Separation process 2,, Summaries of Chemical Separation Processes

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SEPARATION

PROCESS II

Drying of solids

Mechanism of drying

• (^) Drying may be defined as the vaporization and removal

of water or other liquids from a solution, suspension, or

other solid-liquid mixture to form a dry solid.

• (^) It is a complicated process that involves simultaneous

heat and mass transfer, accompanied by

physicochemical transformations.

• (^) Drying occurs as a result of the vaporization of liquid by

supplying heat to wet feedstock, granules, filter cakes

and so on.

• (^) Based on the mechanism of heat transfer that is

employed, drying is categorized into direct (convection),

indirect or contact (conduction), radiant (radiation) and

dielectric or microwave (radio frequency) drying.

• (^) Heat is transferred to the product to evaporate liquid, and mass is transferred as a vapor into the surrounding gas.
• (^) The drying rate is determined by the set of factors that affect heat and mass transfer.
• (^) Solids drying is generally understood to follow two distinct drying zones, known as the constant-rate period and the falling-rate period.
• (^) The two zones are demarcated by a break point called the critical moisture content.
• (^) In a typical graph of moisture content versus drying rate and moisture content versus time (Figure 1), section AB represents the constant-rate period.
• (^) In that zone, moisture is considered to be evaporating from a saturated surface at a rate governed by diffusion from the surface through the stationary air film that is in contact with it.
• (^) This period depends on the air temperature, humidity and speed of moisture to the surface, which in turn determine the temperature of the saturated surface.

• (^) At the end of the constant rate period, (point B, Figure 1), a break in the drying curve occurs.
• (^) This point is called the critical moisture content, and a linear fall in the drying rate occurs with further drying.
• (^) This section, segment BC, is called the first falling-rate period.
• (^) As drying proceeds, moisture reaches the surface at a decreasing rate and the mechanism that controls its transfer will influence the rate of drying.
• (^) Since the surface is no longer saturated, it will tend to rise above the wet bulb temperature.
• (^) This section, represented by segment CD in Figure 1 is called the second falling-rate period, and is controlled by vapor diffusion.
• (^) Movement of liquid may occur by diffusion under the concentration gradient created by the depletion of water at the surface.

• (^) The capacity of the air (gas) stream to absorb and carry away moisture determines the drying rate and establishes the duration of the drying cycle.
• (^) The two elements essential to this process are inlet air temperature and air flowrate.
• (^) The higher the temperature of the drying air, the greater its vapor holding capacity.
• (^) Since the temperature of the wet granules in a hot gas depends on the rate of evaporation, the key to analyzing the drying process is psychrometry, defined as the study of the relationships between the material and energy balances of water vapor and air mixture.

Types of dryers

• Adiabatic dryers are the type where the solids are dried by direct contact with gases, usually forced air.
• (^) With these dryers, moisture is on the surface of the solid.
• Non-adiabatic dryers involve situations where a dryer does not use heated air or other gases to provide the energy required for the drying process Dryer classification can also be based on the mechanisms of heat transfer as follows:
• Direct (convection)
• Indirect or contact (conduction)
• Radiant (radiation)
• Dielectric or microwave (radio frequency) drying

Batch dryers: The following are descriptions of various types of batch dryers.

• Tray dryers : This dryer type operates by passing hot air over the surface of a wet solid that is spread over trays arranged in racks.
• (^) Tray dryers are the simplest and least expensive dryer type.
• (^) This type is most widely used in the food and pharmaceutical industries.
• (^) The chief advantage of tray dryers, apart from their low initial cost, is their versatility.
• (^) Drying times are typically long (usually 12 to 48 h).
• Vacuum dryers : Vacuum dryers offer low-temperature drying of thermolabile materials or the recovery of solvents from a bed.
• (^) Heat is usually supplied by passing steam or hot water through hollow shelves.
• (^) Drying temperatures can be carefully controlled and, for the major part of the drying cycle, the solid material remains at the

• Freeze dryers : Freeze-drying is an extreme form of vacuum drying in which the water or other solvent is frozen and drying takes place by subliming the solid phase.
• (^) Freeze-drying is extensively used in two situations: (1) when high rates of decomposition occur during normal drying; and (2) with substances that can be dried at higher temperatures, and that are thereby changed in some way.

• (^) Microwave vacuum dryers: High frequency radio waves with frequencies from 300 to 30,000 MHz are utilized in microwave drying (2,450 MHz is used in batch microwave processes).
• (^) Combined microwave convective drying has been used for a range of applications at both laboratory and industrial scales.
• (^) The bulk heating effect of microwave radiation causes the solvent to vaporize in the pores of the material.
• (^) Mass transfer is predominantly due to a pressure gradient established within the sample.
• (^) The temperature of the solvent component is elevated above the air temperature by the microwave heat input, but at a low level, such that convective and evaporative cooling effects keep the equilibrium temperature below saturation.
• (^) Such a drying regime is of particular interest for drying temperature-sensitive materials.
• (^) Microwave-convective processing typically facilitates a 50% reduction in drying time, compared to vacuum drying.

• (^) Spray dryer. The spray-drying process can be divided into four sections: atomization of the fluid, mixing of the droplets, drying, and, removal and collection of the dry particles Atomization may be achieved by means of single-fluid or two-fluid nozzles, or by spinning-disk atomizers.
• (^) The flow of the drying gas may be concurrent or countercurrent with respect to the movement of droplets.
• (^) Good mixing of droplets and gas occurs, and the heat- and mass- transfer rates are high. In conjunction with the large interfacial area conferred by atomization, these factors give rise to very high evaporation rates.
• (^) The residence time of a droplet in the dryer is only a few seconds (5–30 s).
• (^) Since the material is at wet-bulb temperature for much of this time, high gas temperatures of 1,508 to 2,008°C may be used, even with thermolabile materials.
• (^) For these reasons, it is possible to dry complex vegetable extracts,