A considerable amount of time is usually required to restart a cryogenic air separation plant following a shutdown or interruption in operation. The shutdown or interruption may be caused by a number of reasons such as power interruption, equipment problem, oversupply of non-condensable gas to a column, or an economic choice brought about by high power rates. The non-productive period from restart until product purities are re-established is costly, consuming the same amount of power as during normal production, but not providing desired product. In addition, product may have to be delivered by other means during downtime at considerable cost and burden on a product delivery system. At many plants the number of unscheduled shutdowns or interruptions in operation makes this non-productive period a significant factor.
A vertical composition profile exists throughout a distillation column. The liquid held up at various levels in the column becomes progressively richer in the more volatile component in ascending order in the column. Examples of composition profiles typical of air separation plants may be found in published literature (e.g. R. Latimer, "Distillation of Air", Chemical Engineering Progress, Volume 63, Number 2, pp. 35-59, 1967). A distillation column in an air separation plant promotes separation of the components of air when the air, in liquid and vapor form, contacts internal material situated in the distillation column. The internal material may be any of the known materials used to promote separation in distillation columns such as trays or packing. Rising vapor in the column serves to hold up liquid on internal material in the column. When vapor flow ceases in a distillation column the majority of liquid, which was held up on the internal material by the vapor, will drain from the internals to the bottom of the column or column sump. The resulting liquid pool is of a composition intermediate between the column's top and bottom compositions during normal operation.
To restart the operation of the air separation column(s), liquid retained at the bottom or sump of the column(s) is normally drained or is reprocessed to reestablish desired purity. By draining liquid from the sump, refrigeration provided by that liquid to the plant is lost, representing a power penalty. By reprocessing liquid from the sump, upon restart, significant time is required to re-establish purity of liquid, resulting in costly power consumption.
By draining liquid from the sump in argon producing plants, a considerable quantity of argon is lost. Because argon is such a minor component in air, a costly time delay occurs before argon purity is reestablished when the plant or argon column is restarted.
After an interruption in operation, it would be desirable and advantageous to efficiently re-start a plant by reducing the time required to achieve desired purity on an air separation plant or a distillation column in a plant, in a cost effective and simple manner.
Accordingly, it is an object of this invention to provide a system for restarting a cryogenic air separation plant in a faster and less costly manner than is possible with conventional practice.