How does the presence of olefins in the feed stream affect the performance of an isomerization unit?

The presence of olefins in the feed stream can negatively affect the performance of an isomerization unit. Isomerization is a process that converts straight-chain paraffins (n-paraffins) into branched-chain paraffins (isoparaffins) to improve the octane number of gasoline. An isomerization unit typically uses a catalyst, often containing a metal like platinum supported on an acidic support, to facilitate this conversion. Olefins are unsaturated hydrocarbons containing one or more carbon-carbon double bonds. They are more reactive than paraffins and can undergo several undesirable reactions in the isomerization unit, leading to catalyst deactivation and reduced product quality. One major issue is that olefins can polymerize or oligomerize on the catalyst surface, forming heavier compounds and coke. Coke is a carbonaceous deposit that blocks the active sites on the catalyst, reducing its activity and selectivity. This coking process leads to a decline in the isomerization activity, requiring more frequent catalyst regeneration or replacement. Olefins can also undergo hydrogenation reactions, saturating the double bonds and converting them into paraffins. While this might seem beneficial, it consumes hydrogen and can shift the equilibrium away from isomerization, reducing the yield of isoparaffins. The heat released during hydrogenation can also lead to temperature control issues in the reactor. Furthermore, olefins can react with acidic sites on the catalyst, forming strongly adsorbed species that block the active sites and reduce the catalyst's effectiveness. These adsorbed species can also promote other side reactions, such as cracking and aromatization, leading to the formation of unwanted byproducts. To minimize the negative impact of olefins, the feed stream to an isomerization unit is typically pretreated to remove olefins. This can be achieved through processes like hydrotreating, where the olefins are reacted with hydrogen over a catalyst to convert them into paraffins. The effectiveness of the pretreatment step is critical to the performance and longevity of the isomerization catalyst. For example, if an isomerization unit is processing a feed stream with a high olefin content without adequate pretreatment, the catalyst will quickly deactivate due to coking, leading to a decline in octane number and increased operating costs. Therefore, minimizing the presence of olefins in the feed stream is essential for maintaining high activity, selectivity, and stability in an isomerization unit.