December 11, 2014
ChE 211
2nd Midterm
Instructions: This is an open book, open notes exam.
Photocopies are not allowed. You are not allowed to
detach papers. Duration is 115 minutes. Whenever you
use data from your book, indicate page and table
number. Always show your work and give a brief
explanation of what you are doing.
Question 1:
/50
Question 2:
/50
NAME : __________________________________
PAY ATTENTION TO SIGNIFICANT FIGURES!!!!
Question # 1 :
Fresh air at 40oC with 20 % RH is mixed adiabatically with recycled air and is fed to a drier to
decrease the moisture content of a wet solid from 20 % by weight to 5% by weight. The air
leaving the drier is at 24.5 oC with 80 % RH. The flow rate of wet solid is 1000 kg/hr and enters
the drier at 22oC..
a)
b)
c)
d)
e)
f)
What is the rate of moisture removal from the solid in kg/hr?
What is the moisture content of fresh air in kg H2O per kg dry air?
What is the moisture content of air leaving the drier in kg H2O per kg dry air?
What is the flow rate of fresh air (dry air + moisture) in kg/hr?
What is the enthalpy of fresh air in kJ/kg dry air?
What is the enthalpy of air leaving the drier in kJ/ kg dry air?
One half (50 %) of the air leaving the drier is recycled,
g)
h)
i)
j)
k)
What is the moisture content of air entering the drier?
What is the temperature of air entering the drier?
What is the specific enthalpy of air entering the drier?
What is the relative humidity of the air entering the drier?
What is the amount of heat in kJ/hr that must be supplied to the drier?
Assume that the enthalpy changes undergone by the unevaporated liquid water and the dry
solid are negligible.
Question # 2 :
Cumene (C6H5C3H7) is produced by reacting benzene (C6H6) with propylene (C3H6). A liquid
feed containing 75% propylene and 25% n-butane (C4H10) and a second liquid stream which is
pure benzene are fed to a reactor. This second stream consists of fresh and recycled benzene
mixed in a ratio of 3 (recycle) to 1 (fresh). This pure benzene stream is heated with the reactor
effluent in a heat exchanger. The reactor effluent enters the exchanger at 4000F and leaves at
2000F. The reactor effluent is also a liquid.
The reactor effluent is sent to a distillation column after passing through the heat exchanger. In
this column all of the n-butane and unreacted propylene are separated from unreacted benzene
and cumene. Unreacted propylene and n-butane is the first product stream. Unreacted benzene
and cumene are then sent to a second distillation column where benzene is separated from
cumene. Benzene leaving this column is recycled and cumene is the second and main product
stream.
The production rate is 1200 lbm/hr of cumene. 20 % of propylene fed to the reactor does not react.
a) Draw a flow chart of the process
b) Calculate the molar flow rates of each species in the streams fed to the reactor, the reactor
effluent and the product streams. Fill in the table below.
c) Calculate the temperature of the benzene stream fed to the reactor.
Assume that the following liquid heat capacities are constant.
Species
Benzene
Propylene
n-Butane
Cumene
Mol. Wt. (lbm/mole)
78.00
42.00
58.00
120.00
Heat Capacity (Btu/lb-0F)
0.4500
0.5700
0.5500
0.4000
TABLE : Molar flow rates (moles/hr)
Species
Benzene
Propylene
n-Butane
Cumene
Reactor Feed
1
? 12.5
?4.17
-
Reactor Feed
2
10 +30?
-
Reactor
Effluent
?30
?2.5
?4.17
?10
First
Product
?2.5
?4.17
-
Main Product
?10
Question # 1
1,000 kg/hr
20% moisture
400C
20 % RH
24.50C
80 % RH
5% moisture
Mass of dry solid is constant.. Therefore moisture content leaving the drier is given by:
0.05 = x/(800 + x) → X = 42.1kg
Therefore rate of moisture removal is 200 – 42.1 = 157.9 kg/hr
Moisture content of fresh air = 0.0092 kg/kg dry air
Moisture content leaving the drier = 0.0153 kg/kg dry air
Flow rate of fresh dry air = 157.9/(0.0153 – 0.0092) = 25,885 kg/hr dry air
Flow rate of fresh air = 25,885x1.0092 = 26,123 kg fresh air.
e) Enthalpy of fresh air = 64.3 kJ/kg dry air
f) Enthalpy of air leaving the drier = 64.9 kJ/kg dry air
g) The amounts of fresh and recycled dry air are equal.
Therefore moisture content is (0.0092 + 0.0153)/2= 0.01225 kg /kg dry air
a)
b)
c)
d)
h) Temperature of air entering the drier = (40 + 24.5 )/2 = 32.250C
i) Specific enthalpy entering the drier = 64.6 kJ/kg dry air
j) Relative humidiry is 41 %
Question # 2:
Reactor
Hex
Therefore moles of propylene entering the reactor is 10/0.8 = 12.5 lbmoles/hr
Moles of n-butane entering the reactor = 12.5/3 = 4.17 lbmoles/hr
Moles of fresh benzene = 10.0 lbmoles/hr
Moles of recycled benzene = 30.0 lbmoles/hr
Energy balance around heat exchanger: