Distillation of Air to Recover Nitrogen

In most industrial applications, nitrogen is used as an inert gas. Cryogenic air separation can easily produce nitrogen gas with concentrations of oxygen below 5 p.p.m. Until the 1950s, the demand for nitrogen was low. Supply could easily be met by withdrawing a portion of nitrogen vapour from the top of the high pressure column as a co-product of a double-column process for oxygen production (for example, see Figure 6). Generally, up to 30% of the feed air can be recovered as high pressure nitrogen product from the top of the high pressure column. When needed, a portion of the nitrogen-rich vapour stream from the top of the low pressure column can also be recovered as a useful product.

In the 1960s industrial demand for nitrogen increased, and this led to a need for plants that were designed solely for nitrogen with no co-production of oxygen. For most applications, nitrogen product is required at a pressure between 6 and 10 atm. There are two basic schemes for nitrogen separation from air: one uses a single column while the other uses two columns (similar to the double-column process for oxygen production). The single-column process is used for relatively small size nitrogen plants (up to about 500 tons per day of nitrogen) and two-column processes are used for larger size plants. Cold boxes can now be designed to produce as much as 10 000 tons per day of nitrogen in a single train.

A single-column process for nitrogen separation is shown in Figure 7.

Feed air is compressed to a pressure in excess of about 5 atm absolute, cleaned of impurities in the molecular sieve purifier and cooled to near its dew-point in the main heat exchanger by heat exchange against the returning streams. The cooled air stream is then fed to the bottom of a single column. Sufficient separation stages are used in this column to attain the desired purity at the top of this column. A portion of the nitrogen vapour from the top is withdrawn and warmed in the main heat exchanger to provide the desired nitrogen product. The rest of the nitrogen vapour stream is condensed in a reboiler-condenser and returned as reflux to the column. The ratio of liquid to vapour flow rates in the column is in the neighbourhood of 0.6. The crude liquid oxygen stream from the bottom of the column is reduced in pressure and vaporized in the reboiler-condenser. The vaporized stream is partially warmed in the main heat exchanger and then expanded in a turbo-expander to near atmospheric pressure to provide the needed refrigeration for the plant. The expanded stream is then warmed to near ambient temperature in the main heat exchanger and eventually discharged as an oxygen-rich waste stream. The concentration of oxygen in the waste stream is approximately 35%. The flow rate of the nitrogen product stream is about 40-50 mol per 100 mol of the feed air.

The main problem with the single-column process is that the crude liquid oxygen leaving from the bottom of the column is at best, in thermodynamic equilibrium with the feed air. This means that there is a lower limit to the concentration of nitrogen in the crude liquid oxygen stream. This limits the recovery of nitrogen. For higher recoveries of nitrogen and more efficient processes, it is essential that the crude liquid oxygen stream be further distilled to recover the contained nitrogen. Figure 8 shows such a two-column process.

The major difference between this process and the single-column process of Figure 7 is that now

Figure 7 A single distillation column process for nitrogen production.
Figure 8 A two-column process fo nitrogen production.

crude liquid oxygen from the bottom of the high pressure column is fed to an intermediate location of a low pressure column for further distillation. A low pressure nitrogen vapour stream is recovered from the top of the low pressure column as a second product stream. Another portion of this low pressure nitrogen stream is condensed in the top reboiler-condenser and returned as the major reflux stream to the top of the low pressure column. Optionally, a minor nitrogen reflux stream can also be provided to the low pressure column from the high pressure column. An oxygen-rich liquid stream containing about 70% oxygen is withdrawn from the bottom of the low pressure column, reduced in pressure and vaporized in the top reboiler-condenser. The vaporized stream is then warmed in the main heat exchanger and eventually discarded as a waste stream. Note that the refrigeration for the plant is met by expanding a portion of the gaseous feed air stream to the low pressure column in a turbo-expander. In this process, the feed air is compressed to about 8-9 atm absolute and the pressure of the low pressure column is about 3 atm absolute. A nitrogen compressor is generally used to compress the low pressure nitrogen product stream and then it is combined with the high pressure nitrogen product stream. The typical flow rate of the combined nitrogen product stream is about 72 mol per 100 mol of the feed air stream. This two-column nitrogen generator and its variations are particularly attractive for enhanced oil recovery where a very large quantity of nitrogen is injected in the wells to maintain pressure.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment