Ind. Eng. Chem. Res. 1994, 33, 171-173
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RESEARCH NOTES Thermal Cracking of Butadiene Harrie A. M. Duisters DSM Research b.v., P.O. Box 18, 6160 MD Geleen, The Netherlands This paper presents experimental data on the thermal cracking of butadiene in a pilot plant, under conditions representative of industrial operation. The product distribution of pure-butadiene cracking is shown. Results from cocracking experiments in naphtha and C4-raffinate are also presented. It is shown that butadiene cracking can be an interesting outlet for the increasing butadiene overcapacity in steam crackers. Some aspects of coke formation during butadiene pyrolysis are addressed as well.
Introduction Butadiene is principally produced as a byproduct of steam cracking. In the steam cracking process about 0.5-4 wt % of 1,3-butadiene is formed, depending on feedstock type and cracking severity. In recent years there has been a sharp decline in the price of butadiene. This is caused by a shift toward heavier feedstock cracking, especially in the US, producing more butadiene and increasing overcapacity in steam-cracker butadiene production. The latter is caused by the high ethylene production growth rate of about 4%/yr and the slow growth in the use of butadiene in the rubber (tires) business (2% /yr). The easiest outlet, besides burning, for steam crackers is cocracking. Industry estimates suggest that up to 400 000 metric tons of crude44 stream was cocracked last year by 10 producers. Butadiene cocracking was put into practice in the past year by at least three European producers. DSM Research b.v. has carried out a thorough study for DSM's two naphtha crackers in Geleen on the product yield of butadiene (colcracking and on the rate of coke formation during this cracking. The experiments were performed in a pilot plant steam cracker and in a laboratory-scale setup for fouling rate measurements.
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A pilot cracker is used for product yield evaluations. Coke formation can also be studied in this pilot unit, but this can only be done off-line by means of a well-controlled burn-off procedure with COz analyses of the off-gas. Since this is not a very accurate method and no distinction can be made between the effects of temperature, run length, and place in the reactor, another setup was used for cokeformation studies. This setup consists of a microreactor. This technique is described in detail by Froment (1990). In this reactor a metallic cylinder of the cracker tube material is suspended from a wire that is coupled to an electrobalance with an accuracy of about 0.1 mg. After preheating of the feedstreams the reactor is continuously fed with hydrocarbon and steam and cracking is performed at elevated temperatures of 800-850 "C. The off-gas of the microreactor can be analyzed for conversionlselectivity calculations. Coke formation is measured continuously and can be plotted by the computer. In Figure 1such a plot is shown for cracking naphtha and it can be seen that the coke formation process can be separated into two phases. Phase one, lasting for about 8 h, is a catalytic
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Figure 1. Weight of carbon deposited versus time during pyrolysis of naphtha for 0-10 h and for 0-250 h.
coking process in which the bare metal surface catalyzes the formation of coke. After this initial catalytic coking the coking rate reaches a steady state, the so-called asymptotic coking rate, with coke being deposited on existing coke. Since the run length of a commercialfurnace is of the order of 40 days, the amount of coke formed in the cracker tubes is mainly determined by this asymptotic coking rate. The microreactor with the electrobalance is very useful in determining the coking rate of various feedstocks. In the present study it was used for measuring the coking tendency of butadiene cracking.
0888-5885/94/2633-0171$04.50/00 1994 American Chemical Society
172 Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994
Cracking of P u r e Butadiene
boiling aromatic products are rather high. This can be explained from the fact that butadiene is a very reactive species and can react via a Diels-Alder addition with other unsaturates to form cyclic compounds that can be dehydrogenated to form aromatics. To evaluate the cracking value of butadiene, one has to compare the product yields with the product of naphtha cracking. Depending on market prices for the various products and the type of naphtha used as a basis for comparison, it can be concluded that butadiene has a cracking value of about 0.90 times that of naphtha. In spite of the low olefins yields, butadiene can be an interesting cracker feedstock due to its low methane and high aromatics yields.
Under standard cracking conditions with coil outlet temperatures varying from 780 to 850 "C pure butadiene was cracked in the pilot plant. The major product yields are presented in Table 1. Butadiene cracking gives low yields of methane, ethylene, and propylene. The product yields of the higher
Cocracking of Butadiene i n Naphtha or Ca-Raffinate It is well-known that interaction between various components in the cracking of a mixture of feedstocks can affect product selectivities. The cocracking yields of
Table 1. Product Distribution for the Major Products from the Cracking of Pure Butadiene at Various Coil Outlet Temperatures COT (OC) main products (wt % ) 800 820 835 840 (after ethane recycle) methane ethylene propylene butadiene pyr gasoline (180 OC)
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Figure 2. (a-d) Butadiene cocracking yields in C1-raffinate-11. (e, f) Butadiene cocracking yields in naphtha.
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Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994 173 Table 2. Butadiene Cocrack Streams Tested in the Pilot Plant Full-Range Naphtha (wt %) (Density 0.700 kg/L) paraffins 41.5 isoparaffins 33.0 olefins 0.0 naphthenes 17.3 aromatics 8.2 total 100.0 C4-Raffinate-I1(wt %) isobutane 6.8 n-butane 25.4 isobutene 1.1 butane-1 40.7 butene-2 25.5 butadiene 0.5 total 100.0
hydrocarbons can be different from the yields resulting from the mixing of the effluents of separate cracking. This is why experiments on the thermal cracking of mixtures of butadiene with other hydrocarbons have been performed in the pilot plant under conditions representative of industrial operation. In Table 2 two butadiene cocrack fractions are listed. First of all a full-range naphtha (FRN) boiling between 32 and 167 OC with a density of 0.700 kg/L was tested. Also tests were performed with C4raffinate-11, a stream often used as cocrack stream in industrial pyrolysis. It is also possible to cocrackthe entire C4 cut. In this way it is not necessary to perform a butadiene extraction. This option is probably not very realistic in practice because the isobutene is recycled as well and no methyl tert-butylether production is possible. The butadiene cocrack concentrations were varied from 0 to 50 wt 7%. A large amount of experimental data are not listed in this report. Some summarizing results are shown in Figure 2. In these figures the yields of some key compounds are shown as a function of the butadiene concentration in the feed. The experiments with pure butadiene are also plotted in these figures. This was done for various coil outlet temperatures. Since the product yields seem to be depending linearly on the butadiene concentration, it can be concluded that there are only minor cocracking effects.
Coke Formation during Butadiene Pyrolysis In the literature (Zimmermann et al., 1986) coking models are used in which the coking rate is related to the butadiene concentration in the pyrolysis gas. A possible
mechanism is the cyclization of butadiene with ethylene, propylene, or itself, which can be followed by a dehydrogenation step yielding benzene, polycyclics,and eventually coke. Cocracking experiments with FRN and butadiene in the microreactor showed that butadiene has an increasing effect on the coking rate. For example, 5 w t % of butadiene gave a 25 % higher coking rate. The addition of 50 wt % increased the coking rate, compared to FRN, by 125%. These experiments were performed at 835 "C. In addition, a test was done at 625 "C to check for butadiene fouling in the convection section of the cracker furnace. At this temperature there was no effect of butadiene. This higher coking rate will result in shorter run lengths of the pyrolysis furnaces when cocracking higher (>5% ) butadiene concentrations. If furnace capacity is limiting the ethylene capacity of the plant, one has to take this shorter run length into account in evaluating the economic merits of butadiene cracking. Industrial practice has shown a significant decrease in furnace run length only in cases where butadiene concentrations of more than a few percent are cocracked.
Conclusions Butadiene cocracking can be an interesting outlet for the increasing butadiene overcapacity in steam crackers. Butadiene cracking yields of methane, ethylene, and propylene are low. Due to cyclization reactions high yields of benzene and aromatic pyrolysis gasoline are produced. A cracking value of butadiene of about 0.9 times that of naphtha can be calculated. Due to ita unsaturated character, and possible cyclization reactions, butadiene enhances coke formation in the pyrolysis process, and will therefore reduce furnace run length when cocracking at higher (>5 % ) concentrations. DSMs industrial experience in cocracking butadiene in its naphtha crackers shows good agreement between pilot plant/microreactor and large-scale practice. Literature Cited Froment, G. F. Coke formation in the thermal cracking of hydrocarbons. Rev. Chem. Eng. 1990,6(4), 293-328. Zimmerman, G.; et al. Znt. J. Chem. Kinet. 1986,18, 159. Received for review February 16, 1993 Revised manuscript received September 13, 1993 Accepted October 26, 1993.
* Abstract published in Advance ACS Abstracts, December 1, 1993.
