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 the734 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 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