What is the commonly used method for calculating reaction heat?
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I’m curious to know what methods other designers use to obtain data, and how large the discrepancy is between the calculated results and the actual values Are there also some uncertain data on how to estimate and obtain them? What is the impact of the discrepancy between the design results and actual conditions on production? For example, in terms of reactor cooling and the selection of heat exchangers. . .
If there is a need to calculate the reaction heat, I can help with that.
The way to find out is not to want to know the result
There are estimation methods based on the reactant groups. It is recommended to check the book \"Chemical Data\" published by Sinopec Press; it introduces several such estimation methods. The standard enthalpy of formation for most organic compounds can be estimated, with the exception of those containing special functional groups
Just look at Chapter 4 on heat transfer in university textbooks; it’s explained very clearly there
I first use the standard enthalpy of formation to calculate the change in enthalpy under standard gaseous conditions before and after the reaction, and then determine the enthalpy change under the reaction conditions based on the heat of vaporization and specific heat. This type of calculation is fairly accurate, especially if there are precise data on the vaporization heat and specific heat of the reactants and products; in such cases, the accuracy can generally exceed 5%. If accurate data are not available, simulation using Aspen Properties is the only option, and the errors resulting from this approach are relatively large. Especially in cases where hydrogen bonds are involved, the errors in the thermodynamic data themselves can exceed 10%, making the simulations less accurate.
The way to find out is not to want to know the result
Simply put, it involves using Hess’s law to break down a reaction into multiple steps, calculating the enthalpy change for each step separately, and then determining the total enthalpy change, which gives the enthalpy change for the entire reaction. In the entire reaction, the most important parameter is the enthalpy change of the reaction itself, which can be easily calculated using standard enthalpies of formation or bond energies; ASPEN PLUS also makes it straightforward to carry out such calculations. The method is as follows: 1. Convert the materials in the reaction state to the standard state and calculate the enthalpy change. 2. Calculate the standard enthalpy of formation and then the enthalpy change. 3. Convert the products under standard conditions to the final state and calculate the enthalpy change. 4. Add up the enthalpy changes from these three steps to obtain the heat of reaction. The accuracy depends on the accuracy of the thermodynamic data you have.
Even calculating it manually by using the enthalpy of formation is rather troublesome, as the enthalpy of formation data are generally provided under standard conditions. In practice, temperature changes also need to be taken into account, that is, the effect of Kirchhoff’s law. Using simulation software is quite convenient. Additionally, it is preferable to use experimental data; this may be more accurate than the empirical formulas built into simulation software.
Additional note: Some manuals provide enthalpies of formation at different temperatures, which can be used as a reference.