Ind. Eng. C h e m . Res. 1987,26, 2393-2397 Figures 11 and 12 show the relation between
and
Tcat 300 or 1000 rpm of the agitator for NH4Br-NH3 and NH41-NH3 systems. For both systems, a t 1000 rpm waa superior to that at 300 under the operating conditions examined. These figures showed that the agitation speed in the absorber had a important effect on the coefficient of Derformance.
2393
Q G = heat
quantity supplied from heating water in the generator, kJ QH = net heat WmtitY h n ~ f e r r e d in the generator ( 8-~83, kJ
21 et^,^^^;^,
Tg= temperature in the generator, K
Greek Symbol
Conclusion In the performance test on the absorption cooling apparatus proposed in this work, the system of NH4Br-NH3 was superior to the NH41-NH3 system. The coefficient of performance was about 55% -65% of the estimated one calculated from the enthalpy-concentration chart. These values are not sufficient in practical use, but the usefulness for the batch-type absorption cooling apparatus for use of utilizing low-level energy has been confirmed.
t
= coefficient of performance
Subscripts i = initial state of the solution f = final state of the solution E-A = evaporating-absorbing process G-C = generating-condensing process est. = estimated value obs. = observed value Registry No. NH3, 7664-41-7; NH,Br, 12124-97-9; NHJ, 12027-06-4.
Acknowledgment
Literature Cited
We A*Kurata for his Toyoda for his experimental work.
Fujiwara, I.; Sato, M. Reito 1985, 60,24. Jeager, F.; Fox, E. C. D.O.E. Report CS-30248-T-1, 1981; D.O.E., New . - . York. - -. .. Toyoda, T.; Kurata, A.; Sanga, S. Preprints of the 48th Annual JaDan, Kyoto, Meeting of The Society of Chemical Engineers, 1983, p43. Yamamoto, H.; Kurata, A.; Sanga, S. Kagaku Kogaku Ronbunshu 1984, 10, 421.
discussion and T*
Nomenclature H = enthalpy of ammonia gas, kJ/kg h = enthalpy of ammonia solution, kJ/kg M = mass, kg N = speed of revolution, rpm QR = heat quantity of air cooling in the evaporator, kJ
Received for review March 31, 1986 Accepted August 10, 1987
Tendencies of Aromatization in Steam Cracking of Hydrocarbons Frank-Dieter Kopinke,* Gerhard Zimmermann, and Bernd Ondruschka Central Institute f o r Organic Chemistry, Academy of Sciences of GDR, Department of Basic Organic Materials, 7050 Leipzig, GDR
T h e formation of aromatics from nonaromatics during steam cracking of naphtha is described quantitatively. To get realistic data, the tracer technique was used on the basis of about 40 '%-labeled hydrocarbons as constituents of a naphtha fraction. These model compounds are representative of pyrolysis feedstocks, reaction intermediates, and reaction products. Characteristic aromatization yields are given for different types of C atoms and essential molecules. The formation of aromatic hydrocarbons from nonaromatic ones is unavoidable under the conditions of industrial steam cracking of straight-run or hydrocatalytically pretreated crude oil fractions. The liquid fractions resulting from the pyrolysis of such feedstocks are cracked gasoline and cracked fuel. Both fractions are highly aromatic. On the one side, cracked gasoline is a valuable feedstock for producing pure benzene and motor fuel; on the other side, alkylbenzenes and especially oligocyclic aromatics are precursors in the formation of coke-like deposits in the pyrolysis coils and the transfer line heat exchangers (e.g., Trimm (1983), Lohr and Dittmann (1978)). Further, aromatization reactions decrease the yield of olefins, the main object of the process. In most cases, the fuel fraction can be used for heating only. For these reasons, the extent of aromatization has to be limited by suitable means, such as special temperature profiles along the coils, short residence times, high speeds of product quenching, and optimum steam dilutions. Since the paper of Kossiakoff and Rice (1943),the very complex reaction taking place in the thermal degradation 0888-5885/87/2626-2393$01.50/0
of hydrocarbons has been interpreted on the basis of a radical chain mechanism. The generation of aromatic structures is attributed to what is called secondary reactions, about which only little relevant knowledge is available now. Recent results obtained by us on the mechanism of aromatics formation will be presented elsewhere (Kopinke et al., 1987). In principle aromatics are formed under the condition of pyrolysis via two routes: by synthesis reactions of lower hydrocarbon species and by degradation reactions. The first route is the only one in the pyrolysis of paraffins; the second path dominates in the pyrolysis of oligocyclic naphthenes (Zimmermann et al, 1985; Cypres and Bredael, 1980; Korosi et al., 1979). An analysis of published results leads to an important conclusion. Sufficient fundamental knowledge concerning thermal aromatization does not exist under conditions relevant to industrial steam cracking which would permit a kinetically and mechanistically founded quantitative description or even modeling of this important process. Accordingly, the concepts and kinetic data used in pyrolysis models like SPYRO (e.g., Dente et al. (1979)) or 0 1987 American Chemical Society
2394 Ind. Eng. Chem. Res., Vol. 26, No. 11, 1987 Table I. Standard Pyrolysis Conditions reactor temp, OC 810 ca. 0.4 residence time, s run time, h 1 feedstock gh-l naphtha 16.3 gh-' water 11.4 methene:mpropene 2.5 f 0.2 pressure, MPa 0.1
Figure 1. 14C-Labeledhydrocarbons used in this investigation.
PHENICS (Mol, 1980) are dubious as to their chemical substance in detail. In such a situation, the phenomenological description of aromatics formation can well be advantageous. The question is how to obtain quantitative information on rates of aromatization or on yields of aromatics formed from different types of hydrocarbons. Results from experiments with pure individual nonaromatic hydrocarbons do not permit us to draw conclusions on the yields of aromatics to be expected from the same compounds if they are constituents of a complex hydrocarbon mixture available as industrial feedstock. Strong interactions between the feedstock components, the radicals and the intermediates generated, and the products result in significant differences in reaction behavior of one and the same hydrocarbon despite comparable external parameters. This is particularly true for the type of aromatics involved in reactions proceeding in a synthesis route. An appropriate means of overcoming these difficulties is provided by the tracer technique: 14C-labeled model hydrocarbons are added to an industrial pyrolysis feedstock (naphtha) in such small amounts that its reaction behavior is not influenced significantly. The yields of product components or fractions formed from the labeled hydrocarbon can be calculated immediately from the analysis of radioactivity distribution in the pyrolysis products. This paper gives information on such tracer-based data on thermal aromatization obtained as additional results to extensive investigations dealing with coking problems.
Experimental Section Materials. All pure hydrocarbons used as feedstocks were synthesized according to literature instructions and had a gas-chromatographicpurity of more than 98%. The 14C-labeledhydrocarbons depicted in Figure 1 were synthesized using 14C precursors from Isocommerz GmbH, Berlin (Kopinke, 1986). Their radio gas-chromatographic purity amounted to 398% in most cases; the specific activities were in the range from 3 to 100 MBq-g-l. For the tracer experiments we employed straight-run naphtha (provenance Romashkino, br 60-180 "C, d = 0.72-0.73 g - ~ m -content ~, of aromatics 10 vol %, content of sulfur 0.03 wt % ) containing small amounts (0.1-1 w t % ) of I4C-labeled hydrocarbons. Apparatus and Procedure. All pyrolysis runs were carried out in laboratory-scale equipment including electrically heated reactor tubes made from stainless steel or quartz (surface to volume ratio 10 and 4 cm-', respectively), schematically shown in Figure 2. The standard pyrolysis conditions applied are characterized in Table I. They correspond to high severity cracking on an industrial scale. The liquid pyrolysis products were condensed at different
Table 11. Formation of Aromatics from Nonaromatic C Atoms during Naphtha Cracking (wt %) labeled* C atoms gasolinea fuel" or molecules benzene toluene (CB-C8) (IC,) methane ( 5 7 0 ) ~ 0.6 0.5 ethane (45%) 1.2 1.1 R*CH,' 3.5 1.3 6.5 3.5 4 8 R*CHzR' 4.5 1.4 R*CHR'R" 13.5 7 2.5 8.5 9 12 cyclopentane 16 2 2 8 12 cyclohexane 16 x32d methylcyclopen tane [methyl-14C]methylcyclox19 hexane [ring-'4C]ethylcyclohexane E31 20 10 [ 7-14C]ethylcyclohexane [ 8-14C]ethylcyclohexane E13 27 16 16 7.5 [9-14C]decalin 29 26 8 13.5 [9-14C]tetradecahydroanthracene 4 3 0.7 2.5 ethene (33%) 1.8 8.5 6 5 [2-14C]propene (75 70) 1.4 7 5 4 [ 1,3-14C]propene 19 [ 1-14C]- 1,3-butadiene (85% ) 3.5 12.5 10 I RCH=*CHz 13 RCH=*CHR 14.5 10.5 21 3.5 RCH=CR'R" 19 14 6.5 32 cyclopentene 29 18 5.5 40 cyclopentadiene 32 19 6 50 8 8 27 40 [ methyl-14C]methylcyclopentadiene 5 [ 7J4C]ethylcyclopentadiene 9.5 cyclohexene 4 37 30 10 70 1,3-cyclohexadiene 5 60 10 [ 7-14C]methylenecyclohexane 18 35 12 10 11.5 17 [methyl-14C]-1,3-methyl21 40 cyclohexadiene ethyne 16 4.5 23 19 [ 3-14C]propyne 11 15 6 23 [1,2-14C]-l-butyne E55 [5-l4C]-1-pentyne 4 6.5 [ 3-14C]- 1-hexyne 22 14.5 34 8 Separated by atmospheric microscale distillation. *Degree of conversion if
