Feb-2009

Improving the production of renewable diesel by co-processing

Haldor Topsøe A/S has developed a new process allowing Preem AB to co-process light gas oil and raw tall diesel (RTD), producing a renewable diesel meeting all EN 590 specifications.

Stefan Nyström, Preem, Niels Høygaard Michaelsen and Rasmus Egeberg,
Haldor Topsøe

Article Summary

RTD is a refined tall oil, which is a low-value byproduct from Kraft paper mills. In contrast to other feedstocks used for renewable diesel production, it is non-edible, so the process does not add to the problems of global food shortage or high food prices. The basic engineering for applying the process in Preem’s refinery in Gothenburg has been concluded recently by Haldor Topsøe and the unit is expected to start up in early 2010.

Challenges in hydrotreatment of organic material
Renewable organic material is a general term denominating a wide host of energy resources that are a result of recent biological processes and can be replaced within a relatively short time horizon contrary to fossil fuels. Vegetable oils and animal fats typically consist of more than 95 wt% triglycerides, which can be seen as a triple ester (product of the condensation of an alcohol R-OH and a carboxylic acid R-COOH), where the alcohol is trialcohol glycerol. These oils and fats are the main constituents in the human diet and it has therefore been debated whether the production of transportation fuels in so-called first-generation technologies can result in food scarcity and higher food prices. It is therefore desired to develop technologies that can process other types of feedstocks.

Preem has partnered with Sunpine, which produces RTD based on tall oil from Kraft paper mills in Northern Sweden. Tall oil mainly consists of resin acids and free fatty acids as well as a number of contaminants in smaller concentrations. Through a transesterification process, the majority of free fatty acids are converted to fatty acid methyl esters (FAMEs), while the resin acids are left unconverted.

For a number of years, it has been known how to convert vegetable oils into normal paraffins in the gasoline or diesel boiling range by employing a hydrotreating process. In this process, the renewable organic material is reacted with hydrogen at elevated a temperature and pressure in a catalytic reactor. Although RTD is significantly different in chemistry from that of typical vegetable oils, the major difference from fossil fuels is the high content of oxygen (approximately 10 wt%). This means that a new family of highly exothermic and hydrogen-consuming reactions will take place in the hydrotreater.

Although it is known how to hydrotreat conventional mineral oil in combination with significant amounts of renewable organic material, the current industrial practice involving the hydrotreatment of a feed comprising oil and renewable organic material has often been limited to the use of small amounts of the latter, normally below 5 vol%. As it was attractive for Preem to treat higher volumes of RTD without building a new unit, there were a number of issues that needed to be addressed. Preem therefore approached Topsøe, which has previously revamped some of Preem’s refinery units in Lysekil and Gothenburg and supplied catalysts for these units. Topsøe’s R&D division was already active in developing new catalysts for biodiesel applications, and a development agreement was made between Preem and Topsøe with the purpose of inventing a new hydrotreating technology for the production of green diesel.

A major concern was the feed handling and the mineral/renewable feed blending system. The high amount of resin acid and free fatty acids with a high total acid number (TAN) conveys the penalty of increasing corrosion in pipes, heat exchangers and fired heaters upstream of the hydrotreating reactor. So far this has imposed a limitation on the industrial applicability of the attractive concept of hydrotreating mixtures of conventional mineral oil with significant proportions of tall oil or tall oil-derived material.

To address this problem, a new RTD feed system was invented by Preem and Topsøe, and the mixing with the mineral feed is done in several stages. Part of the RTD is introduced at an injection point after the fired heater and prior to entering the reactor. This way, all existing process equipment upstream of this injection point is not affected. Another part of the RTD feed is introduced between the first two beds in the reactor to control the temperature profile, but also to control the TAN and thereby minimise corrosion (see Figure 1).

Another concern is the large amount of heat that is released due to the hydrogenation of the tall oil-derived FAME taking place in the hydrotreating reactor as per the two reaction routes shown in Figure 2. The upper hydrodeoxygenation (HDO) route forms water and the decarboxylation route forms CO2. Both reactions are fast, consume hydrogen and are highly exothermic (create heat), thereby potentially deteriorating the hydrotreating catalyst and giving rise to fouling of the catalyst bed either by coke formation and/or gum-forming polymerisation reactions.

The decarboxylation route uses less hydrogen compared to the HDO route and may therefore appear to be the preferred reaction route, but, as will be explained below, the CO2 formed can react with hydrogen, forming CO and, in turn, methane. Moreover, CO2 and CO can cause a number of other problems, including catalyst inhibition, and are more difficult to remove from the loop than water. Normally, both reaction paths will be active and the distribution between the two routes is determined by the specific type of feed, the operating conditions and type of catalytic system.

In order to control the heat release, the effluent from the first catalytic bed in the hydrotreating reactor is mixed with fresh RTD feed, as described above. It is therefore possible to limit the use of hydrogen in order to control the heat release in the first bed, since quenching is provided by RTD. Furthermore, this provides a higher hydrogen partial pressure upstream of the reactor, preventing gum formation and corrosion.

The splitting of RTD in several streams and delaying the mixing of the mineral feed with renewable organic material prior to hydrotreating thus serves several purposes. It allows for eliminating the risk of corrosion, particularly in upstream equipment, and at the same time acts as a liquid quench, giving control of the heat release from the exothermic reactions and thereby lengthening the lifetime of the hydrotreating catalysts significantly.