1. CONTROL OF VOLATILE ORGANIC COMPOUNDS (VOCs)
VOCs and hydrocarbons are not identical
VOCs:are liquid or solids that contain organic carbon bonded to carbon, hydrogen, nitrogen, oxygen or sulfur which vaporize at significant rates
Hydrocarbons contain only hydrogen and carbon.
Acetone is a VOC (CH3-CO-CH3) not a hydrocarbon.
Hydrocarbons are slightly soluble in water.
Polar VOCs which contain oxygen or nitrogen in addition to carbon and hydrogen are much soluble in water.
Polar VOCs can be removed from a gas stream by scrubing with water.

2. VOCs VOCs are a large family of compounds.
Some of them toxic and carcinogenic to humans (benzene, toluene)
They participate in smog formation
NO+VOC+O2+sunlight NO2+O3
Some VOCs are strong IR absorbes contributing global warming problem

3. The Clean Air Act Amendments of 1990 List of Hazardous Air Pollutants
CAS
Number Chemical
Name
75070 Acetaldehyde
60355 Acetamide
75058 Acetonitrile
98862 Acetophenone
Acetylaminofluorene
Acrolein
79061 Acrylamide
79107 Acrylic acid
Acrylonitrile
Allyl chloride
Aminobiphenyl
62533 Aniline
90040 o-Anisidine
Asbestos
71432 Benzene (including benzene from gasoline)
92875 Benzidine
98077 Benzotrichloride
Benzyl chloride
92524 Biphenyl
Bis(2-ethylhexyl)phthalate (DEHP)
Bis(chloromethyl)ether
75252 Bromoform
,3-Butadiene
Calcium cyanamide
Caprolactam(See Modification)
Captan
63252 Carbaryl
75150 Carbon disulfide
56235 Carbon tetrachloride
Carbonyl sulfide
Catechol
Chloramben
57749 Chlordane
Chlorine
79118 Chloroacetic acid
Chloroacetophenone
Chlorobenzene
Chlorobenzilate
67663 Chloroform

4. National emissions of VOCs
34% of the total
40% of the total

5. Principal uses of VOCs Motor fuels Solvents
More than 80% come from solvent usage
Solvents and motor fuels are mostly derived from
petroleum
Most VOC emissions are of refined petroleum
products used as fuels and solvents

6. Vapor Pressure Liquid Vapor
The vapor pressure of a liquid is the partial pressure exerted by the vapor when it is in dynamic equilibrium with the liquid at a constant temperature.
vaporization
Liquid Vapor
condensation

7. Liquid–Vapor Equilibrium
… until equilibrium is attained.
More vapor forms; rate of condensation of that vapor increases …

8. Vapor pressure increases with temperature; why?

9. Vapor pressure, equilibrium vapor content, evaporation
Normal boiling point is the temperature at which vapor pressure equals the atmospheric pressure, liquid converts to a vapor by bubble formation.
At room temperature vapor pressure of water is atm (Water does not boil but evaporates!!!
1 atm= 760 torr=101.3 kPa=14.7 psia

10. Propane, methyl chloride, butane have vapor pressures above atmospheric pressure at room temperature. They must be kept in closed pressurized containers because they boil at room temperature

11. Vapor Pressure of a Solution and Raults Law
The vapor pressure of solvent above a solution is less than the vapor pressure above the pure solvent.
Raoult’s law: the vapor pressure of the solvent above a solution (Psolv) is the product of the vapor pressure of the pure solvent (P°solv) and the mole fraction of the solvent in the solution (xsolv):
Psolv = xsolv ·P°solv
The vapor in equilibrium with an ideal solution of two volatile components has a higher mole fraction of the more volatile component than is found in the liquid.

12. Raults Law Yi= Xi *Pi/P Yi= mole fraction of component i in the vapor
Xi= mole fraction of component i in the liquid
Pi= vapor pressure of pure component i
P= total pressure Xi

13. Why Rault’s law is revisited??
VOC are widely used in solvents and fuels
They are volatile
The rate of evaporation is proportional with vapor pressure
Composition of vapor is important in prevention and control of emissions (Rault’s law)

14. Example
At 25 oC, the vapor pressure of pure benzene and pure toluene are 95.1 and 28.4 mmHg. A solution is prepared that has equal mole fractions of benzene and toluene. What is the composition of the vapor in equilibrium with the benzene toluene solution.
Pben=95.1 mmHg
Ptol=28.4 mmHg.

15. Control approaches for VOCs
1.CONTROL BY PREVENTION
2. CONTROL BY CONCENTRATION AND RECOVERY
3.CONTROL BY OXIDATION

16. 1. CONTROL BY PREVENTION Substitution Process modification
Leakage control

17. Substitution
Substitution of more volatile ones with less volatile or nonvolatile solvents.
e.g:Oil based paints, coatings etc. contain volatile solvents.
substitution of solvent based paints with water based paints.
Less toxic solvents can be substituted for a more toxic solvent

18. Process modification
Process is modified so that formation of VOC is prevented or reduced.
Replacing electric powered vehicles with gasoline powered vehicles is a process modification.
Use of public transport instead of private cars is a kind of substitution.

19. Leakage control 1. Filling, breathing and emptying losses
2. Displacement and breathing losses for gasoline
3. Seal leaks

20. Filling, breathing and emptying losses
Vapor out
Liquid in
vapor
Liquid
Vapor out
Liquid in
vapor
Liquid
Figure 1

21. How to calculate working losses
VOC mass emission= volume of air (VOC mis expelled from the tank)x (concentration of VOC mix in that mixture)
mi= V Ci
Where mi= mass emission of component i
ci= concetration
ci= yi Mi/V (molar, gas)
mi/V = Xi Pi Mi/RT

22. Vapor out
Liquid in
vapor
Liquid
Example 10.4
The tank in Fig 1 contains pure benzene at 20oC which is in equilibrium with the air benzene vapor in its headspace. If we now pump in liquid benzene, how many kg of benzene will be emitted in the vent gas per cubic meter of benzene liquid pumped in? What fraction of this liquid benzene pumped into the tank? (density of benzene=0.864 kg/l

23. Displacement and breathing losses fro gasoline
Gasoline is a complex mixture containing 50 different hydrocarbons.
A typical gasoline has an average formula of about C8H17 and molecular weight of about 113.
Composition of the gasoline varies with seaon of the year and from refinery to refinery.

24. For zero percent vaporized p=6 psia and M=60
Vapor pressure at 20oC of the remaining liquid, at this %vaporized psia
For zero percent vaporized p=6 psia and M=60

25. For large scale storage
Large amounts of liquid like high pressure gasoline never stored in simple, cone roof like container.
A floating roof tanks can be used

26. Floating roof tank
Liquid in

27. Floating vs fixed roof tanks

28. Transfer of gasoline from tank trucks to underground storage
Vent line
Storage tanks are placed underground to save space and to reduce fire hazard.

29. Vapor from customer tanks is forced back to the storage tank

30. Seal leaks Small emissions of VOC occur as leaks at seals Static seals
Compression seals
Elastomeric seals
All of the pumps and valves that process VOCs have a leakage problem

31. Example 10.5
The tank in figure 1 is heated by the sun to 100 F; both vapor and liquid are heated to this temperature. How many pounds of benzene are expelled per cubic foot of tank?Assume that initially the tank was 50% by volume full of liquid , 50% by volume full of vapor?

32. Control by condensation
Most VOC’s are valuable
VOC can be condensed as liquid and can be separated from gas stream.
For large VOC containing gas streams it is economical

33. Difficulties with simple condenser-phase separator
Low Temperature needed for condensation requires multiple stage refrigerators
Material freezes on the cooling coils, requires frequent defrosting.
If the gas contains significant amounts of water vapor, it will condense and freeze on the cooling coils, requires defrosting. Water may also contaminate recovered liquid.
The cleaned gas leaving the system is very cold; refrigeration work to cool is wasted

34. Example 10.9
We wish to treat an airstream containing mol fraction(0.5%= 5000ppm) toluene moving at a flow rate of 1000 scfm at 100 F and 1 atm, so as to remove 99% of the toluene by cooling, condensation, and phase change. To what temperature must we cool the airstream?

35. Two stage condenser separator for recovering VOC from displacement of vapors of a gasoline tank truck
Water is eliminated
Gasoline is recovered

36. QUESTION 10.10
Displacement loss from the returning tank should not exceed 35 mg of VOC per litter of gasoline filled. Assume that vapor leaving the track is 200C= 68F in equilibrium with remaining gasoline in the track and that of vapor is 1 mol% water vapor.
A) How cold must the second chiller cool the displaced vapor before discharging it to the atmosphere. Discharge vapor is is in equilibrium with liquid gasoline at that temperature and gasoline MW is 60 and vapor pressure is given by lnP(psi)=11.24-( R)/T
B)What fraction of the gasoline will be removed in the first stage which cools the gas about 32F
C) What is the ratio of ice formed to gasoline condensed in the second stage
REFER two stage condenser separator

37. Adsorption
3 adsorbent beds, steam desorption and gravity separation

38. Adsorption curves (Isotherms)50 50
Saturation parameter 10
T= temperature (R)
L= liquid density (g/cm3)
M= molecular weight (g/mol)
s= fugacity of the adsorbate at vapor liquid equilibrium, vapor pressure
= fugacity of the adsorbate in the gas stream, partial pressure
W’=lb/lb or equivalent
Weight adsorbed per weight of adsorbent, 100w*/’L
Saturation parameter

39. Example 10.11
(1% toluene in the gas)

41. Question
A painter is working in a paint-spraying operation where the temperature is 20 ºC and the toluene concentration is 1000 ppm. She is breathing in at a rate of 15 kg of air per 24 hours. For protection, she is wearing a mask containing 100 g of charcoal. The vapor pressure of toluene at 68 °F (20°C) is atm How often should the mask be changed? Follow isotherm F for charcoal (M toluene= 92 g/mol and toluene= g/cm3, Mair= 28.6 g/mol)

42. Example
For the air stream in example 10.9 we wish to remove all the toluene. If the bed operate 8 hour between regenerations, how many pounds of activated carbon must it have?
If it is only used once and then thrown away?
If it is regenerated to an outlet stream toluene content of 0.5 percent?

44. Dimensionless breakthrough plot for a fixed bed adsorber

45. absorption Solvent should be able to dissolve VOC
But remainder of the contaminated gas should be insoluble in this solvent

46. Absorption (Scrubbing)
Solute gas out
Gas liquid separator

48. The absorption solvent should
have reasonable solubility for the material to be removed
have low vapor pressure at absorber temperature
have low vapor presure at the higher temperature of the stripper.
be stable at the conditions in the absorber and the stripper
Have low molecular weight (though this causes high vapor pressure )

50. yi: mol fraction of the gas of the component
yi*: concentration of absorbable component that would be in equilibrium with the liquid absorbent
yi is maximum at the bottom of the column and minimum at the top of the column (stripper outlet)
yi* is minimum at the top of the column and maximum at the bottom of the column
The transfer of the absorbable component will be from gas to liquid as long as yi > yi*

51. Henry’s Law xi = Py*/Hi P: absolute pressure Hi: Henry’s law constant
(Does it remind you of something?)

52. Film theory

53. Liquid Gas out in Any point in the column Interface Liquid out Gas in
Bulk gas phase
Bulk liquid phase
Gas film
liquid film
Interface

55. Material balance of the transferred component
mols of i transferred from gas
= mols of i transferred to the liquid
= mass transfer capacity per unit volume.d volume
-GdYi = LdXi = (KaP)(yi-yi*) A dh
G: molar gas flow
L: molar liquid flow
Yi: gas content of the transferred component
Xi: liquid content of the transferred component
Yi and Xi: mol transferred component / mol nontransferred components)
K: mass transfer coefficient
a: interfacial area for mass transfer

56. Absorption in a counter current design.
Liquid in
Gas out
Liquid out
Gas in

57. Control by combustion

58. Explosive limits: UEL and LEL
When a gas is “too rich” or “too lean”, combustion will not occur
Combustion will take place only between the Upper Explosive Limit (UEL) and the Lower Explosive limit (LEL)

59. Fuel Gas
"Lower Explosive or Flammable Limit" (LEL/LFL) (%)
"Upper Explosive or Flammable Limit" (UEL/UFL) (%)
Acetaldehyde 4 60
Acetone
2.6
12.8
Acetylene
2.5 81
Ammonia 15 28
Arsine
5.1 78
Benzene
1.35
6.65
n-Butane
1.86
8.41
iso-Butane
1.80
8.44
iso-Butene
1.8
9.0
Methane 5

60. Table 7.1 in your book (Combustion data for hydrocarbon fuels) is used in LEL and UEL estimation.
There is no direct way to calculate these limits. They are determined by experimental data.

61. Example:
At what percentages of its stochiometric combustion level does methane have its UEL and LEL? (i.e. Find the limits on Table 7.1 of the book by using the table provided 2 slides before)

62. Combustion kinetics Decrease in concentration of A per unit time
r: reaction rate
n: reaction order
Decrease in concentration of A per unit time
A and E are experimental constants
(A is related to frequency of collisions, E is related to bond energies)
Table 10.4 in your book

63. First-Order Reactions
In a first-order reaction, the exponent in the rate law is 1.
Rate = k[A]1 = k[A]
The integrated rate law describes the concentration of a reactant as a function of time. For a first-order process: ln
[A]t
[A]0
= –kt

64. Rate constant increase with temperature
Some of the compounds are not easy to burn at relatively lower temperatures.
Benzene alone can not be oxidized easily but if it was burned together with another substance such as hexane , it will be easily oxidized due to radical attack and the amount oxidized is higher than the calculated based on rate law.

65. Example 10.18
Estimate the time required for removing 99% of toluene in a waste gas by combustion at 1000ºF, 1200ºF and 1400ºF.
(Assume first-order reaction)
Example 2

66. Flares
Flaring is a combustion process in which VOCs are piped to a remote location and burned in either an open or an enclosed flame. Flares can be used to control a wide variety of flammable VOC streams, and can handle large fluctuations in VOC concentration, flow rate, and heating value.

67. FLARES

68. Amount of fuel is high
Heat is wasted
Heat exchanger reduced the amount of fuel
Expensive and prone to corrosion
Catalysts reduce the destruction reactions to occur at lower temperatures
Reduce the lower expolosive limit (no additional fuel maybe.)
Fuel saving

69. Recuperative thermal oxidizer

70. Two-chamber regenerative

71. Two-chamber regenerative oxidizer emissions

72. Three-chamber regenerative

73. Biofiltration (biological oxidation)
ORGANISMS
ORGANISMS
ORGANISMS
ORGANISMS
ORGANISMS
ORGANISMS