In fixed bed catalytic reaction, the shape of catalyst has plenty role to control the kinetics of the reaction. People have used trilobe and quadralobe cross section of catalyst in preference to cylindrical or spherical cross section. What is the exact advantage in using such dimension of catalyst on the transport phenomenon of reactants inside the reactor over catalyst particles? How does RTD depends on the extrude shape?
Some catalyst vendors claim that catalyst contacting is improved by multi-lobe catalyst in mixed phase (vapor- liquid) reactors and that diffusional/catalyst contacting limitations are minimized. Many of the most active Hydrotreating catalysts available are multi-lobe catalyst. It is unclear how much (if any) the shape of the catalyst actually contributes to actual increased activity. The most common claim is that it improves catalyst contacting and therefore apparent activity.
In a vapor phase reactor catalyst shape advantages are less likely because catalyst conatacting is not likely to be a problem.
Hi Indrajit. If the reaction occurs in the kinetic regime, the shape of the catalyst does not have an effect on the rate observed. If the reaction is transport limited, then I agree that the shape of the catalyst has influence on the reaction rate observed, or observed kinetics if you prefer to call like that.
If the limiting step is the mass transfer of a species from the bulk of the fluid to the external surface of the catalyst, the advantage of trilobe or quadralobe versus spherical or cylindrical catalysts is related to the surface area to volume ratio of the catalyst, let´s call this value ai = (S/V)i. For simplicity of the argument, let us only compare a sphere with a trilobed. Their surface to volume ratio have the behavior (S/V)s < (S/V)t, where s = sphere and t = trilobed. What does it mean for the reaction? it means that trilobed catalysts have more external surface area available "to receive" species from the bulk phase compared to a spherical particle. Please be aware that the mass transfer coefficient is not changing, the only change is an increase in surface area that enhances the mass transport rate observed.
To summarize the idea. Imagine you have the reaction A --> B which is limited by the mass transfer of species A from the bulk phase to the surface of the catalyst. The observed reaction rate would be: r = kG*ai*(C_A - C_A,ext); where, r is the rate, kG is the mass transfer coefficient, ai was defined above and, C_A and C_A,ext represent the concentration of species A in the bulk phase and the external surface of the catalyst. A change on the catalyst shape that would allow you to increase ai will have a positive impact on the reaction rate observed, since the rate (in these conditions) is directly proportional to the surface area to volume ratio of the catalyst particle, ai.
I hope this helps with your doubt.
Hi Carlos Ortega, do you have any reference material for this topic that i can read.
Hi, Carlos. Thanks very much for your explanation. I agree to your clarification.
Hi Blessing Mahuhushe. First, my apologies for the late reply (I took my end-of-the-year holidays rather seriously and had little online interactions…) I would refer you to text books that deal with kinetic measurement and kinetic descriptions of reactions. They should, in general, explain what I have said in the initial reply to this post. My favorite book would be "Mass Transfer in Heterogeneous Catalysis" (1970), from Charles Satterfield. In my opinion, the best book dealing with this topic. If you have further comments and/or questions about it, please let me know. I'm really passionate about this topic.
Best wishes to all in 2020!
Hi Carlos Ortega,
Thank you for the feed-back, its appreciated.
Since you left the door open for further equiry, may i have your email adress so that i can ask your help at any stage, mine is [email protected]
Happy new year.
Hi Blessing Mahuhushe,
I would be happy to discuss further catalysis related topics with you. I have sent you a short email from my personal email account. The subject of the email is "Carlos Ortega - ResearchGate contact for catalysis related discussions". Could you please confirm that you have received it? You could confirm directly with another email, to not disturb the rest of the people in this thread of questions/answers.
Best regards,
Carlos
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