Explain the connection between column and flash distillation processes

Multi-Stage Column Distillation

Unfortunately, unless the relative volatility of the components to be separated is vastly different from 1, a single flash operation is unlikely to achieve the required separation. (What would happen if the relative volatility was equal to 1?).

One option of improving the degree of separation is to use a number of flash drums in series.

Column 1

In this configuration, the liquid from the flash drums "below" the main feed are partially vaporized and used as feed to yet "lower" drums, while the vapor streams "above" the main feed are partially condensed to form the feed streams in "higher" drums. This leads to a single vapor stream concentrated in the more volatile component at the "top" and a single concentrated stream of the less volatile at the "bottom"; however, we also make several streams of intermediate concentration (that might not be useful to anyone).

An alternative to this is to simply take those intermediate streams and vaporize/condense them as needed to form a second feed stream to each of the drums, as shown.

Column 2

This configuration is far superior since we now get a single stream that is concentrated in each of our two components (for a binary mixture). the only small issue is that the total vapor flowrate decreases as we move "up" the column, while the total liquid flowrate decreases as we move "down" the column. A commonly used method of fixing this issue (and making design/analysis of these columns much simpler) is to use reflux and boilup streams

DEFINITION

A reflux stream is a stream that essentially recycles a portion of the vapor product stream back to the "highest" drum (stage).

DEFINITION

A boilup stream is a stream that essentially recycles a portion of the liquid product stream back to the "lowest" drum (stage).

A final process improvement can be made by recognizing that the energy needed to vaporize one stream can be obtained directly through the condensation of the other stream. Therefore, instead of using partial condensers/vaporizers all along the column, we simply allow the intermediate streams to physically intermix so that they can exchange energy efficiently. In this way, we change our energy requirements in such a way that we simply need a single boiler at the bottom, and a single condenser at the top. This final design is typically called a column distillation tower or simply a distillation tower:

distillation

NOTE:

Each of our "flash drums" is now called an equilibrium column stage, since they are no longer separate process units (although we will do balances as if they were).

OUTCOMES:

Explain the connection between column and flash distillation processes