Vertical basket centrifuges are routinely used for separating solids from liquids and are a suitable alternative to microfiltration. Their benefits were recognised at the turn of the 20th century (Richards, 1908). They are part of a general group of centrifuges that rely on centrifugal forces to produce a separation of solids based on relative differences in density.
Basket centrifuges can have the drive motor at the bottom or the top of the centrifugal casing. They can also be found as horizontal basket centrifuges.
General Principles
Dewatering requires the sedimentation of particles in the feed slurry. This slurry separates into two phases: a supernatant and a cake. dewatering occurs the supernatant permeates through the cake (Ginting et al., 2015).
If the cake has poor permeability, it takes longer for supernatant to pass through the pores and be discharged. Once the supernatant is fully removed, the cake starts to consolidate with a reduction in its moisture content.
The supernatant flow-rate Qc is:-
Qc = [2 π* rm* h/μ] * [ΔP/ {α*W/Ac + Rm}]
where rm is the distance from the rotational axis to the end of the cylindrical filter medium, h is the filter chamber height, u the viscosity of the liquid, alpha av is the average specific filtration resistance of the cake, W is the solid mass of the cake and Rm the cylindrical filter medium resistance. Delta P represents the centrifugal potential drop across the liquid, cake and filter medium which serves as the driving force of the liquid discharge and Ac is the effective filtration area.
Based on some practices, the supernatant is discharged by either suction using a skimming pipe (Matsumoto and Hashimoto, 2002) from
the inner side of the basket or by increasing the rotational speed. However, these methods are not the perfect solutions.
In the skimming pipe method, the pipe cannot have any physical contact with the cake, which makes it impossible to completely discharge the liquid. On the other hand, it is very inefficient to dewater highly compressible materials under high rotational speed.
As rotational speed increases, the centrifugal force and pressure drop
across the filtration system increased, resulting in an increase in
separation efficiency. However, at a very high rotational speed, the
efficiency decreases due to a short retention time of slurry
on filtering screen. Increasing basket radius provides the rise in
centrifugal force. The filtering screen length was divided into 3 sections; feed, transition, and drainage section. An increase in centrifugal force acting on deposited cake decreased the cake thickness and permeability; hence specific cake resistance increased. Higher L/S ratio or low solid content in feed slurry results in a higher separation efficiency due to the facilitation of water passing material through the deposited cake and filtering screen.
Applications
Basket centrifuges of all types are used in the food, environmental waste, pharmaceutical and biotechnology industries.
Separation of Whey
Whey is produced when curd is manufactured. Whey and curd are protein rich fractions, each with functional and nutritional benefits. The curds and whey are separated usually warmed to between 25 and 30ºC before they are processed further. Separation of the whey is best achieved with a solid bowl basket centrifuge running at 1100 rpm. Large amounts of curd can be centrifuged this way in one batch. Roughly 120kg of curd-whey can be treated for 90 minutes. The alternative is to use a horizontal centrifuge.
Winterizing is a process for removing oils and waxes. In one example it is helped by holding liquid milkfat at 20ºC for 3 to 5 days and removing the solid fraction with a basket centrifuge. Large scale gel filtration of whey can be by either the centrifugal process or the column process (Richert, 1975).
The centrifugal procedure was introduced by Morr et al. (1968). In the example they cite, it involves a bed of Sephadex G-25 mounted in a basket centrifuge. There are several steps to a cycle.
First, the void volume is removed by running the centrifuge at approximately 1000 x g. The beads remain hydrated. The centrifuge then is slowed to approximately 60 × g and the feed material which is partially whey concentrate with lactose removed applied to fill the void volume.
The low molecular weight solutes diffuse into the gel matrix while proteins are excluded. The void volume which is the product is removed at 1000 × g minus some of its low molecular weight solutes. The centrifuge is then slowed to 60 x g and the bed eluted with water to remove the low molecular weight solutes. This cycle has been automated for commercial use.
Other Applications In Food
Coconut milk can also be processed with the involvement of a basket centrifuge. The coconut milk is filtered either through a cloth filter or centrifuged at low speed using this device.
Sugar crystals are separated from the crystallization liquor when this has been precipitated. Basket centrifuges is also used for separating grape juice from must for wine production, for cleaning up waste vegetable oil, biofuel, for removing crushed rape seed and treating biodiesel.
The removal of waste biological solids can be estimated from pilot-scale operation. An application using potato wastewater produced an 80 to 85% solids recovery and a cake with 5.5% solids concentration. This was for sluidge from a mixed liquor with an SVI of over 250. Chemical additions or reduced SVI levels would allow a higher feed rate or a longer cycle and possibly a higher solids cake because of better clarification of the centrate (Richter et al., 1973).
Issues with Basket Centrifuges
The same issues that affect all centrifuges are excessive vibration which results in noise. Some vibration has to be expected but any excessive vibration is dangerous.
The first check in a batch or continuously operated system is to operate the centrifuge without any feed in it and check if it is correctly balanced. Most centrifuges will vibrate slightly as feed enters the bowl for example in a continuous operation. Worn spindles, damaged bearings and surfaces within the bowl, debris and dirt both inside and outside the bowl are typical culprits. Failure too to connect the rotor to the drive is linked to vibration.
The next check is to see how the centrifuge performs when feed is pumped into the centrifuge. It is a careful balance between feeding the liquor into the bowl versus its exit because the ideal is to have a cake that evenly coats the internal surface of the bowl. usually discharge the collected oil – automatically.
Vibration processes generated during a centrifuge operation provide information which helps diagnose the device technical condition and identify most faults which occur in the course of its operation, without switching it off (Ligier, 2008).
The best way to identify most faults of the rotating system elements
such as rolling bearings, coupling is spectral analysis, owing to its universality and the abundant literature which describes it.
Different fault states are identified. These include:-
- lack of balance in the rotating system
- loss of elasticity in any flexible couplings
- loss or lack of balance in the cooling fan or engine rotor
- defects in the centrifuge shaft bearings
- defects in the engine bearings
References
Ginting, A. N., Fukuyama, R., Jami, M. S., Tanaka, T., & Iwata, M. (2015). Improving slurry dewatering performance of basket centrifuge: Discharge of supernatant using bypass filter medium. Journal of Chemical Engineering of Japan, 48(12), pp. 966-969
Ligier, K. (2008). Methods of diagnosing an acww 1000 sugar centrifuge with the use of vibration processes. Technical Sciences, 11, pp. 289-300. .
Matsumoto, T. and Hashimoto, S. (2002) Method for Centrifugal Separation, Centrifugal Separator and Device for Detecting Cake Face for the Same (in Japanese), Japanese Unexamined Patent Application,
Publication No. JP2002-143721
Mort, C. V., M. A. Neilsen, and S. T. Coulter. (1967). Centrifugal Sephadex procedure for fractionation of concentrated skim milk, whey, and similar biological systems. J. Dairy Sci. 50 pp. 305
Richards, T. W. (1908). The Use of the Centrifuge. Journal of the American Chemical Society, 30(2), 285-286.
Richert, S. H. (1975). Current milk protein manufacturing processes. Journal of Dairy Science, 58(7), pp. 985-993 (Article) .
Richter, G. A., Lynnsirrine, K., & Tollefson, C. I. (1973). Conditioning and Disposal of Solids from Potato Wastewater Treatment. Journal of Food Science, 38(2), pp. 218-224.
Seow, C. C., & Gwee, C. N. (1997). Coconut milk: chemistry and technology. International Journal of Food Science & Technology, 32(3), pp. 189-201.
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