Lubricating Oils

Report
Dr Fatma Ashour
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Lubricating oils (or “Lube Oils
are heavy petroleum fractions
used
to
lubricate
metallic
surfaces which are in relative
motion with respect to one
another.
The essential aim of these oils is
to minimize the friction between
the moving surfaces and hence,
prevent mechanical wear in the
metals, and to absorb the heat
of friction.
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General steps followed in producing finished lubricating oils are:
A-The crude oil is initially fractionated under atmospheric
pressure to produce the conventional top products (gases and
gasoline) side products (kerosene, gas oil, and perhaps diesel oil)
and a bottom product, usually known as the “Long Residue”
(known as Mazot).
B-The long residue is then fractionated under vacuum to
produce a top product used for diesel oil, and side products
which could be used for producing finished lube oils. These side
products of the vacuum tower are known as “Wax Distillates” The
bottom product of the vacuum tower, known as the “short
Residue”, may be asphalt on heavy oil.
In either case, this bottom product could be used for producing
heavy lubricating oils, which are then called “Residual Oils”.
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The products obtained from the vacuum distillation
tower (cannot be marketed as such. They should be
refined to produce the required grade of finished
lube oils.
Deasphalted to remove any asphalt they may
contain

Solvent refined to decrease the effect of
temperature on its viscosity (i.e. improve its
Kinematic Viscosity Index-KVI)

Dewaxed” and possibly “percolated through clay” to
remove any solid particles or coloring material
 Neutralized”, especially if they have been previously
subjected to acid treatment.
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In general, there are FOUR basic “Lube Oil Stocks” - three
of which are distillate oils and one is a residual oil.
As a matter of fact, more “basic stocks” could be
produced by the vacuum distillation of one or more of the
previously obtained oils.
Basic stocks are then “blended” with one another, in
various proportions, to produce the optimum viscosity
range necessary for the market.
The lube oil is finally “compounded” by the addition of
various additives to improve the quality of the oil. After
compounding, the finished lube oil is ready for selling on
the market.
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Viscosity:
Lube oils should have a “suitable viscosity”. As
a matter of fact, for a certain lubricating job, the
lube oil must have a definite optimum viscosity.
Below this optimum viscosity, the lube oil cannot
properly prevent the friction between the moving
metallic parts; while above this optimum
viscosity, the lube oil itself requires a greater
power to overcome its high resistance to motion,
and consequently this is a waste of power.
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To determine the optimum viscosity required of a
certain lubrication job, use is made of the fact
that the “Coefficient of Friction” between any
moving surfaces depends upon the following
“dimensionless group”[ N/P]
Where,
 = viscosity of the lube oil.
N = relative speed of the moving surfaces.
P = stress between the moving surfaces.
Since the values of N and P are fixed by the
operating conditions, then for minimum friction,
the value of  is also fixed.
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For minimum friction, higher viscosity oils are
required for low speeds of rotation and for high
stresses
between
the
moving
surfaces.
Conversely, lower viscosity oils are required for
high speeds of rotation and low stresses between
the moving surfaces.
Examples of the choice of the optimum viscosity
of lube oils are given below:

In the case of lubricating, textile spindles, the
pressure load is very low and the speed of
rotation is very high consequently, the proper
lube oil to be used should have a very low
viscosity. For this reason, lube oils with the
lowest viscosity are known as “spindle oils”.
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In the case of lubricating cylinders
of big engines, the speed is relatively
low and the pressure is high, thus
the proper lube oil to be used should
have a relatively high viscosity. Such
lube oils are known as “Cylinder
oils”.
In case of lubricating gears, the
stresses on the gears are very high,
while
the
relative
motion
is
significantly low; accordingly, lube
oils of very high viscosities are
therefore needed, and these are
called “Gear Oils”.
N.B: Gear oils and cylinder stocks are
usually manufactured from residual
oils.
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The K.V.I is a measure of the variation of
viscosity with temperature.
As a rule, the viscosity of a lube oil decreases
with increase in temperature. The variation of
viscosity with temperature depends upon:
- The type of crude
- The extent of refining
As a matter of fact, paraffinic hydrocarbons
change their viscosities to a smaller extent
than naphthenic ones.
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This scale has been developed on the basis of Two “Reference
Crudes” namely:
i. Pennsylvanian Crude (PC) - highly paraffinic crude.
ii. Gulf Coast Crude (GCC) - highly Naphthenic crude.
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Any lube oil derived from the first reference crude would have a
relatively small variation of viscosity with temperature, and is
arbitrarily given a K.V.I value of 100.
On the other hand, any lube oil derived from the second reference
crude would show large variations of viscosity with temperatures,
and is therefore given a K.V.I value of zero.
To find the K.V.I of any lube oil, its kinematic viscosity at two
standard temperature levels of 210˚and 100˚F is measured.
Generally lube oils are broadly classified into the
following TWO grades according to their K.V.I
values:

Premium grade oil having a K.V.I of 85 or
higher.
 Regular grade oils having a K.V.I of 80 or less.
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N.B. Lube oils having K.V.I values between those
2 grades are sometimes called “Intermediate
grade oils”. Furthermore, lube oils are not
marketed unless their K.V.I. value is at least 70
(not less).
Some lubes have a K.V.I value higher than 100;
this means that the change of their viscosities
with temperature is less than the change in
viscosity of the corresponding lube oil from the
first reference crude.
 Such a lube could only be obtained by:
 Starting from crude which is more paraffinic
than the Pennsylvanian crude (PC)
 Subjecting the lube to intensive solvent refining
 Adding, certain additives known to increase the
K.V.I such additives are known as “VI-improvers”.
N.B. Similarly oil with a (-ve ) K.V.I indicates that
the variation in viscosity (kinematic viscosity)
with temperature is greater than the variation of
the kinematic viscosity of the Gulf Coast
reference crude (GCC).
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Stricter environmental requirements and mileage
standards for new cars have created a growing
demand for high-quality multigrade motor oils
which have lower volatility and oil consumption
characteristics and reduced thickening of the oil by
oxidation during service which increases fuel
consumption.
Solvent refined oils have difficulty in meeting the
new standards and are increasingly being replaced
with hydrocracked and poly-alpha-olefin based
oils, especially in the lower viscosity grades such as
5W-30 and 10W-30 multigrades.
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The low-viscosity multigrade oils are typically
blended from low-viscosity mineral-based oils
which have volatilities and tendency to high
oil-consumption and rapid thickening by
oxidation during service.
Hydrocracking of base stocks, followed by
solvent
extraction
to
remove
partially
hydrocracked aromatic compounds, offers a
more cost-effective route than production of
poly-alphaolefins
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Wax particles in lube oils are undesirable for the
following reasons:
A high wax content results in a high pour point,
hence the lube will tend to solidify in cold weather
conditions (causing friction).
Wax is susceptible to cracking at high T & P. Hard
solid particles deposited on the lubricated surfaces
will increase the friction.
Wax is easily oxidized and will result in the breaking
of the chain into shorter molecules, and hence the
viscosity is lowered.
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N.B. Wax was formerly removed from lube
oils by “Chilling” operations, wherein the wax
solidifies and could then be filtered off, or
left to SWEAT.
More modern dewaxing methods are now
adopted using “Solvent Refining” methods,
(e.g. using propane or ketones especially
methyl ethyl ketone – MEK). As a matter of
fact, the oil is soluble in these liquid solvents,
and thus the wax could be filtered off.
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Asphaltic particles are undesirable because they would tend to
stick to the lubricated surface and increase the friction between
them.
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Deasphalting is therefore one of the main processes adopted in
lube oil refining.
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Sulphuric acid treatment or “percolation through active clay” to
adsorb the asphaltic particles.
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Solvent extraction processes.
Typical solvent used is propane- ie the “propane deasphalting”
methods. In this process, the propane dissolves the oil and
leaves the asphaltenes (hard asphaltic particles) to deposit and
could be separated by filtration.
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The most important refining operations to which these raw fractions
are subjected to are outlined in the following steps:
1- Deasphalting: to remove any solid or asphaltic particles which
deposit on the lubricating surfaces causing friction. Deasphalting is
accomplished by the following methods:
A- H2SO4 treatment (98%): carried out in a series of agitation and
settling tanks at temperatures ranging from 35-65 C and not
higher because of possible cracking.
B- Clay treatment: the raw fractions are basically percolated through
active clay.
C- Solvent extraction: the raw fractions are essentially dissolved in
a suitable solvent
(e.g. propane) whereby asphaltic particles are deposited and can
thus be removed.
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2- Solvent Extraction: to improve the KVI of
the lube oil through the removal of ring
compounds,
namely
naphthenes
and
aromatics
whose
viscosities
change
substantially with tempt.
This is achieved by a solvent extraction
process using special “selective solvents”
such as phenol, chlorex, and above all
furfural which is presently the most widely
used solvent.
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Solvent refining process is carried out to remove
naphthenes & aromatics (in addition to acids) thus
resulting in the improvement of the KVI of the
finished lube oil.
The most widely used solvent is Furfural (a liquid
aldehyde of B.P = 180C, heavier than water but
miscible with it, obtained as a byproduct in the starch
industry-especially when starch is produced from
maize).
N.B. Such a solvent, called a “Selective solvent”, only
dissolve the ring structures which DO NOT have a
LONG side chain (i.e. : those which lower the KVI),
this is because such ring structures with a long side
chain could break to n-paraffins that have a high KVI.
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Flash Point: Normally, the flash point of lube oil is not
of particular importance. It is sometimes essential
however, to have lube oils with high flash points.
Such oils could be obtained by “steam refining” (ie. by
flashing the lighter constituents in the oil using
superheated steam). These oils having high flash
points are referred to as “steam refined oils”.
For example, lubricating oil given the grade (600 SR)
indicates oil which has been steam refined and whose
flash point is 600˚F.
N.B. The flash point of lube oil is used to detect
contamination with lighter volatile hydrocarbons.
Ex: Crank-case oil when it is contaminated with
gasoline. Flash point determination is the easiest
method to detect such a contamination (or cheating).
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Acidity: Acidity in the lube oil resulting from
mineral or organic acids, leads to corrosion of
the lubricated surfaces. Hence, any acidity in
the lube oil must be neutralized before
marketing. Solid lime is usually used for that
purpose.
Emulsification: When the lube oil is to be
used in high speed steam turbines, it usually
has the tendency to form emulsions with the
water. Such emulsions destroy the lubricating
properties of the oil. For this reason, certain
additives are used to reduce such a tendency.
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Furfural is a liquid aldehyde, which is susceptible to oxidation,
hence the lube oil charge (feed) must be deaerated before
mixing it with the solvent, by passing it through a “Deaerator”
which is subjected to a vacuum system.
The deaerated lube is then introduced into an Extraction tower
where it meats a counter current flow of the furfural solvent. The
raffinate will be obtained through the top of the extractor while
the extract will be obtained through the bottom.
N.B. Both the raffinate and the extract contain some solvent
which must be recovered. Accordingly, both streams are
subjected to a series of fractionation Process followed by steam
stripping to recover the furfural solvent.
The solvent furfural, recovered from the various parts of the
plant, is collected then mixed with the make-up solvent and
finally recycled to the extractor tower
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Dewaxing process is carried out by:
Clay treatment
Chilling operation and most widely by solvent extraction. In the
latter method, the solvents generally used are ketones of different
types (e.g.: acetone, methyl – ethyl ketone MEK, propyl – ethyl
ketone or diethyl ketone)
N.B. It has been found that “Ketones” are excellent dewaxing
agents whereby they dissolve the oil and precipitate the wax.
Furthermore, it has also been found that when aromatics are mixed
with ketones (i.e.: in the form of a “Solvent Mixture”) then, the
aromatics reduce the solubility of the wax in oil resulting in a higher
efficiency of dewaxing (greater than the propane dewaxing process).
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The most widely used dewaxing process
is the “MEK Dewaxing Process” where the
solvent ketone is methyl ethyl ketone
(MEK) and the mixed aromatics are
essentially benzene and toluene in
approximately the following proportions:
MEK
= 65%
Solvent Mixture : Benzene = 25%
Toluene = 10%
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Propane deasphalting is based on the fact that liquid propane is
capable of dissolving the lube oil & precipitating the asphaltic
particles.
N.B. The rate of precipitation (i.e. the extractive power) of
asphaltic particle is increased by increasing the temperature.
It has been found the optimum operating temp is about 70C
(not higher otherwise the paraffin would crack) using a high
press of 450 psi (to maintain the propane in the liquid phase).
The lube oil and liquid propane are contacted in vertical drums
which are heated by internal steam coils. These drums are
maintained at the required operating conditions (70C & 450
psi). The asphalt is obtained from the bottom while the lube oil,
containing the main bulk of the propane solvent, is withdrawn
from the top.
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N.B. Both the “raffinate” (lube oil) and the “extract”
(asphalt) contain propane. This propane is recovered
by heating and evaporation using a TSH for asphalt
and a reboiler for the lube oil. The reboiler still
contains some propane which is removed by a
stripper (i.e: using S.H.S).
Asphaltic particles are obtained as a bottom product
from a stripper.
N.B. The propane obtained from the various parts
of the process units, is collected, compressed and
mixed with make-up propane then finally recycled
with the feed.
Advantages of this Process:
a) Propane could be obtained from the refinery at a cheap price.
b) Efficiency of separation is very high.
c) Only asphaltenes are removed hence the oil, originally
present with the asphalt, is recovered with the lube oil.
d) This method could be applied to any lube oil.
Disadvantages of this Process:
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The only disadvantage is that hard asphaltenes have No use.
This was overcome by any of the following procedures:
i) Adding them to soft asphalt to increase its hardness.
ii) Adding them to viscous Fuel Oil to convert it to ordinary
F.O.
iii) Adding them to soft asphalt then blowing with hot air to
produce much harder grade of asphalt, known as “Air blown
asphalt”.

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