APC - LPG

A tailored application for LPG Units:

The LPG advanced control package combines Axens Horizon' 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

De-ethanizer.
De-propanizer.
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 fluid flows in the column, reducing risk of flooding
Room for increased throughput

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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 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.

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