CONTROL OF VOLATILE ORGANIC COMPOUNDS (VOCs) VOCs and hydrocarbons are not identical VOCs:are liquid...

CONTROL OF VOLATILE ORGANIC COMPOUNDS (VOCs) VOCs and hydrocarbons are not identicalVOCs: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.

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

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

Prentice Hall © 2005Hall © 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, Perry

Chapter Eleven

6

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 Vaporcondensation

Prentice Hall © 2005Hall © 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, Perry

Chapter Eleven

7

Liquid–Vapor Equilibrium

More vapor forms; rate of condensation of that

vapor increases …

… until equilibrium is attained.

Prentice Hall © 2005Hall © 2005General Chemistry 4th edition, Hill, Petrucci, McCreary, Perry

Chapter Eleven

8

Vapor pressure increases with temperature; why?

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

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.

Vapor Pressure of a Solution and Raults Law

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

XiXi

YiYi

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

Vapor out

Liquid in

vapor

Liquid

Vapor out

Liquid in

vapor

Liquid

Filling, breathing and emptying losses

Figure 1

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

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

Vapor out

Liquid in

vapor

Liquid

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.

Vap

or p

ress

ure

at

20oC

of

the

rem

aini

ng

liqui

d, a

t th

is %

vapo

rized

psi

a

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

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

Vent line

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)50

f

fs

M

T L log8.1

'Saturation parameter

f

fs

M

T L log8.1

'Saturation parameter

50

Wei

ght

adso

rbed

pe

r w

eigh

t of

ads

orbe

nt,

100

w*/’

L

10T= 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

Example 10.11

f

fs

M

T L log8.1

'23.2

01.0

07.0log

928.1

782.0*560

Ro

(1% toluene in the gas)

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/cm3, Mair= 28.6 g/mol)

Example 10.12.

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?

a) If it is only used once and then thrown away?

b) 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)

Gas liquid separator

Solute gas out

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 )

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

*

Henry’s Law

xi = Py*/Hi

P: absolute pressure Hi: Henry’s law constant

(Does it remind you of something?)

Film theory

Liquid in

Liquid out

Gas out

Gas in

Any point in the column

Bulk gas phase

Bulk liquid phase

Gas film

liquid film

Interface

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 flowL: 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 coefficienta: interfacial area for mass transfer

Absorption in a counter current design.Liquid in

Liquid out

Gas 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

"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 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

)exp(RT

EAk

nA

A kcrdt

dC

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

r: reaction rate

n: reaction order

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.

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 1000ºF, 1200ºF 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

Heat exchanger reduced the amount of fuel

Expensive and prone to corrosion

Amount of fuel is high

Heat is wasted

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/cm3, Mair= 28.6 g/mol)