In this blog post, we’ll use the analogy of a chemical plant as a giant living organism to take an easy-to-understand look at the types of chemical reactors and their respective characteristics.
Why is a chemical plant likened to a giant monster?
If you’ve ever driven out of the city into open fields while heading home for the holidays or going on vacation, you’ve probably seen this at least once: a massive, monstrous structure. With its overwhelming size, billowing white steam, and the low rumble of machinery, a chemical plant viewed from afar feels just like a giant, living organism. Normally, you might pass by a chemical plant without a second thought or simply look at it with curiosity. But have you ever wondered how this giant monster “eats” and “lives”?
This monster has a metabolism similar to that of an animal. An animal’s metabolism consists of a mouth that takes in food, digestive organs that break down the food, a brain that regulates digestion, and organs that store the digested nutrients. Similarly, a chemical plant consists of a feed section that introduces raw materials (hereinafter referred to as “reactants”), a reactor that converts reactants into products, a control system that regulates this process, and a processing line that transforms the products into the final goods. Among these components, the reactor—which corresponds to the digestive system—is arguably the most critical part. Just as an animal cannot sustain life if it fails to digest food properly no matter how much it eats, a chemical plant cannot produce the desired products if the reactor does not function properly. Just as an animal’s digestive system is divided into the mouth, stomach, and intestines, chemical reactors are also broadly classified into three types.
Why is a tank reactor similar to an animal’s mouth?
The first type of reactor is the tank reactor (batch reactor). A batch reactor consists of a sealed tank with no material inflow or outflow, and a stirrer that uniformly mixes the material inside the tank. This reactor is operated by an operator who directly adds the reactants and then removes the products after a set period of time. In this respect, it resembles an animal’s “mouth,” where digestion begins even before the food is chewed and swallowed.
The most significant characteristic of this reactor is that, when the concentration of reactants is the same, there is no direct correlation between the size of the reactor and the conversion rate—which indicates the extent to which reactants have been converted into products. For example, even if a portion of the contents of a batch reactor is transferred to a beaker and set aside, the reaction rate will remain the same as long as the concentration of reactants is identical. This follows the fundamental principles of chemical reactions. Since chemical reactions occur when reactant particles collide with one another, the reaction rate increases as the number of reactant particles in a given space increases—that is, as the concentration of reactants rises. However, just as seawater tastes similarly salty no matter where it is drawn from, the concentration of reactants in a tank reactor remains constant even if the volume changes. Therefore, neither the reaction rate nor the conversion rate is affected by volume.
Ultimately, the conversion rate in a batch reactor is determined by reaction time rather than the amount of reactants. The longer the reactants remain inside the reactor, the higher the conversion rate becomes; with sufficient time, a very high conversion rate can be achieved. Because of these characteristics, batch reactors are primarily used to produce high-value-added chemical products, such as pharmaceuticals and fine chemicals, which require high purity.
How do continuous stirred-tank reactors and tubular reactors differ?
Unlike tank reactors, the other two types of reactors are called flow reactors because the material continuously moves within them. Additionally, unlike tank reactors, flow reactors have the characteristic that as the reactor volume increases, the residence time of the reactants inside increases, generally resulting in a higher conversion rate.
The second type of reactor is the continuous stirred-tank reactor (CSTR). While this reactor is similar to a tank reactor in that it consists of a tank and a stirrer, it differs in that it has an inlet and an outlet that allow reactants to continuously enter and exit at a constant rate. Due to this structure, a CSTR is similar to an animal’s “stomach,” which constantly receives food, digests it, and then sends it to the next organ.
The most significant feature of the CSTR is that the concentration of the reactants is maintained almost uniformly throughout the reactor due to the agitator. To put it somewhat exaggeratedly, the composition of the reactants just entering the reactor is nearly identical to that of the reactants being discharged immediately afterward. Consequently, the concentration of the reactants throughout the reactor is the same as that at the outlet, and since the concentration is relatively low, the reaction rate is also slow. Although increasing the reactor volume improves the conversion rate, actual equipment is limited in size, making it difficult to achieve a high conversion rate compared to other reactors. However, unlike batch reactors, once operation begins, continuous operation is possible for extended periods, making it highly suitable for mass production. For this reason, CSTRs are widely used in the production of various liquid chemical products that require mass production.
The third type of reactor is the tubular reactor (PFR, Plug Flow Reactor, and PBR, Packed-Bed Reactor). Although PFRs and PBRs are sometimes classified as distinct reactors, PBRs are essentially PFRs packed with solid catalysts, and since their basic operating principles are very similar, they will be discussed together here. Tubular reactors have a structure in which reactants enter at one end of a long tube and products exit at the opposite end. The appearance of a long tube with multiple bends is reminiscent of an animal’s “small intestine.”
The most significant characteristic of a tubular reactor is that, because it lacks a stirrer, the concentration of reactants varies depending on location. While the concentration is nearly uniform throughout a CSTR, in a PFR, the reactant concentration is highest near the inlet and gradually decreases toward the outlet. Consequently, the average concentration inside the reactor is higher than in a CSTR, resulting in a faster reaction rate. Furthermore, since multiple long tubes can be connected in series, it is relatively easy to construct reactors with large volumes, which is advantageous for achieving high conversion rates. Due to these advantages, PFRs are still widely used today in various chemical processes to produce both liquid and gaseous products.
This may raise the question: from the perspective of a chemical plant operator, wouldn’t it be sufficient to use only PFRs, which have a higher conversion rate, and eliminate the need for CSTRs? However, this is not actually the case. First, in a PFR, “hot spots”—areas where the temperature rises abnormally—can occur at the bends in the tubes. Agitators not only maintain a uniform concentration of reactants but also dissipate the heat generated during the reaction. In contrast, because PFRs lack agitators, heat can become concentrated in specific locations; this phenomenon is particularly pronounced in liquid-phase reactions when fluid mixing is insufficient. Hot spots are a critical issue because they not only interfere with the desired reaction but, in severe cases, can also compromise equipment safety.
Furthermore, as the length of the PFR increases, friction between the fluid and the tube wall increases, which can lead to a greater pressure drop; this, in turn, may impose constraints on throughput or operating conditions. In other words, when comparing a CSTR and a PFR of the same volume, depending on the operating conditions, the CSTR may be able to process a larger volume of reactants more reliably. For this reason, CSTRs are often more suitable for the mass production of products that do not require extremely high purity.
To compensate for these drawbacks of PFRs, many chemical plants use both types of reactors in combination. The process is designed so that the reaction proceeds rapidly in the CSTR to raise the conversion rate to a certain level, after which the process continues in the PFR to reach the final target conversion rate. This approach allows for high production efficiency while reducing the length of the PFR. This process closely resembles the way our bodies digest food: food first passes through the stomach before undergoing full digestion in the intestines.
In this way, chemical plants—which may appear to be giant monsters at first glance—are actually like a single, massive living organism equipped with a “mouth” (the tank reactor), a “stomach” (the continuous stirred-tank reactor), and “intestines” (the tubular reactor). Chemical plants, too, operate by efficiently utilizing these three digestive organs, each performing a distinct role, to produce more products. Given that the chemical industry plays a vital role in various fields today, if we think of chemical plants not merely as massive industrial facilities but as familiar entities that operate on principles similar to our own, we may come to view them from a whole new perspective.
