Mass Transfer Operations
Assignment 1
U21CH062
Divyansh Verma
Interfaces are formed above and below the heavy liquid diffuser and light liquid diffuser in a
spray column for liquid-liquid extraction or similar processes due to the differences in density
and interfacial tension between the two liquid phases involved. Let's explore why interfaces form
in these locations:
1. Above the Heavy Liquid Diffuser:
- Heavy Liquid Phase: The heavy liquid phase is typically used as the stationary phase or the
continuous phase in a liquid-liquid extraction process. It has a higher density compared to the
lighter liquid phase.
- Differences in Density: The heavy liquid tends to settle at the bottom of the column due to its
higher density. As the lighter liquid is sprayed or introduced from above, it must displace the
heavy liquid to create a space for itself.
- Interface Formation: This displacement of the heavy liquid by the lighter liquid results in the
formation of an interface or boundary between the two liquids. The interface forms at the point
where the two liquids meet, with the lighter liquid floating on top of the heavier liquid.
2. Below the Light Liquid Diffuser:
- Light Liquid Phase: The light liquid phase is typically introduced from below as a dispersed
phase in the form of droplets or a continuous phase in a countercurrent extraction process. It
has a lower density compared to the heavy liquid phase.
- Differences in Density: The light liquid phase rises in the column because it is less dense
than the heavy liquid below. As it rises, it displaces the heavy liquid, creating space for itself
within the column.
- Interface Formation: Similar to above the heavy liquid diffuser, an interface forms below the
light liquid diffuser as the light liquid phase displaces the heavy liquid. This interface marks the
boundary between the two phases, with the lighter liquid suspended above the heavier liquid.
The formation of interfaces is essential in liquid-liquid extraction processes as it allows for the
intimate contact and mass transfer between the two immiscible phases. The interfaces provide
a large interfacial area where the transfer of solutes or target components from one phase to the
other can occur. The efficiency of the extraction process depends on the effectiveness of these
interfaces in facilitating the transfer of desired compounds while minimizing the mixing of the
two phases.
In processes involving heavy and light liquid phases in a spray column or liquid-liquid extraction
system, there are specific reasons for withdrawing the heavy liquid from above (often referred to
as the overflow) rather than from the bottom. This design choice is primarily driven by the
physical properties and behavior of the two immiscible liquids and the requirements of the
separation process. Here's why heavy liquid is typically taken from above rather than from the
bottom:
1. Density Difference: The primary reason is the difference in density between the heavy and
light liquid phases. Heavy liquids have a higher density, causing them to settle at the bottom
naturally, while lighter liquids tend to rise. If you were to withdraw the heavy liquid from the
bottom, it would be challenging to maintain a clear separation between the two phases.
2. Minimizing Mixing: Withdrawal from the bottom could lead to turbulent mixing of the two
liquids. This mixing would reduce the efficiency of the liquid-liquid extraction process because
you want to maintain a well-defined interface between the two phases to maximize mass
transfer and separation efficiency.
3. Preserving the Interface: To ensure effective mass transfer between the two phases, it's
essential to maintain a stable interface. Withdrawal from the top (overflow) allows you to
maintain this interface and prevents significant disruption of the boundary between the heavy
and light liquids.
4. Avoiding Contamination: Taking heavy liquid from the bottom may result in entrainment of
the lighter liquid phase, leading to contamination of the heavy liquid phase. This can be
undesirable in cases where the heavy liquid is the product or where purity is critical.
5. Process Control: Withdrawal from the top is more controllable and allows for better
monitoring and adjustment of the liquid-liquid interface. It is easier to maintain consistent
operating conditions and separation performance when withdrawing the heavy liquid from
above.
In summary, taking the heavy liquid from above in a liquid-liquid extraction system is a practical
design choice that helps maintain the separation between the two immiscible phases, enhances
separation efficiency, minimizes mixing, and allows for better control of the process. It ensures
that the heavy liquid remains relatively undisturbed at the bottom, while the lighter liquid can be
introduced and withdrawn from the top to optimize the separation process.