CONTER CURRENT EXTRACTION

 

Countercurrent Extraction - Craig Apparatus.

 

Theory

A method of multiple liquid-liquid extractions is countercurrent extraction, which permits the separation of substances with different distribution coefficients (ratios). A clever design known as Craig apparatus is used for this purpose (Lyman C. Craig, 1943).



Craig apparatus consists of a series of glass tubes (r: 0, 1, 2..) that are designed and arranged such that the lighter liquid phase is transferred from one tube to the next. The liquid-liquid extractions are taking place simultaneously in all tubes of the apparatus which is usually driven electromechanically. In the following animated picture of a single glass tube the typical "extraction/transfer" cycle is shown.


The lower (heavier) phase of the two-phase solvent system (e.g. water, blue layer in the picture) is the "stationary phase", whereas the upper (lighter) phase (e.g. hexane, red layer in the picture) is the "mobile phase".

In the beginning, tube #0 contains the mixture of substances to be separated in the heavier solvent and all the other tubes contain equal volumes of the same solvent. The lighter solvent is added to tube #0, extraction (equilibration) takes place and the phases are allowed to separate. The upper phase of tube #0 is then transferred to tube #1 and fresh solvent is added to tube #0, and the phases are equilibrated again. The upper layers of tubes #0 and #1 are simultaneously transferred to tubes #1 and #2 respectively. This cycle is repeated to carry on the process through the other tubes of the apparatus. Obviously, substances with higher distribution ratio move faster than those with a lower distribution ratio.

It is interesting to examine the distribution of a substance A in each tube after a given number of equilibration/transfer cycles.

Comments

Post a Comment

Popular posts from this blog

BATCH AND CONTINOUS EXTRACTION

Image
  Difference between Continuous and Batch Proce For a long time, the food industry has relied on a large-scale continuous process. At the outset, the production facilities for new products used to be a batch process produced on a larger scale because of reduced investment costs. As the economy of scale has become a main factor, food process industries and process engineering, in general, emphasized on developing and designing continuous processes for mass production of goods. However, specialty products and product diversification, with more emphasis on customer requirements and their need for food security and high quality, as well as trace ability requirements, mean that equal focus has shifted to batch processes. In fact, almost half of today’s processes in the food industry are actually batch processes. Industrial processes can actually be classified based on the process’ output as: Batch processes Continuous processes So, what are these two and how are they different from one ...
Read more

SOLVENT EXTRACTION

 Solvent extraction, also called liquid-liquid extraction    Immiscible liquids are ones that cannot get mixed up together and separate into layers when shaken together. These liquids are usually water and an organic solvent. •   Solvent extraction is the process in which a compound transfers from one solvent to another owing to the difference in solubility or distribution coefficient between these two immiscible (or slightly soluble) solvents. • It is a method of quantitative separation of compounds. • When extracting solvent is stirred with solution containing solute then solute from original solvent gets transferred into an extracting solvent. • When stirring is stopped extracting solvent form separate layer and now it contains solute of interest. • Compared with other separation methods, it gives a better separation effect than chemical precipitation, and a higher degree of selectivity and faster mass transfer than the ion exchange method. • Compared with distil...
Read more