* Deethanizer. * Depropanizer. * Feed drum (if any). * Stabilizer or other columns can be included.
Typical benefits:
- Top and bottom product qualities on specification - Reduction of reboiler duty - Optimal traffic in the column, reducing risk of flooding - Room for increased throughput
Generic control strategy for the columns:
Objective: Simultaneous top and bottom quality control (percentages of impurities),
Manipulated variables: - Reflux flowrate. - Reboiler duty. - Pressure, when economic incentives are identified.
Optimization opportunities: - Maximum recovery of one product. - Minimum energy consumption.
Inferentials: - Used when online quality analyzers are not available. - Estimation of impurities from internal sensitive temperatures (plus top or bottom temperature when product is a non-binary mixture).
Other features: - The gains of the controller are reset on line when required. - Constraints on internal liquid-vapor traffic are taken into account. - Feed analysis not required. - When the manipulated variables hit their bounds, the two qualities are altered according to a pre-specified ratio.
This generic approach is modified to cope with the specific design of the low level controllers (top and bottom levels, pressure).
Deethanizer control is usually only a bottom quality control with minimum propane loss. Stabilizer bottom quality for can be alternatively described in terms of RVP.
Generic control strategy for the feed drum: Minimization of the output flowrate variations (the output flowrate is the only manipulated variable) with bounds enforced on the level. Level setpoint (somewhere in-between the specified bounds) slowly reached, to create room and better compensate for future disturbances.
