An ethylene glycol column is studied in this paper, where the main objective is to describe the bifurcation diagram with respect to the reboiler boilup ratio. The bifurcation analysis reveals the existence of a unique equilibrium point at low reboiler heat inputs and three equilibrium points (two stable and one unstable) at high reboiler heat inputs. Moreover, the existence of input multiplicities at moderate and high product purity is also revealed.

The simple reactor/separator process with recycle has recently been used as a benchmark case to gain insights into the problem of plantwide control structure selection. This paper explores the use of a balanced control scheme aimed at improving the disturbance rejection capabilities of the controlled process. The idea is to change the operating conditions in both the reactor and the separator so that the composition control effort in the presence of production rate changes is distributed. To this end, a parallel control structure is proposed where the product composition is regulated by means of simultaneous feedback manipulations of the vapor boilup rate and the reactor temperature. In this way, the use of the reactor temperature as a secondary control input reduces the snowball effects reflected in large vapor boilup rate changes in response to relatively small production rate changes. Nonlinear simulations show that effective composition control can be obtained with moderate vapor boilup control efforts.

In this work, an output-feedback control for the regulation of distillate purity via manipulations of the reflux ratio in reactive batch distillation is designed. The approach is based on an approximate model of the composition dynamics and makes use of a reduced-order observer to estimate the modeling error. An input/output linearizing feedback is proposed where the estimated modeling error is included to achieve robust tracking of a composition reference. It is shown that the resulting controller has the structure of a proportional-integral derivative (PID) controller with antireset windup. The controller performance is tested using a simulation example including strong uncertainties in the reaction model. An interesting finding is that the required reflux ratio policy to reach asymptotically a constant reference resembles the reflux ratio policy obtained from posing an optimization technique (

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The aim of this paper is to propose a novel proportional-integral-derivative (PID) control configuration based on an observer structure. Batch distillation is used as the base case study where the regulated output is the distillate composition. The proposed PID control law is derived in the framework of robust nonlinear control with modeling error compensation techniques. A reduced-order observer is proposed to estimate both the derivative of the regulated output and the underlying modeling error. These observations are subsequently used in a control loop to feedback variations of distillate composition (derivative feedback) and to counteract the effects of modeling errors. It is shown that, under certain conditions, the resulting control law is equivalent to a classical PID controller with an antireset windup scheme. Moreover, the tuning of the controller is performed very easily in terms of a prescribed closed-loop time constant and an estimation time constant. Numerical results are provided for binary and multicomponent separations. Sampled/delayed measurements and several sources of uncertainties are considered in order to provide a realistic test scenario for the proposed control design procedure.

Steady-state input multiplicities in vapor−liquid flash separation processes are common when a product mole fraction is specified and places restrictions on the selection of controlled variables such that more than one set of conditions can produce the same result; if this happens, then potential problems during the process operation can arise. The equations that describe isobaric flash operation with a product composition specified are analyzed to propose a fast method for determining the existence of steady-state input multiplicities. The ability of the method is shown via numerical examples using homogeneous nonazeotropic multicomponent mixtures. The most attractive features of the proposed method are that it is easy to use, has a high reliability in determining the presence of steady-state input multiplicities, and requires only the separation factors of each component at the bubble and dew points at the pressure of the flash separation process.

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