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CHE 205 Chemical Process Principles Section 1

Final Examination. Open bookclosed notes. Use one side of each page, begin each numbered problem on

a new page, and box your answers. Number the pages and put your initials at the top of each page. When

done, fold the pages vertically and put your name, section, and Final Exam on the outside.

The instructor has permission to post my final grade identified by the last six digits of my student ID number.

(Signature) __________________________________

1. (15) The following problems can be done quickly using data in your 205 text. If you dont see a quickway to do them, outline a long method but dont do any calculations.

(a) (5) Two 205 students trying to calculate a volumetric flow rate of pure n-butane vapor at 425 K and150 atm are arguing about whether or not ideal gas behavior is a good assumption. The first student

reasons that at a temperature as high as 425 K the vapor is bound to be ideal, and the second replies

that at 150 atm the vapor will be nonideal, no matter how hot it gets. You are called on to settle the

problem. Estimate the ratio Vreal/Videal. With which students statement would you agree? (First,

second, both, neither)

(b) (5)A gas contains 10.0 mole% water in argon (noncondensable) at 100oC and 800 torr. (i) What is thedew point of the gas? (ii) If the gas were compressed isothermally, at what pressure (torr) would

condensation begin?

(c) (5) On a clear spring day a weather report indicates that the temperature is 55oF (notethatsFahrenheit) and the relative humidity is 40%. In the afternoon the temperature begins to drop and by

evening a fog sets in. Estimate the temperature at which the fog first forms.

2. (15) One hundred grams of pure liquidn-hexane completely fills an expandable chamber. The chamber isslowly heated from 25oC to 120oC at a constant pressure of 1 atm.

(a) (10) Calculate the chamber volume (liters) at the beginning and end of the process.(b) (5) Sketch a plot of temperature versus time from the beginning to the end of the process. (Assume

all segments are linear.) Label the initial and final temperatures and the value of any intermediate

temperature where the slope of the plot abruptly changes.

3. (15) An open-ended mercury manometer is connected to the top of a flask containing liquid Species A inequilibrium with its vapor on a day when atmospheric pressure is 760 torr. When the temperature in the

flask is 52.5oC, the mercury level in the arm connected to the flask is 300 mm higher than the level in the

open arm. When the temperature is raised to 77.5oC, the mercury levels in the two arms are equal.

(a) (3) Sketch a phase diagram of A and label the values of the (T,P) coordinates of two points on thevapor-liquid equilibrium curve based on the given data.

(b)(2) What is the normal boiling point of A?

(c) (10) Estimate the heat input (kW) that would be required to vaporize 10 mol/s of A at its normalboiling point, using only data from the first paragraph of this problem. If you cannot figure out how

to do so, assume that the molecular formula for A is C2H5O2 and use another method for partial credit.

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4. (15) A liquid mixture containing 60.0 mole% benzene and the balance toluene at 125 oC andPfeed(torr) isfed to an adiabatic flash evaporator at a rate of 150 mol/s. Liquid and vapor streams leave the evaporator

in equilibrium at 1 atm and temperature Te. Data: pC6H6o C* ( )125 = 2530 torr, pC7H8

o C* ( )125 = 1124 torr.

(nv mol / s)

y (mol C6H6(v)/mol)150 mol/s (1y)( C7H8(v)/mol) Equilibrium

0.600 mol C6H6(l)/mol (nl mol / s) Te(oC), 1 atm

0.400 mol C7H8(l)/mol x (mol C6H6(l)/mol)

125oC, Pfeed (1x)( C7H8(l)/mol)

(a) (5) What is the minimum value ofPfeed required to assure that the feed stream does not vaporize in thefeed line? (In other words, ifPfeedwere lowered, at what value would the first bubble of vapor form?)

(b) (5) Without doing any calculations, use information in the text to specify a range within which theoutlet temperature Te must fall if both liquid and vapor phases are present. The narrower the range

you specify correctly, the more points you will get.

(c) (5) Briefly explain why Te must be less than 125oC if any vaporization takes place. (The fact that theevaporator is adiabatic should be part of your reasoning.) What would be the effect on Te of

lowering the evaporator pressure? (Lower Te, higher Te, no change, cant tell without more

information)

5. (40) A gas stream contains CH3OH (methanol) vapor [mole fraction y0 (mol CH3OH/mol)] and anoncondensable mixture of CO and H2 in a ratio 2 mol H2/mol CO at 100

oC and 1000 torr. The stream

flows at a rate of 500 SCMH (standard cubic meters per hour). The relative saturation of methanol vapor

(which should be considered the only condensable species) in the mixture is 5.00%. The gas is fed to a

condenser. Vapor and liquid streams in equilibrium at 0oC and 1 atm emerge from the condenser.

Vapor pressures of methanol: p pMo

MoC) = 29.8 mm Hg, C) = 2598 mm Hg* *( (0 100

(a) (10) Draw and label a flow chart (include the heat duty on the condenser, Q (kJ/h), in the labeling),state the number ofindependentspecies in the process, and carry out the degree-of-freedom analysis.

(b) (15) Write in an efficient order the complete set of equations you would solve to calculate thepercentage recovery of methanol [mol CH3OH condensed/mol CH3OH fed]. In each equation or set

of simultaneous equations, circle the variable(s) for which you would solve. Dont worry about unit

conversions.

(c) (15) Prepare an inlet-outlet enthalpy table and write a complete set of equations to calculate Q . Theonly variables in your equations should be the ones labeled on your flow chart and specific enthalpies.

When determining the specific enthalpies, choose as references CO(g) and H2(g) at 25oC and

CH3OH(v) at 0oC and determine all values (such as 0) that can be determined without doing any

calculations. Show the limits of all integrals and the values of all heats of vaporization, and clearlyindicate the species and phases for all heat capacities [e.g. (Cp)CH3OH(v)] but do not substitute formulas

for them. Ignore effects of pressure on enthalpy.