ERT 313

Bioseparation Engineering

SOLID-LIQUID EXTRACTION

(LEACHING) Zulkarnain Bin Mohamed Idris

e-mail: zulkarnainidris@unimap.edu.my

COURSE OUTCOME (CO) Ability to APPLY principles of leaching, ANALYZE

leaching equipment and CALCULATE material balance and number of ideal stages for multistage leaching!

OUTLINES 1. Introduction to leaching 2. Principles of continuous countercurrent leaching 3. Calculations of material balance and number of ideal stages

for multistage leaching 4. Equipment for leaching

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

2) Extraction of coconut milk from coconut

• The amount of soluble material removed is often greater than in ordinary filtration washing

• The granular feed solids may disintegrate in pulp or mush wheb their content of soluble material is removed.

LIQUID-LIQUID EXTRACTION vs SOLID-LIQUID EXTRACTION

Liquid-Liquid Extraction

Solid-Liquid Extraction

PRINCIPLE OF CONTINUOUS

COUNTERCURRENT LEACHING

• The most important method of leaching is the continuous

countercurrent method suing stages.

• The solid is not moved physically from stage to stage

• The liquids is being moved from stage to stage

• 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

• 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 xe = y.

EQUILIBRIUM

OPERATING LINE • Total solution:

• Solute:

• Operating line equation:

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

• The equilibrium line is on 450 line.

EXAMPLE 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 in Table 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.

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 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.

Leaching Equipment: Bollman extractor

• Contains a bucket elevator in a closed casing

• The buckets are loaded with flaky solids such as

soybeans

• The solids are sprayed with appropriate amount of

half miscella as they travel downward.

• Half miscella is the intermediate solvent containing

some extracted oil and some small solid particles.

• As solids and solvent flow concurrently down the

right-hand side of the machine, the solvent extracts

more oil from the beans.

Fig. 23.1 (a) Bollman extractor

Rotocel extractor • A horizontal basket is divided into walled

compartments with a floor that is permeable to the

liquid

• The basket rotates slowly about a vertical axis

• Solid are admitted to each compartment at feed

point

• The compartments then pass a number of solvent

sprays, a drainage section and a discharge point.

• To give countercurrent extraction, the fresh solvent

is fed only to the last compartment before the

discharge point.