Fluids

Fluids

Thermal profile of packed bed reactor

    • Ric_Rin_DTU
      Subscriber

      Good evening,

      I am simulating the fluid dynamics of a packed bed reactor for methanol production and comparing them with some previous results I have obtained with simplified models. My CFD model comprises a "slice" (30°) of the tubular reactor; the mixture (mainly CO, H2, CO2 and small amounts of inerts) enters the unit at 473K and reacts along the reactor. The cooling medium is vapour at a constant temperature of 240°C (I have just put the wall temperature equal to 240°C). The unit has been modelled as a porous domain (made of Aluminum).

      The simple CFD model I have implemented seems to work with no problem, and predicts the productivity of methanol well (if compared to the previous models I have used). In spite of this, the predictions of the thermal profiles along the reactor are absolutely not reliable: I would expect a strong increase of temperature in proximity of the entrance of the reactor (due to the exothermic reaction involved) up to a peak, and then a progressive decrease of temperature up to a plateau (i.e., cooling medium temperature, 240°C). In other words, I would expect a steep augmentation of T in the first 10% of reactor length, mainly due to the exothermicity of the chemical reaction; and then a milder thermal decrement of T, as the reaction reaches the chemical equilibrium and no reaction heat is produced any longer (but the cooling effect of vapour prevails).

      In my simulations, though, it seems that the fluid is progressively heated up to the cooling medium temperature (i.e., it goes from 473 K to 240°C = 513 K), with no temperature peaks, as if no reaction heat were released. I have implemented the kinetics through a series of expressions, that I have then utilized as "Sources" in a subdomain (called "Reaction Domain"). Similarly, I have put the reaction heat as energy source inside the aforementioned subdomain. Even after many checks, it seems that the reactions and the expression for reaction heat are properly written. The strange thing is that the reaction kinetics are properly predicting my system behaviour (and their values are in line with those of my previous models), and, similarly, the value of reaction heat is comparable with that obtained in simpler models, but in the CFD model the released heat somehow "disappears" from the thermal profile of the reactor.

      Has anybody suggestions on where the problem could be?


      Thanks a lot,

      Riccardo

    • Rob
      Ansys Employee
      If you turn off the heat source what happens? How is the heat source linked to the mass sink/source?
    • Ric_Rin_DTU
      Subscriber
      Thanks a lot for your answer.
      Minimal changes occur by turning on and off the reaction heat (the thermal profiles are basically identical).
      As for the link between mass and heat source, the heat released by the reaction is related to the reaction rates with the following formula (j is the reaction number):
      HEAT RELEASED = sum(reaction_rate(j) * Enthalpy of reaction(j)).
      In other words, the reaction heat is the sum, weighted on reaction rates, of reaction enthalpies.
      The reaction heat is "distributed" along the reactor differently in the CFD model with respect to what takes place in my other models (that are correct, as confirmed by all previous literature works). In the CFD code, the highest value of reaction heat is achieved at reactor entrance; then, the number diminishes and then it augments again, before finally decreasing (i.e., the reaction heat profile has a minimum and a maximum). What one normally would expect is to have just one maximum (low value at the entrance, then steep increase and then progressive decrease). The formulas utilized in the "Expression" setup are correct, since they provide identical results to my model if I use the function "Evaluate" and put the same input values of my model.
      What could be the reason of such discrepancies?

      Thanks,
      Riccardo
    • Rob
      Ansys Employee
      Is the heat related to the sum of the reactions or to the actual mass source/sink? Ie are you applying the right amount of heat to the whole zone rather than into specific bands?
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