Thread: Vacuum Evaporation

Hello Science Forum Members,I am building a proto evaporationtank to evaporate a sugared liquid from 3% to 6%.The evaporation tank will sendvapour to a shotgun condenser then to a catch tank – I want to save the waterfor reconstitution- I am formulating a pop/cola.A venturi will be installedbetween the evaporation tank and the condenser to pull a vacuum at 1”HG/25.4mmHG/0.03 atm – that should allow me to evaporate somewhere around 25 deg C. Aswell a venturi will be placed between the catch tank and the condenser to pumpthe condensate into it – 10”HG/254 mmHG/0.33 atm.Some specs:225 liter round evaporation tank/flatbottom – 45 deg down pumping pitch blade impeller at center bottom – turbulentflow with baffles – top surface area (liquid level full) – 0.292 sq meters- turbulent flows should actuallyhelp with evaporation.Liquid – 3% sugar reduced to 6%sugar - approx. 1 to 150 cP, p ≈ 1 - 1.3g/cm³ ≈ 1.3 kg/L , μ ≈ 1.14 -1.19 mP_a/s- approximately half/112.5 liters of water to beremoved.In-tank heaters will be used: notto boiling but rather to 80-90 C to apply a slow pasteurization method. Thiswill take place during evaporation by applying heat to batch contents from thebeginning starting at 25 C – with any luck, arriving at this temp by the timethe evaporation stage is complete. And of course this will raise pressures.I realize that there are a numberof variables here. I’ve tried to cover as many specifications as possible – Iam not a calculus guy – but can punch numbers in a calculator as good as any.This is where my questions come in;Is there a formulation I coulduse to calculate the evaporation flow rate and the time to spec(cfm/scfm/liters per min or hour) – 3%to 6% sugar of evaporation tank?Is there a formulation I coulduse to calculate the dew point or points temperature required of the condenser– the required temp of the circulating fluid to allow condensing? It is a shotgun type condenser (tube in shellwith cold fluid pumped through it).Could you provide an exampleusing the above specs ?Do you foresee any problems withthis setup or have any recommendations?Empirical formulations are fineas from what I’ve been reading it is not an exact science – but close would benice. My efforts to use equilibrium equations - Clausius–Clapeyron, etc.. - have gone no where.Well, that’s a mouth full.Appreciate any help.Regards,David S.

The equation you need to calculate the evaporation rate is the Langmuir equation. There is a discussion of it here Maruza, sharing for a better world: Water Evaporation Rate Per Surface Area remember you will need to calculate the ambient partial pressures using the reduced pressure of the system. As for the temperature of the condenser as long as it is cooler than the boiling point of water at the pressure you are operating at (data for this should be readily available try the CRC handbook or here. Pressure and Boiling Points of Water ) the water will condense. You can calculate how much water needs to be lost to achieve the concentration you want and then use the Langmuir equation to calculate how long this will take.

Cheers PhDemon,

I was on to Langmuir- and a few others- just to unsure - the need to be told.

Really appreciate the help.

Regards,

David S