Separation Recycle Systems Research Articles

Synthesis and Optimization of Reaction–Separation–Recycle Systems with Complex Distillation Sequences

In this work, a novel synthesis superstructure for reaction-separation-recycle systems with complex distillation sequences is proposed. In the superstructure, complete interconnections among reacto...

Design of Reactor–Separator–Recycle Systems Based on the Optimization of Estimations of the Domain of Attraction

In this paper, an optimization-based methodology is applied to the design of a reactor–separator–recycle system based on a measure of the extension of the domain of attraction of the operating equilibrium. The approach consists in maximizing the radius of a ball in the state space contained in the region of negative definiteness of the time derivative of a quadratic Lyapunov function. A two-level optimization strategy is proposed that solves a deterministic nonconvex global optimization problem at the inner level. To cope with the non-differentiability introduced by the inner problem, we applied a stochastic algorithm to manipulate the design variables at the outer level.

Steady state analysis of snowball effects for reaction–separation–recycle systems with thermally coupled distillation sequences

It is known that the steady and dynamic characteristics of processes with recycles can be significantly different from those without recycle streams. Particular attention has been given to the reactor–separator–recycle (RSR) system; because of the existence of snowball effects, a small load change can translate into a large change in the recycle flowrate, which propagates throughout the whole system. Previous works have analyzed the influence of separators on snowball effects using flash units and conventional distillation columns. This work explores the existence of snowball effects in steady state conditions under the implementation of thermally coupled distillation sequences in RSR systems. In particular, the implementation of coupled systems with side columns (sequences with side rectifiers or side strippers) is considered. A simplified process for ethylbenzene production is taken as a basis, and the effects of implementing thermally coupled systems on snowball effects are compared to those generated by conventional distillation systems.

Design and Control of Integrated Styrene-Aniline Production Plant

This paper illustrates the operational difficulties arising from simultaneously performing exothermic and endothermic reactions, and demonstrates that a plant can be built and safely operated by integrating the design and plantwide control issues. The behaviour of reactor – separation – recycle systems carrying the coupled reactions A ?P + Q (endo) and B + Q ? R (exo) is investigated. Irrespective of the control structure, state multiplicity cannot be removed if the intermediate component Q is recycled. Therefore, the chemical reactor should be designed such that the recycle of Q can be avoided without economic penalty. The theoretical findings are applied to the design and control of a plant coupling ethylbenzene dehydrogenation and nitrobenzene hydrogenation for simultaneous production of styrene and aniline. After plant design, a rigorous dynamic model is developed using AspenDynamics. A plantwide control structure is implemented and shown to be able to achieve stable operation. Production rate changes of reasonable magnitude can be achieved by changing the reactor-inlet flow rates or bed-inlet temperature.

Steady-state behaviour of PFR-separation-recycle systems with simultaneous exothermic and endothermic, first-order reactions

A systematic investigation of plug-flow reactor (PFR)-separation-recycle systems where first-order exothermic and endothermic reactions are simultaneously performed is presented. The nonlinear behaviour is analyzed for two flowsheet alternatives and four plantwide control structures. It is shown that the system can exhibit a complex nonlinear behaviour. For the parameter values used, regions of unfeasibility, two or three multiple steady states and branches of isolated solutions were found. The undesired nonlinear phenomena can be avoided by fixing the reactor-inlet flow rates of each reactant or, when this is impossible due to the flowsheet structure, by providing sufficient cooling capacity.

State multiplicity in PFR–separator–recycle polymerization systems

This article explores the non-linear behaviour of isothermal and non-isothermal plug-flow reactor (PFR)–separator–recycle systems, with reference to radical polymerization. The steady-state behaviour of six reaction systems of increasing complexity, from one-reactant first-order reaction to chain-growth polymerization, is investigated. In PFR–separator–recycle systems feasible steady states exist only if the reactor volume exceeds a critical value. For one-reaction systems, one stable steady state is born at a transcritical bifurcation. In case of consecutive-reaction systems, including polymerization, a fold bifurcation can lead to two feasible steady states. The transcritical bifurcation is destroyed when two reactants are involved. In addition, the thermal effects also introduce state multiplicity. When multiple steady states exist, the instability of the low-conversion branch sets a lower limit on the conversion achievable at a stable operating point. A low-density polyethylene process is presented as a real plant example. The results obtained in this study are similar to CSTR–separator–recycle systems. This suggests that the behaviour is dictated by the chemical reaction and flowsheet structure, rather than by the reactor type.

State multiplicity in CSTR–separator–recycle polymerisation systems

This article continues earlier work (Comput. Chem. Eng. 24 (2000) 209) concerning the design and control of isothermal reactor–separator–recycle systems. The multiplicity behaviour of six reaction systems of increasing complexity, from one-reactant, first-order reaction to chain-growth polymerisation, is investigated. Below a critical value of the plant Damkohler number, Da< Da cr, the only steady state involves infinite flow rates. Feasible steady states become possible if the critical value is exceeded, Da> Da cr. For one-reaction systems, one stable steady state is born at a transcritical bifurcation. For consecutive-reaction systems, including polymerisation, a fold bifurcation can lead to two feasible steady states. Moreover, the transcritical bifurcation is destroyed when two reactants are involved. If the gel-effect is included, a maximum of four steady states are possible. When multiple steady states exist, the achievable conversion is constrained by the instability of the low-conversion branch. This has practical importance for polymerisation systems when the radicals’ quasi-steady state assumption is not valid or the gel effect is significant.

Nonlinear behavior of reactor—separator—recycle systems

The nonlinear behavior of reactor—separator—recycle systems is studied. The dimensionless mole-balance equations are parameterized by the plant Damkohler number (Da) and the separation specifications. Reactant accumulation does not occur if Da > Da cr. Da Cr corresponds to a bifurcation point of the mole-balance equations and depends on separation performance. For one reactant recycle and high purity separation, Da cr = 1. When two reactants are recycled, multiple steady states are possible. In this case, Da cr depends also on the control structure. Close to Da cr, control structures must change the reaction volume or temperature in order to reject effectively the disturbances.

Synthesis of isothermal reactor—separator—recycle systems

A systematic synthesis approach is presented for isothermal reactor—separator—recycle systems. The approach proposes a general superstructure of different reactors and separation tasks and features all the potential interconnections among the proposed units. The synthesis problem based upon the proposed superstructure results in a mixed integer nonlinear programming (MINLP) formulation in which the objective function involves both integer and continuous variables and is subject to a nonlinear set of constraints. A variety of objectives was selected for the synthesis problem such as the minimization of the total annual cost of the plant and the maximization of its profit, as well as objectives traditionally used for optimizing the performance of a reactor network such as the product yield and selectivity. Discussion of the results and comparison among the different solutions obtained provided the ground for conclusions related to the potential trade-offs and the performance of the isothermal chemical systems under consideration.