Hygroscopic_and_Nonhygroscopic_Material.pptx

7/30/2019 Hygroscopic_and_Nonhygroscopic_Material.pptx

http://slidepdf.com/reader/full/hygroscopicandnonhygroscopicmaterialpptx 1/9

Hygroscopic and Nonhygroscopic Material

Most of the agricultural (food) products arehygroscopic materials which always have aresidual moisture content.

The moisture in hygroscopic materials may be

Bound moisture that remains in the materials due

to closed capillaries or due to surface forces or

Unbounded moisture that remains in the materialsdue to surface tension of water.

A hygroscopic materials, on getting exposed to

air, will either absorb moisture or desorb moisturedepending on the relative humidity of air.



Solar Drying of Agricultural Products

The basic essence of drying is to reduce the moisturecontent of the product to a level that presentsdeterioration within the ‘SAFE STORAGE PERID’.

Drying is a dual process of

Heat transfer to the product from the heating source,

and Man transfer of moisture from the interior of the

product to its surface and from the surface to thesurrounding air.

Drying involves the extraction of moisture from the

product by heating and the passage of air massaround it to carry away the released vapour.




Under ambient conditions the process of extraction of moisture continues until thevapour pressure of the moisture held in theproduct equals the partial pressure of themoisture held in the atmosphere.

In such a case the rate of the moisturedesorption from the product to theenvironment and absorption from theenvironment are in equilibrium.

The moisture in the crop in such a condition isknown as the Equilibrium Moisture Content.




The drying process involves simultaneous Heat transfer from the surrounding to the surface of

the product being dried combined with heattransmission within the material, and

Mass transfer from inside the product to its surface;

followed by external transport of moisture to thesurroundings.

Parameters of the product such as the physicalproperties (size, density etc.) moisture content andmass-heat transfer coefficient between the air and

the agricultural product all vary during the dryingprocess.




This is further influenced by the conditionexternal to the product such as temperature,humidity and flow rate of the drying air andalso by changes in the chemical compositionof the agricultural product during the dryingprocess.

Each agricultural product can tolerate amaximum temperature dependent on itstype(vegetable, fruit, cereal etc.) use of the

product, moisture content and degree of maturity.



Issues in Promoting the Use of Solar Dryers

The level of losses with traditional dryingpractices has often been exaggerated.

Accurate estimation of losses in difficult

Worst case experience (rather than average

experience ) has influenced the perception. Benefits of small scale solar dryers over

estimated.




This is further influenced by the conditionexternal to the product such as temperature,humidity and flow rate of the drying air andalso by changes in the chemical compositionof the agricultural product during the dryingprocess.

Each agricultural product can tolerate amaximum temperature dependent on itstype(vegetable, fruit, cereal etc.) use of the

product, moisture content and degree of maturity.



Issues in Promoting the Use of SolarDryers

• Monetary values of loss prevention is often overestimated.

• Sensitivity of prices to consumer observable quality

characteristics for common commodities is quitelow.

• The market may not bear a sufficient premium for ahigh quality product to cover the additional drying

costs.• The solar dried products may have to be aimed at

an urban market large scale caterers or middleclass consumers.




There is significant seasonal variability in thedemand for dryer services.

Poor capacity utilization of the dryer.

For dietary staples the major part of the

production is for home consumption. No cash benefits from quality preservation of

products.

Very little financial motivation to use a dryer.

Hygroscopic_and_Nonhygroscopic_Material.pptx

Documents

Transcript of Hygroscopic_and_Nonhygroscopic_Material.pptx