Distillation of Air to Recover Oxygen

The basic steps of any cryogenic air distillation process are shown in Figure 1.

N2-rich product

N2-rich product

Figure 1 Basic steps in a cryogenic air distillation plant.

Air is first compressed in a multistage compressor and cooled to near ambient temperature. Given the boiling point temperatures of nitrogen and oxygen in Table 1, it is clear that air has to be cooled to extremely low temperatures before it can be distilled. It follows that a number of impurities present in air and which would freeze at such cryogenic temperatures must be removed to avoid plugging of heat exchange and separation equipment. Typical impurities that are not listed in Table 1 but are present in air include: water (after compression air is saturated); carbon dioxide (about 375p.p.m.); hydrocarbons such as acetylene (0.1-1 p.p.m.), methane (2-10p.p.m.) and some higher hydrocarbons in varying concentrations (ethylene, propylene, ethane, etc.); carbon monoxide; nitrogen oxides and sulfur compounds. Therefore, in the second step, compressed air is sent through a purification system at least to remove impurities such as water, carbon dioxide, acetylene, nitrogen oxides and sulfur compounds.

In the third step, the compressed and cleaned air is cooled to near its dewpoint by heat exchange. Finally, the cooled air is sent to an appropriate distillation column system. Here air is distilled into at least two product steams - one stream is enriched in oxygen and the other is enriched in nitrogen. Both of these streams are then warmed to near ambient temperature by countercurrent heat exchange with the incoming air. When a product stream is required at a higher pressure, it is further compressed. Liquid products such as liquid oxygen, liquid nitrogen or liquid argon can also be produced from the distillation column system and sent to liquid storage for later distribution.

The heat exchanger and the distillation system are enclosed in a well-insulated enclosure called the cold box. Despite the insulation, there is heat leakage and therefore, refrigeration is provided to the cold box to keep the inside equipment cold. For this purpose, modern cryogenic plants employ turbo-expanders that are also enclosed in the cold box. These turbo-expanders produce work out of the cold box and keep all the equipment at the desired cryogenic temperatures.

Now that the basic steps that are common to all the air distillation plants have been described, attention will be paid to the distillation of air after it is cooled to near its dewpoint temperature. The distillation of air is at the heart of an air separation plant and its arrangement varies with the number, quantity and purity of products being produced.

The early developments in air distillation to produce oxygen were propelled by the invention of the oxyacetylene blow torch for welding and steel cutting. In 1902, Carl von Linde introduced the first air distillation process using a single distillation column.

A simplified sketch of this single column arrangement is shown in Figure 2.

Compressed, cleaned and cooled air that is near its dewpoint temperature is first condensed in a reboiler-condenser located in the bottom (sump) of the distillation column. The condensed liquid air is then reduced in pressure across a valve and fed to the top of the distillation column. The stream provides both the feed and the liquid reflux. From Table 1 it can be seen that, of the three major components in air, nitrogen is the most volatile, oxygen the least volatile and argon is of intermediate volatility. As a result, the liquid descending the distillation column becomes enriched in oxygen. The vapour needed for distillation is provided by boiling the oxygen-enriched liquid in the sump by heat exchange against the condensing air in the reboiler-condenser. A portion of the vapour rising from the sump is collected as gaseous oxygen product while the rest is allowed to travel up the distillation column. As vapour ascends the distillation column, it becomes enriched in nitrogen and finally leaves from the top of the column as a nitrogen-enriched waste stream. If needed, a liquid oxygen product stream is collected from the sump of the distillation column. Even when liquid oxygen is not a desired product, a very small quantity of liquid oxygen is continuously withdrawn from the bottom of the distillation column to avoid accumulation of hydrocarbons in the sump. Both the gaseous oxygen product stream and nitrogen-rich vapour stream are then warmed to near

Liquid 02

Figure 2 A single column to produce oxygen.

ambient temperature by heat exchange against the incoming air stream (Figure 1). While the early plants produced oxygen at a purity of 80-90%, the single-column process can provide oxygen at any desired higher purity. Generally, the purity of oxygen used in metal welding and cutting is 99.5% or greater.

The problem with the single distillation column process shown in Figure 2 is that the recovery of oxygen is low. The reason is that the minimum concentration of oxygen in the nitrogen-rich vapour stream leaving the top of the distillation column is limited to that value which is in equilibrium with the liquid air that is fed at the top. Since the concentration of oxygen in air is fairly high, a sizeable fraction of the oxygen in the feed air leaves in the nitrogen-rich vapour stream. To illustrate this for a distillation column operating at 1.4 atm and producing 99.5% oxygen, the pressure of feed air is about 5 atm and a vapour stream in thermodynamic equilibrium with the liquid air stream (at 1.4 atm) will be 6.9% oxygen. A typical oxygen recovery from such a distillation column would only be in the neighbourhood of 14 mol of oxygen per 100 mol of feed air.

It is clear that, for higher recoveries of oxygen, the concentration of oxygen in the nitrogen-rich vapour stream leaving the top of the distillation column must be low. In other words, both product streams should be relatively pure. This requires that the liquid reflux to the top of the distillation column should also be relatively pure. It seems that Georges Claude was the first to provide the solution by using his dephlegma-tion equipment in 1903. However, it was Carl von Linde's double distillation column of 1910 that revolutionized the industry and is still the workhorse of the modern cryogenic oxygen plants.

A typical double distillation column configuration is shown in Figure 3.

In this arrangement, compressed, cleaned and cooled air is now sent to a high pressure distillation column that operates at about 6 atm. As the vapour rises up this high pressure column, it is enriched in nitrogen and at the top of the column, the concentration of oxygen has been reduced to an extremely low level. The nitrogen vapour is condensed by heat exchange in a reboiler-condenser. Of this condensed nitrogen stream, about 60% is returned back to the top of the high pressure column as liquid reflux; approximately 40% of the flow is sent to the top of a low pressure column that operates at around 1.4 atm. The liquid descending the high pressure column becomes enriched in oxygen to produce crude liquid oxygen leaving the bottom (typically around 35% oxygen). This crude liquid oxygen is reduced in pressure across a valve and fed to an intermediate location in the low pressure column. In the low pres-

sure column, crude liquid oxygen is distilled to produce a nitrogen-rich vapour stream at the top and an oxygen product stream at the bottom. The boil-up at the bottom of the low pressure column is provided by vaporizing the liquid oxygen stream in the sump by heat exchange against the condensing nitrogen vapour stream from the top of the high pressure column. A portion of the vapour from the reboiler-condenser is recovered as gaseous oxygen product while the rest rises to perform distillation in the low pressure column. When needed, some liquid oxygen can also be recovered from the sump as product.

In a double-column arrangement, the main purpose of the high pressure column is to distil and provide two saturated liquid streams from the feed air - a liquid nitrogen reflux and a crude liquid oxygen feed. It is in the low pressure column that the crude liquid oxygen is distilled to provide the needed oxygen product stream. The liquid nitrogen stream provides the much needed liquid reflux at the top of the low pressure column. By using sufficient stages of separation in the high pressure column, the concentration of oxygen in the liquid nitrogen stream can often be reduced to p.p.m. level. Therefore, the concentration of oxygen in the nitrogen-rich vapour stream from the top of the low pressure column is reduced to extremely low levels. This not only allows the potential to recover the nitrogen-rich vapour stream as a useful product stream, but also makes very high recoveries of oxygen possible. For a double-column process, production of 99.5% oxygen in excess of 20.5 mol per 100 mol of feed air (maximum oxygen content being 20.95 mol) is quite common.

For most uses, gaseous oxygen is needed at pressures greater than atmospheric pressure. This pressure can range from about 2 atm absolute pressure for glass-making to pressures in the range of 30-80 atm for the gasification of hydrocarbons such as coal and petroleum residuum. One obvious method to deliver pressurized oxygen is to compress gaseous oxygen to the desired pressure after it exits the cold box. However, safety considerations tend to make the equipment associated with oxygen compressors expensive. In certain applications, where both oxygen and nitrogen are needed at higher pressures, one has the option of increasing the pressure of the distillation columns and directly produce both products at elevated pressures. Unfortunately, the low pressure column is seldom operated at pressures greater than 8 atm absolute. This is because the pressure of the high pressure column is typically greater than two to three times the pressure of the low pressure column and distillation in the high pressure col umn must be conducted at a pressure that is sufficiently lower than the critical pressures of nitrogen and oxygen. A third method that is becoming more popular is the use of a pumped liquid oxygen process. This method is also sometimes referred as internal oxygen compression. A schematic of such a plant is shown in Figure 4.

In the pumped liquid oxygen process of Figure 4, air is compressed in a multistage air compressor (MAC) to about 6 atm absolute pressure, cooled to near ambient temperature and then sent to a molecular sieve purifier. About 70% of the cleaned air is directly fed to the cold box for cooling in the main heat exchanger. From an intermediate location of the main heat exchanger, corresponding to 100°C to — 130°C, approximately 10-20% of this flow is withdrawn and expanded in a turbo-expander to a pressure slightly above atmospheric pressure and fed to an intermediate location of the low pressure column. The work extracted from the turbo-expander provides the needed refrigeration for the cold box. The air that remains after the withdrawal of expander feed is cooled to near the dewpoint temperature and is fed to the bottom of the high pressure column. The arrangement of the double distillation column process is the same as discussed in Figure 3, with only two differences. The first is that the liquid nitrogen stream from the top of the high pressure column is cooled in a subcooler heat exchanger against the nitrogen-rich vapour streams.

Figure 4 Pumped liquid oxygen flowsheet.

This increases the fraction of liquid in this stream as its pressure is reduced to that of the low pressure column. This technique, which is commonly used in any oxygen plant to increase liquid nitrogen reflux to the low pressure column, has the beneficial effect of increasing the purity and recovery of products.

The second major difference is the withdrawal of oxygen product from the low pressure column as liquid and its subsequent vaporization. The liquid oxygen is pumped in a liquid oxygen pump to the desired oxygen product pressure. This pumped liquid oxygen is then vaporized in the main heat exchanger. In order to maintain refrigeration balance, it is essential that another stream be condensed through heat exchange as the pumped liquid oxygen is being vaporized. For this purpose, about 30% of the cleaned air is further boosted to a higher pressure in a booster compressor. The pressure of the boosted air is chosen such that it would easily condense through heat exchange with the vaporizing oxygen stream. Generally the pressure of the condensing air stream is much greater than the oxygen stream. The condensed liquid air from the main heat exchanger is appropriately fed to either one or both of the distillation columns. The warmed gaseous oxygen stream provides the desired pressurized oxygen product.

While the early oxygen plants produced only a few tons of oxygen per day, modern plants are capable of producing in excess of 3000 tons per day of oxygen in a single train.

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