Abstract: The scaling problem of polymer in the de-butane column is one of the main factors that restrict the long-term operation of ethylene plants. Recently, the newly-built unit has achieved significant results in preventing polymer scaling in the de-butane column by reducing the operating pressure. Taking the de-butane column as an example, the feasibility, economy and risk points of the step-down process were analyzed and summarized, the process parameters after step-down operation were simulated and optimized by using the chemical process simulation software Aspen Plus, and the optimization results were applied to the actual production process. The results showed that the depressurisation limit of the de-butane column was 350 kPa, the temperature of the whole column decreased about 4 ℃ after depressurisation, the C₄ mole fraction of the crude cracked gasoline in the column kettle decreased by 0.32 percentage point, the low pressure steam was saved by 1.3 t/h, and the energy consumption and the loss of the C₄ product of the plant were reduced. The pressure drop of the circulating water side, the flow rate of the circulating water in the heat exchanger and the temperature difference of the logarithmic heat transfer were all within a reasonable range.

Key words: de-butane tower, polymerization scale, process simulation, step-down operation, ethylene