Ind. Eng. Chem. Res. 1987,26, 1633-1638
1633
Free-Radical Reactions of High Molecular Weight Isoalkanes Yury V. Kissin* Gulf Research and Development Company, Pittsburgh, Pennsylvania 15230
Free-radical cracking of three isoprenoid alkanes, pristane, phytane, and squalane, was studied in the liquid phase at 250 "C. Application of high-resolution capillary GC allowed complete identification of all reaction products, the equimolar mixtures of isoalkanes and isoolefins. Product yields a t 250 " C for 24 h were low, 3-6 w t 7%, and the cracking processes could be adequately described by three reaction steps only: formation of parent radicals in hydrogen abstraction reactions, @-scissionreactions of the parent radicals with the formation of isoolefins, and chain-transfer reactions involving the substrate molecules and all radicals present in the reaction systems, both parent and formed in the scission reactions. Quantitative analysis of all reaction products provided information on two reactions, @-scissionof C-C bonds and the equilibrium formation of various radicals. Reactivities of C-C bonds in the scission reactions depend on the types of the bonds. Probabilities of the scission of the CHz-CH(CH3), CHz-CH,, and CHz-CH3 bonds are 1.5:l:O.l. Relative populations of radicals formed in different positions of isoprenoid chains are given in Chart I. Studies of thermal cracking of isoalkanes play an important role in several areas, including high-temperature catalytic cracking, delayed coking, manufacture of olefins, degradation of polymers, and geochemistry. Although principal chemical reactions occurring under relatively mild conditions are well-known (Kossiakoff and Rice, 1943; Kochi, 1973; Benson, 1960; Ranzi et al., 1983), detailed information about cracking of large branched molecules is lacking from the literature, mostly because of analytical problems in identifying the reaction products, various branched alkanes and olefins in the carbon atom range cs-c20+. The present paper discusses products and principal reaction stages of free-radicalcracking of pristane, phytane, and squalane in the liquid state under mild conditions (250 "C, 24 h). Quantitative information on the distribution of the reaction products provided data for the analysis of reactivities of various C< bonds in the p-scission reactions and reactivities of various C-H bonds in the hydrogen abstraction reactions.
Experimental Section Thermocracking of pristane, phytane, and squalane at 250 "C for 24 h was carried out in the liquid state in small glass ampules sealed under vacuum. Under these conditions, the reactions yielded equimolar mixtures of isoalkanes and isoolefins. A Hewlett-Packard 5880A gas chromatograph equipped with the flame ionization detector, operated in the split injection mode with a split ratio 100:1, was used to obtain chromatographic data. The column used was a 50-m, 0.2-mm-i.d., fused silica capillary, coated with 0.50-pm film of cross-linked methylsilicone. Helium carrier gas was used at a flow rate of 1 mL/min. The column oven was programmed from 40 to 300 "C a t a rate of 5 "/min and held at 300 "C until complete elusion of a sample. Sample size was 1.0 FL. Detector and injector temperatures were held a t 300 "C. Samples were diluted with CS2before injection. Analysis of several alkane-olefin mixtures indicated that relative response factors for the peak areas of olefins and alkanes in the Cs-Cls range are very close. The techniques used for assignment of alkane and olefin peaks in gas chromatograms of the reaction products were *Present address: Mobil Chemical Co., Research & Development, Edison, N J 08818.
0888-5S8~/87/2626-1633$01.50/0
Chart I CH~-CH-CH~-CH~-CH-CH~-CHZ-CH~-CH
0.30
0.70 0.15 1.00 0.04 0.16 0.90 0.05
-
0.98
discussed earlier (Kissin and Feulmer, 1986; Kissin et al., 1986; Kissin, 1986). They are based on the application of a modified additivity principle (Spivakovskii et al., 1977) which allows quantitative estimation peak positions for complex molecules (multibranched alkanes and olefins) from the data on peak positions of more simply built molecules (monobranched alkanes, linear olefins). Separation of peaks of alkanes and olefins in gas chromatograms was achieved by addition of a mineral clay (bentonite) to some of the ampules with isoalkanes prior to their thermocracking. The clay converts olefinic products of the reaction into a complex mixture of various secondary products while keeping the alkanes intact. Comparison of the gas chromatograms of the cracking products obtained with and without the clay assists in identification of branched alkanes. A general procedure in presenting the retention times for peaks of hydrocarbons in gas chromatograms is the calculation of their Kovats factors (KF) from their retention times, RT (Kovats, 1961): KF(isoa1kane CnH2,,+.J = 100(n - 1) + 100[RT(isoalkane CnH2n+2) RT(n-Cn-1H2n)1 / [RT(n-C,&,+J - RT(n-Cn-1H2n)I (1) For example, the peak of an isoalkane situated in the middle between peaks of n-C12Hzsand n-C13H2shas KF = 1250. However, in the case of complex hydrocarbon mixtures with numerous closely spaced peaks in gas chromatograms, a different parameter was found to be more convenient, designated as relative retention factor RRF (Kissin and Feulmer, 1986): RRF(isoa1kane CnH2,+J = [KF(isoalkane CnH2n+2) KF(Fz-C,H~,+~)] / 100 = [RT(isoalkane CnH2n+2)RT(n-CnH2n+J]/ [RT(n-CnHzn+z)- RT(n-Cn-iH2n)l (2) This factor, as a rule, is a negative number representing normalized relative precedence of the peak of a hydrocarbon with respect to the peak of the normal alkane with the same carbon atom number. 0 1987 American Chemical Society
1634 Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987 Table I. Alkanes Formed in Thermocracking Reactions of Pristane, Phytane, and Saualane at 250 O C " position of radical attack alkane pristane phytane squalane RRF (exptl) RRF (calcd) 12 n-C4 5 5 5 iso-C, 6 6, 11 6 2-Me- C 6',7 6',7 6',7 2-Me-C, 10 3-Me-C, 9,lO' 3-Me-C6 8 8 8 2-Me-C7 9 9 9 -0.700 -0.700 2,6-Me2-C7 8 3-Me-C8 -0.644 6 -0.636 10 7,lO 2,6-Me2-C8 lO',ll -0.747 -0.736 5,lO' lO',ll 2,6-Me2-Cg -0.568 -0.578 6 3,7-Me2-Cg -0.695 -0.674 5,6' 3,7-Me2-Clo 4 12 -0.812 12 -0.833 2,6-Me2-C11 3 12 -1.252 -1.175 13 2,6,10-Me3-Cll -0.753 4 -0.788 3,7-Me2-C12 2,14 11 -1.199 -1.114 3,14 2,6,10-Me3-C12 1,2' -1.254 14',15 10 -1.353 2,6,10-Me3-C13 2 -1.150 -1.076 3,7,11-Me3-C13 9,lO' -1.430 -1.329 2,6,10-Me3-C,, 1 -1.210 -1.300 3,7,11-Me3-C1, 16 -1.477 -1.386 2,6,10-Me3-Cl, -1.407 8 -1.531 2,6,10-Me3-CI6 -1.940 -1.788 2,6,10,15-Me4-Cl6 6 -1.862 -1.705 2,6,10,15-Me4-C17 -2.102 -1.880 5,6' 2,6,10,15-Me4-C18 4 -2.115 -2.063 2,6,10,15-Me,-C20 3 -2.345 -2.464 2,6,10,15,19-Me5-Cz0 2 -2.408 -2.320 2,6,10,15,19-Me5-Czl 1 -2.608 -2.455 2,6,10,15,19-Me5-C2,
ARRF, 70
0.0
1.2 -1.5 -1.7 -3.0 -2.5 -6.1 -4.5 -7.1 -7.3 -6.4 -7.1 -7.0 -6.2 -8.1 -7.9 -8.5 -10.6 -2.5 5.1 3.8 6.2
3
(I
Amounts of methane, ethane, and propane cannot be determined quantitatively. These alkanes are not reported in the table.
Scheme I formation of a parent radtcal (hydrogen abstractlon reaction): R' Pristane 12,6,10,l4-tetramethylpentadecane)
t 2.6.10,14-tetramethylpentadecane
3c
JC/\4c/
5c
7c
gC
llc
13c
lC/ \ac/ hc' A,/
15
Lc/Lc
I
CH3
CH3
Results and Discussion Numbering of carbon atoms in the skeletons of the three isoprenoid molecules used in the study is shown in Figure 1. Conversions in the thermocracking reactions (250 "C, 24 h) were for pristane, 6.0%; for phytane, 2.7%; and for squalane, 6.4%, Under these mild conditions, only three chain propagation reactions of the overall chain scheme should be considered: formation of a parent radical, fission of the radical with the formation of an olefin and a smaller alkyl radical, and the chain-transfer reaction yielding a lower molecular weight alkane and regenerating the parent radical. One example of these three reactions involving a radical attack on the sixth position of the pristane molecule and the p-scission of the C7-C8 bond in it is given in Scheme I.
CH3
+
I CH3
-
CH3
~ H ~ C H ~ C H C H ~ C H ~ C H Z (4) CHCH~
I CH3
I
CH3
( 2 , 6 - dimethyl- 1 -heptene) chain- transter reaction:
I
Squalane 12,6.10,15,19,23-hexamethy1tetracosane)
CH3
I
CH3CHCH2CH2CH2C=CH2
'CH2CH2CHCH2CH2CH2CHCH3
Figure 1.
CH3
I
I 3c 5c 7c gC 1lC 13c 15cI 17c 1gCI 2lC 23cI \ZC / \4c / \6c / \EC / \l o c / \1 2 c/ \14c/ \1 b C/ \lec/ \2OC/ '2ZC/ \ 24c
(3)
I
CH3CHCH2CH2CH2kH2CH2CH2CHCH2CH2CH2CHCH3
CH3 lC
I
I
8-scission reaction:
CH3 Thytane (2,6,10,14-tetramethylhexadecane)
+
RH
CH3CHCH2CH2CH2~CH2CH2CH2CHCH2CH2CH2CHCH,
CH3 lC
-
CH3
+ 2.6.10.14 - tetramethylpentadecane
-
CH3 CH3CH2CHCHSH2CH2CHCH3 t
I CH3
I
CH3
(2 ,6-dimethyloctane) R'
(5)
Reactions 3 and 5 in Scheme I are, of course, the same reaction of hydrogen abstraction and are separated here to emphasize chemistry of reaction production formation. Tables I and I1 list all possible alkanes and olefins which are formed in the principal thermal cracking reactions of pristane, phytane, and squalane, experimental and calculated values of chromatographic RRF for the alkanes and olefins, and positions of radical attacks resulting in the formation of corresponding hydrocarbons. Peak areas for the products were used as the basis for the evaluation of reactivities of various bonds in two reactions-hydrogen abstraction (reaction 3) and @scission (reaction 4).
Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987 1635 Table 11. Olefins Formed in Thermocraoking Reactions of Pristane, Phytane, and Squalane at 250 OC" Dosition of radical attack olefinb pristane phytane squalane RRF (exptl) RRF (calcd) 1
c3-
1-c4-
2-C4' iso-C4' 2-Me-l-C4' 3-Me-1-C4' 3-Me-1-C5' 4-Me-1-C5' 4-Me-1-C6' 6-Me-1-C7' 6-Me-2-C7' 6-Me-1-C8' 6-Me-2-C8' 2,6-Mez-1-C,' 2,6-Mez-1-C8' 3,7-Mez-1-Cg' 3,7-Mez-l-Cg' 4,8-Mez-1-C?4,8-Mez-1-C1,' 6,10-Me2-1-Cll' 6,10-Mez-2-Cll' 6,10-Me,-2-Clz' 2,6,10-Me3-1-Cll' 3,7,11-Me3-1-Clz' 2,6,10-Me3-l-Clz= 4,8,12-Me3-1-C13: 3,7,11-Me3-l-C13 4,8,12-Me3-1-C14' 5,9,13-Me3-1-C14' 6,10,14-Me3-1-C15' 6,10,14-Me3-2-C15' 2,10,14-Me3-5-C151 2,10,14-Me3-6-CI56,10,14-Me-3-1-C16' 6,10,14-Me3-2-C16' 2,10,14-Me3-5-CI6' 2,10,14-Me3-6-C16' 3,11,15-Me3-6-c1,,' 3,11,15-Me3-7-C16' 7,11,15-Me3-1-Cle= 7,11,15-Me3-2-cl6' 7,11,15-Me3-3-C16' 2,6,10,14-Me4-1-C16' 2,7,11,15-h&-l-C16' 3,8,12,16-Me4-1-C17' 4,9,13,17-Me4-1-C18' 6,11,15,19-Me,-1-Czo~ 6,11,15,19-Me4-2-CzoP 2,6,11,15,19-Me5-2-Czo~
2 3 4 6' 5 6 7 8 6' 7 6 5 4
1 14' 14' 2 14 3 13 4 12 6' 5 10' 11 6 10 7 9 8 8 6',10' 9 7 10 11 6 12 5 4
2 3 4 6' 5 6 7 8 10' 9 10 11
12 12
1 3 5 7
-0.228 -0.520 -0.455 -0.896 -0.850 -0.796 -0.915 -0.799 -0.751 -1.047 -1.459 -0.992 -1.436 -1.398 -1.379 -1.505
14' 13 1
3 5 7 11 9 10 15 13
ARRF, %
1
11
14 10 9 8 6' 7 6
-1.583 -1.530 -1.712 -2.114 -2.00 -2.14
0.518 -0.455 -0.880 -0.803 -0.846 -0.785 -0.916 -0.812 -0.740 -1.008 -1.328 -0.920 -1.338 -1.287 -1.301 -1.420 -1.474
-1.409 -1.326 -1.241 -1.241 -1.227 -1.227 -1.494 -1.411 -1.525 -1.562 -1.909 -1.963 -2.19 -2.13 -2.18
0.4 0.0 1.8 0.5 1.4 0.1 1.6 4.1 3.7 9.0 7.2 6.8 7.9 5.6 5.6
5.6 7.7 8.8 9.7 2.0 1.9
a Olefins Cze-Czgin the products of squalane thermocracking were not identified quantitatively and are not reported in the table. *The notation ' is used to show the double bond (propylene: C3', etc.).
&Scission Reactions. Information on relative reactivities of various C-C bonds in p-scission reactions is very scarce, mostly because the principal studies of these reactions were concerned with high-temperature pyrolysis of low molecular weight compounds with only one p-scission route possible. It was found that splitting off of a methyl radical from a secondary alkyl radical requires E,, of ca. 1.5 kcal/mol higher than splitting of a higher alkyl radical (Ranzi et al., 1983): C H j t CH2 =CHCH2CH2Ch
.Fact = 32.5 kcal/mol CH~CH~~HCH~CHZCH~ CH~CH; t CH2 =CHCH&Ha .Esct = 31.O kcal/mol
With the preexponential factors of these two reactions being the same, the difference in E , translates into an ca. 4-fold ratio of the corresponding rate constants at 250 "C. In the majority of cases in the isoprenoid cracking, several 0-scission reactions are possible for any radical
formed in reaction 3 (except for radicals in positions 2 in all isoprenoids and in position 16 for phytane). Analysis of yields of reaction products listed in Tables I and I1 allowed estimations of probabilities of various scission reactions. @-Scissionin Tertiary Radicals. Tertiary radicals in positions 6 and 10 are situated in very symmetrical environments. As a consequence, probabilities of p-scissions of the adjacent bonds are virtually equal. The ratios of products formed in the C4-C5 and C7-C8 bond scissions (radicals in positions 6) are in the 1.06-0.98 range for all isoprenoids. The same ratios (1.08-0.99) were found for C8-C, and C11-C12 bond scissions in phytane and squalane (radicals in positions 10). On the other hand, the probability of the CH2-CH2 (C12-C13) bond scission in the phytane radical in position 14 is 10.6 times higher than that for the CH2-CH3 bond (C15-C16) which corresponds to the Eactdifference of ca. 2.5 kcal/mol which is significantly higher than reported by Ranzi et al. (1983) for these reactions.
1636 Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987 Table 111. Relative Populations of Radicals Formed in Thermocracking of Pristane, Phytane, and Squalane carbon atom position 5 6 6' 7 8 9 10 10' 11 12 13 1 2 3 4 Pristane no. of groups 4 2 2 2 2 2 2 2 1 radical pop" 0.31 0.65 0.15 1.00 -0.02 0.16 0.43
14
15
Phytane no. of groups radical popn
2
no. of groups radical popa
4
1
1
0.27 2 0.31
1
1
0.74 2
20.13
1.00
1 -0.04
2 0.16
2 1.00
2 -0.05
2
0.77
1
1
1
1
1
0.16
0.90
0.16
0.97
Squalane 2 2 20.16 0.95
2 0.16
2 0.99
1 -0.05
20.13
1
1 0.70
2 -0.05
2 0.12
2 0.43
1
1
1
0.48
0.14
"pop = population.
j3-Scission in Secondary Radicals. Similar to the previous case, if a secondary radical is situated in a symmetrical environment (positions 8 in all isoprenoid molecules, position 12 in phytane), the probabilities of the two @-scissionreactions are identical (with the mean deviation of 4%), according to expectations. However, even a slight deviation from the symmetry, in positions 4, resulted in deviations from the equality. The ratio between the probabilites for the scission of C&3 and c54&is ca. 0.63. When secondary radicals are in positions 7 (or positions 9 in phytane and squalane), the scissions involve different chemical bonds. In these cases the ratios of scission rates for the CH2-CH2 bonds (CB-C9for radicals in position 7) and for the CH2-CH bonds (c5-CC) are ca. 0.71 (f7%). The same ratio (scission of the CI2-Cl3 vs. C9-Clo bonds) for the phytane radical in position 11 is 0.58. Hydrogen Abstraction Reactions. Chemical patterns of radical thermocracking reactions depend strongly on the relative probabilities of the two reactions involving alkyl radicals formed in the 0-scission reaction (reaction 4). The first of these reactions is the chain-transfer reaction producing the parent radical (in the case of low conversion reactions)-reaction 5. From the kinetic point of view, the most important feature of the last reaction is rapid substitution of the highly active primary radicals formed in reaction 4 by more stable secondary or tertiary radicals. If the thermocracking reactions are carried out in a gas phase, an often regarded alternative to the chain-transer reaction is monomolecular radical isomerization which, for the primary radical formed in reaction 4, can be envisaged as (Kossiakoff and Rice, 1943; Ranzi et al., 1983) in eq. 6.
'CH2CH2CHCH2CH2CH2CHCH3
I
I
CH3
CH3
-
/CH2\
'CH2
sion, 4-methyl-1-hexene which is not present in the products of pristane cracking. Similar conclusions can be drawn from the analysis of possible products expected from other secondary radicals produced in reactions 6. Reaction 7 should produce, in the case of branched alkanes, large yields of light olefins, ethylene, propylene, and isobutene. However, total gas formation in our experiments was relatively small and can be adequately explained by reactions 3-5. These findings,as well as low total conversions in the thermocracking reactions, allowed a conclusion that under the reaction conditions employed, the chain-transfer reactions (reaction 5) involve mostly the substrates and the parent alkyl radicals (formed in reaction 3) and result in the equilibrium distribution of the parent radicals. In such a case, relative yields of the stable products resulting from 0-scission of a parent radical are proportional to the relative concentration of the radical. Table I11 lists relative populations of parent radicals formed in thermocracking reactions of pristane, phytane, and squalane under mild conditions. In this table the relative population of the products formed from the radical in the sixth position is used as a reference point. These data can be interpreted if one applies the steady-state principle to the reaction of hydrogen abstraction between the parent radical R,' of a substrate molecule, S (i = the spin position in the chain) and the C-H bond in the jth position of the substrate molecule: R,'
-+
+ So')
k,
S
R,'
(8)
The steady-state expression for the concentration of the Rk' radical participating in reactions 8 and 4 is
CHCH3
I
_c
H \CH/CH2
1 c4 H9
CH3CH2CHCH26HCH2CHCH3 (6) CH3
CH3
An alternative route to primary radical disappearance in reactions 5 and 6 is the 0-scission reaction of the primary radical:
-
CH~CH2CH(CH&H2CH2CH2CH(CH3)CH3
CH2=CHz + CH3CHCHzCHZCHzCH(CH,)CH3 (7) Under reaction conditions employed in this work, reactions 6 and 7 proved to be relatively insignificant compared to reactions 3-5. Occurrence of reaction 6 can be evaluated by the yields of stable proddcts formed in @-scissionreactions following reaction 6. For example, the secondary radical formed in reaction 6 should produce, after 0-scis-
The first term in this equation represents the sum of the Rk' formation rates in reactions of any radical Riaand the kth position of the substrate (mk is the number of the equivalent positions, listed in Table 111);the second term represents loss of the Rk*radical in its reactions with various C-H bonds of the substrate, j ; and the last term represents loss of the Rk' radical in 0-scission reactions. Under mild conditions, the last term is much smaller than the first two and can be neglected. Then the expression for the Rk' concentration in the system is
with Ck,ln[Rk'] = constant (the steady-state total radical concentration in a system). The numerator in eq 10 is the sum of reactivities of various radicals Ri' in the hydrogen abstraction reaction involving the C-H bond in the kth
Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987 1637 position of the isoalkane chain. In a particular reaction system (or, in the first approximation, in such similar systems, as pristane, phytane, and squalane which have comparable distributions of tertiary, secondary, and primary carbon atoms), the numerator reflects reactivity of a particular C-H bond. The denominator in eq 10 represents reactivity of the Rk’ radical in its exchange reactions with various C-H bonds of a substrate. In similar systems (the systems with comparable distribution of tertiary, secondary, and primary hydrogen atoms), the denominator reflects reactivity of a particular radical. As a rule, the higher the reactivity of a particular C-H bond, the more stable (less reactive) radical is formed after its dissociation. As a result, relative radical populations listed in Table I11 represent very sensitive parameters reflecting radical stability. However, detailed interpretation of the data in terms of C-H bond energies or alkyl radical reactivities is not straightforward, as follows from eq 10. Hydrogen Abstraction from Tertiary C-H Bonds. Tertiary C-H bonds are the most active in radical hydrogen abstraction reactions as emphasized by their low C-H bond energies of ca. 90 kcal/mol (Kochi, 1973; Benson, 1960). It follows from Table I11 that population of tertiary radicals in positions 6 and 10 of all isoprenoids is equal within a few percent. The relative populations of tertiary radicals derived from the end and internal C-H bonds are internal-C*
I I
(positions 6 and 10)
.
: RCCH2CHa : RCCH3
I
I
CH3
,343
(position 14 in phytane)
= 1:0 4 8 : 0 30
(positions 2 )
The direction of the change indicates that the main reasons for the differences are electronic rather than steric: increase in radical population parallels the total inductive effect of the substituents. Hydrogen Abstraction from Secondary C-H Bonds. Populations of radicals derived from secondary C-H bonds depend strongly on the type of the neighbors surrounding CH2groups. The first group of radicals are CH2CHCH(CH,) (positions 5, 7, 9, and 11 in Table 111). Population of all these radicals is the same and is equal to 0.12-0.16 compared to the population of the internal tertiary radicals. The second group includes CH2CHCH2radicals (positions 4 and 8 as well as position 12 in phytane) with the relative population in the 0.43-0.95 range. In all three cases, these radicals, when positioned in the central part of a molecule (position 8), are slightly more abundant than the same radicals positioned near the molecule’s ends (position 4, position 12 in phytane). This example demonstrates ability of the employed technique to distinguish minor differences in C-H bond reactivities and in radical stabilities. Hydrogen Abstraction from Primary C-H Bonds. Information on relative population of primary radicals is limited due to low content of the products of their decomposition. Judging by the data on primary radical populations in positions 6’ and 10‘in phytane and squalane all (CH)CH3groups have the same reactivity. Possible Interpretation of Hydrogen Abstraction Data. A recent review of radical reactions with alkane participation (Ranzi et al., 1983) provides information on reactivities of various C-H bonds and on reactivities of different alkyl radicals (Table IV). Comparison of these two sets of estimations indicates, that, in the first approximation, reactivities of C-H bonds are inversely proportional to reactivities of radicals derived from them (with
Table IV. Reactivities of C-H Bonds and Radicals at 250 O C (per aroud Reactivities of C-H Bonds
-
R*
R’-cH(R”)-R”
k
-2-
kp:kl = 6 6 k3:kl = 25 7
t ‘CH2-A’
R-H
kl
ICH3R‘ t R‘-CH2-R” \
R-H f R ’ - E H - R C
h
-P R - H
+ R/-E(RJ/)-R/~
k 3 : k p = 4.0
Reactivities of Radicals
h4
/R‘-CH;a R-CH3 f Rc?H-R‘lb
‘R’-E(R”)-R”C
R’-CH3
t ‘CH2-R
R’-CH2-R”
Ir(l
t’CH2-R
R‘-CH(R”)-R”
=69
k,:k,
t.CH2-R
k,:k6=36 2
ks:ks = 5 2
OR’ = CH3, CzHs, C3H7. bR’ = CH,; R” = CH3, C2HP cR’ = R” = CH3.
r q ,
054
084
038
019
040
09,
040
099
022
r(C-H)
054
042
019
006
320
OLa
020
099
007
-0 90 -1 72
-2 9 1
-_ 67
Ho (kcal/ml)
-0 Y
- 76
-1 64
0
0
-2 75
Figure 2.
a precision of ca. &15%). If these (limited to small radicals) estimations are applied to larger radicals, eq 10 can be approximated in the following way. Its numerator, as discussed earlier, reflects an average reactivity of a given alkyl group, r(CH,), and its denominator represents the reactivity of a derived radical, r(R*). If, as the estimations in Table IV suggest, r(CH,) l / r ( P ) , relative populations of various radicals formed in thermocracking of pristane, phytane, and squalane (given in Table 111) can be regarded as proportional to r(CH,)2, with mktaken into account as in eq 10. This approach allows an approximate estimation of reactivities of various C-H bonds, r(C-H), in isoprenoid alkanes (assuming, as in Table 111, reactivity on the CH group in position 6 to be equal 1)-see Figure 2. Although these evaluations form a predictable pattern, relative reactivities of the CH2 groups in positions 4 and 8 are unexpectedly high.
-
Registry No. n-C,, 106-97-8; iso-C,, 75-28-5;2-Me-C4,78-78-4; 2-Me-C,, 107-83-5; 3-Me-C5,96-14-0; 3-Me-C6,589-34-4; 2-Me-C7, 592-27-8; 2,6-Me2-C7,1072-05-5;3-Me-C8, 2051-30-1; 2,6-Mez-C8, 2051-30-1; 2,6-Mez-C9, 17302-28-2; 3,7-Mez-Cg,17302-32-8; 3,7Mez-Clo, 17312-54-8; 2,6-Mez-Cll, 17301-23-4; 2,6,10-Mez-Cl2, 82144-67-0; 2,6,10-Me3-Clz,3891-98-3; 2,6,10-Me3-C13,3891-99-4; 3,7,11-Me3-C13,55521-32-9; 2,6,10-Me3-CI4, 14905-56-7; 3,7,11Me3-CI4,36084-05-6; 2,6,10-Me3-CI5,3892-00-0; 2,6,10-Me3-ClG, 55000-52-7; 2,6,10,15-Me4-C16,108560-44-7; 2,6,10,15-Me4-C17, 54833-48-6; 2,6,10,15-Me4-C18,108560-45-8; 2,6,10,15-Me4-Cz0, 108560-46-9;2,6,10,15,19-Me5-Cm,73303-36-3; 2,6,10,15,19-Me5-Czl, 108560-47-0; 2,6,10,15,19-Me5-Czz,108560-48-1; C3=, 115-07-1; l-C4=, 106-98-9;2-C4=, 107-01-7;iso-C,=, 115-11-7;2-Me-1-C4=, 563-46-2; 3-Me-1-C4=,563-45-1; 3-Me-1-C5=,760-20-3; 4-Me-1C5=, 691-37-2; 4-Me-1-C6=, 3769-23-1; 6-Me-1-C7=, 5026-76-6; 6-Me-1-C8=, 13151-10-5; 6-Me-2-C7=,73548-72-8; 6-Me-2-C8=, 108560-49-2; 2,6-Mez-l-C7=,3074-78-0; 2,6-Mez-1-C8=,6874-29-9; 3,7-Mez-1-C8=, 4984-01-4; 3,7-Me2-1-C9=, 108560-50-5; 4,8Me2-1-C9=, 104256-34-0; 4,8-Mez-u-C10=,104256-35-1; 6,lOMez-l-CI1=, 104256-36-2; 6,lO-Mez-2-Cll=, 104256-37-3; 6,lOMez-2-Clz=, 104256-38-4; 2,6,10-Me3-1-Cl1=,32765-42-7; 3,7,11Me3-1-C12=, 1189-36-2; 2,6,10-Me3-1-Cl2=, 89505-05-5; 4,8,12Me3-1-C13=,76152-14-2; 3,7,11-Me3-1-C13=,104256-39-5;4,8,12Me3-1-C14=, 104256-40-8; 5,9,13-Me3-1-C14=, 108560-51-6; 6,10,14-Me3-1-C1,=,108560-52-7;6,10,14-Me3-2-CI5=,108560-53-8; 2,10,14-Me3-5-C15=,108560-54-9;2,10,14-Me3-1-C16=,108560-56-1; 6,10,l4-Me3-2-Cl6=,108560-58-3;2,10,14-Me3-6-C16=, 108560-59-4;
Ind. Eng. Chem. Res. 1987,26, 1638-1645
1638
Kissin, Y. V.; Feulmer, G . P. J. Chromatogr. Sci. 1986,24, 53. Kissin, Y. V.; Feulmer, G. P.; Payne, W. B. J. Chromatogr.Sci. 1986,
3,11,15-Me3-6-CI6=,108560-60-7; 3,11,15-Me3-7-C16=,108560-61-8; 7,11,15-Me3-l-C18=,104256-41-9;7,11,15-Me3-2-C,,=, 104256-42-0; 7,11,15-Me3-3-CI6=,108560-62-9; 2,6,10,14-Me4-1-C15=,2140-82-1; 2,7,11,15-Me,-l-C1,=, 104256-43-1; 3,8,12,13-Me4-1-C17=, 104256-44-2; 4,9,13,17-Me4-1-C18=, 104256-45-3; 6,11,15,19Me4-1-C,o=, 108560-63-0; 6,11,15,19-Me4-2-C20=, 108560-64-1; 2,6,11,15,19-Me,-2-Czo=,108560-65-2; pristane, 1921-70-6;phytane, 638-36-8; squalane, 111-01-3.
24, 164.
Kochi, J. K., Ed. Free Radicals Wiley: New York, 1973; Vols. I and 11. Kossiakoff, A.; Rice, F. 0. J . Am. Chem. SOC.1943, 65, 590. Kovats, E. 2.Anal. Chem. 1961, 181, 351. Ranzi, E.; Dente, M.; Pierucci, S.; Biardi, G. Ind. Eng. Chem. Fundam. 1983,22, 132. Spivakovskii, G . I.; Tishchenko, A. I.; Zaslavskii, I. I.; Wulfson, N. S . J. Chromotogr. 1977, 144, 1.
Literature Cited Benson, S. W. The Foundations of Chemical Kinetics; McGraw-Hill: New York, 1960. Kissin, Y. V. J . Chromatogr. Sci. 1986, 24, 278.
Received for review June 30, 1986 Accepted April 27, 1987
Temperature Front Sensing for Feed Step Control in Pressure Swing Adsorption M i c h a e l J. M a t z and K e n t
S. Knaebel*
Department of Chemical Engineering, The Ohio State University, Columbus, Ohio 43210
Analysis of the progression of adsorbent bed temperatures provides a means for controlling the duration of steps in a pressure swing adsorption (PSA) cycle. This technique was tested for separation of oxygen from air with zeolite 5A. I t was found t h a t the concentration and temperature fronts coincided during breakthrough experiments at fixed pressures from ambient to 4 atm for temperatures of 5 , 25, and 45 "C. Furthermore, when all steps of the PSA cycle were combined, it was possible t o control the duration of the feed step, regardless of whether the pressure was constant or varied linearly during that step. It was also possible to predict, a priori, the trajectories of the concentration fronts by a simple equilibrium theory.
I. Introduction Pressure swing adsorption (PSA) systems consist of columns which contain solid adsorbents that are synchronously pressurized, fed, depressurized, and purged. The process exploits the tendency for the uptake of components to increase in different proportions as pressure rises, which gives rise to selective adsorption of certain components over a pressure range. When the selectivity and/or pressure range are sufficiently large, a PSA system is able to produce high-purity products. Common separations are hydrogen from hydrocarbons and oxygen from air (Cassidy and Holmes, 1984). Pressure swing adsorption systems are relatively simple to operate, i.e., in timed cycles, when flow rates, compositions, and the pressure range are fixed and when the adsorbent is maintained a t maximum adsorbent capacity. Performance may suffer, however, when the adsorbent capacity is diminished or when operating or ambient conditions vary significantly. In such cases, one would employ sophisticated composition-monitoringinstruments to synchronize steps in the cycle to maintain product purity. The purpose of this paper, however, is tQ suggest that it may be possible to use relatively simple instruments, e.g., thermocouples, to compensate for adsorbent capacity loss and variable operating conditions without inordinate effects on performance. A partial explanation, in the context of oxygen separation from air, follows. Heat evolves in a packed bed of zeolite 5A as air displaces oxygen due to the uptake of nitrogen and release of oxygen by the molecular sieve. This is caused by the higher adsorption capacity and higher heat of adsorption of nitrogen relative to oxygen. Simultaneous axial bulk flow, adsorption, and heat release lead to composition and thermal waves that propagate toward the product end of the column. Assuming that these fronts coincide, the penetration of nitrogen into the bed can be determined 0888-5885/87/2626-1638$01.50/0
by measuring the temperature profile within the bed. In operating a PSA system, it is of signal importance for such waves to approach but not breach the end of the column. Recovery and/or purity of the product is reduced by either insufficient or excessive axial displacement of the composition wave. Accordingly, those aspects of PSA performance can be enhanced by terminating the feed step when breakthrough is imminent. In previous related work, Pan and Basmadjian (1970) derived approximate criteria for combined, constant pattern thermal and composition wave fronts. Kowler and Kadlec (1972) considered experimental and theoretical aspects of PSA control but restricted their attention to direct composition control, with a cell model as the basis of their theory. Chihara and Suzuki (1983), in a numerical simulation of air drying with activated alumina, predicted that thermal waves were attenuated from the feed end toward the product end. For air drying with a variety of adsorbenta, Carter and Barrett (1973) experimentally observed varying degrees of sharpness but uniform coincidence of the composition and thermal waves. Similarly, Yoshida and Ruthven (1983) obtained a solution to a model of adiabatic adsorption for a rectangular isotherm. They presented data for air drying with three adsorbents in which, while their shapes are not sharp, the composition and thermal waves coincided. Sircar et al. (1983) considered the effect of heat loss through the column wall, as well as other effects in both experimental and theoretical studies with ethane in helium on zeolite 5A. They found a minor effect of heat loss on the velocities of both the composition and thermal waves but found that the waves coincided regardless of the extent of heat loss. Finally, Kaguei et al. (1985) employed a detailed mathematical model and experimental temperature and concentration profiles in order to estimate parameters of adsorption. Specifically,they determined the 0 1987 American Chemical Society
