Triethyl Phosphite Additive-Based Fouling Inhibition Studies

For fresh coupons, the asymptotic rate of fouling was found proportional to the iron content of coke. A major decomposition product of the additive ha...
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Ind. Eng. Chem. Res. 1999, 38, 1364-1368

Triethyl Phosphite Additive-Based Fouling Inhibition Studies† Shekhar Kumar* R&D Section, Reprocessing Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India

Coke formation during naphtha pyrolysis has been studied in the presence of triethyl phosphite in a jet-mixed reactor at the temperature of 1088 K. The quantitative effect of the addition of triethylphosphite has been reported. Concentrations of iron and phosphorus in the formed coke have been analyzed by chemical methods. For fresh coupons, the asymptotic rate of fouling was found proportional to the iron content of coke. A major decomposition product of the additive has been identified by chemical analysis of an aqueous effluent and a possible mechanism is proposed for the inhibition of coke formation. Introduction

Table 1. Characteristics of Naphtha Feed

In the steam-cracking process, naphtha or feed hydrocarbon is diluted with steam to lower the hydrocarbon partial pressure and is heated to an elevated temperature in radiant heaters. In this process, large hydrocarbon molecules break into smaller molecules such as ethane, ethylene, propylene, and aromatics. The successive deposition of coke over the inner surface of radiant tubes results in increased thermal resistance across the tube wall, leading to high tube skin temperatures. Because of deposits, the effective crosssection area of radiant tubes decreases, resulting in a higher pressure drop across cracking coils. Deposits are also formed in transfer line exchangers (TLEs) employed for rapid quenching of cracked gases. Often partial blockage of TLEs is a starting point for plant shutdown, resulting in economic penalties. Coke formation is mainly due to two mechanisms, catalytic coking and asymptotic coking. In the catalytic coking, base metal constituents (mainly Fe and Ni) catalyze the dehydrogenation of hydrocarbons at elevated temperatures. This dehydrogenation of hydrocarbons yields a product rich in carbon. This product deposits on the inner tube skin surface and finally degrades to filamentous deposits of coke. Catalytic coking rates are roughly proportional to the bare base metal area. As the metal area is covered progressively, the second mechanism takes over. The asymptotic coking is mainly due to gas-phase coking reactions. However, to some extent it is influenced by the texture and composition of the top coke surface. In the present study, triethyl phosphite was used as an additive for inhibiting coke formation in naphtha pyrolysis. Reaction products were analyzed for phosphorus and iron. An attempt has been made to explain these results on the basis of existing as well as emerging literature on the subject. Experimental Section The pyrolysis experiments were conducted in a jetmixed stainless steel reactor. The preheater assembly * Phone: +91-4114-40399. Fax: +91-4114-40207. E-mail: [email protected]. † Studies described in this communication were performed at the Petroleum Engineering Laboratory of the Department of Chemical Engineering, I. I. T. Kanpur 208 016, India.

physical properties (i) density (293.15 K): 718 kg‚m-3 (ii) ASTM distillation: IBP ) 336 K 50% ) 377 K FBP ) 431 K total recovery ) 96.5% (iii) PONA analysisa (wt %) total paraffins ) 61.1% total olefins ) negligible total naphthenes ) 23.9% total aromatics ) 15.0% average mol. wt. ) 102.3 a PONA analysis was carried out at the Indian Institute of Petroleum, Dehradoon, India.

was made of 0.025 m i.d. SS-316 tubing. The reactor length and volume were 0.023 m and 11.3 × 10-3 L. The plate separating the reactor and reheater assembly had 32 1-mm φ holes uniformly distributed. To provide good mixing, the preheater chamber was filled with crushed ceramic beads. The preheater-reactor assembly was heated in a temperature-controlled furnace. Temperature could be controlled within (2 K by a temperature indicator-controller (CT-806, Century Instruments). The sensor was a chromel-alumel thermocouple. Naphtha and water were pumped by a duplex micro pump (SR-BD7, V. K. Pump Industries, Bombay) which could be adjusted for various flow rates ranging from 0.2 to 30 mL/min. Steam, used as an inert gas, was generated in a vaporizer (1.5 kW). Diluent steam was mixed with naphtha before inserting it in the preheater (1.5 kW). The steam-naphtha mixture was heated in this preheater to about 770 K. To start the experiment, the preheater-reactor assembly was heated to 15 K higher than the setpoint desired. After the stabilization of temperature, the reactor was flushed out with steam for about 30 min to have an inert atmosphere in the setup. After the reactor was flushed out, naphtha feed (Table 1) was started. Because of the endothermic nature of a pyrolysis reaction, the temperature dropped by approximately 10 K. Often a minor readjustment of the controller was required. After the restabilization of temperature, a small rectangular coupon of Inconel-600 was suspended by a thin wire into the center of the reactor. This coupon was removed every 30 min and resuspended after being weighed on an electrobalance. A small purge flow of

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nitrogen was maintained. Weighing and resuspension, in 30 min intervals, was continued until constant coking rates were obtained. Preparation of Samples for the Determination of the Iron and Phosphorus Content of Coke. The procedure of a normal run was repeated except with two variationssa larger coupon was used and it remained inside the reactor until asymptotic coking rates were obtained. The time required for these runs was known from normal runs. After the lapse of this time, steam was stopped and the coupon was allowed to cool in a nitrogen atmosphere. When the coupon cooled to room temperature, deposited coke was scrapped with a nonmetallic straight edge. Determination of the Phosphorus Content of Coke. Phosphorus present in coke might have compounded to coke; oxidation with strong acids was required to break C-P and C-O-P bonds. Determination was a two-step procedure. In the first step, coke was oxidized with strong acids and in the second step, phosphorus was determined by using the chromatogenic-spectrophotometric technique. Acid Digestion. In this process, a weighed amount of coke was digested with a 5:1 mixture of concentrated HNO3 and concentrated H2SO4. The reaction mixture was digested to one-sixth of its original volume by heating it in a Pyrex tube on a sand bath for approximately 24 h. After the mixture cooled to room temperature, reaction products were neutralized with 1 N NaOH. When not in use, this Pyrex tube along with all other glassware required for chemical analysis was kept immersed in dilute HCl solution to prevent any stray pickup of phosphorus compounds. All the reagents used in the analysis were of ANALAR grade. Chromatogenic Spectrophotometric Method for the Determination of Phosphorus. Although a number of methods were available, the stannous chloride method was chosen because of its simplicity and adequate accuracy. This method involved the formation of molybdophosphoric acid, which was then reduced to intensely colored complex molybdenum blue by stannous chloride. The minimum detectable concentration was about 3 ng/mL of the phosphorus. In this method, to the 10 mL of sample solution, 0.4 mL of ammonium molybdate (25 g/L) and 0.05 mL of stannous chloride solution (2.5 g/100 mL of glycerol) were added. After a specified interval of 10 min, the color was measured photometrically at 690 nm using a single-beam spectrophotometer (Systonics 105 Mk I). The spectrophotometer was calibrated using blank stock solution to which appropriate reagents were added. Stock solution was prepared by dissolving 21.950 g of potassium dihydrogen phosphate into 1 L of water. Determination of the Iron Content of Coke. The iron content of the coke was determined by precipitating Fe as Fe(OH)3 and drying the precipitate at 375.15 K to a constant weight. Results, for the absence of phosphorus, obtained by gravimetry (≈0.6 wt %) were in good agreement with those by EDAX (≈0.4 wt % reported by Das et al.1 and 0.56-0.78 wt % reported by Albright and Marek2). Chromium and nickel were not analyzed in the present work. Results and Discussion Effect of the Surface Composition of a Base Metal/Alloy. The surface finish also plays a major part

in controlling the fouling rates (Crynes and Crynes3). For freshly polished coupons, the asymptotic fouling rate was reported to be lower, and as the number of coking/ decoking cycles increased, it started to climb up (Bach et al.4). Albright and Marek5 reported that asymptotic fouling rates for alonized Inconel-800 coupons were essentially identical to those for quartz coupons. Brown et al.6 observed that, during steam oxidation, the surface oxide layer was rich in silica. They studied coke formation on silica-coated (deposited through chemical vapor deposition from tetraethoxysilane) coupons and reported a significant reduction in fouling. However, coating and surface enrichment using combinations of Al, Si, and Cr have a short life in a pyrolysis environment, rendering these options impracticable. Passivation through CS2- or H2S-based “pre-sulfiding” was also reported to be effective in reducing coke formation (Albright and McConnel,7 Bajus et al.,8-9 and Trim and Turner10). Ngan and John11 reported surface modification by Cr2O3 film generation. Parks and Schillmoller12 underlined a commercial procedure for dimethyl sulfide based presulfiding. Effect of Additives on the Fouling Rate. Inhibition of fouling during naphtha pyrolysis by the addition of sulfur compounds is discussed in detail by several researchers (Albright and McConnel,7 Trim and Turner,10 Bajus et al.,8-9 Depeyere et al.,13 Sahu and Kunzru14). However, Trim and Turner10 reported that the addition of H2S to feed had different effects on stainless steel and nickel surfaces. While on stainless steel, it inhibited the coke formation; on nickel surfaces, it resulted in higher rates of coke formation. A negative impact of H2S addition to feed on coke formation was also reported by Valenyi et al.15 for inconel-600 surfaces. The use of phosphorus-containing additives has also been reported well in the literature for the inhibition of coke formation. Naberezhnova et al.16 and Boone17 reported successful usage of phosphorus-containing additives. Das et al.1, Ghosh and Kunzru,18 Vaish and Kunzru,19 and Chowdhury and Kunzru20 discussed phosphite-based inhibition of coking. In the studies reported in this paper, fouling inhibition was studied using triethyl phosphite. Coking runs were similar to those reported by Ghosh and Kunzru.18 However, Ghosh and Kunzru18 emphasized on kinetics issues and no effort was made to identify decomposition products of the additive. In this paper, a major decomposition product of the additive was identified by chemical analysis. Figure 1 shows the temporal variation of the coking rates for different levels of additive addition. Figure 2 shows variation in asymptotic rates with phosphorus addition in the feed. The iron content of coke was measured by the method described in the Experimental Section. It could be observed from Figures 2 and 3 that the iron content of coke nearly responds to the asymptotic rate of coking at all phosphorus addition levels. Decomposition of a Phosphorus-Bearing Additive in a Pyrolysis Environment. Phosphorus-based additives used in earlier studies were mainly esters and ester derivatives such as triethyl phosphite, triphenyl phosphite, benzyl diethyl phosphite, and so forth. Ghosh and Kunzru18 and Vaish and Kunzru19 stated that the phosphorus additive might have decomposed, yielding a metal-phosphorus complex. No effort was made to ascertain decomposition products.

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Figure 1. Temporal variation of fouling rates for fresh Inconel600 coupons: effect of phosphorus (as TEP) addition.

Figure 4. Effect of phosphorus addition in the naphtha feed on the phosphorus content of coke deposited on the coupon.

(Et-O)3-P + H2O f H2 + (Et-O)3dP

Figure 2. Effect of phosphorus addition on asymptotic coking rates (kg‚m-2‚h-1).

Figure 3. Effect of phosphorus addition in the naphtha feed on the iron content of coke deposited on the coupon.

A glance through the literature suggested that phosphite-based additives, when heated in a pyrolysis environment in the presence of water, might decompose into phosphate, which might be converted further into phosphonates. Beach et al.21 reported phosphonates as major reaction products from low-temperature pyrolysis of dimethyl hydrogen phosphite. It was reported earlier (Hackspill and Weiss, 1932) that pyrolysis products of triethyl phosphite were phosphine and phosphates in the absence of water. When water was present, the products were mainly hydrogen and phosphates. Since the phosphorus atom has one free loan pair of electrons, it readily combines with oxygen. In the presence of water (steam), triethyl phosphite undergoes decomposition in the following way:

(1)

This reaction is fairly rapid at temperatures above 525 K. At still higher temperatures, phosphate thus produced undergoes condensation polymerization to yield long-chained polyphosphates. A part of the phosphates is also converted to thermally stable polyphosphonates, which may further undergo polymerization. Since phosphates have a good adhesion to a metal surface, the main cause of fouling inhibition could be taken as covering of the metal surface by a polymeric network of phosphates and other derivatives. This protective cover reduces metal atom migration across the coke surface. The surface texture of deposited coke is also smoothened, thus decreasing surface activity and the macro radical nature of the top surface. However, to have the fouling rate limited to a lower value, continuous injection of the additive is required to maintain smoothness of the freshly deposited coke layer. The phosphorus content of coke was almost linear to the phosphorus content of the naphtha feed as shown in Figure 4. Temporal Variation of the Fouling Rate When Phosphorus Addition Is Stopped. It was reported earlier (Ghosh and Kunzru,18 Vaish and Kunzru19) that a memory effect was observed when phosphorus addition was abruptly stopped by switching over to phosphorus-free naphtha. In both instances, authors stated that the new asymptotic fouling rate was higher than that for phosphorus-containing naphtha but lower than what would have been observed with having phosphorusfree feed throughout. A similar run was conducted for a 10 h duration, the phosphorus-containing feed being fed for the first 4 h. Visual observation of coke samples revealed no major difference between surface morphologies of coke from switched feed and that from feed without any phosphorus fed throughout. Thus, the socalled memory effect could be explained on the basis of texture modification by phosphorus addition. In the presence of phosphorus, deposited coke has a smooth texture with lesser surface activity. As soon as phosphorus addition is stopped, coke texture becomes rough and surface activity increases because of the macroradical nature of deposited coke. Thus, the fouling rate starts to climbup, and when a sufficient thickness of coke has been deposited to nullify the earlier addition of phosphorus, the fouling rate reaches the old asymptote of coking for phosphorus-free naphtha. Fate of Phosphorus in the Absence of Pyrolysis Reactions. To determine the fate of phosphorus in the

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absence of pyrolysis reactions, a special run was conducted with naphtha feed remaining off. An Inconel600 coupon was suspended in a 40% H2O/60% N2 atmosphere. Triethyl phosphite was added to water to the tune of 1000 ppm. This run lasted 2 h. Visual inspection of the coupon indicated a golden yellow smooth deposit over it. This deposit was scrapped, and after being dissolved in hot water, was tested for the presence of phosphates by the chromatogenic-photometric technique described earlier in the Experimental Section of this paper. A qualitative determination revealed the presence of water-soluble phosphates in the deposit. Fate of Phosphorus during a Pyrolysis Reaction. Although triesters of phosphorus form several compounds with iron/nickel at higher temperatures such as C15H30FeO9P2 (tricarbonyl bis(triethyl phosphite-p)iron), C17H40FeO9P3 ((1,2,3,-n)-2-cyclooctene-1-yl)tris(trimethyl phosphite-p)iron), C24H62FeO12P4 (dihydrotetrakis triethyl phosphite-p)iron), C21H17FeO6P (tricarbonyl dihydro(triethyl phosphite-p)iron), C12H36NiO12P4 (tetrakis(trimethyl phosphite-p)nickel), and C16H37NiO6P2+ (1,2,3-n)-2-butene-1-yl)bis(triethyl phosphite-p)nickel(1+), formation of these compounds in the presence of water and at around 1000 K is clearly ruled out. As stated earlier, the likely route of a phosphorus additive is to undergo decomposition, phosphate and hydrogen being the major products. Earlier phosphates were detected in a leachate from deposits formed in the absence of naphtha feed. Phosphorus should follow the same route in the pyrolysis reaction. To ascertain validity of this assumption, pyrolysis reaction products were analyzed for the presence of phosphorus. Straightforward determination of phosphorus in the coke was difficult. Thus, phosphorus present in the coke was converted to water-soluble orthophosphates by acid digestion as listed in the Experimental Section and determined quantitatively using the chromatogenicphotometric method. The phosphorus content of the coke was a function of the phosphorus content of the feed as shown in Figure 4. Aqueous condensate from the condenser was also tested for the presence of phosphorus. Since condensate was often cloudy because of the presence of phenolic/naphthenic derivatives, in this case only qualitative estimation was done. In a special run with an additive concentration of 1000 ppm to the naphtha feed, temporal phosphorus concentration in the aqueous condensate was measured. As stated above, this estimation was approximate because of the turbid/ cloudy nature of the condensate and related interference in the photometric procedure. Since phosphates have good adhesion to wetting surfaces, any attempt to filter the condensate through filter paper/glass frit would have resulted in further inaccuracies. These results are shown in Figure 5. There is no reliable method for phosphorus determination in the condensed organic product, which already has many hydrocarbons/derivative compounds. Thus, organic analysis was not attempted. If a P32-labeled additive is used, then analysis would be easier. Conclusions Coke formation during naphtha pyrolysis has been studied in the presence of triethyl phosphite in a jetmixed reactor at atmospheric pressure at the temperature of 1088 K and the quantitative effect of the addition of triethyl phosphite has been reported. Con-

Figure 5. Temporal variation of the phosphorus content (ppm) of an aqueous condensate for a 1000 ppm phosphorus addition in the naphtha feed.

centrations of iron and phosphorus in the formed coke have been analyzed. For fresh coupons, the asymptotic rate of fouling was found to be roughly proportional to the iron content of coke. The major decomposition product of the additive has been identified as phosphate by chemical analysis of the aqueous effluent. A possible mechanism is proposed for the inhibition of coke formation by the inclusion of phosphate into the deposit matrix. Acknowledgment Author sincerely acknowledges the valuable guidance and helpful supervision by Prof. Deepak Kunzru during his stay at I.I.T. Kanpur. Sincere thanks are due to Prof. Jitendra Kumar of the Advanced Centre for Material Sciences (I.I.T. Kanpur) and Prof. Malay Choudhury of the Department of Environmental Engineering. (I.I.T. Kanpur) for allowing use of their facilities. Help provided in the analytical work by Mr. M. Jawed of the Environmental Engineering Laboratory (I.I.T. Kanpur) is acknowledged. Author wishes sincere thanks to Dr. S. B. Koganti, Head, R&D Section, Reprocessing Group, IGCAR and Dr. Placid Rodriguez, Director, IGCAR for valuable guidance and encouragement. Literature Cited (1) Das, P.; Prasad, S.; Kunzru, D. Organophosphorus Compounds as Coke Inhibitors during Naphtha Pyrolysis. Effect of Benzyl Diethyl Phosphite and Triphenylphosphine Sulfide. Ind. Eng. Chem. Res. 1992, 31, 2251. (2) Albright, L. F.; Marek, J. C. Mechanistic Models for Formation of Coke in Pyrolysis Units Producing Ethylene. Ind. Eng. Chem. Res. 1988, 27, 755. (3) Crynes, L. L.; Crynes B. L. Coke Formation on Polished and Unpolished Incoloy 800 Coupon during Pyrolysis of Light Hydrocarbons. Ind. Eng. Chem. Res. 1987, 26, 2139. (4) Bach, G.; Zimmermann, G.; Kopinke, F. D.; Barendregt, S.; Oosterkamp, P.; Woerde, H. Transfer-Line Heat Exchanger Fouling during Pyrolysis of Hydrocarbons. 1. Deposits from Dry Cracked Gases. Ind. Eng. Chem. Res. 1995, 34, 1132. (5) Albright, L. F.; Marek, J. C. Analysis of Coke Produced in Ethylene Furnaces: Insights on Process Improvements. Ind. Eng. Chem. Res. 1988, 27, 751. (6) Brown, D. E.; Clark, J. T. K.; Foster, A. I.; McCarroll, J. J.; Sims, M. L. Inhibition of Coke Formation in Ethylene Steam Cracking. In Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K., Eds.; ACS Symposium Series, American Chemical Society: Washington, DC, 1982; Vol. 202, p 23-43.

1368 Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999 (7) Albright, L. F.; McConnel, C. F. Deposition and Gasification of Coke During Ethane Pyrolysis. Presented at the 175th National Meeting of the American Chemical Society, Anaheim, CA, March 1978. (8) Bajus, M.; Vesely V.; Baxa, J.; Leclercq, P. A.; Rijks, J. A. Steam Cracking of Hydrocarbons. 5. Effect of Thiophene on Reaction Kinetics and Coking. Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 741. (9) Bajus, M.; Baxa, J.; Leclercq, P. A.; Rijks, J. A. Steam Cracking of Hydrocarbons. 6. Effect of Dibenzyl Sulphide and Dibenzyl Disulphide on Reaction Kinetics and Coking. Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 335. (10) Trim, D. L.; Turner, C. J. The Pyrolysis of Propane. II. Effect of Hydrogen Sulphide. J. Chem. Technol. Biotechnol. 1981, 31, 285. (11) Ngan, D. Yuk-Kwan; John, R. C. High-temperature Gas Treatment for Chromizing of Stainless Steel used in Reactors. Chem. Abstr. 1991, 114, 190140p. (12) Parks, S. B.; Schillmoller, C. M. Use Alloys to Improve Ethylene Production. Hydrocarbon Proc. 1996, March, 53. (13) Depeyre, D.; Flicofeaux, C.; Blouri, B.; Osseby, J. G. Pure n-Nonane Steam Cracking and the Influence of Sulfur Compounds. Ind. Eng. Chem. Proc. Des. Dev. 1985, 24, 920. (14) Sahu, D.; Kunzru, D. Effect of Benzene and Thiophene on Rate of Coke Formation during Naphtha Pyrolysis. Can. J. Chem. Eng. 1988, 66, 808.

(15) Velenyi, L. J.; Song, Y.; Fagley, J. C. Carbon Deposition in Steam Pyrolysis Reactions. Ind. Eng. Chem. Res. 1991, 30, 1708. (16) Naberezhnova, G. N.; Nurtdinov, S. K.; Belyakova, L. D. Organophosphorous Compounds as Inhibitors of Coke Formation during Pyrolysis of Liquid Hydrocarbons. Chem. Abstr. 1983, 99, 25105v. (17) Boone, K. Coke Control Extends Furnace On-Stream Time. Oil Gas J. 1983, 81, 83. (18) Ghosh, K. K.; Kunzru, D. Reduction of Coke Formation Using Triethyl Phosphite. Ind. Eng. Chem. Res. 1988, 27, 559. (19) Vaish, S.; Kunzru, D. Triphenyl Phosphite as a Coke Inhibitor during Naphtha Pyrolysis. Ind. Eng. Chem. Res. 1989, 28, 1293. (20) Chowdhury, S. N.; Kunzru, D. Benzyl Diethyl Phosphite as a Coke Inhibitor during Naphtha Pyrolysis. Tubular Reactor Studies. Can. J. Chem. Eng. 1993, 71, 873. (21) Beach, L. K.; Drogin, R.; Shewmaker, J. E. Pyrolysis of Dimethyl Hydrogen Phosphite. Ind. Eng. Chem. Prod. Res. Dev. 1963, 2, 145.

Received for review June 3, 1998 Revised manuscript received December 15, 1998 Accepted December 15, 1998 IE980349J