17 Drying Barry Fox, Giovanni Bellini, and laura Pellegrini SECTION I: INDIRECT DRYING (by Giovanni Bellini and laura Pellegrini) 1.0 INTRODUCTION indt The drying operation is often the final step of manufacturing process rect drying will be discussed in this section; it is the process of removing iquid by conductive heat transfer. Sometimes drying is a part of the manufacturing process itself, as in the case of seasoning of timber or in paper making, but generally, the reasons for carrying out a drying operation are transport To ensure a prolonged storage life To make a material more suitable for handling To avoid presence of moisture that may lead to corrosion To provide the product with definite properties The type of raw material is of extreme importance in the drying process; for instance, to retain the viability and the activity of biological materials such as blood plasma and fermentation products, the operation is carried out at very low temperatures, while more severe conditions can be applied to foodstuffs 706
Drying Barry Fox, Pellegrin i Giovanni Bellini, and Laura SECTION I: INDIRECT DRYING (bu Giovanni Bellini and Laura Pellegrini) 1.0 INTRODUCTION The drying operation is often the final step of a manufacturing process. Indirect drying will be discussed in this section; it is the process of removing liquid by conductive heat transfer. Sometimes drymg is apart ofthe manufacturing process itself, as in the case of seasoning oftimber or in paper making, but generally, the reasons for carrying out a drying operation are: To reduce the cost of transport To ensure a prolonged storage life To make a material more suitable for handling To avoid presence of moisture that may lead to corrosion To provide the product with definite properties The type of raw material is of extreme importance in the drying process; for instance, to retain the viability and the activity of biological materials such as blood plasma and fermentation products, the operation is carried out at very low temperatures, while more severe conditions can be applied to foodstuffs. 706
Drying 70 If it is possible to remove moisture mechanically, this will always be more economical than removing it by evaporation. However, it will be assumed in the following that, for the type of raw material and its final use, the removal of volatile substances is carried out by heat 2.0 THEORY Drying Definition. Drying is a unit operation in which a solvent generally water, is separated from a solution, semisolid material or cake/solid pastes by evaporation In the drying process, the heat is transferred simultaneously with mass, but in the opposite direction Drying process Description. The moisture content of a material usually expressed as a weight percentage on a dry basis. The moisture may esent as Free moisture. This is the liquid in excess of the equilibrium moisture content for the specific temperature and humidity condition of the dryer. Practically, it is the liquid content removable at a given temperature and humidity Bound moisture. This is the amount of liquid in the solids that xhibits a vapor pressure less than normal for the pure liquid In the drying of materials it is necessary to remove free moisture from the surface as well as bound moisture from the interior. The drying characteristics of wet solids can be described by plotting the rate of drying against the corresponding moisture content. A typical drying curve is shown in Fig. I and it can easily be seen that this is subdivided into four distinct sections The curved portion, AB, is representative of the unsteady state period during which the solid temperature reaches its steady state value, ts. AB may occur at decreasing rate as well as at the increasing rate shown The critical moisture content is thus identified as the average moistur content of the solid at the instant the first increment of dry area appears the surface of solid The critical moisture co the ease of moisture movement through the solid, and hence, upon the pore structure of the solid, sample thickness and drying rate. Segment BC is the constant-rate per During this period, the drying is controlled simultaneously by heat and mass transfer applied to a liquid- gas interface in dynamic equilibrium with a bulk
Drying 707 If it is possible to remove moisture mechanically, this will always be more economical than removing it by evaporation. However, it will be assumed in the following that, for the type of raw material and its final use, the removal of volatile substances is camed out by heat. 2.0 THEORY Drying Definition Drying is a unit operation in which a solvent, generally water, is separated from a solution, semisolid material or cakeholid pastes by evaporation. In the drymg process, the heat is transferred simultaneously with the mass, but in the opposite direction. Drying Process Description. The moisture content of a material is usually expressed as a weight percentage on a dry basis. The moisture may be present as: Free moisfure. This is the liquid in excess ofthe equilibrium moisture content for the specific temperature and humidity condition of the dryer. Practically, it is the liquid content removable at a given temperature and humidity. Bound moisture. This is the amount of liquid in the solids that exhibits a vapor pressure less than normal for the pure liquid. In the drying of materials it is necessary to remove free moisture from the surface as well as bound moisture from the interior. The drying characteristics of wet solids can be described by plotting the rate of drying against the corresponding moisture content. A typical drying curve is shown in Fig. 1 and it can easily be seen that this is subdivided into four distinct sections: The curved portion, AB, is representative of the unsteady state period during which the solid temperature reaches its steady state value, ts. AB may occur at decreasing rate as well as at the increasing rate shown. The critical moisture content is thus identified as the average moisture content of the solid at the instant the first increment of dry area appears on the surface of solid. The critical moisture content depends upon the ease of moisture movement through the solid, and hence, upon the pore structure of the solid, sample thickness and drying rate. Segment BC is the constant-rate period. During this period, the drying is controlled simultaneously by heat and mass transfer applied to a liquid-gas interface in dynamic equilibrium with a bulk gas phase
708 Fermentation and Biochemical Engineering Handbook A Mass of liquid mass of dry solid Figure 1. Drying rate curve Moisture flow from within the material to the surface is fast enough to maintain a completely wet surface. The surface temperature reaches the wet bulb temperature. The rate of drying can be expressed where dw/dp is the rate of drying, i. e, change in moisture with time; Kp is the mass transfer coefficient, ps is the saturation vapor pressure of the liquid at the surface temperature, ts; and pa is the partial pressure of water vapor In addition, the following equation also applies dw ha do 1
708 Fermentation and Biochemical Engineering Handbook T A I PON XI 6 IC Y M E A f Mass of liquid / mass of dry solid Figure 1. Drying rate curve. Moisture flow from within the material to the surface is fast enough to maintain a completely wet surface. The surface temperature reaches the wetbulb temperature. The rate of drying can be expressed as: Eq. 1 where dW/i+ is the rate of drying, Le., change in moisture with time; Kp is the mass transfer coefficient, ps is the saturation vapor pressure of the liquid at the surface temperature, fs; and pa is the partial pressure of water vapor. In addition, the following equation also applies: Eq. 2 (fa - ts) = Kp (ps - pa) dW ha dO A -=-
Drying 70 where a is the latent heat of vaporization, ha is the heat transfer coefficient, ta is the dry bulb temperature of the air and ts is the temperature of the product By integrating Eq. 2, it is possible to derive the drying time in the constant rate period. Equation 2 is derived for heat transfer to the material eing dried by circulating air. When large metal sheets or trays are close to the product, it is not possible to ignore the conduction and radiation contribution to heat transfer. In this case, the solid temperature is raised above the air wet-bulb temperature and eq 2 becomes 3W:加A(-)+x-(-)+-2°(4-s where Al, A2, A3 are the solid surfaces, respectively convection conduction and radiation heat-transfer, tc is the temperature of the heat surface for conductivetransfer, Fis a view factor, depending on the geometry E is the emissivity of the surface, 8 is the Stefan-Boltzmann constant, Tr is the absolute temperature of the radiating surface and Ts is the absolute temperature of the product surface. The increase in Ts allows the drying at anincreased rate, both during the constant rate and the first falling rate period At the end of the constant-rate period, the movement of the liquid to the solid surface becomes insufficient to replace the liquid being evaporated. The critical moisture content is thus identified as the average moisture content of the solid at the instant the first increment of dry area appea the surface of the solid. The critical moisture content depends upon the ease of moisture movement through the solid and, hence, upon the port structure of the solid, sample thickness and drying rate Segment CD is the first falling- rate drying period. It is the period between the appearance of the first dry area on the material surface and the disappearance of the last liquid-wet area; drying occurs at a gradually reduced rate. At point D, there is no significant area of liquid saturated surface During the phase CD, Eq. 2 is still applicable to the moisture removal rate, provided that ts and ps are suitably modified and account is taken of the partial dryness of the surface Segment DE is the second falling-rate. The moisture content continues to fall until it reaches the equilibrium moisture content, E. The equilibrium moisture content is reached when the vapor pressure over the solid is equal to the partial pressure of vapor in the atmosphere. This equilibrium condition is independent of drying rate. It is a material property. Only hygroscop materials have an equilibrium moisture content
Drying 709 where A is the latent heat of vaporization, ha is the heat transfer coefficient, ta is the dry bulb temperature ofthe air and ts is the temperature ofthe product surface. By integrating Eq. 2, it is possible to derive the drying time in the constant rate period. Equation 2 is derived for heat transfer to the material being dried by circulating air. When large metal sheets or trays are close to the product, it is not possible to ignore the conduction and radiation contribution to heat transfer. In this case, the solid temperature is raised above the air wet-bulb temperature and Eq. 2 becomes: dW A1 hc A2 FA3 E6 Eq. 3 - = ha-(ta- ts)+ -(tc- ts)+ -(Tr4 - Ts4) do a a a where Al, A2, A3 are the solid surfaces, respectively, for convection, conduction and radiation heat-transfer, tc is the temperature of the heat surface for conductive transfer, Fis a view factor, depending on thegeometry, E is the emissivity of the surface, S is the Stefan-Boltztnann constant, Tr is the absolute temperature of the radiating surface and Ts is the absolute temperature of the product surface. The increase in Ts allows the drying at an increased rate, both during the constant rate and the first falling rate period. At the end of the constant-rate period, the movement of the liquid to the solid surface becomes insufficient to replace the liquid being evaporated. The critical moisture content is thus identified as the average moisture content of the solid at the instant the first increment of dry area appears on the surface of the solid. The critical moisture content depends upon the ease of moisture movement through the solid and, hence, upon the port structure of the solid, sample thickness and drying rate. Segment CD is the first falling-rate drying period. It is the period between the appearance of the first dry area on the material surface and the disappearance of the last liquid-wet area; drying occurs at a gradually reduced rate. At point D, there is no significant area of liquid saturated surface. During the phase CD, Eq. 2 is still applicable to the moisture removal rate, provided that ts andps are suitably modified and account is taken of the partial dryness of the surface. Segment DE is the second falling-rate. The moisture content continues to fall until it reaches the equilibrium moisture content, E. The equilibrium moisture content is reached when the vapor pressure over the solid is equal to the partial pressure of vapor in the atmosphere. This equilibrium condition is independent of drying rate. It is a material property. Only hygroscopic materials have an equilibrium moisture content
710 Fermentation and Biochemical Engineering Handbook the equilibrium moisture content is essentially zero at all temperatures and humidities. Equilibrium moisture content is particularly important in drying because it represents the limiting moisture content for given conditions of humidity and temperature. The mechanisms of drying during this phase are not completely understood, but two ideas can be considered to explain the physical nature of this process one is the diffusion theory and the other the capillary theory Diffusion Mechanism. In relatively homogeneous solids, such as wood, starch, textiles, paper, glue, soap, gelatin and clay, the movement of moisture towards the surface is mainly governed by molecular diffusion and, therefore follows Ficks La Sherwood and Newman gave the solution of this equation in the hypothesis of an initial uniform moisture distribution and that the surface is dry; the following expression is derived ( for long drying times) Eq1.4 Bm两 where dw/dois the rate of drying during the falling rate period, D is the liquid diffusivity of the solid material, L is the total thickness of the solid layer thickness through which the liquid is diffusing, W is the moisture content of the material at time, o, and we is the equilibrium moisture content under the prevailing drying conditions. Equation 4 neglects capillary and gravitational Capillary Model. In substances with a large open-pore structure and in beds of particulate material, the liquid flows from regions of low concentration to those of high concentration by capillary action. based on this mechanism, the instantaneous drying rate is given dw h(ta-ts)(W-We Eq. 5 Do 2pL IWo-w where o is the density of the dry solid and wo is the moisture content when diffusion begins to control Most biological materials obey Eq 4, while coarse granular solids such as sand, minerals, pigments, paint, etc, obey Eq. 5 Shrinkage and Case Hardening. When bound moisture is removed from rigid, porous or nonporous solids they do not shrink appreciably, bur colloidal nonporous solids often undergo severe shrinkage during drying This may lead to serious product difficulties; when the surface shrinks against
710 Fermentation and Biochemical Engineering Handbook For non-hygroscopic materials, the equilibrium moisture content is essentially zero at all temperatures and humidities. Equilibrium moisture content is particularly important in drying because it represents the limiting moisture content for given conditions of humidity and temperature. The mechanisms of drymg during this phase are not completely understood, but two ideas can be considered to explain the physical nature of this processone is the diffusion theory and the other the capillary theory. Diffusion Mechanism. In relatively homogeneous solids, such as wood, starch, textiles, paper, glue, soap, gelatin and clay, the movement of moisture towards the surface is mainly governed by molecular diffusion and, therefore, follows Ficks' Law. Sherwood and Newman gave the solution of this equation in the hypothesis of an initial uniform moisture distribution and that the surface is dry; the following expression is derived (for long drying times): Eq. 4 where dW/dq+is the rate ofdrying during the falling rate period, D is the liquid difisivity of the solid material, L is the total thickness of the solid layer thickness through which the liquid is diffusing, W is the moisture content of the material at time, 0, and We is the equilibrium moisture content under the prevailing drying conditions. Equation 4 neglects capillary and gravitational forces. Capillary Model. In substances with a large open-pore structure and in beds of particulate material, the liquid flows from regions of low concentration to those of high concentration by capillary action. Based on this mechanism, the instantaneous drying rate is given by: Eq. 5 dW h (ta- ts) (W- We) D0 2p L 1 (Wo- We) -- - where 9 is the density of the dry solid and Wo is the moisture content when diffusion begins to control. Most biological materials obey Eq. 4, while coarsegranular solids such as sand, minerals, pigments, paint, etc., obey Eq. 5. Shrinkage and Case Hardening. When bound moisture is removed from rigid, porous or nonporous solids they do not shrink appreciably, but colloidal nonporous solids often undergo severe shrinkage during drying. This may lead to serious product difficulties; when the surface shrinks against
Drying 711 a constant volume core, it causes the material to warp, check, crack or therwise change its structure. Moreover, the reduced moisture content in the hardened outer layer increases the resistance to diffusion. In the end, the superficial hardening, combined with the decrease in diffusive movement, make the layer on the surface practically impervious to the flow of moisture, either as liquid or vapor. This is called case hardening All these problems can be minimized by reducing the drying rate, thereby flattening the moisture gradient into the solid. Since the drying behavior presents different characteristics in the two periods--constant-rate and falling-rate--the design of the dryer should recognize these differences, i.e substances that exhibit predominantly a constant-rate drying are subject to different design criteria than substances that exhibit a long falling-rate period Since it is more expensive to remove moisture during the falling-rate period than during the constant-rate one, it is desirable to extend as long possible the latter with respect to the former. Particle size reduction is a practical way to accomplish this because more drying area is created An analysis of the laws governing drying is essential for a good dryer design, therefore, it is important to note that, due to the complex nature of solid phase transport properties, only in a few simple cases can the drying rate (and drying time)be predicted with confidence by the mathematical expres sions reported above. In these cases, one usually deals with substances that exhibit only, or primarily, constant-rate drying For materials that present a non-negligible falling-rate period, the of specific mathematical equations is subject to a high number of uncert ties and simplifying assumptions are generally required It is clear that the purely mathematical approach for designing a drying plant is not possible, given the present state of knowledge 3.0 EQUIPMENT SELECTION Several methods of heat transfer are used in the dryers where all the heat for vaporizing the solvent is supplied by direct contact with hot gases and heat transfer by conduction from contact with hot boundaries or by radiation from solid walls is negligible, the process is called adiabatic, ordirect drying In indirect or nonadiabatic drying the heat is transferred by conduc tion from a hot surface first to the material surface and then into the bulk This chapter discusses only indirect drying The problem of equipment selection can be very complex; different factors must be taken into consideration, for example, working capacity, ease of cleaning, hazardous material, dryer location and capital cost(see Fig. 2)
Drying 711 a constant volume core, it causes the material to warp, check, crack or otherwise change its structure. Moreover, the reduced moisturecontent in the hardened outer layer increases the resistance to diffusion. In the end, the superficial hardening, combined with the decrease in diffusive movement, make the layer on the surface practically impervious to the flow of moisture, either as liquid or vapor. This is called case hardening. All these problems can be minimized by reducing the drylng rate, thereby flattening the moisture gradient into the solid. Since the drying behavior presents different characteristics in the two periods-constant-rate and falling-rate-the design of the dryer should recognize these differences, Le., substances that exhibit predominantly a constant-rate drying are subject to different design criteria than substances that exhibit a long falling-rate period. Since it is more expensive to remove moisture during the falling-rate period than during the constant-rate one, it is desirable to extend as long as possible the latter with respect to the former. Particle size reduction is a practical way to accomplish this because more drying area is created. An analysis of the laws governing drying is essential for a good dryer design, therefore, it is important to note that, due to the complex nature of solid phase transport properties, only in a few simple cases can the drying rate (and drying time) be predicted with confidence by the mathematical expressions reported above. In these cases, one usually deals with substances that exhibit only, or primarily, constant-rate drying. For materials that present a non-negligible falling-rate period, the use of specific mathematical equations is subject to a high number of uncertainties and simplifying assumptions are generally required. It is clear that the purely mathematical approach for designing a drying plant is not possible, given the present state of knowledge. 3.0 EQUIPMENT SELECTION Several methods of heat transfer are used in the dryers. Where all the heat for vaporizing the solvent is supplied by direct contact with hot gases and heat transfer by conduction from contact with hot boundaries or by radiation from solid walls is negligible, the process is calledadiabah'c, or direct drying. In indirect or nonadiabatic drying, the heat is transferred by conduction from a hot surface, first to the material surface and then into the bulk. This chapter discusses only indirect drying. The problem of equipment selection can be very complex; different factors must be taken into consideration, for example, working capacity, ease of cleaning, hazardous material, dryer location and capital cost (see Fig. 2)
712 Fermentation and Biochemical Engineering Handbook The first step refers to the choice of continuous versus batch drying and depends on the nature of the equipment preceding and following the dryer as well as on the production capacity required. In general, only batch dryers will be considered in the following Batch dryers include Fluidized-bed dryers. These may be used when the average particle diameter is s0. 1 mm. (The equipment required to handle smaller particles may be too large to be feasible. Inert gas may be used if there is the possibility of explosion of either the vapor or dust in the air It is easy to carry out tests in a small fluid-bed dryer. Shelf dryers. Theys are usually employed ofr small capacities and when the solvent doesn t present particular problems Vacuum dryers. These are the most-used batch dryers Vacuum dryers are usually considered when Low solids temperature(<40C)must be maintained to prevent heat causing damage to the product or changing its When toxic or valuable solvent recovery is required When air combines with the product, during heating, causing Before starting work on selecting a dryer, it is good practice to collect all the data outlined in Table 1 In vacuum drying, the objective is to create a temperature difference or driving force"between the heated jacket and the material to be dried. To accomplish this with a low jacket temperature, it becomes necessary toreduce the internal pressure of the dryer to remove the liquid/ solvent at a lowervapor pressure. Decreasing the pressure creates large vapor volumes. Economic considerations arising from concems of leakage, ability to condense the solvent, size of vapor line and vacuum pump, affect the selection of the operating pressure. Materials handled in vacuum dryers may range from slurries to solid shapes and from granular, crystalline product to fibrous solids. The characteristics of each type of vacuum dryer is discussed below to help make a proper choice Vertical Vacuum Pan Dryers. The agitated vertical dryer(Fig 3 has been designed for drying many different products which may come from centrifuges or filters. Generally, the body is formed by a vertical cylindrical casing with aflat bottom flanged to the top cover head. The unit is fully heated by an outside half-pipe jacket welded on the cylindrical wall, the bottom and the top head
712 Fermentation and Biochemical Engineering Handbook The first step refers to the choice of continuous versus batch drying and depends on the nature of the equipment preceding and following the dryer as well as on the production capacity required. In general, only batch dryers will be considered in the following. Batch dryers include: Fluidized-bed dryers. These may be used when the average particle diameter is I 0.1 mm. (The equipment required to handle smaller particles may be too large to be feasible.) Inert gas may be used if there is the possibility of explosion of either the vapor or dust in the air. It is easy to carry out tests in a small fluid-bed dryer. Shelf dryers. Theys are usually employed of? small capacities and when the solvent doesn’t present particular problems. Vacuum dryers. These are the most-used batch dryers. Vacuum dryers are usually considered when: Low solids temperature (< 40°C) must be maintained to prevent heat causing damage to the product or changing its nature When toxic or valuable solvent recovery is required When air combines with the product, during heating, causing Before starting work on selecting a dryer, it is good practice to collect all the data outlined in Table 1. In vacuum drying, the objective is to create a temperature difference or “driving force” between the heated jacket and the material to be dried. To accomplish this with a lowjacket temperature, it becomes necessary to reduce the internal pressure of the dryer to remove the liquidsolvent at a lower vapor pressure. Decreasing the pressure creates large vapor volumes, Economic considerations arising from concerns of leakage, ability to condense the solvent, size of vapor line and vacuum pump, affect the selection of the operating pressure. Materials handled in vacuum dryers may range from slurries to solid shapes and from granular, crystalline product to fibrous solids. The characteristics of each type of vacuum dryer is discussed below to help make a proper choice. Vertical Vacuum Pan Dryers. The agitated vertical dryer (Fig. 3.) has been designed for drying many different products which may come from centrifuges or filters. Generally, the body is formed by a vertical cylindrical casing with a flat bottom flanged to the top cover head. The unit is fully heated by an outside half-pipe jacket welded on the cylindrical wall, the bottom and the top head. oxidation or an explosive condition
Drying 713 soivent toxic sold 7 air oxidation flammable medium agitation 7 vacuu fHuld-bed vacuum shell vacuum shelf dryer pan dryer dryer Figure 2. Flowchart for selection of a batch dryer Table 1. Data To Be Assessed Before Attempting Drying Selection Production capacity(kg/h) Initial moisture content Particle size distribution Drying curve Maximum allowable product temperature Explosion characteristics(vapor/air and dust/air) Toxicological properties already gained Moisture isotherms Contamination by the drying gas Corrosion aspects Physical data of the relevant materials
Drying 713 nuximum product temperature .1 nirn 7 flarnniable vapour 7 /q fkr1d:bed '01 LtationrequiredlM gentle agitation 712 ?I shelf dryer medium agitation 7 ,Ino * G3 , vacuum tumbler/ paddle Figure 2. Flowchart for selection of a batch dryer. Table 1. Data To Be Assessed Before Attempting Drying Selection - Production capacity (kgh) - Initial moisture content - Particle size distribution - Drying curve - Maximum allowable product temperature - Explosion characteristics (vapodair and dust/air) - Toxicological properties - Experience already gained - Moisture isotherms - Contamination by the drying gas - Corrosion aspects - Physical data of the relevant materials
714 Fermentation and Biochemical engineering handbook Heated ch。pper Scrape/Agitator Figure 3. Multidry-EV Pan Dryer(Courtesy of COGEM Spa)
71 4 Fermentation and Biochemical Engineering Handbook tator i' I! Figure 3. Multidry-EV Pan Dryer (Courtesy of COGEIMSpA)
Drying 715 The dished head is provided with the appropriate nozzles for feed inle instrumentation, heating or cooling medium, vapor outlet, lamp and rupture disk. The dished head and the cylindrical body are separated by means of a hydraulic system to provide easy access to the vessel for inspection or eaning. A high powered agitator having two crossed arms located at different heights, is designed for processing products that go through a viscous transition phase(high viscosity). The same dryer can be provided with a different agitator, high speed, which is applicable for low to medium viscous products. The agitator can be totally heated. To eliminate possible agglomerates or lumps formed during the drying process, and discharge problems, a chopper device is supplied. The shaft sealing can be either tuffing-box or a mechanical seal a bottom discharge valve for the dry product is hydraulically driven and located in a closed hatch. The geometri l volume of these vertical dryers ranges from a few liters to approximat 500 liters(see Table 3) Table 2. Standard Pan Dryers-Multidry-EV* Cylindrical Geometrical AgitatorInstalled Diam. mm height, mm volume, m3 speed rpm power kW 500 0.3 0.6 5-55 1.2 5-40 1400 2.0 1600 3.0 2-30 l800 1400 5.0 2-28 Courtesy of COGEIM SpA Materials having average-low density(100-500 kg/m)and low- medium viscosities, which require perfect mixing of the dried product, could require another type of vertical dryer Here, a dryer having a truncated-cone casing is used(Fig. 4). The agitator is supplied combining
Drying 715 Dim. mm 700 900 1200 1400 1600 1800 The dished head is provided with the appropriate nozzles for feed inlet, instrumentation, heating or cooling medium, vapor outlet, lamp and rupture disk. The dished head and the cylindrical body are separated by means of a hydraulic system to provide easy access to the vessel for inspection or cleaning. A high powered agitator having two crossed arms located at different heights, is designed for processing products that go through a viscous transition phase (high viscosity). The same dryer can be provided with a different agitator, high speed, which is applicable for low to medium viscous products. The agitator can be totally heated. To eliminate possible agglomerates or lumps formed during the drying process, and discharge problems, a chopper device is supplied. The shaft sealing can be either a stuffing-box or a mechanical seal. A bottom discharge valve for the dry product is hydraulically driven and located in a closed hatch. The geometrical volume ofthese vertical dryers ranges from a few liters to approximately 500 liters (see Table 3). Table 2. Standard Pan Dryers - Multidry-EV* Cylindrical Geometrical height, mm volume, m3 500 0.3 600 0.6 700 1.2 95 0 2.0 1100 3.0 1400 5.0 Agitator speed rpm 10-80 5-55 5-40 3-35 2-3 0 2-28 Installed power kW 11 15 18 30 45 75 Materials having average-low density (1 00-500 kg/m3) and lowmedium viscosities, which require perfect mixing of the dried product, could require another type of vertical dryer. Here, a dryer having a truncated-cone casing is used (Fig. 4). The agitator is supplied combining:
