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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

  • 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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