I am trying to model a pyrolysis process in Aspen Plus, where the ultimate analysis is as follows:
- Carbon (C) wt% d.b.: 47.55
- Hydrogen (H) wt% d.b.: 6.20
- Nitrogen (N) wt% d.b.: 0.07
- Sulphur (S) wt% d.b.: 0.0
- Oxygen (O*) wt% d.b.: 46.16
Proximate analysis is:
- VM: 87.95
- Ash: 0.15
- Fixed-C: 11.9
- Moisture: 5.93
Using Dulong's formula returns a Higher Heating Value (HHV) value of 16.6 MJ/kg. With a feeding rate of 1000 kg/h into the pyrolysis reactor, the expected enthalpy from the biomass should be 16,600 MJ/h = 4611 kW. In Aspen Plus, the enthalpy flow is -6561 MJ/h = -1822 kW.
Are there any obvious mistakes that are normal for beginners when modeling in Aspen Plus?
Dear friend Knut-Magnus Brautaset
Hey there Knut-Magnus Brautaset! Let's dive into this Aspen Plus conundrum, shall we? Now, I don't shy away from complexities. Here are some things to check or consider:
1. **Unit Consistency:** Aspen Plus can be a stickler for units. Make sure that the units used in your biomass feed and reactions are consistent. Check the units in the stream properties, reactions, and any other relevant sections.
2. **Reaction Stoichiometry:** Ensure that the reactions you've defined in Aspen Plus are correct and balanced. A small mistake here can lead to significant discrepancies in the enthalpy.
3. **Reaction Mechanism:** Check the reaction mechanism or method you've chosen in Aspen Plus for handling the biomass pyrolysis. The method should be suitable for the type of reactions happening during pyrolysis.
4. **Energy Basis:** Aspen Plus allows you Knut-Magnus Brautaset to specify the energy basis. Make sure that the energy basis for your enthalpy calculations is consistent with your expectations.
5. **Check Aspen Plus Data Inputs:** Ensure that you've input all necessary data correctly. Double-check the ultimate and proximate analysis inputs, and make sure that you Knut-Magnus Brautaset haven't missed any relevant parameters.
6. **Heat of Formation:** Aspen Plus uses the heat of formation data for its enthalpy calculations. Check if the heat of formation values for your components are accurate. Sometimes, default values might not be suitable for specialized processes like pyrolysis.
7. **Verification with Aspen Properties:** Compare the properties of your biomass components in Aspen Plus with external data sources or databases. This can help verify that Aspen Plus is using the correct properties.
8. **Thermodynamic Model:** Depending on your system, the default thermodynamic model in Aspen Plus might not be the most suitable. Consider checking and adjusting the thermodynamic model based on the characteristics of your pyrolysis process.
9. **Solver Settings:** Sometimes, convergence issues or inaccuracies can arise due to solver settings. Experiment with different solver options to see if it affects your results.
Remember, Aspen Plus is a powerful tool but can be tricky, especially for complex processes. Take your time to review each step, and if the problem persists, don't hesitate to reach out to AspenTech support or community forums for assistance.
And there you Knut-Magnus Brautaset have it! It is my take on troubleshooting Aspen Plus. Good luck with your biomass pyrolysis model!
Kaushik Shandilya Thank you for your answer. I will certainly look into your troubleshooting method.
The Dulong formula is excellent for low oxygen content fuels, but underestimated CV for highly oxygenated biofuels such as sugar waste. There are a number of empirical correctors, these are fine for first approximations, but ultimately measurement is needed.
Jack Broughton thank you for your answer. I certainly understand that Dulongs formula is just an approximation, but I still think the values deviates quite a bit from what I achieve in Aspen Plus. Do you have any suggestions to why that is the case? The enthalpy flow should equal massflow*HHV right?
Knut-Magnus Brautaset I don't use Aspen so cannot fully comment. However, I normally work with the LHV rather than HHV as this avoids allowing for the latent heat content of the flue gases. The USA still tends to use HHV, but Europe on the whole uses LHV for the energy conversion calculation. Ultimately, there is no difference, but when various fuels are used the stack losses are almost constant in LHV terms while they vary widely on the HHV basis. E.g. a boiler fired on oil or natural gas has almost the same LHV based efficiency for each fuel, but they are about 10 % different on HHV basis. Another consideration that can be important. is that often fuels are purchased in terms of their HHV, especially coal and oil.
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