CEE537 Problem set 6 2017 Air Stripping PDF

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air stripping practice problems...

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CEE 537 Fall 2017

Name: ___________________________

Reference reading for air stripping AWWA 6th edition  Chapter 6, Packed towers MWH 3rd edition 

Chapter 14: Air stripping and aeration

CEE 537 Fall 2017

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Practice Problem Set #6 (Air Stripping) 1. The Water Factory 21 facility in Orange County, California was designed to treat effluent (pH = 7.3) from a wastewater treatment plant and use the treated water for groundwater recharge. One of the processes used in the original plant consisted of steam stripping towers designed to remove ammonia (see schematic). Assuming that ammonia in the off-gas exiting at the top of the tower has reached approximately equilibrium with the dissolved ammonia, estimate the gas-to-liquid flow rate ratio QG/QL that would be required to remove 90% of total ammonia (i.e., [NH 4+] + [NH3]) from the water. Assume that the wastewater is well buffered and thus the pH remains constant inside the tower, and that the Henry’s law constant for ammonia is m = 0.0009 at the temperature of the gas-liquid interface at the top of the tower. Notes: (1) pKa = 9.3 for ammonia acid-base equilibrium: NH 4+ ↔ NH3 + H+; (2) Henry’s law expression: Cg = mCl.

CEE 537 Fall 2017

Name: ___________________________

CEE 537 Fall 2017

Name: ___________________________

2. Air stripping is used to remove hydrogen sulfide H 2S (Henry’s law constant m=36, and dissociation constants pKa,1=7 and pKa,2=14, all at 25°C) from reverse osmosis permeate (permeate pH≈4). A permeate flow rate QL is treated in a single air-stripping tower with air flow rate QG. These flow rates together with a packing height L resulted in total sulfide concentration CT,S (CT,S = [H2S]+[HS−]+[S=]) removal f of approximately 99 percent at 25°C. The membrane originally used in the RO unit (a positively charged polyamide) is then substituted by a negatively charged polyamide membrane. As a result, the permeate pH increases to pH=8. Estimate the new air flow rate QG required to keep total sulfide concentration CT,S removal f at 99 percent at 25°C. Notes: (1) assume you can neglect gas phase resistance to overall mass transfer; (2) assume you can neglect changes in permeate pH due to H 2S volatilization; (3) you might find the following expressions useful for your analysis:

H OL

1 U   H L   L L 46   L

S

mU G mQ G  UL QL

NL 

 S f 1  S ln 1 S  f  S 

L  N L  HOL

  

0.25

 L      L DL 

f  1

0.5

C l,out Cl,in

CEE 537 Fall 2017

Name: ___________________________

CEE 537 Fall 2017

Name: ___________________________

3. A drinking water utility uses a counter-current air stripping tower for the removal or hydrogen sulfide (m = 0.36 for H 2S at 25°C) from groundwater. The groundwater is supersaturated with CO2, which volatilizes, and as a result the pH of the groundwater increases with axial distance x in the direction indicated in the schematic. Provide the normalized mass balances representing the concentration Cg of H2S in the gas phase and total concentration of hydrogen sulfide CL (i.e., CL = [H2S] + [HS−]) in water as a function of dimensionless axial distance z assuming that the pH profile in z, i.e., pH(z), is known. Explain fully your approach. Notes: (1) assume that the change in pH with z is known from a previous calculation of CO2 stripping, and that the hydrogen sulfide concentration is much lower than that of carbon dioxide and thus that although pH affect hydrogen sulfide stripping, hydrogen sulfide stripping does not affect the pH; (2) you are NOT asked to integrate any resulting differential equations; (3) you might find the following normalized expressions for countercurrent air stripping at constant pH useful for your analysis:



 cg dc L  N L'   cL dz m

   0 

 cg  d   '  m   N L  cg  c   0 L  dz S  m  where

z

x L

k OLa ' L UL U S m G UL N L' 

CEE 537 Fall 2017

Name: ___________________________

CEE 537 Fall 2017

Name: ___________________________

4. A wastewater contaminated with a certain volatile organic compound (VOC) is currently being treated by air stripping. A constant wastewater flow rate of QL is treated in a single air-stripping tower with a constant air flow rate QG. These flow rates together with the VOC Henry’s law constant correspond to a stripping factor S = 10. A consistently constant wastewater temperature results in a constant VOC removal f = 90 percent. There are three alternatives to improve the removal of the VOC are: (a) Install a second identical air-stripping tower in parallel to the existing one. The wastewater flow rate to each tower would be QL/2, and the air flow rate to each tower would be QG (the new tower would include a new blower). (b) Install a second identical air-stripping tower in series to the existing one. The wastewater flow rate to each (a) tower would be QL (the wastewater effluent from the QL/2 QL/2 first tower would be the influent to the second tower), and the air flow rate to each tower would be QG (the new tower would include a new blower). (c) Double the height of packing inside the existing L L tower. The wastewater air flow rates to the tower would be QL and QG. QG

Calculate the removal fraction f for (a), (b) and (c), and determine which of the three alternatives would be optimum on the basis of highest f. Justify your answer fully.

QG

(b) QL

Notes: (1) the VOC of interest if very volatile and therefore gas phase resistance to overall mass transfer can be neglected; (2) Cg, in = 0; (3) you might find the following expressions useful for your analysis:

H OL  H L 

S

mU G

NL 

UL



1 U L  L  46   L

mQ G QL

   

0.25

 L     D   L L

f  1

0.5

L

QG

QL

C l,out Cl,in 2L

QG

L

QG

(c)

S  S f 1  ln 1 S  f  S 

L  N L  HOL

QL

CEE 537 Fall 2017

Name: ___________________________

CEE 537 Fall 2017

Name: ___________________________

CEE 537 Fall 2017

Name: ___________________________

5. The expression to describe the number of mass transfer unit, NL, for a counter-current air stripping packed tower was developed in the class. QL cl,in

 S f  1  S NL  ln   1  S  f  S  mUG mQG  UL QL c f  1  l, out c l, in

QG cg,out

x

S

L

Develop the expression of NL for co-current air stripping packed tower.

QG cg,in=0

QL cl,out

CEE 537 Fall 2017

Name: ___________________________

CEE 537 Fall 2017

Name: ___________________________

CEE 537 Fall 2017

Name: ___________________________

6. High volatility contaminants can be removed in spray reactors. Water is sprayed into small droplets that are carried by the turbulent gas flow without significant segregation. The droplets are removed in a demister located at the outlet of the reactor. Provide the equations of continuity for the volatile contaminant in the gas and liquid phases (you are NOT asked for initial and/or boundary conditions; you are NOT asked to integrate any resulting differential equations).

Contaminated Water Injection QL, Cl,in Air QG Cg,in=0

Demister

x Water Droplet

Water Out QL, Cl,out

Air Out QG Cg,out

CEE 537 Fall 2017

Name: ___________________________

CEE 537 Fall 2017

Name: ___________________________

7. An air stripping tower operated under conditions corresponding to a stripping factor S=30 provides a target removal of 90 percent ( f=0.90) for 1,1,2-trichloro-1,2,2-trifluoroethane ( m=10, DL/DL,O2=0.39) at the temperature of interest). The air-stripping tower is treating groundwater from a deep aquifer with dissolved oxygen concentration below detection. New regulations for tower effluent disposal into surface water require that the dissolved oxygen ( m=30 at the temperature of interest) concentration be within 95 percent of the saturation value. Estimate if the tower effluent meets the new regulatory requirement. Note: You might find some of the following expressions useful for your analysis:

HOL

1  HL  46

U L L     L 

0.25

  C g,inS  mC  S  f  1 S l, in  NL  ln  C g,in S  1 S   f  S  mC  l, in   S

mU G UL

f  1



C l,out Cl,in

L  N L  H OL

mQ G QL

0.5

 L     L D L 

CEE 537 Fall 2017

Name: ___________________________

CEE 537 Fall 2017

Name: ___________________________

CEE 537 Fall 2017

Name: ___________________________

8. A company uses an air stripping tower designed to remove 90% (f = 0.9) of the volatile organic contaminant (VOC) fluorotrichloromethane (mVOC = 3; DL,VOC/DL,O2 = 0.4) from anoxic groundwater (Cl,in,O2 = 0). The tower is operated at a stripping factor SVOC = 20. Estimate how close the concentration of oxygen (mO2 = 30; DL,O2/DL,O2 = 1) in the tower effluent water would be to equilibrium with oxygen in ambient air. Note: (1) Assume negligible gas side resistance for fluorotrichloromethane and oxygen gas transfer; (2) f = 1- (Cl,out/Cl,in).

H OL

1 U    H L   L L  46   L 

0.25

Q Gc G, in  S  f  1 QLcL, in   1 S NL   exp  Q Gc G ,in  S  f S  QLcL ,in

NL

S

S 1

mQG QL

0.5

 L      L DL 

é S × ( f 1) ù lnê ú S ë f S û

L  N L  HOL

CEE 537 Fall 2017

Name: ___________________________

CEE 537 Fall 2017

Name: ___________________________...