Thermal Decomposition of C10−C14 Normal Alkanes in Near-Critical

574

Ind. Eng. Chem. Res. 1997, 36, 574-584

Thermal Decomposition of C10-C14 Normal Alkanes in Near-Critical and Supercritical Regions: Product Distributions and Reaction Mechanisms Jian Yu and Semih Eser* Fuel Science Program, Department of Materials Science and Engineering, 209 Academic Projects Building, The Pennsylvania State University, University Park, Pennsylvania 16802

Thermal decomposition of C10-C14 n-alkanes was studied under near-critical and supercritical conditions. The primary products were C1-Cm-2 n-alkanes and C2-Cm-1 1-alkenes, and the secondary products were cis- and trans-2-alkenes, n-Cm-1, n-Cm+1, and Cm+2-C2m-2 normal and branched alkanes, where m is the number of carbon atoms in the reactant. The relative yields of the primary and secondary products were dependent upon the reaction conditions. Product distributions exhibited large pressure dependence in the near-critical region. The observed product distributions and changes in product composition with reaction conditions were explained by a modified free radical mechanism. Table 1. Critical Properties of Three Model Compoundsa and Some Petroleum-Derived Jet Fuelsb

Introduction This investigation was undertaken in relation to future jet fuel thermal stability problems. The fuel in an advanced aircraft is the primary sink for the heat generated on board. It is used to absorb heat from various aircraft components such as engine lubricating oil, hydraulic fluid, environmental control system, electrical system, and air frame. Under thermal stress, the fuel may attain high temperatures and undergo thermal decomposition, resulting in the formation of solid deposits. At present, the fuel temperature is limited to 163 °C bulk temperature and 205 °C wetted wall temperature (Edwards and Zabarnick, 1993). With the development of high-speed aircraft, it is expected that the future fuel system will be operating at much higher temperatures because of the increased thermal management requirements. The Air Force has established a long term goal which is to increase the fuel thermal stability temperature limit to 480 °C (Edwards, 1993). Typical aircraft fuel system pressures are between 3.4 and 6.9 MPa (Edwards and Zabarnick, 1993). These temperatures and pressures are above the corresponding critical values of typical jet fuels (Yu and Eser, 1995). The chemistry of fuel thermal decomposition and the mechanism of deposit formation are very complex. Currently, there is very limited information on the thermal stability behavior of hydrocarbon fuels under supercritical conditions. This is not surprising since the fuel degradation process under subcritical conditions is still not well understood. It is expected that the fuel degradation chemistry under supercritical conditions would be different from that under subcritical conditions. In this work, the thermal decomposition of three model compounds, n-decane (n-C10), n-dodecane (n-C12), and n-tetradecane (n-C14), was studied under nearcritical and supercritical conditions. These model compounds were selected because they are typical components found in petroleum-derived jet fuels (Lai and Song, 1995). In addition, their critical properties are comparable with those of jet fuels (Yu and Eser, 1995). This allows carrying out the stressing experiments under conditions relevant to those for the future jet fuel thermal reactions. Table 1 shows the critical properties S0888-5885(96)00392-2 CCC: $14.00

n-C10 n-C12 n-C14 JP-8P JP-8P2 Jet A Jet A-1 JP-7 JPTS a

Tc, °F

Tc, °C

Pc, psia

Pc, MPa

652 725 786 740 757 752 732 761 719

345 385 419 393 403 400 389 405 382

304 262 228 355 330 345 340 305 340

2.10 1.81 1.57 2.45 2.27 2.38 2.34 2.10 2.34

Teja et al. (1990). b Yu and Eser (1995).

of the three model compounds (Teja et al., 1990) and some typical petroleum-derived jet fuels (Yu and Eser, 1995). Thermal decomposition of hydrocarbons has been the subject of interest since the beginning of petroleum industry. Extensive literature exists on the thermal cracking of paraffinic hydrocarbons. Most studies involved low-molecular-weight alkanes and were conducted at relatively high temperatures, above 500 °C, and near atmospheric pressure (Fabuss et al., 1964; Mallinson et al., 1992), in relation to industrial thermal cracking. It has been generally accepted that the thermal decomposition of paraffinic hydrocarbons proceeds via a free radical chain mechanism, as formulated by Rice and his co-workers (Rice, 1931, 1933; Rice and Herzfeld, 1934; Kossiakoff and Rice, 1943). Only limited work has been done on the thermal decomposition of high-molecular-weight alkanes containing more than eight carbon atoms. Most of the work on the thermal cracking of heavy alkanes involved n-hexadecane, either at high temperatures (g500 °C) and low pressures (near atmospheric) (Voge and Good, 1949; Groenendyk et al., 1970; Rebick, 1981; Depeyre et al., 1985b) or at low temperatures (330-450 °C) and high pressures (0.6-14 MPa, autogenic or nitrogen pressure) (Mushrush and Hazlett, 1984; Blouri et al., 1985; Ford, 1986; Khorasheh and Gray, 1993). The studies on other long-chain alkanes were rare, including n-nonane (Kunzru et al., 1972; Depeyre et al., 1985a), n-decane (Billaud and Freund, 1986), n-dodecane (Zhou and Crynes, 1986), and n-tetradecane (Song et al., 1994). The n-nonane and n-decane were cracked at high temperatures and atmospheric pressure, while n-dode© 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 575

cane and n-tetradecane were pyrolyzed at low temperatures and high pressures. At high temperatures and low pressures, the main products from the thermal decomposition of heavy straight-chain alkanes are methane, ethane, all 1-alkenes with carbon numbers less than that of the parent reactant, and small amounts of hydrogen and propane (Depeyre et al., 1985a,b; Billaud and Freund, 1986; Rebick, 1981, 1983). The yield of ethylene is always the most abundant. It has been shown that the product distributions from the thermal decomposition of longchain alkanes at high temperatures and low pressures can be predicted reasonably well by the Rice-Kossiakoff (R-K) mechanism (Kunzru et al., 1972; Depeyre et al., 1985a; Zhou et al., 1987; Voge and Good, 1949; Rebick, 1981). For the thermal cracking of heavy alkanes at high pressures, significant amounts of n-alkanes larger than ethane were observed in addition to the products stated above (Zhou and Crynes, 1986; Song et al., 1994; Voge and Good, 1949; Doue and Guiochon, 1969; Fabuss et al., 1962; Mushrush and Hazlett, 1984; Blouri et al., 1985; Ford, 1986; Khorasheh and Gray, 1993). Fabuss, Smith, and Satterfield (F-S-S) (1964) modified the R-K mechanism to account for the product distributions from the thermal cracking of n-hexadecane at high pressures. Clearly, the R-K mechanism and the F-S-S mechanism are simply the two extremes of the same free radical mechanism, as indicated by Poutsma (1990) and Khorasheh and Gray (1993). It should be mentioned that neither mechanism is directly applicable to high conversions since they do not take into account the secondary reactions of primary products. In the present study, the thermal reaction experiments of n-C10, n-C12, and n-C14 were carried out at temperatures from 400 to 450 °C and pressures from 1 to 10 MPa (initial pressure). The temperatures and pressures used in this study belong to low-temperature, high-pressure category as compared with industrial thermal cracking. While the present study is mainly related to future jet fuel thermal stability problems, the information obtained is also relevant to petroleumprocessing processes, such as mixed-phase cracking, which is known to operate at temperatures between 400 and 480 °C and pressures above 2.4 MPa (Bland and Davidson, 1967). A companion paper presents the results on the kinetics of supercritical-phase thermal decomposition of C10-C14 normal alkanes and their mixtures (Yu and Eser, 1997). Experimental Section The n-C10, n-C12, and n-C14, all with 99+% purity, were obtained from Aldrich and were used as received. The thermal reaction experiments of n-C10, n-C12, and n-C14 were carried out in a Pyrex glass tube reactor with a strain point of 520 °C. The glass tube used is similar to that employed in the critical temperature measurement experiments (Lyons, 1985; Yu and Eser, 1995). The sample section of the tube is made of nonprecision one-quarter-inch outer diameter and 1 mm bore capillary tubing with a 5.25 in. length. To obtain the desired loading ratio, defined as the ratio of the initial sample volume at room temperature to the reactor volume, a certain amount of sample was loaded into the tube using a glass syringe with a 6 in. long needle and a band was marked around the tube at the sealing position with a permanent marker. The loaded and marked sample tube was evacuated using a vacuum pump and then

sealed off under vacuum using an oxygen-gas torch with the marked band as a guide. For n-C14, the thermal stressing experiments were also conducted in a 316 stainless steel tubing bomb reactor, under both nitrogen and air atmospheres. The body of the tubing bomb reactor consists of a Swagelok port connector with caps at both ends. One end of the reactor body is connected to a small diameter extension tube via a Swagelok fitting. The other end of the extension tube is connected to one end of a union tee, the remainder two ends of which are connected to a pressure gauge and a needle valve which is connected to a female quick connector. The volume of the reactor body is about 22 mL, and the total volume of the reactor system is about 28 mL. The configuration of the tubing bomb reactor has been described by Song et al. (1993). After being loaded with a certain amount of sample, the tubing bomb reactor was sealed and leak tested. For the reaction carried out under nitrogen atmosphere, the reactor was purged with UHP-grade nitrogen at 6.9 MPa five times to remove oxygen and the final head space nitrogen pressure was maintained just above atmospheric pressure. For the experiment conducted under air atmosphere, the head space pressure was atmospheric. A fluidized sand bath was used to heat the reactors. Before an experiment was started, the sand bath was preheated to the desired temperature. The reactor (either the glass tube which was fastened onto a holder or the tubing bomb) was, then, plunged into the bath. The heat-up period for the glass tube to reach 450 °C was less than 2 min., and the corresponding value for the tubing bomb was about 4-5 min. It was found that the temperature of the sand bath was very uniform and was always within (1 °C of the desired temperature after the heat-up period. It should be mentioned that the average reaction temperature for a tubing bomb experiment was slightly lower than the nominal temperature (the temperature of the sand bath) because part of the top stem (the extension tube) was not immersed in the sand. After a given reaction time, the reactor was removed from the bath and was cooled down using pressurized air (the glass tube) or quenched in cool water (the tubing bomb). The gaseous products from the tubing bomb experiments were analyzed quantitatively using a Perkin Elmer AutoSystem gas chromatograph (GC) equipped with two different columns and detectors. One stainless steel column packed with 80/100 Chemipack C18 was used to determine the yields of C1-C6 gases with a flame ionization detector (FID). The other stainless steel column packed with 60/80 Carboxen-1000 was used to determine the yields of H2, CO, CO2, CH4, C2H2, C2H4, and C2H6 with a thermal conductivity detector (TCD). The column with the FID was programmed from 35 to 205 °C at a rate of 10 °C/min with an initial isothermal period of 10 min at 35 °C, while that with TCD was programmed from 40 to 220 °C at 32 °C/min with an initial isothermal period of 7 min at 40 °C and a final isothermal period of 15 min at 220 °C. The gaseous products were identified and quantified by using standard gas mixtures. The gaseous products from the glass tube experiments were not analyzed because of the difficulty in collecting the gas samples due to extremely low gas yields. The liquid products were analyzed quantitatively by a Perkin Elmer 8500 GC equipped with a DB-17 capillary column and an FID. The column temperature

576 Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997

Figure 1. Chromatogram of liquid products from n-C14 at 425 °C for 15 min with a loading ratio of 0.36.

was programmed from 40 to 280 °C at 4 °C/min with an initial isothermal period of 5 min at 40 °C and a final isothermal period of 10 min at 280 °C. The identifications of n-alkanes, 1-alkenes, and cis- and trans-2alkenes were made by running standard mixtures. The liquid products were also identified by gas chromatography-mass spectrometry (GC-MS) using a Hewlett Packard (HP) 5890 II GC connected with an HP 5971A mass selective detector. The GC column and temperature program were the same as those used in the Perkin Elmer 8500 GC. The yield of a reaction product was expressed as the number of moles obtained per 100 moles of the reactant converted. The conversion, on the other hand, was defined as the mole fraction of the reactant converted. Different conversion levels were obtained from the experiments carried out for different reaction times. Reproducibility tests showed that the relative uncertainties for the conversion and the yields of major products, except the C6 products, were lower than 8%. The relative uncertainties for n-hexane and 1-hexene were 15% because the GC peaks for the two compounds were not completely separated. Results and Discussion In the following discussion, all the results relate to the experiments with the glass tube reactor unless noted otherwise. The data from the tubing bomb experiments will be identified as such. Product Distributions. The product distributions were obtained from the thermal decomposition of n-C10, n-C12, and n-C14 under different loading ratios, temperatures, and conversions. All experiments were conducted using a fixed loading ratio of 0.36 except for those dealing with the effect of the loading ratio. The reaction products from the thermal decomposition of C10-C14 n-alkanes under near-critical and supercritical conditions can be divided into two categories: those from the primary reactions and those from the secondary reactions of the primary products. The primary products include n-alkanes from C1 to Cm-2 (m ) number of carbon atoms in the reactant) and 1-alkenes from C2 to Cm-1. The secondary products include cisand trans-2-alkenes, n-Cm-1, n-Cm+1, and Cm+2-C2m-2 normal and branched alkanes. There are also small amounts of cyclopentanes and cyclohexanes. The yields of branched alkanes lighter than the reactant are not significant until high conversions are reached. The relative yields of the primary and secondary products

Figure 2. Product yields versus loading ratio from n-C14 at 425 °C for 15 min.

are dependent upon reaction conditions (loading ratio, temperature, and residence time or conversion). Figure 1 shows the chromatogram of liquid products from the thermal decomposition of n-C14 at 425 °C for 15 min with a loading ratio of 0.36. Since the product distributions from the thermal decomposition of n-C10, n-C12, and n-C14 are very similar, the following discussion on the effects of loading ratio, temperature, and conversion will be focused on n-C14. The effects of chain length will be discussed at the end of this section. Figure 2 shows the changes in overall molar yields of C6-C13 n-alkanes, C6-C13 1-alkenes, C6-C12 2-alkenes, and C14+ normal and branched alkanes with the loading ratio from the thermal decomposition of n-C14 at 425 °C for 15 min. It can be seen that the overall yield of C6-C13 n-alkanes increases and that of C6-C13 1-alkenes decreases as the loading ratio increases. While the overall yield of C6-C13 n-alkanes is lower than that of corresponding 1-alkenes at low loading ratios, their overall yield exceeds that of C6-C13 1-alkenes at high loading ratios. The overall yields of C6-C12 2-alkenes and C14+ alkanes are low at low loading ratios, but their yields become significant as the loading ratio increases. Since it is desirable to know the effects of pressure on the product distributions, the product yields in Figure 2 were plotted as a function of initial reduced pressure, as shown in Figure 3. The initial reduced pressure (Pr ) P/Pc) was calculated at the given temperature and loading ratio using the Soave-Redlich-Kwong equation of state (Soave, 1972). One can see from Figure 3 that the large changes in product distributions with pressure occur in the near-critical region. This result can be

Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 577

Figure 3. Product yields versus Pr from n-C14 at 425 °C for 15 min.

Figure 5. Effects of loading ratio on product distributions from n-C14 in the tubing bomb reactor at 425 °C for 30 min.

Figure 4. Effects of loading ratio on product distributions from n-C14 at 425 °C for 15 min.

attributed to the unique properties of the fluid near its critical point, such as large partial molar volume, local anisotropy, and high compressibility, as will be discussed further in the next section. Figure 4 shows the same trend with the increasing loading ratio/pressure for individual C6-C13 n-alkanes and 1-alkenes. The increases in n-alkane yields and decreases in 1-alkene yields with the increasing loading ratio can be explained by the increased rates of bimolecular reactions (hydrogen abstraction and radical addition) and decreased number of decomposition steps of radicals with the loading ratio. The decrease in product yield with the increasing carbon number arises from the increased decomposition tendencies of large radicals. Figure 5 shows the carbon number distributions of n-alkane and 1-alkene products as a function of the loading ratio from the thermal decomposition of n-C14 in the tubing bomb reactor at 425 °C for 30 min. The

yields of C5 products were not determined because n-C5 and 1-C5 coeluted from the GC column. It can be seen that while the yields of C6-C13 n-alkanes increase, those of methane, ethane, and propane decrease as the loading ratio increases. The yields of all olefinic components decrease with the increasing loading ratio. Among the gaseous products, the yields of C2 and C3 gases decrease significantly while the yield of methane remains within a narrow range as the loading ratio increases. The yield of methane is always lower than those of ethane and propane, and the yield of ethylene is always lower than that of propylene in the loading ratio range examined. At the lower loading ratios, the yields of ethane, propane, and propylene are high and equivalent while those of methane and ethylene show intermediate levels. In supercritical region (loading ratio > 0.15), ethylene is formed in the lowest yield as compared to other gaseous products and very low ethylene yields are obtained at the high loading ratios. The low ethylene yields indicate that the reaction mechanism under the conditions used in this study is different from that under low-pressure and high-temperature pyrolysis conditions which give very high ethylene yields (Kunzru et al., 1972; Depeyre et al., 1985a,b; Billaud and Freund, 1986; Voge and Good, 1949; Rebick, 1981). Examination of Figures 1-5 reveals that the major reaction products from n-C14 thermal decomposition are C1-Cm-3 nalkanes and C2-Cm-2 1-alkenes. While n-Cm-2, n-Cm-1, and 1-Cm-1 are also present in the products, their yields are considerably lower than those of the major products. The same conclusions can be drawn for the thermal decomposition of n-C10 and n-C12. Figure 6 shows the effects of temperature on product distributions from the thermal decomposition of n-C14 for similar conversions. At similar conversions, the formation of 1-alkenes is favored at higher temperatures while the production of n-alkanes is facilitated at lower

578 Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997

Figure 8. Product yields versus conversion from n-C14 at 425 °C.

Figure 6. Effects of temperature on product distributions from thermal decomposition of n-C14.

Figure 9. Effects of conversion on product distributions from n-C14 at 425 °C. Figure 7. Effects of temperature and conversion on molar ratio of C6-C13 n-alkanes to corresponding 1-alkenes from thermal decomposition of n-C14.

temperatures. Figure 7 shows that the selectivity for n-alkanes is affected by temperature more significantly at higher conversions. Figure 8 shows the changes in overall molar yields of C6-C13 n-alkanes, C6-C13 1-alkenes, C6-C12 2-alkenes, and C14+ normal and branched alkanes with the conversion from the thermal decomposition of n-C14 at 425 °C with a loading ratio of 0.36 (Pr ) 1.57). While the overall yields of C6-C13 n-alkanes and C14+ alkanes increase gradually, that of C6-C13 1-alkenes decreases rapidly with the increasing conversion. The overall yield of C6-C12 2-alkenes increases quickly at lower conversions and then remains at a stable level with a further increase in the conversion. Figure 9 shows the same trend with the increasing conversion for individual C6-C13 n-alkanes and 1-alkenes. The effects of conversion on product distributions will be discussed further in the next section.

Figure 10 shows the carbon number distributions of n-alkane and 1-alkene products as a function of the conversion from the thermal decomposition of n-C14 in the tubing bomb reactor at 425 °C. The relationship between the yield of liquid component and the conversion is consistent with that observed in the glass tube reactor experiments, that is, the higher the conversion, the higher the yields of C6-C13 n-alkanes and the lower the yields of C6-C13 1-alkenes. The yields of C1-C3 gases decrease with the increasing conversion. In the conversion range examined, the yields of gaseous products decrease in the order of propane > propylene > ethane > methane > ethylene. Figure 11 shows the effects of chain length on product distributions from the thermal decomposition of C10C14 n-alkanes at 425 °C for similar conversions. It can be seen that the yields of n-Cm-1, n-Cm-2, and 1-Cm-1 are significantly lower than those of other n-alkane and 1-alkene products. The yields of individual n-alkane and 1-alkene with the same carbon number, except for n-Cm-1, n-Cm-2, and 1-Cm-1, increase as the chain length

Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 579

the same compound will be higher for the lighter reactant than for the heavier reactant. Reaction Mechanisms for Thermal Decomposition of n-Alkanes. Thermal decomposition of nalkanes can be described by free radical chain reactions (Rice, 1931, 1933; Rice and Herzfeld, 1934; Kossiakoff and Rice, 1943). Some modifications must be made on the R-K mechanism to account for the formation of significant amounts of secondary products. According to the product distributions and the changes in product composition with reaction conditions, the following reaction scheme has been developed for the thermal decomposition of n-alkanes under near-critical and supercritical conditions

n-CmH2m+2 f Ri1• +Rj1•

(1)

Ri1• + n-CmH2m+2 f n-CiH2i+2 + Rmx•

(2)

Rmx• f Rmy• f 1-CkH2k + Rn1• (|y - x| g 4; 2 e k e m - 1; n e m - 2) (3) Rn1• + n-CmH2m+2 f n-CnH2n+2 + Rmx•

(4)

Rn1• f C2H4 + Rn-21• (3 e n e 5)

(5a)

Rn1• f Rnx• f 1-CkH2k + Rn-k1• (n g 6; x g 5) (5b) Figure 10. Effects of conversion on product distributions from n-C14 in the tubing bomb reactor at 425 °C.

Figure 11. Effects of chain length on product distributions from thermal decomposition of n-alkanes at 425 °C.

of the reactant decreases. This is because the total number of compounds found in the products from the lighter reactant is less than that from the heavier reactant. Therefore, at similar conversions the yield of

Rn1• + 1-CkH2k f Rn+kx•

(6)

Rm1• + 1-CkH2k f Rm+kx•

(7a)

Rmx• + 1-CkH2k f Rm+kb• (x * 1)

(7b)

1-CkH2k f 2-CkH2k

(8)

Rn1• + Rmx• f products

(9)

where Rn1• is the primary radical with a carbon number of n, Rmx• is the primary or secondary parent radical with a free electron at site x of the carbon skeleton, Rm+kx• is the heavy normal alkyl radical, and Rm+kb• is the heavy branched alkyl radical. The initiation reaction occurs by homolytic carboncarbon bond cleavage to produce two primary radicals (eq 1). These radicals then abstract hydrogen atoms from the surrounding reactant molecules to form various parent radicals (eq 2), of which most are secondary radicals, since the removal of a secondary hydrogen atom requires a lower activation energy. The parent radical isomerizes and then decomposes by β-scission to form a 1-alkene and a lower primary radical (eq 3). This lower primary radical could undergo one of the following reactions: hydrogen abstraction from the reactant molecule (eq 4), decomposition to form a 1-alkene and a smaller primary radical (eqs 5a and 5b), and, except for fairly low conversions, addition to the terminal carbon of a 1-alkene to give a higher radical (eq 6). For the radical with carbon number larger than five, isomerization occurs before decomposition (eq 5b). Under the conditions used, the following reactions also occur: the addition of a primary parent radical or a secondary parent radical to a 1-alkene to produce a heavy n-alkyl radical (eq 7a) or a heavy branched alkyl radical (eq 7b) and the double-bond isomerization of 1-alkene to 2-alkene (eq 8). The termination reaction

580 Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997

Figure 12. Expanded chromatogram of liquid products from n-C14 at 425 °C for 15 min with a loading ratio of 0.36.

occurs mostly by recombination of a lower primary radical and a parent radical (eq 9). The recombination reactions between lower primary radicals and between parent radicals are less important because of relatively low concentrations of the lower primary radicals and low-oriented collision frequencies between the parent radicals. It should be mentioned that the isomerization of the lower primary radicals only occurs as an intermediate step during their decomposition (eq 5b). This assumption is reasonable on the basis of the observed product distributions. If the isomerization of these lower primary radicals occurred immediately after their formation, one would expect much more lower secondary radicals than lower primary radicals before they underwent further reactions, according to the R-K mechanism (Kossiakoff and Rice, 1943). If this were the case, one would observe significant amounts of lighter branched alkanes (20%). It is desirable to investigate the effects of reactor type on the thermal decomposition of model compounds. In this work, comparative experiments using the glass tube reactor and tubing bomb reactor were conducted under otherwise identical conditions. Figure 13 shows the change in conversion with reaction time from the thermal decomposition of n-C14 in two different reactors at 425 °C for a loading ratio of 0.36. It can be seen that the reaction rates obtained from the glass tube reactor are much higher than those obtained from the tubing bomb reactor. The lower reaction rates in the tubing bomb experiments result from two factors: longer heat-up time (4-5 min) and slightly lower average temperature because of the existence of a low-temperature region in the top stem of the tubing bomb reactor. Figure 14 shows the product distributions from the thermal decomposition of n-C14 in two different reactors at 425 °C with a loading ratio of 0.36 and at similar conversions. It can be seen that the changes in product

582 Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997

Figure 14. Product distributions from thermal decomposition of n-C14 in two different reactors at 425 °C with a loading ratio of 0.36.

Figure 15. Product distributions from thermal decomposition of n-C14 in the tubing bomb reactor at 425 °C for 60 min under two different atmospheres.

distributions with the carbon number are sharper for the tubing bomb reactor than for the glass tube reactor. The molar yields of n-C9-n-C13 are lower and the molar yields of n-alkanes with carbon number below 8 are higher in the tubing bomb reactor than the corresponding values in the glass tube reactor. The molar yields of all 1-alkene components are higher in the tubing bomb reactor than in the glass tube reactor. These results can also be explained by the lower temperature in the top stem of the tubing bomb reactor. The lower temperature in the top stem results in the lower pressure in the tubing bomb reactor than in the glass tube reactor under otherwise comparable conditions. Since the low pressure favors unimolecular radical decomposition reactions and thus enhances the production of 1-alkenes and correspondingly depresses the formation of larger n-alkanes, higher yields of 1-alkenes and lower yields of long-chain n-alkanes are observed in the tubing bomb reactor. The sharper product distribution versus carbon number curves in the tubing bomb reactor can be attributed to nonuniform distributions of the primary products which affect the secondary reactions at higher conversions (≈25%). During the reaction, the light primary products will stay longer in the low-temperature region in the top stem and, thus, undergo fewer secondary reactions while the heavier primary products will mainly stay in the high-temperature region in the bottom reactor body and thus undergo significant secondary reactions. This, of course, will result in the sharper changes in product distributions with the carbon number. Effect of Oxygen on Thermal Decomposition. In the glass tube reactor experiments, no specific technique was used for complete removal of air in the tube. It is important to know the effect of oxygen on thermal decomposition. In this work, the parallel experiments

for n-C14 stressing were conducted in tubing bombs under nitrogen and air atmospheres. Figure 15 shows the product distributions from the thermal reaction of n-C14 at 425 °C for 60 min under both nitrogen and air atmospheres. The figure also shows the conversion values. It can be seen that there is no significant difference between nitrogen and air atmospheres in terms of conversion and main product distributions. It should be mentioned that there might be some differences in minor product distributions. While the liquid product from nitrogen atmosphere was almost colorless, the liquid from air atmosphere exhibited obvious yellow color with small amounts of suspended gum. The yellow color probably comes from small amounts of hydroperoxides and peroxide-catalyzed olefin oligomer (Ginosar and Subramaniam, 1995; Clark and Subramaniam, 1996). Conclusions The primary products from the thermal decomposition of C10-C14 n-alkanes under near-critical and supercritical conditions include C1-Cm-2 n-alkanes and C2-Cm-1 1-alkenes. The secondary products include cis- and trans-2-alkenes, n-Cm-1, n-Cm+1, and Cm+2-C2m-2 normal and branched alkanes. The relative yields of the primary and secondary products are dependent upon the reaction conditions (pressure/loading ratio, conversion, and temperature). As pressure increases, the yields of C6-Cm-1 n-alkanes and Cm+ alkanes increase and the yields of 1-alkenes and C1-C3 n-alkanes decrease because of the increased rates of bimolecular reactions (hydrogen abstraction and radical addition). The large changes in product distributions with pressure occur in the near-critical region. The effects of the conversion on the product distributions are similar to those of

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pressure. The high temperature favors the formation of 1-alkenes because radical decomposition reactions, which result in the formation of 1-alkenes, are promoted at higher temperatures due to higher activation energies. The observed product distributions and changes in product composition with reaction conditions can be explained by a modified free radical mechanism. Acknowledgment This work was supported by the U.S. Department of Energy, Pittsburgh Energy Technology Center, and the Air Force Wright Laboratory/Aero Propulsion and Power Directorate, Wright-Patterson AFB. Funding was provided by the U.S. DOE under Contract DE-FG2292PC92104. We express our gratitude to Prof. Harold H. Schobert of Penn State for his support. We also thank Mr. Douglas A. Smith for the fabrication of the glass tubes and Mr. Ron Copenhaver for the fabrication of the tubing bombs. We thank Mr. W. E. Harrison III and Dr. T. Edwards of AFWL/APPD and Dr. S. Rogers of PETC for many helpful discussions. Appendix. Pressure Calculations Using the Soave-Redlich-Kwong Equation of State The initial pressure at the specified temperature and loading ratio was calculated using the Soave-RedlichKwong equation of state as shown below (Soave, 1972):

P)

a RT V - b V(V + b)

(A1)

The parameters a and b are expressed as

a ) 0.42747

R2T2c R(T) Pc

(A2)

RTc Pc

(A3)

b ) 0.08664 where R(T) is given by

2 R(T) ) [1 + m(1 - T0.5 r )]

(A4)

m ) 0.480 + 1.574ω - 0.176ω2

(A5)

with

In above equations, Tc, Pc, and ω are the critical temperature, critical pressure, and acentric factor, respectively, and Tr is the reduced temperature (T/Tc). The molar volume V is calculated by the following equation:

V)

Mw Fr

(A6)

where Mw is the molecular weight, F is the density of the sample at room temperature, and r is the loading ratio. Literature Cited Allara, D. L.; Shaw, R. A Compilation of Kinetic Parameters for the Thermal Degradation of n-Alkane Molecules. J. Phys. Chem. Ref. Data 1980, 9, 523-559. Billaud, F.; Freund, E. n-Decane Pyrolysis at High Temperature in a Flow Reactor. Ind. Eng. Chem. Fundam. 1986, 25, 433443.

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Received for review July 10, 1996 Revised manuscript received December 5, 1996 Accepted December 6, 1996X IE960392B

X Abstract published in Advance ACS Abstracts, January 15, 1997.