ERT 313BIOSEPARATION ENGINEERING

LEACHING

Prepared by:Miss Hairul Nazirah Abdul Halim

Introduction• Leaching / solid extraction – methods of removing one constituent

from a solid by means of a liquid solvent.• Examples:

1) Extraction of oil from soybean flakes

Leaching Equipment

Principle of Continuous Countercurrent Leaching

• the quantitative performance of a countercurrent system can be analyzed by utilizing an equilibrium line and an operating line, and as before, the method to be used depends on whether these lines are straight or curved.

• 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

Operating Line• Total solution:

• Solute:

• Operating line equation:

Constant & Variable Underflow

Number of Ideal Stages for Constant Underflow• Use McCabe Thiele Method if the operating line is straight• In leaching, the operating line is always straight

Tutorial 4EXAMPLE 23.1.

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 inTable 23.1.

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.

SolutionLet 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 23.1, is 0.505 kg/kg. Then

From Table 23.1, the solution retained is 0.507 kg/kg:

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

At the solid inlet,

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.

Overall material balance:

Oil balance:

• The point x1 = 0.60, y2 = 0.408 is at one end of the operating line for the remaining stages.

• To determine an intermediate point on the operating line, choose xn = 0.30.

• By an overall balance,

• An oil balance gives

• The points xn , xn+1 , xa , ya and xb , yb define a slightly curved operating line, as shown in Fig. 23.3.

• Four ideal stages are required.