Plantwide Control Structure Research Articles

Integrated Gasification Combined Cycle Dynamic Model: H2S Absorption/Stripping, Water−Gas Shift Reactors, and CO2 Absorption/Stripping

Gasification could potentially emerge as the premier unit operation in the energy and chemical industries. In the future, plants are predicted to be a hybrid between power and chemical with the ability to handle unavoidable swings in both power demand and biomass feed composition without a loss of efficiency. The coupling of a power plant with a chemical plant provides an additional control degree of freedom, which fundamentally improves the controllability of the process. The coupling of an integrated gasification combined cycle (IGCC) power plant with a methanol chemical plant handles swings in power demand by diverting hydrogen gas from a combustion turbine and syn gas from the gasifier to a methanol plant for the production of an easily stored, hydrogen-consuming liquid product. This paper presents an extension of the dynamic gasifier model, which uses a high-molecular weight hydrocarbon (with a 1:1 hydrogen to carbon ratio) as a pseudo-biomass feed stock. Using this gasifier model, the downstream units of a typical IGCC can be modeled in the widely used process simulator Aspen Dynamics. Dynamic simulations of the H2S absorption/stripping unit, water−gas shift (WGS) reactors, and CO2 absorption/stripping unit are essential for the development of stable and agile plantwide control structures of this hybrid power/chemical plant. Because of the high pressure of the system, hydrogen sulfide is removed by means of physical absorption. SELEXOL (a mixture of the dimethyl ethers of polyethylene glycol) is used to achieve a gas purity of less than 5 ppm H2S. This desulfurized synthesis gas is sent to two water−gas shift reactors that convert a total of 99% of carbon monoxide to hydrogen. Physical absorption of carbon dioxide with Selexol produces a hydrogen-rich stream (90 mol % H2) to be fed into combustion turbines or to a methanol plant. Steady-state economic designs and plantwide control structures are developed in this paper.

Design and Control of the Acetone Process via Dehydrogenation of 2-Propanol

Acetone is produced via several alternative processes, one of which is the dehydrogenation of 2-propanol (IPA). The endothermic gas-phase reaction converts IPA to acetone and hydrogen. The process consists of a vaporizer, heated tubular reactor, flash tank, absorber, and two distillation columns. The liquid fresh feed is a mixture of IPA and water. It is combined with a small IPA/water recycle stream, vaporized, and fed into the vapor-phase reactor, which is heated by high-temperature molten salt. Reactor effluent is cooled and fed to a flash tank. The gas from the tank contains most of the hydrogen but also some acetone. This gas is fed to an absorber in which a water stream is used to recover acetone. The liquids streams from the base of the absorber and the flash tank are fed to the first distillation column, which produces high-purity acetone out the top. There is also a vapor vent stream leaving the reflux drum of this column to remove the small amount of hydrogen dissolved in the feed. The second distillation column produces a high-purity water bottoms and a distillate with a composition near the IPA/water azeotrope, which is recycled back to the vaporizer. There are a number of interacting design optimization variables in this process, which illustrate some interesting design trade-offs. Removing the hydrogen without losing too much product acetone is the main challenge. Losses can occur in both the absorber off-gas and the column vent. Raising absorber pressure decreases off-gas losses but increases vent losses. Raising absorber pressure has several other effects. It raises the vaporizer pressure, which raises the required temperature and cost of the vaporizer heat source. It adversely affects kinetics because the reaction is nonequimolar and conversion decreases with increasing pressure. A higher reactor temperature is required to achieve the desired conversion. The purpose of this paper is to develop the economically optimum design considering capital costs, energy costs, and raw material costs and then to develop a plantwide control structure capable of effectively handling large disturbances in production rate.

A Systematic Approach for the Design of Optimal Monitoring Systems for Large Scale Processes

In this work a new concept for designing an efficient monitoring system for large scale chemical plants is presented. It is considered that the monitoring problem must be solved integrated with the optimal sensor location together with the plant-wide control structure design. The solution of these problems involves deciding among a great number of possible combinations between the input-output variables. It is done supported by the application of genetic algorithm (GA). The key new idea is to propose an adequate objective function, within the GA, that takes into account a fault detectability index based on combined statistics. Additionally, by using a specific penalty function, it is possible to drive the search to the less expensive structure, that is by using the lowest number of sensors. The well-known benchmark case of the Tennessee Eastman plant (TE) is chosen for testing this methodology and for discussion purposes. Since several authors have studied the TE case, the results obtained here can be rigorously compared with those already published. All of the previous works considered that every TE output variables were available for the abnormal events detection for designing the monitoring system.

DYNAMIC PERFORMANCE-BASED DESIGN FOR PLANTWIDE CONTROL STRUCTURE

This article presents a plantwide control structure design using dynamic performance-based optimization. The controlled and manipulated variable sets were obtained heuristically. The main plantwide control structures were established via dynamic optimization and the rest established via heuristic rules. The optimization problem was formulated as dynamic mixed integer nonlinear programming with objective function of measuring the control performance (ITAE of CVs) and the cost of manipulated variables (ITAE of MVs). The integer matrix in the formulation represents the plantwide control structure explicitly. The Tennessee Eastman (TE) process was selected to illustrate the proposed design procedure. The obtained control structures were evaluated via dynamic simulation in the face of various disturbances and set-point changing. The results were then compared with the control structure proposed by Luyben et al. (1999).

Design and Control of Integrated Styrene-Aniline Production Plant

This paper illustrates the operational difficulties arising from simultaneously performing exothermic and endothermic reactions, and demonstrates that a plant can be built and safely operated by integrating the design and plantwide control issues. The behaviour of reactor – separation – recycle systems carrying the coupled reactions A ?P + Q (endo) and B + Q ? R (exo) is investigated. Irrespective of the control structure, state multiplicity cannot be removed if the intermediate component Q is recycled. Therefore, the chemical reactor should be designed such that the recycle of Q can be avoided without economic penalty. The theoretical findings are applied to the design and control of a plant coupling ethylbenzene dehydrogenation and nitrobenzene hydrogenation for simultaneous production of styrene and aniline. After plant design, a rigorous dynamic model is developed using AspenDynamics. A plantwide control structure is implemented and shown to be able to achieve stable operation. Production rate changes of reasonable magnitude can be achieved by changing the reactor-inlet flow rates or bed-inlet temperature.

Closed-loop control of a polymerisation plant using production performance indicators (PIs)

The synthesis of plant-wide control structures is recognised as one of the most important production-management design problems in the process industries. This article proposes a closed-loop control structure utilising production performance indicators (pPIs) as a possible solution to simplify this problem. pPIs represent the translation of operating objectives, such as the minimisation of production costs, to a set of measurable variables that can then be used in a feedback control. The idea of closed-loop control at the production-management level using pPIs as referenced controlled variables was implemented on the procedural model of a production process for a polymerisation plant, and two types of controllers were tested: an experimental controller based on look-up tables, and an advanced model predictive controller (MPC). Preliminary results show the usefulness of the proposed methodology.

Plantwide Control for Throughput Maximization: A Case Study

The impact of the basic plantwide regulatory control structure on maximizing the throughput using an explicit optimizing controller is quantitatively evaluated for a simple process module. The module consists of the reaction A + B → C occurring in a continuously stirred tank reactor followed by a distillation column separating the product from the recycled unreacted reactants. The column vapor boilup hitting a maximum acts as the bottleneck constraint limiting the throughput due to column flooding. Through an evaluation of reasonable plantwide control structures, the location of the throughput manipulator, the composition analyzer for overall component inventory balancing and the “local” column reflux management policy are shown to significantly affect the maximum throughput. Quantitative results show that locating the throughput manipulator close to and where possible at the bottleneck constraint reduces the throughput derating necessary due to disturbances. Designing the plantwide regulatory control sys...

Application and Evaluation of Three Methodologies for Plantwide Control of the Styrene Monomer Plant

Plantwide control (PWC) methodologies have gained significant importance given the high and increasing degree of integration in chemical processes due to material recycle, energy integration, and stringent product quality control, all of which though economically favorable, pose tough challenges to smooth plant operation. As part of the continuing search for more effective PWC system design methods, an integrated framework of heuristics and simulation was proposed [Konda et al. Ind. Eng. Chem. Res. 2005, 44, 8300−8313]. The basic idea behind this development is to make effective use of rigorous process simulators to aid in decision-making during the development of the heuristic-based PWC structure. Konda and co-workers have successfully applied the procedure to the toluene hydrodealkylation process. Though the integrated framework is promising, there is still a need to test its applicability to other complex industrial processes. The present contribution considers the development of a PWC structure for th...

Plantwide dynamics and control of processes with crystallization

Recently Ward et al. [Ward, J. D., Doherty, M. F., & Yu, C. C. (2007). Plantwide operation and control of processes with crystallization. AIChE Journal, 53, 2885–2896] presented new shortcut expressions for the steady-state operation of plantwide processes with crystallization. The method allows the engineer to predict the steady-state gain between the production rate and certain other key process variables for various choices of controlled variables. The result is a simple analytical model that shows how the process operates for various choices of control structure. The previous work of Ward et al. considered only steady-state process operation. In this contribution we apply the method and results from our previous work to develop plantwide control structures, and test the structures using a rigorous non-linear dynamic process model. We demonstrate that the method produces workable plantwide control structures, and that the steady-state behavior of the dynamic process is consistent with the predictions of our previous work.

An Industrial and Academic Perspective on Plantwide Control

The purpose of this paper is to stress the importance of selecting the right plantwide control structure and the need for a formalized approach that can encompass the many issues that arise in plantwide control design. Since the concept of process control design based on a holistic view of the process came about, the variety of procedures and approaches to the design problem has illustrated the difficulty of a unified approach. Using examples, the need and advantages of using a systematic approach based on considering the plant economics are highlighted. The examples deal with disturbance rejection, throughput maximization and economic optimization of plants consisting of parallel units.

Analysis of the control structures for an integrated ethanol processor for proton exchange membrane fuel cell systems

The aim of this work is to investigate which would be a good preliminary plantwide control structure for the process of Hydrogen production from bioethanol to be used in a proton exchange membrane (PEM) accounting only steady-state information. The objective is to keep the process under optimal operation point, that is doing energy integration to achieve the maximum efficiency. Ethanol, produced from renewable feedstocks, feeds a fuel processor investigated for steam reforming, followed by high- and low-temperature shift reactors and preferential oxidation, which are coupled to a polymeric fuel cell. Applying steady-state simulation techniques and using thermodynamic models the performance of the complete system with two different control structures have been evaluated for the most typical perturbations. A sensitivity analysis for the key process variables together with the rigorous operability requirements for the fuel cell are taking into account for defining acceptable plantwide control structure. This is the first work showing an alternative control structure applied to this kind of process.

On the application of model reduction to plantwide control

This paper proposes an approach for obtaining reduced-order models of chemical processes with application to the design of plantwide control systems. The approach is based on the inherent structure that exists in a chemical plant and identifies units or groups of units which determine the steady-state and dynamic behaviour of the plant. Specific reduction techniques with different accuracy are applied to each unit, followed by the coupling of the reduced models, according to the process flowsheet structure. By means of a case study, the assessment of plantwide control structures for the iso-butane–butene alkylation plant, the effectiveness of the approach is proven. Moreover, it is shown that the approach is able to retain the nonlinearity of the original plant model and preserves the significance of important model variables.

Steady-state behaviour of PFR-separation-recycle systems with simultaneous exothermic and endothermic, first-order reactions

A systematic investigation of plug-flow reactor (PFR)-separation-recycle systems where first-order exothermic and endothermic reactions are simultaneously performed is presented. The nonlinear behaviour is analyzed for two flowsheet alternatives and four plantwide control structures. It is shown that the system can exhibit a complex nonlinear behaviour. For the parameter values used, regions of unfeasibility, two or three multiple steady states and branches of isolated solutions were found. The undesired nonlinear phenomena can be avoided by fixing the reactor-inlet flow rates of each reactant or, when this is impossible due to the flowsheet structure, by providing sufficient cooling capacity.

Selectivity versus conversion and optimal operating policies for plants with recycle

AbstractA simple and fast procedure is developed for determining optimal steady state operating policies for chemical plants with recycle. This article improves upon the previous findings by Griffin et al. (Ind Eng Chem Res. 2006;45:8056‐8062) and now provides a more general classification procedure for process chemistries that have reversible reactions or reactions of unequal overall order or both. Selectivity vs. conversion relationships are indirectly determined herein, requiring only the reaction orders of reaction species. The effect of these relationships allows process chemistries to be classified into two equivalence classes: bounded and nonbounded. This classification has important implications for the optimal operation and plantwide control structure of a flexible plant with spare equipment capacity. For instance, using the classification procedure and decision tree developed in this article, it can be determined quickly whether or not it is economically optimal to operate the reactor at the holdup constraint under all operating conditions (bounded) or away from the constraint with a variable holdup operating policy (nonbounded). © 2008 American Institute of Chemical Engineers AIChE J, 2008

Synchronizing Field and Control-Room Operators for Process Operating Transients

Process systems engineering is rapidly moving from steady state simulation towards operator training simulation, based on dynamic models, automated procedures, and predictive systems. Conventional operating conditions are well-known and easily controlled by field operators as well as control-room operators, whereas other situations are not yet. This is the case of more unusual circumstances, such as plant start-ups as well as planned and emergency shutdowns.The implementation of detailed dynamic mathematical models allows simulating the behavior of single process units as well as industrial plants, with the possibility to study unusual scenarios, especially for two reasons. First of all, it is necessary to train the operator and prepare him/her to face several events. Secondly, there is the need for evaluating the best automatic procedure to manage either unconventional or critical situations, without waiting for them to occur. In this sense, the safety approach is changing from reacting to predicting, thanks to the spread of tools that offer the possibility to simulate accidental events, unit and/or valve malfunctions, and significant process transients.The paper investigates and compares different procedures to control process transients, described by a sequence of actions, in order to coordinate synergistically the actions dictated by control-room operators and the operations to be accomplished manually by field operators. The main objective is to improve process start-up and shutdown reliability as well as the corresponding plant safety of an existing plant-wide control structure, without requiring any structural modification. A propane/butane splitter is adopted as a case study.

Optimum Economic Design and Control of a Gas Permeation Membrane Coupled with the Hydrodealkylation (HDA) Process

This paper studies the design and control of a modified hydrodealkylation (HDA) process that uses a membrane to reduce hydrogen losses in the methane purge stream. A dynamic, counter-current membrane module is written using Aspen Custom Modeler to capture both the steady-state and dynamic performance of the membrane. The feed to the membrane is the purge stream from the conventional HDA process without a membrane. The hydrogen-rich permeate is compressed and recycled back into the process to reduce the demand for fresh hydrogen feed. The methane-rich retentate is the new purge stream from the system. A steady-state economic analysis is performed to determine the optimum membrane area and permeate pressure. Increasing the area and decreasing the pressure result in lower hydrogen consumption; however, the capital investment and energy costs increase. The return on the incremental investment is used to select the best design. The final design reduces the required flow rate of fresh hydrogen feed by approximately one-third. This results in an estimated annual savings of almost $400 000 in hydrogen feed by investing approximately $836 000 in capital. A plantwide control structure is developed for the new process. A composition controller manipulates the flow rate of the membrane retentate, which acts as the new purge stream, to control the composition of methane in the large total gas recycle stream. A second composition controller is used to control the composition of the hydrogen being lost in the retentate purge by manipulating the power input to the permeate compressors. Dynamic simulations performed in the Aspen Dynamics environment show that the control structure is effective in rejecting disturbances in throughput and hydrogen fresh feed composition.

Process alternatives for methyl acetate conversion using reactive distillation. 1. Hydrolysis

In a polyvinyl alcohol (PVA) plant, reaction stoichiometry indicates that equal molar of methyl acetate is generated for every mole of PVA produced. This work explores an alternative to convert methyl acetate back to acetic acid (raw materials of PVA plant), methyl acetate (MeAc) hydrolysis. The design and control of methyl acetate hydrolysis using reactive distillation is studied. Because of the small chemical equilibrium constant ( ∼ 0.013 ) and unfavorable boiling point ranking (MeAc being the lightest boiler), the reactive distillation exhibits the following characteristics: (1) total reflux operation and (2) excess reactant (water) design. The proposed flowsheet consists of one reactive distillation column with a reactive reflux drum, two separation columns, and one water-rich recycle stream. A systematic design procedure is used to generate the flowsheet based on the total annual cost (TAC). Two dominate design variables are: recycle flow rate (for the degree of excess in water) and the overhead impurity level of acetic acid in the product column (to avoid tangent pinch). Finally, the operability of the hydrolysis plant is evaluated. A plantwide control structure is developed followed by process identification and controller tuning. The results show that reasonable control performance can be achieved using simple temperature control for feed flow and feed composition disturbances.

CLOSED-LOOP PRODUCTION CONTROL OF POLYMERIZATION PLANT USING PRODUCTION KPIs

A synthesis of plant-wide control structures is recognized as one of the most important design problem in production management in process industries. This paper proposes a closed-loop control structure with the utilisation of production Key Performance Indicators (KPIs) as possible solution to this problem. Production KPIs represent the translation of operating objectives, such as minimization of production costs, to a set of measurable variables that can then be used in a feedback control. The idea of production feedback control using production KPIs as referenced controlled variables was implemented on a procedural model of a production process for a polymerisation plant. Some preliminary results show usefulness of proposed methodology.

Performance Assessment of Plantwide Control Systems of Industrial Processes

Performance assessment of control systems has been receiving growing attention in the recent past to improve operability and profit margins of the process. However, such studies, and also the tools available to carry out such studies, from the plantwide perspective are rather limited. In this regard, a new dynamic performance index, called dynamic disturbance sensitivity (DDS) is proposed in this paper. It is then used to assess the performance of three plantwide control (PWC) structures (CS1, CS2, and CS3) for an industrially important petrochemical process, namely, the hydrodealkylation (HDA) of toluene to produce benzene. The three control structures are distinctly different from the throughput manipulator standpoint: CS1 uses an internal manipulator (e.g., reactor temperature), CS2 uses a fixed-feed control strategy, and CS3 uses an on-demand control strategy to control throughput; consequently, the rest of the control structure decisions are significantly different. By analyzing the results from rig...

Valve Positioning Control for Process Through-put Maximization

The application of a valve positioning controller (VPC) on top of a basic regulatory plant-wide control structure for maximizing the process through-put to increase plant profitability is demonstrated. The input to the VPC is a measurement of the bottleneck constraint and its output is the set-point of the through-put manipulator. The hydrodealkylation (HDA) of toluene to benzene process is used as an example case study with the feed hydrogen compressor and the furnace considered as bottleneck constraints that limit production. Results show that the automatic adjustment of the through-put manipulator allows the process to be operated close to (or at) the bottleneck constraint. De-rating of the plant through-put due to the possibility of disturbances is then avoided resulting in enhanced through-puts and plant operating profit. The increase in the through-put and plant profit over the de-rated operation is quantified with respect to variation in the magnitude of the principal disturbance that causes the bottleneck constraint to be hit. Results show that significant enhancement of more than 10% increase operating profit can be achieved. VPC is thus demonstrated to be a simple and effective means for increasing plant profitability by maximizing through-put.