Continuous Counter-Current Extraction in Craig Method

Cornish et al., as well as Randall and Longtin, did early work in the creation of liquid-liquid separation techniques utilising a process called “systematic fractional distribution.” Still, Lyman C. Craig is unquestionably the primary player. Lyman Craig developed counter-current distribution by examining the distribution of medicine called mepacrine (atabrine) across the two layers of a biphasic solvent solution made up of ethylene dichloride, methanol, and aqueous buffer. The composition of the solvent system and the pH of the buffer affected the distribution coefficient (Kc) of atabrine. In the following article, Craig was motivated by Martin and Synge’s work with partition chromatography to create an apparatus to separate chemicals depending on their distribution constant.

In this post, we will look into the definition of counter-current extraction in the Craig method and throw some light on the types of the extraction process and what is the continuous extraction process.

Definition Of Counter-Current Extraction

The counter-current extraction can be defined as a type of liquid-liquid extraction in which two components of distinct solubility in the phases of two immiscible liquids are separated. In this method, two immiscible liquids flow in opposite directions.

This is accomplished by using a sophisticated design known as the Craig device. The Craig apparatus is made up of a succession of glass tubes built and positioned to transmit the lighter liquid phase from one tube to the next. The liquid-liquid extractions take place in all tubes of the electromechanically operated device simultaneously. Tube A holds the combination of chemicals to be separated in the heavier solvent at first, while all the other tubes contain the same solvent in equal volumes. The lighter solvent is supplied to tube A, and the phases are allowed to separate after extraction (equilibration). The top phase of tube A is then transferred to tube B, and a new solvent is introduced to tube A, re-equilibrating the phases. The upper layers of tubes A and B are transmitted simultaneously to tubes B and C.

This cycle is repeated to continue the process through the apparatus’s additional tubes. Substances which have a higher distribution ratio move more quickly than those with a lower distribution ratio. The “stationary phase” of a two-phase solvent system is the lower (heavier) phase, while the “mobile phase” is the upper (lighter) phase.

Both phases are continually introduced (or changed) and flow in opposite directions as the extraction advances in counter-current extraction. The technique becomes a crosscurrent extraction when only one phase is constantly added. The feed is repeatedly (continuously) touched or rinsed with extracting solvent in both countercurrent and crosscurrent extractions. Using solvents with favourable distribution coefficients or raising the solvent-to-feed ratio maximises the number of theoretical stages. Up to eight theoretical steps are commonly used in these processes. Counter-current extractions are beneficial since they employ only solvent, no sorbents, and relatively mild conditions, even though chromatographic procedures have mostly superseded them. When the distribution coefficient is small, they are preferred.

Counter-current extractions, on the other hand, utilise a lot of solvents and aren’t ideal for isolating huge amounts of solute. Because the extraction steps are performed step-by-step rather than as continuous extractions, the Craig counter-current device, which is often used to research partition chromatography, is a discontinuous, differential migration process.

Applications Of Continuous Counter-Current Extraction (CCE)

Having understood the definition of counter-current extraction, let’s now look at some of its applications.

Many significant separations have used the CCE technique, such as bile acids, polycyclic hydrocarbons, penicillin, taxol, ribonucleic acids, etc. Aromatic compounds are recovered from the petroleum oil’s paraffin fraction, and chemical compounds are isolated from aqueous systems using modest amounts of organic solvents in the synthesis of synthetic pharmaceuticals and intermediates.

Types Of Extraction Process

There are many types of extraction processes. Some of them are listed below:

• Supercritical fluid extraction

• Ultrasound-assisted extraction

• Heat reflux extraction

• Mechanochemical-assisted extraction

• Maceration

• Instant controlled pressure drop extraction

• Perstraction

• Liquid-liquid extraction

• Solid-phase extraction

• Acid-base extraction

• Microwave-assisted extraction

What Is The Continuous Extraction Process?

Now that you are aware of the definition of counter-current extraction, let’s now discuss what the continuous extraction process is.

Continuous countercurrent extraction, also known as soxhlet extraction, is extracting chemicals from solid materials. For the determination of crude fat content, soxhlet extraction is used. Fat is found in abundance in many plants’ seeds and fruits. Fat content can be used as a criterion for determining the quality of a product. The extraction procedure is widely used across the world. In China, the analysis of oil and grain is done by a traditional method, known as Soxhlet extraction, and it is the chosen standard method. This process requires a lot of time and is usually done with a fat extractor in a lab.

To extract the solid matter, a pure solvent can be used during the usage of the syphon method and solvent reflux. This leads to high extraction efficiency. However, it is also important to note that a solid substance must be discreetly ground before the extraction process. It is important for maximising the area of liquid immersion.

Assemble the device as directed. When the solvent reaches boiling temperature, the vapour rises through the air tube and condenses into a liquid, which drips into the extractor. The phenomenon of syphoning happens when the liquid level surpasses the highest point of the syphon.

Conclusion

We successfully studied the definition of counter-current extraction, what the continuous extraction process is and the types of the extraction process. Counter-current extraction uses a serial extraction of both the sample and the extracting phases, as opposed to a sequential liquid-liquid extraction, which repeatedly extracts the sample containing the analyte. Although counter-current separations are no longer widely used because chromatographic separations are significantly more efficient in terms of resolution, time, and ease of use, the theory underlying them is nevertheless valuable as an introduction to the theory of chromatographic separations.