DRYING TECHNOLOGY. Drying is the separation of the liquid from a solid-liquid-compound by means of evaporation or steaming.

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1 DRYING TECHNOLOGY Drying is the separation of the liquid from a solid-liquid-compound by means of evaporation or steaming. 1. Why are products dried? - Product must be as dry as possible for the further processing (granulate/powder) or for the purpose of final sale (durability) - Adaption of packaging volumes for storage and transportation - For the processing in pelletizing machines or calcination plants - To reduce undesired by-products e.g. solvents to a minimum residual value (Example: pharmaceutical and chemical products, plant extracts etc.) - Production of pure distillates - Recuperation of solvents 1.1. Drying Methods - Mechanical drying without phase change by filtration, centrifuging or pressing - Chemical drying by means of special drying agents, adsorption with auxiliaries (molecular sieve) or reaction with auxiliaries (chemical compounds) -Thermal drying by means of heat application 1.2. Thermal Drying This drying method is characterized by the simultaneous exchange of heat and material Phase 1 : Heat supply by contact heating conduction (Convection dryer/ Radiation) Generates heat directly in the product (High frequency / Microwaves) Phase 2 : Separation of dry substance and moisture i.e. to remove moisture and condensate it outside of the dryer Drying by Convection A hot gas flushes around the product and enters into a heat and mass transfer with the product. This method transfers the required heat by convection to the wet product. Heat transfer medium is the preheated drying gas i.e. normally air. To intensify the drying, the drying gas does not only stream above the wet product surface but is penetrating into the product from the outside, whirling it up and conveying it pneumatically. Brief-D.dot 1/ Westendstrasse 21 a Fax +49(0)7763 / Bankverbindungen:

2 Heat- and Mass Transfer between drying air and product surface result from the following relations: Heat Transfer : Q =k A ( T L - T T ) Q = Heat stream ( W ) k = Heat transitional rate ( W / m2 K ) T T = Temperature of boundary layer on product surface ( C ) T L = Temperature of drying air ( C ) A = Product surface ( m2 ) Material Transfer : M = K A ( X T - X L ) M = Mass flow ( kg / h ) K = Material transitional rate of evaporation ( kg / m2 h ) X T = Water vapour content of product surface ( kg / kg ) X L = Water vapour content of drying air ( kg / kg ) Contact Drying The product is in touch with a heated surface and is either stationary or being constantly brought into new contact with the effective heating surface by stirring or agitating resp. revolving. This method transfers the heat from the heated walls and interior parts of the dryer generally thru a heat conduction system but also via radiation over the wet product. If the dryer is operated under vacuum, the heat is supplied exclusively by the heated components of the dryer. The moisture evaporates and the wet exhaust vapours are being removed by suction. Heat Transfer : Q = K A ( T H - T P ) Q = Heat stream ( W ) K = Heat transitional rate ( W / m2 K ) T H = Temperature of heating medium ( C ) T P = Temperature of product ( C ) A = Heat exchange surface ( m2 ) Drying by Radiation Important heat flux densities are being transferred to the drying goods by radiation which results in very short drying periods. Part of the electromagnetic radiation energy is being absorbed by the wet product. The absorbed energy serves to warm up the product as well as for the evaporation of the moisture. The heating process becomes active not only on the product surface but has also an effect on the inside so as to allow a faster drying without fissuring or cracking on the surface and avoids formation of exterior skin. Heat Transfer : Q = C Ψ A [ ( T 1 /100 ) 4 - ( T 2 /100 ) 4 ] C = Radiation exchange constant Ψ = Radiation rate ( = f ( Setup radiation volume and product surface )) A = Radiation surface T1 = Temperature of radiator T2 = Temperature of wet product Drying by High Frequency or Microwaves This drying method is treating the wet product dielectrically between the electrodes of a plate capacitor exposing it to high frequent electric fields with frequencies from 2 up to 100 Mhz. Part of the field energy is absorbed for the displacement of ions in areas of lower conducting capacities in the product and for directional movements of dipoles. A direct consequence of the displacement and directional movements is the intensive molecular friction and, as a result, the increasing heat inside the product which is used to warm up the product and to evaporate the moisture. Drying lecture.doc 2 /

3 Freeze Drying The freeze drying or lyophilisation is a vacuum sublimation drying method where the moisture is extracted under vacuum from the deep frozen product at temperatures below 0 C thus changing directly from the solid to the vapour condition Summary The drying equipment is operating often in a combination of both convection and contact drying systems. The extremely different qualities and structures of the various products to be processed is determining the large number of processing systems as well as the numerous types of dryer constructions. Since the condition of the wet product is changing during the drying process, a calculation based on theoreticals is almost impossible. Both experimental values and material testing are necessary for the calculation of drying plants. 2. The Drying Process The drying process largely depends on how the moisture (mostly water) is bound to the solid matter. In the best case, the moisture adheres in thin layers to the products surface and is being evaporated by the drying air lying above the product and will then be carried along as water vapour until saturation is reached. Except from adhering to the surface, the water can also be contained in capillaries, pores or crystals in which cases the molecular and capillary barriers must first be overcome if the moisture content should be brought to the surface and subsequent evaporation. The thermal drying process is subdivided in two steps i.e. in the first place, heat is either led from outside via conduction, convection or radiation to the product or can be generated inside. This heat is used to vaporize moisture from the product whereafter a separation is made between product and exhaust vapours which are carried away and condensed outside the dryer. The necessary drying period to bring the product from its initial moisture content to the ultimately required humidity depends on the drying speed which is never constant but decreases in relation with the products humidity. The drying curve is being divided in an area with constant drying speed and another one with decreasing drying speed. The drying speed remains constant until the surface water is evaporated whereafter it depends on the diffusion speed of the vapour inside the product. The drying curve must be evaluated by trials based on which the drying period can be determined. 3. Capacity of a Drying Plant The capacity of a drying plant is characterized by its drying speed (change of moisture per unit of time). Several factors influence the drying speed, the most important ones are: a. Physical product characteristics b. Product layout and influence on heat exchange against wall (contact) or with gaseous medium (convective) c. Ambience conditions during drying process and conception of drying plant d. Product conduct during drying process e. Features of drying equipment a. Physical Product Characteristics Possible types of products to be dried: Solid Matter Pourable powder or granulate Solid Matter In lumps, pellets or flat-shaped disks, cubes, flakes Pulp or Paste Pasty-slurry, liquefied, very viscous or muddy Pulp or Paste Pasty-non slurry, thick-pasty or kneadable Liquid Solution Liquid Suspension Drying lecture.doc 3 /

4 Following types of humidity in the product may be distinguished : Adhesive liquid = Water in thin layers on exterior product surface Capillary liquid = Water is wettening the interior pore surface of the porous solid. This water can be conveyed to the product surface by means of capillary energy during the drying process. Well Water = Water is wettening the product surface being integral part of product. Drying involves shrinking of product. Crystal water = Water is firm and formative part of product. Removing ( cracking ) can be done only after exceeding a crystal-specific rate of decomposition temperature. The liquid diffusion from the product core to the surface has the most important impact on the drying speed. The humidity evaporates very quickly at the particles surface. This process can be even more accelerated by using vacuum or a flow medium or by revolving the product. b. Product layout and influence on heat exchange The product must be evenly distributed in thin layers because thick layers are drying slower and the drying is less uniform. c. Environmental conditions during drying process The variety of different drying processes can be classified according to following criteria: 1. Pressure and temperature in the drying room: Outer air drying Drying air temperature = Room temperature Normal pressure-excess temperature drying Drying air temperature = Room temperature Vacuum-Low temperature drying Drying at depression and low temperatures 2. Type of energy supply: Convection drying Temperature, degree of humidity and speed of drying medium (air or gas) have a decisive influence on the drying capacity Contact drying Pressure, temperature and flow time are of prime importance for the efficiency of the drying process Examples for heat exchange coefficient of various dryers: Dryer Type Heat transmission ( W / m2 C ) Vacuum drying cabinet 5 Vacuum belt dryer Double cone dryer Rotary dryer Fluidized bed dryer Drum type dryer d. Product behaviour during the drying process The most important factors during the drying are : 1. Structure and composition of raw materials. The kind of the water s texture in the product is very important as we distinguish between free and bound types of water. The so-called free waters evaporate easily and leave the product at constant drying speed while bound waters involve a braking of the drying speed. Drying lecture.doc 4 /

5 2. Shrinkage of product Except for the freeze drying, almost all drying processes are characterized by a strong change of the structure. Products with very strong shrinkage have bad instant qualities i.e. they are difficult to be solved or only slowly in cold or warm liquids. 3. Loss of light volatile components e.g. flavours strongly depending on the product composition, bond structure and drying period. 4. Browning reaction A tanning resp. browning effect may appear during the drying due to high temperatures or during storage due to oxydation and/or enzymatic influence. 5. Water absorption and rehydration ( Sorption-Isotherms ) The study about the water absorption features ( Hygroscopicity ) is important for the determination of storage conditions and the max. storage period. The humidity which leads to microbiologic changes or to enzymatic activities is measured as limiting value. Sorption Isotherms Products are dried in order to extend their durability and to improve their storage stability. The durability, expressed in sensory features such as taste and smell but also the preservation of ingredients, structure and colours, is mainly influenced by the volume of water content. Storage under too much oxygen and light exposure may also reduce the durability. The detrimental changes can be caused by oxydation, chemical and enzymatic reactions, microbiologic deterioration and physical conversion. The water activity or relative air humidity of a product is determined by measuring its sorption isotherms. Depending on the amount of relative air humidity, the following changes in the product can be noticed: -Growth of microorganisms This can be avoided by humidity elimination. -Enzymatic activities These are accelerating with increasing water content. -Chemical changes The non-enzymatic browning reaction, the so-called Maillard-reaction, plays an important part with regard to quality decrease between sugar and amino groups. Exception: controlled Maillard-reaction for flavour enhancing of several products (e.g. soup condiments, meat extracts, malted drinks) -Physical changes Resolving capacity of reconstitution, formation of lumps, inner solution process, recrystallization etc. same as the evaporation of primary flavour ingredients during the storage must be considered as decisive factors for deteriorative physical change and are, more or less, also depending on the volume of water content and temperature. The dehydrogenation by heat impact increases the durability of the foodstuff provided that the activity of the residual moisture in the dried product corresponds with the stipulated requirements. Therefore, the drying can be seen as a conservation process during which the storage properties of the products are stabilized taking into account the surrounding temperatures. Drying lecture.doc 5 /

6 Criteria for the selection of drying process and dryer type Properties of the product to be dried Kind, shape, humidity texture, hygroscopicity, density, viscosity, sorption equilibrium behaviour Caloric conductibility Initial and ultimate moisture content, desired quality of dried product Thermal and mechanical consistency Corrosion behaviour Inflameability, risk of explosion, toxicity Kind and behaviour of moisture Product throughput Investment cost Operation cost Space requirements Operating safety Environmental impact Procedure for layout and optimization of driers Assessment and judgement of product characteristics and drying behaviour, product quality Product appearance and thermal sensitivity Heat conducting behaviour Diffusion behaviour Recording of sorption isotherms Recording of drying behaviour curve, determination of necessary drying time Recording of moisture profiles in the product Conversion of drying behaviour curves to other drying conditions Assessment of operating type Selection of dryer construction type Assessment of phase matching If required, evaluation of necessary drying period in semi-technical trial dryer under operation-like conditions Dimensioning of dryer Heat and gas demand analysis Calculation of dryer s measurements (diameter, length, area of heat exchange contact, (see also under below-listed dryer construction types) Optimization of geometry and operating conditions Layout of peripheral plant components (gas heater, fans, cyclone etc.) Drying lecture.doc 6 /

7 Survey of technically important dryer construction types Convection Dryer Product is lying on fixed bed/base Drying kiln Drying cabinet, drying chamber with horizontal and cross air guidance Nozzle dryer Product lying on mobile supporttunnel dryer Belt dryer (one and multiple belt dryer, turbine belt dryer, spiral belt dryer, vertical belt dryer, Hanging or moving belt dryer, short festoon dryer, swing type dryer, tower dryer) Product is moved by agitator tools Plate dryer Ring type rack dryer Ring wheel turbine dryer Product is moved by force of gravity Drum dryer Drip type dryer Turbine shaft dryer Product is moved by inertial energy Vibration dryer Groove type dryer Product ist moved by drying medium Suspension dryer Fluidbed dryer Stream dryer Helical stream dryer Spray dryer (for liquid products) Contact Dryer Product lying on mobile base Drum dryer Product is moved by agitator tools Plate dryer Trough dryer Screw type dryer Rotary paddle dryer Product is moved by force of gravity Mill dryer Vacuum Dryer Vacuum drying cabinet Vakuum belt dryer Vacuum drum dryer Tumble dryer Plate dryer Spray dryer Infrared Dryer Belt dryer with infrared radiators High Frequency Dryer Belt dryer for high frequency drying Freeze Dryer Drying lecture.doc 7 /