Greek Letters
B = aeration factor
r = r function 0 = dimensionless time = m(GhdLM) p = viscosity, eq 32, g/cm s u = kinematic viscosity, eq 33, cm2/s p = density, g/cm3 or lb/ft3 u = surface tension, eq 32, dyn/cm u2 = dimensionless variance ut2 = time based variance, s2 4 = froth density factor on the tray $?Id = froth density factor in downcomer Literature Cited Bakowski, S., Brit. Chem. Eng., 8, 473 (1963). Barker, P. E., Self, M. F., Chem. Eng. Sci., 17, 541 (1962). Barrett, P. V. L.. Ph.D. Thesis, University of Cambridge, 1966. Bronibrilla,A., Nardini, G., Nenecetti, G. F., Zanelli, S., International Symposium on Distillation, Brighton. England, 1969. Bubble Tray Design Manual, A.I.Ch.E., 1958. Calderbank. P. H., Trans. Inst. Chem. Eng., 34, 78 (1956); 37, 133 (1959). Calderbank. - - -..- , P. H.. Rennie. J.. Trans. inst. Chem. Em.. 40. 3 11962). Calderbank, P. H.: Moo-Young, M. E., Chem. Eng. &i., 16, 39 (1961). Danckwerts, P. V.. "Gas-Liquid Reactions", McGraw-Hill, New York, N.Y., 1970. Danckwerts, P. V., Sharma, M. M., Trans. Inst. Chem. Eng., 44, 244 (1966). Davies, J. A., Pet. Refiner, 29, 93 (1950). De Goederen, C. W. J., Chem. Eng. Sci., 20, 1115 (1965). Dillan, G. B., Harris, I. J., Can. J. Chem. Eng., 307 (Dec 1966). Eduljee, H. E.. Chem. Age India, 7, 17 (1966). Ellis, S. R. M., Moyada, K. M., Brit. Chem. Eng., 47, 10 (1949). Finch, R. N., Van Winkle, M.. Ind. Eng. Chem., Process Des. Dev., 3, 106 (1964). Foss, A. S., Gerster, J. A,, Chem. Eng. frog., 52, 28 (1956).
Foss. A. S..Gerster, J. A., Pigford, R. L., A./.Ch.E. J., 4, 231 (1958). Gerster, J. A,, Chem. Eng. frog., 59, 3 (1963). Gupta, R. K., Sharma, M. M., Trans. Inst. Chem. Ens., 45, 169 (1967). Haq, M. A,, Ph.D. Thesis, University of Surrey, England, 1972. Harris, I. J., Roper, I., Can. J. Chem. Eng., 40, 245 (1962); 41, 158 (1963). Liebson. J., Kelley, R. E., Bullington, L. A., Pet. Refiner, 36 (2), 127 (1957). Mayfield. F. D., Church, W. L., Green, A. C., Rasmusson, R. W., Ind. Eng. Chem., 44 (9), 2238 (1952). McNeil, K. M.. Can. J. Chem. Eng., 48 (June 1970). Millington, A., Ph.D. Thesis, University of Surrey, England, 1972. Pasink-Bronikowska, W., Cbem. Eng. Sci., 24, 1139 (1969). Poherecki, R., Chem. Eng. Sci., 23, 1447 (1968). Porter, K. E., Davies, J. T., Wong, P. E. Y., Trans. Inst. Chem. Eng., 45, 265 (1967). Prince, R. G. H., International Symposium on Distillation, Brighton, England, 1960. Sargent, R. W. H., Bernard, J. D. T., Trans. Inst. Chem. Eng., 44, 314 (1966). Sater, V. E., Levenspiel, O., Ind. fng. Chem., Fundam., 5, 86 (1966). Smith, B. D., "Design of Equilibrium Stage Processes", (Chapters by W. L. Bolles and J. R. Fair), McGraw-Hill, New York, N.Y., 1963. Smith, R., Wills, G. B.. Ind. Eng. Chem., Process Des. Dev., 5, 39 (1966). Thomas, W. J., A.B.C.M. Distillation Symposium, London, England, 1964. Thomas, W. J., Shah, A., Trans. Inst. Chem. Eng., 42, 71 (1964). Thomas, W. J., Campbell, M., Trans. Inst. Chem. fng., 45, 53 (1967). University of North Carolina, Final Report A.1.Ch.E. "Tray Efficiencies", 1959. University of Delaware, Final Report A.I.Ch.E., "Tray Efficiencies", Dec 1958. University of Michigan, Final Report A.I.Ch.E., "Tray Efficiencies", Dec 1958. Walter, J. F., Sherwood. T. K., Ind. Eng. Chem., 33, 493 (1951). West, 8. F., Gilbert, W. G., Shimizu, T., Ind. Eng. Chem., 44, 2470 (1952).
Received for review September 30, 1975 Accepted June 21,1976 Supplementary Material Available. Supplementary tables of sieve t r a y data (16 pages). Ordering i n f o r m a t i o n is given o n any curr e n t masthead page.
Coke Formation during Thermal Cracking of n-Octane Yatish T. Shah," Edward 6. Stuart, and Kalapi D. Sheth Department of Chemical Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 7526 1
The coking phenomenon as a function of temperature space time and surface to volume ratio ( s h ) of the 304 stainless steel reactor was investigatedduring pyrolysis of +octane in a tubular flow reactor. The experiments were carried out within the temperature range of 750-800 OC and space times up to about 1 s. The effect of surface treatments, namely air and H2Streatment, was studied. During all runs except those which were preceded by the H2S treatments, it was observed that in the beginning for about an hour the rate of coking was very high and after that it achieved a lower, steady-state value. The coking rates exhibited curious maxima with respect to the space times. Based on the rate data, reaction models for coking phenomena are proposed. The rate data are correlated by severity factor type correlations.
Introduction The problem of coke formation on the inner walls of a cracking furnace has been a major concern in thermal cracking of hydrocarbons, since the coke deposits not only interfere with the heat flow through the tube walls, but the pressure drop across the tube also increases substantially. The coking usually necessitates expensive shutdowns of the cracking furnace. When cracking heavier liquid feed stocks such as naphtha and crude oils, the furnace might require shutdowns for decoking after each 100-150 h of onstream line. Several studies have been reported on the mechanism of formation of pyrolytic carbon. Slater (1916) suggests that the decomposition of hydrocarbons on a heated surface takes place as a direct decomposition of hydrocarbon molecules into carbon and hydrogen. Lanett et al. (1917) and Conroy et al. 518
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 4, 1976
(1959) suggest a mechanism such as hydrogenation of hydrocarbons with the formation of complex polymer aromatic compounds. According to Grisdale et al. (1951), the hydrogenation proceeds in the gaseous phase, the higher polymer compounds formed being deposited on the surface of the substrate. Syskov and Jelikhovaskya (1967) studied the formation of pyrolytic carbon from methane. According to these authors, the pyrolytic carbon or coke is mainly composed of high-molecular hydrocarbons of aromatic nature and their free radicals. They reported the elementary composition of the pyrolytic coke as C = 99.22%, H = 0.16%, impurities = 0.62%. The composition varied only slightly with the temperature of pyrolysis. The purpose of this study was to evaluate experimentally the coking characteristics during the pyrolytic cracking of
i " " " " ' 1
1
A.648 x IO" s e c - ' E.56.400 cals/g-mole
;8 0 -
0
W
A
Temp,"C 650 670 700
0
725
5 n-octane K P r o d u c l s
c
E
Symbol
1
'
I
1
60-
40-
1
20-
i
-
z
v)
2 -06-
0
-I 0-
20 40 CONVERSION
60 80 OF OCTANE
100
Figure 2. Gas weight percent vs. octane conversion as obtained by Kunzru (1972). I O
IO2
104
+
,,io3
106
108
I I
'K-'
Figure 1. Arrhenius plot for the rate constant for n-octane cracking (Kunzru (1972)).
n-octane. The coke deposition was measured in the temperature range of 750-800 "C and a space time range of approximately 0 to 0.9 s. The results show some unusual and interesting behavior. Models are suggested to explain the experimental data. Previous Work on n-Octane Cracking Dintzes and Frost (1934) studied the thermal cracking of n-octane in the temperature range of 396-572 "C with conversions varying from 2 to 60%. However, the gaseous products were only partially analyzed and no attempt was made t o analyze the liquid products. Marschner (1938) decomposed n-octane between 538 and 572 "C and made a complete analysis of the liquid and gaseous products. He did not find any substantial difference in the product distribution with temperature or conversion. He obtained an activation energy of 60.1 kcal/g-mol for the overall decomposition compared to 64.5 kcal/g-mol reported by Dintzes and Frost. Over the temperature and conversion range studied, the overall decomposition was first order and the rate constant a t 571 O C s-l compared to 6.4 X 10-3 s-l was reported as 15.4 X found by Dintzes and Frost. However, unlike the findings of Dintzes and Frost, Marschner (1938) did not find the rate constant to decrease with conversion. Recently, Doue and Guiochon (1969) cracked octane a t a relatively lower temperature of 443 "C and a pressure range of 2-10 bars with conversions varying from 1to 4%. They were able to explain the cracking products of octane with a slight modification of Rice's theory (1933). No activation energies or rate constants were reported by them. Very recently Kunzru (1972) studied the octane cracking in the temperature range of 650-725 "C and the space time range of approximately 0-2 s. Kunzru (1972) found that above 725 "C significant coke deposition occurred. The temperature range for the large coking rate selected in this study was based on his observation. Since Kunzru's data were also obtained in a 304 stainless steel reactor and they lay background on the cracking mechanism, we briefly outlined them here. As noted in our earlier publication (Shah e t al., 1973), the decomposition of n-octane is a pseudo-first-order reaction. The Arrhenius plot for the decomposition rate constant obtained by Kunzru (1972) is shown in Figure 1. At higher temperatures, these rate constants were lower than the ones predicted from the correlation of Voge and Good (1949). The average molecular weight of the gaseous products was 27.2, a value independent of the temperature and conversion. The constant molecular weight reflects the fact that once formed, there is little further cracking of C4 and lower gases
K
u c
z 4
610
630
SEVERITY
650
c
53 3 22 1 LL iL
' 'ZH4 A
24
0
670
FACTOR ( S : T T " ~ ' ;
690 T
710 IN 'C,
730
750
r IN SECONDS)
b
C3H6 CH4
K
e c K
z $
8
0 610
630
SEVERITY
650 FACTOR
670
690
( S = T ~ 0 0 6 ,T
710
730
750
IN 'C, r I N SECONDS)
Figure 3. a. Yields of hydrogen, hexene, and ethane in octane cracking vs. severity factor (Kunzru (1972)).b. Yields of methane, ethylene, and propylene in octane cracking vs. severity factor (Kunzru (1972)).
since the rate of cracking increases greatly with molecular weight. As shown in Figure 2, the gas weight percentage in the reactor effluent was found to increase with the conversion. At the same conversion, however, the difference between the gas weight percent values a t two different temperatures was negligible. The octane conversions and the gas/liquid ratios in the ranges of .temperature and space time used in the present study can be estimated from the results shown in Figures 1 and 2. The detailed product distribution for some of the Kunzru's runs are illustrated in Table I. As shown in Tables I1 and 111, these products distributions agreed reasonably well with the ones predicted from the theory of Rice (1933) and Kossiakoff and Rice (1943) and the model of Woinsky (1961). Detailed discussions on these models are given by Kunzru (1972). The experimentally measured product distributions were also fairly well correlated by the severity factor correlations. Typical correlations of this type are shown in Figures 3a and 3b. Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 4, 1976
519
Table I. Product Distribution in Octane Cracking 670 670 650 1.95 0.31 2.0 14.6 44.8 12.1 23.2 97.0 98.5 98.1 99.99 3.9 2.8 1.5 2.5 Moles/100 Mole of Octane Cracked 18.5 17.5 27.1 14.7 67.2 48.2 77.2 46.5 110.2 89.5 117.9 80.1 28.9 34.5 28.0 31.4 31.5 46.8 45.1 30.1 3.3 3.9 3.8 4.7 28.2 14.2 29.1 16.5 6.4 1.6 2.5 7.4 6.7 2.4 2.6 8.8 0.8 0.21 0.24 0.84 6.4 2.8 2.9 6.5 3.9 1.1 5.95 2.4 1.8 1.05 3.9 1.0 0.6 0.2 1.26 0.45 0.3 1.0 0 0.65
650 0.4
Temperature, "C Residence time, s Conversion, % Material balance, % Unidentified material, % H2 CH4 C2H4 C2H6 C3Hs C3H8 l-CdH8 C4HlO l-C,iHlo CsHiz I-CsH12 CSH14 l-C'iH14 C7H16 Benzene
700 0.12 14.5 97.5 4.7
700 1.34 51.2 96.9 6.1
725 0.21 27.2 97.7 5.4
725 1.35 82.8 95.1 10.7
25.8 52.5 89.5 30.1 35.5 3.9 14.2 6.1 8.5 0.8 7.8 3.8 1.4 0.7
32.0 74.5 122.5 34.0 48.5 5.7 28.1 1.4 1.3 0.2 2.5 0.7 0.4 0.18
33.5 62.1 110.5 30.4 41.5 4.3 19.9 4.7 5.7 0.6 5.7
0.1
0.9
40.2 83.5 134.2 35.2 55.2 5.5 30.4 0.3 0.4 0.04 0.7 0.2 0.2 0.02 1.8
2.1
0.8 0.56 0.2
Table 11. Calculated and Measured Product Distribution in the Decomposition of n-Octane Moles of product/100 moles octane cracked Calcd from Rice-Kossiakoff theory
Exptl 650 "C = 0.4 s
Component
725 "C = 0.2 s
Hz CH4 C2H4 C2H6 C~HG C3Hs 1-CdHs C4H10 1-CjHlo CsHiz 1-CsH12 C6H14 1-C:H14 C7H16
14.7 46.5 80.1 28.0 30.1 3.3 16.5 7.4 8.8 0.8 6.5 6.0 3.9 1.3 253.9
725 "C
625 "C
T
T
33.5 62.5 110.5 30.4 41.5 4.3 19.9 4.7 5.7 0.6 5.7
0
0
64.61 125.1 35.46 43.62
64.79 126.6 35.06 44.22
0
0
19.88
19.62
0
0
14.37 0 14.37
2.1
0.8 0.6 322.4
14.03 0
14.03
0
0
6.62 0 324.03
6.54 0 324.89
Table 111. Experimental Data vs. Predictions of Woinsky Model Exptl value at minimum residence time
Predictions from Pertinent ratio
650 "C
670 "C
700 "C
650 "C = 0.4 s
T
H2ICH4 0.31 0.38 0.49 0.32 C2Hs/CH4 0.85 0.83 0.78 0.6 Ps/CH4" 0.22 0.18 0.15 0.4 a Note: Ps = a paraffin lighter than the feed but heavier than C2Hfj. Experimental Section Apparatus. A flow reactor system was used in the present study. The experimental setup used in this work is shown schematically in Figure 4. A tubular reactor made of 304 stainless steel having 0.25 in. 0.d. and 0.18 in. i.d. was used for most of the work. The feed was supplied from a reservoir by a Lapp Microflow metering pump. The feed was preheated before its entry to the main reactor. The preheater coil was made out of 0.25-in. 0.d. copper tubing and it was placed 520
Ind. Eng. Chem., Process Des. Dev., Vol. 15,No. 4, 1976
T
670 "C = 0.31 s 0.37 0.6 0.32
-
T
700 "C = 0.12 S 0.49 0.57 0.29
within the heated section of the Lindberg Hevi-Duty furnace F1. The furnace had a matching controller by which the temperature could be controlled up to 1000 "C within fl "C. Just before the preheater entrance, there was a tee with provision to introduce nitrogen or air as needed. The main furnace Fz was also a Lindberg Hevi-Duty furnace with the maximum temperature limit of 1200 "C. The temperature could be controlled within fl "C. The reactor temperature was measured by the thermocouples imbedded into the walls of the reactor in a manner similar to the one de-
_i' J
NITROGEN
..
5oo TO FUME
HOOD
I
Run No I 8 was with fresh stainless steel Temperature :7 8 O O C Run T i m e = 5 0 minutes Air treatment done between two runs s / " = 8.75 ~ m - 1
tube
I
LIQUID
TRAP
Figure 4. Experimental apparatus I8
scribed by Kunzru (1972). In all the runs, the removable test section of the reactor was essentially isothermal (within fl "C). The reactor effluent was passed through a heat exchanger to condense liquid products. The outlet from the heat exchanger was led into a conical liquid trap to collect the liquid product. The gaseous products were vented to the fume hood. A pressure gauge (range of 0-30 psig) was attached between the pump and the preheater to indicate any pressure buildup due to the clogging of the tube. All the experiments in the present study were made at atmospheric pressure. A tee was fitted between the pressure gauge and valve Vs. A drain pipe was attached to the tee with the valve VI at the end to serve as an outlet for the liquid for calibration of the pump. The valve V2 was used to check the liquid feed when running air or nitrogen through the system. Valves V3 and Vq were fitted in the line at the furnace (F2)inlet and outlet, as near the furnace as possible. They served to isolate the reactor completely from the flow system at the end of any run, so that no more reactants or air can enter the reactor interior during the cool-dqwn period. The n-octane used in the present study was obtained from Philips Petroleum Co. and was used without further purification. It was 95 mol % minimum purity with approximately 3 to 5 mol % as the non-n-octane components. Procedure. The objective of this study was to measure the amount of coke deposited in the reactor as functions of run time, reactor space time, and temperature. Since the kinetics of n-octane has been studied previously by Kunzru (1972),the outlet liquid or gas samples were not analyzed in this study. The system as shown in Figure 4 was assembled with a pre-weighed reactor. The furnaces were started. During the 20-30-min heat-up period, nitrogen was passed through the system to exclude air and to maintain an inert atmosphere in the system. The pump, adjusted for any required flow, was then started and when the vaporized feed started passing through the reactor, the stopwatch was started to mark the beginning of the run. After the needed amount of run time the pump and the furnaces were shut off and the valves V3 and V4 were closed to isolate the reactor from reactants. The reactor was disconnected from the system and taken out of the furnace, the valves V3 and V4 still being connected to the reactor and closed. After complete cool-down, the test section of the reactor was gently removed and the weight of the carbon deposited inside this section was noted. To use the reactor for the next run, the deposited carbon was burned off at 750 "C with air. The duration of the burnout varied from 15 min to 1 h until all the deposited carbon was burnt off. Although the air flow rate was not measured accurately, in most experiments it was of the order of 10 cm3/s. This oxidized tube could not be used directly for the next run for reasons discussed in the Results and Discussion section.
RUN
19 NUMBER
20
21
Figure 5. Effect of air treatment on coking.
After oxidation (Le., burning off carbon with air), the tube was pickled in an acid bath (3%HC1,3% H2S04) at about 70 "C for 30 min to remove surface oxides and expose fresh metal surface for reasons of reproducibility. The reactor was then washed with water and dried. Then it was used for the next run. The preheater temperature was maintained at 500 "C in all the runs. Results and Discussion (A) Surface Effects. As indicated previously, the reactor was made of 304 stainless steel. The effect of the surface conditions and nature has been a rather controversial subject for discussion. Marschner (1938) reported the same conversion of n-octane in both stainless steel and Pyrex tubes when the stainless steel tube was cleaned by burning out deposited carbon by air between two runs. Crynes and Albright (1969) observed an increase in activity of the stainless steel reactor with oxygen treatment. In a recent study, Kunzru et al. (1972) reported that during the cracking of octane, the stainless steel tube exhibited catalytic activity for a short time after it was treated with oxygen. In the present study, it was decided to evaluate the effect of air treatment which was used to burn off the coke deposited between two runs. Starting with a fresh stainless steel tube, several runs were repeated at exactly the same experimental conditions. It was observed that the amount of coke formed increased with each successive air treatment, thus showing clearly that the air treatment activates the surface. The results are qualitatively in agreement with those reported by Crynes and Albright (1969) and Kunzru et al. (1972). The nature of these results is shown in Figure 5 . A similar trend was observed a t 750 "C. Crynes and Albright (1969) report that hydrogen sulfide and sulfur treatments "passivated" the reactor surfaces by forming a protective metal sulfide film. In the present study it was decided to investigate the effect of HPS treatment on coking from n-octane. The reactor was treated at 700 "C with H2S for about 30 min. It was found that the amount of coking was extremely low when an H2S treated reactor was used. Thus the surface was indeed "passivated" with respect to coke formation. However, once again the coking rate depended on the time of H2S treatment and no reproducible results were obtained. In order to obtain reproducible results for the effects of temperature, run time, and space time on the coke deposition rate, the need for a constant surface property was evident. According to Crynes and Albright (19691, 304 stainless steel forms complex metal oxides at 700 "C or above. To obtain reproducible results, the tube was pickled a t 70 "C in acid Ind. Eng. Chem., Process Des. Dev., Vol. 1 5 , No. 4, 1976
521
Temperature: 7 7 5 ’ C r:Space T i m e , irecondsl
0
02
0
SPACE
04 TIME
06
08
I O
(SECONDS1
Figure 8. Typical steady-state coking rate vs. space time.
RUN
TIME
(HOUR)
Figure 6. Typical data of coke deposition vs. run time.
Temperature = 775’ C Z4
.
s/v:8
t
X-S/Y=
1
75 CM-’ 4 2 4 CM-‘
1
02
0 4 SPACE
06
08
10
12
14
TIME (SECONDS1
Figure 7. Initial coking rate vs. space time.
area to volume ratio was changed by changing the diameter of the reactor keeping the length constant. Figure 8 illustrates the “steady state” coking rate against space time. From the kinetic constant at 750 “C estimated from Figure 1, the conversion levels for the data shown in Figure 8 are estimated to lie between approximately 55% (at the lowest space time) and 85% (at the highest space time). Hence the reactor could not be treated as a differential reactor. The rates (calculated as total cokeltotal time-total internal area of test section) illustrated in Figures 7 and 8 are overall integral rates for the entire reactor. As illustrated in Figures 7 and 8, both unsteady- and steady-state coking rates showed a curious behavior with respect to space time. The coke accumulation does not depend entirely upon how long the reactants spent within the reaction zone. At all temperatures studied, the curves of the coking rate vs. space time showed a maximum. The maximum may be explained by one of the following two mechanisms reactants
-
coke
--t
some products
(1)
A
1 L coke ~
solution (3% HC1 and 3% HzS04), taking guidelines from Shannon (1930). Pickling of the oxidized tube probably dissolved all or most of the surface oxides and exposed fresh steel surface each time the tube was pickled. Thus the subsequent runs could be made considerably reproducible. The surface role, if any, could be assumed constant from run to run. Each experimental run for the measurement of coke deposition rate was repeated twice. The data reported in plots are the arithmetic averages of the ones obtained in these repeated runs. The maximum deviation in the coke deposition rates for two repeated runs were found to be approximately 7%. (B) Models of Coking. The experimental data were obtained for three run times, namely 1 , 3 , 5 h at each of the three temperatures 750,775, and 800 “C studied. At each temperature, four different space times were investigated. Typical results obtained at 775 “C are illustrated in Figure 6. Similar results were obtained at 750 and 800 “C. The nature of the results shown in Figure 6 is interesting. In the initial period of the run time, the rate of coke formation appears to be rather rapid and after some time the rate of coking becomes more or less constant and achieves a steady state. The higher coking rate at the earlier time is probably due to the surface effects of the reactor tube walls. After a certain amount of time the surface becomes covered with a coke layer and the surface effect presumably diminishes. The coke formed during the later “steady state” period is most probably thermal coke, Le., formed as a result of homogeneous reaction rather than that formed on the surface. Figure 7 illustrates the initial “unsteady state” coking rate as a function of space time for reactors of two different surface area to volume ratio (henceforth denoted as s l u ) . The surface 522
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 4, 1976
B In reaction scheme 1, the term “reactants” means all substances from which coke is formed. This could include octane itself and the products formed by its cracking. According to this reaction scheme, the coke which is mainly composed of high-molecular hydrocarbons of aromatic nature and their free radicals (Syskov and Jelikhovaskya, 1967) probably decomposes by itself or further reacts with some of the components of the reactor gases to give some other products. The second reaction involving consumption of coke may take place entirely in the gas phase, entirely on the surface, or the coke on the surface may react with some gaseous reactants, or any combination of the above. From Figure 7 it is observed that the rates based on unit internal surface area of the reactor in case of s / u = 4.24 cm-l are considerably lower than those obtained with s/u = 8.75 cm-l. This implies that for the high s / u reactor ( s / u = 8.75 cm-l) the initial rapid “unsteady state” coking is mainly due to the surface effects and not due to thermal, homogeneous coking. Thus, the assumption made to explain the initial fast coking rate in Figure 3 appears plausible. It should be noted, however, that even a trace of oxygen or oxygenated species in the gas phase can also cause initial rapid coking until completely removed. For the lower s / u ratio reactor, the maximum has evidently shifted to the right with respect to space time. The shift to the right indicates that for a reactor having high s / u ratio the second reaction resulting in consumption of coke is more predominant than that with a reactor having a lower s / u ratio (Le,, higher tube diameter). This in turn, implies that the second reaction takes place largely on the surface for the higher s / u ratio reactor.
1
i
't
X
-2*
0 8
i
i I
Lz
I
/
i
i
0 6
t"
T = Temperature, O K r = S p o c e Time, Seconds For r < 0 35
40
i
20 n
i
I
Shows Maximum
SPACE
TIME (SECONDS)
Figure 9. Influence of surface area to volume ( s / u ) ratio on the contact time dependence of the rate of pyrolytic carbon formation during methane pyrolysis at 1050 "C (Figure 3 of Makarov and Pechik (1974)). Curves are obtained from eq 24 of Makarov and Pechik.
In reaction scheme 2, species A and B could be n-octane and the cracking products, respectively. Here two different species form coke at different rates and the relative rate constants for the various reaction steps determine the coke formation rate. Makarov and Pechik (1974) studied coking in the pyrolysis of methane and presented a complex model for the reactions. They studied reactors of various s / u ratios. Figure 3 in their article illustrates the rate of coking as a function of contact time with s / u ratio as parameter. This figure is duplicated here in Figure 9. I t is evident from their experimental data points a t s / u = 20.0 cm-l that a maximum is exhibited. At lower s / u ratio, the maximum is not observed. This leads further support to reaction 1 in which the second reaction takes place mostly on the surface of the reactor with high s / u ratio. Preliminary runs were made at 900 "C. The amount of coke deposited at this temperature was extremely low as compared to lower temperatures. It was also observed that the coke formed was much fluffier and the effluent liquid products coming out of the reactor were of dark color. The clear liquids were obtained with lower temperature runs. This observation indicates that at higher temperatures, a large amount of coke is carried away by the flowing stream and the properties of the formed coke depend significantly upon the reaction temperature. With the experimental limitations of the present study, reliable results at temperatures higher than 800 "C were not obtainable. The rate data were also correlated with the gashiquid ratio of the products. The gadliquid ratios for the present experiments were estimated from the data described in Figures 1to 3 and other similar results reported by Kunzru (1972). When plotted they had a similar nature as in Figure 8 where gas/ liquid ratio replaces space time. The reason for the similar nature is that gashiquid ratio increased with an increase in space time. (C) Severity Correlation of the Rate Data. Because of the lack of knowledge of' an exact mechanism of cracking of hydrocarbons, yields of products from cracking furnaces have often been correlated by a severity factor (Linden and Peck, 1955; White et al., 1970). For the present study the severity factor is defined as
S =Tm~n where T is the absolute temperature (K) and
7
is the space
x Experimental
I20 80
I
I
0
1
I
2
3
4
5
6
7
S = T - 2 4 4 O" 1 x ~ ~ - 7 4 1
Figure 10. Severity factor correlations.
time (seconds). The severity factors fitted to the present data X for T < 0.35 and (b) were as follows: (a) S = T9.4T0.4 S = T-24.4T-0'8 X for T < 0.35. The above severity factors were correlated to the total amount of coke produced between the run times of 1and 5 h using the multiple regression technique. The results are shown in Figures 10a and lob. These correlations are applicable for the reactor tube with s / u = 8.75 cm-l and a volume of 1.668 cm3. The results indicate that such correlations are possible for a particular laboratory or commercial cracking reactor. Conclusions As a result of the present study, the following conclusions are made for n-octane pyrolysis in a 304 stainless steel reactor. (1)Air (or oxygen) treatment of the reactor activates the reactor surface to induce more coking than obtained with the untreated tube. The coking rate appears to be dependent upon the total time of the oxygen treatment. H2S treatment passifies the surface. (2) When the amount of coking is plotted as a function of run time, initially the rate of coking is very high. After about 1 h of run time, the coking rates achieve a steady state. The coke formed may either undergo further chemical reaction during cracking or it may be formed by two competitive reactions. (3) The overall steady-state coking rate for any given reactor can be correlated by severity factor S = T m ~where n T is temperature in degrees centrigrade and T is reactor space time in seconds. A much better fit for 7 > 0.35 was obtained than for 7 < 0.35. Literature Cited Conroy, J. S., Slysh, R. S., Murphy, D.B.,Kinney, C. R., Proceedings of theThird Conference on Carbon, p 395, Pergamon Press, Oxford, 1959. Crynes, 6. L.. Albright, L. F., lnd. Eng. Chem., Process Des. Dev., 8, 25 (1969). Dintzes. A. I., Frost, A. V., J. Gen. Chem. U.S.S.R.,4, 610 (1934). Doue, F., Guiochon, G., J. Phys. Chem., 7 3 , 2804 (1969). Grisdale, R. 0.. Pfister, A. C., Van Roesbroeck, W., Bell System Tech. J., 30, 271 (1959).
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Kossiakoff. A.. Rice, F. 0.. J. Am. Chem. SOC., 65, 590 (1943). Kunzru, D., Ph.D. Thesis, University of Pittsburgh, 1972. Kunzru, D., Shah, Y. T., Stuart, E. B., lnd. Eng. Chem., Process Des. Dev., 11, 605 (1972). Lanett, J. E., Egloff, G., lnd. Eng. Chem., 9, 350 (1917). Linden, H.R., Peck, R. E., Ind. Eng. Chem., 47, 2470 (1955). Makarov, K. I., Pechik, V. K., Carbon, 12, 391 (1974). Marschner, R. F.. lnd. Eng. Chem., 30, 554 (1938). Rice, F. 0.. J. Am. Chem. SOC., 55, 3035 (1933). Shah, Y. T., Stuart, E. 6..Kunzru, D., Ind. Eng. Chem., Process Des. Dev., 344 (1973).
Shannon, R. W., “Sheet Steel and Tin Plate,” The Chemical Catalog Company, Inc., New York, N.Y., 1930. Slater, W. E., J. Chem. SOC., 109, 160 (1916). Syskov. K. I., Jelikhovaskaya, Carbon, 5, 201 (1967). Voge, M. M., Good, G. M., J. Am. Chem. SOC., 71,593(1949). White, L. R., Davis, H. G., Keller, G. E.. Rife, R. S.,paper presented at the 63rd AlChE Meeting, Chicago, Ill., Nov Nov 1970. Woinsky, S. G.. lnd. Eng. Chem., Process Des. Dev., 7, 529 (1961).
Received f o r review October 6 , 1975 Accepted June 7,1976
Continuous Dialytic Decontamination of Dissolved Fuel Elements by Solvent-Polymeric Membranes Zwi Ketzinel, Zwi Boger, and Henry Cikurel Nuclear Research Center, Negev, lsrael
David Vofsi, Joseph Jagur-Grodzinski,’ and Saul Gassner The Weizmann lnstitute of Science, Rehovot, lsrael
A continuous decontamination process, based on a selective dialytic permeation of uranyl ions through “solventpolymeric” membranes, has been devised. Spiral-wound permeators were constructed and tested for their efficiency in separating uranium from its fission products and from aluminum. Solutions, similar in composition to those obtained by dissolving spent fuel elements of research type “swimming-pool’’ reactors, were used in the experiments. The overall y-decontamination factors for a two-stage decontamination experiment were -5000, while the total uranium losses were in the range of 0.2-0.8% (depending on flow rate). The effect of irradiation on the permeators’ performance was checked by submitting them to y-irradiation from a cobalt-60 source, up to a total dose of 6.3 Mrads. The performance of the permeators did not deteriorate after three months of operation and 18 months of storage. The experimental results indicate that the tested novel separation technique could replace the standard “decontamination” procedure based on solvent extraction.
Introduction The use of solvent-polymeric membranes (Vofsi and Jagur-Grodzinski, 1974) for the selective transport of uranyl nitrate (UNT) from aqueous solutions has been previously described (Bloch et al., 1967; Bloch, 1969). I t was shown that, when using a polyvinyl chloride (PVC) membrane plasticized with dicresylbutyl phosphate (DCBP), it was possible to transport UNT from an acid solution (pH -2), containing a large excess of aluminum ions, into another aqueous phase (pure water) with a high selectivity with respect to that latter ion. More recently, (Jagur-Grodzinski et al., 1973) the mechanism of transport through such membranes was elucidated, and the diffusion coefficient of the permeating uranium species was evaluated for a given set of conditions. The high rate and specificity of uranium transport through this membrane prompted us to investigate its possible appiication for the separation of enriched uranium from the ionic species present in solutions which are being obtained by dissolving spent research reactor fuel elements in nitric acid. Reactors of the swimming-pool type are usually fueled by elements made of an enriched uranium (90%235U)-aluminum alloy (7.5%U). After about 35% of the uranium has fissioned, these elements are being withdrawn for regeneration. Conventionally, they are dissolved in concentrated nitric acid after the “cooling-down” period, and the aqueous acid solution is then solvent-extracted with tributyl phosphate (TBP). In this 524
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way the uranium is separated from the radioactive fission products-a process termed ‘‘decontamination’’-and then converted into a new fuel element. The reprocessing of fuel elements is often effected in places quite remote from the reactor site and their dispatch to these centers is rather costly. I t would, therefore, appear desirable to decontaminate the uranium-containing solution a t the reactor site by some compact and easily manageable unit, and then dispatch the uranium concentrate for further purification and re-enrichment, while disposing locally the radioactive contaminants by means of accepted procedures. The present work is aimed a t developing such a unit. Experiments are described on decontamination of radioactive model solutions by continuous passage through a membrane module, under conditions that closely simulate an actual situation. Experimental Section Membrane Preparation. The PVC-DCBP mixture was prepared as described in the literature (Bloch, 1969; Marian et al., 1970).The resin solution was then put onto paper strips (6 X 200 cm) by means of a “doctor” blade that enabled us to place a uniform layer on the paper base. The strips were then left exposed to an air draft in the hood for 8 h. The coating operation was repeated twice and the membranes thus obtained were left in the hood for 48 h until practically all of the solvent was evaporated; 80-100 thick homogeneous films
