Feed air to a cryogenic air separation plant is precleaned of higher boiling impurities such as carbon dioxide, water vapor and hydrocarbons prior to passage of the feed air to the distillation column or columns. This precleaning is generally done by passing the feed air through molecular sieve prepurifiers or through reversing heat exchangers followed by gel traps. This precleaning results in a feed air stream having very low concentrations of higher boiling impurities.
Over time, however, some hydrocarbons will accumulate in the oxygen-enriched liquid produced in the rectification column(s). This is because, of the three major components of air, nitrogen, argon and oxygen, oxygen has the lowest volatility and thus these higher boiling impurities will go with the oxygen. Moreover as the oxygen-enriched liquid is reboiled, the higher boiling impurities will preferentially remain in the liquid rather than be boiled off. Such an increasing concentration of hydrocarbons in liquid oxygen poses a safety problem.
Those skilled in the art have addressed this problem by passing oxygen-enriched liquid from a reboiler through an adsorbent bed to remove the hydrocarbons from the liquid. The liquid is pumped from the reboiler and through the adsorbent bed by a liquid pump. A conventional liquid pumped adsorbent bed is illustrated in "Oxygen: 2000 Tons/Day", Mechanical Engineering, January, 1978, R. L. Shaner and W. E. Sweeney.
While this system for cleaning reboiler liquid of higher boiling impurities has been satisfactory, it does have some disadvantages. For example, the use of a pump, as with any piece of rotating equipment, introduces some unreliability and potential hazard to the cryogenic air separation process because of the potential for failure of the moving parts. In addition, the pump uses energy and introduces heat input into the cryogenic process requiring a corresponding increase in refrigeration generation resulting in a process inefficiency.
Accordingly it is an object of this invention to provide an improved method for the cryogenic separation of air wherein oxygen-enriched reboiler liquid is cleaned of higher boiling impurities without need for a liquid pump.