734 Fermentation and Biochemical Engineering Handbook SECTION II: DIRECT DRYING(by Barry Fox) 1.0 INTRODUCTION The purpose of this chapter is to review various forms of solids dryers and auxiliary components. It is intended to be a practical guide to dryer selection(as opposed to the theory of drying, which is addressed in various technical manuals referenced in the bibliography). From a microscopic iewpoint,the process is simple: water or solvent basically evaporates leaving the solid behind. When viewed macroscopically, it is apparent that the drying process is extremely complicated with many interdependent forces that combine in various dryers to achieve the end result. The information in this article can also help the reader become more familiar with the drying process from beginning to end Drying is the process of removing a liquid from a solid The liquid to be dried can be water or a hydrocarbon based solvent. The solids are usually classified as organic or inorganic, either of which can be completely or partially soluble in the liquid medium. The inorganic materials are generally called salts because they ally soluble in water. Organic materials are more difficult to dry due to temperature sensitivity. When drying, organic materials can stick to walls and cling to themselves resulting in a tacky consistency. Direct drying is the process of removing this liquid via the mechanism of convective heat transfer. The heat input usually takes the form of preheating a carrier medium(such as air, evaporated solvent or an inert gas) that transfers the sensible heat and acts as an absorbent to take away the liquid in the vapor form. The carrier medium can hold a fixed amount of liquid (saturation) at its defined temperature. The solids release the liquid to the carrier medium as a function of saturation and equilibrium In essence, the heated gas has a higher saturation affinity for the liquid in the vapor formthan does the solid at the gas temperature Typical examples of conventional direct dryers are spray, fluid bed, flash, rotary, belt and continuous tray type. In the former three types, the wet solids are suspended in the carrier medium. In the latter three types, the medium passes slowly across the bed of solids. Additionally, there exists some minor tumbling of the solids through the gas stream(carrier medium) There is a nonconventional form of direct drying that is often over looked or possibly unknown to the designers of the process. It is applicable to almost any of the forms of dryers mentioned in this chapter. The method is to use the solvent or liquid that is being dried as the carrier medium for the
734 Fermentation and Biochemical Engineering Handbook SECTION 11: DIRECT DRYING (by Barry Fox) 1.0 INTRODUCTION The purpose ofthis chapter is to review various forms of solids dryers and auxiliary components. It is intended to be a practical guide to dryer selection (as opposed to the theory of drying, which is addressed in various technical manuals referenced in the bibliography). From a microscopic viewpoint, the process is simple: water or solvent basically evaporates leaving the solid behind. When viewed macroscopically, it is apparent that the drying process is extremely complicated with many interdependent forces that combine in various dryers to achieve the end result. The information in this article can also help the reader become more familiar with the drying process from beginning to end. Drying is the process of removing a liquid from a solid. The liquid to be dried can be water or a hydrocarbon based solvent. The solids are usually classified as organic or inorganic, either of which can be completely or partially soluble in the liquid medium. The inorganic materials are generally called salts because they are usually soluble in water. Organic materials are more difficult to dry due to temperature sensitivity. When drying, organic materials can stick to walls and cling to themselves resulting in a tacky consistency. Direct drying is the process of removing this liquid via the mechanism of convective heat transfer. The heat input usually takes the form of preheating a carrier medium (such as air, evaporated solvent or an inert gas) that transfers the sensible heat and acts as an absorbent to take away the liquid in the vapor form. The carrier medium can hold a fixed amount of liquid (saturation) at its defined temperature. The solids release the liquid to the carrier medium as a function of saturation and equilibrium. In essence, the heated gas has a higher saturation affinity for the liquid in the vapor form than does the solid at the gas temperature. Typical examples of conventional direct dryers are spray, fluid bed, flash, rotary, belt and continuous tray type. In the former three types, the wet solids are suspended in the carrier medium. In the latter three types, the carrier medium passes slowly across the bed of solids. Additionally, there exists some minor tumbling of the solids through the gas stream (carrier medium). There is a nonconventional form of direct drying that is often overlooked or possibly unknown to the designers of the process. It is applicable to almost any of the forms of dryers mentioned in this chapter. The method is to use the solvent or liquid that is being dried as the carrier medium for the
Drying 735 heat transfer. In essence, the moisture that is evaporated from the product is recycled and reheated. It replaces air or inert gas and this hot vapor is used to strip off additional liquid from the wet product. The excess vapors are removed via a vent condenser outside of the closed vapor loop. This procedure can be applied in any of the drying processes mentioned here. One advantage to this method of drying is that the product sees only the vapor with which it is already in contact in the liquid state. a possible reason for using this method is product oxidation when air drying. This method may reduce oxidation if the solvent is used. This method is also more energy efficient when solvents are present since the inert gas that is recycled in the former method needs to be reheated after it has been cooled down to condense the solvents 2.0 DEFINITIONS Absolute Humidity--the ratio of mass of vapor(moisture) to mass present in the carrier gas stream. Example: 0.02 pounds of water per pound of air. This number can be used to find the relative humidity on the psychrometric charts. It is also useful for cu quantities in a stream due to such items as products of combustion (when a gas fired heater is used), and evaporation and ambient quantities. This is necessary for calculating condenser or venting amounts Bound Moisture--liquid which is bound to a solid by chemical bonds or physical adsorption in the molecular interstices of the solids Capillary flow-the flow of liquid through the pores of a solid Critical moisture content-the average moisture in the solids when the constant rate drying period ends Diffusion--the process of mass transfer of the liquid from the interstices of the solid to the surface of the solid Dry basis--means of measuring moisture content in terms of moisture content per quantity of dry product, forexample, pounds of water per pound of dry product. (Also see Wet Basis.) Equilibrium moisture content-the limiting moisture content to which a product can be dried under fixed conditions such as temperature humidity and pressure Evaporative cooling-when drying a solid with free or bound moisture the effect of a phase change from the liquid state to the vapor state removes energy from the liquid-solid mass. This results in a reduction of temperature in a nonadiabatic operation, whereas in an
Drying 735 heat transfer, In essence, the moisture that is evaporated from the product is recycled and reheated. It replaces air or inert gas and this hot vapor is used to strip off additional liquid from the wet product. The excess vapors are removed via a vent condenser outside of the closed vapor loop. This procedure can be applied in any of the drying processes mentioned here. One advantage to this method ofdrying is that the product sees onlythe vapor with which it is already in contact in the liquid state. A possible reason for using this method is product oxidation when air drying. This method may reduce oxidation ifthe solvent is used. This method is also more energy efficient when solvents are present since the inert gas that is recycled in the former method needs to be reheated after it has been cooled down to condense the solvents. 2.0 DEFINITIONS Absolute Humid@-the ratio of mass of vapor (moisture) to mass present in the carriergas stream. Example: 0.02 pounds ofwater per pound of air. This number can be used to find the relative humidity on the psychrometric charts. It is also usehl for cumulative quantities in a stream due to such items as products of combustion (when a gas fired heater is used), and evaporation and ambient quantities. This is necessary for calculating condenser or venting amounts. BoundMoisture-liquid which is bound to a solid by chemical bonds or physical adsorption in the molecular interstices of the solids. Capillaryflow-the flow of liquid through the pores of a solid. Critical moisture content-the average moisture in the solids when the constant rate drying period ends. Dijfision-the process of mass transfer ofthe liquid from the interstices of the solid to the surface of the solid. Dry basis-means of measuring moisture content in terms of moisture content per quantity of dry product, for example, pounds of water per pound of dry product. (Also see Wet Basis.) Equilibrium moisture content-the limiting moisture content to which a product can be dried under fixed conditions such as temperature, humidity and pressure. Evaporative cooling-when drying a solid with free or bound moisture, the effect of a phase change from the liquid state to the vapor state removes energy from the liquid-solid mass. This results in a reduction of temperature in a nonadiabatic operation, whereas in an
736 Fermentation and Biochemical Engineering Handbook adiabatic operation of constant heat input, the temperature may drop or more likely it will maintain a level (pseudo-wet bulb) temperature Falling rate period-this is the period of drying where the instantaneous drying rate is constantly decreasing Feed material-this is the description of the material being dried before Final moisture content-the desired product moisture level required after completion of the drying process Free flowing-refers to the feed and product characteristics, as in a free flowing powder. This is the state in which the material being dried would not cling to itself, forming large chunks or possibly bridging in a hopper Free moisture--liquid which is promptly removable due to its availabil- ity at the interface between the surface of the particles(solids) and the gas stream Hygroscopic material-solids having an affinity for liquids due to a hemical or physical attraction between the solids and the liquid Initial moisture contenf-the average moisture contained in the wet material before the start of the drying process. If given in percent, specification of wet or dry basis is necessary Plug flow-a term used to describe the breakup of a continuous process into small batch segments. The term may originate from a reactor tube being filled or plugged with small quantities of material using a piston pump The reactor would process the volume of material in each piston cavity like a small batch, yet when the material is viewed as a large quantity, it appears homogene he term is used in conjunction with semi-continuous operations Product-this is the description ofthe solid material after it has been Relative humidity-the percentage of water vapor in a gas stream relative to its saturation level. Example: 100% relative humidity is the complete saturation of a carrier gas stream, whereby any further vapor cannot be absorbed by the gas and will condense or precipitate out in the liquid phase. There is an equilibrium between the liquid solid mass and the gas stream( carrier medium). This equilibrium is a result of a combination of saturation capability of the medium at a given temperature. At higher temperatures, the carrier medium has
736 Fermentation and Biochemical Engineering Handbook adiabatic operation ofconstant heat input, the temperature may drop or more likely it will maintain a level (pseudo-wet bulb) temperature. Falling rateperiod-this is the period of drying where the instantaneous drying rate is constantly decreasing. Feed material-this is the description ofthe material being dried before it enters the dryer. Final moisture content-the desired product moisture level required after completion of the drying process. Freeflowing-refers to the feed and product characteristics, as in a free flowing powder. This is the state in which the material being dried would not cling to itself, forming large chunks or possibly bridging in a hopper. Free moisture-liquid which is promptly removable due to its availability at the interface between the surface of the particles (solids) and the gas stream. Hygroscopic material-solids having an affinity for liquids due to a chemical or physical attraction between the solids and the liquid. Initial moisture content-the average moisture contained in the wet material before the start of the drying process. If given in percent, specification of wet or dry basis is necessary. Plugflow-a term used to describe the breakup of a continuous process into small batch segments. The term may originate from a reactor tube being filled or plugged with small quantities of material using a piston pump. The reactor would process the volume of material in each piston cavity like a small batch, yet when the material is viewed as a large quantity, it appears homogeneous. The term is used in conjunction with semi-continuous operations. Product-this is the description of the solid material after it has been dried. Relative humidi-the percentage of water vapor in a gas stream relative to it’s saturation level. Example: 100% relative humidity is the complete saturation of a camer gas stream, whereby any further vapor cannot be absorbed by the gas and will condense or precipitate out in the liquid phase. There is an equilibrium between the liquidsolid mass and the gas stream (carrier medium). This equilibrium is a result of a combination of saturation capability of the medium at agiven temperature. At higher temperatures, the camer medium has
Drying 737 a higher saturation limit and therefore, a lower relative humidity, given the same absolute humidity Wet basis--means of measuring moisture content in terms of quantity of moisture per quantity of wet material. For example, if we have 1000 lbs of wet cake with 200 lbs of water our moisture content is 20% on a wet basis and 25% on a dry basis. (See Dry basis. Wet bulb temperature-the dynamic equilibrium temperature attained by a water surface when the rate ofheat transfer by convectionequals the rate of mass transfer away from the surface 3.0 PSYCHROMETRIC CHARTS There are many forms of psychrometric charts available from various technical sources as well as many manufacturers of process equipment who have tailored the chart for use with their equipment. These charts are usef for determining moisture content in the air at a given temperature and relative humidity or wet bulb temperature. The type of information obtainable from these charts depends upon which chart one uses, because each is designed differently. Usually, accompanying the chart is a set of instructions for Please see the references for examples of these charts and the various forms in which they exist 4.0 DRYING THEORY The process of drying solids is usually quantified into three phases 1. Initial adjustment period--this is the stage at which the wet feed material heats up or cools down to the starting drying temperature which is basically referred to as the wet cake temperature. For example, the wet feed is introduced to the heated dryer at ambient temperature During this period the material temperature will start to rise to the wet bulb temperature which may be different from the initial feed temperature. The reason the tempera ture of the wet cake remains low relative to the gas temperature is a phenomenon known as evaporative 2. Constant rate period--this is the stage at which the free moisture is evaporating from the solids at a constant rate
Drying 737 a higher saturation limit and, therefore, a lower relative humidity, given the same absolute humidity. Wet basis-means of measuring moisture content in terms of quantity of moisture per quantity of wet material. For example, if we have 1000 Ibs. of wet cake with 200 Ibs. of water, our moisture content is 20% on a wet basis and 25% on a dry basis. (See Dry basis.) Wet bulb temperature-the dynamic equilibrium temperature attained by a water surface when the rate ofheat transfer by convection equals the rate of mass transfer away from the surface. 3.0 PSYCHROMETRIC CHARTS There are many forms of psychrometric charts available from various technical sources as well as many manufacturers of process equipment who have tailored the chart for use with their equipment. These charts are usehl for determining moisture content in the air at a given temperature and relative humidity or wet bulb temperature. The type of information obtainable from these charts depends upon which chart one uses, because each is designed differently. Usually, accompanying the chart is a set of instructions for use. Please see the references for examples of these charts and the various forms in which they exist. 4.0 DRYING THEORY The process of drying solids is usually quantified into three phases: 1. Initial adjustment period-this is the stage at which the wet feed material heats up or cools down to the starting drying temperature which is basically referred to as the wet cake temperature. For example, the wet feed is introduced to the heated dryer at ambient temperature. During this period the material temperature will start to rise to the wet bulb temperature which may be different fromthe initial feed temperature. The reason the temperature of the wet cake remains low relative to the gas temperature is a phenomenon known as evaporative cooling. 2. Constant rate period-this is the stage at which the free moisture is evaporating from the solids at a constant rate
738 Fermentation and Biochemical Engineering Handbook If one were to measure the temperature of the bed or individual particles of wet solids at this point, thetempera ture would be the wet cake temperature. After the free moisture has evaporated, the cake temperature rises, an indication of the end of the constant rate period. Several stages of this period can occur due to the existence of bound moisture. If bound moisture exists, the energy stream as heat input. As the bonds break, the bound moisture is released and is removed as surface moisture described above. The quantity of molecules of hydration and the temperature which the product must reach in order to break these bonds affect the overall constant drying rate period. One can generally observe a rise in the wet temperature, after the free moisture has evaporated temperature which is required to break the bonds. Thi emperature then becomes the next wet bulb level or isotherm. Several levels of bound moisture may exist during the drying process. (Note: In general, this bound moisture phenomenon occurs mostly with inorganic sal and therefore may not be a major concern of the pharma- ceutical or biochemical industry. 3. Diffusion, or falling rate periodthis is the stage where the rate at which the liquid leaves the solid decreases. The liquid which is trapped inside the particles diffuses to the outside surface of the particle through capillary action The random path which the liquid must take slows down the drying process at this stage. (See Fig 9 for typical graph. 5.0 FUNDAMENTAL ASPECTS OF DRYER SELECTION The starting point in determining how to dry certain products is first to ascertain whether the process will be a batch or continuous operation If the product is manufactured in relatively small quantities and identification of particular size lots is required, than batch mode is usually the route taken. Full accountability may be achieved when batch processing with the proper controls and procedures in place. The full batch of material to be dried must be enclosed in the dryer, a necessity for the equipment should be that the product dries uniformly
738 Fermentation and Biochemical Engineering Handbook If one were to measure the temperature of the bed or individual particles ofwet solids at this point, the temperature would be the wet cake temperature. After the free moisture has evaporated, the cake temperature rises, an indication of the end of the constant rate period. Several stages of this period can occur due to the existence of bound moisture. If bound moisture exists, the energy required to break the bonds is absorbed from the gas stream as heat input. As the bonds break, the bound moisture is released and is removed as surface moisture described above. The quantity of molecules of hydration and the temperature which the product must reach in order to break these bonds affect the overall constant drying rate period. One can generally observe a rise in the wet cake temperature, after the free moisture has evaporated, to the temperature which is required to break the bonds. This temperature then becomes the next wet bulb level or isotherm. Several levels of bound moisture may exist during the drying process. (Note: In general, this bound moisture phenomenon occurs mostly with inorganic salts and therefore may not be a major concern of the pharmaceutical or biochemical industry.) 3. Diffusion, or falling rate period-this is the stage where the rate at which the liquid leaves the solid decreases. The liquid which is trapped inside the particles diffuses to the outside surface of the particle through capillary action. The random path which the liquid must take slows down the drying process at this stage. (See Fig. 9 for typical graph.) 5.0 FUNDAMENTAL ASPECTS OF DRYER SELECTION The starting point in determining how to dry certain products is first to ascertain whether the process will be a batch or continuous operation. If the product is manufactured in relatively small quantities and identification ofparticular size lots is required, than batch mode is usually the routetaken. Full accountability may be achieved when batch processing with the proper controls and procedures in place. The full batch of material to be dried must be enclosed in the dryer. A necessity for the equipment should be that the product dries uniformly
x= lb moisture/Ib dry solid Figure 9. Typical rate-of-drying curve, constant drying conditions When manufacturing large quantities of a product which does not require tight batch controls, a more efficient operation(usually less expen sive)results by drying the product in a continuous or semi-continuous fashion. The product, in one case, can be batch-stored in large vessels and fed at a continuous rate to the dryer. The product is usually dried in small quantities thus requiring a long time to process the entire amount in a smaller, more efficient piece of equipment. In another situation, ideal for continuous operation, the product would be manufactured upstream of the dryer in atrue plug flow manner and transferred to the dryer at a constant rate. In other words, the dryer's capacity matches that of the upstream equipment. This is the most efficient manner 5.1 Batch Direct Dryers Most direct batch dryers are fluid bed types such as those which retain the batch on a screen while pneumatically fluidizing the product. Mechani cally agitated or tumble rotary dryers also exist. If the product is temperature
Drying 739 Figure 9. Typical rate-of-dqing curve, constant drying conditions. When manufacturing large quantities of a product which does not require tight batch controls, a more efficient operation (usually less expensive) results by drymg the product in a continuous or semi-continuous fashion. The product, in one case, can be batch-stored in large vessels and fed at a continuous rate to the dryer. The product is usually dried in small quantities thus requiring a long time to process the entire amount in a smaller, more efficient piece of equipment. In another situation, ideal for continuous operation, the product would be manufactured upstream ofthe dryer in a true plug flow manner and transferred to the dryer at a constant rate. In other words, the dryer’s capacity matches that of the upstream equipment. This is the most efficient manner. 5.1 Batch Direct Dryers Most direct batch dryers are fluid bed types such as those which retain the batch on a screen while pneumatically fluidizing the product. Mechanically agitated or tumble rotary dryers also exist. Ifthe product is temperature
740 Fermentation and Biochemical Engineering Handbook sensitive, the user should consider a vacuum dryer as an alternative. Vacuum or lower pressure can be utilized to assist in drying the product. However, since most of the mass transfer occurs as a result of the heat input transferred ia conduction through the walls of the dryer' s jacket, that is considered to be an indirect dryer. For more information on indirect dryers please refer to the first section of this chapter. 5.2 Batch Fluid Bed Dryers In the category of fluid bed dryers, there are two types of processes commonly used to suspend the material-pneumatic and mechanical fluidi zation 1. Pneumatic Fluid Bed Dryers. In the pneumatic fluidiza- tion process, the wet cake is placed in the dryer and dry heated gas is introduced at a very high velocity(under the bed of product)through a fine screen or a porous plate in order to fluidize the product. There is a visible layer of material which is sustained as the gas passes through the bed. The wet gas leaves the chamber through a sock or bag type dust collector which removes the fines and retums them to the batch. [ Morerecently, stainless steel cartridge filters are becoming very popular because they can be cleaned-in-place(CIP). This has been developed by the Aeromatic-Fielder Division of Niro. If the carrier me dium is air containing only clean water vapor, the gas can then be exhausted to the atmosphere if it contains clean water vapor. If the medium is an inert gas it can be recycled back to the dryer while removing contaminants or solvents via a condenser and filter. However, this inert gas must then be reheated to the proper inlet temperature 2. Mechanically Agitated Fluid Bed Dryers. In the fluidi zation process, the wet cake is gently lifted by rotating paddle type agitators thus blending the product into the gas stream creating an intimate mixing of the wet solids with the dry gas stream. This results in a efficient exposure of the wet product's surface area advantage of such a dryer is a faster drying time and a lower total energy input due to lower overall energy requirements(see Fig. 10)
740 Fermentation and Biochemical Engineering Handbook sensitive, the user should consider a vacuum dryer as an alternative. Vacuum or lower pressure can be utilized to assist in drying the product. However, since most of the mass transfer occurs as a result of the heat input transferred via conduction through the walls of the dryer’s jacket, that is considered to be an indirect dryer. For more information on indirect dryers please refer to the first section of this chapter. 5.2 Batch Fluid Bed Dryers In the category of fluid bed dryers, there are two types of processes commonly used to suspend the material-pneumatic and mechanical fluidization. 1. Pneumatic Fluid Bed Dryers. In the pneumatic fluidization process, the wet cake is placed in the dryer and dry heated gas is introduced at a very high velocity (under the bed of product) through a fine screen or a porous plate in order to fluidize the product. There is a visible layer of material which is sustained as the gas passes through the bed. The wet gas leaves the chamber through a sock or bag type dust collector which removes the fines and returns them tothe batch. [More recently, stainless steel cartridge filters are becoming very popular because they can be cleaned-in-place (CIP). This has been developed by the Aeromatic-Fielder Division of Niro.] If the carrier medium is air containing only clean water vapor, the gas can then be exhausted to the atmosphere if it contains clean water vapor. If the medium is an inert gas, it can be recycled back to the dryer while removing contaminants or solvents via a condenser and filter. However, this inert gas must then be reheated to the proper inlet temperature. 2. Mechanically Agitated Fluid Bed Dryers, In the fluidization process, the wet cake is gently lifted by rotating paddle type agitators thus blending the product into the gas stream creating an intimate mixing of the wet solids with the dry gas stream. This results in a very efficient exposure of the wet product’s surface area. The advantage of such a dryer is a faster drying time and a lower total energy input due to lower overall energy requirements (see Fig. 10)
Air distribulion Figure 10. Typical batch fluid bed dryer 5.3 Batch Rotary Dryers Ina rotary dryer, a horizontal cylinder is used to contain the batch while heated air is passed across the length of the cylinder. A jacket can be placed on the outside of the cylinder where steam or hot water is introduced to aid in heat transfer via conduction through the walls. Sometimes tator shaft with paddle arms, either heated or unheated, are included in design to assist in heat transfer and product discharge 5.4 Ribbon Dryers This type consists of a long, jacketed, horizontal cylinder, or a"U shaped trough, which contains an agitator shaft positioned down the length of the bowl. The purpose of the cylindrical-shaped vessel may be for operation under pressure or vacuum. The agitator spokes are intermittently
Drying 741 Exhaust air Exhwt duct R Filk h I oT tort,& filters - Material / contoiner Air preporation Material , unit Figure 10. Typical batch fluid bed dryer. 5.3 Batch Rotary Dryers ha batch rotary dryer, a horizontal cylinder is used to contain the batch while heated air is passed across the length of the cylinder. A jacket can be placed on the outside of the cylinder where steam or hot water is introduced to aid in heat transfer via conduction through the walls. Sometimes an agitator shaft with paddle arms, either heated or unheated, are included in the design to assist in heat transfer and product discharge. 5.4 Ribbon Dryers This type consists of a long, jacketed, horizontal cylinder, or a “U” shaped trough, which contains an agitator shaft positioned down the length of the bowl. The purpose of the cylindrical-shaped vessel may be for operation under pressure or vacuum. The agitator spokes are intermittently
742 Fermentation and Biochemical Engineering Handbook mounted on the shaft which support inner and outer rows of ribbon flights pitched so as to move the product. The outer ribbon flights usually move the product towards one side of the vessel and the inner ribbons move the product towards the other side. This design would have the discharge port at one end of the dryer. An alternative to this design is to have a center discharge, where the ribbons on one half of the dryer are pitched at 90 to the ribbons on the opposite end The drying here is achieved by means of exposing the product to the surface area of the jacketed vessel. The jacket is a shell of metal(usually carbon steel) welded onto a stainless steel vessel body. This design can include a heated shaft for increased surface area exposure. The heat transfer medium used here is generally steam, hot oil, or hot water. Ports must be provided so as to vent the evaporated vapors being removed from the product 5.5 Paddle dryers Whereas a gaseous medium can be used to transfer heat to the product, in most cases the paddle type is considered to be an indirect dryer. It is similar in design to the ribbon dryer. The differences exist when heated(hollow) paddles are used as opposed to flat blades. (See the previous section on indirect drying )Also, shoe-like paddles or plows can be used which tend to disperse or smear the product against a heated, horizontal, cylindrical wall The advantage of a heated paddle design is that the surface area exposure to the product being dried has been expanded thus increasing the overall heat transfer rate. Most paddle dryers are designed for use under vacuum whicl can supplement the indirect drying process 5.6 Agitated Pan Dryers An agitated open pan dryer is somewhat more complicated mechani cally. This is a short cylinder whose axis and agitator are vertical. The agitator can enter from either the top or the bottom As with the paddle dryers, these are mostly considered to be indirect dryers since heat transfer is from the jacket. If the product is a sticky, pasty material one may wish to use this design. the advantage of the pan dryer is the availability of several heated agitator designs which improve the overal heat transfer rate appreciably over a simple heated jacket; the reason is the same as mentioned in the previous section on paddle dryers. As mentioned earlier, venting of the dryer is necessary to remove the evaporated vapors
742 Fermentation and Biochemical Engineering Handbook mounted on the shaft which support inner and outer rows of ribbon flights pitched so as to move the product. The outer ribbon flights usually move the product towards one side of the vessel and the inner ribbons move the product towards the other side. This design would have the discharge port at one end ofthe dryer. An alternative to this design is to have a center discharge, where the ribbons on one half of the dryer are pitched at 90" to the ribbons on the opposite end. The drying here is achieved by means of exposing the product to the surface area of the jacketed vessel. The jacket is a shell of metal (usually carbon steel) welded onto a stainless steel vessel body. This design can include a heated shaft for increased surface area exposure. The heat transfer medium used here is generally steam, hot oil, or hot water. Ports must be provided so as to vent the evaporated vapors being removed fromthe product. 5.5 Paddle Dryers Whereas a gaseous medium can be used to transfer heat to the product, in most cases the paddle type is considered to be an indirect dryer. It is similar in design to the ribbon dryer. The differences exist when heated (hollow) paddles are used as opposed to flat blades. (See the previous section on indirect drying.) Also, shoe-like paddles or plows can be used which tend to disperse or smear the product against a heated, horizontal, cylindrical wall. The advantage of a heated paddle design is that the surface area exposure to the product being dried has been expanded thus increasing the overall heat transfer rate. Most paddle dryers are designed for use under vacuum which can supplement the indirect drying process. 5.6 Agitated Pan Dryers An agitated open pan dryer is somewhat more complicated mechanically. This is a short cylinder whose axis and agitator are vertical. The agitator can enter from either the top or the bottom. As with the paddle dryers, these are mostly considered to be indirect dryers since heat transfer is from the jacket. If the product is a sticky, pasty material one may wish to use this design. The advantage of the pan dryer is the availability of several heated agitator designs which improve the overall heat transfer rate appreciably over a simple heated jacket; the reason is the same as mentioned in the previous section on paddle dryers. As mentioned earlier, venting of the dryer is necessary to remove the evaporated vapors
743 5.7 Continuous dryers Continuous types of direct dryers are spray, flash or rotary designs, where the product enters in a form suitable to be handled properly by that dryer. Spray dryers can accommodate a feed stream in a slurry or solution form, whereas a flash dryer is intended to take a feed cake which can be broken up into individual pieces without coalescing. The feed characteristics uired for flash dryers are that the product must break up if introduced as a cake. If introduced as a paste, it is necessary for the feed to be backmixed with previously dried material so as to firm up the cakess consistency Rotary dryers are more flexible in that they can handle a wide variety of feed consistencies 5.8 Spray Dryers Spray dryers are large cylindrical chambers with a cone or flat bottom They also appear in the form of a large cube or box referred to as a box dryer, Small nozzles are located in the chamber walls through which the feed material, in the form of a slurry or solution, is atomized to a fine droplet. the droplet comes into contact with a hot gas stream and dries to a powder in the time that it takes for it to fall to the bottom of the chamber. Typical residence times in a chamber are 12 to 30 seconds. due to the evaporative cooling effect, the inlet temperature on this type of dryer is normally quite high relative to the dry products temperature limitations. Typical inlet tempera tures for spray dryers range from 400oF to 1000F depending on the application. The higher temperatures are normally for inorganic salt drying and the lower temperatures are normally for organic temperature sensitive products. The resultant individual product is always spherical in shape due to the initial droplet and it will tend to be extremely porous and fracture easil Further processing of spray dried product has been done to achieve instantizing by creating agglomerates of these spheres which retain a relatively high surface area compared to the individual particles. The action of adding this powder to water results in a release of energy from the agglomerated bonding forces and the capillary effect of the water traveling into the porous spheres. The net effect is one of quick dissolution of the agglomerated powder. This is highly desirable when searching for a means of instantizing a product In order to consider spray drying, certain criteria about the material must be met. The feed slurry viscosity must be low enough whereby it can be pumped through either a rotary atomizer, a two fluid nozzle or a high
Drying 743 5.7 Continuous Dryers Continuous types of direct dryers are spray, flash or rotary designs, where the product enters in a form suitable to be handled properly by that dryer. Spray dryers can accommodate a feed stream in a slurry or solution form, whereas a flash dryer is intended to take a feed cake which can be broken up into individual pieces without coalescing. The feed characteristics required for flash dryers are that the product must break up if introduced as a cake. If introduced as a paste, it is necessary for the feed to be backmixed with previously dried material so as to firm up the cakes's consistency. Rotary dryers are more flexible in that they can handle a wide variety of feed consistencies. 5.8 Spray Dryers Spray dryers are large cylindrical chambers with a cone or flat bottom. They also appear in the form of a large cube or box referred to as a box dryer. Small nozzles are located in the chamber walls through which the feed material, in the form of a slurry or solution, is atomized to a fine droplet. The droplet comes into contact with a hot gas stream and dries to a powder in the time that it takes for it to fall to the bottom ofthe chamber. Typical residence times in a chamber are 12 to 30 seconds. Due to the evaporative cooling effect, the inlet temperature on this type of dryer is normally quite high relative to the dry products' temperature limitations. Typical inlet temperatures for spray dryers range from 400'F to lOOOOF depending on the application. The higher temperatures are normally for inorganic salt drying and the lower temperatures are normally for organic temperature sensitive products. The resultant individual product is always spherical in shape due to the initial droplet and it will tend to be extremely porous and fracture easily. Further processing of spray dried product has been done to achieve instuntizing by creating agglomerates of these spheres which retain a relatively high surface area compared to the individual particles. The action of adding this powder to water results in a release of energy from the agglomerated bonding forces and the capillary effect of the water traveling into the porous spheres. The net effect is one of quick dissolution of the agglomerated powder. This is highly desirable when searching for a means of instuntizing a product. In order to consider spray drying, certain criteria about the material must be met. The feed slurry viscosity must be low enough whereby it can be pumped through either a rotary atomizer, a two fluid nozzle or a high