AIMI ATHIRAH BINTI AZNAN SCHOOL OF BIOPROCESS ENGINEERING, UNIMAP ERT 356 UNIT OPERATIONS DRYING PREPARED BY: AIMI ATHIRAH BINTI AZNAN LECTURER SCHOOL OF BIOPROCESS ENGINEERING, UNIMAP

Topic 2: Drying In this topic, you will learn on: Basic drying theory Heat and mass transfer in drying Equilibrium moisture content Air drying Conduction drying Drying equipment Lab 1: Moisture content in different system

INTRODUCTION Drying meat & fish under the sun Drying One of the oldest methods of preserving food Still important in food preservation Stored for long periods Drying meat & fish under the sun

WHY DRIED FOOD CAN BE STORED FOR LONG PERIOD? Microorganisms Cause: Promote undesired changes in the chemical composition of the food Cause: Food spoilage & decay Unable to grow & multiply In the absence of sufficient water Malfunction without water

INTRODUCTION cont. Drying is accomplished by vaporizing the water in the food by supplying the latent heat of vaporization. Process-controlling factors that enter into the unit operation of drying: Transfer of heat to provide the necessary latent heat of vaporization. Movement of water vapor through the food and then away from it to separate the water from food.

INTRODUCTION cont. 3 categories of drying processes. Sublimed: Change directly into vapor when heated

BASIC DRYING THEORY

THREE STATES OF WATER What can you describe on: Point O Line A – A’ 3 state of pure water (Solid, liquid, vapor) Each state at any time depends on temperature and pressure condition. What can you describe on: Point O Line A – A’ Line B – B’ Line OP: Liquid & vapor coexist in equilibrium Line = vapor pressure/temperature line. Vapor pressure = the measurement of the tendency of molecules to escape as a gas from liquid. The Triple Point 0.0098ºC, 0.64kPa

THREE STATES OF WATER

THREE STATES OF WATER cont. Boiling occurs when: water vapor pressure = total pressure on the water surface Boiling point at atmospheric pressure? At pressures above or below atm pressure, water boils at corresponding temperatures abv or below 100ºC. 1 atm = 101.325 kPa

HEAT REQUIREMENTS FOR VAPORIZATION Energy supplied to vaporized the water at any temperature depends on the required temperature. If from liquid = LATENT HEAT OF VAPORIZATION; The quantity of energy required per kg of water. If from solid = LATENT HEAT OF SUBLIMATION. The heat energy required can be calculated from latent heats value given in the steam table in Appendix 8. http://www.nzifst.org.nz/unitoperations/appendix8.htm

EXAMPLE 1: Heat Energy in Air Drying A food containing 80% water is to be dried at 100°C down to moisture content of 10%. If the initial temperature of the food is 21°C, calculate: The quantity of heat energy required per unit weight of the original material for drying under atm pressure The energy requirement/kg water removed Latent heat of vaporization of water at 100°C at standard atm pressure = 2257kJ/kg Specific heat capacity of the food = 3.8kJ/kg.°C Specific heat capacity of water= 4.186kJ/kg.°C

HEAT REQUIREMENTS FOR VAPORIZATION cont. Heat Energy in Vacuum Drying Using the same material as in previous example, if vacuum drying is to be carried out at 60°C under the corresponding saturation pressure of 20kPa abs. Calculate the heat energy required to remove the moisture per unit weight of raw material.

HEAT TRANSFER IN DRYING The rates of drying are generally determined by the rates at which heat energy can be transferred to the water or ice. All of the 3 mechanisms of heat transfer (conduction, radiation and convention) may enter into drying. Rate of heat transfer in air drying (conduction) is: q = Heat transfer rate (Js-1) hs = Surface heat transfer coefficient (Jm-2s-1ºC) A = Area through which the heat flow (m2) Ta = Air temperature (ºC) Ts = Drying surface temperature (ºC) q = hsA(Ta – Ts)

HEAT TRANSFER IN DRYING Rate of heat transfer in roller dryer (conduction) is: q = Heat transfer rate U = Overall heat transfer coefficient A = Area of drying surface on the drum Td = Drum temperature Ts = Surface temperature of the food q = UA(Td – Ts) Moist material is spread over the surface of the heated drum

HEAT TRANSFER IN DRYING By radiation, the surface temperature of the food may be higher than the air temperature. Estimation of surface temperature can be made using the relationships developed for radiant heat transfer. In freeze drying, energy must be transferred to the surface at which sublimation occurs. However, the energy must be supplied at such rate as not to increase the temperature at the drying surface above the freezing point. In most freeze drying application, the heat is transferred mainly by conduction. As drying proceeds, the character of the heat transfer changes. Dry material begins to occupy the surface layers and conduction must take place through these dry layers which are poor heat conductors so that heat is transferred to the drying region progressively more slowly.

MASS TRANSFER IN DRYING Mass is transferred under the driving force provided by a partial pressure or concentration difference. The rate of mass transfer is proportional to the potential pressure or concentration difference and to the properties of the transfer system characterized by a mass transfer coefficient. 𝑑𝑤 𝑑𝑡 = kg’ A ∆Y dw/dt = mass (moisture) being transferred (kg/s) in time A = Area through which the transfer is taking place kg’ = mass transfer coefficient (kgm-2s-1) ΔY = humidity difference (kg/kg)

The study of moist air or mixture of dry air & water vapor PSYCHROMETRY The study of moist air or mixture of dry air & water vapor Humidity (Y): Measure water content of the air. Absolute humidity or Humidity Ratio: Mass of water vapor per unit mass of dry air (kg/kg) Saturated air: Air is saturated with water vapor at given T and P if it achieved maximum humidity. If add >water; it appear as liquid water (mist/droplets) At saturation; Partial pressure of water vapour in the air = saturation vapor pressure of water at that T Total Pressure: Sum of the pressure of its constituents (Partial Pressures) such as air & water vapor

PSYCHROMETRY cont Relative humidity (RH): Ration of the partial pressure of the water vapor in the air (p) to the partial pressure of saturated water vapor at the same T (ps). %RH = p/ps Humidity (Y): Measurement of water content in the air. It is related to water partial pressure (pw) in air vapor by: Y = 18 pw / [29(P-pw)] P: Total pressure Dew point: The T at which moisture starts to condense out of the air. Also known as saturation T.

WET BULB TEMPERATURE Wet-bulb temperature: Temperature recorded by thermometer when the bulb is enveloped by cotton wick saturated with water.

Psychometric Chart 3. Read RH 2. Dry Bulb (25°C) 4. Read Enthalpy 1. Wet Bulb (20°C) EXAMPLE: If the wet-bulb temperature in a particular room is measured and found to be 20°C in air whose dry-bulb temperature is 25°C (that is the wet-bulb depression is 5°C) estimate the relative humidity, the enthalpy and the specific volume of the air in the room. 5. Read Sp Vol

EXAMPLE If the air in previous example is then to be heated to a dry blub temperature of 40°C, calculate the rate of heat supply needed for a flow of 1000m3/hr of this hot air for a dryer, and the RH.

MOISTURE CONTENT & EQUILIBRIUM MOISTURE CONTENT

Moisture Content of a Material Wet basis (MCwb) Dry basis (MCdb) Mass of water/Mass of wet product (Undried) Mass of water/Mass of dry product

Conversion of Moisture Content MC are often expressed as percent (multiplying by 100)

EQUILIBRIUM MOISTURE CONTENT Under a given vapor pressure of water in the surrounding air, a food attains a MC in equilibrium with its surroundings WHEN there is no exchange of water between the food and its surroundings.

Example: For potato, at temp 20°C of RH 30%, the equilibrium MC is _______. It would not possible to dry potatoes below 10% using an air dryer with air at 20°C and RH 30% In equilibrium MC graph, mostly the curse is presented by sigmiod (S-shape) MC at low humidity is high for food of dry solids contained high protein, starch or other high molecular weight polymers. MC at low humidity is low for foods with high soluble solids. (Ex: Fats and crystalline salts and sugar)

DRYING CURVE http://moisturecontrol.weebly.com/drying-curve.html

Typical Drying Curve Drying curve usually plots the drying rate versus drying time or moisture contents. Three major stages of drying can be observed in the drying curve. Transient early stage, during which the product is heating up (transient period) Constant rate period, in which moisture is comparatively easy to remove Falling rate period, in which moisture is bound or held within the solid matrix

Typical Drying Curve Critical moisture content: The moisture content at the point when the drying period changes from a constant to a falling rate.

The drying behaviours of food materials depend on the porosity, homogeneity, and hygroscopic properties. 1 Hygroscopic food materials enter into the falling rate faster compared to non-hygroscopic food materials. 2

Air Drying

AIR DRYING Rate of Water Removal Air Condition Food properties Type of Dryer

AIR DRYING Water is held by forces, whose intensity ranges from very weak forces to a very strong chemical bonds. In drying, loosely bonding liquid can be removed easily. Thus, drying rate decrease as MC decreases, with the remaining water bonded are stronger. In many cases, a substantial part of the water is loosely bound such as the free water on the surface of a food.

Most cases; a substantial part of water is loosely bound. Free water at the surface Sand > free water Meat > bound water Comparison between sand and meat drying curve shows the effect of water bond on drying rates. Water at a free surface Constant drying rate

Calculation of Constant Drying Rate Water evaporated at free water surface during drying rate. Rate of removal of water can be relate to rate of heat transfer (If no change in T of the material) All heat energy transferred is used to evaporate water Rate of removal of water is also the rate of mass transfer Mass transfer based on drying force of the diff of water partial pressure between food & air. Alternatively, driving force maybe be expressed in term of humidity driving forces. Numerical values of the mass transfer coeff are linked to partial pressure/humidity relationship as Eq 7.4 & Eq 7.5. Food Air MASS TRANSFER

Calculation of Constant Drying Rate Psychrometric table with humid air properties: http://www.engineeringtoolbox.com/moist-air-properties-d_1256.html

Equation 7.5 kg’ = hc/1000 Or hc= 1000 x kg’ kgm-2s-1 ≈ Jm-2s-1 ˚C Use Lewis Relationship to find mass transfer coefficient (kg’) & heat transfer coefficient (hc) kg’ = hc/1000 Or hc= 1000 x kg’ kgm-2s-1 ≈ Jm-2s-1 ˚C

Calculation of Drying Time Drying Rate Drying Time Drying Equipment + Operations Experiment/ Theory Assume T and RH of the drying air are constant

Drying Time at Constant Rate t = w (Xo – Xc) / (dw/dt)constant Where (dw/dt)constant = kg’A(Ys – Ya) Xo : Initial MCdb Xc : Final MCdb (Critical MC) w : Amount of dry material in food (dw/dt)constant : Constant drying rate Mass Transfer

DRYING EQUIPMENT The principles of drying may be applied to any type of dryer that are available in the industry. Major problem in calculation of real dryers: Condition changes as the drying air and the drying solids move along the dryer in a continuous dryer or change with time in the batch dryer.

EXAMPLE OF DRYING EQUIPMENT Food is spread out in thin layer on the trays. Heating maybe by air current across trays, conduction from heated tray or shelves or radiation from heated surface. Food is spread over the surface of heated drum. Drum rotate with food on the surface. Food remain on the drum surface and then being scraped off. Video URL: https://youtu.be/JGb77K_K55M

What about other types of drying equipment? Liquid/fine solid material in a slurry is sprayed in fine droplets into a current of heated air. Dryer body usually large enough for the dried particles to drop down without sticking on the walls. Commercial dryers are large in size; may be in 10 m (diameter) x 20m (high). Video URL: https://youtu.be/KL4-SpP-Ghk Foodstuff is heated by the air flow through the horizontal inclined cylinder or by heat conduction from the walls. Video URL: https://youtu.be/RSjNRwiR1iU What about other types of drying equipment?

Summary

Summary