Cryogenic Air Separation Units (ASUs) play a vital role in producing large quantities of high purity oxygen, nitrogen and argon gas and/or liquid phase products.
ASU distillation columns includes high pressure (HP), low pressure (LP), crude argon (CAR) and pure argon, operating at a temperature between 80 and 100K. To minimize refrigeration losses, all cold parts are arranged inside a "cold box" built from a steel framework and surfaces filled with insulating materials.
The height of the cold box is associated with heat leak into the cryogenic system — the higher the size, the higher the heat leakage. Liquid transfer from the HP to the LP column preferably uses the pressure difference between these columns. It may require liquid pumping or vapor lift if the columns are too tall.
Today, these columns are generally equipped with structured packings. To benefit from savings in operation costs, fast response to changes in operating conditions, and a broad operating range, using structured packings is inevitable. In switching from tray to structured packings, shorter internals are desired, and therefore customized.
After a certain bed height, the well-distributed liquid tends to segregate, resulting in liquid maldistribution and differences in liquid concentration across the column. Therefore, a liquid redistribution system used within a long-packed bed must serve two primary functions: equalizing liquid flow and concentration.
The long beds exist in the CAR, HP and lower section of the LP columns. Up to 10 sets of such a system are required for the cold box columns. A novel customized redistribution system for ASU developed by Sulzer integrates the support, collector and distribution into one without sacrificing liquid distribution or mixing. The system is shorter by half a meter in height compared with the standard system.
The LP column typically has two side vapor draws, for example, waste nitrogen and crude argon draw, resulting in significant variations of vapor traffic in the column. The vapor nozzle is larger than the liquid nozzle due to the small gas density, and height must be reserved in conventional distillation units to accommodate the vapor draw nozzle. Additional height is required below and above the draw nozzle for proper vapor distribution. Thus, the resulting total height for the vapor draw is significant. Sulzer optimizes the elevation of vapor draw nozzles by taking advantage of changes in vapor traffic immediately above the nozzle so the required height for vapor draw can be reduced.
Height optimization, advanced liquid redistribution systems and customized internals offer significant benefits for energy savings, improved operational response and cost reduction. As ASU technology advances, collaboration between ASU partners and Sulzer will drive further innovation in column design for enhanced performance and sustainability.
For more information, visit sulzer.com.
