To distillate the oil out of the lemongrass, the moisture from the lemongrass has to evaporate from the bone-dry material. This means the lemongrass has to be heated to 100 degrees Celsius, to cook the water. We assume the average temperature in Valaramkunnu is T1= 25 °C, and the cooking temperature of water is off course T2=100 °C. With these numbers we can calculate how much heat is needed to distillate the lemongrass, with the following equation:
The m is the mass, h is the specific heat and ΔT is the difference in temperature. Only for the latent heat, it is not necessary to multiply by the ΔT. In this equation the specific heat of water hwater= 4184 J/kg K and the latent heat of water hlatent= 2.27*106 J/kg. The specific heat of dry grass hgrass= 1500 J/kg K[1]. We obtained a total heat of Eneeded = 13.4*106 J. We are using steam distillation, so this same heat is needed for the required steam.
However, we have not yet calculated the heat loss. Heat disappears by conduction through the walls of the still. This heat loss can be calculated by the following equation:
In which U= 40 W/m2K (?), A=1,0 m2 is the surface area of the distiller (only this area the heat is lost due to conductivity), ΔT=75 K is the temperature difference between the wall and the outside temperature (the wall temperature is the same as the inside temperature of the distiller) and t is the time in seconds. The time is a variable, because we want to know the optimal time for the batch to distil, but we estimate it at t=90 min= 5400 sec. This gives us a Qloss=16.2*106 J.
This heat lost is just a waste of energy, so a decision was made to insulate the walls of the distiller. The insulation material would be glass fiber, because this is cheap and easy to apply. We estimated the insulation thickness to be 2 inch, so the total diameter with insulation Dinsulation=0.368 m. The thermal conductivity of insulation hinsulation= 0.029 W/mK, which is found in scientific literature. The rate of heat loss was already required, so the total heat loss with insulation Eloss,insulation=0.72*106 J. This is a very big difference in comparison with no insulation.
Now we know the total heat content required for the steam. This is found by adding Eneeded and Eloss,insulation. The total heat Etot=14.2*106 J.
The total steam required is obtained by dividing Qtot by the latent heat of water, which gives us Msteam=6.3 kg. Then the flow rate of steam is calculated by Msteam/time = Q = 4.17 kg/hr. This has to be disparged within the still. Eventually we obtain a total power required for the heater of Pheater=3.01 kW.
[1] Heat capacity measurements of various biomass types and pyrolysis residues, Capucine Dupont – Rodica Chiriac – Guillaume Gauthier, Article In Fuel, January 2013: The highest value from Fig 1 is taken to avoid undervaluation of the energy needed.