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According to page P628 of <<Sulfuric Acid Production Technology>>, there are formulas for the molar heat capacity of gases (similar formulas can be found in other sources). CPSO2 = 25.74 + 5.80*10^-2T – 38.1*10^-6T^2 + 0.861*10^-8T^3; CPSO3 = 15.09 + 15.20*10^-2T – 120.7*10^-6T^2 + 3.62*10^-8T^3; CPO2 = 25.74 + 1.30*10^-2T – 3.86*10^-6T^2; CPN2 = 27.18 + 0.591*10^-2T – 0.338*10^-6T^2. On page P633, as an example in the heat balance calculations, the average molar heat capacities of SO2, O2, and N2 at 430°C are 44.32, 30.616, and 29.659 respectively, while at temperatures ranging from 0 to 589°C they are 47.38, 67.38, 31.61, and 30.2 respectively. I would like to ask whether T in the above formula should be used as an absolute temperature or how it should be substituted; I’ve tried various values but it still doesn’t work. SO2 SO3 O2 N2: Use 430 (589) directly. 44.32 61.01 30.62 29.66 48.44 70.14 32.06 30.54. Using absolute temperature: 703 = 430 + 273, giving 50.68, 74.87, 32.97, 31.17; 862 = 589 + 273, giving 52.94, 79.61, 34.08, 32.02. If the average value is used: 351.5 = 703/2, giving 41.79, 55.18, 29.83, 29.22; 431 = 862/2, giving 44.35, 61.08, 30.63, 29.66. None of these sets of values match those in the example. So which temperature should I use in the formulas? Or is my approach completely wrong? Also, for heat exchangers, there is the CPt value (average specific heat at constant pressure). Where can this value be found? It is still calculated using the heat transfer rate of the heat exchanger / (mass * temperature difference). I seek guidance from the experts. Thank you so much! This post was last edited by shen*rong2006 on 2009-4-10 08:21.]
CPSO2 = 25.74 + 5.80*10^-2T – 38.1*10^-6T^2 + 0.861*10^-8T^3; CPSO3 = 15.09 + 12.50*10^-2T – 120.7*10^-6T^2 + 3.62*10^-8T^3; CPO2 = 25.74 + 1.30*10^-2T – 3.86*10^-6T^2; CPN2 = 27.18 + 0.591*10^-2T – 0.338*10^-6T^2. The absolute temperature should be used in these formulas. The average molar heat capacity has not been determined (unless someone has already published it), as the constant pressure heat capacity varies at different temperatures. The formulas above are used to calculate the constant pressure heat capacity at a specific temperature, while the average constant pressure heat capacity is the mean value of such capacities over a certain temperature range. For example, for the range of 0 to 400 degrees, one can calculate the value at 0 degrees and at 400 degrees, and then take their average
The constant-pressure heat capacity of some substances can be found, and then you can calculate the average using those values. Hehe, don’t forget to score it!
You used EXCEL to do the calculations, right? It’s been quite some time since then; back then, calculators were probably used, which could lead to some errors. If you want to verify the results, set a certain number of decimal places for rounding each value – the final result might then satisfy you, hehe. 1# shen*rong2006
The constant-pressure specific heat of pure components can be found in property databases or property manuals, and the relationship with temperature can also be determined through regression analysis, which can be done using Excel. The mixture gas is approximated as an ideal gas.
Thank you to everyone above; I used EXCEL to do the calculations, but after checking the formulas again, the data seems a bit similar, and it appears that this formula isn’t the right one.