Heat and Mass Transfer

SIMULTANEOUS HEAT AND

MASS TRANSFER

(SHMT)

Department of Chemical Engineering

Drying Fundamentals

Drying is unit operation

Involves the removal of relative small amounts of water or another liquid from solid material. In drying Heat & Mass are transferred

Purpose of Drying

To reduce the cost of transport.

To make the material more suitable for handling. To provide definite properties to the solid.

Action in Dryers

Liquid removal

Mechanically

Moisture:

Moisture = mass of liquid / mass of solid 1. Dry basis

General Terminology

Equilibrium moisture Free moisture

Bound moisture Unbound moisture

General Terminology

Classification of Drying

Equipments

 Mode of operation  Heat input type  State of material  Operating Pressure  Drying Temperature  Drying medium  Relative motion  Stages

Gas-Solids Contacting - Direct dryer Cross-circulation Drying Through-circulation Drying Showering of Solid Fluidized Entrained

Gas-Solids Contacting – Indirect Dryer

Solid is spread at stationary surface Solids are moved at heated surface.

Solids slide by gravity on inclined heated surface or carried upward for a time then slide

Temperature pattern in Dryers

Temperature variation in dryer depends upon

o Nature of feed

o Liquid content of feed

o Temperature of heating media o Drying time

For Batch Dryer

 Heating media is at constant temperature

 Temperature of wet solid rises from initial value to vaporization temperature

 Non adiabatic dryer

Tv = B.P of liquid

 Adiabatic dryer

For Continuous Dryer

Solid – same pattern as in Batch Steady state operation

Tv is constant

Gas enter Thb & low humidity

o Cools rapidly at first then more slowly o Humidity rises steadily

For flow through fixed or fluidized

bed of solid particles

Problem

Flourspar (CaF₂) is to be dried from 6 to 0.4 percent moisture dry basis in a countercurrent adiabatic rotary dryer at a rate of 18000 lb/hr of bone dry solids. The heating air enters at 1000 o_{F with a humidity of 0.03 and}

a wet bulb temperature of 150 o_{F. The solids have a}

specific heat of 0.48 Btu/lb-o_{F; they enter the dryer at 70} o_{F and leave at 200} o_{F. The maximum allowable air}

velocity is 2000 lb/ft2_-h.

(a) Assuming eq….. applies , what would be the diameter and length of the dryer if N_t = 2.2? Is this a resonable design?

General Terminology : Again

Equilibrium moisture Free moisture

Bound moisture Unbound moisture

Equilibrium Moisture

Equilibrium moisture content is the limiting moisture to which a given material can be dried under specific conditions of air temperature and humidity.

Free Moisture

Moisture which is in excess of equilibrium moisture content.

Free moisture content is that liquid which is removable at a given temperature and humidity.

Bound Moisture

Bound moisture in a solid is that liquid which exerts a vapor pressure less than that of the pure liquid at the given temperature.

Substance containing bound water are called hygroscopic substance

Liquid may become bound

o by retention in small capillaries, o by solution in cell or fiber walls,

o by homogeneous solution throughout the solid, and o by chemical or physical adsorption on solid surfaces.

Unbound Moisture

Unbound moisture in a hygroscopic material is that moisture in excess of the equilibrium moisture content corresponding to saturation humidity.

All water in a non-hygroscopic material is unbound water.

Cross Circulating Drying

Mechanism of drying:

o Nature of solid o Gas solid contact

Solids

o Crystalline o Porous

Rate of Drying

Constant Rate Period Falling Rate Period

Constant Rate Period

Mass transfer based or Heat transfer based,

Critical Moisture

Critical moisture content is the moisture content when the constant-rate period ends.

Critical moisture content varies with

Thickness of material Rate of drying

Falling Rate Period

Rate of drying depends on:

o Nonporous solid and Diffusion theory o Porous solid and Capillary theory

Assignment

Word + power point slide

Each member of the group must show his/her part. (by writing Regd # on the respective

pages)

Direct (convective) dryers:

a. Direct contacting of hot gases with the solids is employed for solids heating and vapor removal.

b. Drying temperatures may range up to 1000 K, the limiting temperature for most common structural metals.

c. At gas temperatures below the boiling point, the vapor content of gas influences the rate of drying and the final moisture content of the solid. With gas temperatures above the boiling point throughout, the vapor content of the gas has only a slight retarding effect on the drying rate and final moisture content. Thus, superheated vapors of the liquid being removed (e.g., steam) can be used for drying.

d. For low-temperature drying, dehumidification of the drying air may be required when atmospheric humidities are excessively high.

e. The lower the final moisture content, the more fuel per pound of water evaporated, that a direct dryer consumes.

Indirect (conductive) Dryers:

1. Heat is transferred to the wet material by conduction through a solid retaining wall, usually metallic.

2. Surface temperatures may range from below freezing in the case of freeze dryers to above 800 K in the case of indirect dryers heated by combustion products.

3. Indirect dryers are suited to drying under reduced pressures and inert atmospheres to permit the recovery of solvents and to prevent the occurrence of explosive mixtures or the oxidation of easily decomposed materials.

4. Indirect dryers using condensing fluids as the heating medium are generally economical from the standpoint of heat consumption, since they furnish heat only in accordance with the demand made by the material being dried.

5. Dust recovery and dusty materials can be handled more satisfactorily in indirect dryers than in direct dryer

SELECTION OF DRYING EQUIPMENT

 Properties of the material being handled  Drying characteristics of the material  Flow of material to and from the dryer  Product qualities

 Recovery problems

Drying Equipment

Dryers for Solids and Pastes

TRAY DRYER

• It consists of a rectangular chamber whose walls are insulated.

• Trays are placed inside the heating chamber. The number of trays may vary with the size of the dryer. Each tray is rectangular or square and about 1.2 to 2.4 meters square in area .

• Trays are usually loaded from 10.0 to 100 millimetres deep.

Alternately the trays can be placed in trucks on wheels , which can be rolled into and out of chamber. Two such trucks can be arranged inside dryer.

Dryer is fitted with a fan for circulating air over the trays. Electrically heated elements are provided inside (rather than outside) to heat the air.

In the corner of the chamber, direction

vanes are placed to direct air in the expected path.

MERITS:

• The method is operated batch wise. • Attrition is not observed.

• Loading and unloading can be done without losses.

DEMERITS:

• Only a fraction of the solid particles is directly exposed.

• The method is costly and time consuming.

USE :

Sticky materials, plastic substances, granular mass or crystalline materials, precipitates and paste can be dried in a tray dryer.

`

Fluidized bed dryers

Principle: Systems in which the solid particles are partially suspended in an

upward moving gas stream. In this dryer hot air (gas) is passed at high pressure through a perforated bottom of the container

containing granules to be dried.

The hot gas is surrounding every granule to completely dry them. Thus materials or

Two types of bed dryers are available, vertical fluid bed dryer and

MERITS:

1. Efficient heat and mass transfer give high

drying rates, so that drying times are shorter than with static bed dryers.

2. The temperature of a fluidized bed is

uniform throughout and can be controlled precisely.

3. The free movement of individual particles eliminates the risk of soluble materials.

4. The unit has a high output from a small floor space.

5. The thermal efficiency is 2 to 6 times than tray dryer.

6. It can be used either batch type or continuous type.

7. Used for mixing the ingredients and its mixing efficiency is also high.

DEMERITS:

1.The turbulence of the fluidized state may cause

excessive attrition of some particles, with damage to some granules and the production of too much dust.

2.The vigorous movement of particles in hot dry air can lead to the generation of static electricity

charges and suitable precautions must be taken.

USE :

This dryer is properly used for drying of granules in the production of tablets. It can be used for three

DRUM DRYER or ROLLER DRYER

The drum dryer consists of a horizontally mounted hollow steel drum of 0.6 to 3.0 metres diameter and 0.6 to 4.0 metres

length, whose external surface is smoothly polished.

Below the drum, feed pan is placed in

such a way that the drum dips partially

into the feed.

On one side of the drum a spreader is

placed and on the other side a doctor’s

knife is placed to scrap the dried

material.

A storage bin (or a conveyor ) is placed

connecting the knife to collect the

Steam is passed inside the drum. Heat transfer coefficient of the drum metal is high. Heat is transferred by conduction to the material.

Simultaneously drum is rotated at the rate of 1-10 revolutions per minute.

The liquid material present in the feed pan

adheres as a thin layer to the external surface of the drum during its rotation.

The materials are completely dried during its journey in slightly less than one rotation

(from one side to another side of the drum). The dried materials are scrapped by the

doctor’s knife, which than falls into a storage bin.

The time of contact of the material with hot metal is 6 to 15 seconds only.

Therefore processing conditions such as film thickness, steam temperature are closely

MERITS:

• The method gives rapid heat drying and mass transfer are higher.

• The entire material is continuously exposed to heat source.

• The equipment is compact.

• Heating time is short being only a few seconds.

• The product obtained is completely dried and is in the final form.

DEMERITS:

• Operating conditions are critical. • Attrition is not possible.

• Skilled operators are essential to control feed rate ,film thickness, speed of rotation and temperature.

• Maintenance cost of a drum dryer is higher than spray dryer.

• It is not suitable for solutions of salts with less solubility.

USE :

Drum dryer is used for drying solution slurries suspensions etc.

• The products dried are milk products,

starch products, ferrous salts, suspensions of zinc oxide, suspension of kaolin, yeasts, pigments, malt extracts, antibiotics, DDT, calcium, insecticides and barium

A filter cake 24in square and 2in thick, supported on a screen , is dried from both sides with air at a wet bulb temperature of 80o_{F and dry bulb temperature of 120}o_F.

The air flows parallel with the faces of the cake at a velocity of 3.5ft/s.The dry density of the cake is 120lb/ft3_.

The equilibrium moisture content is negligble Under the conditions of drying the critical moisture content is 9 percent , dry basis.

(a) What is the drying rate during constant rate period? (b) How long would it take to dry this material from an

initial moisture content of 20 percent (dry basis) to a final moisture content of 10 percent?

Equivalent diameter D_e is equal to 2 ft.

Numerical

Planks of wood 25.4mm thick are dried from an initial moisture content of 25 percent to a final moisture content of 5 percent using air of negligible humidity. If D’_v for the wood is8.3x10-6

The filter cake of Ex 24.1 is extruded onto a screen in the form of cylinder 1/4in diameter and 4in long. The solids loading is 8 pounds of dry solids per square foot of screen surface. The bed porosity is 60 percent. Air at 120oF

(dry-bulb) and with a wet bulb temperature of 80o_{F is}

passed through the bed at asuperfical velocity of 3.5ft/s. How long will it take to dry solids from 20 percent to 10 percent moisture

Calculate the diameter and length of a an adiabatic rotary dryer to dry 2800 lb/h of a heat sensitive solid from an initial moisture content of 15 percent to a final moisture content of 0.5 percent , both dry basis. The solids have a specific heat of 0.52Btu/lb-o_{F. Heating air is}

available at 260o_{Fand a humidity of 0.01lb of}

water per pound of dry air. The maximum allowable mass velocity of the air is 700 lb/ft2_-h.