For immiscible species here Water vapor and hydrocarbons  :

P(total) = P (HC) + P (Steam)

So constant total pressure, increasing of P (Steam) will result in lower HC partial pressure

Adding steam reduced  the partial pressure of the individual hydrocarbons in vapor pressure above a mixture according to  Dalton’s Law.

If the hydrocarbon compound is nearly immiscible with the water that results from condensed steam, the non-condensed part of the injected steam keeps the phase separated liquid agitated, so that each of the two liquid phases exerts its own vapour pressure as if the other liquid phase were not present. So, the vapour pressures of both phases, organic and aqueous, add themselves to increase the vapour pressure of the mixed liquid phases when compared with that of the isolated organic phase if no steam was injected. Boiling occurs when the temperature is such that the sum of the vapour pressures of the two immiscible liquids ― which are a function of temperature ― equals or exceeds the absolute vacuum pressure, as it is maintained by the vacuum system that assists distillation. Such a vacuum system may consist of a steam ejector or not.  

Dear Mohammed

If the effective pressure is lowering means the total pressure lowering and according to Henry’s law of partial pressure and Dalton’s law, If P1 is the pressure of steam, P2 is the pressure of Hydrocarbon, P1+ P2 =P(Total pressure)If P1 (mass of steam increase)increase according to these laws P2 automatically decreased because the total pressure is constant.

Any fractionation column consists of re boilers, which provide the heat duty required for separation. In case there are chances of the process fluid degrade at certain temperatures, option of re boiling such fluid at high temperature need to be replaced with some other mechanisms in order to achieve vaporization. (e.g. higher feed preheating or Steam distillation) Injecting steam serves two purposes, first and foremost is that steam lowers the partial pressure of the process fluid (steam & fluid are totally immiscible) and hence the boiling point. Therefore, the process fluid can start vaporizing at much lower temperature (well below its degradation point) Secondly, hot steam is source of heat energy and this heat is transferred to the fluid to create partial vaporization. 

Regards,

Prem Baboo

Dear Mr. Mohammed

All of them have explained rightly.  I think Mr. Dalton will explain you in more details. So please try to understand his law.

Prem Baboo Does adding steam also help in vacuum distillation where addition of steam increases the total pressure proportionately instead of being constant ?

The fact is that if the partial pressure of some components equals the total pressure a saturation point accures. If the partial pressure is less than the total pressure, then components begin to compensate by their phase change to make a balance between the total and partial pressures. Now, when some components with higher partial pressure flys, they will cause afew components with lower partial pressure flys too. The ratio actually depends on the molecular mass of the components.