Drying of solid.pdf

Chapter One – Drying of Solids……………………… 1 ……………………Unit Operations 1.1- INTRODUCTION Drying of various feedstocks is needed for one or several of the following reasons: need for easy-to-handle free-flowing solids, preservation and storage, reduction in cost of transportation, achiev...

Chapter One – Drying of Solids……………………… 1 ……………………Unit Operations 1.1- INTRODUCTION Drying of various feedstocks is needed for one or several of the following reasons: need for easy-to-handle free-flowing solids, preservation and storage, reduction in cost of transportation, achieving desired quality of product, etc. Before proceeding to the basic principles, it is useful to note the following unique features of drying which make it a fascinating and challenging area for R&D: Product size may range from microns to tens of centimeters (in thickness or depth) Product porosity may range from zero to 99.9 percent Drying times range from 0.25 sec (drying of tissue paper) to five months (for certain hardwood species) Production capacities may range from 0.10 kg/h to 100 t/h Product speeds range from zero (stationary) to 2000 m/s (tissue paper) Drying temperatures range from below the triple point to above the critical point of the liquid Operating pressure may range from fraction of a millibar to 25 atmospheres Heat may be transferred continuously or intermittently by convection, conduction, radiation or electromagnetic fields Clearly, no single design procedure that can apply to all or even several of the dryer variants is possible. It is therefore essential to revert to the fundamentals of heat, mass and momentum transfer coupled with a knowledge of the material properties (quality) when attempting design of a dryer or analysis of an existing dryer. The drying of materials is often the final operation in a manufacturing process, carried out immediately prior to packaging or dispatch. Drying refers to the final removal of water, or another solute, and the operation often follows evaporation, filtration, or crystallisation. drying is carried out for one or more of the following reasons: (a) To reduce the cost of transport. (b) To make a material more suitable for handling as, for example, with soap powders, dyestuffs and fertilisers. (c) To provide definite properties, such as, for example, maintaining the free- flowing nature of salt. (d) To remove moisture which may otherwise lead to corrosion. One example is the drying of gaseous fuels or benzene prior to chlorination. Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 2 ……………………Unit Operations 1.2- BASIC PRINCIPLES Drying is a complex operation involving transient transfer of heat and mass along with several rate processes, such as physical or chemical transformations, which, in turn, may cause changes in product quality as well as the mechanisms of heat and mass transfer. Physical changes that may occur include: shrinkage, puffing, crystallization, glass transitions. In some cases, desirable or undesirable chemical or biochemical reactions may occur leading to changes in color, texture, odor or other properties of the solid product. In the manufacture of catalysts, for example, drying conditions can yield significant differences in the activity of the catalyst by changing the internal surface area. Drying occurs by effecting vaporization of the liquid by supplying heat to the wet feedstock. As noted earlier, heat may be supplied by convection (direct dryers), by conduction (contact or indirect dryers), radiation or volumetrically by placing the wet material in a microwave or radio frequency electromagnetic field. Over 85 percent of industrial dryers are of the convective type with hot air or direct combustion gases as the drying medium. Transport of moisture within the solid may occur by any one or more of the following mechanisms of mass transfer: Liquid diffusion, if the wet solid is at a temperature below the boiling point of the liquid Vapor diffusion, if the liquid vaporizes within material Knudsen diffusion, if drying takes place at very low temperatures and pressures, e.g., in freeze drying Surface diffusion (possible although not proven) Hydrostatic pressure differences, when internal vaporization rates exceed the rate of vapor transport through the solid to the surroundings Combinations of the above mechanisms The moisture content of a material is usually expressed in terms of its water "liquid" content as a percentage of the mass of the dry material, though moisture content is sometimes expressed on a wet basis. If a material is exposed to air at a given temperature and humidity, the material will either lose water "if the air have lower humidity than that corresponding to the moisture content of the solid " or gain water "if air has more humid than the solid in equilibrium with it, the solid absorbs moisture from the air" until an equilibrium condition is established. Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 3 ……………………Unit Operations 1.3- Wet Solid Group 1.4- Terminology and Definitions Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 4 ……………………Unit Operations Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 5 ……………………Unit Operations Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 6 ……………………Unit Operations Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 7 ……………………Unit Operations Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 8 ……………………Unit Operations Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 9 ……………………Unit Operations 1.5- Humidity Measurement 1.6- Humidity Data for Air – Water system Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 10 ……………………Unit Operations Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 11 ……………………Unit Operations Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 12 ……………………Unit Operations 1.7- Rate of Drying Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 13 ……………………Unit Operations Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 14 ……………………Unit Operations Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 15 ……………………Unit Operations 1.8- Calculation Method of Drying Rate and Time Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 16 ……………………Unit Operations Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 17 ……………………Unit Operations Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 18 ……………………Unit Operations Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 19 ……………………Unit Operations Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 20 ……………………Unit Operations Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 21 ……………………Unit Operations 1.9- Mechanism of Moisture Movement in Wet Solid Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 22 ……………………Unit Operations Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 23 ……………………Unit Operations Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 24 ……………………Unit Operations 1.10- Material and Balance for Continuous Dryers Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 25 ……………………Unit Operations 1.11- Rate of Drying for Continuous Direct Heat Driers Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 26 ……………………Unit Operations Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 27 ……………………Unit Operations Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 28 ……………………Unit Operations Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 29 ……………………Unit Operations 1.12- Drying Equipment Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 30 ……………………Unit Operations Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar  Chapter One – Drying of Solids……………………… 31 ……………………Unit Operations Fourth Class …Chemical Engineering Department ------- Dr. May Ali alsaffar