Method and apparatus for producing nitrogen from air by cryogenic distillation

Abstract

Nitrogen gas at a single pressure is produced from a two-column cryogenic distillation of air. The bottoms liquid product from the high pressure column is divided into portions, at least one of which does not enter the low pressure column as a feed stream. By these means, a portion of an oxygen-rich stream is removed from the distillation, further enhancing nitrogen recovery and achieving low specific energy consumption for nitrogen product.

Description

[0001] This application is entitled to the benefit of Provisional Patent Application Ser. No. 60 / 186,572 filed Mar. 2, 2000.[0002] This is a Continuation-in-part of Ser. No. 09 / 775,362, Feb. 1, 2001, now abandoned.[0003] The present invention is directed to the cryogenic separation of air by distillation for the production of primarily gaseous nitrogen.[0004] Nitrogen is among the most heavily produced and used chemicals. It finds application in the petroleum, glass, foods, electronics, pharmaceutical, and metals industries. Cryogenic separation of air is a principal means of producing nitrogen. Cryogenic air separation plants, chiefly for the production of gaseous nitrogen, exist in a number of configurations. These, in turn, group around single distillation column and double distillation column designs. There are many variations of these designs in each category. In most cases the objective is to produce nitrogen at the lowest energy consumption for any given delivery pressure; bu...

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Benefits of technology

[0006] An object of the invention is to provide a process for a two-column cryogenic distillation of air which achieves high nitrogen recovery, low unit energy consumption, and, though nitrogen is produced by each distillation column operating at different pressures, the product gaseous nitrogen is delivered at a single pressure, a desirable and convenient feature, while maintaining high nitrogen recovery and low unit energy consumption.

[0011] It can be seen then that such a system described above becomes easier to effect as the pressure ratio of the pressure of the high pressure column to the pressure of the stream vaporizing in the condenser of the low pressure column becomes greater. This pressure ratio, when coupled with the quantity of nitrogen actually recovered, has a direct impact on the requisite energy to produce a nitrogen product at a given delivery pressure. A greater pressure ratio indicates a higher energy consumption for a given product delivery pressure than other processes which have lower corresponding pressure ratios. For energy reduction, improvements in this process strive to reduce this pressure ratio and the related pressure ratio of the pressure of the high pressure column to the pressure of the low pressure column.

[0013] The current invention improves on this process by conducting the condensation of vapors at the pressure of the high pressure column, all of which may be the overhead vapor from the high pressure column, in at least two stages of coolant vaporization in series. The composition of the boiling stream becomes richer in oxygen as the extent of vaporization increases. At essentially a constant temperature of vaporization, the first stage of vaporization occurs at a higher pressure of the vaporizing stream and the second stage at a lower pressure of the vaporizing stream. The vapor from the first stage is both richer in nitrogen and higher in pressure than the vapor from the second stage, and constitutes a feed to the low pressure column. Therefore, the pressure of the low pressure column is maximized--a desirable effect for a given high pressure column pressure, and oxygen is preferentially rejected from the column system from the second stage condenser. Because the composition of the liquid bottoms from the low pressure column are related to the composition of the vapor feed to the bottom of the low pressure column, these bottoms are richer in nitrogen and vaporize at a colder temperature when transferred to the low pressure column condenser and reduced in pressure, which reduces the ratio of the pressure of the high pressure column to the pressure of the low pressure column. The low pressure column condenser coolant can operate just above atmospheric pressure; but in alternative designs may operate at higher pressure, retaining the energy-reduction benefits of the invention. The effects of reducing the pressure ratio of the operating pressures of the two distillation columns, and rejecting an oxygen-rich mixture from the second or last stage of the high pressure column condenser lead to lower compression energy and higher nitrogen recovery, which minimize unit energy expenditure for the nitrogen produced at a specified delivery pressure.

[0020] The vaporized rich liquid from separator 16 is fed to the bottom of the low pressure column 20. This rich liquid vapor was vaporized at essentially the operating pressure of the low pressure column. The balance of the rich liquid which was passed to condenser 18 is vaporized, is partially warmed in subcooler 19 and main heat exchanger 11 and turboexpanded in 12 to produce refrigeration. The turboexpander exhaust gas 109 is warmed in subcooler 19 and main heat exchanger 11 and may be used elsewhere or vented to atmosphere. This is a stream of elevated oxygen content; and therefore, its disposition in this manner assists in the separation of the air to make the nitrogen product.

[0025] In other respects the process embodiment in FIG. 2 is similar to that of FIG. 1. Still another embodiment of the invention (not shown) achieves elevation of the low pressure column pressure and the high pressure column pressure by means of elevation of the vaporization pressure of the low pressure column condenser coolant. In these cases said vaporized coolant may also be turboexpanded to produce refrigeration. Another advantage of such operation is that the delivery pressure of the nitrogen product from the high pressure column can be efficiently raised to meet a specified product delivery pressure, while maintaining low energy requirements inherent in the process invention.

Problems solved by technology

Nevertheless, simple two-column systems do not necessarily have lower unit energy requirements than improved single column systems.

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