Spiral Jet, Pancake Mill Case Studies

Fluid Energy Mill Overview

Jet milling or fluid-energy grinding via a spiral jet mill is a common method of comminution or pulverizing granular material into a fine powder with a narrow size distribution.

The spiral jet mill, also called a pancake mill, is shown in the drawing to the left. This design was first described by NH Andrews in 1936 (US2032827)1. Several nozzles are placed in the outer perimeter of the mill through which the grinding medium, gas or steam, enters.

The solid particles are introduced into the grinding chamber through a venturi feed injector. The outlet is placed in the center of the mill chamber. Grinding and static classification both occur within the cylindrical chamber. The vortex formed by the jets causes coarse particles within the mill to be transferred to the outer zone, as fines exit through the central outlet.

The Micro-Jet pancake mill is a horizontally oriented jet mill for most applications but the orientation can be adjusted for space and processing considerations. Spiral jet mills, like the Fluid Energy Micro-Jet, are notable for their robust design and compactness. Their direct air operation avoids the need for separate drive units. Capable of grinding dry powders to 0.5-45-micron averages, this mill generates a maximum number of “fines” resulting in high surface areas in the final product.2 As with all Fluid Energy jet mills, the grinding process occurs with no heat buildup when using ambient gases.

A few case studies of the Micro-Jet spiral jet grinder are described below.

Pigment and Abrasives Grinding

The Model 42 Micro-Jet was made to grind a chemical intermediate to a 6-micron average, at a rate of 2 metric tons an hour.

The initial raw feed size was 325 mesh (~45 microns) material. This model also features replaceable silicon carbide liners for all product contact surfaces to provide maximum abrasion resistance. The mill utilizes 4,000 cfm of air compressed to 100 psi.

In a high-temperature configuration, the Model 42 Micro-Jet utilizes super-heated steam as the grinding fluid. Steam requirements range from 4,000-9,000 lbs/hr at 150 psig. The steam is typically super-heated from 550ºF (290ºC) to 1,000ºF (540ºC). These units have been designed for Titanium Dioxide (TiO2) applications producing a particle size of 0.5 microns at a rate of 4000 lbs/hr. The same unit has also been used for grinding red and yellow iron oxide pigments.

Polymer Grinding

The Model 24 Micro-Jet grinds fluoropolymers to a particle size range of 10-15 microns at a rate of 200-400 lbs/hr. The system is equipped with a feed impact chamber for reducing larger particles prior to entry into the mill.

The unit is constructed in type 304 stainless steel with replaceable nozzles and liners. Fluid Energy has ground many types of fluoropolymers including virgin and reprocessed grades.

Sanitary Micro-Jet and Pharmaceutical Production System

The Model 8 sanitary Micro-Jet meets USDA and pharmaceutical guidelines for cleaning and sterilization. This unit is equipped with FDA approved O-rings and gaskets.

All sanitary designs have mirror polish and provide easy access to product and gas contact areas for cleaning and sterilization. There are no threaded connections in product contact areas. Sanitary models can be made in sizes from Model 4 up to Model 24.

The Model 8 Micro-Jet System was designed to grind penicillin to less than a 10-micron average at a rate of 50 lbs/hr. This system is equipped with a cyclone for primary sterile collection and a baghouse for secondary product collection.

A HEPA filter with an exhaust fan was also included to provide maximum particulate capture of the system exhaust. All product contact areas are constructed in type 316 stainless steel with pharmaceutical polish and sanitary O-Rings. The system utilizes 100 scfm of air compressed to 100 psig.

Individual Micro-Jet systems may be tailored to optimize both the desired particle size and the production rate. The airflow rate, air pressure, and grinding pattern are easily adjusted by means of interchangeable grinding nozzles and liners. Abrasive, sticky, and contamination-sensitive products can all be processed by means of specialized Micro-Jet liners. Specialized liners include:

• Alumina
• Tungsten Carbide
• Silicon Carbide
• Urethane
• Polyethylene

These case studies are just a few examples of granular solids that can be jet-milled in a spiral or pancake mill. A list of additional product applications can be found here on the Micro-Jet equipment web page.

1. Andrews, NH, 1936, Method of And Apparatus for Providing Material in Finely Divided Form, US2550390.
2. R.H. Perry and D.W. Green, Chemical Engineering Handbook, 8th Ed., 2007, p. 21-61.

Frequently Asked Questions: Jet Milling and Particle Classfication

Loop Mill (JET-O-MIZER) Pancake Mill (MICRO-JET) Fluid Bed Mill (ROTO-JET)

Below are several frequently asked questions on jet milling and particle size classification. If your question is not answered here, feel free to contact us using this link.

1.What is jet milling?
2. What is the size classification?
3. What are the different types of classifiers?
4. How does a jet mill classify particles?
5. How is the output particle size controlled?
6. When is jet milling used?
7. What are typical raw feed sizes and/or size distributions?
8. What is the basic difference between a pancake mill (e.g. MICRO-JET) and a loop mill (e.g. JET-O-MIZER)?
9. How does the fluid bed jet mill (e.g. ROTO-JET) differ from the pancake mill and loop mill?
10. What applications are best suited for a fluid bed mill design?
11. Are jet mills for continuous or batch operation?
12. What components are required for a jet milling system?
13. When is the cyclone included in a jet milling system?
14. What are the typical materials of construction?
15. What types of liners are available for a Fluid Energy MICRO-JET?
16. What types of liners are available for a Fluid Energy JET-O-MIZER?
17. What types of liners are available for a Fluid Energy ROTO-JET?
18. What are the typical compressor requirements for a jet milling system?
19. What compressible fluids are used in jet milling systems?
20. Can the jet-milling system be designed for closed-loop gas operation?
21. What system design features are employed for explosive applications?
22. What are the typical raw materials used in jet-mills? What is the particle size of the feed and the final product?


1. What is jet milling?

Jet milling is the term commonly applied to fluid energy milling. It is a process utilizing the potential energy of a compressible fluid and converting it to kinetic energy within the mill grinding chamber. This occurs when compressed gas is injected through specially designed nozzles. As the gas exits the nozzle it rapidly expands and creates a high-velocity stream within the mill. Raw feed material enters the same chamber through a venturi eductor. As the feed becomes entrained in this high-velocity stream the particles collide. It is this inter-particle collision that is the basis for the comminution of the material.


2. What is size classification?

Size classification is one of the important unit operations used in industries and/or applications involved with processing differently sized solid particles. It is a solid-solid separation process separating particles into two or more fractions. It is commonly used in combination with grinding and comminution of solid materials.


3. What are the different types of classifiers?

For dry particles, classification is predominately performed on screens or sieves and air classifiers. Screens and sieves are predominately used on particles larger than 200 mesh (70 microns) and very seldom on particles smaller than 325 mesh (44 microns). Air classifiers are used predominately on fine powders but can be used on coarse particles up to 1000 microns in diameter.

Air classifiers employ various airflow techniques including elutriation, free vortex, and forced vortex, either separately or in combination, to achieve the desired classification. Elutriation is a gravity-based particle classification that rinses coarse particles in a flowing stream of air or other gas. It is a relatively crude method that separates the feed’s fine and coarse particles by introducing the feed into the flowing air. The airflow carries the fines to a fines discharge outlet, while the heavier coarse particles fall with gravity against the airflow to a coarse outlet. Increasing or decreasing the airflow adjusts the cut size.

The flow in a free vortex classifier moves in a decaying spiral pattern toward a discharge outlet point, as found in a cyclone. A tangential inlet for the feed and air establishes a circular airflow that forces coarse particles to the chamber’s periphery where they flow to a coarse outlet. The fines remain with the airflow and are carried to a central outlet where they are discharged with the air. The cut size is controlled by adjusting the airflow and physical dimensions of the cyclone.

A forced vortex classifier is more complex and achieves a more precise classification than either the elutriation or free vortex units. In this unit, the airflow is pulled from the chamber’s outer circumference and a classifying wheel sets the airflow, with entrained feed particles, into a rotational motion (forced vortex) around the wheel. The centrifugal force of the wheel pushes the coarse particles out of the open vanes, but the fines pass thought the vanes and are carried to the outlet with the air discharge. The cut size is controlled by varying the wheel speed and airflow through the classifier. These two variables can be adjusted independently and thus allows an infinitely adjustable classification.


4. How does a jet mill classify particles?

A jet or fluid energy mill can have a static (MICRO-JET, JET-O-MIZER) or dynamic (ROTO-JET) classifier.

Static classification occurs as the gas and product recirculate within the mill. This recirculation creates a centrifugal force that directs the larger particles to the outer periphery and away from the mill exhaust. The finest particles, less influenced by this centrifugal force, are too small to overcome axial forces created by the exhaust and are carried out of the mill with the spent gas. It is this process that creates a stratification of particles throughout the milling chamber. In a jet mill with static classification, the air volume is particularly important. Each model has a designed nominal air volume at which the mill is most efficient in classification. Too much or too little airflow will have an adverse effect on classification.
Dynamic classification is provided by a variable speed classifying rotor. The speed of the rotor prevents specific size particles from passing through the rotor and rejecting these particles back to the fluidized zone for additional grinding.


5. How is the output particle size controlled?

In a jet mill with static classification (MICRO-JET, JET-O-MIZER), the particle size is controlled by a combination of gas pressure and feed rate. With a constant feed rate of granular solids, increased inlet gas pressure results in higher velocities and greater kinetic energy, therefore a finer grind. Lower inlet gas pressures result in less energy and larger particle size. With a constant gas inlet pressure, faster feed rates result in higher mill loading, which reduces residence time, therefore producing a larger particle size. As you lower the feed rate the particle size decreases.

By varying these two parameters you may produce the desired particle size.
In a jet mill with dynamic classification (ROTO-JET), particle size is controlled by a combination of inlet gas pressure and classifier rotor speed. Higher inlet gas pressures in combination with the highest classifier speeds result in the finest average particle size and the finest top size. Lower inlet gas pressures in combination with lower classifier speeds result in larger average particle size and a larger top size. The variable speed classifier allows for a wider range of adjustments to tailor the particle distributions to your exact specifications.


6. When is jet milling used?

Jet milling is typically used to grind a dry powder to a range of 0.5 to 45 microns. Customers generally require smaller particle sizes to increase the surface area of their product. Maximizing surface area provides some of the following benefits:

• Improved reaction rates for catalysts and explosives
• Improved absorption in the body for pharmaceuticals
• Improved suspension of specialty chemicals
• Improved color characteristics for pigments
• Smoother textures for pigments or foods
• Improved strength qualities of products

For particle sizes above 45 microns, a mechanical mill is more energy-efficient and should be used except when heat and contamination are of concern.


7. What are typical raw feed sizes and/or size distributions?

A raw feed to a jet mill is usually, but not necessarily, the product of a mechanical mill such as a hammer, air classifying (ACM), or roller mill. Raw feed sizes vary from granular or flake to the low micron range. When selecting a desired feed to a jet mill, remember a finer raw feed will yield a finer product or a faster rate. Similarly, the narrower the distribution of the raw feed, the better the distribution of the jet-milled product.

Raw feed sizes also vary with the actual size mill being used. For example, a Model 00 JET-O-MIZER has a venturi throat size of 1/8″ and therefore all raw feed should be in the range of table sugar or smaller. While the Model 0808 JET-O-MIZER has a venturi throat size of 1-1/2″ and can accept a larger feed size, it is still recommended that the feedstock be in powdered form in order to obtain a similar fineness in the final powder.


8. What is the basic difference between a pancake mill (e.g. MICRO-JET) and a loop mill (e.g. JET-O-MIZER)?

A pancake mill is a horizontally designed jet mill, while a loop mill is a vertically oriented jet mill. Both are equipped with static classification. The basic differences are highlighted below:

• While both mills are capable of grinding to 1-10 microns, a pancake mill (e.g. MICRO-JET) will grind a few microns finer (assuming the same processing conditions). As such, a pancake mill should be used if you are looking to obtain maximum grinding and more surface area.
• A pancake mill due to its simple geometry is easy to line with a variety of materials such as ceramics or special alloys for abrasive products; or PTFE or UHMW polyethylene for sticky products.
• A pancake mill typically produces a particle size distribution with a high concentration of fine particles.
• A loop mill (e.g. JET-O-MIZER) is simple to feed with a wide variety of materials. It does not experience the “blow-back” problems that could occur when feeding very light bulk density materials into a pancake mill.
• Because the loop mill is easier to feed with all materials, there is no required ramp up of pressure when starting the mill. In effect, the operating pressures can be set before feed is introduced into the mill.
• The JET-O-MIZER models have the widest range of throughput, from 1-gram batches to rates as high as 3 tons per hour.
• A loop mill typically produces a particle size distribution with a normal bell-shaped curve and less fines.


9. How does the fluid bed jet mill (e.g. ROTO-JET) differ from the pancake mill and loop mill?

As indicated above, a pancake mill and loop mill are both jet mills that employ static classification. These mills are also similar in that they have a tangential grinding nozzle pattern. Here are the basic differences of the fluid bed design:
• The fluid bed mill employs a variable speed classifier wheel.
• The fluid bed mill grinding nozzles are directly opposed rather than tangential.
• The fluid bed system can be totally automated through a PC or PLC.


10. What applications are best suited for a fluid bed mill design?


If you cannot achieve the required distribution with a pancake mill or loop mill, then a fluid bed design (e.g. ROTO-JET) should be used. With its advanced, dynamic classification it will produce a narrower distribution with the finest top size. Also, any material that has a high aspect ratio (flakes or needles) and is difficult to classify would be better suited for a fluidized bed jet mill.

An example of such an application is toner powder. Toner manufacturers typically require a very specific top size (15-20 microns) while generating minimal fines (less than 4 microns).


Since a fluid bed jet mill has opposed grinding nozzles, “wall effects” that occur in the pancake mill and loop mill are eliminated. The grinding occurs in the center of the milling chamber, preventing direct impingement with mill surfaces.


Any material that is difficult to fracture, such as polymers, alloys, or some ceramics, typically requires more residence time in the grinding stream than a pancake mill or a loop mill can provide. In these cases, the fluid bed jet mill excels because the classifier wheel prevents the particles from exiting prematurely and rejects them back into the grinding zone.
In a pancake mill and loop mill the raw feed distribution is critical, because the particle size is controlled by a combination of feed rate and gas pressure. In these static mills, if any aspect of the feed distribution changes (mean or top size) then the final product will be affected unless the controlling parameters are changed. A fluid bed jet mill, due to the classifying rotor, will consistently produce in-spec product regardless of fluctuations in the raw feed particle sizes.


11. Are jet mills for continuous or batch operation?

Typically, jet milling systems are designed for continuous operation. However, pancake and loop lab units can be operated in a batch mode.


12. What components are required for a jet milling system?


• Volumetric or gravimetric screw feeder
• Compressor package
• JET-O-CLONE cyclone (optional)
• PLC/PC based controls (primarily ROTO-JET — optional)
• Baghouse
• HEPA (optional)
• Exhaust fan (optional)


• Volumetric or vibratory feeder
• Compressor package or compressed gas cylinders
• Filter collection bag *, **
• JET-O-CLONE cyclone (optional—extends batch time)
• HEPA filter (optional)
* Must not be used with materials that have explosion potential
** Recommend placement in a fume hood


13. When is a cyclone included in a jet milling system?

A cyclone (e.g. JET-O-CLONE) is a static solids collection device that can provide primary solids collection. It does not eliminate the baghouse requirement. Fluid Energy cyclones are the most efficient in the industry with capture efficiencies as high as 95%-98%. Listed below are applications where a cyclone is typically utilized.


A JET-O-CLONE cyclone is used in sterile pharmaceutical applications where the customer prefers that the product not contact the filter media due to the potential of fibers contaminating the product. In these cases, the cyclone is a sanitary collection point that can be steam sterilized or placed in an autoclave.


A JET-O-CLONE cyclone is a quick-disconnect (where possible), easy-to-clean component as compared to a baghouse, which can be very labor-intensive to disassemble and clean. When a customer is processing many materials in the same system the cyclone is used as the primary collection point. Any cyclone exhaust that is conveyed to the baghouse is considered waste. During changeovers, a customer experiences much less downtime by cleaning the cyclone and not the baghouse.


In applications involving mill exhaust temperatures above 300° F, an EVAPORATIVE COOLER is used to cool down the process stream prior to entering the baghouse. Because the EVAPORATIVE COOLER atomizes water into the exhaust it is recommended that a JET-O-CLONE cyclone be used to remove the solids from the process stream prior to the cooler.
In applications where powders tend to “cake” and “blind” filter bags, causing excessively high-pressure drop across the baghouse, a JET-O-CLONE cyclone is used to reduce the solids loading within the baghouse. In these applications, the cyclone will alleviate the problem or prevent the need for excessively high filter cloth area in the baghouse


14. What are the typical materials of construction?

All of our equipment is available in carbon steel; type 304 or 316 series stainless; or exotic alloys except for the following mills, which are stocked in type 316 stainless steel:
• Model 00 Jet-O-Mizer (also available in hardened 630 SS)
• Model 0101 Jet-O-Mizer (also available in hardened 630 SS)
• Model 0202 Jet-O-Mizer (also available in hardened 630 SS)
• Model 4 Micro-Jet (Sanitary or Standard designs)
• Model 8 Micro-Jet (Sanitary or Standard designs)


15. What types of liners are available for a Fluid Energy MICRO-JET?

A complete set of liners for the MICRO-JET typically includes a top liner, bottom liner, manifold liner ring, outlet extension, and venturi. On some of the production scale, MICRO-JETS the bottom liner and manifold liner ring are an integral piece. In abrasive applications, these liners do not wear at the same rate. The bottom liner is replaced most frequently, followed by the manifold liner ring and venturi.

The following is a list of liner materials commonly used:

• Series 300 stainless steel for corrosive materials
• Hardened Series 630 stainless steel for mildly abrasive materials
• Hardened steel for abrasive materials
• Alumina used for light-colored abrasive materials
• Tungsten carbide for dark-colored abrasive materials
• PTFE for sticky materials
• UHMW polyethylene for sticky materials
• Silicon carbide for abrasive applications (production-sized MICRO-JETS)

Note: Fluid Energy supplies all liners as solid components constructed in the materials listed above. We do not epoxy tiles to the base plate unless requested by the customer. Tiles are not as durable as a solid liner when exposed to the harsh environment inside the milling chamber.


16. What types of liners are available for a Fluid Energy JET-O-MIZER?

Only the Model 0405 and 0808 JET-O-MIZERS are available in replaceable liner design. These liners can be cast in NiHard, carbon steel with hard facing, or stainless steel with hard facing. All the JET-O-MIZERS can be provided with a spray or weld applied hard facing on the internal surfaces subject to wear. Hard facing can be carbon-based or stainless based on applications where corrosion is of concern.


17. What types of liners are available for a Fluid Energy ROTO-JET?

As discussed previously in this section, due to the opposed orientation of the grinding nozzle, the ROTO-JET mill does not experience the “wall effects” typical in the MICRO-JET or JET-O-MIZER mills. It therefore does not require liners on the internal chamber, although in a high purity application where no amount of metal contamination is tolerable a polymeric coating may be applied.

In the ROTO-JET design, it is the classifying rotor that experiences some wear, although minimal compared to the MICRO-JET and JET-O-MIZER. In these applications the classifying rotor would be constructed in the following materials:

• Hastelloy
• Stellite
• Ceramic-steel duplex
• Tool steel


18. What are the typical compressor requirements for a jet milling system?

Centrifugal, reciprocating, and rotary screw compressors have all been used in jet milling operations, with the latter being the most common. It is recommended that the compressor deliver a minimum of 100 psig to a maximum of 140 psig to the mill. The higher the grinding pressure, the finer the grind.

The quality of the air is at the discretion of the customer. Oil-free or food-grade lubricated compressors may be necessary for pharmaceutical or specialty chemical or pigment application. Most installations have a compressor package, which consists of a compressor with an aftercooler, refrigerated or desiccant air drier, coalescing filter and a high-pressure receiver. With a refrigerated dryer, a typical dewpoint should be in the 35°-39° F range. Air dryers are not essential but are recommended for preventing condensation and eventual product build-up within the system.


19. What compressible fluids are used in jet milling systems?

Compressed air is the most common source of energy. In explosive or pyrophoric applications, nitrogen or argon may be utilized. Heated air or superheated steam may be used were tolerated by the product. Higher enthalpy of superheated steam results in higher nozzle velocities and a finer grind or improved rates.


20. Can the jet-milling system be designed for closed-loop gas operation?

If a jet milling application requires an inert atmosphere, the system gas can be designed for a once-through or a continuous closed-loop recycling operation. The question of which type of design should be used will depend primarily on usage. In a recycling system, a HEPA filter is required before reintroducing the gas back into the compressor. A sealed exhaust blower and low-pressure receiver are also required for this operation. Automated controls with oxygen and pressure sensors can also be provided.


21. What system design features are employed for explosive applications?

The following methods have been used either separately or in conjunction to handle explosive materials:


Because of the low velocities and fine particle concentrations, the baghouse is the component most susceptible to dust explosions. A relief vent would be sized based on the Kst value of the material and included on the baghouse. Venting is the most common method employed for handling explosive powders. Explosion vents are ducted to a safe area outside the building. It is recommended that the baghouse be positioned as close to an exterior wall as possible. Ducting should be a straight run and be as short as possible.


As discussed above, the system can be operated with an inert atmosphere, eliminating the possibility of an explosion.


Equipment can be provided to sense a pressure rise within the baghouse and release a chemical reagent into the baghouse, suppressing the explosion.


When the above options are not feasible, a baghouse may be designed for explosion containment. For several pharmaceutical applications, we have supplied baghouses that have been designed for 10 bar containment. Since the entire system cannot be designed for this pressure, explosion isolation valves are included at the inlet and exhaust connections of the baghouse. As with the suppression equipment, a sensor is placed in the system to detect a pressure rise. Once detected, the valves are automatically shut, isolating the explosion.

Loop Mill Case Studies

Fluid Energy Mill Overview

The original invention of loop or donut mill is credited to Cleo Harold Kidwell and Nicholas NK Stephanoff (founder of Fluid Energy Processing and Equipment).1

An early application of the Fluid Energy Jet-O-Mizer was for pulverizing solid, heating fuels.2

Since that time, pulverization applications have expanded to other granular solids.

Here we share a few examples of material comminution with this type of jet mill.


Propellant Grinding

A Fluid Energy Model 0304 Jet-O-Mizer was designed to grind solid rocket propellant to a 2 m average particle size at a rate of 500 lbs/hr.

When designing systems for propellants/explosives, special construction features are utilized.

In these cases, Fluid Energy Processing applies procedures that are found in US government specifications relating to the fabrication of milling systems for propellant/explosives use.

Due to its grinding characteristics, the Jet-O-Mizer is most frequently used in the milling of RDX, HMX and black powder. Our Micro-Jet is typically used to grind oxidizers, such as Ammonium Perchlorate.

Mineral and Pigment Grinding

A Fluid Energy Model 0808 Jet-O-Mizer utilizes 1,200-2,200 SCFM of air compressed to 100 psig for mineral and pigment grinding.

It is equipped with replaceable cast Ni Hard liners for superior abrasion resistance. The outlet is equipped with adjustable vanes for fine-tuning the particle size distribution.

The Model 0808 is routinely used to grind carbon black, talc, calcium carbonate and kaolin.

When grinding carbon black or minerals used in the filler industry, super-heated steam is typically used as the grinding fluid. Typical steam rates range from 2,000-7,000 lbs/hr. Steam pressure and temperature requirements can range up to 200 psig and 750ºF (400ºC).

Sanitary Drug Grinding System

A sanitary Model 0101 Tabletop Jet-O-Mizer System was designed for the continuous production of a topical anesthetic. The Model 0101 produces 5 lbs/hr of product at a less than 5 m average particle size.

The system includes all integral piping and instruments, a screw feeder, filter silencer, cyclone collector and laboratory baghouse. Product collection cans were provided for cyclone and baghouse collection.

All components are constructed in type 316 stainless steel with a pharmaceutical polish and are completely assembled on a 4-foot square, portable table. The system utilizes 15-30 scfm of air or nitrogen compressed to 100 psig.

These case studies are just a few examples of granular solids that can be jet-milled in a loop mill. A table of additional applications can be found here on the Jet-O-Mizer equipment web page.

The Jet-O-Mizer has been developed with many distinct design features to consume less power, provide a greater range of throughput (50 gram/hr to 20,000 lb/hr) and ensure exceptional finished product quality. Thorough application engineering allows the determination of the ideal operating conditions for specific raw materials. Production requirements are integrated into a complete Jet-O-Mizer system.

1. R.H. Perry and D.W. Green, Chemical Engineering Handbook, 8th Ed., 2007, p. 21-62.
2. Stephanoff, Nicholas NK, 1951, Method for Treating Fuel, US2550390.

Pneumatic Ring Dryer Case Studies

The Fluid Energy Thermajet Flash Dryer

Pneumatic conveyor dryers often referred to as flash dryers, consist of a duct carrying gas at high velocity.

The solids can be fed by various methods:  screw feeders, venturi sections, high-speed grinders, and dispersion mills.

Selecting the correct feeder to properly disperse the solids in the gas is critical for efficient drying.1

A Fluid Energy ThermaJet style ring dryer is a variant of the flash or pneumatic dryer.  An advantage of our ring style pneumatic dryer is a short retention time.

In a conventional, straight tube flash dryer, the residence time is basically fixed for all particle sizes of material, even though larger particles may take longer to dry.

This limits the application to materials in which the drying mechanism is not diffusion-controlled (i.e. constant rate drying, not “falling rate” drying) and where a range of moisture within the final product is acceptable.

Advantages of the Flash Dryer

A ring configuration offers two advantages over the straight tube. First, residence time varies with the particle size of the material.

Finer particles, which dry quickly, will be carried out with the airflow sooner, while larger particles, which dry more slowly, will make multiple cycles in the outer part of the dryer loop due to their heavier weight and the centrifugal forces created by the airflow within the ring.

The larger particles will now have a longer residence time for drying.

Second, although the dryer typically operates at lower pressures than a loop or torus jet mill, cycling through the ring can grind the material while drying. Even if grinding doesn’t occur, the internal forces are strong enough to break apart (de-agglomerate) wet cakes and clumps. This unit operation can control product particle size and moisture in the same process step.

Alumina Slurry Drying

A Fluid Energy Model 4 ThermaJet was built to dry alumina slurry from an initial moisture content of 80% to a final level of 0.2%.

The dryer includes a rotary valve feed inlet, replaceable abrasion-resistant liners, and a high-temperature manifold with inlet expansion joints.

Additional system components include a process blower, direct-fired natural gas heater, cyclone, baghouse and exhaust fan.

A slurry pump, screw conveyor, and continuous mixer were added to handle slurry feed. It recycles a percentage of dry product through a paddle mixer, increasing the solid content of the slurry to form a filter cake-like consistency for feeding through the inlet rotary valve.

Agricultural Chemical Drying

A Fluid Energy Model 14 ThermaJet, shown in the drawing at the left, was designed in a horizontal configuration in order to meet a customer’s space requirements.

The unit dries a heat-sensitive agricultural product from an initial moisture content of 10% to a final level of 0.2%.

Since some solvent is driven off in the drying process, the ThermaJet system operates with a closed-loop, nitrogen recycling package using a steam coil heat exchanger.

A condenser has been included in the system to remove solvent and water vapor from the process stream prior to recycling.

Drying & Calcining of Iron Oxide

The Model 10 ThermaJet, shown in the picture to the right, dries yellow oxide from an initial moisture content of 40% to 10% bound moisture.

The Model 10 was combined with a Model 12 to provide calcination of the dried oxide. The system begins with a yellow iron oxide wet cake and produces a dry, fine red iron oxide pigment.

Drying temperatures run from an inlet of 800ºF (430ºC) to an outlet of 210ºF (99ºC). Calcining temperatures include an inlet of 900ºF (480ºC) and an outlet of 650ºF (340ºC)

The calcining portion of the system includes an evaporative cooler located after a cyclone to cool down the process stream before entering the baghouse.


Silicate Drying

The Model 26 ThermaJet was designed to process a silicate filter cake from an initial moisture content of 40% down to 10% bound moisture. The Model 26 includes a separate hot air manifold to operate at 1200ºF (650ºC) with inlet nozzle expansion joints.

This unit also accepts direct feed from a volumetric screw feeder. Additional system components include a process blower, a direct-fired natural gas heater, and a reverse pulse collector & exhaust fan with actuated damper. Process controls were integrated into an existing PC-based control system.

These case studies summarize a few applications of the ThermaJet dryer. Additional applications can be found here on the ThermaJet webpage. The case studies demonstrate the versatility and capability of flash drying systems:

Capable of processing heat-sensitive and volatile materials. Able to process materials including wet powders, slurries, sludges, and filter cakes.
• Flexible feed capacities for laboratory, pilot, and full-scale production with feed rates ranging from 10 to 100,000 pounds per hour.
• Produces dry, discrete, deagglomerated products from raw feeds containing up to 95% moisture, often eliminating the need for additional grinding.
• Incorporate a broad range of operating temperatures and pressures for specific drying applications.
• Allows for calcining operations with immediate response time and instantaneous changes in product characteristics.
• Capable of integration with virtually any air heating system and offered with a variety of control options for customized drying applications.
• Able to operate continuously with superior reliability and minimal maintenance.

1. Czeslaw Strumillo and Tadeusz Kudra, Drying: Principles, Applications, and Design, Gordon and Breach Science Publishers, 1986.
2. Irene Borde and Avi Levy, 16-Pneumatic and Flash Drying, Handbook of Industrial Drying, Editor Arun S. Mujumdar.