Drying Rate Curve-Calcium Carbonate

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Drying is defined as removal of small amount of water or other liquid from a material by application of heat.

Drying involves both heat and mass transfer operations. Heat must be transferred to the material to be dried in order to supply the latent heat required for vaporization of the moisture. Mass transfer involves the diffusion of water through the material to the evaporating surface and subsequent evaporation of water from the surface.1,2

Rate relationship can be studied considering a simple model, which mimics the conditions of a dryer. In this model, the wet slab to be dried is placed in a tray whose bottom and sides are insulated. The air is blown over the solid under the constant drying conditions ( air velocity, temperature, humidity and pressure). The superficial water diffuses through the surrounding air film and is carried away rapidly by the moving stream. Then water diffuses from the interior of the solid to the surface. This process continues until bound water gets evaporated. Then the materials attain equilibrium moisture content.

Rate of drying of this process can be determined by periodically weighing the calcium carbonate slurry. The difference in the weights of two successive weightings gives the loss of moisture content, i.e., amount dried. The following equation is used to calculate rate of drying:

Rate of drying = weight of water removed/weight of dry powder*time of drying * surface area exposed.

A graph is plotted by taking FMC on x-axis and drying rate on y-axis. The curve so obtained is called as drying rate curve. It represents different changes occurring during drying, where AB is constant rate period, BC is first falling rate period, and CD is second falling rate period. The FMC at the end of constant rate period is known as critical moisture content (CMC).

Aim: To dry calcium carbonate slurry and plot the rate of drying curve.

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Apparatus: Hot air oven


Stainless steel plate


Chemicals: Calcium carbonate

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The stainless steel plate was weighed and the weight was recorded as well.15 g of calcium carbonate was transferred into a beaker. Water was added slowly to prepare slurry. The calcium carbonate slurry was transferred into the stainless steel plate. Filling must be done in such a way that 3/4th of the volume of the volume of the stainless steel plate was filled with slurry. The weight of steel plate plus slurry was taken and recorded. The plate containing slurry was placed in hot air oven, whose temperature must be maintained at 600c.The time was noted soon after placing plate containing slurry in hot air oven. After 15 minutes, the weight of plate with slurry was taken. The weight was recorded. Again the steel plate containing slurry was placed in the dryer. Steps 8 and 9 were repeated until constant weight was obtained. The rate of drying was calculated. A graph was plotted by taking free moisture content on x-axis and rate of drying in y-axis.

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  1. The drying rate curve was plotted by taking FMC on x-axis and rate of drying on y- axis.
  2. Total moisture content removed from calcium carbonate slurry (30 – 3n) =———-g

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  1. Subrahmanyam CVS, Setty JT, Suresh S. Laboratory manual of pharmaceutical engineering (Unit operations). Delhi Vallabh Publications.
  2. Pharmaceutical Engineering by K.Sanbamurty (New Age International, New Delhi)
  3. Cooper and Gunn’s Tutorial Pharmacy Edited by S.J.Carter (CBS Publishers, Delhi)