Pyrolysis propane

Reduced elementary reaction model of the propane pyrolysis... 

Chemical vapor deposition (CVD) of carbon from propane is the main reaction in the fabrication of the C/C composites [1,2] and the C-SiC functionally graded material [3,4,5]. The carbon deposition rate from propane is high compared with those from other aliphatic hydrocarbons [4]. Propane is rapidly decomposed in the gas phase and various hydrocarbons are formed independently of the film growth in the CVD reactor. The propane concentration distribution is determined by the gas-phase kinetics. The gas-phase reaction model, in addition to the film growth reaction model, is required for the numerical simulation of the CVD reactor for designing and controlling purposes. Therefore, a compact gas-phase reaction model is preferred. The authors proposed the procedure to reduce an elementary reaction model consisting of hundreds of reactions to a compact model objectively [6]. In this study, the procedure is applied to propane pyrolysis for carbon CVD and a compact gas-phase reaction model is built by the proposed procedure and the kinetic parameters are determined from the experimental results. 

Experiments of propane pyrolysis were carried out using a thin tubular CVD reactor as shown in Fig. 1 [4]. The inner diameter and heating length of the tube were 4.8 mm and 30 cm, respectively. Temperature was around 1000°C. Propane pressure was 0.1-6.7 kPa. Total pressure was 6.7 kPa. Helium was used as carrier gas. The product gas was analyzed by gas chromatography and the carbon deposition rate was calculated from the film thickness measured by electron microscopy. The effects of the residence time and the temperature... 

Fig. 3. Reduced elementary reaction model of the propane pyrolysis. (1010°C, r= 20 ms)...

Fig. 5. Effects of residence time on the gas composition fiom propane pyrolysis.

By reducing an elementary reaction model taken fi om the database, a comprehensive gas-phase reaction model of propane pyrolysis was derived objectively. The reaction rate constants that were not accurate under the conditions of interest were found and refined by fitting with the experimental results. The obtained reaction model well represented the effects of the gas residence time and temperature on the product gas composition observed in experiments under pyrocarbon CVD conditions. 

Product distribution for propane pyrolysis. [From Schutt, Chemical Engineering Progress, 50 (415), 1954. Used with permission.]... 

Equation (48) e ees with experimental results in some circumstances. This does not mean the mechanism is necessarily correct. Other mechanisms may be compatible with the experimental data and this mechanism may not be compatible with experiment if the physical conditions (temperature and pressure etc.) are changed. Edelson and Allara [15] discuss this point with reference to the application of the steady state approximation to propane pyrolysis. It must be remembered that a laboratory study is often confined to a narrow range of conditions, whereas an industrial reactor often has to accommodate large changes in concentrations, temperature and pressure. Thus, a successful kinetic model must allow for these conditions even if the chemistry it portrays is not strictly correct. One major problem with any kinetic model, whatever its degree of reality, is the evaluation of the rate cofficients (or model parameters). This requires careful numerical analysis of experimental data it is particularly important to identify those parameters to which the model predictions are most sensitive. 

Molecular reaction schemes have a long history of use in the design of pyrolysis coils. Since the pioneer works of Myers and Watson [46] and Schutt [47] on propane pyrolysis, improved by Snow and Schutt [48], molecular reaction schemes have been applied to the modelling of the pyrolysis of light and heavy hydrocarbons [38, 49—56]. Froment and co-workers have extensively promoted molecular reaction schemes in a series of papers [57—61] a brief account can be found in a book by Froment and Bischoff [25]. 

C. A. McDowell and J. B. Farmer, Fifth Symposium (International) on Combustion, p. 453, op. ciL, have shown the formation of peracetic acid as the principal initial product in the photosensitized and thermal oxidation of acetaldehyde. (See also earlier papers of McDowell and Farmer.) J. Grumcr, ibid, p. 447, also showed that, at low O2 content, C2H4, C He, CO, CH4, CH,OH, CHsCHO, and CH,CH2CHO were important products from propane pyrolysis in the range 350 to 475 C. He also found considerable amounts of acetic acid from the oxidation of CHaCHO in mixtures at 130 to 450 C having about 3 per cent O2. Such I0W-O2 mixtures are, of course, ideal for observing sensitized pyrolysis reactions. 

T he literature on pyrolysis of paraffin hydrocarbons is extensive. A recent review (1) of propane pyrolysis lists 103 references covering a period from 1928 to 1976. The study indicates a remarkable lack of quantitative agreement on energies of activation and on the effect of product inhibition on rate of decomposition. Energies of activation, from reputable experimental efforts, cover a range from 40 to 80 kcal/mol. 

In the annular reactors, propane pyrolysis was studied at atmospheric pressure and at 775°, 800°, 825°, and 850°C, with reaction times of 0.005-0.113 sec and conversions from 10 to 82%. Percent conversion as a function of time and temperature is shown in Table I. 

Figure 7. Propane pyrolysis in Type II reactor. Fraction unconverted corrected to plug flow vs. value calculated by Equation 8 (Xp vs. XcaIJ.

Starting with the pioneering work of Myers and Watson (9) on propane pyrolysis, this approach has been successfully applied to virtually all light hydrocarbons (10,11,12,13,14) and extended up to C8 normal and branched paraffins (15,16). Fewer studies have been reported on mixture pyrolysis (17,18,19), especially for heavier feedstocks (20). This type of modeling will be illustrated later with an example. 

Staveley and Hobbs and Hinshelwood studied the inhibition of the reaction by nitric oxide, and found under certain conditions that some 13 % was uninhibitable. Several investigations have shown that this residual reaction is not a molecular reaction. Stevenson et al studied the pyrolysis of propane containing radioactive carbon, and concluded that isotopic mixing took place at the same rate, relative to the pyrolysis rate, when the reaction was completely inhibited as when it was uninhibited Hinshelwood et al obtained a similar result. Poltorak and Voievodsky showed that when propane is pyrolyzed in the presence of D2, D atoms appear in the hydrocarbon fraction at a rate, relative to the rate of decomposition, that is independent of the amount of nitric oxide present. All of these results show that free radicals are still important in the reaction occurring in the presence of nitric oxide, and provide no support for the view that a molecular mechanism plays a significant role in the propane pyrolysis. Evidence reading to the same conclusion is provided by the experiments of Niclause et which show that in certain reaction vessels the propane pyrolysis is completely... 

There does not, at the present time, seem to be much doubt about the main features of the mechanism of propane pyrolysis. Some of the features of the mechanism need, however, to be checked and made more quantitative by careful measurements of rates of formation of the major and minor products. 

Molecular reaction schemes have a long history of use in the design of pyrolysis coils. Since the pioneer works of Myers and Watson [46] and Schutt [47] on propane pyrolysis, improved by Snow and Schutt... 

Table III shows the effect of shifting furnace operation from propane fresh feed to ethane. Data are from Schutt and Zdonik (54). The reduction of propylene yield from ethane to negligible levels in favor of increased ethylene production cannot be done if a plant has propylene commitments. Because propylene requirements cannot be satisfied with ethane feed, Ericsson (14) has concluded that propane will continue to be the preferred feedstock to make ethylene. Actually, 85% of the U.S. ethylene plants are located in the Gulf Coast area so that they can obtain and operate on economical ethane and propane feeds. The need for propane pyrolysis has resulted in a renewal of experimental interest in this area, and in-depth studies have been made by Crynes and Albright (17) and by Buekens and Froment (7).

Figure 9-2. Kinetics of the double-step pyrolysis of hydrocarbons in plasma-chemical jet reactor. Step 1, methane pyrolysis inplasmajet of hydrogen Step 2, propane pyrolysis, injection of propane is delayed with respect to start of the process (1) 0.17 ms, (2) 0.37 ms, (3) 0.73 ms. Propane injection temperature 293 K propane-to-methane flow rate ratio 1 2.

Process characteristics Pyrolysis of methane Pyrolysis of propane Pyrolysis of benzene... 

An attempt was made to calculate the product distributions of propane pyrolysis on the basis of the reaction model considering both inhibition and acceleration effects observed in the pyrolysis of propane-propylene mixtures. The reaction model for propane pyrolysis used in this work is shown in Table 4. The rate constants given in Table 4 were measured in our previous works (3, 4). At the initial stage of propane pyrolysis, the formation of the primary products such as methane, ethylene and propylene is predominant but as the reaction proceeds, the consecutive decomposition of each product is also remarkable. Therefore, a reaction model was postulated which consisted of major stoichiometric reactions for propane (i) - (iv), for propylene (v), (vi) and... 

Figure 5. Comparison between experimental product distributions and calculated values for propane pyrolysis, Tmaa = 900°C, (CcsHgh = 10%...

Crynes, B. L., Ph.D. Thesis, "Surface Effects During Propane Pyrolysis in Tubular Flow Reactors," Purdue University (1968). 

Harriott, G., Eckert, R., and Albright, L., "Kinetics of Propane Pyrolysis," 68th National Meeting AIChE, Houston, Texas (February, 1971). 

The model shown in Table II also represents reasonably well, propane pyrolysis data, as will be discussed in the next chapter of this book (9). 

Product Composition and Carbon Yield for Propane Pyrolysis in Different Reactors (800 C, 502 steam in feed)... 

Both metal reactors were treated with H-S for 15 minutes and then used for several hours of pyrolysis of wet ethane before being used for propane pyrolysis. 

Surface reactions are thought to be relatively unimportant when propane is pyrolyzed in a Vycor glass reactor. This conclusion is based on two factors. First, surface reactions in the Vycor reactor were of fairly minor importance for ethane pyrolysis (1) and are considered to be even less significant for propane pyrolysis. Secondly, as was shown in both table I and Figure 1 and as will be described later in this paper, there was good agreement between the experimental results for the Vycor reactor and the predictions of the mechanistic model described earlier (1). 

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