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Used oil is defined as any oil originating in crude oil or petroleum with physical or chemical impurities as contaminants from its use.
The United States produces over 2 Billion gallons of used oil annually. This oil has considerable value as a heating fuel and for reblending into lube oil after recycling.
Used oil has mineral-based origins, as in crude oil or synthetic oil. Used engine oil, transformer oil, turbine lube oil, etc., are examples of used oil.
On the other hand, Waste oil is an oil with organic origin such as WVO (Waste Vegetable Oil), Used Cooking Oil, yellow grease, etc.
Used oil reclamation is a thriving industry that collects and purifies used oil in large quantities. These industries use different types of separation techniques that are listed below in the methods section.
The commercial reclamation of used oil has multiple benefits from a commercial and environmental perspective. Some of these benefits are listed below.
Used oil discarded in landfills is a significant source of environmental pollution. It contaminates groundwater and soil. Recovery of used oil prevents this pollution.
Refined used oil is a feedstock for the production of virgin lubricating oil. This process allows for the reuse of used oil as a low-cost and environmentally friendly alternative.
A considerable amount of used oil finds use in heating industrial buildings. Though contaminated, used oil has a high calorific value and is ideal for heating fuel.
Purified used oil is also known as black diesel. This name comes from the use of this oil as fuel for diesel engines. The blend of diesel and used oil is allowable by the manufacturers of large diesel engines.
A used oil filtration system includes tanks, heaters, multi-bank filters, pumps, etc. The typical process involves the following steps.
An industrial oil centrifuge combines most of the recovery mentioned above steps into one device that uses highly amplified centrifugal force to separate the oil from water and sediment. These centrifuges are continuous processing devices that offer high separation efficiency with a small footprint.
A used oil centrifuge is an industrial disc-stack centrifuge that exerts over 7,000 Gs of centrifugal force on the oil. This force is sufficient to separate all free water and sediment as small as 0.5 microns. The self-cleaning feature of the centrifuge allows it to eject the separated solids intermittently while the separated water exits the centrifuge continuously.
Read our new article: 6MGY Used Oil Centrifuge Plant Layout
A used oil centrifuge works on the principle of differential settling of the different phases in used oil based on their respective specific gravity. The contaminated oil enters the rotating centrifuge bowl through an inlet pipe and passes through the rotating stack of discs within the bowl. The rotating discs impart the rotational velocity to the oil and its contaminants.
The heavier sludge moves towards the bowl wall due to the high centrifugal force. This sludge displaces the lighter oil towards the center from where the separated oil enters an exit chamber before discharge from the bowl.
The sludge and water (if any) accumulate at the bowl wall, and the centrifuge automatically ejects the sludge periodically. The centrifuge bowl has a passage for the separated water, which allows the water to discharge through.
A used oil centrifuge needs to be a three-phase centrifuge to separate the water and solid particles simultaneously. A two-phase centrifuge will only separate the solids from used oil but not the water.
A used oil centrifuge is very similar to a waste oil centrifuge. Both are three-phase centrifuges capable of separating oil from water and sludge. A used oil centrifuge may require some modifications to accommodate chemicals such as acids in the process.
A used oil centrifuge can separate oil emulsions to a certain extent. Disc stack centrifuges cannot separate chemically bonded emulsions without the use of demulsifiers.
A used oil centrifuge (disc-stack) can separate metal particles down to 0.5-microns. Non-metal particle separation efficiency is typically around 2-microns.
Disc stack centrifuges for used oil do not use any filters or media. These centrifuges use the amplified force of gravity (centrifugal force) to separate water and sludge from used oil.
Used oil centrifuges of the disc-stack type can remove some of the ash content from used motor oil. Ash particles are microscopic carbon particles, and the centrifuge's separation ability is limited to such tiny particles separation.
One of the advantages of using an industrial centrifuge for waste oil separation is the separation efficiency of these centrifugal separators.
A disc-stack centrifuge has a separation efficiency of 0.5 microns for metal particles and 2 microns for non-metal particles.
In addition to particle removal, a used oil centrifuge also simultaneously separates all free water from the oil. This centrifuge can also break oil-water emulsions and recover more oil under certain conditions as an added benefit.
Ash particles impart a black color to used oil. Though these particles are sub-micron level in size and light, the high centrifugal force in a disc-stack centrifuge can separate a considerable amount of ash from used oil. Flocculants enhance this ash separation ability further as they solidify some finer ash particles into larger particles that the centrifuge can separate.
Used oil often carries an emulsion phase that is difficult to separate by gravity settling without chemicals. Disc stack centrifuges can break oil emulsion to a certain extent based on the emulsion type.
Chemically bonded emulsions need acid treatment or demulsifiers to separate the oil from the water.
Industrial centrifuges with high centrifugal force can extract the microparticles that act as nucleoids, thus assisting in emulsion breakdown.
There are several benefits to using centrifuges for used oil purification compared to gravity settling and filtration. The following is a list of the advantages.
Disc stack centrifuges separate water and sludge simultaneously from used oil. This ability is referred to as three-phase separation and is unique to disc-stack centrifugal separation.
Filters and gravity separation are only helpful for separating sludge from used oil but not water.
Disc stack centrifuges have a relatively smaller footprint than conventional devices for used oil purification. A used oil centrifuge has a fraction of the size of filters and settling tanks for similar capacity.
Used oil centrifuges use gravitational force to affect oil-water separation. These centrifuges do not use media for separation, thus eliminating the media and related labor costs. Therefore, industrial centrifuges have very low operating costs as compared to filters and tanks.
The particle separation efficiency in the case of settling and filtration depends on the filter media. Filter media with a tiny mesh size tends to need frequent replacement, and the related costs add up.
Oil centrifuges separate a range of particle sizes that would otherwise require multiple filters of different sizes. The smallest particle size rating for a centrifuge is 0.5 microns.
In addition to microscopic particle size separation, disc stack centrifuges can often break oil-water emulsions by extracting the tiny particles that bind mechanical emulsions. The emulsion-breaking ability depends on the emulsions and may require chemical demulsifiers to assist in the separation.
Industrial centrifuges incorporate heavy-duty construction that allows these centrifuges to last for decades of operation. In addition, with periodic maintenance, these centrifuges operate continuously over extended periods without needing a stoppage. In other words, it is not uncommon for industrial centrifuges to operate for months without the need for shutdown or operator involvement.
The following list of the most prominent disadvantages of used oil centrifuges.
The initial capital expenditure for centrifuges is relatively higher than for filters and tanks. However, the lower operating costs and labor cost savings quickly recover the additional expense.
Used oil centrifuges are mechanically driven and therefore require utilities such as electricity and water for operation. Filters are primarily non-moving media that do not require utilities for operation.
The rotating components in an Alfa Laval used oil centrifuge create noise from the gears and drive motors as well as the air movement within the centrifuge also generates noise. Filters, on the other hand, are relatively quiet during operation.
All mechanical devices that operate at high rotational speeds require periodic maintenance. This upkeep ensures reliable operation of the equipment as well as equipment longevity.
Filters only need to be replaced with new media on an ongoing basis with little maintenance.
Specific design considerations apply to used oil centrifuges. Some of the important considerations are as follows.
Used oil processors use acids to dissolve and remove metals and break emulsions. The low pH value of the oil necessitates the use of corrosion-resistant material in the centrifuges and related piping. Centrifuges with all stainless steel wetted parts are available for such applications.
The provision of chemical injection ports upstream of the centrifuge allows adding chemicals such as demulsifiers or flocculants. Metering chemical pumps as an option are well suited for these applications.
The standard disc stack centrifuges design uses large volumes of oil with a small proportion of water. However, the water proportion can be relatively high in the case of used oil.
Some internal centrifuge modifications allow for the discharge of this larger volume of water to allow the centrifuge to process maximum volumes.
Adding chemicals or inherent used oil properties may not be compatible with ordinary elastomers. In such cases, the centrifuges have special elastomers with specific compatibility for the process fluid.
The following are typical specifications of used oil centrifuges manufactured by Alfa Laval. These are disc-stack-type centrifuges with self-cleaning ability.
| Centrifuge Model >>> | DMPX-040 | DMPX-070 |
| 40 GPM | 68 GPM | |
| Capacity (Heavy Used Oil) | 25 GPM @ 180 F | 40 GPM @ 180 F |
| Motor Power | 10 HP | 15 HP |
| Bowl Speed | 5,200 RPM | 4,200 RPM |
| Weight | 3,200 Lbs | 5,100 Lbs |
| Dimensions | 4' x 5' x 7' (H) | 5' x 6' x 8' (H) |
| Sludge Handling | Self Ejection (Automatic) | Self-Ejection (Automatic) |
| Liquid Phase Discharge | Light & Heavy Phase - Pressurized | Light & Heavy Phase - Pressurized |
| Bowl Wetted Parts | 316 Stainless Steel / Marine Grade Bronze (Nickel Plated) | |
| Operating Voltage | 230 or 460 VAC - 3-Phase - 60 Hz (50 Hz Available) | |
The following diagram shows a large-scale used-oil centrifuge processing plant layout using our three-phase centrifuges. This layout includes a decanter centrifuge that is a preprocessor to remove bulk sludge from the used oil if needed.
