Acknowledgment Work on fast fluidization is supported by Grant AER72-03426 A No. 4 from the RANN Program ("Research Applied to National Needs") of the National Science Foundation. Valuable discussions with L. Reh are gratefully acknowledged. M. J. Gluckman supervised the installation of the compressor and assisted generously with advice. The City College Chemical Engineering Shop under J. Bodnaruk constructed the experimental setup. A. E. McIver assisted in the installation of the equipment. Literature Cited Braca, R. M.,Fried, A. A,, "Operation of Fluidization Processes," in "Fluidization," D. F. Othrner. Ed., Reinhold. New York, N.Y., 1956. Geldart, D.. Powder Techno/., 7 , 285 (1973). Kiang, K. D., Liu, K. T., Nack, H.. Oxley, J. H., "Heat Transfer in Fast Bed", paper presented at the International Conference on Fluidization, Asilornar. Calif., June 1975.
Kehoe, P. W. K., Davidson, J. F.. Inst. Chern. Eng. (London) Syrnp. Ser., 33, 97 (1971). Kunii, D., Levenspiel. O., "Fluidization Engineering," Chapter 2, Wiley, New York, N.Y., 1969. Lanneau, K. P., Trans. Inst. Chern. fng., 38, 125 (1960). Lewis, W. K., Gilliland. E. R.. U.S. Patent No. 2,498,088 (Feb 21, 1950). Massirnilla, L., AIChE Syrnp. Ser., 69, No. 128. 11 (1973). Matsen, J. M.. "Some Characteristics of Large Fluid Solids Circulation Systerns", paper presented at the International Conference on Fluidization, Asilornar, Calif., June 1975. Reh, L., Chern. €ng. Prog., 67(2), 58 (1971). Reh, L., personal communication, 1972. Saxton, A. L., Worley. A. C., OiIGasJ., 68(20), 82 (1970). Schmidt, H. W., "Combustion in a Circulating Fluid Bed", in Proceedings of third International Conference on Fluidized-Bed Combustion, Environmental Protection Technology Series, EPA-65012-73-053, p 11-1-1, Dec 1973. Wainwright, M. S.,Hoffman, T. W., Adv. Chern. Ser., 133, 669 (1974). Yousfi, Y., Gau, G., Chern. Eng. Sci.. 29, 1939 (1974).
Received for reuiew September 17;1974 Accepted July 21,1975
Degradation and Isomerization Reactions Occurring during Alkylation of lsobutane with Light Olefins Bharat Doshi and Lyle F. Albright* Purdue University, West Lafayette, Indiana 47907
Trimethylpentanes and dimethylhexanes degrade and isomerize when contacted with sulfuric acid at conditions similar to those used in commercial alkylation reactors. The products obtained include isobutane, all isoparaffins formed during the alkylation of isobutane with light (C,-C,) olefins, and hydrocarbons (or conjunct polymers) that dissolve in the acid phase. lsobutane also slowly reacts in the presence of sulfuric acid to form the same isoparaffins. Kinetic equations have been developed for each C8 isoparaffin and isobutane. Degradation reactions are apparently important relative to acid consumption, and suggestions to minimize them are presented. Important features of the alkylation mechanism have also been clarified.
Trimethylpentanes, whose research octane numbers vary from 100 to almost 110, degrade and isomerize when contacted with concentrated sulfuric acid. Reactions of 2,2,4and 2,3,4-trimethylpentanes (2,2,4-TMP and 2,3,4-TMP) were investigated a t 25OC by Hofmann (1964) and by Kramer (1967a), respectively. About 19% of 2,2,4-TMP reacted during a 10-min run producing isobutane, light ends (C, to C7 hydrocarbons), other trimethylpentanes (TMP's), dimethylhexanes (DMH's), and heavy ends (C, and higher hydrocarbons). The isoparaffins formed were the identical ones produced during alkylation. Kramer found that 2,3,4T M P degraded a t a faster rate than 2,2,4-TMP. He also found that increasing the water content of the acid from 1.5 to 11.1%caused lower rates of degradation. The degradation products from both 2,2,4- and 2,3,4T M P often had similar compositions implying that a common intermediate, probably a trimethylpentyl cation (TMP+), was formed. Sulfuric acid is a sufficiently good oxidizing agent (Kramer, 1967b) so that the ion intermediates will be formed from a trimethylpentane (TMP) as follows TMP
+ 4H,SO,
TMP*
--+
+ 2H,O+ +
3HS01- + SO2(1)
Although these investigations give some indications of the reactions that probably occur during alkylation, the conditions that have been investigated are relatively differ-
ent from those employed in commercial alkylation reactors in at least three respects. (a) 25OC is significantly higher than 10 to 15OC usually employed commercially. (b) The sulfuric acids employed as acid feedstocks contained 1.5 to 11 wt % water but no dissolved hydrocarbons. The acids employed in commercial units always contain some (up to several percent) dissolved hydrocarbons which when separated from the acid (Miron and Lee, 1963) are often called conjunct polymers. The amount of both dissolved hydrocarbons and water in the sulfuric acid have been found to be of considerable importance during alkylation in affecting the quality of the alkylate produced (Mosby and Albright, 1966; Li et al., 1970; and Albright et al., 1972). Furthermore, there is reason to believe that dissolved hydrocarbons were formed in the investigations of both Hofmann and Kramer since the hydrogen-to-carbon ratios in the degradation products reported were higher than that of the feed TMP. These dissolved hydrocarbons that have a lower hydrogen-to-carbon ratio would explain this observation. (c) In commercial units, the alkylate is mixed with isobutane that is used in large excesses. Isobutane may have an effect on the degration reactions. More information is needed relative to degradation and isomerization reactions for at least three reasons. First, degradation and consequent decrease in the quality of the alkylate may be of significance in a t least some commercial Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 1, 1976
53
units. Second, these reactions may contribute significantly to the buildup of dissolved hydrocarbons in the sulfuric acid. Third, additional information that could be obtained may be most helpful in clarifying the mechanism of alkylation. In the present investigation, degradation and isomerization reactions were studied over a wide range of conditions including those present in commercial alkylation units. Important operating variables were identified, and degradation reactions apparently are more important in the overall alkylation process than previously realized.
Experiment Details The reactions between sulfuric acid and alkylate compounds were conducted in glass reactors provided with an efficient agitator and baffles. Impeller speeds were varied as desired from about 450 to 1600 rpm. The two reactors used were essentially test tubes with working capacities of 30 and 100 cm3, respectively. A cork was used as a closure for the top of each reactor. The shaft on which the impeller was mounted entered the reactor through a hole in the cork. The glass reactors were immersed in a refrigerated bath maintained a t any desired temperature from about -20 to 25'C. The hydrocarbon liquid to be investigated was first added to the reactor. Sulfuric acid at room temperature was then added to the reactor immersed in the constanttemperature bath. The agitator was started immediately, emulsifying the two immiscible liquid layers and starting the batch run. The volumes of both the sulfuric acid and the hydrocarbons were measured before being added to the reactor and hence the volumetric ratio of acid to hydrocarbon in the reactor was known. At desired intervals during the run that usually lasted at least 3 hr, the agitator was stopped for 1 min or less to allow at least partial separation of the phases. A cold microsyringe was employed to obtain hydrocarbon samples, approximately 15 pl, which were injected within 5-10 sec into the gas chromatographic equipment (Li et al., 1970) so that all major Cd through Clo isoparaffins could be measured with high accuracy. At the end of a degradation run, the acid phase was titrated, and the decrease in the acidity was considered to be equal to the amount of dissolved hydrocarbons produced during the run. Each T M P investigated was at least 99.5% pure. 2,2,4and 2,3,4-TMP's were research grade products obtained from Phillips Petroleum Co. 2,2,3- and 2,3,3-TMP's obtained from Chemicals Sample Co. of Columbus, Ohio showed no impurities when analyzed. The DMH mixture used was also obtained from Phillips Petroleum Co. Except for limited runs described later, the hydrocarbon-to-acid ratio employed was 2:l and the emulsion formed was then hydrocarbon-continuous. Fresh acids are defined as those acids in which water is the only impurity. Fresh acids varying from approximately 95 to 99% acidity were prepared by blending sulfuric acid as purchased with either water or fuming sulfuric acid. Used alkylation acids with acidities of 88.2 and 89.4% were obtained from American Oil Co. and Standard Oil Co. (Ohio), respectively. These latter acids after being received were refrigerated (-22'C) in order to minimize reactions that occur during storage. At room temperatures, reactions that occur include oxidation reactions that result in the evolution of sulfur dioxide, increases in the molecular weight of dissolved hydrocarbons, and darkening of the acid color. The American Oil Co. used acid employed contained 3.0% water and 8.8%dissolved hydrocarbons (Sumutka, 1971). Acids with differing water and dissolved hydrocarbon contents were prepared in two ways. First, used acids were 54
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 1. 1976
PERCENT
TOTAL
CONVERSION
Figure 1. Drop in acidity (or buildup of dissolved hydrocarbons) of acid as function of conversion of trimethylpentane a t 10°C and a trimethylpentane-to-acid ratio of 2:l.
blended with fresh acids of known composition. Second, 1butene was bubbled a t about 10'C through fresh acids of known composition; the decrease in acidity caused by this bubbling was assumed to be equal to the amount of dissolved hydrocarbons produced.
Experimental Results This investigation indicates that trimethylpentanes react in the presence of sulfuric acid to form both C4 to C9 or Clo isoparaffins noted in previous investigations and hydrocarbons that dissolve in the acid phase (often called conjunct polymers). The latter compounds cause both decreased acidity of the acid phase and often the development of a yellowish or even orange color in the acid phase. In most 180-min runs, the acidity of the acid phase decreased from about 1 to 3% because of the dissolved hydrocarbons. Figure 1 shows a plot of the decrease in acidity as a function of the percent total T M P that reacted (both degraded and isomerized). The experimental data shown in Figure 1 correlate reasonably well regardless of the composition of the sulfuric acid used; these results are, however, limited to runs made using pure TMP's (i.e., no isobutane was added to the hydrocarbon phase), at lO'C, with a 2:l volumetric ratio of T M P to acid, and with moderately high levels of agitation. (Insufficient data were obtained to determine how this correlation may vary as a function of operating conditions used. Possibly it would vary significantly with the TMP-to-acid ratio and perhaps also with temperature and isobutane.) At this ratio of T M P to acid, a 1%decrease in acidity is approximately equal to 1.3% reaction of the TMP. In many runs and especially those using fresh acids, the degradation products formed in the initial stages of the run were predominantly dissolved hydrocarbons. More than 50% of the degradation products were dissolved hydrocarbons a t conversion levels of T M P up to perhaps 2.5%. As the conversion levels increased, the degradation products were increasingly isoparaffins that accumulate in the hydrocarbon phase; a t conversion levels of 50%, over 90% of the degradation products were these isoparaffins. Figures 2 and 3 indicate how the various degradation and isomerization products formed in runs a t l0'C using 2,2,4T M P and using 94.5% and 98.85% fresh acid, respectively. As indicated by comparing the results of these two runs, considerably more products were formed with the more concentrated acid. From 1 2 to 30 times as many degradation and isomerization isoparaffins that accumulate in the hydrocarbon phase were formed with the 98.85% acid, but only about three times as many dissolved hydrocarbons were produced. In runs with fresh acids, a yellowish (or even orange) color developed in the acid phase as the run progressed. In runs with used acids (which contained large amounts of dissolved hydrocarbons), the rate of formation of addi-
n W
1.5,
1 94 5 %
Fresh
Acid American
2 I
oa
1
I
H y d r o c a r b o n - to-Acid Volumetric R a t i o
/
1
2
I O O N
Vsad
Acld
I Hydrocarbon - to - A c i d Volumetric Ratio
5r.i >
I
I
200
2 50
/ I
=J
Wt-
E
/’
t-z 0 Y
IO0
J
Figure 2. Degradation of 2,2,4-trimethylpentane in contact with 94.5% fresh acid (run 97).
1 5-
2 I Hydrocarbon Volumetric
n 50
I50
IO0
T I M E , MINUTES
Figure 4. Degradation of 2,2,4-trimethylpentane in contact with used alkylation acid (runs 37 and 81).
$
9 8 8 5 % Fresh Acid
c
a
200
I50
T I M E , MINUTES
100
0.5
0
70
T E M P : IO’C
-
l o - Acid Ratio 2.2.4 - T M P
u Y 4
$ 40
2.3.3-TMP 2,3.4-TMP
3
300
30
60
90 TIME,
120
150
180
MINUTES
Figure 5. Comparison of degradation of four trimethylpentanes a t 10°C with 98.85% acid.
50
IO0
150
I80
T I M E , MINUTES
Figure 3. Degradation of 2,2,4-trimethylpentane in contact with 98.85% fresh acid (run 94). tional amounts of dissolved hydrocarbons was reasonably constant during the entire run. Figure 4 indicates how the degradation products formed as a function of time for a run using 2,2,4-TMP and the American Oil Co. used acid. As noted in Figure 4, some sulfur dioxide was also present in the hydrocarbon phase (n-hexane was later found to extract sulfur dioxide from the used acid). As degradation runs progressed, sulfur dioxide slowly decreased in amount presumably because it vaporized and was lost to the air. The amounts of sulfur dioxide were found to be essentially proportional to the amount of dissolved hydrocarbons present in the acid. The rate of degradation (and isomerization) for the feed T M P was found for each run to be essentially first order relative to the amount of the feed T M P
Plots of the logarithm of the amount of the feed trimethylpentane, (TMP)feed,vs. time resulted in straight lines after a start-up or induction period that occurred in most runs. In making these plots, all degradation products, including those in both the hydrocarbon and acid phases, were taken into account. Figure 5 shows the results for comparable runs with the four TMP’s. The value of k’ for each run is equal to the negative slope of the straight line portion of the plot; k‘ is considered to be an apparent or pseudo-firstorder reaction rate constant, as will be discussed later.
Positive induction periods up to 20 min occurred in runs using 2,3,4- and 2,3,3-TMP’s, but negative induction periods occurred with 2,2,4- and 2,2,3-TMP’s. The induction period was determined by extrapolating the straight line portion of plots to the amount of TMP present a t the start of the run. Negative induction periods indicate that the value of k’ decreased during the initial stages of the run. Induction periods with negative values are not considered to be very accurate since they occurred in runs indicating little degradation and hence small change in the slope of the line results in large changes in the value reported. If the analytical results for only the hydrocarbon phase are considered, “apparent” positive induction periods were noted for all runs made using relatively weak fresh sulfuric acids; Figure 2 shows the results for such a run. Effect of Acid Composition a n d Trimethylpentane. The composition of the feed acid used and the specific trimethylpentane employed each have very significant effects on the kinetics of degradation reactions. Tables I and I1 summarize the data obtained after 180 min for runs made using TMP’s and DMH’s a t 10°C in the small reactor with a Cs hydrocarbordacid ratio of 2:l. The amounts of degradation and isomerization products decreased significantly as both the water and dissolved hydrocarbon content increased. The conversion of 2,2,4-TMP varied after 180 min of the run from about 3 to 60% depending on the composition of the acid used, as shown in Table I. The following equation was found to be applicable for correlating k’ values of a given T M P for all runs at a given temperature, rate of agitation, and TMP/acid ratio
log K’ = 2[(-H,J - 0.065PI
+
C
( 3)
where C = constant for given T M P at given temperature, level of agitation, and TMP-to-acid ratio, Ho = Hammett Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 1, 1976
55
Table I. Effect of Acid Composition on the Degradation of 2,2,4-Trimethylpentane ( l O T , Small Reactor, 2:l TMP/Acid, 180 min) ~~
~
~~
~
Run
36
Acid composition ?c H2S04 % Organic
96.5 0.0
39, 78
37, 81
46
72
73
95.0
88.2 8.8
92.5 2.5
91.6 4.4
92.4 4.4
0.0
82
94 102 104
88.0
98.85
7.0
0.0
95
93.4 4.4
96
91.85
7.0
97
94.5 0.0
99
95.7 2.15
Rate constant 110 34 14 18 24 64 6 760 110 140 29 3CO ( x i o ? ) ,sec-‘ 1.5 1.2 1.0 (1.1‘) 1.0 1.6 (0.9) 3.0 1.9 (1.7‘) 1.15 2.0 A(acidity), wt % Induction period, -9 -15 435 -145 -26 -17 -175 4 -70 -110 -17 -80 min 24.96 24.94 25.05 24.99 24.94 24.82 25.02 24.98 25.01 Negative C value 25.04 24.99 25.03 Cf 56.2 16.0 22.7 3.9 37.6 1.4 10 Total TMP conv. 12.2 4.9 3.8 3.3 3.0 7.4 Product ratios i-Cj/2,3-DMB
LE/DMH LE/TMP DMH/TMP LE/HE
1.6 5.4 1.4 0.27 4.0
1.5 5.0 1.8 0.35 8.5
(2.3) (5.0) (1.8) (0.35)
--
1.9 5.0 1.8 0.35 7.0
1.5 5.0 1.6 0.32
-_
1.5 5.0 1.5 0.3 4.6
(2.8) (5.0) (0.6) (0.1)
-_
1.6 5.2 1.15 0.22 4.8
1.8 4.8 1.4 0.29 4.5
1.6 4.3 1.2 0.28 4.4
1.5 4.1
1.7 0.4 4.1
1.5 4.8 1.15 0.25 5.5
Values at 75 min.
acidity function; (This value is assumed to be identical with the value for fresh sulfuric acid with the same water content as the acid being used.), and P = percent dissolved hydrocarbons in the acid. The Hammett acidity function varies as the dissolved hydrocarbon content of the acid changes, but acidity values are not available for such acids. The term [(-Ho) - O.O65P] probably is the approximate value of the Hammett acidity for used acids; i.e., the term -0.065P is the term for correcting the acidity of fresh acid to those of used acids. The above equation is an expansion of the equation found applicable by Kramer (1967a) for correlating k’ values obtained in runs employing fresh acids. Since the composition of the acid changes during a run, a key question is what composition should be used in correlating the k’ values by eq 3. I t was assumed that the final acid composition is more representative during most of the run, and hence it was used. In general, there was only a relatively small change in the acid composition during the last 75% of all 3-hr runs of this investigation; this is the portion of the run after any type of induction period. C values for all four trimethylpentanes (TMP’s) a t 1O0C, a 2:l volumetric ratio of T M P to acid, and the same rate of agitation (in the small glass reactor) are listed in Table 111. The DMH mixture investigated contained about 51% 2,4-DMH, 44% 2,5-DMH, and 5% 2,2,3-TMP. Degradation runs a t 10°C with this mixture resulted in the formation of isobutane, all higher isoparaffins normally formed during alkylation, and dissolved hydrocarbons. Apparently eq 2 and 3 were also applicable for the mixture, and the approximate C value is reported above. Isobutane was contacted with 98.85% sulfuric acid a t room temperature (about 25OC) in a small steel tube that was vigorously shaken during the run. Light ends, TMP’s, and DMH’s were formed in small amounts as determined by tests after 24 and 42 hr. The dissolved hydrocarbon content of the acid was 1.15 and 2.0%, respectively, for these two runs. The C value a t 10°C reported in Table I11 was calculated using eq 2 and 3 and using the same apparent energy of activation, as will be discussed later. 58
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. l , 1976
The relative rate of degradation of two hydrocarbons, A and B, can be calculated a t 10°C using the above C values as follows.
(4) Equation 4 was employed to calculate the ratio of k’ values tabulated above. Because of the low rates of reaction for 2,2,3-TMP, DMH mixture, and isobutane, the results for these hydrocarbons are considered to be relatively less accurate. Within experimental accuracy, the composition of the degradation and isomerization isoparaffins for all runs with a specific T M P were identical (as indicated by the reasonably constant ratios between various specific compounds or families of compounds), as shown in Tables I and 11. For these runs in which little degradation occurred, there is however more uncertainity in the exact composition of the products. The light ends produced in this investigation had a similar if not identical composition regardless of the T M P , DMH, or isobutane used. Yet for runs with different TMP’s, the ratios a t 10°C of the light ends (LE’S)to the dimethylhexanes (DMH’s) differed, being about 5:l for 2,2,4-TMP and about 6:l to 1O:l for 2,3,4- and 2,3,3TMP’s. Effect of Additives t o Trimethylpentanes. The effects of adding isobutane, n-butane, or a Cq olefin t o TMP’s were investigated. Several comparative runs were made a t 0°C and -10°C to determine the effect of adding isobutane to 2,2,4-TMP or 2,3,4-TMP a t volumetric ratios of isobutane to T M P up to 2.0:l. The fraction of T M P that reacted to form isomerization and degradation products that accumulated in the hydrocarbon phase decreased significantly as increased amounts of isobutane were added as diluents. The k’ values decreased by factors of 1.5-3.5 and by almost 30 as the isobutane-to-TMP ratio increased from 0:l to 0.3:l and from 0:l to 2:1, respectively. The amount of dissolved hydrocarbons formed tended to increase with increased isobutane contents. Presumably part of these dissolved hydrocarbons were produced be-
Table 11. Effect of Acid Composition on the Degradation of Other Trimethylpentanes (2,3,4-TMP, 2,2,3-TMP, and 2,3,3-TMP) and DMH's ( 10°C, 2:l Ca Hydrocarbon/Acid, 180 min) Run
44
TMP's (DMH's)
47
101
2,3,4-TMP
107
108
109
2,2,3 -TMP
110
48
2,3,3-TMP
93
DMH's
~
Acid composition % H2S04 70 Organic Rate constant (xlO'), sec-' A(acidity), wt % Induction period, min Negative C value % Total TMP conv.
95.0 0.0
135 1.8 6 24.34 13.4
92.5 2.4 70
3.0 6 24.31 10.3
98.85
95.5
98.85
98.85
95.5
95.0
98.85
0.0
0.0
0.0
0.0
0.0
0.0
0.0
140 1.25 4 24.58 14.3
5 0.5
69 1.2 -1 0 26.24 7.4
3200 3.0 20 24.38 64.7"
5 0.95
77
1.15 -9
9 .
26.07 1.4
26.14 8.2
1850 2.15 20 24.67 43. 8"
..
26.18 0.65
Product ratios
.. .. .. ..
l.lU 1.15 1.5 1.47 1.7" 1.15 1.65 7.0 5.5" 6.9" 7.7 LE/TMP 6.2 0.93" 1.8 1.45" 3.8 0.43 DMH/TMP 0.16 0.17" 0.22" 0.5 5.5 4.0" 5.6 1.8 LE/HE 5.0 4.4 Change of acidity determined by use of Figure 1.Values shown in parentheses are only approximations; in some cases the level of degradation i n t h e run was very low.
i-C5/2,3 -DMB LE/DMH
1.4 10.0 5.0 0.2 5.3
.. .. .. ..
Table I11
Hydrocarbon 2,3,4 -TMP 2,3,3-TMP 2,2,4-TMP 2,2, 3-TMP Mixture 2,4- and 2,5-DMH Isobutane
No. of degra dation runs
3 2 12 2 2 2
Av C at 10°C
Rate of degradation relative to rate for 2,2,4-TMP
-24.34 -24.62 -24.98 (-26.1) (-26.2)
4.8 2.4 1.0 (0.09) (0.08)
(-2 8.0 1)
(0.001)
cause of degradation of isobutane. Production of DMH's and heavy ends were in particular suppressed by the presence of isobutane. Isomerization to form other TMP's was decreased, however, to only a rather limited extent. Based on these results, degradation would be relatively small at higher isobutane-to-TMP ratios such as normally present in commercial alkylation reactors. n-Butane apparently acted only as an inert diluent in a run in which a 0.3:l mixture of n-butane and 2,2,4-TMP was used. The value of k' for this run was equal within experimental accuracy to the value for a comparable run using pure 2,2,4-TMP; the composition of the degradation products formed was not significantly changed because of the n-butane. Both isobutylene and 1-butene significantly increased the initial rates of degradation and isomerization of 2,2,4TMP. This conclusion is based on four runs made with olefin-to-TMP ratios of 0.01:1, 0.04:1, and 0.1O:l. The rates of reaction were especially fast within the first 20-30 min of the run and then decreased to relatively constant values for the remainder of the run. Although the results of this investigation could not definitely determine the exact fate of the olefins, apparently the following are occurring. (a) About 50% of the olefins reacted to form isoparaffins of essentially the same composition as the degradation and isomerization products that accumulate in the hydrocarbon phase. This conclusion is based in part on the results of Li
.. ..
..
.. ..
et al. (1970). (b) The remainder of the olefins reacted to form dissolved hydrocarbons that were produced in higher amounts in these runs. Effect of Agitation. A series of four runs were made in which the speed of the impeller was varied in the larger reactor. These runs were made using 2,2,4-TMP and 96.1% fresh acid at 10°C. In all cases, the two phases appeared to be well emulsified, and the appearance of the emulsion was uniform throughout the reactor. Increasing the speed of the impeller in the range of 450 to 1600 rpm caused significant increases in k' values. Plotting the logarithm of k' vs. the logarithm of the rpm of the agitator resulted in a straight line with a slope of about 0.70-0.75. The composition of the degradation products formed were similar if not identical for these runs. Runs made in the smaller reactor resulted in a level of degradation and isomerization equivalent to an impeller speed of about 1200 rpm in the big glass reactor. Effect of TMP-to-Acid Ratio. All runs of this investigation were made using a 2:l volumetric ratio of T M P to acid except for three runs made a t 0.33:l to 0.51 ratios. In these latter runs, an acid-continuous emulsion was obtained but a hydrocarbon-continuous one was formed a t the 2:l ratio. Comparative runs made with both 95 and 98.85% acid in contact with pure 2,2,4-TMP indicated that the rates of degradation were about three times as great with the 0.5:l ratio as compared to runs with 2:l ratio; the amount acid used for a given amount of 2,2,4-TMP hence varied by a factor of 4. Temperature Effect. Degradation and isomerization reactions decrease rapidly as the temperature is lowered in the range from 25 to -1OOC. This conclusion is based on the results of the runs made in this investigation (see Table IV) and also on the results of Hofmann (1964) and of Kramer (1967a). The k' values as determined in this investigation must be considered as only pseudo-first-order reaction rate constants since they vary with agitation and the volumetric ratio of acid to hydrocarbons. Temperature affects to a significant extent both the viscosity of the liquid phases and the interfacial surface tensions; hence the efficiency of agitation or the interfacial surface area between phases varies with temperature even though the speed of the agitator remains unchanged. The k' values shown in Table IV were, however, used to calculate the apparent Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 1, 1976
57
Table IV. Effect of Temperature on Degradation of TMP's in Contact with Sulfuric Acid (2:l TMP/Acid, Small Reactor, 180 min)
Run
TMP
Acid strength, wt %
Temp, "C
Apparent rate constant k', sec-' x
io7
-1 0 19 96.5 +10 96.5 110 88.2 -1 0 5 88.2 +10 14 88.2 +20 27 98.85 -1 0 460 98.85 +10 3200 98.85 +10 77 98.85 +20 155 98.85 -1 0 235 98.85 A10 1850 n R u n of 75 min. Note: Fresh acids were employed except for American Oil Co. used acid employed in runs 33,37,and 105. 32 36 33 37 105 106' 101' 108 112 113 109
2,2,42,2,42,2,42,2,42,2,42,3,42,3,42,2,32,2.3 2,3,32.3.3-
energies of activation using the Arrhenius equation. Within experimental accuracy, all TMP's had an energy of activation equal to 14 f 1 kcal/mol. (An activation energy of 14 kcal/mol was used to correct the k value for isobutane at ambient temperature, about 25OC, to 10°C. The C value reported earlier at 10°C was calculated using eq 3.) As a first approximation, the true energies of activations for the four TMP's are probably also essentially equal. Mechanism of Degradation The overall degradation phenomenon is complicated involving numerous chemical and physical transfer steps. The reactants and products must be transferred to and from the main reaction site. Possible sites are the hydrocarbon phase, the acid phase, or the interfacial area between phases. The hydrocarbon phase obviously is not the main site since the rate of degradation is almost directly proportional to the amount of acid employed for a given amount of hydrocarbon; this finding was indicated by comparable runs with different acid-to-hydrocarbon ratios. The results of this investigation were next analyzed to determine whether the main reactions occurred more or less uniformly throughout the acid phase. Assuming for the moment that such a postulate is correct, the hydrocarbons that degraded would need to be transferred from the hydrocarbon to the acid phase where they would dissolve. Since agitation had a large effect on the rates of agitation, it would then be necessary to conclude that mass transfer steps for these hydrocarbons were rate controlling. Agitation would then be expected to have the largest relative effect on the rates of degradation for runs in which rapid rates occur, Le., runs using 2,3,4-TMP and/or strong sulfuric acid. Such a finding was, however, not noted in this investigation for these runs, and it seems safe to conclude that the main reactions do not occur uniformly throughout the acid phase. As still another test of this latter conclusion, estimates were made of the rates of mass transfer that could be expected between the two liquid phases. These calculations suggest that the level of agitation employed would be more than sufficient to keep the acid phase essentially saturated at all times with dissolved isoparaffins that then degrade. Now considering the possibility that the main reactions occur at or a t least near the acid-hydrocarbon interface of 58
Ind. Eng. Chem., Process Des. Dev., Vol. 15,No. 1, 1976
the emulsion, the correlation of Rodger et al. (1956) was used to determine how changes in agitation and in the ratio of hydrocarbons to acid used would affect the interfacial area. It was found that the estimated areas were linearly proportional to the k' values. Such a finding strongly supports the hypothesis that the main reactions do occur at the interfaces. Dissolved hydrocarbons that have surfactant characteristics may accumulate at the acid-hydrocarbon interface where they apparently have important roles relative to degradation. It was of special interest to note that these hydrocarbons are often, if not always, the main degradation product during the initial stages of a run. As a side note, dissolved hydrocarbons are also formed in relatively large amounts during the start-up of alkylation runs using fresh acid. These hydrocarbons that are always partially ionized are probably involved in the production of a t least part of the TMP+, as will be indicated later by the reverse step of eq 7. Positive induction periods, as noted in some runs using fresh acids, were likely the periods during which the concentrations of trimethylpentyl ions and/or dissolved hydrocarbons increased to values that led to fairly rapid rates of reaction. The acidity (and hence the catalytic reactivity) of the acid decreased as the dissolved hydrocarbons were formed during a degradation run. Hence a negative induction period was possible, as occurred in other runs. The above discussion further emphasizes that k' values as determined for each run should be considered as only apparent reaction rate constants; they should be used only with caution in making any theoretical conclusions. It is of considerable interest that the isoparaffin products formed during degradation reactions are the same ones produced during conventional alkylations. Furthermore, the light ends formed have essentially the same if not identical compositions as produced during the main alkylation reactions. This evidence further confirms the complicated nature of alkylation reaction. Albright and Li (1970) reported earlier that the mechanism proposed by Schmerling (1945, 1946, 1964) does not explain important features of alkylation. They presented evidence, for example, that much, if not all, of the olefins initially react (and dissolve) in the acid as a primary step of the overall sequence. Secondary reactions then occur in which the isobutane and organic compounds dissolved in the acid react to form TMP's. Degradation and isomerization reactions, as found in this present investigation, should be considered as tertiary reactions. The degradation mechanism proposed earlier by Hofmann (1964) and by Kramer (1967a) for TMP's has been expanded to account for the additional results of this investigation. First, a TMP+ is formed by oxidation of T M P as outlined earlier in eq 1. This ion fragments as follows to start the degradation sequence TMP*
t-C,Hi
+
C 4 olefin (probably isobutylene)(5)
When, however, isobutane or dissolved hydrocarbons are present, TMP+ can be converted to a considerable extent back to T M P by the following reactions TMP' + i-C,H,, TMP + t-C,H,' (6) TMP' + dissolved f TMP + dissolved ( 7) hydrocarbons hydrocarbon ion As a result, the rate of T M P degradation is significantly reduced. n-Butane, however, is much less reactive (with TMP+) since it contains no hydrogen atoms attached to tertiary carbon atoms. Dissolved hydrocarbons or conjunct polymers also apparently complex with the acid causing
both reduced acidity and reduced oxidative abilities; as a result the kinetics of TMP oxidation by eq 1 are lower with used acids than with fresh acids. Presumably 2,2,4-TMP+ is an intermediate ion formed from all TMP’s, and it is this ion that leads to similar products ranging from C4-CIo isoparaffins plus also dissolved hydrocarbons. This is the only TMP+ that will easily fragment by @-scissionto form tert- butyl cations and a butene (that is probably always isobutylene). The question can be raised why both 2,3,4- and 2,3,3-TMP’s degrade so much faster than 2,2,4-TMP and why 2,2,3-TMP degrades much slower. In the case of 2,3,4-, 2,3,3-, and 2,2,3-TMP’s, at least two reaction steps seem necessary to obtain 2,2,4TMP+ 2,3,4-TMP 2,3,4-TMP’ 2,2,4-TMP’ (8) 2,3,3-TMP 2,2,3-TMP
= 2,3,3-TMP+ * 2,2,3-TMP’
2,2,4-TMP’
(9)
2,2,4-TMP* (10)
Possibly both 2,3,4- and 2,3,3-TMP are easily oxidized to form 2,3,4- and 2,3,3-TMP+, respectively, and these latter two ions also isomerize rather readily to 2,2,4-TMP+. Based on this line of reasoning, 2,2,3-TMP probably is oxidized only slowly to 2,2,3-TMP+. In the case of 2,3,4-TMP, its rapid rate of degradation may be caused by the three hydrogen atoms that are attached to tertiary carbon atoms; all other TMP’s have only one such hydrogen atom. I t is most improbable that any DMH’s was formed by reactions between 1-butene and tert-butyl cations; such a mechanism was in the past considered important during alkylation. Instead the present results appear consistent with the two mechanisms for production of DMH’s as proposed by Hofmann and Schriesheim (1962). By their first mechanism, isobutylene molecules react as follows.
7
7
hydnde transfer
7 c=cc+
c=cc
-
c
7
c=cc+ + c=cc
(11)
c c
c=ccckc
+
(12)
This latter unsaturated ion after suitable hydride and proton transfer steps forms DMH’s. Their second mechanism for production of DMH’s involved the formation and @-scissionof various pentamethylheptyl ions, and one such example is shown below TMP’
+
Z-C,H,
c c +
cccy2cc k c c
c7+
c
cccyccc c c
7 7 cc=cccc
--+
668
- cc+ e
-c
cccf.‘ccc c c
cr:
cy+ + cc=cccc
(13)
(14)
(15)
C
proton and
cc+
CCCCC C
q c
cccckc
(16)
(2,4-dimethylhexane) Isomerization of the intermediate DMH+ formed after proton transfer to the dimethylhexene above would result in the production of other DMH’s. Other pentamethylheptyl ions (i-C12+) can be postulated that would by P-scission result in isoalkyl cations and branched olefins. These latter cations and olefins would lead to the formation of all CS to C7 isoparaffins found in the light end fraction (Hofmann and Schriesheim, 1962). In further support of the above hypothesis involving a common i-C12+, Li et al. (1970) and Albright et al. (1972) transfer
obtained information indicating that as the light ends content of an alkylate increased the DMH content also generally increased in rather direct proportion. Presumably DMH’s are oxidized fairly rapidly forming DMH+’s by reactions such as shown in eq 1; the DMH’s employed each contain two hydrogen atoms attached to tertiary carbon atoms. The mechanism for the degradation of DMH+ is not known at present, but the following are possibilities. First, DMH+’s formed may isomerize to TMP+’s and specifically 2,2,4-TMP+;such structural isomerization seems unlikely, however. Second, at least some DMH+’s scission forming t-C*Hg+ and a Cq olefin (possibly 1-butene) similar to the reaction shown by eq 5 . Third, Cls+’s are produced from DMH+’s and dimethylhexenes (formed from DMH+’s); these CIS% then scission eventually forming all degradation products noted. The presence of a small amount of 2,2,3-TMP in the DMH mixture would not be sufficient, however, to cause the appreciable degradation noted. Reactions between isobutane and sulfuric acid undoubtedly first result in the formation of t-C4H9+ by an oxidation step similar to eq 1. A portion of the t-C*Hg+ probably decomposes forming isobutylene (with the emission of a hydride ion). Reactions between other t-C*Hg+ and isobutylene lead to the entire spectrum of isoparaffins formed during alkylation. Den0 et al. (1964) have also reported that some t-CqHg+’s form cyclopentyl cations that are likely precursors to conjunct polymers that contain many Cj-Cg ring compounds (Miron and Lee, 1964). Application of Results The results of this investigation significantly expand the information available relative to the secondary (or even tertiary) reactions that occur in an alkylation reactor. TMP’s, DMH’s, isobutane, and probably also other isoparaffins react to a significant extent when contacted with concentrated sulfuric acids. Fortunately at operating conditions conducive to the production of high quality alkylates, the rates of degradation reactions are slow. It seems safe to conclude that in a well-operated commercial alkylation unit there will in general be only an insignificant decrease in the quality (or octane number) of the alkylate. The present results indicate, however, that acid consumption may be affected to a considerable extent by degradation reactions. This conclusion is based on the fact that the acid phase is often recycled on the average 250 or more times in a commercial unit. (Calculations were made for several operating conditions. When acid consumption is 0.4 pound of acid per gallon of alkylate produced, the acid is recycled almost 250 times when the unit operates with an isobutane-to-olefin ratio of 9:l in the feed and with a volumetric ratio of 1:l for the acid-to-hydrocarbon phases. When these two ratios are higher, as sometimes occurs, the acid would on the average be recirculated more times, sometimes as much as 300 to 500 times.) Typically the amounts of dissolved hydrocarbons in the acid phase are allowed to form up to 5-7.5% before the acid is discarded; 2-4% water also generally accumulates in the acid during this time. Hence the average amount of dissolved hydrocarbons formed per pass in the reactor is in the range of 0.020.03% when the acid is recycled on the average of 250 times. Isoparaffins dissolve or entrain in the alkylation acid in the approximate range of about 0.1 to 0.2 wt %. In many commercial units, the average residence time of the acid in the separator (that separates the acid and hydrocarbons phases by decanting) is a t least 1 hr. Often the temperature in these separators rises above that of the reactor, and the isoparaffins dissolved or entrained in the acid phase would Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 1 1976
59
be expected to degrade to a t least some extent during this period of time. Based on the results obtained when TMP's were mixed with isobutane, a significant amount of the dissolved hydrocarbons were produced from isobutane. Such a conclusion is not surprising since isobutane is more soluble in the acid phase than heavier hydrocarbons including Cg isoparaffins. Calculations indicate that up to 60-70% of the dissolved hydrocarbons produced in a commercial alkylation unit are probably formed during the separation of the phases and during the recycling of the acid. If the acid phase was quickly separated and recycled, acid consumption might be reduced by perhaps 30-50'70. Two techniques that should be considered to obtain decreased acid consumption are as follows: first, separate the phases by centrifuging; second, when separation is by decanting, the residence time of the acid in the separator should be significantly decreased. As a result, incomplete separation will occur and the acid phase that is recycled will include some unseparated hydrocarbon droplets. Such a technique has been used commercially to at least a limited extent.
Literature Cited Albright. L. F., Houle, L., Sumutka, A. M., Eckert, R. E., Ind. Eng. Chem., Proc e s s & ~ . Dev., 11, 446(1972). Albright. L. F., Li, K. W., Ind. Eng. Chem.. Process Des. Dev., 9,447 (1970). Deno. N. C., Boyd, D. B., Hodge, J. D., Pitman, C. U.. and Turner, J. O., J. Am. Cbem. Soc., 86, 1745 (1964). Hofmann. J. E.. J. Org. Chem., 29,3627 (1964). Hofmann. J. E., Schriesheim. A.. J. Am. Chem. Soc., 84, 953, 957 (1962). Kramer, G. M.. J. Org. Chem., 32, 920 (1967a). Kramer. G. M., J. Org. Chem., 32, 1916 (1967b). Li, K. W., Eckert, R. E.. Albright, L. F., Ind. Eng. Chem.. Process Des. Dev., 9, 434, 441 (1970). Miron, S.. Lee, R. J., J. Chem. Eng. Data, 8 , 150 (1963). Mosby, J. F.. Albright, L. F., lnd. Eng. Chem., Process Des. Dev., 5, 183 (1966). Rodger, W. A., Trice, V. G., Rushton, J. H., Chem. Eng. Progr., 52 (12), 515 (1956). Schmerling, L., J. Am. Chem. Soc., 67, 1778 (1945). Schmeriing, L..J. Am. Chem. Soc., 68, 275 (1946). Schmerling. L., "FriedLCrafts and Related Reactions," Vol. II, "Alkylation and Related Reactions," G. A. Olah. Ed., Part 2, p 1075, Interscience. New York. N.Y.. 1964. Sumutka. A. M., M.S. Thesis, Purdue University, Lafayette, Ind., 1971.
Received for reuiew November 13,1974 Accepted July 7,1975 Universal Oil Products Co. provided financial support. This paper was presented at the 169th National Meeting of the American Chemical Society, Philadelphia, Pa., Apr 7-11, 1975.
The Leaching of Cupric Sulfide in Ammonia lrvine G. Reilly Scbool of Engineering, Laurentian University,Sudbury, Ontario, Canada
Donald S. Scott' Depatfment of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada
The rate of dissolution of synthetic covellite (CuS) was measured in ammoniacal solutions under oxygen pressure for conditions when chemical reaction rate controlled. Temperatures from 50 to 85OC, oxygen pressures to 68 atm, and ammonia concentrations from 2.5 to 7.0 M were investigated. A simplified mechanism similar to one proposed for dissolution of metallic copper was found to describe the rate results satisfactorily. Low yields reported previously in the literature for temperatures over 100°C were found to be due to hydrolysis of the soluble cupric ammine salt. Rate results can also be fitted equally well at any given temperature by an empirical power law equation, in which ammonia shows a one-half order and oxygen a one-third order.
Introduction Ammoniacal leaching of sulfide ores under oxidizing conditions has been practiced commercially for many years. The best known application of direct reaction of ammonia and sulfides is probably the process pioneered by the Sherritt-Gordon Co., which is used commercially for treating nickel sulfide concentrates. The chemistry of this process has been described in a number of articles and patents by Forward (1953, 1955) and Forward and Mackiw (1955). These same authors have also described the application of ammonia leaching techniques to copper sulfides, particularly chalcopyrite concentrates. Other workers have also developed techniques for leaching sulfide copper ores in oxidizing ammoniacal solutions, and described these over the past 20 years. While in a number of articles, graphical or numerical results have been presented showing the rates of dissolution a t various reaction conditions of copper ore particles in am60
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 1, 1976
monia solutions in the presence of oxygen, there does not appear to be any published study which attempted to develop a quantitative expression for rates of leaching. In 1966, Stanczyk and Rampacek explored the maximum yields obtainable in aqueous ammonia and oxygen for the common copper sulphide minerals. In 1974, Kuhn et al. described a low pressure process for leaching various copper sulfides (Arbiter process), and discussed some of the thermodynamic and kinetic factors influencing reaction conditions. These workers also investigated the importance of agitation for oxygen transport and for removal of the iron oxide product from the reaction zone when chalcopyrite is being leached. However, although some rate information is presented, no rate equations are given. The present work was undertaken to develop functional relationships describing the rate of dissolution of copper sulfides in aqueous oxidizing ammonia systems. Because covellite (CuS) is readily leached in such systems and has
