I would like to ask the experts: what kind of reactor is used in FTO synthesis to produce low-carbon olefins?

Reactor types for F-T synthesis: 1. The early ARGE process used tubular fixed-bed reactors, in which the solid catalyst was placed inside tubes with an inner diameter of about 5 cm and a length of 12 meters, with these tubes arranged vertically. Thousands of reaction tubes make up a reactor; water is present between the reaction tubes as well as inside the reactor, to absorb the heat released by the reactions. In fact, each reaction tube is itself an independent reactor; a larger reactor is simply formed by arranging multiple tubular reactors side by side. Both the upper and lower ends of the tube bundle are welded to massive steel plate supports. 2. The Kellogg process in the United States uses a circulating fluidized bed reactor; after improvements were made in Sasol, South Africa, it was given a new name: Synthol, which is a term meaning synthetic oil, formed by combining the words \"synthetic\" and \"oil\". The academic term is still circulating fluidized bed, abbreviated as CFB. 3. In the 1980s and 1990s, a fixed fluidized bed FFB was developed, and Saso gave it its own name: the Saso Advanced Synthetic Oil Reactor, abbreviated as SAS. These are several types of reactors, but the F-T synthesis is generally not used to produce low-carbon olefins. The current technology for producing low-carbon olefins is MTO/MTP, and there are also two types of reactors for this process: fixed-bed and fluidized-bed.

Thanks for the reply from above! MTO/MTP can also be used to directly produce low-carbon olefins, but methanol is required for this process, and methanol is generally produced from syngas. I know that there are many reports on the use of fixed-bed reactors for the production of low-carbon olefins via Fischer-Tropsch synthesis; however, there are almost no studies on the use of fluidized-bed reactors for this purpose. Why is that? Is it because the selectivity for low-carbon olefins is low, making separation difficult?

FTO, or Feito synthesis for the production of low-carbon olefins, represents an application and extension of the Feito synthesis reaction. The literature reports only the use of fixed-bed reactors and no fluidized-bed reactors; this is not a decision based on the reaction or design, but rather on cost considerations and practical operational factors. At the laboratory scale, fixed-bed reactors require the least investment and are easier to operate. Setting up a fluidized-bed reactor for FTO in a laboratory involves high costs and a long time frame, unless the laboratory already has experience in working with fluidized-bed reactors. FTO has been quite popular in recent years, with an increasing number of articles on it. However, research has focused mainly on catalysts, with the development of engineering and processes lagging behind relatively. FTO currently relies primarily on iron catalysts (at least for now, no catalysts better than iron are available); operation at high temperatures necessitates catalyst regeneration. However, the pursuit of high activity and selectivity for low-carbon olefins during catalyst development has resulted in catalyst strengths that are not sufficient for regeneration purposes. As of now, I believe the only iron catalysts prepared by the coprecipitation method that can be used in fluidized-bed processes for the production of low-carbon olefins are those supplied by Sasol (the catalysts developed by Southern Chemical for them), although their main purpose is not the production of low-carbon olefins. Personally, I consider the difficulty of FTO to be as follows: 1. Reaction pressure – low pressure is favorable for the selectivity of lower-carbon olefins; therefore, one approach must be to find a way to achieve high selectivity for lower-carbon olefins even at high pressures ； 2. The selectivity of the catalyst itself for low-carbon olefins ； 3. The combination of catalyst and reactor: While it is not impossible to use a fixed-bed system for FTO in industrial applications, it is a rather challenging task. The fixed-bed MTP technology used by Lurgi can serve as a reference, but implementing it in practice is even more difficult due to the high reaction temperature rise and the narrow suitable range for reaction temperatures ； The technology of domestic circulating fluidized beds is quite good, but fixed fluidized beds have almost no application or development. 4. To ensure that the coprecipitated iron catalyst has a strength sufficient for use in fluidized beds, extensive research is required, as well as a good understanding of the catalyst itself ； 5. The reaction process inevitably produces hydrocarbons with slightly longer chains (with a selectivity of around 50%); how should these hydrocarbons be dealt with?

This post was last edited by fossil-zhang on 2012-10-12 at 21:09. Some questions have already been answered above, so I won’t repeat them. What the original poster is referring to is likely the fact that articles generally report that fixed-bed reactors are used most often for FTO synthesis to produce olefins, while fluidized-bed reactors are used less frequently; this is quite normal, as explained by the person who posted above. At present, no such technology at an industrial or pilot-scale level exists; most research is carried out in the laboratory. Once it is put into industrial use, it is not necessarily limited to fixed-bed systems – fluidized beds can also be used. The key is to determine which process is more economical, practical, and efficient. We considered using the Fischer-Tropsch process to produce low-carbon olefins a long time ago; in fact, many people have wanted to do this since the invention of Fischer-Tropsch. However, in the past it was more cost-effective to produce oil, and the production of low-carbon olefins was dominated by traditional petrochemical industries, which is why few individuals or companies put this idea into practice. In fact, the Fischer-Tropsch process is designed to produce low-carbon olefins; only, its selectivity is too low. Moreover, in the past, it was not desired for the selectivity of low-carbon-number products to be too high – the lower, the better. How should existing large-scale industrial plants deal with these small amounts of low-carbon olefins? They can either send them back to the reactor to increase the selectivity for long-chain hydrocarbons, or separate them out to use as LPG, or use them in more advanced processes for olefin oligomerization in order to produce products with higher added value. In any case, any way to earn more money is beneficial, as the main purpose of these existing plants is to produce oil. SASOL’s process uses a fluidized-bed high-temperature iron-based catalyst, which offers a higher selectivity for low-carbon products; therefore, they sought ways to make use of these products in order to increase their added value and thus boost the overall economic efficiency of the process. They did this very well. These concepts and overall strategies are more valuable than those specific processes. It’s a shame that such concepts and strategies are considered by some experts in China to be outdated technology; as a result, they opted for the slurry-bed low-temperature cobalt-based catalyst technology, which those experts consider to be more advanced. Yet even this still cannot compete with domestic technologies that have similar pricing but a lower actual technical level. But that’s beside the point. Why has the use of FTO synthesis for producing low-carbon olefins gained renewed attention recently? It is thanks to the surge in crude oil prices, which has led to the rise of new types of coal-based chemical processes. As for MTO/MTP, it is a technology that has existed for a long time but was not given much attention abroad; however, it is now seeing significant development in China. After the ideas related to MTO/MTP were exhausted, people thought of Fischer-Tropsch synthesis; since it wasn’t possible to produce oil with this method, it made sense to focus on producing low-carbon olefins with higher added value, and that’s how research in this area began to develop. As mentioned above, we considered this approach a long time ago and conducted some research. The conclusion we reached was that if one wants to use the Fischer-Tropsch process to produce low-carbon olefins, based on the principles of this synthesis reaction, there are two key aspects to focus on: catalysts and the process itself. As for what type of reactor to use, it again comes down to what is most suitable, what works best in combination with the catalysts and the process; there is no such thing as advanced or outdated options. We are experts in catalysts, so we’ll talk about that later; first, let’s discuss the unprofessional processes. Fischer-Tropsch synthesis is a reaction in which carbon monoxide and hydrogen combine, under the action of a catalyst, to form long-chain hydrocarbons: CO + 2H2 → CH2 + H2O. It should be noted that these are long-chain hydrocarbons; under traditional catalysts and reaction conditions, this reaction tends to produce C5+ products. In other words, more than 80% of the hydrocarbon products consist of C5+ compounds, while C1-C4 compounds account for only 20% of the total hydrocarbon output. Methane constitutes the largest proportion among these C1-C4 compounds, with saturated hydrocarbons being predominant among C2-C4 compounds, and very few alkenes present in this range. Due to the mechanism of Fischer-Tropsch synthesis, these C2 alkenes also participate in the reaction, but only a small amount of them end up in the final products. As a result, most of the low-carbon alkenes in the products are C3 and C4 compounds, and the selectivity for such alkenes is quite low. So, what should be done to improve the selectivity for C2-C4 olefins? The only way is to deviate from the traditional carbon chain elongation reactions and keep the reaction at a lower carbon level as much as possible. In other words, when the carbon count reaches C4, chain termination reactions should occur more frequently, while chain elongation reactions should occur less often. As a result, the reaction products will naturally consist mainly of compounds with lower carbon counts. This sounds simple, but in reality it’s not that easy; it’s very difficult to break away from traditional rules. Someone said, \"You’ve said so much yet still haven’t gotten to the point – how can this idea be implemented?\" I’m setting the stage here; if these issues aren’t clarified, it will be impossible to move on. As I mentioned earlier, on one hand, the approach lies in optimizing the process parameters. How to do this? It’s simple: reduce the reaction pressure, increase the reaction temperature, and raise the hydrogen-to-carbon ratio. These are the only methods I can think of that can improve the selectivity for low-carbon compounds. Why is this? The principle behind the Fischer-Tropsch process is that higher pressure facilitates the formation of long-chain hydrocarbons, while higher temperature promotes the formation of low-carbon hydrocarbons. An increased hydrogen-to-carbon ratio also helps to produce low-carbon hydrocarbons. As for why these effects occur, it involves more complex issues, so I won’t go into detail here; otherwise, it would amount to a lecture on catalytic chemistry. In any case, it’s sufficient to remember these rules regarding the impact of various parameters. But can these methods truly achieve a significant increase in the selectivity for low-carbon hydrocarbons? Perhaps they can, but the economic benefits are likely not comparable to those of other processes; after all, the use of FTO is intended to improve economic efficiency. Firstly, there is a stress-reducing effect, but it is not yet possible to obtain products that are entirely low-carbon hydrocarbons; only an increase in the amount of low-carbon products is seen compared to high pressure conditions, with methane possibly increasing even more. Additionally, lower pressure results in lower production efficiency. Moreover, fluidized beds are more suitable at low pressures, while fixed beds can also be used but are more complicated to operate. The reactors are relatively short and stout; to increase the processing capacity, they cannot be made longer, as this would increase the bed resistance, so horizontal expansion is the only option. A fluidized bed can increase the space velocity to boost throughput, whereas a fixed bed has limitations in this regard. Raising the temperature is also rather troublesome, but it must be done. If the catalyst is not improved, a much higher temperature is required to achieve a significant increase in the selectivity for low-carbon hydrocarbons; however, methane production may increase even more as a result. The most important issue arising from increased temperature is the stability of the catalyst. At temperatures of 350 degrees or higher, cobalt can no longer be used – only molten iron can be employed (it is said that Zhejiang University in China has developed a high-temperature catalyst made of precipitated iron, which possesses both the strength and stability necessary for use in traditional high-temperature processes). But aside from stability and strength, iron has one major problem related to the water-gas shift reaction: increased temperature raises the activity of this reaction, causing CO to turn into carbon dioxide – so what’s the point then? What’s left is to increase the hydrogen-to-carbon ratio. Raising this ratio can improve the selectivity for low-carbon compounds, but methane production is likely to increase even more. Moreover, the source of hydrogen presents a problem; using natural gas to produce syngas – if natural gas is available – would be better, as it would be more efficient to use methane for ethylene production rather than resorting to the FTO process. Use coal to produce syngas; increasing the hydrogen ratio means wasting carbon monoxide (hydrogen extraction via water-gas shift). So, from a process perspective, there are limited possibilities for improving the selectivity of low-carbon products in Fischer-Tropsch synthesis, and the costs may not even be lower. Well, only the catalyst can be improved, and that’s my area of expertise; yet even though it’s my specialty, I find it quite challenging. There are two methods we’ve thought of for improving catalysts. One is to minimize the size of the active metal in the catalyst; from the perspective of catalytic reaction principles, reducing the microscopic size of the active metal increases the steric hindrance in the carbon chain growth reaction. This might be difficult for many people to understand, and this is as far as I can go on this topic. Going any deeper would mean returning to lessons on catalytic chemistry. As usual, it’s sufficient to remember that steric hindrance helps in the formation of lower-carbon hydrocarbons. However, this brings two problems: firstly, methane production increases further, and secondly, the catalyst’s activity is very low, and it’s easy for the active metal in the catalyst to become sintered and lose its activity, especially at high temperatures. Why is the activity so low? Logically, the smaller the active metal, the more surface active centers there should be, which should lead to higher activity. The Fischer-Tropsch process is somewhat special – when the size of the active metal becomes too small, the reaction capacity of each individual active center actually decreases (due to steric hindrance). So this path is no longer viable. Then there is one remaining option, which is to improve the catalyst using molecular sieves; this approach is also the most commonly used for catalyst improvement at present. Why? Actually, I explained it in detail in previous posts. Why are SAPO-34/ZSM-5 used as catalysts in MTO/MTP processes? It is because the pore structures of these two types of molecular sieves are quite special, with pore sizes that are approximately equal to those of C2/C3 olefin molecules. This is the function of molecular sieve selection – through the selectivity of their unique pore structures and their acidity, they enable improved selectivity for C2/C3 olefins. Although the reaction mechanism of MTO/MTP differs from that of the Fischer-Tropsch synthesis, the Fischer-Tropsch synthesis also produces -CH2- intermediates similar to those in the MTO/MTP reactions. It is possible to use the selective properties of molecular sieves to improve the selectivity for C2-C4 olefins in Fischer-Tropsch synthesis, and it has been proven that this can indeed be achieved with good results; we have also carried out some research in this area. Since this type of molecular sieve lacks Fischer-Tropsch catalytic activity and has very small pores, it is necessary either to modify the molecular sieve by metal doping in order to endow it with such catalytic activity, or to encase the molecular sieve in a membrane around a conventional Fischer-Tropsch synthesis catalyst, thereby allowing the products of Fischer-Tropsch synthesis to undergo selective separation through the molecular sieve and improving the selectivity for low-carbon olefins (a similar approach has already been successfully used to significantly enhance the selectivity for gasoline and isomeric hydrocarbons in conventional Fischer-Tropsch synthesis catalysts). The former method destroys the structure of the molecular sieve, the amount of doping that can be used is limited, and the catalytic activity remains low; therefore, the latter method holds the greatest potential for development. Nevertheless, this latter method remains quite difficult. Because despite improvements in the catalysts, changes to the process parameters are still necessary—specifically low pressure, high temperature, and a high hydrogen-to-carbon ratio. The high temperature is the most problematic factor; to significantly increase the selectivity for low-carbon olefins, it is necessary to use temperatures even higher than those in traditional high-temperature Fischer-Tropsch synthesis. Iron catalysts can be used, but combining molecular sieves with iron catalysts reduces their strength, which leads to stability issues related to the catalysts. This is the biggest obstacle to the industrial application of this technology. A fixed-bed reactor is a better choice, and this is one of the reasons why many studies use fixed-bed reactors. Under high-temperature conditions, the wear on the catalysts caused by fluidized beds poses serious stability problems. A separate molecular sieve and separate molten iron might be fine on their own, but combining the two could pose significant problems. So, after saying all this, the point is to illustrate that perhaps the Fischer-Tropsch process is the most suitable for producing low-carbon olefins in a fixed-bed system; although I think a fluidized-bed system would be more preferable, catalyst technology needs to be improved first. Who knows, maybe one day someone will invent a new catalyst that can directly and easily synthesize C2-C4 olefins from syngas with high selectivity. Think about it – being a catalyst isn’t easy either; sometimes one has to be prepared for people to call one’s ideas fanciful.