Pusher centrifuge

A pusher centrifuge is a type of filtration technique that offers continuous operation to de-water and wash materials such as relatively in-compressible feed solids, free-draining crystalline, polymers and fibrous substances. It consists of a constant speed rotor and is fixed to one of several baskets. This assembly is applied with centrifugal force that is generated mechanically for smaller units and hydraulically for larger units to enable separation.

Pusher centrifuges can be used for a variety of applications. They were typically used in inorganic industries and later, extensively in chemical industries such as organic intermediates, plastics, food processing and rocket fuels.

A suspension feed enters the process to undergo pre-acceleration and distribution. The subsequent processes involve main filtration and intermediate de-watering, after which the main filtrate is collected. Wash liquid enters the washing step and final de-watering follows. Wash filtrate is extracted from these two stages. The final step involves discharge of solids which are then collected as the finished product. These process steps take place simultaneously in different parts of the centrifuge.

It is widely accepted due to its ease of modification, such as gas-tight and explosion protection configurations.

Applications

Pusher centrifuges are mainly used in chemical, pharmaceutical, food (mainly to produce sodium chloride as common salt) and mineral industries. During the twentieth century, the pusher centrifuge was used for desiccation of comparatively large crystals and solids.

Although pushers are typically used for inorganic products, they appear in chemical industries such as organic intermediates, plastics, food processing and rocket fuels. Organic intermediates include paraxylene, adipic acid, oxalic acid caprolactam, nitrocellulose, carboxymethylcellulose, etc.

In food processing, pusher centrifugation is used to produce monosodium glutamate, salt, lysine and saccharin.

Pusher centrifugation is also used in the plastic industry, contributing to products such as PVC, polyethylene and polypropylene, and a number of other resins.

Individual products

• Soda Ash—Particle size is commonly beyond 150  µm. Feed slurry usually has 50% solids by weight, and discharged cake has about 4% moisture.

• Sodium bicarbonate—Feeds usually contain more than 40% of solids in weight with and crystals generally beyond the particle size of 45 µm. Cake production usually has only 5% water. To achieve such high efficiency of desiccation, requires device modifications.

• Paraxylene—Fed as frozen slurry with a particle size ranging from 100 to 400 µm. Purity of 99.9% is available using a single stage long basket design. Considerations and measurements have to be taken to avoid contamination of paraxylene and oil. Lip seals and rod scrapers are used on the shaft seal to eliminate cross-contamination. The feed is purified using a funnel. Vents integrated into process housing ensure that gases moves uninhibited, preventing contamination.

• Adipic acid—Undergoes repeated process of crystallisation, centrifugation and remelting to achieve the required purity. Adipic acid crystals are generally larger than 150 µm. nitric acid is reduced from 30% in the feed to 15 ppm in the cake produced. Separation of nitric acid from adipic acid is essential for further treatment.

• Cotton seed delinting—Cotton seeds contain fibres that grow and form a ball of lint. This is separated using sulphuric acid, where the lint may be used to produce cotton fibre. Adding sulphuric acid causes the lint to become brittle, hence ensuring that in the subsequent tumbling process de-linting occurs effectively.

Advantages and limitations

Advantages

• Pushers offer higher processing capacities than batch filtering centrifuges such as vertical basket and inverting filter.
• Provides the best washing characteristics of any continuous centrifuge due to control of retention time and uniform cake bed.
• Gentle handling makes pushers better suited for fragile crystals.

Limitations

• Pushers require a constant feed flood due to their continuous nature.
• Although high capacities may be preferred, this may result in longer residence time.
• Typical particle sizes must be at least 150 μm and average 200 μm.
• A high viscosity feed lowers throughput.
• Pushers have a limited liquid filtration capacity and requires fast-draining materials, since it must form a cake within the period of one stroke.

Designs

The designs for pusher centrifuge are as follows:

Pushers come with eithermechanical and/or hydraulic drive units. Speed can vary.

Single-stage

Single-stage units can be cylindrical or cylindrical/conical with a single long basket and screen

• Can maximise solids volumetric capacity
• Resulting cake can shear or buckle due to unstable operation of the longer screen length
• Capacity may be slightly less than with multistage units
• Lesser fine losses due to small contact of particles with the slotted screen and no reorientation of crystals between stages
• Used to achieve stability for low-speed operation

Multi-stage

Multistage (two-, three-, or four- stage designs): cylindrical and cylindrical/conical

• Most common
• Greater flexibility due to higher filtration capacity
• Reorientation can enhance wash effect on the latter portion of the first stage and through transition onto second stage

Three-and-four stage

• Used for largest sizes with long baskets
• Recommended for materials with high friction coefficients, low internal cake shear strength, or high compressibility, e.g., processing high rubber ABS
• Lower capacity affects performance due to correspondingly thin cakes and short retention time

Cylindrical/conical

Feed distributor design: conical/cylindrical or plate

• Optionally applied for single- and two stage- designs.
• The feed section is cylindrical combined with a sloping design towards the discharge end
• Axial component of force in the conical end aids solids transport
• Lower production costs compared to that of baskets

Process characteristics

The important parameters of centrifugal filtration are screen area, acceleration level in the final drainage zone, and cake thickness. Cake filtration affects the residence time and volumetric throughput rate. Residence time for pushers on the screen is controlled by the length and diameter of the screen, thickness of the cake and frequency and length of stroke of the cake.

Feed

Pusher centrifuge utilises the cake layer to act as a filtration hence the feed to the pusher centrifuge normally contains high solid concentration containing fast draining, crystalline, granular or fibrous solids. The solid concentration ranges from 25-65 wt% solid. The size of the mean particle suitable for this type of centrifuge must be at least 150 µm. The capacity depends on the diameter of the basket ranging from 1 ton/h to 120tons/h.

Operations

The cake is under centrifugal force which becomes drier as it progresses in the basket and discharged from the pusher basket into the solid discharge housing (pusher centrifuge operation). The stroke length ranges from 30 to 80 mm and the stroke frequency is between 45 to 90strokes/min.

The push efficiency is defined as the distance of the forward movement of the cake ring divided by the stroke length. The push efficiency is a function of the solid volumetric loading which results in a self-compensating control of varying rates. Up to 90% push efficiency is achievable depending on the cake properties.

Filtration rate

The equation for the Filtration rate, Q:

(1) Q = (πbρKΩ^2(r_b^2 - r_p^2))/(μ([r_b/r_p])+(KR_m)/r_b)

(2) αKp_s = 1

Where µ and ρ are viscosity and density of liquid respectively, Ω is the angular speed, K is the average permeability of the cake which is related to equation 2, r_p,r_c, and r_b are the radius of the liquid surface, cake surface and filter medium adjacent to the perforated bowl respectively, R_m is the combined resistance, α is the specific resistance and ρs is the solid density.

The numerator of the equation describes the driving force of the centrifuge which is due to the hydrostatic pressure difference across the wall and the liquid surface. The denominator describes the resistance due to the cake layer and the filter medium.

Process variables

The performance of the pusher centrifuge is a function of many parameters. Some of the important variables for the pusher variable are the particle size, viscosity, solid concentration and the cake quality.

Particle size and porosity

The particle size has to be as large as practically possible to create the cake layer. Larger particle size will increase the porosity of the cake layer and allow feed liquid to pass through. The shape of the particle is equally important because it determines the surface area per unit mass. As it decreases, there is less surface area for the moisture to bind to hence providing a drier cake.

Viscosity

In addition to porosity, filtration rate is a function of the viscosity of the feed fluid. From equation 1, the relationship of the filtration rate is inversely proportional to the viscosity. Increasing viscosity means adding resistance to the flow of the fluid which will make it more difficult for the separation of the fluids from the slurry. Consequently, the throughput of the pusher is de-rated due to the viscosity of feed fluid.

Solid concentration

In most cases, the solids discharge capacity or the hydraulic capacity is not the limiting factor. The usual limitation of the pusher centrifuge is the filtration rate. Therefore, more solids can be processed by increasing the feed slurry concentration.

Cake quality

The cake quality is determined by the purity and the amount of volatile matter.

Purity

Wash liquid is introduced on the cake in order to displace the mother liquor along with the impurities. Cake wash ratio is normally between 0.1 to 0.3 kg wash/kg solids which displace at least 95% of the feed fluid and impurities within the normal residence time of the wash zone.

Amount of volatile matter

Amount of volatile matter present in the discharge is a function of the centrifugal force (G) and the residence time at that force. Separation increases with G and hence favours the filtration rate as illustrated in equation 3.

(3) G = (Ω^2)r/g

Where G is the centrifugal force, Ω is the angular speed, r is the radius of the basket, and g is the gravitational force. By relating equation 3 to equation 1, the relationship of the centrifugal force is proportional to the filtration rate. As the pusher centrifuge are suited to deal with fragile crystals, considerations need to be made to the movement of the pusher plate along with the acceleration in the feed funnel because it can cause some of the particles to break. In addition to the movement plate, G can cause breakage and compaction, and volatile matter in discharge cake will increase. The gentle conveying of cake in the low G, single stage, long basket design of pusher centrifuge results in low particle attrition(Furthermore, as more solids are processed through, residence time of solids on basket decrease which increases volatile matter in discharge cake.

Possible heuristics to be used during design of process

The heuristics of pusher centrifuge is concerned with the selection of the suitable size of equipment, the operation sequence and recycle structure to deliver an optimum performance of the process.

Design process

The overall approach to the design process is as follows:

1) Define the problem

2) Outline process conditions

3) Assess types of pusher centrifuge design and make preliminary selections

4) Develop a test program

5) Take samples of materials to be processed

6) Do simple testing

7) Adjust process conditions if required

8) Consult equipment manufacturers

9) Make final selection and obtain quotations

Equipment sizing

Variables considered in sizing equipment:

• Feed rate
• Feed concentration
• Cake thickness
• Bulk density
• Long and short baskets
• Single and two-stage baskets
• Individual drive for rotor and hydraulic system
• Easy accessibility for maintenance
• Energy consumption
• Previous applications

Equipment selection

Equipment selection is based upon evidence testing, references from similar existing or previous processes, and experience and considered in terms of:

• Cost, quality and productivity
• Financially modelled, over the anticipated lifetime of the process.

Optimising performance

For designs with the conical and cylindrical assembly, the cone slant angle should not exceed sliding friction angle of the cake otherwise it would result in high vibration and poor performance.

In order to optimise capacity and performance, it is desirable to pre-concentrate the feed slurry as high as possible. Some designs have a short conical section at the feed end for prethickening within the unit, but generally it is preferable to thicken ahead of the centrifuge with gravity settlers, hydrocyclones, or inclined screens. The feed should also have a higher concentration of solids.

The volumetric throughput for multistage designs can be increased by increasing the forced cake height while still retaining acceptable push efficiency.

Design selection

Selection of designs is usually done by scale-up from lab tests on materials to be processed. The analysis of the data from the tests should be rationalised and consequently scale-up of equipment is implemented. Computer software can be used to assist in design and scale-up. Pilot-testing should follow after.

Waste stream

Production of waste stream

The majority of liquid contained within the mixture would be drawn out at an early stage, which is the feed zone of the slot screen and will be discharged into the filtrate housing. After formation of solid cakes, the main by-product produced would be water, which may be used in all sorts of industrial usage. Filtration cakes will be washed using nozzles or waste baskets.

Post-treatment systems

Due to the wide range of utilisation of this particular method, it is impossible to list out all the methods of treating waste product. However, each by product produced can be treated accordingly to its property.

New developments

In the last few decades there have been several improvements for the pusher centrifuges. Design modifications have been made on them to enhance performance and to broaden their range of applications. These features and modifications include multiple stages, push hesitation, horizontal split process housing, an integral hydraulic system, seals, pre-drained funnels, and an integrated thickening function.

Multiple stages

B&P Process Equipment and Systems (B&P) has the largest single-stage pusher centrifuge which they claimed to be superior compared to multistage pusher centrifuges. According to their research in multistage pusher centrifuge, there are additional impurities going to the liquid housing due to the tumbling of the particles in each successive stage. However recent development shows the problem can be overcome by installing shorter inner basket with smaller diameter between the pusher plates and the basket and also enabling pusher movement to take place between the pusher plate and the basket as well as between the inner basket and the outer basket. Compared to single-stage pusher centrifuges that only have pusher movement between pusher plate and the basket, multistage centrifuges have the advantages that the height of the cake is reduced, lower filtration resistance and lesser pushing force required.

Push hesitation

Push hesitation holds the pusher plate in the back stroke, allowing the cake to build up on itself. The cake acts as the filtering media that can even capture finer solids. This reduces the loss of solid passing through the wedge slots. Although this modification reduces capacity, it has helped improved the solid capture efficiency and make the pusher centrifuges applicable to smaller particles.

Horizontal split process housing

This allows the removal of the rotating assembly without disassembling the basket and pusher centrifuge from the shafting assembly.

Integral hydraulic system

Enhanced with automatic mechanism and the system operate independently.

Seals

Shaft seals are introduced to the system to eliminate the possibility of cross- contamination between the hydraulic and process ends. Options are available for decision making, including a centrifugal liquid ring seal and a non-contacting inert gas purged labyrinth seal that allows zero seal leakage.

Pre-drained funnel

The pre-drain funnel removes a portion of the feed fluid through a puncture surface. This feature helps to make the feed more concentrated, which is especially important for drainage-limited applications. However the funnel cannot be back-washed therefore this feature is only limited for crystals that tend not to back-crystallise.

Integrated thickening function

According to Ferrum's "Centrifuge Technology Solution", by integrating the thickening function enables the machine to be loaded with solid-liquid mixture that has as low as 30-35% wt of solid. It also reduces process costs of solid-liquid separation by as much as 20%.

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