Butanes isomerization equilibria

To illustrate the value of the method we will again calculate the equilibrium constant for the butane isomerization reaction at 800 K using the free-energy function data in Table 5.4. 

Most feeds contain some olefin as an impurity moreover many sulfated zirconia catalysts contain traces of iron or other transition metal ions that are able to dehydrogenate hutane. In the presence of such sites, the olefin concentration is limited by thermodynamics, i.e a high pressure of H2 leads to a low olefin concentration. That aspect of the reaction mechanism has been proven in independent experiments. The isomerization rate over sulfated zirconia was dramatically lowered by H2. This effect is most pronounced when a small amount of platinum is deposited on the catalyst, so that thermodynamic equilibrium between butane, hydrogen and butene was established. In this way it was found that the isomerization reaction has a reaction order of +1.3 in -butane, hut -1.2 in hydrogen [40, 41]. The byproducts, propane and pentane, are additional evidence that a Cg intermediate is formed in this process. As expected, this kinetics is typical for butane isomerization only in contrast pentane isomerization is mainly a monomolecular process, because for this molecule the protonated cyclopropane ring can be opened without forming a primary carbenium ion [42]. 

For iso-butane, two calorimetric studies. Prosen et. al. [25] and Pittam and Pilcher [24] and two isomerization equilibrium results, Scott [32], and Pines [23], are in moderate agreement. The four values have been averaged. Rossini s value, reported in 1935 [30], is not in agreement and has been discarded. 

In units that isomerize n-pentane and n-hexane, the reactions are catalyzed either by Pt/alumina or Pt on zeolite. The zeolite catalysts require higher temperatures, but they are less sensitive to water. As with butane isomerization, the reactions are controlled by equilibrium, so lower reaction temperatures favor branched isomers. The high temperatures required by zeolite catalysts reduce the octane of the product relative to products made at lower temperatures with chlorided alumina catalysts. 

Example 4.3 In elementary chemistry we learn the law of mass action, which says that for a reaction like the iso-butane-normal-butane isomerization the concentrations at equilibrium can be represented by an equation of the form... 

The isomerization of butane to iso butane in superacids is illustrative of a protolytic isomerization, where no intermediate olefins are present in equilibrium with carbocations. 

Equilibria. The equilibrium distributions of butane, pentane, and hexane isomers have been experimentally determined (5, 16) and are diagrammed in Figure 2. In each case, lower temperatures favor the more highly branched structures. At the approximately 200° F. temperature usually employed for isomerization, the butane equilibrium mixture contains about 75% isobutane. That for pentane contains about 85% isopentane.. In the case of hexane, the equilibrium product contains about 50% neohexane and has a Motor octane rating of about 82. In all cases, of course, the yield of the desired isomers can be increased by fractionation and recycle. 

The first precise evaluation of the anomeric effect was realized by Descotes and co-workers in 1968 (22). These authors have studied the acid catalyzed isomerization of the cis and trans bicyclic acetals 6 and 6 and found that, at equilibrium, the mixture contains 57% ci s and 43% trans at 80°C. The cis isomer is therefore more stable than the trans by 0.17 kcal/mol. The cis isomer 5 has one (stabilizing) anomeric effect whereas the trans isomer 6 has none. Steric interactions in cis acetal 5 were estimated tobel.65 kcal/mol (one gauche form of ri-butane, 0.85 kcal/mol and an OR group axial to cyclohexane, 0.8 kcal/mol). By subtracting an entropy factor (0.42 kcal/ mol at 80°C) caused by the fact that the cis acetal S exists as a mixture of two conformations (cis decalin system), they arrived at a value of 1.4 kcal/mol for the anomeric effect. 

The following facts are the basis for butene isomerization (I) There is a basic similarity in the composition of alkylates produced from all four butene isomers. (2) Alkylate molecules, once formed, are relatively stable under alkylation conditions and do not isomerize to any appreciable extent alkylate fractions having the same carbon number ore not equilibrated (see Table I). (3) Thermodynamic equilibrium between the butene olefins highly favors isobutene formation at alkylation temperatures. (4) Normal butenes p>roduce only small and variable amounts of normal butane, thus indicating only a small and variable amount of chain initiation from normal butenes. Yet the alkylate composition shows a high concentration of trimethylpentanes and a low concentration of dimethylhexanes. (5) A few of the octane isomers can be explai.ned only by isomerization of the eight-carbon skeletal structure this isomerization occurs while isobutene dimer is in ionic form. For example, 2,3,3- and 2,3,4-trimethylpentanes... 

Besides the limiting of isomerization by equilibria, the rate at which equilibrium is approached is also important. Butanes and pentanes... 

Example 11-7 The rate of isomerization of o-butane with a silica-alumina catalyst is measured at 5 atm and 50°C in a laboratory reactor with high turbulence in the gas phase surrounding the catalyst pellets. Turbulence ensures that external-diffusion resistances are negligible, and so Q = Q. Kinetic studies indicate that the rate is first order and reversible. At 50°C the equilibrium conversion is 85%. The effective diffusivity is 0.08 cm /sec at reaction conditions, and the density of the catalyst pellets is 1.0 g/cm, regardless of size. The measured, global rates when pure n-butane surrounds the pellets are as follows ... 

The reaction order in buta-1,3-diene is close to zero, indicating that the fraction of vacant sites is very low, and at the total consumption of buta-1,3-diene the mole fractions of butenes are not equal to zero. The assumption of equilibrium adsorption of the intermediate compound (but-l-ene) in the case of irreversible butadiene hydrogenation and but-l-ene isomerization and hydrogenation cannot explain the latter observation. Therefore, adsorption/desorption steps for buta-1,3-diene but-l-ene, but-2-ene are thought to be reversible and have an "adsorption-assisted desorption " nature. The desorption of butane step 15 is assumed to be irreversible and fast. For conformational isomerization (step 2) a quasi-equilibrium approximation will be used. 

Little has been published about the commercial results obtainable by the use of dual-function catalysts. However, it can be stated that, when either n-butane or n-pentane is charged, isomerization proceeds to thermodynamic equilibrium, with selectivities in the range of 90-95%. 

Recent experiments (Beeck el al., 25a) on the isomerization of propane-1-C to propane-2-C further demonstrate that the isomerization proceeds through an intramolecular rearrangement. No propane containing more than one C carbon atom per molecule was foimd. These experiments were made by contacting vapors of propane at approximately 25 and 450 mm. pressure with a catalyst prepared by the addition of 0.023 g. water to 0.40 g. anhydrous aluminum bromide for periods of time varying from 0 to 1074 hours. It was found that the rate of isomerization of propane-1-C to propane-2-C is comparable to the rate of isomerization of n-butane to isobutane under similar conditions. The equilibrium distribution of the propanes-l-C and -2-C was found to be statistical, that is, propane-l-OVpropane-2-C = 2. 

An overview of theoretical studies of the equilibrium properties for this system has been given by Jorgensen A In the following section, we shall discuss the first simulation of this reaction in a full solvent, a simulation that first used the rare event computational technology in a solution system. A model system involving a similar type of cis-trans isomerization has been studied by Robinson and co-workers °> however, we shall concentrate here on simulations involving more realistic representations of the butane molecule. 

D. Brown and J. H. R. Clarke, /. Chem. Phys., 92, 3062 (1990). A Direct Method of Studying Reaction Rates by Equilibrium Molecular Dynamics Application to the Kinetics of Isomerization in Liquid -Butane. 

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