Material Classification in Drying Operations


Written by Joe Scholl, July 18, 2016

The word “classify” has different meanings, depending on the context in which the word is used. In bulk solids handling applications, it is generally taken to mean the separation of one type of material from others, even though the materials may be substantially similar.  For example, a material is discharged from a drying operation and is then sent to a screening operation to remove over-sized (“overs”) or under-sized (“unders”) material from the “on-spec” material of a desirable particle size or particle size distribution (“PSD”).  With respect to the various bulk solids drying technologies available, only a fluidized bed operation can be used as means to classify overs and unders from the on-spec material to some degree and without a further screening or separation operation.

Material Classification Fluid Bed Drying

(Here is an example of different classifications, from left to right are unders, on-spec, and overs).

Examples of the most prevalent drying technologies available today include spray and/or flash, heated or hollow flight, belt, rotary drum, and fluidized bed dryers.  While each of these technologies has applicability in various drying applications, some provide no material classification whatsoever, with others providing a small extent of classification, and others providing even more material classification.  Certainly, there are many different equipment configurations that can be developed for any given application that can influence the material classification effect.  However, as will be discussed in the following, only a fluid bed operation can specifically be designed and/or operated to achieve material classification within the drying operation itself.

In spray drying, a solution of suspended or dissolved solids is sprayed into a drying chamber that receives hot drying gas (“gas” because it may be air, nitrogen, carbon dioxide, or some other heated gas suitable for the application).  As the sprayed liquid droplets contact the hot gas, the water evaporates, leaving a residual solid behind.  The velocity of the drying gas (typically in counter-current flow to the solids) is controlled such that the dried residual solids fall via gravity to the bottom of the drying chamber.  The solids may be collected and discharged intermittently or continuously from the unit.  As there is some gas movement that is counter-current to the solids flow, it is possible that some very small particles (“fines”) are removed from the drying vessel and collected in exhaust gas dust removal equipment.  While, technically, this is a small degree of classification, it is more the consequence of a gas flowing in opposition to the direction of the dried solids and, with the gas velocity intentionally set at low values, the object of the unit operation is more to conserve mass from being removed from the vessel to exhaust gas de-dusting equipment and maintain the highest rate of solids flow toward the bottom of the unit operation for further processing.  In other words, the unit is not designed to provide classification to any significant degree.  Rather, it is designed to minimize classification of material within the vessel.  In some configurations, the spray drying vessel directly feeds a fluid bed dryer unit operation to complete the drying process.

Flash drying is similar to spray drying in the sense that a solution of suspended or dissolved solids is sprayed into the drying vessel, where it also contacts a hot drying gas.  However, in this case, the hot gas and spray/residual solids are typically arranged to be in co-current fashion.  In this manner, the hot gas acts not only as the drying media, but also the pneumatic transport mechanism by which the dried material is removed from the drying vessel.  The product is then recouped via dust removal equipment (cyclone, bag filter, etc.).  In this case, it can also be seen that the technology is not a means by which a portion of the material is segregated or classified from the balance of material.

Heated or hollow flight drying technologies utilize one or more (typically two) screw conveyor-like shafts and flights that are fabricated such that there are internal spaces within the shafts/flights for the movement of a heated thermal transfer fluid (hot water, thermal oil, steam, etc.).  With this particular drying technology, the wet material is introduced into the drying operation and the movement of the heated flights move, turn, mix, and (via direct contact with the heated flights in a conductive energy transfer fashion) dry the material.  Typically, heated flight drying technology does not use a “sweep” gas (a gas passed through the internal volume of the dryer to prevent high moisture vapor content in the vessel and, therefore, to prevent internal moisture condensation), or uses very little sweep gas for this same purpose.  As such, there is little, if any, material removed from the main bulk of the product to downstream dust removal equipment.  Therefore, the unit has little or no material classification ability (and is not designed to do so).

In belt drying, the wet material to be dried is deposited on to a moving belt.  The belt may be arranged in a single-, double- or even greater number of “passes” within the dryer unit in a serpentine fashion.  The belt typically is of a mesh or perforated design such that the hot drying gas may pass through the belt and material residing on the belt.  The gas flow may be co-current, counter-current, or some other arrangement (passes through the wet material vertically/perpendicularly).  In all configurations, the drying gas is meant only to impart the drying energy necessary and is not meant to carry smaller particles away from the unit with it.  As such, it can also be seen that belt drying operations do not present an opportunity for material classification within the drying operation itself.  In fact, fines generated during the drying process may pass through the belt perforations and wind up accumulating in the bottom of the dryer requiring periodic removal.

With rotary or drum drying, a heated drying gas is introduced into a rotating shell (the “drum”) that is equipped with internal vanes or flights.  The material is fed to the dryer and is “lifted” within the vessel by the internal flights.  As the drum rotates, the internal flights reach a point where the material falls off the flight and drops via gravity toward the bottom of the dryer vessel in a “curtain-like” shower of falling material.  As the material falls, it directly contacts the hot drying gas, thereby receiving the necessary drying energy.  The material and drying gas are typically in counter-current flow to each other, although it is possible to have a co-current arrangement as well.  Since the material is falling through a moving gas stream, smaller particles (“fines”) of sufficiently-small size may be entrained in the drying gas stream and be removed from the drying vessel in a process known as “elutriation” (the separation of small particles from the main bulk of larger-sized material and exhausted with the gas).  This “dust” is then removed from the exhaust gas stream via downstream dust-recovery equipment.  While this is a form of material classification, it is not generally the intent in a rotary drum drying application to do so.  It is merely the consequence of the smaller particles being captured by the moving exhaust stream, with the movement (velocity) of the gas stream within the vessel more designed for providing the necessary gas mass rate to (a) effect proper heat transfer for the drying operation and (b) have sufficient moisture-carrying capacity for the moisture removed from the product such that internal condensing conditions are avoided.  In this sense, it can be seen that, while a rotary drum unit can achieve some small degree of material classification, this is not the intent of the operation and the unit operation is not specifically designed to classify material.

With fluidized bed drying, however, it is possible to exert a significant influence on the classifying effect via the selection of a fluidizing velocity that will result in classification of smaller particles (“unders,” “fines,” “dust,” etc.).  Additionally, design features can be incorporated into the fluid bed dryer unit that can also assist in the separation of larger-sized particles from the main bulk of material (i.e. “overs” separation/classification).  In this sense, the fluid bed dryer operation can act as a triple-deck screener (separating unders, overs, and on-size material into three distinct material streams), while concurrently acting as a drying operation.  To further illustrate this point, one should consider the act of material fluidization itself.

In fluidization operations, a fluidizing gas (heated in the case of a drying operation) enters the dryer through its lower “plenum” section.  The gas then passes through a gas distributor to evenly-distribute the fluidizing gas over the entire fluidized area.  The fluidizing gas then passes through the bulk solids, exerting “drag force” on the surface of the particles and, with proper velocity selection, suspending the particle in a “cushion” of gas, thereby fluidizing the material.  Were all of the particles of the exact same size, shape, density (or particle specific gravity), etc., no particles would be removed from the others, nor elutriated or classified from the main bulk of material.  This, however, is almost never the case, as the vast majority of applications involve materials having distinct particle size distributions, shapes, etc. (i.e. all particles do not have the exact same fluidizing characteristics).  Therefore, at a constant fluidizing velocity, the “right-sized” particles will be suspended and the “fines”/“unders” will necessarily be classified from the balance of material, since the fluidizing velocity being used is higher than the necessary “transport” velocity of the fines/unders.  Therefore, they will be removed from the dryer unit with the exhaust gas for recovery in downstream dust-recovery equipment.  Should the fluidizing velocity be increased from the previous velocity, particles that were previously too large to be elutriated are now elutriated, establishing a new larger “cut point” particle size for classification.  In general, the lower the fluidizing velocity, the smaller the cut-point particle size, and vice-versa.  As such, the fluidizing velocity can be adjusted to directly-affect the size of particle removed (classified/elutriated).

As discussed above, at a given fluidizing velocity, the “on-size” material is properly fluidized and the fines classified from the bulk of material.  However, it is also likely that the particle size distribution of the material is such that there are particles larger than those that would be suspended by the set fluidizing velocity.  These “overs” may not fluidize as well as the on-size material and drop toward the top of the gas distributor.  As Schwing Bioset utilizes a “directional flow” gas distributor design, these larger particles are essentially “pushed” toward the discharge end of the unit, with the gas distributor acting as a transport mechanism for these larger particles.  When these larger-sized particles reach the discharge end of the unit, they may be discharged via an “underflow” arrangement, which may constitute a separate discharge stream from the dryer unit and may be kept separate from the main bulk of material discharged, thereby establishing a distinct “overs” stream from the unit.  With the on-size material discharged from the dryer via a second discharge point (typically on “overflow” weir), one can see that there are now three distinct material streams from the dryer unit – the “unders” elutriated with the exhaust gas stream, the “overs” discharged via a separate underflow discharge point, and the “on-size” material stream discharged from the overflow weir.

With proper fluidization velocity selection and system design, it can be seen that a fluidized bed unit operation can be designed and operated such that, while acting predominately as a drying operation, can also be used to exert influence on the material such that it can be classified into two (or more) material streams.  In this respect, it can be seen that only a fluidized bed drying operation, relative to other drying technologies, can also “double” as a means by which material classification may be achieved, potentially eliminating further downstream screening steps or, at a minimum, reducing the design requirements of such downstream processing equipment.

To learn more about material classifying and our Fluid Bed Technology, please contact a Schwing Bioset Regional Sales Manager by calling 715-247-3433, email us, and/or visit our website here.


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