Dynamic equation settings in Aspen
Thread Content
In the kinetic equation for methanol synthesis, KCO=exp×104×(1/T-1/503.2). Does this mean that A=-1.9023 and B=1.5632×104? How should the term -1/503.2 in the formula be handled?
This K of yours is a kinetic factor, right? It’s not the complete expression of the kinetic equation. For methanol, the kinetic model in LHHW form is commonly used. The parameters such as ABCD you mentioned can take two forms: one is the ABCD values related to the driving force, and the other is the ABCD values related to the adsorption term; it’s important to distinguish between them. 503.2 should be the reference temperature in the expression for the kinetic factor
Yes, this is part of the formula for the adsorption term. Could you tell me what A, B, C, and D represent in this formula? It doesn’t respond as I set it according to my own understanding.
If KCO=exp, does that mean A=-1.9023 and B=1.5632×104?
In ASPEN, the adsorption terms all appear with a plus sign: A = -1.9023, B = -1.5632×10^4. Note that B is multiplied by 10^4, not 104. Additionally, the term -1/503.2 is not in the standard LHHW form, and ASPEN is unable to represent it
For this expression with a reference temperature, is it okay to ignore -1/503.2 during simulation? Could the error be very large? How should one address simulations with reference temperatures in such expressions?
Should we just switch to a different reactor model for simulation? As a beginner, I’m struggling with simulating the reaction of synthesizing methanol from CO, CO2, and hydrogen.
It shouldn’t be possible to use this approach; it’s also semi-empirical. The temperature for methanol synthesis is around 200°C. 200 + 273 equals almost 503, so ignoring 503 would result in a significant error. You can calculate exactly how large the error is. There isn’t a good way to address this issue – either use a standard kinetic model and look up relevant literature, or define the kinetics yourself, but that would require custom development using the Fortran language
Upon closer inspection, it seems that A and 503 can be combined. . .
Yes, I was also thinking about whether the constants at the end could be directly added to A.
You said that the adsorption terms in ASPEN are all in the form of positive values; then why are A and B negative? Could you please help me explain it?