Oxygen is supplied through the cryogenic separation of air within an air separation plant. As well known in the art, the air is separated in such a plant by compressing, purifying and cooling then air to a temperature suitable for the distillation thereof and then separating the air in a distillation column system. The distillation column system typically utilizes a higher pressure column and a lower pressure column that are thermally linked. Within the higher pressure column, the air is separated into a nitrogen-rich vapor column overhead and an oxygen-rich liquid column bottoms known as kettle liquid or crude liquid oxygen. The column bottoms is further refined in the lower pressure column to produce an oxygen-rich liquid as a column bottoms. The linkage between the columns can be effected by means of a condenser reboiler situated within the lower pressure column to vaporize part of the oxygen-rich liquid column bottoms against condensing the nitrogen-rich vapor of the higher pressure column. The condensed nitrogen-rich vapor can be used as reflux for both of the columns and the resulting heated oxygen-rich liquid serves as boil-up for the lower pressure column.
The oxygen is supplied from the air separation plant by heating an oxygen-rich stream composed of the oxygen-rich liquid column bottoms produced in the lower pressure column within a heat exchanger used in cooling at least part of the air. Where the oxygen is desired at high pressure, the oxygen-rich liquid stream can be pumped before being heated either to produce a high pressure vapor or a supercritical fluid after having been heated. There are many applications where the operator of the air separation plant has to guarantee the supply of oxygen. In certain applications, even intermittent interruptions of the oxygen supply are not permissible. A supply failure can be occasioned by a cessation of the normal operation of an air separation plant that can be caused by a failure of a key component, for instance, a compressor or turbine trip. In such case, the air separation plant warms and the liquid within the distillation columns falls to bottom regions thereof. When the plant is able to be restarted, the purity of the oxygen may not be high enough to use in the particular contracted application. As such, even after a plant restart, there is a further delay until the plant can be brought back on line, one again, supplying the oxygen.
In order to assure the delivery of the oxygen at pressure, it is known to accumulate a portion of the liquid oxygen produced within the plant within a remote storage tank or other reservoir. During a transient event when the plant operation is interrupted, the oxygen can be pumped from the storage tank to an auxiliary vaporizer in order to supply the oxygen during the transient. Typically, a set of pumps is provided in the cold box of the air separation plant for pumping the oxygen-rich liquid during normal operations and another set of pumps associated with the storage tank is also provided for pumping the oxygen-rich liquid during the transient. This represents a considerable capital expense given that two sets of pumps and associated valves and instrumentation must be purchased, operated and maintained and that these pump must be specified for high pressure oxygen service.
Another arrangement is shown in United States Patent Application No. 2008/0184736. In this case, liquid flows from the low pressure column sump to an external storage tank from which it is pumped and sent to the plant heat exchanger. The problem with this type of plant design is that liquid is continually exported from the plant to the storage tank and along with such export, refrigeration that would otherwise maintain the plant in balance due to heat leakage into a cold box used in containing the distillation columns and warm end losses from the plant heat exchanger. This is exacerbated by the fact that the large liquid reservoir contains also the liquid to be used for extended back-up during transients and due to its size may be located some distance from the column cold box. Thus, there are losses in this type of installation that are compensated by supplying increased refrigeration at an increase in the overall power consumption of the plant.
As will be discussed, the present invention provides a method and oxygen supply system to be used in connection with an air separation plant in which, among other advantages, can be effectuated that does not use extra pumps that are solely associated with supplying oxygen during a plant transient event and that inherently operates in a more energy efficient manner.