After a long-term operation of the air separation plant, the components in the low-temperature equipment and pipelines of the distillation system will be blocked due to the internal precipitation of icing or mechanical powder, which will increase the resistance of the fractionating column. Therefore, after the normal operation of the air separation plant for about 2 years, the fractionating column system should be heated and thawed to remove these sediments. When the molecular sieve adsorber and the main heat exchanger have water or other faults, they must also be stopped for heating. The following describes several heating and thawing operation methods:
1.Heating and thawing operation whennormal shutdown:
After normal shutdown of the air separation plant, large-scale liquid discharge of the equipment shall be carried out. After liquid discharge, the equipment shall be kept for 24 hours, and then the air compressor, pre-cooling system and purification system shall be started. The dry air shall be led into the lower column through the main heat exchanger, and all liquid discharge valves of the lower column shall be opened, so that the impurities accumulated in the main heat exchanger and the lower column can be heated and then discharged out of the cold box. It should open the liquid-air joint flow into the upper column valve V1 and the liquid nitrogen joint flow into the upper column valve V3, guide the heating gas into the upper column, and open the upper column blow off valve for heating according to the flow path. When the inlet and outlet temperature of the heated gas are basically the same, the heating is completed.
2.Heating and unfreezing operation whenfault shutdown:
a.When the air separationplant is shut down due to the water inflow into the molecular sieve adsorber: first, judge the amount of water inflow. When a small amount of water enters the molecular sieve adsorber, which results in the reduction of carbon dioxide adsorption layer and the increase of carbon dioxide content entering the main heat exchanger and distillation column, the molecular sieve can be activated to restore the adsorption ability. If the amount of water entering the molecular sieve is too large, resulting in molecular sieve pulverization, the molecular sieve must be replaced. When heating the distillation column, it is necessary to adhere to the principle of usinglower column firstly and then upper column. After the lower column is completely heated, the heating gas shall be led into the upper column and the main cooling through valves V1, V3 and valve of main cooling liquid nitrogen back to the lower column.
b.In case of water inlet failure of the main heat exchanger, the main heat exchanger can be heated in two ways: forward heating or reverse heating. Forward heating refers to that the heating gas enters the main heat exchanger from the molecular sieve adsorber through the normal channel and is discharged from the blow off valve of the lower column. Reverse heating refers to that the heating gas from the outside enters the lower column from each blow off valve and is discharged from the molecular sieve adsorber after the main heat exchanger is blown back. Normal comprehensive heating cannot be used for heating operation. It must adoptthe method thatusing lower column firstly and then upper column. Valves V1, V3 and valve of main cooling liquid nitrogen back to the lower column shall be closed to blow out the water from the main heat exchanger through the lower column, and then the gas from the lower column shall enter the upper column for heating. The booster pipeline in the expander system shall also be thoroughly heated and blown out.
Whether the heating and thawing operation of the air separation plant is complete or not directly affects the operation stability and energy consumption of the air separation plant after heating. In different cases, the heating operation methods of the air separation plant are different, but they should be based on the principle of not polluting other equipment, and ensure the stable, slow and even recovery of each part during heating, so as to avoid the formation of stress due to excessive temperature difference, which would damage to the equipment and conduit.
