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