Novel Approaches for Retrofitting Heat Exchanger Networks Subject to Varying Operating Conditions

Licentiate thesis, 2020

The process industry is responsible for a significant share of the final industrial energy use in Sweden. In particular, the pulp and paper industry accounts for more than 50 %. In this context, a number of studies have shown that there is a substantial potential for energy savings in the pulp and paper industry. In particular, increased heat recovery is an important measure for increasing energy efficiency. Industrial process plants usually include heat recovery systems that are built to transfer heat between different process streams. Such heat recovery systems consist of Heat Exchanger Networks (HENs) which can be characterized by a high degree of complexity, e.g. through stream splits, recycle and closed circulation loops.

Increasing heat recovery, e.g. by redesigning (retrofitting) the existing heat recovery system, can contribute significantly to meeting energy efficiency improvement targets for industrial process plants. One issue to consider when screening retrofit design options is that industrial heat recovery systems must be able to handle external variations, e.g. in inlet temperatures or heat capacity flow rates, in such a way that operational targets are reached. Consequently, there is a need for systematic retrofitting methodologies applicable to HENs subject to variation in operating conditions. The aim of this thesis is to propose new approaches for retrofitting HENs operating in multiple periods.

Three different approaches have been developed and published in three papers appended to this thesis which allow for retrofitting HENs operating in multiple periods. These approaches can be applied independently and are applicable to HENs commonly present in process industry, e.g. pulp and paper industry. All approaches have in common that they require structural retrofit design proposals as input. For the structural proposal generation, graphical approaches (e.g. Pinch-based) may be utilized in order to take advantage of the designer interaction during the design process. The three approaches propose different strategies to evaluate and ensure feasibility of the design proposals when operating conditions vary, e.g. by means of design modifications. In this context, feasibility is achieved if predefined target values, e.g. stream target temperatures, can be reached for the entire span of variations. Furthermore, different strategies help the designer to identify the most promising proposal (or proposals) among the provided ones with respect to a defined objective, e.g. most energy-efficient or most cost-efficient.