What Is a Centrifugal Concentrator?
A centrifugal concentrator is generally a decompression concentration and drying device that combines a centrifuge and a vacuum dryer.
While centrifugal force suppresses sudden boiling, the boiling point is lowered by depressurization, and the heat of vaporization is supplemented by heating to evaporate the solvent and concentrate the target sample.
In addition to being used for the analysis of trace samples, this equipment is also being introduced into the concentration process, which has a significant impact on sludge treatment systems in light of the worsening deposition of sewage sludge. Since it can be operated continuously and almost unattended, it is expected to become more widely used in the future.
Applications of Centrifugal Concentrators
Because centrifugal force prevents solvent from boiling or foaming, sample loss can be minimized, making it suitable for concentrating samples for trace analysis.
They are used, for example, to concentrate DNA, RNA, peptides, and other applications that require concentration and drying.
On the other hand, centrifugal thickeners are also increasingly being used in sewage sludge treatment due to their ease of operation, automatic operation, and maintenance.
Principle of Centrifugal Concentrators
Centrifugal concentrators use the “three states of matter” to concentrate samples. Specifically, the process of solid ⇔ liquid ⇔ gas is concentrated by low-temperature drying under reduced pressure.
First, the boiling point of the sample (solvent) is lowered by reducing the pressure applied to the sample. At the same time, heating accelerates the volatilization of the solvent and prevents the freezing of the solvent that occurs during depressurization.
The overall system consists of a centrifugal concentrator, cooling trap, and vacuum pump. The vacuum pump reduces the pressure in the centrifugal concentrator to concentrate the sample. The cooling trap then condenses and collects the vaporized solvent.
The centrifugal thickener for sludge concentration concentrates excess sludge by centrifugal force. The outer and inner cylinders rotate around a sludge feed pipe while maintaining a constant difference in rotation. Sludge is continuously fed from the sludge feed pipe into the outer drum, and centrifugal force causes the solids in the sludge to settle and accumulate on the inner wall of the outer drum. The sludge is then discharged out as concentrated sludge by the action of a screw installed in the inner drum.
Sludge Concentration by Centrifugal Thickener
Among sewage sludge, sludge in the first sedimentation tank settles easily and can be concentrated by gravity sedimentation, but sludge in the final sedimentation tank (especially excess sludge) does not settle well, so it is not easy to concentrate the sludge by gravity sedimentation. Therefore, it is difficult to concentrate sludge by gravity settling. Mechanical concentration methods include atmospheric pressure floating thickeners and belt-type filtration thickeners, and centrifugal thickeners are one of the mechanical concentration methods.
Sludge is first pumped to the centrifugal thickener and supplied. The structure of the centrifugal thickener is largely divided into an outer bowl and an inner bowl. Before the sludge is introduced into the centrifugal thickener, flocculants are added. By mixing the added flocculant, the sludge mixed with the flocculant is discharged into the end of the bowl. The outer bowl rotates at high speed, and the sludge is subjected to the centrifugal force of 800-1200 G, which initiates sedimentation separation due to the difference in specific gravity between water and solids.
The centrifugal force causes the feed sludge to settle and separate faster than gravity settling. The settled and separated concentrated sludge is transported to the concentrated sludge outlet by a screw conveyor attached to the inner bowl, and the separated liquid is transported to the separation liquid outlet by the flow.
Thus, sludge with a feed sludge concentration of 0.6-0.8% is concentrated and discharged as 4-6% concentrated sludge.
Differential Velocity of Centrifugal Thickeners
The structure of the centrifuge is largely divided into an outer bowl and an inner bowl.
The supplied sludge is fed into the bowls, where it is subjected to centrifugal force, stuck to the outer bowl, sedimented, and separated. The supernatant is discharged as a separated liquid, and the sedimented material as concentrated sludge. The outer bowl and inner bowl of the centrifuge do not rotate at the same speed but rather have a rotational difference of about 3 to 8 revolutions, with the inner bowl, to which the screw conveyor is attached, rotating slightly slower. This difference in the rotation is called “differential speed.” This difference in the rotation is the force that transports the concentrated sludge to the discharge outlet by the screw conveyor.