Retention Time in a Liquid-Liquid Vessel

Retention time, also called residence time, in a liquid-liquid separation vessel is the amount of time that a liquid phase spends inside the vessel.  In the context of liquid-liquid separators, this is the time it takes for the dispersed phase (often droplets of one liquid) to coalesce and separate from the continuous phase.  Retention time is typically designed based on experience or calculated using the flow rates and physical properties of the liquids being separated.  Adequate retention time ensures efficient separation in processes like oil-water separation, solvent extraction, or chemical reactions involving immiscible liquids.

Importance of Retention Time

The immiscible liquids have sufficient time to separate based on density differences.
The settling or coalescence process is complete before the liquids exit the vessel.

Factors Influencing Retention Time

Fluid Properties  -  The densities and viscosities of the liquids affect how quickly they separate.
Flow Rate  -  Higher flow rates reduce retention time, while lower flow rates increase it.
Vessel Design  -  The volume and internal geometry of the vessel play a role in determining the retention time.
Emulsion Stability  -  If the liquids form a stable emulsion, longer retention times may be necessary for effective separation.

 

Retention Time in a Liquid-Liquid Vessel Formula
\( T \;=\;  A \; \eta \;/\;  \gamma_b - \gamma_t \)
Symbol	English	Metric
\( T \) = Retention Time in a Liquid-Liquid Vessel	\(h\)	\(h\)
\( A \) = Separator Constant (varies from 0.05 to 1.0)	\(dimensionless\)	\(dimensionless\)
\( \eta \)  (Greek symbol eta) = Viscosity of Predominant Phase	\(lbf - sec \;/\; ft^2\)	\( Pa - s \)
\( \gamma_b \) = Specific Gravity of Bottom Phase	\(dimensionless\)	\(dimensionless\)
\( \gamma_t \) = Specific Gravity of Top Phase	\(dimensionless\)	\(dimensionless\)