The seven stages of chemical engineering technology from concept to industrialization (Issue 21/100) -- Catalyst and material selection

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This post was last edited by xiouxingzhe on 2026-6-23 at 15:27. The seven stages of chemical technology from concept to industrialization (Issue 21/100). Technology research and development: Catalyst and material selection. Dear friends: Hello everyone! In the previous issue, we discussed the proof-of-concept experiment; the core reaction was able to occur, so the direction chosen was correct. The next question to address is: among the many candidate systems, which one is most suitable for industrialization? This issue focuses on catalysts and the selection of key materials. If your reaction does not involve a catalyst, the methods in this section are also applicable to the selection of equipment materials and separation media. I. List of candidates: It’s not the case that more is better; rather, each candidate should have a reason for being included. The first step in the selection process is to create a list of candidates. Many people create lists by “including everything that has been reported in the literature.” The list resulting from this approach might be long, but a considerable portion of it is not targeted and merely adds to the workload. A good list of candidates does not depend on the large number of candidates, but rather on the fact that each candidate has a clear reason for being on that list. This reason may have several sources. In-depth analyses of a certain mechanism in the literature suggest that a particular structure might be effective; experience gained in other systems indicates that a certain material is worth trying; theoretical deductions regarding the structure of active sites point in a specific direction. But if a candidate’s reason is simply “others have used it, so I’ll give it a try,” it probably shouldn’t make it onto this list. Without attempts at in-depth understanding, the success rate is low; even if the right choice is made by chance, it is difficult to explain why it is correct or under what conditions it will fail. The list should not be too long. From my own experience, 5 to 10 candidates are usually sufficient to cover the main directions. There are too many things that distract attention, and the screening process takes too long ； Too few may result in missing good directions. II. Activity and selectivity: Both indicators must be considered simultaneously. After the list of candidates is compiled, a preliminary screening is carried out under uniform conditions. It’s important to have uniform conditions. All candidates are tested under the same temperature, pressure, and ratio, so that the comparisons resulting from this are meaningful. The purpose of the preliminary screening is not to find the “best performance” among all candidates, but to determine who performs best under the same conditions. There are two evaluation criteria: conversion rate and selectivity. Both must be considered simultaneously; neither can be omitted. I have encountered situations where a catalyst had extremely high activity, achieving conversion rates in the 90s, but its selectivity was only around 70%. Most of what is produced are by-products – you spend money on raw materials to create a bunch of waste, and then have to spend more money to deal with that waste, ending up losing money in both cases. Catalysts with high activity and low selectivity have little practical value in industry. Unless your by-products also have market value, or there are inexpensive methods to recover and reuse them—but such cases are after all rare. Conversely, high selectivity with low activity is also not the most ideal. The selectivity is over 90%, but the conversion rate is only around 40-50%; a large amount of raw material circulates within the system, and this circulation implies energy consumption, higher equipment costs, and increased operational complexity. Therefore, during the preliminary screening stage, active selective cross-evaluation is crucial. The usual approach is to create a 2D graph with conversion rate on the horizontal axis and selectivity on the vertical axis, to identify which candidates fall in the upper right corner. These candidates proceed to the next round of evaluation. III. Preliminary assessment of stability: Lifespan issues cannot wait until pilot testing to be revealed. For candidates that perform well in the initial screening, multiple repeated use tests must be conducted thereafter. Why is this step important? Some catalysts perform well when used for the first time, but their performance declines significantly after the second and third uses. If it isn’t detected during the pilot stage and is only discovered on a pilot plant or even an industrial scale – where the catalyst becomes ineffective after just one month of operation – the cost will be far more than simply replacing the catalyst in the laboratory. Stability testing requires attention to the following indicators. Has the conversion rate decreased after each use, and by how much? Has the selectivity changed, and is the distribution of by-products drifting? Have there been any changes in the catalyst’s appearance, specific surface area, and content of active components? The trend in these indicators is more important than the absolute value of a particular batch. Of course, the pilot stage can only provide a preliminary life assessment. A true long-term life assessment requires to be conducted on a pilot plant, as only a pilot plant can simulate real raw material fluctuations and operating conditions. However, the preliminary judgments from the pilot phase can eliminate those candidates with significantly insufficient lifespan at an early stage. If the initial stability performance is good, accelerated aging methods can be tried next to conduct further evaluation. The principle of accelerated aging is to speed up the deactivation of the catalyst under more stringent parameters than normal conditions, and then use kinetic models to extrapolate the expected lifespan under normal conditions. Common acceleration methods include increasing temperature, raising space velocity, and increasing impurity concentration. It is important to note here that accelerated aging may trigger inactivation mechanisms that do not exist under normal conditions—for example, when the temperature is high enough, catalyst sintering becomes the main factor leading to inactivation, whereas under normal conditions the primary cause of inactivation is carbon deposition; the kinetic laws for these two processes are completely different. Therefore, after the accelerated aging test, it is necessary to take several samples and return them to normal conditions for verification, to confirm that the degradation patterns under normal and accelerated conditions are consistent. IV. Cost assessment: It’s not enough to consider only the unit price; the overall cost must also be taken into account. Even if the performance meets the requirements and stability seems feasible at first glance, there is one more factor that cannot be ignored – cost. The cost assessment of catalysts certainly cannot rely solely on the unit price. I’ve seen a project where the unit cost of Catalyst A was twice that of Catalyst B, but the lifespan of Catalyst A was three times longer than that of Catalyst B – calculated this way, the total cost over its entire life cycle was actually lower for Catalyst A. Therefore, cost assessment needs to be considered from several aspects comprehensively. What is the purchase unit price, what is the expected service life, what are the downtime costs associated with each replacement, and does the disposal of waste catalysts incur costs related to hazardous waste treatment? The unit price multiplied by the number of replacements, plus the downtime losses and the costs associated with handling the waste catalyst, divided by the total production volume – this is the actual catalyst cost per ton of product. While conducting cost estimates, it is also necessary to prepare at least one alternative supplier for each candidate. If a certain catalyst can only be produced by one supplier, any problems with supply will force the entire plant to shut down. The diversity of supply sources and the reliability of supply also need to be taken into consideration in the evaluation. In the catalyst selection for various projects, we typically consider three factors—performance, stability, and cost—on a comprehensive basis before making a final recommendation. Sometimes, a catalyst with slightly lower performance but stable supply and controllable costs is more advisable than one with better performance but higher costs and limited supply. V. Three-dimensional Trade-offs and Decision-Making: After evaluating the three aspects of performance, stability, and cost, it’s time to make a comprehensive judgment. It is rare for all three dimensions to be optimal at the same time. More commonly, A has good performance but is expensive; B offers good value for money but its stability is questionable; C has excellent stability but lower activity. How to choose? There is no universal formula. My approach is to convert all three dimensions into economic indicators, calculate the catalyst cost per ton of product for each option, and then take into account the effects of performance differences and supply risks. Finally, ask yourself a question: if you make the wrong choice, what will be the cost? Is it easy to replace? If it’s relatively easy to switch, you can take appropriate risks ； If changing the catalyst means altering the design of the entire device, then a more conservative choice is required. After comprehensive evaluation, it is often necessary to engage in technical discussions with the catalyst supplier to understand potential areas for further improvement. Some catalysts perform moderately at the laboratory stage, but the supplier promises to improve their performance by optimizing the production process. These factors also need to be taken into account in decision-making. VI. After screening: Identifying optimization directions from the results. The results of catalyst screening not only tell you \"who to choose\", but also indicate \"what to optimize next\". If screening reveals that the conversion rates of all candidates are unsatisfactory, it may be necessary to start from the reaction mechanism and reconsider the approach to catalyst design. If the stability of all candidates is poor, it may be necessary to check whether impurities in the raw materials are causing catalyst poisoning. If the selectivity of the best candidate is only at a moderate level, then strategies to suppress side effects may need to be considered. These are the inputs for optimizing pilot-scale conditions, and they are also the topics to be discussed in the upcoming episodes. Preview for the next issue: Issue 22 – Optimizing reaction conditions: Finding the operational range rather than a single point. Once the catalyst has been selected, the next step is to determine the optimal reaction conditions. Temperature, pressure, ratio, residence time — how to systematically find the optimal operating range? Why does industrial production require a “window” rather than a “point”? To be continued in the next issue. Table of Contents link: Seven-Stage Model of Chemical Technology from Idea to Industrialization (Issue 100) – Table of Contents (updated in real time)