Three main phenomena are necessary to be considered in a heterogeneous catalytic process:

1.       External mass (and heat) transport, i.e. reactants from the bulk of fluid (gas or liquid) to the surface of a catalyst, and products from the surface to the bulk of fluid

2.       Internal mass transport , i.e. diffusion of reaction species in pores of a porous catalyst

3.       Catalytic reaction (chemisorption of at least one of reactants, surface reaction, desorption of products)

In heterogeneous reactors external mass transport can be increased by a better mixing of the fluid. A higher flowrate also contributes to a better mixing. Under the better mixing a laminar film around the catalyst particle is thinner and more reactants can be transported to the surface. Consequently, the overall conversion of reactants can be increased in spite of lowering of the residence time.

A good explanation of phenomena occurring in heterogeneous catalytic systems can be found in many textbooks. According to my experience from teaching “Heterogeneous Catalysis in Practice” written by Charles N. Satterfield is one of the most comprehensible.

Dr. Kralik, should be the conversion a constant in spite of the flow?

I agree with Dr. Kralik. However, you should also consider the following:

I presume you mean with activity, the amount of reactant converted per unit mass of catalyst (i.e. the integral rate of reaction). The rate of reaction is a function of the concentration (or partial pressure) of the reactants/product. In a plug flow reactor, the concentration of the reactants decrease with increasing bed length. Increasing the flow rate will result in a lower concentration gradient in the catalyst bed, and thus the integral rate of reaction. At low levels of conversion, a low concentration gradient, the integral rate is independent of the flow rate (note: the conversion is not constant when changing the flow rate of the reactants, but increases linearly with the flow rate at low conversion)  

In order to ascertain whether external mass transfer limitations affect your observed integral rate, you will need to vary the lienar veloctiy through the catalyst bed keeping the space time constant. This can be achieved by carrying out the reaction using a mass m1 of the catalyst and a flow rate V1 of the reactants; then load 2.m1 of catalyst and feed the reactants with a flow rate of V2=2.V1. Keep on increasing the flow rate of the reactants and the mass of catalyst, unitl the conversion is no longer a function of the volumetric flow rate of the reactants.

Many Thanks Professor Van Steen.

In fact, it also depends on your reaction, we have the same problem: during the study on the reaction: CO2 + H2 --> CH3OH + H2O

When we increase the flow rate (increase GHSV), the methanol production increase. But in the reation CO + H2 --> CH3OH + H2O

The increase of flowrate with decrease the catalyst performance.

Pls read the article in attached file

Dear Mr. Daza,

In addition to the perfect answers of Prof. Kralik and Prof. van Steen, I would like to point out another interesting effect of flow systems, which is the shear stress exerted on the catalyst surfaces. The shear stress - which depends directly on the superficial velocity - can help increasing the conversion over fixed catalysts by introducing a "self-clean" effect; that is, the cleaning of the catalyst surface by shear-induced removal of adsorbed by-products that block the catalytic sites. This can become particularly important for flows occurring in channels with small hydraulic diameters.

Best regards! 

Thanks Le Phuc and Bruno.

I would like to recommend the following article: "Experimental methods in catalytic kinetics" (C. Perego, S. Peratello, Catalysis Today, 52 (1999) 133-145). It describes  the effects of transport phenomena due to gas flow, catalyst and reactor geometry. It also explains how to design an experiment in order to avoid mass transport limitations and to ensure chemical kinetics control (a must for catalytic activity measurements). I hope that this might help answer your question.

Kind Regards

Dear Lucia, Thank you, I read yesterday. Excellent paper!

I agree with everyone and would like to add also that when comparing catalysts on a reaction, we should know whether there are mass transfer limitations present or not. To be totally honest in comparing catalytic activities, we should design in order to eliminate mass transfer phenomena, however the knowledge of conversions achieved per unit mass of catalyst with mass transfer limitations has its own value in many cases.

Dear Dr. Daza,

you should indicate which type of system you are utilizing.

Mass transfer limitations can be important in some cases. In the case of a suspension of photocatalytic powders in aqueous systems (slurry reactors), mass transfer does not limit the observed reaction rate except at very low flow rate (indicatively at Reynolds number less than 250). We have studied this aspect in a few papers: Camera-Roda et al., Study and optmization of an annular photocatalytic slurry reactor, Photochem. Photob. Sci., 8 (2009), 712-718; Camera-Roda & Santarelli, A rational approach to the design of photocatalytic reactors, Ind. Eng. Chem Res., 46 (2007) 7637-7644; Camera-Roda & Santarelli, Design of a pervaporation photocatalytic reactor for process intensification, Chem. Eng. Technol., 35 (2012) 1221-1228.

In the case that the photocatalyst is immobilized as a film on suitable supporting materials, mass transfer limitation of the observed rate are much more likely, especially in gaseous systems but also in aqueous systems. So, high flow rates or fluidized beds have to be adopted to avoid this type of limitation. For gas system, I can suggest to read the article: Minero et al., On the standardization of the photocatalytic Gas-Solid tests, Int. J. Chem. React. Eng., 11 (2013) 717-732.

Kind regards

Of course, my previous observations were referred only to photocatalytic systems.

Regards

Dear Dr. Camera Roda, thank you for your answer. We have worked, mainly, in methane decomposition, dry reforming of methane and oxidative reforming of ethanol.

From my own experience, diffusion limitation can be control by reactor design, and the flow rate normally affect the conversion. The higher the flow rate the lesser the contact time/ residence time and the lesser the conversion but catalysis generally is a function of what you are working on. I respect most of the contribution. When you increase the mass of catalyst the residence time increases and the conversion increases.

The reaction rate is a function of reactant concentrations and usually decreases with conversion since main reactants are consumed. When you increase flow rate conversion decreases due to shorter residence time and therefore observed activity increases. You must also verify the presence of external mass transport limitations,  according to procedure suggested previously. In heterogeneous catalysed processes the maximal reaction rate is determined by mass transport. If you have very active catalyst and want to operate at high conversions, then you will usually face with the mass transport resistances and also with the unisothermality of fixed bed reactor unless you use high dilution ratio, however this will also increase pressure drop.

Hi Dr Ramos, I find your speculation very interesting. It is very relevant to my current research investigation and I am experiencing a similar result where the rate of reaction dramatically increases exceeding a certain space velocity (W.H.S.V.). I was wondering if you might be able to supply a reference source which is supportive of this phenomena.

Gorazd Bercic Do you know why residence time decreases when flowrate increases?

Residence time = Volume/Flow rate { = liters/[liters/minute]= minute }

In the case of particular reactor the volume is constant and therefore residence time decreases when flowrate incrases.