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[Leecontractor]

​Hello,

 

I have been using Chemcad for the last one week to model a simulation for some reactors, and other process equipment.  The process uses organic components that do not exist in the databank. However, that is not my issue here.

 

My issue is with how Chemcad is defining heat of formation data for components. A screenshot attached (Att1) shows that the heat of formation can only be input in ideal gas or solid state.  My components are in a liquid state at STP and I am using correlations from literature to estimate heat of formation data from group contributions.  However, I don't know how to input the heat of formation data for the liquid phase. And therefore, not sure how to make sure my reactor can calculate the correct heat of reaction.

 

 

Something, I haven't checked but I am theorizing - Does chemcad somehow interrelate gas state heat of formation with liquid state heat of formation through heat of vaporization (This I am theorizing based on a conversation with some tech support person, however, I am not sure if there is any sound theoretical basis for that and that tech support might just be shooting in the dark or talking about something else)

 

Another related problem is that liquid heat of reaction calculation for the kinetic reactor has units of kJ/hr which does not even make sense since units should be kJ/mol.  I also tried to input the value manually (see attachment Att2) but program re-calculates it to be some kind of value in kJ/hr (Att3).  I am clueless as to what Chemcad is doing behind the scenes. 

 

The tech support seems to be clueless in solving this issue.  So any help would be greatly appreciated.

 

Thanks,

Lee

Attached Files

•  Att1.png   306.38KB   4 downloads
•  Att2.png   310.73KB   1 downloads
•  Att3.png   310.87KB   1 downloads

[PingPong]

I don't have experience with Chemcad, but I have some remarks:

 

- You say that your components do not exist in the databank, but from the screenshot it looks like that Methoxyozonidese is a component in Chemcad. Or did you add that name et cetera yourself?

 

- You say that your components are liquid at STP, but on the screenshot it says that the melting point of that Methoxyozonidese is 46 ^oC so it is a solid at STP.

 

- You say that you estimated the heat of formation data from group contributions, but you do not mention the method used. For what physical state does that method estimate heat of formation?

In any case, how accurate do you think such estimation method is anyway? I would not put much value on it.

 

- Heat of formation difference between gas and liquid is indeed heat of vaporization, and heat of formation difference between liquid and solid is heat of fusion.

 

- Heat of reaction in kJ/h is probably based on the feed flowrates (kg/h or kmol/h) that you have entered as input.

Note also that you obviously cannot input heat of formation for all components as well as heat of reaction.

 

You do not mention what the purpose of the simulation is, but you should realise that a process simulator is not smarter than you, it is only better in number crunching than you. If your components are not in its database, and you don't know either what the correct properties of your components are, then the simulation becomes rather useless.

If this is just a student exercise accuracy does not really matter, but if this is real life you may need to rethink what you are doing.