Industrial centrifuge types, operation and applications:
A machine designed to apply the concept of centrifugal force generated as a direct result of rotation of any mass, for large scale industrial applications, is an industrial centrifuge. You can think of an industrial centrifuge being a scaled up version of a lab centrifuge (test-tube) except at a much bigger scale with a flow-through design. So in an industrial centrifuge, the separated solids and liquid(s) to continuously exit the centrifuge while the centrifuge is rotating. Industrial centrifuges are used for continuous separation of liquid(s) from solids. These centrifuges can be further categorized as sedimentation type and filtration type centrifuges.
In a filtration type centrifuge, a rotating ‘basket’ with perforations allows the liquid to pass through and flow out while the solids are accumulated and removed from the basket. A relatively lower rotational speed (lower centrifugal force) is typically adequate for such filtration type centrifuge which are predominantly used for large amounts of coarse solids from a liquid. An example would be separation of crystallized sugar from syrup.
A sedimentation type centrifuge has a solid bowl (without any perforations or filter media). In this case the solids accumulate along the bowl wall or periphery while the lighter liquids are separated and exit the centrifuge through a liquid outlet passage. This ‘solid bowl’ feature allows for an additional benefit of these ‘sedimentation’ centrifuge, namely three phase separation or liquid/liquid/solid separation. In other words, solid bowl or sedimentation type centrifuges are well suited for Solid/Liquid; Liquid/Liquid & Solid/Liquid/Liquid or three-phase separation applications across various industries.
Solid bowl centrifuges can be further sub categorized as horizontal bowl (also known as decanter or scroll centrifuges) and conical plate (also known as vertical bowl or or disc-stack centrifuges). More information about decanter centrifuges construction, operation and applications can be accessed here.
Conical plate (disc-stack centrifuges) are available in 2 main configurations. Self-Cleaning bowl and manual-cleaning bowl types. The design, operation and applications of these centrifuges are summarized in the following section.
Types of conical-plate centrifuges
Self-cleaning bowl (Auto de-sludging bowl)
In a self-cleaning centrifuge, the feed is sent into the rotating bowl through the inlet tube. The oil phase moves towards the center of the bowl and is discharged by a paring disc (4). The water phase leaves the bowl over the top disc (5) and through a paring disc aka centripetal pump (6). The heavier solids phase is collected at the bowl periphery, from where it is discharged automatically (intermittently), as explained below.
The solids discharge is achieved by a hydraulic system below the separation space in the bowl, which at preset intervals forces the sliding bowl bottom (7) to drop down, thus opening the solids ports (8) at the bowl periphery.
Intermittent sludge ejection enables these self-cleaning centrifuges to be used for three-phase separation with solids up-to 5% (%v/v). Higher solids concentration require frequent sludge ejection cycles which is not desirable. Proportion of liquid phases may be in any ratio as the separated liquid phases are continuously discharged from the bowl.
Typical applications of a self-cleaning bowl centrifuge include:
- Separation of machining coolant from tramp oil & sediment.
- Crude oil dewatering & polishing.
- Used engine oil – water & sludge separation.
- Chemical solvent clarification.
- Water & solids removal from diesel fuel.
- Wine and beer clarification.
- Large scale algae cell harvesting.
Needless to say these types of centrifuges are applicable to many other industrial applications wherein two or three phase separation is desired with limited solids concentration.
Manual-cleaning bowl (Solid retaining bowl)
In a manual-cleaning centrifuge the feed is introduced to the rotating centrifuge bowl from the top (1) and is accelerated in a distributor (2) before entering the disc stack (3). It is between the discs that the separation takes place. The water and the heavier solids are forced towards the bowl wall where the solids accumulate and the water proceeds over the top disc (4) to an open outlet (5) clarifier version.
The light phase (oil) moves towards the center and leaves the bowl through a paring disc (6). The centrifuge needs to be stopped at intervals for manual cleaning of separated solids. The bowl is mounted on a vertical spindle (7) driven by a horizontally mounted motor, via a worm gear or a pulley and belt drive system.
The manual-cleaning (solids retaining) design of these centrifuge bowls allows for a simpler design without the sludge ejection mechanism. Therefore these centrifuges do not require additional operating water resources leading to simpler installation and operation. However, this design also limits the applicability of manual-cleaning centrifuge primarily to liquid/liquid separation with very small amount of solids.
Typical applications of a manual-cleaning bowl centrifuge include:
- Separation of moisture from steam turbine lube oil.
- Purification of diesel from water with minimal solids.
- Biodiesel glycerin separation.
- Small engine lube oil cleaning.
- Water removal from jet fuel.
- Corn oil separation from corn syrup.
Manual-cleaning centrifuges are suitable for most application where two immiscible liquids need to be separated on a continuous basis.