Questions and Answers
Drying of solids is needed for easy-to-handle free-flowing liquids.
False
Product size in drying can range from microns to tens of meters.
False
Drying times can range from 0.25 sec to five months.
True
Heat can only be transferred by convection or conduction in drying.
Signup and view all the answers
Drying is often the first operation in a manufacturing process.
Signup and view all the answers
Drying refers to the removal of air or gas from a material.
Signup and view all the answers
One of the reasons for drying is to increase the cost of transport.
Signup and view all the answers
Drying is carried out for one or more of the following reasons: preservation and storage, reduction in cost of transportation, achieving desired quality of product, etc.
Signup and view all the answers
Drying is a simple operation involving permanent transfer of heat and mass.
Signup and view all the answers
The chapter is about Drying of Liquids.
Signup and view all the answers
Physical changes that may occur during drying include shrinkage and melting.
Signup and view all the answers
The chapter is part of the Unit Operations course.
Signup and view all the answers
The chapter is written by Dr. May Ali alsaffar.
Signup and view all the answers
Drying can occur by effecting vaporization of the liquid by supplying cold to the wet feedstock.
Signup and view all the answers
Most industrial dryers are of the conduction type with hot air or direct combustion gases as the drying medium.
Signup and view all the answers
Humidity Measurement is discussed in section 1.5.
Signup and view all the answers
The chapter covers Humidity Data for Air – Gas system.
Signup and view all the answers
Desirable chemical or biochemical reactions may occur during drying, leading to changes in product quality.
Signup and view all the answers
Drying can be used to remove impurities from a material.
Signup and view all the answers
Rate of Drying is discussed in section 1.6.
Signup and view all the answers
Drying involves only heat transfer.
Signup and view all the answers
The chapter discusses Calculation Method of Drying Rate and Time in section 1.9.
Signup and view all the answers
The chapter is for Third Class Chemical Engineering Department.
Signup and view all the answers
Drying conditions can affect the internal surface area of the final product.
Signup and view all the answers
Moisture content of a material is usually expressed in terms of its water vapor content as a percentage of the mass of the wet material.
Signup and view all the answers
Vapor diffusion occurs when the liquid does not vaporize within the material.
Signup and view all the answers
Surface diffusion is a proven mechanism of mass transfer.
Signup and view all the answers
A material will always lose water if it is exposed to air with any humidity.
Signup and view all the answers
Liquid diffusion occurs at any temperature regardless of the boiling point of the liquid.
Signup and view all the answers
Knudsen diffusion occurs at high temperatures and pressures.
Signup and view all the answers
Moisture content is always expressed on a dry basis.
Signup and view all the answers
Mass transfer mechanisms in drying of solids always occur independently.
Signup and view all the answers
Moisture movement in a wet solid is a complex process.
Signup and view all the answers
Continuous dryers are always operated in a batch mode.
Signup and view all the answers
Material balance is an important consideration for batch dryers only.
Signup and view all the answers
The rate of drying in a continuous direct heat drier is always constant.
Signup and view all the answers
Drying of solids is only used in the chemical engineering industry.
Signup and view all the answers
Drying is only used to remove water from a material.
Signup and view all the answers
Heat transfer by radiation is not possible in drying.
Signup and view all the answers
Drying is always the final step in a manufacturing process.
Signup and view all the answers
Study Notes
Introduction to Drying of Solids
- Drying of various feedstocks is necessary for easy handling, preservation, and storage, reducing transportation costs, and achieving desired product quality.
- Drying is a complex operation involving transient transfer of heat and mass, along with physical or chemical transformations that may affect product quality and heat and mass transfer mechanisms.
Unique Features of Drying
- Product size can range from microns to tens of centimeters in thickness or depth.
- Product porosity can range from zero to 99.9 percent.
- Drying times can range from 0.25 seconds (tissue paper) to five months (certain hardwood species).
- Production capacities can range from 0.10 kg/h to 100 t/h.
- Product speeds can range from zero (stationary) to 2000 m/s (tissue paper).
- Drying temperatures can range from below the triple point to above the critical point of the liquid.
- Operating pressure can range from a fraction of a millibar to 25 atmospheres.
- Heat can be transferred continuously or intermittently by convection, conduction, radiation, or electromagnetic fields.
Reasons for Drying
- To reduce transportation costs.
- To make materials more suitable for handling (e.g., soap powders, dyestuffs, and fertilizers).
- To provide definite properties (e.g., maintaining the free-flowing nature of salt).
- To remove moisture that may lead to corrosion (e.g., drying of gaseous fuels or benzene prior to chlorination).
Basic Principles of Drying
- Drying involves transient transfer of heat and mass along with physical or chemical transformations that may affect product quality and heat and mass transfer mechanisms.
- Physical changes that may occur include shrinkage, puffing, crystallization, and glass transitions.
- Chemical or biochemical reactions may occur, leading to changes in color, texture, odor, or other properties of the solid product.
Heat and Mass Transfer Mechanisms
- Heat can be supplied by convection (direct dryers), 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 liquid diffusion, vapor diffusion, Knudsen diffusion, surface diffusion, hydrostatic pressure differences, or combinations of these mechanisms.
Moisture Content and Equilibrium
- Moisture content is usually expressed as a percentage of the mass of the dry material.
- If a material is exposed to air at a given temperature and humidity, it will either lose water (if the air has lower humidity) or gain water (if the air has higher humidity) until an equilibrium condition is established.
Studying That Suits You
Use AI to generate personalized quizzes and flashcards to suit your learning preferences.
Description
This quiz covers the introduction to drying of solids, reasons for drying, and its importance in unit operations.
