*3.2. Integrated and Newly Developed Heat Exchangers*

Wang et al. [108], in their paper entitled "An Extended Grid Diagram for Heat Exchanger Network Retrofit Considering Heat Exchanger Types", developed a new approach to heat exchanger network retrofit that determines the retrofit design and selects the most cost-effective heat exchanger types. It targeted some specific features, as noted in a recent review [49]. The approach is based on the pinch method and uses the visualisation of an extended grid diagram previously developed by Yong et al. [109] to identify possible alternatives for improving the network. The method developed for this Special Issue allows choosing between six common industrial types of heat exchangers and estimates their capital cost. In addition to graphical methods, mathematical programming (MP) can also be used for a similar purpose, as shown by Soršak and Kravanja [110], who developed a mathematical model for network synthesis including the selection of heat exchanger types. However, as the presented paper demonstrated, selection of heat exchanger types for the retrofit should consider that the temperatures of the process streams should be within the temperature ranges applicable for each of the six types of heat exchangers. For this reason, the calculated heat transfer area is required to be within the recommended area range of the specific exchanger type. For the identified alternative retrofit plans, the investment in heat exchangers and the utility cost were assessed, and the optimum retrofit design was selected. The implementation of the proposed methodology to a network with six hot and one cold streams was presented based on the problem presented by Yong et al. [111]. The SRTGD-STR (Shifted Retrofit Thermodynamic Grid Diagram with the Shifted Temperature Range of Heat Exchangers) appears to provide valuable visualisation, representing a considerable advantage compared to MP for retrofitting heat exchanger networks (HENs). A reduction in utility cost was achieved, while the right choice of heat exchanger typed enabled relatively low investment cost.

In the next article, written by Langner et al. [112] and entitled "A Framework for Flexible and Cost-Efficient Retrofit Measures of Heat Exchanger Networks", the design of a retrofitted heat exchanger network was discussed, taking into account uncertain input data. Uncertainty is an important aspect that influences the flexibility of the network, which should operate optimally even with fluctuating input data. The design of processes under uncertainty conditions is a significant challenge because a process model must be solved for several scenarios at the same time [113]. The presented model was extensive, and the computational effort for its solution can be high. Langner et al. [112] proposed a multi-step methodology in which they reduced the complexity of the problems they addressed in each step. The application of the methodology was demonstrated using the example of a network with two hot and two cold streams and eight uncertain parameters, i.e., related to inlet temperatures and heat capacity flow rates. The procedure started with the initial structure of the network, for which several reconstruction proposals were derived using graphical methods, with the aim of reducing the consumption of the hot utility. The flexibility of the given retrofit proposals was checked, and alternatives that did not reach the required value of the flexibility index were rejected. For feasible alternatives, critical points were determined using methods from the literature [114]. By considering critical points, a multi-period optimisation mixed-integer nonlinear programming (MINLP) problem that selects the network retrofit proposal with the lowest total annual cost was solved. Finally, the flexibility of the selected solution was reviewed. In the proposed methodology, graphical methods to generate alternative retrofit proposals and mathematical programming methods to select an optimal design of heat exchanger network were combined. They improved and automated the procedure for determining critical points. The authors suggested that their methodology could be suitable for retrofitting larger industrial networks.

The synthesis of heat exchanger networks (HENs) is usually performed separately from utility system design. The authors of the third article within this topic, Sheng et al. [115], pointed out the advantage of simultaneous synthesis of a heat exchanger network and a steam generation system in their paper entitled "Simultaneous Synthesis of Heat Exchanger Networks Considering Steam Supply and Various Steam Heater Locations". The steam generation system was based on the Rankine cycle, which generates multi-level saturated and superheated steam, and the power [116]. The authors of this SI paper chose a multi-stage superstructure of a HEN as a basis, which they combined with a utility system. The mathematical model of a composite system corresponded to the MINLP optimisation model, which contained mass and energy balances, feasibility constraints for the temperature and utility system, and an objective function based on the total annual cost. Binary variables were used to select the heat matches; continuous variables represented temperatures, heat flows, surface areas of heat exchangers, amounts of generated steam, and power. The developed model was illustrated using a case study with four hot and four cold streams connected to a Rankine steam system. The selection of multi-level steams and their use at the end of the streams and/or between the stages allowed greater flexibility in optimising steam distribution, power generation, and fuel consumption. Better economic parameters of the overall system can be achieved. The simultaneous synthesis of the network and the utility system makes it possible to establish interactions between the investments, the fuel cost, and the revenues from the electricity generated, leading to better solutions than if the two systems were considered separately.

Haber–Bosch ammonia synthesis is a well-established mature technology, but is still a challenge due to the demanding operating conditions and the highly exothermic reaction that requires an efficient heat transfer system. Processes for the synthesis of ammonia under milder conditions and through more environmentally friendly reaction paths are under development [117]. Reactors with built-in heat recovery systems are most commonly used for traditional ammonia synthesis [118], in which hot reaction products heat the reactants to the required temperature. The authors of the final paper in this theme, Tovazhnyanskyy et al. [52], reported on "Optimal Design of Welded Plate Heat Exchanger for Ammonia Synthesis Column: An Experimental Study with Mathematical Optimisation". They investigated heat transfer in a plate heat exchanger with a specially welded construction for use in ammonia synthesis. Plate heat exchangers (PHEs) are one of the high-efficiency types of compact heat exchangers with intensified heat transfer [119]. The main construction features and principles of

operation and design for PHE have been well discussed in publications (see, e.g., Klemeš et al. [120]). The exchanger consists of round, corrugated plates on which criss-cross channels are arranged to allow cross-flow of streams. The authors carried out an experiment in a laboratory to determine the correlations between heat transfer and pressure drop in a single-pass heat exchanger at high temperature and high pressure. With this data, they were able to develop a mathematical model for the design and optimisation of individual parts of the heat exchanger with a multiple-pass flow regime. The model was validated on an industrial device that confirmed better heat transfer properties than the tubular heat exchanger commonly used in ammonia synthesis. They carried out an optimisation of the exchanger surface, in which the height of a rib and the number of passes were optimisation variables. The validity of this model was confirmed by the results of industrial tests performed with the prototype WPHE (Welded Plate Heat Exchanger) installed in the operating column of ammonia synthesis at temperatures of about 500 ◦C and pressure of about 32 MPa. The tests confirmed the reliability of WPHE and its efficiency compared to a tubular heat exchanger. It has a significantly lower weight and occupies a smaller volume, which increases the ammonia production capacity by up to 15%. In addition, it has a higher heat transfer efficiency. The developed optimisation model allows for the optimal design of the exchanger plates and flow regime for specific operating conditions in an ammonia synthesis reactor.
