What's a Process Unit?

If a refinery is compared to the human body, then process units are the organs. They each have a purpose. They have inputs and outputs and most have chemical reactions.

In a refinery, a process unit is a group of equipment such as pumps, vessels, and heat exchangers that are interconnected and have a common purpose. The purpose is typically to re-arrange molecules, separate them, or both.

The diagram below is called a “block flow diagram” or BFD. It is used extensively in the chemical process industries to provide a high-level overview. I made this BFD to show a generic process unit in a refinery. It is very typical to have three sections:

1) Feed Treatment (also called feed preparation)

2) Reaction

3) Separation

• Inputs: The materials entering the unit. This includes fuels such as natural gas.

• Feed Treatment: Some process units require impurities to be removed from feed to protect reactor catalyst or prevent corrosion.

• Reaction: Molecules are rearranged. There is a desired product, and there could be undesired products. Engineers design and then monitor the process to maximize the desired product.

• Reactor Effluent: Sometimes called just “effluent”. This is the process stream immediately after it leaves the reactor (prior to separation).

• Separation: The desired product needs to be purified, which means it must be separated from the other streams that make up the reactor effluent.

• Products: Separated reactor effluent streams. Process units have targets for product purity and acceptable amounts of contaminants in each product.

Now, let’s apply these definitions to an actual refinery process unit - the Butamer. This process unit (licensed by UOP) is designed to isomerize n-butane to isobutane (hence the name, butane isomerization…butamer). The chemical reaction is aided by a platinum catalyst. The catalyst is poisoned (deactivated) by water. Both the n-butane and hydrogen that do not react are recycled to the reaction section. They have already passed through the dryers for water removal.

If we allow the metaphor of the body to continue, then it follows that the process units (like organs) are interconnected. As you learn more about process units, you start to dig into where the feeds come from (possibly from other process units) and where the products go and how they are used. For example, the desired product of the butamer unit is isobutane. Isobutane is a major feed for the alkylation reaction, which makes high-octane gasoline blend components from LPG.

I encourage you to explore different process units. I have found it helpful to think of the generic BFD when learning a new process. It’s important to think about what feed treatment is required, where the reaction takes place, and how the effluent is separated into products.