Balancing design and control of an olefin metathesis reactive distillation column through reactive section distribution

In this contribution, balance of design and control is investigated through reactive section distribution in an olefin metathesis reactive distillation column. Four designs with different strategies for reactive section distribution are studied and these include one with all stages as reactive ones (design-I), one with the distribution of reactive section according to the principle of internal mass integration (design-II), and ones with the extension of the reactive section of the design-II by five stages to either the stripping section (design-III) or the rectifying and stripping sections simultaneously (design-IV). The design-II appears to be more thermodynamically efficient than the design-I but with considerable degradation in process controllability. The design-III and design-IV retain most of the economical advantages of the design-II with only slight deterioration in controllability as compared with the design-I. This outcome demonstrates the fact that the strategy for reactive section distribution could be an effective decision variable for the balance of design and control in the development of reactive distillation columns.     

Controllability and energy efficiency of a high-purity divided wall column

The divided wall column system is a promising energy-saving alternative for separating multi-component mixtures. However, high energy efficiency and stable operations can only be achieved with careful design of steady state operation and control scheme. In this study, the effects of liquid split and vapor split ratios on the energy efficiency and controllability of a divided wall column system for separating ethanol, n-propanol, and n-butanol were investigated. A region with high energy efficiency was identified. However, relative gain analysis found that the performance of multi-loops composition control would be very poor. Dynamic tests showed that multi-loop temperature control cannot return the product compositions to the desired values in case of feed composition disturbances. Outside this region, composition control can compensate for external disturbances such as feed flow rate and feed composition changes but not changes in operating region caused by internal variations such as liquid and vapor splits. Offsets in the product purities were found if temperature controls were used and there are disturbances in feed composition or changes in operating region caused by upsets in liquid and vapor splits. There is a trade-off between energy efficiency and controllability. A composition + temperature cascade scheme was proposed to stabilize the operation with high energy efficiency. The proposed scheme was able to maintain high product purity and reject external disturbances in feed flow and composition changes as well as internal disturbances such as changes in liquid and vapor splits.     

Steady‐state process optimization with guaranteed robust stability under parametric uncertainty

Interest in chemical processes that perform well in dynamic environments has led to the development of design methodologies that account for operational aspects of processes, including flexibility, operability, and controllability. In this article, we address the problem of identifying process designs that optimize an economic objective function and are guaranteed to be stable under parametric uncertainties. The underlying mathematical problem is difficult to solve as it involves infinitely many constraints, nonconvexities and multiple local optima. We develop a methodology that embeds robust stability constraints to steady‐state process optimization formulations without any a priori bifurcation analysis. We propose a successive row and column generation algorithm to solve the resulting generalized semi‐infinite programming problem to global optimality. The proposed methodology allows modeling different levels of robustness, handles uncertainty regions without overestimating them, and works for both unique and multiple steady states. We apply the proposed approach to a number of steady‐state optimization problems and obtain the least conservative solutions that guarantee robust stability. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Simultaneous process and control system design for grade transition in styrene polymerization

We present a comprehensive approach to the simultaneous design and control of a continuous stirred tank reactor (CSTR) for styrene solution polymerization that must be able to produce different polymer grades. The resulting tool allows simultaneous selection of the polymerization equipment, the multivariable feedforward-feedback controller's structure and tuning parameters, the steady states and the transition paths between them. For this purpose a multiobjective optimization is implemented to minimize the annualized reactor cost, the operating costs, the production of off-specification polymer and the transition time between steady states. Trade-offs between the sometimes conflicting objectives are dealt with by the optimization. Unlike many previous grade transition studies, steady states are not known a priori. The only known parameters are the target molecular weights to be produced at each steady state. We have analyzed three different scenarios, and propose practical criteria for selecting the most reasonable optimum when the solution is not unique.     

Optimization and control of vertical double wall dividing-wall column for separating a quaternary system

In this paper, the new separation structure (VDWDWC) for separation of a quaternary system is proposed for the first time, which has lower energy consumption and higher separation efficiency than the traditional three-column and Kaibel column. Sensitivity analysis and response surface optimization (RSM) are applied to the structural design and parameter optimization of VDWDWC. In addition, the dynamic control of VDWDWC is also investigated. Specifically, the performances of temperature-composition cascade control (TC-CC) with and without feed-forward ratio control are compared and analyzed. The results suggested that the TC-CC structure can achieve outstanding controllability for VDWDWC, when the feed flow rate and feed composition is disturbed. In particular, TC-CC with feed-forward ratio control has better dynamic response: the maximum deviation was reduced by 53.7%, and the settling times are significantly shortened, while the steady state deviation of product purity was slightly reduced.     

Optimal Retrofit Design of Crude Distillation Units for Processing Shale Gas/Natural Gas Condensate Oil

The increasing production of condensate oil due to shale gas/natural gas booming has economically motivated refiners to add this valuable and abundant crude source into their feedstock. Many refineries, however, were originally built to process heavier crudes, whose design must be retrofitted to enable the processing of the changed feedstock. The conceptual retrofit design of crude distillation units for processing condensate oil has been studied. Four retrofit designs are proposed and simulated including facilities of preflash column, atmospheric distillation unit, and vacuum distillation unit. All retrofit designs are comprehensively evaluated by steady‐state modeling for feasibility check, energy consumption analysis for operating cost evaluation, and retrofit cost evaluation. The retrofit design with preflash columns in sequence potentially could be the most economical case.     

Modeling,Control, and Optimization of Ideal Internal Thermally Coupled Distillation Columns

Internal Thermally Coupled Distillation Columns (ITCDIC) are the frontier of energy saving distillation research. In this paper, the ideal ITCDIC is considered. A novel mathematical model and a related simulation algorithm are proposed. The dynamic responses of open‐loop, PID controllers and the responses of closed‐loops are carried out. The results show that the ITCDIC is a self‐balance process and could be operated smoothly with two PID controllers; the steady‐state optimization met the need of ITCDIC optimization. Furthermore, a steady‐state optimization model of the operation parameters is presented, which can be used to directly obtain the optimal operation parameters simultaneously guaranteeing not only the product quality and the maximum energy savings but also the dynamic operability and controllability. The benzene‐toluene system is studied as an illustrative example.     

Multiobjective batch process design aiming at robust performances

The most common batch design approach in practice and literature is a deterministic one. However, given the uncertainties prevailing in early stages of process design, a deterministically calculated productivity is not sufficient to select one of the large number of optional designs. Therefore, we propose a Tabu Search multiobjective optimization framework, which allows to approximate the Pareto-optimal set of designs while considering uncertain variables in the initial recipe. As a novel technique, we include performance robustness as a separate objective function within the multiobjective optimization alongside with productivity of a design, thus obtaining not only designs with high productivity or solely robust designs, but both high productivity and robust designs in one set of solutions. We examined several robustness criteria as a possible quantification of performance deviations under uncertain recipe variables. The implementation of a Tabu Search framework in combination with Monte-Carlo simulation and Latin Hypercube sampling provides a huge flexibility in the problem specification, in particular in the definition of parameter uncertainties. As a result we successfully demonstrate that metaheuristic optimization techniques are capable to approximate the Pareto-optimal set under uncertainty and are able to capture potentially antagonistic solution qualities such as high productivity and robustness by multiobjective optimization. With the help of this approach, parameters can be identified that have to be put into the focus of process research and development efforts in order to obtain high performance batch process designs.     

Design and control of an olefin metathesis reactive distillation column

This paper considers the design and control of a reactive distillation column in which one reactant is consumed and two products are formed (A?B+C). The volatilities are α_B>α_A>α_C, i.e. the reactant is intermediate boiling between the two products. The metathesis of 2-pentene is considered as the demonstrative example. The column has a single feed of the intermediate boiling reactant. The distillate contains mostly light component and the bottoms mostly heavy.Three designs are considered: the base case (low-conversion/low-pressure), a low-conversion/high-pressure case and a high-conversion/high-pressure case. The base design is obtained from the literature, and the other two steady-state designs are optimized with respect to the total annual cost. All the designs are found to be openloop stable. Five control structures are studied for the base design. Then the best two structures are applied to the remaining two designs. This category of reactive distillation exhibits less challenging problems than other categories since it uses a single feed, which eliminates the need for the control structure to perfectly balance two fresh feeds.Simulation results demonstrate that effective dynamic control is provided by a control structure that uses two temperatures to maintain the purities of both product streams. No internal composition measurement is required. This structure is found to be robust and stable and rejects loads and tracks setpoints very well.     

Towards Integrated Process and Control System Synthesis for Heat‐Integrated Plants

Most chemical processes are networks of different pieces of equipment, as reactors, distillation columns, compressors, heat exchangers, etc. Process integration is an area of chemical engineering that deals with the optimal design of these networks, from the point of view of energy efficiency, capital costs, emissions reduction, waster water minimization, and raw materials usage. Until recently, engineers developed conceptual process designs by experience and intuition, however, with the establishment of process integration methodologies, this activity can be performed systematically. One of the subjects that have received the most attention from researchers in this area is the steady state design of Heat Exchanger Networks (HENs). Several tools have been developed and are in use; however, the development of a tool for synthesis of HENs that takes into account network controllability is not available. Hence, the purpose of this paper is the development of a new methodology for design of heat‐integrated chemical processes, particularly HENs where controllability and energy recovery are both balanced during the design synthesis stage.     

Towards further internal heat integration in design of reactive distillation columns—Part III: Application to a MTBE reactive distillation column

According to the principle introduced in the first two papers of this series, seeking further internal heat integration between reaction operation and separation operation during the synthesis, design, and operation of a reactive distillation column synthesizing methyl tertiary butyl ether (MTBE) from methanol and isobutylene is investigated. Although the MTBE reactive distillation column is characterized by complicated thermodynamic properties and multiple steady states, a substantial reduction of energy requirement and capital investment can still be achieved with the consideration of further internal heat integration between the reaction operation and the separation operation in the two existing steady states. Dynamics and operation of the resultant process designs are then examined in terms of static and dynamic analysis and sharp improvement in process dynamics and controllability is clearly identified through intensive comparison against the simple process design without the consideration of further internal heat integration between the reaction operation and the separation operation involved. It is demonstrated that the more synergistic relationship evolved during the reinforcement of internal heat integration should account for the dramatic improvement in process dynamics and controllability.     

Analysis and optimization of two-column cryogenic process for argon recovery from hydrogen-depleted ammonia purge gas

Traditionally, high-purity argon recovery from air is considerably difficult owing to the boiling point of argon close to that of oxygen. Recently with the increasing demands for argon, another attractive source of ammonia purge gas has been paid more attention. In this paper with an objective of minimizing energy consumption per argon product, the two-column process for recovering argon from hydrogen-depleted ammonia purge gas is analyzed and optimized in detail on the ASPEN PLUS platform. Firstly, the model of two-column process is set up using the standard unit operation blocks and PENG-ROB property method of ASPEN PLUS, in which validation of PENG-ROB property method is carried out by comparison with a total 623 experimental data from three aspects: vapor-liquid equilibrium, liquid phase density, and enthalpy. It is followed by the thermodynamic and simulation and sensitivity analysis, which on the one hand can reduce the number of decision variables related to optimization problem, and on the other hand can obtain reasonable parameter specification, variables initial values and ranges, thus effectively ensuring the later optimization algorithm converges quickly and accurately. Finally the built-in sequential quadratic programming (SQP) solver of ASPEN PLUS is adopted to solve the minimum energy consumption optimization problem of two-column process. On the processor of 2.66 GHz Intel(R) Core (TM)2 Duo CPU with 4 GB RAM, the whole optimization only takes CPU times 10 s or so to accomplish. The optimal results show that thermal state of feed to demethanizer is a very efficient and valuable means to reduce system energy consumption which at T_C05 = 103 K is only 87.4% of that at T_C05 = 109 K where T_C05 is the temperature of feed to demethanizer directly reflecting its thermal state. The condensing pressure of hydrogen-depleted ammonia purge gas also plays a vital role in reducing system energy consumption which is less at higher condensing pressure, whereas it almost has no influence on the yield and purity of argon recovery. The optimal operating pressure of flash separator used to remove the residual hydrogen in the feed hydrogen-depleted ammonia purge gas is 0.4-0.6 MPa (A); the most economical reflux ratio of argon distillation column is 1.15, and that of demethanizer varies from 0.33 to 0.45 depending on thermal state of feed to demethanizer.     

A path constrained method for integration of process design and control

Integration of process design and control (IPDC) has been the holy grail of process systems engineering since the introduction of heat and mass integration. A proper combination of these separate yet connected tasks carries the promise of achieving superior designs that cannot be realized with conventional procedures. In this work, a bi-level dynamic optimization approach is introduced for achieving IPDC in its true sense. The principal idea proposed here is to utilize an optimal controller (a modified linear quadratic regulator) to practically evaluate the best achievable control performance for each candidate design during process design. The evaluation of complete, closed-loop system dynamics can then be meshed with a superstructure-based process design algorithm, thus enabling considering both cost and controllability in design of a process. The practicality of the introduced approach enables a solution of this complex dynamic optimization problem within reasonable computational requirements, as demonstrated in an evaporator case study.     

Interpreting the dynamic effect of internal heat integration on reactive distillation columns

In this work,the impact of internal heat integration upon process dynamics and controllability by superposing reactive section onto stripping section,relocating feed locations,and redistributing catalyst within the reactive section is explored based on a hypothetical ideal reactive distillation system containing an exothermic reaction:A + B ←→ C + D.Steady state operation analysis and closed-loop controllability evaluation are carried out by comparing the process designs with and without the consideration of internal heat integration,For superposing reactive section onto stripping section,favorable effect is aroused due to its low sensitivities to the changes in operating condition,For ascending the lower feed stage,somewhat detrimental effect occurs because of the accompanied adverse internal heat integration and strong sensitivity to the changes in operating condition.For descending the upper feed stage,serious detrimental effect happens because of the introduced adverse internal heat integration and strong sensitivity to the changes in operating condition.For redistributing catalyst in the reactive section,fairly small negative influence is aroused by the sensitivity to the changes in operating condition.When reinforcing internal heat integration with a combinatorial use of these three strategies,the decent of the upper feed stage should be avoided in process development.Although the conclusions are derived based on the hypothetical ideal reactive distillation column studied,they are considered to be of general significance to the design and operation of other reactive distillation columns.     

Integrated process design and control of divided-wall distillation columns using molecular tracking

This work explores the dynamic and control behavior of dividing wall distillation columns from two different steady-state design approaches (molecular tracking and optimization method) for three different mixtures. The controllability of the six design cases was evaluated using singular value decomposition and the closed-loop performance was evaluated using integral absolute error in Aspen Dynamics. The results demonstrate that the side-draw location obtained by molecular tracking (MT) provides optimal controllability. As a result, there is a slight advantage in control properties while obtaining designs by reducing the time to find the optimal solution through the MT method.     

An overview on controllability analysis of chemical processes

Controllability is one of the most important aspects of chemical process operability, because it can be used to assess the attainable operation of a given process and improve its dynamic performance. The purpose of this article is to outline the main methodologies that have been developed to deal with the assessment of process controllability and the improvement of its controllability characteristics. Several existing controllability assessment methods are reviewed and discussed. For improving the controllability characteristic of a process, there are two main design methods: the optimization‐based method and the controllability indices‐based anticipating sequential method. Advantages and disadvantages of these techniques are discussed. It has been emphasized that bifurcation analysis, as a powerful nonlinear analysis tool, could provide important guidance for making processes more controllable by eliminating or avoiding some undesirable behaviors of processes. Further challenges and developments in the field of process controllability are identified. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

New measures for robustness in linear multivariable feedback systems

New measures for robustness are proposed for linear multivariable control systems. A new theorem is presented to account for the controllability of systems that contain an integrating element, in the presence of additive uncertainties. The theorem leads to the definition of a sufficient condition for robust integral controllability: robustness margin (RM). The ratio of robustness margins for competing control or processs structures can be used to discriminate between these structures. These measures are useful, especially in design stages, since their calculation depends only on steady state information. Applications to several published distillation systems show the merits of these robustness measures.     

Latent variable projections of sensitivity data for experimental screening and kinetic modeling

In microkinetic modeling the number of kinetic parameters is large and the precision of the “known” parameters is often very low. The standard approach is then to fit only the most uncertain parameters while regarding the fixed parameter as “true”. This assumption will have consequences also on the fitted parameters since the correlation structure often is quite significant. In this study we have taken the approach to fit many parameters and then try to use more efficient experimental designs to break the correlation structure and thus obtain more precise parameter estimation despite the large number of fitted parameters. After performing sensitivity analysis of many candidate experiments, a latent variable model (PCA) is made from the resulting sensitivity matrix and the score matrix is used as a candidate set prior to experiment selection. Due to the correlation structure in the sensitivity matrix, the number of components from the PCA model is fewer than the number of parameters. The columns in the score matrix are furthermore orthogonal whereas the columns in the original sensitivity matrix are not. Different designs were generated using the original sensitivity matrix, the score matrix as well as using a space-filling design and performing a sequential approach. Both steady state and transient experiments were evaluated. These different designs were used to fit kinetic parameters to a simulated dataset made using published parameter values. The results show no significant difference when using the original sensitivity matrix or the score matrix. However, since the score matrix has fewer columns than the full sensitivity matrix, the use of designs based on the score matrix enables more efficient designs when few experiments are required. The number of components in the PCA model also gives the rank of the parameter space induced by the candidate experiments. This is useful information when fitting many parameters in a microkinetic model and provides an assessment of the value of every candidate experiment before it is even performed.     

Sustainable process design by the process to planet framework

Sustainable process design (SPD) problems combine a process design problem with life cycle assessment (LCA) to optimize process economics and life cycle environmental impacts. While SPD makes use of recent advances in process systems engineering and optimization, its use of LCA has stagnated. Currently, only process LCA is utilized in SPD, resulting in designs based on incomplete and potentially inaccurate life cycle information. To address these shortcomings, the multiscale process to planet (P2P) modeling framework is applied to formulate and solve the SPD problem. The P2P framework offers a more comprehensive analysis boundary than conventional SPD and greater modeling detail than advanced LCA methodologies. Benefits of applying this framework to SPD are demonstrated with an ethanol process design case study. Results show that current methods shift emissions outside the analysis boundary, while applying the P2P modeling framework results in environmentally superior process designs. Future extensions of the P2P framework are discussed. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3320–3331, 2015     

Control structure comparison for three-product Petlyuk column

The focus of this paper is to investigate different control structures (single-loop PI control) for a dividing wall (Petlyuk) column for separating ethanol, n-propanol and n-butanol. Four control structures are studied. All the results are simulations based on Aspen Plus. Control structure 1 (CS1) is stabilizing control structure with only temperature controllers. CS2, CS3 and CS4, containing also composition controllers, are introduced to reduce the steady state composition deviations. CS2 adds a distillate composition controller (CCDB) on top of CS1. CS3 is much more complicated with three temperature-composition cascade controllers and in addition a selector to the reboiler duty to control the maximum controller output of light impurity composition control in side stream and bottom impurity control in the prefractionator. CS4 adds another high selector to control the light impurity in the sidestream. Surprisingly, when considering the dynamic and even steady state performance of the proposed control structures, CS1 proves to be the best control structure to handle feed disturbances inserted into the three-product Petlyuk column.