Distillation columns are generally classified into two major categories: sieve-tray columns and packed columns. Packed columns are further divided into random (dumped) packing and structured packing. Although sieve-tray columns are relatively simple in structure, highly adaptable, and easy to scale up, and have therefore been widely used in air separation units, with the development of gas–liquid heat and mass transfer technology and the research on high-efficiency structured packing, the development of some high-efficiency structured packings with low pressure drop and low liquid holdup has shown a trend of gradually replacing sieve-tray columns over the past decade or so.
Structured packing consists of metal corrugated sheets approximately 0.22 mm thick. Arranged piece by piece, these corrugated sheets allow cryogenic liquid to form a liquid film on each packing surface, contacting the rising vapor for heat and mass transfer. The specific metal surface area of structured packing is about 30 times that of sieve trays, and the liquid oxygen holdup is only 35%–40% of that in sieve-tray columns. Moreover, because the cross-sectional area of a structured packing column is about one-third smaller than that of a sieve-tray column, and the packing is arranged vertically, there is no horizontal concentration gradient. As long as the liquid distribution is uniform, the rectification efficiency is high and the pressure drop is small. The pressure difference for gas passing through the liquid film on the packing is much smaller than that for passing through the liquid layer on sieve trays-approximately only 50 Pa. The reduction in pressure at the bottom of the upper column inevitably leads to a reduction in lower column pressure, which in turn lowers the outlet pressure of the main air compressor, thereby reducing the energy consumption of the entire air separation unit. At the same time, because structured packing has low liquid holdup, its adaptability to load changes is relatively strong.
In summary, compared with sieve-tray columns, structured packing columns have the following advantages:
Very low pressure drop. Gas-phase heat and mass transfer occur by convection at the liquid film surface within the packing; there is no resistance from a clear liquid layer on trays or from sieve holes. Under normal conditions, the resistance of structured packing is only 1/5 to 1/6 of that of the corresponding sieve-tray column.
Sufficient heat and mass exchange, high separation efficiency, leading to improved product extraction rates.
Large operating flexibility, with no weeping or flooding, so the load adjustment range is wide and adaptability is strong. The load adjustment range can be 30%–110%, whereas that of sieve-tray columns is 70%–100%.
Low liquid holdup, enabling fast startup and load adjustment.
Energy saving. Because of the low resistance, the air inlet pressure to the column can be reduced by about 0.07 MPa, thereby reducing air compression energy consumption by about 6.5%.
Smaller column diameter.
In addition, after applying structured packing, because the pressure drop per equivalent theoretical stage is reduced, full rectification argon production becomes possible, and the argon extraction rate can be increased by 10%–15%.
Structured packing distillation columns are generally divided into 3 to 5 packing sections. Between each section there are liquid collectors and redistributors. The traditional tray spacing in sieve-tray columns is 110–160 mm, whereas the height equivalent to a theoretical plate (HETP) for structured packing is 250–300 mm; therefore, the height of a packed column will increase.
Aluminum is generally chosen as the material for structured packing, which can reduce weight and cost, but the residual lubricating oil on the packing metal surface must be controlled to less than 50 mg/m². Under this condition, aluminum packed columns and aluminum sieve-tray columns can be considered equally safe for oxygen rectification.
Of course, the cost of structured packing is higher than that of sieve-tray columns, and the column body is also taller. However, its advantages are outstanding; therefore, since the 1990s, many air separation equipment manufacturers have first replaced sieve-tray columns with structured packing columns in the upper column and argon column, and there is a further trend to adopt them in the lower column as well.
