Bottom Product Compositions Research Articles

Inferential Composition Control of a Distillation Column Using Active Disturbance Rejection Control with Soft Sensors

This paper presents the integration of active disturbance rejection control (ADRC) with soft sensors for enhancing the composition control performance in a distillation column. Static and dynamic soft sensors are developed to estimate the top and bottom product compositions using multiple tray temperatures. In order to cope with the collinearity issues in tray temperature measurements, static and dynamic principal component regression is used in developing the soft sensors. The soft sensor outputs are introduced as the feedback signals to ADRC. This control scheme is termed as "inferential ADRC control". Static control offsets are eliminated through mean updating in the soft-sensor models. The effectiveness of the proposed control scheme is demonstrated on a benchmark simulated methanol-water distillation column.

Controllability Evaluation for Reactive Distillation Columns with Multiple Reactive Sections Disproportionating Trichlorosilane to Silane

In our recent work, the arrangement of multiple reactive sections is found to greatly enhance the steady-state performance of reactive distillation columns disproportionating trichlorosilane to silane. In the current research, process dynamics and controllability are studied through in-depth comparative analysis of open-loop and closed-loop behaviors between three reactive distillation columns with, respectively, a single reactive section (RDC-SRS), double reactive sections (RDC-DRS), and triple reactive sections (RDC-TRS). In the regulation path, although the arrangement of double and triple reactive sections reduces slightly the process gains between the bottom product composition and reboiler heat duty in the RDC-DRS and RDC-TRS, it helps to alleviate the coupling between the bottom product composition and reflux flow rate, thereby gaining increases in process controllability as compared with the RDC-SRS. In the disturbance path, the arrangement of double and triple reactive sections results in no inte...

Inferential Active Disturbance Rejection Control of a Distillation Column

Distillation columns are the major energy consumers in petrochemical and chemical industry and their efficient operation is essential for energy saving and product quality enhancement. This paper presents an inferential active disturbance rejection control (ADRC) method for product composition control in distillation columns. The proposed control strategy integrates ADRC with inferential feedback control. Tray temperatures are used to estimate the top and bottom product compositions which are difficult to measure on-line without time delay. In order to overcome the colinearity in the tray temperature data, principal component regression (PCR) is used to build the soft sensors, which are then integrated with ADRC. In order to overcome static control offsets caused by the discrepancy between soft sensor estimations and the true compositions, intermittent mean updating is used to correct PCR model predictions. The proposed technique is applied to a simulated methanol-water separation column.

Neural-network-based composite disturbance rejection control for a distillation column

Binary distillation columns are essentially multi-variable systems with couplings, non-minimum phase characteristics, model mismatches and various external disturbances. To get the desired top (distillate) and bottom product composition, a composite disturbance rejection control strategy using a radial basis function network (RBFN) is proposed in this paper. The composite controller includes neural network inverse controller (NNIC) and neural network disturbance observer (NNDOB) both using the inverse model of system which is identified by the RBFN. The stability of the identified inverse model is proved, and a rigorous analysis is also given to show why the NNDOB can effectively suppress the disturbances. Performances of the proposed scheme are compared with PID and NNIC without disturbance compensation in three cases by simulation studies. The simulations demonstrate the feasibility, effectiveness and disturbance rejection property of the proposed method in controlling the product composition of the binary distillation columns.

Dual Composition Control in Continuous, Middle-Vessel Distillation Columns, with a Draw Stream in the Middle Vessel

The dual composition control problem for a class of continuous middle-vessel distillation, which accepts a draw stream in the middle vessel, is studied in this work. This distillation configuration is advantageous when the separation goal is to recover high-purity light and heavy components from a multicomponent mixture with sufficiently large separation between relative volatilities. By using an estimator of the input–output modeling error, a first-order output-feedback compensator is designed which is shown to be equivalent to a multi-input multi-output proportional–integral (MIMO PI) controller. Tuning issues in terms of two time constants are given, and the ability of the controller to regulate the distillate and bottom product compositions in the face of sampled/delayed measurements is shown via numerical simulations.

Design and Control of a Complete Azeotropic Distillation System Incorporating Stripping Columns for Isopropyl Alcohol Dehydration

As demonstrated by Pham and Doherty [Chem. Eng. Sci.1990, 45, 1823] in an ethanol dehydration process and by Arifin and Chien [Ind. Eng. Chem. Res.2007, 46, 2535] in an isopropyl alcohol dehydration process, a three-column heterogeneous distillation sequence which contains a preconcentrator column, an azeotropic column, and an entrainer recovery column is more energy saving than two-column and four-column sequences. The three-column sequence for the isopropyl alcohol (IPA) dehydration process, according to Arifin and Chien, is more energy-saving than the two-column sequence, but the capital cost of the entire sequence is more expensive than that of the two-column sequence. Because the reflux ratio used either in the preconcentration column or the recovery column of the three-column IPA dehydration system is quite small, these two conventional distillation columns are both replaced by a stripping column, and in this study a new separation scheme is thereby developed. The economic analysis of the proposed system shows there is more saving in energy and capital costs than the two- and three-column sequences aforementioned. Further, in developing a plant-wide control structure, a tray temperature control loop is implemented in each of the three columns which regulates reboiler duty in order to maintain the bottom product compositions for the new scheme, and ratio control of the organic reflux flow to the feed flow rate of the azeotropic column is used to reject feed rate disturbance. The purities of water achieved in the preconcentrator and entrainer recovery columns are greater than 99.88 mol %, and the IPA purity in the product stream is greater than 99.99985 mol %. Closed-loop responses to ±20% changes in fresh feed rate and feed H2O composition show that the proposed strategy has good control performance.

Column configurations of continuous heterogeneous extractive distillation

AbstractIncreasing regulations and constraints have renewed interest for more efficient distillation process in separating azeotropic mixtures and close boiling components. We investigate the feasibility of heterogeneous extractive distillation process in a continuous column considering several feed point strategies for the entrainer recycle stream and for the main azeotropic feed. Depending on these choices, the heterogeneous distillation column is composed of one, two, or three column sections. A differential mass balance model enables to compute continuous liquid composition profiles of the rectifying, extractive, and stripping sections. Unlike homogeneous extractive distillation, reflux policy composed by a single or both decanted liquid phases is considered as well as the external feeding influence on the composition of the top column liquid stream. Limiting operating conditions of key parameters like the entrainer/feed flowrate ratio and reflux ratio required to obtain a target top and bottom product compositions are obtained. For illustration, separation of acetonitrile–water mixture using butyl acetate as a heavy heterogeneous entrainer is selected. In this case, withdrawal of a saddle binary heteroazeotrope is the main difference of this process compared with the well‐known heterogeneous azeotropic distillation process where the top vapor product is the lower boiling point of the ternary system. © 2007 American Institute of Chemical Engineers AIChE J, 2007

Offset‐Free Inferential Feedback Control of Distillation Compositions Based on PCR and PLS Models

AbstractAn offset‐free inferential feedback control strategy for distillation composition control using principal component regression (PCR) and partial least squares (PLS) models is presented in this paper. PCR and PLS model based software sensors are developed from process operational data so that the top and bottom product compositions can be estimated from multiple tray temperature measurements. The PCR and PLS software sensors are then used in the feedback control of the top and bottom product compositions. With this strategy the problem of substantial time delay in composition analyzer based control and of substantial bias in single tray temperature control can be overcome. A practically very important issue in software sensor based feedback control is that static control offsets often exist due to a static estimation bias, especially when the process operating condition changes. A technique for eliminating the static estimation bias and the resulting static control offsets through mean updating of process measurements is proposed in this paper. Applications to a simulated methanol‐water separation column demonstrate the effectiveness of this control strategy.

Experimental study of feasibility in kinetically‐controlled reactive distillation

AbstractBifurcation studies predict limited ranges of feasibility for products in certain reactive distillations. These are closely related to the bifurcations in the singular points of dynamic models for simple reactive distillation (isobaric open evaporation with liquid phase reaction). A new dynamic model is described with constant vapor rate together with an experimental study for the reactive distillation of acetic acid with isopropanol to produce isopropyl acetate, catalyzed by Amberlyst‐15 ion‐exchange resin. An experimental apparatus with real‐time measurement of liquid compositions based on Fourier transform infrared (FTIR) spectroscopy is described, and used to follow the composition dynamics at several initial conditions and Damköhler numbers (Da). The experimental results match model predictions that show four regions of behavior. For Da ≈ 1, these show a stable node at acetic acid and several other fixed points as saddles. However, near Da ≈ 2, both isopropanol and acetic acid are stable nodes and a quaternary singular point appears. The presence of two stable nodes requires the presence of a distillation boundary and, therefore, a limited feasibility for the bottom product compositions from continuous reactive distillation. For the reaction rates studied, the model predictions are closely consistent with the experimental findings, and are robust to variations in the vapor rate. These experiments are among the first to analyze the dynamics and feasibility in a kinetically‐controlled reactive distillation and are consistent with previous studies for the reaction equilibrium limit, indicating the formation of a reactive azeotrope. © 2005 American Institute of Chemical Engineers AIChE J, 51: 464–479, 2005

Singular points of reactive distillation systems

AbstractFor the conceptual design of countercurrently operated reactive distillation columns, fast methods are needed to estimate potential top and bottom products. The possible column bottom product composition can be determined from the stable singular points of a batch reactive reboiler. In a similar manner the top product composition can be obtained from the stable singular points of a batch reactive condenser. Geometrically, the singular points of both batch processes are located on a common potential singular point surface (PSPS) whose trajectory depends on the reaction stoichiometry and the phase equilibria. At the singular points, the PSPS intersects a reaction kinetic surface that is dependent on the reaction rate expression and the phase equilibrium of a reference component. Based on the singularity analysis, a single‐feed reactive distillation column can be designed. Several hypothetical and real reaction systems are analyzed to illustrate the singularity analysis and the design procedure. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2866–2876, 2004

A graphical method for designing reactive distillation columns. II. The McCabe-Thiele method

This paper shows how to construct a McCabe–Thiele diagram for binary reactive distillation. We consider two types of reaction that are consistent with binary mixtures: isomerization (R1 ⇔ P1) and decomposition (R1 ⇔ 2P1). We can account for reaction by tracking only two features of the diagram while constructing it. First, the intersection point of the operating line with the y = x line can be called the reactive cascade difference point, and it moves in a manner directly determined by the ratio of the molar extent of reaction (or reaction turnover flowrate) to the product flowrate. Second, the heat supplied (removed) by reaction decreases (increases) the slope of the operating line while not changing the location of the reactive cascade difference point. By tracking these two features, one can quickly sketch such a diagram. We also show how to construct a McCabe–Thiele diagram if the reaction zone is only at the feed stage. Finally, we discuss a case in which the reactive cascade difference point moves past the feed composition and the bottom product composition is less pure than the feed composition.

The product composition regions of single-feed azeotropic distillation columns

We show how the limiting operating conditions, total reflux and thermodynamically optimum operation, can be used to determine the feasibility of a desired separation by continous distillation. To assess feasibility, so-called pinch point trajectories are established as the limits of separations achievable in each column section. These trajectories may be determined graphically using residue curve maps. The feasibility criterion is generalized to result in a method to establish the ranges of top and bottom product compositions achievable by a single-feed distillation column for a given ternary feed. One particularly interesting application is to reveal where and to what extent distillation boundaries for azeotropic mixtures, derived for total reflux, can be surpassed in columns operated at finite reflux ratios. We present a criterion to estimate the maximum reflux for such separations and illustrate process schemes that exploit the possibility of crossing of total reflux boundaries to separate azeotrope-forming mixtures. Finally, we demonstrate how intermediate heat exchangers can be used to improve separations of azeotropic mixtures across total reflux boundaries

On‐line multivariable adaptive control of a binary distillation column

AbstractAn experimental evaluation of the simultaneous control of top and bottom product compositions of a binary distillation column utilizing multi‐variable self‐tuning control algorithms is presented. The study was carried out using an 8‐tray, 22.86 cm diameter methanol–water pilot scale column with continuous capacitance analysis of top product composition and bottom composition analyzed on a 3 minute cycle, by a gas chromatograph. Column control is studied for ±25% step disturbances in feed flow rate. Terminal composition regulation using both ‘positional’ and ‘incremental’ forms of a generalized minimum variance self‐tuning control algorithm is compared with that achieved using a conventional digital PI/PID multiloop control strategy.

Multivariable Self-tuning Control of a Binary Distillation Column

Control of distillation columns remains a key concern of industry because of the high energy consumption associated with distillation separations. The reported results, from a continuing program of experimental studies directed to improved distillation column control, are concerned with the evaluation of multivariable self-tuning control algorithms for the simultaneous control of both top and bottom product compositions. Evaluation of the performance of multivariable self-tuning control algorithms has been performed by experimental testing using a 8-tray, 30.5 cm diameter methanol-water pilot scale column. The column is controlled using a DLC LSI 11/03 microcomputer with continuous capacitance analysis of top product composition with bottom composition analyzed on a 3 min cycle, by gas chromatograph. Column control is studied for 25% step increases and decreases in feed flow rate. The performance of the multivariable algorithms is compared with that achieved using conventional digital PI/PH) control algorithms. The results have demonstrated that use of self-tuning control will provide better control performance for feed flow rate disturbances than can be achieved using well-tuned conventional multiloop PI/PTD control.

Control of a distillation column using self‐tuning regulators

AbstractSelf‐tuning regulators were applied to the multivariable control of a pilot distillation unit. Both the top and the bottom product compositions were simultaneously controlled. The multivariable interactions were compensated for by different strategies. It was shown that a proper choice of the control variables made it possible to use a regulator with a smaller number of parameters. The evaluation of the results shows that the self‐tuning regulator is a valuable tool in multivariable control of distillation columns.

A1.3 : Application of Self-Tuning Regulators to the Control of Distillation Columns

Self-tuning regulators were applied to the control of distillation columns. Both singlevariable and multivariable control of the bottom product and top product compositions were studied. Also control systams containing feedforward action from measured disturbances in the feed stream were evaluated. The performances of the different control systems were evaluated both by the way of simulation on a digital computer and by real time control experiments on an eleven tray pilot distillation unit.Multivariable self-tuning regulators showed satisfactory performances in cases of simultaneous control of the two product compositions. It was found that only the most important couplings in the column had to be accounted for in the regulator structure. Also a proper pairing of the control variables made it possible to control the multivariable distillation process using a pair of isolated single-input, single-output self-tuning regulators.