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Used engine oil is a term used to describe used lubricating oil from Internal Combustion (IC) engines. Lubricating oil tends to lose its lubricating properties with use.
Hence, the oil is periodically replaced to prolong engine life and prevent breakdowns. The oil that is replaced is then referred to as used engine oil.
Automotive oil change shops to lubricating oil replacement in large marine engines are all sources of used engine oil.
Used engine oil has several use cases in its afterlife, including serving as a source of fuel or as feedstock for the production of fresh lubricating oil.
When used as fuel, used engine oil is burnt as heating oil in residential and commercial heating. It is a low-cost source of heating, especially for producers of such oil.
Diesel fuel blended with used engine oil produces ‘black diesel’. Black diesel can often be used as fuel even in engines designed to operate on straight diesel fuel. Engine manufacturers allow a certain percentage of blended fuel in their engines. This use of blended fuel does not affect the manufacturer’s warranty under certain conditions.
Furthermore, after being purified and distilled, used oil from engines can be used as feedstock for fresh lubricant manufacturing.
Contaminants in engine oil can take the form of either liquids or solids. The liquid contaminant could be water or glycol from the automotive engine coolant. Condensation in storage tanks can also introduce water (moisture) to the oil.
Solid contaminants are categorized as non-metallic or metallic. Non-metallic contaminants include sludge, dirt, carbon, ash, etc. Metallic particles include bearing and engine wear metal particles.
The most prominent example of non-metallic engine oil contaminant is ash, a carbon-based particle generated as a by-product of combustion. Ash suspended in used engine oil causes the color of the oil to appear black.
A major source of metallic contaminants in engine oil is heavy metals present in fuels. Heavy Fuel Oil (HFO) is the fuel of choice in large engines due to its low cost compared to diesel. HFO has high levels of heavy metals such as Chromium, Vanadium, Nickel, and Lead. These heavy metals can easily contaminate the engine lubricating oil.
The other source of heavy metals in engine oil is metal wear particles. As engine bearings and other parts wear due to friction, they release metal particles into the engine lube oil.
As an example, the following table compares the metal content in virgin engine oil compared to used engine oil from marine engines.
Engine Oil Metal Contaminants – Before & After Use
|Sample Type||Lead (ppm)||Iron (ppm)||Nickel (ppm)||Copper (ppm)||Chromium (ppm)|
|Used Engine Oil||132.0||785.0||47.0||125.0||65.0|
Water removal is necessary for several reasons. Used oil with water reduces the sale value of used oil as fuel. The processing of used oil for lubricant feedstock is hampered by the presence of water. Used oil emulsion is caused by water and is not desirable for many reasons.
Removal of heavy metals and ash from used engine oil is essential for the following reasons.
When used engine oil is burnt as fuel, it releases the heavy metals into the environment through the combustion gases. This is a cause of environmental air pollution.
Air polluted with heavy metal compounds is detrimental to health. Polluted air introduces toxic metals into the oceans which affect marine life. Soil and groundwater contamination is also a direct result of this pollution.
Ash in used engine oil causes processing challenges during the conversion of used oil into lubricant feedstock. Ash residue causes distilling equipment to get gummed up with thick carbon sludge. This leads to frequent cleaning and low process efficiency.
An industrial disc-stack centrifuge uses mechanical separation to remove heavy metal and ash particles from used motor oil.
A disc-stack centrifuge exerts centrifugal force up to 10,000 g on the process fluid. In the case of used motor oil, this force is sufficient to extract fine metal and carbon particles from the oil.
Under this force, the heavier metal particles get pushed outward towards the centrifuge bowl wall. The ash particles flocculate to form bigger solids which separate under the high g-force.
Disc-stack centrifuges are also uniquely equipped to handle three-phase separation. The centrifuge can separate the water phase from the used oil, while simultaneously removing any solid contaminants. The separated water discharges through a discreet heavy phase outlet on the centrifuge.
Oil temperature is inversely proportional to the viscosity of the oil. Thus, hotter the oil, the lower the viscosity. Low viscosity oil offers less resistance to the ash and metal particles from moving out of the oil.
Oil temperature can be raised by adding an inline heater to the centrifuge system. This is in addition to the customer’s heated tanks.
Centrifugal Force (G-Force)
Centrifugal force is directly proportional to the separation efficiency of a centrifuge. A disc-stack centrifuge produces the highest level of centrifugal force among industrial centrifuges.
Flow-rate of the used oil through a centrifuge is inversely proportional to retention time. Retention time is the duration each particle of used oil is exposed to the high centrifugal force.
Thus, the lower the flow-rate, the higher the retention time, which leads to better separation efficiency.
Specific Gravity Differential
A disc stack centrifuge magnifies the difference in specific gravity between phases. This magnification causes accelerated separation of the phases. So, the heavy metal particles separate from the oil quicker than the lighter ash particles.
A used oil collection and processing company based on the East coast collected used engine oil. Their collection came from cruise ships, naval vessels, and inland oil change shops.
They processed the used oil by using filters and heating the oil to boil off the water. They then vacuum distilled the cleaned oil to produce lube oil feedstock.
The company faced many challenges while using this process. Listed below are some of these challenges.
The customer then analyzed the clean oil sample from our test. There was a significant reduction in heavy metal and ash contamination of the used engine oil.
Dolphin Centrifuge recommended an over-sized centrifuge. This was based on the flow-rate reduction required for ash removal. The following table summarizes the technical specifications of the centrifuge supplied by Dolphin.
Used Engine Oil Centrifuge Specifications
|Type of Centrifuge||Self-Cleaning, Disc Centrifuge|
|Manufacturer and Model||Alfa Laval MOPX207|
|RCF (Relative Centrifugal Force)||~ 6,800 G|
|Rated Separation Efficiency||< 1 µ Metal Particles|
|Rated Flow-Rate||28 GPM on Diesel Fuel|
|Flow on Used Engine Oil||10 GPM @ 180 F|
|Drive Motor||7.5 HP|
The centrifuge system supplied by Dolphin Centrifuge produced the following results on used engine oil purification at the customer site.
|Test Parameter||Units||Before Centrifuge||After Centrifuge|
|Ash Content||wt %||1.17||0.09|
Using a disc stack centrifuge to remove heavy metal and ash particles from used engine oil is efficient and cost-effective. This method is especially suitable for large scale processing of used engine oil.