THERMAL PROCESS ENGINEERING DRYING AND EVAPORATION

BASIC KNOWLEDGE BASIC KNOWLEDGE DRYING EVAPORATION

In general, drying refers to the removal of moisture from solids, gases A wide range of process engineering In the context of thermal The aim of evaporation can be Different evaporator designs are or liquids. For drying gases and liquids, adsorption is normally used. principles are used in drying, due to process engineering, evapora- to obtain the solvent, to create a used depending on the aim of the The food technology industry is an example of where drying solids on the variety of industrially moisture tion is understood to be the concentrate solution or to precipitate separation process. Essentially a large scale is important. containing materials. These mate- separation of a solvent from a the dissolved solid by crystallisa- they are heat exchangers in which rials can have extremely different solution. An example of a solu- tion. steam is normally used as the heat- Thermal drying of solids involves removing moisture from the behaviours. tion is salt water, in which salt ing medium. The solution can pass material by vaporisation or evaporation. The drying characteristics (the dissolved solid) is present Industrial applications of evapora- through the evaporator tubes once depend on how the moisture is retained within the material. In the The following unit operations can in the solvent, i.e. water. The tion include: (straight-through evaporator) or first instance, the liquid adhering to the surface of the material to be distinguished: addition of heat exclusively several times (circulation evapora- be dried can be removed by vaporisation or evaporation. Once this evaporates the pure solvent Increasing the concentration of tor). For solutions containing temper- liquid has been removed, drying of the moisture contained within Convection drying (water in this example) from solutions i.e., salts, alkalis, acids, ature-sensitive substances, thin film the capillaries and pores of the material begins. The drying speed A flowing gas transfers the heat the solution and carries it away. plastic solutions, fruit and vegetable evaporators are used. These limit reduces due to the need to overcome capillary forces and diffusion necessary for drying to the material The remaining solution thus juices, milk etc. the retention time of the solution in resistance. Crystal water which is bonded into the crystal structure of by convection. As well as delivering has a higher concentration areas with high temperatures. the material, can only be removed by intense heating in addition to heat, the gas is also used to remove of dissolved solid than before Obtaining products i.e., sugar low drying speed. the moisture given off by the mate- the addition of heat. from juices, salt from brine, drinking rial. water from sea water.

Contact drying The material is placed on or is passed over heated surfaces. Heat is predominantly transferred to the material by conduction.

Radiation drying The material absorbs emitted elec- tromagnetic radiation from sources of radiation (e.g. infrared radiators). Heating and evaporation occur not only at the surface of the material but also within it.

Freeze drying The frozen material is placed in a vacuum below its triple point. Mois- ture is removed from the material, by changing it directly from a solid to a gaseous state.

High frequency drying Drying characteristics of a solid with division into drying sections (I-IV): The material is exposed to high Principle of increasing the concentration of a solution by evaporating the solvent: dX/dt drying rate, X moisture content [kg (water)/kg (dry solid)], A solution before evaporation of solvent, frequency electrical fields between t drying time; B more concentrated solution after evaporation of solvent, the electrodes of a plate capacitor. . I surface moisture, II capillary moisture, III pore moisture, Q addition of heat, 1 dissolved solid, 2 solvent IV moisture in crystal structure A part of field energy is absorbed by the material resulting in internal heating and removal of moisture.

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