(329d) Improved Control Strategies for Semicontinuous Distillation

Improved
Control Strategies for Semicontinuous Distillation

Pranav Bhaswanth
Madabhushi, Thomas A. Adams II

Department of Chemical
Engineering McMaster University, 1280 Main St. West, Hamilton, Ontario, Canada,
L8S 4L8.

Semicontinuous distillation
processes have undergone certain design and control configuration changes in
the past decade that significantly improved their controllability and lowered
the total annualized cost. Adams
and Seider, 2008, modified the traditional Distillate-Bottom
(DB) control configuration to include the control of the side stream flow rate
()
using a model-based heuristic called the “ideal side draw recovery arrangement
(ISR)” for reducing cycle time (T). Although this heuristic soon became the
standard in the studies that followed for almost a decade, its effect on the
cycle time and on the system dynamics were not well understood.

In the current study, we compare
the performance and dynamic characteristics of two new heuristics that were
introduced to reduce cycle time. The effect of the heuristics was demonstrated
on a case study involving the separation of benzene, toluene and, o-xylene.
Results show that they performed better in comparison to the status quo, with a
remarkable 25% reduction in total annualized cost per total annual production
of the toluene (TAC) while maintaining product purities. This shows that the
ideal side draw recovery heuristic is inherently sub-optimal even though it is effective.
To find the optimal side stream flow rate function, dynamic optimization was
used with the objective of minimizing TAC without compromising product purities
and cycle periodicity. It was however found that dynamic optimization did not
return better results when compared to the two newly introduced heuristics, which
were used as initial guesses.

References

·        
Adams, T.A., Seider,
W.D., 2008. Semicontinuous distillation for ethyl lactate production. AIChE
Journal 54, 2539–2552. doi:10.1002/aic.11585

·        
Pascall,
A., Adams II, T.A., 2013. Semicontinuous separation of dimethyl ether (DME)
produced from biomass. The Canadian Journal of Chemical Engineering 91,
1001–1021. doi:10.1002/cjce.21813

Figure 1 Semicontinuous
distillation system with the control configuration as suggested by Pascall and
Adams, 2013. CC represents concentration control, LC represents level control
and PC represents pressure control.

Figure 2 Phase plane plot
when ideal side draw recovery heuristic is used to control the side stream
flowrate. PR, RQ and QP representing the Charging, Separating and Discharging
modes.