Ind. Eng. Chem. Res. 1987,26, 1340-1344
1340
Thermodynamics and Kinetics for the Manufacture of Tetrafluoroethylene by the Pyrolysis of Chlorodifluoromethane Percy B. Chinoy and Pharokh D. Sunavala* Department of Chemical Engineering, Indian Institute of Technology, Bombay 400 076, India This paper presents a complete thermodynamic and material balance analysis of the pyrolysis of chlorodifluoromethane at atmospheric pressure, using steam as a diluent in a diluent ratio range of 0-8. Reported expressions for the rate constants have been used to derive a rate equation for the above reaction as a function of the conversion and temperature. One of the reported rate constants has been corrected to bring about a good consistency between the thermodynamic and kinetic analyses. 1. Introduction Tetrafluoroethylene (TFE) is the monomer of the important plastic poly(tetrafluoroethylene), commercially known as PTFE or Teflon. This plastic has excellent corrosion and heat-resistant properties and is inert to most chemical reagents. It is highly resistant to wear and tear and has good antistick properties. As a result, Teflon has wide-ranging chemical, mechanical, and electrical applications in industry. A number of processes for the manufacture of TFE have been reported in literature (Renfrew and Lewis, 1946; Park et al., 1947; Venkateswarlu and Murti, 1970; Shingu and Hisazumi, 1966). The most widely used process for the commercial production of TFE is the pyrolysis of chlorodifluoromethane (CHC1F2). 2. Thermodynamics and Material Balance Material balance equations can be written about the reactor, and the composition of the reactor products can be calculated for different conditions of temperature and pressure. If byproducts other than hexafluoropropylene (C3F6)are neglected, the overall reaction can be written as CHClF2 + RHZO += aC2F4 + bCHClF2 + CC3Ftj + dHCl + RH20 (1) where R is the diluent ratio (the diluent being considered here is steam). The carbon balance is 1 = 2a + b 3c (2)
+
The hydrogen balance is l = b + d Equilibrium reaction for the pyrolysis is 2CHC1F2 = C2F4+ 2HC1
(3) (4)
Since the reaction is completely in the gaseous phase and the temperatures are very high, near ideal conditions can be assumed to exist. If P is the total pressure in the reactor, we can write Kpl =
b2(a +
ad2 b +c
Kp2=
b3(a + b
+ c + d + R)P
(7)
Substituting eq 2 and 3 in eq 5 and 7, we get
(Kpl)b3- [ ( a + 4
+ 3R)K,,
- 3aP]b2-
(6aP)b + 3aP = 0 (8)
and b2 + (2a - 3X - 2)b
+ (1 - 2
~ =) 0
(9)
where
x = K-PZ KP1
Expressions for Kpland Kp2were derived as functions of temperature using the thermodynamic properties of C2F4,CHC1F2, C3F6,and HC1 and shown in Table I. The final expressions obtained for Kpl and Kp2are In KP1= -24.33 - 17777.1/T + 6.033 In T (10) In K,, = -16.64 - 16665.8/T
+ 4.438 In T
(11)
Table I1 shows the variation of the equilibrium constants, Kpl and Kp2,as a function of temperature. It is seen that Kpland Kp2values continuously increase with temperature, with Kplalways being greater than Kp2in the given range. Equations 8-11 have been solved for P = 1atm because although subatmospheric pressures in the reactor favor the formation of C2F4, they lead to an increase in the equipment and operating costs and create problems of leakage of air into the system during operation. A flow chart giving the procedure for solving these equations on a computer is shown in Figure 1. The computer program was run for temperature values ranging from 500 to 1000 "C and R = 0,0.5,1.0,2.0, 7.5, and 8.0. The conversion and yields were calculated as % conversion = [(moles of CHCIFz converted into products) /(total moles of CHC1F2 taken)]100 7'0 yield = [(mass of C2F4 formed)/(total mass of
CHClF, taken)]100
+ d + R )P
Equilibrium reaction for the pyrolysis is 3CHClF2 = CSF6 + 3HC1 Similarly we can write
* Author
cd3
(5)
(6)
to whom all correspondence should be addressed.
0888-5885/87/2626-1340$01.50/0
The results of the computer program are shown in Figures 2-5. Figures 2 and 3 show that the CHCIF, content in the reactor off-gases decreases with increasing temperatures, while the HCI and CzF4 content increases. The C3F6 content is very small and mostly decreases with an increase in temperature. Figures 4 and 5 show that the conversion and yield increase with increasing temperatures, the values being higher for higher diluent ratios. 0 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 7, 1987 1341
0
Table I. Thermodynamic Properties of C2F,, CHC1F2,C3H8, and HCl
START
c?
TAKE b = 0.1 CALCULATE LHS EQUATION 8
b BV NEWTON RAPHSO METHOD NO
-
f
A
w CALCULATE LHS
-
t CALCULATE NEW
OF
OF
NO
-
NEWTON RAPH
compd CZF4 CHClFz CBF6 HCl
CP
AG;:
A&:
kcal/g-mol -157.40 -119.90 -257.80 -22.06
kcal/g-mol -149.10 -112.47 -240.64 -22.78
cal/ (g-mol.K)b CY lo2@ 6.929 5.439 4.132 3.865 1.172 9.920 6.700 0.084
a and AG; are at 298 K and 1 atm absolute. * C,
=
CY
+ @T.
Table 11. Equilibrium Constants K , , and K , , as a Function of Temperature temp, O C Kpl,atm Kpz,atm temp, OC KpI,atm Kp2,atm 500 O.OOO74 0.00017 800 3.320 0.3013 600 0.02150 0.00344 900 23.312 1.6810 700 0.3353 0.03960 1000 125.540 7.377
N
I
R E D i l u e n l Ratio =
Moles C H C l F 2
7 0
IVES ALCULATE C AND BVEWATIONS 2 AND 3
AN0
VIELO
Figure 1. Flow chart for the computer program.
~
500
550
600
6%
100
750
Temp.
800
850
IIO0
950
1000
("C)
Figure 3. Effect of temperature on composition of product gases ( R = 1.0).
Figure 2. Effect of temperature on composition of product gases ( R = 0). lot
Since the curves for conversion and yield begin to flatten out at very high temperatures, the operating temperature should be chosen to lie in the range 700-850 OC. At temperatures lower than this, the separation costs will be very high because of low yields, and at temperatures higher than this, the slight increase in conversion and yield may not offset the additional costs of energy required. The diluent
Figure 4. Effect of temperature on conversion ( R = 0,0.5, 1.0, 2.0, 7.5, and 8.0).
ratio is usually never chosen greater than 2 because although it leads to a higher conversion and yields, the re-
1342 Ind. Eng. Chem. Res., Vol. 26, No. 7, 1987
500
I 550
I 650
I
600
I
I
I
I
I
759
I
800
850
900
950
1000
I
100 4
Figure 5. Effect of temperature on yield ( R = 0, 0.5, 1.0, 2.0, 7.5, and 8.0). Table 111. Typical Material Balance for the Pyrolysis of Chlorodifluoromethane for P = 1.0 atm, T = 800 "C, and R = 1.o
input CHClF, HzO
mol 1.000 1.000
86.50 18.00
ka
total
2.000
104.50
outuut CzF4 CHClFz C3Fs HCl
HZO total conversion yield
mol
kg
0.3964 0.1828 0.0082 0.8172 1.OOOO
39.64 15.81 1.23 29.83 18.00
2.4046
104.51
81.72% 79.28%
actor volume will have to be increased by a large amount to accommodate this diluent steam apart from the very high steam costs. A typical material balance is shown in Table 111. 3. Kinetics 3.1. Reaction Mechanism and Rate Constants. Edwards and Small (1965) studied the pyrolysis of chlorodifluoromethane (CHClFJ on a bench-scale unit and proposed the following reaction mechanism: main reactions (12)
CF2 + H C 1 2 CHClF2
(13)
k3
C2F4
(14)
CzF4 2CF2 byproduct (high boiler) reactions
(15)
ki
CF2 + CzF4 * CF2:CF*CF,
Figure ti. Comparison of the equilibrium conversion-temperature plots obtained from thermodynamics and from kinetics for diluent ratio = 1.0.
total pressure have been reported (Edwards and Small, 1965): kl (s-1) = 1013.84 exp(-55.79 (kcal-mol-l)/ R T ) (20)
k 2 (L-mol-l-s-') =
exp(-6.21 (kcal.mol-l)/RT) (21)
k, (L.mo1-l-s-l) = k,
=
10'6.66
exp(-70.36 (kcal.mol-')/RT)
(16)
C2F4 + HC1+ H*(CFz)&l (17) 2C2F4F= C4F8 (18) CHClF2 + C2F4 + H*(CF2)&1 (19) If the byproduct reactions are neglected, the volume expansion of the reaction mixture is taken into account, the activation energy of reaction 14 is assumed to be zero, the following expressions for the rate constants at 1-atm
(22) (23)
A rate equation was derived by using the above mechanism, and a computer program was run to check its thermodynamic consistency. To do this, the equilibrium conversions (see eq 41) were determined at the same temperatures as those used in the thermodynamic analysis (section 2). The two sets of values of equilibrium conversion were found to be very much different, the error at times being greater than loo%! Since the thermodynamic analysis had a sound theoretical basis, it was concluded that for the given reaction mechanism, one or more of the rate constants were wrongly reported. Consider the combination of reactions 12 and 13: k
CHCIFz & CFz + HC1 kl
CF2 + HC1
CHClF,
2CF2
50
TQmP(c)
(24)
Since the difference in the activation energies of the forward and backward reactions is equal to the net heat of reaction 55.79 - 6.21 = 22.1 + aHf(CF2) - 119.9 .. aHf(CF2)= -48.3 kcal/g-mol This value of the heat of formation of the difluoromethylene radical matched the value reported by Gozzo and Patrick (1964). Now consider the reactions 14 and 15: k
2CFz
k4
CzF4
(25)
Similarly,
..
0 - E4 = -157.4 - 2(-48.3) E4 = 60.8 kcal/g-mol
This value was different from the reported value of 70.36 given in eq 23.
Ind. Eng. Chem. Res., Vol. 26, No. 7, 1987 1343
0 61
650
I
1
I
700
750
800
Temp.
('Cl
1 850
__t
Figure 7. Comparison of the equilibrium conversion-temperature plots obtained from thermodynamics and from kinetics for diluent ratio = 7.5.
The value of E4was corrected, and the rate equation was again checked for thermodynamic consistency. Once again the values of equilibrium conversion were found to be different but with lesser errors than before. Hence, it was concluded that one or more of the ko values were wrongly reported. Since E4had already been proved wrong, it was most likely that the value of k4 was wrong. However, it should be noted that it makes no difference to the rate equation whether k,, k3, or k4 is concerned (Le., increased or decreased, as the case may be, by the same factor) because these three values are in the form of a ratio in the original rate equation (see eq 33). After the computer program was run to get a good consistency (within about 3% ) between thermodynamics and kinetics, the value of k4 obtained was 1012.3(instead of the reported value of 1016.66).See Figures 6 and 7. The final corrected expression for k4 is k4 = 1012.3exp(40.8 (kcal.mol-')/RT) (26) 3.2. Rate Equation. Based on the mechanism given above and if all the side products are neglected, a rate equation can be derived as shown below. Consider eq 24 and modified eq 25,
50
Figure 8. Temperature-conversion plot for different reaction rates and for diluent (steam) ratio = 1.0.
Now consider the overall reaction on the basis of 1mol of CHClFz CHClF, + '/2CzF4 + HC1 (34) If NAo are the initial moles of CHClF, taken, (35) NCHClF2 = NAO(l - xA) where XAis the conversion of CHClF,
N c ~ =F YZNAOXA ~
(36)
and NHCl
(37)
= NAOXA
From eq 35-37,
= NAO(1 + R + 0.5XA) (38) Assuming the ideal gas law to be valid, NTRgT P v = N~o(14- R 0.5X~)O.o82T (39) Equation 39 shows the variation of volume with conversion as well as with temperature. From eq 33,35-37, and 39, 8.439 X 1014exp(-28077/T)(l - X A ) -~CHCIF~ = (1 + R 0 . 5 x ~ ) T 1.545 x 10" e X p ( - 1 8 4 2 5 / T ) X ~ ~ ~ ~ (40) (1 R A t equilibrium, NT
v=-
+
By steady-state approximation, rCF2
..
[CFZI =
=0
k,[CHClF,] + k4[CzF4]1/2 k,[HC1] + k3
(30)
*'
546.214exp(-9652/T)
():;
- = (1
+ R +XA0.5XA)T
1"'
(41)
Ind. Eng. Chem. Res. 1987,26, 1344-1351
1344
ious values of -rCHCIF2 ranging from 0 to 2.0 (g-mol of CHClF,/s)/(L.reactor vol). The results were plotted as shown in Figures 8 and 9. The conversions increase with increasing temperatures (endothermic reaction), the curves being steeper at lower temperatures and flatter at higher temperatures. 4. Conclusion
A combination of elemental mass balances and equilibrium constants has been employed to calculate the thermodynamic equilibrium composition of the reactor effluents for the pyrolysis of monochlorodifluoromethane. The results have been presented in the form of plots for varying diluent ratios in the range 0-8.0. A kinetic study has also been presented for developing a rate equation for the formation of tetrafluoroethylene based on available expressions for the rate constants. To get a good consistency between the thermodynamic and kinetic analyses, it was found necessary to correct one of the reported expressions for the rate constants. Registry No. TFE, 116-14-3;CHClF,, 75-45-6; C3Fs,166-15-4; HC1, 7647-01-0.
Literature Cited
I
li i
j, 700
p
J,
/
1 4 -
800
750
TEMPERATURE
I
(‘C)
’ 85P
W
Figure 9. Temperature-conversion plot for different reaction rates and for diluent (steam) ratio = 7.5.
Equation 41 gives the equilibrium conversion of CHCIFz for given values of diluent ratio and temperature. Equation 40 was solved on a computer for diluent ratios = 1.0 and 7.5, temperature range = 650-850 “C, and var-
Edwards, J. W.; Small, P. A. Ind. Eng. Chem. Fund. 1965, 4 , 396-400. Gozzo, F.; Patrick, C. R. Nature (London) 1964, 202,80. Park, J. D., et al. Ind. Eng. Chem. 1947, 39, 354. Renfrew, M. H.; Lewis, E. E. Ind. Eng. Chem. 1946, 38, 870. Shingu, H.; Hisazumi, M. British Patent 1041738 (cl C O~C),Sept 7, 1966, p 4; Appl. April 9, 1963. Venkateswarlu, Y.; Murti, P. S. Chem. Process. Eng. 1970, Oct, 25.
Received for review January 27, 1986 Revised manuscript received March 2, 1987 Accepted April 10, 1987
Thermodynamics of a Single Electrolyte in a Mixture of Two Solvents Ani1 Rastogi* and Dimitrios Tassios New Jersey Institute of Technology, Newark, New Jersey 07102
It is demonstrated that t h e Debye-Huckel (D-H) term when applied t o single electrolyte-binary solvent mixtures predicts salting in when salting out is observed. An empirical extension of the D-H term is proposed t h a t eliminates this problem. When combined with an NRTL term to account for the long-range forces, the resulting expression provides successful correlation of mean ionic activity coefficients and vapor-phase compositions of several ternary systems. Prediction results for y* are in the right direction, but vapor-phase compositions do not reflect the electrolyte effect. Vapor-liquid equilibrium in electrolytic solutions is of theoretical and industrial importance in various chemical, biological, pollution control, and electrochemical processes. Early in the century Debye and Huckel(l923) proposed the classical excess Gibbs free energy expression for strong electrolytes in a single solvent, but applicable only to dilute solutions. Guggenheim and Turgeon (1935) extended the range of validity of the Debye-Huckel equation to 0.1 m for aqueous solutions. Recently many workers have proposed semiempirical correlations for concentrated aqueous *Present address: Real-Time Simulation, Inc., 27 W. 47th St., New York. NY 10036.
electrolytic solutions (Bromley, 1972, 1973; Meissner and Kusik, 1972; Pitzer, 1973, 1977; Pitzer and Mayorga, 1973, 1974; Pitzer and Kim, 1974; Cruz and Renon, 1978; Meissner et al., 1972). Correlation and prediction in multicomponent systems-the typical industrial case-has been studied by several authors. For weak volatile electrolytic systems, such as NH3-CO2-H,S-H20, considerable work has been carried out by Prausnitz and co-workers (Prausnitz et al., 1975, 1978). For strong electrolytic solutions, correlation and prediction in ternary systems-one electrolyte in two solvents-has been discussed among others by Hala (1969),
0888-5885/87/2626-1344$01.50/0 0 1987 American Chemical Society
