In this work, we compare the behavior of a stand-alone reactor with that of a coupled nonlinear reactor-separator system. The coupling between the two units arises because of the recycle of the reactant-rich stream from the downstream separator. The reaction considered is an elementary autocatalytic reaction of the form A + 2B → 3B. The reactor is assumed to be isothermal. Three different modes of operation of the coupled system corresponding to three different control strategies are investigated. The nonlinear system behavior is analyzed for these cases using singularity theory and the D-partition method. We obtain the preferred control strategy as that in which the reactor effluent flow rate and the fresh feed flow rate are flow-controlled and the reactor holdup is allowed to vary.

Subramaniam Pushpavanam

Department Of Chemical Engineering

Ajit Arjun Sagale

catalysis

Flow of fluids

Separation

ISOTHERMAL REACTIONS

Chemical reactors

catalyst

CONTROL METHOD

Coupling

EFFLUENT TRANSPORT

flow velocity

reactor

SEPARATOR

article

flow rate

mathematical analysis

methodology

reaction analysis

steady state

Theory

Thermal analysis

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IIT Madras

Industrial and Engineering Chemistry Research

In this work, the behavior of a couple'd nonlinear reactor-separator system is analyzed. The reactor is modeled as a continuous stirred tank reactor (CSTR) that sustains a first-order reaction of the form A→B. The separator is modeled as a flash. The effluent from the reactor is fed to the separator. Here, we assume that the liquid stream from the separator is recycled back to the reactor. The primary interest is to investigate the system when the vapor-liquid equilibrium (VLE) has an azeotrope. Under these conditions, the recycle stream can be either reactant-rich or reactant-lean depending on the feed composition to the flash. The focus is on pure mass recycle, as the two units are assumed to be decoupled energetically via heat exchangers. Two different modes of operations are considered. In the first mode of operation, the fresh feed flow rate is fixed. In the second mode of operation, the effluent flow from the reactor is fixed. In practice, these different modes of operation can be achieved using a suitable control strategy. When fresh feed to the network is flow-controlled, there is a region of dynamic instability in the solution branch corresponding to the recycle of the reactantlean stream. In addition, coexistence of states with the recycle of reactant-rich and reactant-lean streams is possible. Our analysis indicates the presence of large regions of static and dynamic instabilities when the reactant-lean stream is recycled to the reactor and the effluent from the reactor is flow-controlled. Our results imply that the region in which the separator operates is very important in determining the behavior of the coupled system. Consequently, startup of the system is critical when the VLE of the system has an azeotrope. © 2006 American Chemical Society.

Nonlinear behavior of reactor - Separator systems with azeotropic mixtures

Previous studies on reactor-separator networks have focused largely on first- and second-order kinetics. A study of the effect of autocatalytic kinetics has been restricted to cases where the separator is a single-stage process. In this work, the effect of autocatalytic kinetics on the design and operation of a reactor-separator system, where the separator is a multistage distillation column, is investigated. Apart from steady-state design and stability, dynamic aspects such as response to disturbances and set-point changes are studied in detail. It was found that when the reactor holdup is maintained constant, for stable operation, the exit reactor concentration (mole fraction of the reactant A) has to be <1/2 for quadratic autocatalysis and <1/3 for cubic autocatalysis. Further, in the stable regime, the ability of the system to absorb changes in the inlet flow rate or the composition decreases as the domain of instability is approached. Regarding dynamics, we observe that the response of the system becomes slower with an increase in the base steady-state value of the exit reactor composition in the stable regime. This is correlated with the appropriate time constant, which shows a qualitatively different behavior for autocatalytic kinetics as compared to first-order or second-order kinetics. Snowball effects, which refer to sharp changes in the distillate rate due to small changes in the inlet flow rate, were also found to be dominant. For the case of variable reactor holdup, the range of stable operation is increased for quadratic autocatalysis. In general, settling times are larger when compared to the case of constant holdup, but the percentage deviations from steady-state values due to changes in the inlet flow rate and the composition are smaller. Further, snowball effects are negligible. © 2005 American Chemical Society.

Effect of autocatalytic kinetics on the steady state and dynamics of a reactor-separator system

In this paper we analyze the behavior of a coupled reactor-separator system with reactant recycle. The reactor is represented by a continuously stirred tank reactor, and the separator is represented by a flash unit. The reactor is operated isothermally and sustains a first-order reaction A → B. The individual units always possess a unique, stable, and feasible steady state. Surprisingly, even for the simple model system considered here, more complex patterns of behavior-involving infeasibility, multiple steady states, and limit cycles-can be observed when the recycle is closed. It is shown that the behavior crucially depends on the flow and the flash control strategy. Stability criteria are derived for different flow and flash control strategies. They depend on the operating conditions and on the basic physicochemical properties of the mixture. Potential sources for instability are systematically identified and illustrated.

Nonlinear behavior of reactor-separator networks: Influence of separator control structure

In this work we investigate the behavior of a coupled reactor - separator system. The reactor considered is a continuous stirred tank reactor, and the coupling between the two units occurs via recycle of the reactant-rich stream (assumed to be the bottoms) from the downstream flash to the upstream reactor. We investigate two reactions: (i) an irreversible isothermal first-order reaction A → B and (ii) an elementary cubic autocatalytic isothermal reaction of the form A + 2B → 3B. The separator is modeled as a flash unit operating isothermally and isobarically. The feed to the flash is restricted to be a binary mixture. This uniquely fixes the composition of the two effluent streams, leaving the flash unit. The main focus in this work is to study the effect of delay arising from the transportation lag from the reactor to the separator. The behavior of the system is governed by delay differential equations. We want to study how the dynamic instability induced by delay crucially depends on the flow control strategy of the coupled reactor - separator system. It is shown that delay does not induce any instability if the fresh feed flow rate F0 is flow-controlled and the molar holdup is controlled using the reactor effluent flow rate F. In the second control strategy where the reactor effluent flow rate F is flow-controlled and the holdup is controlled using fresh feed flow rate F0, delay above a threshold value can induce instability for the two reaction systems considered. In the third control strategy, both F and F0 are flow-controlled and the holdup in the reactor is allowed to vary. Here again delay beyond a critical value can introduce a new region of instability for the isothermal first-order reaction, and it increases the region of dynamic instability for the cubic autocatalytic reaction. The main result of the paper is to demonstrate that some of the preferred control structures of Luyben that are stable can be rendered unstable by sufficiently large delay.

Effect of delay on the stability of a coupled reactor-separator system

Who's Who

Levin, David Saul, (born 28 Jan. 1962), Chief Executive, McGraw-Hill Education, New York, since 2014

Chemical Engineering Science

Nonlinear behavior of an ideal reactor separator network with mass recycle

Industrial & Engineering Chemistry Research

Limit Cycles in Homogeneous Azeotropic Distillation

Experimental Study of Multiple Steady States in Homogeneous Azeotropic Distillation

Journal of Process Control

Dynamic behaviour of integrated plants

A comparison of control strategies for a nonlinear reactor-separator network sustaining an autocatalytic isothermal reaction

Journal	Data powered by TypesetIndustrial and Engineering Chemistry Research
Publisher	Data powered by TypesetAmerican Chemical Society
ISSN	08885885
Open Access	No