Abstract

Butylenes are C₄H₈ mono-olefin isomers: 1-butene, cis-2-butene, trans-2-butene, and isobutylene (2-methylpropene). These isomers are usually coproduced as a mixture and are commonly referred to as the C-4 fraction. These C-4 fractions are usually obtained as by-products from petroleum refinery and petrochemical complexes that crack petroleum fractions and natural gas liquids. Since the C-4 fractions almost always contain butanes, it is also known as the B–B stream. The linear isomers are referred to as butenes. The carbon–carbon double bond is the distinguishing feature of the butylenes and, as such, controls their chemistry. The carbon–carbon bond, acting as a substitute, affects the reactivity of the carbon atoms at the alpha positions through the formation of the allylic resonance structure. This structure can stabilize both positive and negative charges. Reactions of butylenes can be divided into two broad categories: those that take place at the double bond itself, destroying the double bond; and those that take place at alpha carbons. The C-4 isomers are almost always produced commercially as by-products in a petroleum refiner–petrochemical process. There are other commercial processes available for the production of butylenes. However, these are site- or manufacturer-specific. Any of these processes can become an important source in the future. Several technologies are emerging for the production of isobutylene to meet the expected demand for isobutylene: deep catalytic cracking; superflex catalytic cracking; dehydrogenation of butanes; and the Coastal process of thermal dehydrogenation of butanes. There are four types of separation methods available: selective removal of isobutylene by polymerization and separation of 1-butene; use of addition reactions with alcohol, acids, or water to selectively produce pure isobutylene and 1-butene; selective extraction of isobutylene with a liquid solvent, usually an acid; and physical separation of isobutylene from 1-butene by absorbents. There are three important processes for the production of isobutylene; the extraction process using an acid to separate isobutylene; the dehydration of tert-butyl alcohol, formed in the Oxirane process; and the cracking of MTBE. Handling requirements are similar to liquefied petroleum gas (LPG). Butylenes are not toxic. The effect of long-term exposure is not known; hence, they should be handled with care. They are volatile and asphyxiants. Care should be taken to avoid spills because they are extremely flammable.