Metathesis is a general term for a reversible reaction between two olefins, in which the double bonds are broken and then reformed to form new olefin products. In order to produce propylene by metathesis, a molecule of 2-butene and a molecule of ethylene are combined to form two molecules of propylene.
The process described in this article is similar to the following licensed technologies:
- Olefin Conversion Technology (licensed by Lummus Technology)
By the 1960s, Phillips Petroleum developed the first commercial process of olefin metathesis. The focus, at that time, was to convert propylene into ethylene and 2-butene. This technology was developed in an effort to increase ethylene and butene production from “low value” cracker-derived propylene to meet the growing market demand for polyethylene and polybutadiene.
The fact that metathesis is a reversible reaction, and that the demand for polymer grade (PG) propylene grew from the 1970s on, led to the use of the Phillips Triolefin process in a reverse way. This reverse process is known as Olefin Conversion Technology (OCT), and is now offered for license by Lummus Technology, a CB&I Company.
The raw materials for the production of propylene via metathesis reaction are ethylene and 2-butenes. Both components are mainly supplied from steam cracker units (olefins plants) or fluid catalytic cracking (FCC) units.
Steam cracker units are facilities in which a feedstock such as naphtha, liquefied petroleum gas (LPG), ethane, propane or butane is thermally cracked through the use of steam in a bank of pyrolysis furnaces to produce lighter hydrocarbons. The products obtained depend on the composition of the feed, the hydrocarbon-to-steam ratio, and on the cracking temperature and furnace residence time.
After the pyrolysis process, the olefins are separated from the other by-products by distillation.
In some regions, the supply of ethylene is tight and/or ethylene is expensive, making the building of a conventional metathesis unit unfeasible without subsidies.
There are disadvantages of conventional metathesis which are: intensive use of energy (using cryogenic refrigeration for ethylene recirculation loop) and feedstock loss (removing butadiene by hydrogenation from the butenes feed represents a feedstock loss of 10%+).
Although the yield of propylene is high in the conventional metathesis process, the aforementioned disadvantages motivated the development of a different process, in which a metathesis reaction occurs with butenes as the only feedstock. This process is called butenes auto-metathesis, or self-metathesis.
In the process, a stream comprised of 1-butene plus 2-butene is admixed with recycled butenes and pentenes in the metathesis reactor. The stream leaving the reactor is sent to a separation unit, composed of distillation columns. The stream can contain C4 paraffins, but the amount of isobutene should not exceed 2% of the feed mixture.
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