Counter current extraction

THE PRESENTATION ABOUT COUNTER CURRENT EXTRACTION GIVINGBDETAIL ON ITS INTRODUCTION, THEORY, EQUATION, EXAMPLES ARE SIMPLIFIED IN EASY WAY TO THE STUDENTS AND READERS LEARNING A TOPIC AT A PLACE ACCORDING TO PCI SYLLABUS FOLLOWED BY CHEMISTRY STUDENTS OF OTHER BRANCHES.

1. COUNTER CURRENT EXTRACTION

2. COUNTER-CURRENT EXTRACTION Counter-current extraction is a refinement of basic liquid-liquid extraction that permits separation of substance with very similar portioning behaviour The term counter-current indicates that the two phases move in opposite direction actually in the CCD procedure it is usually used one phase is stable and other moves hence the two phases are in relative motion

3. Principle of counter-current extraction The distribution of a single solute between two immiscible liquids the fraction p distribution into he upper phase is a function of the partition coefficient k and the U of upper to lower phase volume p=KU­­­­­­­/KU+1 The fraction of solutes in the lower phase at equilibrium q is given by, q=1/KU+1 since p+q =1 The greater the value of KU the larger the fraction p of solutes that passes into the upper phase

4. A liquid-liquid extraction posses in which the solvent and the process stream in contact with each other flow in a opposite direction screw extactor and carousel extractor are the two types of equipments used for counter-current extraction

5. Process of Counter-current Extraction In these extraction wet raw material is pulverised using toothed disc disintegrators to produce the fine slurry The material to be extacted is moved in the one direction (generally in the form of fine slurry) within a cylindrical extractor where it comes in contact with extraction solvent The further the starting material moves the more concentrated the extract becomes Finally, sufficiently concentrated extract comes out at one end of the extractor while the marc falls out the other end

6. 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).

7. 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.

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

9. 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.

10. Normal Approximation to the Binomial Distribution Counter-current extraction is a discontinues process that is it occurs in discrete countable steps its exact mathematical description the distribution is therefore a discontinuous function The procedure in finding the function is to express n by equation which gives a close approximation when n is large

11. This equation is variously called as the normal distribution , normal error curve this derived equation it is an approximation to the binomial distribution it is an important distribution function in many physical contexts

12. The Normal Curve

13. Advantages A unit quantity of plant material can be extracted with much smaller volume of solvent as compared to other methods like maceration, decoction, and percolation CCE is a commonly done at room tempreture which spare the thermoliabile constituent from exposure to heat which is employed in most other techniques As the pulverization of the drug is done under wet conditions the heat generated during communication is neutralized by water this again spares the thermoliabile constituent from exposure to heat The extraction procedure has been related be more efficient and effective than continuous hot extraction

14. Applications DNA purification Food industry Examples citrus oils unsaturated fatty acids An important application is citrus oils processing An important subject in perfumes and food industry

15. Extraction With Agitation Agitation is the movement of one or more components of a mixture to improve contact Putting into motion by shaking or stirring often to achieve mixing

16. Mechanically Agitated Extractors rotary-agitated columns, Reciprocating columns Examples of rotary-agitated columns include: Scheibel columns, rotating disk contactor (RDC), Oldshue -Rushton Columns, Kuhni columns, etc. An example of the reciprocating column is the Karr columns

17. Because of the mechanical advantages of rotary agitation, most modern extractors employ this method. Mechanical agitation helps improve the formation of new droplets and increase interfacial turbulence, resulting in greater efficiency. These extractors are typically proprietary devices for which complete design procedures are not publicly available.

18. Scheibel Extractor This is probably the oldest of column having agitators. It operates on the mixer-settler principle. There are many versions of this design, an example of which is shown in the Figure. The agitators are mounted at fixed intervals on a central vertical shaft, and wire-mesh packings are installed to improve coalescence and separation of the phases

19. Rotating Disk Contactor (RDC) In this system, horizontal disks are used as agitating elements, which are mounted on a centrally supported shaft. Mounted on the column wall and offset against the agitator disks are the stator rings, whose have aperture is greater than the agitator disk diameter. This device uses the shearing action of the rapidly rotating disks to inter-disperse the phases.

20. Asymmetric Rotating Disk (ARD) Contactor The column consists of an asymmetrically located mixing zone and a transfer-settling zone partially separated from each other by means of a vertical baffle. The mixing zone contains a number of compartments, each of which is equipped with a disk-type mixing impeller mounted on a common rotor shaft. The transfer-settling zone consists of a series of compartments, separated by means of annular horizontal baffles. Its design is aimed at retaining the efficient shearing action of the RDC by using rotating disks to produce dispersion while using the coalescence re-dispersion cycle produced in the separated transfer-settling zone to reduce back-mixing.

21. Karr Reciprocating-Plate Extractor This is a "descendant" of the pulse column with fixed plates. Essentially, the reciprocating-plate column has sieve plates moving up and down (i.e. reciprocating motion) driven by an overhead motor. Thus its operation involves "pulsing" the plates in a steady flow liquid medium. The Karr Extractor consists of a stack of perforated plates and baffles which have a free area of about 58%. The central shaft which supports the plates is reciprocated by means of a reciprocating drive mechanism located at the top of the column. The amplitude is adjustable generally from 3 to 50-mm and the speed is variable up to 1000 stokes/ Perforated trayes moves up and down More uniform drop size distribution

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