chapter 1 - leaching continuous countercurrent

7/27/2019 CHAPTER 1 - LEACHING Continuous Countercurrent

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LEACHING


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Introduction

Leaching / solid extraction methods of removingone constituent from a solid by means of a liquidsolvent.

Examples:1) Extraction of oil from soybean flakes2) 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 isremoved.


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Principle of ContinuousCountercurrent 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


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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


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Equilibrium

Equilibrium is attained when the solute iscompletely dissolved and the concentration of the

solution 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.


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Operating Line

Total mass balance:

Solute:

Operating line equation:


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Number of Ideal Stages for ConstantUnderflow

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.


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EXAMPLE 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 exhaustedsolid) 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.


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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.


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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. IfXb= 0.1, the solution retained, from Table 23.1,

is 0.505 kg/kg. Then


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

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


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e) Determine the inlet and exit concentrations for the first stage and

locate the operating line for the remaining stages.

Sincex1= ya= 0.60, solution retained is 0.595 kg/kg solid.

Overall material balance:

Oil balance:


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The pointx1= 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


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The pointsxn,xn+1,xa, yaandxb, ybdefine a slightly curved

operating line, as shown in Fig. 23.3.

Four ideal stages are required.

chapter 1 - leaching continuous countercurrent

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