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 (CH 3 -CO-CH 3 ) 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.

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+O 2 +sunlight NO 2 +O 3 Some VOCs are strong IR absorbes contributing global warming problem

                                         

The Clean Air Act Amendments of 1990 List of Hazardous Air Pollutants

CAS Number Chemical Name 75070 Acetaldehyde 60355 Acetamide 75058 Acetonitrile 98862 Acetophenone 53963 2-Acetylaminofluorene 107028 Acrolein 79061 Acrylamide 79107 Acrylic acid 107131 Acrylonitrile 107051 Allyl chloride 92671 4-Aminobiphenyl 62533 Aniline 90040 o-Anisidine 1332214 Asbestos 71432 Benzene (including benzene from gasoline) 92875 Benzidine 98077 Benzotrichloride 100447 Benzyl chloride 92524 Biphenyl 117817 Bis(2-ethylhexyl)phthalate (DEHP) 542881 Bis(chloromethyl)ether 75252 Bromoform 106990 1,3-Butadiene 156627 Calcium cyanamide 105602 Caprolactam(See Modification) 133062 Captan 63252 Carbaryl 75150 Carbon disulfide 56235 Carbon tetrachloride 463581 Carbonyl sulfide 120809 Catechol 133904 Chloramben 57749 Chlordane 7782505 Chlorine 79118 Chloroacetic acid 532274 2-Chloroacetophenone 108907 Chlorobenzene 510156 Chlorobenzilate 67663 Chloroform

National emissions of VOCs

34% of the total 40% of the total

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

Vapor Pressure

• 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 condensation Vapor 6 Chapter Eleven Prentice Hall © 2005

General Chemistry

4 th edition, Hill, Petrucci, McCreary, Perry

Liquid –Vapor Equilibrium

More vapor forms; rate of condensation of that vapor increases … … until equilibrium is attained.

7 Prentice Hall © 2005

General Chemistry

4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven

Prentice Hall © 2005

Vapor pressure increases with temperature; why?

General Chemistry

4 th edition, Hill, Petrucci, McCreary, Perry Chapter Eleven 8

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 0.023 atm (Water does not boil but evaporates!!!

1 atm= 760 torr=101.3 kPa=14.7 psia

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

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 (

P

solv ) is the product of the vapor pressure of the pure solvent (

P °

solv ) and the mole fraction of the solvent in the solution (

x

solv ):

P

solv =

x

solv ·

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.

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 Yi

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)

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.

Control approaches for VOCs

1.CONTROL BY PREVENTION 2. CONTROL BY CONCENTRATION AND RECOVERY 3.CONTROL BY OXIDATION

1. CONTROL BY PREVENTION

 Substitution  Process modification  Leakage control

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

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.

Leakage control

 1. Filling, breathing and emptying losses  2. Displacement and breathing losses for gasoline  3. Seal leaks

Figure 1

Filling, breathing and emptying losses

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

vapor Liquid Vapor out

Example 10.4

Liquid in  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

Displacement and breathing losses fro gasoline

 Gasoline is a complex mixture containing 50 different hydrocarbons.

 A typical gasoline has an average formula of about C about 113.

8 H 17 and molecular weight of  Composition of the gasoline varies with seaon of the year and from refinery to refinery.

For zero percent vaporized p=6 psia and M=60

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

Floating roof tank

Liquid in

Floating vs fixed roof tanks

http://www.sandborn.ca/Page.asp?pID=66

Transfer of gasoline from tank trucks to underground storage

Vent line Storage tanks are placed underground to save space and to reduce fire hazard.

Vapor from customer tanks is forced back to the storage tank

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

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?

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

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

Example 10.9

 We wish to treat an airstream containing 0.005 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?

Two stage condenser separator for recovering VOC from displacement of vapors of a gasoline tank truck

Water is eliminated Gasoline is recovered

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-(5236.5 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

Adsorption

3 adsorbent beds, steam desorption and gravity separation

Adsorption curves (Isotherms)

Saturation parameter 

T

1 .

8  '

L M

log  

fs f

  10 T= temperature (R)  L= liquid density (g/cm 3 ) 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 Saturation parameter 

T

1 .

8  '

L M

log  

f fs

 

Example 10.11

 (1% toluene in the gas) 

T

1 .

8  '

L M

log  

fs f

   560

o R

* 0 .

782 1 .

8 92 log 0 .

07 0 .

01  2 .

23

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 0.029 atm How often should the mask be changed? Follow isotherm

F

for charcoal (

M toluene = 92 g/mol and



toluene =0.8669 g/cm 3 , M air = 28.6 g/mol)

Example 10.12.

 a) b) 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?

Dimensionless breakthrough plot for a fixed bed adsorber

absorption

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

Absorption (Scrubbing)

Solute gas out Gas liquid separator

The absorption solvent should

•have reasonable solubility for the material to be removed •have low vapor pressure at absorber temperature •have low vapor presure of the stripper.

at the higher temperature •be stable stripper at the conditions in the absorber and the •Have low molecular weight high vapor pressure  ) (though this causes

y i

: mol fraction of the gas of the component

y i * :

concentration of absorbable component that would be in equilibrium with the liquid absorbent

y i

is maximum at the bottom of the column and minimum at the top of the column (stripper outlet)

y i *

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 y i > y i *

Henry’s Law

 x i = Py * /H i  P: absolute pressure  H i : Henry’s law constant  (Does it remind you of something?)

Film theory

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

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 -GdY i

G:

= LdX i = (K molar gas flow a P)(y i -y i * ) A dh

L:

molar liquid flow

Y i : X i :

Y i gas content of the transferred component liquid content of the transferred component and X i : mol transferred component / mol nontransferred components)

K:

mass transfer coefficient

a:

interfacial area for mass transfer

Absorption in a counter current design.

Liquid in Gas out  Liquid out Gas in

Control by combustion

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)

Fuel Gas Acetaldehyde Acetone Acetylene Ammonia Arsine Benzene n-Butane iso-Butane iso-Butene Methane "Lower Explosive or Flammable Limit" (LEL/LFL)

(%)

4 2.6

2.5

15 5.1

1.35

1.86

1.80

1.8

5 " Upper Explosive or Flammable Limit" (UEL/UFL)

(%)

60 12.8

81 28 78 6.65

8.41

8.44

9.0

15

 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.

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)

Combustion kinetics

Decrease in concentration of

A

unit time

k

 per  

dC A

exp( 

dt E A RT

) 

r



kc A n

r: reaction rate n: reaction order 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

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

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.

Example 10.18

 Estimate the time required for removing 99% of toluene in a waste gas by combustion at 1000 ºF, 1200 ºF and 1400ºF.

 (Assume first-order reaction)

Example 2

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.

FLARES

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

Recuperative thermal oxidizer

Two-chamber regenerative

Two-chamber regenerative oxidizer emissions

Three-chamber regenerative

Biofiltration (biological oxidation)

ORGANISMS ORGANISMS ORGANISMS ORGANISMS ORGANISMS ORGANISMS

.

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 0.029 atm How often should the mask be changed? Follow isotherm

F

for charcoal (

M toluene = 92 g/mol and



toluene =0.8669 g/cm 3 , M air = 28.6 g/mol)