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ENV 6519: Physical and Chemical Processes in Environmental Engineering

Spring 2012 University of South Florida Examination Civil & Environmental Eng. Thurs., March 22, 2012 J. A. Cunningham Instructions: 1. You may read these instructions, but do not turn the page or begin working until instructed. 2. Answer all questions in the exam booklet provided, and write your name conspicuously on

the exam booklet. 3. You are allowed one sheet of 8.5-by-11-inch paper (or A4 paper) with hand-written notes.

You may write on both sides of that paper. However, mechanical reproductions (photocopying, laser printing, scanning, etc.) are not allowed; all notes must be hand-written.

4. A calculator is recommended, but it may not be pre-programmed with formulae from the class.

5. Time limit: 70 minutes. Stop working when asked. If you continue working after time has been called, you will be penalized at a rate of 1 point per minute.

6. Show all work and state all assumptions in order to receive maximum credit for your work. 7. Make sure your answers include units if appropriate. Watch your units!!!! 8. This exam contains 3 questions, all with multiple parts. Answer question 1. Then, choose

either question 2 or question 3. 9. The total point value for the exam is 70 points -- one point per minute. Gauge your time

accordingly! 10. Use a reasonable number of significant digits when reporting your answers. You are likely

to be graded down if you report an excessive number of significant digits. In some cases, the problem may indicate the precision to which you should report your answer.

11. Don't cheat. Cheating will result in appropriate disciplinary action according to university policy. More importantly, cheating indicates a lack of personal integrity.

12. Pages 2–3 of this exam contain background information, data, constants, and conversion factors that might be helpful to you as you complete the exam. I recommend that you read page 2 carefully, especially the background information, as it is likely that you will need some of that information to complete the problems on the exam.

13. Additional pages, photocopied from your course text, are provided to give you additional information (pp 4–6 of this exam). You might or might not find the information useful.

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Background Information A site is contaminted with 1,1,2-trichloroethane (abbreviated TCA henceforth). The site owner hired a consulting firm to design three possible clean-up technologies for the contaminated site. The consulting firm was requested to design technologies to treat 104 gal/min (equivalent to 0.00656 m3/s) at an influent TCA concentration of 405 g/L, and to reach a treatment objective of no more than 5 g/L in the effluent (treated) water. This corresponds to 98.8% removal of the TCA. Unfortunately, after all three designs were completed, it turned out that the consulting firm had been given incorrect information. The good news is that the contaminated water actually contains only 207 g/L of TCA – about half of what was believed previously. Hence, only 97.6% removal is required to meet the treatment objective of 5 g/L. The bad news is that contaminated water must be pumped at a rate of 208 gal/min, not 104 gal/min, in order to prevent the contamination from migrating off-site (which would result in a lawsuit). Properties of 1,1,2-TCA are given below. The temperature of the groundwater is 18 °C. Properties of 1,1,2-TCA: Molecular formula: C2H3Cl3 Molecular weight: 133.41 g/mole Liquid density: 1.44 g/cm3 = 1440 kg/m3 Vapor pressure, Psat: 3980 Pa at 25 °C Aqueous solubility: 0.034 mol/L Octanol-water partition coefficient: 102.34 = 220 Reaction rate constant with •OH: 1.1×108 L/(mol•s) according to text Table 8-8 Heat of vaporization, Hvap: 9.10 kcal/mol = 38.1 kJ/mol estimated from Staudinger and

Roberts [2001] Potentially useful constants:

Ideal gas constant, R: 8.314 Pa•m3•mol–1•K–1 = 82.0610–6 atm•m3•mol–1•K–1 Gravitational acceleration, g: 9.81 m/s2 Molecular weight of water, H2O: 18.01 g/mole Density of water at 18 °C: 0.9985 g/mL = 998.5 kg/m3 Viscosity of water at 18 °C: 1.06×10–3 Pa•sec Density of air at 18 °C: 1.21 kg/m3 Viscosity of air at 18 °C: 1.81×10–5 Pa•sec

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Potentially useful conversion factors: Pressure: 1 atm = 760 mm Hg = 760 torr = 101325 Pa

Mass: 1 kg = 1000 g = 106 mg = 109 g Temperature: 25 °C = 298.15 K Volume: 1 m3 = 1000 L = 106 mL = 106 cm3 1 gal = 3.785 L Other : 1 Pa = 1 N/m2 = 1 kg/(m•sec2) Atomic Masses: H = 1.008 g/mole C = 12.011 g/mole N = 14.007 g/mole O = 15.999 g/mole Cl = 35.453 g/mole Br = 79.904 g/mole

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1. (40 pts) One of the technologies designed by the consulting firm was air stripping via packed-tower aeration. The original design of the air stripping tower looked like this:

Tower diameter, d = 1.2 m Packing type = 2” plastic saddles Air flow rate, Qa = 0.630 m3/s Required height of packing material, L = 8.8 m [based on no safety factor – OK for here] Pressure drop in tower, P/L = 100 Pa/m Liquid-side mass transfer coefficient, estimated by Onda correlation, kL = 1.41×10–4 m/s Gas-side mass transfer coefficient, estimated by Onda correlation, kG = 1.04×10–2 m/s Overall mass transfer coefficient, KL = 9.06×10–5 m/s Wetted surface area of packing, estimated by Onda correlation, aW = 48.7 m2/m3

a. (5 pts) Estimate Henry’s constant for 1,1,2-TCA at 18 °C. Hint: you can use the information on p 2, but there is an easier way, based on the information given above.

Suppose you built the air stripping tower according to the original design. Afterwards, you

found out about the new treatment conditions (Q = 208 gal/min, and influent concentration C0 = 207 g/L). Let’s see if the tower you built will still work. Suppose you keep the same air flow rate as in the original design. If you do that, then the Onda correlation predicts the following for the revised treatment conditions:

Liquid-side mass transfer coefficient, kL = 1.97×10–4 m/sec Gas-side mass transfer coefficient, kG = 1.04×10–2 m/sec Wetted surface area of packing, aW = 59.2 m2/m3

b. (4 pts) For the revised treatment conditions, estimate/calculate the liquid loading rate, LM, in the tower, in units of kg/(m2•s). Is the value reasonable? Explain briefly (one sentence should probably suffice).

c. (20 pts) Estimate/calculate the concentration of TCA you would expect to see in the effluent water under the revised treatment conditions. (Hint: Use L = HTU*NTU. You are given L and you can estimate everything else you need.) Is the treatment objective reached under the new conditions?

d. (8 pts) Estimate/calculate the pressure drop through the tower at the higher water flow rate. (Hint: find the point on the Eckert curve that corresponds to the original design conditions. How will that point change for the new design conditions?) Is the value acceptable? Explain briefly (a sentence or two).

e. (3 pts) Can you still use the tower you built to achieve your objective? Why or why not? Explain briefly.

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2. (30 pts) One of the technologies designed by the consulting firm was adsorption onto granular activated carbon (GAC). The original design of the GAC system was for two GAC contactors to be operated in series. Each contactor would have a diameter of 1.75 m and a length of 2.5 m. The contactors would be filled with Calgon Filtrasorb 300 (F300) activated carbon, which has a bulk density f = 480 kg/m3. The adsorption of TCA onto Calgon F300 carbon can be described with a Freundlich isotherm. The Freundich parameters are KF = 5.8 and 1/n = 0.60, where the units on KF are (mg/g)/(mg/L)1/n.

a. (8 pts) For the original design parameters (Q = 104 gal/min and C0 = 405 g/L), estimate

how many bed volumes can be treated in one of the contactors before the GAC must be changed. Also estimate the carbon usage rate in units of g GAC required per m3 water treated. Do the values appear reasonable? Explain briefly (a sentence should suffice).

Suppose you built the GAC system according to the original design. Afterwards, you found

out about the new treatment conditions (Q = 208 gal/min, and influent concentration C0 = 207 g/L). Let’s see if the system you built will still work.

b. (5 pts) Estimate/calculate the superficial velocity, v0, and the empty-bed contact time,

EBCT, for the new treatment conditions. Are the values acceptable? Explain briefly (a sentence or two).

c. (4 pts) Re-calculate the bed-volumes treated and the carbon usage rate for the new treatment conditions. Do the values appear reasonable? Explain briefly as before.

d. (4 pts) Suppose the cost of Calgon F300 is $3.00 per kg. When you switch from the original treatment conditions to the revised treatment conditions, how much does your estimated annual cost of GAC change? Be sure to specify if the change is a cost increase or a cost decrease.

e. (5 pts) Can you still use the GAC contactors you built to achieve your objective? Why or why not? Explain briefly.

f. (4 pts) Which process is more robust to changes in operating conditions – air stripping (from problem 1) or adsorption onto GAC (from this problem)? Explain briefly.

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3. (30 pts) One of the technologies designed by the consulting firm was advanced oxidation by ozone and peroxide. The consulting firm designed the process to take place in a plug-flow reactor (not a completely-mixed flow reactor). They have a special design where the reaction takes place in a long, serpentine pipe, which induces plug-flow conditions. Ozone and peroxide are added at multiple points along the pipe, which enables the concentration of hydroxyl radicals to remain approximately constant along the duration of the reactor. The consulting firm specified a pipe with a cross-sectional area of 0.20 m2. They estimate that the concentration of hydroxyl radicals is at least 10–10 mol/L in the reactor.

a. (8 pts) For the original design conditions (Q = 104 gal/min and C0 = 405 g/L), how long

would the pipe have to be in order to achieve the treatment objective? Hint: for a PFR

with first-order reaction at steady state, CE = C0 exp(–k1 where k1 is the first-order reaction rate coefficient, and is the residence time of the reactor.

Suppose you built the AOP reactor according to the original design. Afterwards, you found

out about the new treatment conditions (Q = 208 gal/min, and influent concentration C0 = 207 g/L). Let’s see if the reactor you built will still work.

b. (5 pts) Suppose you maintain the same concentration of hydroxyl radicals as above.

What will be the concentration of TCA in the effluent water? Did you meet the treatment objective?

Suppose in part (b) you found that the reactor cannot meet the treatment objective under the new treatment conditions. To improve the reactor performance, you want to increase the concentration of hydroxyl radicals by 70%. You think that perhaps you can achieve this by pre-treating the water: either by softening the water or by removing organic carbon from the water prior to the AOP. The concentration of bicarbonate in the water is 183 mg/L (as HCO3

–) and the concentration of dissolved organic carbon is 3.6 mg/L (as C). c. (8 pts) Which pre-treatment would you recommend: removal of DOC, or softening to

remove HCO3–? Why? About how much removal would be required to increase the

hydroxyl radical concentration to its desired level? Hint: estimate QR, the quenching factor, before and after pre-treatment.

d. (5 pts) Can you still use the reactor you built to achieve your objective? Why or why not? Explain briefly.

e. (4 pts) Which process is more robust to changes in operating conditions – air stripping (from problem 1) or advanced oxidation (from this problem)? Explain briefly.

END OF EXAMINATION