The influence of different thermodynamic methods
Thread Content
An atmospheric pressure concentration tower is used to separate dimethyl carbonate, methanol, and methyl carbamate. 30wt% dimethyl carbonate and 70wt% methanol form an azeotrope at atmospheric pressure. It was found that by using the NRTL thermodynamic approach, as the methyl carbamate feed concentration increased (from 0 to 20 WT%), the separation of dimethyl carbonate in the tower became more effective, and the reboiler load decreased. When comparing this with the thermodynamic approach of Wilson, it was found that as the concentration of methyl carbamate in the feed increases (from 0 to 20 WT%), the tower’s ability to separate dimethyl carbonate decreases; in other words, the higher the content of methyl carbamate in the feed, the worse the separation efficiency for dimethyl carbonate. The vapor-liquid equilibrium data for three groups (dimethyl carbonate, methanol, and methyl carbamate) were regressed using these two methods respectively, and the experimental VLE data showed little difference from the calculated VLE data. Why do the results obtained from different thermodynamic methods be exactly opposite? I need everyone’s advice urgently!
The tower setup is as follows: the feed rates of dimethyl carbonate and methanol remain constant, while the concentration of methyl carbamate is gradually increased to 10 wt% and 20 wt%. Specify the flow rate of dimethyl carbonate at the top of the tower, and the reflux ratio of the tower. The results show that: 1. When the NRTL method is used and the feed does not contain methyl carbamate, the content of dimethyl carbonate at the tower top can only reach 21 wt%, with a relatively high load ； When the feed contains 20 wt% methyl carbamate, the dimethyl carbonate content at the tower top can reach 30 wt%, and the load is also relatively low. When the Wilson method is used, the situation is exactly the opposite: the higher the amount of methyl carbamate, the worse the separation efficiency.
I used two thermodynamic methods to regress the pairwise interaction coefficients for dimethyl carbonate–methanol, methyl carbamate–methanol, and methyl carbamate–dimethyl carbonate. The vapor-liquid equilibrium data obtained through these methods did not differ significantly; yet why is the trend observed during the separation process exactly the opposite? If the results from the thermodynamic methods differ so much, how should one choose among them?
The choice of thermodynamic method is crucial for the calculation results; if the wrong method is selected, the simulation, even if it runs successfully, will be highly inaccurate, as the original poster saw. Regarding the selection of thermodynamic methods, Kai Chuan Shang has provided a dedicated introduction; the original poster might want to download it for reference. In the example above, methanol is a polar substance; therefore, only the activity coefficient method can be used for simulation, and NRTL must be chosen
““The obtained vapor-liquid equilibrium data do not differ significantly.” So, how significant does a difference have to be considered significant? Perhaps this difference is amplified in multiple gas-liquid equilibrium calculations. Are there any actual testing devices? For the calculation of complex polar substances, the software’s capabilities are limited; perhaps both property calculation methods will yield inaccurate results.