Q & A > Question Details
How does the introduction of Superheated Steam into a CDU disturb the partial pressures thus lowering boiling point?
30/01/2018 A: Peter Marsh, XBP Refining Consultants Ltd, peter.marsh@xbprefining.co.uk
When steam and hydrocarbon vapours are mixed together, the total pressure exerted is equal to the sum of their partial pressures (Dalton's Law). So if the total pressure (tower top pressure) is maintained constant by the control system (assumes adequate overhead condensing capacity available), then increasing the partial pressure of steam (stripping steam flow) will decrease the partial pressure of the hydrocarbons, thereby increasing vapourisation.
30/01/2018 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
The addition of steam to the column increases the total moles of vapor. The partial pressure of any vapor phase component is the mole fraction x total pressure. The addition of steam reduces the mole fraction of hydrocarbons, and thus their partial pressure. The steam and hydrocarbon behave independently of each other as far as boiling points, the reduction in partial pressure that results from adding steam lowers the hydrocarbon boiling point. As a side note the water dew point can be estimated from its partial pressure using something like Antoine's equation or steam tables (The dew point temperature is the temperature that corresponds to saturated steam conditions at the the steam partial pressure.)
29/01/2018 A: Stefan van der Wal, Mettler-Toledo Process Analytics, stefan.vanderwal@mt.com
Water and hydrocarbons are typically immiscible.
Each component in a mixture exerts ist own vapor pressure, regardless of the other components present.
Vapor pressure increases with temperature.
The total vapor pressure of the mixture is the sum of the individual vapor pressure of each component in the mixture (proportional to their mole fractions).
Adding steam (water) is adding another vapor pressure component which contributes to the total vapor pressure of the mixture.
A mixture starts boiling when its vapor pressure exceeds the atmospheric pressure.
Thanks to the extra component water, the total vapor pressure of the mixture exceeds the atmospheric pressure sooner; in other words: it boils at lower temperature.
(As water and hydrocarbons are immiscible, separating them is not an issue.)
26/01/2018 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
At a given temperature, more will flash if the pressure is reduced. Mixing water vapor with the hydrocarbon in the flash zone reduces the pressure that the hydrocarbon is influenced by. Due to the large difference in relative quantities of hydrocarbon and steam, the flash temperature is only negligibly changed. The hydrocarbon partial pressure is the mole fraction of hydrocarbon times total pressure.