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ID: 124784.0, MPI für Dynamik komplexer technischer Systeme / Process Synthesis and Process Dynamics
Nonlinear behavior of reactor separator networks : influence of separator control structure
Authors:Zeyer, K.-P.; Pushpavanam, S.; Kienle, A.
Date of Publication (YYYY-MM-DD):2003
Title of Proceedings:4th European Congress of Chemical Engineering; Vol. 8, Topic 9
Sequence Number:0-9.1-002
Name of Conference/Meeting:4th European Congress of Chemical Engineering
Place of Conference/Meeting:Granada, Spain
(Start) Date of Conference/Meeting
End Date of Conference/Meeting 
Review Status:not specified
Audience:Experts Only
Intended Educational Use:No
Abstract / Description:Chemical process plants are primarily made up of two basic units, i.e. reactors and separators. In a typical plant fresh reactants are partially converted to products in a reactor and the mixture of reactants and products is then transferred to a separator unit for purification. For economic reasons unreacted material or expensive catalysts must be recycled back to the reactor unit. We investigate the dynamic behavior of a coupled reactor separator system. The upstream reactor is modeled as a continuous flow stirred tank reactor (CSTR) and the downstream separator as a binary one-step distillation unit (flash). The outflow of the CSTR is used as the input of the flash unit. Coupling arises due to the recycle of the liquid stream (the bottoms) of the separator to the reactor. The CSTR is operated under isothermal and ideally mixed conditions. A simple first order reaction A to B is studied. The flash is nonreactive and the mixture of A and B is assumed to be homogeneous and ideal. In particular, three different types of isobaric flash control strategies are considered: a) fixed flash temperature, b) fixed flash heating rate (including an adiabatic flash), and c) fixed flash vapor flow rate. For each of these flash operation modes we discuss three different flow control strategies for the CSTR: a) fresh feed flow controlled, b) reactor effluent flow controlled, and c) both of these flow controlled (variable CSTR holdup). We can show that the individual units always possess unique, stable and feasible steady states. Surprisingly, even for this very simple model system, more complex dynamics involving infeasibility, multiple steady states and stable oscillations can be observed in many cases as soon as the recycle is closed. It is shown that the behavior crucially depends on the flow control as well as on the flash control strategy. Stability criteria are derived and instabilities are systematically identified to predict the conditions for possible pitfalls a practicing engineer may encounter in operation of this type of reactor separator system, which is a basic unit in many industrial plants.
External Publication Status:published
Document Type:Conference-Paper
Communicated by:Achim Kienle
Affiliations:MPI für Dynamik komplexer technischer Systeme/Process Synthesis and Process Dynamics
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