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A Series Reaction Approach to VOC Incineration
0957±5820/98/$10.00+0.00 Institution of Chemical Engineers Trans IChemE, Vol 76, Part B, November 1998
A SERIES REACTION APPROACH TO VOC INCINERATION A. O’REILLY Chemical Engineering, University of Teesside, Middlesbrough, UK
T
hermal incineration of VOCs has been evaluated by means of a two step series reaction approach, VOC CO CO2 , so that the rates of VOC and CO oxidation may both be considered. Temperature and concentration pro® les have been obtained for oxidation of several VOCs as functions of space time t. Appropriate conditions for 99.99% removal of VOC and 99.5% oxidation of intermediate CO appear to be 930 to 980 K inlet temperature, 0.05 to 0.06 kmol auxiliary fuel per kmol dry waste air and 90 to 130 % excess air for auxiliary fuel, without heat recovery. These conditions yield approximately 0.4 seconds space time and an L/di ratio for the oxidation section of the incinerator of approximately 2 to 3. Heat loss from the incinerator should be less than 2% of the total heat from auxiliary fuel and VOC combustion and incinerator design should therefore be optimized with respect to fuel and insulation costs. The approach outlined in this work can easily be adapted to estimate auxiliary fuel savings via heat recovery. Keywords: atmospheric pollution; VOCs; CO; thermal incineration.
INTRODUCTION
facility and those obtained in practice from a 36 i.d. full scale reactor. These results show the bene® t of careful mixer design and have been discussed elsewhere1,2 . The calculation procedure described in this work has therefore assumed that at least comparable mixer designs are available. The proposed two step series reaction scheme VOC CO CO2 is itself greatly simpli® ed. VOC and CO oxidation consist of many individual steps, the most important of which for CO is CO + OH CO2 + H 7 . The OH reaction step could be included in the calculations but use of global kinetics should be suf® cient for design purposes. Predicted results should be compared with experimental data and the kinetics then re® ned if necessary, possibly by inclusion of the CO + OH reaction. Although the data sources for the kinetics3,4,7 appear somewhat dated, De Nevers’ recent review of VOC incineration8 indicates that they are de® nitive. Moreover, Barnes7 explicitly recommends Hemsath and Susey’ s data for CO oxidation for afterburner design. The Appendix sets out the design procedure proposed in this work, which may be summarized as follows.
Thermal incineration of VOCs and CO has been evaluated for the individual species1,2 by means of the following reaction rate expressions3,4 . dCVOC dt d pco /PT dt
A exp
E CVOC RT
k0 exp
E RT
pco /PT n , n
1 1.5
2
Thermal incineration of VOCs must ensure that intermediate CO is also oxidized to high conversion before release of the waste gas into the atmosphere. However, the rate of oxidation of CO to CO2 may be appreciably slower than its rate of formation from VOC 4 , especially if at least 99.5% removal of intermediate CO is required 2 . It is therefore preferable to evaluate the oxidation of both VOC and CO according to a series reaction of the form VOC CO CO2 , rather than as a single overall reaction VOC CO2 . This work describes such a procedure.
1. Initialization of the temperature pro® le in the oxidation section, equation (10). 2. Initialization of space time t pro® le for VOC oxidation to CO, equations (27)±(30). 3. Initialization of concentration pro® le for VOC as partial pressure pVOC , equation (30). 4. Initialization of concentration pro® les for CO, CO2 as partial pressures pCO , pCO2 , equations (37) to (39).
VOC INCINERATION AS A SERIES REACTION, VOC CO CO2 Figures 1 and 2 show respectively the thermal incineration process schematically and a simpli® ed incinerator cross section. Table 1 summarizes typical operating data5 8 . Barnes 7 indicates further that oxidation sections of incinerators typically have L/di ratios of approximately 2 to 3. Hemsath and Susey4 and Carleton5 have shown how mixer design must achieve as near to plug ¯ ow as possible for maximum conversion of VOC. Hemsath and Susey have reported very good agreement between the results for incineration of toluene predicted from their small scale test
Equation (38) cannot be solved analytically and the Runge Kutta method has been used for XVOC # 0.9999. See equations (43) to (45). 5. Iteration to determine consistent values for temperature 302
A SERIES REACTION APPROACH TO VOC INCINERATION
303
estimated 1,2 and the other values are assumed. The incinerator is assumed to have an i.d. of 0.8 m, a wall thickness of 0.0125 m and a layer of insulation brick of thickness 0.3125 m. tMIX 0.3 s for all calculations5 . Incremental step sizes for the calculations have been chosen as follows:
Figure 1. VOC incineration, schematic ¯ owsheet.
For 0.0 # XVOC # 0.9999 Range of XVOC DXVOC 0.0000 to 0.9000 0.0500 0.9000 to 0.9950 0.0050 0.9950 to 0.9990 0.0020 0.9990 to 0.9995 0.0005, single step 0.9995 to 0.9999 0.0004, single step
For XCO when XVOC 0.9999 Range of ppm CO Dppm CO 0 to 40 1 to 5 40 to 100 5 to 10 100 to 500 10 to 50 500 to 5000 50 to 200
Reducing the above step sizes for DXVOC to 0.0250, 0.0025, 0.0010, 0.00025, 0.0002, respectively has little effect on the magnitude of VOC incineration space time t. COMPARISON WITH AVAILABLE EXPERIMENTAL DATA
Figure 2. VOC incineration vessel, schematic diagram.
pro® les for adiabatic operation, equation (13) and non adiabatic operation, equation (20). 6. Iteration to determine consistent values for concentration pro® les for XVOC # 0.9999. 7. Iteration using equations (13), (20) to (40) and (46) for residual pCO pro® le until XCO $ 0.995. Table 2 gives the basic data for the calculations and Table 3 summarizes thermodynamic and kinetic data for each of the VOCs considered for the calculations. U values of 0.5, 1.0 and 2.0 W m 2 K 1 have been used for evaluation of non adiabatic conditions in order to determine the effect of signi® cantly increasing heat loss on incinerator design and operation. The value of U 0.5 W m 2 K 1 has been Table 1. Operating conditions for industrial gas incinerators. Actual gas velocity, m s 1 Incineration time, s Temperatures: Odour control Hydrocarbons CO
7.5 to 15.0 0.2 to 1.0 750 to 1000 K 750 to 920 K 920 to 1075 K
Trans IChemE, Vol 76, Part B, November 1998
The calculated results have been compared with the experimental data of Hemsath and Susey4 for incineration of toluene and hexane. Figures 3 and 4 reproduce their concentration pro® les against space time and show the calculated concentration pro® les. Two calculated pro® les have been generated for toluene. One uses the values for the kinetic parameters as reported. The other introduces a correction factor for the deviation of the particular experimental data point from the Arrhenius regression line, which is signi® cant in this case. Hemsath and Susey indicated that the temperature rise in their test facility during VOC incineration was approximately 20°F. It has therefore been assumed for these calculations that the temperature ranges during incineration were 1440°F to 1460°F or 1067 K for hexane and 1505°F to 1525°F or 1103 K for toluene. The calculations have then been carried out by assigning two constant temperature intervals of equal magnitude for the incineration of each of the VOCs. This approximation gives mean temperatures for use in equations (33a) and (38a) of 1445°F and 1455°F for hexane and 1510°F and 1520°F for toluene. This simpli® cation accounts for the slight discontinuity in the calculated pro® le for ppm hexane at a space time of 0.06 seconds. Equation (46), slightly modi® ed, has been used for overall estimates of CO conversion and corresponding space times. See Table 4. Agreement between the experimental and calculated concentration pro® les is good for each of the VOCs if the correction factor is used for toluene, see above. Although trends are similar, agreement between the experimental and calculated concentration pro® les for CO is poor. (The former suggest some initial CO contamination of the waste air used in the experiments.) However, the experimental data shows a considerably greater increase in CO concentration than would be possible from stoichiometry. The maximum increase in CO concentration which could be achieved by conversion of the inlet toluene is 1400 ppm. Figure 3 shows an increase of almost 2000 ppm. Figure 4 shows an equivalent increase in CO concentration of 1500 ppm, whereas stoichiometry predicts a maximum increase of 1200 ppm. It is this apparent discrepancy which also yields poor agreement between the initial segments of the experimental and calculated pro® les for CO2 concentration,
304
O’ REILLY Table 2. Incineration of VOC to CO2 and H2 O, basic data. Waste gas stream containing VOC Total dry volumetric ¯ owrate at STP Initial temperature of fuel, air, waste, T0 Required VOC removal by incineration Auxiliary fuel Composition DH°c 298.15 K for natural gas CO oxidation reaction Kinetic data for CO combustion: T # 1041 K T $ 1041 K
0.5% VOC in dry air, with 0.0116kmol H2 O (kmol dry air) 0.35 m3 s 1 , 0.01559kmol s 1 298.15 K 99.99%, plus 99.5% of CO formed from VOC Natural gas CH4 90%, C2 H6 8%, CO2 1%, N2 1% 0.83631 GJ kmol 1 CO 0.5O2 CO2 k0 , mol fraction 0.5 s 1 E, 105 kJ mol 1 24 1.0 ´ 10 4.1846 2.5 ´ 1011 1.6747
Table 3. Incineration of VOC to CO2 and H2 O, 0.5% VOC in 1 kmol dry waste gas, 0.35m3 s Ethyl mercaptan C2 H6 S
VOC: Overall reaction TIN , K DH°c TIN , to CO2 GJ kmol 1 DH°c TIN , to CO GJ kmol 1 DH°c TIN , CO to CO2 , GJ kmol A, s 1 k0 , m3 kmol
1
E, 105 kJ kmol
1
s 1
1
C2 H6 S + 4.5O2 2CO2 + 3H2 O +SO2 930.0 1.7632 1.1972 0.2830 5.20 ´ 105
Acrolein C3 H4 O C3 H4 O + 3.5O2 3CO2 + 2H2 O 930.0 1.5933 0.7443 0.2830 3.30 ´ 1010
1.9701 ´ 108
1.2502 ´ 1013
0.6155
1.5031
although the agreement improves as ® nal CO2 concentration is approached. The results for toluene incineration are 3250 ppm and 2380 ppm respectively. The equivalent results for hexane incineration are 2400 ppm and 2600 ppm respectively. Agreement between experimental and predicted results for conversion of residual CO to CO2 is nevertheless good. Table 4 shows that the error between experimental and calculated values for space time increment Dt is 6 14% but the error for CO conversion is only 6 4%. (The error obtained using the calculated CO concentration pro® le arises from the effect of residual VOC incineration.)
1
1
at STP, thermodynamic and kinetic data.
Toluene C7 H8
Chlorobenzene Cl C6 H5
Benzene C6 H6
C7 H8 + 9O2 7CO2 + 4H2 O 930.0 3.7763 1.7952 0.2830 2.28 ´ 1013 (Ref 3) 6.56 ´ 1013 (Ref 4) 2.9612 ´ 1016 0.8638 ´ 1016 2.3655 (Ref 3) 2.4493 (Ref 4)
C6 H5 Cl + 7O2 6CO2 + 2H2 O + HCl 980.0 2.9889 1.2910 0.2829 1.34 ´ 1017
C6 H6 + 7.5O2 6CO2 + 3H2 O 960.0 970.0 3.1696 3.1696 1.4716 1.4716 0.2829 0.2829 7.43 ´ 1021
5.0767 ´ 1019
2.8149 ´ 1024
3.2071
4.0151
Hemsath and Susey’ s kinetic data for CO oxidation should therefore be appropriate for the incinerator design procedure proposed in this work. CALCULATED RESULTS FOR TOLUENE INCINERATION, SERIES REACTION APPROACH Table 5 and Figures 5 to 7 show the results for evaluation of toluene incineration by means of the series reaction approach, with kinetic parameters from both Lee et al.3 and Hemsath and Susey4 . Results from the single reaction approach1 serve as a basis for comparison.
Figure 3. ppm toluene CO CO2 during incineration, 1525°F, 1102.6K exit temperature.
Trans IChemE, Vol 76, Part B, November 1998
A SERIES REACTION APPROACH TO VOC INCINERATION
305
Figure 4. ppm hexane CO CO2 during incineration, 1460°F, 1066.5K exit temperature.
Table 4. Incineration of CO from VOC to CO2 , 0.18 # Exit Temperature: VOC A, s 1 E, 105 kJ kmol 1 Results, XVOC < 1.0: Estimated mean T, K ppm CO in ppm CO out XCO t Observed, s t Calculated, s % Error for t ppm CO out by t Observed XCO by t Observed % Error for XCO
1460 °F
1460 °F
C6 H14 4.51 ´ 1012 2.1981 Experiment Series 1063.7 1063.7 1000.0 314.2 306.0 116.5 0.694 0.629 0.031 0.050 0.034 0.049 10.0 2.0 333.3 113.8 0.667 0.638 3.9 1.4
On this basis, the series reaction approach shows that appreciably higher exit temperatures are required in order to ensure that the required conversion of CO is achieved. The inlet temperature has therefore had to be increased slightly from 922 K to 930 K and the excess air requirements must be decreased from 2.0 to 1.8. The fuel requirement decreases slightly from 0.09 to 0.08 kmol (kmol dry waste) 1 in order to satisfy energy balance. Space time and in turn L/di ratio for the oxidation section increase rapidly as the heat loss from the incinerator exceeds approximately 2% of the total heat from fuel and VOC combustion. This suggests that incinerator design should aim to achieve as near to adiabatic conditions as possible and that the allowance of 10% heat loss6 is rather generous. Consider the third set of results in Table 5, for which the space time is 0.4 seconds under adiabatic conditions. Using the data of Table 2, it may be shown that the total heat in is 1037 kW, of which 294 kW is from the combustion of VOC and intermediate CO. If one assumes a heat loss of 10%, the total heat into the incinerator must be increased to 1152 kW, of which 858 kW must be provided by fuel, which is an Trans IChemE, Vol 76, Part B, November 1998
PO2 , M # PT
0.21.
1525 °F
1525 °F
C 7 H8 6.56 ´ 1013 2.4493 Experiment Series 1099.8 1099.8 838.1 102.3 98.0 35.3 0.883 0.655 0.056 0.050 0.048 0.050 14.3 0.0 79.1 35.2 0.906 0.655 2.5 0.02
increase in auxiliary fuel requirement of 15.5%. Most of this increase could be avoided by incineration at as close to adiabatic conditions as possible. There is clearly an incentive for optimization with respect to minimization of fuel, vessel and insulation costs. The results for the evaluation of L/di ratio show that the range of values recommended by established practice 7 are achieved for toluene incineration only by space times of 0.4 seconds or less. Similar results for toluene incineration have been obtained from each set of kinetic parameters, although those of Hemsath and Susey4 have yielded a lower space time than those of Lee et al.3 for the same quantities of fuel and excess air. Nevertheless, the agreement should be considered satisfactory, given that the data sources are completely independent. Lee et al. have provided kinetic parameters A and E for a wide range of VOCs which have therefore been used in the subsequent calculations, together with Hemsath and Susey’ s parameters for CO oxidation. Inlet temperature for incineration has been determined using the data of Table 1 as a guide. Table 5 reveals that
306
O’ REILLY Table 5. Incineration of toluene to CO2 and H2 O, calculated results. Reaction steps XAIR mFUEL pO2 M , bara t, s, U:
One4
Two4
Two4
Two3
0.0 0.5 1.0 2.0
2.036 0.093 0.147 0.62 0.70 n.a. n.a.
1.800 0.080 0.143 0.66 0.75 0.88 1.44
1.275 0.057 0.139 0.40 0.45 0.51 0.70
1.800 0.080 0.143 0.76 0.88 1.05 1.96
0.0 0.5 1.0 2.0
n.a. n.a. n.a. n.a.
0.62 0.71 0.84 1.40
0.38 0.43 0.50 0.68
0.74 0.86 1.03 1.92
TIN INCIN , U: 0.0 0.5 1.0 2.0
922 920 n.a. n.a.
930 928 925 921
930 928 925 920
930 928 925 921
1062 1055 n.a. n.a.
1094 1086 1077 1046
1159 1154 1147 1131
1094 1086 1075 1032
tVOC , s, U:
TOUT , U:
0.0 0.5 1.0 2.0
% QLOSS , U: 0.5 1.0 2.0
1.0 n.a. n.a.
1.0 2.2 6.4
0.7 1.5 3.5
1.1 2.5 8.3
L/di , U:
n.a. n.a. n.a. n.a.
6.5 7.4 8.8 14.2
2.8 3.2 3.6 4.9
7.5 8.7 10.3 19.4
0.0 0.5 1.0 2.0
Figure 5. Toluene incineration, TIN
for the chosen value of 930 K, toluene is one of the more refractory VOCs, requiring 95% of the total space time for 99.99% removal. Figures 5 to 7 show this feature very clearly. There is an extended lag time corresponding to slow conversion of both VOC and intermediate CO until a temperature is achieved at which the rates of both main reactions increase rapidly, yielding a rapid rise in both CO2 concentration and in stream temperature. Figure 7 displays the S shaped curves characteristic of this type of kinetic behaviour. Stream temperature begins to increase sharply at
930 K, XAIR
1.275, U
0.0.
approximately 1020 K, close to the temperature of 1045 K which was found to be suitable for incineration of CO in isolation 2 . Figure 7 also shows the very noticeable effect of increasing heat loss on space time. Figures 5 and 6 show a slight discontinuity in the concentration pro® le for CO which occurs just after the maximum. This arises from the change in the magnitude of the kinetic parameters for CO oxidation which is assumed for simplicity to take place at a single temperature of 1041 K, although the transition is actually more gradual. Trans IChemE, Vol 76, Part B, November 1998
A SERIES REACTION APPROACH TO VOC INCINERATION
Figure 6. Toluene incineration, TIN
930 K, XAIR
1.275, U
2.0 W m
Figure 7. Toluene incineration temperature vs. space time, TIN
CALCULATED RESULTS FOR INCINERATION OF VOCS, SERIES VS. SINGLE REACTION SCHEMES Table 6 shows the results for the series reaction approach to incineration of several VOCs for space times of 0.4 and 0.7 seconds under adiabatic conditions. Results for the single reaction approach for a space time of 0.7 seconds under adiabatic conditions 1 have been included for comparison. Each approach yields similar values for inlet temperature, fuel and excess air requirements for the more refractory VOCs, such as toluene, chlorobenzene and benzene. The single reaction approach may therefore be suitable for preliminary design estimates for species such as these, which could later be re® ned by means of the series reaction approach. The results for ethyl mercapatan and acrolein indicate that preliminary design estimates for VOCs which are relatively easily oxidized should be based on oxidation of intermediate CO. Figures 8 and 9 clearly illustrate how the space time for an easily oxidized VOC is used mainly for CO removal. The wide variation in A and E values 3 and the possibility that mixtures of VOCs may interact during incineration8 therefore strongly suggest kinetic data speci® c Trans IChemE, Vol 76, Part B, November 1998
2
307
K 1.
930 K.
to the stream to be incinerated should ideally be available for ® nal design. The recommended L/di ratios of 2 to 3 are only achieved for a space time which < 0.4 seconds for the VOCs under consideration, so these results are of greater interest. Higher space times, combined with heat losses in excess of 2% may not achieve the required fractional oxidation of VOC and intermediate CO, particularly for the more refractory VOCs such as benzene and chlorobenzene. (This is why Table 6 shows different values for heat generated by VOC oxidation at the different adiabatic space times. Excessive heat loss in the case of benzene incineration may prevent any appreciable oxidation of CO. See **** entries in Table 6.) This result con® rms the desirability of shorter space times and as near as possible to adiabatic conditions for thermal incineration operations. Even at a space time of 0.4 seconds, 10% heat loss increases fuel requirement by 13% for ethyl mercaptan and acrolein, 16% for toluene, 15% for chlorobenzene and 14% for benzene. Auxiliary fuel requirements for a space time of 0.4 seconds vary between 0.05 and 0.06 kmol (kmol dry waste gas) 1 while excess air varies between 90% to 130%. These
0.40 0.43 0.48 0.83
0.0 0.5 1.0 2.0
0.0 0.5 1.0 2.0
t, s, U:
tVOC , s, U:
0.5 1.0 2.0
TMAX , U:
0.995
2.0
0.0 0.5 1.0 2.0
XCO , U:
L/di , U:
2.5 2.7 3.0 5.2
0.8 1.7 4.9
658 138
1048 1043 1034
1053 1047 1040 1018
930 927 925 920
% QLOSS , U: 0.5 1.0 2.0
Fuel heat, kW VOC heat, kW
0.0 0.5 1.0 2.0
TOUT , U:
TIN, INCIN , U: 0.0 0.5 1.0 2.0
0.144 0.050
pO2 , M , bara mFUEL
0.05 0.05 0.05 0.05
1.065
XAIR
5.1 6.6 9.7 10.7
0.983
1.3 3.5 7.5
763 137
1030 1024 1013
1036 1027 1011 983
930 928 925 920
0.05 0.05 0.05 0.05
0.70 0.91 1.34 1.48
0.145 0.059
1.345
Two
Reaction steps
Two
C 2 H6 S
VOC:
n.a. n.a.
n.a.
0.8
924 137
698
700 697
650 649
n.a. n.a.
0.70 0.71
0.178 0.071
5.270
One
2.3 2.6 3.1 6.4
0.995
0.8 1.9 6.0
612 124
1043 1038 1026
1049 1043 1036 1006
930 927 925 920
0.04 0.05 0.05 0.05
0.40 0.45 0.54 1.11
0.145 0.047
0.916
Two
4.7 6.1 9.0 9.9
0.976
1.3 3.6 7.5
701 123
1027 1020 1001
1034 1025 1009 981
930 928 925 920
0.05 0.05 0.05 0.06
0.70 0.92 1.36 1.49
0.146 0.054
1.191
Two
C3 H4 O
n.a. n.a.
n.a.
1.0
1206 124
845
849 844
800 798
n.a. n.a.
0.70 0.75
0.163 0.093
3.168
One
2.5 2.8 3.2 4.4
0.995
0.7 1.4 3.4
659 294
1182 1176 1159
1188 1182 1176 1159
930 927 925 920
0.39 0.44 0.51 0.70
0.40 0.45 0.51 0.70
0.138 0.051
1.043
Two
6.6 7.6 8.9 15.3
0.996
1.0 2.3 7.0
991 294
1095 1085 1053
1103 1095 1085 1050
930 928 925 921
0.68 0.79 0.94 1.61
0.70 0.81 0.95 1.63
0.142 0.076
1.730
Two
C7 H8
n.a. n.a.
n.a.
1.1
1150 294
1062
1070 1062
922 920
n.a. n.a.
0.70 0.80
0.149 0.088
1.976
One
Table 6. Incineration of VOC to CO2 and H2 O, 0.5% VOC in 1 kmol dry waste gas, 0.35 m3 s
980 977 974 969
0.39 0.46 0.55 0.90
0.40 0.47 0.56 0.90
0.136 0.057
0.950
Two
2.7 3.2 3.8 6.2
0.995
0.8 1.7 4.6
744 233
1169 1162 1138
7.3 9.1 11.4 36.2
0.992
1.2 2.9 14.9
1128 232
1101 1089 1024
1110 1101 1088 996
980 977 975 970
0.70 0.85 1.09 3.49
0.70 0.87 1.10 3.49
0.140 0.087
1.565
Two
C6 H5 Cl
n.a. n.a.
n.a.
1.2
1307 234
1084
1093 1084
980 977
n.a. n.a.
0.70 0.85
0.145 0.100
1.728
One
at STP, case study data, T in K.
1176 1169 1162 1138
1
3.1 3.7 4.6 9.1
0.995
0.8 1.7 5.5
830 247
1135 1127 1098
1141 1135 1127 1098
960 957 955 950
0.35 0.43 0.54 1.11
0.40 0.48 0.59 1.18
0.139 0.064
1.268
Two
10.8 12.9 16.4 ****
****
1.2 2.9 ****
1663 247
1054 1044 ****
1062 1054 1042 ****
970 968 965 ****
0.55 0.68 0.90 ****
0.70 0.84 1.06 ****
0.143 0.128
2.013
Two
C 6 H6
n.a. n.a.
n.a.
1.3
1416 247
1047
1056 1047
950 948
n.a. n.a.
0.70 0.91
0.148 0.109
2.004
One
308 O’ REILLY
Trans IChemE, Vol 76, Part B, November 1998
A SERIES REACTION APPROACH TO VOC INCINERATION
Figure 8. Acrolein incineration, TIN
930 K, XAIR
0.916, U
0.0.
® gures depend mainly on the concentration of VOC in the original waste stream, although fuel savings could be achieved by means of recuperative or regenerative designs. The former incur high heat transfer area requirements and possible corrosion problems8 but the procedure outlined in this work may readily be adapted to evaluate potential savings in auxiliary fuel for either strategy. The inlet waste stream and/or combustion air temperature simply needs to be increased to a speci® ed target value. The molar gas ¯ ow in the incineration section must remain essentially constant for the same operating temperature range. Waste or combustion air preheat would therefore require a slight adjustment of combustion air ¯ ow, which the calculation procedure would accommodate. Another strategy for fuel saving is catalytic oxidation8 , which the calculation procedure could easily accommodate if global kinetic expressions of the form of equations (1) and (2) were available. CONCLUSIONS 1. Evaluation of thermal incineration of VOCs by a series
reaction approach is more appropriate than evaluation by means of a single, overall reaction approach because the kinetics of oxidation of intermediate CO may explicitly be considered. Comparison of concentration pro® les for VOC, CO and CO2 vs. space time with available experimental data shows reasonable agreement for VOC and CO2 but more precise data is needed for CO. 2. Oxidation of intermediate CO has been shown to dominate the incineration process for readily oxidized VOCs such as ethyl mercapatan and acrolein, which further vindicates the series reaction approach. Preliminary estimates of incineration requirements may be obtained by means of the single reaction scheme for VOCs such as toluene, benzene and chlorobenzene, which are more dif® cult to oxidize but these estimates should be checked by more detailed calculations using the series approach. The apparent agreement between results obtained by the series and single reaction approaches for the more refractory VOCs rests upon the assumption of 99.5% oxidation of intermediate CO, not 99.99%. The latter conversion cannot be achieved for space times of less than 0.5 seconds unless inlet temperatures are very high 2 . 3. Heat losses from the incinerator have a very signi® cant effect on VOC incineration and auxiliary fuel consumption. Heat losses should if possible be maintained at less than 2% of the heat in from combustion of auxiliary fuel and the VOC itself. With recommended inlet temperatures of 930 to 980 K, space times of 0.4 seconds are required for the VOCs studied in order to achieve recommended L/di ratios of 2 to 34 . Rather high exit temperatures of 1050 to 1190 K are required in order to ensure that 99.5% oxidation of intermediate CO is achieved. 4. Recuperative and/or regenerative heat recovery schemes could be considered in order to offset additional fuel consumption imposed by heat losses or to determine potential fuel savings. The calculation procedure described in this work can easily be adapted to address these schemes. The procedure can also be adapted to evaluate fuel savings by means of catalytic oxidation, if global kinetic expressions are available in the appropriate form.
Figure 9. Acrolein incineration temperature vs. space time, TIN
Trans IChemE, Vol 76, Part B, November 1998
309
930 K.
310
O’ REILLY APPENDIX Incinerator Design Procedure for Series VOC Incineration
Consistent values of T2 and pCO are obtained by iteration. Non adiabatic conditions, with fuel and air at 298.15 K. Equation (12) is easily modi® ed to yield the energy balance for non adiabatic conditions.
Heat Balance, Adiabatic conditions, with fuel and air at 298.15 K. Over the combustion chamber, Figures 1, 2 QFUEL
DHc,0 298 K
mFUEL
mCG CpCG TA
yVOC XVOC, 2
T0 3
mCG a b XAIR mFUEL The data of Table 2 yields a TA
4 11.0597 and b
T0
a
b XAIR CpCG
.
b XAIR CpCG TA
mWASTE CpWASTE TIN
T0
QLOSS, 2 mWASTE
T2
UDA2
T1
U AMIX TIN , INCIN
T0
T1
T00
6
DA1
mWASTE CPWCG TIN
T0
7
Over the incineration section, for the single step reaction scheme mVOC XVOC
DHc,0 TIN
mWASTE yVOC XVOC mFUEL a
´ CpWCG T2 DHc,0 298.15 K
mIN mWASTE CpWCG TIN U AMIX TIN, INCIN
OUT yVOC b XVOC
TIN
8
CpWCG TIN T0 a b XAIR CpWCG TIN
T0
where mIN AMIX DA
9 T2
yVOC XVOC DHc,0 TIN m CpWCG
TIN
10
where T2 is stream temperature for any value of XVOC and mFUEL OUT m 1 yVOC b XVOC a b XAIR . 11 mWASTE For the series reaction scheme, the heat balance over the incineration section becomes mVOC XVOC
2pCO DHc,0 CO, TIN 2PT pCO
mWASTE yVOC XVOC
mFUEL a
yVOC XVOC TIN
OUT yVOC b XVOC CpCWG T2
DHc,0 TIN
m
.
TIN, INCIN
m for XVOC pd02 1 4 4n0
17
T0
K2 2
0. Note that K
4 n0
d0 tMIX di2
d0 Dt . di2
18
19
This gives on rearrangement T1 T0 QLOSS, 1 2 m mWASTE CpWCG . UDA2 2m mWASTE CpWCG UDA2
T2, ADIABATIC T2 1
20
12 Evaluation of Residence Time for Series VOC Incineration: Kinetic Expressions
b XAIR
mWASTE 1 T2
DHc,0 TIN
2pCO DHc,0 CO, TIN 2PT pCO
mWASTE m
T0
Dt is obtained from the kinetic expressions for oxidation of VOC for XVOC # 0.9999 and from the expression for the oxidation of CO2 until XCO $ 0.995. T2, ADIABATIC is T2 in equation (13) and consistent values of T2 and Dt are obtained by iteration. Heat recovery bene® ts may be evaluated by increasing the value of T0 for waste and/or combustion air.
DHc,0 TIN
mWASTE m
16
An arithmetic mean temperature driving force for heat transfer has been found to yield the same numerical values as the more rigorous LMTD. Subscripts 00,1 and 2 refer to successive increments for XVOC . TIN , INCIN is de® ned by energy balance over the mixing section
b XAIR
mWASTE 1
mFUEL mWASTE
DHc,0 TIN
QLOSS, 1 .
2
or, combining combustion and mixing sections, DHc,0 298 K mFUEL a b XAIR
T0
2
15 QLOSS, 1
mFUEL
14
where
5
TIN
2pCO, 2 DHc,0 CO, TIN 2PT pCO, 2
m
TIN
10.0197 and
Over the mixing section, before incineration takes place mFUEL a
m CpWCG T2
QLOSS, 2
DHc,0 298 K
DHc,0 TIN
TIN
2pCO DHc,0 CO, TIN 2PT pCO
m CpWCG 13
Assuming plug ¯ ow, a differential material balance for VOC oxidation yields p rVOC dV 21 n0 VOC, IN dXVOC RTIN Trans IChemE, Vol 76, Part B, November 1998
A SERIES REACTION APPROACH TO VOC INCINERATION rVOC
k0
reaction scheme.
E CVOC RT
A exp
22
pVOC pO2 E exp RT RT 2
For differential and ® nite volume increments, respectively dV dt 23 n0 DV . 24 Dt n0 Second order pre exponential factor means of the approximation k0
AR
T p O2
3,4
k0 is obtained by
.
25
MEAN
The range of temperatures over which the values of A and E have been determined experimentally are 1000°F to 1400°F, or 810 K to 1033 K 3 and 1390°F to 1574°F or 1028 K to 1130 K 4 . Variation in oxygen concentration is 18% to 21% of the VOC stream4 . A simple arithmetic mean for the ratio T/pO2 has been used in this work for the evaluation of k0 . Variation in VOC oxidation rate with temperature over the interval DXVOC is expressed by means of an integral average reaction rate coef® cient kM T2
kM
k0 TIN
1
dpVOC dt
kM, VOC pVOC
pVOC
pVOC, IN exp
26
T2
pVOC
pVOC,IN exp
dpCO dt
rCO
exp
E RT2
exp
E RT1
.
27
28
e voc may be introduced into equations (21) and (22) by means of the expression pVOC, IN
1
XVOC . e VOC XVOC
29
However, the value of e VOC is approximately 10 3 to 10 2 throughout the calculations. It can be neglected without introducing appreciable error into the evaluation of space time t. Therefore Dt
33
A exp
E t . RT
33a
k0 PT exp
E RT
pCO PT
1.5
.
34
dt
TIN dt T
35
an integral average for variation of reaction rate coef® cient with temperature 2 is used
k0 TIN T P0.5 T
kM,CO
1 E exp dT RT T2
T1
,
T2
36
T1
kM pO2 , MEAN .
1
kM,VOC t .
dT
The value of pO2 has been found in this work to vary only slightly between 0.13 and 0.15 bara and a mean value has been used for each evaluation. The reaction rate coef® cient is therefore
pVOC
31
Noting that for non isothermal operation
dT,
k0 TIN E T2 T1
kM, VOC
.
The rate expression used for oxidation4 of CO to CO2 is
T1
kM
2
For isothermal conditions, as in Figures 3, 4, equation (33) becomes
T2
T1
e VOC pVOC pVOC, IN 1 e VOC
Neglecting volumetric expansion, equation (31) becomes d pVOC kM,VOC pVOC 32 dt from which one obtains
E E exp dT RT RT 2
E
311
1 1 ln km,VOC 1
XVOC, 1 . XVOC, 2
30
Equation (30) refers explicitly to the initial oxidation of VOC to CO, not the overall oxidation to CO2 and the rate expressions must be transformed as follows for the series Trans IChemE, Vol 76, Part B, November 1998
k0 RTIN T2 T1 E exp RT 2E T2 T1 P0.5 2 T
kM, CO
exp
E RT1
. 37
The rate of formation of CO according to the series reaction scheme may now be written dpCO dt
mIN k p mOUT M,VOC VOC
c
mOUT
2PT
kM,CO p1.5 CO
2PT m . pCO, OUT
38
39
For isothermal operation, equation (38) becomes, using equation (33a) dpCO dt
c
mIN E A exp p mOUT RT VOC k0 E 1.5 exp pCO . RT P0.5 T
38a
De® ning a term to describe the overall conversion of CO to CO2 , one obtains XCO
1
pCO, OUT mOUT OUT p c XVOC VOC, IN mIN
40
312
O’ REILLY pCO, OUT 1
OUT XVOC
mIN mOUT
2PT 2m 1 XCO c yVOC
41
yVOC P . mIN T 42 The Runge Kutta, or other numerical method, must now be used, giving for XVOC # 0.9999 pCO2 , OUT
pCO t
Dt
pCO2 , IN
pCO t
DpCO Dt
DpCO2 Dt
c DpVOC Dt
pCO2 t
pCO2 t
Dt
OUT XCO c XVOC
DpCO Dt
DpCO2 Dt .
43 44 45
2
Equation (46) may then be used for oxidation of residual CO with assigned values of DpCO , Dt
2 1 0.5 kM, CO pCO, 2
1 . 0.5 pCO, 1
46
kM, CO is replaced by k0 / PT for isothermal evaluation. Iteration of the above expressions is carried out until convergence is achieved for T2 . Equations (40) to (42) are used to check the overall material balance. L/di for the incineration section is obtained from XVOC 0.9999, XCO 0.995
t
Dt XVOC 0
Vi n0
pdi2 L 4n0
47
4n0 t pdi3
48
pCO2 pO2 , pO2 , M PT pVOC QLOSS R t T T1,2 T0 TA TIN TIN, INCIN t tMIX tVOC U Vi v0
XCO XVOC DXVOC OUT XVOC yVOC
where n0
mFUEL mFUEL mCG mVOC mWASTE pCO
XAIR
from which L di
L/di m
0.01559 ´ 1.0115
mIN RTIN . PT
49
ratio of incinerator section length to internal diameter kmol of incinerated waste and combustion gas (kmol humid waste gas) 1 kmol s 1 of auxiliary fuel to process kmol auxiliary fuel (kmol dry waste gas) 1 kmol s 1 of combustion gas kmol s 1 of VOC kmol s 1 of waste gas partial pressure of CO, bara, subscripts IN, OUT are before/ after incineration partial pressure of CO2 , bara, subscripts IN, OUT are before/ after incineration partial pressure of O2 , mean partial pressure of O2 during incineration, bara total pressure, 1.01325 bara partial pressure of VOC, bara, subscript IN is before incineration rate of heat loss from incinerator, kW universal gas constant, kJ (kmol K) 1 or bar m3 (kmol K) 1 time, s temperature, K stream temperature into and out of increment of incinerator volume DV , K standard temperature, 298.15 K adiabatic ¯ ame temperature, K initial temperature of combined stream to incinerator, K initial temperature for incineration, K, identical to TIN for adiabatic operation space time for incineration of VOC and residual CO, s space time in mixing section, s space time for incineration of VOC, s overall heat transfer coef® cient, W m 1 K 1 internal volume of incineration section, m3 volumetric ¯ owrate of combined waste and combustion gas at TIN , m3 s 1 factor for excess combustion air, XAIR 0.0 for stoichiometric air fractional oxidation of CO formed from oxidation of VOC fractional oxidation of VOC increment of fractional oxidation XVOC, 2 XVOC, 1 fractional oxidation of VOC exit incinerator mol. fraction of VOC in original waste gas 0.005 in DRY air
REFERENCES NOMENCLATURE a, b A AMIX DA b Cp CG Cp, WASTE Cp, WCG CVOC c di , d0 e VOC DH 0 C 298 K DH 0 C, TIN E K k0 kM, CO kM, VOC
stoichiometric coef® cients for fuel combustion, dependent on fuel composition pseudo ® rst order pre exponential factor, s 1 outside area of mixing section of incinerator including dished end, m2 increment of outside area of incineration section of incinerator, m2 mol change in VOC incineration reaction, (kmol VOC oxidized) 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 kmol CO formed (kmol VOC oxidized) 1 inside, outside diameter of incinerator respectively, 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 activation energy, kJ kmol 1 or bar m3 kmol 1 ratio of depth to radius for dished end, value 0.5 pre exponential factor, m3 kmol 1 s 1 for VOC, mol fraction 0.5 s 1 for CO reaction rate coef® cient over increment DpCO , (bar mol fraction) 0.5 s 1 reaction rate coef® cient over increment of oxidation DXVOC , s 1
1. O’ Reilly, A., 1997, Estimation of residence time in VOC incineration, Trans IChemE, Part B, 75(B1): 33±42. 2. O’ Reilly, A., 1998, Estimation of residence time in CO incineration, Trans IChemE, Part B, 76(B2): 166±176. 3. Lee, K. C., Morgan, N., Hansen, J. L. and Whipple, G. M., 1982, Revised model for the prediction of the time-temperature requirements for thermal destruction of dilute organic vapours and its usage for predicting compound destructibility, 75th Ann Meet Air Pollution Control Association, New Orleans, Louisiana, June 20 to 25. 4. Hemsath, K. H. and Susey, P. E., 1974, Fume incineration kinetics and its applications, Recent Advances in Air Pollution Control, AIChE Symposium, No. 137, 70: 439±449. 5. Valentin, F. H. H. and North, A.. A., 1980, Odour Control, A Concise Guide, (D.O.E. Warren Spring Laboratory), 51±60. 6. Ledbetter, J. O., 1974, Air Pollution, Part B: Prevention and Control, (Dekker Inc.) 138. 7. Barnes, R. H. et al, 1979, Chemical aspects of afterburner systems, US Environmental Protection Agency, Report No. EPA 600/7 79 096, (US Government Printing Of® ce, Washington, DC), 4. 8. De Nevers, N., 1995, Air Pollution Control Engineering, (McGraw Hill, Inc.) 300.
ADDRESS Correspondence concerning this paper should be addressed to Dr A. O’ Reilly, Chemical Engineering, University of Teesside, Middlesbrough, TS1 3BA, UK. The manuscript was received 17 February 1998 and accepted for publication after revision 9 September 1998.
Trans IChemE, Vol 76, Part B, November 1998
A SERIES REACTION APPROACH TO VOC INCINERATION A. O’REILLY Chemical Engineering, University of Teesside, Middlesbrough, UK
T
hermal incineration of VOCs has been evaluated by means of a two step series reaction approach, VOC CO CO2 , so that the rates of VOC and CO oxidation may both be considered. Temperature and concentration pro® les have been obtained for oxidation of several VOCs as functions of space time t. Appropriate conditions for 99.99% removal of VOC and 99.5% oxidation of intermediate CO appear to be 930 to 980 K inlet temperature, 0.05 to 0.06 kmol auxiliary fuel per kmol dry waste air and 90 to 130 % excess air for auxiliary fuel, without heat recovery. These conditions yield approximately 0.4 seconds space time and an L/di ratio for the oxidation section of the incinerator of approximately 2 to 3. Heat loss from the incinerator should be less than 2% of the total heat from auxiliary fuel and VOC combustion and incinerator design should therefore be optimized with respect to fuel and insulation costs. The approach outlined in this work can easily be adapted to estimate auxiliary fuel savings via heat recovery. Keywords: atmospheric pollution; VOCs; CO; thermal incineration.
INTRODUCTION
facility and those obtained in practice from a 36 i.d. full scale reactor. These results show the bene® t of careful mixer design and have been discussed elsewhere1,2 . The calculation procedure described in this work has therefore assumed that at least comparable mixer designs are available. The proposed two step series reaction scheme VOC CO CO2 is itself greatly simpli® ed. VOC and CO oxidation consist of many individual steps, the most important of which for CO is CO + OH CO2 + H 7 . The OH reaction step could be included in the calculations but use of global kinetics should be suf® cient for design purposes. Predicted results should be compared with experimental data and the kinetics then re® ned if necessary, possibly by inclusion of the CO + OH reaction. Although the data sources for the kinetics3,4,7 appear somewhat dated, De Nevers’ recent review of VOC incineration8 indicates that they are de® nitive. Moreover, Barnes7 explicitly recommends Hemsath and Susey’ s data for CO oxidation for afterburner design. The Appendix sets out the design procedure proposed in this work, which may be summarized as follows.
Thermal incineration of VOCs and CO has been evaluated for the individual species1,2 by means of the following reaction rate expressions3,4 . dCVOC dt d pco /PT dt
A exp
E CVOC RT
k0 exp
E RT
pco /PT n , n
1 1.5
2
Thermal incineration of VOCs must ensure that intermediate CO is also oxidized to high conversion before release of the waste gas into the atmosphere. However, the rate of oxidation of CO to CO2 may be appreciably slower than its rate of formation from VOC 4 , especially if at least 99.5% removal of intermediate CO is required 2 . It is therefore preferable to evaluate the oxidation of both VOC and CO according to a series reaction of the form VOC CO CO2 , rather than as a single overall reaction VOC CO2 . This work describes such a procedure.
1. Initialization of the temperature pro® le in the oxidation section, equation (10). 2. Initialization of space time t pro® le for VOC oxidation to CO, equations (27)±(30). 3. Initialization of concentration pro® le for VOC as partial pressure pVOC , equation (30). 4. Initialization of concentration pro® les for CO, CO2 as partial pressures pCO , pCO2 , equations (37) to (39).
VOC INCINERATION AS A SERIES REACTION, VOC CO CO2 Figures 1 and 2 show respectively the thermal incineration process schematically and a simpli® ed incinerator cross section. Table 1 summarizes typical operating data5 8 . Barnes 7 indicates further that oxidation sections of incinerators typically have L/di ratios of approximately 2 to 3. Hemsath and Susey4 and Carleton5 have shown how mixer design must achieve as near to plug ¯ ow as possible for maximum conversion of VOC. Hemsath and Susey have reported very good agreement between the results for incineration of toluene predicted from their small scale test
Equation (38) cannot be solved analytically and the Runge Kutta method has been used for XVOC # 0.9999. See equations (43) to (45). 5. Iteration to determine consistent values for temperature 302
A SERIES REACTION APPROACH TO VOC INCINERATION
303
estimated 1,2 and the other values are assumed. The incinerator is assumed to have an i.d. of 0.8 m, a wall thickness of 0.0125 m and a layer of insulation brick of thickness 0.3125 m. tMIX 0.3 s for all calculations5 . Incremental step sizes for the calculations have been chosen as follows:
Figure 1. VOC incineration, schematic ¯ owsheet.
For 0.0 # XVOC # 0.9999 Range of XVOC DXVOC 0.0000 to 0.9000 0.0500 0.9000 to 0.9950 0.0050 0.9950 to 0.9990 0.0020 0.9990 to 0.9995 0.0005, single step 0.9995 to 0.9999 0.0004, single step
For XCO when XVOC 0.9999 Range of ppm CO Dppm CO 0 to 40 1 to 5 40 to 100 5 to 10 100 to 500 10 to 50 500 to 5000 50 to 200
Reducing the above step sizes for DXVOC to 0.0250, 0.0025, 0.0010, 0.00025, 0.0002, respectively has little effect on the magnitude of VOC incineration space time t. COMPARISON WITH AVAILABLE EXPERIMENTAL DATA
Figure 2. VOC incineration vessel, schematic diagram.
pro® les for adiabatic operation, equation (13) and non adiabatic operation, equation (20). 6. Iteration to determine consistent values for concentration pro® les for XVOC # 0.9999. 7. Iteration using equations (13), (20) to (40) and (46) for residual pCO pro® le until XCO $ 0.995. Table 2 gives the basic data for the calculations and Table 3 summarizes thermodynamic and kinetic data for each of the VOCs considered for the calculations. U values of 0.5, 1.0 and 2.0 W m 2 K 1 have been used for evaluation of non adiabatic conditions in order to determine the effect of signi® cantly increasing heat loss on incinerator design and operation. The value of U 0.5 W m 2 K 1 has been Table 1. Operating conditions for industrial gas incinerators. Actual gas velocity, m s 1 Incineration time, s Temperatures: Odour control Hydrocarbons CO
7.5 to 15.0 0.2 to 1.0 750 to 1000 K 750 to 920 K 920 to 1075 K
Trans IChemE, Vol 76, Part B, November 1998
The calculated results have been compared with the experimental data of Hemsath and Susey4 for incineration of toluene and hexane. Figures 3 and 4 reproduce their concentration pro® les against space time and show the calculated concentration pro® les. Two calculated pro® les have been generated for toluene. One uses the values for the kinetic parameters as reported. The other introduces a correction factor for the deviation of the particular experimental data point from the Arrhenius regression line, which is signi® cant in this case. Hemsath and Susey indicated that the temperature rise in their test facility during VOC incineration was approximately 20°F. It has therefore been assumed for these calculations that the temperature ranges during incineration were 1440°F to 1460°F or 1067 K for hexane and 1505°F to 1525°F or 1103 K for toluene. The calculations have then been carried out by assigning two constant temperature intervals of equal magnitude for the incineration of each of the VOCs. This approximation gives mean temperatures for use in equations (33a) and (38a) of 1445°F and 1455°F for hexane and 1510°F and 1520°F for toluene. This simpli® cation accounts for the slight discontinuity in the calculated pro® le for ppm hexane at a space time of 0.06 seconds. Equation (46), slightly modi® ed, has been used for overall estimates of CO conversion and corresponding space times. See Table 4. Agreement between the experimental and calculated concentration pro® les is good for each of the VOCs if the correction factor is used for toluene, see above. Although trends are similar, agreement between the experimental and calculated concentration pro® les for CO is poor. (The former suggest some initial CO contamination of the waste air used in the experiments.) However, the experimental data shows a considerably greater increase in CO concentration than would be possible from stoichiometry. The maximum increase in CO concentration which could be achieved by conversion of the inlet toluene is 1400 ppm. Figure 3 shows an increase of almost 2000 ppm. Figure 4 shows an equivalent increase in CO concentration of 1500 ppm, whereas stoichiometry predicts a maximum increase of 1200 ppm. It is this apparent discrepancy which also yields poor agreement between the initial segments of the experimental and calculated pro® les for CO2 concentration,
304
O’ REILLY Table 2. Incineration of VOC to CO2 and H2 O, basic data. Waste gas stream containing VOC Total dry volumetric ¯ owrate at STP Initial temperature of fuel, air, waste, T0 Required VOC removal by incineration Auxiliary fuel Composition DH°c 298.15 K for natural gas CO oxidation reaction Kinetic data for CO combustion: T # 1041 K T $ 1041 K
0.5% VOC in dry air, with 0.0116kmol H2 O (kmol dry air) 0.35 m3 s 1 , 0.01559kmol s 1 298.15 K 99.99%, plus 99.5% of CO formed from VOC Natural gas CH4 90%, C2 H6 8%, CO2 1%, N2 1% 0.83631 GJ kmol 1 CO 0.5O2 CO2 k0 , mol fraction 0.5 s 1 E, 105 kJ mol 1 24 1.0 ´ 10 4.1846 2.5 ´ 1011 1.6747
Table 3. Incineration of VOC to CO2 and H2 O, 0.5% VOC in 1 kmol dry waste gas, 0.35m3 s Ethyl mercaptan C2 H6 S
VOC: Overall reaction TIN , K DH°c TIN , to CO2 GJ kmol 1 DH°c TIN , to CO GJ kmol 1 DH°c TIN , CO to CO2 , GJ kmol A, s 1 k0 , m3 kmol
1
E, 105 kJ kmol
1
s 1
1
C2 H6 S + 4.5O2 2CO2 + 3H2 O +SO2 930.0 1.7632 1.1972 0.2830 5.20 ´ 105
Acrolein C3 H4 O C3 H4 O + 3.5O2 3CO2 + 2H2 O 930.0 1.5933 0.7443 0.2830 3.30 ´ 1010
1.9701 ´ 108
1.2502 ´ 1013
0.6155
1.5031
although the agreement improves as ® nal CO2 concentration is approached. The results for toluene incineration are 3250 ppm and 2380 ppm respectively. The equivalent results for hexane incineration are 2400 ppm and 2600 ppm respectively. Agreement between experimental and predicted results for conversion of residual CO to CO2 is nevertheless good. Table 4 shows that the error between experimental and calculated values for space time increment Dt is 6 14% but the error for CO conversion is only 6 4%. (The error obtained using the calculated CO concentration pro® le arises from the effect of residual VOC incineration.)
1
1
at STP, thermodynamic and kinetic data.
Toluene C7 H8
Chlorobenzene Cl C6 H5
Benzene C6 H6
C7 H8 + 9O2 7CO2 + 4H2 O 930.0 3.7763 1.7952 0.2830 2.28 ´ 1013 (Ref 3) 6.56 ´ 1013 (Ref 4) 2.9612 ´ 1016 0.8638 ´ 1016 2.3655 (Ref 3) 2.4493 (Ref 4)
C6 H5 Cl + 7O2 6CO2 + 2H2 O + HCl 980.0 2.9889 1.2910 0.2829 1.34 ´ 1017
C6 H6 + 7.5O2 6CO2 + 3H2 O 960.0 970.0 3.1696 3.1696 1.4716 1.4716 0.2829 0.2829 7.43 ´ 1021
5.0767 ´ 1019
2.8149 ´ 1024
3.2071
4.0151
Hemsath and Susey’ s kinetic data for CO oxidation should therefore be appropriate for the incinerator design procedure proposed in this work. CALCULATED RESULTS FOR TOLUENE INCINERATION, SERIES REACTION APPROACH Table 5 and Figures 5 to 7 show the results for evaluation of toluene incineration by means of the series reaction approach, with kinetic parameters from both Lee et al.3 and Hemsath and Susey4 . Results from the single reaction approach1 serve as a basis for comparison.
Figure 3. ppm toluene CO CO2 during incineration, 1525°F, 1102.6K exit temperature.
Trans IChemE, Vol 76, Part B, November 1998
A SERIES REACTION APPROACH TO VOC INCINERATION
305
Figure 4. ppm hexane CO CO2 during incineration, 1460°F, 1066.5K exit temperature.
Table 4. Incineration of CO from VOC to CO2 , 0.18 # Exit Temperature: VOC A, s 1 E, 105 kJ kmol 1 Results, XVOC < 1.0: Estimated mean T, K ppm CO in ppm CO out XCO t Observed, s t Calculated, s % Error for t ppm CO out by t Observed XCO by t Observed % Error for XCO
1460 °F
1460 °F
C6 H14 4.51 ´ 1012 2.1981 Experiment Series 1063.7 1063.7 1000.0 314.2 306.0 116.5 0.694 0.629 0.031 0.050 0.034 0.049 10.0 2.0 333.3 113.8 0.667 0.638 3.9 1.4
On this basis, the series reaction approach shows that appreciably higher exit temperatures are required in order to ensure that the required conversion of CO is achieved. The inlet temperature has therefore had to be increased slightly from 922 K to 930 K and the excess air requirements must be decreased from 2.0 to 1.8. The fuel requirement decreases slightly from 0.09 to 0.08 kmol (kmol dry waste) 1 in order to satisfy energy balance. Space time and in turn L/di ratio for the oxidation section increase rapidly as the heat loss from the incinerator exceeds approximately 2% of the total heat from fuel and VOC combustion. This suggests that incinerator design should aim to achieve as near to adiabatic conditions as possible and that the allowance of 10% heat loss6 is rather generous. Consider the third set of results in Table 5, for which the space time is 0.4 seconds under adiabatic conditions. Using the data of Table 2, it may be shown that the total heat in is 1037 kW, of which 294 kW is from the combustion of VOC and intermediate CO. If one assumes a heat loss of 10%, the total heat into the incinerator must be increased to 1152 kW, of which 858 kW must be provided by fuel, which is an Trans IChemE, Vol 76, Part B, November 1998
PO2 , M # PT
0.21.
1525 °F
1525 °F
C 7 H8 6.56 ´ 1013 2.4493 Experiment Series 1099.8 1099.8 838.1 102.3 98.0 35.3 0.883 0.655 0.056 0.050 0.048 0.050 14.3 0.0 79.1 35.2 0.906 0.655 2.5 0.02
increase in auxiliary fuel requirement of 15.5%. Most of this increase could be avoided by incineration at as close to adiabatic conditions as possible. There is clearly an incentive for optimization with respect to minimization of fuel, vessel and insulation costs. The results for the evaluation of L/di ratio show that the range of values recommended by established practice 7 are achieved for toluene incineration only by space times of 0.4 seconds or less. Similar results for toluene incineration have been obtained from each set of kinetic parameters, although those of Hemsath and Susey4 have yielded a lower space time than those of Lee et al.3 for the same quantities of fuel and excess air. Nevertheless, the agreement should be considered satisfactory, given that the data sources are completely independent. Lee et al. have provided kinetic parameters A and E for a wide range of VOCs which have therefore been used in the subsequent calculations, together with Hemsath and Susey’ s parameters for CO oxidation. Inlet temperature for incineration has been determined using the data of Table 1 as a guide. Table 5 reveals that
306
O’ REILLY Table 5. Incineration of toluene to CO2 and H2 O, calculated results. Reaction steps XAIR mFUEL pO2 M , bara t, s, U:
One4
Two4
Two4
Two3
0.0 0.5 1.0 2.0
2.036 0.093 0.147 0.62 0.70 n.a. n.a.
1.800 0.080 0.143 0.66 0.75 0.88 1.44
1.275 0.057 0.139 0.40 0.45 0.51 0.70
1.800 0.080 0.143 0.76 0.88 1.05 1.96
0.0 0.5 1.0 2.0
n.a. n.a. n.a. n.a.
0.62 0.71 0.84 1.40
0.38 0.43 0.50 0.68
0.74 0.86 1.03 1.92
TIN INCIN , U: 0.0 0.5 1.0 2.0
922 920 n.a. n.a.
930 928 925 921
930 928 925 920
930 928 925 921
1062 1055 n.a. n.a.
1094 1086 1077 1046
1159 1154 1147 1131
1094 1086 1075 1032
tVOC , s, U:
TOUT , U:
0.0 0.5 1.0 2.0
% QLOSS , U: 0.5 1.0 2.0
1.0 n.a. n.a.
1.0 2.2 6.4
0.7 1.5 3.5
1.1 2.5 8.3
L/di , U:
n.a. n.a. n.a. n.a.
6.5 7.4 8.8 14.2
2.8 3.2 3.6 4.9
7.5 8.7 10.3 19.4
0.0 0.5 1.0 2.0
Figure 5. Toluene incineration, TIN
for the chosen value of 930 K, toluene is one of the more refractory VOCs, requiring 95% of the total space time for 99.99% removal. Figures 5 to 7 show this feature very clearly. There is an extended lag time corresponding to slow conversion of both VOC and intermediate CO until a temperature is achieved at which the rates of both main reactions increase rapidly, yielding a rapid rise in both CO2 concentration and in stream temperature. Figure 7 displays the S shaped curves characteristic of this type of kinetic behaviour. Stream temperature begins to increase sharply at
930 K, XAIR
1.275, U
0.0.
approximately 1020 K, close to the temperature of 1045 K which was found to be suitable for incineration of CO in isolation 2 . Figure 7 also shows the very noticeable effect of increasing heat loss on space time. Figures 5 and 6 show a slight discontinuity in the concentration pro® le for CO which occurs just after the maximum. This arises from the change in the magnitude of the kinetic parameters for CO oxidation which is assumed for simplicity to take place at a single temperature of 1041 K, although the transition is actually more gradual. Trans IChemE, Vol 76, Part B, November 1998
A SERIES REACTION APPROACH TO VOC INCINERATION
Figure 6. Toluene incineration, TIN
930 K, XAIR
1.275, U
2.0 W m
Figure 7. Toluene incineration temperature vs. space time, TIN
CALCULATED RESULTS FOR INCINERATION OF VOCS, SERIES VS. SINGLE REACTION SCHEMES Table 6 shows the results for the series reaction approach to incineration of several VOCs for space times of 0.4 and 0.7 seconds under adiabatic conditions. Results for the single reaction approach for a space time of 0.7 seconds under adiabatic conditions 1 have been included for comparison. Each approach yields similar values for inlet temperature, fuel and excess air requirements for the more refractory VOCs, such as toluene, chlorobenzene and benzene. The single reaction approach may therefore be suitable for preliminary design estimates for species such as these, which could later be re® ned by means of the series reaction approach. The results for ethyl mercapatan and acrolein indicate that preliminary design estimates for VOCs which are relatively easily oxidized should be based on oxidation of intermediate CO. Figures 8 and 9 clearly illustrate how the space time for an easily oxidized VOC is used mainly for CO removal. The wide variation in A and E values 3 and the possibility that mixtures of VOCs may interact during incineration8 therefore strongly suggest kinetic data speci® c Trans IChemE, Vol 76, Part B, November 1998
2
307
K 1.
930 K.
to the stream to be incinerated should ideally be available for ® nal design. The recommended L/di ratios of 2 to 3 are only achieved for a space time which < 0.4 seconds for the VOCs under consideration, so these results are of greater interest. Higher space times, combined with heat losses in excess of 2% may not achieve the required fractional oxidation of VOC and intermediate CO, particularly for the more refractory VOCs such as benzene and chlorobenzene. (This is why Table 6 shows different values for heat generated by VOC oxidation at the different adiabatic space times. Excessive heat loss in the case of benzene incineration may prevent any appreciable oxidation of CO. See **** entries in Table 6.) This result con® rms the desirability of shorter space times and as near as possible to adiabatic conditions for thermal incineration operations. Even at a space time of 0.4 seconds, 10% heat loss increases fuel requirement by 13% for ethyl mercaptan and acrolein, 16% for toluene, 15% for chlorobenzene and 14% for benzene. Auxiliary fuel requirements for a space time of 0.4 seconds vary between 0.05 and 0.06 kmol (kmol dry waste gas) 1 while excess air varies between 90% to 130%. These
0.40 0.43 0.48 0.83
0.0 0.5 1.0 2.0
0.0 0.5 1.0 2.0
t, s, U:
tVOC , s, U:
0.5 1.0 2.0
TMAX , U:
0.995
2.0
0.0 0.5 1.0 2.0
XCO , U:
L/di , U:
2.5 2.7 3.0 5.2
0.8 1.7 4.9
658 138
1048 1043 1034
1053 1047 1040 1018
930 927 925 920
% QLOSS , U: 0.5 1.0 2.0
Fuel heat, kW VOC heat, kW
0.0 0.5 1.0 2.0
TOUT , U:
TIN, INCIN , U: 0.0 0.5 1.0 2.0
0.144 0.050
pO2 , M , bara mFUEL
0.05 0.05 0.05 0.05
1.065
XAIR
5.1 6.6 9.7 10.7
0.983
1.3 3.5 7.5
763 137
1030 1024 1013
1036 1027 1011 983
930 928 925 920
0.05 0.05 0.05 0.05
0.70 0.91 1.34 1.48
0.145 0.059
1.345
Two
Reaction steps
Two
C 2 H6 S
VOC:
n.a. n.a.
n.a.
0.8
924 137
698
700 697
650 649
n.a. n.a.
0.70 0.71
0.178 0.071
5.270
One
2.3 2.6 3.1 6.4
0.995
0.8 1.9 6.0
612 124
1043 1038 1026
1049 1043 1036 1006
930 927 925 920
0.04 0.05 0.05 0.05
0.40 0.45 0.54 1.11
0.145 0.047
0.916
Two
4.7 6.1 9.0 9.9
0.976
1.3 3.6 7.5
701 123
1027 1020 1001
1034 1025 1009 981
930 928 925 920
0.05 0.05 0.05 0.06
0.70 0.92 1.36 1.49
0.146 0.054
1.191
Two
C3 H4 O
n.a. n.a.
n.a.
1.0
1206 124
845
849 844
800 798
n.a. n.a.
0.70 0.75
0.163 0.093
3.168
One
2.5 2.8 3.2 4.4
0.995
0.7 1.4 3.4
659 294
1182 1176 1159
1188 1182 1176 1159
930 927 925 920
0.39 0.44 0.51 0.70
0.40 0.45 0.51 0.70
0.138 0.051
1.043
Two
6.6 7.6 8.9 15.3
0.996
1.0 2.3 7.0
991 294
1095 1085 1053
1103 1095 1085 1050
930 928 925 921
0.68 0.79 0.94 1.61
0.70 0.81 0.95 1.63
0.142 0.076
1.730
Two
C7 H8
n.a. n.a.
n.a.
1.1
1150 294
1062
1070 1062
922 920
n.a. n.a.
0.70 0.80
0.149 0.088
1.976
One
Table 6. Incineration of VOC to CO2 and H2 O, 0.5% VOC in 1 kmol dry waste gas, 0.35 m3 s
980 977 974 969
0.39 0.46 0.55 0.90
0.40 0.47 0.56 0.90
0.136 0.057
0.950
Two
2.7 3.2 3.8 6.2
0.995
0.8 1.7 4.6
744 233
1169 1162 1138
7.3 9.1 11.4 36.2
0.992
1.2 2.9 14.9
1128 232
1101 1089 1024
1110 1101 1088 996
980 977 975 970
0.70 0.85 1.09 3.49
0.70 0.87 1.10 3.49
0.140 0.087
1.565
Two
C6 H5 Cl
n.a. n.a.
n.a.
1.2
1307 234
1084
1093 1084
980 977
n.a. n.a.
0.70 0.85
0.145 0.100
1.728
One
at STP, case study data, T in K.
1176 1169 1162 1138
1
3.1 3.7 4.6 9.1
0.995
0.8 1.7 5.5
830 247
1135 1127 1098
1141 1135 1127 1098
960 957 955 950
0.35 0.43 0.54 1.11
0.40 0.48 0.59 1.18
0.139 0.064
1.268
Two
10.8 12.9 16.4 ****
****
1.2 2.9 ****
1663 247
1054 1044 ****
1062 1054 1042 ****
970 968 965 ****
0.55 0.68 0.90 ****
0.70 0.84 1.06 ****
0.143 0.128
2.013
Two
C 6 H6
n.a. n.a.
n.a.
1.3
1416 247
1047
1056 1047
950 948
n.a. n.a.
0.70 0.91
0.148 0.109
2.004
One
308 O’ REILLY
Trans IChemE, Vol 76, Part B, November 1998
A SERIES REACTION APPROACH TO VOC INCINERATION
Figure 8. Acrolein incineration, TIN
930 K, XAIR
0.916, U
0.0.
® gures depend mainly on the concentration of VOC in the original waste stream, although fuel savings could be achieved by means of recuperative or regenerative designs. The former incur high heat transfer area requirements and possible corrosion problems8 but the procedure outlined in this work may readily be adapted to evaluate potential savings in auxiliary fuel for either strategy. The inlet waste stream and/or combustion air temperature simply needs to be increased to a speci® ed target value. The molar gas ¯ ow in the incineration section must remain essentially constant for the same operating temperature range. Waste or combustion air preheat would therefore require a slight adjustment of combustion air ¯ ow, which the calculation procedure would accommodate. Another strategy for fuel saving is catalytic oxidation8 , which the calculation procedure could easily accommodate if global kinetic expressions of the form of equations (1) and (2) were available. CONCLUSIONS 1. Evaluation of thermal incineration of VOCs by a series
reaction approach is more appropriate than evaluation by means of a single, overall reaction approach because the kinetics of oxidation of intermediate CO may explicitly be considered. Comparison of concentration pro® les for VOC, CO and CO2 vs. space time with available experimental data shows reasonable agreement for VOC and CO2 but more precise data is needed for CO. 2. Oxidation of intermediate CO has been shown to dominate the incineration process for readily oxidized VOCs such as ethyl mercapatan and acrolein, which further vindicates the series reaction approach. Preliminary estimates of incineration requirements may be obtained by means of the single reaction scheme for VOCs such as toluene, benzene and chlorobenzene, which are more dif® cult to oxidize but these estimates should be checked by more detailed calculations using the series approach. The apparent agreement between results obtained by the series and single reaction approaches for the more refractory VOCs rests upon the assumption of 99.5% oxidation of intermediate CO, not 99.99%. The latter conversion cannot be achieved for space times of less than 0.5 seconds unless inlet temperatures are very high 2 . 3. Heat losses from the incinerator have a very signi® cant effect on VOC incineration and auxiliary fuel consumption. Heat losses should if possible be maintained at less than 2% of the heat in from combustion of auxiliary fuel and the VOC itself. With recommended inlet temperatures of 930 to 980 K, space times of 0.4 seconds are required for the VOCs studied in order to achieve recommended L/di ratios of 2 to 34 . Rather high exit temperatures of 1050 to 1190 K are required in order to ensure that 99.5% oxidation of intermediate CO is achieved. 4. Recuperative and/or regenerative heat recovery schemes could be considered in order to offset additional fuel consumption imposed by heat losses or to determine potential fuel savings. The calculation procedure described in this work can easily be adapted to address these schemes. The procedure can also be adapted to evaluate fuel savings by means of catalytic oxidation, if global kinetic expressions are available in the appropriate form.
Figure 9. Acrolein incineration temperature vs. space time, TIN
Trans IChemE, Vol 76, Part B, November 1998
309
930 K.
310
O’ REILLY APPENDIX Incinerator Design Procedure for Series VOC Incineration
Consistent values of T2 and pCO are obtained by iteration. Non adiabatic conditions, with fuel and air at 298.15 K. Equation (12) is easily modi® ed to yield the energy balance for non adiabatic conditions.
Heat Balance, Adiabatic conditions, with fuel and air at 298.15 K. Over the combustion chamber, Figures 1, 2 QFUEL
DHc,0 298 K
mFUEL
mCG CpCG TA
yVOC XVOC, 2
T0 3
mCG a b XAIR mFUEL The data of Table 2 yields a TA
4 11.0597 and b
T0
a
b XAIR CpCG
.
b XAIR CpCG TA
mWASTE CpWASTE TIN
T0
QLOSS, 2 mWASTE
T2
UDA2
T1
U AMIX TIN , INCIN
T0
T1
T00
6
DA1
mWASTE CPWCG TIN
T0
7
Over the incineration section, for the single step reaction scheme mVOC XVOC
DHc,0 TIN
mWASTE yVOC XVOC mFUEL a
´ CpWCG T2 DHc,0 298.15 K
mIN mWASTE CpWCG TIN U AMIX TIN, INCIN
OUT yVOC b XVOC
TIN
8
CpWCG TIN T0 a b XAIR CpWCG TIN
T0
where mIN AMIX DA
9 T2
yVOC XVOC DHc,0 TIN m CpWCG
TIN
10
where T2 is stream temperature for any value of XVOC and mFUEL OUT m 1 yVOC b XVOC a b XAIR . 11 mWASTE For the series reaction scheme, the heat balance over the incineration section becomes mVOC XVOC
2pCO DHc,0 CO, TIN 2PT pCO
mWASTE yVOC XVOC
mFUEL a
yVOC XVOC TIN
OUT yVOC b XVOC CpCWG T2
DHc,0 TIN
m
.
TIN, INCIN
m for XVOC pd02 1 4 4n0
17
T0
K2 2
0. Note that K
4 n0
d0 tMIX di2
d0 Dt . di2
18
19
This gives on rearrangement T1 T0 QLOSS, 1 2 m mWASTE CpWCG . UDA2 2m mWASTE CpWCG UDA2
T2, ADIABATIC T2 1
20
12 Evaluation of Residence Time for Series VOC Incineration: Kinetic Expressions
b XAIR
mWASTE 1 T2
DHc,0 TIN
2pCO DHc,0 CO, TIN 2PT pCO
mWASTE m
T0
Dt is obtained from the kinetic expressions for oxidation of VOC for XVOC # 0.9999 and from the expression for the oxidation of CO2 until XCO $ 0.995. T2, ADIABATIC is T2 in equation (13) and consistent values of T2 and Dt are obtained by iteration. Heat recovery bene® ts may be evaluated by increasing the value of T0 for waste and/or combustion air.
DHc,0 TIN
mWASTE m
16
An arithmetic mean temperature driving force for heat transfer has been found to yield the same numerical values as the more rigorous LMTD. Subscripts 00,1 and 2 refer to successive increments for XVOC . TIN , INCIN is de® ned by energy balance over the mixing section
b XAIR
mWASTE 1
mFUEL mWASTE
DHc,0 TIN
QLOSS, 1 .
2
or, combining combustion and mixing sections, DHc,0 298 K mFUEL a b XAIR
T0
2
15 QLOSS, 1
mFUEL
14
where
5
TIN
2pCO, 2 DHc,0 CO, TIN 2PT pCO, 2
m
TIN
10.0197 and
Over the mixing section, before incineration takes place mFUEL a
m CpWCG T2
QLOSS, 2
DHc,0 298 K
DHc,0 TIN
TIN
2pCO DHc,0 CO, TIN 2PT pCO
m CpWCG 13
Assuming plug ¯ ow, a differential material balance for VOC oxidation yields p rVOC dV 21 n0 VOC, IN dXVOC RTIN Trans IChemE, Vol 76, Part B, November 1998
A SERIES REACTION APPROACH TO VOC INCINERATION rVOC
k0
reaction scheme.
E CVOC RT
A exp
22
pVOC pO2 E exp RT RT 2
For differential and ® nite volume increments, respectively dV dt 23 n0 DV . 24 Dt n0 Second order pre exponential factor means of the approximation k0
AR
T p O2
3,4
k0 is obtained by
.
25
MEAN
The range of temperatures over which the values of A and E have been determined experimentally are 1000°F to 1400°F, or 810 K to 1033 K 3 and 1390°F to 1574°F or 1028 K to 1130 K 4 . Variation in oxygen concentration is 18% to 21% of the VOC stream4 . A simple arithmetic mean for the ratio T/pO2 has been used in this work for the evaluation of k0 . Variation in VOC oxidation rate with temperature over the interval DXVOC is expressed by means of an integral average reaction rate coef® cient kM T2
kM
k0 TIN
1
dpVOC dt
kM, VOC pVOC
pVOC
pVOC, IN exp
26
T2
pVOC
pVOC,IN exp
dpCO dt
rCO
exp
E RT2
exp
E RT1
.
27
28
e voc may be introduced into equations (21) and (22) by means of the expression pVOC, IN
1
XVOC . e VOC XVOC
29
However, the value of e VOC is approximately 10 3 to 10 2 throughout the calculations. It can be neglected without introducing appreciable error into the evaluation of space time t. Therefore Dt
33
A exp
E t . RT
33a
k0 PT exp
E RT
pCO PT
1.5
.
34
dt
TIN dt T
35
an integral average for variation of reaction rate coef® cient with temperature 2 is used
k0 TIN T P0.5 T
kM,CO
1 E exp dT RT T2
T1
,
T2
36
T1
kM pO2 , MEAN .
1
kM,VOC t .
dT
The value of pO2 has been found in this work to vary only slightly between 0.13 and 0.15 bara and a mean value has been used for each evaluation. The reaction rate coef® cient is therefore
pVOC
31
Noting that for non isothermal operation
dT,
k0 TIN E T2 T1
kM, VOC
.
The rate expression used for oxidation4 of CO to CO2 is
T1
kM
2
For isothermal conditions, as in Figures 3, 4, equation (33) becomes
T2
T1
e VOC pVOC pVOC, IN 1 e VOC
Neglecting volumetric expansion, equation (31) becomes d pVOC kM,VOC pVOC 32 dt from which one obtains
E E exp dT RT RT 2
E
311
1 1 ln km,VOC 1
XVOC, 1 . XVOC, 2
30
Equation (30) refers explicitly to the initial oxidation of VOC to CO, not the overall oxidation to CO2 and the rate expressions must be transformed as follows for the series Trans IChemE, Vol 76, Part B, November 1998
k0 RTIN T2 T1 E exp RT 2E T2 T1 P0.5 2 T
kM, CO
exp
E RT1
. 37
The rate of formation of CO according to the series reaction scheme may now be written dpCO dt
mIN k p mOUT M,VOC VOC
c
mOUT
2PT
kM,CO p1.5 CO
2PT m . pCO, OUT
38
39
For isothermal operation, equation (38) becomes, using equation (33a) dpCO dt
c
mIN E A exp p mOUT RT VOC k0 E 1.5 exp pCO . RT P0.5 T
38a
De® ning a term to describe the overall conversion of CO to CO2 , one obtains XCO
1
pCO, OUT mOUT OUT p c XVOC VOC, IN mIN
40
312
O’ REILLY pCO, OUT 1
OUT XVOC
mIN mOUT
2PT 2m 1 XCO c yVOC
41
yVOC P . mIN T 42 The Runge Kutta, or other numerical method, must now be used, giving for XVOC # 0.9999 pCO2 , OUT
pCO t
Dt
pCO2 , IN
pCO t
DpCO Dt
DpCO2 Dt
c DpVOC Dt
pCO2 t
pCO2 t
Dt
OUT XCO c XVOC
DpCO Dt
DpCO2 Dt .
43 44 45
2
Equation (46) may then be used for oxidation of residual CO with assigned values of DpCO , Dt
2 1 0.5 kM, CO pCO, 2
1 . 0.5 pCO, 1
46
kM, CO is replaced by k0 / PT for isothermal evaluation. Iteration of the above expressions is carried out until convergence is achieved for T2 . Equations (40) to (42) are used to check the overall material balance. L/di for the incineration section is obtained from XVOC 0.9999, XCO 0.995
t
Dt XVOC 0
Vi n0
pdi2 L 4n0
47
4n0 t pdi3
48
pCO2 pO2 , pO2 , M PT pVOC QLOSS R t T T1,2 T0 TA TIN TIN, INCIN t tMIX tVOC U Vi v0
XCO XVOC DXVOC OUT XVOC yVOC
where n0
mFUEL mFUEL mCG mVOC mWASTE pCO
XAIR
from which L di
L/di m
0.01559 ´ 1.0115
mIN RTIN . PT
49
ratio of incinerator section length to internal diameter kmol of incinerated waste and combustion gas (kmol humid waste gas) 1 kmol s 1 of auxiliary fuel to process kmol auxiliary fuel (kmol dry waste gas) 1 kmol s 1 of combustion gas kmol s 1 of VOC kmol s 1 of waste gas partial pressure of CO, bara, subscripts IN, OUT are before/ after incineration partial pressure of CO2 , bara, subscripts IN, OUT are before/ after incineration partial pressure of O2 , mean partial pressure of O2 during incineration, bara total pressure, 1.01325 bara partial pressure of VOC, bara, subscript IN is before incineration rate of heat loss from incinerator, kW universal gas constant, kJ (kmol K) 1 or bar m3 (kmol K) 1 time, s temperature, K stream temperature into and out of increment of incinerator volume DV , K standard temperature, 298.15 K adiabatic ¯ ame temperature, K initial temperature of combined stream to incinerator, K initial temperature for incineration, K, identical to TIN for adiabatic operation space time for incineration of VOC and residual CO, s space time in mixing section, s space time for incineration of VOC, s overall heat transfer coef® cient, W m 1 K 1 internal volume of incineration section, m3 volumetric ¯ owrate of combined waste and combustion gas at TIN , m3 s 1 factor for excess combustion air, XAIR 0.0 for stoichiometric air fractional oxidation of CO formed from oxidation of VOC fractional oxidation of VOC increment of fractional oxidation XVOC, 2 XVOC, 1 fractional oxidation of VOC exit incinerator mol. fraction of VOC in original waste gas 0.005 in DRY air
REFERENCES NOMENCLATURE a, b A AMIX DA b Cp CG Cp, WASTE Cp, WCG CVOC c di , d0 e VOC DH 0 C 298 K DH 0 C, TIN E K k0 kM, CO kM, VOC
stoichiometric coef® cients for fuel combustion, dependent on fuel composition pseudo ® rst order pre exponential factor, s 1 outside area of mixing section of incinerator including dished end, m2 increment of outside area of incineration section of incinerator, m2 mol change in VOC incineration reaction, (kmol VOC oxidized) 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 kmol CO formed (kmol VOC oxidized) 1 inside, outside diameter of incinerator respectively, 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 activation energy, kJ kmol 1 or bar m3 kmol 1 ratio of depth to radius for dished end, value 0.5 pre exponential factor, m3 kmol 1 s 1 for VOC, mol fraction 0.5 s 1 for CO reaction rate coef® cient over increment DpCO , (bar mol fraction) 0.5 s 1 reaction rate coef® cient over increment of oxidation DXVOC , s 1
1. O’ Reilly, A., 1997, Estimation of residence time in VOC incineration, Trans IChemE, Part B, 75(B1): 33±42. 2. O’ Reilly, A., 1998, Estimation of residence time in CO incineration, Trans IChemE, Part B, 76(B2): 166±176. 3. Lee, K. C., Morgan, N., Hansen, J. L. and Whipple, G. M., 1982, Revised model for the prediction of the time-temperature requirements for thermal destruction of dilute organic vapours and its usage for predicting compound destructibility, 75th Ann Meet Air Pollution Control Association, New Orleans, Louisiana, June 20 to 25. 4. Hemsath, K. H. and Susey, P. E., 1974, Fume incineration kinetics and its applications, Recent Advances in Air Pollution Control, AIChE Symposium, No. 137, 70: 439±449. 5. Valentin, F. H. H. and North, A.. A., 1980, Odour Control, A Concise Guide, (D.O.E. Warren Spring Laboratory), 51±60. 6. Ledbetter, J. O., 1974, Air Pollution, Part B: Prevention and Control, (Dekker Inc.) 138. 7. Barnes, R. H. et al, 1979, Chemical aspects of afterburner systems, US Environmental Protection Agency, Report No. EPA 600/7 79 096, (US Government Printing Of® ce, Washington, DC), 4. 8. De Nevers, N., 1995, Air Pollution Control Engineering, (McGraw Hill, Inc.) 300.
ADDRESS Correspondence concerning this paper should be addressed to Dr A. O’ Reilly, Chemical Engineering, University of Teesside, Middlesbrough, TS1 3BA, UK. The manuscript was received 17 February 1998 and accepted for publication after revision 9 September 1998.
Trans IChemE, Vol 76, Part B, November 1998