-
=
average molal: heat capacity of drop number j
c, acceleration average gas molar heat capacity of gravity ‘Pdi
= = ht5 = mass transfer coefficient for component
SUPERSCRIPT
0
zero mass flux value
=
g
i to drop j heat transfer coefficientfor drop number j M = total number of different representative drops M f = molecular weight of component i J7, = average molecular weight of the gas m, = total spray mass flow n = number of gas-phase species n = number flow rate of spray droplets nc = number of components in the spray phase N f j = molar flux of component i toward drop numberj Ntot5 = total molar flux toward drop number j P = pressure P,, P d = defined in eq 17 and 16, respectively pi* = vapor pressure of pure component i Qi: = molar gas flow rate of component i in the direction of spray flow R, = gaslawconstant T, = gas temperature Tdi = temperature of drop number j V , = gas velocity in the spray direction Vdj = velocity of drop j x, = diameter of drop j y = gas mole fraction of component i yi,* = gas mole fract,ion of species i a t the interface with liquid drop j W f j , - mole fraction o’f component i in representative drop II z = distance from spray nozzles; positive in the direction of spray flow hTi =
GREEKLETTERS a = 7t j = qj = xi5
=
sprayholdup moles of compoinent i in drop j total number of moles for all components in drop j partial molar heat of vaporization of species i in
drop j
fiC
pc Pd
45 $ij
gasviscosity = gasdensity = spraydensity = number fraction of representative drop j in the spray = partial molar v’olumeof component i in drop j =
SUBSCRIPT 0 = evaluatedatz
=
Literature Cited
Adler, D., Lyn, W. T.,Int. J. Heat Mass Transfer, 14, 793 (1971).
Amelin, A. G., “Theory of Fog Condensation,” 2nd ed, Israel Program for Scientific Translations Ltd., Jerusalem, 1967. Baltas, L., Gauvin, W. H., AIChEJ., 15, 764 (1969). Bird, R. B;! Stewart, W. E., Lightfoot, E. N., “Transport Phenomena, Wiley, New York, N. Y., 1960. Bose, A. K., Pei, D. C. T., Can. J. Chem. Eng., 42,259 (1964). Carstens, J. C., Zung, J. T., J. Colloid Interface Sci., 33, 299 (1970).
Dickinson, D. R., Marshall, W. R., Jr., AIChE J., 14, 541 (1968). Dlouhy, J., Gauvin, W. H., Can. J. Chem: Eng., 38, 113 (1960). Elliott, D. G., Weinberg, E. “Acceleration of Liquids in TwoPhase N o z z l ~ , ’NASA ~ d?echnical Report No. 32-987, Jet Propulsion Laboratory, July, 1968. Fraser, R. P., Eisenklam, P., Trans. Inst. Chem. Eng., 34, 294 11956).
Frker, R. P., Dombrowski, N., Johns, W. R., Brit. Chem. 8,390 (1963).
Gal-Or, B., Int. J.Heat Mass Transfer., 1 1 , 551 (1968). Galloway, T. R., Sage B. H., Chem. Eng. Sci., 22,979 (1967) Habib, I. S.,Int. J. Heat Mass Transfer, 13, 1378 (1970). Hortig, H. P., C h m . Ing. Tech., 42, 390 (1970). Kim, K. Y., Marshall, W. R., Jr., AIChE J., 17,575 (1971) Laskowski, J. J., Ph.D. Thesis, University Minneapolis, 1 QRR
L&k&ski, J. J., Ranz, W. E., AIChE J.,16,23 802 (1970). Marshall, W. R., Jr., “Atomization and Spra Drying,” CEP Monograph Series, AIChE, New York, N. 1954. Marshall, W. R., Jr., Trans. ASME, 77, 1377 (1955). Miesse, C. C., A p p l . Mech. Rev., 9, 321 (1956). Mugele, R. A., Evans, H. D., Ind. Eng. Chem., 43, 1317 (1951). Mugele, R. A., AIChE J.,6 , 3 (1960). Nienow, A. W., Brit. Chem. Eng., 12, 1737 (1967). Perry, J. H., “Chemical Engineering Handbook,,’ 4th ed, BIcGraw-Hill, New York, N. Y., 1963. Rawson, E., Chinn, J. S., Stevens, W. R., AIChE J., 7, 448
?,
(1961).
Reid, R. C., Sherwood, T. K., “The Properties of Gases and Liquids,” 2nd ed, McGraw-Hill, New York, N. Y., 1966. Rowe, P. N., Claxton, K. T., Lewis, J. B., Trans. Inst. Chem. E’ng., 43, T14 (1965).
Zung, J. T., J. Chem. Phys., 46, 2064 (1967a). Zung, J. T.,J. Chem. Phys., 47,3578 (196713). RECEIVED for review September 18, 1972 ACCEPTEDFebruary 16, 1973
0
Thermal Cracking of 2-Pentene Deepak Kunzru, Yatish T. Shah,* and Edward B. Stuart Department of Chemical Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 16.213
I n a recent paper Kunzru, et al. (1972), reported an experimental study of thermal cracking of n-nonane in a flow reactor. I n this paper we report a similar study of thermal cracking of an important, olefinic compound, namely, 2-pentene. A number of investigators have previously studied the pyrolysis of 2-pentene but the information is still very sketchy. Norris and Reuter (1927) studied the decomposition of 2pentene at 600’ and a residence time of 15 sec. The decomposition was 6670 and no1 hydrogen was detected in the exit gas. Methane was the on1.y saturated paraffin lower than the
feed. Butadiene was found in appreciable quantities (16 mol yo on the basis of pentene decomposed). Pease and Morton (1933) decomposed 2-pentene in a batch reactor between 525 and 560’ and reported the activation energy to be 61 kcal/mol. However, the product distribution was not measured. Hurd and coworkers (1936) cracked 2-pentene a t 550, 580, and 600’. They found the major product to be methane and smaller yields of butadiene than reported by Norris and Reuter (1927). KO rate constants or activation energies were reported. Gorin and coworkers (1946) studied the pyrolysis of 2-pentene between 750 and 850” and pressures Ind. Eng. Chern. Process Des. Develop., Vol. 12, No. 3, 1973
339
50
z 40
t
2-
P v)
30 2. I
u 0
20 67OoC
A 700.C
IO
8 725' C
"
0
.2
A
.6
.8
1.0
1.2
1.4
r, SECONDS Figure 1 . Conversion of pentene vs. residence time, T
of 0.1 and 0.2 atm. The main products were found to be methane, propylene, ethylene, butene, and 1,3-butadiene. Small amounts of pentadiene, butane, and ethane were also reported. Approximately 2 mol of product per 1mol of pentene cracked were formed. They concluded that Rice's theory (1935) would have to be modified to account for all the primary reactions observed. Hepp and Frey (1949) pyrolyzed 2pentene between 778 and 850' in a quartz reactor and found the first-order decomposition rate constant a t 780' to be 13 sec-'. They also found the main products to be methane, butadiene, butene, propylene, and ethylene. Again, for 1 mol of pentene cracked 2 mol of product were formed. No activation energies were reported and no theory was put forward to explain the product distribution. Experimental Section
The experimental apparatus used in this study is the same as the one described by Kunzru, et al. (1972). The liquid 2pentene (96y0 pure) was obtained from Eastman Chemicals and was not purified further. The cis and trans content of 2pentene was not analyzed. However, it is most likely that the kinetics and product distribution of the two forms would be very similar. The analysis of liquid products was carried out in the same manner as one described by Kunzru, e t al. (1972). The gaseous products were analyzed in a 20-ft long, 0.25-in. diameter copper column. The first 10 ft of the column were packed with silica gel, 60-200 mesh (J. T. Baker Chemical Co.), and the remaining 10 ft with 48-100 mesh activated alumina (Alcoa, F-1 grade). The column was operated isothermally a t 170' and nitrogen flow rate was kept at 100 cc/min. The experimental reactor was made of stainless steel. There are conflicting reports on catalytic effects of stainless steel tubes in pyrolysis reactions. For thermal decomposition of octane Marschner (1938) reported exactly the same decomposition products in stainless steel tubes as in Pyrex under the similar reaction conditions, as long as the walls of the metal tube were thoroughly cleaned between runs with air or oxygen. In rapid thermal cracking of n-hexadecane a t elevated pressures, Fabuss, e t al. (1962), did not report catalytic effect of stainless steel tube due t o reactor wall regeneration period. On the other hand, Crynes and iilbright (1969) found that the oxygen treatment increases the activity of the stainless steel tube. They reported, however, that for decomposition of propane, when pyrolysis started, this activity decreased by 90% in 30 min and reached the level of the untreated reactor 340 Ind. Eng. Chem. Process Des. Develop., Vol. 12, No. 3, 1973
in 100 min. During this unsteady state period, they observed peaks of carbon dioxide in their chromatograms. They also reported the similar activity for the decomposition of olefinic compounds such as ethylene and propylene. However, the data for decrease in activity with time for these cases are not given. In the present study we assumed that the catalytic activity time relationship given by Figure 6 of Crynes and Albright (1969) apply to the decomposition of 2-pentene. Air was passed after each run with an interval of about 45-50 min between the start and sampling of the run. No peaks of carbon dioxide were observed in the chromatograms, thus ensuring that the catalytic effect of the reactor wall on 2pentene cracking was insignificant. At this stage it is worth pointing out that although the stainless steel reactor causes problems of wall catalytic activity, in the present study we used a stainless steel reactor because most of industrial reactors are made of stainless steel. Hence our results will be more representative of what one would obtain in an industrial process. The detailed descriptions of experimental apparatus and the analysis of products are given by Kunzru (1972). Results
As outlined by Kunzru, et al. (1972), the effect of the wallgas temperature difference and the laminar temperature gradients within the tube on cracking was less than 5%. We will present the experimental results in two separate sections. First we will evaluate the overall kinetics of cracking. Subsequently, we analyze the product distribution. Kinetics of Cracking
2-Pentene (96.0y0 pure) was cracked a t 670, 700, and 725' with conversions varying from 0 to 50%. The flow rate of 2-pentene was varied from 0.2 to 1.4 g/min. The reactor was operated a t 1 atm pressure. The frequency factor and the activation energy were determined by assuming the decomposition of 2-pentene to be first order. For a first-order reaction
A
+
vB
(1)
it can be shown that k~
=
Y
1 In - -
1-x
(Y
- l)X
where Y, the moles of product per mole of 2-pentene cracked, was determined experimentally. The material balance varied from 100% a t low conversions (
