Nitin Kaistha

Process intensification in duplex pressure swing adsorption

Abstract As an alternative to the Pressure Swing Adsorption (PSA) based on the Skarstrom cycle or its variants, a novel two-bed PSA – called duplex PSA – has been proposed by Hirose and independently by Leavitt to get both products of high purities. A modified duplex PSA has been presented to achieve process intensification, that is, to enhance the product purities and productivities. Simulation studies were carried out to explore the attainable product purities and possible process intensification for CO2 capture with the original and modified duplex PSA. The volume reduction of beds that can be realized with modified duplex PSA is about 100–50 times the original duplex PSA depending upon the product purities.

A Statistical Process Control Framework for the Characterization of Variation in Batch Profiles

An statistical process control framework for the characterization of the systematic variability in a historical database of batch profiles is proposed. The framework is geared toward facilitating an understanding of the sources of variability affecting the process. The overall variability in the profiles is categorized into two parts, systematic and unsystematic. The former is further divided, as along the time axis and the measurement axis. Scaling methods are applied to the profiles to obtain scale parameters that characterize the systematic time and measurement axis variability. The profile scaling is proposed so that each parameter has a very specific meaning in terms of the type of variability explained. Multivariate SPC charts on the scale parameters and also on the residuals remaining after scaling are developed for process monitoring. Profiles from two simulation examples, a simulated methyl methacralate polymerization reactor and a nylon-6,6 reactor, are used to demonstrate the application of the SPC framework. The examples demonstrate that a systematic study of the correlation structure of the scale parameters can reveal the signature of the primary disturbances affecting the process. Besides providing meaningful scale parameters, the framework also retains the power of projection methods for subtle special cause detection. The demonstration also highlights the importance of using the time variability information for final product quality predictions in batch data mining.

An efficient algorithm for rigorous dynamic simulation of reactive distillation columns

An efficient algorithm is developed for solving the system of differential algebraic equations (DAE) describing the dynamics of a reactive distillation (RD) column using the equilibrium tray model. Unlike existing algorithms, the rate of change of tray specific enthalpy is obtained analytically. This allows direct calculation of the instantaneous vapor rate leaving a tray from the dynamic energy balance. Application to an example methyl acetate RD column shows that the developed algorithm is twice as fast as the algorithm of Jhon and Lee [Jhon, Y. H., & Lee, T. H. (2003). Dynamic simulation for reactive distillation with ETBE synthesis. Separation and Purification Technology, 31, 301–317].

Simple Rules for Economic Plantwide Control

Abstract In this work, we consider the systematic economic plantwide control design procedure proposed by Skogestad (2004) and from this we derive practical rules that can be used to devise close-to-optimal control structures based on engineering insight. We attempt to present these rules in an easy-to-understand fashion and exemplify them on a simple but quite famous reactor-separator-recycle plant case study. We successfully demonstrate that while Skogestad’s procedure requires an optimization of the plant model under various disturbances in order to be fully utilized, using its practical rules, combined with a good engineering insight of the process, can facilitate the design of a close-to-optimal control structure by suggesting what should be controlled and how.

Plantwide Control of a Cumene Manufacture Process

Abstract 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. Two modes of operation are considered: (I) given feed rate and (II) optimized throughput.

Online batch recipe adjustments for product quality control using empirical models: Application to a nylon-6,6 process

The application of batch profile characterization tools to enhance process understanding by uncovering the signature of the primary disturbances on the profiles and its effect on the product quality is illustrated on a nylon-6,6 process. The historical profile data for the fixed recipe operation are systematically studied to understand the primary disturbances affecting the process, and it is shown that good online predictions of the final product quality are possible much before the completion of the batch from the available measurement profiles. A simple online recipe adjustment strategy based on the predicted quality deviation from the target is proposed. Results show that the recipe adjustments significantly reduce the variation in the final product quality. Issues in the use of empirical prediction models from recipe-based data are discussed.

Continuous diisobutylene manufacturing: Conceptual process design and plantwide control

The complete process design cycle encompassing flowsheet synthesis, design and controllability evaluation is studied for continuous DIB manufacturing. The residue curve map tool is applied to synthesize two new flowsheets, FS1 and FS2, exploiting pressure swing distillation. A unique feature of these is the use of a decanter for recovery and recycle of water, which allows maintaining the reactor tert-butyl alcohol content at the desired level for suppressing the side reaction. Unlike the literature flowsheet (FS0), this innovation makes it possible to achieve both high conversion and high yield for an economically superior design. Between FS1 and FS2, FS1 is found to be superior with significantly lower capital and energy costs as well as lower fresh water consumption. A “smart” control strategy for holding the single-pass reactor conversion and the overall process yield towards economic process operation is also developed. Rigorous dynamic simulations demonstrate the controllability of FS1 and FS2.

Plantwide Control Design of the Monoisopropylamine Process

Abstract Plantwide regulatory control system design for a monoisopropylamine (MIPA) process is presented from the perspective of plantwide transient variability propagation. The process consists of a hot reaction section followed by a cold separation section with three columns and a decanter with two liquid recycles. Two control structures with the throughput manipulator (TPM) at, respectively, the limiting reactant fresh feed (CS1) and the decanter feed (CS2) are evaluated. The decanter level control scheme dramatically affects the overall plantwide response speed. The response is extremely sluggish when total decanter hold-up is regulated using the organic outflow (Scheme 1) and is significantly speeded-up (> 3 times) when the total (organic + aqueous) outflow (Scheme 2) is manipulated instead. The reason is traced to Scheme 2 effectively propagating water imbalance transients out of the plant to the water by-product stream and not to the organic recycle stream, which disturbs the side-product recycle-to-extinction balance. The case-study affirms the heuristic of structuring the plantwide control system to propagate transients out of a recycle loop for improved dynamic performance.

Plantwide Control for Economic Operation of a Recycle Process

Abstract Plant-wide control system design for economically optimum operation of a recycle process with side reaction is studied. The process consists of a liquid phase CSTR followed by two simple distillation columns. The exothermic irreversible reactions A + B → C (main reaction) and C + B → D (side reaction) occur in the CSTR. The reactor effluent is distilled in the recycle column to recycle the light reactants (A and B) back to the CSTR. The column bottoms is further distilled in the product column to produce nearly pure C as the overhead product with side-product D leaving from the bottoms. For a base-case design, the steady-state operating degrees of freedom are optimized to maximize operating profit for two modes of operation - Mode I: Given fresh A feed rate and Mode II: Maximum through-put. The set of active constraints at the economic optimum significantly simplifies the plant-wide control design problem by forcing structural decisions for process operation close to and where possible, at the active constraints. The economic performance of the control structure so synthesized is compared with other reasonable regulatory structures with and without a supervisory optimizing constraint controller. Quantitative process operation back-off results show that the incorporation of economic considerations in plantwide control system design can significantly improve profitability.

Temperature Inferential Dynamic Matrix Control of Reactive Distillation Systems

Abstract Abstract Two-temperature inferential control of the ideal and the methyl acetate double feed reactive distillation (RD) systems operated neat is evaluated using constrained dynamic matrix control (CDMC) and traditional decentralized control. For the ideal RD system, significant improvement in the stripping tray temperature control and the transient deviation in the bottoms purity is observed using CDMC. For the methyl acetate system, CDMC results in significant improvement in the control of the two tray temperatures as well as transient deviations in both the distillate and bottoms purity. Results also show that the magnitude of the maximum through-put change for which the control system fails is noticeably higher using CDMC.