Distillation under vacuum is a substitute for steam distillation
Flash distillation, differential distillation & steam distillation are all continuous distillation processes
In differential (simple) distillation, the vapour is removed as soon as it is formed without appreciable condensation
In flash (equilibrium) distillation, a definite fraction of liquid is vaporised in such a way that the evolved vapour & residual liquids are in equilibrium with each other
EXAMIANS