Single-end Control Research Articles

Dynamics and control of thermal- versus electrical-driven pressure-swing distillation to separate a minimum-boiling azeotrope

The implementations of heat integration (HI, thermal-driven), vapor recompression (VRC, electrical-driven), and self-heat recuperation (SHR, electrical-driven) in pressure-swing distillation (PSD) are available for providing energy saving and CO2 emission reduction over the conventional design (CONV, thermal-driven), exemplified by a minimum-boiling tetrahydrofuran/water. Approximately a 20 % decrease in total annual cost (TAC) and over 25 % CO2 emission reduction can be reached by using these measures. However, these steady-state advantages sometimes go along with decreases in controllability, so it is necessary to investigate the dynamics and control of these highly integrated and interacting processes. Robust temperature control structures based on decentralized proportional-integral (PI) feedback controllers are developed to tackle ± 20% disturbances in throughput and feed composition. For the thermal-driven processes, the PSD-CONV is easily controlled with the single-end control strategy. Based on that, the effective control of PSD-HI is achieved by either pressure compensation or bypass strategy. Regarding the complicated electrical-driven arrangements, the dynamic responses of PSD-VRC and PSD-SHR indicate their controllability is comparable to those of a regular sequence. These results demonstrate that there are no conflicts between the steady-state advantages and dynamic controllability for this specific separation. Meanwhile, these electrical-driven processes can be considered to extend to other minimum-boiling azeotropes owing to their economic and environmental advantages.

Dynamic control analysis of partially heat-integrated pressure-swing distillation for separating a maximum-boiling azeotrope

Dynamic control behavior of partially heat-integrated pressure-swing distillation process is investigated by taking a separation of maximum-boiling ethylenediamine/water azeotrope as the example. The feasibility of using a single-end control strategy is affirmed by the method of feed composition sensitive analysis, and a series of novel control structures are developed to eliminate or attenuate the snowball effect, wherein the special control loops are that the flow in bottoms recycle line is controlled by regulating the set-point of a flow controller placing in recycle loop through two alternative ratio controllers including flow and thermal ratios. Both the robust and stable regulatory control are achieved with dual-product composition restored to their steady-state design specifications for handling large (20%) step changes in throughput and feed composition. The effectiveness of control structure with flow ratio option works better than that of another alternative taking the comprehensive consideration of product transient deviations, product offsets and settling time. Besides, there is no product offset for distillate ethylenediamine composition, although only the temperature control loops are used in the low-pressure column for control structure with thermal ratio option, which is the advantage of this control structure since the dual-end product composition control loops are required for that with thermal ratio. Of the particular importance is that the severer and stronger oscillatory effect of product composition is weakened by taking the proportional-only cascaded temperature controller for facing throughput perturbations.

Heat-integrated pressure-swing distillation process for separation of the maximum-boiling azeotrope diethylamine and methanol

Abstract The steady-state economics and dynamic controllability of heat-integrated pressure-swing distillation (PSD) processes are explored by taking the separation of maximum-boiling azeotrope diethylamine and methanol as demonstrating example. Compared to the conventional process, the B-E configuration (hot stream preheating the feed of high-pressure column) is attractive than others due to reducing 36.83% in energy consumption, 20.49% in CO2 emissions and 25.69% in total annual cost (TAC) and enhancing 49.48% in thermodynamic efficiency, respectively. Dynamic controllability of the economic efficient backward-integrated processes is also investigated by teaching the terminology of “simultaneous design”. The effectiveness of single-end control strategy is determined by the method of feed composition sensitive analysis, and a series of control structures are devised and assessed by the perturbations of feed flowrate and composition. The robust control scheme (CS5) with the ratio strategy to manipulate recycle stream flowrate is constructed to eliminate snowball effect and oscillation phenomena for partially case. It is also used to the combinational heat-integrated processes, and the differences of integral absolute error (IAE) are small between the alternative arrangements, demonstrating that the performance of single-end control scheme with the feedforward strategy of molar ratio reflux-to-feed (R/F) is robust and efficient.

Design and control of extractive distillation process for separation of the minimum-boiling azeotrope ethyl-acetate and ethanol

Design and control of extractive distillation process is explored by taking the separation of minimum-boiling azeotrope ethyl-acetate and ethanol as an example. The two evaluation indicators of second-law efficiency and CO2 emissions are employed to evaluate different alternatives, which consist of conventional case, F-E process (hot solvent stream to preheat fresh feed of extractive column), B1-E process (hot solvent stream to preheat feed of recovery column), and F-B1-E process (hot solvent stream to preheat feed of extractive and recovery column). The conventional case can reduce 32.23% in total annual cost (TAC), 28.81% in energy-saving comparing to economically optimum fully heat-integrated pressure-swing distillation process. Besides, it can further cut 4.38% in TAC and 9.79% in steam cost by heat integration B1-E configuration while others are 1.11% (1.30%) in TAC and 10.45% (12.11%) in steam cost, where, data in the brackets are for F-B1-E process. Furthermore, the control of extractive distillation process is explored since the interaction of parameters is complicated. For conventional process, the effectiveness of single-end control strategy is determined by the method of feed composition sensitive analysis. The appropriate control scheme (CS3) is screened by the indicator of integral absolute error (IAE) since its deviations of product purities to specifications are the least. A new control scheme of bypassing portion of hot stream around economizer with dual-point temperature control strategy is proposed for the efficient economics thermal integration alternative, and it can still achieve robust control performance at facing feed flowrate and composition disturbances.

Heat-integrated pressure-swing distillation process for separating the minimum-boiling azeotrope ethyl-acetate and ethanol

Abstract Design and control of the forward integration of pressure-swing distillation processes is explored with the aid of the tools of Aspen plus and Aspen dynamics taking the separation of the minimum-boiling azeotrope ethyl acetate and ethanol as an example. The optimized separation configuration is obtained by taking the minimization of total annual cost (TAC) as an objective function. The two key performance indicators of second law efficiency (η) and carbon dioxide emissions are employed to evaluate the three operations, which consist of conventional non-heat integration and the corresponding partially and fully heat integration processes. Compared to the non-heat integration process, the fully heat integration process is more attractive since it can achieve 33.33% energy consumption saving, reduction of 31.33% CO 2 emissions and 26.64% TAC. Furthermore, the control of the PSD processes with heat integration are explored since the interaction of parameters are complicated for this neat configuration. The effectiveness of the single-end control strategy is evaluated and determined by method of the feed composition sensitive analysis, and a series of control structures are devised and assessed by the perturbations of the feed flow rate and feed composition. It is concluded that the small disturbances are addressed for fully heat integration in comparison with the partially heat integration owing to the degree of freedom reduction. In addition, the efficient control strategy with the ratio scheme to manipulate the cascade flow controller is developed to eliminate the snowball effect for PSD with partially case.

Robust Design and Control of Extractive Distillation Processes under Feed Disturbances

A mixed-integer nonlinear programming optimization framework based on the steady-state model of the extractive distillation process for benzene/acetonitrile azeotropic mixture separation with dimethyl sulfoxide solvent is developed. Two single-end control structures are evaluated for the designed plant for the nominal feed composition. Because the adoption of inferential control strategies cannot guarantee the specification of the products, benzene feed composition is assumed to be a Gaussian variable. Then, the optimization problem is reformulated taking the set-points of the controllers together with the design parameters as decision variables which leads to a closed-loop stochastic optimization problem in which unscented transform is used as the uncertainty propagation tool. Investigation of the steady-state properties, as well as the dynamic responses of the optimal solution under feed composition disturbances, reveals that the use of the control scheme with fixed reflux-to-ratio for the extractive an...

Design and control of methyl acetate-methanol separation via heat-integrated pressure-swing distillation

Design and control of pressure-swing distillation (PSD) with different heat integration modes for the separation of methyl acetate/methanol azeotrope are explored using Aspen Plus and Aspen Dynamics. First, an optimum steady-state separation configuration conditions are obtained via taking the total annual cost (TAC) or total reboiler heat duty as the objective functions. The results show that about 27.68% and 25.40% saving in TAC can be achieved by the PSD with full and partial heat integration compared to PSD without heat integration. Second, temperature control tray locations are obtained according to the sensitivity criterion and singular value decomposition (SVD) analysis and the single-end control structure is effective based on the feed composition sensitivity analysis. Finally, the comparison of dynamic controllability is made among various control structures for PSD with partial and full heat integration. It is shown that both control structures of composition/temperature cascade and pressure-compensated temperature have a good dynamic response performance for PSD with heat integration facing feed flowrate and composition disturbances. However, PSD with full heat integration performs the poor controllability despite of a little bit of economy.

Use of Mass or Molar Reflux-to-Feed Ratios In Distillation Single-End Control Structures

Many distillation columns use single-end control structures in which one temperature (or composition) is controlled while the other control degree of freedom holds either the reflux ratio or the reflux-to-feed ratio constant. In most chemical processes, flow rates are determined from pressure drops over orifice plates. This means that the flow signal reflects the mass flow rate since density affects pressure drop. In computer simulations, flow rates can be studied in either molar or mass units. This paper considers the issue of using mass or molar flow ratios in single-end control structures. Results illustrate that using molar ratios can improve product quality in some systems. The practical question of how to estimate molar flow rates from plant measurements is addressed.

Performance Assessment of Different Control Structures for Generic Reactive Distillation Using Linear and Nonlinear Process Models

This paper compares the closed-loop performance of three control structures using an approximate linear model to that when a rigorous nonlinear process model is used for a generic reactive distillation. It is shown that an approximate linear model behaves essentially similarly to a nonlinear model in a closed-loop system when the deviation of process variables resulting from the disturbance is within the region of the base steady state. Responses based on the linear model for various control structures show a good agreement when compared to those based on the nonlinear model under the same conditions. It is found that the linear process model could be used to develop a robust control system when the control valves are conservatively designed. However, it becomes inadequate if the system is open-loop pseudostable or unstable. The performance of the linear model is shown to be better in a single-end control system than in a dual-end control system because of the nonlinearity effect, which is more pronounced in the latter.

Control Configuration Selection for the Inferential Control of a High Purity Distillation Column

The regulatory control structure to achieve the best product quality 1s established by utilizing the recent control configuration selection methods for the Inferential control of an industrial, multicomponent, high-purity ethylbenzene distillation column In the dual composition control, (L, V) configuration with (L-y D ;V-x B ) scheme 1s found as the best one among different schemes considering RI, RGA, JEC and RDG methods. However, in .Inferential control studies, SVD analysis revealed that high-purity end of the column cannot be utilized for temperature measurements. Thus, the possible use of single-end control is investigated within the context of Elgenstructure concept. It is found that for feed flow rate and composition disturbances column can be successfully controlled with a single-end structure. The unsteady-state simulation studies favored the [L-y D ;V=f(F)] scheme. However, for the Inferential control, [L=f(F);V-x B ] Is selected by taking into account the practical difficulties in measuring the high purity compostions. The results obtained for the existing industrial column has indicated that the control performance can be significantly improved by the proper choice of the size of the drums at an earlier stage of the design.

Analysis of a System of Mutually Synchronized Oscillators

An analysis of the performance of a synchronous digital communications system in which the clock rate of each station is established as the average of the clock rates of all incoming signals to each station is presented. Using both-end control, the system frequency is the average of the free-running frequencies of the phase locked oscillators, weighted by the number of inputs to each station. In single-end control, the frequency is a similar average plus a term, which is a function of changes in transmission path delay. Rules are derived defining how a transmision path delay change will be reflected in changes of total phase difference at each transmission path terminal. These phase changes reflect requirements for elastic storage of data at each transmission path termination. The dynamic stability of a simple two station network is analyzed mathematically. A subsequent technical paper [7] will report on the extensive analysis of the dynamic behavior of various system configurations using a computer simulation of the time history response to various perturbations.