CBEMS 130 Midterm Exam, Feb 18 2015.
This is a closed book/note test You may use the following items: Pen, Pencil, Ruler, Eraser, and Calculator
The TA’s have additional paper if needed.
When you are finished with your you r test, take a couple of minutes to write your name and today’s date on each sheet. Also make sure you number each page and indicate the total number of pages below.
Good luck and stay calm.
Name (please print): __________________________ ________________________________________ ________________________ __________________ ________
Signature: ________________________ __________________________________ _________________________ __________________________ __________________ _______
Total number of pages: _______
Question 1. Please write ‘true’ or ‘false’ after each of the following statements: (1p each)
a) For a 2-phase system in equilibrium the concentration of all species are equal in both phases. b) Separation systems can be designed to be environmentally friendly, perfectly safe, and economical. c) Small droplet size in a separation system will improve kinetics of phase transfer. d) In x-y diagrams it is conventional to plot the values of the more volatile component. e) Compressing a gas will always make it precipitate (i.e. a liquid will form). f)
Equilibrium constants are not constants (they are functions).
g) At the dew point a saturated vapor exists. h) Isothermal and adiabatic flash distillation is the same thing. i)
Co-current extraction is more efficient than cross current.
j)
Increasing number of stages in counter-current extraction may give perfect extraction (100%).
Please provide short answers to the following questions Question 2. Give 4 examples of different types of separation processes. (4p)
Question 3. Give 3 examples of when separation processes are required in industry or for different applications. (6p)
Question 4. In a liquid-liquid extraction process what assumption can we make that allows us to disregard ternary phase diagrams? (5p)
Question 5. What is the impact and significant of an azeotropes in a vapor-liquid separation system? (5p)
Problem 1: (25p)
An ethanol-water mixture is to be flash distilled at 1 atm. The vapor stream from the flash drum contains 51 mol% ethanol. a) Draw a diagram and label all streams. (5p) b) What is the liquid stream composition? (5p) c) Derive an equation that relates the compositions in all three streams (x,y, and z) to the fraction vaporized in the flash drum V/F. (5p) d) If the feed is 25 mol% ethanol, what fraction of the liquid feed must be vaporized? (5p) e) If the feed is 100kmol/hr. Find V and L. (5p) Molecular weights for ethanol and water are 46.07 and 18.01 g/mol respectively.
Y-X Diagram for Ethanol-Water at 1 atm 1 0.9 0.8 0.7 0.6
l o n a h t e y
0.5 0.4 0.3 0.2 0.1 0 0
0.1
0.2
0.3
0.4
0.5 x ethanol
0.6
0.7
0.8
0.9
1
Problem 1 answer sheet.
Problem 2: (25p)
A liquid mixture of 25mol n-pentane, 45 mol n-hexane and 30 mol n-heptane initially at 69°C and a higher pressure is fed into a flash drum and isothermally flashed by dropping the pressure to 1.013 bar. The vapor pressure for each component is given by Antoine’s equation as follows:
ln
ln
ln
= 10.422 −
= 10.456 −
= 11.431 −
26799 29676 35200
Where P is in bar, T is in Kelvin, R= 8.314 J/mol*k and subscripts 5, 6, 7 represent pentane, hexane and heptane respectively.
a) Using the Gibbs Phase Rule prove that this system is indeed solvable. (5p) b) Find the K values for each component. (5p) c) Set up the system of equations necessary to find the total amount of the liquid and vapor phases AND compositions and write down how you would go about solving for them (L, V, x 5, x6, x7, y5, y6, y7). PLEASE NOTE, you do not need to calculate any specific values for these variables. (15p)
Problem 2 answer sheet.
Problem 3: (20p)
Acetic acid is extracted from water into isopropyl ether (IPE) in a single stage extraction system. The ternary phase diagram is given below with the phase transition line and the tie lines. The values on each axis are in wt%. The extract in this system is the IPE-rich phase and the raffinate is the water rich phase. a) A feed of 250 kg of water with 400 kg of acetic acid is mixed with a solvent of 350 kg/h of isopropyl ether. What is the composition of the extract phase and the raffinate phase and how large is each of the outgoing phase? (10p) b) Use the data provided in the phase diagram below to draw an equilibrium graph for the extraction .
system (i.e. . vs ). In this case x refers to wt%. Use the blank grid provided on the separate page. Comment on the shape of the curve and the value of the distribution constant. (10p)
Problem 3 answer sheet.
