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Comment PDF Separation Processes

Filtration Centrifuges: An Overview

By Peter Schmidt Andritz KMPT GmbH |

To solve the task of solid-liquid (S-L) separation in the chemical process industries (CPI), a wide variety of methods are invariably used. Many of these methods are not mutually exclusive and include a selection of technologies that can be expediently combined to provide an efficient operating system.

A good understanding of the options available and details of the individual system modules can enhance the efficiency of the overall concept. For example, mechanical separation using pusher centrifuges has proved to be a very expedient intermediate stage for the dewatering of sodium bicarbonate downstream of the vacuum filter and before calcination. By reducing the residual moisture between filter discharge and centrifuge discharge, some 38% of the required evaporation heat can be saved; and just a fraction of the saved energy is required to operate the centrifuge. This helps to protect the environment and also saves costs.

This article presents a basic overview of different types of centrifuges with descriptions of how they operate and where they are applied in the CPI.

 

Some definitions

To begin with, a distinction is made between thermal and mechanical separation of solid products from liquids. Whereas thermal S-L separators can be grouped under the general term “dryer”, it is not so easy to classify the equipment used in the first stage of mechanical S-L separation. For a finer distinction it is necessary to consider the flow directions of the solid and the liquid phases. If these are both in the same direction one talks of filtration; in the case of opposing directions the process is referred to as sedimentation (Figure 1). There is also the special case in which the flow directions of the solid and liquid phases are at right angles to each other, this is referred to as cross flow filtration.

 Figure 1. A classification scheme of the various types of equipment available
for solid-liquid separation is shown here

 

In most sedimentation processes, the difference in density between the solid and liquid phase is utilized, but it is also possible to use electric or magnetic fields for separating purposes. To some extent, the natural sedimentation in the earth’s gravitational field is used to this end, for example, by gravity thickeners. This natural sedimentation is enhanced by also superimposing a centrifugal field, as is done by cyclones and centrifuges.

Filtration, on the other hand, uses a filter medium that retains the solid phase while allowing the liquid phase to flow through.

Another way to distinguish filtration methods is to consider the driving potential that moves the liquid phase, or filtrate, through the filter medium. Separation can take place in the simplest of ways: using a screen in the earth’s gravitational field or by imposing a pressure gradient. Such pressure gradients can be generated by applying a vacuum to the filtrate side. Here, however, natural limits are quickly revealed.

Far greater potentials are to be found when the pressure gradient is applied to the solids side using overpressure. This brings us to the focus of the article, filtration centrifuges, which are used when a cake-forming filtration is required.

 

Filtration centrifuges

At the most basic level, nearly everyone is familiar with filtration centrifuges; this is the separation principle used in common appliances, such as washing machines and salad spinners.

The design of a centrifuge is usually always the same: a basket holds the filter medium, and the mixture to be separated rotates about an axis. Filtrate is spun outwards through the filter medium and the solids remain in the basket for easy discharge. In the simplest of centrifuges this operation is performed manually. On an industrial scale, however, the discharge is nearly always performed automatically.

Filtration centrifuges can also be further distinguished. On the one hand there are discontinuously operating centrifuges where the processes are carried out in batchwise steps, and then there are the continuous operating centrifuges where all process steps are carried out simultaneously.

In addition to this, there are many other features and variations in centrifuge design, for instance, the horizontal or vertical alignment of the rotation axis. In the various filtration centrifuge types, different combinations of these additional features have proven their worth in numerous applications. The following sections describe the evolution of such features.

 Figure 2. A cross section of a horizontal peeler
centrifuge with screen basket is shown here

 

Batch centrifuges

    Peeler.  Chronologically, the discontinuous or batch-operating peeler centrifuge represents the first generation, with the first horizontal peeler centrifuge launched in 1928 by Krauss-Maffei. As shown in Figure 2, these machines feature a cantilever-mounted basket that rotates in a closed process housing. The door on the front side houses all the fittings required for operating a process run. The drive part mounted at the rear contains, among other things, the motor for driving the basket shaft.

In a typical batch run, the S-L mixture (feed) is first fed to the basket. Centrifugation then proceeds through the following sequence of steps (Figure 3): cake formation on the filter medium in the supersaturated state (sedimentation); dewatering of the cake up to saturation (intermediate spinning); washing of the cake with a new supply of liquid (washing); spinning of the liquid up to the desired residual-moisture level (dry spinning); and finally, the discharge of the cake from the basket (peeling) using a peeler blade.

Attention is drawn to the fact that during the peeling off of the cake — this taking place at full speed in high quality machines — the peeler blade cannot be swung in flush with the filter medium (usually a mesh of plastic fibers or fine metal wires), as otherwise there is a risk of the mesh being damaged or peeled off. As a result, a residual heel is left in the basket that can be used again several times with many products. However, after a certain period of time, the heel becomes clogged with fine particles. Because of the increased filtration resistance, continued use becomes uneconomical and the residual heel has to be removed. This can be done pneumatically with pressure blows or hydraulically by dissolving and breaking up the cake through swirling action.

 Figure 3. Batch process for a peeler centrifuge involves several steps

 Figure 4. A horizontal peeler centrifuge with siphon basket
backwashes the heel while adding pressure for higher filtration efficiency

   

Siphon peeler.  The siphon-peeler centrifuge (Figure 4) is a design variant of the peeler centrifuge that enhances the performance. This design uses a rotary siphon that enables the residual heel to be backwashed from the filtrate side, which frees the capillaries of fine particles. In this way, the residual heel can be used much longer without necessitating its time-consuming removal.

A further advantage of the siphon variant is that, besides the applied filtration pressure generated by the mass of the liquid, there is an additional vacuum created behind the filter medium (through the low level in the siphon chamber) that increases the filtration rate.

Because it is possible to extend the length of the siphon-peeler pipe, feeding into a liquid pool is also possible with this type of machine.

Horizontal peeler centrifuges — both standard and with siphon basket — are very flexible in use. They can be used for starch, herbicides or fine chemicals, to name just a few application examples.

Vertical-basket peeler.  Another variant of the peeler centrifuge is the vertical-basket peeler centrifuge. The operating principle is practically the same as the horizontal peeler, except that the product is not discharged after peeling via a chute or conveyor screw, but rather drops through the openings in the bottom of the basket during the peeling process. To do this, the centrifuge is decelerated to enable a controlled product discharge. This, however, extends the batch processing time and consequently reduces the throughput rate.

Nevertheless, this vertical variant reduces capital costs due to its compact design. A typical application of vertical-basket peeler centrifuges is for the dewatering of gypsum produced in fluegas desulfurization units.

In general, the throughput of peeler centrifuges is somewhat restricted in comparison with the other types of filtration centrifuges. This is where the continuously operating centrifuges, in which all process steps are carried out simultaneously, enables a huge increase in the throughput rate.

 

 Figure 5. This photo shows the
basket of a baffle ring centrifuge

 

Continuous centrifuges

In continuous centrifuges a further distinction is made with regard to the conveying mechanism used for discharging product.

    Sliding discharge.  The sliding discharge centrifuge uses the simplest possible conveying mechanism. Instead of a cylindrical shell common to peeler centrifuges, the horizontally arranged basket has a conical shell that opens out towards the discharge of the centrifuge. The solids in the basket have a product-specific friction angle that has to be taken into account when selecting the opening angle of the basket. Through the slope resistance of the centrifugal force and the pressure of the following solids, the product slides through the basket without requiring any mechanical tool. To keep the friction angle of the product and the basket angle small, a fine-hole sheets or slotted screens rather than a mesh, are used in continuous centrifuges. The slotted screens are made of profiled wires between which the screen slots point in the transport direction.

    Baffle ring.  A special variant of the sliding discharge centrifuge is the baffle ring centrifuge (Figure 5). Here it is not the cake transport as a closed product layer that is intended, but rather a dewetting of individual particles. To this end, steps are arranged on the shell surface so that the particles are in a free fall for a short period of time. When the individual particles hit the next step, the surface moisture is spun off the particle. If, in the closed cake layer, there is still interstitial water between the particles that are in direct contact, this impact effect can attain cake residual-moisture levels of less than 0.1 wt.%. This application example, however, is almost solely restricted to coarse synthetic granules.

    Vibrating basket.  The next modification of the sliding discharge centrifuge is the vibrating basket centrifuge. Here, the opening angle of the conical basket is first selected in such a way that no sliding action or product transport will take place. However, by superimposing an axial vibrating movement to the rotation, the friction can be overcome for a short while through the impulse that is transferred to the product by the vibration of the shaft and the basket. This results in a stepwise transport through the basket.

The vibrating basket centrifuges stand out with their substantial solids throughput rates of up to 350 ton/h. The centrifugal force (C-value), however, is limited to approximately 150 times the earth’s gravitational acceleration (150 g), as otherwise the transport impulse can no longer be generated by means of vibration.

The main field of application of vibrating basket centrifuges is in the treatment of fine coal.

 Figure 6. The basket of a tumbler centrifuge is mounted on a U-joint, which
results in a tumbling motion that enables continuous product discharge

 

Tumblers.  The most complex way to generate a transport impulse is found in the tumbler centrifuge (Figure 6). Here again basically the same principle is applied as in a sliding centrifuge, using a conical basket. In this case, however, the basket is mounted via a U-joint and an articulated hollow shaft at a slight angle to the actual axis of rotation. Through a different speed of the inner drive shaft to that of the outer hollow shaft, the angular offset wanders independent of the rotation. The movement of the basket can be seen to wobble like a child’s spinning top.

In doing so, every point on the basket periodically passes through the area of the slightest inclination and the area of the greatest inclination, relative to the surface of the screen. In the area of the greatest inclination, the product starts to slide; in the area of the slightest inclination, the product is slowed down again until it stops. There is therefore no cake that is moved as an intact surface, as in the vibration basket centrifuge, or as a cake ring, as in the pusher centrifuge. Instead, there is a cake that is constantly subjected to a rotating wave movement.

Since the intensity of the transport impulse increases to the same degree as the centrifugal acceleration, the tumbler centrifuge is not subjected to the constraints of a vibrating basket centrifuge. C-values of up to 8,000 g are possible, and throughput rates of up to 300 ton/h can be achieved.

The tumbler centrifuge is used today for dewatering carnallite, for example.

 Worm/screen.  The conical-shaped basket used in the above centrifuges are also used in the worm/screen centrifuge. In the conical basket, there is also a conical screw that rotates in the same direction as the basket, but at a slighter differential speed, resulting in a forced transport of the product in the centrifugal field.

In this case, it is not necessary to exactly coordinate the basket angle to the product friction angle because the screw takes on a regulating function. Where the screw retains the still liquid slurry at the smaller diameter, it pro-actively conveys the product at the large diameter, the friction angle of which has increased due to the reduced residual moisture.

As a result of this effect, and through the forced product transport, the worm/screen centrifuge is relatively insensitive to feed fluctuations and interruptions in the product supply.

Typical applications are the filtration of iron sulfate and polystyrene.

Pusher.  The pusher centrifuge also has a forced product transport. This centrifuge has no conical basket. Its cylindrical basket is fixed to a hollow shaft and has a plate on the basket bottom that is mounted on a pusher rod, which runs inside the hollow shaft. The pusher rod oscillates axially so that the plate, known as the pusher plate, performs a pushing motion at the bottom of the basket.

 Figure 7. This inside view of a two-stage pusher centrifuge shows the rotating
parts in green and the rotating and oscillating parts in red. The bearing housing is blue

 

The product that is fed centrally through a rotating feed system, dewaters in the feed zone on the screen. The cake ring that then forms is pushed towards the discharge by the pusher plate. The vacant space that appears when the pusher bottom is driven back is again filled with new product, allowing a new cake ring to develop that, during the forward motion, pushes the previous one further forward.

For a continuous product transport, the pusher centrifuge requires a constant topping up with product. The pusher centrifuge therefore relies on the feed conditions being kept as constant as possible. Through the cake ring that is moved over the screen as a compact block, the throughput of particles is very low for a continuous operating filtration centrifuge, as most of the fine particles are retained in the cake. In a modern pusher centrifuge throughput rates of up to 150 ton/h can be reached.

After the single-stage pusher centrifuge was introduced, there soon followed two-stage pusher centrifuges (Figure 7). Here, a second, shorter inner basket with smaller diameter is installed between the pusher plate and the basket. In the two-stage pusher centrifuge the pusher movement — that in the single-stage machine only takes place between the pusher plate and the basket — also takes place between the inner basket and the outer basket. With two shorter baskets instead of the one long basket, the two-stage centrifuges have the advantages that the height of the cake is reduced, the filtration resistance is lower and the pushing force required is less than in single-stage machines. Even if the single-stage pusher centrifuges provide a higher throughput rate due to the higher cake volume in the basket, the two-stage machines have become established as the more popular version today. Machines are also available with three and more stages for special applications (Figure 8).

 Figure 8. Closeups of the interior of
two-stage (above) and three-stage (below)
pusher centrifuges

The main fields of application are crystalline products with average particle sizes of around 80 µm up to 10 mm, and solids feed concentrations from around 25 wt.%. Some examples of applications are the dewatering of ammonium sulfate, sodium chloride and adipic acid.

Final remarks

To select the right type of centrifuge it is most important to maintain close contact with the equipment manufacturer. For the coordination of the full system, it is also expedient for the manufacturer to not only be able to supply the complete range of centrifuges, but also the equipment integrated in the process line upstream or downstream of the centrifuge.

  Edited by Gerald Ondrey  

 

Author  

Peter Schmidt is sales engineer for pusher and other continuous centrifuges at Andritz KMPT GmbH (Industriestrasse 1–3, 85256 Vierkirchen, Germany; Phone: +49-8139-80-299-113; Fax: +49-8139-80-299-150; Email: peter.s.schmidt@andritz.com). He has been with KMPT since 1997, first as a student intern. From 2000–2008, he was KMPTs project manager for pusher centrifuges. Schmidt has a Diplom in mechanical engineering from the Berufsakademie Ravensburg.  

 

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