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Thoughts on Reading “Chemical Process Design” IV – Heat Exchanger Networks. In chemical process design, once the reaction, separation, and circulation systems are determined, the material and energy balances of the system are also established as a result. However, it requires careful consideration of how to design the external heat exchange network and utility systems in order to maximize economic benefits. At the same time, the design of the outer heat exchange network and utility systems is not an isolated system either; it enables process design to be improved from the outside in, allowing for gradual optimization of the inner design. This facilitates designers to refine the design of the reactors, separators, and circulation systems in the inner layer, thereby improving the energy efficiency and reducing the investment costs of the outer layers. The optimal utilization of energy is a key focus for industrial facilities. According to statistics, most companies use 50% more energy than is actually necessary. How to save energy and reduce consumption remains an eternal challenge for the survival and development of enterprises. When the performance of reaction, separation, and recycling systems is at the same level, the quality of the heat exchange network design directly determines a company’s competitiveness and its ability to survive and thrive. The benefits it brings are truly astonishing; the author has learned that in some companies with excellent performance, a large portion of their annual profits comes from energy-saving and consumption-reducing technological upgrades. A small number of particularly excellent companies even establish dedicated specialized teams to carry out systematic energy optimization across the entire facility, which leads to revolutionary improvements. I. Energy Objectives 1. For the analysis of heat exchange networks using combined curves, the first step is to determine the heat source (referred to as the heat flow stream) and the heat sink (referred to as the cold flow stream) based on mass and energy balances. The hot stream can be cooled using circulating water or other cooling media, while the cold stream can be heated using steam or other heating media. To maximize economic benefits and achieve sustainable industrial development goals, heat recovery should be carried out as much as possible to reduce energy consumption. The amount of heat recovery can be determined by plotting the flow streams on a temperature-humidity diagram. Heat transfer between the two streams is possible only when the temperature of the hot stream is higher than that of the corresponding cold stream at all points. The temperature and enthalpy change of the stream cannot be changed; therefore, its slope on the temperature-enthalpy diagram also remains unchanged. However, since the reference enthalpies of the two streams can be different, they can move horizontally on the graph, and their relative positions can change. In other words, the temperature difference between the two streams can be adjusted; the smaller this temperature difference, the more heat can be recovered, and the less energy will be required for heating and cooling systems. For cases with multiple hot streams and multiple cold streams, all the hot streams are plotted on a temperature-humidity diagram and combined within the same temperature range; the division of these temperature ranges is determined by the points at which the total rate of enthalpy change varies with temperature. In each temperature range, all heat flux streams are combined into a single combined heat flux stream. The CP of the combined heat flux stream within any temperature range is the sum of the CPs of all the individual heat flux streams in that range, and its enthalpy change is also the sum of the enthalpy changes of each stream. This combined heat flux profile is the combined curve of the heat flux profiles. Similarly, this method can also be used to construct the composite curve of cold streams. By plotting the hot combination curve and the cold combination curve on the same temperature-humidity diagram, and by setting a specific value for the minimum heat transfer temperature difference ΔTmin between the hot/cold combination curves, the relative positions of these two curves are determined; the overlapping area between the curves represents the maximum amount of heat that can be recovered. The amount beyond the overlapping portion constitutes the heat and cold utility volume, and the energy in this portion cannot be recovered. Once the heat utility volume, cold utility volume, or ΔTmin are specified, the relative positions of the two curves are fixed. When the two combined curves are in exact contact, there is no driving force for heat transfer at some point during the process; this requires an infinitely large heat transfer area, and as a result the investment cost for the equipment becomes infinite. As the energy target increases (ΔTmin between the two curves increases), the investment cost decreases, while the energy cost increases accordingly. Therefore, there is a trade-off between equipment investment costs and energy costs, that is, an economic efficiency for energy recovery exists. At the same time, when setting ΔTmin, attention must be paid to the constraints involved in its implementation; in shell-and-tube heat exchangers, ΔTmin generally cannot be lower than 10°C, as there is periodic cross-flow in the shell side flow, making it impossible to achieve a purely counterflow operation. In plate heat exchangers, the minimum temperature difference can reach 5°C. In plate-fin heat exchangers, the minimum temperature difference can reach 1–2°C. Of course, these constraints apply only to the design at the point where the two curves are closest. If vaporization or condensation occurs here, additional constraints will arise. 2. The rational configuration of the heat recovery pinch curve is determined by a trade-off between energy costs and investment costs, which corresponds to an economically optimal minimum heat transfer temperature difference ΔTmin. Once ΔTmin is determined, the relative position of the combined curve is fixed, and thus the energy target is also determined. The minimum heat transfer temperature difference ΔTmin exists at only one point between the cold and hot profile curves, and this point is known as the heat recovery pinch point. To achieve the energy target set by the combined curves, designers must prevent heat from passing through the pinch point; that is, they should avoid: (1) heat exchange between the processes above and below the pinch point ； (2) Improper use of utilities. Meanwhile, the heat transfer temperature difference for each heat exchanger shall be no less than ΔTmin. 3. The threshold issue is that not all problems have a pinch point that allows the process to be divided into two parts. When the cold and hot combined curves are aligned at the hot end, the combined curves are brought closer together, which further reduces the amount of cooling utility required at the cold end. However, a new cooling utility is needed at the hot end, and the amount required is equal to the reduction in cooling utility needed at the cold end. That is: although the two combined curves move closer together, the total amount of utility required remains unchanged. This is a “threshold” position; problems that exhibit “threshold” characteristics are simply referred to as “threshold problems”. In the threshold problem, some have no demand for hot utility services, while others have no demand for cold utility services. The optimal total cost point cannot occur below the threshold value; it can only be equal to or greater than this threshold value. 4. Process constraints: The analysis presented above is based on the assumption that, as long as an allowable temperature difference exists, any heat flow stream can, in principle, be matched with any cold flow stream. However, this is not the case in reality; it often cannot be achieved due to certain constraints. For example, when two fluids of different types are mixed for heat exchange in the same heat exchanger, leakage can lead to direct contact between the two fluids, resulting in unacceptable consequences. At this point, a constraint was added, prohibiting heat exchange between these two flow streams. In addition, there are many other constraints; for example, if the two flow streams are too far apart, it will result in long-distance transportation through pipelines ； Heat exchange between different functional areas can lead to a lack of flexibility in the system. Potential control, start-up and shutdown capabilities, operational flexibility, safety, etc., will all impose constraints, necessitating the abandonment of some heat recovery. For this reason as well, most engineering and technical personnel simply gave up on the heat exchanger network design method. The author believes that by fully taking these constraints into account and combining them with heat exchanger network design methods, an ideal outcome will surely be achieved. In some process constraints, if the cost of satisfying those constraints is high, it may be advisable to give up independence and have the process operate as a system ； Or remain independent and find another way to overcome this constraint. This can usually be achieved through indirect heat exchange between the two processes. For example, in ethylene glycol distillation, the steam generated in the high-temperature vapor phase at the top of the tower is fed into a steam main, from which other users draw steam for heat exchange; in this case, the utility system acts as a buffer. II. Investment Costs and Total Cost Targets The factors that influence the investment costs of heat exchange networks include: the number of heat exchange units, heat exchange area, material of the equipment, number of shell sides, pressure rating, and type of equipment. Generally speaking, the final heat exchange network design should aim to minimize the number of heat exchange units in order to reduce equipment investment costs; that is: minimum number of heat exchange units = total number of flow streams – 1 ； If there is a pinch point in the network, the aforementioned formula should be applied on both sides of the pinch point; that is: minimum number of heat exchange units = (number of streams above the pinch point – 1) + (number of streams below the pinch point – 1). Heat exchange area: The heat transfer film coefficient is obtained from empirical data, and then the total heat exchange area of the heat exchange network is calculated using empirical formulas; however, it is difficult to obtain the relevant data using this method. The author generally models using the process simulation software ASPEN, while utilizing Aspen Energy Analyzer to calculate and obtain relevant data. The total cost is the sum of the investment cost and the operating cost. If the energy consumption during the process increases, the temperature difference available for heat recovery increases, thereby reducing the required heat exchange area. Therefore, there is an optimal point for energy consumption at which the total cost is minimized. It is important to note that different projects exhibit regional differences, and generalizations cannot be made. For example, compared to the central regions, the costs of utility services vary significantly in Xinjiang and Inner Mongolia, and the optimal solutions also change accordingly. III. Heat exchange network design: During the project design process, since most products are quite mature and their processes are well-established, the design of the heat exchange networks is also carefully considered. For some products, the volume of heat and cold flow involved is relatively low, allowing heat recovery to be determined directly based on the process. In addition, there are many cases where heat recovery is limited by process constraints. For these reasons, the vast majority of process designers, including many process developers, have simply given up on designing heat exchange networks, considering them to be of no practical value. The author does not agree with this view; in fact, scientific methods for heat exchange network design are of great significance. They play an irreplaceable and vital role both in further improving existing product technologies and in optimizing those of newly developed products. Firstly, there’s its own design; secondly, its close interrelation with reaction, separation, and recycling systems, etc. It is a choice made after comprehensively weighing multiple factors. For heat exchanger network design, a good initial approach is to assume that the heat transfer temperature difference for all heat exchangers is greater than ΔTmin. To achieve the energy targets, heat transfer between process streams must not cross the pinch point, and there must be no improper use of utility streams above or below the pinch point. 1. The pinch design method: As discussed above, it involves the trade-offs between the investment costs and energy costs of heat exchanger networks. The minimum total cost corresponds to an optimal ΔTmin value. The value of ΔTmin is generally set based on experience; it usually falls between 10 and 20°C. For certain industries, a lower ΔTmin value can be chosen. Some recommended empirical values for ΔTmin are as follows: petroleum refining – 20~40℃ ； Petrochemicals – 10~20℃ ； Chemicals – 10~20℃ ； Low-temperature process — 3~5°C. (1) Start the design from the pinch point; the pinch point is the most constrained area in the entire problem. The initial match is determined based on the most constrained part of the entire problem, which generally does not pose difficulties for subsequent matches ； (2) For a single matching CP inequality, to ensure the feasibility of pinch point matching, above the pinch point: CPH ≤ CPC, and below the pinch point: CPH ≥ CPC. The CP inequality applies only to pinch point matching; once away from the pinch point, the temperature difference increases, so it is no longer necessary to adhere to the CP inequality. Cold utility services must not be used above the pinch point, and hot utility services must not be used below it ； (3) Flow distribution: To ensure an appropriate process matching, above the pinch point: SH ≤ SC ； Below the grip point: SH≥SC ； When the above requirements cannot be met, or when the CP inequality cannot be satisfied, the method of stream splitting can be considered to solve the problem ； (4) “Pruning” heuristic rule: utilize this rule to keep the number of units to a minimum. To eliminate one stream, each heat exchange unit should be as large as possible, so that the stream with the smaller heat load among the two matching streams is completely matched. After leaving the grip point, there are more degrees of freedom for the designer to choose the matching relationship. At this point, one can use their own experience, judgment, and knowledge of process design to find a solution. 2. Multi-intercept problems: It is rare for two process intercepts to occur in a single problem; multi-intercept problems are usually caused by the introduction of utility systems, which result in utility intercepts. For multi-clamp problems, when designing between two clamps, one should start from the most constrained clamp. Once the initial heat exchange network structure is determined, the network cost can be adjusted through loops, utility paths, and stream splitting. During this process, there are no constraints imposed by a temperature difference greater than ΔTmin or by the absence of heat flow across the junction. The only goal is to achieve a design with the lowest total cost. 3. Utilizing Aspen Energy Analyzer to perform pinch analysis on heat exchanger networks: In the process of pinch analysis and optimization, a large number of cold and hot streams along with their respective thermodynamic properties are required. After simulation using Aspen Plus, the results can be directly transferred to Aspen Energy Analyzer, where heat exchanger network design can be carried out. The greatest advantage of this software is that it **improves efficiency and provides comprehensive material property data, enabling rapid comparison of different options. The specific analysis methods and the principles for designing heat exchange networks are the same as those mentioned above. The heat exchanger network design method described above is a form of thermal integration based on pinch analysis. It has been widely applied in various sectors of the process industry and power sector. The approach involves taking into account the different heat sources and heat sinks within a process, and proposing solutions for heat exchange within that process in order to achieve optimal energy utilization.