Spinning Success: Unleashing the Power of the Centrifuge
What are the basic principles of separation?
The purpose of separation can be:
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- To free a liquid from solid particles (for example, sludge/soot in lube oil)
- To separate two mutually insoluble liquids with different densities while removing any solids present at the same time (for example, marine diesel oil)
- To separate and concentrate solid particles from a liquid (for example, the content in a sludge tank)
Product-related factors that influence the performance of a separation are:
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- Density Difference: The basic pre-condition for separation is the density difference between the particle and the liquid. The minimum density difference is approx. 0,01-0,03g/ml. The particle in the liquid sinks if its density is greater than that of the liquid (sedimentation). On the other hand, if the density of a substance is less than the density of the surrounding liquid, the particle will rise and settle on the surface.
- Particle Size: Any increase in particle diameter increases the capacity. This can be achieved, for instance, by adding flocculants in order to agglomerate several suspended solid particles or colloidal particles.
- Viscosity
- pH Valve
- Separation Temperature: The single most important factor to consider with regard to efficiency is the separation temperature because the temperature will affect the density and viscosity of the oil.
- Particle size distribution
- Surface tension
- Nature of the solids to be discharged (for example, dull, pasty, etc.)
Separation by Gravity
A liquid mixture in a stationary cylinder will clear slowly as the heavy particles in the liquid mixture sink to the bottom under the influence of gravity. A lighter liquid (for example, oil) rises while a heavier liquid (for example, water) and solids sink. Heavier, denser particles in the liquid mixture will settle and form a layer on the tank bottom.
The settling speed can be calculated according to Stoke’s law. It depends on the density difference, particle diameter, and viscosity of the liquid. All separation methods that use gravity for centrifugal force occur because there is a density difference between the continuous media (MDO, lube oils) and the media that is to be separated (water, soot, contaminants, ash). The density difference is the main factor that makes centrifugal separation work.
Centrifugal Separation
Centrifugal separation is the process of concentrating the natural forces that act on all particles of different densities to accelerate the natural process of separation. In a rapidly rotating centrifugal bowl, the force of gravity is replaced by centrifugal force, which can be thousands of times greater. With sedimentation centrifuges, the centrifugal force splits solids from liquids. The centrifugal force in the separator bowl can be achieved in a few seconds, which takes many hours in a tank under the influence of gravity.
What is a Centrifuge?
A centrifuge is a mechanical device that generates centrifugal force to separate solids and liquids or immiscible liquids with different specific gravities like water and oil. Using centrifugal force, the G (gravity) can be multiplied many thousands of times. Centrifuges increase the particle settling velocity. Centrifuges are machines used for separating dispersions consisting of two or more phases of different densities. One of these phases is a liquid, and the other phases may be liquids and/or solids. If these phases are to be separated, they must not be soluble in each other. In general, the solids phase has a higher density.
To learn more about common types of industrial centrifuges, visit our Industrial Centrifuge blog.
What is the primary principle behind centrifugation?
The primary principle is centrifugal force, which separates particles based on their density and size differences.
How does a centrifuge work?
Centrifuge separation means separating substances with different specific gravities by means of centrifuge force. The centrifugal force (relative centrifugal force or g forces) upon an object is determined by rotating distance from the rotating center (radius of rotation) and rotating speed.
Gravitational Acceleration
Substances are separated through natural sedimentation if they have different specific gravities (for example, water and oil). However, it takes a longer time to settle because the sedimentation takes place under a gravity of only 1 G. On the other hand, this substance can be settled and separated in a shorter time with the centrifugal force generated by rotation. The centrifuge works by accelerating gravity, causing denser substances and solids/particles to move outward in a radial direction. At the same time, objects that are less dense are displaced and move towards the center.
Disc Stack Centrifuge
Similar to a settling tank, a disc stack centrifuge uses the force of gravity to separate liquid with a specific density from other liquids and solids. Unlike a settling tank, which uses retention time as the main criteria to divide different liquid densities into layers and solids to precipitate in the tank, disc stack centrifuges use mechanical force to separate fluids and solids with different densities from each other.
A disc stack centrifuge separates solids and one (1) or two (2) liquids simultaneously. This is a single continuous process. The centrifuge uses extremely high centrifugal forces to separate the liquids and solids into phases. The sedimentation area in a disc-type centrifuge is increased by as large a quantity as possible of conical discs (disc stack). This enables the throughput capacity to be increased. The number of bowl discs depends on the height of the bowl design (rotor design), which is determined by the design requirements. The height of the space between the discs is determined by the product.
Centrifugation is the method of separating the components of a substance by subjecting it to a centrifugal force. Centrifuges can provide clarification or separation. In the case of liquid/liquid/solids mixtures, centrifuges clarify and separate. In the case of mineral oil applications, the expression ‘purification’ is used instead of separation.
Clarification = Liquid / Solids
Separation = Liquid / Liquid
Clarification + Separation = Purification = Liquid / Liquid / Solids
To learn more about common types of disc stack centrifuges, visit our Industrial Centrifuges blog.
How does a disc stack centrifuge work?
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Decanter Centrifuge
The decanter centrifuge is a major processing tool in various liquid/solids separation (clarifying decanter) and liquid/liquid/solids (separating decanter) applications. A decanter centrifuge comprises a solid cylindrical bowl rotating at high speed. Inside the bowl is a scroll (screw conveyor) rotating at a slightly different speed. The differential speed between the bowl and scroll provides the conveying motion to collect and remove the solids, which accumulate at the bowl wall.
The decanter centrifuge operates mainly by sedimentation. The centrifugal acceleration force (g-force) separates the solids from the liquid. The different variety of decanter separation applications can be grouped into these broad types:
Clarification (of liquid) = provide a high degree of clarification
Classification (of solids in liquid suspension) = single cut required between two sizes of solid particles
Dewatering = washed solids to a high degree of dryness
Recovery = removing a valuable solids from its suspension in a liquid
Thickening = producing a clear liquid and a more concentrated slurry
To learn more about different decanter separation phases, visit our Industrial Centrifuges blog.
How does a decanter centrifuge work?
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Filter Centrifuge
A filter centrifuge functions as a clarifying centrifugal separator (liquid/solids separation). A clarifier (clarification separation) uses centrifugal force (g force) to separate solid materials from a liquid phase. The solid material collects against the centrifuge bowl wall (solid wall). A proper clarifier has a stack of conical discs (disc stack) for creating a large equivalent clarification area in a relatively small centrifuge bowl. Filter centrifuges with bowl disc technology have an increased separation efficiency when separating particles by their density. Therefore, these disc filter centrifuges can remove significantly smaller, fine particles faster than any oil centrifuge without bowl disc technology.
Clarification = Liquid / Solids
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How does a filter centrifuge work?
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