Post on 24-Dec-2015
Lubrication of synovial joints
• The extremely low value of coefficient of friction found in bone joints (~0.01) is due to the combined effect of the cartilage tissue as well as the synovial fluid present.
• Cartilage tissue at the ends of bones are soft, slippery and deformable.
• Synovial fluid contains certain molecules that cause the low COF.
http://www.shoppingtrolley.net/images/anatomy/synovial-joint.jpg
Lamina splendens
Collagen fibers
Proteoglycan aggregate
Chondrocyte cells
Calcified cartilage
Subchondral bone
Superficial tangential zone (STZ)
Middle zone
Deep zone
Zone of calcified cartilage
• Cartilage is made up of a porous material.• It contains 70% by volume water.• It is therefore considered to be a bi-phasic material.• Within the cartilage there are hydrophilic molecules that hold
the water within.
Lubrication regimes in a bone joint
• The bone joint undergoes a combination of lubrication regimes.
• Due to the deformable nature of cartilage, there is elastohydrodynamic lubrication (EHL).
• Under standing conditions there is normal squeeze film lubrication.
• When there is heavy load, the cartilage undergoes weeping lubrication.
• In weeping lubrication, the fluid that is present within the cartilage is squeezed out into the joint space to provide lubrication.
Synovial fluid- characteristics• The lubricant in a bone joint is called synovial fluid.
• Syn means like and ovial means egg white.
• The lubricant is therefore similar to egg white.
• It is non-newtonian in nature.
• This non-newtonian behavior is provided by a molecule known as Hyaluronic acid containing a chain of the COO- group approximately 1.5 nm apart.
• In a healthy joint, the molecular weight of the hyaluronic acid is approximately 5 x 105 and its concentration is 0.1-5 mg/ml.
• The hyaluronic acid of diseased joints gets depolymerized and the fluid does not have non-newtonian behavior.
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Atmospheric distillation
• In an oil refinery, the principal process for converting crude oil into a range of marketable products is distillation
• After removal of gaseous products, the crude is distilled in a column at atmospheric pressure
• All materials boiling at temperatures below 340 oC (approximately) are vaporized in stages depending on their boiling temperature
• These materials are then condensed in order of increasing boiling range, at successively lower points in the column, and are drawn off
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Atmospheric distillation (contd.)
• The distillates obtained are the raw materials for production of gas oils, kerosines and motor oils.
• The residue from the bottom of this column contains materials boiling above about 340 oC, and is the raw material for manufacture of lubricating oils, fuel oil, waxes and bitumen.
• This residue is generally known as ‘atmospheric residue’.
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Vacuum distillation• The temperature to which the residue from the atmospheric
column can be raised is limited by its tendency to decompose or ‘crack’.
• Thus, further separation is effected in a vacuum distillation column where boiling is achieved at moderate temperatures by operating under reduced pressures.
• The products from the vacuum distillation column comprise a series of three or four distillate fractions separated according to boiling range, and a vacuum residue.
• The distillate fractions provide a number of materials with different viscosities, and an oil with a higher viscosity than the distillates is produced from the residue.
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Vacuum distillation (contd.)
• Some heavy gas oil is also produced in this process and is removed from the top of the column.
• The distillate and residual fractions from the vacuum column are not, at this stage, suitable for blending into finished lubricating oils, but require to be treated to remove undesirable constituents.
• The treatments applied vary according to the properties of the crude.
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Bitumen• Bitumen is a mixture of organic liquids that are highly viscous, black,
sticky, entirely soluble in carbon disulfide, and composed primarily of highly condensed polycyclic aromatic hydrocarbons.
• Naturally occurring or crude bitumen is a sticky, tar-like form of petroleum which is so thick and heavy that it must be heated or diluted before it will flow.
• At room temperature, it is much like cold molasses. Refined bitumen is the residual (bottom) fraction obtained by fractional distillation of crude oil.
• It is the heaviest fraction and the one with the highest boiling point, boiling at 525 °C (977 °F).
• Bitumen is primarily used for paving roads.
• It is also used for water proofing products, blending with other oils, and preservation of fossils.
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Solvent de-asphalting
• The residue from the vacuum distillation column can be used as a blend component for fuel oils, or for bitumen manufacture.
• It can also be processed to yield a high viscosity lubricating oil.
• For lubricant production, the vacuum residue is treated with liquid propane, which dissolves the oily and waxy components and leaves the asphaltic residue as a precipitate.
• After separating the precipitate, the propane is stripped off for re-use and the treated residual oil proceeds to further processing, after which it is generally called ‘Bright stock’.
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Solvent extraction or acid treatment
• All of the oils, including the de-asphalted vacuum residue, contains resins and aromatic hydrocarbons which impair stability and oxidation resistance and lower the viscosity index.
• These materials are removed, either by treatment with concentrated sulphuric acid, or by the more modern process of extraction with solvents.
• The principal solvents used in the extraction process are furfural and phenol.
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Solvent extraction
• The disadvantage of the sulphuric acid process is that acid sludges are formed, which present a disposal problem.
• Extract from the solvent treatment can be stripped to recover solvent and the residue is suitable for fuel oil blending.
• Very heavy acid treatments are used to produce speciality products of highly paraffinic character such as medicinal ‘liquid paraffin’.
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Solvent dewaxing
• Wax refers to crystalline substance in crude oil which are solids at room temperature.
• The waxes are normally dissolved in the lubricating oil stocks, but are readily precipitated at low temperatures to form a solid matrix which holds the liquid oil in the interstices.
• Thus, wax has the undesirable property of preventing oil from flowing freely at low temperatures, and is therefore removed.
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Solvent dewaxing
• In this process the oil is dissolved in a mixture of solvents, such as methyl ethyl ketone and toluene, and chilled
• The wax is precipitated as a crystalline solid and is filtered off
• The solvent is stripped from the dewaxed oil for re-use, and the wax is treated separately to produce a marketable product
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Finishing treatments• The lubricating oil cuts receive a final treatment to
neutralize or remove any acidic materials, to improve the color and increase the oxidation stability.
• Normally the oil is either percolated (passed through porous substance or small holes) through a bed of clay or mixed with finely divided clay, heated and filtered.
• The spent clay is removed and either discarded or reactivated for further use, and the oil is passed to storage.