• Author / Uploaded
• Himanshu Panchal

Citation preview

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/265144952

41 Drying of Polymers Article · November 2006 DOI: 10.1201/9781420017618.ch41

CITATIONS

READS

0

4,348

2 authors, including: A. S. Mujumdar McGill University 795 PUBLICATIONS   12,822 CITATIONS    SEE PROFILE

Some of the authors of this publication are also working on these related projects:

dehydration of veggie ingredients View project

Food Drying View project

All content following this page was uploaded by A. S. Mujumdar on 18 November 2015. The user has requested enhancement of the downloaded file.

41

Drying of Polymers Arun S. Mujumdar and Mainul Hasan

CONTENTS 41.1 41.2

Introduction ......................................................................................................................................... Common Polymerization Processes ..................................................................................................... 41.2.1 Bulk Polymerization ................................................................................................................ 41.2.2 Solution Polymerization .......................................................................................................... 41.2.3 Suspension Polymerization...................................................................................................... 41.2.4 Emulsion Polymerization ........................................................................................................ 41.2.5 Gas-Phase Polymerization....................................................................................................... 41.3 Dryer Classification.............................................................................................................................. 41.3.1 Classification by Mode of Heat Transfer ................................................................................ 41.3.1.1 Indirect Dryers ...................................................................................................... 41.3.1.2 Direct Dryers ........................................................................................................ 41.3.2 Classification by Residence Time ............................................................................................ 41.3.2.1 Short Residence Time ........................................................................................... 41.3.2.2 Medium Residence Time....................................................................................... 41.3.2.3 Long Residence Time............................................................................................ 41.3.3 Other Considerations .............................................................................................................. 41.3.4 Common Polymer Dryers ....................................................................................................... 41.3.4.1 Rotary Dryers ....................................................................................................... 41.3.4.2 Flash Dryers.......................................................................................................... 41.3.4.3 Spray Dryers ......................................................................................................... 41.3.4.4 Fluidized Bed Dryers ............................................................................................ 41.3.4.5 Vibrated Fluidized Beds........................................................................................ 41.3.4.6 Contact Fluid-Bed Dryers..................................................................................... 41.3.4.7 Paddle Dryers........................................................................................................ 41.3.4.8 Plate Dryer............................................................................................................ 41.3.4.9 DRT Spiral Dryers ............................................................................................... 41.3.4.10 Miscellaneous Dryers ............................................................................................ 41.4 Typical Drying Systems for Selected Polymers .................................................................................... 41.4.1 Drying of Polyolefins............................................................................................................... 41.4.1.1 Polypropylene ....................................................................................................... 41.4.1.2 High-Density Polyethylene.................................................................................... 41.4.2 Drying of Polyvinyl Chloride .................................................................................................. 41.4.2.1 Emulsion Polyvinyl Chloride ................................................................................ 41.4.2.2 Suspension Polyvinyl Chloride.............................................................................. 41.4.2.3 Vinyl Chloride–Vinyl Acetate Copolymer ............................................................ 41.4.3 Drying of Acrylonitrile–Butadiene–Styrene ............................................................................ 41.4.4 Drying of Synthetic Fibers ...................................................................................................... 41.4.4.1 Nylon .................................................................................................................... 41.4.4.2 Polyester................................................................................................................ 41.4.5 Miscellaneous .......................................................................................................................... 41.5 Drying of Polymer Resins .................................................................................................................... 41.5.1 General Observations .............................................................................................................. 41.5.1.1 Nonhygroscopic Resins.........................................................................................

ß 2006 by Taylor & Francis Group, LLC.

954 954 954 955 955 955 956 956 956 956 956 956 956 956 957 957 957 957 957 958 958 958 959 959 959 960 961 963 963 963 965 965 965 966 968 968 970 970 971 971 971 972 972

41.5.1.2 Hygroscopic Resins................................................................................................. Drying Methods ...................................................................................................................... 41.5.2.1 Drying with Heat as Transfer Medium................................................................... 41.5.2.2 Drying without a Heat Transfer Medium............................................................... 41.6 Drying of Selected Polymers ................................................................................................................ 41.7 Conclusion ........................................................................................................................................... Acknowledgments .......................................................................................................................................... References ...................................................................................................................................................... 41.5.2

41.1 INTRODUCTION Spurred by continually escalating energy costs, along with the advent of new competitive polymers accompanied by new and extended applications of polymers and plastics, interest in the energy-intensive operation of drying of polymers has been on the rise in recent years. Drying is one of the prime polymer recovery operations performed before transfer to the compounding plant or packaging for direct use. It is the part of the process in which the polymer is handled essentially as a solid and liquid and gas streams become relatively minor. In polymer production, other recovery operations include salvation of the unreacted monomer and solvent, coagulation and precipitation, concentration and devolatilization, and liquid–solid separation [1]. Although drying is the oldest and most commonly encountered of all unit operations of chemical engineering, it is one of the most complex and least understood operations. One of the prime reasons for this state of affairs is the enormous diversity of drying equipment; over 100 clearly identifiable different types of dryers are in commercial use around the world. Depending upon the nature of the processing mode, physical state of the feed, mode of heat and mass transport, operating temperature and pressure, and other factors, one can classify existing dryers into so many different types that it is impossible to develop or hope to develop generalized procedures for analysis of all types of dryers [2]. This chapter provides a few guidelines for the selection of polymer dryers and discusses the alternatives available. Picking the best dryer for a specific polymer application is beyond the scope of this chapter because of many variables involved in such selection. Since most of the work in this area is proprietory in nature with very little information available in the open literature, it is therefore believed that this chapter will help to overcome the initial agonies of a polymer engineer in selecting dryers for water- or solvent-wet granular polymer particles. For better understanding of the subject for a nonspecialist in polymer technology, a brief survey

ß 2006 by Taylor & Francis Group, LLC.

972 972 972 973 974 976 978 978

is given about the various polymerization techniques employed commercially before discussion of the drying equipment.

41.2 COMMON POLYMERIZATION PROCESSES The selection of dryers for polymer drying depends to a large extent on the upstream operations, e.g., polymerization, since the behavior of a polymerization reaction and the properties of the resulting polymer can vary greatly according to the nature of the physical system in which the polymerization reaction is carried out. Several processes are used commercially to prepare polymers. Each process has its advantages, usually depending on the type and final use of the polymer. The following types of polymerization processes based on physical systems are considered briefly in this section: (1) bulk; (2) solution; (3) suspension; (4) emulsion; and (5) gas-phase polymerization [3].

41.2.1 BULK POLYMERIZATION The polymerization of the pure monomer without diluent is called bulk polymerization or mass polymerization. The monomer (e.g., styrene, vinyl chloride (VC), vinyl acetate (VA), acrylic esters, butadiene, or acrylonitrile) is first purified to remove oxygen or other inhibitors (by bubbling nitrogen through it, by distillation, or by evacuation) and then the polymerization is started through heating, ultraviolet (UV) radiation, or the addition of an initiator (e.g., peroxides, azo compounds, and others). Usually, after a short period of heating, the reaction mixture continues to heat by itself, and therefore it is necessary to remove the heat by cooling. With increasing conversion, because of the rapidly increasing viscosity of the polymer–monomer mixture, this becomes more and more difficult. With large amounts of monomer, bulk polymerization often takes very turbulent and even explosive form as a result of the rapidly increasing temperature. The violence of the reaction is even further increased by the increase in the radical concentration that occurs with increasing viscosity.

Because of the difficulty in heat removal, bulk polymerization is only carried out in a few cases. However, in the cases in which it is used, it is done on a very largescale, e.g., the bulk polymerization of styrene or ethylene (high-pressure process). Since in this type of polymerization the possibility of chain transfer is relatively small and because of self-acceleration, other polymers with high molecular weights are found. One of the characteristics of this process that is always a technical advantage is the great purity of the polymer resulting from the lack of additives during polymerization.

41.2.2 SOLUTION POLYMERIZATION For solution polymerization, a solvent inert to the monomer is used to control the polymerization. High exothermicity is limited by dilution, causing the reaction rate to be slowed owing to solvent addition. The solvent is recycled after cooling and is sent back to the polymerization reactor. The concentration of the solvent is chosen in such a way that the polymerization mixture can still be stirred after complete conversion. Solution polymerization has been employed almost exclusively in cases in which the polymer is then used in the form of solutions (50 to 60%) for lacquers, adhesives, impregnation materials, and other products. Obtaining the pure polymer by distilling off the solvent is complicated because the hard polymer cannot be taken out of the vessel after evaporation of the solvent. Through construction of extruders with vacuum distillation zones and by using other special evaporators, it is possible to separate the polymer from the solvent. In this process the choice of the solvent’s chain transfer constant is very important because this influences the molecular weight to a considerable extent. Because of chain transfer with the solvent and because of the lower monomer concentration, the molecular weight of polymers prepared by solution polymerization is usually lower than that of the corresponding bulk polymers. Commercially, solution polymerizations are not carried out to high conversions (near 100%) but continuously at a constant monomer concentration. The unreacted and evaporated monomer is recycled together with the solvent. This type of production process has two advantages. The reactor always works in a range of high polymerization rates, and the molecular weight distribution curve is not so broad as it is with polymers produced in a discontinuous process with high conversions.

41.2.3 SUSPENSION POLYMERIZATION In the suspension polymerization process, water is used to control heat generation. A catalyst is dis-

ß 2006 by Taylor & Francis Group, LLC.

solved in the monomer, which is dispersed in water. A dispersing agent is incorporated to stabilize the suspension formed. For any nonpolar monomers, this method offers a method of eliminating many of the problems encountered in bulk and in solution polymerization, especially the heat dissipation problem in the former and solvent reactivity and removal of the latter. Another attractive feature of large batch preparations is that the polymeric products obtained from a suspension polymerization, if correctly carried out, are in the form of finely granulated beads that are easily filtered and dried. On a technical scale, suspension polymerization is used in the production of polyvinylchloride, polystyrene, polymethyl methacrylate, and others. For the production of rubbery, sticky, polymers (e.g., the polyacrylates), this is less suitable.

41.2.4 EMULSION POLYMERIZATION The emulsion polymerization process is similar to suspension polymerization. This process is also carried out in a water medium. An emulsifier, either anionic soap or cationic soap, is added to break the monomer into very small particles. The initiator is in solution in the water. After polymerization, the polymer can be precipitated, washed, and dried, or the mixture can be used directly (e.g., latex paint). Emulsion polymerization is superficially related to suspension polymerization, but the kinetic relationships are entirely different. The major causes of the differences are: first, the monomer droplets in the latter system are approximately 0.1 to 1 mm in size and the particles in the former are approximately 10 7 to 10 6 mm in size; and second, the catalyst is dissolved in the aqueous phase in the latter but is incorporated directly into the droplets in the former. In all cases in which the presence of an emulsifier is not disturbing, emulsion polymerization is advantageous. In comparison with other polymerization techniques, it has the following advantages: (1) the polymerization heat can be removed very easily and (2) the viscosity of the lattices, even with high concentration (up to 60%), is low in comparison with corresponding solutions. One large-scale use of this process is in the production of synthetic rubber and, on a small-scale, in the production of polyvinyl chloride (PVC) and polystyrene. The other large-scale use is in the production of plastics dispersions used as such (without first coagulating them) for the production of paints, pigments, inks, coatings, and adhesive paste (e.g., polyvinyl acetate, polyvinyl propionate, and polyacrylic ester dispersions).

41.2.5 GAS-PHASE POLYMERIZATION The term gas-phase polymerization is a misnomer in that it refers only to a polymerization reaction initiated on monomer vapors, generally by photochemical means. High-molecular weight polymer particles are not volatile, so a fog of polymer particles containing growing chains quickly form and the major portion of the polymerization reaction occurs in the condensed state. High-uniformity polyethylene (PE) can be manufactured by passing gaseous ethylene through an active chromium-containing catalyst bed. Other monomers that have been polymerized successfully in the gas phase include methyl methacrylate, VA, and methyl vinyl ketone [4].

41.3 DRYER CLASSIFICATION [5–7] 41.3.1 CLASSIFICATION

BY

MODE OF HEAT TRANSFER

41.3.1.1 Indirect Dryers Indirect dryers, also called nonadiabatic units, separate the heat transfer medium from the product to be dried by a metal wall. These dryers are subdivided on the basis of heat applied by radiation or through heat transfer surface and also by the method in which volatile vapors are removed. Heat transfer fluids may be of either the condensing type (e.g., steam and diphenyl fluids, such as Dowtherm A) or the liquid type (e.g., hot water and glycol solutions). Because of low film coefficients of the noncondensing gaseous system, it is seldom used as the heating medium. Indirect dryers have several distinctive operating features: (1) the risk of cross-contamination is avoided since the product does not contact the heating medium; (2) since a limited amount of gas is encountered, solvent recovery is easier than with an adiabatic dryer; (3) dusting is minimized because of the small volume of vapors involved in indirect drying; (4) dryers allow operation under vacuum or in closely controlled atmospheres that can avoid product degradation; and (5) explosion hazards are easier to control. Typically, indirect dryers are used for small- or medium-size production. The product from such a unit has a higher bulk density than the same material processed in direct dryers. Particle size degradation usually can be minimized by proper selection of agitator speed or design. The common indirect heated dryers are tubular dryers (with or without vacuum), drum dryers (atmospheric, vacuum, horizontal or rotary vacuum, and others), hollow disk dryers, paddle

ß 2006 by Taylor & Francis Group, LLC.

dryers, mechanically fluidized bed dryers, pneumatically conveyed dyers, cone or twin-shell dryers, and others. 41.3.1.2 Direct Dryers Direct dryers or adiabatic or convective dryers transfer heat by direct contact of the product with the hot gases. The gases transfer sensible heat to provide the heat of vaporization of the liquid present in the solid. Direct dryers may use air, inert gas, superheated vapor, or products of combustion as the heating medium. Combustion gases are seldom used in polymer drying because of possible product contamination. Inert gas eliminates the explosion and fire hazard and may be desirable to prevent oxidation of polymers prior to the introduction of stabilizers. Use of superheated vapor as a heat carrier is highly desirable when solvent is vaporized in the dryer and has to be recovered. Commonly used direct dryers in polymer plants are rotary warm air, fluidized bed, flash, spray, tunnel, and various vibrating and spouted bed (SB) types. All these have a common disadvantage. The amount of air or hot gas required is fairly large, which causes the auxiliary equipment needed (e.g., air heaters, blowers, and dust collectors) to be sized accordingly; the thermal efficiency is also lower than that of indirect dryers. Although this classification of dryers has some importance, it is quite difficult to apply it in more than a general way. Both types of dryers are commonly used in polymer-drying processes. Often a combination of direct and indirect drying is economically the most efficient solution to some polymer-drying problems.

41.3.2 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. 41.3.2.1 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. 41.3.2.2 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).

41.3.2.3 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.

41.3.3 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, individual or closely related drying curves, properties of the emitted volatiles, limitations on temperature, and particle 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 consistency 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 product 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 tonnage 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.

41.3.4 COMMON POLYMER DRYERS 41.3.4.1 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 countercurrent with the granular polymer, can be introduced at either end of the cylindrical shell.

ß 2006 by Taylor & Francis Group, LLC.

Solids move through the dryer by the effect of gravity, 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 industries 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. 41.3.4.2 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 product, 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 heatsensitive polymers. High mass and heat transfer rates are obtained because of the high relative velocity between feed and inlet gas and a large exposed product surface area. The method of feeding wet polymer to FD is very important. Granular products are relatively free flowing 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 concept 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 particles 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 fractions 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 deflectors, 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 removing both surface and bound moistures, as well as solvent removal and recovery. 41.3.4.3 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., emulsionpolymerized PVC. In polymer industries, wherever spray dryers are used they are primarily used as predryers of a multistage system. Final drying is normally done in a fluid bed, which is either stationary or vibrated type. Stationary 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 temperature and lower outlet temperature operation in such a system give improved dryer thermal efficiency and increased dryer capacity without product quality degradation. 41.3.4.4 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,

ß 2006 by Taylor & Francis Group, LLC.

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 carrier and fluidizing medium (i.e., a superheated vapor) has proved a feasible and beneficial design. Its advantages 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 foreign 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 consequently 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 economic 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, heatsensitive applications; (3) when a plug-flow, rectangular 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). 41.3.4.5 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

wet, sticky, and granular media and has been used successfully for drying polymers. It is often used as a second-stage dryer after a flash or spray dryer in many polymer-drying applications. Benefits achieved from such dryers are: (1) uniform residence time distribution regardless of particle size; (2) ability to handle polydisperse solids; (3) ability to operate at low aeration rates and hence lower pressure drops; (4) gentle handling of product; (5) higher heat transfer and drying rates, and others. Since the equipment is mounted on resonance springs, the power consumption for vibration is minimal for well-designed VFB [6]. The vibration vector is typically applied at a small angle to the vertical to permit conveying of the solids in the long direction at the desired rate. This permits control of residence times and also better control of the drying or heating rates as the material progresses downstream. 41.3.4.6 Contact Fluid-Bed Dryers Contact FB units are characterized by the residence time distribution of the individual particles inside the unit. A broad residence time distribution is obtained in a back-mixed FB in which the length/width ratio of the bed is relatively small. The narrow residence time distribution is obtained in a plug-flow FB in which the length/width ratio of FB is very large. This corresponds to a long, narrow FB. Alternatively, this can be obtained by compartmentalizing FB and is the usual practice followed in the industry. Compared with the plug-flow FB, a back-mixed FB has a significant advantage inasmuch as the back-mixed FB can accept a feed material that is not readily fluidizable. This is possible owing to the vigorous mixing inside FB and that the material inside the bed acts as a large reservoir in which incoming feed material will be dispersed and the surface moisture will be flashed off, making the product fluidizable. This characteristic makes the back-mixed FB concept well suited as the predrying stage in many polymer-drying systems. The plug-flow FB drying concept is particularly suitable for drying bound moisture from heat-sensitive materials since the residence time is controlled within narrow limits. In the typical polymer application, this means that the bound moisture can be removed from the polymer product at the lowest possible product temperature. These two concepts, along with heating of the bed indirectly by immersed heat exchange surfaces, are jointly utilized in contact FBD. In FB applications for polymers with indirect heating, the temperature of the heating panels is typically limited by the softening point of the polymer.

ß 2006 by Taylor & Francis Group, LLC.

41.3.4.7 Paddle Dryers The paddle type of dryer, marketed by Nara Machinery Company of Japan, is an indirect dryer for granular or powdery material that dries such materials by bringing them into contact with revolving, cuneiform hollow heaters (paddles) without using gas as a heating medium. The paddles revolve at a low speed (10 to 40 rpm) inside the grooved trough fitted with a jacket (Figure 41.1). The heating medium passes inside the hollow paddle so that the entire surface of the paddles and shafts acts as the heat transfer surface. The cuneiform blade enhances agitation of the material and at the same time prevents the powder from adhering to the heat transfer surface. For greatest heating efficiency the dryer is tilted slightly in the direction of product flow and is designed so that the material contacts all heated surfaces, both front and back. The wet product is fed continuously at the top of the dryer at one end. As the powder is agitated slowly by the heated rotating paddles, the moisture generated is conveyed out by a flow of hot air or other gas. The main features of the paddle dryer are: (1) it is compact; (2) has high heat transfer coefficient and good thermal efficiency; (3) the paddles have an interplay for self-cleaning; (4) it is easy to control; and (5) a small amount of gas is required that minimizes dusting and other problems. Paddle dryers have been successfully used in drying such polymers as VC resin, nylon pellets, and polypropylene (PP), as well as polyethylene. Operated in a closed-cycle mode they can recover organics from such solvent-laden products as polyethylene or PP and can reduce the air volume requirement to only 5 to 10% of that used in direct dryers. Energy requirements for such dryers are also lower. It is seen that 1300 to 1500 Btu is required to dry 1 lb of moisture with the paddle dryer compared with 3000 Btu/lb for a suspended air unit. Because of the smaller air volume needed, the sizes of downstream condensers and refrigeration system units are reduced. 41.3.4.8 Plate Dryer The plate dryer (PD) is an indirect dryer in which heat transfer is accomplished by conduction between the heated plate surface and the product. It comes under three major variations, e.g., atmospheric, gas tight, and vacuum. In these dryers, the product to be dried is metered and continuously fed onto the top plate. A vertical rotating shaft provided with radial arms and selfaligning plows conveys the product in a spiral pattern

FIGURE 41.1 Paddle dryer.

across stationary plates. The plates are heated by a liquid medium or steam. Small plates with internal rims and large plates with external rims are arranged in an alternating sequence (Figure 41.2). This arrangement makes the product drop from the outside edge of the small plate down to the large plate, on which it is conveyed to the inside edge, and then drops to the following smaller plate, where it is conveyed again toward the external edge. This design of the conveying system ensures plug flow of the product throughout the entire dryer. Each plate or

1 +

3 4

2

2 1. Product

2

2 2. Heating or cooling medium 2 3. Shell housing

5

2 2

2 4. Conveying system

2

2 5. Plate

2

2 +

FIGURE 41.2 Plate dryer.

ß 2006 by Taylor & Francis Group, LLC.

group of plates may be heated or cooled individually, thus offering precise control of the product temperature and the possibility of adjusting a temperature profile during the drying process. Thermal degradation of sensitive materials can thus be avoided, and cooling subsequent to drying can be achieved. In the plate dryer, the product layer is kept shallow (approximately 10 mm). The entire plate surface is utilized for heat transfer. The product surface exposed to the surrounding atmosphere is even larger than the actual ‘‘wetted’’ heat exchange surface. The design of the product-conveying system ensures product turnover numbers in the range of 200 to 1500. A thin product layer on a large heat exchange surface coupled with high product turnover improves both heat and mass transfer rates. From vacuum plate dryers, the evaporated volatiles are removed by evacuation. Solvents can be recovered economically by simple condensation [8]. Plate dryers are typically fabricated in a modular design; this yields a wide range of dryer sizes with a heat exchange surface between 3.8 and 175 m2. 41.3.4.9 DRT Spiral Dryers DRT is a recent innovation among the nonadiabatic contact dryers. It utilizes heat from a jacketed wall and transmits it to a thin, fast-moving product film rising in a spiral path along the inner wall surface (Figure 41.3). A very small quantity of the conveying medium is required to move the vapor from the dryer

11 1 2 3 4 5 6 7 8 9 10 11

Conveying gas Bottom bearing/support unit Rotating tube Air guide plates Moist product Product film Flow channel Heating or cooling jacket Dry product and conveying gas Head Drive for rotating tube

9

10

8 6 7 4 4 3

41.3.4.10 1

5 2

FIGURE 41.3 DRT spiral dryer.

since the heat transfer rate is very high and the crosssectional gas flow area is small. The jacketed outer cylinder rests on a base, which also embodies the product inlet. Product film moves spirally up the inner wall, and dry powder is discharged at the top. A steam-heated concentric displacement body is placed within this cylinder, which rotates slowly by an external-geared motor. Many segmented air guides are provided on the outer surface of the cylinder and are arranged at a suitable angle. The distance between these plates and the inner wall must be greater than the product film thickness. Conveying gas is blown tangentially by means of a blower into the tube base entering opposite a wet-feed metering screw. As a result, the gas disperses the wet feed by intense mixing. Product film is created by the inertial force, which threads its way upward in a spiral path until it reaches the exhaust port. During this flight through the dryer, the conveying gas and the product are heated by the jacketed tube wall and the inner displacement body surface. Therefore, both gas and product temperature increase in the cylinder with a concomitant increase in the gas absolute humidity. This ensures that product moisture is lowered up to the discharge point, despite increasing moisture partial pressure. This increasing driving force guarantees lower final moisture contents when compared with a convectional flash dryer.

ß 2006 by Taylor & Francis Group, LLC.

DRT is suitable for water-wet and solvent-wet chemicals, polymers, flour, and other products that are in a powdery form. For removing low-boiling solvents from polymeric products, it acts best as a predryer to a fluid-bed postdryer [9]. Among the advantages claimed are: (1) increased thermal efficiency; (2) lower power consumption; (3) compactness; (4) gentle drying; (5) reduced product holdup; (6) quick turnaround; (7) capability of using low-pressure waste steam for drying; (8) low product moisture content; (9) simple operation; and (10) minimum dust explosion potential. The unit also features low gas flow rates, short residence times (3 to 10 s), and high throughput (up to 20 t/h). Miscellaneous Dryers

A number of proprietory dryers suitable for various polymer-drying operations are available in the market. Among them are the Solidaire, Continuator, Torusdisc, and Thermascrew (Figure 41.4) [10]. Solidaire is a continuous dryer consisting of a mechanical agitator rotating with a cylindrical housing, usually jacketed for indirect heating. The agitator is equipped with a large number of narrow, flat, adjustable-pitch paddles that sweep close to the inner surface of the housing. Residence time can be varied from seconds to 10 min by changing either the pitch of the paddles or the speed of the rotor. High paddle speed breaks up agglomerates and continually exposes new surface to the heat. It has been successfully used for drying ABS, PC, polyvinyl alcohol, polyolefins, and other polymers. The Continuator is used primarily for removing tightly entrapped volatiles and for process applications requiring a long residence time. The mild agitation employed in this device provides gentle product mixing that minimizes ‘‘short-circuiting’’ while reducing particle breakup. This type of dryer can process polyethylene, PP, PVC, and other polymers. The Torusdisc is another proprietory design particularly useful in processes that require high-capacity heating or cooling. Its chief advantage is its versatility. A single unit can be varied over a wide range of heat transfer coefficients, residence times, and temperature profiles. It consists of a stationary horizontal vessel with a tubular rotor on which are mounted the doubled-walled disks. These hollow disks provide approximately 85% of the total heating surface. It has been used commercially for drying ABS, PCs, polyolefins, and other polymers. Thermascrew is a hollow screw, jacketed trough dryer that provides three to four times more heat transfer surface than simple jacketed screw conveyors and six times more than water-cooled drums. In either

(a) Solidaire

(b) Continuator

(c) Torusdisc

(d) Thermascrew

FIGURE 41.4 Some recent proprietary polymer dryers. (From Bepex Corporation, CEP, 79(4):5 (1983). With permission.)

continuous or batch operation, it provides efficient and uniform heating, cooling, evaporating, or other processing. It can operate in either a pressure or a vacuum environment. Polyester and polyolefins are among the materials dried with good thermal efficiency in such devices. Among recent developments in dryers is the Yamato band FBD [11]. This is a modified FBD having all the components of a standard FBD with an additional carriage means with multiple blades mounted thereon and projecting there from for effective fluidization and transportation of materials (Figure 41.5). The carriage includes a crank mechanism for effecting a circular or linear movement of the blades. It is driven in such a manner that the blades scratch

and fluidize the material being treated in cooperation with a heated gas. The fluidized bed is thus carried or conveyed toward the outlet port. The blades on the carriage extend in close proximity to the surface of the gas distributor plate. The blades may be straight, curved, or T-shaped. Such dryers can be used to process a variety of difficult-to-treat materials, e.g., slurries and materials containing solidified portions, as well as those having a high degree of cohesion or adhesion and/or containing lumps. The spouted beds (SBs) can also be used to dry polymer beads. It is an efficient solid–gas contactor. In the conventional SB there is dilute-phase pneumatic transport of particles entrained by the spouting jet in the central core region and dense-phase downward motion of the particles along the annular region

X

XI

XI IIc

17

2c

X

FIGURE 41.5 Yamato band fluidized bed dryer.

ß 2006 by Taylor & Francis Group, LLC.

bounded by the cylindrical wall. Thus, the particle– gas contact is cocurrent in the core (or spout) and countercurrent in the downcomer or annulus. This characteristic recirculatory motion of the particles enables one to control the residence time of particles within wide limits by letting the particles go through a desired number of cycles prior to their withdrawal. With both batch and continuous operations possible along with the various modifications available, these beds have a strong potential as postdryers in polymer drying [12].

41.4 TYPICAL DRYING SYSTEMS FOR SELECTED POLYMERS This section discusses briefly the drying of selected large-scale polymers. It is important to note the data in Table 41.1, which gives permissible moisture levels in various commodity resins [13].

41.4.1 DRYING OF POLYOLEFINS 41.4.1.1 Polypropylene PP is produced by a variety of processes, most of them by a diluent phase propylene polymerization utilizing a Ziegler–Natta-activated titanium trichloride catalyst in the presence of low- to high-boiling hydrocarbons. Residual catalyst removal followed by hydrocarbon slurry centrifugation is the immediate upstream operation prior to thermal drying. Hexane is the solvent used in the major PP processes in operation today. As a result these polymers are solvent wet. Many plants operate with two resin varieties, e.g., homopolymers and copolymers. Each requires

a different drying approach owing to different centrifuge cake-handling characteristics. Homopolymer cakes, although somewhat tacky, are much less than high-ethylene-content copolymer cakes, which tend to agglomerate, form lumps, adhere to surfaces, and so on. Considering capital cost, it is desirable to have a single dryer line for both resins. Consequently, initial dryer selection becomes a critical issue because of the feed flexibility required [14]. Both polymer centrifuge cakes are discharged hot (50 to 608C), with diluent contents as high as 35 (wb) or 53.2% (db). Between 35 (wb) and 5% (wb), most homopolymers and copolymers exhibit constant-rate drying characteristics; i.e., all moisture evaporation is from the particle surface. Drying is rapid, and residence time is heat transfer-dependent. Since the product temperature limit for these polymers is 100 to 1108C, the solvent boiling point has a definite effect on dryer selection. Historically, again, rotary dryers are used. During the 1960s and early 1970s, a twostage system of paddle-type dryers was used successfully. This consisted of a first stage or surface solvent dryer, with the characteristics of very high agitation, high heat transfer, and short residence time. The second stage, or bound moisture dryer, consisted of a device with low agitation, low heat transfer, and long resistance. Each dryer is provided with a recycle purge gas system to aid in controlling dew points and increase dryer efficiency. The gas flow is minimized; the amount used is that required to give a partial pressure necessary to achieve required product moistures. With the emergence of very high capacity PP polymer lines and high-boiling point solvents, various types of dryers and drying systems have evolved. One reason for the advent of the new technologies in PP

TABLE 41.1 Percentage by Weight of Permissible Moisture (db) in Some Selected Polymer Resins Material

ABS resin Acrylic Cellulosics Ethyl cellulose Nylon Polycarbonate Polyethylene Low density High density Polypropylene Polystyrene Vinyl

ß 2006 by Taylor & Francis Group, LLC.

Permissible Moisture Injection (%)

Extrusion (%)

0.10–0.20 0.02–0.10 Max. 40 0.10 0.04–0.08 Max. 0.02 — 0.05–0.10 0.05–0.10 0.05 0.10 0.08

0.03–0.05 0.02–0.04 Max. 30 0.04 0.02–0.06 0.02 — 0.03–0.05 0.03–0.05 0.03–0.10 0.04 0.08

Drying Temperature (8C)

77–88 71–82 66–88 77–88 71 121 — 71–79 71–104 71–93 71–82 60–88

drying is the economic recovery of the flammable hydrocarbon solvents. Another reason is that PP has to be dried to a very low volatiles level and the final drying requires the drying gas to have an extremely low dew point. Usually nitrogen gas is used as a drying gas in a closed-cycle drying system. Figure 41.6 shows this by low and high gas dew point product moisture points indicating relative drying times. Reduction in recycle gas dew point is required to remove evaporated solvent first and, especially in mass transfer limited drying, reduce solvent partial pressure to increase the overall drying rate. Accordingly, the low dew point case normally uses 208C hexane dew point recycle nitrogen gas yielding the lowest residence time but at a higher energy expense than the higher dew point case. Since this is an expensive part of the flow sheet, refrigeration costs become a factor and recycled gas should be minimized. It is in this region that residence times vary from 30 min to over 1 h, depending on recycle gas dew point and polymer-drying characteristics; consequently, a controlled residence time dryer is desired. It is also desirable that this postdryer has an independent recycling loop to minimize energy consumption and maximize process control. (0.532) Dry basis

Based on these fundamentals, a two-stage flash or fluid-bed drying system has been developed for PP drying. The flash dryer disperses the feed cake in a venturi throat, with hot recycled gas breaking the cake and drying it to about 5% (wb) level. Final drying is carried out in the fluid bed in a nitrogen atmosphere. The solvent is recovered by a scrubber– condenser system. The PP, after being dried in the fluid bed, contains an extremely low level of solvent (e.g., hexane or heptane), typically 500 ppm. A very recent development in terms of heat economy and corrosion control is the use of a spiral DRT dryer (Drallrohr Trocking) as a predryer in place of the flash predryer [9] in flash–plug-flow FBD systems. The gas/solid ratio is approximately 0.2 in these types of dryers, compared with 1.0 in the flash dryers. Another important advantage of DRT dryer as a predryer in PP drying is its suitability in a corrosive environment. A persistent problem often seen in PP and high-density polyethylene (HDPE) manufacturing plants is the deterioration of the equipment due to free chlorides. The chlorides result from the deactivation of the activated catalysts with alcohol. Stress corrosion cracking is the most common corrosion phenomenon that results from the catalyst’s chloride

Flash 0.35

0.30

Post fluid-bed dryer, plug flow with gas Dew point control DRT/backmix FB

Fraction hexane. wet basis

0.25

0.20

0.15

Minimum fluidization homopolymer

0.10

Minimum fluidization copolymer

0.05

Break point: constant rate/falling rate Product diluent

0.02 0.0005

Low dew point High dew point drying gas drying gas Drying rate is mass transfer controlled

Drying rate is heat transfer controlled Time

FIGURE 41.6 Typical polymer-drying curve of polypropylene.

ß 2006 by Taylor & Francis Group, LLC.

remnants. The corrosion rate becomes remarkable when the product contains a very small amount of water. To prevent such corrosion, a neutralization liquid is condensed. Also, part of the equipment is sometimes coated with an acid-proof resin. Despite the best neutralization techniques, chlorides are always present and cause significant equipment deterioration unless precautionary measures are taken prior to the drying system design. Therefore, it is essential that the constant-rate drying period be run in an atmosphere that precludes potential hexane vapor condensation. DRT has advantages in such a corrosive environment since it operates with low gross heat input and product inventory [15]. 41.4.1.2 High-Density Polyethylene HDPE is usually presented to the drying system from a decanter centrifuge, either water wet or wet with solvent (e.g., hexane or heptane). The product temperature limit for this polymer is in the range of 1008 to 1108C. This influences dryer selection. Similar to PP drying, HDPE drying technology progressed along the same route because of similarity in the upstream physical operations prior to drying, as well as similar physical characteristics. Similar to PP, drying of HDPE is best done in a multistage system, especially on FD/CFBD centrifugal FBD system (Figure 41.7).

41.4.2 DRYING OF POLYVINYL CHLORIDE 41.4.2.1 Emulsion Polyvinyl Chloride Historically, spray dryers were used because of their ability to produce a constant quality product under full operational control. Normally, emulsion PVC (E-PVC) is water wet in a slurry and dried to a powder in one single-pass operation with high capacities. The slurry is atomized using a rotary wheel or nozzle. Evaporation takes place under constant and falling rate conditions. Rapid evaporation maintains a low temperature of the spray droplets so that high dry gas temperature can be applied without affecting polymer quality. Conical spray-dryer chambers are commonly employed. An improvement over the conventional opencycle adiabatic spray dyers for E-PVC is the recycle exhaust spray dryer. In this type, up to 50% of the exhaust stream is recycled to preheat the supply air makeup from the atmosphere. An improvement with respect to thermal efficiency is the two-stage dryer. This involves operating a spray dryer with a fluid-bed afterdryer. By adopting a two-stage layout with a fluid bed, powder is taken out of the spray dryer at a lower outlet temperature with higher moisture content. The cooler but higher moisture content powder is transferred to the fluidized bed, where the drying is completed to the desired

Fan Wet condenser

Solvent cooling system Fan Heater

Feed inlet Solvent cooling system

Solidaire

Wet condenser

Continuator Product discharge

FIGURE 41.7 Drying system for polypropylene and polyethylene.

ß 2006 by Taylor & Francis Group, LLC.

Heater

extent by controlling the residence time. The overall heat consumption of the two-stage process is reported to be about 20% lower than the corresponding singlestage dryer. A recent improvement over the above-mentioned two-stage drying system for drying E-PVC is a spray dryer with an integrated fluid bed. The basic concept in this type of dryer is to avoid contact of the wet powder with any metal surface in the primary drying stage by transferring wet powder directly into a fluidized powder layer (second drying stage). To achieve this requirement, the fluid bed is integrated at the base of the spray-drying chamber. Another improvement in the design of dryers for drying E-PVC and polyethylene is a dispersion dryer that operates on what is known as the jet-drying principle and is offered by Fluid Engineering International (London) under the name Jet-ODryers. It is a pneumatic dryer of toroidal design developed from jet-milling principles. It has no moving parts. It is claimed to offer the following advantages over conventional flash drying: (1) much shorter drying times and (2) combination of drying and fine grinding in a single operation to deagglomerate the materials.

41.4.2.2 Suspension Polyvinyl Chloride Suspension-grade PVC (S-PVC) and its copolymers have many possible drying options. Since polymerization of this polymer is done by using water as the dispersion liquid, water and some monomers are present in the wet cake. Usually, wet cake with 20 to 25% water (wb) is obtained after centrifuging slurries. Most of the water contained in the centrifuge cakes is typically free moisture, with only a minor part bound moisture. Moreover, the bound moisture in typical S-PVC is held relatively loosely and is fairly easy to dry off. Traditionally, a rotary dryer system was applied to achieve a final moisture content of 0.2%. Rotary dryers for the purpose are typically 1 to 2 m diameter and 15 to 30 m long, rotating at 4 to 8 rpm. Centrifuged S-PVC is introduced at the upper and cocurrent with the hot gas flow. Gas flow contact is enhanced by the use of longitudinal lifting flights attached inside the drum wall, the purpose of which is to shower the material through the hot gas stream. Recently, a two-stage flash fluid-bed system has appeared in the market that is preferable to a rotary drying system (Figure 41.8). Most of the surface water is removed in the flash dryer stage within

Cyclone

Flash dryer Fluid bed

Feed Heater

Heater

Blower

FIGURE 41.8 Flash fluid-bed dryer for suspension-grade polyvinyl chloride.

ß 2006 by Taylor & Francis Group, LLC.

Air filter

Blower

seconds; some surface and all of the bound moisture are removed in the fluid-bed stage by holding the product at suitable drying temperatures for about 30 min. Usually, wet cake with 22 to 25% water (wb) is fed to the dryer by a screw conveyor and enters by a special mill that deagglomerates the feed material, disperses it into the drying airstream, and accelerates it to duct velocity. The mill should handle the feed gently, as PVC is sensitive to high shear. The flash dryer stage discharges the product to the fluid bed between 2 and 8% water, with the intermediate moisture chosen according to the basis of optimization used. Flash dryer air temperature may be typically 1808C at the inlet and 608C at the outlet, depending on moisture content and the drying characteristics of the particular resin. As already mentioned, S-PVC is sensitive to shear; for this reason, dry duct velocities are kept low (around 15 m/s) and care is exercised in handling the dried product. It is possible to arrive at the required product final moisture content by flash drying alone but, because of the residence time available in the flash dryer, the high temperatures required give an unsatisfactory product. Moreover, there is a very wide range of S-PVC homopolymers, varying in molecular weight, particle size, and other properties, and all have different dewatering and drying characteristics. The benefits achieved in a two-stage system are its ability to handle upsets in inlet moisture in the flash dryer, a lower energy cost, and a relatively simple scale-up.

Modest improvements with respect to the most economical drying of S-PVC are a continuous, single-stage, contact fluidized bed dryer, as shown in Figure 41.9. In this type of dryer, the concepts of back-mixed fluidization and plug-flow fluidization are advantageously combined in a single unit. A broad residence time distribution is obtained in a back-mixed fluid bed in which the bed itself has a relatively small length/width ratio. In performance it can be compared with an agitated tank provided with overflow, inasmuch as the vigorous mixing inside the fluid bed will result in a uniform temperature and constant average moisture content of the particles throughout the entire bed. The product discharged from this back-mixed fluid bed has the same temperature and moisture content as the bulk material inside the fluid bed. Further, because of the excellent heat and mass transfer between the fluidized particles and the drying air, equilibrium is reached between the exhaust air and the product inside the bed. This type of fluid-bed drying concept is found to be very suitable for drying surface moisture when residence time has no impact on the drying performance. After the mixed-bed section, a plug-flow section is provided in which the final drying of PVC takes place. This section is fairly small compared with the backmixed section and is usually obtained by dividing the fluid bed into compartments. This concept is particularly advantageous for drying bound moisture from heat-sensitive materials since the residence time is controlled within the narrow limits and a distinct

Cyclone

Contact fluid bed (back-mixed part)

Air filter

Plug flow

Heater Product

FIGURE 41.9 Contact fluidizer for suspension-grade polyvinyl chloride.

ß 2006 by Taylor & Francis Group, LLC.

moisture profile can be obtained along the length of the unit because of a very low degree of back-mixing. In this type of drying system for S-PVC, wet PVC cake is usually transported from the decanter centrifuge by a screw feeder to the product distributor of the back-mixed fluid-bed section. It then flows through an overflow weir into the plug-flow section where the final drying takes place. Finally, the product is discharged through the discharge weir arrangement. The back-mixed section of the unit is provided with heating panels; no heating panels are provided in the plug-flow section, partly because the cost cannot be justified and partly because of the tendency for electrostatic deposits on the heating panel encountered with PVC at low moisture content to decrease the heat transfer coefficient. The contact fluidized bed provided with heating panels appears to have proven to be superior to the flash fluid-bed drying system from the point of view of heat economy and overall savings. The contact fluidizer does have a few limitations. First, it is mandatory that the polymer material be readily fluidizable at a moisture level well above the moisture level in the back-mixed section to avoid defluidization of the bed during upset conditions. Second, the centrifuge cake should not be too sticky and have too much tendency to form agglomerates of the individual polymer particles. In such a case, a flash dryer is better suited as the predrying stage as better disintegration takes place in the venturi section of a flash dryer than in a back-mixed fluid bed. Although a fluid bed as a second-stage dryer gives accurate product temperature control while providing adequate residence time, depending on the predryer load, evaporative load in this stage may be small. This results in a low airflow requirement and makes fluidization more difficult. In such cases, a vibrating fluid-bed design is a better alternative. Here, PVC is conveyed by vibration, permitting varying gas speeds without affecting the conveying rate or residence time. Also, with the low airflow rates of the vibrating fluid bed, the fines pickup problem (normally associated with high gas flow rates) is minimized and, as the vibration is at a low frequency, the overall effect of the gas and vibration is to transport the product gently, minimizing damage. The vibrating FBD must be among the most important but underutilized dryer of all granular products. During fluidization of PVC, electrostatic charges arise of such magnitude that they affect the hydrodynamics of the system. This is disadvantageous for transfer processes in the bed, e.g., for heat transfer between the heating surface and the bed. This is a difficult problem in a fluidized bed because of intensive movement of particles and frequent interparticle

ß 2006 by Taylor & Francis Group, LLC.

and particle–wall contact. Although charge generation cannot be prevented, one can limit its magnitude (and try to increase its dissipation) by changing process conditions. One method is the addition of a small portion of fines to the bulk; this results in the splitting of agglomerates and disappearance of the particulate layer at the walls. As a result, the bed regains its original parameters, which assure intensive running of processes in the bed. 41.4.2.3 Vinyl Chloride–Vinyl Acetate Copolymer There is a wide difference in the difficulty of drying vinyl chloride–vinyl acetate (VC–VA) copolymers according to the degree of VA content in polymer and extent of polymerization. If the heat-resisting property of polymer is too low to use the hot air at a high temperature (even if the hydroextracting degree in the former stage is generally good, e.g., 13 to 17% wb), then it is difficult to remove VA monomer. As a result, the necessary retention time becomes longer compared with that of PVC-homo. The equipment recommended for this application is single-stage batch fluidized bed dryer (B-FBD) or a flash B-FBD system. For proper selection it is necessary to make a detailed study on the basis of specified conditions. An important factor that should be taken into account while drying PVC is the corrosion of the equipment due to the monomer chloride. Monomer chloride, which is always present in the wet cake, induces pitting corrosion and stress corrosion cracking in parts where the powdery materials are processed. Those parts, therefore, are made of AISI316L and are partially coated with an acid-resistant coating. It is indispensable to make periodic inspection of the corrosion condition and to make timely replacement of the necessary spare parts. Preventive maintenance is imperative to successful operation. Another important consideration in drying PVC is the emission of VC. U.S. EPA emission limitations of