Isobutene boiling point

The industrial reactions involving cis- and trans-2-butene are the same and produce the same products. There are also addition reactions where both 1-butene and 2-butene give the same product. For this reason, it is economically feasible to isomerize 1-butene to 2-butene (cis and trans) and then separate the mixture. The isomerization reaction yields two streams, one of 2-butene and the other of isobutene, which are separated by fractional distillation, each with a purity of 80-90%. Table 2-3 shows the boiling points of the different butene isomers. 

Butenes or butylenes are hydrocarbon alkenes that exist as four different isomers. Each isomer is a flammable gas at normal room temperature and one atmosphere pressure, but their boiling points indicate that butenes can be condensed at low ambient temperatures and/or increase pressure similar to propane and butane. The 2 designation in the names indicates the position of the double bond. The cis and trans labels indicate geometric isomerism. Geometric isomers are molecules that have similar atoms and bonds but different spatial arrangement of atoms. The structures indicate that three of the butenes are normal butenes, n-butenes, but that methylpropene is branched. Methylpropene is also called isobutene or isobutylene. Isobutenes are more reactive than n-butenes, and reaction mechanisms involving isobutenes differ from those of normal butenes. 

Most butenes are produced in the cracking process in refineries along with other C-4 fractions such as the butanes. Butenes are separated from other compounds and each other by several methods. Isobutene is separated from normal butanes by absorption in a sulfuric acid solution. Normal butenes can be separated from butanes by fractionation. The close boiling points of butanes and butenes make straight fractional distillation an inadequate separation... 

Triisobutylaluminum is a colourless transparent liquid (the boiling point is 138 °C at 6.6 GPa), which dissolves well in hydrocarbons. When heated up to 140-160 °C in vacuum (the residual pressure is 35 GPa), triisobutylaluminum disintegrates into isobutene and diisobutylaluminum hydride. Unlike lower trialkyl derivatives of aluminum with the direct carbon chain (e.g. triethylaluminum), aluminumtrialkyls with a branched chain (triisobutylaluminum is one of them) are monomeric. 

The main use of MTBE is as an octane booster in gasoline formulations. Table 2.1 (above) compares octane number and boiling points of some tertiary ethers and hydrocarbons. The volatility is another important property of gasoline components. In fact, the lower volatility of ETBE is an advantage with respect to MTBE. Another (smaller scale) application of MTBE is the synthesis of high purity isobutene by cracking MTBE over amorphous silica-alumina. This isobutene serves as a monomer for polyisobutene. 

Stagewise condensation is one method obvious due to the large difference in boiling points. A drawback is the high cost of electricity. Also, countercurrent condensation-vaporisation could be performed for higher ptuity of isobutene. 

After the isobutene is consumed by the reaction, the resulting mixture of n-butene, methanol, and MTBE has vapor-liquid equilibrium properties that are nonideal with two binary azeotropes. At a pressure of 11 bar, which is the operating pressure of the column, the pure component boiling points are 76.1, 140.6, and 152.8 °C, respectively. 

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