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Figure 23: Process efficiency results employing the new In Situ control logic
CONCLUSIONS

The newly proposed In Situ control logic has been presented in detail. The core
principle is quite simple: the values of both the dissolved alumina concentration and the
ACD can be estimated at the end of a 5 to 10 minute no feed observation period only by
using the standard cell amperage and voltage data. With this information, it is possible to
employ a PID controller to regulate the rate of alumina feeding using only the normalized
cell voltage to calculate the error between the estimated dissolved alumina concentration
and a target value for an extended period of time.

In Situ
control logic has been coded in Fortran and imbedded in the Dyna/Marc
kernel as an additional control subroutine and tested. It has worked exactly as designed.
The dissolved alumina concentration is effectively controlled with an impressive
accuracy only using standard line amperage and cell voltage signals as feedback control
inputs. The drastic reduction of the process variation of the concentration of the
dissolved alumina, promotes the possibility of reducing the averaged value without
increasing the risk of anode effect. This new idea produced in the Dyna/Marc simulator
a positive impact on both current efficiency and the power efficiency.