Equilibrium Moisture Content - Methods of Determination of EMC, Models of EMC Determination, Importance of EMC, Hysteresis effect.

Definition of Equilibrium Moisture Content, Factors that effects EMC, Methods to determine EMC by Static & Dynamic methods, Models like Kelvin equation, Chung-frost equation, Henderson equation, Hurkins-Jura equation, it's importance and Hysteresis effect.

1. APE 311 PROTECTED CULTIVATION AND SECONDARY AGRICULTURE (1+1) EMC – METHODS FOR DETERMINATION OF EMC, EMC MODELS AND IMPORTANCE, HYSTERESIS EFFECT Presented By … Guhan . C, B.Sc.( Hons ). Agri , Id no : 2018033029 , JKKMCAS , Erode , T N .

2. EQUILIBRIUM MOISTURE CONTENT Moisture content attained by a grain with respect to set of atmospheric temperature & relative humidity is called EMC of a grain. Vapour pressure of water in grain > Vapour pressure of water in air = Water in grain vaporises & diffuses in atm. Vapour pressure of water in grain < Vapour pressure of water in air = Water in grain absorbs moisture from atm.

3. Temp ↑ & RH ↓  Water in the grain vaporises , loose moisture and dries . The property of gaining or loosing of moisture as per atmospheric condition is known as Hygroscopisity .

4. Methods… Static Method Dynamic Method Static method - Grains are left in humid condition till they attain equilibrium. Dynamic method – Humid air is agitated or moved by mechanical method and the grain attain equilibrium. METHODS OF DETERMINATION OF EMC

5. To bring atm air to desired RH levels different conc of Hcl & H2SO4 are used. Time consuming process, 3 – 4 weeks needed for attaining grain equilibrium. High RH & High Temp  High attack of molds, decomposition & structure change of grain are possible. Temp is maintained by Air-oven . RH is maintained by Acid solution . VP of acid depends upon conc. of chemical & temp. STATIC METHOD…

6. Handling of conc. solution is not ea sy, using salts are relatively safer. Hall (1957) and Lal (1969) collected info on use of salts for EMC determination .

7. Desorption Method. The property of dry air to absorb moisture from moist grains is employed. Moist grain put in air tight container, when air attains equilibrium to grain its RH is measured by Electric hygrometer or Hair hygrometer. Container has little quantity of air, reach equilibrium in short period. DYNAMIC METHODS…

8. ii. Isotenoscopic method. Absorption of moisture by dry air to determine grain EMC. In this method arrangement is available to measure directly the vapour pressure exerted by the moist grains.  Isotenoscope is a U Tube filled with liquid of negligible vapour pressure.

9. Determination of EMC of agri products many theoritical , semi- theroitical and experimental models have been proposed. But none of the theoritical equations are found to predict correct EMC values. Some of the models are… Kelvin Equation Harkins-Jura Equation Chung- Pfost Equation Henderson Equation EMC MODELS…

10. Kelvin Equation Given in 1871. Model of moisture absorption by solid materials. Phenomenon of capillary condensation in pores of solid materials was considered. Relationship b/w VP of water present in capillary and saturated VP at same temp is the basis of Capillary condensation theory.

11. Kelvin equation Grain EMC determination is limited to RH > 95% when the capillary condensation takes place.

12. 2. Harkins-Jura equation. Proposed in 1944 Based on theory of existence of a potential field above surface of solid materials. Work required to absorb or desorb water molecules = Work required to overcome vapour molecules to come on surface + work necessary for condensation. Harkins-Jura equation doesn’t predict accurate EMC values When RH > 30%. d & e = product constant depend on temp.

13. 3. Chung- Pfost Equation. Proposed an equation based on potential field theory. EMC values are predicted when RH b/w 20 – 90%.

14. 4. Henderson Equation. Proposed in 1952. Equation is very popular and based on Gibbs’ absorption equation.

15. Where, M e = EMC, % ( db ) rh = relative humidity, decimal T = temperature, Kº C and n = constant, dependent on crop type and temperature

16. EMC gives idea of gain or looses of moisture at particular atmospheric condition . Gives Rate of moisture removal . Helps to determine drying characteristics . Helps to predict the moisture level in which materials can be dried with heated air. IMPORTANCE OF EMC

17. Some agri product loose moisture and attain EMC with surroundings – Desorption EMC . When dry products gain moisture from surroundings and attain EMC – Absorption EMC . At same temp & RH there is a m eaningful difference b/w desortion and absorption EMC . The desorption values are higher than absorption values. The difference b/w desorption and absorption curves is known as Hysteresis effect. HYSTERESIS EFFECT