Intelligent systems in process engineering : A review

Distillation column dynamics and control have been viewed by many as a very mature or even dead field. However, as is discussed in this paper significant new results have appeared over the last 5-10 years. These results include multiple steady states and instability in simple columns with ideal thermodynamics (which was believed to be impossible), the understanding of the difference between various control configurations and the systematic transformation between these, the feasibility of using the distillate-bottom structure, for control (which was believed to be impossible), the importance of flow dynamics for control studies, the fundamental problems in identifying models from open-loops responses, the use of simple regression estimators to estimate composition from temperatures, and an improved general understanding of the dynamic behavior of distillation columns which includes a better understanding of the fundamental difference between internal and external flow, simple formulas for estimating the dominant time constant, and a derivation of the linearizing effect of logarithmic transformations. These issues apply to all columns, even for ideal mixtures and simple columns with only two products. In addition, there have been significant advances for cases with complex thermodynamics and complex column configurations. These include the behavior and control of azeotropic distillation columns, and the possible complex dynamics of nonideal mixtures and of interlinked columns. However, both for the simple and more complex cases there are still a number of areas where further research is needed.

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Dynamics and control of distillation columns : a critical survey

This work aims at identifying the effects of the main operational spray-dryer variables on the milk powder quality. Experiments have been performed in a pilot spray-dryer following the full-factorial design technique to provide data and correlations that predict the whole powder properties as function of the main operational variables of the spray-dryer. The emulsion feed flow rate, the atomization rotation and the inlet air temperature have been chosen as the independent variables while the residual moisture content, the tapped bulk density, the cohesion force enhancement between particles as well as size distribution of agglomerate and its morphology are the response variables that quantify the powder quality. Correlations obtained are analyzed and incorporated into a mathematical model previously developed for simulating the spray-drying of whole milk emulsion.

Effect of Spray-Dryer Operating Variables on the Whole Milk Powder Quality

A second generation nonequilibrium model for computer simulation of multicomponent separation processes

This work presents a simulation study of the spray dryer operation using the whole milk suspension as the emulsion to be dried. Two approaches are used to obtain a general description of this operation. The first approach comprises a population balance model, in which drops and particles make up the discrete phase and are distributed into temporal compartments following their residence time in the dryer. Air is the continuous and well-mixed phase. Mass and energy balances are developed for each phase, taking into account their interactions. Constitutive model equations to describe the drop swelling and drying mechanisms as well as the heat and mass transfer between particles and hot air are also analyzed. The set of algebraic-differential equations obtained in this approach is solved by DASSL numerical code. The second approach involves a three-dimensional model solved by a computational fluid dynamic (CFD) code. The continuous air phase follows the time-averaged Navier-Stokes equations coupled w...

Spray Dryers: Modeling and Simulation

Experimental studies have been made with the help of laboratory-scale distillation columns of the startup and product switchover operations of distillation columns. The object of these studies was to minimize the time necessary from initial startup or product switchover for steady-state operations to be attained. It has been shown that the time needed can be clearly reduced by a specific mode of operation. Theoretical studies support the experimental findings. Dynamic simulation has proved to be a suitable aid for the preparation and planning of experimental studies. As a result, the time needed to reach steady state can be considerably reduced, and consequently the costs.

Experimental and theoretical studies of the dynamics of startup and product switchover operations of distillation columns

On presente l'etat actuel de la simulation dynamique et on decrit quelques applications typiques. On etudie les demandes pour la simulation dynamique et les simulateurs. On presente les limitations courantes d'un point de vue industriel

Dynamische Prozeßsimulation in der industriellen Praxis

A spray dryer simulation program is described. It is based on the known conservation laws and permits determination of temperature and moisture profiles and droplet trajectories inside spray dryers. An industrial spray dryer used in the manufacture of detergents serves as an example to demonstrate the applicability of the program; this approach required prior experimental determination of the flow field and an axial temperature profile. On the basis of a rough estimate of the drying rate in the spray dryer, good agreement was observed between experiment and simulation.

Estimation of the drying rate and moisture profiles in an industrial spray dryer by means of experimental investigations and a simulation study

In order to achieve optimal energy-saving, load-dependent operation of rectification columns, development and implementation of model-based control concepts are necessary. This paper reports on the design and step-by-step testing of a state observer for the estimation of two state variables which cannot be measured directly, namely the vapour rate and the location of mass transfer zone (temperature front) in the stripping section of a production column. The procedure is described, and the relevant simulation and operation results are presented and discussed. Initial experience, gained when the concept was put into practical operation, is reported.

Design and analysis of a state observer for the temperature front of a retification column

For the simulation of steady- and non-steady-state behaiour of separation columns, a simulation program was developed and employed for the calculation and optimization of methanol-water separation. The demand for increasingly higher purities and energy-saving optimization of plants results in complex column configurations and thus in complex control schemes. The number of possible control schemes was reduced on the basis of heuristic rules. By simulating the process, the optimum control and manipulative variables and measuring points were determined from the reduced number of control concepts. A control scheme was introduced. The model was verified by simulation of the dynamic behaviour of the process. The investigation included the effects of computing technique, time interval, model reduction and tray hold-up on the results. Thus, various effects and interactions were verified theoretically with respect to the robustness of the model. This was followed by a simulation of the non-ontrolled and controlled column. By implementing the controllers in the existing column program, the computer can predict the control parameters. These parameters were checked and corrected when the metahnol-ater column was put in operation. In the start-up phase, major problems were caused by the sensitivity and failure rate of density, throughput and pump measuring sensors. Feed impurities were another source of problems. Operation with high product purities is possible, with a side stream to discharge impurities.

Günter Wozny

Papers

Experimental and theoretical studies of the dynamics of startup and product switchover operations of distillation columns

Dynamische Prozeßsimulation in der industriellen Praxis

Estimation of the drying rate and moisture profiles in an industrial spray dryer by means of experimental investigations and a simulation study

Design and analysis of a state observer for the temperature front of a retification column

Dynamics of distillation with high product purities