(202c) External Reset Feedback for Constrained Economic Process Operation

External
Reset Feedback for Constrained Economic Process Operation

Rahul
Jagtap¹, Nitin Kaistha¹ and William L Luyben²

¹
Department of Chemical Engineering, Indian Institute of Technology Kanpur,
Kanpur 208016, India

²
Department of Chemical Engineering, Lehigh University, Bethlehem PA 18015, USA

ABSTRACT:

Today's
fierce competition for maximizing profit and strict environmental norms dictate
plant operation at the optimum steady state, where typically multiple process constraints
are active. This set of constraints changes depending upon the production
objectives, which include the production rate (throughput) and the product
grade. Override control schemes, where two or more controllers compete through
a low/high select for the manipulation of a control input, are conventionally
employed to manage the switching between the constraints.

Generally
the competing controllers in an override scheme have integral action which
results in reset windup in the unselected controllers causing large swings
(bumps) in controller output as control is transferred from one controller to
another. To avoid the large swings, the use of ?external reset feedback? was proposed
by Shinskey/Buckley^1,2 for smooth (bumpless) switching between
active constraints over 45 years ago. Even as the concept has existed for over
four decades, there are surprisingly very few quantitative studies in the open
literature on the dynamic control and consequent economic benefits of external
reset feedback as applied to typical constraint scenarios in continuous
chemical processes. Commercial dynamic process simulators such as Aspen / Hysys
also do not provide external reset feedback controllers.

The
objective of this work is to quantitatively demonstrate the significant
improvement in dynamic control and consequent process economic performance
(compared to internal reset)with the help of three process case studies of
increasing complexity. In the process we also illustrate how to implement external
reset feedback in Aspen dynamics, a popular commercial simulator.

The
first case study investigates a compressor gas-header (Figure 1) supplying
pressurized gas to multiple downstream users. At low downstream demand, the
compressor work is manipulated for maintaining the gas-header at maximum
pressure. As downstream demand exceeds the maximum permissible flow without
damaging the compressor drive, the manipulation of the compressor work is taken
over by a maximum flow controller.  The second example is a ternary
distillation column(Figure 2) separating a benzene-toluene-xylene mixture with
column flooding hard constraining the maximum achievable throughput. In the last
case study, the override control scheme on a complete ethyl benzene process
(Figure 3) is evaluated for maximum throughput operation with flooding in the first
(recycle) column acting as the bottleneck constraint.

Rigorous
dynamic simulations in Aspen plus are used to compare the dynamic control and
economic benefit of external reset feedback over conventional internal reset
feedback in override control structures. Results show
that the overall transient response is significantly smoother, which allows
process operation to be driven closer to hard process constraints. This usually
leads to one or more economic benefits in terms of significantly higher maximum
achievable throughput, improved energy efficiency and tighter product quality
control. Override controllers have a wide area of application since the optimum
operation of most processes involves operating at constraints. External reset
controllers should therefore be of considerable industrial and academic
importance.

References:

1.       Shinskey
FG. Process Control Systems. New York: McGraw-Hill, 1967.

2.       Buckley
PS, Luyben WL, Shunta JP. Design of Distillation Control Systems. Instrument
Society of America, 1985.