Basics in base oil and lube
Basics in base oil and lube
This section is designed to bring together information that is useful to those working in the areas of base oils and lubricants.
As such it will be regularly updated and changed as needed.
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What is lubrication?
Friction exists in all moving parts. It can be beneficial or destructive, depending on how it affects the moving parts.
In a moving vehicle, friction is applied to make it come to a halt. In this case, friction is applied constructively and there is no need to reduce or minimise it.
In a machine, friction between moving parts causes wear, and if allowed to continue, can cause damage to the parts. In this case, friction is considered destructive, and it needs to be minimised to prolong the life of the machine.
Applying a thin film of a suitable lubricating oil to the parts helps to prevent them from rubbing directly against each other. The continuous presence of an oil film between the moving parts is referred to as lubrication.
What is a lubricant?
A lubricant is made from a base oil and additives. The quality of a lubricant, therefore, depends on the quality of the base oil and additives used in the formulation. Additives, although used in relatively small quantities, play a very important role in the performance and composition of the lubricants.
Many types of additives used including:
——Pour-point depressants to decrease the pour point of the oil
——VI improvers to increase the viscosity index, i.e. to reduce the viscosity at low temperatures
——Anti-wear additives to decrease the engine/unit wear
——Anti-oxidants to extend lubricant life by reducing degradation by oxygen from the air
——Ashless dispersants to keep particles or deposits in suspension
What is a base oil?
The oils blended with additives to produce lubricants are the base oils.
They are produced by processing feedstocks (crude oils) that have varying compositions of paraffinic, naphthenic, and aromatic compounds. Some of the properties of the base oils have a direct influence on the performance of the finished lubricant.
Base oils are made of:
——Paraffinic compounds. These include normal (straight-chain) and iso (branched-chain) paraffins
——Naphthenic compounds. These contain one or more saturated rings of 5 to 6 carbon atoms to which paraffinic-type side branches are attached
——Aromatic compounds. These are molecules containing unsaturated carbon rings with paraffinic side branches.
There are two types of conventional base oil – naphthenic and paraffinic – depending on the composition of the crude oil from which they are produced. There is a larger number of different unconventional base oils – XHVI, PAO, esters, etc. which are made from a wide range of materials.
Naphthenic base oils have low viscosity index (VI) (generally 40–80) and a low pour point. They are produced from feedstocks rich in naphthenes and low in wax content. They are used mainly for lubricants in which colour and colour stability are important, and VI and oxidation stability are of secondary importance.
Paraffinic base oils have higher VI (generally >95) and a high pour point. They are produced from feedstocks rich in paraffins, and are used for lubricants for which VI and oxidation stability are important.
Each of these types has several grades, according to its viscosity.
Base oils used to produce lubricants must have the following important properties clearly defined:
Quality of base oils
The viscosity of an oil is a measure of its resistance to flow, and depends on the temperature at which the measurement is made. It has high values at low temperatures, and low values at high temperatures. A viscosity measurement has no value unless it is associated with a temperature.
In the past, viscosity was expressed in Engler (Europe), Redwood (UK) and Saybolt (USA). Nowadays, centistoke (kinematic viscosity) and centipoise (dynamic viscosity) are commonly used units.
As viscosity depends on temperature, a measure of its sensitivity to temperature is expressed by its viscosity index (VI) value. VI is determined by comparing the change of viscosity with temperature to two reference oils – one of which changes very little with temperature and is given a VI = 100, and another which changes a lot and is given a VI = 0.
VI = 100 x (L – U)/(L – H)
L = viscosity at 100°F of reference oil with VI = 0
H = viscosity at 100°F of reference oil with VI = 100
U = viscosity at 100°F of the base oil
V = viscosity at 210°F of the base oil
An oil with VI = 95 changes its viscosity less with temperature than one with a VI = 90. The VI of an oil depends on the market needs. [see “classification of base oils”]
Most oils, when exposed to air over time, react with oxygen. The Turbine Oil Stability Test (TOST) and the resulting TOST life are measures of the oxidation stability, i.e. how much it degrades in the presence of air.
Oils used to blend lubricants must have high oxidation stability, otherwise, they become discoloured during storage. When base oils with poor oxidation stability are used in engines, the high temperature causes them to form corrosive acids and insoluble sludge. This can hamper the engine’s performance by forming a hard layer in the grooves of the piston rings.
Pour point and cloud point
The cloud point of an oil is the temperature at which the first trace of wax starts to separate out, causing it to become turbid or cloudy. If the temperature is reduced further, more wax will crystallise out until a point is reached when the oil and wax crystallise together as a whole, and will not flow when poured. The temperature at which this just happens is the pour point of the oil. Oils used for lubricants must have a low pour point so that in areas where the temperature is very low, they remain as fluids. A good cloud point ensures that they remain clear and bright in such areas.
The flash point of an oil is the temperature at which its vapour ignites when exposed to a flame. A minimum flash point is normally specified for safety reasons.
The colour of a base oil has no influence on its performance properties. However, because it is easy to measure it is often used to provide indications of consistency, i.e. of possible contamination. The colour is determined via an ASTM test converting the colour of an oil into a unitary scale:
Pale = 4.5 ASTM colour or lighter
Red = darker than 4.5 ASTM
Dark = darker than 8.0 ASTM
The heavier the base oil, the darker the colour. A dark colour may also be indicative of oxidative degradation having taken place (see colour stability).
To prevent discoloration when stored for a period of time, the oils must have good colour stability.
The carbon residue of an oil is what remains after it is evaporated. This is usually measured by the Ramsbottom carbon residue (RCR) test. Oils used for lubricants must have low carbon residue so that they do not leave residue deposits when evaporated after being subjected to the heat of an engine.
Base oil manufacture
Naphthenic and paraffinic base oils are produced via different processes since they come from feedstocks with different compositions.
Earlier plants for the processing of naphthenic base oils use sulphur dioxide extraction, sulphuric acid and clay treatment. Modern plants use less hazardous hydroprocessing to upgrade the distillates.
Paraffinic base oils require steps that include vacuum distillation, propane and furfural extraction, and solvent dewaxing. Hydroconversion (hydroprocessing and catalytic dewaxing) is applied in newer plants
Base oil plant configurations
There are three main steps in processing base oils from vacuum distillate and de-ashphalted oil:
——Refining to increase VI
——Dewaxing to reduce pour point
——Finishing to improve colour and oxidation stability
In addition, distillations are often used before, between and/or after refining, dewaxing and finishing steps to adjust viscosity, volatility and flash point.
A typical base oil plant, consisting of a high-vacuum unit (HVU), propane de-asphalting unit (PDU), furfural extraction unit (FEU) and methyl-ethyl-ketone (MEK) dewaxing unit (MDU), is referred to as a solvent extraction (solvex) configuration. The FEU can be replaced by a hydroconversion unit (HCU). In this case, the plant is said to have a hydrocatalytic (hycat) configuration.
If, instead of replacing the FEU, an HCU is added downstream to it, the base oil plant is said to have a hydroconversion after extraction (hydrex) configuration.
Hydrex configuration is effectively a composite of solvex and hycat configurations. If different modes are employed – one grade by solvex mode, another by hycat mode and the rest by hydrex mode – it is referred to as hybrid configuration.
An approved crude oil must undergo several processing steps before base oils of high viscosity index (HVI) are finally produced.
It is first processed in an atmospheric distillation unit where different petroleum fractions – ranging from tops, naphtha, kerosene and gas oil to bottom product, called long residue (LR) – are produced. For HVI base oils manufacture, the first step is to recover the useful distillates that still remain in the long residue coming from the atmospheric distillation unit. This is done in a high vacuum unit (HVU).
In the HVU, LR is distilled in conditions of high vacuum and temperature. The high vacuum condition is necessary as the residue would require a much higher temperature at atmospheric pressure. Such a high temperature would crack the LR into light fractions, making the recovery of distillates impossible. Also, the temperature would be too high for thermal stability of the oils. Employing a high vacuum, therefore, makes it possible to apply a relatively low temperature in the HVU without cracking or causing instability to the distillates recovered.
Recovered from LR in the HVU are:
——Light distillate, or spindle oil (SPO)
——Medium distillate, or light machine oil (LMO)
——Heavy distillate, or medium machine oil (MMO) and a remaining
Each distillate is recovered within a viscosity range corresponding to its base oil viscosity. They are normally stored in intermediate tanks to await further processing.
The product coming out of the column bottom is the short residue (SR). It still contains useful oil that is not distilled, even at the high-vacuum conditions of the HVU. To recover this oil requires a higher temperature, which would jeopardise the thermal stability of the distillates, which would rapidly turn dark upon storage. To avoid this problem, the oils remaining in SR are recovered by other means.
De-asphalting for preparation of bright-stock
Short residue from the HVU still contains a significant amount of heavy oil, which can be used for producing heavy base oil. This class of base oil is often referred to as bright-stock.
To recover the heavy oil in SR requires a much higher temperature than that which causes the oil to crack. In most base oil plants, this is achieved by solvent extraction in a propane de-asphalting unit (PDU), where the solvent applied is liquid propane.
There are three main sections in a PDU – the extraction section and two product recovery sections. In the extraction section, SR is routed into the upper level of an extraction column. This is because SR is heavier than propane, which is routed to the lower level of the same column. Extraction is assisted by rotor and stator discs installed in the column to improve liquid contact. Extraction is effected by density difference utilising counter-current flow of SR and liquid propane. In some cases, efficiency of extraction is improved by rotating the rotor discs.
Typically, the amount of propane required ranges from two to four times that of SR. If too little is used, the extraction may not be sufficient, resulting in low yield and low viscosity. If too much is used, undesirable heavier oils are also extracted which results in higher yield of heavier and poorer quality oil.
Liquid propane is chosen for its high selectivity and solubility in oil compared to the heavier (asphalt) material. Temperature affects the extraction performance. It is kept at a level according to the quality of oil desired, namely, viscosity and colour.
After going through two recovery sections, the propane is recovered for re-use in the process. The de-asphalted oil (DAO) is then stored in an intermediate tank to wait further processing in downstream unit. The by-product, asphalt, is used for bitumen manufacture or blended into the refinery fuel-oil pool.
Upgrading of the distillates
This step involves removing aromatic compounds from the oils. This is done in an FEU or in an HCU, or in a combination of both units.
Solvent refining uses furfural, phenol or N-methyl pyrrolidone to extract undesirable components such as low-VI aromatics, naphthenes and some hetero-atoms. The resulting products are higher VI raffinate and a highly aromatic extract with relatively low value, which can be blended into fuel oil or further processed in a fluid catalytic cracker (FCC), or a fuels hydrocracker.
Feed to the FEU is distillate from the HVU, or DAO from the PDU. The feeds are processed one grade at a time – called blocked-out operation – for a period ranging from three to seven days. There are three main sections in the FEU – the extraction section, and the two product recovery sections.
In the extraction section, feed is routed to the bottom level of an extraction column while furfural, whose density is higher, is routed to the top level. The column is equipped with stator and rotor discs, which improve the extraction by increasing liquid contact between the two streams. The extraction process is effected by density difference utilising counter-current flow of feed and furfural. In some cases, the rotor discs are rotated to improve the efficiency of the extraction.
The amount of furfural applied ranges from two to five times that of the feed, depending on grade processed. If too little is used, extraction may not be sufficient, and the recovered raffinate will have a high yield of poor VI. If too much is used, the over-extraction results in low raffinate yield.
The main purpose of extraction is to remove aromatic compounds from the feed. These are undesirable components since they have a very poor VI. The furfural dissolves them preferentially out of the feed, leaving aromatic-free (paraffin-rich) saturates – called raffinate – with improved VI. In addition, some other undesirable compounds are also removed which could otherwise affect the oxidation stability of the product.
FEU is used, therefore, where the VI and oxidation stability of raffinate are improved before further processing into base oils. The raffinate is kept in an intermediate tank to await further processing. The by-products – extracts (aromatics) – are used as feed for some other refinery process, or as dilutents in refinery fuel oil pool.
In a hydro-conversion unit, aromatics in feed are converted into useful base oil materials using hydrogen in the presence of catalysts under severe temperatures and pressures. Depending on the configuration of the plant, feed to the HCU is either distillate from the HVU, DAO from the PDU or raffinate from the FEU. The feeds are processed in the HCU one grade at a time – in blocked-out operations.
The feed is first pre-mixed with hydrogen-rich gas, recycled from within the unit, before being heated in a furnace to the reaction temperature. It enters the top of a reactor which consists of beds of catalyst. The catalysts promote several different kinds of reaction at the same time. These are usually strongest in the first bed, and gradually less so in subsequent beds. The temperature rises in the catalyst beds since the reactions are mainly exothermic in nature. Safety is a major concern due to the nature of gases (hydrogen and hydrogen sulphide) and the possibility of uncontrolled temperature increase in the catalyst beds – referred to as temperature runaway.
Reactions in the catalyst beds are rather complex. It is sufficient to say that the final product, after the separation sections of the unit, contains very few aromatics and an appreciable amount of light materials which need to be removed. The removal of light materials is usually done in a re-distilling unit (RDU) where the light ends are removed and the viscosity of the final product controlled.
The HCU is where the VI of product is improved, while the RDU is where the correct viscosity is maintained. Treated product is stored in an intermediate tank to await further processing in the next unit.
Wax removal step
The last step involves removing paraffin waxes from the raffinates so that the base oils produced have a sufficiently low pour point. This is done in a methyl-ethyl ketone (MEK) dewaxing unit (MDU) where the various grades are also processed in blocked-out operations.
Solvent dewaxing uses solvent or solvent mixtures (e.g. MEK or MEK/MIBK) to remove waxy (high pour) hydrocarbons by crystallisation and filtration. This yields solvent dewaxed oil (SDWO) and slack wax (SW). Solvent mixture is injected into the raffinate to facilitate the rejection of crystals (mainly n-paraffins) from the feed by freeze-crystallisation. The crystals are separated from the oil and solvent by rotary drum filters.
The MDU has a number of sections for the feed to pass through before the base oil is finally produced.
The cooling and chilling section is first, where feed is pre-diluted with solvents in stages as it is gradually chilled to a sub-zero temperature. Chilling is achieved by vaporising liquid propane in double-piped heat exchangers. These exchangers consist of two concentric tubes through which the two streams exchange heat. Solvents used are methyl-ethyl ketone (MEK) and toluene. MEK helps to crystallise and precipitate the wax, while toluene ensures the oil remains in solution at the sub-zero temperature. In these conditions the waxes appear as crystals, making it easier for filtration to take place in the next section.
Filtration is the second section, which consists of a number of rotary vacuum filters for dewaxing by separating the wax from the base oil. The plus solvent – called slack wax – and base oil are routed to separate recovery sections, where solvents are recovered for re-use in the process.
The base oils coming from the MDU have a very low pour point. They are kept in product tanks as finished base oils. The slack wax is fed to other refinery units or, if suitable, routed to tanks to be sold as wax.
In the past, plants have included an additional processing step: the MDU. This is the hydrofinishing step, where the stability and colour of base oil are improved by treating with hydrogen in the presence of catalyst. Due to modern severe extraction and hydroconversion processes, this step is no longer necessary.
Catalytic dewaxing (CDW)
Catalytic dewaxing is a process to selectively convert wax catalytically in the presence of hydrogen. It produces low pour point lubricating base oils from either hydrocracked or synthetic (Fischer Tropsch products) stocks or from raffinates from furfural/NMP solvent extraction units. Very low pour point speciality oils such as those required for transformer and refrigeration oils, and for greases can be produced. By-products include a range of light hydrocarbons and middle distillate fractions.
The process is well-suited to integration with the various luboil hydroprocessing routes or with Hydrogen Finishing technology, in which case a single unit can simultaneously achieve product pour point and other specifications, such as S (and N) and saturate content specifications.
The feedstock is mixed with hydrogen, heated to the desired reactor inlet temperature and charged to a fixed bed hydro-dewaxing (HDW) catalytic reactor. The hydro-dewaxing catalyst reduces the pour point of the oil by selectively converting the waxy components into oil, middle distillate and light products. The hydro-dewaxed effluent is fed to an hydrotreating reactor to stabilize the dewaxed oil.
The hydro-dewaxed base oil product is flashed to remove unreacted hydrogen (and its reaction products (H2S, NH3) which is recycled back to the feed system. Light hydrocarbon by-products are removed by distillation and/or steam stripping.
The pour point of the base oil is controlled by the HDW reactor inlet temperature. As the catalyst ages, this temperature is increased to compensate for decreased catalyst activity. Periodic rejuvenation of the dewaxing catalyst by flushing with hot hydrogen optimizes the useful life of the catalyst.
Traditional ways of lube oil finishing include adsorbent clay (which removes some of the undesirable molecules, e.g. solvents or solvent compounds) or base-metal-catalysed hydrofinishing (which saturates aromatics in a hydroprocessing unit).
Fuels hydrocracking examples – making VHVI from fuels hydrocracker bottoms, e.g.
——BP HC VHVIs, Lavera and Singapore
——Yukong, Ulsan, Korea
——Neste, Porvoo, Finland
Lube oil hydrocracking examples – Gp (I), II or III
——Shell Petit Couronne, France (Hycat process)
——Shell Geelong, Australia (Hydrex)
——Mobil Jurong, Singapore, Petro-Canada Minnissauga, Canada
——Chevron Richmond LOP, USA
——Excel Paralubes, Lake Charles, USA
Typical hydroprocessing route
VDU – affects viscosity and volatility
HTU – removes catalyst poisons (sulphur, nitrogen) affects oxidation stability, colour and surface properties
HCU – affects viscosity index and engine performance
Distillation – into required viscosity grades
CatDW or iso-DW – affects low-temperature performance and pour point
HFU – affects colour and surface properties
An overall classification of base oils is in terms of their viscosity index (VI) (link to Glossary), LVI, MVI, HVI and XHVI denote low, medium, high and extra-high viscosity indices respectively.
LVI oils are made from naphthenic crude feedstocks. They have very low wax contents and therefore do not need dewaxing. These oils have poor viscosity/temperature characteristics (viscosity indices are about zero) and poor oxidation behaviour. They are manufactured by the hydrotreating of (un)neutralised naphthenic distillates or mild furfural extraction neutralised distillates. Neutralisation is by caustic soda wash. LVI oils are used in processing oils and greases, although their use is discouraged because of their potentially hazardous properties (certainly if manufactured via conventional extraction).
MVI oils can be produced from naphthenic or paraffinic crude distillates and are classified accordingly as MVIN or MVIP.
MVIN oils are made in a similar fashion to LVI oils, albeit at a more severe extraction level.
Both MVIN and LVI oils remain fluid at low temperatures and have good solvency.
MVIP oils require dewaxing. The oils are produced by high-vacuum distillation of long residue, followed by mild extraction or hydrotreating, with subsequent dewaxing of the distillates. The manufacturing of MVIP 1300 (a popular heavy grade) is done via propane de-asphalting of a paraffinic short residue followed by dewaxing.
* MVIP oils have poorer low temperature fluidity than MVIN oils
? but becoming excellent after addition of additive
Shell Base Oils uses a standard approval process to ensure that only fit-for-purpose base oils are used in the lubricant products of the customers of the Shell Global Solutions base oil group.
The base oils from Shell’s base-oil manufacturing plants are produced according to ISO-9001, or local equivalent, quality standards. On top of this they are also monitored regularly to provide a back-up to the quality systems. Base oils from non-Shell manufacturing plants are also monitored, largely at the request of customers who wish to use these base oils to optimise supply economics or are obliged by local legislation to use base stocks of local origin.
A large number of non-Shell base oils are produced to lower standards than those demanded of Shell base oils. Therefore a clear and effective approval process exists to ensure that non-Shell base oils meet our customers’ brand standards and to assure fitness-for-purpose. Approvals are granted on the basis of the performance of base oils in question. For marginal base oils, approved use may be limited to a few, specified lubricants and/or quality changes suggested.