Feedstocks for steam crackers: key considerations

In plastic production, the steam cracker is king. And, to produce the polymer, we must meet its every requirement.

The feedstock possibilities for plastics are ever-widening — our extensive R&D into renewable feedstocks has allowed us to process many more types in the past decade than was possible before.

But these feedstocks must still be transformed into naphtha (or another product with a suitable boiling point range) that meets the specific requirements of a given steam cracker.

In this article, we examine the process enabling a large variety of renewable or waste-derived feedstocks to be used to arrive at an output suitable for steam cracking and, ultimately, plastic production.

Feeds for thought

But first we will review what feeds are available. There is a variety of types of feedstocks, including:

Liquid feedstocks

• Tall oil
• Tallow
• Vegetable oils
• Waste oils
• Animal fats
  
  

Solid feedstocks

• Biomass
• Waste (including municipal waste)
• Tires
• Plastic

The process for hydrotreating fatty acid-based feedstocks, such as vegetable oils and animal fats, is already well understood. In fact, refineries all over the world have been hydrotreating them for many years.

Newer developments include solid-waste items like tires, plastic, and biomass. These have previously proven too difficult to process due to the many more complex contaminants and impurities within the feeds. Topsoe´s PureStep™ technology now makes it possible to create feedstock for a steam cracker from solid waste streams to create “circular plastic.”

This occurs via a liquefaction process (pyrolysis or hydrothermal liquefaction) that converts the waste to an oil, which is then purified and upgraded so it can be used as a feedstock.   

                                  The process for converting a wide range of feedstocks into plastic

Command of carbon chains

Plastics, as all polymers, are made from monomers, and the steam cracking process is essential for producing monomers for plastic production. Naphtha steam crackers require an extremely pure hydrocarbon feedstock which we can now produce from a variety of waste and renewable sources.

For example, when processing fatty acid-based feedstocks in the hydroprocessing unit, you produce primarily C16, C17, and C18 straight chain hydrocarbons. Reducing the chain length can convert the hydrocarbons to paraffinic naphtha, perfect for a steam cracker. Oils from liquified solid-waste streams however differ greatly in their properties, so tailor-made hydroprocessing technologies and catalysts are needed to produce suitable steam-cracker feedstock.

Detailed knowledge of the vast number of contaminants and the hydrocarbon structure of these oils is essential to complete the task. Whether it is pyrolysis/HTL oils from waste plastic, tires or biomass, Topsoe has the dedicated technology and catalysts to upgrade these oils and reduce the end boiling point.

The contaminants and the hydrocarbon structure of your feedstocks define what kind of hydroprocessing process layout you will need.

The steps to purity

Contaminants are a key factor in advanced chemical recycling. Different feeds have different characteristics; for example, some may contain unwanted elements such as silicon, phosphorous, or a wide range of other elements. These impurities are not familiar to conventional fossil hydroprocessing. Fortunately, through internal efforts and expertise, we at Topsoe know how to handle and remove all these contaminants.

As such, it´s crucial that an operation utilizes robust hydroprocessing catalysts and technology that can handle these new contaminants in extremely high levels.

End-point reduction

Purifying the oils could be enough to solve the contaminants challenge and produce a clean hydrocarbon product. But sometimes the vegetable oil, animal fat and waste plastic pyrolysis oils are too heavy for the steam cracker.

This is where “end-point reduction” comes into play. This process reduces the hydrocarbon chain length and converts the feed oil into naphtha by adding hydrogen. Minimizing hydrogen consumption and reducing CO₂ emissions is, of course, a key priority throughout the entire hydrotreatment process.

Refinery refinements

Because of the sheer diversity in feedstocks and steam cracker requirements, a refinery´s unit design i is crucial to achieving the desired results. PureStep can be deployed as a green-field plant setup, or implemented within existing refineries or upstream-integrated petrochemical complexes.

Other factors must also be considered outside of the feedstock´s physical properties. With volatile markets and fast-changing legislation, the price and availability of certain feedstocks can vary tremendously.

Therefore, having feedstock flexibility within your setup is essential. Your hydrotreating unit should be able to process feedstocks of varying quality and type, while still meeting stringent steam cracker specifications. 

Hybrid hydroprocessing

It is also likely that different renewable feedstocks will need to be combined to deliver a high enough volume. This “hybrid feedstocks” approach can only be enabled by a partner who has worked with all the different types and understands how each works and which ones should and should not be blended.

Naturally, selecting and loading the right catalysts is also vital. These must be adapted for the type of feedstock, targeted end product specifications and the unit layout.

Taking stock

Each feedstock creates its own challenges, and no two projects are exactly the same. But the possibility exists for all the above-mentioned feedstocks to be processed and for the previously highlighted challenges to be solved.

To meet steam cracker specifications requires a hydrotreating process that is optimized for CO₂ efficiency, product quality, high yields, price, and your business.

Topsoe has extensive experience from hydroprocessing and working with a multitude of renewable and waste feedstocks. PureStep is a technology that can be adjusted for the requirements of your business, so you can create pure value no matter the feedstock.