Jul-2011

Industrial-scale production of renewable diesel

A revamp has made possible the large-scale production of clean diesel with a significant percentage of renewable content

Rasmus Egeberg and Kim Knudsen, Haldor Topsøe
Stefan Nyström, Eva Lind Grennfelt and Kerstin Efraimsson, Preem

Article Summary

A renewable diesel has emerged as a drop-in substitute for today’s mineral diesel fuels. Its production is not limited to seed oils, but is flexible in that it can use a number of different feedstocks with very little change in operating conditions yet still produce a renewable and clean product. Renewable diesel is produced by hydrotreating, whereby the oxygenates in bio-feedstocks are hydrogenated into hydrocarbons. Hydro-treating takes place in existing diesel hydrotreaters in a co-processing scheme, where the biofeed component is deoxygenated and the fossil diesel component is simultaneously desulphurised, or it can take place in a standalone unit processing 100% biofeed. In both cases, conversion occurs over hydrotreating catalysts and in conditions similar to those used in hydrotreaters today. The product can, in some cases, require its cold flow properties to be upgraded by, for instance, catalytic dewaxing.

Haldor Topsøe has supplied catalyst for a number of operating units producing renewable diesel and has also licensed process technology for standalone units and revamps to enable co-processing. An earlier article in PTQ (Q2 2010, p101) discusses the chemistry, catalyst technology and challenges of renewable diesel hydrotreating. This article describes how innovative technology solutions were implemented at the Preem Gothenburg refinery. Operating data from the current cycle explain how the revamp at Gothenburg has made possible the large-scale production of clean diesel with a significant percentage of renewable content.

Rebuilding for renewables
This case study shows how the challenges of renewable diesel hydrotreating can be overcome through technology solutions and the use of a specifically designed catalyst series. It also serves as an example of how a non-edible and low-value stream can be upgraded into a high-value transportation fuel by hydrotreating. Preem contacted Topsøe in order to scope out the possibilities of revamping an existing mild hydrocracking unit to a green hydrotreating unit, where large quantities of raw tall diesel (RTD) could be co-processed together with light gas oil. The capacity of the unit is approximately 10 000 b/d, and a target was set of co-processing up to 30% RTD in the feed. A development agreement was signed, stipulating that Topsøe would carry out the basic design of the unit.

RTD is produced from tall oil by Sunpine from Kraft paper mills in the north of Sweden. Tall oil mainly consists of resin acids and free fatty acids, as well as a number of contaminants in smaller concentrations. Through an esterification process, the majority of the free fatty acids are converted to fatty acid methyl esters (FAME), while the resin acids are left almost unconverted. RTD differs from other feedstocks used for renewable diesel production in that it is non-edible, and thus this technology does not affect the global food shortage negatively or add to food prices. The composition of RTD varies, depending on the type of wood used in the paper mill and the severity of the esterification process. Unlike other bio-oils, RTD also contains several hundred ppm sulphur. Thus, flexibility was needed to ensure that all feed types could be processed. This was verified in the pilot plant phase, where several different compositions of RTD were successfully converted, confirming the performance of the catalyst and the revamp design.

The design has enabled Preem to co-process 30% RTD with light gas oil to produce a renewable diesel meeting all ULSD EN 590 diesel specifications. A level of 30% is a relatively high fraction of biofeed in an existing unit not built to handle this type of feed, and unique challenges were encountered, in particular related to hydrogen consumption, exotherm, catalyst selection and corrosion, both upstream and downstream of the reactor. The revamped unit was started up in 2010.

Handling of corrosive feed and high exotherm
The introduction of RTD poses a special challenge with regard to procedures for material selection, as this is a very unconventional feed with a very high total acid number (TAN). It contains many unconverted free fatty acids, as well as resin acids, which may increase the corrosion rate in pipes, heat exchangers and the fired heater upstream of the hydrotreating reactor, where these compounds are converted. The corrosion issue has, in other cases, been design-limiting when deciding the amount of tall oil or derived material that can be co-processed. Therefore, a high priority was to design the feed system to overcome these challenges.

A special RTD feed system was designed for this purpose by Preem and Topsøe. Injection of RTD and mixing with the mineral feed is carried out in multiple steps (see Figure 1). Part of the RTD is introduced at an injection point after the fired heater and prior to entering the reactor. In this way, all existing process equipment upstream of this injection point is left unaffected. The remaining part of the RTD feed is introduced between the first two beds of the reactor to control the temperature profile, but also to control the TAN and thereby minimise corrosion. Since RTD is only injected after the fired heater and as a liquid quench to the second reactor bed, the hardware has very limited exposure to the highly corrosive RTD. Thus, only minor changes to the material selection were necessary. These changes meant that the unit is prepared for, and has already experienced, operation with an even higher fraction of RTD feed.

Following its revamp to renewable diesel production, the unit operates at a lower average temperature, but with significantly higher hydrogen consumption. As a result of exothermic hydrodeoxygenation reactions, the heater duty and fuel consumption of the unit are lower compared with what is seen in normal hydrodesulphurisation mode. This means that the added reaction heat is also utilised in fossil light gas oil hydrodesulphurisation, so this operation is less energy consuming. However, the large amount of heat released due to the hydrogenation of RTD in the reactor must be controlled by a proper quench system. Although the main purpose of splitting the RTD into several streams and delaying the mixing is to eliminate the risk of corrosion, another equally important purpose is to provide a liquid quench, which makes it possible to control heat release from the exothermic reactions, thereby lengthening the lifetime of the hydrotreating catalysts by a significant degree. With fresh RTD used as a liquid quench between the first two beds, more hydrogen can be used upstream of the reactor to increase the hydrogen partial pressure. This measure helps prevent gum/coke formation and corrosion, thereby ultimately delivering higher unit reliability and lower investment cost.

Selection of catalysts
The implementation of new process technology solutions goes hand in hand with the selection of catalysts, as catalyst activity and selectivity determine the necessary reaction conditions and the composition of reactor effluent. The conversion of high levels of RTD in the feed constitutes a very fast reaction, consuming substantially higher amounts of hydrogen than in the case of conventional hydrotreating, and this requires specialised catalysts for the conversion of renewable material. The correct balance between activity and stability depends on the feed and conditions and has important implications for the operation of the unit.