thermal cracking of ethylene, propylene and light hydrocarbon

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The selectivity of a component I in the cracking of a mixture A-B is defined as : mol9 of I from A + mols of I from B YI = mols of A cracked + mois of B cracked When there is no interaction, Froment et (7) showed that equation 1 is identical with : This predicted value of the selectivity will be called the non- interaction selectivity. * The pure additivity selectivity : yI = yI,*YA + yI,Bph 3), used in the literature so far, is obviously a very special case of 2) valid only for xA,jJ = x ~ , ~ ~ . It is clear from Fig.rce 12 that 2) allows a far better prediction of the experimental results than 3). Analogous figures were plotted for different conversions and for all binary mixtures mentioned in the experimental program. From all these curves, the following rule can be deduced : "The experimental selectivities deviate from the pure additivity lines in the same direction as the non-interaction selectivitiesll. This means that the effect of interaction can be predicted when the selectivities from the individual Conponents and the relative rates of cracking are Icno~m. The selectivities for cracking of ternary mixtures may be represented in diagrams of the type shown in figure 13,. In this figure the ethylene selectivity at conversions ~~,]\,~=40 F, $ ; x ,-88 $ is plotted with respect to the ternary feed composition. i'ttfi-ternary mixtures the pure additivity, non interaction and experimental lines of Pig. 12 become surfaces. Here too the experimental selectivities deviate from the pure additivity surface in the same sense as the non-interaction surface. 4. Kinetics The kinetics of the cracking of a component X, are derived from the experiments by means of its continuity equation. For ethane in a binary mixture ethane-propylene e.g., cracked in a tubular reactor with plug flow conditions, the continuity equation may be written : n E FE,OdxE,p = A e d - m ) . ( C E , p ) dV 4) E =expansion= (total molar flow rate)exj-t-(mols G 2II 6 +molsC mols C kI cracked + iiiols C3F16 cracked 2 6 6 = molar ratio propylene/etilylene = dilation factor : mols 112O/mol hydrocarbon In 4) the experimental temperature and total preswwe profile are also accounted for. Tlie kinetics can now be derived froin 4) as such or by inaking use of the equivalent reactoz volume concept. 137
* The-es_ulval,en4_reac4or_volwne_conceEt In this approach, the non-isothermal, non-isobaric data are first reduced to isothermality and constant pressure. This leads to an e uivalent reactorvolume V- rather than the physical volume V in 47,while T(z) is replaced gy a reference temperature and pt(Z) by a reference pressure (6,7,8). The kinetic parameters of the thermal cracking of individual alkanes were determined by means of this concept. Figure 14 shows the Arrhenius plot of the first order rate constants for ethane, propane, n.butane and i.butane. The concept was also used to calculate the kinetic coefficients of the cracking of the individual components in the mixture, assuming first order. The integration of 4) then requires a relation xp E'f(XE p). From the experimental data, this relation was found in'all caees to be parabolic, so that equation 4 could be integra- ted analytically. Figure 15 shows the value of kE p versus the feed composition. The inhibiting effect of propylkne on the ethane rate coefficient is very pronounced. The same procedure was followed to calculate the individual rate coefficients for all the components of the different feed mixtures. Table 3 summarizes the relative influences, caused by cocrackinr. TABLE 3 Effect of the addition of various components on the rate coefficient for the cracking of ethane, propane, n.butane, i.butane and propylene. I I Effect on rate coefficient Bddi t ion I Of C& ', C3HB nC4H1 0 iC4H,0 C3H6 I kC2H6 kC3H8 I knC4H10 1 kiC4H10 1 kC3H6 - r r no data .r 4 - .1 no data f .1 .1 .I no data no data no data f - no data J no data no data - 1 The individual rate coefficients in the ternary mixture ethane- propane-n.butane were calculated in an analogous way. Figure 16 shows the effect on the rate coefficient for ethane cracking of the addition of one or two components in various ratios. Analogous diagrams could be shown for kp Ti and kN M. All three of them clearly illustrate the effect bn the rate coefficients of interaction between reaction partners. The equivalent reactorvolume concept requires an initial guess of the activation energy, valid over the whole range of investigated temperatures. This is no problem with alkanes but, as will be discussed in the next section, the ethylene and propylene cracking experiments cover a transition zone in which polymerization processes with activation energies of 2 35 kcal/mol and decomposition processes with activation energies of 2 70 kcal/mol simultaneously occur. In such cases it is preferable to resort 138 N
Page 1 and 2: Thermal cracking of ethylene, propy
Page 3: TABLS 1 Influence of the partial pr
Page 7 and 8: 5. Decomposition and pol.wnerizatio
Page 9 and 10: From the mechanistic point of view
Page 11 and 12: 7) Froment G.F., Van de Steene B.O.
Page 13 and 14: .l 2 I u 1 CLASS 1 0 CLASS 2 A CLAS
Page 15 and 16: N u . I? Y - CLASS 1 0 CLASS 2 A CL
Page 17 and 18: CLASS 1 CLASS 2 CLASS 3 CLASS L CLA
Page 19 and 20: CLA5S 1 0 CLASS 2 A I,LA-\sS 3 + CL
Page 21 and 22: CLASS 1 0 CLASS 2 a CLASS 3 + CLASS
Page 23 and 24: SE 1 .a .6 .4 .2 a ECTl V IT Y Figu
Page 25 and 26: -Nb 3 -1 2 1 0 -2 ( Figure 14. I I
Page 27 and 28: I Figure 16. TERNARY MIXTURES N.BUT
Page 29: 2 , 2 D ECOM PO S I T ION ZONE .. .

thermal cracking of ethylene, propylene and light hydrocarbon

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