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Reflections on “Chemical Process Design” – Part 2: Reactions. The reaction process is the very essence of the entire chemical processing system. Its complex mechanisms and numerous influencing factors have shrouded it in a veil of mystery. In particular, for most engineering professionals, there is a deep sense of awe toward it; they feel that they are unable to fully master it. In fact, for most reactions, there are patterns to follow and they can be predicted. In engineering practice, it is common that when a reaction involves a catalyst, all data related to the reaction is provided by the catalyst manufacturer; engineering technicians then focus only on issues such as separation and heat exchange. This is indeed a viable approach, but it makes it difficult to achieve overall optimization of the entire system’s progress, and thus it is not possible to identify the optimal process. Because when a reaction is incorporated into the entire system, it becomes intricately linked with the system as a whole; it is no longer merely an isolated reaction, and thus must be evaluated and considered in a comprehensive manner. The better an engineer’s ability to control the reaction, the better the optimization of the entire system will be, and the greater the likelihood of finding the optimal process. Of course, the mechanisms of some reactions are also very complex; although they have been proven through industrial applications, there are still many mysteries surrounding these reactions that remain unsolved to this day, such as the combustion reactions in sulfur recovery, the hydrogenation reactions of coal tar, and coal gasification reactions. When a cycle involves numerous reactions and multiple components, among which there are hazardous substances as well as wastes that could cause environmental pollution, it is necessary to consider not only the efficiency of the process (low energy consumption, short process length) but also its safety and environmental compatibility. In such cases, it becomes **more difficult to determine the optimal process and parameters; the esterification and coupling cycle systems in the synthesis of ethylene glycol from syngas are a typical example of this. When it comes to reactions, the fundamental principles remain constant. Let’s follow the original author to understand, one by one, how various factors affect different reactions, thereby enhancing our ability to control them. By considering things from an overall perspective, we can select the appropriate reactions and parameters, ultimately achieving optimization of the entire system. I. Reaction pathways: Depending on the requirements of the product, there are usually multiple pathways available for producing a given product. For example, sulfuric acid can be produced by using acidic gases, sulfur paste, or desulfurization solutions ； Ethylene glycol products can be produced via the ethylene route or via the syngas route ； Symptom coke can be produced using the petroleum method or the coal tar method, etc. The choice of different pathways is primarily considered from the following aspects: 1. Whether the technical approach is reliable. Although there are always those who are the first to adopt each technical approach on an industrial scale, it is necessary to carefully examine whether the foundational work is solid, and in particular whether there are any bottlenecks in the reaction process. The difficulty in developing new reaction pathways often lies in the lack of suitable catalysts, such as those for converting carbon dioxide into methanol ； 2. Cost-effectiveness, that is, considering economic factors and comparing costs ； 3. Raw material supply: For the same product production process, different raw material supply methods can have a significant impact. Examples include methanol production from coke oven gas, methanol production via gasification, and methanol production from natural gas. Factors such as the stability of the raw materials, the difficulty of purifying them, future price fluctuations, and the compatibility between different processing units all have a substantial effect on the quality of the final product ； 4. Safety: Whether it is safe and reliable to operate. In short, the optimal reaction pathway is one in which the reaction is easy to carry out, the raw materials are inexpensive and readily available, and minimal by-products are produced. II. Reaction Performance and Types Reaction performance is commonly expressed by three parameters: conversion rate, selectivity, and yield. In describing reactor performance, the significance of selectivity is often greater than that of yield; the latter is based on the reactants in the feed, while the former is based on the reactants that have been consumed. Obviously, a portion of the reactants may be recycled material rather than fresh feed. Therefore, the reaction yield does not take into account the separation and recycling of unreacted materials. The reaction yield is meaningful only when, for some reason, the unreacted material is no longer recycled back to the reactor inlet. When describing the entire factory, the overall process yield is a very important parameter. The author has personal experience in this regard: in a certain acid production technique, the yield achieved using the sulfur dioxide conversion reactor was around 96%, whereas in similar technologies the yield for such reactors is approximately 99.5%. In both cases, if the exhaust gases are released directly, they do not meet the required standards; therefore, exhaust gas washing is necessary to achieve ultra-clean emissions. By using appropriate detergents, the composition of the washing solution can be part of the raw material composition. The author adopted a liquid-phase recycling approach (with the amount of washing solution being relatively smaller than that of the raw material solution), and the overall yield reached around 99.99%. Compared to similar technologies, this method offers significant advantages in terms of the sulfur dioxide conversion reactor – the required yield per cycle is lower, less catalyst is needed, and there is no need for separation, cooling, or absorption steps in the reactor. As a result, the entire process is simplified, with lower energy consumption, reduced equipment costs, and easier operation. Since the unconverted material generally needs to be separated and recycled, the conversion rate of the reaction cannot be determined when selecting a reactor; the final conversion rate will be determined during the subsequent design process. But in order to move the design forward, some assumptions need to be made first; of course, the reaction conversion rate chosen in this way will likely require multiple adjustments. The by-products formed as a result of the reaction often cannot be converted back into products or raw materials. These by-products increase the consumption of raw materials. Although some of them may have certain economic value, this requires additional costs for separation and purification, resulting in poor economic benefits. Most by-products lead to increased environmental protection costs and pose difficulties in downstream processing. Therefore, the highest selectivity should be achieved for the conversion rate of the selected reaction. For a single reaction, the design goal is to achieve the desired conversion rate for that reaction, while minimizing the capital investment required for the reactor. To increase the reaction conversion rate, it is necessary to increase the reactor volume, which raises the cost of the reactor; however, this can reduce the costs of other aspects of the process, such as subsequent separation, heat exchange, and the treatment of by-products. Therefore, for a single irreversible reaction, the reaction conversion rate should first be set at around 95% ； For a single reversible reaction, the conversion rate is about 95% of the equilibrium conversion rate. For parallel reactions, the design goal is to minimize the formation of by-products at the desired conversion rate, that is, to achieve the highest selectivity for the reaction. When selecting reaction conditions, it is necessary to consider the characteristics of the kinetics of the main and side reactions as well as the reaction equilibrium, so as to favor the formation of the product while minimizing the formation of by-products, thereby increasing the selectivity of the reaction. Due to such reactions, the factors that affect the reaction conversion rate also influence the reaction selectivity; therefore, it is difficult to determine the reaction conversion rate. If the order of the side reaction is greater than that of the main reaction, the selectivity of the reaction increases as the conversion rate increases. If the order of the side reaction is lower than that of the main reaction, the selectivity of the reaction decreases as the conversion increases. If selectivity increases with increasing conversion rate, the reaction conversion rate should initially be set at around 95%. For reversible reactions, the conversion rate is selected to be around 95% of the equilibrium conversion rate. If selectivity decreases as the reaction conversion increases, it becomes difficult to make a choice. For irreversible reactions, a conversion rate of around 50% is chosen; for reversible reactions, a conversion rate of around 50% of the equilibrium conversion rate is selected. Of course, these are just preliminary options; the reaction conversion rate will be adjusted after a better understanding of the entire process is obtained. Series reaction: In a series reaction system, as the conversion rate increases, the selectivity of the reaction is likely to **decrease**. The selectivity of the reaction increases as the concentration of side reaction reactants decreases, which means that operating the reactor at low product concentrations, i.e., low conversion rates, is favorable for increasing selectivity. Therefore, it is difficult to determine an appropriate reaction conversion rate for this reaction system. In the preliminary stage, for irreversible reactions, it is advisable to choose a reaction conversion rate of around 50%; for reversible reactions, a reaction conversion rate of approximately 50% of the equilibrium conversion rate is more suitable. When the raw material contains impurities, side reactions may sometimes occur, which should be avoided as much as possible. Typically, this side reaction is limited by purifying the raw materials, rather than changing the reaction conditions. As in the ethylene glycol process, the feed gas carbon monoxide often contains hydrogen, and in this reaction hydrogen has a stronger ability to occupy certain sites compared to methyl nitrite and carbon monoxide. Therefore, it is necessary to keep the hydrogen content within acceptable limits in the upstream units. In cases where deep cryogenic separation is used upstream, the requirements are generally met; however, in smaller-scale units, when PSA or membrane separation is employed for carbon monoxide purification, the resulting concentration may not meet the requirements. In such situations, it is necessary to carry out dehydrogenation of carbon monoxide within the reactor itself, otherwise it will affect the reaction process. Furthermore, impurities in the raw materials often cause catalyst poisoning or lead to adverse effects such as an increase in the amount of circulating gas and vent gas. The author has conducted repeated comparative analyses and found that purification of the raw materials is a good method. After initially determining the reaction conversion rate and the maximum selectivity achievable at this conversion rate, the next step is to determine the type of reactor. III. Ideal Reactor Models The three most commonly used ideal reactor models are as follows: The first is the ideal batch model, in which the materials are in a fully mixed state; after a certain period of reaction, the product is obtained. The concentration within the reactor changes over time, and full mixing ensures that both the concentration and temperature inside the reactor are uniform. This model is frequently utilized in the production of fine chemicals ； The second type is the continuous completely mixed flow model, in which both the raw materials and products are continuous. Since the materials are fully mixed, a fluid unit may exit the reactor immediately upon entering it or it may remain inside the reactor for some time; as a result, the residence time of different fluid units in the reactor varies greatly ； The third is the plug flow model, in which the reaction stream flows uniformly with no mixing in the direction of flow, and all fluid elements have the same residence time. The above three reactor models are suitable for different reaction systems respectively. For a single reaction, the higher the concentration of the reactants, the higher the reaction rate. In a continuous fully mixed reactor, the reactants are diluted by the products formed as soon as they enter the reactor, resulting in lower reactant concentrations; these concentrations are equal to those at the reactor outlet. Therefore, the reaction rate is lower than that in an ideal batch reactor or a plug flow reactor. Therefore, for a single reaction, an ideal batch reactor or a plug flow reactor is advisable. For parallel reactions, the smaller the ratio of the rate of the side reaction to the rate of the main reaction, the higher the reaction selectivity. When the order of the main reaction is greater than that of the side reaction, the higher the concentration of the reactants, the more favorable it is for the main reaction; in such cases, an intermittent reactor or a plug flow reactor is advisable ； When the order of the main reaction is lower than that of the side reaction, the lower the reactant concentration, the more favorable it is for the main reaction; in such cases, a continuous fully mixed flow reactor is suitable. If there are multiple reactants in the reaction, the reaction rate equation is used to make a decision; when it is necessary to keep the concentrations of all reactants low, a continuous fully mixed flow reactor is chosen ； When it is necessary to maintain high concentrations of the reactants, an intermittent reactor or a plug flow reactor should be used ； When it is necessary to maintain a higher concentration of one reactant and a lower concentration of another, one reactant is added first, while the other reactant is added continuously as the reaction proceeds. For a series reaction, when the reaction conversion is constant, the residence time of the reactants is a constant value. For continuous fully mixed flow reactors, the residence time distribution of reactants and products is wide; the residence time of reactants may be longer or shorter than the predetermined value. The reaction selectivity is low, so for series reaction systems, batch reactors or plug flow reactors are more suitable. For parallel-series mixed reactions, we mainly consider the case where the order of the main reaction is less than that of the side reaction. To achieve better reaction selectivity, it is advisable to use a combination of continuous completely mixed reactors and plug-flow reactors. Examples include: series combinations of multiple continuous stirred-tank reactors, piston-flow reactors with circulation, and series combinations of piston-flow reactors and continuous stirred-tank reactors. To achieve high selectivity in this reaction system, it is necessary to conduct a thorough design and economic analysis of the reaction system. In summary, when the conversion rate is fixed, the goal in selecting a reactor is to minimize the reaction volume for single reactions and maximize the selectivity for complex reactions. IV. Concentration – Single irreversible reaction: For such reaction systems, selectivity is generally not an issue; the decision regarding which reactant should be used in excess is based mainly on the availability of the starting materials, their toxicity, and the difficulty of subsequent separation. For a single reversible reaction, according to Richard’s principle, if changes in certain conditions of the equilibrium system disrupt that equilibrium, the new equilibrium will shift in a direction that reduces such changes. The maximum conversion rate of a reversible reaction is limited by the equilibrium conversion rate; therefore, reaction conditions are typically adjusted to increase this equilibrium conversion rate, such as by changing the feed ratio, the concentration of inert substances, or removing the product at intermediate stages of the reaction. The two-stage conversion and two-stage absorption process in traditional dry acid production technology is a classic example. Reaction systems with parallel and sequential side reactions: For reaction systems that involve parallel side reactions, in order to improve reaction selectivity, it is necessary to determine which reactant is in excess based on the reaction order. If the side reactions are reversible and there is a change in the number of moles before and after the reaction, the formation of by-products can be reduced by increasing or decreasing the amount of inert components. For all reversible side reactions, adding the byproduct to the system can suppress the equilibrium of the side reaction. Therefore, in some industrial plants, the by-products are separated and then recycled back to the reactor, with the aim of suppressing their formation. For reaction systems involving series-side reactions, to minimize the formation of by-products, this can be achieved by lowering the product concentration; specific methods include altering the molar ratios in the feedstock and implementing intermediate separations. V. Temperature: For a single reaction, if it is an endothermic reversible reaction, increasing the reaction temperature can raise both the conversion rate and the reaction rate. Therefore, in such cases, the reaction temperature should be set at the highest safe operating temperature for the material and the catalyst. For a single reaction, if it is an irreversible exothermic reaction, the reaction temperature shall be set at the highest allowable temperature ； If the reaction is a reversible exothermic reaction, a higher conversion rate can be achieved at low temperatures; however, the reaction rate is low at such temperatures. Therefore, at the start of the reaction, a higher temperature is used to increase the reaction rate, and as the reaction approaches equilibrium, the temperature is reduced to increase the conversion rate. In other words, the reaction temperature gradually decreases as the reaction conversion increases. For complex reactions, it is often more important to improve selectivity; therefore, a decision is made based on the rates at which the reaction rate constants of the main and side reactions increase with temperature. When the reaction rate constant of the main reaction increases faster than that of the side reactions, the highest allowable reaction temperature is chosen ； When the reaction rate constant of the main reaction increases more slowly than that of the side reaction, the reaction can achieve greater selectivity at low temperatures. However, at low temperatures the reaction rate is low and the reactor volume is large; therefore, an economic trade-off between the increased amount of product and the additional capital investment in equipment must be considered to determine the optimal reaction temperature. VI. Pressure: For irreversible gas-phase reactions, increasing the reaction pressure can increase the reaction rate and reduce the volume of the reactor. For liquid-phase reactions, the effect of pressure is minimal. For gas-phase reversible reactions, the effect of reaction pressure on the reaction depends on the change in the number of moles during the reaction process, as well as whether it is a single reaction or a complex reaction. For a single reaction, if the number of moles involved in the reaction decreases, increasing the reaction pressure can raise the reaction conversion rate and reaction rate as well as reduce the volume of the reactor; in such cases, high-pressure reactions should be employed. Of course, this is also subject to constraints such as the capacity of compressors, mechanical manufacturing costs, and safety considerations. For a single reaction, if the molar amount of reactants increases, reducing the reaction pressure helps to increase the reaction conversion rate; however, carrying out the reaction at low pressures slows down the reaction rate and increases the volume of the reactor. Therefore, at the start of the reaction, a higher reaction pressure should be used, and this pressure should be reduced once equilibrium is reached in order to increase the reaction conversion rate. The ideal reaction pressure should gradually decrease as the conversion rate increases. Specifically, the partial pressure of the reactant components can be reduced by lowering the absolute pressure of the operating system or by adding a diluent (usually an inert gas) to the reaction system. For complex reactions, the effect of pressure on the reaction is similar to that in single reactions; however, apart from the reactor volume, reaction selectivity is even more important, that is: pressure is adjusted to minimize the ratio of the reaction rates of side reactions to those of the main reaction. For liquid-phase reactions, pressure has little effect on the reaction conversion rate and selectivity. When selecting a pressure, the following factors should be considered: 1. Preventing the product from vaporizing ； 2. Vaporize the liquid in the reactor, allow it to condense, and then return it to the reactor to carry away the heat of reaction ； 3. Vaporize a certain component in the reaction to increase the conversion rate of the reversible reaction. VII. Catalysts: Catalysts can increase the reaction rate, but they do not alter the chemical equilibrium; their chemical composition and quantity remain unchanged before and after the reaction. For systems composed of multiple reactions, catalysts have different effects on the reaction rates of these various reactions; this makes it possible to develop catalysts that increase the rate of the main reaction (compared to side reactions). Therefore, the choice of catalyst has a significant impact on reaction selectivity. Catalysts are used in the vast majority of chemical processes, and these catalysts are well-known; therefore, their selection is very important and relies to a large extent on experience. Catalytic processes are divided into homogeneous catalysis and heterogeneous catalysis. Homogeneous catalysis involves reactions taking place in the same phase; the catalyst participates in the reaction to alter its mechanism, and then it is recycled after being regenerated. Typically, it is difficult to separate and recycle the catalyst, leading to significant losses that increase production costs and cause environmental pollution. The process of removing hydrogen sulfide by wet method using complexed iron as a desulfurizing agent is one such example. Heterogeneous catalysis involves catalysts and reactants being in different phases; typically, the catalyst is solid while the reactants are in the gas or liquid phase. The reaction generally goes through seven steps: external diffusion, internal diffusion, adsorption, reaction, desorption, internal diffusion, and external diffusion. The entire reaction process involves issues related to rate-controlling steps; some reactions are controlled by external diffusion, some by internal diffusion, and others by intrinsic kinetics. For reactions controlled by external diffusion, the rate can be improved by increasing the linear velocity of the reactants; for reactions controlled by internal diffusion, the reaction rate can be increased by reducing the size of the catalyst. Catalyst deactivation: Generally, over time, the performance of a catalyst gradually deteriorates. For a given reactor, a decrease in the reaction rate leads to reduced production capacity. This is generally compensated for by raising the reaction temperature; however, excessively high temperatures accelerate the deactivation of the catalyst and reduce reaction selectivity, resulting in an increase in by-products. Factors that cause catalyst deactivation include the following: 1. Physical loss. Primarily due to losses during the catalyst separation and recycling process in homogeneous reactions, as well as wear and fragmentation of catalyst particles in fluidized beds that result in losses during recycling ； 2. Surface deposition. In liquid-phase reactions, the deposits on the surface of solid catalysts are mostly insoluble reaction by-products ； In gas-phase reactions, the deposits on the surface of solid catalysts are mostly non-volatile reaction byproducts ； A typical example is the formation of carbon deposition on the catalyst surface during hydrocarbon reactions (with severe cases resulting in coking). The formation of coking can be suppressed by making appropriate adjustments to the feed composition; an example of this is the addition of methanol, the reaction product, to the feed in the hydrogenation of oxalate esters. Once coking forms, the catalyst is often regenerated by oxidizing the carbon deposits with air at an increased temperature. 3. Sintering. During gas-solid reactions at high temperatures, sintering of the carrier or active material may occur. Sintering is a molecular rearrangement that occurs at temperatures below the melting point of the active material; it can take place at half the melting point of the catalyst, resulting in a reduction in the catalyst’s effective surface area. If poor heat transfer in the catalyst bed or uneven mixing of the reactants leads to local hot spots, the sintering phenomenon will become more severe. 4. Poisoning. Poisons generally originate from impurities or corrosive products in the raw materials; poisoning occurs when certain substances react chemically with the catalyst or form strong chemical bonds, thereby damaging the catalyst and reducing its activity. When selecting a catalyst, in addition to factors such as conversion rate, selectivity, raw material cost, and recovery, its service life is also an important consideration. Frequent regeneration or replacement of the catalyst can cause significant problems, and since it is not possible to recycle the catalyst in its entirety, it may also lead to environmental pollution. The above analysis examines the selection of reactions and the impact of various factors on different reactions. In actual reaction processes, these effects are often the result of multiple factors working together, which requires additional human analysis and judgment; this in turn demands that designers have a thorough understanding of the entire system. For example, in the hydrogenation reaction of dimethyl oxalate, this is a reaction system that involves both parallel and sequential side reactions. The reaction is influenced by factors such as the hydroxyester ratio (the molar ratio of the reactants), temperature, pressure, and the catalyst used; in addition, there are issues such as a narrow operating temperature range, tendency to coking, and easy degradation of the catalyst. It is worth pondering how to achieve the best reaction outcome among numerous influencing factors! Most of the theories mentioned above are taken from the original book, with appropriate additions based on the author’s own understanding. Critics and suggestions from industry professionals are welcome, and those who are interested are also invited to engage in discussions and exchanges!