Composition Control Of Distillation Column Research Articles

Selection of Controlled Variables: Maximum Gain Rule and Combination of Measurements

The appropriate selection of controlled variables is one of the most important tasks in plantwide control. In this paper, we consider the selection of secondary temperature measurements for indirect composition control of distillation columns. The maximum gain rule (maximize minimum singular value of scaled gain matrix) is compared to the exact local method, and it is found that the gain rule should be used with care for ill-conditioned plants like distillation columns. We also consider the use of optimal combinations of measurements to further reduce the effect of disturbances and implementation errors.

Evaluation of control configurations for a depropaniser

Distillation columns are highly coupled and non-linear, and have major impact on the utilities consumption and product quality. By proper design of their controls, energy consumption, product variability, operator intervention and equipment downtime can be reduced. Thus, selection of proper controls for distillation columns is both challenging and critical. Although extensive literature have been published on various aspects of distillation control, viz., level controller tuning, ratioing manipulated variables and turndown operation, there is no comprehensive study on control evaluation considering all aspects and rigorous simulation. In particular, turndown operation has received little attention in control research. This work deals with the composition control of distillation columns considering depropaniser as an example and using rigorous simulation. Effect of level controller tuning, ratioing the manipulated variable and turndown operation on the performance of several control structures to reject step disturbances in feed flow rate and composition, and sinusoidal disturbance in feed composition, is studied. Both single- and dual-ended composition control of the depropaniser are considered. Results of this study show the need and importance of a comprehensive and rigorous analysis including column operation far away from the design conditions, for optimal design of column control.

Composition control of distillation columns with a sidestream by using gas chromatographs

Distillation columns with liquid or vapor sidestreams are utilized to accomplish a separation in one column which would otherwise require two or more columns and which will usually require higher utility usage. While sidestream columns offer the economic incentives of reduced capital and operating costs, they also pose a more challenging control problem than ordinary distillation columns. Columns with a liquid or vapor sidestream drawoff have an additional degree of freedom and, as a result, have an additional dimension of coupling. Therefore, these columns are, in general, more difficult to control than standard distillation columns. This paper describes a concept for composition control of all product streams of a distillation column with a liquid sidestream. All product compositions are determined by on-line gas chromatographs and are characterized by high dead times. The resulting problem of sampled-data feedback control was investigated by using a dynamic simulator. Plant data, both steady-state and dynamic were used to validate steady-state and dynamic model of the column. The developed control concept and the diagnostic system for the plant operators were implemented in the DCS. Both elements show good performance and ensure product quality in operation.

A nonlinear profile observer using tray temperatures for high-purity binary distillation column control

A new, concise nonlinear profile position observer using tray temperatures instead of tray compositions is proposed. Composition measurement has been one of the major difficulties associated with the composition control of distillation columns because the on-line analyzers still suffer from large measurement delays, high investment/maintenance costs and low reliability. One of common alternatives to the analyzers is to use the secondary measurement such as tray temperatures which is able to infer the tray compositions. The performance of the nonlinear control scheme which consists of this nonlinear profile observer and the model-based controller for profile position control of high-purity distillation column proposed by Han and Park (1993, A.I.Ch.E. Journal, 39(5), 787–796) has been tested and compared with that of Han (1993, Control of high-purity distillation columns using a nonlinear wave theory, Ph.D. thesis, KAIST) in simulation of high-purity distillation column. The results of this new control scheme is very similar to Han and Park (1993). The control scheme is insensitive to the noise of the temperature measurements and so simple that it can be directly applied to the real high-purity distillation columns.

A comparative experimental study of the performance of non-linear model predictive controllers for dual composition control of distillation columns

Conventional PID controllers as well as linear model predictive controllers often do not perform satisfactorily for dual composition control of distillation columns because of severe loop interactions and a high degree of nonlinearity. Attention has recently been diverted to the development of non-linear controllers for such difficult-to-control systems. Only limited experimental applications have been reported in the literature. The performance of the non-linear analytical model predictive control (NAMPC) has been compared with that of the generic model control (GMC), for the control of a pilot-scale distillation column for both servo and regulatory problems. Two single-loop GMC controllers with feedforward were found to perform satisfactorily when subjected to small servo and load disturbances. Multivariable GMC as well as NAMPC were successful in tracking setpoint changes as well as eliminating load disturbances. Both algorithms were robust and stable even in the presence of large disturbances.

Analysis and Design of CASCADE Control Schemes for Distillation

Different issues related to the introduction of a cascade loop in dual composition control of distillation columns are addressed. In particular it is pointed out how static and dynamic interactions can be reduced by the introduction of the cascade and a measure of this reduction is defined. Two other disturbance-specific parameters are defined in order to address the choice of the loop to which cascade is added and of the pilot tray. Applications to different columns show that the use of the defined parameters can guide advantageously in the analysis and design of cascade control for distillation.

Online Optimization and Dual Composition Control of a Distillation Column Using a Nonlinear Controller

The nonlinear model predictive controller (NAMPC) reported earlier has been extensively tested on a pilot scale distillation column and its performance compared with other controllers such as generic model control (GMC) and proportional-integral (PI). The algorithm has also been extended for dual composition control of the distillation column. With the use of a transport phenomena based analytical model at the supervisory level, the disturbances in the secondary variables generate an automatic feedforward signal which significantly reduces their destabilizing effects. A comparison of NAMPC with GMC for a single input- single output system showed that while the former is somewhat slow because of gradual change in manipulated variable, the latter takes more drastic control action which may result in undershoots and overshoots. Dual composition control of distillation column using NAMPC was stable and smooth for both regulatory and servo problems for which PI was found to be oscillatory and unacceptably poor

Remarks on Some Applications of Nonlinear Control Techniques to Chemical Processes

Abstract This paper is a collection of heterogeneous remarks on some typical nonlinear control problems of chemical plants. The time-scale control method used in [20] for dual composition control of distillation columns constitutes a simple way to avoid singularity of the decoupling matrix. For several continuous stirred tank reactors (CSTR) [17, 2], the linearizing outputs [13] corresponding to dynamic or static feedback linearization have a clear physical interpretation. The stability of the zero dynamics, associated with temperature control of CSTR [19] via input-output linearization techniques, is investigated: new necessary conditions, ensuring the global asymptotic stability, are given; they are directly expressed on the reaction scheme and kinetics. Saddle-node bifurcation control [7] of an exothermic CSTR is recalled. These remarks illustrate the crucial role played, for applications, by the qualitative analysis of the process dynamics.