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 <40T 7 solvent 7 air oxidation ? no r, average paate sire >.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: