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