Question
problem #6
DHmwk 8-F17.pdf X/ was process path a. × av/pid-2679320-dt-content-rid-12803305 1/courses/ENCH215 6406 FA2017/Hm 0-%20F 17.pdf 3. Steam flowing at 10 feet/sec is throttled down from 900 to 145 psia by passage through an insulated needle valve. If the initial temperature is 620°C, what is the temperature after expansion? 4. Using a psychrometric chart, answer the following: a) find the energy (Q) required to heat humid air at 60F and 60% relative humidity to 90F. Find the following for the initial state and the final state: dew point and humid volume. b) Describe what happens when you cool the humid air at 60°C and 60% relative humidity to 40°F. Assume constant pressure (1 atm). 6 of a mixture of 12 5. Use data in Table B.8 to calculate the heat that must be extracted from 1000 ft mol% CO2, 8 mol% 02 and 80 mol% N2 when it is cooled from 1200°C to 500°C 0 6. Show a process path and calculate the energy (in kJ/mol) required to carry out each of the following a) heat iron from 1400°C to 2000°C, and b) heat cyclohexane from 0°C to 100°C. Assume cp of solid cyclohexane is 156 kJ/mol 7. Calculate specific change in enthalpy for the following: a) oxygen being compressed from 1 atm to 1.16 atm at a constant temperature of 298K, and b) bromine being compressed from 1 atm to 5 atm at 9 9 8 9 a constant temperature of 273K.
Explanation / Answer
a)
heating iron from 1400°C to 2000°C
note that iron melts --> 1,538
then
Qsolid = m*Csolid*(Tmelt-Ti)
Qmelting = mass *Latent heat of melting
Qliquid = m*Cliquid *(Tf-Tmelt)
substitute
assume mass = 1 g
Qsolid = 1*0.45*(1538-1400) = 62.1
Qmelting = 1*126 = 126
Qliquid = 1*0.611 *(2000-1538) = 282.282
Qtotal = 62.1+126+282.282 = 470.382 J (per gram)
b)
cyclohexane --> MP = 6.5 ; BP = 80.7
Qsolid = m*Cs*(Tf-Ti)
Qmelt = m*LHmelt
Qliq = m*Cl*(Tb-Tm)
Qvap = m*LHvap
Qgas = m*Cgas*(Tf-Tb)
calcualte:
Qsolid = 1*156/84.162 *(6.5 -0) = 12.04
Qmelt = 1*2.68*1000/84.162 = 31.84
Qliq = 1*2.6769/84.162 *(80.7-6.5)= 2.36
Qvap = 1*31000/84.162 = 368
Qgas = 1*120/84.162*(100-80.7) = 27
Qtotal = 12.04+31.84+2.36+368+27 = 441 J per g approx
