q2 b) What is the enthalpy of n-hexane liquid at 20 C and 1.0 atm relative to n-
ID: 636384 • Letter: Q
Question
q2
b) What is the enthalpy of n-hexane liquid at 20 C and 1.0 atm relative to n-hexane vapor at 200 C and 2.0 atm? c) Beginning with the value of A calculated in part (a) and still assuming ideal gas behavior, determine the specific internal energy of the vapor at 200 C and 2.0 atm. Once again, state where you used the ideal gas assumption. 2) A mixture of n-hexane vapor and air leaves a solvent recovery unit and flows through a 70 cm diameter duct at a velocity of 3.00 m/s. At a sampling point in the duct the temperature is 40 C the pressure is 850 mm Hg, and the dew point of the sampled gas is 25 C. The gas is fed to a condenser in which it is cooled at constant pressure, condensing 60% of the hexane in the feed. a) Perform a degree-of-freedom analysis to show that enough information is available to calculate the required condenser outlet temperature (C) and cooling rate (kW). b Perform the calculations to acquire the required condenser outlet temperature (C) and cooling rate (kW). 3) A liquid is placed in a well-insulated container. which is then sealed. Initially, the container and its contents (the liquid and pure nitrogen) are at 93 C and I atm: the liquid volume is 70 cm and the gas volume is 3.00 L. The liquid partially evaporates, and the system cools down and 2 entually comes to thermal equilibrium at 85 C with liquid still present. Physical property data for the liquid and its vapor are: ?0.-20 kcal/mol at 90 C CJ (Ci 20 cal/(mol. C) 10 cal/(mol. C(C-8 cal/(mol. C) (SG):…-090 42Explanation / Answer
ans)
From the given data,
the molar flow rate of mixture can be calculated. The dew point gives the partial pressure of hexane vapro. From the equation
Mole of hexane/total moles = partial pressure of hexane/ total pressure
Moles of hexane vapor can be calculated.
Since 60% of hexane is condensed, moles of hexane at out let can be calculated. Hence the partial pressure of hexane ( which in turn will be equal to vapor pressure , sicne air is saturated at the outlet of condenser), temperature corresponding to this partial presure = vapor presure can be calculated.
Volumetric flow rate of the gas = cross sectional area* velocity = (PI/4)* (70/100)2* 3
=1.155 m3/s
Molar flow rate of gas,
n= PV/RT
P= 850/760 atm =1.118 atm,
T= 40+273.15K= 313.15K,
R =0.0821 L.atm/mole.K
Total moles = 1.118*1.155*1000L/(0.0821*313.15)
=50.23 moles/sec
Dew point at the inlet= 25 deg.c
Dew point is the temperature at which partial pressure of vapor= vapor pressure of hexane liquid
So vapor pressure of hexane liquid = 151.2 mm Hg at 25 deg,c= partial pressure of hexane
At inlet
Mole of hexane/total moles = partial pressure of hexane/ total pressure
Moles of hexane = 50.23* 151.2/(850-151.2)
=10.87 moles /s of hexane at inlet
Mole of air = 50.23-10.87=39.36 moles/s.
This does not change during condensation
60% of the hexane is condensed in the condenser, mole of hexane condensed ( unknown)
= 10.87*0.6= 6.522 moles/sec
Moles of hexane uncondensed = 10.87-6.522
=4.348 moles/s
Total moles at outlet = 4.348+39.36
=43.71 moles/se
At outlet of condenser , partial pressure of hexane = (moles of hexane/ total moles)* total pressure
= (4.348/43.71)* 850 =84.55 mm Hg
At the out let of condenser, the air is assumed to be saturated with hexane.
So 84.55 mm Hg partial pressure of hexane correspond to vapor pressure at that temperature.
So temperature corresponding to 84.55 mm Hg partial pressure
= 12.1 deg.c
Load on the condenser = (moles of hexane condensed/s * latent heat of vaporizatino f hexane = 6.522*43.85 Kj/mo
=286 Kilo joules/sec = 286 Kw
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