1. Field of the Invention
This invention relates to an air liquefaction and separation process and equipment, and more particularly to a method and equipment by which liquefied air is purified and separated at a pressure significantly lower than the source air pressure in the recently prevailing overall low pressure air liquefaction and separation method.
2. Description of the Prior Art
Since the advent of the air liquefication and separation method investigations of purification and separation have been directed toward energy saving in purifying and separating products. Typical examples of those investigations include improvements in process for minimizing the amount of residual air for ensuring coldness necessary for liquefaction and separation of air, improvements in rectifying towers for ensuring a higher yield of products and reducing energy necessary for purification and separation and improvements by which to enhance operating efficiencies of various machines and components in separators. As a result, the air liquefaction and separation equipment has undergone changes from the early type wherein a pre-cooling auxiliary air system with about 200 Kg/cm.sup.2 G of pressure is incorporated into an air system with about 5 Kg/cm.sup.2 G of source air pressure to the overall low pressure type wherein no auxiliary air system is required through the low pressure type wherein an auxiliary air system of about 10 Kg/cm.sup.2 G pressure is employed. The latest overall low pressure type air liquefaction and separation equipment is constructed as shown in FIG. 1. Having compressed to about 5 Kg/cm.sup.2 G and cooled to approximately its liquefying point, source air is fed through a passage 1 to a lower tower 3 (high pressure tower) in a multiple rectifying tower 2. While traveling upward within the lower tower 3, the source air conducts exchange of substance with liquefied nitrogen flowing from an evaporator 4. The circulating liquid bears highly purified nitrogen and exhibits an increase in oxygen content, respectively, when traveling upward and downward in the tower. Accordingly, the liquefied air containing high density oxygen at the bottom of the lower tower 3 is fed to an upper tower 7 (low pressure tower) through a passage 5 and an expansion valve 6. Nitrogen gas moving upward along the lower tower 3 exchanges heat with liquid oxygen resting on the bottom of the upper tower 7, evaporating the liquid oxygen and condensing by itself. The resultant liquefied nitrogen is supplied as circulating liquid nitrogen to the lower tower 3 with the part thereof being directed to the upper tower 7 via a passage 8 and an expansion valve 9. The source material supplied to the upper tower 7 is then separated into nitrogen and oxygen through rectification with product nitrogen discharged via the top of the upper tower and product oxygen being discharged via the bottom thereof through passages 10 and 11, respectively. In FIG. 1, an impure nitrogen drain is labeled 12.
The overall low pressure air liquefaction and separation method has been well advanced thanks to a highly efficient multiple rectifying tower system to the extent that it can almost completely rectify and separate oxygen and nitrogen. Furthermore, an attempt to reduce power conventional units by meamns of an improvement in providing coldness has attained its maximum through the utilization of a reactionary expansion turbine, etc. More advanced energy-saving purification and separation seems impossible or impractical without a drastic innovation introduced in connection with separation technique.
With ever-increasing demand for oxygen and in view of a resource-saving requirement, energy necessary for separation and purification should be as low as possible. The present-day overall low pressure air liquefaction and separation process, however, appears to be a way to attain a high yield with the highest reliability and is not expected to decrease an electrical energy requirement for purification and separation to a minimum (typically, below 0.45-0.47 KWH/Nm.sup.3 in connection with high purity oxygen) no matter how a large-sized system is designed. Lowering the pressure of the source air is deemed as an effective and practical approach to reduce energy consumption due to the historical fact that the pressure of an auxiliary system has been gradually decreased. Since a differential pressure between the lower tower (say, 4.5-5.0 Kg/cm.sup.2 G) and the upper tower (say, 0.2-0.45 Kg/cm.sup.2 G) of the multiple rectifying tower system insures a differential temperature of about 1.degree.-3.degree. C. necessary for evaporating the liquefied oxygen at the bottom of the upper tower and condensing the gaseous nitrogen at the top of the lower tower, an attempt to decrease the pressure of the lower tower results in decreasing the internal temperature of the lower tower and failing to insure the differential temperature between the top of the lower tower and the bottom of the upper tower. However a high performance evaporator is designed and manufactured, it is essentially impossible to decrease the pressure of the source air below about 5 Kg/cm.sup.2 G and save energy for purification and separation.