with the flowing liquid. There are two basic kinds of packing (1) regular or structured and (2) random. The former is fixed by design and typically in a honey comb-like structure made of sheet metal. Random packing is a mass of loose of material such as glass beads, ceramic rings or shaped pieces of metal that can be dumped or poured into the column.
Yet another type of liquid-gas contacting structures are called trays. A series of trays, one above another, in a column hold shallow pools of liquid through which the ascending gas is bubbled.
While packing, both structured and random, had been demonstrated as useful for cryogenic air separation and other distillation processes, it was not until the 1980's that packing, especially structured packing, became preferred contacting structures. The desirable properties of packing were known in that less power was required to compress the air being fed to the distillation column than when trays are employed. This is because a greater difference in pressure is necessary between the top and bottom of the column where the ascending gas has to be bubbled through a number of pools of liquid on trays as opposed to the passing over liquid films on the surfaces of packing. Moreover, the cost of electrical power is about one-third of the total operating cost of a cryogenic air separation plant so that even a percentage or two reduction in that power cost is significant.
Despite the known operating cost savings of using structured packing, such savings were offset by the additional capital cost of the packing. Also, tray design had been highly developed to give precise and predictable results. Circumstances changed, however, in the 1970's and early 1980's. The cost of generating electricity increased dramatically in part because of actions by the OPEC cartel and to a lesser extent as a result of environmental cost burdens. At the same time (perhaps stimulated by the increased cost of electricity) more efficient and less costly structured packing was developed and used commercially in the petrochemical and petroleum refining industry where the vast majority of distillation equipment is employed. Commercial use in cryogenic air separation followed.
Notwithstanding the improvements in structured packing, it remains more expensive as a capital item than trays. Random packing, on the other hand, is cheaper than structured packing and will operate at lower pressure drop than trays like structured packing, but it has serious performance deficiencies for air separation. The primary deficiency is lack of uniform flow of the descending liquid over the entire cross section of the column when a section or bed of random packing has a depth that is normal for air separation. This is a problem since an economically viable air separation system requires precise operational control in order to make a nearly complete separation of oxygen, nitrogen and argon because their boiling points are within a relatively narrow range (-360.degree. F., -383.degree. F. and -368.degree. F. respectively at normal atmospheric pressure).
Therefore, it is an object of this invention to provide a method of cryogenic air separation employing random packing as liquid-gas contacting structures.
Another object of the invention is to provide a distillation column for cryogenic air separation containing alternate sections of random and structured packing.