(61p) Energy Flow Redistribution for Optimal Operation of Heat Exchanger Networks

Strong impetus on the implementation of sustainable practices has increased the attention of process industries towards process and energy integration strategies. Heat exchanger networks (HENs) are designed to implement energy integration at a processing site. The design of HEN has received tremendous attention in the past few decades and can be considered a fairly matured field. However, the optimal operation of HEN, especially in the context of dynamic market conditions, is a challenging and an open problem. Some of the existing approaches to address this include design of flexible HENs [1], real time optimization (RTO) and control [2] or self optimizing control [3].

This work focuses on the optimal operation of heat exchanger networks using a concept of energy flow redistribution (EFR). The EFR methodology uses the structure of the HEN to direct the energetic impact of a disturbance along a favorable (e.g. minimum utility) path. Specifically, when a disturbance enters the HEN, the duties of the process-to-process heat exchangers are adjusted, using internal degrees of freedom like exchanger bypass or stream split fraction, such that the net impact on the utility exchangers is minimized. To this end, the governing equations of an individual heat exchanger and energy balance for the entire HEN are restructured to explicitly quantify the effect of disturbance on the overall performance of the HEN, and subsequently back-calculate the change needed in the values of the manipulated inputs (utility flow, exchanger bypass or stream split fraction) to achieve optimal operation. Additional operational constraints, such as limits on utility usage, absence of bypass on any exchanger can also be incorporated. The EFR strategy can be implemented in an openloop (similar to RTO) or closed-loop (similar to economic MPC) configuration as shown in the figure below.

Using multiple illustrating examples, it is shown that the proposed approach successfully handles commonly encountered (measured and unmeasured) disturbances (such as feed flowrate or temperature, target specifications) as well as plant-model mismatch (exchanger fouling or complex flow patterns), and achieves better performance as compared to the existing methodologies. The proposed approach offers additional benefits such as flexible configuration (in terms of degrees of freedom and performance objective) and ability to incorporate trade-off between optimality, flexibility and computational efficiency.

References:

[1] Floudas CA and Grossmann IE, Synthesis of flexible heat exchanger networks with uncertain flowrates and temperatures, Comput. Chem. Eng., 11(4), 319-336, 1987.

[2] Aguilera N and Marchetti JL, Optimizing and controlling the operation of heat exchanger networks, AIChE J., 44, 1090-1104, 1998.

[3] Jaschke J and Skogested S, Optimal operation of heat exchanger networks with stream split: Only temperature measurements are required, Comput. Chem. Eng., 70, 35-49, 2014.