Save It For Later - Catalyst Recycling Gains as Refiners Focus on Profitability, Environmental Performance

Refinery distillation units separate crude oil into light, medium and heavy fractions. After that, refiners start performing chemical reactions using catalysts — materials that accelerate chemical reactions — to change the oil’s natural molecules into the forms needed in modern fuels. In recent years, refiners have stepped up their efforts to recycle those catalysts to improve their profitability and environmental performance. In today’s RBN blog, we explain how catalysts, which were formerly disposed of as hazardous waste, are increasingly being recycled and reused in refineries. 

To produce the fuels they need, refineries employ catalytic cracking, which cracks heavier molecules into lighter gasoline molecules, increasing the volume of gasoline produced from a barrel of crude, and catalytic hydrogenation, which removes sulfur atoms embedded in the molecules, thereby reducing pollution when fuel is combusted. These reactions do not occur at measurable rates except in the presence of a catalyst.

To understand the importance of a catalyst, let’s start with a look at a sulfur compound commonly found in gasoline before it is desulfurized, known as 2,4-diethylthiophene. It consists of a carbon-ring structure containing a single sulfur atom with two attached hydrocarbon side chains containing two carbon atoms each (see Figure 1 below). The sulfur atom (S) is a problem because when this molecule is burned in engines, the sulfur atom ends up as part of sulfur oxide, which impairs the performance of a vehicle’s catalytic converter and contributes to air pollution.

Tighter regulations forced the virtually complete removal of sulfur from 21st century gasoline. Today, this molecule is desulfurized at the refinery in a hydrogenation reactor, where it reacts with hydrogen to form a sulfur-free gasoline molecule that goes into the gasoline pool. The sulfur atom instead ends up in the byproduct hydrogen sulfide (H2S), which goes to another unit at the refinery where it is converted into elemental sulfur. That desulfurization reaction, and ones like it, are occurring continuously in hydrogenation reactors in every modern refinery today and are the single biggest reason modern fuels cause less pollution.