leaching

SOLID LIQUID EXTRACTION

(LEACHING PROCESS)

Solid-Liquid Extraction

Definition- In order to separate the desired solute constituent or

remove an undesirable solute component form the solid phase, the solid is contacted with liquid phase

- Such a process is also referred to as liquid solid leaching

or simply leaching

- In leaching when an undesirable component is removed from a solid with water, the process is called washing

Solid-Liquid Extraction

Basic Concept

Other example :Gaharu extraction, Ginger extraction

CALCULATION FOR SINGLE STAGE

V + L = M ( total mass kg)Lo + V2 = L1 + V1 = M (general)

For component ALoYAO + V2XA2 = L1YA1 + V1XA1 = MxAM

For component C (Solid) B = NoLO + 0 = N1L1 + 0 = NMM

eg : 12.9-1 (Geankoplis)

Leaching

V1 X1

B LO

No Yo

V2 X2

B L1

N1 Y1

CALCULATION FOR MULTISTAGE

• Usually Lo and VN+1 are known and yAN (exit conc)is set

• Then the coordinates of point M, ie NM and xAM are calculated from the following equations

* Overall total solution (solute A + solvent C) balance

* Component balance on A

* Total solids balance B

MVLVL NNo 11

MNNoo NLNLNB

AMAANNANNAOo MxxVyLxVyL 1111

Solid-Liquid Separation - Leaching

Operating line equation:L1

V2 Vn+1VnV3

Ln-1L2 LnLN-1

VN

1 2 n N

VN+1

xN+1

yN,+1

LN,,B

V1

x1

yo, No

Lo, Bo

leached solidsfeed solids

leaching solventexit overflow

on

oon

non LVL

xLxVy

LLVx

1

11

11 /1

1


Graphical determination of the number of stages for multistage

countercurrent leaching

coordinate for the operating point, , is given by the following equation

* x coordinate

* N coordinate

• The point is located graphically as the intersection of lines LOV1 and LNVN+1

• To graphically determine the number of stages,

* Start at LO, draw line LO to locate V1. A tie line through V1 gives locates L1.

* Line L1 is drawn given V2. A tie line gives L2

* This is continued until the desired LN is reached

1

11

1

11

NN

ANNANN

o

AAooA VL

xVyL

VL

xVyLx

11 VL

LN

VLB

No

oo

o


Example 1: Countercurrent Leaching of oil from meal

A continuous countercurrent multistage system is to be used to leach oil from meal by benzene solvent. The process is to treat 2000 kg/hr of inert solid meal (B) containing 800 kg oil (A) and also 50 kg benzene (C). The inlet flow per hour of fresh solvent mixture contains 1310 kg benzene and 20 kg oil. The leached solids are to contain 120 kg oil. Settling experiments similar to those on the actual extractor show that the solution retained depends upon the concentration of oil in the solution. The data are tabulated below as N kg inert solid B/kg solution and yA kg oil A/ kg solution

N yA N yA

2.00 0 1.82 0.4

1.98 0.1 1.75 0.5

1.94 0.2 1.68 0.6

1.89 0.3 1.61 0.7

Calculate the amounts and concentration of the stream leaving the process and the number of stages required.


Solution

• The underflow data from the table are plotted as N versus yAin the following

figure,

• For the inlet solution with the untreated solid,

LO = 800+50 = 850 kg/h

yAO = 800 / (800+50) = 0.941

B = 2000 kg/h

NO = 2000 / (800+50) = 2.36


Solution

• For the inlet leaching solvent,

VN+1 = 1310 +20 = 1330 kg/h

xAN+1 = 20 / (1330) = 0.941

• The points VN+1 and LO are plotted.

• At the point LN, the ratio NN/ yAN = (kg solid/kg solution)

= kg solid/ kg oil

= 2000/120 =16.67= slope

• Hence a dashed line through the origin at yA=0 and N=0 is plotted with a slope of

16.67, which intersects the N vs yA line at LN. The coordinates of LN at this

intersection are NN = 1.95 kg solid/kg solution and yAN = 0.118 kg oil/kg solution

• The point M (xAM, NM)is determined from overall material balance and component balance on A;

* for xAM;

• MVL No 218013308501

AMANNAOo xxVyL 2180015.01330941.085011

376.0 AMx


Solution

• For NM,

• The point M is plotted with the the coordinates xAM=0.376 and NM = 0.918

• The line VN+1MLO is drawn, as is line LNM, which intersects at point V1 where

xA1=0.600

• The amounts of streams V1 and LN are calculated by solving the following

equations simultaneously

• Hence, LN = 1016 kg solution / h in the outlet underflow stream and V1=1164 kg

solution/ h in the exit overflow stream

• The operating point , is obtained as the intersection of lines LOV1 and LNVN+1

• The stages are stepped off as shown, where the fourth stage for L4 is slightly

past the desired LN

Þ Hence, about 3.9 stages are required

•

918.021802000 MMM NNMNB

21801 MVLN

376.02180600.0118.0 111 VLxVyL NAANN


Equipment for leaching

3 distinct processes usually involved in leaching operations:

• dissolving the soluble constituent

• separating the solution , so formed, from the insoluble solid residue

• washing the solid residue in order to free it of unwanted soluble matter or to obtain, as much of the soluble material as possible



A) Batch plant for extraction of oil from seeds

• . Consists of a vertical cylindrical vessel divided into two sections by slanting partition

• Upper section is filled with the charge of seeds which is sprayed with fresh solvent via a distributor

• Solvent percolates into the bed of solids and drains into the lower compartment



Extraction from cellular materials;

B) Bollman extractor

• .series of perforated baskets, arranged as in a bucket elevator

• solid is fed into top basket on the downward side and is discharged from the top basket on the upward side

• Solvent sprayed on to the solid which is about to be discarded, and passes downwards

• Solvent is finally allowed to flow down through the remaining baskets in co-current flow



Leaching of coarse solids;

A) Dorr rake classifier

• .Solid is introduced near the bottom of a sloping tank and is gradually moved up by means of a rake

• Solvent enters at the top and flows in the opposite direction to the solid, and passes under a baffle and finally discharged over a weir

• Operates satisfactorily, provided the solid does not disintegrate & the solids are given an ample time to drain before discharged


SUPERCRITICAL FLUID EXTRACTION

Supercritical Fluid Extraction

• Lower viscosity than liquid

• Gas like permeation of solid structures

• Higher density than gas

• Higher solubility than gas

Supercritical Fluid Extraction with C O2

• Replace Organic Solvents with CO2• Substances easily degraded by heat

can be extracted• Non-toxic for use in food products

Non-reactive and Non-flammable Easy processing

• Environmentally safe

Example Process Flow

After Extraction Process• Change Conditions to alter Phase

Behavior• Reduce Solubility by lowering the

pressure• Equilibrium Phase comes out of

solution• Another example of this is

crystallization

leaching

Documents

liquid solid leaching

solid phase

countercurrent leaching

inert solid bkg solution

component c solid b

liquid phase

inlet solution

solution n ya n ya