Question: Is the heat transfer of large-particle catalysts generally better than that of small-particle catalysts? Is there a theoretical basis to explain this?

Large-particle catalysts are affected by external diffusion, which generally results in poorer heat transfer and a higher temperature gradient. I’m not sure what type of reaction system the original poster is working with

Large-particle catalysts have a larger specific surface area; they are heated more evenly, are less prone to thermal decomposition, possess a larger heat transfer area, and thus exhibit higher activity. Small-particle catalysts are unstable and prone to loss. Just my personal opinion; let’s have more discussions.

At the level of an individual catalyst (on a microscopic scale), larger particles offer better heat transfer, while for the entire catalyst bed, smaller particles are preferable, although the issue of hot spots arises.

The specific surface area is related to the pore structure and has no direct relation to the size of the catalyst particles; moreover, for solid spheres, the larger the particle size, the smaller the specific surface area should be

This post was last edited by fossil-zhang on 2010-8-3 at 12:34. Regarding this question, is the original poster from the Institute of Geology? Do Fischer-Tropsch synthesis? Fixed-bed reactor? In a fixed-bed system, this phenomenon does indeed occur: larger particles tend to have better heat transfer properties than smaller particles. What was said on page 4 is incorrect; when considering individual particles, it’s clear that larger particles have poorer heat transfer capabilities than smaller ones. However, this issue must be viewed in terms of the entire catalyst bed. In such a bed, the bulk density of larger particles is lower than that of smaller particles. Heat transfer depends not only on 1) the catalyst itself but also on the axial heat conduction between the catalyst and the walls of the reaction tubes, as well as the radial heat conduction throughout the bed. For 1), heat transfer related solely to the catalyst itself requires consideration of the particle size and pore structure of the catalyst. As for 2), it depends on factors such as the diameter of the reaction tubes, the bulk density of the catalyst particles, the particle size of the catalyst, the space velocity of the reactant gas, and the proportion of inert gases. When the tube diameter and reaction conditions remain constant, heat transfer is primarily influenced by the catalyst’s bulk density and particle size. If the catalyst particles are too small, there are fewer gaps between them (resulting in a high bulk density), and heat may be less able to conduct through them; therefore, there is an optimal size for the catalyst particles in a fixed-bed system. As for how to consider this specifically, one can look at the three transfers and one reaction in chemical engineering; these are the most fundamental concepts.

This post was last edited by *aojungnft on 2010-8-3 14:36. First of all, it’s definitely referring to a fixed-bed reactor, right? Second, it’s unclear whether the uniform temperature distribution mentioned by the poster refers to the radial or axial direction Third, whether the temperature distribution is uniform or not is not necessarily directly related to the quality of heat transfer; in other words, there is not necessarily a direct causal relationship between the two. Could you be more specific? Because based on your current description, it’s not possible to provide any suggestions. Furthermore, it is generally believed that for the fixed-bed Fischer-Tropsch synthesis catalysts currently in use, under normal reaction conditions, the temperature difference between the inside and outside of the catalyst particles is not significant; therefore, in most cases it is ignored.

It is divided into two parts: 1. Individual catalyst particles, 2. Catalyst bed. For 1. Individual catalyst particles, the reaction heat is transferred from within the catalyst particle to its outer surface through thermal diffusion, and then carried away by the fluid; the influence of particle size can be determined using the Biot number. Qualitatively, the larger the particles, the greater their thermal resistance, which hinders heat transfer. Heat tends to accumulate within the particles, causing the temperature there to be higher than that of the fluid; as a result, methane selectivity is high. Of course, when the particles are larger, the mass transfer effect also leads to an increase in methane. Under normal Fischer-Tropsch synthesis conditions, the Bi number is not high, and the temperature within the particles can be considered uniform; thus, the effect of particle size on heat transfer can be ignored. What needs to be considered is the impact of mass diffusion. The young imperial son-in-law was also mentioned above. 2. Heat transfer in the bed layer. Heat transfer between the fluid and the catalyst bed is highly dependent on particle size; the Reynolds number is one of the quantitative indicators used to describe this relationship. The larger the diameter of the particles, the higher the heat transfer coefficient. Depending on the application, the influence of particle diameter ranges roughly between an exponent of 0.5 and 1.0. Although there is currently no unified theoretical formula for this relationship, only empirical insights exist, theoretical guidance is still available in a qualitative sense: as the fluid flows over the curved outer surface of the particles, the separation of the boundary layer leads to turbulence, which reduces the thermal resistance and thereby increases the heat transfer coefficient. This is why I* observed that large particles are beneficial for heat transfer in the bed.

The heat transfer of single-particle catalysts is the first issue that must be addressed when studying heat transfer in fixed-bed systems. As mentioned by the person above, an empirical constant, namely the Biot number, is usually used as a basis for making judgments. For catalysts with a high Biot number, such as methanation catalysts with a diameter of around 1 cm, under normal reaction conditions, the temperature difference between the center of the catalyst and its surface can exceed 15 degrees ； For a Fischer-Tropsch synthesis catalyst with an effective particle size of around 3 mm, under normal reaction conditions, the temperature difference between its interior and exterior is only about 1-2 degrees (estimated); such a temperature difference is essentially negligible for Fischer-Tropsch synthesis. The original poster mentioned that catalysts can vary in size, but in the case of Fischer-Tropsch synthesis catalysts, the difference in temperature between the inside and outside is very limited. Additionally, due to diffusion effects, the average reaction rate of larger catalyst particles is lower than that of smaller particles; therefore, any differences in temperature distribution within the catalyst bed are not primarily caused by the catalyst itself. Next, let’s talk about the bed layer. The original poster mentioned the size of the catalyst particles; however, for fixed-bed reactors, the porosity of the bed layer remains roughly constant, with variations possibly ranging from 0.43 to 0.45 – a very small difference indeed. When the carrier is the same, the thermal conductivity of catalyst particles of different sizes is also essentially the same. Therefore, the thermal conductivity of the bed layer does not change significantly depending on the size of the catalyst particles (this is true provided that the particle sizes remain within a reasonable range, as the differences in particle size for catalysts used in Fischer-Tropsch synthesis are quite limited). Under Fischer-Tropsch synthesis conditions, the composition of the bed layer is relatively complex due to the presence of liquid, which differs from normal gas-solid heat transfer. Traditional gas-solid heat transfer models, whether they are one-dimensional pseudo-homogeneous models (considering only the radial direction) or two-dimensional pseudo-homogeneous models (considering both axial and radial directions), need to be modified when applied to fixed-bed Fischer-Tropsch reactors. This is mainly because, when the reaction rate is relatively high, the thermal conductivity of the bed layer varies in different parts of the bed layer, as the proportions of gas, liquid, and solid phases differ in those areas. Next, we will discuss the influence of fluid flow on heat transfer. This is a model similar to a trickle bed, but with a relatively small amount of liquid. It is generally believed that the flow rate of liquids is less than 1 cm/s, and for gases, at typical Fischer-Tropsch synthesis operating speeds, the flow rate is also in the cm/s range. Based on these estimates, the flow of fluids within the bed is far from the turbulent region; the phenomenon of the boundary layer separating and causing turbulence was mentioned, and under Fischer-Tropsch synthesis conditions, this may or may not occur – this has not been estimated. I conducted experiments in the laboratory and found that the effect of slip flow on heat transfer is very small, both in the radial and axial directions (under Fischer-Tropsch synthesis conditions). I’m still not quite clear what exactly is meant by uniform temperature distribution in the bed layer as described by the original poster. Does it refer to the presence of hot spots, that is, significant discontinuities in the temperature distribution along the axial and radial directions? Or does it mean that there are differences in the radial temperature gradient between two different catalysts, or differences in the axial temperature gradient? There are a total of 3 possible influencing factors that come to mind: 1, catalysts with different particle sizes result in varying edge effects between the catalyst bed and the inner wall of the reactor ； 2. Different catalysts have varying reaction rates, which leads to differences in heat transfer per unit area and thus affects the temperature gradient ； 3. The temperature gradient affects the variations in the liquid composition within the reactor bed, as well as the differences in the gas-liquid-solid ratios in different areas; this in turn leads to changes in the thermal conductivity throughout the bed, which in turn influences the temperature distribution. What is said are merely my own understanding and opinions; if there are any mistakes, please point them out.

Reply: 5# fossil-zhang is absolutely correct; the specific surface area has nothing to do with particle size. As for the claim that it has a high heat transfer efficiency, I believe it is because larger particles result in less gas resistance and a faster gas flow rate, which in turn allows heat to be transferred more quickly.

The uniform temperature distribution of the bed mentioned refers to the axial temperature distribution, with fewer hot spots and smaller particles. After reading everyone’s discussions, I think heat transfer must be related to particle size. Larger granule piles are smaller, resulting in a higher porosity of the bed layer and lower gas resistance, which facilitates the removal of heat. This is just my personal opinion; please feel free to correct me.