2 COURSE OUTCOMESCOAPPLY principles of leaching. ANALYZE leaching equipments. CALCULATE material balance and number of ideal stages for multistage leaching.
3 OUTLINES Introduction to leaching. Leaching equipments. Principles of Continuous Countercurrent Leaching.How to CALCULATE material balance and number of ideal stages for multistage leaching.
4 INTRODUCTION TO LEACHING Leaching / solid extraction – methods of removing one constituent from a solid by means of a liquid solvent.Examples:1. Making coffee from ground coffee beansand tea from tea leaves.(The complex mixture of chemicals that give coffee and tea their odor, taste, and physiological effectsare leached from the solid by hot water)2. Extraction of oil from soybean flakes3. Extraction of coconut milk from coconutThe 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.
7 STATIONARY SOLID BEDSIt 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.
10 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.
13 ROTOCEL EXTRACTORA 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.
15 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.
16 PRINCIPLES OF CONTINUOUS COUNTERCURRENT LEACHING Ideal Stages In Countercurrent LeachingEquilibriumOperating LineConstant And Variable UnderflowNumber of ideal stages for constant underflowNumber of ideal stages for variable underflow
17 IDEAL STAGES IN COUNTERCURRENT LEACHING 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 NConcentrated solution overflow from Stage 1
18 EQUILIBRIUMEquilibrium 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.
19 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.
20 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.
21 NUMBER OF IDEAL STAGES FOR CONSTANT UNDERFLOW Use McCabe Thiele Method if the operating line is straightIn leaching, the operating line is always straightThe equilibrium line is on 450 line.
22 NUMBER OF IDEAL STAGES FOR VARIABLE 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….
23 QUESTION 1EXAMPLE 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.
24 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.
25 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 kg/kg. Then;
26 From Table 23.1, the solution retained is 0.507 kg/kg: Benzene in the underflow at Lb is = 447 kg/h.At the solid inlet,
28 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 kg/kg solid. Overall material balance: Oil balance:
29 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
30 The points xn , xn+1 , xa , ya and xb , yb define a slightly curved operating line, as shown in FigFour ideal stages are required.
31 QUESTION 2EXCERCISE1 (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.
32 Mass Fraction Oil (Solute) in Solution TABLE 1: Test Data For Extraction Of Oil From Meal With BenzeneMass Fraction Oil (Solute) in SolutionMass Fraction Underflow (Rafffinate)yAxAxDxS0.670.3330.10.03360.6640.3040.20.06820.660.2720.30.10390.65410.2420.40.14190.2130.50.18170.63660.60.2240.62680.14920.70.2680.61720.1148
33 QUESTION 2 EXCERCISE1 (LEACHING) * Keywords (Answer): The calculation procedure for countercurrent leaching operations is exactly the same as for LLE.Plot the equilibrium data.Plot the locations of known points.Find mixing point, M.Locate EN.Find the ∆ point.Step off stages.