ChE 210 Exam 2 - Spring 2004
1
Understanding that academic integrity is expected of all Iowa State University students, at all times, I pledge that
I have neither given nor received unauthorized aid on this examination.
Signature:
Date:
Instructions: Open book; open notes. Use one side of each page, begin each numbered problem on a new page,
show all your work, and box your answers. Number and put your initials at the top of each page. When done,
fold all the pages (exam questions and your solutions) vertically and put your name and “Test 2” on the outside.
(Note: The numbers in parentheses next to each part of each problem are point values for that part.)
1. (45 points) Ethane is chlorinated in a continuous reactor:
C2 H6 + Cl2 → C2 H5 Cl + HCl
Some of the product monochloroethane (C2 H5 Cl) is further chlorinated to form dichloroethane (C2 H4 Cl2 )
in an undesired side reaction:
C2 H5 Cl + Cl2 → C2 H4 Cl2 + HCl
The percent conversion of ethane is 12% and the selectivity of monochloroethane to dichloroethane is 14 mol
C2 H5 Cl/1 mol C2 H4 Cl2 . Assume the feed contains only ethane and chlorine and that all of the chlorine fed
to the reactor is completely consumed by the two reactions. Your objective is to determine the fractional
yield of monochloroethane in this reactor.
Feed
Products
Reactor
(a) (10) Take a basis of 100 mol C2 H5 Cl produced and completely label the flowchart for the reactor.
Indicate the units assigned to each variable and value you add to the chart. You may write on the
diagram above or draw your own.
(b) (10) Peform a degree-of-freedom (DoF) analysis to demonstrate you have enough information to calculate all the unknowns labeled on the flowchart. Do not calculate anything in this part.
(c) (15) Write a system of equations for determining the unknown variables. Identify what each equation
represents, (e.g. balance on
, etc.). Outline an efficient procedure for solving the equations by
numbering the equations in the order you will solve them and circling the variables for which you will
solve. No algebra or numerical calculations.
(d) (10) Write an equation to calculate the fractional yield, Y , of monochloroethane, in terms of the
variables listed on the flowchart. Take Cl2 as the limiting reactant. Show all work and include the
appropriate units. No calculation required.
2. (15 points) The pressure rating on a gas cylinder indicates the maximum pressure that the cylinder can
withstand before rupturing. The pressure rating on a 565 L cylinder is 2.00 × 103 psig. If the cylinder
holds 2.20 × 103 moles of nitrogen (N2 ) gas, use the truncated virial equation of state to determine the
temperature, Tmax (◦ C), at which the cylinder will rupture. Data for N2 : B = -0.113 L/mol (assume the
second virial coefficient B is independent of temperature), Tc = 126.2 K, Pc = 33.5 atm, MW = 28.0 g/mol.
ChE 210 Exam 2 - Spring 2004
2
3. (40 points) In a process for producing chlorinated polyvinyl chloride (CPVC), 100.0 kg/h of CPVC wetcake
(90.0 wt% CPVC(s), 10.0 wt% NaCl(aq)) is fed to a rotary dryer where the moisture content of the wetcake
is reduced to 0.1 wt% H2 O. A fresh air stream at 27.0◦ C, 1.00 atm and 60.0% relative humidity is mixed with
an air stream recycled from the dryer exhaust, and the combined stream is heated and fed to the dryer. The
exhaust air leaves the dryer at 110◦ C, 1.3 atm, and 20.0% relative humidity. Ninety percent of the exhaust
air is recycled and the rest is vented to the atmosphere. A labeled flowchart depicting this process is given
below.
(a) (5) Draw a dashed line boundary to show the streams included in the overall process.
(b) (10) Perform a degree-of-freedom (DoF) analysis to demonstrate you have enough information to solve
for the unknowns appearing in the overall process you identified in part(a). Do not calculate anything
in this part.
(c) (20) Write a complete set of equations for determining the unknown variables in the overall process.
Identify what each equation represents, (e.g. balance on
, etc.). Fill in all known values and
necessary unit conversions but do no arithmetic. When vapor pressures are needed, just write p∗H2O (T )
where T is the temperature you intend to use in the equation. Outline an efficient procedure for solving
the equations by numbering the equations in the order you will solve them and circling the variables
for which you will solve. No algebra or numerical calculations.
(d) (5) Briefly explain the reason for venting a fraction of the exhaust air stream to the atmosphere instead
of recycling the entire exhaust air stream to the dryer.
60% relative humidity
27.0 oC
1.00 atm
Fresh air
V1 (m3/h)
20% relative humidity
110.0oC
1.30 atm
Exhaust air recycle
20% relative humidity
110.0oC
1.30 atm
Exhaust air to atmosphere
0.90 n 3 (kmol/h)
y 3 (kmol H 2O(v)/kmol)
(1 − y3) (kmol dry air/kmol)
0.10 n 3 (kmol/h)
y 3 (kmol H 2O(v)/kmol)
(1 − y3) (kmol dry air/kmol)
Exhaust air
n3 (kmol/h)
y 3 (kmol H 2O(v)/kmol)
(1 − y3) (kmol dry air/kmol)
20% relative humidity
110.0oC
1.30 atm
Heater
n1 (kmol/h)
y 1 (kmol H2O(v)/kmol)
(1 − y1) (kmol dry air/kmol)
147.4oC
n4 (kmol/h)
y 4 (kmol H 2O(v)/kmol)
(1 − y4) (kmol dry air/kmol)
Rotary
dryer
Wetcake
100.0 kg/h
0.90 kg CPVC(s)/kg
0.10 kg NaCl(aq)/kg
0.0011 kg NaCl/kg NaCl(aq)
0.9989 kg H 2O/kg NaCl(aq)
Dried cake
m 2 (kg/h)
(0.999 − x2) kg CPVC(s)/kg
0.001 kg H 2O/kg
x 2 kg NaCl/kg
ChE 210 Exam 2 - Spring 2006
1
Understanding that academic integrity is expected of all Iowa State University students, at all times, I pledge
that I have neither given nor received unauthorized aid on this examination.
Signature:
Date:
Instructions: Open book; open notes. Use one side of each page, begin each numbered problem on a new
page, show all your work, and box your answers. Number and put your initials at the top of each page.
When done, fold all the pages (exam questions and your solutions) vertically and put your name and “Test
2” on the outside. (Note: The numbers in parentheses next to each part of each problem are point values for
that part.)
1. (45 points total) A vapor mixture of n-butane (B) and n-hexane (H) contains 50.0 mole% butane at
120◦ C and 1.0 atm. A stream of this mixture flowing at a rate of 150.0 L/s is cooled and compressed,
causing some but not all of the vapor to condense. Liquid and vapor product streams emerge from the
process in equilibrium at T (◦ C) and 1100 mm Hg. The vapor product contains 60.0 mole% butane. A
labeled flowchart for this process is given below.
(a) (10) Write a basis of calculation for the flowchart.
(b) (10) Peform a degree-of-freedom (DoF) analysis to demonstrate you have enough information
to calculate all the unknowns labeled on the flowchart. Remember to count T as an unknown.
Do not calculate anything in this part.
(c) (15) Write a complete set of equations for determining the unknown variables. Identify what
each equation represents, (e.g. balance on
, etc.). Fill in all known values and necessary
unit conversions but do no arithmetic. If vapor pressures are needed, just write p∗i (T ) where i
is the species and T is the temperature you intend to use in the equation. Outline an efficient
procedure for solving the equations by numbering the equations in the order you will solve them
and circling the variables for which you will solve. No algebra or numerical calculations.
(d) (10) State three assumptions you made that could lead to errors in the calculated quantities.
Feed
150.0 L/s, 120 C, 1 atm
n1 (mol/s)
0.50 mol B/mol
0.50 mol H/mol
Vapor, T(C), 1100 mm Hg
n2 (mol/s)
0.60 mol B/mol
0.40 mol H/mol
Liquid, T(C), 1100 mm Hg
n3 (mol/s)
x3 mol B/mol
(1-x3) mol H/mol
ChE 210 Exam 2 - Spring 2006
2
2. (10 points total)
(a) (5) Water is in a gaseous state at 800 K and 1 atm. Would it be classified as a vapor or a gas?
Explain.
(b) (5) If water at 800 K and 1 atm is compressed isothermally to 300 atm, will a condensate form?
What term might you use to refer to the water at its final condition?
3. (45 points total) Sorbitol is an ingredient in “sugar-free” candy. It is sweet but does not promote tooth
decay because bacteria cannot metabolize it for food, and it is considered diet food because humans
do not metabolize it well either. Sorbitol (C6 H14 O6 ) is made from glucose (C6 H12 O6 ) and hydrogen.
100 kg/day of a 30 wt% glucose solution is mixed with a stoichiometric flow rate of hydrogen and
sent to a reactor; 80% of the glucose is converted to sorbitol. The hydrogen is then separated from the
sugar solution as a gas stream. A labeled flowchart is given below.
(a) (10) Write a basis of calculation for the flowchart.
(b) (15) Peform a degree-of-freedom (DoF) analysis to demonstrate you have enough information
to calculate all the unknowns labeled on the flowchart. Do not calculate anything in this part.
(c) (20) Write a complete set of equations for determining the unknown variables. Identify what
each equation represents, (e.g. balance on
, etc.). Fill in all known values and necessary
unit conversions but do no arithmetic. If vapor pressures are needed, just write p∗i (T ) where i
is the species and T is the temperature you intend to use in the equation. Outline an efficient
procedure for solving the equations by numbering the equations in the order you will solve them
and circling the variables for which you will solve. No algebra or numerical calculations.
Hydrogen
m1 (kg H2/day)
Glucose solution
100 kg/day
G = glucose
S = sorbitol
m4 (kg H2/day)
Reactor effluent
Reactor
0.30 kg G/kg
0.70 kg H2O/kg
Exit hydrogen
m2 (kg G/day)
70 kg H2O/day
m3 (kg S/day)
m4 (kg H2/day)
Separator
Sorbitol solution
m2 (kg G/day)
70 kg H2O/day
m3 (kg S/day)