Decanter centrifuge
A centrifuge is a device, which employs a high rotational speed to separate components of different densities. This becomes relevant in the majority of industrial jobs where solids, liquids and gases are merged into a single mixture and the separation of these different phases is necessary. A decanter centrifuge separates solid materials from liquids in slurry and therefore plays an important role in wastewater treatment, chemical, oil and food processing industries. There are several factors that affect the performance of a decanter centrifuge and some design heuristics to be followed which are dependent upon given applications.
The operating principle of a decanter centrifuge is based on separation via buoyancy. Naturally, a component with a higher density would fall to the bottom of a mixture, while the less dense component would be suspended above it. A decanter centrifuge increases the rate of settling through the use of continuous rotation, producing a force equivalent to between 1000 and 4000 G's. This reduces the settling time of the components by a large magnitude, whereby mixtures previously having to take hours to settle can be settled in a matter of seconds using a decanter centrifuge. This form of separation enables more rapid and controllable results.
The main application of decanter centrifuges is to separate large amounts of solids from liquids on a continuous basis. They are also used to wash and dry various solids in industry, such as polystyrene beads, clarify liquids and concentrate solids. Table 1.0 displays various examples of the utilisation of decanter centrifuges in various industries.
Generally the decanter centrifuge has more advantages than disadvantages; however there are some limitations when compared to other processes.
Advantages:
Limitations:
The main types of decanter centrifuges are the vertical orientation, horizontal orientation and Conveyor/Scroll.
In vertical decanter centrifuges the rotating assembly is mounted vertically with its weight supported by a single bearing at the bottom or suspended from the top. The gearbox and bowl are suspended from the drive head, which is connected to the frame. The vertical decanter allows for high temperature and/or high-pressure operation due to the orientation and the rotational seals provided at one end. However, this makes the device more expensive than the horizontal decanter centrifuge, which is non-pressurised and open. The advantage of the vertical machine over the horizontal machine is that the noise emitted during production is much lower due to less vibration.
In horizontal decanter centrifuges, as shown in figure 1, the rotating assembly is mounted horizontally with bearings on each end to a rigid frame, which provides a good sealing surface for high-pressure applications. The feed enters through one end of the bearings, while the gearbox is attached to the other end and is operated below critical speed. Capacities range up to 40,000 pounds (18,000 kg) of solids per hour with liquid feed rates of up to 300 US gallons (1.1 m3) per minute. The horizontal machine is arranged in a way that slurry can be introduced at the centre of a rotating horizontal cylindrical bowl. The scroll discharge screw forces the solids to one end of the bowl as it is collected on the walls. This orientation is the most common design implemented in industry.
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