Ind. Eng. Chem. Res. 1987,26, 2139-2144
pressure) and one for a supercritical solvent (or an oil solution, supercritical with respect to the solvent critical pressure). In both cases, an imaginary process is shown in P-T coordinates, where a low-temperature fluid (1)is heated until it reaches a high temperature (2) along a 1-2 temperature-pressure path. The supercritical pressure path shown in the right part of Figure 10 does not cross the VLL region. Therefore, VLL phase transitions (VL for pure solvent) can be avoided. As a result, the corresponding enthalpy curve (see below in Figure 10) is smooth with no discontinuities or inflection points. In general, such smooth enthalpy curves are known to be desirable in heat integration schemes because they are easy to fit for different enthalpy exchanging fluids. The subcritical pressure path, on the other hand, makes enthalpy exchanging fluids undergo phase transitions, as can be seen in the left part of Figure 10. As a result, the corresponding enthalpy curve (see below in Figure 10) is not smooth; the solid part of this curve illustrates a pure solvent enthalpy, whereas the dashed part of this curve illustrates an oil solution enthalpy. In general, such enthalpy curves are known to be less desirable in heat integration schemes because they are difficult to fit for different enthalpy exchanging fluids.
Conclusion Oil solutions in light hydrocarbon solvents can form VL, LL, and VLL equilibria which have been found to depend primarily on the degree of molecular size asymmetry. Specifically, low molecular weights of solvents but high molecular weights of oils favor large VLL regions. The equilibrium two-phase compositions can be explained by utilizing retrograde phenomena concepts and regular P-X and T-X diagrams. P-T phase diagrams, qualitatively similar to those known for simple binary mixtures of hydrocarbons, allow us to evaluate enthalpy H-T diagrams for various oil-solvent fluids.
2139
Acknowledgment Acknowledgment is made to E. Niessen for his contribution in the measurements and to Exxon Research and Engineering Company for permission to publish this paper.
Literature Cited Brule, M. R.; Corbett, R. W. Hydrocarbon Process. 1984,63(6), 73. Cotterman, R. L.; Dimitrelis, D.; Prausnitz, J. M. In Supercritical Fluid Technology; Penninger, J. M. L., Radosz, M., McHugh, M. A., Krukonis, V. J., Eds.; Elsevier Science: Amsterdam, 1985. Cotterman, R. L.; Prausnitz, J. M. AZChE J. 1986, 32, 1799. Cotterman, R. L.; Schwartz, B. J.; Prausnitz, J. M. AZChE J. 1986, 32, 1787. Irani, C. A.; Cozewith, C.; Kasegrande, S. S. US.Patent 4 319021, 1982. McClellan, A. K.; Bauman, E. G.; McHugh, M. A. In Supercritical Fluid Technology; Penninger, J. M. L., Radosz, M., McHugh, M. A., Krukonis, V. J., Eds.; Elsevier Science: Amsterdam, 1985. Peters, C. J.; Lichtenthaler, R. N.; de Svaan Arons, J. Fluid Phase Equilibr. 1986,29, 495. Poetmann, F. H.; Dean, M. R. Chem. Eng. Prog. 1949,45, 636. Radosz, M. In Supercritical Fluid Technology; Penninger, J. M. L., Radosz, M., McHugh, M. A., Krukonis, V. J., Eds.; Elsevier Science: Amsterdam, 1985. Radosz, M. Fluid Phase Equilib. 1986,29, 515. Radosz, M.; Cotterman, R. L.; Prausnitz, J. M. Znd. Eng. Chem. Res. 1987, 26, 731. Rowlinson, J. S.; Swinton, F. L. Liquids and Liquid Mixtures, 3rd ed.; Butterworths Scientific: London, 1982. (Type A and B diagrams in Figure 2 correspond to type I and type V diagrams, respectively, in Rowlinson and Swinton (1982, p 194). We make this small change in terminology because we do not discuss other types of VLL phase diagrams, such as I11 and IV, and to avoid confusion with another classification of ternary phase diagrams, types I and I1 in Radosz (1986).) Starling, K. E.; Khan, M. A,; Watanasiri, S. In Supercritical Fluid Technology; Penninger, J. M. L., Radosz, M., McHugh, M. A., Krukonis, V. J., Eds.; Elsevier Science: Amsterdam, 1985. Wilson, R. E.; Keith, P. C.; Haylett, R. E. Ind. Eng. Chem. 1936,28, 1065. Received for review J u n e 9, 1986 Accepted J u n e 11, 1987
Coke Formation on Polished and Unpolished Incoloy 800 Coupons during Pyrolysis of Light Hydrocarbons Lawrence L. Crynes and Billy L. Crynes* School of Chemical Engineering, Oklahoma S t a t e University, Stillwater, Oklahoma 74078-0537
T h e objective of this study was t o determine if polishing Incoloy 800 metal coupons significantly reduced coke formation during pyrolysis of light hydrocarbon feedstocks. All experiments were conducted a t 700 O C , 1 atm, and l - h contact and using methane, ethane, ethene, propane, propene, and isobutane. The ratio of carbon formed from the unpolished coupons to that from polished varied from a low of 5.6 for isobutane up t o 28.1 for ethene; most were between 5.6 and 7.9. T h e polishing effect had little influence on the extent of gas conversion and major product yields. Our results suggest that polishing results in lower surface temperatures (radiation effects), less mechanical surface defects, and possibly changes in surface chemistry. Less carbon, and an inactive carbon, is created under the new conditions, and this prevents or greatly reduces catalytic, filamentous carbon formation. Our intent in this study was to determine if polishing Incoloy 800 metal coupons significantly reduced coke formation during pyrolysis of light hydrocarbon feedstocks a t 700 "C. Production of olefins is a major petrochemical operation in which the reactor capacity is limited by coke accumulation on the reactor tube walls and within transfer line exchangers. Although the primary pyrolysis reactions are considered to be homogeneous, coke formation and the influence of 0~88-~8~5/87/2626-2139$01.50/0
reactor wall material have been recognized as significant factors for several years (Marek and Albright, 198213; Albright and McGill, 1986; Zimmerman et al., 1986; Crynes and Albright, 1969; Tsai and Albright, 1976; Brown et al., 1982;LaCava et al., 1982; Ghaly and Crynes, 1976). Research has shown that certain surfaces minimize coke formation, whereas others promote it. Quartz is generally considered a noncatalytic surface with respect to coke or carbonaceous material formation during high-temperature 1987 American Chemical Society
2140 Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987
reactions of hydrocarbons. Surfaces that are reported to inhibit coke formation include those that are silica- or manganese-rich, and surfaces coated with titanium, molybdenum, tantalum, and aluminum (Alonized) (Baker and Chludzinski, 1980; Clark and Foster, 1979; Brown et al., 1982; Albright and McGill, 1986). Chemical treatment has been shown to passify some surfaces with respect to carbon formation. Hydrogen sulfide passed through a high-temperature stainless steel reactor formed metal sulfides which significantly reduced carbon (Crynes and Albright, 1969; Ghaly and Crynes, 1976; Shah et al., 1976). In contrast, treatment with oxygen activated the surfaces and resulted in increased coke formation. Marek and Albright (1982a,b) investigated other pretreatments and found that the amount and characteristics of the coke that was formed during pyrolysis varied significantly depending on the nature of the pretreatment. Experimental results indicated that Ni and Fe on reactor surfaces were primary catalysts for coke formation (Suzuki et al., 1986; Albright and McGill, 1986). An interesting study, more directly related to our work, indicated that the simple, mechanical operation of polishing a surface reduced coke formation during acetylene pyrolysis (Marek and Albright, 1982b). Gregg and Leach (1966) demonstrated reduced coke formation when Ni surfaces were electropolished and then subjected to catalysis of carbon monoxide. In a previous study in our laboratory, polishing 304 stainless steel and Incoloy coupons demonstrated significant carbon reduction (by 80% or more) during pyrolysis of both isobutane and n-butane (Chen, 1987). Durbin and Castle (1976) polished various metal samples to study carbon deposition from pyrolysis of acetone. They reported no effect from this polishing, presumably because of surface annealing as the temperature of the samples was raised. In contrast, Emsley and Hill (1977) in pyrolysis of methane reported advantages of thermal annealing in reducing the surface roughness resulting from cold-worked manufacture of their test sample. A rather extensive amount of literature has been dedicated to the study of carbon formation under a variety of conditions; Trimm (1983) presents an extensive summary of catalytic coke formation. There is not yet a mechanism of universal acceptance. During the pyrolysis of hydrocarbons, carbon may be formed in the gas phase, itself a rather complex procedure, or carbon may be catalyzed by surface reactions (Albright and Marek, 1986a; LaCava et al., 1982; Marek and Albright, 1982). The type of feedstock and its purity have major influences on carbon formation: acetylenes, polycyclic forming aromatics, and diolefins are noted carbon precursors. There have been a t least seven types of carbons named, although the literature does not necessarily agree upon a common classification for these. As expected, temperature is a significant parameter having a major influence on not only the rate but on the type of carbon that is produced (Albright and Marek, 1986a; Chen, 1987; Baker et al., 1972, 1982; Trimm, 1983). There lacks little agreement on exactly what the metal surface chemistry is. An "active site", for instance, has been called atomic metal, a metal carbide, a metal oxide, a surface defect (mechanically or chemically roughened area or other disorder), grain boundary, some types of carbon itself, and displaced metal particles from the surface as filament tips or by metal dusting corrosion or metal crystallites (Albright and Marek, 1986a,c; Albright and McGill, 1986; Albright and Tsai, 1983; Audier et al., 1983; Baird et al., 1974; Baker et al., 1972, 1973, 1975, 1982;
TEMPERATURE AND WEIGHT RECORD
ELECTROBALANCE CHAMBER
FLOWMETER
PURGE
FURNACE
COUPON
FINGER
OXYGEN FREE HYDROCARBON FEED
i
OXISORB
MlCRlCON CONTROLLER
Figure 1. Experimental apparatus.
Bradley et al., 1985; Brown and Albright, 1976; Brown and Hill, 1982; Castle and Durbin, 1975; Chen, 1987; Emsley and Hill, 1977; Ghaly and Crynes, 1976; Gregg and Leach, 1966; Holmen et al., 1982; LaCava et al., 1982; Lobo and Trimm, 1973; Marek and Albright, 1982a,b; Sacco and Caulmare, 1982; Suzuki et al., 1986; Trimm, 1983; Tsai and Albright, 1976; Zimmerman et al., 1986). The complex chemistry that exists at high temperatures plays a significant role. This is true especially of equilibria and kinetic chemistry such as metal-carbon reactions, metal-oxide reactions, and carbon solubility in metals and their compounds. The type of metal packing is important, for example, for iron, the X state or a state-face- or body-centered cubic lattice (Brown and Hill, 1982; Baker and Chludzinski, 1980). The history of the surface (pretreatment, previous operational conditions, manufacturing) is important (Albright and Marek, 1986a,c; Marek and Albright, 1982b; Brown and Hill, 1982; Baker et al., 1975). The gas-phase residence time and even geometric factors such as the shape and position of the metal surface have been shown to be influential (Albright and Marek, 1986b; Holmen et al., 1982; Marek and Albright, 1982a). Clearly, the overall process is one that challenges an understanding and a full description. From the literature we found that polishing metal surfaces reduced coke during the pyrolysis of acetylene, isobutane and n-butane, but we did not know the effect of polishing on other feedstocks. The mechanism of coke formation is complex and poorly understood, and our objective was to evaluate other feedstocks and provide data for better understanding.
Experimental Section The primary component of the experimental apparatus was a Cahn System 113 thermal gravimetric analyzer (TGA),Figure 1. The various feed gases were introduced into the TGA after mixing with the appropriate flow of helium (60:120 cm3/min feed gas to helium). The reactor effluent passed through an ice trap to remove condensables and then into the gas chromatograph (GC) sampling valve. A small flow of helium (15 cm3/min) was used to purge the top of the TGA to protect the electronic elements from product gases. An oxygen and moisture trap was located before the TGA on the feedstock lines. Table I lists grades and vendors for the feedstocks and helium; GC analyses of the feedstocks were made to verify their purity. The GC used in this study was a Varian Model 3700 with a flame ionization detector and a 2.8-m Porapak Q column programmed for 40-120 "C (20 "C/ min). All experiments were conducted at 700 "C, essentially 1atm and 1-h contact of the sample coupon in the reaction
Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 2141 Table I. Gases, Grades, and Vendors gas grade vendor Scientific Gas Co. CP methane Scientific Gas Co. research ethane Scientific Gas Co. ethene polymer Scientific Gas Co. propane CP polymer Scientific Gas Co. propene instrument Matheson Gas Products isobutane Matheson Gas Products O2free helium (feed flow) high purity Matheson Gas Products helium (GC) helium (TGA purge) ultra purity Matheson Gas Products
.
J
0,451
i 0 I-
zone. This temperature was chosen as suitable for giving definitive conversions and carbon production on the coupons for a range of feedstocks. Coupons of Incoloy 800 cut to nominal dimensions of 9.7 X 4.0 by 1.6 mm were used. These coupons were suspended from a platinum wire (which had been shown to be nonreactive) within the 700 OC reaction zone. Coupons of this size and mass (0.37 g) were chosen to accommodate the capacity of the TGA and provide a measurable quantity of coke. Coupons were used as received (Alon Processing Inc.) and after being polished. Polishing was achieved by the use of fine S i c paper followed by use of a jeweler's polishing wheel to achieve an average finish of 6 rms as measured by profilometer (VJ3 Mototracer Model 3). Unpolished coupons measured an average of 71 rms. Following the polishing treatment, the coupons were placed in an ultrasonic cleaner for 0.17 h. All coupons were rinsed with acetone and subsequently handled only with tweezers. One experiment was conducted on methane using an unpolished coupon, and two experiments (one each for polished and unpolished coupons) were conducted on each of the gases ethane, ethene, propane, and propene. Three replicate experiments were conducted using isobutane.
Results Data from this study revealed that the simple mechanical operation of polishing the Incoloy 800 coupons resulted in a significant reduction in carbon deposited during pyrolysis of various feedstocks. Figure 2 illustrates the effect for the results from propane pyrolysis. For propane, a marked reduction of carbon (one-sixth of the unpolished value) was achieved after 1h of coupon contact. Except for methane (no measurable amount of carbon), similar results were obtained from the other feedstocks. Table I1 shows the coke accumulation after 1h for both polished and unpolished coupons and the ratio of these accumulations for the respective gases. The ratio of carbon formed on the unpolished coupon to that from the polished varied from a low a 5.6 for isobutane up to 28.1 for ethene.
4
0.30
2
0.15
,1
UNPOLISHED 7
GAS-COUPON CONTACT, minutes
Figure 2. Carbon deposition from propane on polished and unpolished coupons.
6
0.60
Q i
0.451
ISOBUTANE
5
GAS- COUPON CONTACT, minutes
Figure 3. Carbon deposition from various gases on unpolished coupons.
Most ratios were between 5.6 and 7.9; the reason for the unusually high value for ethene is not yet clear. Experiments were conducted on methane, but at the temperature and contact times used in this study, no measurable quantity of coke could be detected. A comparison of the coke formation on unpolished coupons for the various feedstocks is shown in Figure 3. The more stable, smaller hydrocarbons, ethane and methane (not shown), resulted in the lowest coke formation. The amount of coke increased with an increase in the size of the feedstock compound. Ethene and propene produced essentially the same quantity of coke. This general trend of rate of coke formation for feedstock type is expected (Zimmerman et al., 1986). The curve for isobutane represents the average of three experiments. The polishing effect had little influence on the extent of conversion of the gases and on their major product yields (Table 111). Differences in conversion between the polished and unpolished coupon experiments were only
Table 11. Coke Accumulation (ma/cm2) for 1 h isobutane unpolished polished ratio @AfterTGA repair.
methane 0 b
ethane
ethene
propane
propene"
1
2
3"
mean
0.099 0.016 6.3
0.267 0.0095 28.1
0.337 0.044 7.7
0.273 0.035 7.9
0.449 0.080 5.6
0.838
0.692
0.660
b
b
Not run.
Table 111. Major Gas Products-Unpolished (Polished) products, mol% ethane ethene propane methane 0.219 (0.326) 0.24 (0.162) 7.660 (6.989) ethane 1.372 (0.965) 1.377 (0.939) ethene 18.945 (22.134) 16.963 (12.368) propane propene 1.440 (1.051) 18.025 (15.685) isobutene 2.957 (2.370)
propene 0.861 (0.582) 2.54 (1.530) 1.338 (0.901)
isobutane 8.322 0.269 2.779 0.786 14.622 14.807
(8.144) (0.295) (2.829) (0.805) (14.530) (15.878)
conversion 0.64 19.42 5.67 44.90 4.90 41.53
(22.87) (4.08) (36.83) (3.28) (42.45)
2142 Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 30
1
3 GAS
PHASE PRECURSER TRANSPORT
/-
\ 3 SITE 3 J
.ACTIVE
*
ADSORPTION
f
/
/ONFILAMENTOF 3cARBON
&
A -\
SdFliACE REACTIONS C A 9 6 0 N FORMATION
METAL CRYSTALLITE [ : M E T A L OXIDE OP C A q B I D E S U r i i A C E 3lSORDEP
v%
3 SOLUTIOY
FlLAMENTOdS CARBON NJCLEATION A N 2 DRECIPlTATI3N
‘5 CARBON DlFFJSlON
Figure 5. Carbon formation steps.
00
0
10
20
30
40
50
60
GAS-COUPON CONTACT minctes
Figure 4. Rate of carbon deposition from isobutane on unpolished coupon. Table IV. Isobutane Experiments (Unpolished Coupons; Coke (mg/cm2)) expt t. min 1 2 3 mean 0.060 0.071 0.074 0.035 3 0.122 0.155 0.144 0.068 6 0.202 0.177 0.090 0.229 9 0.275 0.345 0.321 0.159 15 0.407 0.351 0.207 0.438 21 0.420 0.515 0.479 27 0.265 0.539 0.476 0.305 0.585 33 0.594 0.533 0.355 0.649 39 0.649 0.587 0.401 0.710 45 0.696 0.637 0.449 0.765 51 0.739 0.684 0.493 0.821 57 0.759 0.708 0.515 0.850 60
1-3.4%, except those for propane differed by 8.1% . The percentage yields of the various products from the respective feedstocks were essentially the same irrespective of the use of polished or unpolished coupons. Again, the greatest difference appeared in propane. The two primary products from propane pyrolysis (ethene and propene) were lower for the polished coupon by about 2.4-4.7 %. These similarities between the polished and unpolished coupons experiments for conversions and yields are to be expected since the predominance of the pyrolysis reactions occurs in the gas phase, and the quantity of coupon surface is much too small to significantly influence this homogeneous phase. Except for ethane, the initial rates of carbon formation were high with all feedstocks, and the rates fell toward lower values with time. This can be seen from the slopes of the curves in Figure 3 and is demonstrated in Figure 4 for isobutane. This curve is typical of the rate decay for most other feedstocks, except for ethane. Three isobutane experiments were made incorporating different operators and different days (at the beginning and at the end of the study) and after a repair of the TGA, Table IV. Isobutane experiment 1gave results that are low compared to experiments 2 and 3. We feel that the average of these three runs is the most representative curve for isobutane performance. Careful error analysis suggests that the small quantity of coke formed, 0.01 mg/min, and the possible variations in coupon surface result in the range of data shown in Table IV. Discussion Our results can best be considered after first looking at a generalized, qualitative mechanism which incorporates
several features from the literature, as shown in Figure 5. In the figure and what follows, gas-phase carbon formation is excluded, as is carbon and tar deposition on surfaces (Trimm, 1983). An “active site” exists on the bare metal surface, on a metal particle at the tip of a carbon filament, or on certain types of carbon. The carbon precursors in the gas phase must diffuse or otherwise be transported to active sites (Albright and Marek, 1986a; Trimm, 1983; Graff and Albright, 1982). The gaseous species adsorb and surface reaction occurs by one or multiple steps to form carbon (Albright and Marek, 1986a; Suzuki et al., 1986; McCarty et al., 1982). This carbon may vary in its morphology and its properties (Albright and Marek, 1986a,c; Baker et al., 1972, 1982; McCarty et al., 1982; Trimm, 1983). Carbon that exists on the surface may move about the surface (Baker et al., 1972, 1982), or it may dissolve within the metal (LaCava et al., 1982; Brown and Hill, 1982; Baker et al., 1972). The solution equilibria and super saturation dissolution of carbon in metal and metal compounds are themselves complex phenomena (Trimm, 1983; Emsley and Hill, 1977; Baker and Chludzinski, 1980). The carbon in solution can diffuse (LaCava et al., 1982; Trimm, 1983; Lobo and Trimm, 1973; Baker et al., 1973; Baker and Chludzinski, 1980) to an “active site” where nucleation can occur to create metal particles (Baker et al., 1975, 1982; Trimm, 1983). Precipitation occurs and active metal particles can now exist at the surface, and these can be covered with various types of carbon (Brown and Hill, 1982; Trimm, 1983). Under certain temperatures and conditions, filamentous carbon can grow by a continuation of steps 1-6 (Baker et al., 1982). As filamentous carbon grows, the growth occurs from the bulk metal surface outward, carrying a metal particle a t the tip of the filament. In limited cases, filamentous carbon can grown with the metal particle remaining on the support surface (Trimm, 1983). Mechanisms have been postulated where the filament growth occurs at the metal particle base at the filament tip (Baker et al., 1972). These metd-tipped filaments have been confirmed many times (Albright and Marek, 1986a; Albright and McGill, 1986; Chen, 1987; Baker et al., 1972, 1973; Baird et al., 1974; Sacco and Caulmare, 1982; Brown and Hill, 1982; Evans et al., 1973). Such metal particles greatly increase the activity because of the increased surface area and the number of particles that are created. The filamentous carbon with the active metal tips are particularly active and continue to create carbon until the tip itself is capped with a nonreactive carbon (Albright and Marek, 1986a; Albright and McGill, 1986; LaCava et al., 1982; Brown and Hill, 1982; McCarty et al., 1982; Trimm, 1983; Baker et al., 1972, 1973). Even this complex set of steps is an oversimplification. Furthermore, the various steps of the mechanism all have their own rates of transfer or reaction (activation energies) which in turn have separate dependencies upon the temperature, pressure, and chemistry of the environment.
Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 2143 When one surveys the complex mechanism in search of an explanation for our observed phenomenon of polishing, a number of candidates seem reasonable. In our studies, one obvious contribution results from heat transfer by radiation. The emittance, which influences radiation and therefore the temperature of the coupon, may vary from below 0.1 for a polished surface up to 0.9 for a plain or a carbonized surface (Siege1 and Howell, 1981). In considering our surface emittances, as received and highly polished, one might reasonably expect that polished coupons with low emittances operate as much as 50-100 “C below that of the gas temperature (Albright and McGill, 1986; Chen, 1987). Accordingly, a coupon that is 50-100 “C below the gas temperature can be operating in a regime that significantly reduces the carbon formation for no other reason than the slower chemical reaction rate and phase changes in solution equilibria that occur a t these lower temperatures. Also a shift in carbon morphology with temperature changes is known (Baker et al. 1982; Albright et al., 1979). Temperature alone is not sufficient to explain the differences observed. Mechanically polishing a surface will result in a reduction of the general roughness or number of disorders that have resulted from manufacturing (Marek and Albright, 1982b; Chen, 1987; Brown and Hill, 1982; Gregg and Leach, 1966). This was demonstrated in our study by a marked reduction in the surface roughness from 71 rms for the as-received coupon to 6 rms for polished. Furthermore, if active metal particles already exist on the surface (Marek and Albright, 1982a),they will be removed or reduced in size and number with polishing (Audier et al., 1983; Chen, 1987). One other consideration is the change in surface chemistry. As manufactured, a metal surface will exist as a layer of oxides and other chemical species that will be somewhat different than the bulk or subsurface chemistry (Marek and Albright, 1982a; Castle and Durbin, 1975). By polishing there is a change in chemistry of the surface which is exposed to the reacting gas (Marek and Albright, 1982b). Not only the reaction chemistry but the phase behavior relating to carbon solution at the surface will have been changed by polishing. In reviewing other studies, polishing metals of different chemical compositions usually results in rates of carbon formation similar to that of quartz (Chen, 1987). The possible exception is when temperatures are high, say 900 “C and above, and then the polishing effect is less pronounced or nonexistent (Albright and Marek, 1986~;Marek and Albright, 1982b). With the chemistry of a range of metals all yielding the same low carbon production rates, perhaps the most significant effects of polishing are not changes in surface composition but are the lower temperature and marked decrease in those active sites from mechanical defects and from metal particle removal. Our studies show that a range of feedstocks pyrolyzed over different polished surfaces have unusually low rates of carbon formation similar to those on a quartz surface, which is generally considered most inert. With lower surface temperatures (radiation effects), less mechanical surface defects, and possibly a change in chemistry, less carbon and an inactive carbon is created under the new conditions, and this prevents or greatly reduces filamentous carbon formation (Baker et al., 1982; McCarty et al., 1982). Therefore, with reduced filamentous carbon, the active metal particles cannot continue to be generated and/or be exposed to further catalyze carbon formation. Registry No. Methane, 74-82-8; ethane, 74-84-0; ethene, 7485-1; propane, 74-98-6; propene, 115-07-1; isobutane, 75-28-5;
carbon, 7440-44-0; Incolog 800, 11121-96-3.
Literature Cited Albright, L. F.; Marek, J. C. Purdue University, “Mechanistic Model for Formation of Coke in Pyrolysis Units Producing Ethylene”, unpublished paper, 1986a. Albright, L. F.; Marek, J. C. Purdue University, “Role of Residence Time and Geometry on Coke Formation During Pyrolysis”, unpublished paper, 198613. Albright, L. F.; Marek, J. C. Purdue University, “Coke Formation During Pyrolysis As a Function of Time of Operation”,. unpublishedpaper, -1986~. Albrieht. L. F.: McConnel. C. F.: Welther. K. In Thermal Hvdroca;bon Chemistry; Oblad, A. G., Davis,’ H. G., Eddinger, ?. E., Eds.; ACS Symposium Series 183; American Chemical Society: Washington, DC, 1979; Chapter 10. Albright, L. F.; McGill, W. A. Presented a t the Spring National Meeting American Institute of Chemical Engineers, New Orleans, April 6-10, 1986. Albright, L. F.; Tsai, T. C. H. In Pyrolysis: Theory and Industrial Practice; Albright, L. F., Crynes, B. L., Corcoran, W. H., Eds.; Academic: New York, 1983; Chapter 10. Audier, M.; Bowen, P.; Jones, W. J. Cryst. Growth 1983, 63, 125. Baird, T.; Fryer, J. R.; Grant, B. Carbon 1974, 12, 381. Eaker, R. T. K.; Barber, M. A.; Harris, P. S.; Feates, F. S.; Waite, R. J. J. Catal. 1972, 26, 51. Baker, R. T. K.; Chludzinski, J. J., Jr. J . Catal. 1980, 64, 464. Baker, R. T. K.; Harris, P. S.; Henderson, H. J.; Thomas, R. B. Carbon 1975, 13, 17. Baker, R. T. K.; Harris, P. S.; Thomas, R. B.; Waite, R. J. J. Catal. 1973, 30, 86. Baker, R. T. K.; Yates, D. J. C.; Dumesic, J. A. In Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K., Eds.; ACS Symposium Series 202; American Chemical Society: Washington, DC, 1982; Chapter 1. Bradley, J. R.; Chen, Y. L.; Sturner, H. W. Carbon 1985, 23, 715. Brown, S. M.; Albright, L. F. In Industrial and Laboratory Pyrolysis; Albright, L. F., Crynes, B. L., Eds.; ACS Symposium Series 32: American Chemical Society: Washington, DC, 1976; Chapter 17. Brown, D. E.; Clark, J. T. K.; Foster, A. I.; McCarroll, 3. J.; Sims, M. L. In Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K., Eds.; ACS Symposium Series 202; American Chemical Society: Washington, DC, 1982; Chapter 2. Brown, A. M.; Hill, M. P. In Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K., Eds.; ACS Symposium Series 202; American Chemical Society: Washington, DC, 1982; Chapter 10. Castle, J. E.; Durbin, M. J. Carbon 1975, 13, 23. Chen, C. T. Ph.D. Dissertation, Oklahoma State University, Stillwater, 1987. Clark, J. T. K.; Foster, A. I. Br. Patent 1552284, 1979. Crynes, B. L.; Albright, L. F. Ind. Eng. Chem. Process Des. Deu. 1969, 8, 25. Durbin, M. J.; Castle, J. E. Carbon 1976, 14, 27. Emsley, A. M.; Hill, M. P. Carbon 1977, 15, 205. Evans, E. L.; Thomas, J. M.; Thrower, P. A.; Walker, P. L. Carbon, 1973, 1 2 , 441. Ghaly, M. A.; Crynes, B. L. In Industrial and Laboratory Pyrolysis: Albright, L. F., Crynes, B. L., Eds.; ACS Symposium Series 32; American Chemical Society; Washington, DC, 1976; Chapter 13. Graff, M. J.; Albright, L. F. Carbon 1982,20, 319. Gregg, S. J.; Leach, H. F. J . Catal. 1966, 6 , 308. Holmen, A.; Lindvaag, 0. A.; Trimm, D. L. In Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K., Eds.; ACS Symposium Series 202; American Chemical Society: Washington, DC, 1982; Chapter 3. LaCava, A. I.; Fernandez-Raone, E. D.; Isaccs, L. L.; Caraballo, M. In Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K.,Eds.; ACS Symposium Series 202; American Chemical Society: Washington, DC, 1982; Chapter 5. Lobo, L. S.; Trimm, D. L. J . Catal. 1973, 29, 15. Marek, J. C.; Albright, L. F. In Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K., Eds.; ACS Symposium Series 202; American Chemical Society: Washington, DC, 1982a; Chapter 7. Marek, J. C.; Albright, L. F. In Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K., Eds.; ACS Symposium Series 202; American Chemical Society: Washington, DC, 198213; Chapter 8. McCarty, J. G.; Hou, P. Y.; Sheridan, D.; Wise, H. In Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K., Eds.;
2144
I n d . Eng. Chem. Res. 1987, 26, 2144-2148
ACS Symposium Series 202; American Chemical Society: Washington, DC, 1982; Chapter 13. Sacco, A.; Caulmare, J. C. In Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K., Eds.; ACS Symposium Series 202; American Chemical Society: Washington, DC, 1982; Chapter 9. Shah, Y . T.; Stuart, E. B.; Sheth, K. D. Ind. Eng. Chem. Process Des. Dev. 1976, 15, 518. Siegel, R.; Howell, J. R. Thermal Radiation Heat Transfer; McGraw-Hill: Washington, DC, 1981. Suzuki, G.; Uchida, M.; Ohsaki, K.; Onodera, T.; Umemura, T.; Sundaram, K. M. Presented a t the Spring National Meeting, American Institute of Chemical Engineers New Orleans, April, 1986.
Trimm, D. L. In Pyrolysis: Theory and Industrial Practice; Albright, L. F., Crynes, B. L., Corcoran, W. H., Eds.; Academic: New York, 1983; Chapter 9. Tsai, C. H.; Albright, L. F. In Industrial and Laboratory Pyrolysis; Albright, L. F., Crynes, B. L., Eds.; ACS Symposium Series 32; American Chemical Society: Washington, DC, 1976; Chapter 16. Zimmerman, G.; Kopinke, F. D.; Nowak, S. Presented at the Spring National Meeting American Institute of Chemical Engineers, New Orleans, April 1986. Received for review December 24, 1986 Revised manuscript received J u n e 8, 1987 Accepted July 15, 1987
A New Photoimaging System Based on Singlet Oxygen David S. Breslow,* David A. Simpson, Brian D. Kramer, Robert J. Schwarz, a n d Norman R. Newburg+ Hercules Research Center, Wilmington, Delaware 19894
Singlet oxygen chemistry has been used to develop a highly photosensitive lithographic plate which can be imaged by projection or in a camera and which shows no reciprocity failure when laser imaged. T h e system consists of a polymeric film former, a monomer capable of free-radical polymerization, a singlet oxygen acceptor, a singlet oxygen sensitizer, and a redox metal catalyst. Exposure is carried out in air, the plate is processed in the absence of oxygen to cross-link the exposed areas, and an aqueous alkaline developer is used t o dissolve the uncross-linked areas. Plate sensitivity is about 0.2 m J cm-?-, 2-3 orders of magnitude more sensitive than commercial wipe-on plates. Commercial feasibility has been demonstrated. Photopolymerization represents an excellent approach
to amplifying the effect of a single photon. Walker et al. (1970) estimated that quantum yields for photopolymerization systems can range from IO3 to IO6 double bonds consumed per photon. High efficiency can only be obtained under optimum conditions. The classic problem with photopolymerization systems is inhibition by oxygen. Oxygen can quench photosensitizers and can interact with free radicals to inhibit chain growth. We decided to use the normally deleterious quenching reaction, which can generate singlet oxygen, to form a novel latent image. Activation of this latent image in the absence of oxygen results in polymerization, allowing eventual image development. Our ultimate goal was to develop a photopolymerization system which would be sufficiently sensitive to be imaged by projection, in a camera, or by a low-power laser. For these imaging methods, certain requirements must be met. (1)The system must be sensitive to low light levels and not show reciprocity failure during laser imaging. (2) It must be sensitive to visible light. Ultraviolet light would require quartz optics in a projector or camera, which would be very expensive. In addition, inexpensive, reliable lasers emit only in the long-wavelength region. (3) A projection plate must be negative working, Le., the exposed areas must be ink receptive and printing. A camera plate would have to be positive working, although there are methods which allow reversal. Laser imaging is compatible with negative or positive working systems. Although various possibilities exist for utilization of singlet oxygen chemistry, we decided to investigate lithography first. Lithography is a printing process based on the ability of a surface to accept ink or water. A li*Present address: Breslow Associates, Wilmington, D E 19803. Deceased.
thographic printing plate is prepared by a photographic process in which the ink-receptive, or oleophilic, area is usually a cross-linked polymer (2-10 pm thick); the hydrophilic area, usually aluminum, rejects ink when wet by water. During printing, the press applies water and ink alternately; the ink adhering to the oleophilic areas is transferred to a rubber roller and then to paper. Because of this intermediate ink transfer, the process is often referred to as “offset” printing.
Discussion Ground-state oxygen is a triplet, possessing two unpaired electrons with parallel spins; this accounts for its radical-like activity. Singlet oxygen, however, has its electrons paired; as a result, its chemistry is quite different. Only 22 kcal(92 kJ)/mol is required to convert triplet oxygen to the first excited singlet state, ‘Ag. Thus, sensitizers activated by visible light have sufficient energy to effect the conversion. The ‘Ag state is a relatively long-lived species, allowing for efficient chemical reaction in solution (Adams and Wilkinson, 1972). Singlet oxygen reacts with olefins possessing an allylic hydrogen by an “ene-type”reaction to form an unsaturated hydroperoxide; the reaction is favored by alkyl groups on the double bond. For example, 2,3-dimethyl-3-hydroperoxy-1-butene is formed from 2,3-dimethyl-2-butene, eq 1 (Foote et al., 1968). 1,2-Dimethylcyclohexene,since it (1)
contains two different types of allylic hydrogen, yields a mixture of two hydroperoxides in an approximately 9: 1 ratio, eq 2 (Foote, 1968). The relative reactivities of 1,2dimethylcyclohexene and 2,3-dimethyl-2-buteneto ‘02are 0.53:l (Kopecky and Reich, 1965). 1987 American Chemical Society
