John Heapy
Thermal Products Application Engineer Michael White
Director of Technology Bepex International, LLC
INTRODUCTION
Particle agglomeration is an important processing step in the food, chemical and mineral industries, where it plays a key role in de-dusting, particle size control, density control and the performance of bulk solid products. Drying systems play an equally important role in various parts of the agglomerating process; selecting the right drying system can have a significant impact on investment and processing costs, finished product quality and the product’s ultimate performance in a given application.
Selecting the right drying process requires a thorough evaluation of the many available technologies, often with the help of a trusted and experienced equipment supplier.
Whether used earlier or later in the agglomeration process, drying involves the removal of unwanted volatile compounds from a material. Examples of bulk solid materials include chemical salts, foodstuffs or feed components, polymers or mineral products and any number of byproducts. The volatile component could be water, solvents or other type of liquid medium. For our purposes, we will define drying as a separation process in which the liquid portion of a mixture of solids and liquids is removed by non-mechanical means. This generally requires heating the mixture to volatilize or evaporate the liquid.
Constant-rate drying vs falling-rate drying
When heat is applied to a material to be dried, the rate at which the volatile component is removed can vary depending on a number of factors. A lot can be learned by simply bench-testing the material. The rate at which the material loses its moisture can be determined in one of these tests. This test results in a drying curve (Figure 1).
Combining the drying curve data with general physical information about the material will often lead to a good estimate of which dryer system to consider. As a first step, it is important to determine whether the volatile material is being removed at a constant or falling rate.
During constant-rate drying, the thermal energy input results in evaporation that takes place either at or just below the surface of the particle, removing what is commonly called “free moisture.” This rate of evaporation is primarily controlled by the rate of heat transfer from the energy source to the material. As such, the material being dried will have a constant product temperature consistent with constant thermal energy input.
In falling-rate drying, the thermal energy input to evaporate the liquid decreases as the surface moisture is removed, and instead goes into increasing the heat in the remaining solids that may have entrapped or bound volatiles. At this point the drying is now typically limited by diffusion. That is, the drying rate is proportional to the rate of diffusion of trapped moisture to the particle surface. As such, the drying rate is affected by the vapor concentration surrounding the particles, the porosity residence time, the particle temperature and the particle size.
Selecting the right drying system
for agglomerating bulk solids
How to evaluate for optimal cost and performance
Factors for selecting optimal direct- and indirect-drying systems for particle agglomeration include heat transfer requirements and temperature sensitivity of material. In addition, the physical properties of the finished product often drive the choice of agglomeration and dryer technology.
Methods of heating
There are three primary methods of heating materials:
direct (convective) heating, indirect (conductive) heating, and radiant heating (Figure 2).
In convective (or direct) drying, the heat is transferred through direct contact between the heat transfer medium (usually a gas such as air or nitrogen) and the material.
Some direct dryers will employ mechanical agitation to increase the mixing of the material with the heat transfer medium. Many direct dryers will utilize pneumatic conveyance to move the material through the process.
In conductive (or indirect) drying, the heat is transferred to the material indirectly through contact with a heated surface. In this way, the material is separated from the heat transfer medium by a wall, usually steel, and the heat is conducted through the material. In conductive drying processes, the heat transfer medium is typically steam or hot oil, but other examples include water, glycol solutions, electrical resistance or molten salts. Mechanical agitation helps keep the material moving across the heated surface in order to distribute the heat evenly. This mechanical agitation often provides conveyance.
In radiant drying, heat is transferred via radiation from an external energy source with no physical contact between the energy source and the material being heated. A crude form of this is solar or sun drying. Industrial processes use radiation sources such as microwaves or UV waves.
Although radiant-type drying systems find success in some specialized applications, agglomeration systems most often benefit from either direct or indirect dryers.
Therefore, we will concentrate the balance of the paper on these two technologies.
Depending on the materials involved, each method offers advantages and disadvantages. While more than one drying process solution for your application may exist, it is important to consider both the required capital investment and operational expenditures before deciding on a path forward. Along with this, the impact of the dryer on the final agglomerated product is key. What might seem like the lowest cost from the standpoint of initial capital investment could end up costing more in operating costs over the life of the dryer or through lost or off-spec product production.
Figure 1: Typical drying curve — weight loss over time.
Figure 2: Primary methods of heat transfer
Moisture Content Wt. (%)
Time (minutes)
Direct drying Indirect drying Radiant drying
DIRECT THERMAL PROCESSING VS INDIRECT THERMAL PROCESSING
In terms of system setup and operation, there are many differences between indirect dryers and direct dryers.
The amount of ancillary equipment, dryer design, overall system footprint, utility costs and capital costs can differ widely. Since the interaction between different dryer types and the agglomerated material will vary, it is important to take all of this into account when selecting a dryer.
As stated previously, direct drying involves direct contact between the product and the heat transfer medium. In most cases, the heat transfer medium is air. Since any continuous dryer will have a wet product inlet and a dry product outlet, an efficient natural tendency is to use the heat transfer medium as the means to convey the product between the dryer inlet and outlet. In its most basic form, a so-called flash dryer mixes hot air with wet product while conveying. Variations of this are dryers that incorporate a dispersion unit in the hot air stream to better mix the wet solids and hot air. The dispersion unit could be a simple paddle mixer or a high intensity mill. Other examples of direct dryers are rotary dryers, fluid bed dryers, belt dryers, tray dryers and spray dryers. Each type has a unique design and operation and all have been used in aggregation processes.
Direct dryers have additional benefits that derive naturally from their design. For example, if the volatile liquid to be removed is water, direct drying equipment could be economical to procure. This is especially true when drying a powder or fine granules. In addition, if the solid material is heat sensitive, evaporative cooling that effectively transfers heat from the particle to the liquid being carried off helps protect the material from overheating. If the product has a sticky phase between the “wet” and
“dry” phase, most direct dryer designs perform well.
For example, in the fluid bed design there is internal material back-mixing that naturally occurs between the partially dried product and the fresh wet feed that helps to push past this sticky phase. Spray dryers will flash off the surface volatiles relatively quickly and almost instantaneously move through the sticky phase.
Heat transfer coefficients dictate design
Most solid materials will have a heat capacity of 0.1 to 0.4 Btu/(lb°F) while that of most liquids ranges from 0.4 to 1.2 Btu/(lb°F). Water has a specific heat value of 1.0. In drying applications, most of the thermal energy is consumed in the liquid-to-gaseous phase transition of the volatile where the latent heat of evaporation can range from 100
to 1000 Btu/(lb°F). Air, on the other hand, typically has a constant pressure heat capacity somewhere around 0.24 Btu/(lb°F). This means that most direct drying applications will typically require a much higher rate of heated airflow than the combined rate of solids plus liquids in the feed material.
Due to the required high rate of airflow, the process will need one or more fans pulling air from the surrounding environment. This air must be heated, either directly with a burner or through heat exchangers. The amount of air required can be minimized as the air temperature increases. The maximum air temperature will be material dependent. Due to the particle size distributions of typical products, there will always be some amount of fine material conveyed away from the dryer in the gas exhaust stream, requiring a dust collector/separator to prevent the material from reaching the atmosphere. This dust collector could be a cyclone or bag/cartridge filter with either an airlock or series of valves to discharge the material from the vessel. In some cases, this is the final product; in others, this creates a separate recycle stream. Even though there are numerous components required for assembling a direct drying system, many are commercially available from reputable suppliers. The cost of a direct dryer is usually, but not always, less than that of an indirect dryer. However, the total cost of the dryer plus ancillary equipment needed to run the process should always be part of the analysis.
Operational costs matter
In addition to the total equipment acquisition costs, it is also important to consider the system’s operating cost over the life of the process. Utility energy costs for a direct dryer system with high airflows are typically higher than those of an indirect dryer. When compared to an indirect drying system, a direct drying system typically has a larger footprint, which adds to overall installation costs. If the exhaust gasses can be directly vented to the atmosphere, direct drying is always a viable choice.
However, if the exhaust gases require treatment for odor or pollution control, a direct dryer with its higher airflow will result in a larger and more costly system. Depending on the properties of the materials involved, this could weigh heavily against a direct drying option.
Since indirect dryers isolate the heat transfer medium from the processed material, there is little or no gas flow through the machine, requiring some mechanical means for moving the material. Mechanical agitation can be provided by screws, ploughs, paddles or other sorts of machined surfaces that rotate to push the material past the heat transfer surface. Depending on the application,
indirect dryers can be designed to be operated with only a thin film of product or a thick bed of material. These agitation devices could be detrimental to freshly formed or green agglomerates, so an indirect dryer might not be the best choice in post-drying in an agglomeration process.
Having the heat transfer medium separated from the product offers many benefits. First, contamination is minimized since the heat transfer medium is not in contact with the product. Further, if a gas is used to sweep the environment (to keep the dryer atmosphere below the saturation point), the amount needed is minimal and can effectively provide additional drying in some indirect dryer designs. Minimal use of process gas lowers capital expenses for ancillary equipment such as blowers, gas heaters, condensers, filters, and exhaust treatment. This reduces the overall system footprint when compared to direct drying options, thus lowering installation costs.
Because the amount of sweep gas is low in indirect dryers, the use of inert gas is much more economical and can be used to process materials when the volatile is not water.
Solvent-laden gases can be passed through a solvent recovery condenser, which are often installed in a closed loop to further reduce inert gas use.
In general, indirect dryers typically have higher thermal efficiencies than direct dryers because the heat transfer fluids in indirect systems are also typically operated in a closed loop (for example, boiler or hot oil heater systems).
The total surface area is smaller, resulting in lower heat losses to the atmosphere. Direct drying systems will often employ a heat exchanger prior to the air inlet heater that uses recycled exhaust heat to preheat the air going into the system. However, even with this, the thermal efficiency of a direct dryer will still not approach that achievable in an indirect dryer.
Mechanical agitation helps dry and convey material While most indirect dryers have heated bodies, the mechanical agitation and conveyance is provided by a variety of rotor styles that can be chosen to match the application. For example, hollow parallel disc rotors allow the dryer to operate practically full and yields a high amount of heat transfer surface in a compact design. Hollow screws not only transfer heat but convey the material.
High-speed paddles or ploughs will force material against the heated shell and are better suited for sticky materials when compared to a disc or screw-type rotor design.
Mechanical agitators will sometimes input small amounts of additional energy into the product, thus decreasing the thermal load for drying (and inversely requiring additional
cooling for cooling applications). Nearly all indirect technologies can operate in continuous or batch modes.
Indirect dryers may or may not have an effect on the particle size distribution of the product being processed, but this is an important consideration for agglomerated products. Paddles, plows, hammers, ribbons, etc., are typically designed to only impart enough force to keep the material moving past the heat transfer surface. While the environment might be too aggressive for curing of green agglomerates, these devices have been used for cooling. Except in zones of great acceleration with abrasive material, wear is minimized on the parts in an internal indirect dryer, resulting in lower maintenance costs.
On extremely large heat load applications, a single indirect dryer may not make sense from a capital expenditure or an operating expense standpoint. In that case, two-stage drying may be a more economical solution, as is often the case when driving volatile levels down to very low ppm levels. In this case, a combination of direct and indirect dryers can be used.
Table 1 summarizes factors of direct and indirect drying to consider. Certainly, every material will introduce its own challenges, and the requirements for drying of powders and fine granules will be different from those required for drying pellets or briquettes, but in many cases, these general rules apply.
Table 1: Key points of direct vs indirect thermal technologies
Consideration Direct Indirect
Operating Costs Higher Lower
Contact w/ Thermal Fluid Yes No
Process Gas Usage Large Minimal
to None Overall Heat Transfer Lower Higher Non-Ambient Pressure? Not Typical Yes Material Agitation None /
Mechanical / Process Fluid
Mechanical
Capital Costs Lower Higher
Installation Space Larger Smaller
Table 2 offers some basic points to consider when choosing a technology. Again, each product or process will have its own peculiarities, but these considerations can serve as a starting guide. Often, a given project may require additional steps to overcome known shortcomings.
As an example, a belt dryer for curing briquettes can be quite large, but there may be no other feasible way to achieve the desired results.
Table 2: Comparison — direct and indirect thermal technologies.
Direct Indirect
Pros 1. Temperature-sensitive material (added heat offset by evaporative cooling).
1. Product not
temperature-sensitive (high wall temperature minimizes equipment sizing).
2. Large evaporation rate. 2. Evaporation rate can be large or small.
3. The moisture is water. 3. Moisture is not water.
4. Application requires non-ambient pressures.
Cons 1. Solvent is present. 1. Product is
temperature-sensitive (localized heating causing melting or product degradation).
2. Gas scrubbing is required (odor control).
2. Excessive feed moistures may require high gas flow, resulting in custom design.
3. Larger space requirements.
3. Possibility exists of fouling mechanical elements.
4. Drying is diffusion- limited.
4. Moving parts could result in particle size degradation.
5. Utility cost considerations.
5. At very high capacities retention times are limited.
6. Noise considerations.
Table 3 summarizes different dryer types that can be considered in an agglomeration process. For example, since some dryers are available as continuous units, they would not be considered where batch control is required, as in the pharmaceutical industry. Also remember that the air in a direct dryer can be heated directly, by burner, or indirectly as required for sanitation.
Table 3: Basic overview of different dryer types.
Equipment Type Indirect Direct Continuous Batch
Rotary X X X
Rotating Hollow Disc X X X
Screw X X
Plough & Screw X X X
Paddle X X X
Steam Tube X X X
Blender X X
Stacked Tray X X
Dispersion Mill / Flash X X
Traveling Bed X X
Fluid Bed X X X
Purge Vessel/Hopper X X X
And finally, thorough testing of material in a bench lab or in some cases even a cursory examination of the material to be dried can provide clues to which type of dryer might be best for the application. In general, actual testing with the help of reliable vendors will define the type and design of the dryer. Working with a single source supplier often provides the best match of the agglomerating device and the dryer.
Along with the data obtained from the drying curve, bench testing results will yield particle size, total moisture, stickiness/tackiness or the tendency to re-agglomerate, density and friability or hardness. The data in Table 4 offers a starting point.
Table 4: Guidelines for dryer selection
Incoming material properties
Recommendation
Solvent present / odor concerns
Indirect drying
Large particle size (pellets/briquettes)
Traveling bed / stacked tray/
rotary
Sticky materials Consider back-mix-fluid bed or paddle dryer
High moisture/water Direct drying-fluid bed/
spray dryer/flash dryer Tendency to agglomerate Direct fluid bed
Friable granules Screw-, belt- or tray-type, or fluid bed
Extended drying time (indirect)
Batch or two stage
Extended drying time (direct)
Rotary, belt or tray
Size reduction desired Dispersion / milling flash type Heat sensitive material Direct / indirect vacuum
DRYERS IN AGGLOMERATION PROCESSES Dryers are key components in many agglomeration systems. Much like selecting a dryer for a powder or granular materials, many factors affect agglomeration system design. Although final product properties will lead toward a dryer choice, it is important to understand that the dryer can also influence these properties.
The common approach is to divide the project into two steps: drying and agglomeration. It is more desirable to consider the process as a whole, because the dryer itself could impact the final properties of the product. Several combinations of dryer/agglomerator can be considered to meet requirements. A full understanding of the agglomeration step, the drying step and the combination of the two allows for the most efficient operation.
First, fully quantify the feed material and the final product.
Identify the volatile that must be removed. If the volatile is water, hot air or sweep gas can be the drying fluid. In removing solvents, an inert sweep gas would be used.
Next, examine the particle size and shape, inlet and final moisture content and the rate of moisture removal. Also consider characteristics such as heat sensitivity and how a certain dryer type might affect the final particle.
Some dryer types may have a limited capacity. Long drying times combined with low capacity may be best suited to batch dryers. Review site conditions for available space and utilities. Product contamination concerns will determine if the air is heated directly or indirectly.
Additional equipment, such as scrubbers, might be required if odor emissions are a concern.
In most cases, the material leaving an agglomeration device and entering a dryer will be soft, since the process relies on drying for curing. Many products, like briquettes, cannot tolerate any breakdown during drying. Granules, on the other hand, can enter the dryer with a slightly larger overall particle size distribution, with the expectation that some particle polishing and size reduction will occur. In any case, avoid heavy impact by mechanical parts.
In a majority of agglomeration systems, the volatile being removed is water. In general, this would suggest direct drying. Although agglomeration systems receive feed material in a variety of forms such as powders, wet cakes, slurries or even solutions, a post dryer will receive a formed product, such as a pellet, briquette or granule. The size, shape, final moisture and strength (both green and final) define the dryer type.
After drying, cooling may or may not be required. However, materials often leave a direct dryer at temperatures above that required for safe packaging. Coolers could be stand- alone units but are very often supplied as an additional zone in the dryer itself.
EXAMPLES:
The following examples describe generic processes for selecting a drying system based on application need and parameters.
1. Producing larger particles using a binder.
Several devices can convert a powder of fine granular material to particles over 12 mm: roll presses, ring and die pellet mills or high-pressure screw extruders. Although many materials put through these devices will form under pressure alone, some materials benefit from the use of a binder or simply additional water.
A roll press converts the material to the final product: 15 to 300mm and almond- or pillow-shaped. Most materials discharged from the briquetting rolls are soft in their green or uncured state, so gentle handling is required. Very likely, the binder is water or water-based, which suggests direct drying. Because of the larger product size, diffusion of the moisture from the center to the surface will require a longer drying time. Drying temperature could be limited, both to preserve product shape and to avoid product degradation or combustion. Additionally, cooling before packaging or stockpiling might be required.
Direct dryer choices include flash, fluid bed, direct rotary and traveling belt. In this case, a flash dryer would make it difficult to entrain the large particles in an air stream. A rotary motion would result in product breakdown since the green briquettes would be subjected to numerous drops and collisions. A vibrating fluid bed could be made to work, but it would be difficult to fluidize large particles.
Long drying times would necessitate very large machines and the turbulent action in a fluid bed could lead to particle breakdown. A continuous tray dryer provides a gentle drying action, but tray dryers can be very tall, causing the product to fall numerous times within the device as it goes from tray to tray. The logical choice, then, would be a belt dryer.
Example 1 shows a flowsheet of a typical belt dryer system. The material to be briquetted is mixed, usually continuously, with either water or the binder ingredients.
Binders could be in the form of powders or be premixed with water and added as a solution. In some pelleting applications, the moisture could be added in the form of steam. The heat of the steam will also soften or condition the material, especially when the materials are grains or biomass.
The mixed material drops to the briquetting press where the product is formed. A vibrating screening device
mechanically removes fines. Since these fines are relatively wet, they can be recirculated directly to the briquetting press. Milling of the fines, especially if they are large, will create a more homogenous blend in the briquetter. Finally, the briquetted product is gently conveyed to the belt dryer.
The belt dryer incorporates a porous belt and is designed to ensure proper airflow through the bed. The air could be directly or indirectly heated, based on the application or the available utilities. A swinging apron conveyor ensures even distribution across the drying belt. The bed depth is controlled by rate and belt speed and is usually selected to allow for sufficient air flow through the bed. The drying time is then controlled by the length of the belt. Exhaust air is passed through a baghouse and the small amount of collected fines can be recycled. Fines that fall through the belt are also recycled. Because these fines are dry, they would normally be directed back through the mixer to be rewetted.
The belt dryer can be divided in zones to control drying efficiency. Often, these dryers incorporate a cooling zone so the discharged product can be immediately packaged.
Note that the drying principles would be the same for any products that are briquetted, extruded or pelletized.
However, the operating conditions are product specific.
Material degradation is limited because of the relatively slow product movement and handling, so the belt dryer is the best option for larger briquette drying.
Examples of materials that would employ this or similar flowsheets are charcoal, coal, animal feeds, cereals, alloying agents and various ores.
Example 1: A belt dryer is designed to ensure proper airflow throughout the bed, and can be divided into zones to control efficiency.
2. Particle size enlargement of a solvent-laden powder.
Solvent-based processes are used in the production of many materials, especially when reactions are involved.
In other cases, byproducts of filtered materials contain solvents, causing handling or disposal issues. In all cases, the fine particle size and low bulk density create the need for agglomeration.
Defining the properties of the final product will suggest processing options. Granules could be produced by a variety of methods, such as agitation or extrusion. These are wet processes, and granulation would be performed on the wet cake itself. Roll compaction would normally be accomplished starting with a dry powder. The complicating factor here is the solvent. As discussed earlier, it is
desirable to use indirect drying for solvent laden materials in order to minimize the amount of sweep gas. Therefore, inert gases can be used.
Certainly, direct dryers can be built with inert environments, but because of the high gas volumes, the operation might not be economical. Collection and recovery equipment would also become prohibitively large. In fact, the overall space required for a direct drying system will be much larger. Therefore, indirect drying would make the most sense.
As previously stated, most indirect dryers rely on mechanical agitation since there is insufficient gas flow to move the material. These agitators take the form of a rotating device in most cases. Indirect dryers employ screws, paddles or ploughs to move the material.
Unfortunately, these agitators tend to break down agglomerated materials, especially in the freshly formed or green state. When using an indirect dryer, therefore, agglomeration is best done after drying. See Example 2 for a dryer processing solvent-based materials.
To make the required final product from the dried powder, processes like extrusion or agitation could be considered but these processes require rewetting. This means a second drying step. Besides adding additional processing costs, repeated heat cycles could be detrimental to some materials. Taking all things into account, in this case, indirect drying followed by high pressure roll pressing makes the most sense.
There are a variety of indirect dryer types: paddle, plough, disc and tube. There are even indirectly heated rotary machines, such as drums or pans. A disc dryer, like the unit shown in Example 2, offers extremely large surface area in a small space. Coupled with the low volume of sweep gas, this results in a very compact system that is economical to operate. Moisture levels are controllable, but residual solvent levels in the parts-per-million range are possible.
These solvents can be recovered, leading to minimal environmental impact.
Agglomeration is accomplished in a roll press. If the desired product requires granules of controlled size, a conventional compaction/granulation system is used.
For simple densification, a once-through system with roll compaction followed by milling is used. Where briquettes or larger pieces are required, briquetting rolls are used.
In all cases, controlled pressure is applied to the powder, resulting in the formation of medium to hard masses.
Screening and optional milling control final size.
Examples of materials that would employ this or a similar process are polymers such as PPO, PPE, PE and fluoropolymers; industrial sludges and specialty chemicals such as resins.
Example 2:
Disc dryers offer large surface area in a small space.
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3. Water-dispersible granules from powders.
Often, the powder already exists in a dry form, especially in situations where the agglomeration system is used as a product enhancement. It could also be that the product is sold in two forms, powders and granules. Or, fine milling of the powder prior to granulation is required to meet the needs of the end use. In these cases, a stand- alone granulation system makes sense. Because of the requirement for dispersible granules, roll compaction is best avoided due to its tendency to make the final product too hard and dense.
This leaves the options of mixing or low-pressure extrusion methods, both of which are used to produce dispersible granules. Both methods require mixing a powder and liquid. The final product size may be in the range of 100- 2,000 microns and the mixer and can discharge directly into a dryer. In other cases, the mixed material could be further processed in a low-pressure extruder and then dried or spheronized before drying. The binding agent is typically water or a water-based solution. In order to grow granules in a mixer, the fine material is wetted, causing the individual particles to stick together. These freshly formed granules are very soft and cohesive on the surface.
With the moisture being water and the particles being very soft, direct drying is optimal. Options might be belt or tray dryers, but the sticky material could lump up or form a sintered cake. It is important to keep the granules separate.
A rotary type dryer is an option, but the cascading action could result in particle breakdown or larger ball forming as dust or wet feed particles continue to agglomerate. Flash dryers will surely break down the particles. An attractive option is a fluid bed, which offers gentle agitation, effluent gas contact and moisture stripping.
A typical “wet” agglomeration system is shown in Example 3. In many cases, the agglomeration mixer or extruder is mounted directly on the inlet of the fluid bed. The fluid bed uses air of sufficient velocity to keep the bed in a state of turbulence. The fluid bed box can be fixed, or vibratory.
A vibratory bed uses mechanical action to move material and would normally operate with lower air velocity. This requires more screen area, but these units can handle a wider range of particle sizes. However, the vibratory action creates the need for a more robust structure to absorb the vibration. Fixed beds, on the other hand, transfer only weight to the structure. However, since higher air velocities are required to move the material, the fan power requirements are higher. Normally, the fluid bed is divided into zones.
In the first zone, the wet granules must remain separated and are case hardened. The air in the first zone might have higher velocity and temperature. Evaporative cooling limits material temperatures. Wet agglomerates enter a bed of partially dried granules, which prevents further product growth. As the moisture is removed, the granules become lighter and directional screens move the granules toward the second zone. As new granules enter, partially dried granules overflow to the next zone.
The second zone uses lower temperature air and relies on a longer retention time to fully cure the granules. The final zone provides additional drying or if necessary for packaging, cooling. The action in the fluid bed promotes particle polishing and the high airflow through the beds removes dust. As granules exit, they can be screened and sized or very often are clean enough for packaging.
Fines from the screener and dust removed in the bed are recycled to the agglomerating device. Oversized material can be pulverized for re-agglomeration or crushed to size and screened. Examples of materials processed through this or similar systems are feed additives, drink mixes, detergents, ag chemicals, nutraceuticals, pigments or dyes.
In a variation of the process described above,
agglomerates may form with a high solids solution or slurry, as the feed material eliminates the need for first drying. Typically, solutions or thin slurries are dried in spray dryers. Although spray dryers produce “microspheres,”
these particles are extremely fine. Spray-dried materials are notorious for their dustiness and low bulk density.
Example 3:
A fluid bed dryer offers gentle agitation, effluent gas contact and moisture stripping.
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Many spray-dried materials require some form of agglomeration. The usual choice for retrofitting
agglomeration is roll compaction; however, roll compaction might not be desired, as the product will be hard and dense.
Starting with spray-dried powders, a wet agglomeration process can easily produce granules. Unfortunately, this results in rewetting the powder followed by another drying step. This is an inefficient and costly process and could even lead to product degradation due to an extra heat cycle. Sometimes, agglomerators, such as paddle or blade mixers, have been retrofitted to spray towers. Dry product is back-mixed with a spray dryer feed solution. However, this hybrid system often results in compromised product quality since the spray dryer is not designed to be a granule dryer. An alternative is to bring the solution directly to the agglomerator.
During agglomeration, dry solids are mixed with the solution or slurry to build granules. The dry material comes from the conventional recycle stream in the system: fines from screening, pulverized oversize from screening and dust from the fluid bed baghouse. In some circumstances, a portion of the product is back mixed. The remainder of the system stays the same.
4. Granules from wet cakes.
Many materials exit a process in the form of a wet cake.
Wet cakes are produced by mechanically removing water using a filter or centrifuge. These wet cakes must be dried, but in many cases the desired product is an agglomerate, either briquettes, pellets or granules.
There are many ways to process wet cakes. They can be extruded or conditioned in pin mixers. They can be rolled in drum or pan agglomerators. Since the product size and shape of these different products varies so much, consider different dryer types for different agglomerators. Direct dryers would include drums, fluid beds, tray type or belt type; each has its place. Agglomeration processes and drying solutions include:
• Pin mixer ➝ Fluid bed
• Drum or pan granulator ➝ Drum dryer
• Small extrudate ➝ Fluid bed
• Large extrudate ➝ Belt dryer
But there may be a situation where the end product must be a granule and a powder. A portion of the agglomerated material could be pulverized, but this might impact product performance. Unless there is a reason to install two complete processes, the situation again calls for predrying all of the material followed by post granulation of a portion of it. Since the powder will be dry, the agglomeration
methods available are high-pressure extrusion or high- pressure compaction. High-pressure compaction could be in the form of roll pressing, slugging or tableting. The choice will depend somewhat on the material, but for the most part, roll compaction systems are best suited to produce granules.
Any number of dryers could be used to transform the wet cake to powder, especially when removing water: rotaries, flash dryers, dispersion dryers, belt dryers or tray dryers.
Fluid beds do not normally perform well on fine particles as the particles can fall through the screens. Each type of dryer has its unique benefits and drawbacks. Most, like rotaries, belts and trays, will result in product lumping, which introduces the need for a de-lumping device after drying. Flash dryers overcome this with slinging devices, but to ensure a powder is discharged, a dispersion dryer is the best alternative.
A dispersion dryer is based on a flash dryer but uses a low-intensity mill to break up lumps and disperse the wet cake into the hot air stream. Because of the high moisture, high temperature air can be used, and evaporative cooling tends to control product temperature. These factors lead to increased dryer efficiency.
Example 4:
Roll compacting systems are best suited to produce granules.
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The system is set up with a feeder delivering wet cake to the dispersion unit. The feed may or may not incorporate recycle, depending on the tendency of the feed material to build up. In either case, the wet feed is drawn with heated air through the dispersion chamber where a medium- intensity rotor breaks up lumps and combines the streams.
This air/powder mixture is then drawn through cyclones or baghouses where the product is separated from the air. The air can be directly or indirectly heated.
Product from the baghouse can be flash-cooled for
packaging, if required, or diverted to the granulation system.
The granulation system includes a roll compactor, which presses the material into sheets or flakes. The flakes are then milled and screened to the required size. Larger capacity systems may incorporate multiple milling steps to increase yield. A dust collection system prevents dust from escaping, and the dust is also recycled.
Examples of materials processed on this or similar systems are feed additives such as amino acids, vitamins and antibiotics; lithium carbonate; organic fertilizers and protein powders.
SUMMARY
Just as there are many options for agglomeration there are many types of dryers to go along with them. It is important to evaluate the process requirements as a whole and to avoid the temptation of dividing the project into two parts (drying and agglomeration). Designing as a whole creates the most efficient system. Most often the dryer type in an agglomeration system will be a direct type because water is the liquid to be removed. Indirect dryers are preferred when any solvent is present.
Consider all variables in the process: volatile content and type, wet material and dry material consistency and properties most important in the end product. Define ambient conditions and footprint requirement and include these parameters in the overall system design. Different dryers can sometimes be made to work, but it is important to consider the characteristics of the feed material and the product requirements when selecting a dryer. Even if a dryer is already on-site or available, it may not provide the best long-term process solution.
In defining a process, the end results can be reached much more quickly by dealing with suppliers that are skilled in both dryer and agglomerator designs.
About Bepex International
Bepex International, with roots dating back to 1897, serves the global food, chemical, and polymer markets by providing process development services and custom-designed industrial scale process systems and equipment. With a wide array of proprietary platform technologies, including thermal, size reduction, compaction/agglomeration, mixing and blending, and mechanical dewatering, Bepex custom-designs each piece of process equipment to the exacting requirements of each process and customer, reducing time-to-market and increasing processing efficiency.
