O
O
Saturation of double bonds in FA chain
O
O
CH O
R2
CH O
R2(=)
C
C
+ x H
CH
O
C CH O
C
R1
R1(=)
Guard
CH O R3 C
C
CH O R3(=)
O
O
O
Zone 1
O
CH O
R2
C
Triglyceride decomposition to fatty acids
R1, 2, 3
O
+ 3 H
3
C CH O
C
+ C H
R1
O R3 C
CH
HO
O
+ 3 H
Zone 2
R-CH + 2H O HDO -
Hydrodeoxygenation
O R
Fatty acids decomposition to n-para ns
RH + CO
C
DCX - Decarboxylation
OH
+ H
RH + CO + H O DCN -
Decarbonylation
Zone 3
+ H
Reverse- water-gas- shift and methanation
CO + H O
RWGS - Reverse water gas shift
CO
+ 3 H
CO
CH + H O
MTN - Methanation
Figure 7 Main reactions of triglycerides and fatty acids and where they occur in a co-processing distillate hydrotreater. For the definition of the operating zones, refer to the high-pressure distillate hydrotreating section and to Figure 1
as refiners can vary renewable feedstock intake based on market conditions and hydrotreater operating needs. Renewable feedstocks are mainly composed of triglycer - ides and free fatty acids (FFA). They also contain mono- and di-glycerides, phospholipids, sterols, and traces of metals. The main goal of hydrotreating renewable feedstocks for ULSD production is to decompose triglycerides and fatty acids into n-paraffins in the diesel range (C16-C24 ). First-generation biofeeds like palm, soybean, and sun - flower oils are easy to hydrotreat but are not sustainable as they compete with food production. Therefore, their use in Europe is being phased out under the Renewable Energy Directive. Second-generation biofeeds include UCO and animal fats. UCO is obtained mostly from restaurants and households. It is promoted as being cheap and a waste product, but it requires quite severe pretreatment due to impurities from cooking. Animal fats, in particular, contain high nitrogen and metals, which can inhibit and deactivate hydrotreating catalysts. The suitability of biofeeds for hydrotreating depends on several factors, including their chemical composition, avail - ability, environmental impact, and cost. Key properties to consider in this respect are reactivity, acidity, and the pres - ence of impurities. Reactivity depends on the concentration of the double bonds in the triglycerides (unsaturation level) and on their position. Highly reactive biofeeds need to be effectively managed with a specific grading system to avoid ‘gum’ formation and minimise the risk of reactor plugging. High acidity caused by the presence of FFAs is an addi - tional problem, leading to corrosion upstream of the reac - tors. Chloride levels must also be controlled, as hydrochloric acid formed from organic chloride can react with ammonia, forming ammonium chloride salt, which also causes fouling and corrosion.
UCO and animal fats, in particular, can contain impurities that cause tank sludge and clogging. Unsaponifiable impu - rities (sterols, tocopherols), insoluble impurities (mechani - cal, mineral, resins), and metal contaminants should also be properly managed as they deactivate hydrotreating cata - lysts and lead to pressure drop (dP). One of the main factors to control when hydrotreating renewable feedstock is the impact of phosphorus, present in the form of phospholipids, which often carry alkali and alkaline earth metals (K, Na, Ca, and Mg). If not properly trapped in the guard section of the reactor, phosphorus deposits in the main catalysts’ pores, causing severe deac - tivation and plugging. Therefore, when co-processing biofeeds, it is imperative to always apply a proficient phos - phorus-trapping catalyst. Reactions of renewable feedstocks and impact on operation Figure 7 summarises the main reactions in the conversion of triglycerides and fatty acids and where they occur in the operating zones of a distillate hydrotreater during co-pro - cessing. The operating zones are defined in line with the high-pressure distillate hydrotreating section and were depicted earlier in Figure 1. Note that Zone 3 is reached in the reactor only if nitrogen is completely removed from the conventional and renewable feed blend. The first reaction step involves double bond saturation in the fatty acid chains of the triglycerides. This saturation occurs very rapidly at the reactor top, even at low temper - ature. When multiple double bonds are present, the sat - uration process is even faster, leading to polymerisation reactions, gum formation and potentially coking. To prevent pressure drop issues as a result of these reactions, a dedi - cated guard catalyst is necessary.
24
PTQ Q4 2025
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