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Title: Plantwide Control of a Cumene Manufacture Process (June 2011)

Summary: In This work, we tried to answer a question which has been on the minds of control engineers for ages: which variables need to be controlled?  Prof sigurd’s group has been able to come up with several satisfactory answers including a technique called ‘Self-optimizing Control’.

“Self-optimizing control is when acceptable operation under all conditions is achieved with constant setpoints for the controlled variables.” (taken from sigurd’s website) 

This work describes the application of the plantwide control design procedure of Skogestad (Skogestad, 2004) to the cumene production process. A steady state top down analysis is used to select the set of self-optimizing primary controlled variables which when kept constant lead to acceptable economic loss without the need to reoptimize the process when disturbances occur.

  • Book Publication Computer Aided Chemical Engineering, Volume 29, 2011. Co-authors: Mehdi Panahi, Sigurd Skogestad, Nitin Kaistha

Title: Design and control of a vapor-phase conventional process and reactive distillation process for cumene production (June 2010, March 2011)

Summary: Over the past two decades, reactive distillation (RD), which combines reaction and separation in the same unit, has matured into an alternative for conventional “reaction followed by separation” processes. The technology is particularly attractive when significant reaction rates can be achieved at the tray bubble temperature and the component relative volatilities allow recycle of reactants through stripping/rectification. The process integration then yields a much more compact and significantly less expensive plant. On the flip side, fewer valves are available for regulating both the reaction and the separation along with pronounced nonlinear effects that may compromise control performance. Unlike conventional processes, controllability evaluation is then an integral part of the design cycle for RD technology-based processes where the economic process design itself may require alteration for acceptable controllability.

The purpose of this article was to quantitatively compare the economics and controllability of the RD process with a conventional vapor-phase alkylation process for cumene manufacture. 

The results show that the economic optimum design for the RD process is about 47% cheaper than the corresponding conventional process design. This is primarily due to process integration in the RD column with 61% lower capital cost and 27% lower energy cost. The plantwide control results show that both the processes effectively reject throughput change and feed C3 composition change disturbances. The RD process for cumene production is thus an attractive alternative to the conventional vapor-phase alkylation process. 

Title: Economic Plantwide Control of a Cumene Process (Dec 2012)

Summary: In practice, depending on the prevailing business scenario, a plant must be operated significantly above or below the design throughput for sustained periods, possibly including process operation at maximum throughput. As throughput is increased, equipment sequentially hit (hard) capacity constraints and crucial control tasks such as product quality control or inventory stabilization may be lost. The active constraint set then expands and the Self-optimizing variable (SOV) set contracts due to the loss in degrees of freedom.

Conventionally, such additional constraints are handled by providing appropriate over-ride controllers that, alter the material balance control structure from the (hard) constrained unit operation till the process feed. To avoid such cumbersome over-rides and simplify the overall plantwide control system, through a series of recent case studies, Kaistha and co-workers have propounded developing a robust control system for process operation at maximum throughput, where the highest number of constraints are active, and then adapting it for taking up SOV control at lower throughputs, where additional setpoints become available for manipulation due to constraints becoming inactive. They have proposed a systematic five-step procedure for top-down synthesis of such an economic plantwide control system that gives (near) optimal process operation over a wide throughput range.

In this work, we demonstrate the application of this procedure to the cumene process. The main contribution of the work is in demonstrating that the top-down approach to plantwide control system design results in an unconventional plantwide control system providing acceptable regulatory control (stabilization) with significant economic benefit due to tight economic variable control. Further, the additional complexity of over-ride controllers that alter the material balance structure for constraint handling is avoided.

  • Journal Publication (I&EC Research). Co-author: Nitin Kaistha
  • My M. Tech. Thesis at IIT Kanpur (Attached below).
Vivek Gera,
Jul 9, 2012, 4:12 AM