After the distillation and the cooling down of the oil and the water, the oil needs to be separated from the water. To establish this, a separator unit is needed. To accomplish the best results, two different separator designs were made, both based on the same law. This law is called Stokes’ law, as stated here:
The vertical velocity can be calculated easy because all the numbers are known. ρw (density of water)= 1000 kg/m3, ρo (density of oil) = 920 kg/m3, (dynamic viscosity of oil) = 0,00132 , g (gravitational acceleration) = 9,81 m/s2 and R (radius of oil droplet) = 0,002 m. So the vertical velocity is 0,13212 m/s. The maximum allowable horizontal velocity is 15 times the vertical velocity: 1,9818 m/s. While distilling the lemongrass, 4,13 litres of oil has to be produced in the CSP system. During the distillation process 3,33 litres/hour is extracted from the lemongrass. So the inlet flow from the separator is 4,13 + 3,33 = 7,46 litres/hour. So now it is possible to calculate the minimal design requirements. The minimum horizontal area is the flow rate divided by the vertical velocity, which equals 0,000251 m2. The minimum cross-sectional area is the flow rate divided by the maximum allowable horizontal velocity, which equals 1,67*10-6 m2. Of course these numbers are too small to really use, so the design will be bigger than the minimal requirements. In the circuit only 4,13 litres/hour is needed, so the remaining 3,33 litres need to be discharged somewhere. There are various moments in the circuit where that can be done. That’s where the two different designs are based on.
Design Qin = Qout
In this design the inlet flow of the separator is the same as the outlet flow. The inlet flow is 7,4 litres/hour. The outlet flow that goes back in the circuit has to be equal to 4,13 litres/hour. The outlet flow that will be discharged has to be equal to 3,33 litres/hour. These two outlet flow rates will be regulated with pumps. The design used for the prototype will have a length of 0.5 m, a width of 0,04 m and a depth of 0.25 m. If we use the 10 kg batch, approximately 70 ml of oil will be produced. This means a layer of 0,00007 / (0,5 * 0,04) = 0,0035 m = 3,5 mm of oil will grow at the top of the separator. An outlet with a valve starting at the underside will regulate the oil outlet and will make sure almost no water will come along.
Design Qin > Qout
In this design the inlet flow will be bigger than the outlet flow, so the water level will rise in the separator. That is why a reservoir is needed on top of the separator. After one distillation cycle there will be 5 litres of liquid in the reservoir. A small top layer of oil will appear on top of the reservoir that can be removed the same way as in the other design. After the oil is removed, the rest of the water can be removed from the reservoir. After this the distillation process can start again.
After some discussions the Qin=Qout design was chosen. This is because in the Qin>Qout design some oil will be sucked into the outlet stream. Because of the relatively big depth of the Qin=Qout design this will not happen there.