4510 Lec 8 - Drying PDF, Food Process Engineering

CENTRAL LUZON STATE UNIVERSITY Science City of Munoz, Nueva Ecija 3120 PHILIPPINES  ABEN 4510 FOOD PROCESS ENGINEERING Melba Domes Denson Department of Agricultural and Biosystems Engineering College of Engineering Central Luzon State...

CENTRAL LUZON STATE UNIVERSITY Science City of Munoz, Nueva Ecija 3120 PHILIPPINES  ABEN 4510 FOOD PROCESS ENGINEERING Melba Domes Denson Department of Agricultural and Biosystems Engineering College of Engineering Central Luzon State University Basic Drying Theory ✓ Drying is one of the oldest methods of preserving food. ✓ Primitive societies practiced the drying of meat and fish in the sun long before recorded history. ✓ Dried foods can be stored for long periods without deterioration occurring. The principal reasons for this are that the microorganisms which cause food spoilage and decay are unable to grow and multiply in the absence of sufficient water and many of the enzymes which promote undesired changes in the chemical composition of the food cannot function without water. Basic Drying Theory ✓ two important process-controlling factors that enter into the unit operation of drying: (a) transfer of heat to provide the necessary latent heat of vaporization, (b) movement of water or water vapour through the food material and then away from it to effect separation of water from foodstuff. Movement of moisture during drying Bound water – water molecules that are tightly bound or associated with other components within the food Free water/ Surface moisture – readily available and contributes directly to the water activity (aw) of the food, influencing its texture, microbial stability, and overall quality 3 Categories of Drying: ✓ Air and contact drying under atmospheric pressure. In air and contact drying, heat is transferred through the foodstuff either from heated air or from heated surfaces. The water vapour is removed with the air. ✓ Vacuum drying. In vacuum drying, advantage is taken of the fact that evaporation of water occurs more readily at lower pressures than at higher ones. Heat transfer in vacuum drying is generally by conduction, sometimes by radiation. ✓ Freeze drying. In freeze drying, the water vapour is sublimed off frozen food. The food structure is better maintained under these conditions. Suitable temperatures and pressures must be established in the dryer to ensure that sublimation occurs.  Basic Drying Theory ✓ Pure water can exist in three states, solid, liquid and vapour. The state in which it is at any time depends on the temperature and pressure conditions and it is possible to illustrate this on a phase diagram, as shown in the figure. Principle of Drying moisture 1. Drying happens when VP VP product product is > VP ambient. heat 2. Adsorption of moisture happens VP ambient air when VP product < VP ambient. 3. What happens when VP product = VP ambient? The condition of product is in equilibrium level. www.mecmargroup.com Psychrometry Equilibrium Moisture Content ✓ The equilibrium moisture content (EMC) refers to the moisture content at which a material is in a state of balance with the surrounding environment. This means that the material neither gains nor loses moisture because it has reached a point where its internal moisture content matches the humidity and temperature of the air around it. www.researchgate.net  Drying Curve I. Preheating period – drying I II III rate is slowly increasing II. Constant-rate period- drying rate is constant in time - removal of surface water III. Falling-rate period – drying rate declines over time - transfer of internal moisture to the surface Source: Ip & Wan, 2012  Heat Requirements for Vaporization ✓ The energy, which must be supplied to vaporize the water at any temperature, depends upon this temperature. ✓ The quantity of energy required per kg of water is called the latent heat of vaporization, if it is from a liquid, or latent heat of sublimation if it is from a solid. ✓ The heat energy required to vaporize water under any given set of conditions can be calculated from the latent heats given in steam tables. Sample Problem: Heat energy in 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 atmospheric pressure. The latent heat of vaporization of water at 100°C and at standard atmospheric pressure is 2257 kJ kg-1. The specific heat capacity of the food is 3.8 kJ kg-1 °C-1 and of water is 4.186 kJ kg-1 °C-1. Find also the energy requirement/kg water removed. Solution: Heat Transfer in Drying Air Drying: q = hsA(Ta - Ts) where q is the heat transfer rate in J s-1, hs is the surface heat-transfer coefficient J m-2 s-1 °C-1, A is the area through which heat flow is taking place, m2, Ta is the air temperature and Ts is the temperature of the surface which is drying, °C. Heat Transfer in Drying - in a roller dryer where moist material is spread over the surface of a heated drum, heat transfer occurs by conduction from the drum to the foodstuff Drum/Roller Dryer: q = UA(Ti– Ts ) where U is the overall heat-transfer coefficient, Ti is the drum temperature (usually very close to that of the steam), Ts is the surface temperature of the food (boiling point of water or slightly above) and A is the area of drying surface on the drum. Dryer Efficiencies ✓ it is a simple ratio of the minimum energy needed to the energy actually consumed ✓ useful when assessing the performance of a dryer, looking for improvements, and in making comparisons between the various classes of dryers which may be alternatives for a particular drying operation. h = (T1 - T2)/(T1 - Ta) for spray dryers where T1 is the inlet (high) air temperature into the dryer, T2 is the outlet air temperature from the dryer, and Ta is the ambient air temperature. The numerator, the gap between T1 and T2, is a major factor in the efficiency. Dryer Equipment Tray Dryers ✓ food is spread out, generally quite thinly, on trays in which the drying takes place ✓ Heating may be by an air current sweeping across the trays, by conduction from heated trays or heated shelves on which the trays lie, or by radiation from heated surfaces. Most tray dryers are heated by air, which also removes the moist vapours. Dryer Equipment Tunnel Dryers ✓ These may be regarded as developments of the tray dryer, in which the trays on trolleys move through a tunnel where the heat is applied and the vapours removed. In most cases, air is used in tunnel drying and the material can move through the dryer either parallel or counter current to the air flow. Sometimes the dryers are compartmented, and cross-flow may also be used. Dryer Equipment Roller or Drum Dryers ✓ the food is spread over the surface of a heated drum. The drum rotates, with the food being applied to the drum at one part of the cycle. The food remains on the drum surface for the greater part of the rotation, during which time the drying takes place, and is then scraped off. Drum drying may be regarded as conduction drying. Dryer Equipment Fluidized Bed Dryers ✓ the food material is maintained suspended against gravity in an upward- flowing air stream. ✓ There may also be a horizontal air flow helping to convey the food through the dryer. Heat is transferred from the air to the food material, mostly by convection. Dryer Equipment Spray Dryers ✓ liquid or fine solid material in a slurry is sprayed in the form of a fine droplet dispersion into a current of heated air. Air and solids may move in parallel or counterflow. Drying occurs very rapidly, so that this process is very useful for materials that are damaged by exposure to heat for any appreciable length of time. The dryer body is large so that the particles can settle, as they dry, without touching the walls on which they might otherwise stick. Commercial dryers can be very large of the order of 10 m diameter and 20 m high. Dryer Equipment Pneumatic Dryers ✓ the solid food particles are conveyed rapidly in an air stream, the velocity and turbulence of the stream maintaining the particles in suspension. Heated air accomplishes the drying and often some form of classifying device is included in the equipment. In the classifier, the dried material is separated, the dry material passes out as product and the moist remainder is recirculated for further drying. Dryer Equipment Rotary Dryers ✓ The foodstuff is contained in a horizontal inclined cylinder through which it travels, being heated either by air flow through the cylinder, or by conduction of heat from the cylinder walls. In some cases, the cylinder rotates and in others the cylinder is stationary and a paddle or screw rotates within the cylinder conveying the material through. Dryer Equipment Trough Dryers ✓The materials to be dried are contained in a trough-shaped conveyor belt, made from mesh, and air is blown through the bed of material. The movement of the conveyor continually turns over the material, exposing fresh surfaces to the hot air. Dryer Equipment Bin Dryers ✓ the foodstuff is contained in a bin with a perforated bottom through which warm air is blown vertically upwards, passing through the material and so drying it. Dryer Equipment Belt Dryers ✓ The food is spread as a thin layer on a horizontal mesh or solid belt and air passes through or over the material. In most cases the belt is moving, though in some designs the belt is stationary and the material is transported by scrapers. Dryer Equipment Vacuum Dryers ✓ Batch vacuum dryers are substantially the same as tray dryers, except that they operate under a vacuum, and heat transfer is largely by conduction or by radiation. The trays are enclosed in a large cabinet, which is evacuated. The water vapour produced is generally condensed, so that the vacuum pumps have only to deal with non-condensible gases. Another type consists of an evacuated chamber containing a roller dryer. Dryer Equipment Freeze Dryer ✓ The material is held on shelves or belts in a chamber that is under high vacuum. In most cases, the food is frozen before being loaded into the dryer. Heat is transferred to the food by conduction or radiation and the vapour is removed by vacuum pump and then condensed. In one process, given the name accelerated freeze drying, heat transfer is by conduction; sheets of expanded metal are inserted between the foodstuffs and heated plates to improve heat transfer to the uneven surfaces, and moisture removal. The pieces of food are shaped so as to present the largest possible flat surface to the expanded metal and the plates to obtain good heat transfer. A refrigerated condenser may be used to condense the water vapour. Dryer Equipment THANK YOU!