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Rate of Drying Curve

A representative curve for each product can be determined at specific temperatures, velocities, and pressures.

Rate of Drying Curve

A representative curve for each product can be determined at specific temperatures, velocities, and pressures. The drying curve of a specific product is referred to as a drying curve. Depending on the carrier velocity and temperature, the curve will exhibit significant variations. Understanding the curve is extremely helpful when it comes to understanding unusual drying behavior. The process of drying can be subdivided into three phases:
  • First falling drying rate period
  • Second falling rate period
  • Constant drying rate period.
Material or a mass of material, in a constant period, contains much water, resulting in a liquid surface that dries like an open body of water. As long as surface conditions remain saturated, evaporation occurs at a constant rate due to diffusion of moisture from within the droplet. Moisture content (MC) typically decreases when a solid is continuously dried. The graph begins linearly, then curves downward, then eventually plateaus. Once free moisture is visible on the surface, the drying rate will be progressively reduced until the constant rate (B-C) is reached. Drying at constant rates ends with CMC. During a constant rate event, moisture migrates from the interior to the surface via various means and is evaporated.

As the moisture content of the surface decreases, the rate at which the sediment migrates to the surface decreases as well. At too high a temperature, the surface shrinks into tightly packed cells that become sealed together. The barrier prevents moisture from migrating out and keeps it inside. Case hardening is the result of this. It involves a constant drying rate regardless of the moisture content. When the solid is so wet during this time that it persists a continuous film of water over its entire surface, lowering the rate of drying! As soon as a wet surface is wet, it reaches the wet-bulb temperature.

Web bulb temperature - It is the temperature when the bulb of the thermometer, which is covered with a wet wick, evaporates water into a high-speed air stream. The evaporation of water does not affect the temperature or humidity of the air stream. A high-velocity air stream is used to evaporate the water from the wick (minimum 300 m/min). Heat is required to cause evaporation. The glass bulb of the thermometer absorbs the heat. As the glass bulb temperature decreases, the bulb temperature also decreases. This heat is caused by the temperature difference between the small and large glass bulb. The transfer of mass and heat is simultaneous. Water vaporizing causes a change in thermal energy.

Falling Rate Periods

When evaporation from the surface exceeds migration from the interior of the surface, the constant rate period ends. 'Failing rate period' refers to the period after the critical point. A rapid decrease in the drying rate occurs after this point as the surface temperature rises. Even though moisture removal may be much less during the falling rate period, it takes far longer than during the constant rate period. At some equilibrium moisture content, the rate of drying slows down to zero. There are two processes involved in drying during the falling rate period:
  • Surface moisture removal.
  • Moisture is transported to the surface from within the material.
It depends on whether the solid is porous or non-porous how drying rates and drying times are estimated in the falling rate period. The rate at which a non-porous material can be dried depends on how fast bulk moisture diffuses to the surface after it removes superficial moisture. Porous materials, on the other hand, tend to dry from the inside out, and not just at the surface.

First Falling Period

'Critical moisture content' refers to the moisture at the end of the constant rate period (point c). Currently, the solid's surface is not saturated. As the moisture content drops, the rate of drying decreases. A full evaporation of the surface moisture film occurs at point C, where the rate of moisture movement through the solid determines the drying rate as moisture content decreases.

Second Falling Period

Drying occurs at a relatively constant rate during period C to D, regardless of the external conditions. Liquid diffusion and capillary movement are the most common methods of moisture transfer.

Effect of Shrinkage

As moisture is removed from the solid, shrinkage is one of the factors that greatly affects the drying rate. The shrinkage of rigid solids is not very pronounced; however the shrinkage of colloidal and fibrous materials is quite pronounced. In particular, the surface develops an impervious hard layer that accelerates drying by interfering with the flow of liquids and vapors. A layer of tightly packed, shrunken cells forms at the surface, which is sealed together, when materials are dried at high temperatures. This layer prevents moisture from evaporating. Material warping and structural changes are also caused by shrinkage. It is sometimes beneficial to dry with moist air in order to reduce these shrinkage effects. Therefore, the shrinkage effect on warping or hardening at the surface is greatly reduced since the drying rate is reduced.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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