Co-processing renewables in a hydrocracker

Understanding and controlling the impact on catalyst performance, yields, and product properties is key to renewables co-processing in hydrocracking units.

Peter Andreas Nymann and Pronit Lahiri

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Article Summary

The demand for having part of the product slate from refineries consisting of renewable materials is increasing. Hydrotreating units have been the primary choice so far, but hydrocracking units are also quite suitable and will be unavoidable in the future, especially when processing more demanding second- and third-generation feedstocks.

Extensive pilot plant testing and a fundamental understanding of chemistry supported by industrial operation from several units worldwide have put Topsoe in a position to support refineries in this challenging effort. Topsoe has been collaborating with refineries navigating through the energy transition to reduce CO2 emissions originating from the use of fossil-based feedstocks.

Renewable feedstock challenges
The world energy sector is going through an energy transition and looking for solutions to decarbonise energy production. Part of this is facilitated by replacing fossil feedstocks with renewable resources and treating these in the existing refinery framework to produce fuels that are fully compatible with the hydrocarbon fuels in the market today.

Processing of renewable feedstocks, like first-generation feedstocks such as palm oil and other triglyceride-based hydrocarbons, has therefore been widely applied in recent years. Several projects constructing new standalone units or revamping existing hydroprocessing units to either co-process or solely process renewable feedstocks have materialised.

Co-processing renewable feedstocks in hydrotreating units is now well established, and refineries are looking for more ways to include renewable feedstocks in their existing units. Furthermore, the increased demand for including renewable feedstocks in the energy system and the availability of renewable feedstocks directs the industry to incorporate second- and third-generation renewable feedstocks. This introduces new and more demanding requirements to convert these challenging feedstocks into on-spec fuels.

One way is to co-process renewables in hydrocracking units. The hydrocracker has advantages and disadvantages when it comes to co-processing renewables. If the economic uplift from processing renewable feedstocks is lower than that of normal hydrocracker feedstock, or the renewable feedstock should cause severe performance deficits in the hydrocracker, co-processing in the hydrocracker might not be feasible.

Understanding the impact on the performance of the hydrocracking unit when introducing new feedstock types, such as first-, second- and third-generation renewable feedstocks, is therefore of utmost importance for this evaluation.

Feedstocks and chemistry
In recent years, the most applied renewable feedstocks in refinery units have been triglycerides. These typically originate from what is referred to as virgin oils, more commonly known as first-generation feedstocks. More recently, second-generation feedstocks like used cooking oil (UCO), palm oil mill effluent (POME), and others have been considered. Second-generation renewable feedstocks are typically byproducts from the production of virgin oils. Going forward, third-generation feedstocks like pyrolysis oils derived from municipal sewage waste, byproducts from the paper and pulp industry, wood, plastic, or tyres are being investigated.

The triglycerides are characterised by consisting of three glycerol chains with a typically even carbon number and a backbone of propane connected by oxygen atoms (see Figure 1). Hydrotreating reactions include saturation of double bonds inside chains, breaking of triglyceride into propane from the backbone and paraffins from the side chains, and the reaction of oxygen atoms with hydrogen, forming water, CO or CO2. They occur readily over hydrotreating catalysts and are well known in several hydroprocessing units worldwide.

Many second- and third-generation feeds are more complex than the first-generation feeds and hold more complex molecules. Consequently, they result in a different boiling range of the products as well as different properties. Unlike the triglycerides that generate straight chained molecules boiling in the distillate boiling range, the second- and third-generation feedstocks contain ring structures with different heteroatoms that need more hydrogenation and eventually cracking to bring them into the transportation fuel boiling range and to ensure that these products fulfil other product specifications, such as cold flow properties and densities. 

Hydrotreating of triglycerides requires significant amounts of hydrogen, and the reactions are highly exothermic. The product will be boiling in the middle distillate range and suitable for diesel. If jet fuel is needed, the longer normal paraffin chains need to be converted to lower boiling material by either cracking or isomerisation. Normal paraffins have low density and high cetane index, but also high cloud point. Isomerisation provided by a hydrocracking catalyst is therefore sometimes needed to achieve cloud point specifications.

Hydrotreating of aromatics and oxygen-containing ring structures produce naphthenes with higher density and boiling range than what is suitable for diesel. Selective hydrocracking reactions involving ring opening will therefore be needed for processing these more demanding second- and third-generation feedstocks to produce high yields of high-quality jet and diesel products.

Overcoming operational challenges
Operational challenges are better overcome in hydrocrackers. Considering the high content of oxygen in renewable feeds, the H2 consumption associated with these reactions is in the range of 300-500 Nm3/m3, which is orders of magnitude greater than what is normally seen in hydrotreaters. However, compared to hydrocrackers, the difference is less, and the impact from processing in a hydrocracker would therefore be less. Likewise, the associated heat release originating from these reactions is also more in the same order of magnitude as in hydrocrackers designed to handle high temperature rises by having multiple beds.

Contaminants present in the renewable feeds pose a challenge to the hydrotreaters, and significant amounts of bulk catalysts need to be replaced to manage the contaminants. In hydrocrackers, the VGOs and HCGOs often processed also have considerable amounts of contaminants, and hydrocrackers are therefore normally designed to include volume for grading catalysts. They may therefore allow greater quantities of renewables to be processed without compromising catalyst cycle length.

Different types of renewable feeds may produce different renewable products, and the fractionation section in hydrocrackers makes it possible to separate these different products. The products from renewables often require more than hydrotreating, either to achieve better cold flow properties by isomerisation or, in the case of second- and third-generation feedstocks, hydrogenation of rings and ring opening. Hydrocracking catalysts are multifunctional and may be selected to include both isomerisation and selective ring opening activity to ensure the fulfilment of all required end-product properties.

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