Argon extraction in the double-tower process

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How to allocate the amount of air flowing into the lower column and through the expander in a double-column process is a very important issue. However, textbooks on cryogenic air separation do not discuss this extremely crucial topic in sufficient detail, and I would like to offer some insights on this matter. The most important technical indicators for cryogenic air separation units are the purity of oxygen and the oxygen recovery rate! As for nitrogen purity, it has never been an important issue (only when high-purity nitrogen is to be produced does nitrogen purity and the nitrogen extraction rate become issues); moreover, the concept of nitrogen extraction rate does not exist. When the double-tower process was first introduced, the only way to achieve cryogenic air separation cooling was through isothermal enthalpy difference. At that time, the pressure at the outlet of the air compressor was very high; all of the pressurized air was throttled to reduce its pressure before entering the lower tower. The heat load required by the main condenser per unit of oxygen produced was at its maximum, while the liquid-gas ratio of the reflux stream in the stripping section of the upper tower, as well as the amount of liquid nitrogen, oxygen-enriched liquid, and air required per unit of oxygen produced in the distillation section of the upper tower, were also at their maximum values. At this point, the purity of oxygen is around 95%, and the oxygen extraction rate cannot reach 100%! The reason why the oxygen purity can only reach 95% is mainly due to the insufficient number of trays in the stripping section of the upper column and the difficulty in separating oxygen from argon through distillation. Even when the amount of liquid nitrogen is at its maximum, the oxygen extraction rate still cannot approach 100%, and this is due to an insufficient number of theoretical trays in the distillation section, as well as the fact that the actual reflux ratio in that distillation section below the outlet for contaminated nitrogen gas is lower than the minimum required reflux ratio! By adding so-called efficiency towers, the purity of the oxygen product and the oxygen extraction rate can be improved under the same conditions. The so-called efficiency tower is actually a simplified version of the crude argon condenser tower! Later, expansion refrigeration was introduced; as a result, the amount of air entering the lower column decreased. The liquid-to-gas ratio of the reflux stream in the distillation section of the upper column dropped, and the amounts of liquid nitrogen and oxygen-enriched liquid air also decreased. Due to the constraints imposed by the distillation conditions in the upper column, the amount of air used for expansion refrigeration could be no more than 30% of the total air volume, and sometimes it couldn’t even all enter the upper column to participate in the distillation process. Later, structured packing was introduced, which brought about significant improvements in the distillation process in the upper column. Firstly, the expanded refrigerated air could now enter the upper column to participate in the distillation process, thereby increasing the oxygen extraction rate. This energy-saving effect is the most noticeable and direct! At the same time, the increase in the number of trays in the stripping section of the tower leads to a continuous improvement in oxygen purity, which reaches over 99.5%! In fact, at this point, a significant change occurred in the air separation tower without anyone noticing; it changed from a nitrogen-oxygen-argon distillation and separation tower to a nitrogen-argon-oxygen distillation and separation tower. Whether the air separation tower is used for separating nitrogen, argon, and oxygen or nitrogen, oxygen, and argon depends on the gas-liquid ratio in the reflow stream of the stripping section, as well as on the number of tray levels and the purity requirements for the oxygen product. The emergence of structured packing also made full distillation for argon recovery possible. After the transition phase using efficiency-enhancing columns, the introduction of an argon recovery process brought about further significant changes in the upper column of the two-column system, establishing a trend toward transforming the nitrogen-argon-oxygen separation column into a nitrogen-argon-oxygen separation column. Essentially, the upper column has become a combination of two distillation columns optimized using a baffle model for the distillation separation of an oxygen-nitrogen-argon ternary mixture; above the argon fraction outlet in this upper column operates the nitrogen-argon-oxygen distillation column, which functions similarly to an oxygen-nitrogen distillation column, while below the crude argon condenser and argon fraction outlet lies the nitrogen-argon-oxygen distillation column. The argon extraction process in the double-column system generally works as follows: An argon-rich stream containing 10% argon is drawn from the stripping section of the air separation column (at a point 25 theoretical plates above the bottom of the upper column). This stream undergoes condensation, rectification, and separation in the crude argon condenser column. Crude argon gas (process argon) is obtained from the top of this column. The crude argon condenser installed at the top of the crude argon column uses oxygen-enriched liquid air from the lower column as a cooling medium. The resulting oxygen-enriched air, after vaporization, is returned to the upper column at a point below the inlet for oxygen-enriched liquid air but above the outlet for the argon-rich stream, so that it can participate in the rectification process. The liquid at the bottom of the crude argon condenser column also returns to the upper column. In fact, there can be another process scheme for argon recovery in the double-column process, described as follows: An argon-rich fraction containing 10% argon is drawn from the stripping section of the upper column (above the bottom portion, covering 25 theoretical plates). This fraction then undergoes nitrogen-argon-oxygen separation via rectification in a crude argon rectification column. The crude argon rectification column operates under a conventional rectification process. The pressurized air coming from the main process enters a heater located at the bottom of the crude argon rectification column (this heater is a pressurized air condenser; in standard rectification terminology it is referred to as a reboiler, while in air separation terminology it is called a condenser). This heater heats the liquid oxygen at the bottom of the column, causing it to vaporize. Part of this vaporized oxygen serves as rising vapor for the stripping process, while the remaining portion is extracted as product oxygen. The pressurized air is liquefied into liquid air, which is then sent to the crude argon condenser at the top of the crude argon condensation tower; there it evaporates, thereby liquefying the process argon at the top of the crude argon distillation tower and forming a reflux stream. The vaporized air merges with the expanded refrigeration air and returns to the upper tower at a position 3–4 theoretical tray levels below the oxygen-enriched liquid air inlet, where it participates in the distillation process. The oxygen product with a purity of 99.5%, drawn from the bottom of the upper column and the bottom of the crude argon rectification column, is combined, then reheated in the main heat exchanger before being withdrawn from the air separation unit. Of course, under such conditions, the amount of air entering the lower tower only needs to be 60%-70% of the total air volume. Compared to the existing double-tower process, which is a crude argon condensation-based argon recovery process, what are the differences between this double-tower process and the crude argon distillation tower process? First, it is the argon extraction process in the crude argon distillation column; the existing argon extraction process for the twin-column system is the crude argon condensation column process. Secondly, its reboiler uses pressurized air as the heat source, while its condenser uses liquid oxygen as the cooling source; in contrast, the existing crude argon condensation tower process does not have a reboiler (evaporator), and its condenser uses oxygen-enriched liquid oxygen as the cooling source. Third, in the existing double-tower process, the liquid at the bottom of the crude argon condenser tower is returned to the upper tower, whereas there is no liquid returning to the upper tower from the argon fraction outlet of the crude argon distillation tower. Fourth, the amount of argon fraction drawn from the crude argon distillation column is only about 40% of that from the crude argon condensation column. 5. In the crude argon condenser of the crude argon condensation tower, the vaporized air returns to the upper column above the argon fraction outlet and below the inlet for the oxygen-enriched liquid air in that column; the vaporized air from the condenser of the crude argon distillation tower, along with the air coming from the expander, merges together and then enters the upper column. In the current argon extraction process for the crude argon condenser in the double-tower process, oxygen-enriched liquid air is used as the cooling source for the crude argon condenser. The vaporized oxygen-enriched air returns to the upper tower above the argon fraction outlet and below the inlet of the oxygen-enriched liquid air. Six, in the process of extracting argon using a crude argon distillation column, the oxygen product is drawn out from the bottom of the air separation tower and from the bottom of the crude argon distillation column; these two streams are then combined before being taken out of the air separation plant, with a ratio of approximately six to four. It is important to note that in addition to the existing process scheme of using a crude argon condenser for argon extraction in the two-tower process, there is also a process scheme that involves a crude argon distillation tower. The significance of this lies in the fact that by comparing these two approaches, it becomes possible to gain a deeper understanding of the advantages as well as the shortcomings of the current process scheme for argon extraction in the two-tower process. It is now more clearly understood that, when argon is removed, the upper column of the so-called double-column process is actually composed of two distillation columns! In fact, by introducing an argon component, the air separation unit is no longer involved in the distillation separation of an oxygen-nitrogen binary system, but rather in the distillation separation of an oxygen-nitrogen-argon ternary system. For the distillation separation of the oxygen-nitrogen-argon ternary system, there are no dual-column processes or new single-column processes; rather, only distillation process schemes for this ternary system that are based on dual-column processes or new single-column processes exist. If the air separation unit is aimed at producing nitrogen or gas with a purity of 95% (4.5% argon and 0.5% nitrogen), then distillation separation based on an oxygen-nitrogen binary system is still applicable (with argon treated as oxygen). However, when the goal is to produce argon with a purity of 99.5% (0.5% argon and trace amounts of nitrogen), it is no longer a case of distillation separation using an oxygen-nitrogen binary system, nor even an approximation of such a system; rather, it involves complete distillation separation based on an oxygen-nitrogen-aragon ternary system. There is a significant difference between the two!