A module is available in Comflow® with which fixed bed reactors can be simulated. This allows the computation of chemical reaction and flow in fixed catalyst beds within a 2-D Comflow® CFD model. The heat and mass transfer from the fluid to the catalyst particles is computed as well as the diffusion inside the particles. Thus, the effectivity of the catalyst particles is calculated as a function of the local fluid velocity, temperature and composition.
As an example, consider a simple (fictitious) packed-bed deNOx reactor. The deNOx catalyst, 4 mm diameter spherical particles, are packed in a tube with a diameter of one meter. This tube has an inlet with a diameter of 240 mm. The other end of the reactor is open to the surroundings. At the inlet, a mixture of 80 mole% nitrogen, 10 mole% water, 5 mole% ammonia and 5 mole% NO enters the reactor with a velocity of 4 m/s and a temperature of 480 degrees Celcius. The wall of the reactor is ideally isolated.
In the catalyst bed (located at 2.5 m from the inlet and 2 m deep), NO is reduced to N2 in the (fictitious) reaction:
6 NO + 4 NH3 --> 5 N2 + 6 H2O
with a first-order reaction rate
-rNO = k0 exp(-Ea/RT) cNO; k0=0.58; Ea = 12520
Note that this is just an example; almost any (set of) chemical (equilibrium) reactions with power law or Langmuir Hinshelwood kinetics can be used in the reactor module.
The reactor has rotation symmetry so we can calculate the flow in a pie-shaped 2-dimensional cross-section of the reactor. The geometry and calculated flow field can be seen below. The reactor inlet is on the left side, the outlet to the right. The vectors display the flow direction; larger vectors depict a larger flow rate; the color of a vector gives the flow rate in m/s. In addition, streamlines are drawn; these give the path of a weightless particle that is released in the flow; the color gives the time required to reach that point of the streamline.
It can be seen that a large eddy is predicted in the area next to the inlet. This eddy may cause a maldistribution of flow over the catalyst bed.
The next figure gives the NO profile in the reactor (in mole %). It can be seen that the NO concentration decreases from 5 to 0.2 mole% inside the catalyst bed due to the deNOx reaction.
|Temperature (adiabatic reactor)|
The temperature profile inside the tube can be seen in the next figure.
|Temperature (cooled reactor)|
When, instead of isolated, the reactor wall is cooled to a fixed temperature of 0 degrees Celcius, the temperature profile looks like this:
What can be seen is that the maximum temperature inside the catalyst bed has increased instead of decreased compared to the adiabatic case (and the difference would be larger if the conversion of NO would not have been so high).
Of course, this reactor case is just a simple example with a limited number of cells. This example along with the calculation results are included in the Comflow® package. To recreate this example on your own computer, download the Comflow® Reactor tutorial (Adobe Acrobat Reader required).
© 2008 Inudent
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