MISS. RAHIMAH BINTI OTHMAN

(Email: rahimah@unimap.edu.my)

CHAPTER 5:

SOLID-LIQUID

EXTRACTION

(LEACHING)

COURSE OUTCOMES

CO APPLY principles of leaching.

ANALYZE leaching equipments.

CALCULATE material balance

and number of ideal stages for

multistage leaching.

� Introduction to leaching.

� Leaching equipments.

� Principles of Continuous

Countercurrent Leaching.

� How to CALCULATE

material balance and

number of ideal stages for

multistage leaching.

OUTLINES

� Leaching / solid extraction – methods of

removing one constituent from a solid by

means of a liquid solvent.

� Examples:

1. Making coffee from ground coffee beans

and tea from tea leaves. (The complex mixture of chemicals that give coffee

and tea their odor, taste, and physiological effects

are leached from the solid by hot water)

2. Extraction of oil from soybean flakes

3. Extraction of coconut milk from coconut

� The amount of soluble material removed is

often greater than in ordinary filtration

washing.

� The coarse, hard or granular feed solids may

disintegrate in pulp or mush when their

content of soluble material is removed.

INTRODUCTION TO LEACHING

Stationary Solid

Beds

LEACHING

EQUIPMENTS

Moving-bed

Leaching

Dispersed-solid

Leaching

DESIRED OUTPUT

Stationary Solid

Beds

LEACHING

EQUIPMENTS

Moving-bed

Leaching

Dispersed-solid

Leaching

DESIRED YIELD

STATIONARY SOLID BEDS

It is done in a tank with a perforated false bottom to support the solids and permit drainage of the solvent. Solids are loaded into the tank, sprayed with solvent until their solute content is reduced to the economical minimum. In some cases the rate of solution is so rapid that one passage of solvent through the material is sufficient, but countercurrent flow of the battery of tanks are more common. A series of tank is called as extraction battery. [The solid in any one tank is stationary until it is completely extracted]

Shanks process – other tanks in the battery are kept in countercurrent operation by advancing the inlet and draw off tanks one at a time as the material is charged and removed. Diffusion battery – A series of such pressure tanks operated in countercurrent solvent flow.

Stationary Solid

Beds

LEACHING

EQUIPMENTS

Moving-bed

Leaching

Dispersed-solid

Leaching

DESIRED YIELD

MOVING-

BED

LEACHING

Bollman

Extractor

Rotocel

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.

BOLLMAN EXTRACTOR

Fig. 23.1 (a) Bollman extractor

MOVING-

BED

LEACHING

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.

ROTOCEL EXTRACTOR

� 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.

PRINCIPLE OF CONTINUOUS

COUNTERCURRENT LEACHING

PRINCIPLES OF CONTINUOUS

COUNTERCURRENT LEACHING

Ideal Stages In Countercurrent Leaching

Equilibrium

Operating Line

Constant And Variable Underflow

Number of ideal stages for constant

underflow

Number of ideal stages for variable

underflow

� 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

IDEAL STAGES IN COUNTERCURRENT LEACHING

� 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:

� As usual, the operating line passes through the points (xa, ya) and

(xb, yb), and if the flow rates are constant, the slope is L/V.

CONSTANT AND VARIABLE

UNDERFLOW � Two cases are to be considered.

� If the density and viscosity of the solution change considerably with solute concentration, the solids from the lower-numbered stages may retain more liquid than those from the higher-numbered stages.

� As shown in Eq. 23.3, the slope of the operating line varies from unit to unit.

� If the mass of solution retained by the solid is independent of concentration, then Ln is constant and the operating line is straight. – called as constant solution underflow.

� If the underflow is constant, so is the overflow.

� Constant underflow and variable underflow are given separate consideration.

� 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.

NUMBER OF IDEAL STAGES FOR

CONSTANT UNDERFLOW

� When the underflow and overflow vary from stage to stage, a modification of the McCabe Thiele graphical method may be used for calculation.

� The terminal point on the operating line are determined using material balances.

� Will be discussed later….

NUMBER OF IDEAL STAGES

FOR VARIABLE UNDERFLOW

QUESTION 1

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.

EXCERCISE1 (LEACHING)

We wish to treat 1000 kg/hr (wet basis) of meal (D)

that contains 0.20 wt frac oil (A) and no benzene (S).

The inlet solvent is pure benzene and flows at

662 kg/hr. We desire an underflow product that is

0.04 wt frac oil. Temperature and pressure are

constant, and the equilibrium data are given in Table

1. Find the outlet extract concentration and the

number of equilibrium stages needed in a

countercurrent leaching system.

QUESTION 2

Mass Fraction Oil

(Solute) in Solution Mass Fraction Underflow (Rafffinate)

yA xA xD xS

0 0 0.67 0.333

0.1 0.0336 0.664 0.304

0.2 0.0682 0.66 0.272

0.3 0.1039 0.6541 0.242

0.4 0.1419 0.6541 0.213

0.5 0.1817 0.6366 0.1817

0.6 0.224 0.6268 0.1492

0.7 0.268 0.6172 0.1148

TABLE 1: Test Data For Extraction Of Oil From Meal With Benzene

QUESTION 2

EXCERCISE1 (LEACHING)

* Keywords (Answer):

The calculation procedure for countercurrent leaching operations is exactly the same as for LLE.

1. Plot the equilibrium data. 2. Plot the locations of known points. 3. Find mixing point, M. 4. Locate EN. 5. Find the ∆ point. 6. Step off stages.

ASSIGNMENT 1

QUESTION: EXERCISE 1

DATELINE: MONDAY

14 FEBRUARY 2011

(BEFORE 5 PM)

TUTORIAL 2

QUESTIONS: 23.1, 23.2

Reference Book;

McCabe et al. 2004 “Unit Operations Of Chemical

Engineering”

Prepared by,

MISS RAHIMAH OTHMAN

THANK YOU