Sedimentation is one of the most versatile separation processes. Pieralisi Vertical Separators (Disc- Stack Separators) are ideally suited to separation processes where the physical characteristics of sedimentation can be applied thanks to density difference.
Centrifugal sedimentation is often a very good and economical alternative to other separation processes such as:
The basic principle of separation by sedimentation allows for the simultaneous separation of two or three different, immiscible substances. PIERALISI Vertical Separators that make use of this latter separation regime are called three-phase separators. None of the alternative principles mentioned above can achieve this result. Sedimentation is the physical process whereby suspensions and emulsions (or a combination of both) are exposed to gravitational force, either the earth’s gravity or by using a centrifuge. By applying g-force, the droplets and/or particles acquire settling velocity (vg) and are sent to the bottom/wall of the container/centrifuge bowl.
Settling velocity (vg) increases with:
Settling velocity can be estimated via a well-known equation known as Stokes’ Law
Sedimentation is used in many industrial processes where it is necessary to separate substances that are mixed together (dispersion). With a suitable recipient and the time needed to complete the sedimentation process, the two or more substances with different densities will be separated (stratified) within the recipient, the lightest going to the surface, the heaviest to the bottom.
But what can we do if we do not have enough time or if we want to increase capacity? Going back to Stokes’ Law, to obtain an higher settling velocity and consequently to accelerate the sedimentation process, we can replace gravitational acceleration “g” by centrifugal acceleration “ˆ2 r”. The centrifugal acceleration “ˆ2 r” produced in a Pieralisi Vertical Separator is up to ten thousand times higher than the ordinary force of gravity “g”. As a consequence, settling velocity is up to ten thousand times faster, and separation can be achieved in a very short time and/or using very small volumes.
Pieralisi Vertical Separators combine very high rotational speed, a unique design and special materials. They generate a very high centrifugal force which, when used for sedimentation purposes, ensures extremely efficient equipment. The mixture to be separated is forced through a conical disc stack, where multi-layer sedimentation takes place. The separation happens very quickly as the mixture passes through the parallel thin channels of the disc stack. A medium-sized Disc-Stack Separator, when using high centrifugal force, can be compared to the efficiency of a sedimentation tank with a surface area of 60,000 m2, the same size as 10 football fields. In technical terms, this separator hosts an “equivalent settling area” of 60,000 m2.
Thousands of “g”s are produced in the bowl of the Disc Stack Separator, the centrifugal force being the product of the high rotational speed and the large size of the bowl. The rotation is generated by an electric motor connected to the horizontal shaft by means of an elastic coupling or clutch. Thanks to the gear ratio produced by specially-designed gears, the movement is transferred to the vertical shaft. The bowl is assembled on the conical top-end of the vertical shaft. The processing liquid is fed into the centre of the bowl through the stationary feed pipe. Subsequently the inflow is accelerated by means of specially-designed accelerators, forced in the first instance to the periphery of the bowl, and then through the discs where multi-layer separation takes place. The separated liquid(s) flow to the top of the bowl where they are discharged by overflow or by centripetal pumps. The solids captured via the described settling process are collected at the periphery of the bowl, and then discharged intermittently. This discharge is activated by a hydraulic system located underneath the separation area. In essence, the solids are discharged via openings at the periphery of the bowl which are opened and closed by the down- and upwards movement of either a sliding piston or bowl bottom (see picture).
The structural parts of the bowl are made of special stainless steel, the inside parts in contact with the product are made of suitable materials for the processing environment: stainless steel, brass or bronze. The housing of the Vertical Separator is in stainless steel or aluminium and cast iron. The gearbox is made of painted cast iron or cast iron coated with stainless steel. The gasket material is always selected according to the process media.
The movement is always generated by an electric motor and is transmitted by means of:
A special transmission version makes use of a high-speed motor coupled directly to the vertical shaft.
Feeding and discharge of process liquids
The mixture generally is fed in through a stationary feed pipe in the centre of the bowl. The separated liquid may be discharged from the Vertical Separator by an overflow or by one or two built-in centripetal pumps.
Specially-designed all-stainless-steel Separators, designed to be used with a CIP cycle for cleaning purposes. This version is designed for the easy sanitization of all parts in contact with the product and requires with no manual operation.
Specially-designed and certified Vertical Separators, designed to be installed in hazardous areas and for the treatment of potentially inflammable products, i.e. Separators complying with II2G EEx cp IIB TB4. These separators are inactivated by means of nitrogen gas to minimise the risk of ignition and to fulfil ATEX requirements.
Hydro-hermetic device installed inside the bowl, designed to ensure that contact with the surrounding environment is avoided, and to prevent oxidation and gas leaks.
Vertical Disc-Stack Separators are normally designed for one type of separation only, but in some cases, more versatile versions are available that can carry out two different types of separation. Below is a list of the range of disc stacks and bowl designs used for different types of separation.
PIERALISI Disc-Stack Separators are able to simultaneously process dispersions of two liquids and one solid. The Pieralisi product range offers three main equipment solutions: one with solids retention (requiring manual cleaning) known as the S-Type, one equipped with nozzles that allow continuous solids discharge, known as the SU Type, and the other with automatic, intermittent solids discharge, known as the FPC-Type. The tables below show types and sizes, with some general indications of dimensions.
Mineral- Fuel and Lube Oils Processes
Dairy Products and Juices