Unrevealed additional WASA additive performance at European refinery
Study of the response of winter and summer grade diesel fuels to static dissipator additive and the effect of other additives on the electrical conductivity of the fuel.
Rosen Dinkov, Ivo Andreev, Dicho Stratiev, Ilian Kolev and Miroslav Atanasov
LUKOIL Neftohim Burgas AD
Katarzyna Grabowska, Brenntag Oil & Gas Research and Application Center
Cobbin Mackenzie, Infineum Business and Technology Centre
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Severe hydrotreating has not only a positive ecological effect on emissions from burning engine fuels; it also decreases the ability of diesel boiling range hydrocarbons to dissipate static charge. Therefore, to turn the electrical conductivity of hydrotreated gasoils back to safe limits (150 pS/m), LUKOIL Neftohim Burgas AD (LNB) uses the most effective way – adding conductivity-improving additives or so-called static dissipator additive (SDA). The response of summer and winter grade diesel fuels to SDA is different from the antagonistic effect of middle distillate flow improver (MDFI) and wax anti-settling additives (WASA). WASA is proven to dissipate static charge in a concentration above 100 ppm. Winter grade low-sulphur diesel fuels with 125 ppm WASA and without dosing SDA are safe (at electrical conductivity above 150 pS/m) to be stored and transported.
Since the introduction of a 10 mg/kg sulphur cap on 1 January 2009 for diesel fuels by Directive 2003/17/EC,1 European refineries have begun to severely hydrotreat their gasoil fractions into lower sulphur content. These severe conditions lead to the deterioration of some other properties2 of diesel fuel, including lubricity,3 cetane number, and electrical conductivity.4 Together with sulphur, many organic compounds containing nitrogen and oxygen atoms and poly-aromatic compounds are also removed.
During hydrotreating, cyclic organic compounds responsible for the electrical conductivity of diesel fuel are removed. Thus, electrical conductivity drastically diminishes and can cause the generation and accumulation of electrostatic charges (static electricity), which can result in static discharges.4 Static discharge poses a serious threat when moving petroleum products through the distribution system. Hydrocarbons are poor conductors of electricity, and static electricity may accumulate and take significant time to leak off to the ground.
There have been cases where such accumulations discharged as high energy sparks have caused fires or explosions under certain air/fuel vapour conditions. This is particularly true for modern ULSD because of their high purity, high pumping rates, and the use of filtration capable of producing a high rate of charge separation and static build-up in the fuel.5 Conductivity of ULSD is very low at 0-2 pS/m against higher sulphur diesel fuels, which exhibits electrical conductivity between 150 to 250 pS/m (depending on the sulphur content)6 and thus static charge at ULSD cannot dissipate quickly.
Ways to cope with a lower rate of static charge dissipation are described in Guide for Generation and Dissipation of Static Electricity in Petroleum Fuel Systems (ASTM D4865).4 It recommends measures to prevent explosions, such as earthing (bonding and grounding), pumping rate limits, and ‘time for charge dissipation’ (relaxation time), before the fuel is exposed to air. Another way to improve the electrical conductivity of fuel is via the implementation of additives.4,7
SDAs (also known as conductivity-improving additives, antistatic additives, or electrical-conductivity additives) are needed to replace lost natural polar components. Besides relaxation time, SDA is the only way to increase conductivity effectively.5 These additives do not hinder electrical charge generation but increase only the rate of charge dissipation by increasing the conductivity of the fuel and thus relaxing static charges.4
Materials and methods
In this project, several gasoils originated from an LNB commercial unit – straight-run (straight-run light gasoil [SRLGO] 180-240°C; straight-run middle gasoil [SRMGO] 200-300°C; straight-run heavy gasoil [SRHGO] 240-360°C; light vacuum gasoil [LVGO]) – and from a secondary origin (FCCPT diesel; FCC light cycle oil [LCO] and H-Oil diesel) are analysed and their properties are summarised in Table 1. These gasoils are hydrotreated in LNB commercial hydrodesulphurisation (HDS) units, and two types of hydrotreated gasoils are produced: light and heavy.
Properties and test methods of light and heavy hydrotreated gasoils are presented in Table 2. SDA is added in such low concentrations that it is extremely difficult to detect by any standard analytical procedure. Therefore, it is controlled by measuring the resultant electrical conductivity of the fuel. The standard field test for electrical conductivity is ASTM D2624/IP 274: Electrical Conductivity of Aviation and Distillate Fuels. Although the method is intended to measure conductivity with the fuel at rest in storage tanks, it can also be used in a laboratory.5
Results and discussions
LNB produces Euro 5 diesel fuel via hydrotreating/HDS several gasoil fractions, of whose properties are described in Table 1. As can be seen from Table 1, the fractions are both straight run and secondary, i.e., produced from conversion units like FCC and ebullated bed hydrocracking of vacuum residue (H-Oil). Their properties vary in a wide range. Density of SRLGO is just 0.801 g/cm³, while gasoil fractions with secondary origin reveal very high density values – 0.872 g/cm3 for the H-Oil diesel fraction and up to 0.930 g/cm3 for FCC LCO.
Poly-nuclear aromatics (PNAs) content follows duly density as significantly high values are registered for FCC LCO and H-Oil diesel fractions – 45.6 wt% and 21.8 wt%, respectively. The opposite is distributed sulphur content – higher values are typical for straight-run fractions (SRLGO, SRMGO, SRHGO, and especially LVGO). Also, due to a certain degree of hydrotreatment in the conversion processes, diesel fractions with secondary origin possess lower sulphur at levels of about 0.2 wt% and less. Gasoil fraction properties should be polished in order for the final diesel fuel to correspond to the requirements of both fuel quality norms8,9 and specific environmental legislation.9,10
Figure 1 presents the origin of all diesel fractions in LNB, obtained from different production units, with gasoil divided into light and heavy fractions. The light gasoils are processed in a dedicated HDS unit, HDS-2 or, when the amount of gasoils is limited due to lower refinery throughput, in HDS-3. HDS-5 is designed to process mainly heavier gasoil fractions, as a dearomatisation section is included in this unit. HDS-5 is capable of improving the heavy gasoil fraction as per the following metrics: decreasing density at 15°C from 0.865 g/cm3 to 0.850 g/cm3 for the hydrotreated product, lowering feed sulphur content from 0.67% (m/m) down to 8 mg/kg, and polycyclic aromatic content from 12% (m/m) to 4% (m/m).
Along with the positive effect of HDS over gasoil fraction properties, removing impurities such as hetero-atoms (nitrogen and oxygen-based polar trace compounds) and metal-containing compounds2 causes some negative effects. With adoption of the HDS process, hydrotreated gasoils lose their inherent lubricity3 and electrical conductivity.4 The physico-chemical properties of two types, light hydrotreated gasoils (LHTGO) and heavy hydrotreated gasoils (HHTGO), at LNB are summarised in Table 2.
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