Aspen simulates the CO2-saturated water content; why is there a large discrepancy between the calculated values and those from HYSYS?
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This post was last edited by zzhaichuan on 2022-10-25 17:06. Calculation 1: The saturated water content of CO2 at 40°C and 25 bar (g). I obtained a value of 3296 ppm using Aspen, while others using HYSYS got 4218 ppm – there is a significant difference. Calculation 2: The saturated water content of CO2 at 40°C and 35.8 bar (g) – I obtained a value of 2627 ppm using Aspen, while others using HYSYS got 3400 ppm; there is a significant difference. Both of the above use the PR equation; could someone please tell me which one yields more accurate results, or is there an issue with a certain step in my calculations? Below is the detailed procedure: 1.1 Enter component 1.2 Select PR equation 1.3 Confirm binary components 1.4 Result flowchart 1.5 Result summary table 1.6 Results from others using HYSYS 2.1 Simulation results at 40 degrees and 35.8 bar 2.2 Others’ HYSYS results Chemical Equipment and Machinery
There’s another detail: the calculation method in Aspen One allows the use of the HYSYSPR equation, and the resulting value is 3583. This differs by 8% from the value obtained using the PR equation, which is 3296; this difference is acceptable. But there is still an 18% difference compared to 4218 in HYSYS, which is unacceptable. Please let me know which one is more accurate.
The situation with HYSYS is unclear. ASPEN can try the PR-BM method
If it doesn’t work in the east, it might work in the west; try something else
I don’t know about 25 bar; it’s 35.8 bar, and the gas phase compositions are different. It cannot be directly compared
The property methods in simulation software are not the original property methods from our Chemical Engineering Thermodynamics*. Different software have made modifications to the PR equation with varying tendencies. Among them, HYSYS, as it is geared toward the simulation of petrochemical hydrocarbons, places more emphasis on the PR equation and has more modifications in this regard. So, although it seems like you are using the same property method, in reality, Aspen’s built-in PR and HYSYSPR are two different things. Using the PR equation to simulate the water-carbon dioxide system in the Aspen system is certainly not very accurate. Add an electrolyte, set a Henry’s law composition, and use ENRTL to obtain results; then compare them with those from HYSYS-PR
Hello, thank you for your guidance. I have added the Henry component, but I don’t understand what you mean by “increasing electrolytes”. Below are the simulation results I have checked according to what you said. The difference between 3882 ppm and 4178 is quite large. There are two questions: 1. Based on the vapor pressure of saturated steam under ideal conditions, the ppm level of water vapor at 40 degrees and 25 bar should be 2837. According to the HYSYS calculation results, 4178 ppm represents a 47% increase compared to the ideal value, while according to the ASPEN simulation it represents an increase of around 37% compared to the ideal value. Which one is closer to the actual value? 2. Isn’t the NRTL equation suitable for low-pressure conditions? Is it appropriate to use the NRTL equation for my operating conditions?
Carbon dioxide reacts with water in the liquid state; although the reaction is weak, it does occur. The carbon dioxide-water system deviates significantly from an ideal state. Electrolytes are what facilitate the electrolytic reaction equation CO2+H2O=H+=HCO3- and a series of related reactions. For physical properties, ENRTL or a variant of ENRTL should be chosen.
Thank you again. I’m not sure where to add the reaction equation for increasing electrolytes as you mentioned. Could you please take a screenshot and show me the steps to do it? I also want to determine the water content in gaseous CO2. Can electrolytic reactions take place in the gas phase, or in a misty state (where water is present)? The attachment is an Aspen V8.4 file
Keep pressing Next until you reach the carbon dioxide-water system; there is also an APISOUR system designed specifically for acidic water stripping – you can give it a try
Thank you again for always patiently answering my questions! The results obtained after adding the electrolyte components as you suggested differ by about 1% from the results using the NRTL-RK equation for the Henry components. This calculation differs by ~10% from the results of the HYSYS simulation; can it be interpreted that the results obtained after adding the electrolyte component are closer to the actual values?