Evaporate, Boil down

Evaporate, boil down

During evaporation, a solution is heated until the solvent has evaporated and the nonvolatile fraction has been enriched or completely separated as a solid. The colloquial term for the same process is boiling down, especially when viscous concentrates are produced.

For a simple mass balance between feed (0) and concentrated product (1), the following applies:

ṁ_0 = ṁ_1 + ṁ_Dampf

ṁ_0 · w_S,0 = ṁ_1 · w_S,1

The heat required for evaporation consists (highly simplified) of sensible heating and latent heat of vaporization:

Q̇ = ṁ_0 · c_p · (T_Sied − T_Ein) + ṁ_Dampf · Δh_V

Here, c_p is the heat capacity, T_(Sied) the boiling temperature, T_(Ein) the inlet temperature, and Δh_V the enthalpy of vaporization.

Meaning of the symbols:

m_0̇: Mass flow rate of the feed
m_1̇: Mass flow rate of the concentrated product
m_Dampḟ: Mass flow rate of vaporized solvent
w_S,0: Mass fraction of solids in the feed
w_S,1: Mass fraction of solids in the product
Q̇: Heat rate
cp: specific heat capacity of the liquid/solution
T_Sied: Boiling temperature in the apparatus
T_Ein: Inlet temperature of the solution
Δ_hV: Enthalpy of vaporization of the solvent

In equipment such as amixon® mixing and drying systems, the liquid can be efficiently removed by evaporation. Optionally also under vacuum. Afterwards, the residual moisture is further reduced by drying until a free-flowing powder is formed. Here, in addition to thermodynamics (boiling point, enthalpy of vaporization), changing material consistencies (liquid, highly viscous, semi-solid, powdery) also play a role. This approach avoids low fill levels in the vacuum mixer-evaporator.