Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 4, 1979 653
High-Temperature Pyrolysis of Hydrocarbons. 2. Naphtha to Acetylene A. Holmen, 0. A. Rokstad," and A. Solbakken SINTEF, The Norwegian Institute of Technology, University of Trondheim, N-7034 Trondheim-NTH,
Norway
Pyrolysis of naphtha has been studied in an electrically heated tubular reactor in the temperature range 1525-1765 O C . High yields of acetylene were obtained at short residence times. Hydrogen dilution and reduced pressure were used to prevent carbon formation during the pyrolysis. The mechanism of product formation is discussed. Ethylene seems to b e an important intermediate in the formation of acetylene from naphtha. The carbon formation was found to be very sensitive to the partial pressure of acetylene in the reaction mixture.
Introduction In a recent paper (part 1) we dealt with the hightemperature pyrolysis of methane to acetylene (Holmen et al., 1976). Other interesting raw materials for production of acetylene are naphtha and heavier feedstocks. The energy needed to convert different hydrocarbons to acetylene decreases from methane to the higher paraffins as shown in Figure l. Thus, the energy consumed by the reaction is about 40% higher per mole of acetylene when acetylene is formed from methane than from nheptane. Thermodynamic considerations of the equilibrium show that acetylene may be formed at a lower temperature from higher paraffins than from methane. The rate constant for cracking of higher paraffins is much greater than for methane. Thus, it should be possible to make acetylene from naphtha at lower temperatures than from methane. A lower reaction temperature is desirable for several reasons. At lower temperatures less energy is lost by quenching of the hot product gas from the reactor, and less energy is lost by cooling of the metal parts of the reactor. The lifetime of the reactor tube is prolonged by operating it a t a lower temperature. The idea of producing acetylene from liquid hydrocarbons is not new. Many processes have been proposed and some have been developed for industrial use (Miller, 1965). The main difference between the processes is the way energy is put into the reaction. In this work naphtha has been pyrolyzed in the same type of electrically heated reaction tube as we previously used for methane (Holmen et al., 1976). A similar reaction system for high-temperature pyrolysis of hydrocarbons has been used by Kunugi and Tamura (19611, and by Happel and Kramer (19661, who also realized the advantage of using hydrogen dilution in high-temperature pyrolysis of hydrocarbons. Compared to other process types the electrically heated reaction tube makes it easier to control the temperature profile in the reactor and thus makes it possible to obtain an approximately isothermal reactor. In systems where the hydrocarbon is cracked in hot combustion gases the reactor is typically adiabatic (Kamptner et al., 1966; Steinratter and Krekeler, 1975), and the product gas mixture is not so simple as in the case of hydrogen dilution. In another type of adiabatic reactors the hydrocarbon is cracked in a hydrogen plasma of very high temperature (Sennewald et al., 1963; Rozanova and Sidorov, 1976). The very high temperatures in plasma processes lead to a higher loss of energy. 0019-7882/79/1118-0653$01.00/0
Table I. Major Components in the Naphthas (wt %)"
2,2-DM-propane 2-M-butane n-pentane 2-M-pentane 3-M-pentane n-hexane M-cyclopentane benzene cyclohexane 2-M-hexane 2,3-DM-pentane 3-M-hexane 1-cis-3-DM-cyclopentane 1-trans-3-DM-cyclopentane 1-trans-2-DM-cyclopentane n-heptane M-cyclohexane toluene 1,1,2-TM-cyclopentane n-octane n-nonane
Esso solvent 4
straightrun naphtha
0.1 0.4 0.4 1.8 1.2 0.3 3.5 9.4 4.4 11.3 2.5 3.2 4.3 24.9 10.0 5.7 2.2 1.7
9.1 14.2 8.3 6.2 14.8 4.5 1.8 2.9 2.6 1.4 3.1 0.8 1.0 1.3 5.3 2.6 1.4 1.0 1.7 1.1
1.0
a Note: M is methyl, DM is dimethyl, and TM is trimethyl.
Experimental Section All the experiments were carried out with the apparatus previously described (Holmen et al., 1976). This was modified for naphtha pyrolysis by using an evaporator to vaporize naphtha into a stream of hydrogen. Rotameters were used to measure the feed streams of hydrogen and liquid naphtha. The reactor tube was 600 mm long and 6 mm inside diameter. It consisted of a graphite tube (ATJ from Union Carbide) with an inside lining of alumina (Alsint type 710 from Haldenwanger). The hot product gas was quenched by direct injection of water immediately after the reactor, and the reaction products were analyzed as described in part 1 (Holmen et al., 1976). The naphtha feed-stocks were analyzed on a Perkin-Elmer F11 gas chromatograph equipped with a flame ionization detector and a temperature programmed oven with a squalane coated capillary column 100 m long. Two naphthas were used in this work. The first (Esso Solvent 4) had a boiling range of 85-115 "C (ASTM) and a specific gravity of 0.73 g cm-3 (15 "C). The second (straight run naphtha) had a boiling range of 30-170 "C (ASTM) and a specific gravity of 0.68 g cm-3 (15 "C). 0 1979 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 4, 1979
654
I N L E T RESIDENCE TIME
;100
-
0
C2H2
I
800 -
400
800
1200 1600 TEMPERATURE I O K I
2000
100
1
200
1
Figure 1. Enthalpy of formation of hydrocarbons.
Analysis of the two naphthas is given in Table I. Both naphthas were found to be completely pyrolyzed, with hydrogen, acetylene, ethylene, methane, and carbon being the main products. Ethane, diacetylene, vinylacetylene, and tars were detected, but in quantities too small to be significant in the mass balance. From the mass balance, the amount of acetylene produced may be written
where W C 2 H reflects the amount of acetylene dissolved in the quenck water, and C,H, is the average formula of the naphtha used. a and P were estimated from gas chromatographic and elementary analysis: a = 7, P = 15 for the medium naphtha and a = 6, P = 13.3 for the straight run naphtha. The yield of acetylene based on the weight of naphtha in the feed is, then (YO)
10
30
20
40
60
50
REACTOR LENGTH l C M l
Figure 2. Temperature profiles of reaction tube at two different inlet residence times.
- 100 z
20
L
1
8
1
1
8
010
020
050
030 040 INLET RESIDENCE TIME I S E C I
Figure 3. Yield of acetylene from medium naphtha a t different temperatures. Feed atomic ratio H:C = 6, total pressure after reactor 80 torr.
Yields of ethylene and methane were calculated in a similar way. Their solubilities in water is much less than acetylene and may be neglected in the mass balance calculation. The yield data were correlated with the inlet residence time, defined as .T=-
VR
FO where VR is the volume of the reactor, and F, is the total inlet flow at 20 "C and 1 atm. An optical pyrometer was used to measure wall temperatures over the visible section of the reactor tube. Examples of measured temperature profiles are shown in Figure 2. The yield data were correlated with the maximum reactor temperature. Results and Discussion The experimental technique described above was used for high-temperature pyrolysis of two types of naphtha, one with a narrow boiling range and the other was a straight run naphtha. The H:C atomic ratio in the total feed (H, + naphtha) was usually about 6, the temperature of the reactor was in the range 1525-1765 "C, and the reactor outlet pressure about 80 torr. After some hours of operation at the usual conditions carbon was observed to build up in the reactor tube, and it was regularly necessary to shut off the naphtha and hydrogen feed and
,
, 010
,
, a20
,
,
,
030 040 INLET RESIDENCE TIME I S E C I
,
i 0 50
Figure 4. Yield of acetylene from straight run naphtha at different temperatures. Feed atomic ratio H:C = 6, total pressure after reactor 80 torr.
carefully burn off the carbon deposit with air. This burning-off process could easily be observed with the pyrometer. Characteristic yield data for acetylene are presented in Figures 3 and 4. At temperatures above 1600 "C acetylene yields of 80 wt % or more are obtainable from naphtha. Both types of naphtha give about the same yields a t the same pyrolysis conditions.
Ind. Eng. Chem. Process Des. Dev., Vol.
18, No. 4, 1979
655
50
024
-
020
-
016
r
s
Y
z
,,,'
1
7
7
r
5
1
-
4
, I200 -
w A W
2LO
z w W
8
&
Y
160
80 -
LO L
-
UO8
004
#
'
L
012 R16 020 OZL a28 NAPHTHA I N FEED I K G / H I
d R32
010
Figure 5. Production of acetylene from medium naphtha as a function of feed rate a t different temperatures. Feed atomic ratio H:C = 6, total pressure after reactor 80 torr, reactor tube i.d. 6 mm.
t
f
E
.~1620°1:
~
010
&
9
~
-
~
020 030 040 I N L E i RESIDENCE TIME I S E C I
-
2Lo
050
160
'
1
120
-
80
-
LO
-
\ ~-
L _ L L
010
050
Figure 6. Yield of ethylene from medium naphtha at different temperatures. Same conditions as in Figure 3.
a20 030 0 LO I N L E T RESIDENCE TIME I S E C I
Figure 8. Yield of methane from medium naphtha a t different temperatures. Same conditions as in Figure 3.
c
1525OC
___-----
I
~
--
-
020 030 040 I N L E T RESIDENCE TIME I S E C l
0 50
Figure 9. Yield of methane from straight run naphtha a t different temperatures. Same conditions as in Figure 4.
The Rice (1931,1933) free-radical theory for the thermal decomposition of hydrocarbons is well established below 1100 K (Appleby et al., 1947; Woinsky, 1968; Kunzru et al., 1972; Bach et al., 1975; Sundaram and Froment, 1978). Even at 2000 K the theory may be used to explain the formation of the main products. At such a high temperature the decomposition of a higher paraffinic hydrocarbon is very fast. This depends on fast initiation of the reaction and rapid subsequent decomposition of the alkyl radicals. Thus, ethylene is easily formed by unimolecular decomposition of primary alkyl radicals. RCH2CH2. R. + CH2=CH, ( 1) Acetylene, becoming more stable at higher temperature (Miller, 1965), is formed from ethylene via vinyl radicals by the chain reaction C2H4 + H. C2H3. + Hz (11) CzH3. C2H2 + Ha (111) Alkyl radicals with methyl side groups may split into a-olefins and methyl radicals or into propylene and a smaller radical RCHCHZ. RCH=CH2 CHB. (IV) -+
010
020 0 30 OLO I N L E T R E S I D E N C E TIME I S E C I
050
Figure 7. Yield of ethylene from straight run naphtha a t different temperatures. Same conditions as in Figure 4.
The acetylene production capacity is of interest for scaling up studies. It is shown for the laboratory reactor in Figure 5. The increase in acetylene production capacity with increasing temperature is due to the increasing yield especially at high feed rates. Other products measured were ethylene and methane. The yield of these decreased with increasing temperature and increasing residence time as shown in Figures 6 , 7 , 8 , and 9. Both ethylene and methane seems to be primary pyrolysis products and they appear to be intermediates in the formation of acetylene. There has to be a maximum in ethylene and methane yield at shorter residence time (as observed at lower temperatures by Rozanova and Sidorov, 1976).
-
+
+
I
CH3
-
+
R. + CH,CH=CHz (VI Higher oc-olefins decompose even faster than the corresponding paraffins, leading to C2H4,C3H6,CH,, and H, as primary products. Propylene decomposes easily according to the mechanism CH,CH=CH2 + H. CH3CH2CH2 (VI) CH3CH2CH2 CH,. + CH2=CH2 (VII)
- -
656
Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 4, 1979
The concentration of propylene is very low because it is less stable than ethylene at high temperatures. Methane is formed in the following reactions involving methyl radicals CH3. + RH CHI R* (VIII)
+ CH4 + He
+
CH3- + H2
+
(IX)
To get an impression of the rate of radical decomposition reactions the rate of reaction VI1 may be compared with the rate of methane-forming reactions.
-
hII
+~
~VIII
I X
kVII(C3H7')
~ ~ I I I ( C H ~ . ) (+R ~HI)X ( C H , * ) ( H ~ )
For reaction VII, Lin and Laidler (1966) gave the following expression for the rate constant
( 3:3
kvII = 3.5 X 1013 exp
s-l
--
The general reaction VI11 may be represented by the reaction CH3.
+ C&5
+
CH4
+ C2H5.
(XI
For this reaction McNesby (1960) found the following expression for the rate constant
(
kx = 2 x
exp) : -:-l
cm3 mol-' s-1
For reaction IX the expression given by Walker (1968) for the rate constant may be used
Applying the above expressions at 2000 K and a total pressure of 0.1 atm
and if With p H 2 = 0.1 atm or (H2) = 6.1 x IO-, mol (C3H7.)is put approximately equal to (CH,.), then rv11
1.3 x 105 + r1x This ratio is probably too high since it has been assumed that the first-order rate constant kwI for the unimolecular decomposition is independent of the pressure. A t high temperatures and low pressure this would not be the case. The calculation shows, however, that decomposition reactions of higher alkyl radicals are extremely fast at 2000 K. The shape of the curves in Figures 8 and 9 indicates that more methane is formed at low conversion when the temperature is higher. The following explanation may be given for this. Assume that a certain amount of methyl radicals are formed rapidly by the decomposition of naphtha hydrocarbons. These methyl radicals may react according to reactions VI11 and IX to form methane, or they may combine to form ethane which subsequently may react further to ethylene and acetylene. 2CH3-+ M C & 3+ M (XI) rv111
-
-+
C2Hs + M CH3.
+ C&
He
+ CaHs
+
2CH3. + M CH4 Hz
+ C2H5.
+ CzHS.
(-XI) (XI (XII)
C2H5.
+M
+
CzH4 + Ha
+M
(XIII)
Addition of methyl radicals to ethylene and propylene may be neglected, because the chemically excited radicals thus formed would dissociate almost immediately at the high temperature to form methyl radical and olefin once again. From the above equations it can be shown that the difference between the total rate of reaction of methyl radicals and the rate of formation of methane is given by TCH:,
-
rCHl
= kXI(CH3*)2*M - ~-XI(C~H,).M
which is the rate of formation of products other than methane from methyl radicals. If reaction XI is far from equilibrium then it may be considered irreversible, and rxI = rCH3 - rCH1 = kxI(CH,.)2.M Methane is formed by the elementary reactions VI11 and IX and the activation energy of these are about 10 to 12 kcal/mol. The activation energy of reaction XI is zero in the high-pressure region and should be negative in the low-pressure region. Thus, the ratio of the rates r c H 4 / r x I is expected to increase with increasing temperature, and relatively more methane should be formed at higher temperature at very short residence time. This conclusion is based on the assumption that reaction XI is far from equilibrium. Unfortunately, it has not been possible to come to any conclusion whether this is true or not. The reactions XI and -XI are probably in the low-pressure region at the high temperatures of these experiments (Burcat et al., 1973). The methane formed during the pyrolysis decomposes according to a stepwise scheme (part 1) leading to an overall increase in the acetylene yield 2CH4 C2HG --+ CZH4 + C2H2
-+
62
H2
k2
During naphtha pyrolysis radicals are formed by unimolecular splitting of C-C bonds. The initiation reaction is thus much easier with naphtha than with pure methane. After the initiation period the rate of decomposition of methane is not expected to be significantly influenced by the naphtha pyrolysis. Attack on methane by hydrogen atoms is expected to be important CH4 + H. CH,. H2 (-1X) +
+
A high yield of acetylene was only possible at low hydrocarbon partial pressure. When the pressure increased above a certain limit at constant H:C ratio in the feed there was a dramatic increase in carbon formation which tended to plug the reactor quickly. Based on a series of experiments at about 1700 "C with varying the HGnaphtha ratio at different total pressures in the range of 75-200 torr the following approximate relation could be set up for a hydrocarbon C,H, fed to the reactor 50 plim== - torr cy
where p l i m is the limiting hydrocarbon partial pressure at the inlet above which excessive carbon formation occurred and cy is the number of C atoms in a hydrocarbon molecule (an average number for naphtha). An exact value for the limiting pressure could not be found, because it was difficult to reproduce and depended on the surface of the reactor tube. Fresh carbon deposit on the reactor wall seemed to lower the limiting pressure. The inlet limiting pressure corresponds to a maximum partial pressure of
Ind. Eng.
acetylene of about 20 torr a t the outlet. The above relationship was also found to apply to methane pyrolysis under similar conditions (part 1). The composition of the product streams is very similar for methane and naphtha high temperature pyrolysis. The main source of carbon deposit is probably the decomposition of acetylene in both cases. Nomenclature Po = volume rate of feed as gas at 20 "C and 1 atm k = rate constant M = molecule participating in elementary reaction M . = molecular weight of component i NdH, = feed rate of hydrogen, mol/h p = partial pressure R = gas constant r = reaction rate T = temperature, K V , = reactor volume W = weight of acetylene produced per hour, kg/h acetylene absorbed in quench water, kg/h WC.H~- feed rate of naphtha, kg/h X i= mole fraction of component i in product gas cy = average number of C atoms in a naphtha molecule fl = average number of H atoms in a naphtha molecule @ c ~ H=~yield of acetylene (based on weight of naphtha) 7 = inlet residence time, s
d,$2_=
Chem. Process Des. Dev., Vol. 18, No. 4,
1979
657
Literature Cited Appleby, W. G., Avery, W. H., Meerbott, W. K., J. Am. Chem. Soc., 68, 2279
(1947). Bach, G.,Nowak, S.,Kalinenko, R. A,, Lavrovskii, K. P., Sevelkova, L. V., Belostockii, M. G.,Felgln, J. A,. Z. Chem., 15. 165 (1975). Burcat, A., Skinner, 0.B., Crossley, R. W., Scheller, K., Int. J . Chem. Kinet.,
5(3),345 (1973). Happel, J., Krarner, L., US. Patent 3227771 (1966). Holrnen, A., Rokstad, 0. A., Solbakken, A,, Ind. Eng. Chem. Process Des. D e v . , 15,439 (1976). Karnptner, H. K., Krause, W. R., Schilken, H. P., Chem. Eng., 73(5), 80,93 (Feb 28, 1966). Kunugi, T., Tamura, T., Chem. Eng. Prog., 57(11),43 (1961). Kunzru, D., Shah, Y. T., Stuart, E. B., Ind. Eng. Chem. Process Des. Dev., 11, 605 (1972). Lin, M. C., Laidler, K. J., Can. J . Chem., 44,2927 (1966). McNesby, J. R., J . Phys. Chem., 64, 1671 (1960). Miller, S.A., "Acetylene, Its Properties, Manufacture and Uses", Vol. I, Ernest Benn, London, 1965. Rice, F. O., J . Am. Chem. SOC.,53, 1959 (1931);55, 3035 (1933). Rozanova, M. V., Sidorov, V . I.,Soviet Chem. Ind., 6(2),92 (1976). Sennewald, K., Schallus, E., Pohl, F., Chem. Ing. Tech., 35(1),1 (1963). Steinrotter, H. H. W., Krekeier, H., Inst. Chem. Eng. Symp. Ser. No. 43 (15),
l(1975). Sundararn, K. M., Frornent, G. F., Ind. Eng. Chem. Fundam., 17, 174 (1978). Walker, R. W., J . Chem. SOC.A , 2391 (1968). Woinsky, S.G.,Ind. Eng. Chem. Process Des. Dev., 7, 529 (1968).
Received f o r review July 18, 1978 Accepted May 1, 1979
The support of this work by the Royal Norwegian Council for Scientific and Industrial Research is gratefully acknowledged.
UNIFAC Parameters from Gas-Liquid Chromatographic Data J. Aleksonis Zarkarian, Frank E. Anderson, John A. Boyd, and John M. Prausnltz' Chemical Engineering Department, University of California, Berkeley, California 94 720
Specific-retention-volume data from gas-liquid chromatographic measurements can be used to obtain activity coefficients at infinite dilution. There are numerous such data in the analytical chemistry literature for a variety of binary systems containing common volatile organic fluids and high-boiling, multifunctional organic substrates. A data-reduction method has been established wherein chromatographic specific-retention-volume data may be used to estimate group-interaction parameters for the UNIFAC correlation to predict activity coefficients. New UNIFAC parameters are reported for 30 group interactions.
Introduction Quantitative phase equilibria for mixtures are frequently needed in chemical engineering design. Often the design engineer must estimate the activity coefficients of systems for which little or no experimental data are available. A useful method for providing such estimates is the UNIFAC group-contribution method (Fredenslund et al., 1977a,b) for nonelectrolyte liquid mixtures. A group-contribution method is attractive because it is applicable to a large number of multicomponent mixtures using only a relatively small number of parameters. A t a given temperature and composition, UNIFAC gives activity coefficients for every component in a mixture provided three types of parameters are at hand. These are group-surface area parameters (QJ, group-volume parameters (&), and group-interaction parameters (arnnand anm, where m and n represent different groups). Volume and surface area parameters are readily available for nearly all groups from crystallographic data. However, groupinteraction parameters, which describe energetic interactions between functional groups in the molecules, must be obtained from experimental mixture data. 0019-7882/79/1118-0657$01.00/0
Application of UNIFAC is limited by the scarcity of reliable experimental data for determining the groupinteraction parameters. This work extends the range of applicability of UNIFAC: first, by reporting a data-reduction procedure which uses experimental gas chromatographic data, and second, by presenting new parameters for 30 group interactions. Background For many years analytical chemists have used specific-retention-volume data from gas-liquid chromatography (GLC) to determine the activity coefficient of a solute at infinite dilution in a stationary solvent phase (Kwantes and Rijnders, 1958; Porter et al., 1956; Locke, 1976). Meanwhile, chemical engineers have tried to use vaporliquid equilibrium data to establish correlations for prediction of phase equilibria. The two professions have heretofore overlapped only slightly, primarily because the solvents employed by chromatographers are high-boiling substances of little interest to chemical engineers. The group-contribution concept provides a link between experimental chromatography and attempts to correlate activity coefficients. This follows from the additivity 0 1979 American Chemical Society
