*3.1. Heat Integration and Heat Transfer*

The first paper on this subject, entitled "Thermal Effects of Natural Gas and Syngas Co-Firing System on Heat Treatment Process in the Preheating Furnace" and authored by Józwiak et al. [ ´ 83], examined the possibilities of partially replacing natural gas with synthesis gas derived from biomass. The system under study was a preheating furnace in the steel industry, for which the authors investigated how the air volume, the distribution of burner power, and the share of bio-based gas influenced heat transfer, temperature, and gas flow in the furnace. The modelling was performed with a computational fluid dynamics tool. Computational fluid dynamics (CFD) tools are widely used to simulate and optimise the processes of heat transfer [84] and energy release from various fuels [85]. The results showed that up to 40% of the natural gas could be replaced by syngas of biogenic origin, while still achieving satisfactory thermal efficiency and temperature characteristics. The authors claimed that GHG emissions could be reduced by 40%. The results showed that the replacement of fossil fuels by renewable fuels needs to be promoted, especially in heat-intensive industrial plants, as satisfactory operational performance can be achieved with significantly lower emissions. However, the economic performance also needs to be analysed and taken into account because renewable fuel production processes are not yet necessarily economically viable; see You et al. [86].

The next paper, "Isomerisation of n-C5/C6 Bioparaffins to Gasoline Components with High Octane Number", authored by Hancsók et al. [87], addresses the challenges of producing fuels from alternative sources such as waste and biomass. These fuels often contain by-products, e.g., light hydrocarbons, especially n-alkanes C5-C7, which reduce the quality and negatively affect the safety properties of the fuel. Light hydrocarbons are formed in the production of bio-gasoline from rice straw biomass [88], Fischer–Tropsch synthesis of syngas from wood chips [89], and in various chemical reactions involving sorbitol [90] and simple sugars [91]. Catalytic reactions of isomerisation and aromatisation were carried out in the experimental device, in which light hydrocarbons were converted into iso-alkanes with a higher octane number. The authors claimed that the yield of liquid products could exceed 98%, and the research octane number could reach 92. Improved reaction pathways and optimised operating conditions for high quality, affordable, and safe end products could facilitate the success of fossil fuel replacements.

One of the measures to increase energy efficiency is to improve heat transfer between fluids in heat exchanger units. The authors of the next paper, Tian et al. [92], entitled "Numerical Study of Heat Transfer in Gravity-Driven Particle Flow around Tubes with Different Shapes", investigated the mechanism of heat transfer in moving bed heat exchangers, in which heat transfer takes place between the solid particle stream and the fluid. This type of heat exchanger is widely used in energy-intensive industries. The influence of tube shape, particle outlet velocity, and diameter on particle movement and heat transfer efficiency were investigated. Modelling was performed for five different geometric tube shapes, such as circular, elliptical, and flat elliptical, using the discrete element method [93]. The model was validated by comparing the authors results with experimental results from the literature [94]. The main contribution of the research was the visualisation of heat transfer parameters and solid particle motion for different tube shapes. The particle velocity distribution around the tubes, the contact time of the particles with the tube, and the heat transfer coefficient as a function of the output velocity of the particles were presented. The heat transfer coefficient was also influenced by the particle size; the smaller the particles, the higher the value of the coefficient. The authors concluded that the elliptical tube was best suited for use in moving bed heat exchangers as it demonstrated the best particle motion and heat transfer properties.

The next article, authored by Létal et al. [95] and entitled "Nonlinear Finite Element Analysis-Based Flow Distribution and Heat Transfer Model", also dealt with fluid flow and heat transfer in large heat exchangers in the process and energy industry. The CFD method, which can be very computationally intensive, is most often used to model various types of heat transfer units, such as compact heat exchangers [96], cross-flow heat exchangers with elliptical tubes [97], or plate solar collectors [98]. In this paper, a simplified model using the finite element method was developed. The model calculated outlet temperatures and pressure drops on the pipe side and the shell side. It was able to handle the laminar and turbulent flow. The model was used for two types of heat exchangers. The results were compared with data from the literature [99], with the results of a commercial computer program [100] and with data from an existing energy plant. Visualisations of temperature profiles for hot and cold streams, in addition to fluid velocities within the exchanger, were presented. The authors argued that their program was easier to use than the commercial program and provided comparable results with less computational effort. They noted that the model still needs to be validated and improved to predict the mechanical stresses that could occur due to uneven thermal loads, which could also result in mechanical failures.

In the next paper, "Comparison of the Evaporation and Condensation Heat Transfer Coefficients on the External Surface of Tubes in the Annulus of a Tube-in-Tube Heat Exchanger", the authors, Tang et al. [101], investigated the influence of the tube surface in a heat exchanger on the efficiency of heat transfer. They compared a tube with a smooth surface and a tube in which the dimples were arranged along the surface in a certain pattern. Several studies conducted in the past have shown that dimpled tubes provide better heat transfer than smooth tubes [102]. However, it was also observed that heat transfer in smooth tubes was better when the refrigerant was condensed on horizontal tubes [103]. Investigations were conducted using an experimental unit with a double-pipe heat exchanger, in which the evaporation and condensation of the refrigerant took place in the intermediate space (the so-called annulus). The influence of mass velocity, annulus width, and steam quality on the heat transfer

coefficient was measured. The results showed that the heat transfer coefficient increases with mass velocity in both types of tubes. Compared to a smooth tube, the tube with a modified surface showed significantly improved heat transfer during fluid condensation. A slight improvement was also observed in the case of boiling under certain conditions, while under other conditions a smooth tube proved to be better. It was concluded that the modified surface used in the study could be suitable to improve heat transfer during condensation of fluid, but not to enhance the wave-like stratified flow during boiling.

In the last paper on this topic, the authors Gai et al. [104] reported on the "Critical Analysis of Process Integration Options for Joule-Cycle and Conventional Heat Pumps". They examined different types of heat pumps and their heat integration into the process. The analysis included traditional heat pumps, such as the vapour compression heat pump (VCHP) [105] and the transcritical heat pump (TCHP) [106], in addition to a newer Joule cycle heat pump (JCHP) [107]. The aim was to determine which type of heat pump was more suitable for a specific process. The operation of heat pumps was simulated with the Petro-SIM program. To investigate the integration of the heat pump into the process, the authors used the pinch method, in particular the Grand Composite Curve. Four case studies from the food and chemical industry were analysed. The results demonstrated that the slopes of the source and sink curves in the temperature–heat flow diagram were most important for the selection of the most suitable heat pump. If the source and sink process curves are steep, it is advantageous to choose a JCHP that has a low average temperature difference of the working fluid and the source/sink. Its Coefficient of Performance (COP) is consequently higher. For processes with flat sink and source process curves, VCHP is more favourable. The smaller the difference between the inlet temperatures of the sink and the source, the higher is COP with this type of heat pump. The use of TCHP is limited to processes where the slope of the source is relatively small, and the slope of the sink is relatively large. The advantage of the approach proposed is that it allows a quick selection of a suitable heat pump based on the temperature–enthalpy diagram for a particular process.
