Role of the Reactor Surface in Pyrolysis of Light Paraffins and Olefins

17 Role of the Reactor Surface in Pyrolysis of Light Paraffins and Olefins STEVEN M. BROWN and L Y L E F. ALBRIGHT

Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0032.ch017

School of Chemical Engineering, Purdue University, West Lafayette, Ind. 47907

Surface reactions occurring during pyrolysis s t i l l need considerable clarification. These reactions probably include the initiation and termination of free radicals, coking, and the formation of metal oxides, metal carbides, and metal sulfides. Earlier findings at Purdue University (1-4), including those of Tsai and Albright (5), have indicated the importance of these surface reactions during the pyrolysis of light hydrocarbons. More quantitative information is, however, s t i l l needed particularly regarding the surface reactions of light paraffins and olefins as a function of the materials of construction of the reactor. In order to accentuate the surface reactions, the present investigation was limited to relatively low temperatures (450750°C) and to long space times (5-30 s e c ) . For such relatively low temperatures, gas-phase reactions and/or initiation of reactions are slight; surface reactions, however, often occur readily at such conditions. The surface reactions were investigated in four tubular flow reactors: 304 stainless steel, 410 stainless steel, Incoloy 800, and Vycor glass. Four light hydrocarbons were pyrolyzed in this study: ethane, propane, ethylene, and propylene. The results were used to investigate conversions, carbon deposition, and product yields as a function of temperature, of hydrocarbon feed, and of the material of construction, age, and pretreatment of the reactor. Pretreatments were performed with oxygen and hydrogen. Steam was added to the feed in some runs. The equipment and operating procedure employed were similar to those used by Tsai (5), and they are discussed in more detail by Brown (4).

•Present address:

Diamond Shamrock Corp., Cleveland, Ohio. 296

In Industrial and Laboratory Pyrolyses; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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Role of the Reactor Surface

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Experimental Results Valuable new Information have been obtained relative to the surface reactions that are often of Importance during the pyrolys1s of l i g h t hydrocarbons. The material of construction, the history and pretreatment of the reactor surfaces, and the operating conditions employed, a l l had an appreciable effect on these surface reactions that result in the destruction of olefins and in the production of coke and carbon oxides. Effect of Temperature. Experiments in which the temperature of the reactor was increased from about 450 C to perhaps 700 C were helpful in c l a r i f y i n g the Importance of surface reactions in the metal reactors tested. Figures 1, 2 , and 3 indicate that for ethane, ethylene, and propylene, appréciable conversions of the dry feed hydrocarbons begin at 450 -475 C in the metal reactors tested, but do not begin until about 550 -575°C in the Vycor reactor. Propane conversions began at about 450 -475 C 1n a l l reactors Investigated. The composition of the product gases for runs made in the range of about 450 -600 C differed s i g n i f i c a n t l y , depending on the reactor used. Table I shows the products obtained for runs with dry ethane, propane, ethylene, and propylene; the products are arranged in the approximate order of 1 προrtance. In the Vycor reactor, the expected gas-phase products are predominant; for ethane pyrolysis, ethylene and hydrogen are expected to be the major gas-phase products, and for propane pyrolysis, ethy lene, propylene, methane, and hydrogen are expected. In the metal reactors and especially in the 304 stainless steel reactor, surface reactions resulting in coke are of much greater impor tance. Coke was sometimes a major product whereas i t 1s not 1n commercial pyrolysis units that have much lower surface-to-vol ume ratios in the reactor tubes. Hydrocarbon conversions can, in general, be represented f a i r l y well by f i r s t - o r d e r reaction k i n e t i c s , and the conversion levels for runs made with a constant hydrocarbon flow rate as a general rule increased s i g n i f i c a n t l y as the temperature increased. Figures 1, 2, and 3 show typical results for ethane, propane, ethylene, and propylene. Based on f i r s t - o r d e r k i n e t i c s , the activation energies for ethane, propane, ethylene, and propylene were determined in the various reactors tested. In the Vycor reactor, these activation energies were approximately 51, 57, 56, and 66 k cal/g mole respectively. They were much lower in metal reactors especially a f t e r the reactor was oxidized. In a r e l a t i v e l y new and unoxidized Incoloy reactor, the activation ener gies were 15, 47, 27, and 26 k cal/g mole respectively. Two explanations were considered to account for the s i g n i ficant reactions and for the different types of products obtained at lower temperatures (475 - 600 C) in the metal reactors. The f i r s t one considered was that free radicals formed in the gas phase are destroyed to a greater extent at the walls of the Vy-


298


INDUSTRIAL AND LABORATORY PYROLYSES

τ

different reactors

1

1

TEMPERATURE

1

Γ

°C


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Table I.


299

Major Products for the Pyrolysis of Light Hydrocarbons in Three Reactors. (450° to 600°C) Type of Reactor

Vycor Glass

Incoloy 800

304 Stainless Steel


Ethane Feed Ethylene Hydrogen Methane Propylene Coke

Ethylene Hydrogen Methane Propylene Coke

Hydrogen Coke Ethylene Methane Propylene

Propane Feed Propylene Methane Ethylene Hydrogen Ethane Coke

Hydrogen Propylene Ethylene Methane Coke Ethane

Hydrogen Coke Propylene Methane Ethylene Ethane

Ethylene Feed Ethane Acetylene Hydrogen Methane Coke

Hydrogen Coke Ethane Methane Acetylene

Hydrogen Coke Ethane Me thane Acetylene

Propylene Feed Ethylene Methane Hydrogen Coke Ethane

Hydrogen Coke Ethylene Methane Ethane

Hydrogen Coke Ethylene Methane Ethane




300


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301


cor glass reactor as compared to the metal reactors, and hence reduced the reaction rate in the glass reactor. This explanation seems unlikely since metal surfaces are generally considered to be better free-radical terminators than glass surfaces (6). The preferred explanation is that the metal surfaces I n i t i a t e both decomposition and pyrolysis reactions to a much greater extent than Vycor. The hydrocarbon feed probably reacts breaking carbonhydrogen bonds and resulting in essentially the following reac tion: Hydrocarbon

• Coke

+ Hydrogen Radicals ( Η · )

To support this explanation, coke was formed (as a major product) on the metal surfaces in the temperature range of 450 to 550 C. It seems quite l i k e l y that as the carbon-hydrogen bonds are broken on the reactor surface, hydrogen atoms (or free radicals) form and at least some migrate into the gas phase to i n i t i a t e gas-phase reactions. As further suoport of t h i s hypoth e s i s , some products formed in the 450 to 550 C range appear to be products that are formed by gas-phase reactions, namely o l e f i n s , hydrogen, and methane. With propane as the feed hydrocarbon, r e l a t i v e l y l i t t l e d i f ference was noted in the conversions in the different reactors (see Figure 1). Probably gas-phase reactions begin with propane at much lower temperatures than for the other three hydrocarbons; propane i s the only hydrocarbon of those tested that has secon dary carbon-hydrogen bonds. For propane, the metallic reactors did result in s l i g h t l y higher coke and hydrogen yields than the Vycor reactor, but the conversions were s i m i l a r . As the metal reactors were used in this investigation, they became roughened, and the relative importance of surface to gasphase reactions increased. This roughening also resulted in higher reaction rates and lower activation energies, hence i n d i cating the increased relative importance of surface reactions. Deactivation and Activation of Metal Reactors. As coke was formed on a metal reactor, the reactor in general was partly de activated, but never to the low level of the Vycor reactor; de activation i s defined as lower conversions of the feed hydrocar bon. Figure 4 shows results with ethylene feed in the 304 s t a i n less steel reactor. Conversions were highest in the new reactor (Run 1 ) ; a new reactor i s defined as one that has been used for reactions less than 1-2 hours. The conversions in both Runs 2-A and 2-B were lower than those of Run 1 because of the coke formed during Run 1: Run 2-B (made as a continuation of Run 2-A) had even lower conversions than Run 2-A. After Run 2-B, hydrogen was passed through the reactor; the hydrogen reacted with the surface carbon forming methane. Run 3-A was made after the rate of re action between hydrogen and surface coke had decreased to almost zero. Run 3-A indicated an active reactor especially in the early stages of the run (namely up to 525 C). At 550 and 575°C,



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the reactor was s t i l l much more active than in Runs 2-A and 2-B. Apparently, however, the reactor deactivated rapidly during Run 3-A at 575 C, presumably because of the buildup of coke. The reactor was considerably less active in Puns 3-B made after Run 3-A. Figure 5 gives more details on the deactivation of the 304 stainless steel reactor during Run 3 (using dry ethylene feed) and during Run 4 (using dry propylene feed). The reactor had been cleaned using hydrogen (and hence activated) before the start of both runs. The data points for these two runs are numbered chronologically. Deactivation because of coke formation was s i g n i f i c a n t during both runs as a comparison of data points # 4 and # 6 and of # 3 and # 7 indicate. Considerable new information has been obtained concerning the role of metal oxides on the surface of metal reactors. The results indicate that as a reactor oxidized with oxygen i s used,



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the a c t i v i t y of the reactor often passes through a maximum. Figure 6 shows how propylene conversions changed in both oxidized 304 stainless steel and oxidized Incoloy reactors. The data points are again numbered chronologically. In both reactors, conversions in the oxidized reactors were i n i t i a l l y very low with dry propylene feed at temperatures up to about 525 to 550 C. Then at 550-575°C in the Incoloy reactor and at 575-600°C in the 304 stainless steel reactor, s i g n i f i c a n t reactions were noted. Products were obtained of what was apparently both gas-phase freeradical reactions and surface decomposition reactions; coke and carbon oxides were both formed in s i g n i f i c a n t amounts. The production of carbon oxides indicated that the propylene was reacting with surface oxides on the reactor surface. The level and the types of surface oxides were changing (e.g. F e 0 was probably converted to Fe~0, or FeO), hence changing the surface a c t i v ity. o o When the reactors were then cooled to 475 to 525 C, s i g n i ficant propylene reactions were s t i l l noted. Clearly the reac2

0

3

4

Figure 6. Effects of oxidation of stainless steel 304 and Incoloy reactors on propylene conversions In Industrial and Laboratory Pyrolyses; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


304


tors were then in a much more active state than during the i n i t i a l phases of the runs. With continued use of the reactor and with dry propylene feed, both reactors then rather slowly de activated. This l a t t e r deactivation presumably occurred because of the formation of coke. Figure 7 shows the results of a propylene run at 500 C in an oxidized Incoloy reactor. In the f i r s t 75 minutes using dry pro pylene, the conversions passed through a maximum. Wet propylene was then used as a feedstock for the next 60 minutes; propylene conversions were s l i g h t l y higher and they perhaps decreased some what during this period. Of considerable Interest, the subse quent phase of the run with dry propylene indicated that the re actor had been s i g n i f i c a n t l y activated during the wet propylene phase of the run, presumably because of the formation of metal oxides on the surface (5). Continued use of dry propylene re sulted in a rather rapid deactivation of the reactor; the reactor

15141312-

OXIDIZED INCOLOY REACTOR

I I -

50 % STEAM Ν PROPYLENE DRY 'PROPYLENE

Jl

I

I

L

1 0 0 120 140 160 180 2 0 0

ME

(MIN)

Figure 7. Effects of past history and steam in Incoloy reactor on propylene conversions at 500°C



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was s t i l l very active after about 40 minutes of dry propylene feed. When, however, steam diluent was added to ethylene feed in the metallic reactors, the a c t i v i t y remained the same or decreased s l i g h t l y with time. Clearly the effects of a steam are complicated and more study i s needed. Flushing a coke-coated reactor with either helium or n i t r o gen for about 30 minutes was found on several occasions to a c t i vate the reactor in the subsequent run. Certain hydrocarbons were desorbed during the flushing as w i l l be discussed in more detail l a t e r . Materials of Construction. The materials of construction of the reactor c l e a r l y affect the level and probably to some extent the type of surface reactions. Surface reactions are in general much less important in Vycor reactors at least in the range of approximately 450 to 550 C (see Figures 1, 2, and 3 ) , as i n d i cated by the lower hydrocarbon conversions in these reactors. The general level of a c t i v i t y of the Incoloy 800 and the 304 stainless steel reactors depends in some complex manner on the level of the surface oxides and apparently on the hydrocarbon feedstock. Similar conversions were obtained with dry ethane and with dry propane in r e l a t i v e l y new (and unoxidized) Incoloy and 304 stainless steel reactors (see Figure 1). Yet as shown in Figure 2, ethylene was much more reactive in the unoxidized 304 stainless steel reactor than in the unoxidized Incoloy reactor. With ethylene, oxidized Incoloy gave much higher conversion than the reduced new Incoloy; the reverse was found to be true with oxidized and reduced 304 stainless s t e e l s . The 410 stainless reactor showed much lower a c t i v i t y than either of the other two metal reactors. This reactor had a c t i v i ties only s l i g h t l y greater than those of Vycor at temperatures up to about 650 C. Yet the Vycor reactor had higher hydrocarbon conversions than the 410 stainless steel reactor at higher temperatures (between 650 C and 750 C). The 410 stainless steel reactor probably terminated more free radical reactions than did the Vycor surface. Characteristics of Surface Reactions* In general, the rea c t i v i t l e s of the feed hydrocarbons tested in reduced metallic reactors were in the following order: Ethylene

> Ethane

> Propane

>

Propylene

Such a conclusion i s shown for example by comparing the results of Figures 2, 3, and 5. Only the olefins were tested 1n oxidized metallic reactors, and both were very reactive. The o l e f i n feeds were apparently more strongly affected by the oxidized metallic surfaces than the paraffins. The amount of coke formed on the Inner surface of a reactor was in general closely related to the surface a c t i v i t y . With the long residence times employed and with the r e l a t i v e l y small d i ameter reactor used, coke yields based on the moles of feed hydro-


306


carbon reacted were much higher than those normally found in commercial reactors. The relative comparisons for different reactors and hydrocarbons are, however, thought to be r e l i a b l e . Figures 8, 9, and 10 show several comparisons. In a l l cases, the coking a b i l i t i e s of the hydrocarbons were as follows:


Ethylene

> Propylene

> Propane

> Ethane

These results are shown in Figure 8 and by making a comparison of Figures 9 and 10. Lobo et al (7) also found that olefins produced much more coke than paraffins. The greater coking a b i l i t y of ethylene as compared to propylene i s caused primarily by the higher reactivity of ethylene; somewhat higher yields of coke (based on the amount of o l e f i n that reacts), however, occur with propylene as compared to ethylene. A key finding of this investigation was that s i g n i f i c a n t amounts of olefins and heavier hydrocarbons were often absorbed on the inner surface of the reactor, probably mainly on the coke deposited there. About 0.2 millimoles of ethylene were, for



307


17. B R O W N A N D A L B R I G H T

500

550

600

650

700

TEMPERATURE °C Figure 9.

Coke produced from ethylene feed in several different reactors

example, absorbed on the 304 stainless steel reactor that was operated at 530 C. After several runs, helium was used to flush out the reactor; several different hydrocarbons were desorbed and found 1n the e x i t helium stream for up to 1.5 hours. These hydrocarbons Included olefins and heavier hydrocarbons; these l a t ter hydrocarbons were probably 1n the Cr - C range and may have Included some aromatlcs. Adsorbed hydrocarbons presumably are r e l a t i v e l y reactive and would l i k e l y dehydrogenase f a i r l y rapidly forming coke. Appleby (8) and John (9) have also postulated a similar coking mechanism. Clearly propylene and ethylene are important during pyrolysis in the production of coke. The high reactivity of ethylene as compared to propylene 1s thought to be caused primarily by the lower s t a b i l i t y of vinyl r a d i c a l s , as compared to that of a l l y l radicals. Formation of the C - C intermediates possibly occurs by dimerization and/or trimerTzation of these radicals. g

g

g


308



1

450

500

550

'

•

600

TEMPERATURE

ζ}

650 °C

Figure 10. Coke produced from propylene feed in several different reactors

The results of the present investigation are probably ap plicable to commercial units even though s i g n i f i c a n t l y lower temperatures were employed in this investigation. Fewer hydro carbons would adsorb on the reactor surface at the higher tempera tures used commercially, but extrapolation of the low temperature results to higher temperatures seems to indicate that s i g n i f i c a n t adsorption does s t i l l occur at temperatures of commercial interest. Presumably, the absorbed hydrocarbons would decompose rapidly In to coke and hydrogen at higher temperatures. It 1s concluded that coke formation 1s often, i f not always, a two-step process involving f i r s t adsorption and then surface decomposition (prob ably dehydrogenation and hydrogenolysis). A run with dry propylene feed at temperatures between 500 C and 575 C resulted i n almost complete plugging of an unoxidized 304 stainless steel reactor after 7 hours of operation. Even though the metal surface was rather completely covered with car-



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309

bon by the end of the run, propylene conversions were s t i l l higher 1n this reactor than 1n the Vycor reactor at comparable conditions. Propylene or hydrocarbons formed from propylene may have diffused through the porous carbon layer to the metal surface where they reacted to form more coke. Metal atoms 1n the coke such as detected by Hoppstock et al (10) and by Lobo et al (7) may also have been a factor. Miscellaneous Results. After the Vycor reactor was contacted with oxygen for several hours at 800 C, the a c t i v i t y of the reactor was increased s l i g h t l y . Both ethylene and propylene showed detectable reactions for almost 15 minutes at 500 C which 1s 50 C lower than for the untreated reactor. Small amounts of carbon oxides were detected during this time indicating that some oxygen had been adsorbed on the inner Vycor w a l l . After a l l of the oxygen had desorbed, the a c t i v i t y of the reactor returned to a level s i m i l a r to that in the untreated Vycor reactor. A steam treatment of the Vycor reactor at high temperatures did not, however, produce any noticeable Increase in the reactor a c t i v i t y . Surface deposits were noted in the Vycor reactor following the experimental runs. These dark brown or black deposits occurred primarily at the e x i t end of the reactor. These deposits were o i l y and tarry in nature, and were probably coke and condensed heavy hydrocarbons. Hydrogen treatment at 800 C of the Vycor reactor, which contained some coke or tarry material, resulted in the production of some methane. This result was surprising since i t had been thought that methane formation would occur only in the presence of a metal catalyst such as nickel or iron. The 410 stainless steel reactor was oxidized upon completion of a run, and a red r u s t - l i k e powder was formed on the w a l l . Part of this powder was e a s i l y brushed from the reactor. Discussion of Results The present results clearly confirm the importance and complexity of surface reactions during pyrolysis reactions. Obviously, the composition of the inner surface of the reactor i s of importance relative to the level and types of surface reactions, In addition, valuable new information has been obtained concerning the role of coke in affecting more coke formation. Although the deposition of coke on the walls of a metal reactor decreases the a c t i v i t y of the reactor, i t i s of interest that the surface a c t i v i t i e s of coke-covered metal reactors always remained higher than those for the Vycor reactor. Lobo and Tri mm (11) have i n d i cated that carbon without contaminants i s inactive. Based on this finding, metal contaminants were presumably present in the coke formed. Other investigators (10, 11) have found both nickel and iron contamination of various cokes. Furthermore, coke i s sometimes reported to be autocatalytic in nature. The evidence that olefins and other hydrocarbons adsorbed on the surface and



310


presumably on or in the coke would seem to offer at least one ex planation f o r autocatalysis. Currently the major factors considered in the selection of a tube to be used i n a pyrolysis furnace are the physical proper t i e s and the expected longevity of the tube. Corrosion and carburization certainly are of importance in this l a t t e r respect, but the surface a c t i v i t y should also be considered. Based on the present r e s u l t s , increasing the chromium content and decreasing the nickel content of the metal would reduce coking and other undesired surface reactions. A high chromium stainless steel would l i k e l y result in r e l a t i v e l y few surface reactions. Chromizing the inner surface of tube i s a p o s s i b i l i t y that should be considered in attempting to obtain a high-chromium sur face. Aluminizing the surface may also produce r e l a t i v e l y inert surfaces. The surface chromium and aluminum l i k e l y would react with steam or oxygen to form stable oxides that are probably f a i r l y non-reactive relative to undesired surface reactions. Clearly more investigations are needed to c l a r i f y the role of metal oxides on the surface of the reactor. Analysis of the sur face for metal oxides and metals would be most helpful in this endeavor. In addition, the roles of the metals (or metal oxides) and of the coke in i n i t i a t i n g reactions need c l a r i f i c a t i o n . Methods of extrapolating the results of a laboratory unit to com mercial units should also be developed. Acknowledgments Purdue University and the Procter and Gamble Company provided generous financial support. Literature Cited 1.

Crynes, B. L. and Albright, L. F., Ind. Eng. Chem. Proc. Des. Dev. 8, 1, 25 (1969). 2. Herriott, G. E., Eckert, R. E . , and Albright, L. F., AIChE Journal, 18, 1 (1972). 3. Dunkleman, J. J. and Albright, L. F . , This Book, Chapters 14 and 15, 1976. 4. Brown, S. M., M. S. Thesis, Purdue University, May, 1976. 5. Tsai, C. H. and Albright, L. F., This Book, Chapter 16 (1976). 6. Nishyama, Y., Bull. Chem. Soc. J . , 42, 9, 2494 (1969). 7. Lobo, L. S., Trimm, D. L . , and Figuerdo, J. L, Proc. Int. Congr. Catalysis. 5th, 1125 (1973). 8. Appleby, W. G., Gibson, J. W., and Good, G. M. Ind. Eng. Chem. Proc. Des. Dev. 1, 2, 102 (1962). 9. John, T. M. and Wojciechowski, Β. M., J. Catalysis, 37, 240 (1975) 10. Hoppstock, F. H . , Hutchings, D. Α., and Frech, K. J . , This Book, Chapter 12, 1976. 11. Lobo, L. S. and Trimm, D. L . , J. Catalysis, 29, 15 (1973).


Role of the Reactor Surface in Pyrolysis of Light Paraffins and Olefins

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