Controllability of Heat Exchanger Networks
A heat exchanger network (HEN) is the heart of a heat integrated plant, and groundbreaking design methodologies have been developed and introduced to meet lowest-cost-of-operation and environmental constraints/demands leading to heat-integrated plant designs. However, controllability issues are often overlooked during the early stages of a plant design, and this problem may lead to extra work and extra costs. Many authors have demonstrated that good control design is an effective way to ensure that a plant will exhibit flexibility for changes in operating conditions and resiliency to disturbances to operating conditions. Before further design for detailed engineering, analysis and modifications of the synthesised HEN have to be carried out in order to guarantee good flexibility and resiliency of the HEN - since in reality operating conditions are expected to change.
A HEN may be considered a multivariable system problem that sometimes involves several process variables, in this case the target temperatures attained through a process-to-process heat exchanger, and several potential manipulated variables. The concept of a decentralized feedback control system is the simplest approach to multivariable control design as it has three major advantages: flexible operation, simple design, and failure tolerance.
In this paper we present a five-step procedure that involves the use of multivariable control analyses and disturbance analyses and that can be easily implemented to assess both process design and control systems design, with the purpose to assess process design developments and to propose control strategy alternatives appropriate and suitable for a HEN. To do this analysis we only require steady-state information, which can be obtained easily from steady-state models, dynamic models or process identification models.
A simple case study is presented with the purpose to illustrate the easy application of the five-step procedure to determine the controllability of six different HEN arrangements of the same four process streams, and comparing the controllability measurements results we can determine the most controllable HEN of the six alternatives.