Isobutane product compositions

Now suppose the refinery crude unit that contributes feed to the butane splitter suddenly increases the propane content of its butane product. Assume this change raises the propane content in the splitter s feed by 20%. If the tower top temperature is maintained at 140°F, the isobutane product composition would be 13% propane, 66% isobutane, and 21% normal butane. 

The overall reaction is highly exothermic. Depending on the product composition, 82-93 kJ/mol of reacted isobutane are liberated (23). 

Table 5.1. Product composition (wt%) in the alkylation of isobutane with C4 alkenes...

More information is available about orientation, when a second alkyl group is introduced into the aromatic ring, and about relative rates. As might be expected, propene reacts more easily than ethylene [342,346] and isobutene more easily than propene [342]. Normal butenes are sometimes isomerised in the process practically the same product composition, consisting mainly of 2,2,4-trimethylpentane, is obtained in the alkylation of isobutane whether the olefin component is isobutene or 2-butene [339]. In the alkylation of aromatic hydrocarbons, this side reaction is negligible. 

In general, distillation columns should be operated at a low pressure. For example, Fig. 3.3 shows an isobutane-normal butane stripper. This fractionator is performing poorly. A computer simulation of the column has been built. The column has 50 actual trays. But in order to force the computer model to match existing operating parameters (reflux rate, product compositions), 10 theoretical separation stages (i.e., 10 trays, each 100 percent efficient) must be used in the model. This means that the trays are developing an actual tray efficiency of only 20 percent. 

Table 2.4 Comparison of product composition obtained during isobutane/2-butene alkylation on H2S04 and a solid silico-aluminate catalyst...

Step 5. The final isobutane product is the distillate from the DIB column, and we want to keep the composition of the nC4 impurity at 2 mol %. Nothing can be done about the propane impurity. Whatever propane is in the fresh feed must leave in the product stream. Because the separation involves two isomers, the temperature profile is flat in the DIB column. Use of an overhead composition analyzer is necessary. 

Even with propylene feed, a high isobutane-to-olefin ratio influences the product toward predominantly Cg hydrocarbons which have the highest octane number and also Improves yields. Thus, both alkylate quality and yield are found to improve with increasing ratio and olefin dilution. In Table IX, detailed propylene-isobutane alkylate composition data are shown, where the volume ratio was increased from 4.6 to 126. For quick reference, composition data are summarized in Table IV. 

Use of Catalysts Containing Transition Metal Cations. Ethyl -ene being alkylated over certain zeolite catalysts reacts specifically. Ethylene can not, however, be alkylated with Isobutane In the presence of H2SO., because of the formation of stable ethylsulphates. We examined the Isobutane - ethylene alkylation over crystalline aluminosilicates and found that those catalysts containing RE and/or Ca In combination with transition metal cations were most active. The alkylation has resulted In not hexanes as would be expected, but an alkylate containing octane Isomers as the major product (about 80%). Moreover, the product composition was similar to that obtained from n-butene over CaREY. The TMP-to-DMH ratios were 7.8 and 7.1 respectively. 

Isobutane and 1-butene are close boilers and, although they do not form an azeotrope, are difficult to separate by conventional distillation. Using a single stage, a feed stream containing 40% mole isobutane and 60% mole 1-butene is flashed at 520 kPa so that 50% of this stream is vaporized. With no solvent added, the vapor and liquid product compositions are about the same. The ratio of 1-butene to isobutane in the liquid product is about 1.6 (Figure 2.10)... 

The flow rate of the overhead or bottoms product determines roughly which components go mostly in the overhead and which ones in the bottoms. This also defines the key components where the separation takes place. In this example, an overhead rate of 50 kmol/h would include most of the methane, ethane, propane, isobutane, and n-butane. The bottoms product would include most of the hexane, n-pentane, and isopentane. The n-butane is, therefore, considered the light key component and the isopentane, the heavy key component. The product compositions at a reflux ratio of 1.0 are given in Table 7.3. 

The economic data concerning the production of isobutane by vapor phase n-butane isomerization, with and without recycling of unconverted n-butane, and concerning the production of isobutene by dehydrogenation, by the three main current industrial processes, are given in Table 6.5 a. The corresponding feed and product compositions are given in Table 6.5 b. 

This analysis is consistent with the results of Medley and Cooley [33] on the effect of pressure on product composition in the oxidation of isobutane. 

The reaction is often initiated by photolysis of bromine. The hydrogen-abstraction step is rate-limiting, and the product composition is governed by the selectivity of the hydrogen abstraction. The enthalpy requirement for abstraction of hydrogen from methane, ethane (primary), propane (secondary), and isobutane (tertiary), by bromine atoms are -1-16.5, -1-10.5, -1-7.0, and -1-3.5 kcal/mol, respectively. These differences are reflected in the activation energies, and there is a substantial kinetic preference for hydrogen abstraction in the order tertiary > secondary > primary. Structural features that promote radical stability by delocalization, such as phenyl, vinyl, or carbonyl substituents, also lead to kinetic selectivity in radical brominations. 

Trotman-Dickenson reported an activation energy of 6.7 kcal/mol for the reaction of phenyl radicals with isobutane in the gas pha.se. ( ) On the basis of this, we were confident that conformational interconversion (ki,k i) would be rapid with respect to other reactions and that the product composition would be determined by the ratio kj [SH]k-i/kj ki. Recent experimental work by J. P. Lorand,( ) however, shows that the rate of reaction of phenyl radicals with the tertiary hydrogens of alkanes is about 10 times faster than that reported by Trotman-Dickenson. A competition between kR[SH] and ki is, therefore, plausible. 

Products Composition of Oxidation Products, mol % with Respect to Introduced Isobutane ... 

Table I gives the compositions of alkylates produced with various acidic catalysts. The product distribution is similar for a variety of acidic catalysts, both solid and liquid, and over a wide range of process conditions. Typically, alkylate is a mixture of methyl-branched alkanes with a high content of isooctanes. Almost all the compounds have tertiary carbon atoms only very few have quaternary carbon atoms or are non-branched. Alkylate contains not only the primary products, trimethylpentanes, but also dimethylhexanes, sometimes methylheptanes, and a considerable amount of isopentane, isohexanes, isoheptanes and hydrocarbons with nine or more carbon atoms. The complexity of the product illustrates that no simple and straightforward single-step mechanism is operative rather, the reaction involves a set of parallel and consecutive reaction steps, with the importance of the individual steps differing markedly from one catalyst to another. To arrive at this complex product distribution from two simple molecules such as isobutane and butene, reaction steps such as isomerization, oligomerization, (3-scission, and hydride transfer have to be involved.

An interesting variation on sulfated metal oxide type catalysts was presented by Sun et al. (198), who impregnated a dealuminated zeolite BEA with titanium and iron salts and subsequently sulfated the material. The samples exhibited a better time-on-stream behavior in the isobutane/1-butene alkylation (the reaction temperature was not given) than H-BEA and a mixture of sulfated zirconia and H-BEA. The product distribution was also better for the sulfated metal oxide-impregnated BEA samples. These results were explained by the higher concentration of strong Brpnsted acid sites of the composite materials than in H-BEA. 

All Keggin-type POMs exhibit an initial unsteady catalytic behavior, which can last from a few hours up to 100 hours, depending on the composition of the POM and on the method employed for its preparation. The progressive variation of catalytic performance occurring during this equilibration period is shown in Figure 14.5, where the conversion of isobutane and the selectiviy to the products are plotted as functions of the reaction time. The catalyst was a... 

Yields are very difficult to determine commercially when charging propylenes and amylenes, partially because they are always charged in conjunction with butylenes and partially because the composition of the feed stock to commercial units varies almost continually. The greatest difficulty is the determination of the quantity of propane and pentanes produced from the equivalent olefin by the hydrogen transfer reaction. This is also true for the production of normal butane when charging butylenes. Another difficult determination is the isobutane consumed in the reaction because of variations in the quantities of isobutane in the outside isobutane feed stream, the butane-butylene feed stream, and the normal butane-isobutane stream leaving the unit. Also, the quantities of isobutane con-... 

A sidelight to the reaction mechanism of splitting catalysts at different reaction temperatures is furnished by the composition of the gaseous hydrocarbons obtained as by-products. The butane content of the total hydrocarbon off-gas and the isobutane content of the butane fraction are... 

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