Rigorous Dynamic Simulation Research Articles

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].

Design and Control of a Fully Heat-Integrated Pressure-Swing Azeotropic Distillation System

If the composition of a binary homogeneous azeotrope changes significantly with pressure, the chemical components can be efficiently separated by using two distillation columns that operate at different pressures. This process is called pressure-swing azeotropic distillation. This paper updates the work of Abu-Eishah and Luyben (Ind. Eng. Chem. Process Des. Dev. 1985, 24, 132−140). These authors studied a partially heat-integrated two-column system in which an auxiliary reboiler permitted independent adjustment of vapor boilup in both columns. In the present paper, the two columns operate in the “neat” mode without any auxiliary reboiler. The details of achieving rigorous steady-state and dynamic simulations of the neat system in Aspen Plus and Aspen Dynamics are discussed. The condenser duty in the high-pressure column must equal the reboiler duty in the low-pressure column. The heat-transfer rate depends on the temperatures on the hot and cold sides in the condenser/reboiler, which change with varying p...

Operation and control of batch extractive distillation for the separation of mixtures with minimum-boiling azeotrope

Batch distillation is commonly used in the fine chemicals, specialty polymer, biochemical, pharmaceutical, and food industries. For separating mixtures with minimum-boiling azeotrope, a heavy entrainer is frequently added to the top section of the batch column to aid in the separation. This process is called batch extractive distillation. Most of the papers in open literature have only studied the first operating step of the batch extractive distillation which is the recovery of the light component without mentioning the later steps for the recovery of the other component and entrainer. In this paper, two real chemical systems, one separating acetone and methanol using water as entrainer and the other separating isopropyl alcohol (IPA) and water using dimethyl sulfoxide (DMSO) as entrainer, are studied for the feasible operation of the complete batch distillation sequence. The operating variables, including the pre-load amount with the mixture, continuous feed rate of the entrainer, and reflux ratio at each operating step are determined in the operating sequence. The constant reflux ratio and constant entrainer feed rate operating policy and another policy to allow these two operating variables to be varied will be compared in order to further improve the batch operation. All dynamic simulations that are performed directly mimic industrial situations from an empty column using a rigorous dynamic simulator, Aspen Dynamics™.

Rigorous Dynamic Simulation of an Industrial Tray Column, Considered Liquid Flow Regime and Efficiency of Trays

A rigorous model is presented for dynamic simulation of an industrial tray column which uses mass and energy balances together with a proper hydraulic equation. Three different liquid flow regimes can be considered in the industrial tray columns which each of them possesses a specific hydraulic equation. The implemented model by recognizing the type of liquid flow regime is able to use the proper hydraulic equation in the computations. Moreover, by calculating the tray efficiency, the effect of tray geometry on the separation performance is investigated. As the result of this study, the open loop response of the industrial tray column is presented after inputting a step change in reflux flow rate.

Simulations of collective synchronization in Josephson junction arrays

We developed a virtual model of a Josephson junction series array based on a rigorous dynamic simulation and investigated coupling effect of Josephson oscillations. Our virtual model for a superconductor-insulator-normal metal-insulator-superconductor junction array reveals that a time-lag junction instability should be avoided for global synchronization in a long array with a large number of junctions. We found that the coupling between the self-generated Josephson oscillations through a microwave transmission line plays an important role in the collective synchronization of the Josephson array.

MULTIVARIABLE SUBSPACE IDENTIFICATION AND PREDICTIVE CONTROL OF A HEAT-INTEGRATED SUPERFRACTIONATOR RIGOROUS MODEL

The control of a heat-integrated superfractionator is addressed in this paper. A multivariable subspace identification method is proposed to overcome the difficulties associated to the large time constants of the process and identify a linear process model, upon which a constrained predictive controller is developed. The effectiveness of the proposed identification and control algorithm is shown by means of closed-loop rigorous dynamic simulation results.

Effect of Recycle Streams on the Closed-Loop Dynamics of Thermally Coupled Distillation Sequences

ABSTRACT The effects of thermal links on the closed-loop dynamics of thermally coupled distillation sequences for the separation of quaternary mixtures have been studied in this work by using rigorous dynamic simulations. The incorporation of thermal links into conventional distillation sequences can lower the energy consumption up to 40% without introducing additional control problems to the resulting thermally coupled distillation sequences. In some cases, the thermally coupled distillation sequences outperformed the dynamic behavior of the conventional distillation sequences for set point tracking. This result is important to establish that thermally coupled distillation sequences not only can have significant energy savings but also good dynamic properties.

Dynamic compartmental modeling of nitrogen purification columns

The development of nonlinear control strategies for high purity distillation columns requires the availability of suitable nonlinear dynamic models. Relatively simple models are needed for the design of nonlinear model predictive controllers (NMPC) which are implemented on-line via repeated solution of a constrained nonlinear optimization problem. We utilize the compartmental modeling approach to construct control relevant dynamic models of a very high purity nitrogen purification column. Reduced-order nonlinear models are derived from a stage-by-stage balance model by partitioning the column into distinct compartments and applying time-scale arguments to convert most of the ordinary differential equations (ODE) describing each compartment into simpler algebraic equations. The compartmental models are dynamically simulated using the differential–algebraic equation (DAE) solver available in MATLAB. The stage-by-stage and compartmental models are compared to a rigorous dynamic simulator constructed in Aspen Dynamics to assess the merits of the proposed modeling strategy with regard to prediction accuracy and computational efficiency.

Influence of Feed Impurity on the Design and Operation of an Industrial Acetic Acid Dehydration Column

In this work, the design and operation of an industrial column for acetic acid dehydration via heterogeneous azeotropic distillation is investigated. Multiple column feed streams from various parts of the upstream process are fed into this column. The feed components besides acetic acid and water also contain small amounts of methyl acetate and another component. For proprietary reasons, this component will not be specified in this paper. This component during normal operation will not leave the column system through either a top decanter aqueous outlet stream or a column bottom stream. The accumulation of this component inside the column system is illustrated via rigorous dynamic simulation. A side stream is proposed to solve the accumulation problem of this component. The appropriate side-stream location and its flow rate are determined in this paper by total annual cost analysis. There is a great influence of this small feed impurity on the design and operation of the column. With the same purity specifications on both the top and bottom products, the column system with feed impurity requires much larger total annual cost and operating cost in comparison with the design of the no feed impurity case. A very simple overall control strategy is proposed for the column system with feed impurity and continuous side-stream draw-off. Because the optimal flow rate of the side stream is quite small, an alternative operating strategy in industry is to periodically purge out this impurity through the side stream. A practical automatic purging strategy is thus proposed in the paper. This automatic purging strategy does not rely on any online composition measurement but just needs information on some tray temperature measurements. Closed-loop dynamic simulation demonstrates that the proposed purging strategy can successfully keep the column under normal operation with both the top and bottom product purities at their specifications.

A nonlinear model-based control system for distillation columns using rigorous simulator

In this paper we provide a novel model-based control system for nonlinear process for the purpose of minimizing an objective function, followed by similar research areas such as dynamic optimization and model predictive control. Their target processes have large nonlinearity, multi manipulated variables and multi control variables. We then provide our control system scheme and an optimization algorithm. We also illustrate the results obtained from application of a depropanizer distillation column using a rigorous plant-wide dynamic simulator.

プラズマ応用技術におけるシミュレーション研究 3. Kinetic Computer Modeling of Microwave Surface-Wave Plasma Production

Kinetic computer plasma modeling occupies an intermediate position between the time consuming rigorous particle dynamic simulation and the fast but rather rough cold- or warm-plasma fluid models. The present paper reviews the kinetic modeling of microwave surface-wave discharges with accent on recent kinetic self-consistent models, where the external input parameters are reduced to the necessary minimum (frequency and intensity of the applied microwave field and pressure and geometry of the discharge vessel). The presentation is limited to low pressures, so that Boltzman equation is solved in non-local approximation and collisional electron heating is neglected. The numerical results reproduce correctly the bi-Maxwellian electron energy distribution functions observed experimentally.

Rigorous dynamic modeling and simulation of wine distillations

Abstract A novel simulation strategy for dynamic distillation of complex mixtures, such as wine, is proposed and evaluated in terms of computing efficiency and accuracy. The model developed describes wine distillation as a multicomponent reactive batch distillation process. The simulation approach transforms the system of differential algebraic equations (DAE) into a set of ordinary differential equations, by pre-solving the algebraic equations and replacing them with artificial neural networks. This new simulation strategy for wine distillation is 40% faster than the rigorous solution of the DAE system, compared at the same level of accuracy. The model can be applied to the distillation of other spirits or complex mixtures, as well as in other separation processes in which the recovery of aromas is essential.

Application of a plant-wide control design to the HDA process

With the increasing interest in and the need for plant-wide design and control, it is desirable to develop rigorous models for realistic and large-scale processes with recycle streams, energy integration and non-ideal systems, and use them for evaluating plant-wide control schemes. In this study, a rigorous model for the hydrodealkylation of toluene (HDA) process is developed using the commercial software, hysys.plant. After reviewing the reported methods for plant-wide control, a systematic method, namely, Control Configuration Design (CCD) method, is selected for application to the HDA process. The resulting control structures from the application of this method are evaluated and compared through rigorous dynamic simulation. The results show that the CCD method successfully yields workable base-level regulatory control structures for the HDA process. The control structures obtained by the CCD method are also compared with that reported by Luyben et al. (Plant-wide Process Control, McGraw-Hill, 1998) using their nine-step method.

Model Optimization for the Dynamic Simulation of Reactive Absorption Processes

The optimal design of reactive separations is impossible without reliable process models. Especially for the dynamic simulation and the model-based control of complex reactive absorption processes the model development leads to a contradiction between the required model accuracy to reflect the process complexity and the feasibility of process simulations regarding the computation time. In this respect, we have developed a new rigorous dynamic two-phase model based on the two-film theory as a first step, which takes into account the influence of chemical reactions and additional driving forces in electrolyte systems on mass transfer considering thermodynamic nonidealities as well as the impact of column internals on the process hydrodynamics. For a model optimization, we have performed an analysis of different model approaches for complicated industrial absorption processes and determined an appropriate model complexity. Based on results of sensitivity studies, we have accomplished different model modifications leading to a stabilization of the numerical solution without affecting the good agreement between simulation results and the experimental data. This time-optimized model can be considered superior as compared to previous approaches and facilitates for the first time a rigorous dynamic simulation of entire reactive absorption columns and the application within an on-line process control system.

Comparison of Alternative Control Structures for an Ideal Two-Product Reactive Distillation Column

Although steady-state design and open-loop dynamics of reactive distillation columns have been explored in many papers, very few papers have dealt with closed-loop control. Most of these control papers consider reactive distillation columns in which there is only one product, and an excess of one of the reactants is sometimes assumed. This paper explores the closed-loop control of a reactive distillation column in which two products are produced in a single column and stoichiometric amounts of fresh feeds are desired. The reversible reaction is A + B ⇌ C + B. The relative volatilities are favorable for reactive distillation; i.e., the reactants are intermediate boilers between the light product C and the heavy product D. Simple ideal physical properties, kinetics, and vapor−liquid equilibrium are assumed so that the basic control issues of reactive distillation can be explored without being clouded by complexities of a specific chemical system. Six alternative control structures are evaluated via rigorous dynamic simulation. All of the schemes use a composition analyzer in the reactive zone of the column to detect the inventory of one of the reactants so that fresh feed can be manipulated to satisfy the reaction stoichiometry. Therefore, the use of excess reactant is not required. The interaction between design and control is illustrated by the impact of holdup in the reactive zone: increasing holdup (catalyst) improves the dynamic controllability of the process. Single-end temperature control can keep both products at or above specified purity values, even for large disturbances, if reactive-zone holdup is sufficiently large.

Controllability and Resiliency Analysis for Homogeneous Azeotropic Distillation Columns

Abstract Flowsheet controllability and resiliency analysis is a desirable capability in the early stages of process design. When implemented, it is commonly carried out using low-order approximations for the process dynamics, generated on the basis of preliminary flowsheet information. However, processing units that conditionally feature undesirable dynamics such as inverse response and overshoots require more complex models to account for these phenomena. Such models can be generated using a new method that relies on appropriate partitioning of the process into a number of component parts, from which an approximation of the linear dynamics is derived. In this paper, an azeotropic distillation column is analyzed using the new approach, leading to results that concur with and extend those obtained using rigorous dynamic simulation.

An industrial design/control study for the vinyl acetate monomer process

This work presents design details of an industrial process for the manufacture of vinyl acetate monomer. Our purpose is to offer a realistic example that is uniquely suited for academic researchers pursuing simulation, design, and control studies. The vinyl acetate process has common, real chemical components. It contains many standard unit operations in a realistic flowsheet. And it illustrates the types of systems of industrial research interest in the areas of process design, optimization, simulation, and control. Vapor-phase reactions convert ethylene, oxygen, and acetic acid into vinyl acetate with water and carbon dioxide as byproducts. The process contains a packed tubular reactor, a feed-effluent heat exchanger, an absorber, a vaporizer, an azeotropic distillation column with decanter, and both gas and liquid recycle streams. All physical property, kinetic, and flowsheet data have been compiled from sources in the open literature. We detail the flowsheet information required to construct rigorous steady state and dynamic mathematical models of the process and present the process control requirements and objectives. Finally, we briefly describe the rigorous nonlinear dynamic simulation we have constructed for this process using TMODS, DuPont’s in-house dynamic simulator. Models of this process have also been developed by Aspen Technology and Hyprotech in their commercial simulators and are available directly from the vendors.

Steady-state transitions in the reactive distillation of MTBE

The reactive distillation process for MTBE synthesis can produce multiple steady states which reduce column operability and controllability, particularly during start-up. A rigorous dynamic simulation of a MTBE reactive distillation column has been developed for a column which was suspected to exhibit multiplicity. Three distinct steady states were found at the nominal design point. Certain perturbations in the feed composition or the manipulated variables were found to cause transitions between parallel steady states and these were plotted using the dynamic capabilities of the column model. Steady-state transitions were only found when neither product flow rate was used as a column input (i.e. when an energy balance control configuration was used). Appropriate closed-loop control was also found to essentially prevent steady-state transitions. No practical mechanism of controlling a transition from an undesirable steady state to a desirable steady state was found: the controlled perturbations of a manipulated variable, the initiation of closed-loop control and the direct manipulation of the column material balance were all found to be either ineffective or impractical. A catastrophic shift can be contrived by manipulating an input variable but destabilisation of the column operation is a potential consequence of the rapid change in internal flows which occurs during the transition.

A methodology for fault diagnosis in large chemical processes and an application to a multistage flash desalination process: Part I

Abstract This work presents a new strategy for fault diagnosis in large chemical processes (E.E. Tarifa, Fault diagnosis in complex chemistries plants: plants of large dimensions and batch processes. Ph.D. thesis, Universidad Nacional del Litoral, Santa Fe, 1995). A special decomposition of the plant is made in sectors. Afterwards each sector is studied independently. These steps are carried out in the off-line mode. They produced vital information for the diagnosis system. This system works in the on-line mode and is based on a two-tier strategy. When a fault is produced, the upper level identifies the faulty sector. Then, the lower level carries out an in-depth study that focuses only on the critical sectors to identify the fault. The loss of information produced by the process partition may cause spurious diagnosis. This problem is overcome at the second level using qualitative simulation and fuzzy logic. In the second part of this work, the new methodology is tested to evaluate its performance in practical cases. A multiple stage flash desalination system (MSF) is chosen because it is a complex system, with many recycles and variables to be supervised. The steps for the knowledge base generation and all the blocks included in the diagnosis system are analyzed. Evaluation of the diagnosis performance is carried out using a rigorous dynamic simulator.

Rigorous industrial dynamic simulation of a crude distillation unit considered valve tray rating parameters

A rigorous dynamic simulation is performed to find optimal operating conditions of a crude distillation unit with 56 trays and 65,000 barrel per day capacity. The model includes not only mass and energy balance equations but also nonlinear equations capturing the fluid behavior in the column such as downcomer load, jet flooding, hole velocity limit and downcomer backup. Discrete behavior of pump and heater on/off, valve open/close are also included. Hundreds of thousand differential algebraic equations (DAEs) are set up and solved by gPROMS simultaneously. As a result, the dynamic behavior of the unit during the starting and shutdown procedure is fully understood.