the mass, energy, and momentum equations in this analysis by using the predefined interfaces in the Chemical Reaction Engineering Module.
The process of setting up the model can be made more efficient with the thermodynamic properties database, introduced in version 5.3a of the COMSOL Multiphysics® software. An update of this model that uses the thermodynamic properties database will be available soon. (Learn more in this blog post about the thermodynamic properties database.)
Analyzing the Steam Reforming Process
First, we examine the weight fraction of the various species in the reactor bed. The image to the left below shows that the weight fraction of propane is 0.28 at the inlet and decreases to almost 0 at the outlet. By examining a cross section through the center of the reformer (middle image below), we can also visualize the concentration distribution of propane throughout the bed. Since temperature is the main determiner of local reactivity, we conclude that, in this example, the heat supplied by the tubes efficiently uses the entire catalytic bed.
By looking at the weight fraction of every reacting species in the catalytic bed (right image below), we can confirm that the entire length of the bed is actively converting propane.
Mass fraction distribution of propane in the reformer bed (left) and in a cross section through the middle of the reformer bed (middle). The mass fractions of all of the reacting species are a function of the reactor position and are evaluated along the reactor centerline (right).
Next, we take a closer look at the temperature in the reformer. When looking at the system as a whole, we see that the gas enters the heating tubes at 900 K and leaves the tubes at around 674 K. Meanwhile, the gas temperature within the reformer bed begins at 700 K and is reduced to a minimum temperature before rising again and exiting the bed at an average temperature of 795 K.
The plot to the right below visualizes how the endothermic reformation reactions cause the temperature to initially decrease. Then, when a lower temperature and amount of propane cause a reduced reaction rate, the energy supplied by the heating tubes determines the temperature evolution. As a result, the temperature increases again.
The temperature distribution and energy exchange between the bed, heating tubes, and insulating wall (left). The right image shows the temperature of the reformer bed as a function of position and evaluated along the centerline.
Moving on to velocity, let’s analyze the heating and reacting gases. For the heating gas, the flow is laminar and we can clearly observe a parabolic velocity distribution. On the other hand, the reacting gas in the porous bed has a significant increase in velocity, so much so that its velocity at the outlet is around double what it was at the inlet. This increased velocity is caused by the gas expansion from the chemical reaction and the temperature increase.
Velocity field plot, depicting the heating gas in the tubes and the reacting gas in the bed.
The related density variation within the bed, which accounts for temperature and composition, shows that the density decreases through the reformer.
Gas density in the reformer bed.
The goal of this tutorial is to demonstrate that by coupling mass, energy, and flow equations in COMSOL Multiphysics, you can successfully model a steam reformer. To try this example yourself, click on the button below.
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