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