Separating different liquid phases and solids from each other plays a key role in countless industrial processes. Virtually all branches of industry need to separate different liquid and solids at some point in their manufacturing processes. The basic centrifuge idea is based on what happens in a settling tank, in which particles, sediment and solids gradually fall to the bottom, and the liquid phases of different density separate due to the force of gravity. However, such clarification is an extremely slow process and is unable to meet industry’s needs for rapid, controllable results. The general idea behind centrifuges is therefore to ensure that the mechanical separation of different liquid phases and solids can be carried out on a rapid, continuous basis in order to meet the demands associated with modern industrial processes.
Accelerating the process
In essence, a centrifuge is a settling tank whose base is wrapped around a centre line. Rotating this entire unit rapidly means that the effect of gravity is replaced by a controllable centrifugal force that can have an effect up to 10,000 times greater. This force is then used to separate liquids from other liquids and solids efficiently and with great accuracy, and in a manner that is easy to control.
Types of centrifuge
There are several different basic types of centrifuge normally used in industrial separation. Decanter centrifuges are generally used for greater solids concentrations with larger particle sizes. Disc stack separators, on the other hand, are ideal for a wide range of separation tasks that involve lower solids concentrations and smaller particle and droplet sizes. This applies to both liquid–liquid and liquid–solid separation. The most difficult separation tasks can often involve three phases, with hardly any difference in the density of the separate liquid phases and with the particles to be separated very small in size. In such applications, no other technology can compete with disc stack separator technology.
How a disc stack separator works
A disc stack separator separates solids and one or two liquid phases from each other in one single continuous process, using extremely high centrifugal forces. When the denser solids are subjected to such forces, they are forced outwards against the rotating bowl wall, while the less dense liquid phases form concentric inner layers. The area where these two different liquid phases meet is called the interface position. This can be easily varied in order to ensure that the separation takes place with maximum efficiency.
Inserting special plates (the “disc stack”) provides additional surface settling area, which contributes to speeding up the separation process dramatically. It is the particular configuration, shape and design of these plates that make it possible for a disc stack separator to undertake the continuous separation of a wide range of different solids from either one or two liquids. The concentrated solids phase formed by the particles can be removed continuously, intermittently or manually, depending on separator type and the amount of solids involved in the specific application.
The clarified liquid phase (or phases) overflow close to the rotating axis, in the outlet area on top of the bowl. The liquids then flow into separate chambers. Each separated liquid phase then leaves the bowl due to the force of gravity or by means of a paring disc, which is a special pumping device. The chambers can be sealed off from each other to prevent any risk of cross-contamination.
Solids discharge section
There are three basic ways of removing the solids from disc stack separators
● continuous solids discharge, in which solids and liquid exit via nozzles in the periphery
● intermittent solids discharge, in which a carefully designed system opens ports in the bowl periphery at controlled intervals in order to remove the collected solids
● manual removal, in which the machine is stopped and the bowl is opened so that the collected solids can be removed manually.
The solution most appropriate for a particular application depends on a combination of factors. The most important of these are the amount of solids in the liquid, the nature of the particular application and the consistency of the solids once they have been separated.
Naturally, the overall efficiency of a disc stack separator as part of a production set-up is heavily dependent on many other ancillary systems and equipment. Unparalleled experience means that Alfa Laval has a unique capability for providing all the necessary equipment to achieve maximum efficiency in the continuous separation of different liquid phases and solids in countless industrial processes. This can be done on the basis of highly efficient, standardised equipment packages and fully tested modular units, or specially customized disc stack separator installations to meet individual liquid–liquid and liquid–solid separation requirements.
Images of Disc Stack Separator made by EnvitechCorp: