Future fuel - the challenges associated with renewable diesel hydrotreating

As a consequence of the general concern about fossil fuel resources and global warming from CO2 emissions, the use of alternative, sustainable sources of energy for the transportation sector has been increasing.

Per Zeuthen, Haldor Topsoe
Henrik Rasmussen, Haldor Topsoe USA

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Article Summary

Despite this growth in renewable fuels, there has been little integration of renewable fuels into petroleum refineries so far. The two main biofuel products used in transportation fuels are bioethanol, used in gasoline, and fatty acid methyl ester (FAME), used in diesel. There are several compatibility issues with the properties of FAME and the specification of diesel, including a poor stability that causes filter plugging. For these reasons, only limited volumes of FAME can be blended into diesel fuel.

However, a commercially available hydrotreating technology from Topsoe, HydroFlex™, allows renewable feedstocks to be converted into drop in gasoline, jet or diesel. In this process, the renewable organic material is reacted with hydrogen at elevated temperature and pressure in a catalytic reactor. The clear advantage of hydrotreating compared to the use of FAME biodiesel is the fact that the final products from this simple hydroprocessing process (simple paraffins) are the same components as those present in normal fossil diesel.

Most of the existing FAME biodiesel can be characterised as first generation biodiesel, since it relies on vegetable seed oils that normally enter the human food chain. Consequently, this type of fuel may lead to escalating food prices and a shortage of food supply. In contrast, renewable diesel by hydrotreating may be produced from a broad variety of sources, including animal fats and vegetable oils, but also tall oil from the paper and pulp industries, pyrolysis oils, and other non-edible compounds.

The hydrotreating of these feedstocks into renewable diesel utilises specialty designed catalysts compatible with the type of catalysts used for hydrotreating of fossil diesel streams to meet environmental specifications. Thus, a co-processing scheme where fossil diesel and renewable feedstocks are mixed and co-processed is possible, producing a clean and green diesel that meets all ASTM D975 specifications. The hydrotreating may also take place in a dedicated stand-alone unit that produces 100% renewable drop in diesel. In either case, the new feed components give rise to new reactions and to the formation of new byproducts, resulting in a series of challenges. These challenges can be addressed through the use of optimised catalysts and process design.

Processing renewable feeds
Consequences for hydrotreating

Refinery hydrotreaters play an important role in the production of fossil fuels, and before introducing even minor amounts of biomaterial into a diesel hydrotreater, it is important to know the implications and how all potential risk factors can be mitigated.

The main issue is not to achieve full conversion of the renewable feed, because most naturally occurring oxygen containing species are much more reactive than the sulphur compounds in fossil fuels. The challenge in industrial operation is mainly to control the very exothermic reactions and the large amounts of hydrogen consumed, which require higher make-up hydrogen rates and larger quench gas flows, even when co-processing smaller amounts of renewable feedstocks. Thus, the refinery hydrogen balance must be checked, and the unit capacity may be lower than when only processing fossil diesel. The depletion of hydrogen, combined with high temperatures, may lead to accelerated catalyst deactivation and pressure drop build-up. Before these factors can be controlled, tailor-made catalysts and a careful selection of unit layout and reaction conditions must be employed.

Another challenge is the often quite high content of contaminants, such as phosphorous, silicon, sodium, and calcium. This may cause rapid pressure drop build-up or deactivation unless proper guard beds are designed either to prevent the pressure drop or to protect the downstream catalysts. When processing other feed types, such as tall oil or vegetable oils with a high content of free fatty acids, severe corrosion of pipes and other equipment upstream of the reactor will take place.

High amounts of propane, water, CO, CO2, and CH4 are formed as byproducts of the hydrogenation reactions. These gases must be removed from the loop either through chemical transformation, by a gas cleaning step (e.g. an amine wash), or more simply by increasing the purge gas rate. If not handled properly, the gases will build up in the recycle gas loop and reduce the catalyst activity.

Due to these quite severe challenges, the present industrial practice involving co-processing of fossil oil and renewable organic material is usually limited to blends with less than 5 vol% renewable diesel. However, Topsoe has developed solutions to all of these issues that will result in a better economy of the co-processing scheme and allow the proportion of renewable organic material in the feed to be increased up to 35 vol% or more.

Renewable diesel
Feeds, products, and reaction pathways
The purpose of hydrogenating biologically derived (i.e. renewable) feedstocks on an industrial scale is to produce hydrocarbon molecules boiling in the diesel range, which are directly compatible with existing fossil-based diesel and meet all current specifications for ultra-low sulphur diesel (ULSD), as specified in ASTM D973. With the introduction of feedstocks stemming from renewable sources, new types of molecules with a significant content (10–15 wt%) of oxygen are present and must be properly treated by both the hydrotreating process and catalysts.

Although a great variety of renewable feeds exist, the hydrotreating to produce diesel type molecules is somewhat simplified by the fact that the chemistry of vegetable oil or animal fat is rather similar and based on triglycerides with a structure as shown in Figure 1.

Since the paraffins produced from the fatty acid chains are in the diesel boiling range, a diesel hydrotreater is the preferred unit for co-processing of most feeds. Unlike fossil feedstocks, the content of sulphur and nitrogen species in these feedstocks is very low, meaning the hydrodesulphurisation (HDS) conversion required to make ULSD is lower when co-processing renewable feeds.

To investigate how the triglycerides react under typical hydroprocessing conditions, a pilot plant test with a NiMo catalyst was conducted using a blend of 75 vol% Middle East straight-run (SR) light gas oil (LGO) and 25% rapeseed oil. Rapeseed oil is a triglyceride mainly derived of C18 and C22 carboxylic acids. The conversion of triglycerides was confirmed to be 100%. Furthermore, yields of CO (0.6 wt%), CO2 (1.2 wt%), and CH4 (0.1 wt%) were observed. The total liquid product was analysed by gas chromatography (OC), and the results are shown in Figure 2. The total conversion of rapeseed oil is confirmed by the fact that all high boiling components (at retention times over 35 min) are not present in the product. Instead, four normal paraffins are formed with chain lengths of 17, 18, 21, and 22, respectively.

The conversion mechanism consists of several steps. The double bonds and the triglyceride are hydrogenated by one of at least two distinct reaction pathways (Figure 3). The first pathway involves a complete hydrogenation and is usually called the hydrodeoxygenation (HDO) pathway. The other pathway involves a decarboxylation step, which means that CO2 is split off.

The byproducts of both carbon dioxide and water influence the propagation of two additional reactions (also shown in Figure 3); the reverse water gas shift reaction (Equation 1) and the methanation reaction (Equation 2) will both impact the overall hydrogen consumption:
CO2 + H2 → CO + H2O                     (1)
CO + 3H2 → CH4 + H2O                   (2)

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