Solid Liquid Extraction 694y2l

Solid-Liquid Extraction

Introduction • Definition: Methods of removing one constituent from a solid by means of a liquid solvent. • Generally also known as leaching, lixiviation or washing. • Widely used when thermal or mechanical method is not possible

Introduction • Solid liquid extraction procedures includes – appropriate solvent to transfer the soluble constituent into solvent. – Separation and washing of solution from the residual solid.

• Examples: – Extraction – Extraction – Extraction – Extraction

of of of of

sugar from sugar beets. oil from oil bearing seeds. coconut milk from coconut. oil from soy bean flakes.

Comparison Between LLE and SLE •

Figure 1: Liquid-Liquid Extraction

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Figure 2: Solid –Liquid Extraction

Source: http://www.gunt.de//extraction_english.pdf

Equipments used for extraction Figure 4: Figure 3: • Separator Funnel

• Soxhlet Extractor

• Bollman Extractor

• Rotocel Extractor

CONTINUOUS COUNTERCURRENT LEACHING • •

The solid is not moved physically from stage to stage The liquids is being moved from stage to stage

Figure:5: Countercurrent leaching process • V phase = the liquid phase (from stage N to stage 1) • L phase = the liquid carried with the solid (from stage 1 to stage N) • Exhausted solids leave Stage N • Concentrated solution overflow from Stage 1

CONTINUOUS COUNTERCURRENT LEACHING • • • •

Solution retained by entering solid, xa Solution retained by leaving solid, xb Fresh solvent entering the system, yb Concentrated solution leaving the system, ya

Equilibrium Relationship • •

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Equilibrium is attained when the solute is completely dissolved and the concentration of the solution so formed is uniform. The concentration of the liquid retained by the solid leaving any stage is the same as that of the liquid overflow from the same stage. The equilibrium relationship is simply xb= y.

Operating line • Use McCabe Thiele Method if the operating line is straight • In leaching, the operating line is always straight • The equilibrium line is on 45oline.

Example: Oil is to be extracted from meal by means of benzene using a continuous countercurrent extractor. The unit is to treat 1,000 kg of meal (based on completely exhausted solid) per hour. The untreated meal contains 400 kg of oil and is contaminated with 25kg of benzene. The fresh solvent mixture contains 10kg of oil and 655 kg of benzene. The exhausted solids are to contain 60kg of unextracted oil. Experiments carried out under conditions identical with those of the projected battery show that the solution retained depends on the concentration of the solution, as shown in the following table.

Concentrati on, kg Solution oil/kg retained, solution kg/kg solid 0 0.5 0.1 0.505 0.2 0.515 0.3 0.53 0.4 0.55 0.5 0.571 0.6 0.595 0.7 0.62

Find: (a) the concentration of the strong solution, or extract; (b) the concentration of the solution adhering to the extracted solids; (c) the mass of solution leaving with the extracted meal; (d) the mass of extract; (e) the number of stages required. All quantities are given on an hourly basis.

Solution: Let x and y be the mass fractions of oil in the underflow and overflow solutions. At the solvent inlet,

Determine the amount and composition of the solution in the spent solids by trial. If xb= 0.1, the solution retained, from Table, is 0.505 kg/kg. Then

Solution: From Table ,the solution retained is 0.507 kg/kg:

Benzene in the underflow at Lb is 507 - 60 = 447 kg/h. At the solid inlet,

Solution e) Determine the inlet and exit concentrations for the first stage and locate the operating line for the remaining stages. Since x1 = ya= 0.60, solution retained is 0.595 kg/kg solid.

The points xn , xn+1 ,xa , ya and xb, yb define a slightly curved operating line Four ideal stages are required.