Start-up of Empty Cold Reactive Distillation Columns by means of Feedback Control Strategies: the Discontinuous Stage
Systematic methods and tools for managing the complexity
Process Operation, Monitoring & Analysis (T4-2)
Keywords: Reactive distillation, Start-up, Discontinuous stage, Feedback control, Esterification
Title: Start-up of Empty Cold Reactive Distillation Columns by means of Feedback Control Strategies: the Discontinuous Stage
Authors: Steffen Sommer (corresponding author)*, Joerg Raisch**, Kai Sundmacher**
*Otto-von-Guericke Universitaet Magdeburg, IFAT, Uni-Platz 2, 39106 Magdeburg, Germany
email: steffen.sommer@e-technik.uni-magdeburg.de
**Max Planck Institute
Dynamics of Complex Technical Systems
Sandtorstr. 1, 39106 Magdeburg, Germany
email: {raisch, sundmacher}@mpi-magdeburg.mpg.de
Keywords: Reactive distillation, Start-up, Discontinuous stage, Feedback control, Esterification
Reactive distillation (RD) processes integrate chemical reactions and distillation in the same process unit. This has several positive implications, for instance reduction of investment and operational costs and potential improvement of performance. However, the dynamic behaviour of reactive distillation columns (RDC) is quite complex, especially during the start-up stage.
The start-up process of RDC is of environmental and commercial importance, and represents an active field of research. For RD operation, start-up should be finished as fast as possible since all process variables are changing during this stage, and the product has to be disposed. So far, there are only few publications addressing startup of empty cold RDC.
The motivation of this contribution is to solve the problem of the start-up process of the cold empty column by means of feedback control strategies. Thus, an automatic start-up strategy is obtained. The use of feedback implies several potential advantages when compared to the existing open loop strategies: for example the start-up process can be made insensitive against disturbances, operating safety can be improved, and a desired steady state can be reached exactly. Furthermore, automatic start-up from the cold empty column, which describes definite initial conditions, can prevent undesired steady states.
It is convenient to model different stages of the start-up procedure separately and to apply a different control strategy for each stage. The outcome is a switching scheme consisting of several control policies based on different models.
In this contribution, we investigate the first part of start-up. It is called the discontinuous stage and consists of two single stages. Stage I is the fill-up and heating process. After achieving boiling conditions, stage II starts and ends if vapor reaches the condenser. The discontinuous stage is relevant, because in most works on start-up of RDC the trays are filled and warm.
The control strategy is based on servo-control of the process variables temperature and liquid level of the reboiler with switching PI controllers including anti-reset-windup, and is combined with recycling partial bottom product back to the feed (bottom product recycling). The recycle can reduce the amount of waste during start-up.
A corresponding model is developed to describe the discontinuous stage of start-up. For each tray, one total mass balance, one energy balance, and nc-1 component mass balances are formulated (nc = number of components). The generated equations are a rather complex, nonlinear, and implicit differential algebraic equation (DAE) system.
Our start-up strategy has been tested for an example where ethyl acetate and water is produced from ethanol and acetic acid. Promising simulation results have been obtained. The approach with bottom product recycling is recommended for the presented discontinuous stage.
See the full pdf manuscript of the abstract.
Presented Tuesday 18, 08:45 to 09:05, in session Process Operation, Monitoring & Analysis (T4-2).
