(130f) Dynamic Simulation for Distillation Column Start-Ups in an Ethylene Plant

The start-up operation of a distillation column normally presents a challenging task for a chemical plant, because complex heat and mass transfers are coupled in the transition period associated with nonlinear responses/controls such as pressure, temperature, and product compositions.  The reduction of distillation column start-up time has important merits to a plant in terms of raw material and energy savings, susceptible to process disturbances, and flare and downtime reduction.  To effectively reduce start-up time of an distillation column, a common way is to set up the total recycle first before the feed is directed to the column; then ramp up feed input associated with adjustment of controls of reflux rate, heating and cooling duties, and etc.  The shifting from the operating status with total reflux to normal operation status under various disturbances, so as to produce on-spec products, is the most important and also the slowest step during the entire startup operation, which needs extensive studies.  

In this paper, rigorous dynamic simulations are employed to study startups of two distillation columns in an ethylene plant.  The first case is about LP DeC₃ (low-pressure depropanizer) start-up, where the significance (time savings compared with the plant historian start-up operation) of smooth transition from the total reflux status to normal operation status is demonstrated.  In the second case, the start-up optimization of a C3 splitter is investigated, where the operating pressure, heating and cooling capacities are well adjusted under realistic constraints. Both case studies help understand start-up dynamic behaviors of distillation columns and result in improved start-up performances.