CONTINOUS FLOW EXTRACTION

When a substance is to be extracted from as solvent in which it is not very soluble. Into one in which it is, then the use of the separating funnel with 3 batches of fresh solvent is usually sufficient.
A method of continuous extraction is used when solubility are less favorable or when it is necessary to extract all the material as in reaction mechanism ………

Two types of apparatus are used. One for extraction from a less dense solvent into a more dense solvent and the other for extracting from a more dense into a less dense solvent.

The most common method is to extract from a relatively dense solvent such as aqueous alcohol into a less dense solvent such as pentane
Whichever method is used, the solvent chosen for the extraction must not be miscible with the solution that is to be extracted.

To set up the apparatus for extraction with the less dense solvent, start by adding the material to be extracted to the sample reservoir flask.
Add the extracting solvent to the reflux flask .
And place the flask in the heating mantle.
Add a few anti-bumping granules then connect the two flasks to the extraction apparatus
The solvent delivery tube on the right should reach close to the bottom of the sample reservoir.
Clamp the apparatus and secure the flasks with clips.
Finally add a reflux condenser .
The water should be secure with wire or clips since the apparatus may be left refluxing for long periods.
Then start heating.

When the solvent in the reflux flask is boiling, its vapor passes across the side arm up into the condenser above the sample flask.
Condensate from the condenser drips into the funnel shaped top of the solvent delivery tube.
There should be a small space between the funnel and the base of the condenser to allow the vapor free access to the condenser
The extracting solvent collects in the delivery tube until a head is formed which forces droplets of solvent out into the bottom of the sample reservoir.
Here the light droplet of solvent rise up through the denser sample solution extracting solute as they do so and forming an upper solute rich layer.

This upper layer accumulates until it reaches the side arm which acts as a ware controlling the return of  solvent to the reflux flask.
As the extracting solvent circles through the system, the solute is concentrated to the reflux flask
The apparatus can be left running for long periods to ensure complete extraction.
In difficult cases, it can be left running for several days
The time needed depends on relative solubility of the material in the two solvents.

The apparatus for extraction with the solvent that which is denser than the sample solution, is designed to produce a continuous descending stream of solvent droplets.
Commonly, these denser solvents are halogenated compounds such as dichloromethane
First, assemble then extracting apparatus and reflux flask then add the extracting solvent to the sample reservoir until the level is a couple of centimeters below the side arm connecting it to the reflux flask which should already contain a few anti-bumping granules
Then, carefully add the material to be extracted on top of the solvent
As the sample layer builds up, it starts to dry the denser lower layer of solvent cross into the reflux flask.

Take care not to add too much sample
Some solvent should always remain at the bottom of the sample reservoir to prevent the sample solution from entering g the tube to the reflux flask.
The solvent delivery tube has holes at the top to allow vapor access to the condenser.
At its base, it has  a glass fret to break the descending solvent into small droplets.
Finally, add the condenser.
Note that the condenser water supply tube must be secured as the apparatus will often need to be left working for long period of time.
Turn on the water, and start heating.
When the solvent in the reflux flask is boiling, the vapor passes up across the upper side arm into the condenser..
Drops of condensate collect in the delivery tube and are channeled to the frit.

A fret forms small droplets with its relatively large surface area increases the rate of solute extraction as the solvent descends through the sample solution in the reservoir.
For solute rich solvent collects at the bottom and then returns through the side arm to the reflux flask where the extracted material is concentrated.
So in both types of apparatus, on completion, all the extracted material will be in the reflux flask together with some solvent.
Solvent in the sample reservoir should be disposed off in the appropriate waste solvent container.