Spring 2000                                                                

ChE 250  Exam III                       

Name _____________________

 

This is a closed-book exam.  Some data tables and unit conversion factors are provided on the back side of the exam.  State any assumptions you use explicitly to ensure adequate credit.  Write only on the front of each page, and turn in all pages in order with the exam on top. 

 

Problem 1 (35 points)

A heat exchanger uses superheated steam to vaporize and heat a cyclohexane stream.  Steam at 320°C and 5 bar enters at 20 kg/min.  Liquid cyclohexane at 20°C and 2 bar enters at 1000 mole/min.  The water leaves the exchanger as saturated liquid at 5 bar.  Property data for cyclohexane follows. (Steam table is on back of exam.)

            DHvap (p=1 bar, Tbp=80.7°C) = 30.1 kJ/mole

            cp,liq = 130 J/mole°C                  cp,vap = 94 + 0.5 T (°C) J/mole°C

a)      Determine the enthalpy change for the water stream. What fraction of the energy extracted comes from the latent heat change, and what fraction is from the sensible heat change?

b)      Determine the outlet temperature of the cyclohexane vapor.

 

Problem 2 - Short Answer (30 points)

a) What are saturated vapor and subcooled liquid?  When is each found?

b) Write the full Energy Balance.  Describe each term, and when it is significant or negligible.

c)  What is special about intensive properties (also called specific properties) such as
or ? Compare to extensive properties, and give an example of proper use.

d) What is a “state variable”? How is this property of “state-ness” helpful?

 

Problem 3 (35 points)

The air stream leaving the painting booth contains solvent vapors that must be removed, both for environmental and economic reasons. A condensor can be used to remove a stream of liquid heptane from the paint-booth effluent.  The incoming stream is 30 mol % heptane, at 1 atm and 60°C, and flows at 2.5 kgmol/min. You are to cool the vapor stream to 20°C, where the heptane saturation vapor pressure is 0.05 atm and most of the heptane will condense and leave the cooler as saturated liquid at that same temperature.  The condensor uses water provided at 15°C for cooling.

a)   What % of the heptane is recovered in the liquid stream?

b)   What is the total heat removal required?  If the exit water temperature is to be 45°C, what water flow rate do you recommend?

 

air:                    use an average cp = 29.2 J/mol °C

heptane:            DHvap (p=1 bar, Tbp=98.4°C) = 31.69 kJ/mole

                        cp,liq = 242 J/mole °C

                                    cp,vap = 158 + .42 T (°C) J/mole °C

water:               cp,liq = 4.2 J/g °C

 

 

(over for more data)