Nitrogen purity control in the air separation unit of an IGCC power generation system

In a process for the separation of air wherein an air feed stream is introduced into a multiple-column cryogenic distillation system comprising at least a higher-pressure column and a lower-pressure column, wherein a nitrogen-enriched vapor stream is withdrawn from the lower-pressure column, and wherein a nitrogen-enriched liquid stream is withdrawn from the higher-pressure column, reduced in pressure, and introduced into the lower-pressure column as a reduced-pressure nitrogen-enriched liquid reflux stream, a method of operating the cryogenic distillation system which comprises:

• (a) measuring the composition of the nitrogen-enriched liquid stream withdrawn from the higher-pressure column and manipulating the flow rate of the nitrogen-enriched liquid stream as a function of the resulting measured composition, wherein the flow rate of the nitrogen-enriched liquid stream is controlled by pressure reduction across a control valve to yield an intermediate-pressure nitrogen-enriched liquid stream;
• (b) storing nitrogen-enriched liquid in a nitrogen-enriched liquid storage vessel, wherein the storage vessel is in flow communication with the intermediate-pressure nitrogen-enriched liquid stream, thereby yielding a net stream of intermediate-pressure nitrogen-enriched liquid reflux at a flow rate which is equal to, greater than, or less than the flow rate of the intermediate-pressure nitrogen-enriched liquid stream;
• (c) compressing the nitrogen-enriched vapor stream from the lower-pressure column in a nitrogen product compressor, recycling a portion of the resulting compressed nitrogen-enriched vapor stream to the higher-pressure column, and withdrawing the remaining portion of the compressed nitrogen-rich vapor stream as a compressed nitrogen product stream;
• and
• (d) measuring the level of nitrogen-enriched liquid in the storage vessel and manipulating the flow rate of the compressed nitrogen-enriched vapor stream to the higher-pressure column as a function of the level of nitrogen-enriched liquid in the storage vessel.