APC - LPG
A tailored application for LPG Units:
The LPG advanced control package combines Axens' latest generation multivariable controller with inferentials. It is designed to maximize the benefits from the LPG process at the lowest operating cost.
A four-step control strategy is implemented
- Feed drum (if any).
- Stabilizer or other columns can be included.
- Top and bottom product qualities on specification
- Reduction of reboiler duty
- Optimal fluid flows 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.
- 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 flows are taken into account.
- Feed analysis not required.
- When the manipulated variables reach their limits, the two qualities are altered according toa pre-specified ratio.
This generic approach is modified to cope with the specific design of the low level controllers (top and bottom levels, pressure).
De-ethanizer control is usually only a bottom quality control with minimum propane loss. Stabilizer bottom quality 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 limits maintained on the feed drum level. Level set-point (within the specified range) slowly reached to better compensate for future disturbances.