= Primary drying
\b[Primary drying] is the <freeze-drying> stage in which ice is removed mainly by <sublimation>. Frozen <water> becomes <water vapor> without first becoming bulk liquid water. This distinguishes it from <secondary drying>, which addresses residual sorbed water after the main ice-removal stage.<Lyophilization of Parenteral (7 93) by the Food and Drug Administration>[Introduction: freezing, primary drying/sublimation and secondary drying/desorption.]{ref}
A simple physical balance explains why removing frozen water still requires energy. If a mass $m$ of ice sublimes and the relevant specific <latent heat> is $L_s$, the idealized energy requirement is
$$
Q=mL_s.
$$
For an illustrative $m=0.20\,\mathrm g$ and assumed $L_s=2.8\,\mathrm{MJ\,kg^{-1}}$, this gives $Q=560\,\mathrm J$. The example neglects heating of the remaining material and heat losses; it is an energy inventory rather than a drying schedule.
Thus reducing pressure does not remove the need for <heat transfer>. The phase change consumes energy while vapor must leave the material. A decrease in sample mass can establish removal of matter under appropriate assumptions, but identifying <primary drying> also requires evidence that the removed water was initially ice and followed the <sublimation> route.
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