The Practical Re-conditioning of Used Oil
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An Analytical Survey of an important Subject During the Course of Which the Author Makes Reference to the Several Systems and Different Types of Plant Employed for Cleaning Used Oil By j. C. Liddle
A.M.I.Chem.E.
ALL oils contain certain fractions which will oxidize readily under suitable conditions, and cheap oils usually contain a relatively high proportion of such less stable fractions. In an internal-combustion engine, crankcase oil is pounded into a fog, which consists of small drops of oil exposing large surfaces to the air, intimately mixed with traces of .the slightly acid fumes of combustion. Although the temperature of the pistons may not be high enough to carbonize the oil, the conditions are particularly favourable for chemical combination, and unstable fractions of the oil pollimerize, producing a sludge. This product will remain in suspension, particularly during agitation, but a saturation point is quickly reached, when the excess is precipitated in the form of a gummy slime. The fractions which readily oxidize are not of the highest lubricating value, therefore their elimination does not adversely affect the remainder. An excess of sludge, however, is detrimental.
Reasons for Oil Deterioration.
Lubricating oils do not wear out in a mechanical sense. They deteriorate in service from pollution with sludge and because they pick up water, solid particles of carbon, grit, metal, etc., which act as abrasives. When such impurities are removed the oil has less tendency to oxidize, whilst its lubricating qualities will be much the same as when B30
new. It has been proved repeatedly under practical conditions that dirty used oil can, with advantage, be reconditioned and put back into service.
The practice is, in fact, advocated by many engine .manufacturers and oil companies, and although not yet universally popular, it is becoming more generally adopted.
That oil does not " lose its nature !' by long service is proved by the abnormally long periods for which some en
gines are kept working on the same oil. Although in these cases the wear is almost always excessive, it is not because of any breakdown in the oil, but because of the abrasive action of the solids it acquires. In time, these particles become either sufficiently large individually, or numerous enough collectively to bridge the working films of oil, thus destroying the relative floating action between bearing surfaces. It follows logically that if all impurities were removed at intervals the same oil would function satisfactorily for these long periods, and the engine wear would be much less, whilst it is found in practice that oil can be cleaned and used, over and over again, an indefinite number of times without falling off in lubricating value.
Dilution of fresh oil by fuel takes place in an engine rapidly for the first few hours after filling, until a state of equilibrium is reached. About 2 per cent, dilution is usual, and this lowers the oil viscosity by about l2f per cent. A 6 per cent, dilution will cause a drop of about 40 per cent, and, under certain conditions, this is not abnormal. Occasional short runs will cause an increase in dilution and in water condensation, particularly if the engine be allowed to cool off between runs.
Utmecessary Dilution Fears.
Unless carried to excess, dilution iS not of such serious consequence as is sometimes feared. For instance, commercial-vehicle engines working with oil viscosity reduced by dilution by so much as 80 per cent, have not shown signs of excessive wear under service conditions.
In America a well-known company markets an engine oil under the name of Isoviz, which is an extra heavy lubricant deliberately prediluted with paraffin oil, in order to avoid early gumminess, and it is claimed that this oil does not thin down in service.
A new oil quickly loses its bloom in use, and after a few hours becomes almost black, yet its lubrication qualities have not appreciably deteriorated. It is obvious, therefore, that, for practically the whole of its working life the characteristics of an oil are different from those it possessed when in its original state, particularly in regard to appearance, viscosity, flash-point and specific gravity.
Reconditioning by the simple means under review does not bring back the • original bloom, and the colour is restored only to a limited extent. Oil thus cleaned compares favourably with Iubri cant which has been in use for only a few hours, but, on account of its lesser sludge-forming potentialities it may be preferable to a new oil.
Almost any -type of used oil can be reconditioned. There are several systems and many types of plant in daily use for cleaning oil, all of which have their merits.
The most elementary method is by settlement. This action is expedited if the oil be first heated to about 200 degrees F. Owing to the surface tension of the oil molecules, however, colloidal suspension is not overcome, and very fine impurities do not settle out.
Merits of the Centrifugal Method.
By applying centrifugal force, instead of gravity, a better result is obtained. A centrifugal machine will deal with large quantities of dirty oil in a comparatively short time. It is particularly effective in removing water and other heavy impurities, but fine particles in colloidal suspension are not readily extracted. The chief application of this method is, therefore, in dehydration, and for rapidly removing other heavy impurities, prior to further treatment.
A third method of cleaning is by filtration. A highly effective method is the Streamline, which employs laminations of paper, between which the oil is forced under pressure. Heat is employed to reduce viscosity. Suspended solids are retained by the fibrous inequalities of the paper surfaces which are held together by a spring-loaded device.
This method is particularly efficient when the filter packs 'are fresh, When the efficiency falls off, the filter packs can be taken out and cleaned by blowing air through the lamination in the reverse direction to the oil flow. It is claimed that, with ordinary care, the packs often last for over a year before requiring renewal.
Advantages of. a New System.
The application of chemistry to industry is responsible for the development of a comparatively new system of cleaning oil which offers many advantages. This method, developed by the Fox concern, consists of washing with a hot dilute aqueous solution, containing about 2 per cent, of a suitable chemical which acts as a coagulent. The solution is made to percolate through a body of oil. The re-agents employed generally consist of various powdered, sodium phosphate compounds which are readily soluble in water but are insoluble in oil.
An accompanying illustration shows diagrammatically a standard Fox oil cleaner consisting of a cleaning head mounted above storage bins. The head comprises a lagged vessel with a sparge tube in the centre, open at the bottom, and containing an electric immersion heater. Water is placed in the vessel up to the level of a cock. The dirty oil is next introduced on top of the water to within -an inch of the end of the central tube.
A measure full of the chemical preparation is then scattered on top of the oil or, alternatively, this may be dissolved in the water beforehand. On switching on the current, part of the heat from the water in the central tube escapes into the oil, which slowly begins to circulate. After about half an hour the water boils, and some of it is ejected through an orifice in the top of the sparge pipe, overflowing on to the hot oil, through which it sinks by gravity, to be recirculated by thermosiphonic action. The chemical coagulent causes the ultra-fine particles to be drawn together into larger units, which sink through the hot oil, forming a layer between water and oil.
Paradoxical though it may seem, dehydration is also promoted by this method. Any free acid or other water-soluble contaminent present is also entrained and removed with the washing solution.
About 31, to 4 hours' "cooking " is generally required, after which the current is cut off automatically by an adjustable time switch. The contents of the machine are then allowed to cool and settle, after which the clean oil can be run off.
By this means all water, and mechanical impurities, are practically removed. The apparatus will not remove fuel oil, but if petrol diluent is present, a proportion of this is driven off by the heat, This system has no mechanical working parts.
An Economic Proposition.
The oil to be cleaned should, of course, be kept apart from other lubricants. In garage work, however, gear oil may become accidentally mixed with the dirty engine oil. Such a mixture, which is unfit for use in an engine, generally emulsifies when washed, assuming a distinctive appearance which is readily detected, and which enables a check to be kept on supplies. For treating extra heavy engine oils, a pre-heater can be incorporated in the dirty-oil storage tank.
Even if the supplies of used oils available are not great, the installation of a cleaning plant is usually a sound economic proposition. In the case of road-transport concerns there is, additionally, the advantage of being able to change and clean the oil at more frequent intervals and at negligible cost.