Handbook of Industrial Drying

Classification by Residence Time

The pressing need of product quality in the plastics industry also forces one to consider residence time distribution of the product when comparing dryers.

1. 
   Short Residence Time
   

The short residence time category comprises spray dryers, pneumatic dryers, and thin-film dryers in which the residence time may be of the order of several seconds.

2. 
   Medium Residence Time
   

Continuous fluid-bed dryers, steam tube rotary dryers, and rotary dryers can be designed to provide medium residence time (of the order of minutes).

3. 
   Long Residence Time
   

Rotary dryers, batch fluid dryers, continuous or batch tray dryers, hopper dryers, multispouted bed, and vacuum tumble dryers are typical long-residence units used in polymer drying.

3. 
   OTHER CONSIDERATIONS
   

On the basis of the polymerization alone, it is difficult to specify definite dryer selection rules since typically polymer properties differ over a wide range. The choice of dryer is also limited by the physical properties of the polymers, e.g., polymer-handling characteristics, indi- vidual or closely related drying curves, properties of the emitted volatiles, limitations on temperature, and par- ticle size and distribution requirements. Other factors include equipment space limitations, production rates, pollution control requirements, solvent recovery, thermal sensitivity, and product quality specifications. The primary step in specifying a dryer is to define the physical, thermal, and chemical properties of the product and the volatiles present. Often the consist- ency of the feed reduces the choice of dryer. A few guidelines are always helpful in selecting polymer dryers. For example, if a solvent must be evaporated and then recovered, it is usually not desirable to choose a convection dryer. Since solvent must be condensed from a large carrier gas flow, the condenser and other equipment become rather large. If the maximum prod- uct temperature is lower than about 308C, it is possible to specify a vacuum dryer. If the average particle size is about 0.1 mm or larger, a fluidized bed dryer may be considered, or if the feed is a slurry or paste a spray dryer may be a judicious choice. Scaling is another important factor that can dictate dryer selection. For example, if the requirements are to produce high ton- nage of a polymer in one line, it probably would be advantageous to consider a fluid-bed dryer rather than
a mechanical rotating type.
The reader is referred to other chapters of this handbook for details concerning specific dryers discussed.

4. 
   COMMON POLYMER DRYERS
   
   1. 
      Rotary Dryers
      

Historically, rotary dryers (RDs) have been the most popular in polymer-drying operations. A rotary dryer consists of a slowly revolving drum (often fitted with internal flights or lifters) through which both the material and the gas pass. Gas, cocurrent or counter- current with the granular polymer, can be introduced at either end of the cylindrical shell.

Solids move through the dryer by the effect of grav- ity, the rotation of the cylinder, and gas flow (in the case of cocurrent units). Internal scoops, blades, and lifters, which give the solids a showering pattern, are provided for better gas–solid contact. Baffles and dam rings are also available to retard the forward motion of the solids and to increase residence time (5 to 20 min is common; much larger times are also found in drying of certain polymer pellets).
An improvement over the standard rotary dryer is the steam tube rotary dryer. Here, two or three rows of steam tubes are located in concentric circles within the shell, which extend the full length of the cylinder. The tubes together with a series of small radial flights serve to agitate the material for uniform drying. These types of dryers were used in the polymer indus- tries for heat-sensitive polymers requiring indirect heating.
With the advent of the new and energy-efficient dryers, rotary dryers nowadays are seldom used in polymer drying in new polymer plants. Modified fluid-bed dryers as well as novel spouted bed dryers can replace rotary dryers in many applications.

2. 
   Flash Dryers
   

The flash dryer (FD) is a direct-type, cocurrent unit that is essentially a long vertical tube with no moving parts. In polymer drying this is mostly used as a predryer to remove surface moisture.

In FD units, hot inlet gases contact the wet prod- uct, which may be powdery, granular, crystalline, or pasty material, as discharged from a centrifuge or filter press. Providing a short residence time of several seconds, FD is well suited for high evaporative loads. Drying is nearly adiabatic, an advantage with heat- sensitive polymers. High mass and heat transfer rates are obtained because of the high relative velocity between feed and inlet gas and a large exposed prod- uct surface area.
The method of feeding wet polymer to FD is very important. Granular products are relatively free flow- ing when wet polymers are fed with devices such as screw and rotary star feeders; sticky polymers may be best handled with a table feeder. Lumpy or pasty polymer must be broken up or mixed with dry product recycle to produce a more uniform and free-flowing feedstock.
Among the developments in flash drying, the first and the simplest is the ‘‘thermo venturi’’ drying con- cept in which a vertical drying column expands so that coarse particles remain suspended while drying and finer particles travel straight through with the drying air. This is quite effective as long as the par- ticles are relatively spherical and the size spread is not

too great. Similar designs feature ‘‘bicones’’ in which the drying column expands and contracts, possibly with the addition of supplementary hot air, often injected tangentially.
Recent improvements in flash drying include the ring dryer. The heart of this dryer is a centrifugal separator. It combines renewal of the drying air with centrifugal classification. The lightest and finest frac- tions of the product are passed with the spent drying medium into the product collection system; oversize, partially dried material is held in circulation. The split is varied by adjusting the positions of suitable deflec- tors, introduced in the flow loop. This type of FD is available in both multistage and closed-circuit designs with both direct and indirect heating options for re- moving both surface and bound moistures, as well as solvent removal and recovery.

3. 
   Spray Dryers
   

In spray dryers, the feed material, in the form of a solution, suspension, slurry, or paste, is sprayed in a high-temperature gas zone by centrifugal disks or pressure nozzles. Such dryers are used in polymer industries in which the polymers cannot be separated mechanically from the carrier liquid, e.g., emulsion- polymerized PVC.

In polymer industries, wherever spray dryers are used they are primarily used as predryers of a multi- stage system. Final drying is normally done in a fluid bed, which is either stationary or vibrated type. Sta- tionary fluid beds are used when spray-dried powder leaving the drying chamber is directly fluidizable. The vibrated type of fluid bed is used for products that, on leaving the spray dryer, are not readily in a fluidizable state owing to their particle form, size distribution, or wetness.
In such a multistage system, the higher moisture
content powder leaving the spray-drying chamber is transferred to the second stage, which is a fluid bed for completion of drying. The higher inlet temp- erature and lower outlet temperature operation in such a system give improved dryer thermal efficiency and increased dryer capacity without product quality degradation.

4. 
   Fluidized Bed Dryers
   

Fluidized bed dryers (FBDs) involve the suspension of solid particles in an upwardly moving stream of gas, which is introduced through a distribution plate that may be cooled for heat-sensitive polymers. Such a dryer may operate batchwise.

The advantages offered by FBDs are: (1) the even flow of fluidized particles permits continuous,
automatically controlled, large-scale operation with easy handling of feed and product; (2) no mechanical moving parts, i.e., low maintenance; (3) high heat and mass transfer rates between gas and particles— this is well mixed, which also avoids overheating of the particles; (4) heat transfer rates between fluidized bed and immersed objects, e.g., heating panels, are high; and (5) mixing of solids is rapid and causes nearly isothermal conditions throughout the bed, thereby facilitating easy and reliable control of the drying process.
Using the solvent being removed as the heat car-
rier and fluidizing medium (i.e., a superheated vapor) has proved a feasible and beneficial design. Its advan- tages include: (1) reduction in size of condensing and recovery equipment; (2) increase in drying rate due to the elimination of the gas-film resistance of the for- eign vapor; (3) volumetric heat capacity of various vapors is usually greater than that of air; and (4) space velocity for fluidization is lower than with air, which reduces the volumetric vapor flow and conse- quently the size of the dust collector, air moving equipment, and other parts.
Drying of polystyrene beads is a typical example
for industrial use of these dryers because of the close range of bead particle size. Also, the size of the beads permits high fluidizing velocities and therefore eco- nomic dryer sizes.
In recent years, indirect-heated fluidized beds have made inroads in almost all industries. Some of their advantages over the direct-heated FBD are: (1) the indirect heat transfer rate significantly reduces gas flow requirements; (2) there is tremendous leverage gained by the multiple of the heat transfer coefficient, LMTD, and heat transfer surface density permits very high heat inputs into low-temperature, heat- sensitive applications; (3) when a plug-flow, rectan- gular indirect fluid bed or low bed height is used, the solids flow counter to the thermal fluid, behaving like a countercurrent heat exchanger with all its attendant benefits; and (4) since the heat source is decoupled from the fluidizing gas source, vessel diameters and pollution-control equipment are much smaller.
Indirect fluid beds have already proved efficient in drying very heat-sensitive polymers with large constant-rate drying periods, as in drying PVC, polyethylene, acrylonitrile–butadiene–styrene (ABS) copolymers, and polycarbonates (PC).

5. 
   Vibrated Fluidized Beds
   

A vibrated fluidized bed (VFB) is basically a long rectangular trough vibrated at a frequency of 5 to

25 Hz with a half amplitude of a few millimeters (2 to 5 mm). This kind of dryer can be used for drying

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