I n d . E n g . Chem. R e s . 1989, 28, 1293-1299
1293
Triphenyl Phosphite as a Coke Inhibitor during Naphtha Pyrolysis Sanjay Vaish and Deepak Kunzru* D e p a r t m e n t of Chemical Engineering, I n d i a n I n s t i t u t e of Technology, K a n p u r , K a n p u r 208 016, I n d i a
T h e effectiveness of triphenyl phosphite as a coke inhibitor during naphtha pyrolysis has been investigated. Pyrolysis runs were conducted in a jet-stirred reactor at atmospheric pressure in the temperature range 1088-1108 K. Addition of triphenyl phosphite significantly reduced the coke formation without affecting the product yields. Used in the same concentration, triphenyl phosphite is a more effective coke inhibitor than triethyl phosphite or sulfur. A previously proposed model for coke inhibition due t o the formation of a passivating metal-phosphorus complex could satisfactorily correlate the data. Decoking occurred at a faster rate for the coke formed during pyrolysis of phosphorus-containing naphtha than for phosphorus-free naphtha. Pyrolysis of naphtha is a major petrochemical process and is always accompanied by the formation of undesirable carbonaceous deposits on the inner walls of the reactor. Although the amount of coke deposited is only 0.01 90 of the naphtha feed, for large on-stream times, the thickness of the deposit can be as much as 10 mm. This coke deposit reduces the overall heat-transfer coefficient and increases the pressure drop across the reactor. This results in a gradual increase with run time of both the reactor rube metal temperature and the pressure drop across the reactor, necessitating periodic shutdowns for decoking. This decoking operation results in loss of production, affects the coil life, and increases the utility costs. It has been estimated that surface reactions often account for 5-10% profit losses in smaller diameter reactors, which are being increasingly used to enhance olefin selectivity (Albright and Tsai, 1983). During pyrolysis, coke can form both in the gas phase and on the metal surface (Trimm, 1983). Albright and Marek (1988a-c) have suggested that coke can be formed by three mechanisms. These include metal-catalyzed reactions in which metal carbides are intermediate compounds, noncatalytic coke formed from tars, and reaction of small precursors with free radicals on the coke surface. The effect of the composition of the metal surface on the rate of coke formation is well-documented. Baker and Chludzinski (1980) studied the effect of various oxide additives on the growth of filamentous carbon on nickeliron surfaces. Above 620 "C, W03, Ta205,MOO,, and Si02 were found to inhibit the coking rate, with Si02being the most effective. Until recently, very little attention has been given to develop techniques to reduce coke formation during naphtha pyrolysis. A technique commonly used to reduce coke formation is to either presulfide the reactor (Trimm and Turner, 1981; Shah et al., 1976) or to add sulfur compounds to the feed (Bajus et al., 1981, 1983; Sahu and Kunzru, 1988). Aluminized (or alonized) steels have proved to be effective in reducing the coke formation (Albright et al., 1979; Marek and Albright, 1982; Albright and Tsai, 1983). Brown et al. (1982) have shown that the rate of coke formation can be significantly reduced by depositing an inert silica coating over the metal surface. Similarly, Kukes et al. (1984) have patented a process for inhibition of coke formation, which includes treatment of the Inconel 800 with hexachlorodisilane and subsequent oxidation to S O z . Several workers have reported that coking can be reduced if the reactor metal is coated with a film of passivating compound or alloy. Feigin et al. (1985) achieved a significant reduction in the coking rate
* To whom correspondence
should be addressed.
0888-5885/89/ 2628-1293$01.50/0
by coating the reactor walls with a descending film of 60% tin, 30% lead, and 10% cadmium. Feigin et al. (1981) reported that coking can be completely eliminated by producing 10-2-10-5-cm-thick films of molten lead or NaC1-KC1 salt mixture. Porter and Reed (1985) found that coke deposition on metals could be reduced by treating the metals with a compound of tin and aluminium, aluminium and antimony, or antimony, aluminium, and tin. They also developed coke inhibitors containing compounds of tin and phosphorus, antimony and phosphorus, or tin, antimony, and phosphorus (Porter and Reed, 1986). Reed et al. (1983) achieved a reduction in coke deposition by coating the reactor walls with SnO-Sb203 or Sb203Ge02. These oxides most probably catalyze the cokesteam reaction. Another technique reported for reducing the coke formation during hydrocarbon pyrolysis is to reduce the surface roughness of the tubes by polishing the metal surface (Crynes and Crynes, 1987). Recent investigations have shown that phosphoruscontaining additives can significantly inhibit the coking rate. Naberezhnova et al. (1983) found that organophosphorus compounds such as CH3COCH2C(CHJ2PhPS(C2H5) and CH3COCH&!(CH3)2PhPS(OC2H,) were effective in reducing the coke formation without affecting the product yields. The development of propietary coke inhibitors by Nalco Co. (for example, Nalco 5210,5211) containing phosphorus with or without sulfur has also been reported (Boone, 1983). Ghosh and Kunzru (1988) achieved a significant reduction in the rate of coke formation during naphtha pyrolysis by adding triethyl phosphite to the feed. For instance, by adding 100 ppm triethyl phosphite to the naphtha, the coke formation was reduced by 56%. These authors also developed a model for the coke formation 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 the coke simultaneously deposited at different rates on the bare metal surface and the metal-phoshorus complex. However, neither the effect of different phosphorus compounds on the rate of coking nor the rate of decoking of the phosphorus-containing coke was investigated. The objective of this study was to investigate the effect of triphenyl phosphite (a compound larger than triethyl phosphite) on the rate of coke formation during naphtha pyrolysis as well as on the rate of subsequent decoking. A further objective was to model the coke formation in the presence of triphenyl phosphite. Experimental Section The pyrolysis experiments were conducted in a jetstirred reactor, and the details have been described earlier 0 1989 American Chemical Society
1294 Ind. Eng. Chem. Res., Vol. 28, No. 9, 1989
I Phosphor us content ,p p m * O
0
2
L
0
50
d
100
0
1000
6
Time,h
Figure 1. Effect of phosphorus concentration on rate of coke formation ( T = 1088 K, 6 = 0.7, 7 = 0.53 s).
(Kumar and Kunzru, 1985). The feed was a desulfurized naphtha with a boiling range of 362-450 K. The percentage by weight of paraffins, naphthenes, and aromatics was 61.1, 23.9, and 15.0, respectively. The sulfur content was 5 ppm. For studying the effect of triphenyl phosphite on the rate of decoking, a thermogravimetric balance (Ainsworth, Denver, CO) was used. The coke was burnt with pure oxygen either at isothermal conditions or at a linear heating rate.
Results and Discussion To study the coke formation during naphtha pyrolysis, experiments were conducted at atmospheric pressure in the temperature range 1088-1108 K, with steam as an inert diluent. The weight ratio of steam to naphtha, 6, and the space time, 7,were kept fixed a t 0.7 kg/kg and 0.53 s, respectively. The phosphorus content of the feed was varied from 50 to 1000 ppm, by adding triphenyl phosphite to the naphtha. For all the runs, the material of construction of the suspended plate was Inconel 600. Effect of Phosphorus Content of Feed The addition of triphenyl phosphite 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. It was observed that the rate of coke formation was initially high and then decreased asymptotically to a constant value in approximately 2-3 h. As can be seen from this figure, coking rates were significantly reduced in the presence of triphenyl phosphite. For example, for naphtha containing 100 ppm phosphorus, the asymptotic rate of coke formation was reduced by 66%. Visual inspection showed that, when triphenyl phosphite was added to the feed, the metal surface was, at times, not completely covered with the coke. Boone (1983) achieved a reduction of approximately 50% in the coke formation during n-hexane pyrolysis a t 1088 K and a concentration of 200 ppm phosphorus in the feed. Ghosh and Kunzru (1988) studied the effect of triethyl phosphite on the coking rate for the same naphtha, and as shown in Figure 2, for the same phosphorus concentration in the feed, triphenyl
c
Additive 0
10.0
0
200
Triphenyl phosphite
LOO 600 800 Phosphorus content of feed ,ppm
1000
Figure 2. Reduction in asymptotic rate of coke formation with phosphorus content (7' = 1088 K, 6 = 0.7, T = 0.53 s). O
Phosphorus-free naphtha ,N2 as iner Phosphorus-free
naphtha.steam"
"
A
Phosphorus containing naphtha ,N2
A
Fhosphorus-containirq rmphtha,steun
"
"
-
t 2
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6
lime, h
Figure 3. Comparison of rate of coking using steam or nitrogen as diluent (T = 1088 K, 6 = 0.7, T = 0.53 s).
phosphite is a more effective coke inhibitor than triethyl phosphite. Although a detailed product analysis was not carried out, addition of triphenyl phosphite had no effect on the total liquid and gaseous products, in agreement with the earlier results for triethyl phosphite. Although steam does not take part in the main pyrolysis reactions, it can react with the deposited coke to form carbon oxides (Bajus et al., 1979, 1981, 1983). To investigate the importance of coke removal by steam gasification, runs were conducted by replacing steam with nitrogen for identical temperatures, 6, and r . As can be seen from Figure 3, both for phosphorus-free naphtha and naphtha
Ind. Eng. Chem. Res., Vol. 28, No. 9, 1989 1295 Phosphorus content, p p m 0
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