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2. a) Draw a chemical structure diagram for propyne. [2 marks] b) Write a fully balanced chemical equation for the complete combustion of butyne (C4H6) in air under stoichiometric conditions. Assume air is composed of 21% Oxygen and 79% Nitrogen by moles. [3 marks] c) 1 kmol of liquid pentanol is burned completely in pure oxygen under stoichiometric conditions, with temperature and pressure maintained at 298 K and 105 Pa respectively. Calculate the quantity of energy available for external heating, assuming that the water condenses to liquid. Enthalpies of formation are available in Table Q2-1 and you may take the chemical equation for the combustion of pentanol to be: 2 C5H₁1OH + 15 0₂ 10 CO₂ + 12 H₂0 Substance Enthalpy of Formation [kJ kmol ¹] Carbon Dioxide -393,522 Water (liquid) -285,838 Oxygen Pentanol (liquid) 0 -351,900 Table Q2-1:Enthalpy of formation of relevant reactants and products [4 marks] d) If the pentanol was now burned in air under adiabatic conditions at 105Pa, show that the flame temperature lies between 2300 and 2400K. The following data in table Q2-2 shows the total enthalpy of the reaction products at these temperatures. Alternatively, a complete table can be found on page 8. Ideal gas Enthalpies hf + hr/298 at 105 Pa (kJ kmol-¹1) Temperature K CO CO₂ H₂ H₂O N₂ 2300 -42,853 -283,851 63,371 -153,532 67,007 2400 -39,183 -277,734 66,915 -148,223 70,651 Table Q2-2: The total enthalpy of gases at various temperatures. [6 marks] Question 2 continued on the next page
Question 2 continued e) Describe potential pollutants that may arise under the adiabatic combustion of pentanol. [4 marks] f) Draw sketch graphs showing the temperature distributions along a concentric tube heat exchanger for: (i) Parallel flow when the hot fluid has a heat capacity very much larger than the cold fluid. (ii) Counter flow heat exchange when the hot and cold fluids have the same flowing heat capacity. (iii) Counter flow heat exchange when the hot fluid has double the flowing heat capacity as the cold fluid [6 marks]
FORMULA SHEET Combustion CO CO2 H2 H20 h at 105 Pa -110,529 -393,522 0 -241,827 0 33,723 kJ kmol-¹ Temperature (K) HT/298 KJ kmol-1 1,000 21,690 33,397 20,680 26,000 21,463 32,344 1,500 38,850 61,705 48,151 38,405 59,309 72,790 56,137 87,259 77,831 59,748 93,274 83,036 63,371 98,947 36,290 2,000 56,744 91,439 52,951 2,100 60,375 97,500 56,379 2,200 64,019 103,575 59,860 2,300 67,676 109,671 63,371 2,400 71,346 115,788 66,915 2,500 75,023 121,926 70,492 2,600 78,714 128,085 74,090 2,700 82,408 134,256 77,718 88,295 67,007 104,633 93,604 70,651 110,332 98,964 74,312 116,039 104,370 77,973 121,754 109,813 81,659 127,478 2,800 86,115 81,370 115,294 85,345 133,206 120,813 89,036 138,942 126,549 92,715 144,267 137,553 100,161 156,180 148,854 107,608 167,695 160,247 115,081 179,222 140,444 2,900 89,826 146,645 85,044 3,000 93,546 152,852 88,740 3,200 100,998 165,331 96,199 3,400 108,479 177,849 103,738 3,600 115,976 190,405 111,361 3,800 123,495 202,999 119,064 4,000 131,031 215,622 126,874 4,500 149,937 247,354 146,660 4,900 165,126 272,882 162,801 236,380 163,973 5,000 168,932 279,283 166,876 242,313 167,763 Ideal Gas Enthalpies 171,724 122,570 190,761 183,552 130,027 201,859 212,764 148,850 230,756 253,902 259,692 All enthalpies reported at 105Pa N2 NO2