Dynamics and control of heat exchanger networks / Dinamica e controle de redes de trocadores de calor
AUTOR(ES)
Luis Fernando Novazzi
DATA DE PUBLICAÇÃO
2007
RESUMO
Energy integration among chemical process streams can lead to quite complex heat exchanger networks (HEN) with difficulties in terms of controI. In this work HEN dynamics and steady state aspects were addressed with the aim of proposing a control strategy that minimizes utility consumption and satisfies imposed specifications. A steady state model of the HEN was suggested using the effectiveness method and by taking into account some operational conditions like inlet stream temperatures and flow, bypasses position in the heat exchangers and by considering HEN topology, with the presence of heat exchangers, heaters 01 coolers, splitters and mixers. Based on these considerations a linear set of equations was written, solved and outlet stream temperatures were calculated. Shell and tube heat exchanger dynamic model is described by differential partial equations, which were discretized and solved by the finite difference method, in Matlab. When an inlet temperature disturbance occurs, it was verified that the dynamic response of a 1-1 heat exchanger can be represented by a first order transfer function associated with dead time. In the case of flow disturbances, the responses are more intense on the streams directly disturbed. When bypasses positions change, the responses can be approximated by . a lead Ilag system. It was shown that the dynamics of a 1-2 heat exchanger depends on flow configuration, i.e., paralle1 flow I counterflow or counterflow I parallel flow. When a flow disturbance happens in 1-2 exchangers, a temporary inversion on respons~ can take place. The dynamic models of isolated 1-1 and 1-2 heat exchangers were used to, describe the dynamic behavior of HEN and one could conclude that disturbances were damped along the network. The heat exchangers dynamic models were linearized and it was shown that system non linearity is not very accentuated. By, taking into account some heat exchanger design equations and assumptions, it was proved that there are typica1 intervals for the relation between heat exchange area and shell volume and heat exchange area and tubes volume. This is an important result since these relations appear naturally on heat exchangers dynamic models. With the tools developed in this work it was proposed a feedforward optimal control strategy which consisted in lhe minimization of a steady slate °7ve function connected to utility consumption and subjected to constraints in outlet stream temperatures. The solution of the minimization problem resulted in optimal bypasses positions in the HEN, which were dynamic~lly implemented using a ramp function and a step function. It was verified that ramp implementation was better, with tolerable dynamic violations. With the objective of getting better responses of the control system, it was also suggested a feedback / feedforward strategy, promoting energy consumption optimization. In this strategy, input and output variables were paired and rigid control objectives, connected to integral gain controllers, and flexible control objectives, associated with proportional gain controllers, were distinguished. Through this approach utility consumption was optimized, temperatures specifications were satisfied and at the same time a fast dynamic response was got. Key words: 1. Heat Exchanger Networks 2. Modeling 3. Optimal Contro14. Dynamics
ASSUNTO(S)
process control controle de processo modelos matematicos heat exchanger networks dynamics permutadores termicos
