Surface pollution

1) Splashes in the vaporization space

The vapor velocity must be limited in the gas part of vacuum crystallizers, so that the walls are not covered with droplets, which would leave a solid deposit on vaporization.

According to Mersmann [MER 88], the vapor velocity must be such that:

Y: surface tension of the liquid (N.m-1)

pL, pB: density of the liquid and the condensation (kg.m-3)

g: acceleration due to gravity (9.81 m.s-2).

2) Encrustation of surfaces and thermal transfer Encrustation is detrimental to thermal transfer.

The thermal flow density is:

where tsus, tsurf, tpf and tf are, respectively, the temperatures of the suspension, the wall surface (encrusted or not), the metal lining for the thermal fluid and, finally, the thermal fluid (the liquid that heats or cools, gas which condenses, liquid which vaporizes). The thermal flux density is:

By adding the thermal resistances:

eM: thickness of the metal lining: 0.001-0.002 m

XM: metal conductivity (W.m-1.°C-1) (15 for stainless steel and 50 for mild steel)

een: encrustation thickness (m) (zero initially)

Xen : conductivity of deposit (W.m-1.°C-1) (from 1 to 5) asus and af are the forced convection coefficients for the suspension and for the thermal fluid.

If the solution includes a solute that can crystallize on the wall, it is necessary that:

Wall roughness has a detrimental effect for two reasons:

  • - protuberances hook particles from the suspension and can thus hold them to the wall;
  • - the wall surface area is multiplied by the roughness compared with a smooth surface, which facilitates nucleation (see section 4.1.2.).

All deposits are subject to re-entrainment, if the suspension velocity is sufficient.

There are three types of deposits:

  • 1) thickness een increases in a linear way according to the time for hard and resistant deposits;
  • 2) if the deposit is less resistant, the fluid can at least partially drag it. According to time, thickness een follows a law of logarithmic aspect;
  • 3) re-entrainment is equal to the deposit, or when thickness een is such that the global transfer coefficient has decreased sufficiently, encrustation ceases and een moves towards an asymptomatic level.

Note that crystal deposit on the thermal transfer surfaces can be significant, particularly for crystallizers; this deposit is known as a “crust”.

Often, encrustation only begins after a latency period during which the particles are progressively snared by the wall, which occurs more slowly if the suspension velocity is high (the recommended velocity is between 1.5 m.s-1 and 2 m.s-1)

 
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