I n d . E n g . Chem. Res. 1988,27, 559-565
559
Dehmlow, D. V.; Dehmlow, S. S. Phase Transfer Catalysis, Monographs i n Modern Chemistry; Verlag Chemie: Weiheim, 1980; VOl. 11. Hughes, E. D.; Ingold, C. K.; Patel, C. S. J. Chem. SOC.1933, 526. Ingold, C. K. Structure and Mechanism in Organic Chemistry, 2nd ed.; Cornel1 University Press: Ithaca, NY, 1969. Markl, G.; Merz, A. Synthesis 1975, 295. Mathias, L. J. J. Macromol. Sci.-Chem. 1981, A15, 853. Rasmussen, J. K.; Howell, H. K. Polym. Sci. Technol. 1984,24, 105. Sherrington, D. C. Macromol. Chem. 1984, 3, 303. Starks, C. M.; Liotta, C. Phase Transfer Catalysis, Principles and Techniques, Academic: New York, 1978. Streitwieser, A,, Jr. Solvolytic Displacement Reactions; McGraw Hill: New York, 1962. Vollardt, K. P. C. Synthesis 1975, 765. Weber, W. P.; Gokel, G. W. Phase Transfer Catalysis in Organic synthesis; Springer-Verlag: Berlin, Heidelberg, New York, 1977. Wittig, G. Acc. Chem. Res. 1974, 7, 6.
the solvation of the activated complex.
Conclusions The synthesis of BTPPC is the only reaction product. Under appropriate conditions, the conversion can approach 100%. The reaction obeys second-order kinetics. The order of relative activities of solvents is CH30H > CH3CO2H > CHzClz > CH3COCH3 > CZH5OC2H5 > C & j > CH3C6H5. The present study has valuable implications in organic synthesis, especially in the synthesis of stilbene via the two-phase Wittig reaction. Registry NO.BC, 100-44-7; BTPPC, 1100-885; PPh,, 603-350; methanol, 67-56-1; acetic acid, 64-19-7; dichloromethane, 75-09-2; acetone, 67-64-1; diethyl ether, 60-29-7;benzene, 71-43-2; toluene, 108-88-3; water, 7732-18-5.
Literature Cited
Received for review March 20, 1986 Revised manuscript received August 6, 1987 Accepted September 4, 1987
Bunnett, J. F.; Reinheimer, J. D. J.Am. Chem. SOC.1962,85, 3284. Cook, F. L.; Brooker, R. W. Polym. Prepr. 1982,23, 149.
Reduction of Coke Formation during Naphtha Pyrolysis Using Triethyl Phosphite Kalyan K. Ghosh and Deepak Kunzru* Department of Chemical Engineering, Indian Institute of Technology, Kanpur, K a n p u r 208016, India
The reduction in the rate of coke formation during naphtha pyrolysis due to the injection of triethyl phosphite has been investigated in a jet-stirred reactor at atmospheric pressure in the temperature range of 1088-1108 K. Coke formation was significantly reduced in the presence of the additive. The data are consistent with the formation of a metal-phosphorus complex which passivates the metal activity for coke formation. Used in the same concentration, phosphorus was more effective than sulfur in reducing the coking rate. The coke formation could be satisfactorily modeled by assuming that the fraction of the metal surface covered with the complex was in equilibrium with the phosphorus-containing free-radical species in the gas phase and that coke simultaneously deposited a t different rates on the bare metal surface and the metal-phosphorus complex. Pyrolysis of naphtha and other hydrocarbons, which is of primary importance in the manufacture of olefins, is always accompanied by the undesirable formation of coke. This coke deposits on the walls of the reactor, reducing the overall heat-transfer coefficient and increasing the pressure drop across the reactor. This results in a gradual increase with run time of both the reactor tube metal temperature and the pressure drop across the reactor, necessitating periodic shutdowns. Rates of coke formation during hydrocarbon pyrolysis have been investigated by several workers. Because it is well established that the reactor walls catalyze coke formation, most of the methods used t~ reduce coking employ some means to passivate the reactor walls. Presulfiding the reactor walls has been reported to reduce coke formation during the pyrolysis of ethane (Albright and McConnel, 1978), propane (Trimm and Turner, 1981), and n-octane (Shah et al., 1976). Bajus and co-workers (1981, 1983) studied the effect of thiophene, dibenzyl sulfide, and dibenzyl disulfide on reaction kinetics and coking of steam cracking of heptane. The metal sulfide, which passivated the reactor surface, inhibited the coke deposition considerably. Similarly, Sahu and Kunzru (1988) found that the rate of coke formation during naphtha pyrolysis was significantly reduced in the presence of thiophene. Brown et al. (1982) deposited a silica coating on preoxidized steel substrates by the decomposition of an alkoxysilane in a carrier gas. With such an inert coating, coke formation during pyrolysis was reduced by a factor of 10 0888-5885/88/ 2627-0559$01.50/0
in short-term tests and by a factor of 3-4 in longer term tests. Tomita (1977) has developed a sintered catalyst consisting of calcium oxide, aluminum oxide, and less than 0.2 wt 70silicon dioxide. Considerable reduction in coke formation during naphtha pyrolysis has been claimed with this catalyst. Attempts have also been made to reduce the rate of coking by catalytically gasifying the coke with steam during pyrolysis. Showa Denko (1983a,b) reported that incorporating alkali metals and their oxides to Ni-Cr alloys or HK-40 was effective in significantly reducing the rate of coke formation during the cracking of steam-hydrocarbon mixtures. Molten alloy steel was modified with lithium and cast, whereas the HK-40 tubes were either microalloyed with barium or flame-sprayed with a powder of HK-40 and lithium oxide. Mandal and Kunzru (1986) investigated the coking rates during n-hexane pyrolysis on potassium carbonate coated metal surfaces and found that the rate of coke formation was significantly reduced in the presence of K2C03due to the catalytic gasification of coke. Similar results were obtained by Bahadur et al. (1987) for the catalytic gasification of coke during naphtha pyrolysis. However, Bahadur (1986) found that the potassium carbonate was gradually removed from the surface during decoking and the rate of coke formation increased with successive decokings. Phosphorus-containing additives have been shown to be effective in inhibiting coking rates. Naberezhnova et al. (1983) found that the organophosphorus compounds 0 1988 American Chemical Society
560 Ind. Eng. Chem. Res., Vol. 27, No. 4, 1988
CH3COCHzC(CH3)zPhPS(OCzH5) and CH3COCHzC(CH3)2PhPS(OCzH5)were effective in controlling coke formation during pyrolysis of liquid hydrocarbons without affecting the product yields. Boone (1983) reported the development of proprietary products containing phosphorus with or without sulfur which were effective in reducing coking rates during hydrocarbon pyrolysis. The additive provided filming to passivate the metal surface to prevent it from catalyzing the coke formation. The increase in run length varied from 50% to 223%. Moreover, the steam-air decoking time decreased by 540% and cleaning of exchangers was easier because of softer coke. In the above-mentioned studies on inhibition of coke formation using phosphorus additives, no information on the quantitative rates of coke formation has been revealed. Furthermore, very little information is available on the combined effect of phosphorus and sulfur on coking rates. The objective of this study was to investigate the inhibition of coke formation during naphtha pyrolysis using triethyl phosphite as an additive both in the absence and presence of thiophene. A further objective of this study was to model the rate of coke formation during naphtha pyrolysis in the presence of phosphorus. Experimental Section The experiments were conducted in a jet-stirred 316 stainless steel reactor which had provision for measuring the kinetics of cracking as well as coking. The main reactor length was 0.023 m, and the reactor volume was 11.2 mL. For measuring the coke deposition, a small Inconel 600 plate was suspended in the reactor, periodically weighed outside the reactor, and resuspended. To prevent any oxidation of the deposited coke, prior to each weighing the plate was cooled by raising it from the center of the reactor into the relatively cooler portion of the outlet tube. The details of the experimental setup and procedure have been described earlier (Kumar and Kunzru, 1985). The feed was a desulfurized naphtha with a boiling range of 362-450 K. The sulfur content was 5 ppm.
Results and Discussion To investigate the coke formation during naphtha pyrolysis, experiments were conducted at atmospheric pressure in the temperature range of 1088-1108 K, with steam as an inert diluent. For most of the runs,the weight ratio of steam to naphtha, 6, and the space time, T, were kept fixed a t 0.7 kg/kg and 0.53 s, respectively. To study the effect of inlet naphtha partial pressure on the rate of coke formation, for some runs 6 was changed to 0.5 kg/kg. The phosphorus content of the feed was varied from 50 to lo00 ppm by adding triethyl phosphite to the naphtha feed. The sulfur content of the feed was varied by adding thiophene to the feed. Effect of Phosphorus Content of Feed. The presence of triethyl phosphite in the feed reduced the rate of coke formation, and the variation with run time for different phosphorus concentrations in the feed at 1088 K is shown in Figure 1. The rate of coke formation was initially high and then decreased asymptotically to a constant value in approximately 90-180 min. Such a trend of coking rate with run time is well established (Kumar and Kunzru, 1985; Sundaram et al., 1981; Shah et al., 1976). As can be seen from this figure, coking rates were significantly reduced in the presence of triethyl phosphite. For instance, the presence of 200 ppm of phosphorus in the feed reduced the rate of coke formation by approximately 70%. This reduction in coking rate is comparable to the limited data reported by Boone (1983). From her data, the reduction in coke formation during n-hexane pyrolysis at 1088 K and
120c
r\,
I Phosphorus content ,ppm a n
A 0
Y
0
10
-
0
500 1000
-
I
1
I
20
3.0
40
0
" 1 50
Time , h
Figure 1. Effect of phosphorus concentration on rate of coke formation (7' = 1088 K; 6 = 0.7; T = 0.53 s).
Phosphorus content of f e e d , ppm
Figure 2. Reduction in asymptotic rate of coke formation with phosphorus content (2' = 1088 K; 6 = 0.7; T =: 0.53 s).
a concentration of 200 ppm of phosphorus in the feed was estimated to be approximately 50%. The asymptotic rate of coke formation was reduced with an increase in the phosphorus content of the feed but at higher phosphorus concentrations the inhibition in coking rate was less. The decrease in the asymptotic rate of coke formation with increasing phosphorus content of the feed is shown in Figure 2. As can be seen from this figure, the reduction in the rate of coke formation was initially higher and then gradually decreased such that, at high concentrations of phosphorus, the coking rate approached a constant value. The total gas and liquid yields for different phosphorus concentrations were also measured, and these are shown in Table I. The total gas and liquid yields were approximately the same for different phosphorus concentrations. Although a detailed analysis of the liquid and gaseous products was not conducted, the data on the total yields indicate that phosphorus did not have any effect on the primary pyrolysis reactions. To investigate whether the reduction in the rate of coke formation in the presence of phosphorus was due to the
Ind. Eng. Chem. Res., Vol. 27, No. 4,1988 561 Table I. Gas and Liquid Yields during Naphtha Pyrolysis" run 1 2 3 4 5 6 I
8 9
P, ppm
S, ppm
0 100 200 500 1000 50 100 200 0
0 0 0 0 0
100
LOO 100 100
liquid, mL 19.5 20.0 20.0 20.5 19.8 14.5 14.7 15.0 15.0
aTemperature = 1088 K, 6 = 0.7 kg/kg, and
T
n
gas, L 40.6 40.0 39.0 40.2 39.6 44.0 43.0 44.3 43.5
0
3 Phosphorus content, ppm 0 P:O A P.100
E
= 0.53 s.
.I
-d*
0
?l
I
10.0, 0 Steam A Nitrogen
b 7.0-
o
o
y
I
L
0
a
o
Feed switched t o ~nosDhorus-treenophiha
5.0 -
A
0
2 .o
40
6 .O
80
10 0
Time, h
Figure 4. Rate of coking formation when feed switched from phosphorus-containing naphtha to phosphorus-free naphtha (T= 1088 K;6 = 0.7;T = 0.53 s).
is in equilibrium. Assuming that the diethoxy phosphorus radicals are the predominant phosphorus-containing radI I 1 0 ical species, this equilibrium concentration of the surface 1.0 2.0 3.0 40 5.0 complex would depend on the reaction temperature and Time ,h the concentration of diethoxy phosphorus radicals in the Figure 3. Comparison of rate of coking using steam or nitrogen as gas phase. diluent (2' = 1088 K;6 = 0.7; T = 0.53 s, P = 100 ppm). To further check the hypothesis that the metal-phosphorus complex formation reaction was in equilibrium, enhanced rate of the coke-steam reaction, runs were runs were made in which the feed was switched from conducted by replacing steam with nitrogen for the idenphosphorus-containing naphtha to phosphorus-free tical temperature, 6, and space time. As shown in Figure naphtha after the asymptotic coking rate had been at3, the rates of coke formation were the same irrespective tained. A typical result is shown in Figure 4. When of whether steam or nitrogen was used as the inert mephosphorus-free naphtha was introduced, the rate of dium. It was also noted that the total gas and liquid coking gradually increased and approached a constant product yields were the same with either nitrogen or steam value after 120-180 min of introducing the phosphorus-free as the inert. naphtha. However, this asymptotic coking rate was lower The results discussed above show that the phosphorus than that obtained with phosphorus-free naphtha on a neither takes part in the homogeneous reactions nor does fresh Inconel 600 surface. The results show that phosit catalyze the coke-steam reaction. Most probably, the phorus affects the coking rate even after the additive phosphorus containing additive passivates the metal sursupply has been terminated. However, this effect is temface (Boone, 1983). A t high temperatures, triethyl phosporary, and successive runs without phosphorus additive phite can decompose in two ways: do not show a reduced coking rate. Such a behavior has C~H~OPOCZH~ C2HsOPO' + C2H5' (A) also been reported by Boone (1983) for pyrolysis of I feedstocks containing phosphorus additives. Thus, to I 0c2H5 OC2h maintain a reduced coking rate, continuous injection of the or additive is necessary. C ~ H S O P O C ~ H ~ C2HsOP' + 'OC2H5 (B) Effect of Temperature and Inlet Naphtha Partial I I Pressure. Rates of coke formation varied with reactor OC2H5 oc2H5 temperature and inlet naphtha partial pressure. The effect diethoxy phosphorus of temperature was studied by varying the temperature radical in the range of 1088-1108 K, keeping 6 and space time, 7, fixed at 0.70 kg/kg and 0.53 s, respectively. Since the The bond strength of 0-C (1079.4 MJ/kmol) is more than variation of coking rates with run time was similar to that that of 0-P (598.9 MJ/kmol). Therefore, the cleavage of the 0-P bond is more probable. The radicals (C2H50)2PO' shown in Figure 1, the variation of only the asymptotic coking rate with temperature and weight fraction of and (C2H50)2P' thus formed can combine with the metal phosphorus is shown in Figure 5. As can be seen from this surface to form a film. Initially, the metal surface is devoid figure, coking rates increased with increasing temperature of any coke so that the bare surface catalyzes the coke and decreased with increasing weight fraction of phosformation. With increasing run time, a film of the phosphorus. The trend for phosphorus-free naphtha is similar phorus-metal complex covers the surface and the metal to that reported by other investigators (Kumar and activity is gradually reduced. The asymptotic coking rate Kunzru, 1985; Bahadur, 1986; Sahu and Kunzru, 1988). would then correspond to the time when the reaction reA t a phosphorus concentration of 200 ppm in the feed, sulting in the formation of the metal-phosphorus complex I
-
-
562 Ind. Eng. Chem. Res., Vol. 27, No. 4, 1988 Phosphorus and sulphur content ppm
Temp, K 0 1088 J 1098 a 1108
8 0-I
P 0 V 50 100 a 200 0 180
i
S 100 100 100 100 LOO
=
E,
20c
4 ' 0
I
,
I 50 ! 100 150 200 Phosphorus content of feed ,ppm
1
i
Figure 5. Effect of temperature on asymptotic rate of coke formation for phosphorus-containing naphtha (6 = 0.7; T = 0.53 s). l
L . ___A -
18 01
I
Y
10
20
30
L O
5c
Time, h
Figure 7. Effect of phosphorus in presence of sulfur on the rate of coke formation (T= 1088 K; 6 = 0.7, T = 0.53 s).
s.0, P:O
I
2
ob
10
I 2 0
I 30 Time, h
I
40
I 1 50
Figure 6. Effect of inlet partial pressure of naphtha on rate of coke formation (T = 1088 K; T = 0.53 s).
increasing the reactor temperature from 1088 to 1098 K increased the rate of coke formation by 31.0%, whereas an increase in the temperature from 1088 to 1108 K increased the coking rate by 71.0%. These results are in good agreement with the results for coke formation during pyrolysis of n-hexane containing 200 ppm of phosphorus (Boone, 1983). As estimated from the data presented, when the temperature is increased from 1088 to 1098 K, the rate of coking increased by approximately 38%, whereas from 1088 to 1108 K the rate of coke formation increased by approximately 66%. Increasing the temperature not only affects the rate constant for the coking reaction but also changes the equilibrium constant of the surface complex. This is discussed later. The effect of inlet partial pressure of naphtha on the rate of coking at a constant temperature of 1088 K both for phosphorus-containing and phosphorus-free feed is shown in Figure 6. With an increase in inlet partial pressure of naphtha, the concentration of coke-forming species is increased, thus increasing the rate of coke formation. A t this temperature, the coke-steam reaction is not expected to be significant (Biba et al., 1978) and would not contribute to the reduction in the rate of coking with increasing partial pressure of steam. As can be seen from this figure, with a decrease in partial pressure of naphtha, the reduction in coking rate is approximately the same for
both phosphorus-free and phosphorus-containing feeds. As 6 was increased from 0.5 to 0.7 kg/kg, the rate of coke formation reduced by approximately 40% for both feeds. Effect of Phosphorus in t h e Presence of Sulfur. Sulfur compounds are well-known to be effective in reducing coking during pyrolysis of hydrocarbons. Most furnace feedstocks contain sulfur either added or naturally occurring, and these help to control coking. To investigate the combined effect of sulfur and phosphorus on coking rates, thiophene was added to the phosphorus-containing naphtha. It should be noted that the fresh naphtha feed was virtually sulfur-free (sulfur content = 5 ppm). The effect of phosphorus in the presence of sulfur is shown in Figure 7. As can be seen from comparing Figures 1and 7, used in the same concentrations, phosphorus is more effective than sulfur in reducing the rate of coking. Moreover, with sulfur present in the feed, the percentage reduction in the coking rate is less than for sulfur-freefeed. The asymptotic coking rates were reduced with increasing phosphorus concentrations but not as much as when the feed contained no thiophene. For instance, for thiophene-free naphtha, when the phosphorus concentration was increased from 50 to 100 ppm, the rate of coke formation reduced by approximately 26%, whereas the reduction in the coking rate was only 10% when both sulfur and phosphorus were present in the feed. As shown in Table I, for a fixed sulfur concentration in the feed, the total gas and liquid yields did not change with varying phosphorus content of the feed. However, in the presence of sulfur, the yield of the gaseous products was higher, whereas the organic liquid yields were lower. When thiophene is present in the reaction mixture during pyrolysis, the aromatics can react with S' and 'SH-free radicals produced due to thiophene decomposition (Glass and Reid, 1929). It has been reported that addition of sulfur compounds reduced the rate of coking as well as the yields of organic liquid products during pyrolysis of n-heptane (Bajus et al., 1981,1983) and naphtha (Sahu and Kunzru, 1988). Thus, in contrast to phosphorus, sulfur affects both the surface as well as the homogeneous reactions. Modeling of Coke Formation during Pyrolysis of Phosphorus-Containing Naphtha (without Sulfur). Coke formation during naphtha pyrolysis is a complex phenomenon due to the various possible coke-forming
Ind. Eng. Chem. Res., Vol. 27, No. 4, 1988 563 free-radical reactions, and the exact mechanism is still not clear. Kumar and Kunzru (1985) postdated simple models involving either the reactant and/or products to model coke formation during naphtha pyrolysis, and the asymptotic coking rate for chemical control could be represented as In eq 1, ra is the asymptotic coking rate and CA is the concentration of aromatics in the reactor. Sahu and Kunzru (1988) could also model their data on coke formation during naphtha pyrolysis by a similar expression and found that the value of n was different for thiophene-free and thiophene-containing naphthas. The exact mechanism for the reduction of coke formation due to the addition of phosphorus is still not clear, and a simple model has been proposed to explain the observed asymptotic coking rates. Since a phosphorus-containing passivating film forms on the metal, therefore at any time, coke formation occurs simultaneously on two types of surfaces, viz., (1) a surface covered with metal-phosphorus complex and (ii) a metal surface which has not taken part in the complex formation. The asymptotic coking rate corresponds to the time when an equilibrium concentration of the complex has formed on the surface. This equilibrium concentration would depend on the partial pressure of diethoxy phosphorus radicals in the gas phase and the reaction temperature. From the available data, it can be concluded that phosphorus does not effect the gas-phase reactions leading to the formation of the coke precursors and only reduces the rate of the surface reaction. Thus, assuming that coke is only formed from aromatics and that the chemical reaction is the rate-controlling step, the asymptotic coking rate, ra, can be expressed as
Table 11. Variation of Asymptotic Rate of Coke Formation with Temperature and Phosphorus Concentration" asymptotic rate of coking X lo2, temp, K P concn, ppm ktz/(m2)(h) 8.60 1088 0 50 5.12 100 3.80 200 2.85 500 1.64 1000 0.75 0 11.00 1098 50 6.62 100 4.85 200 3.60 1108 0 13.60 50 8.60 100 5.95 200
" 6 = 0.70 kg/kg;
0
T
= 0.53 s.
0
Temp.,K 1108
a
1098
0
1088
I 4.0
1
8.0 (
where yp is the fraction of the metal surface covered with metal-phosphom complex, k p is the rate constant for coke formation on a surface completely covered with phosphorus complex, and k, is the rate constant for coke formation for a phosphorus-free naphtha feed. It should be noted that k,, kp, and yp depend on temperature. Assuming that the rate at which the metal complex is formed is proportional to the product of the diethoxy phosphorus radical concentration in the gas phase and the fraction of surface area occupied by the complex, we obtain
4.40
)
.
16 0
12.0
i n 5 ,k g / l m 2 ) ( h 1
Figure 8. Determination of re and Kp at different temperatures. -1
0,
(3)
IO 3
In eq 3, it has been assumed that the concentration of diethoxy phosphorus radicals is proportional to the weight fraction of phosphorus in the reaction mixture. At equilibrium, dyp/dt = 0 or
IO 2 a r C -
KPP yp = 1 KpP
+
(4)
where Kp, equal to kf/k,, is the equilibrium constant for the complex formation and P is the weight fraction of phosphorus in the reaction mixture. When no phosphorus is present in the feed, yp = 0, and the asymptotic coking rate then can be expressed as (5)
whereas for yp = 1, the surface is completely covered with the complex and
ralyp=l- rap = kpCAn
(6)
01
00
-
90
90
92
1 I 1OL(K-') T Figure 9. Arrhenius plots for rap, rab, and Kp.
rab can be determined experimentally from the data on phosphorus-free naphtha. In contrast, rap cannot be determined experimentally because the concentration of
564 Ind. Eng. Chem. Res., Vol. 27, No. 4, 1988 Table 111. Variation of KP and rG with Temperature temp, K equilibrium const, lo-"& 103rap,kg/(m)2(h) 1088 2.72 7.45 2.62 9.97 1098 2.46 1108 11.35
-
phosphorus at which y p 1 is not known. From eq 2, 5, and 6, we obtain re =
raPYP
+ r a b ( 1 - YP)
or 1 _---YP
rap
r a b - Fa
rab -
ra
Substituting eq 4 in eq 7b, we obtain
--rab rab - ra
-1+-
1
KPp
+-
rap
rab
- ra
or
For a particular temperature, Tab is known from the asymptotic rate of coking at P = 0 and ra is known at different concentrations of phosphorus. Thus, a plot of r a b - r, vs (rab- r a ) / Pshould give a straight line with slope of -l/Kp and an intercept of rab - rap. The asymptotic rate of coke formation at various temperatures and concentrations of phosphorus measured for the data shown in Figures 1and 5 are given in Table 11. It should be emphasized that all these runs were conducted at the same 6 and T, and for a particular temperature, CA would be the same at different phaphorus concentrations. For the same 6 and r, CA would change with temperature because of the change in outlet conversion. However, at these conditions, due to the high outlet conversions, the change in CAwith temperature is not expected to be significant (Sahu and Kunzru, 1988). The plot of rab - ra vs (rab - ra)/Pat three different temperatures is shown in Figure 8. Considering the simplifying assumptions, eq 9 represents the data very satisfactorily for all the runs with thiophene-free naphtha. The values of Kp and rG calculated from the slopes and intercept of Figure 8 are shown in Table 111. rapincreases whereas Kp decreases with temperature, which shows that the formation of the metal-phosphorus complex is an exothermic process and that the fraction of the surface covered with the complex at a constant phosphorus concentration decreases with temperature. The activation energies for k p , k,, and Kp, as determined from Arrhenius plots, Figure 9, were found to be 232.8, 218.5, and 54.0 MJ/kmol, respectively. The activation energy for k , is in good agreement with the value of 212.3 MJ/kmol (Kumar and Kunzru, 1985) and 243.3 MJ/kmol (Sahu and Kunzru, 1988) reported for coke formation during the pyrolysis of phosphorus-free naphtha. This model cannot be extended to determine the kinetics of coke formation in the presence of both sulfur and phosphorus. This is due to the fact that sulfur affects the homogeneous reactions also, and when the sulfur concentration is varied CAalso changes. Extension of this model to account for the presence of both sulfur and phosphorus is in progress. Conclusions The results of this study show that the rate of coke deposition during naphtha pyrolysis can be significantly
reduced by injection of a phosphorus-containing additive together with the naphtha feed. The data suggests that phosphorus inhibits the coking rate by passivating the catalytic activity of the metal for coke formation. To maintain a reduced coking rate, continuous injection of the additive is necessary. Addition of phosphorus does not affect the homogeneous reactions but only affects the surface reaction most probably by forming a metalphosphorus complex which is in equilibrium with the phosphorus-containing free radicals in the gas phase. Phosphorus is more effective than sulfur in reducing the coking rate. The coke formation can be satisfactorily modeled by assuming that the coke is simultaneously depositing at different rates on the bare metal surface and the surface covered with the phosphorus complex. Nomenclature CA = concentration of aromatics, kmol/m3 = equilibrium constant for metal-phosphorus complex formation k f = forward rate constant for complex formation, h-' k , = rate constant for coke formation in the absence of phosphorus, kg of coke/(m2)(h)(kmol/m3)" kp = rate constant for coke formation on a surface completely covered with phosphorus complex, kg of coke/ (m2)(h)(kmol/m3)" k , = reverse rate constant for complex formation, h-l n = reaction order for coke formation P = phosphorus concentration in the reactor, ppm or weight fraction ra = asymptotic rate of coke formation, kg/(m2)(h) rab = asymptotic rate of coke formation at y p = 0, kg/(m2)(h) re = asymptotic rate of coke formation at y p = 1,kg/(m2)(h) T = temperature, K t = time, h y p = fraction of metal surface covered with metal-phosphorus comp1ex K p
Greek Symbols
6 = weight ratio of steam to naphtha, kg/kg = space time, s Registry No. C, 7440-44-0; (C2H,),P03, 122-52-1; thiophene, 110-02-1.
T
Literature Cited Albright, L. F.; McConnel, C. F. "Deposition and Gasification of Coke during Ethane Pyrolysis". Paper presented at the 175th ACS National Meeting, Annaheim, CA, March 1978. Bahadur, N. P. M.Tech. Thesis, Department of Chemical EngiKanpur, 1986. neering, I.I.T., Bahadur, N. P.; Sahu, D.; Kunzru, D. "Reduction of Coke Formation during Naphtha Pyrolysis". Paper presented at the 4th Asian Pacific Confederation of Chemical Engineering Congress, Singapore, May 1987. Bajus, M.; Veseley, V.; Baxa, J.; Leclercq, P. A.; Rijks, J. A. Ind. Eng. Chem. Prod. Res. Deu. 1981,20, 741. Bajus, M.; Baxa, J.; Leclercq, P. A.; Rijks, J. A. Ind. Eng. Chem. Prod. Res. Deu. 1983,22, 335. Biba, V.; Macak, J.; Klose, E.; Malecha, J. Ind. Eng. Chem. Process Des. Deu. 1978, 17, 92. Boone, K. Oil Gas J. 1983,81, 83. Brown, D. E.; Clark, J. T. K.; Foster, A. I.; McCarrol, J. J.; Sims, M. L. ACS Symp. Ser. 1982, 202, 23. Glass, H. B.; Reid, E. E. J. Am. Chem. SOC.1929,51, 3428. Kumar, P.; Kunzru, D. Can. J. Chem. Eng. 1985, 63, 598. Mandal, T. K.; Kunzru,D. Ind. Eng. Chem. Process. Des. Deu. 1986, 25, 794. Naberezhnova, G . N.; Nurtdinov, S. K.; Belyakova, L. D. Chem. Abstr. 1983, 99, 25105~. Sahu, D.; Kunzru, D. Can. J. Chem. Eng. 1988, in press. Shah, Y. T.; Stuart, E. B.; Sheth, K. D. Ind. Eng. Chem. Process Des. Deu. 1976, 15, 518.
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Showa Denko, K. K. Jap. Pat. 58 104990,1983a. Showa Denko, K. K. Jap. Pat. 58 104889, 1983b. Sundaram,K. M.; Van Damme, P. S.; Froment, G. F. AlChE J. 1981, 27, 946. Tomita, T. U.K. Patent 1478899, 1977.
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Trimm, D. L.; Turner, C. J. J. Chem. Tech. Biotechnol. 1981,31,285.
Received for review May 22, 1987 Revised manuscript received November 20, 1987 Accepted December 3, 1987
Chemistry of Olefin Oligomerization over ZSM-5Catalyst Richard J. Quam, Larry A. Green, Samuel A. Tabak,* and Frederick J. Krambeck Mobil Research and Development Corporation, Paulsboro Research Laboratory, Paulsboro, New Jersey 08066
Light olefins (c&) can be converted to a mixture of higher molecular weight olefins via a sequence of acid-catalyzed-shape-selectivepolymerization and isomerization reactions over the ZSM-5 zeolite catalyst. T h e composition and molecular weight of the product are very dependent on reaction temperature and pressure through both thermodynamic and kinetic constraints. Distillate-range olefins having an almost petrochemical-type structure with high-quality fuel properties are produced a t relatively high pressure (30-100-bar) and lower temperature (200-300 "C) conditions. At lower pressure and higher temperature, lower molecular weight products are formed, including aromatics and saturates from olefin condensation and hydrogen-transfer reactions. The polymerization of light olefins to produce higher molecular weight hydrocarbon fuels over acid-type catalysts is a well-known area of chemistry as reviewed by Oblad et al. (1958). The products of acid-catalyzed reactions of olefins may include primarily olefins from straight oligomerization or mixtures of olefins, paraffins, cycloalkanes, and aromatics from what has been termed "conjunct" polymerization (Pines, 1981). The product spectrum is influenced by both reaction conditions and the nature of the catalyst but is generally restricted to the gasoline range (
