Environ. Sci. Technol. 1966, 2 0 , 568-574
Conversion of Chloroform to HCI by Reaction with Hydrogen and Water Vapor Shlen C. Chuang and Joseph W. Borzelll* Department of Chemical Engineerlng and Chemistry, New Jersey Institute of Technology, Newark, New Jersey 07102
Chloroform was used as a model chlorocarbon to investigate the reductive processes of chlorocarbons with hydrogen and the reductive/oxidative reactions with water vapor. The reactions were studied in a tubular reactor at a pressure of 1atm with residence times of 0.005-1.2 s in the temperature range 550-1100 "C. At a reaction temperature above 1100 "C, the major products from the reaction of chloroform with hydrogen were observed as HCl, C(s), CH4,and C2H2: HC1, C(s), C2H2,CO, and C02were, identified as the primary products from the reaction of chloroform with water vapor. Quantitative conversion of chlorine into HCI was obtained from thermal reaction of chlorocarbons under either a reductive atmosphere of hydrogen or a combinationreductive/oxidative atmosphere of water vapor.
Introduction There is presently a great deal of concern about species such as chlorinated plastics, PCB's, chlorinated dry cleaning solvents, flame inhibitors, and other chlorinated chemicals that may undergo partial oxidation or reaction during incineration to produce dangerous toxic or carcinogenic species such as phosgene or dioxin. Certainly the emission of these unwanted combustion products is neither desirable nor beneficial to the environment. The incineration of these chlorocarbons is generally performed in an oxygen-rich environment (1)that contains excess O2and N2,in addition to the C and C1 from the halocarbon, with relatively small amounts of available hydrogen from the limiting fuel operation. In considering products from incineration, the H-C1 bond is the strongest (thermodynamically) and has the lowest Gibbs free energy of formation per chlorine atom. HCI is, therefore, the thermodynamically favored product for chlorine, providing there exists sufficient hydrogen for its stoichiometric formation. It is noted, however, that the 0-H bond in water is stronger than the H-C1 bond, and the 02-rich conditions limit hydrogen availability. The C-C1 bond is the next strongest compared with other possible chlorinated products such as C1-C1, N-C1, or 0-C1 bonds. Consequently, C-C1 may persist in a hydrogen-lean and oxygen-rich atmosphere. This suggests that the emission of toxic chlorine-containing organic products may persist through an oxygen-rich incineration, as it is one of the more stable sinks for the chlorine. In order to obtain quantitative formation of HC1 from chlorocarbons, it might help to convert these chlorocarbons under a more reductive atmosphere of hydrogen or a reductive/oxidative atmosphere of water vapor. The chlorocarbon plus hydrogen system contains only carbon, hydrogen, and chlorine elements and is expected to lead to formation of light hydrocarbons, carbon(s), and hydrogen chloride at the high temperatures where complete reaction occurs. It also does not have wet HC1 in the effluent and is, therefore, not nearly as corrosive as the system with water vapor present. It should be noted that small or limited amounts of oxygen (up to 5%) in the feed stream would be converted to CO/C02 and would not significantly affect the efficient formation of HC1. 566
Environ. Sci. Technol., Vol. 20, No. 6, 1986
Table I. Heats of Reaction and Equilibrium Constants for the Given Equations
-
T, K
298 400 600 800 1000
CHC13+ 3Hz CHC&+ HzO CH4 + 3HC1 CO + 3HC1 AH, AH, K equilibrium K equilibrium kcal/mol constant kcallmol constant -59.87 9.88 X -17.18 1.19 X loz2 -60.66 5.58 X -17.73 1.35 X lozo -62.29 3.69 X loz4 -18.53 1.83 X 10l8 -63.67 9.63 X 10" -19.10 2.14 X loT7 -64.71 2.16 X 1016 -19.25 5.58 X lox6 --L
Water vapor reagent adds the complexity of carbon oxides CO and C02 and possibly O2 to the list of stable products but provides additional exothermicity through production of these species. The problem here is that hot HCI with H20 present is extremely corrosive to metals, but this same problem exists in many incinerator systems, which accept chlorocarbons. Thermodynamicvalues for reaction of chloroform, which is used as a model chlorocarbon in this study, with H2 or HzO are listed in Table I. It is interesting here to consider the overall or global reactions, even though a complex set of many series and parallel free radical reactions must be occurring to form the HC1, CHI, carbon oxides, and carbon (solid). The overall reactions are seen to be highly exothermic with little dependence on temperature. The exothermicity for reactions with hydrogen is due to the similar bond strength between H-H and H-C1 and to the fact that the C-H bond is (in most cases) more stable than that of C-C1. The equilibrium constants for the reactions, as shown in Table I, indicate that the forward direction is strongly favored. Drawbacks associated with using water vapor are the cost associated with vaporizing the water and, of course, the corrosion problem. While acknowledging this, we also point out that the reactions can be made to run in an exothermic regime where fuel energy costs may be minimized and the corrosion problem will exist in any combustion system with chlorocarbon inlet. Recently, Louw et al. (2,3) and Manion et al. ( 4 ) have reported limited product distributions for reaction of hydrogen with chlorocarbons such as dichloroethylene, tetrachloroethylene, chlorobenzene,and polychlorobiphenyl. Remarkably littIe work has been done, however, on kinetics studies of these hydrogen or water vapor reactions with chlorinated hydrocarbons. We have not found any reported kinetic parameters for vapor phase reaction of water vapor or hydrogen with common chlorinated solvents or pollutant species. The most closely related work involves studies designed to investigate reactions of atomic hydrogen with specific halogens where H2was also present. The H atoms served to initiate a series of chain-branching reactions which, in end-product analysis, were observed to terminate in HCl and other halocarbons, usually with a smaller number of chlorines than that of the initial reagent (5-16). The initial reaction in the room temperature H atom studies appears to be a relatively slow abstraction reaction in the case of saturated halocarbons,
0013-936X/86/0920-0568$01.50/0
0 1986 American Chemical Society
where a free radical product formed would then undergo rapid chain-branching and termination reactions. An example of the first step in the reaction of atomic hydrogen with carbon tetrachloride would be He CCll HC1+ CC13.
+
-
These H atom studies are themselves rather limited in scope, and additional research to determine accurate rate constants, activation energies, and mechanisms would certainly be worthwhile. Costes et al. (17) showed that the reaction of H atoms with C C 4 produced carbon atoms, which they used for further reaction studies on C. but not the hydrogen/CC14 reaction. One previous study on gas-phase reactions of water vapor with halocarbons was found in the literature: Gaisnovich et al. (18) have reported on hydrolysis of CC14and COClz to CO and HC1 products in the temperature range from 220 to 550 "C using a static quartz cell reactor. The global activation energies observed were 24.9 and 12.2 kcal/mol for conversion of the respective parent species. This work presents initial experimental data on several reactions of hydrogen and water vapor with chloroform in the temperature range from 550 to 1100 "C using a tubular flow reactor. Product distributions along with preliminary activation energies and rate constants are reported. The equilibrium composition for the reaction of chloroform with hydrogen was calculated to help verify the reaction scheme proposed.
Experimental Section The reactions of hydrogen and water vapor with chloroform have been studied in a high-temperature flow reactor. Two quartz reactor tubes were utilized in this study, one a 2.0 mm i.d. by 30 cm length and the second 4.0 mm i.d. by 30 cm length, with average flow velocities ranging from 0.15 to 5 m/s. The total volume flow, reagent plus carrier, was in a laminar regime; but the axial dispersion number (19) calculates to be less than 0.002, and there is only a small (less than 5%) deviation from plug flow resulting from the laminar velocity profile. This is because radial dispersion is greater than 0.5 (20,21). The reactor can, therefore, be approximated by a plug-flow model if wall reaction is not significant. In addition, a recent kinetic modeling study taking laminar velocity profile, radial dispersion, and wall reactions into account (21)has shown that the reacting system under conditions of this study is well represented by the plug-flow model. Hydrogen was fed into the reactor through two parallel calibrated rotameter assemblies. One of the hydrogen inlet lines included two impingers, in series, where the Hz (room temperature) is saturated with chloroform vapor or other volatile halocarbons for inlet of the chlorinated reagent to the reactor. The measurement of chloroform vapor was calibrated in two ways: first by measuring the total vapor flow before and after the two series impingers and second by measuring the decrease in chloroform volume in an impinger over periods of time while a constant flow of H2 passed through the impinger. Water was fed to the reactor by using constant-flow liquid-metering pumps: an LDC (Lab Data Control Corp./Milton Roy) minipump (0-10 cm3/min) or syringe pump (Sage Instruments Model 341A). Calibration of these pumps was done by measuring the volumetric flow of liquid over a period of time under constant-flow conditions identical with those in the reactor. Liquids were injected into the center of a preheated mixing manifold, concentric capillary, on axis, in the direction of the flow, with either hydrogen or water-vapor flow sweeping the reagent into the reactor.
, The initial experiments utilized a 230-V electric muffle-type furnace 30 cm in length as the reactor oven which was capable of operating at up to 1200 "C. The furnace temperature was controlled by RFL Industries Model 76 proportional controller. The longitudinal temperature profile of the reactor was measured before each individual run with a chromel-alumel thermocouole movable along the length of the reactor. The temperature profile was not isothermal, and the difference in temperature in the central region of the reactor (70% of length) was less than 25 "C. The methods utilized to determine kinetic parameters are included under Results and Discussion. The reactor effluent stream was analyzed by an on-line Beckman Model GC-2 gas chromatograph modified for flame ionization detection and gas sampling. An HP 3370 integrator was incorporated for quantitative peak area measurement. A Pyrex batch-sampling bulb was used for effluent vapor collection and transport to a Varian Mat-44 GC/mass spectrometer. The GC used a 2 m long by 1.2 mm 0.d. stainless steel tube packed with 5% SE 52 on Chromosorb R as the column. The GC/MS used a 60-m OV 101 WCOT fused-silica capillary column with liquid nitrogen cryofocusing on a 20 cm length loop at the front of the flexible column, electron impact ionizhtion, and a dual floppy disk Varian microcomputer data system. HC1 was observed by bubbling the reactor effluent through base solution (impinging) and monitoring the pH through standard titrations or a pH meter. The presence of HC1 product in most cases was verified but not always determined quantitatively. It was assumed that all the chlorocarbon was converted to HC1 when no chlorinated species (other than HC1) could be observed in the GC or GC/MS reaction-product analysis. No chlorine (Cl,) was observed in any of the mass-spectrometric analyses or in the solid carbon. An experimental run consisted of determining the product distribution for the reactant under study at seven different space velocities for a given temperature profile, pressure, and feed composition.
Results and Discussion Reaction of Chloroform with Hydrogen. The reaction of chloroform with hydrogen was studied at average (linear) temperatures of 604,643, and 693 "C at 1atm of pressure and an initial mole ratio of hydrogen to chloroform of 14 to 1 in a 4 mm i.d. reactor. Studies were also done at average temperatures of 599, 631, 851,899,984, and 1046 "C at 1atm of pressure with the initial mole ratio of hydrogen to chloroform being 9 to 1 in a 2 mm i.d. reactor. Mean residence time based on average temperature ranged between 0.02 and 2 s. Figure 1shows the conversion of chloroform in a 4 mm i.d. reactor with an initial mole ratio of hydrogen to chloroform of 14 to 1as a function of mean residence time for several temperatures. The conversion of chloroform consistently increased with increasing temperature and mean residence time. Primary gas-phase product distributions are shown in Figures 2 and 3 for different temperatures as a function of residence time. HC1, CH, (methane, acetylene, and ethylene, which were not well resolved by the GC column), CH3C1,and CHzClzwere the main products observed at temperatures around 600 "C, where up to 40% chloroform conversion and appreciable carbon deposition was observed. Trace quantities of vinyl chloride and 1,l-dichloroethylenewere also observed at this temperature while up to 2% of CHC1, was converted to dichloroethylene at the average temperature of 693 "C. Similar vapor phase product distributions, as shown in Figures 2 and 3 (Hz/CHC1, = 14 and 4 mm i.d. reactor), were observed for reactions in a 2 mm i.d. reactor with the Environ. Sci. Technol., Vol. 20, No. 6, 1986 569
CHC13t h2
0
804%
6
643.C
0
693.C
1.0
0.e 0
0.2 0 I
m
$0.6
g
-,
\
.+
3
g 0.4
u
0
0.1
0.2
I .o
0.5
I .a
2.0
g-
RESIDENCE TIME-SECONDS
Figure 1. Chloroform reaction with hydrogen: CHCI, as a function of residence time at 604, 643, and 693 "C average tempertures, 4 mm i.d. reactor. Initial H,:CHCI, ratio of 14. (CHCI,), indicates lnltial concentration of CHCI,. 0.9
0.7
-0
m
A
CH3CI
0
CH4
0.5
0 I \
F
0 z 0
0.3
I5
10
RESIDENCE TIME-SECONDS
Figure 3. Chloroform reaction with hydrogen: CHCI, and product concentration profiles vs. residence time, 4 mm i.d. reactor. Initial H2:CHCI, ratio of 14 and average temperature 693 "C. CH, may include up to 20% acetylene or ethylene.
0'3
-
u
-
05
0.I
t
CHCI3t H2
1 t
0 cn4
104e.c
0
CH4
B99.C
0 A
CH3CI 899.C
0
BENZENE 899.C
BENZENE 046.C
1 01
0 2
03
0 4
RESIDENCE TIME - SECONDS
0. I
Figure 4. Chloroform reaction with hydrogen: benzene, CH,, and chloromethane concentrations as a function of residence time, 2 mm i.d. reactor. Initial H,:CHCI, ratio of 9 and average temperature of 899 and 1046 "C. CH, is methane but may include small amounts, up to 20%, of acetylene or ethylene.
A .-/
0. 5
#
&
U
M
I.!!
2.5
RESIDENCE TIME- SECONDS Figure 2. Chloroform reactlon with hydrogen: CHCI, and product concentration profiles vs. residence time, 4 mm i.d. reactor. Initial H,:CHCI, ratio of 14 and average temperature of 604 "C.
initial H2/CHC13ratio equal to 9. This lower ratio of Hz to CHC13gave higher carbon deposition. Figure 4 shows gas-phase product distribution at the temperatures above 899 "C in a 2 mm i.d. reactor. The highest concentration hydrocarbon was methane, with estimates of acetylene levels being less than 20% of CH4, at all temperatures in these narrow bore tubular reactors. Formation of CH, (methane, acetylene, and ethylene) is shown to increase with increasing temperature, in addition to observation of benzene. The formation of benzene may be due to the hydrogenation of graphitic carbon formed by pyrolysis of methane or a ring-closure, vapor-phase mechanism with olefinic and/or acetylenic species. The chloromethane concentration as shown in Figure 4 is decreasing with increasing residence time at 899 "C, while it was not observed at 1046 "C. Figure 4 further illustrates that the concentration increases with residence time a t 899 "C and decreases with residence time at 1046 "C. This slight increase 570
Environ. Sci. Technoi., Vol. 20, No. 6, 1986
at 899 "C may be due to a higher rate of CH, formation from chlorocarbon/H2 reaction, while the higher temperature decreases can be attributed to pyrolysis or unimolecular decomposition after most of the chlorocarbons are converted. The CzC14,which is known to be a major product of thermal decomposition of CHC1, (23,24),was not observed in these hydrogen reactions. This suggests that the overall reaction in the presence of excess hydrogen reduces the combination of the C1 chlorocarbon species which forms C2 chlorocarbons. Product distributions of CH,, CH3C1, CH2C12,CzH2Clz and C6H6vs. temperature at an average residence time of 0.3-0.5 s are shown in Figure 5. Formation of CHzClz increases with increasing temperature (not shown in Figure 5) to a maximum near 600 "C and then decreases. Formation of the CH3C1also shows the same trend but with a maximum around 850 "C. This indicates the lesschlorinated methanes are more stable, with chloromethane the most stable chlorocarbon in this reacting system. It is consistent with the bond strengths of C-C1 bonds on C1 chlorocarbonswhich increase with decreasing chlorination (25). The formation of dichloromethane increases proportionally to the decrease in chloroform in the temperature range 550-600 "C, strongly indicating that dichloro-
Table 11. Material Balance for Primary Products: H2
+ CHCIIa
mol formed/100 mol of carbon HCl
cZc14 H Z
mean residence time, s Residence times: 0.3-0.6
599
631
29 104 C0.005 797.8 0.52
68 241 C0.005 800.7 0.58
CHC& input, CHC13:H2 = 1:9, 851 899
at T,"C, of 984
1046
40 281
30 288
53 294
60 300
743.0
883.3 0.38
893.3 0.36
901.6 0.34
0.4
s. CHSCl
FORMED PER CHC13 CHCll+ H2
REACTED
0.29
600
700
AVERAGE
BOO TEMPERATURE
8 00
0
O8l.C
o
ewc
0
89S'C
IO w
n-
C
Figure 5. Chloroform reaction wlth hydrogen: reaction products as a function of temperature, 2 mm i.d. reactor. Initial H,:CHC13 ratlo of 9 and mean residence time of 0.3-0.5 s. CH4 may include up to 20% acetylene or ethylene.
methane is the initial stable product in the thermal reaction of chloroform with hydrogen. Chloromethane is then produced from further reaction of dichloromethane with hydrogen. The overall reaction scheme based on distributions of highest concentration products can be illustrated as follows: CHC13 -* CH2Clp CH3C1- CH, -+
(plus HC1 in each step). It should be pointed out that this reaction scheme is not a complete detailed mechanism, with the actual mechanism obviously including a significant number of free radical reactions (22). Figure 6 shows formation of chloromethaneas a function of residence time for five different temperatures and its complete destruction at 984 "C,at a residence time of 0.37 s. Under 631 "C the formation of chloromethane increased with a residence time from 0 to 0.5 s, while it decreased with residence time at temperatures above 851 "C. The increase in chloromethane with residence time suggests that its rate of formation is faster than its destruction and is another indication that the chloromethane is a stable intermediate product in the overall reaction scheme. The material balance data shown in Table I1 were established to estimate the formation of graphitic carbon and hydrogen chloride along with the consumption of hydrogen. The data indicate that formation of graphitic carbon varies with increasing temperature in this system. It is also noted that while there is some variation in temperature throughout the central region of our reactor, the reagents incurred very large temperature changes upon inlet and exit of the furnace, with preheat inlet temperatures approximately 300 "C and exist lines kept at 150 "C. It was not unusual to observe no carbon in the central region of the reactor with significant C(s) in the inlet and exit regions where the temperatures are somewhat lower than the central, more isothermal, region. Elemental analysis for this graphitic carbon by Galbriath Lab. Inc. (Knoxville, TN), found no measurable chlorine or hydrogen, only
0.2
0.4
08
oe
RESIDENCE TIME- SECONDS
Flgure 6. Chloromethane (CH,CI) formed per mole of chloroform feed, at five temperatures between 631 and 984 "C. Initial H,:CHCI, ratio of 9.
carbon. A substantial amount of hydrogen chloride has been found in GC/MS analysis, but no molecular chlorine has been detected. This shows that chlorine from the halocarbons has been efficiently converted to HCl and is maintained in this form through thermodynamic stability and the equilibrium reaction of Hp C1p s 2HC1
+
and the large hydrogen concentration. Equilibrium Composition Calculation. Thermodynamic consideration of the system is important to the theoretical evaluation of limiting performance characteristics for this multiple-reaction system. A knowledge of the equilibriumcomposition for this reaction system is also essential for the proper interpretation of reaction schemes. Chemical equilibrium is not easy to obtain, experimentally, due to kinetic limitations on the rate of approach to equilibrium of certain reactions in the system. Several techniques for computation of chemical equilibrium in a multiple-reaction system have been discussed in the literature (26-28). The series reactor method proposed by Meissner et al. (28) was found to be an effective and straightforward method for calculating the equilibrium composition in our system. The method is an iterative procedure in which we assume reactors in series for independent reactions, i.e., one reactor for each independent reaction. Simplifying assumptions adopted for the calculation (22) were that all gas-phase compounds were assumed to be ideal gases and trace compounds (less than 0.1 % ), such as chlorobenzene, toluene, and tetrachloroethane, were excluded in the thermodynamic equilibrium composition calculation. Figure 7 shows the equilibrium composition of the primary products for an initial mole ratio of chloroform to hydrogen of 1 to 9 in the temperature range 800-1500 K, and indicates only three products, CHI, HC1, and carbon (solid), are predicted from the thermodynamics. Environ. Sci. Technol., Vol. 20, No. 6, 1986
571
EQUILIBRIUM COMPOSITION OF REACTION:
9t
CHLOROFORM WITH HYDROGEN
CHC13 t H20
8
0 611°C
I
0 665%
6
A 68BoC
. d 5 5
7
" 2 i3
4
- 3
6
2
1
5
0
0.1
3
-
u
e
u
_
u
-
u
u
D
U
CHC13
i
665'~
H20
5 u
$
0 .t
2
I
0
,
__------_ -c(s;
LC
0.E
0
C C l 2 'CC1.2
0
CHClj
a
CHCl
.
CCI2
\
I'
800
IO00
1200 TEMPERATURE
1400
1600
OK
Flgure 7. Equilibrium composition calculation of HCi, CH,, C (solid), and H, as a function of reaction temperature. Initial H,:CHCI, ratio of 9.
The equilibriumcomposition of CH3C1,CH2C12,C,H2C1,, and CBH6were calculated to be less than 0.01% (22). However, the experimental product distribution as illustrated in Figure 5 shows that the concentrations of CH2C12 and CH3C1are significantly greater than their calculated equilibrium values in the temperature range 580-680 "C. This demonstrates that CHzC12and CH3C1are stable reaction intermediate products which will eventually be converted when equilibrium is achieved and that conversion is kinetically limited. It also provides evidence to support the proposed global reaction scheme in the previous section. Reaction of Chloroform with Water Vapor. The reaction of chloroform with water vapor was studied in a 0.4 cm i.d., 30 cm long tubular reactor at the following conditions: average temperatures of 611,665,688,950, and 1050 "C, total pressure of 1atm, space velocity of reactant flow (at 20 "C) 27.55-135.33 reactors/h, and an initial mole ratio of reagents CHC13:H20:Heof 1:10:4. Helium served as carrier gas to vaporize the chloroform in series impingers. The loss of chloroform on the basis of initial concentration vs. means residence time at various temperatures is shown in Figure 8. The destruction of chloroform clearly increases with increasing temperature and residence time. The major product distribution vs. residence time at 665 "C is shown in Figure 9. The primary products observed as a function of temperature are shown in Table 111. A significant amount of CO, C 0 2 , and HC1 were observed by GC/mass spectrometry in the temperature range from 611 to 1058 "C along with other identified products found in specific temperature ranges. Carbon deposition on the wall of the reactor was also observed in 572
0.6
0.5
Flgure 8. Chloroform reaction with water: CHCI, as a function of residence time; temperatures 61 1-688 OC. Initial H,:CHCI, ratio of 10.
HCI
0
Y
0.4
RESIDENCE TIME- SECONDS
4
c -!
0.3
0.2
Envlron. Scl. Technol., Vol. 20, No. 6, 1986
0
-+,
0.4
3
r v U
$
0.2
0
01
0.3
0.2
RESIDENCE
TIME- SECONDS
Flgure 9. Chloroform reaction with water: CHCI, and inltial products as a function of residence time: temperture 665 "C. Initial H20:CHC13 ratio of 10.
Table 111. Primary Product Distributions: CHC13
+ Hz04
mol formed/100 mol of CHCls input, CHCIR:HzO:Hz= 1:10:4,at T, "C, of 611
CHC13b cZc14
68.8 9.0
665
668
3.5 21.6
-e
33.1 0.32 0.60 1.0
950
-
1050
-
CzClH3 0.70 0.17 Xd
