Estimation of Residence Time in VOC Incineration

0957±5820/97/$10.00+0.00 q Institution of Chemical Engineers

ESTIMATION OF RESIDENCE TIME IN VOC INCINERATION A. O’REILLY Division of Chemical Engineering, University of Teesside, Middlebrough, UK

A

calculation procedure has been developed for evaluation of residence time for thermal incineration of volatile organic compounds (VOCs) to CO2 and H2 O which takes into account change in reaction rate with temperature, heat losses through the walls of the incinerator and change in concentration of both oxygen and the VOC. The values for residence time obtained by means of this procedure are typically less than 50% of those obtained by using an arithmetic mean incineration temperature. Use of an arithmetic mean incineration temperature to evaluate residence time is inappropriate because it does not satisfactorily account for the rapid increase in the rate of VOC incineration with temperature, which follows an Arrhenius relationship. An approximate version of the above procedure has been developed for preliminary calculations and gives good agreement with the results obtained by means of the more rigorous model, for both adiabatic and non-adiabatic conditions. Experimental studies to test the model’ s predictions are strongly recommended. Keywords: atmospheric pollution; VOCs; thermal incineration.

INTRODUCTION Thermal incineration is an important method of controlling VOC emissions1,2,3,4 , especially if the concentration of the VOC is too low for its combustion to be self-sustaining. In this case, auxiliary fuel is burnt in excess air to heat the waste gas, typically an air stream contaminated with VOC, to a suf® ciently high temperature for the VOC then to undergo essentially complete oxidation, or incineration. Figure 1 shows the basic process scheme for VOC incineration and Figure 2 schematically depicts a typical VOC incinerator5 . Good turbulent mixing of waste and combustion gases followed by as close as possible an approach to plug ¯ ow in the actual incineration chamber are the optimum conditions for ef® cient incineration of VOCs1,2 . De Nevers1 indicates that good turbulent mixing of the waste and combustion gases can be achieved by imparting a high velocity to each of the inlet streams. Hemsath and Susey6 describe how they achieved uniform mixing of preheated air and hydrocarbon vapour in their experimental facility prior to feeding the gaseous mixture to the tubular reaction chamber from which they obtained their kinetic data. The mixing section of the incinerator in this work has therefore been likened to a well mixed furnace ® rebox, for the purpose of estimating the actual inlet temperature for incineration, in the case of non-adiabatic conditions. This approach should, of course, be tested experimentally but Carleton4 states that `Uniformity of temperature across the (mixing) chamber, at the end of the (mixing) stage, can serve as a good practical indication of adequate mixing’ . He describes various strategies by which a fairly even radial stream temperature pro® le can be achieved prior to incineration, which indicates that likeness of the mixing chamber to a well mixed furnace ® rebox for

the purpose of a heat transfer calculation should not be too unreasonable. Hemsath and Susey6 also appear to have achieved minimal radial temperature and concentration gradients in the reaction chamber of their experimental facility, with Reynolds numbers ranging from 2500 to 15,000, or in other words, essentially plug ¯ ow conditions. They found subsequently that their kinetic results obtained from this facility agreed closely with those obtained from a full-scale incinerator of 36" i.d. equipped with an auxiliary burner, even though the ¯ ow conditions, temperature and concentration gradients in this incinerator were signi® cantly different from those of their test facility. For example, they predicted 99% oxidation of toluene in the full-scale incinerator for a residence time of 0.12 seconds at 1410 F or 1039 K using their experimentally determined kinetics and found that the full-scale incinerator achieved 98 to 99% oxidation, with an average of 98.5%. It has been assumed therefore, for this work, that heat balance and reaction kinetic expressions for VOC incineration can be developed reasonably on the assumption of uniform mixing and plug ¯ ow in the incinerator. Even if these assumptions are not fully realized in practice, Hemsath and Susey’ s work indicates that the effect on the extent of VOC oxidation actually achieved may not be excessive, even in a full scale-incinerator. However, the possible magnitude of this effect should also merit further experimental study. Table 1 lists typical operating conditions1,7 for thermal incineration. These values are of course very dependent on the kinetics of combustion of the particular VOC or VOC mixture7 . Even for a single VOC component consisting of simple molecules, such as methane, CH4 , combustion kinetics are complex, with the initial formation of carbon monoxide, 33

34

O’ REILLY Table 1. Operating conditions for industrial gas incinerators.

Figure 1. VOC incineration, schematic ¯ owsheet, adiabatic process. 1Ð waste gas with VOC. 2Ð auxiliary fuel. 3Ð combustion air. 4Ð VOCfree waste gas.

CO, which must then be oxidized to carbon dioxide, CO2 . This second step, the oxidation of CO to CO2 , is believed to be rate controlling in many incineration processes1,6,7 . However, the kinetics of oxidation for some VOCs can be simpli® ed using overall values for pre-exponential factor A and activation energy E1,6,7 in a pseudo ® rst order rate expression of the form: dCvoc -E - dt = A exp RT Cvoc

( )

(1)

for the complete combustion of the VOC to CO2 and H2 O. Equation (1) may then be integrated to give the familiar logarithmic expression for residence time if the VOC incineration is assumed to take place under essentially isothermal conditions at a speci® ed temperature: 1 C 1 1 s = ln voc, in = ln Cvoc, out A exp -RTE 1 - Xvoc, out A exp -RTE

(

)

(

)

(2)

Actual gas velocity, m s-1

7.5 to 15

Residence time, s

0.2 to 1

Temperatures: Odour control Hydrocarbon oxidation CO oxidation

480 to 7308 C 480 to 6508 C 650 to 8008 C

It would be appropriate to use a suitably weighted or `true’ mean temperature in equation (2), if such a temperature could be estimated, because the incineration of some VOCs may generate suf® cient heat of combustion to yield a signi® cant increase in the temperature of the gas stream, even allowing for heat losses through the incinerator walls. Such a temperature rise may take place even for a VOC in very low concentration of, say, 0.5 mol %. It will in turn have a very signi® cant effect on the rate of VOC oxidation and therefore on the value of residence time s obtained from equation (2), through the presence of the Arrhenius term. This work describes a procedure for determination of the residence time for VOC incineration which takes into account the increase in gas stream temperature as the oxidation of the VOC proceeds, under both adiabatic and non-adiabatic conditions. This work also demonstrates that a simple arithmetic average of the stream temperatures to and from the incinerator cannot be reliably used as a weighted mean temperature of incineration, even for an approximate evaluation of residence time, and proposes an alternative method. The procedures are illustrated by means a case study, ® rst with respect to incineration of toluene and then with respect to incineration of several other VOCs, with widely differing values of kinetic parameters and heats of combustion1 , namely ethyl mercaptan, acrolein, chlorobenzene and benzene.

VOC INCINER ATION: A CASE STUDY Table 2 summarizes basic data for the complete oxidation of toluene in a waste air stream to CO2 and H2 O. The Energy Balance: Matching Residence Time and Temperature Rise An energy balance must be carried out over the process to determine: (1) The quantity of auxiliary fuel required to heat the waste gas to the desired inlet temperature. (2) The quantity of excess air required to ensure that the temperature rise during incineration of the VOC is compatible with a `reasonable’ residence time. These quantities are interdependent and must be evaluated for the subsequent calculations of residence time. The energy balance is set out as follows: Figure 2. VOC incineration vessel, schematic diagram. 1Ð waste gas with VOC. 2Ð auxiliary fuel. 3Ð combustion air. 4Ð VOCfree waste gas.

Adiabatic conditions Equations (9) to (11) enable the quantity of auxiliary fuel to be determined and the increase of stream temperature as Trans IChemE, Vol 75, Part B, February 1997

ESTIMATION OF RESIDENCE TIME IN VOC INCINERATION

35

Table 2. Incineration of toluene to CO2 and H2O1,6 : basic data. Waste gas stream containing VOC

0.5% toluene, C7H8, in dry air, with 0.0116kmol H20 (kmol dry air)- 1 0.35 m3 s- 1 , 0.01559kmol s- 1 298.15 K 922 K, adiabatic operation C7H8 + 9O2 ! 7CO2 + 4H2O 3.7761 ´ 106 kJ kmol- 1 99.99% removal of C7H8 6.56 ´ 1013 s- 1 , 2.4493 ´ 105 kJ kmol- 1 Natural gas CH4 90%, C2H6 8%, CO2 1%, N2 1% 0.83631 ´ 106 kJ kmol- 1

Total volumetric ¯ owrate at STP Initial temperature of fuel, air and waste gas, T0 Initial temperature of incineration VOC oxidation reaction -D H 8 C 922 K for C7H8 VOC removal by incineration Kinetic data for C7H8 combustion6 : A, E Auxiliary fuel Composition -D H 8 C 298.15 K for natural gas

oxidation of the VOC proceeds. See Appendix 1 for the full derivation of these expressions, including equations (1) to (8). mFUEL mWASTE

= -D

HC0 , 298.15 K

Equation (18) gives the incremental change in residence time, D s I N C I N , as a function of incremental VOC oxidation, D XV O C .

CPWCG (TIN - T0 ) - (a + bXAIR )CP,WCG (TIN - T0 ) (9)

yVOC XVOC (-D HC0 ,TIN ) = TIN + m* Cp,WCG

TOUT

D s

m m = 1 + yVOC b XVOC + FUEL (a + bXAIR ) mWASTE *

(11)

æè

s

INCIN

=

4v0

(

TOUT - T0 TIN,INCIN - T0

)

di (TOUT - TIN ,INCIN ) U 4v0 mWASTE T OUT - T0 d2

Trans IChemE, Vol 75, Part B, February 1997

- T1 )

T2 - T0 T1 - T0

(18)

di, m

0.80

Assumed wall thickness of 1.25 cm

do, m

1.45

0.3125m thickness of insulation brick, kw = 0.21 w m- 1 K- 1 (Reference 8) For dished end, see Figure 2

0.5

)

Remarks

0.3

Carleton4 indicates values of 0.3 to 0.5 s and 0.1 to 0.3 s, according to the design of the mixing section

v0, m3 s- 1 at TIN = 922 K

4.643

Consistent with volumetric ¯ owrate of 5 m3 s- 1and gas velocity of 10 m s- 1 (Reference 1) at average gas temperature of 992.2 K, under adiabatic conditions

U, W m- 2 K- 1

0.5

Obtained by separate calculation

s

öø

IN ,INCIN - T0

ln

p ,WCG (T2

Value

MIX,

yVOC XVOC (D HC0 ,TIN ) + m*WASTECp,WCG (TOUT - TIN ) + ln(T

mWAST Ed 20

*

IN

(T2 - T1 )

Parameter

K

(13)

0

Udi

Table 3. Incineration of toluene to CO2 and H2O: additional data for heat loss calculations.

s

(TOUT - TIN,INCIN ) ln

)+mC ( )

0 - XVOC,1 )(- D HC,T

Non-adiabatic conditions: simpli® ed heat transfer expressions Equations (13a), (17a) and (18a) are simpli® ed heat transfer

Non-adiabatic conditions: full heat transfer expressions When heat loss through the walls of the incinerator must be accounted for, the term for residence time s I N C I N occurs in both the heat balance and kinetic expressions which must then be solved simultaneously to yield values for residence time s I N C I N as a function of fractional VOC oxidation XV O C to CO2 and H2 O. Equations (13) and (17) express the quantity of heat loss Q L O S S and residence time s I N C I N as a function of stream temperature TO U T and fractional VOC oxidation XV O C . See Appendix 1 for the full derivation of these expressions.

U AMIX (TIN,INCIN - T0 ) + AINCIN

yVOC (XVOC,2

Data from Table 2 and the literature have been used to estimate the values of terms used in equations (12) to (18) which have then been solved simultaneously with the kinetic expressions over each increment of VOC oxidation D X V O C by means of a Microsoft Excel 5 Spreadsheet. Table 3 gives the data which have been used in equations (12) to (18).

Note that XV O C = 0.9999 in equation (11) such that m* Cp ,W C G is a mean heat capacity ¯ owrate in equation (10) over the range of temperature TIN # T # TOUT . Equations (9) and (10) may be solved simultaneously for adiabatic conditions by selecting a value of XAIR to give a `reasonable’ value for exit temperature TO U T for XV O C = 0.9999. Note that the value of Cp ,W C G is not identical in equations (9) and (10) but is adjusted according to the temperature range over which each expression applies.

=

=

(10)

where

QLOSS

IN, CIN

U AMIX (TIN ,INCIN - T0 ) mWASTE

(17)

36

O’ REILLY

expressions which are to be used subsequently with simpli® ed kinetic expressions for the evaluation of residence time s I N C I N under non-adiabatic conditions. QLOSS

This does not affect the validity of the calculations but imposes an uncertainty on the calculated results for residence time s I N C IN of approximately 6 15% which could only be resolved by further experimental study. Successive values for T1 and T2 have been obtained for the adiabatic conditions by means of applying equations (10) and (11) for successive values of VOC oxidation XV O C . Residence time s I N C I N has in turn been obtained for adiabatic conditions by solving equations (27) to (35) using a Microsoft Excel 5 Spreadsheet.

= U AMIX (TIN,INCIN - T0 )

+ AINCIN s

INCIN

D s

=

INCIN

((

TOUT + TIN,INCIN 2

(- D

yVOC XVOC

)

T0

(13a)

(

)

HC0 ,TIN + m* Cp,WCG TOUT - TIN + 4v0 m

=

)- )

U di 2 WASTE d0

(

yVOC XVOC,2 - XVOC,1 4v0 m

((

TOUT + TIN,INCIN 2

) -T)

U di

2 WASTE d0

*

p,WCG

Assuming plug ¯ ow in the incinerator, a rigorous expression may be derived for evaluation of incremental residence time D s INCIN over increment of VOC oxidation, D XVOC . The expression takes into account the decrease in both VOC and oxygen concentration and the increase in the rate of VOC oxidation with temperature over this increment. The full derivation of the expression may be found in Appendix 1, together with the de® nitions of the terms Z1 to Z6 . INCIN

Z22 Z4 + XVOC,OUT ln Z3 Z4 + XVOC,IN

(

= Z1

)-

Z52 Z3

1 - XVOC OUT ´ In 1 X , - Z6 (XVOC,OUT - XVOC,IN - VOC,IN

(

( T2 - T1 )

(18a)

0

Residence Time for VOC Incineration to CO 2 and H2O: Full Kinetic Expressions

D s

)

Equations (29) and (36) have been solved simultaneously with the heat balance equations (13), (17) and (18) for non-adiabatic operation by means of a Microsoft Excel 5 Spreadsheet to yield successive values of D s I N C I N , T1 and T2 in order to determine s I N C I N . Equation (37) has then been solved in order to obtain TM . Table 4 and Figure 3 show the results of the calculations, for both adiabatic and non-adiabatic operation. Simpli® ed kinetic expressions If the VOC is in low concentration, the kinetic expressions may be simpli® ed by using a mean value for oxygen concentration in the incinerator, as follows:

)

mO2 ,M

=

where D s

INCIN

=

INCIN

=

D s

XVOC=0 .9999

S

D s

INCIN

=

INCIN

=

a mVOC,IN XVOC

mO2 ,IN -

2(1 + e

)

(38)

VOC XVOC )

(

(36)

INCIN

m* mWASTE mO2 MPT RTIN kM

´ (1 + e

XVOC=0

-e

A true mean temperature of incineration TM may then be de® ned by equation (37): s

(

which yields a simpli® ed expression for incremental residence time D s I N C I N :

D V , v0

by de® nition, which yields in turn

s

(17a)

0

) (- D (H( ) + mT C )- ) 0 C,TIN T 2 + T1 2

U AMIX (TIN, INCIN - T0 ) mWASTE

( ) ( (

m* IN mWASTE RTM Z22 X ln 1 + VOC,OUT Z3 Z4 P k exp - E T 0

RTM

Z52 - Z ln 1 - XVOC,OUT 3

(

)-ZX

6 VOC,OUT

) (37)

A mean value for the pre-exponential factor k0 , de® ned by equation (20), has been obtained from the data given in Table 2, noting that the experimental values for oxygen concentration and incineration temperature varied from 18 to 21 mol % and from 1028 to 1130 K maximum respectively6 .

VOC

(

VOC )ln

1 - XVOC,IN 1 - XVOC,OUT

(XVOC,OUT

)

)

- XVOC,IN )

(29a)

from which a simpli® ed de® nition of the true mean temperature of incineration is obtained: s

INCIN

=

m* mWASTERTM

( )

-E mO2 ,M PT k0 exp RT M

(

´ (1 + e

-e

VOC )ln

VOC (X VOC,OUT

(

1 - XVOC,IN 1 - XVOC,OUT

)

)

- XVOC,IN ) (37a)

These equations have ® rst been solved for adiabatic conditions in the same manner as the equivalent but more rigorous equations to yield values of residence time s I N C I N and temperature TM for comparison with the values obtained from Trans IChemE, Vol 75, Part B, February 1997

ESTIMATION OF RESIDENCE TIME IN VOC INCINERATION

37

Table 4. Calculated results for toluene incineration to CO2 and H2O. Result

Non-adiabatic operation

Adiabatic operation

XAIR mFUEL kmol (kmol dry waste gas) - 1 D XVOC, 0.0 # X VOC # 0.9 0.9 # X VOC # 0.995 0.995 # XVOC # 0.9999

2.0364 0.09297 0.0500 0.0050 0.0049

Reducing D X VOC to 0.025, 0.0025 and 0.0024 has negligible effect on the magnitude of s k0, m3 kmol- 1 s- 1 s INCIN, s, full expressions s INCIN, s, simpli® ed expressions s APPROX, s

0.62 0.62 0.67

TIN, IN,CIN, K TOUT, K, full expressions TOUT, K, simpli® ed expressions TOUT, K, approximate

INCIN

2.9612 ´ 1016

0.70 0.70 0.74

922.0 1062.4 1062.4 1062.4

% heat loss, full expressions % heat loss, simpli® ed expressions

919.8 1055.3 1055.3 1055.4

n.a. n.a.

TM, K, full expressions TM, K, simpli® ed expressions TAV, K, full expressions TAV, K, simpli® ed expressions

5.0 5.0

1020.9 1019.9 992.2 992.2

1016.4 1015.4 987.6 987.5

(TM, K/ TAV), full expressions (TM, K/ TAV), simpli® ed expressions

1.0289 1.0279

1.0292 1.0282

k(TM)/k(TAV), full expressions k(TM)/k(TAV), simpli® ed expressions

2.31 2.24

2.33 2.26

those expressions. They have then been solved simultaneously with the simpli® ed heat balance equations, (13a), (17a) and (18a) for the case of non-adiabatic conditions. Table 4 shows the results of these calculations. s

of a detailed spreadsheet. The following expression has therefore been developed from its more rigorous counterparts:

Approximate Estimate of Residence Time s

INCIN

If it is desirable to make an approximate estimate of residence time s I N C I N , say for the purpose of preliminary costing of an incinerator, it is useful to have a simpli® ed expression for this purpose, which does not require preparation

1

APPROX

1

)

1

1 - XVOC 1 1 1 X ln - VOC,2 ´ ln 1 X , + k 1 - XVOC,3 - VOC,2 APPROX,3 1 1 X ln - VOC,3 (39) +k 1 - XVOC,4 APPROX,4

(

Figure 3. Residence time vs VOC oxidation.

Trans IChemE, Vol 75, Part B, February 1997

(

= kAPPROX 1 ln 1 - X VOC 1 + kAPPROX 2 , , ,

)

(

(

)

)

38

O’ REILLY

Table 5. Toluene incineration: comparison of residence time evaluation, De Nevers Kinetics. Adiabatic conditions, TIN = 922 K. XAIR

TAV , K

TM , K

1.250 1.540 1.740 1.976 2.185

1040.6 1022.9 1010.6 995.9 982.9

1061.2 1049.2 1039.3 1025.8 1011.7

s

INCIN , s

s

APPROX,

0.30 0.40 0.50 0.70 1.00

0.29 0.40 0.52 0.75 1.10

s

k(TM )/k(TAV ) 1.70 2.01 2.18 2.29 2.28

where 1

m* m

IN WASTE kAPPROX = mO MPT RTIN kM 2,

(40)

Subscripts 1, 2, 3, 4 in equation (39) refer to values of fractional oxidation of VOC, XV O C of 0.25, 0.50, 0.75 and 0.9999 respectively and the corresponding temperature intervals for the evaluation of kA P P R O X . Successive values of incineration temperature required for estimation of kA P P R O X are easily obtained for adiabatic conditions by means of equation (10). Estimation of approximate residence time s A P P R O X for non-adiabatic conditions is carried out by appropriate modi® cation of the second term in equation (10), yielding a correction factor of 0.90 to 0.95, to represent 5 to 10% heat loss from the incinerator, which has been found to be typical. The slight decrease in inlet temperature to the incinerator incurred by heat loss over the mixing section has been found to have little effect on the calculated value of approximate residence time s A P P R O X and has therefore been ignored for the approximate calculations. Table 4 shows the results of these calculations. Some calculations have also been carried out in order to compare the results of prediction of residence times from the full or rigorous and the approximate expressions, over most of the range recommended for VOC incineration1 , using kinetic data from that source. Table 5 shows the results. The above calculation procedures have then been applied to the other VOCs listed in the Introduction, in order to test their consistency over the very wide range of values of kinetic constants represented by these VOCs. Tables 6 and 7 show the results.

DISC USSION OF CALCUL ATED VOC INCIN ERATION RESULTS Tables 4, 6 and 7 show that consistent predictions of residence time can be obtained from the full and the simpli® ed kinetic and heat transfer expressions for the ® ve VOCs investigated. These VOCs exhibit greatly different values of heat of combustion and activation energy, parameters which have the greatest in¯ uence on rate of incineration. A value of 0.7 s residence time from the full expressions has been chosen as a basis for comparison but Table 5 shows that consistently good agreement is maintained for incineration of toluene over very different values of residence time. Separate calculations have con® rmed that such agreement is observed for incineration of the other four VOCs listed above. If the concentration of oxygen decreases by less than 10 % of its initial value during incineration, then use of the simpli® ed expressions should be quite adequate for the evaluation of residence time for either adiabatic or non-adiabatic conditions. If only an approximate estimate of residence time is required, say for a preliminary costing exercise, then the approximate expression should yield satisfactory results. It is suggested that 10% heat losses should be assumed for evaluation of non-adiabatic conditions using the approximate expressions, in order to be conservative, and this suggestion does have some support from the literature3 . Overall, the approximate expression gives results that are rather higher than those obtained from the more rigorous expressions. This effect undoubtedly stems from the use of the coarser step size in the evaluation, given that the residence time versus fractional oxidation pro® le is very non-linear, especially at very high values of fractional oxidation XV O C . See Figure 3. It should be carefully noted, of course, that the actual calculated value of residence time is very strongly in¯ uenced by the magnitude of the kinetic parameters used. More than just one set of such parameters may be available for any particular VOC 7 . Comparison of the results for toluene incineration in Table 4, which have been obtained from Hemsath and Susey’ s parameters, with the italicized results in Table 6, which are based on De Nevers’ data, show a 25% discrepancy in the calculated value for residence time, with respect to the former. This should be borne in mind if one is evaluating incineration of different VOCs by means of

Table 6. Adiabatic incineration of VOC to CO2 and H2O, De Nevers Kinetics, 0.5% VOC in dry waste gas, 0.35 m3 s- 1 at STP. VOC XAIR kmol fuel (kmol dry waste) - 1 E, bar m3 kmol- 1 s INCIN, s, rigorous s INCIN, s, simpli® ed s APPROX, s TIN, INCIN, K = TIN, K TOUT, K TM, K, rigorous TM, K, simpli® ed TAV, K TM, rigorous/TAV TM, simpli® ed/TAV k(TM)/k(TAV), rigorous k(TM)/k(TAV), simpli® ed

Ethyl mercaptan C2H6S 5.270 0.0709 615.46 0.70 0.69 0.73 650.0 700.4 689.3 689.2 675.2 1.0209 1.0208 1.25 1.25

Acrolein C3H4O

Toluene C7H8

Toluene C7H8

3.168 0.0925 1503.06 0.70 0.69 0.75 800.0 848.9 839.9 839.8 824.5 1.0187 1.0187 1.50 1.49

1.976 0.0882 2365.54 0.70 0.70 0.75 922.0 1069.9 1025.8 1024.6 995.9 1.0299 1.0288 2.29 2.22

2.036 0.0930 2365.54 0.77 0.77 0.84 922.0 1062.4 1021.9 1020.9 992.2 1.0299 1.0289 2.30 2.24

Chlorobenzene C2H5Cl 1.728 0.1003 3207.09 0.70 0.70 0.76 980.0 1092.7 1060.8 1060.1 1036.3 1.0236 1.0230 2.36 2.31

Benzene C6H6 2.004 0.1086 4015.14 0.70 0.71 0.72 950.0 1055.6 1019.9 1019.3 1002.8 1.0170 1.0164 2.24 2.18

Trans IChemE, Vol 75, Part B, February 1997

ESTIMATION OF RESIDENCE TIME IN VOC INCINERATION

39

Table 7. Non-adiabatic incineration of VOC to CO2 and H2O, De Nevers Kinetics, 0.5% VOC in 1 kmol dry waste gas, 0.35 m3 s- 1 at STP. VOC XAIR kmol fuel (kmol dry waste) - 1 E, bar m3 kmol- 1 s INCIN, s, simpli® ed s APPROX, s TIN, INCIN, K TOUT, K, simpli® ed TOUT, K, approximate % heat loss, simpli® ed % heat loss, approximate TM, K, simpli® ed TAV, K, simpli® ed TM/TAV, simpli® ed k(TM)/k(TAV), simpli® ed

Ethyl mercaptan C2H6S 5.270 0.0709 615.46 0.71 0.74 649.0 697.3 697.9 5.9 5.0 687.5 673.2 1.0212 1.26

Acrolein C3H4O

Toluene C7H8

Toluene C7H8

3.168 0.0925 1503.06 0.75 0.82 798.4 843.7 844.0 10.5 10.0 836.6 821.0 1.0190 1.51

1.976 0.0882 2365.54 0.80 0.84 919.7 1062.1 1062.5 5.2 5.0 1019.8 990.9 1.0291 2.25

2.036 0.0930 2365.54 0.88 0.93 919.8 1054.0 1055.4 5.9 5.0 1015.8 986.9 1.0293 2.27

kinetic data which has been obtained from a variety of sources. One of the most striking results to emerge from the tables is the value of the ratio of the rate constants as de® ned by equations (37) and (37a), or indeed by equation (2). This ratio shows that if the arithmetic mean stream temperature is used to estimate residence time for VOC incineration, by means of equation (2), the result may be 25 to over 125% in excess of the values obtained by the other, more detailed methods and the discrepancy is typically more than 100%. This is to be expected because although stream temperature increases almost linearly with fractional oxidation of VOC, the incineration process is essentially kinetically controlled and the Arrhenius term in the kinetic expressions ensures that reaction rate increases exponentially with temperature and therefore with increasing fractional oxidation of VOC. Even though the arithmetic mean is found to be only 2 to 3% less than the `true’ mean temperature of incineration, this small difference nevertheless has a very signi® cant effect on the rate of incineration because the activation energies for the VOC oxidation reactions are so high. It is unfortunate that there does not seem to be any simple way of estimating the value of the true to arithmetic mean temperature ratio in advance. This value must be known very accurately for predictions of residence time to be obtained which are consistent with those evaluated from the more detailed expressions. Moreover, the values of the ratio of reaction rate constants k(TM )/k(TA V ) as a function of increasing activation energy are not considered to be generally applicable for use as correction factors in equation (2). Separate calculations have shown that the trend observed in Tables 6 and 7 is not maintained for values of residence time different from 0. 7 s. Clearly, a great many calculations would have to be carried out with the more detailed expressions in order to generate satisfactory values of correction factors for equation (2) for all VOCs for the whole range of feasible incineration residence times. This would rather defeat the purpose of the exercise but the dif® culty is overcome by use of the approximate expressions given above for preliminary calculations. In effect, the discrepancy shows that the residence time for the incineration process cannot be satisfactorily predicted by means of a single step of size equivalent to 99.99%, with an incineration temperature estimated by heat balance only. Trans IChemE, Vol 75, Part B, February 1997

Chlorobenzene C2H5Cl 1.728 0.1003 3207.09 0.84 0.94 977.4 1083.5 1081.4 8.0 10.0 1054.5 1030.5 1.0233 2.35

Benzene C6H6 2.004 0.1086 4015.14 0.91 0.90 947.6 1046.8 1045.0 8.4 10.0 1013.8 997.2 1.0166 2.20

At least four steps are required, of size equivalent to 25%, for approximate consistency, with mean values of reaction rate constants obtained using the Arrhenius expressions themselves. See Appendix 1. If rigorous consistency is required for prediction of residence time, up to 30 steps may be necessary, according to the more detailed methods of analysis described above. CONCLUSIONS (1) An analytical procedure has been developed for evaluation of residence time for incineration of a VOC for which basic kinetic data is available. The procedure accounts for variation in concentration of both the VOC and oxygen available for incineration, the increase in temperature which takes place as the incineration proceeds and the effect of heat loss through the walls of the containing vessel. Use of rigorous and simpli® ed versions of this procedure shows that it gives consistent results for several VOCs with widely different values of kinetic parameters. This consistency is maintained over a considerable variation in residence time values, from 0.3 to 1.0 s. The simpli® ed heat balance and kinetic expressions may be used satisfactorily if the change in the concentration of oxygen during incineration is less than 10% of its initial value, which would be advisable in practice, in order to ensure complete combustion of the VOC to CO2 and H2 O. (2) An approximate version of the procedure has been developed for preliminary calculations which is easy to use and gives results which agree closely with those obtained by means of the more rigorous procedure. Decrease in oxygen concentration during incineration of less than 10% of its initial value also applies to use of this approximate procedure but this should be a reasonable assumption for any preliminary study. (3) Although the procedure is based on certain assumptions with respect to plug ¯ ow and perfect mixing in the incinerator, experimental results reported by other workers indicate that departures from these conditions in practice may not introduce excessively serious errors with respect to the reliability of the predicted results. See discussion in the Introduction. (4) It has not been found possible to predict a single, accurate mean temperature of incineration in advance of carrying out more detailed calculations. However, the use

40

O’ REILLY

of the approximate calculation procedure described above is believed to be a satisfactory method of overcoming this particular dif® culty, for the purpose of preliminary calculations.

where: m*

= 1 + yVOC b

XVOC +

mFUEL (a + b XAIR ) mWASTE

(11)

Non-Adiabatic Conditions (Fuel and Air at 298.15 K)

APPENDIX 1Ð DERIVATION OF HEAT BALANCE AND REACTION KINETIC EXPRESSIONS Adiabatic Conditions (Fuel and Air at 298.15 K)

Heat losses through the containing walls of the incinerator in both the mixing and incineration sections are likely to occur in practice. Equation (8) is therefore modi® ed as follows to yield the energy balance for non-adiabatic conditions:

Over the combustion chamber, see Figures 1, 2:

= mFUEL(-D HC0 ,298 K ) = mCGCp,CG(TA - T0) mCG a b XAIR mFUEL = + QFUEL

(3)

yVOC XVOC (-D HC0 ,TIN ) = m* Cp,WCG (TOUT - TIN )

(4)

+m WASTE

QLOSS

where a = 11.0597 and b = 10.0197 using the data in Table 2. This yields: TA

= T0 +

- D HC,298 K (a + b XAIR )Cp,CG

where: QLOSS

0

(5)

=U

where XAIR = 0.5 for TA = 1500 C in the combustion chamber. Over the mixing section, before incineration takes place: mFUEL (a + b XAIR )Cp,CG (TA - TIN )

= mWASTE Cp,WASTE (TIN - T0 )

mFUEL (-D HC0 ,298 K ) = (mFUEL (a + b XAIR ) + mWASTE) (7)

= mWASTE yVOC XVOC (-D

where m*IN and:

AMIX AINCIN

= (mFUEL(a + b XAIR ) + mWASTE (1 + yVOCb XVOC ))

s

INCIN

=

(8)

d0

(T OUT - TIN ,INCIN )

ln

(

TOUT - T 0 TIN INCIN - T 0 ,

= -D

TOUT

Cp,WCG (TIN - T0 ) HC0 ,298 .15 K - (a + b XAIR )Cp,WCG (TIN - T0 )

= TIN +

yVOC XVOC (-D HC0 ,TIN ) m* Cp,WCG

TOUT - T0 TIN,INCIN - T0

)

(13)

(14)

=

)

p dO2 ds 1 + K 2 (2 - K ) + 4 v0 i MIX 4 d02

(

= 4v0

di s

INCIN

(15) (16)

d02

U AMIX (TIN ,INCIN - T0 ) mWASTE

(17)

)

Equations (7) and (8) yield respectively: mFUEL mWASTE

(

which gives:

yVOC XVOC (D HC0 ,TIN ) + m*WASTECp,WCG (TOUT - TIN ) + di U 4v0 mWASTE 2

ln

= m* where XVOC = 0

HC0 ,TIN )

´ Cp,WCG(TOUT - TIN )

öø

(TOUT - TIN,INCIN )

= U AMIX (TIN,INCIN - T0 )

Note that in equation (6) the value of XA I R will have been increased and that of TA correspondingly decreased, with respect to equation (5) according to the scheme of Figure 1. Over the incineration section: mVOC XVOC (-D

AMIX (TIN,INCIN - T0 ) + AINCIN

m*IN mWASTE Cp,WCG (TIN - TIN,INCIN )

or, combining combustion and mixing sections:

HC0 ,TIN )

æè

Where TI N , I N C I N is de® ned by energy balance over the mixing section:

(6)

´ Cp,WCG (TIN - T0 )

(12)

(9)

The above expressions may be somewhat simpli® ed by using an arithmetic mean temperature difference for heat transfer: QLOSS

=U ´

(10)

((

AMIX (TIN,INCIN - To ) + AINCIN TOUT + TIN,INCIN 2

)- ) T0

(13a)

s

which gives: INCIN

=

yVOC XVOC

(- D

)

HC0 ,TIN + m* Cp,WCG (TOUT - TIN ) + 4v0

U di

mWASTE d20

((

TOUT + TIN ,INCIN 2

)

- T0

)

U AMIX (TIN ,INCIN - T0 ) mWASTE

(17a)

Trans IChemE, Vol 75, Part B, February 1997

ESTIMATION OF RESIDENCE TIME IN VOC INCINERATION Evaluation of Residence Time for VOC Incineration: Kinetic Expressions

which gives on integration: kM

Assuming plug ¯ ow, a differential material balance for VOC oxidation yields: P v0 VOC,IN d XVOC RTIN,INCIN

= - rVOC d V

k

= ER(T2 0- T1 )

D s

( )

RT

RT

INCIN

( ( ´( -

1 XVOC,OUT 1 XVOC,IN

2

= mWASTEm

p * T

= m*IN (1 + e VOC XVOC)

(22)

Z6 (XVOC,OUT

)

- XVOC,IN )

m*IN mWASTE PT RTIN km

=

Z2

= e VOCmO INVOC - 9mVOC

(31)

= mO IN (1 + e VOC ) - 9mVOC

(32)

Z3

mVOC = pT mWASTE m*

= pVOC,IN

(

1 - XVOC 1 + e VOC XVOC

Z4

9m

(30)

)

Z5

and:

= pO IN - D

p O2

2

=

´ D m O2

(

Z6

mO2 IN PT PT = mWASTE * mIN mWASTE m*

mO2 IN

a mVOC,IN XVOC

1+ e

VOC XVOC

)

(25)

k0 RTIN INCIN PT

= (RT)2m*IN, mWASTE

( ´(

9mVOC,IN XVOC + e VOC XVOC

´ mO2 IN - 1

)

1 - XVOC dV 1 + e VOC XVOC

)

(26)

Equation (26) may be integrated to give INCIN

=

D V v0

over interval of VOC oxidation D XVOC . The variation in temperature T over this interval is taken into account as follows: T2

=

k0

ò

T1

E RT 2

(

)

ER

ò dT T 1

Trans IChemE, Vol 75, Part B, February 1997

= 1 + e VOC

(34) 2 VOC

e

= e VOCmO IN - 9mVOC

(35)

NOMENCLA TURE

exp -RTE dT T2

(33)

Acid gases sulphur dioxide SO2 and hydrogen chloride HCl are formed respectively in the combustion of ethyl mercaptan and chlorobenzene. These gases would have to be removed from the incinerated waste gas stream before it could be discharged to atmosphere. A very likely strategy would be to cool the waste gas by recuperative or regenerative heat exchange with combustion air and then absorb the acid gas components in alkaline solution. This strategy would yield a saving in auxiliary fuel requirement, which should at least partially offset the cost of the heat exchange and scrubbing operations. A proper evaluation of the fuel saving would require a calculation over the incinerator with preheated combustion air and the reduced fuel input but giving the same residence time as for fuel and air at ambient conditions, or no heat recovery. This calculation would be carried out by means of a simple iteration to determine a suitably adjusted value for the multiple of excess air, XA I R . The heat balance calculation procedure presented in this work does exactly that and is therefore eminently suited to investigating the feasibility of heat recovery in incinerator operation.

Equation (18) can now be written: v0 d XVOC

= e VOCmO IN - 9mVOC

APPENDIX 2Ð IMPLICATIONS OF ACID GAS REMOVAL DOWNSTREAM OF VOC INCIN ERATION

(24)

( )

mO2 IN

2

PT mWASTE m *IN

where for incineration of toluene, from Table 2: m O2 a = 9 mVOC STOICHIOMETRIC =

2

2

(23)

kM

)-

)-

Z52 ln Z3

2

pVOC

D s

(28)

Z1 (21)

then:

p O2

E RT1

where:

noting: m*

exp

(29)

and: mj

E RT2

RT RT (20)

pj

exp

Z22 Z X ln 4 + VOC,OUT Z3 Z4 + XVOC,IN

= Z1

where:

( )

( (- ) - (- ))

Equation (25) can now be integrated to give:

(19)

-E C - E pVOC pO VOC = k0 exp - rVOC = A exp

41

a

(27)

a, b A

kmol oxygen required for complete incineration of 1 kmol of VOC stoichiometric coef® cients for fuel combustion, dependent on fuel composition pseudo ® rst order pre-exponential factor, s- 1

42 AMIX AINCIN b

Cp,CG Cp,WASTE Cp,WCG

«

Cvoc di do D

D

VOC H 0C,298 K H 0C,TIN

D s

INCIN

D P O2

D V E K k(T ) k0 kAPPROX km kw m* mFUEL mCG mO 2 , M mO2 ,IN mj mVOC mWASTE PO2 PT PVOC,IN pj QLOSS QFUEL R rVOC t T T1,2 T0 TA

O’ REILLY outside area of mixing section of incinerator including dished end, m2 outside area of incineration section of incinerator, m2 change in number of mols in VOC incineration reaction, (kmol VOC)- 1 mean heat capacity of combustion gas, kJ (kmol K)- 1 mean heat capacity of waste gas, kJ (kmol K)- 1 mean heat capacity of the combined waste and combustion gas, kJ (kmol K)- 1 concentration of VOC, kmol m- 3 inside diameter of incinerator, m outside diameter of incinerator, m expansion factor in VOC incineration standard heat of combustion of fuel, kJ kmol- 1 heat of combustion of VOC, kJ kmol- 1 at TIN incremental change in residence time, s change in partial pressure of oxygen over fractional oxidation of VOC from 0 to XVOC , bar incremental change in incineration volume, m3 activation energy, kJ kmol- 1 or bar m3 kmol- 1 ratio of depth to radius for dished end reaction rate constant at temperature T, m3 kmol- 1 s- 1 pre exponential factor, m3 kmol- 1 s- 1 approximate reaction rate constant, s- 1 reaction rate constant over increment of oxidation D XVOC, kmol bar- 2 m- 3 s- 1 thermal conductivity of insulation brick, kW m- 1 K- 1 kmol of incinerated waste and combustion gas, (kmol original waste gas) - 1 kmol s- 1 of auxiliary fuel to process, reported as kmol (kmol dry waste gas) - 1 rate of combustion gas, kmol s- 1 mean kmol of oxygen for incineration initial kmol of oxygen for incineration kmol of reactant j rate of VOC, kmol s- 1 rate of waste gas, kmol s- 1 partial pressure of oxygen at any instant, bar total pressure, bar initial partial pressure of VOC, bar partial pressure of reactant j, bar rate of heat loss from incinerator under non adiabatic conditions, kW rate of heat release from combustion of fuel, kW universal gas constant, kJ (kmol K)- 1 or bar m3 (kmol K)- 1 rate of incineration of VOC, kmol m- 3 g- 1 time, s temperature, K gas temperature into and out of increment of incinerator volume D V, K standard temperature, 298.15 K adiabatic ¯ ame temperature, K

TAV TIN

s

TIN,INCIN TM TOUT APPROX

s

INCIN

s

MIX

U v0 XAIR XVOC D XVOC yVOC Z1,2,3,4,5,6

arithmetic mean temperature of gas for incineration, K initial temperature for incineration in mixing section, adiabatic operation, K TIN for non adiabatic operation, K true mean temperature of gas for incineration, K ® nal temperature of gas out of the incineration chamber, K approximate residence time for VOC incineration, s residence time for VOC incineration, s residence time in mixing section, s overall heat transfer coef® cient for heat loss from incinerator, kW m- 2 K- 1 volumetric ¯ owrate of combined waste and combustion gas at TIN, m3 s- 1 fraction or multiple for excess combustion air, XAIR = 0.0 for stoichiometric air fractional oxidation of VOC increment of fractional oxidation = XVOC,OUT-XVOC,IN mol fraction of VOC in original waste gas = 0.005 in dry air de® ned by equations (30) to (35)

REFERENCES 1. De Nevers, N., 1995, Air Pollution Control Engineering, 300ff (McGraw Hill Inc). 2. Cheremisinoff, P. N. and Young, R. A., 1977, Air Pollution Control and Design Handbook, Part 1, 449ff, (Marcel Dekker Inc). 3. Ledbetter, J.O., 1974, Air Pollution, Part B: Prevention and Control, 138ff (Marcel Dekker Inc). 4. Valentin, F. H. H. and North, A. A., 1980, Odour Control, A Concise Guide, 51±60 (D.O.E., Warren Spring Laboratory). 5. Caloric Incinerators, Caloric, near Munich, Germany, July 1990. 6. Barnes, R. H. et al., 1979, Chemical Aspects of Afterburner Systems, U.S. Environmental Protection Agency, Report No. EPA 600/7 79 096, page 4 (U.S. Government Printing Of® ce, Washington, DC). 7. Hemsath, K. H. and Susey, P. E., 1974, Fume incineration kinetics and its applications, Recent Advances in Air Pollution Control, AIChE Symposium, No. 137, Vol. 70: 439 to 449. 8. Coulson, J. M. and Richardson, J. R., 1977, Chemical Engineering Volume 1, 3rd edition, 175 (Pergamon Press Ltd).

ADDRESS Correspondence concerning this paper should be addressed to Dr A. O’ Reilly, Division of Chemical Engineering, University of Teeside, Middlesbrough TS1 3BA. The manuscript was received 15 July 1996 and accepted for publication after revision 16 October 1996.

Trans IChemE, Vol 75, Part B, February 1997