*Control of Volatile Organic Compounds (VOCs)

*VOCs are liquids or solids that contain organic carbon (carbon bonded to carbon, hydrogen, nitrogen, or sulfur, but neither carbonate carbon as in CaCO3 nor carbide carbon as in CaC2 or CO or CO2), which vaporize at significant rates.Some VOCs (e.g., benzene) are toxic and carcinogenic, and are regulated individually as hazardous pollutants.

*Most VOCs are believed not very toxic to humans.Our principal concern with VOCs is that they participate in the smog reaction and also in the formation of secondary particles in the atmosphere.Some VOCs are powerful infrared absorbers and thus contribute to the problem of global warming.

*2. VOCsWe may now state, as an approximate rule, that VOCs are those organic liquids or solids whose room temperature vapor pressure are greater than about 0.01 psia (= 0.0007 atm) and whose atmospheric boiling points are up to about 500oF (=260oC).This means most organic compounds with less than about 12 carbon atoms.

*Fig. 10.1 contains data for only a few of the millions of organic chemicals in the VOC vapor pressure range.The Clean Air ACT Amendments of 1990 of the U.S. list 189 compounds that are considered to be health hazards and that are to be regulated to prevent or minimize emissions; most are VOCs.

*The legal definition used for regulatory purposes does not set a lower vapor pressure limition and excludes a large variety of compounds that have negligible photochemical reactivity, including methane, ethane, and most halogenated compounds.The terms VOC and hydrocarbon (HC) are not identical, but often are practically identical.

*Hydrocarbons are only slightly soluble in water.Polar VOCs, which almost all contain an oxygen or nitrogen atom in addition to carbons and hydrogens (alcohols, ethers, aldehydes and ketones, carboxylic acids, esters, amines, nitriles) are much more soluble in water.

*This difference in solubilities makes the polar VOCs easier to remove from a gas stream by scrubbing with water, but harder to remove from water once they dissolve in it.Table 10.3 (next slide) shows some typical values of these solubilities.Within each chemical family the solubility decreases with increasing molecular weight.

*3. Control by PreventionThe ways of doing this for VOCs are substitution, process modification, and leakage control.3.1 SubstitutionWater-based paints are concentrated oil-based paints, emulsified in water.After water evaporates, the small amount of organic solvent in the remaining paint must also evaporate for the paint to harden.

*For many applications, e.g., house paint, the water-based paints seem just as good as oil-based paints.But water-based paints have not yet been developed that can produce auto body finishes as bright, smooth, and durable as the high-performance oil-based points and coatings now used.

*There are numerous other examples where a less volatile solvent can be substituted for the more volatile one.This replacement normally reduces but does not eliminate the emission of VOCs.Replacing gasoline as a motor fuel with compressed natural gas or propane is also a form of substitution that reduces the emissions of VOCs.

*3.2 Process ModificationReplacing gasoline-powered vehicles with electric-powered vehicles is a form of process modification that reduces the emissions of VOCs, as well as emission of carbon monoxide and nitrogen oxides, in the place the vehicle is.On the other hand, it causes other emissions where the electricity is generated.

*Many coating, finishing and decoration processes that at one time depended on evaporating solvents have been replaced by others that do not, e.g., fluidized-bed powder coating and ultraviolet lithography.

*3.3 Leakage Control3.3.1Filling, Breathing, and Emptying LossesTank containing liquid VOCs can emit VOC vapors because of filling and emptying activities as well as changes in temperature and atmospheric pressure.These emissions are called filling or displacement losses, emptying losses, and breathing losses, or, collectively, working losses.Fig. 10.2 (next slide) shows a simple tank of some kind being filled with liquid from a pipeline.

*For all three kinds of working losses, where mi=mass emission of component i ci=concentration in the displaced gas

*Replacing the vapor mol fraction by Raoults law, Eq. (1), replacing the gas molar volume by the ideal gas law, and substituting in Eq. (3), we find

*If the tank is emptied a little at a time, with the incoming air mixing well with the air already in the tank, then the gas forced out the vent will have a benzene concentration close to the saturated value.Here we assume an average of the two, or ci in emitted air = 0.5 ci saturated

*Then using Eqs. (4), (5), and (9), This value is about 5% as large as the filling loss shown in Example 4. #

*Breathing, filling, and emptying losses are minimized by attaching to the vent of the tank in Fig. 10.2 a pressure-vacuum valve, also called a vapor conservation value.These valves remain shut when the pressure difference across them is small, typically 0.5 psi positive pressure or 0.062 psi negative.

*3.3.2 Displacement and Breathing Losses for GasolineThe U.S. uses about 350 million gallons gasoline per day.Gasoline is a complex mixture, typically containing perhaps 50 different hydrocarbons in concentrations of 0.01% or more, plus traces of many others.The smallest molecules have 3 carbon atoms; the largest, 11 or 12.

*A typical gasoline has an average formula of about C8H17 and thus an average molecular weight of about 113.For any mixture of VOCs, like gasoline, both the vapor pressure and molecular weight of the vapor change as the liquid vaporized.This behavior is sketched in Fig. 10.3 (next slide).

*To estimate the displacement and breathing losses for a mixture like gasoline, we first observe that only a small fraction of the gasoline is normally evaporated into the headspace of its containers.So the appropriate vapor pressure and molecular weight are those corresponding to zero percent vaporized, or roughly 6 psia and 60 g/mol at 20oC = 68oF for the gasoline in Fig. 10.3

*For large-scale storage, the petroleum industry store volatile liquids in floating roof tanks, as shown in Fig. 10.4 (next slide).

*The transfer of gasoline from tank trucks to underground tanks at service stations now uses the scheme shown in Fig. 10.5 (next slide).

*The vent line shown in Fig. 10.5 remains open during tank filling to prevent any excessive pressure or vacuum in the system.The vapor return system recovers about 95% of the vapor from the tank being filled, the other 5% exits from the underground tanks vent.

*The same kind of technology is used for the transfer of gasoline from the underground tank to the customers vehicle.The system is sketched in Fig. 10.6 (next slide).

*The vented vapor is normally passed through an incinerator that destroys the VOC before it reaches the ambient air.The numbers in Fig. 10.6 suggest that this system reduces displacement losses by 95%, and spillage losses by 62%.The breathing losses are also reduced because much less fresh air enters the storage tank.

*The vapor pressure of gasoline is specified by the Reid vapor pressure (RVP), which is found by a standard test. The value is close to the true vapor pressure at 100oF.Refiners adjust the RVP of their product by adjusting the ratio of low-boiling components (butanes and pentanes) to higher-boiling components (other hydrocarbons up to about C12).

*In winter they raise the RVP to improve the cold-starting properties of the gasoline.In summer they lower the RVP because cold starting is not a problem, but vapor leak can be.

*Typical winter RVP values in the United States are 9 to 15 psi, with the lowest values in Florida and Hawaii and the highest values in the colder states.Typical summer values are 8 to 10 psi.

*3.3.3 Seal LeaksFig. 10.7 (next slide) shows three kinds of seals. (a)static seal (b)packed seal (c)rotary seal

*4.Control by concentration and RecoveryMost VOCs are valuable fuels or solvents. For large VOC-containing gas streams recovery is often economical, but not often for small streams.We can concentrate and recover VOC by condensation, adsorption, and absorption.

*4.1 CondensationExample 9We wish to treat an airstream containing 0.005 mol fraction (0.5%, 5000 ppm) toluene, moving at a flow rate of 1000 scfm at 100oF and 1 atm, so as to remove 99% of the toluene by cooling, condensation, and phase separation, as sketched in Fig. 10.8 (next slide).To what temperature must we cool the airstream?

*Solution: 99% recovery will reduce the mol fraction in the gas stream to 0.005% = 50 ppm.Assuming the recovered liquid is practically pure toluene (xtoluene 1), we know that we must find from Eq. (1) the temperature at which

*Using the Antoine equation constants for toluene in Appendix A, we find that this corresponds to a temperature of -60 oC. #In spite of many difficulties such devices are used for medium-sized and/or intermittent flow of gas streams containing VOCs.Most often the cooling and condensation occur in stages, with most of the water taken out in the first stage, which operates just above 0oC.

*Example 10The current USEPA requirement for gasoline bulk-loading terminals is that the displacement loss from the returning tank trucks must not exceed 35 mg of VOCs per liter of gasoline filled. One common solution to this problem is sketched in Fig. 10.9 (next slide).

*The first cooling system takes out much of the gasoline and most of the water, and the second, at a lower temperature, takes out most of the remaining gasoline.Assuming that the vapor leaving the truck is at 20oC (= 68oF), in equilibrium with the remaining gasoline in the truck, and that the vapor is 1 mol% water vapor, (a) how cold must the second chiller cool the displaced vapor before discharging it to the atmosphere?

*Assume that the discharged vapor is in equilibrium with liquid gasoline at that temperature, that the gasoline vapor in the air has a molecular weight of 60, and that its vapor pressure is given approximately by which is a fair approximation for 10 RVP gasoline.

*(b)What fraction of the gasoline will be removed in the first stage, which cools the gas to about 32oF?(c)What is the ratio of ice formed to gasoline condensed in the second stage?Solution: Choose a basis 1 mol of vapor leaving the tank in stream and record results as calculated in the following table (next slide).

*From Eq. (10) we estimate the vapor pressure of the gasoline as 6.09 psia, from which we can compute the gasoline mol fraction as (6.09 psia / 14.7 psia) = 0.414.Thus for the assumed basis of 1 mol, the mols of gasoline are 0.414, the mols of water are 0.01, and those of air (1 - 0.414 - 0.01) = 0.576.Thus we have a complete description of stream 1.

*Next we assume that the air passes unchanged through the system.This is equivalent to assuming that no air dissolves in the gasoline or water we remove.Thus we can fill in the row for mols of air in streams 2 & 3 (0.576).

*To find the permitted amount of gasoline in stream 3 we observe that 1 L of gasoline displaces 1 L of vapor (stream 1): We will see later the mols of water in stream 3 are neglible, so we can compute

*We are now able to solve part (a).The vapor pressure of gasoline in stream 3 is equal to the total pressure times the gasoline mol fraction, (14.7 0.024) = 0.350 psia; solving Eq. (10) for the temperature corresponding to this pressure, we find T3 = 410oR = -50.0oF.

*To answer part (b), we estimate the vapor pressure of gasoline at 32oF from Eq. (10), finding 2.95 psia, and look up the vapor pressure of water at 32oF = 0.089 psia.Then we find: And, similarly for water, we find a mol fraction of 0.006.

*Then by difference we find the mol fraction of air (1.0 0.2 0.006) = 0.794, from which the total number of mols in stream 2 is (0.576 / 0.794) = 0.726.Thus the mols of gasoline in stream 2 are (0.726 0.2) = 0.145, and corresponding for water, 0.004.Thus the amount of gasoline removed in the first stage is (0.414 - 0.145) = 0.269 mols, or (0.269 / 0.414) = 65% of the gasoline in stream 1.

*To answer part (c) we must estimate the vapor pressure of ice at -50.0oF.Extrapolating the values from the steam table, we find pice 0.001 psia 0.Then the mols of ice formed are 0.004 0 = 0.004, whereas the mols of gasoline condensed are (0.145 0.014) = 0.131, and the molar ratio of ice to gasoline is 0.004 / 0.131 = 3%. #

*4.2 Adsorption Adsorption means the attachment of molecules to the surface of a solid.In contrast, absorption means the dissolution of molecules within a collecting medium, which may be liquid or solid.Generally, absorbed materials are dissolved into the absorbent, like sugar dissolved in water.Adsorbed materials are attached onto the surface of a material, like dust on a wall.

*Adsorption is mostly used in air pollution control to concentrate a pollutant that is present in dilute form in an air or gas stream.The adsorbent is most often some kind of activated carbon.For large-scale air pollution applications, the normal procedure is to use several adsorption beds, as shown in Fig. 10.10 (next slide).

*When a vessel is being regenerated, steam passes through it, removing the adsorbed VOCs from the adsorbent.The mixture of steam and VOCs coming from the top of the vessel passes to a water-cooled condenser that condenses both the VOCs and the steam.Both pass in liquid form to a separator, where the VOCs, which are normally much less dense than water and have little solubility in water, float on top and are decanted and sent to solvent recovery.

*The steam condensate will be saturated with dissolved VOC.The VOC concentration may be high enough to prevent its being sent back to the steam boiler, or for to be discharged to a sewer.

*4.2.1 AdsorbentsThe catalyst supports typically have surface areas of 100 m2/g, corresponding to internal wall thickness of 100 .Adsorbents like activated carbon often have surface areas of 1000 m2/g, corresponding to an internal wall thickness of 10 . This value is only about four times the interatomic spacing in crystals.

*To design an adsorber of the type shown in Fig. 10.10 we must consider both the adsorbent capacity and the breakthrough performance of the adsorbent.4.2.2 Adsorbent CapacityFig. 10.11 (next slide) shows the capacity of adsorbents in the form suggested by Polanyi.

*Example 11Using Fig. 10.11, estimate the adsorbent capacity curves for toluene on a typical activated carbon at 1.0 atm and 100oF and at 300oF.Solution: From the legend for Fig. 10.11 we see that curves D through I represent various activated carbons.We select curve F, which lies near the middle of this family of curves.

*Then we compute the point on w*-P coordinates for 1 atm, 100oF, and an arbitrarily selected value of 1% toluene in the gas.From that point, we calculate the values for other percent toluene values by ratios.

*Here T = 560oR and M =92 g/mol. We estimate L, the toluene density at the normal boiling point of 110.6oC, as 0.782 g/cm3 from the 20oC density (0.8669 g/cm3) and the typical coefficient of thermal expansion for organic liquids (0.67 10-3/oF).

*At atmospheric pressure, the fugacity f can be replaced by the partial pressure = 0.01 atm and the saturation fugacity fs can be replaced by the vapor pressure.Using the vapor pressure constants in Appendix A we estimate a vapor pressure of 1.03 psia = 53 torr = 0.070 atm.Thus we write:

*From curve F of Fig. 10.11 we read an ordinate of about 41, so that We then repeat the calculation for other values of the mol fraction of toluene in the gas, and for T= 300oC, and plot the results as shown in Fig. 10.12 (next slide). #

*Example 12We wish to treat the airstream in Example 9 to remove practically all the toluene.If the bed must operate 8 h between regenerations, how many pounds of activated carbon must it have (a) if it is only used once and then thrown away, and (b) if it is regenerated to an outlet stream toluene content of 0.5%?

*Solution: Here the incoming air flow is The contained toluene is

*If all the toluene is to be recovered, we must recover m = From Fig. 10.12 we read that for a toluene partial pressure of 0.005 atm at 100oF (38oC), w* = 0.29 lb/lb, and at 300oF (149oC) it equals 0.11 lb/lb.

*Thus, for part (a) we can say that the amount of adsorbent needed is: For part (b) the adsorbent is to be reused and regenerated. The net amount adsorbed per cycle will be 0.29 0.11 = 0.18 lb/lb, and the same calculation leads to an adsorbent requirement of 3180 lb / 1445 kg. #

*4.2.3 Breakthrough PerformanceThe calculation in Example 10.12 assumes that the adsorbent fills up with adsorbed material uniformly.Unfortunately, real adsorbers never work that well, and some of the material to be adsorbed breaks through before the bed has reached its maximum capacity.

*The reason for this early breakthrough is that there is a finite resistance to mass transfer between the gas and the solid, so some finite amount of time is needed for each particle to be loaded with adsorbate.Fig. 10.13 (next slide) compares the ideal breakthrough curve with a typical real breakthrough curve.

*At some time tb less than the ideal time the concentration in the outlet stream reaches the breakthrough value (typically 1% of the inlet value).A reasonable estimate of the breakthrough curve can be made using Fig. 10.14 (next slide).

*Example 13Using the data from Example 12, estimate the breakthrough curve that would be observed if we started with clean adsorbent.Assume that the adsorbent is an activated carbon with a bulk density of 30 lb/ft3 and a particle diameter of 0.0128 ft.The volume of the bed is [1970 lb/ (30 lb/ft3)] = 66 ft3.

*Solution: If we assume a cubically shaped bed, the sides of the bed will be 4.03 ( 4) ft.Then as shown in Appendix E, we may estimate that a = 14.4/ft and b = 7.4/h.Thus on Fig. 10.14, N = ax =14.4/ft 4 ft = 57.6 60, so that the estimated breakthrough behavior of the bed will follow the curve for N = 60.

*From Fig. 10.14, the outlet concentration will become 1% of the inlet concentration at a bt value of about 36, which corresponds to a time of This value is 61% of that for perfect filling of the bed (i.e., no mass transfer resistance). #Fig. 10.13 is similar to Fig. 10.14 except that Fig. 10.13 is on arithmetic coordinates, whereas Fig. 10.14 is on log-normal coordinates.

*4.3 Absorption (Scrubbing)If we can find a liquid solvent in which the VOC is soluble and in which the remainder of the contaminated gas stream is insoluble, then we can use absorption to remove and concentrate the VOC for recovery and re-use, or destruction.Fig. 10.15 (next slide) shows a system of absorption and stripping.

*Normally, bubble caps, sieve trays, or packing is used in the interior of the absorption column to promote good countercurrent contact between the solvent and the gas. The stripper normally is operated at a higher temperature and/or a lower pressure than the absorber.

*4.3.1Design of Gas Absorbers and StrippersIn the device shown in Fig. 10.15, the basic design variables are the choice of selective reagent to be used, the system pressure, the flow rates of the gas and liquid, the gas velocity in the column, and the amount of liquid-gas contact needed to produce the separation.

*In Fig. 10.15 both columns operate in counterflow: the liquid flows down the column by gravity; the gas flow up the column, driven by the decrease in pressure from bottom to top.This design not only utilizes gravity efficiently in moving the liquid but also provides for very efficient contacting. Fig. 10.16 (next slide) shows why.

*In Fig. 10.16, the curve at the right shows the mol fraction in the gas of the component to be absorbed, yi, decreasing from the bottom to the top.The curve at the left shows the concentration of absorbable component that would be in equilibrium with the liquid absorbent, yi*, which increases from top to bottom.

*The relation between yi* and the mol fraction of absorbable component in the liquid absorbent, xi, is complex and differs from one chemical system to another.The simplest description of such absorption equilibria, for slightly solube gases, is Henrys law.

*Henrys law is normally written as Here P is the absolute pressure, and Hi the Henrys law constant for component i, which is normally a strong function of temperature, but a weak function of pressure or concentration.

*In Fig. 10.16 the transfer of the absorbable component will be from gas to liquid as long as yi > yi*, i.e., the actual concentration in the gas at that location in the column is higher than the concentration that would be in equilibrium with the absorbing liquid.In the stripper in Fig. 10.15 the relation is reversed, yi < yi*, and the absorbable component flows from liquid to gas.

*We next perform a material balance on the transferred component for the small section dh of the column shown in Fig. 10.16, finding Here G and L are the molar flows of gas and liquid (mol/s), excluding the flow of the transferred components.

*Yi and Xi are the gas and liquid contents of the transferred component, respectively, expressed in (mol/mol of nontransferred components).Adh is the product of the column cross-sectional area and incremental height, equal to the column volume corresponding to dh.Ka is the product of the mass transfer coefficient and the interfacial area for mass transfer (ft2 of transfer area per ft3 of volume), discussed later, and P is the system pressure.

*Example 14We wish to treat the stream in Example 9, recovering the toluene by absorption in a suitable solvent.Select a suitable solvent, and estimate the required solvent flow rate.Solution: The solubility of toluene in water (see Table 10.3) is low enough that we are unlikely to use water as a solvent.

*Our logical choice is an HC with a higher boiling point than toluene.In Example 9 the permitted toluene concentration in the exhaust gas is 50 ppm.If we assume that we can emit an equal concentration of the solvent, then its vapor pressure at column temperature (100oF in that example) must be no more than 50 10-6 atm.

*From Fig. 10.1, we see that the highest-boiling HC shown, n-decane, has a vapor pressure at 100oF of about 0.06 psi = 0.004 atm, which is 80 times too high.Using the Antoine equation constants, we find that n-tetradecane (C14H30, M =198 g/mol) has an acceptable calculated vapor pressure of 4710-6 atm at 100oF.We would not use a pure HC as absorbent, because pure HCs are expensive.

*But this shows that a hydrocarbon mixture with a vapor pressure comparable to n-tetradecane could be used.This is comparable to the vapor pressure of diesel fuel.However, for the rest of this example we will use n-tetradecane because doing so simplifies the calculations.

*We can also calculate from the Antoine equation that the atmospheric boiling point of n-tetradecane is 490oF, which is the temperature we would expect at the bottom of the stripper.Next we integrate the left two terms of Eq. (12) from the top to the bottom of the column and rearrange, finding

*From the definitions of Y and X that so that for small values of y and x To estimate the Henrys law constant we observe that Eq. (11) is equivalent to Raoults law [Eq. (1)], with the vapor pressure of toluene taking the place of H.

*At 100oF we know from Example 11 that ptoluene 0.070 atm, and we use this as an estimate of H. On Fig. 10.16 we can draw in the values for the inlet gas, yi = 0.5% = 5000 ppm, and for the outlet gas, 50 ppm.

*If we assume that the stripper is 100% effective, then the stripped solvent will have zero toluene (and the leftmost curve will go to yi* = 0 at the top of the column).The maximum conceivable liquid outlet concentration produces a yi* equal to the inlet value of 5000 ppm.On Fig. 10.16 that would correspond to the two curves meeting at the bottom of the column, which means that the concentration difference driving the absorption would be zero at the bottom of the column so that in Eq. (12), dh = 1/0 = .

*To prevent this, we arbitrarily specify that the outlet liquid shall have yi* = 0.8 yi. Thus we can calculate that Then we may write

*The molar flow rate of gas is So the required liquid flow rate is We can also use the Henrys law expression to estimate how thoroughly we must strip the solvent before reusing it.

*At the top of the column we also arbitrarily specify that This is a difficult but not impossible stripping requirement.If we substituted this value of xi top into the above calculation in the place of the assumed value of zero, it would increase the computed value of L/G by 1%, which we ignore. #

*Example 15Estimate the required column diameter in Example 14.Solution: The column diameter is determined almost entirely by the gas flow rate.Although the liquid flow rate in (mol/mol) may be close to that of the gas, the liquid flow rate in (vol/vol) is seldom more than 10% of the gas because of the large difference in densities, and it is practically ignored in sizing the column.

*Typically, absorption columns will operate at a gas velocity that is approximately 75% of the flooding velocity.For most packed absorption columns, the flooding velocity is predicted from a semitheoretical, graphical correlation that can be satisfactorily approximated by log = -1.6798 1.0662 log 0.27098 (log )2 (15) where

*Here is dimensionless, with G = the gas mass velocity = mass flow rate of gas per unit area = GM/A, and L = the liquid mass velocity = LM/A.Since is not dimensionless, the quantities in it must be expressed in the following units (or suitable conversions): G is in lb/(ft2 s), F is a dimensionless packing factor whose values are presented in tables for various packings, is the specific gravity of the liquid, is the liquid viscosity in centipoise, the liquid and gas densities are in lb/ft3, and gc = 32.2

*We know that L/G = 0.087, from which it follows that The column will operate at 1 atm and 100oF, at which temperature the density of the air stream will be about 0.071 lbm/ft3 and that of n-tetradecane about 47 lbm/ft3, so that

*Log = log 0.0073 = -2.14 for the flooded condition.Substituting this value in Eq. (15), we find =0.23.We estimate the viscosity of n-tetradecane at 100oF is 1.6 cp, and its specific gravity is 0.75.For typical packed columns, F 50, and using the units with the dimensions specified for Eq. (15), we compute

*At 75% of flooding G = 0.58 lb/ft2/s.From Example 14 we know that the gas flow rate is gas = 1.25 lb/s. Thus

*Example 16Estimate the required column height for the gas absorption in Examples 14 and 15.Solution: we return to Eq. (12) and rearrange the rightmost two terms to find that

*Ka, product of the mass transfer coefficient and the interfacial area between liquid and gas per unit volume of the absorber, depends on the chemical and physical properties of the liquid and the gas and the geometry of the column internals, whose function is to provide as large a value of Ka as possible with a minimum pressure drop.Kohl and Nielsen present a table of values of Ka observed in industrial practice for a variety of absorption systems. These vary from 0.007 to 20 lbmol/h/ft3/atm.

*For this example we will assume that Ka is a constant = 4.0 lbmol/h/ft3/atm.We have specified that yi* = 0.8 yi at both the top and the bottom of the column.We may rearrange Eq. (12) so that

*Then we apply Eq. (13) twice, replacing X and Y by x and y, solve Eq. (14) for yi*, and substitute into the right side of Eq. (16), finding

*Where N is the number of transfer units, a measure of the difficulty of the separation; xB and yB are the liquid and vapor concentration at the bottom of the column; and yT is the vapor concentration at the top of the column.The required column height is linearly proportional to N.

*

*5. Control by Oxidation The final fate of VOCs is mostly to be oxidized to CO2 and H2O, as a fuel either in our engines or furnaces, in an incinerator, in a biological treatment device, or in the atmosphere (forming ozone and fine particles).VOC containing gas streams that are too concentrated to be discharged to the atmosphere but not large enough to be concentrated and recovered are oxidized before discharge.

*5.1 Combustion (Incineration)Some examples of interests are The nitrogen and sulfur present in compounds being incinerated normally enter the atmosphere partly as N2, NO, or NO2 and SO2.The latter three are pollutants for which we will deal with later.

*For example, In most incinerators, the chlorine content of the material burned will leave the incinerator as hydrochloride acid, HCl.At incinerator temperatures some metals become vapors, e.g., mercury, cadmium, zinc.Their emissions can be a problem.

*5.1.1Combustion Kinetics of the Burning of GasesMost combustion takes place in the gas phase. Liquids and solids mostly vaporize before they burn.For chemical reactions of any kind in any phase (gas, liquid, or solid) the reaction rates are typically expressed by equations of the form:

* wherer=reaction rate k=a kinetic rate constant whose value is strongly dependent on the temperature but is independent of the concentration of the reactants cA=concentration of A n=reaction order

*For most chemical reactions the relation between the kinetic rate constant k and the temperature T is given to a satisfactory approximation by the Arrhenius equation: whereA=frequency factor which is related to the frequency of collisions of the reacting molecules E=activation energy which is related to the bond energies in the molecules R=universal gas constant T=absolute temperatureTable 10.4 (next slide) lists values of A, E, and k, based on n = 1 for the combustion of a variety of compounds.

*A strong simplifying assumption in Table 10.4 is that the concentration of VOC to be burned is much less than the concentration of oxygen in the contaminated air stream.The true kinetic expression is presumably So the k in Eq. (18) is equivalent to k cO2 in Eq. (20).

*Example 17Show the calculation leading to the value of k in Table 10.4 for benzene at 1000oF.Solution:

*Example 18Estimate the time required to destroy 99.9% of the benzene in a waste gas stream at 1000o, 1200o, and 1400 oF.Solution: For n=1, we can integrate Eq. (18) from t=0 to t=t, finding At 1000oF, Repeating the calculation at 1200o and 1400oF, we find 49 s and 0.2 s. #

*From Table 10.4 we see that benzene is one of the more difficult material to burn; it has one of the lowest values of k. We also see that it has the highest value of E, thus showing the highest rate of increase of k with an increase in T.Conversely, ethyl mercaptan has the lowest E and hence the slowest increase of k with an increase in T.

*If a mixture of 0.5% benzene and 0.5% hexane were treated in an incinerator at 1000oF, the percent destruction of hexane might be as predicted with Eq. (21) using the values in Table 10.4, whereas that of benzene would be much larger than the value calculated the same way.The reason is that the free radicals generated in the burning of hexane, which is more easily attacked, will encounter benzene molecules and attack them.

*Barnes et al. suggest the following as typical values of the operating conditions of industrial gas incinerators: The typical ways of carrying out the combustion of VOCs are shown in Fig. 10.17 (next slide).

Gas velocity: 25~50 ft/s Residence time: 0.2 ~ 1 s Temperature: Odor control 900 - 1350oF Oxidize hydrocarbons900 - 1200oF Oxidize CO1200 - 1450oF

*The biggest drawback with the arrangement in Fig. 10.17a is the high cost of the fuel.One way to lower the fuel cost is to put a heat exchanger into the system, as shown in Fig. 10.17b.Unfortunately, the hot-gas-to-cold-gas heat exchangers are expensive and often have severs corrosion problems.

*The second modification of the basic idea is to put an oxidation catalyst in the retention chamber.Such catalysts can cause VOC destruction reactions to occur at much lower temperatures than they would without a catalyst.Barnes et al. state that the operating temperature of an afterburner can typically be reduced from about 1000o to 1200oF to about 600oF if a catalyst is used.

*5.1.2Combustion Kinetics of the Burning of SolidsThe only common fuel that will burn as a solid is charcoal.If a solid has a flat surface and is burning, then we would assume that the burning rate could be expressed in terms like (mass burned) / [(amount of exposed surface) (time)].Fig. 10.18 (next slide) shows the measured burning rates for pure carbon.

*Example 19How large a spherical carbon particle can we completely oxidize in an airstream that is held at 1000 K for 3 s?Solution: From Fig. 10.18 we read that the burning rate at this temperature is approximately r = 0.018 10-3 g/cm2/s.

*If we write a mass balance for a spherical particle whose surface is burning away, we find

*Taking D = D0 at time to and D = 0 at time t, we find that the largest particle that we can completely burn in time t has the following diameter: Using the value of r given earlier and a density of 2 g/cm3 for carbon, we find

*On Fig. 10.18, the coordinates are logarithmic on the ordinate and some other value on the abscissa.One may verify that the abscissa is (1/T) with the origin taken at the right instead of the left.A straight line on such a plot would obey the Arrhenius equations.

*Most other combustible materials (e.g., wood, tars, coal, etc.) will decompose (pyrolyze) on heating, giving off combustible gases.For wood, the sequence is pyrolyze-vaporize-mix with air and oxidize to give the final products, CO2 and H2O.Because of its very low vapor pressure, pure carbon does not give off such gaseous decomposition products.

*5.1.3Mixing in Combustion ReactionsFig. 10.18 makes clear that mixing is important for combustion of solids as well as gases and liquids.In that figure at low temperatures the combustion rate is independent of the rate of air movement across the surface of the burning carbon.But at higher temperatures the combustion rate depends on the air flow rate.

*At low temperatures molecular diffusion moves the air into the solid carbon surface and the carbon dioxide out faster than the chemical reaction can transform them, so the chemical reaction determines the overall reaction rate.At higher temperatures the chemical reaction is so fast that it uses up the oxygen as fast as diffusion can bring it in, and the overall rate is determined by diffusion (mixing).

*5.1.4Application to Boilers, Furnaces, Flares, etc.To get complete combustion with imperfect mixing, one must supply excess air in addition to that needed for stoichiometric combustion in boilers or furnaces.Large industrial furnaces operate with 5 to 30% excess air.

*The mixing problem is especially difficult in flares. These are safety devices used in oil refineries and many other processing plants.All vessels containing fluids under pressure have high-pressure relief values that open if the internal pressure of the vessel exceeds its safe operating value.The outlet of a refinerys relief values are piped to a flare, which is an elevated pipe with pilot lights to ignite any released VOCs.

*Many flares have steam jets running constantly to mix air into the gas being released.These flares handle significant amounts of VOCs only during process upsets and emergencies at the facilities they serve.For a large release the mixing is inadequate, and the large, bright orange, smoky flame from the flare indicates a significant release of unburned or partly burned VOC.

*5.2Biological Oxidation (Biofiltration)The ultimate fate of VOCs is to be oxidized to CO2 and H2O. Many microorganisms will carry out these reactions fairly quickly at room temperature.The typical biofilter (better called a highly porous biochemical reactor) consists of the equivalent of a swimming pool, with a set of gas distributor pipes at the bottom, covered with several feet of soil or compost or loam in which the microorganisms live.

*Typically these devices have soil depths of 3 to 4 ft , void volumes of 50%, upward gas velocities of 0.005 to 0.5 ft/s, and gas residence times of 15 to 60 s.They work much better with polar VOCs, which are fairly soluble in water than with HCs whose solubility is much less.

*6. Choosing a Control TechnologyFig. 10.19 (next slide) shows one guide to choosing technology, based only on flow rate and concentration.Other factors, e.g., the permitted emission regulation and the special properties of the contained VOC are also important.