Taking a spin through centriguges

Compared with alternative methods of solids/liquids separation, centrifugal processing — depending on the type of system selected — provides a number of specific benefits.

Centrifuges can be installed in a relatively small space; have a high washing capability; produce low cake moisture; achieve a high capacity throughput and can provide the user with a totally enclosed and vapour-tight processing facility.

The correct selection of a centrifuge for a specific application, however, is based on two key concerns — economic expectations and technical requirements.

Before considering the various options, it is imperative to define the process clearly — a first step that prospective centrifuge users often overlook due to uncertainty as to what exactly a centrifuge is capable of achieving. An accurate process definition is also of vital importance at initial briefings with the centrifuge supplier, who needs to base screening tests and feasibility studies on relevant data — and not waste time and effort on vague and often misleading assumptions.

Once the process is clearly defined, it is much easier to identify variables which affect the choice of process package. Such variables can include the percentage of suspended solids, volumetric slurry throughput, solids throughput and the required product consistency at discharge. Materials usually exit the centrifuge in a powdered or granular form, but there are some instances in which it can also be discharged as a paste.

The user needs to clearly define exactly what the centrifugal process is to achieve. Does the material require clarification, classification, degritting, thickening, dewatering, washing or separating and repulping, and is the process a solid-liquid, liquid-liquid or a three phase liquid-liquid-solid duty?

Other considerations of equal importance include: the expected G force; allowable cake dryness; allowable solids in the discharge liquors; product temperature; viscosity; specific gravity; pH and whether the process is batch or continuous. Also, if the duty is in the pharmaceutical or fine chemical industry, the centrifuge may have to be manufactured to a GMP design with an integral CIP system.

The decision to use a batch or continuous machine can depend on many factors. Batch centrifuges have very little limitations on the wash function, while continuous machines are mostly limited to a wash-solids ratio of approximately 10 per cent, with no more than a few seconds allocated to the wash zone before completing the operation.

Particle size, distribution and shape are also important factors when determining separation capabilities and whether or not batch or continuous is the best option. Generally speaking, materials that are predominantly 45 microns and above, and relatively incompactable, are highly suitable for separation by filtration equipment, whereas finer or compactable materials lend themselves more to separation by sedimentation.

In circumstances where it is necessary to ensure that there is no cross-contamination between batches, machines can be installed with pre-programmable, validated clean-in-place (CIP) washing systems. These can also eliminate the need to open the casing between cycles for cleaning and maintenance. Such machines are particularly suitable for use on applications within the pharmaceutical and fine chemical industries.

Over the years, stringent codes of practice, rigidly enforced across the international process industries — greatly influenced by the recommendations of authorities like the FDA ‚ have created a need for validatable centrifuge designs. These codes have provided the basic initiative for the development of machines, which combine the very best of good manufacturing practice (GMP) and state-of-the-art PLC controls.

These centrifuges are GAMP compliant with the ability to more than satisfy the requirements of process and environmental integrity. In situations where centrifuges have to handle potentially explosive or flammable products, they can be installed with inert purging gas systems to ensure complete operational integrity. Machines can also be supplied with pressure-tight systems.

Having clearly defined the process, it is now possible to proceed with selecting a specific centrifuge to meet the criteria, combining optimum performance with maximum cost-effectiveness.

Some of the salient features of each type now follow, starting with:

Sedimenting Centrifuges

Horizontal solid bowl decanters

Decanter centrifuges consist of two horizontal concentric rotating elements contained in a stationary casing. The outer rotating bowl is tapered so that solids discharge from a smaller radius than the liquor. Solids are moved to the tapered end of the bowl by the inner element — a hollow hub screw conveyor that rotates at a slightly different speed to that of the outer bowl. Clarified liquor discharges continuously in the opposite direction from adjustable overflow ports. Used as a classifier, the solid bowl decanter centrifuge gives sharp cuts of solids in liquor suspension with materials as coarse as 50 micron, or as fine as one micron.

Solid bowl basket centrifuges

Batch perforated basket centrifuges have, in recent times, been generally looked upon as obsolete technology. Other machines, especially solid bowl decanters, are more in favour for difficult-to-convey solids. But where relatively small volumes have to be processed, and cycle times are not a significant factor, then solid bowl baskets could be considered.

Disc bowl centrifuges Operating at high forces of 3000 to 20 000G, the disc bowl centrifuge provides a continuous clarification system suitable for materials with a solids content of less than 1-2 per cent. Either solid-liquid or liquid-liquid phases can be separated — solids settling on the wall of the bowl and liquid discharging through one or more paring discs. The disc stack greatly increases the effective settling/clarification area, with the liquid and solid phases travelling up or down the disc surfaces.

Tubular centrifuges

A solid tube capped at both ends, the tubular centrifuge is usually fed through a bottom inlet with two liquids of different specific gravities. The heavier phase concentrates against the cylinder wall, with the lighter phase floating against it. The two phases are separated by means of a baffle that discharges them into two distinct flows. Where solid feeds are processed, regular cleaning is necessary, but if there are no suspended solids the process can be continuous.

Filtering Centrifuges

Vertical basket centrifuges Modern basket filtering centrifuges are very adaptable machines, thanks to the wide selection of feed, wash, spin and plough speeds available, through either electrical inverter or hydraulic drives. Producing an exceptionally dry cake, these batch machines have two major advantages — the capability of efficiently washing cake solids, using minimum wash fluids, and an ability to discharge the separated solids at low basket speed, ensuring negligible breakage of delicate crystals.

Given correct feed conditions, feed speed and filter cloth, basket centrifuges can dewater solids from 1 to 10 000 microns. They can also be fully sealed and purged for safe operation, and can operate fully automatically with minimum operator attention.

Horizontal basket peelers

Two types of peeler centrifuge are available — a heavy duty chemical design and a GMP design. Both offer a filtering and decanting capability, and are suitable for processing a wide range of materials in the ultra-clean environmental conditions of the pharmaceutical and fine chemical industries. The machines have perforated baskets and screened membranes for filtering processes or solid bowls for decanting. The peeler has a fully opening front-end casing and, as the name implies, an automatic peeler knife mechanism for cake discharge. It has the additional benefit of an effective 'heel' removal system, a feature that provides complete batch-to-batch containment and dramatically reduces the operating cycle by removing separated solids at high speed.

Due to high G forces and increased discharge speeds, the peeler has short cycle times that can be adjusted to ensure a range of washing capabilities. The machines can be used for applications where the feed slurry has either a low or fluctuating solids concentration. Peelers can be fitted with CIP systems and also built to a 'through the wall' design that allows the centrifuge to be more easily and speedily serviced in a completely separate area of the factory. This totally eliminates the risk of contamination to the front end process side of the system.

Scroll/screen centrifuges The scroll/screen consists of a horizontally driven scroll conveyor, which revolves at an optimum differential speed within a rotating conical basket. Solids discharge takes place by the inclination of the basket and the differential speed of the scroll. At the point of separation, solids are conveyed forwards by the scroll to discharge at the widest open end of the basket, while filtrate passes directly through the screen.

Scroll/screen machines have excellent washing properties and can be used for solids/liquids separation on a diverse range of applications where feed materials have high particle sizes, typically 50 microns and above.

Horizontal screen bowl decanters

Operationally similar to solid bowl decanters, these are designed to provide additional washing efficiency and enhanced moisture removal for applications where crystalline materials such as paraxylene and coal fines are involved. The decanter operates in two stages, combining the clarification and sedimentation advantages of the solid bowl centrifuge with the dewatering benefits of an additional screen section.

Pusher centrifuges

This type of filtering centrifuge, which provides particularly long residence times, operates on a continuous basis, retaining solids as a cake on a wedge-wire basket. Solids are discharged by an oscillating pusher mechanism. Feed solids can be granular, crystalline or fibrous — but should be relatively incompressible. They should also be free-draining with a low aspect ratio and an average size of 200microns.

Vibratory centrifuges

High throughputs of up to 350tonnes/h are possible with this machine. Solids are retained by a sieve and transported by axial vibrations greater than the rotational speed of the centrifuge. These machines are highly suitable for processing high throughput products that can be easily dewatered to the required moisture content.

Inverting bag centrifuges

These are horizontal, automatic filtering machines, incorporating an automatic unloading bag. The front and rear basket walls stroke forward by hydraulic piston to discharge solids. The filter cloth is arranged as a cylinder, with the rear edge secured to the rear basket wall and the front edge to the basket shell at the front rim.

As the piston strokes forward, the cloth is turned inside out and the solids discharge in clumps into the solids collection housing. Primarily used in the pharmaceutical industry, this type of centrifuge provides heel removal after each cycle, though is limited to smaller sizes and capacities.

While the information given above provides a basic guide to making an initial selection of the best centrifuge for a specific duty, in the interests of cost-effectiveness for all concerned it is strongly recommended that the user should consult the centrifuge manufacturer at the earliest possible stages of process design.