(56b) Dynamic Simulation for Optimal Operation of Distillation Column Startups in an Ethylene Plant

The startup operation of a distillation column is a critical task, which needs sufficient cares. Dynamic simulation can be used as a powerful tool to identify optimal operational strategies to start up a distillation column quickly and safely. A column startup in an ethylene plant usually consists of three operating steps: (1) commissioning; (2) building up the total-reflux status; (3) adopting feed and bringing column to normal operation. These startup steps involve complex heat and mass transfer phenomena in different transient time periods, where dynamic simulations are better to be employed to investigate involved operations. Based on dynamic simulations, column startup procedures and control strategies can be optimized in terms of  raw material and energy savings.

In this study, rigorous dynamic simulations are employed to investigate the startup of C2 splitter in an ethylene plant. All the operating procedures during the column startup are programmed and automated in one rigorous simulation model. In Step 1, alternate operations of multiple pressurization and depressurization with pure ethylene vapor are simulated to purge N₂ out of column. In Step 2, pure ethylene liquid is introduced to the reflux drum. When the liquid level attains a certain level, the reflux pump starts running to input liquid into the column. Until the sump level is satisfied, the reboiler would be started to build up the vapor and liquid equilibrium inside the column under total reflux conditions. Through dynamic simulations, different operating control strategies are investigated to achieve the total reflux condition successfully. In Step 3, feed from upstream is introduced into C2 splitter. The effect from feed change upsets is presented in dynamic simulation. Several scenarios are studied in this part to short the startup transition time with operational feasibility. In summary, dynamic simulations performed in this study provides an in-depth understanding for distillation column startup operations.