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{{citation style|date=November 2013}}
{{citation style|date=November 2013}}
[[File:Pusher Centrifuge.jpg|thumb|Pusher Centrifuge]]
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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 (electric)|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.

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]].
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]].
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== Applications ==
== 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.<ref>{{Harvnb|Technologies|2008}}</ref>


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.
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.<ref>{{Harvnb|Technologies|2008}}</ref>


In food processing, pusher centrifugation is used to produce monosodium glutamate, salt, lysine and saccharin.<ref name="Dubal">{{Harvnb|Dubal|2008}}</ref>
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.


Pusher centrifugation is also used in the plastic industry, contributing to products such as [[polyvinyl chloride|PVC]], [[polyethylene]] and [[polypropylene]], and a number of other [[resins]].
In food processing, pusher centrifugation is used to produce monosodium glutamate, salt, lysine and saccharin.<ref name="Dubal">{{Harvnb|Dubal|2008}}</ref>

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


Individual products
Individual products
*Soda Ash—Particle size is commonly beyond 150&nbsp;&nbsp;[[µm]]. Feed [[slurry]] usually has 50% solids by weight, and discharged cake has about 4% moisture.
*Soda Ash—Particle size is commonly beyond 150&nbsp;&nbsp;[[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&nbsp;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&nbsp;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&nbsp;μm. [[nitric acid]] is reduced from 30% in the feed to 15&nbsp;[[parts-per notation|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 (material)|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.


*Sodium bicarbonate—Feeds usually contain more than 40% of solids in weight with and crystals generally beyond the particle size of 45&nbsp;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&nbsp;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&nbsp;µm. [[nitric acid]] is reduced from 30% in the feed to 15&nbsp;[[parts-per notation|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 and limitations ==

=== Advantages ===
=== Advantages ===

*Pushers offer higher processing capacities than batch filtering centrifuges such as vertical basket and inverting filter.
*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.
*Provides the best washing characteristics of any continuous centrifuge due to control of retention time and uniform cake bed.
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=== Limitations ===
=== Limitations ===

*Pushers require a constant feed flood due to their continuous nature.
*Pushers require a constant feed flood due to their continuous nature.
*Although high capacities may be preferred, this may result in longer residence time.
*Although high capacities may be preferred, this may result in longer residence time.
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== Designs ==
== Designs ==

The designs for pusher centrifuge are as follows:
The designs for pusher centrifuge are as follows:


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

=== Single-stage ===
=== Single-stage ===
Single-stage units can be cylindrical or cylindrical/conical with a single long basket and screen
Single-stage units can be cylindrical or cylindrical/conical with a single long basket and screen
*Can maximise solids volumetric capacity
*Can maximize solids volumetric capacity
*Resulting cake can shear or buckle due to unstable operation of the longer screen length
*Resulting cake can shear or buckle due to unstable operation of the longer screen length
*Capacity may be slightly less than with multistage units
*Capacity may be slightly less than with multistage units
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*Reorientation can enhance wash effect on the latter portion of the first stage and through transition onto second stage
*Reorientation can enhance wash effect on the latter portion of the first stage and through transition onto second stage


=== Three-and-four stage ===
==== Three/four stage ====

*Used for largest sizes with long baskets
*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
*Recommended for materials with high friction coefficients, low internal cake shear strength, or high compressibility, e.g., processing high rubber ABS
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Feed distributor design: conical/cylindrical or plate
Feed distributor design: conical/cylindrical or plate
*Optionally applied for single- and two stage- designs.
*Optionally applied for single- and two stage- designs.
*The feed section is cylindrical combined with a sloping design towards the discharge end
*Cylindrical feed section combined with a sloping design towards the discharge end
*Axial component of force in the conical end aids solids transport
*Axial component of force in the conical end aids solids transport
*Lower production costs compared to that of baskets
*Lower production costs compared to that of baskets


== Process characteristics and assessment ==
== Process characteristics ==
The important parameters are screen area, acceleration level in the final drainage zone and cake thickness. Cake filtration affects residence time and volumetric throughput. Residence on the screen is controlled by the screen's length and diameter, cake thickness and the frequency and stroke length of the cake.<ref>{{Harvnb|Schmidt|2010|pp=34–38}}</ref>

In general, the important parameters of centrifugal filtration equipment are screen area, level of centrifugal acceleration in the final drainage zone, and [[cake thickness]]. Cake filtration affects the residence time and volumetric throughput rate. Residence time for pusher centrifuge 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.<ref>{{Harvnb|Schmidt|2010|pp=34–38}}</ref>


=== Feed ===
=== Feed ===
Pushers utilise the cake layer to act as a filter, hence the feed normally contains high solid concentration containing fast draining, crystalline, granular or fibrous solids. The solid concentration ranges from 25-65 wt%.<ref name = "Dubal" /> The mean particle size suitable for pushers must be at least 150&nbsp;μm. The capacity depends on the basket diameter and ranges from 1 ton/h to 120tons/h.<ref name = "Perry">{{Harv|Green|Perry|2008|pp=1056–1065}}</ref>

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.<ref name = "Dubal" /> The size of the mean particle suitable for this type of centrifuge must be at least 150&nbsp;µm. The capacity depends on the diameter of the basket ranging from 1 ton/h to 120tons/h.<ref name = "Perry">{{Harv|Green|Perry|2008|pp=1056–1065}}</ref>


=== Operations ===
=== Operations ===
The cake is under centrifugal force. It becomes drier as it progresses in the basket and is discharged from the pusher basket into the solid discharge housing (pusher centrifuge operation). The stroke length ranges from 30 to 80&nbsp;mm and the stroke frequency is between 45 and 90strokes/min.<ref name="Perry" />


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&nbsp;mm and the stroke frequency is between 45 to 90strokes/min.<ref name="Perry" />
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 self-compensating control of varying rates. Up to 90% push efficiency is achievable depending on the cake properties.<ref name="Perry" /> dQ3ET42T

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.<ref name="Perry" />


=== Filtration rate ===
=== Filtration rate ===
The equation for the filtration rate, Q:<ref name="Perry" />


::(1) <math>Q=(\pi b \rho K \Omega^2(r_b^2 - r_p^2))/(\mu [r_b/r_p])+(KR_m)/r_b)</math>
The equation for the Filtration rate, Q:<ref name="Perry" />
::(2) <math>\alpha K \rho_s = 1</math>


Where <math>\mu</math> and <math>\rho</math> are viscosity and liquid density, respectively. <math>\Omega</math> is the angular speed, <math>K</math> is the average cake permeability, which is related to equation (2), <math>r_p,r_c</math>, and <math>r_b</math> are the radius of the liquid surface, cake surface and filter medium adjacent to the perforated bowl respectively, <math>R_m</math> is the combined resistance, <math>\alpha</math> is the specific resistance and <math>\rho s</math> is the solid density.
::(1) Q = (πbρKΩ^2(r_b^2 - r_p^2))/(μ([r_b/r_p])+(KR_m)/r_b)
::(2) αKp_s = 1


The numerator describes the pusher's driving force, 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.
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<sub>p</sub>,r<sub>c</sub>, and r<sub>b</sub> are the radius of the liquid surface, cake surface and filter medium adjacent to the perforated bowl respectively, R<sub>m</sub> 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 ===
=== Process variables ===
Performance is a function of many parameters, including particle size, viscosity, solid concentration and cake quality.<ref name = "Dubal" />


=== Particle size/porosity ===
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.<ref name = "Dubal" />
To create the cake layer, the particle size has to be as large as practically possible. Larger particle size increases the [[porosity]] of the cake layer and allows feed liquid to pass through. Particle shape is equally important, because it determines the surface area per unit mass. As it decreases, less surface area is available to bind moisture, providing a drier cake.<ref name = "Dubal" />

=== 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.<ref name = "Dubal" />


=== Viscosity ===
=== Viscosity ===
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 fluid flow, which complicates separation of the fluids from the slurry. Consequently, the throughput of the pusher is de-rated.<ref name = "Dubal" /><ref name="Perry" />

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.<ref name = "Dubal" /><ref name="Perry" />


=== Solid concentration ===
=== Solid concentration ===
In most cases the solids discharge capacity/hydraulic capacity is not the limiting factor. The usual limitation is the filtration rate. Therefore, more solids can be processed by increasing the feed slurry 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 ===
=== Cake quality ===

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


==== Purity ====
==== Purity ====
Wash liquid is introduced on the cake in order to displace the mother liquor along with the impurities.<ref name = "Dubal" /> The cake wash ratio is normally between 0.1 and 0.3&nbsp;kg wash/kg solids, which displace at least 95% of the feed fluid and impurities within the wash zone's normal residence time.<ref name="Perry" />


==== Volatile matter ====
Wash liquid is introduced on the cake in order to displace the mother liquor along with the impurities.<ref name = "Dubal" /> Cake wash ratio is normally between 0.1 to 0.3&nbsp;kg wash/kg solids which displace at least 95% of the feed fluid and impurities within the normal residence time of the wash zone.<ref name="Perry" />
The 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).<ref name="Perry" />


::<math>G = (\Omega^2)r/g</math>
==== Amount of volatile matter ====


where <math>G</math> is the centrifugal force, <math>\Omega</math> is the angular speed, <math>r</math> is the radius of the basket, and <math>g</math> is the gravitational force.
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.<ref name="Perry" />


By relating equation 3 to equation 1, the relationship of the centrifugal force is shown to be proportional to the filtration rate. As pushers often deal with fragile crystals, the movement of the pusher plate and acceleration in the feed funnel matter, because they can break some of the particles.<ref name="Perry" /> In addition to the movement plate, G can cause breakage and compaction, and volatile matter in the cake increases. The gentle movement of cake in low G, single stage, long basket designs results in low particle attrition. As more solids pass through, residence time decreases, which increases volatile matter in the discharge cake.<ref name="Dubal" />
::(3) G = (Ω^2)r/g


== Process design heuristics ==
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.
The heuristics of pusher centrifuge design consider equipment size, operation sequence and recycle structure.
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.<ref name="Perry" /> 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.<ref name = "Dubal" />

== 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 ===
=== Design process ===
Overall approach:<ref name="Perry" />

:* Define the problem
The overall approach to the design process is as follows:<ref name="Perry" />
:* Outline process conditions
:1) Define the problem
:* Make preliminary selections
:2) Outline process conditions
:* Develop a test program
:3) Assess types of pusher centrifuge design and make preliminary selections
:* Test sample batches
:4) Develop a test program
:* Adjust process conditions as required
:5) Take samples of materials to be processed
:* Consult equipment manufacturers
:6) Do simple testing
:* Make final selection and obtain quotes
:7) Adjust process conditions if required
:8) Consult equipment manufacturers
:9) Make final selection and obtain quotations


=== Equipment sizing ===
=== Equipment sizing ===

Variables considered in sizing equipment:
Variables considered in sizing equipment:
*Feed rate
*Feed rate
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=== Equipment selection ===
=== Equipment selection ===
Equipment selection is based upon test results, references from similar processes and experience and considered in terms of:

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
*Cost, quality and productivity
*Financial modeling
*Financially modelled, over the anticipated lifetime of the process.


=== Optimising performance ===
=== Optimising performance ===
For conical and cylindrical designs and assembly, the cone slant angle should not exceed sliding friction cake angle. Otherwise it would result in high vibration and poor performance.<ref name="Perry" />


In order to optimise capacity and performance, it is desirable to pre-concentrate the feed slurry as much as possible. Some designs have a short conical section at the feed end for thickening within the unit, but generally it is preferable to thicken before entering the centrifuge with gravity settlers, hydrocyclones or inclined screens, producing a higher concentration of solids.
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.<ref name="Perry" />

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


=== Design selection ===
=== Design selection ===
Selection of designs is usually done by scale-up from lab tests. Test data analysis should be rationalised in preparation for equipment scale-up. [[Computer-aided design]] software can assist in design and scale-up. Pilot-testing and rollout then follows.<ref>{{Harv|Wakeman|Tarleton|1993|pp=530–543}}</ref>


== Waste ==
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.<ref>{{Harv|Wakeman|Tarleton|1993|pp=530–543}}</ref>
=== Production ===
The majority of liquid contained within the mixture is drawn out at an early stage, in the feed zone of the slot screen. It is discharged into the filtrate housing. After formation of solid cakes, the main by-product produced is water, which may be used in all sorts of industrial usage. Filtration cakes are washed using nozzles or waste baskets.


== Waste stream ==
=== Post-treatment ===
Post-treatment processes are a function of the specifics of the waste stream and are diverse.<ref>{{Harvnb|Rotofilt}}</ref>


== Later designs ==
=== Production of waste stream ===
Design advances have enhanced performance and broaden the application range. These include additional stages, push hesitation, horizontal split process housing, integrated hydraulics, seals, pre-drained funnels and an integrated thickening function.


=== Stages ===
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.
B&P Process Equipment and Systems (B&P) makes the largest single-stage pusher centrifuge, which they claimed to be superior to multistage designs.<ref>{{Harvnb|Filtration & Separation|1997}}</ref> They claimed that additional impurities enter the liquid housing due to additional particles tumbling in each stage. The problem can be overcome by using a shorter inner basket with smaller diameter between the pusher plates and the basket and 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 pushers that have pusher movement only between pusher plate and basket, multistage centrifuges have the advantages that the cake height is reduced, filtration resistance is lower and lesser force is required.

=== 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.<ref>{{Harvnb|Rotofilt}}</ref>

== 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.<ref>{{Harvnb|Filtration & Separation|1997}}</ref> 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 ===
Push hesitation holds the pusher plate in the back stroke, allowing the cake to build on itself. The cake acts as the filtering media that can even capture finer solids. This reduces the loss of solids passing through the wedge slots. Although this modification reduces capacity, it has helped improved the solid capture efficiency and make pusher centrifuges applicable to smaller particles.<ref name = "Dubal" />

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.<ref name = "Dubal" />


=== Horizontal split process housing ===
=== Horizontal split process housing ===

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


=== Integral hydraulic system ===
=== Integral hydraulics ===
An automated mechanism allows the system to operate independently.

Enhanced with automatic mechanism and the system operate independently.


=== Seals ===
=== Seals ===
Shaft seals eliminate the possibility of cross-contamination between the hydraulic and process ends. Options include a centrifugal liquid ring seal and a non-contacting [[inert gas]] purged [[labyrinth seal]] that eliminates leakage.

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


=== Integrated thickening ===
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.
Integrating the thickening function enables the pusher to be loaded with mixture with as little as 30-35% wt of solid. It also reduces process costs of solid-liquid separation by as much as 20%.<ref>{{Harvnb|Filtration & Separation|2003|pp=38–39}}</ref>

=== 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%.<ref>{{Harvnb|Filtration & Separation|2003|pp=38–39}}</ref>


== References ==
== References ==

{{Reflist}}
{{Reflist}}


== Bibliography ==
== Bibliography ==

<div class="references-small">
*{{cite book
*{{cite book
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| last = Green | first = Don W. | last2= Perry |first2= Robert H.
Line 244: Line 203:
| pages = 1056–1065
| pages = 1056–1065
}}.
}}.

*{{cite web
*{{cite web
| last = Technologies | first =F.P
| last = Technologies | first =F.P
Line 251: Line 209:
| url = http://www.fenixchemtech.in/fxpusher.html
| url = http://www.fenixchemtech.in/fxpusher.html
| accessdate = 2013-10-14 }}.
| accessdate = 2013-10-14 }}.

*{{cite journal
*{{cite journal
| last = Dubal | first = Gitesh
| last = Dubal | first = Gitesh
Line 260: Line 217:
| pages = 24–27
| pages = 24–27
| date = May 2000
| date = May 2000
| url = http://www.sciencedirect.com/science/article/pii/S0015188200888497
| issn = 0015-1882
| issn = 0015-1882
| doi = 10.1016/S0015-1882(00)88849-7 }}.
| doi = 10.1016/S0015-1882(00)88849-7 }}.

*{{cite journal
*{{cite journal
| last = Ltd
| last = Ltd
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| title = Pusher centrifuges for basic, agricultural and petrochemical industries
| title = Pusher centrifuges for basic, agricultural and petrochemical industries
| url =http://www.ferrum.net/wAssets/pdf/gbz/downloads/prospekte/english/Ferrum-Pusher-centrifuges-en.pdf
| url =http://www.ferrum.net/wAssets/pdf/gbz/downloads/prospekte/english/Ferrum-Pusher-centrifuges-en.pdf
| accessdate = 12/10/2013 }}.
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*{{cite journal
*{{cite journal
| last = Schmidt | first = Peter
| last = Schmidt | first = Peter
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| date = Dec 2010
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| url =http://sirius.library.unsw.edu.au:9003/sfx_local?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:journal&genre=article&sid=ProQ:ProQ%3Aabiglobal&atitle=Filtration+Centrifuges%3A+An+Overview&title=Chemical+Engineering&issn=00092460&date=2010-12-01&volume=117&issue=13&spage=34&au=Schmidt%2C+Peter&isbn=&jtitle=Chemical+Engineering&btitle=&rft_id=info:eric/
| url =http://sirius.library.unsw.edu.au:9003/sfx_local?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:journal&genre=article&sid=ProQ:ProQ%3Aabiglobal&atitle=Filtration+Centrifuges%3A+An+Overview&title=Chemical+Engineering&issn=00092460&date=2010-12-01&volume=117&issue=13&spage=34&au=Schmidt%2C+Peter&isbn=&jtitle=Chemical+Engineering&btitle=&rft_id=info:eric/
| issn = 00092460 }}.
| issn = 0009-2460 }}.

*{{cite book
*{{cite book
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| volume = 1
| year = 1997
| year = 1997
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| url = https://books.google.com/books?id=bqoRAQAAMAAJ
| isbn = 9780471161240 }}.
| isbn = 9780471161240 }}.

*{{cite journal
*{{cite journal
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| last = Wakeman | first = R.J
|last2 = Tarleton |first2 = E.S
|last2 = Tarleton |first2 = E.S
| title = Computer Based Selection of SOlid/Liquid Separation Equipment
| title = Computer Based Selection of Solid/Liquid Separation Equipment
| journal = Process Advances in Filtration and Separation Technology
| journal = Process Advances in Filtration and Separation Technology
| volume = 7
| volume = 7
| pages = 530–543
| pages = 530–543
| publisher =
| location = Chicago
| location = Chicago
| year = 1993
| year = 1993
| url = http://dspace.lboro.ac.uk/dspace-jspui/bitstream/2134/5492/1/Tarleton%201.pdf }}.
| url = http://dspace.lboro.ac.uk/dspace-jspui/bitstream/2134/5492/1/Tarleton%201.pdf }}.

*{{cite web
| title = Pusher Centrifuge
| work = Rotofilt
| publisher = Rotofilt
| year = 2009
| url = http://www.rotofilt.com/pusher-centrifuge.html.
| accessdate = 03/10/2013 }}.

*{{cite journal
*{{cite journal
| title = World's Largest Single-Stage Pusher Centrifuge
| title = World's Largest Single-Stage Pusher Centrifuge
| journal = FIltration & Separation
| journal = Filtration & Separation
| volume =34
| volume =34
| issue =10
| issue =10
| pages =1002
| pages =1002
| date = December 1997
| date = December 1997
| url = http://www.sciencedirect.com/science/article/pii/S0015188297901671
| issn = 0015-1882
| issn = 0015-1882
| doi = 10.1016/S0015-1882(97)90167-1 }}.
| doi = 10.1016/S0015-1882(97)90167-1 }}.

*{{cite journal
*{{cite journal
| title = Innovating the Pusher Centrifuge for Bulk Chemical Separation
| title = Innovating the Pusher Centrifuge for Bulk Chemical Separation
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| pages = 38–39
| pages = 38–39
| date = July–August 2003
| date = July–August 2003
| url = http://www.sciencedirect.com/science/article/pii/S0015188203006347
| issn =0015-1882
| issn =0015-1882
| doi = 10.1016/S0015-1882(03)00634-7 }}.
| doi = 10.1016/S0015-1882(03)00634-7 }}.


[[Category:Articles created via the Article Wizard]]
[[Category:Centrifuges]]
[[Category:Centrifuges]]

Latest revision as of 13:42, 10 July 2024

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

[edit]

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.[1]

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.[2]

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

[edit]

Advantages

[edit]
  • 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

[edit]
  • 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

[edit]

The designs for pusher centrifuge are as follows:

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

Single-stage

[edit]

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

  • Can maximize 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

[edit]

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/four stage

[edit]
  • 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

[edit]

Feed distributor design: conical/cylindrical or plate

  • Optionally applied for single- and two stage- designs.
  • Cylindrical feed section 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

[edit]

The important parameters are screen area, acceleration level in the final drainage zone and cake thickness. Cake filtration affects residence time and volumetric throughput. Residence on the screen is controlled by the screen's length and diameter, cake thickness and the frequency and stroke length of the cake.[3]

Feed

[edit]

Pushers utilise the cake layer to act as a filter, hence the feed normally contains high solid concentration containing fast draining, crystalline, granular or fibrous solids. The solid concentration ranges from 25-65 wt%.[2] The mean particle size suitable for pushers must be at least 150 μm. The capacity depends on the basket diameter and ranges from 1 ton/h to 120tons/h.[4]

Operations

[edit]

The cake is under centrifugal force. It becomes drier as it progresses in the basket and is 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 and 90strokes/min.[4]

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 self-compensating control of varying rates. Up to 90% push efficiency is achievable depending on the cake properties.[4] dQ3ET42T

Filtration rate

[edit]

The equation for the filtration rate, Q:[4]

(1)
(2)

Where and are viscosity and liquid density, respectively. is the angular speed, is the average cake permeability, which is related to equation (2), , and are the radius of the liquid surface, cake surface and filter medium adjacent to the perforated bowl respectively, is the combined resistance, is the specific resistance and is the solid density.

The numerator describes the pusher's driving force, 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

[edit]

Performance is a function of many parameters, including particle size, viscosity, solid concentration and cake quality.[2]

Particle size/porosity

[edit]

To create the cake layer, the particle size has to be as large as practically possible. Larger particle size increases the porosity of the cake layer and allows feed liquid to pass through. Particle shape is equally important, because it determines the surface area per unit mass. As it decreases, less surface area is available to bind moisture, providing a drier cake.[2]

Viscosity

[edit]

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 fluid flow, which complicates separation of the fluids from the slurry. Consequently, the throughput of the pusher is de-rated.[2][4]

Solid concentration

[edit]

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

Cake quality

[edit]

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

Purity

[edit]

Wash liquid is introduced on the cake in order to displace the mother liquor along with the impurities.[2] The cake wash ratio is normally between 0.1 and 0.3 kg wash/kg solids, which displace at least 95% of the feed fluid and impurities within the wash zone's normal residence time.[4]

Volatile matter

[edit]

The 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).[4]

where is the centrifugal force, is the angular speed, is the radius of the basket, and is the gravitational force.

By relating equation 3 to equation 1, the relationship of the centrifugal force is shown to be proportional to the filtration rate. As pushers often deal with fragile crystals, the movement of the pusher plate and acceleration in the feed funnel matter, because they can break some of the particles.[4] In addition to the movement plate, G can cause breakage and compaction, and volatile matter in the cake increases. The gentle movement of cake in low G, single stage, long basket designs results in low particle attrition. As more solids pass through, residence time decreases, which increases volatile matter in the discharge cake.[2]

Process design heuristics

[edit]

The heuristics of pusher centrifuge design consider equipment size, operation sequence and recycle structure.

Design process

[edit]

Overall approach:[4]

  • Define the problem
  • Outline process conditions
  • Make preliminary selections
  • Develop a test program
  • Test sample batches
  • Adjust process conditions as required
  • Consult equipment manufacturers
  • Make final selection and obtain quotes

Equipment sizing

[edit]

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

[edit]

Equipment selection is based upon test results, references from similar processes and experience and considered in terms of:

  • Cost, quality and productivity
  • Financial modeling

Optimising performance

[edit]

For conical and cylindrical designs and assembly, the cone slant angle should not exceed sliding friction cake angle. Otherwise it would result in high vibration and poor performance.[4]

In order to optimise capacity and performance, it is desirable to pre-concentrate the feed slurry as much as possible. Some designs have a short conical section at the feed end for thickening within the unit, but generally it is preferable to thicken before entering the centrifuge with gravity settlers, hydrocyclones or inclined screens, producing 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

[edit]

Selection of designs is usually done by scale-up from lab tests. Test data analysis should be rationalised in preparation for equipment scale-up. Computer-aided design software can assist in design and scale-up. Pilot-testing and rollout then follows.[5]

Waste

[edit]

Production

[edit]

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

Post-treatment

[edit]

Post-treatment processes are a function of the specifics of the waste stream and are diverse.[6]

Later designs

[edit]

Design advances have enhanced performance and broaden the application range. These include additional stages, push hesitation, horizontal split process housing, integrated hydraulics, seals, pre-drained funnels and an integrated thickening function.

Stages

[edit]

B&P Process Equipment and Systems (B&P) makes the largest single-stage pusher centrifuge, which they claimed to be superior to multistage designs.[7] They claimed that additional impurities enter the liquid housing due to additional particles tumbling in each stage. The problem can be overcome by using a shorter inner basket with smaller diameter between the pusher plates and the basket and 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 pushers that have pusher movement only between pusher plate and basket, multistage centrifuges have the advantages that the cake height is reduced, filtration resistance is lower and lesser force is required.

Push hesitation

[edit]

Push hesitation holds the pusher plate in the back stroke, allowing the cake to build on itself. The cake acts as the filtering media that can even capture finer solids. This reduces the loss of solids passing through the wedge slots. Although this modification reduces capacity, it has helped improved the solid capture efficiency and make pusher centrifuges applicable to smaller particles.[2]

Horizontal split process housing

[edit]

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

Integral hydraulics

[edit]

An automated mechanism allows the system to operate independently.

Seals

[edit]

Shaft seals eliminate the possibility of cross-contamination between the hydraulic and process ends. Options include a centrifugal liquid ring seal and a non-contacting inert gas purged labyrinth seal that eliminates leakage.

Pre-drained funnel

[edit]

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

Integrated thickening

[edit]

Integrating the thickening function enables the pusher to be loaded with mixture with as little as 30-35% wt of solid. It also reduces process costs of solid-liquid separation by as much as 20%.[8]

References

[edit]
  1. ^ Technologies 2008
  2. ^ a b c d e f g h Dubal 2008
  3. ^ Schmidt 2010, pp. 34–38
  4. ^ a b c d e f g h i j (Green & Perry 2008, pp. 1056–1065)
  5. ^ (Wakeman & Tarleton 1993, pp. 530–543)
  6. ^ Rotofilt
  7. ^ Filtration & Separation 1997
  8. ^ Filtration & Separation 2003, pp. 38–39

Bibliography

[edit]
  • Green, Don W.; Perry, Robert H. (2008). Perry's Chemical Engineerings' Handbook (8 ed.). New York: McGraw Hill. pp. 1056–1065..
  • Technologies, F.P (2008). "FX Pusher: Pusher Centrifuges". Retrieved 2013-10-14..
  • Dubal, Gitesh (May 2000). "The Pusher Centrifuge: Operation, Applications and Advantages". Filtration & Separation. 37 (4): 24–27. doi:10.1016/S0015-1882(00)88849-7. ISSN 0015-1882..
  • Ltd, F. "Pusher centrifuges for basic, agricultural and petrochemical industries" (PDF). Retrieved 2013-12-20. {{cite journal}}: Cite journal requires |journal= (help).
  • Schmidt, Peter (Dec 2010). "Filtration Centrifuges: An overview". Chemical Engineering. 117 (13). New York: Access Intelligence LLC: 34–38. ISSN 0009-2460..
  • Ruthven, D.M. (1997). Encyclopedia of Separation Technology. Vol. 1. Wiley. ISBN 9780471161240..
  • Wakeman, R.J; Tarleton, E.S (1993). "Computer Based Selection of Solid/Liquid Separation Equipment" (PDF). Process Advances in Filtration and Separation Technology. 7. Chicago: 530–543..
  • "World's Largest Single-Stage Pusher Centrifuge". Filtration & Separation. 34 (10): 1002. December 1997. doi:10.1016/S0015-1882(97)90167-1. ISSN 0015-1882..
  • "Innovating the Pusher Centrifuge for Bulk Chemical Separation". Filtration & Separation. 40 (6): 38–39. July–August 2003. doi:10.1016/S0015-1882(03)00634-7. ISSN 0015-1882..