Liq-Liq & Solid-Liq Separation Notes_3

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Liquid/Liquid Separations (Extraction) Liquid extraction = mass transfer of a solute from a liquid by contact with another liquid. The two bulk liquid streams are relatively insoluble in each other (forming two liquid phases) but the solute is soluble in both liquids. Feed = Diluant (L) liquid phase containing unwanted solute a (or A) Separating agent = (V) liquid phase in which primarily only a is soluable Products = (L) solute-free raffinate + (V) enriched extract streams Principle of separation = preferential solubility Heavy Liquid Outlet Heavy Liquid Inlet Light Liquid Outlet Light Liquid Inlet Separation device = column (either continuous or staged) Extraction is used when combined extraction & solvent recovery are more economical than a more direct separation like distillation….for example when solute is present in very low initial concentration (organic in water) temperature sensitive materials close boiling mixtures with different chemical structures Applications = separation of paraffin from aromatics, manufacture of p-xylene from crude oil, improve purity of lubricating oils, purification of vegetable oil, separation of saturated fats from unsaturated fats, recovery of penicillin Extraction Equipment = goal is to bring two liquid phases into good contact to enhance mass transfer…phases are then separated. In extraction, mixing is not so easy as absorption or distillation….liquid phases are viscous and linear velocities in column are low, therefore mechanical agitation is often used. Mixer-settler Spray column Packed column

Transcript of Liq-Liq & Solid-Liq Separation Notes_3

Page 1: Liq-Liq & Solid-Liq Separation Notes_3

Liquid/Liquid Separations (Extraction) Liquid extraction = mass transfer of a solute from a liquid by contact with another liquid. The two bulk liquid streams are relatively insoluble in each other (forming two liquid phases) but the solute is soluble in both liquids.

Heavy Liquid Outlet

Heavy Liquid Inlet Light Liquid Outlet

Light Liquid

Separation device

=

column (either

continuous or

staged)

Extraction is used when combinethan a more direct separation like

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when solute is present in vtemperature sensitive matclose boiling mixtures wit

Applications = separation of paracrude oil, improve purity of lubrisaturated fats from unsaturated fa Extraction Equipment = goal is tomass transfer…phases are then seabsorption or distillation….liquidlow, therefore mechanical agitatio

Mixer-settler Spray column Packed column

Feed = Diluant (L) liquid phase containing unwanted solute a (or A) Separating agent = (V) liquid phase in which primarily only a is soluable Products = (L) solute-free raffinate + (V) enriched extract streams Principle of separation = preferential solubility

Inlet

d extraction & solvent recovery are more economical distillation….for example ery low initial concentration (organic in water)

erials h different chemical structures

ffin from aromatics, manufacture of p-xylene from cating oils, purification of vegetable oil, separation of ts, recovery of penicillin

bring two liquid phases into good contact to enhance parated. In extraction, mixing is not so easy as phases are viscous and linear velocities in column are n is often used.

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Plate or baffle column ♦ ♦

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Agitated tower Principles of Extraction = Continuous Extraction Processes use countercurrent contact between light phase and heavy phase. Many of the same principles used in absorption & distillation apply to extraction:

Minimum ratio of light phase flowrate/heavy phase Number of ideal stages (or column height) Stage efficiency Column diameter and design of internal components

Differences between Extraction and Absorption or Distillation:

Liquid/Liquid Equilibrium is more complicated than binary distillation or absorption if all three components transfer & are present in each phase

Equilibrium Expressions for Liq/Liq System Notation for Liq/Liq Systems Depicted on the Equilateral Triangular Diagram Below

Solute a in solution with Diluent b Solvent s is used to extract solute a Solvent Rich Stream = Extract = V Diluent Rich Stream = Raffinate = L

Equilibrium for 2 liquid phases containing 3 components (2 bulk liquid components and 1 solute specie) F = 3-2+2 = 3 degrees of freedom Equilibrium data are presented on Triangular Diagrams for 3 component Liq/Liq system. Features of the Diagram 1) Each of the three corners represents a pure component a, b, and s 2) Point M in the interior of the triangle represents a mixture of a, b, and s. The perpendicular distance from M to the base s-b represents the mass fraction of “a” (xa) in the mixture , the distance from M to the base s-a = mass fraction of b and the distance from M to the bas a-b = mass fraction of s.

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3) Common phase diagram where s and b are partially miscible in each other is shown below

4) the area inside the dome shaped region is the 2 phase region. Any overall composition that falls inside this area will separate into two phases - point C=comp of the extract (or solvent) rich layer & point D=comp of the raffinate (diluent) layer 5) Points representing phase equilibrium compositions are joined by tie lines. Point P is the Plait point, where the composition of the extract layer = composition of the raffinate layer 6) When tie lines slope up to the left (extract phase is richer in solute than raffinate phase), solute can be extracted from the diluent using only moderate amounts of solvent. When tie lines slope up to the right, extraction is still possible, but more solvent would be needed. 7) When pure solvent is added to a binary mixture of diluent + solute (b+s) and allowed to come to equilibrium, the composition of the resulting mixture lies on a straight line between the "pure solvent" point s and the point representing the original binary mixture a+b. Single stage extraction calculations can be performed this way.

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Since Triangular diagrams have some disadvantages because of the special coordinates, a more useful method of plotting the three equilibrium data is to use Rectangular coordinates. Two different methods of plotting equilibrium data are shown below.

Upper plot - Mass fraction of Solute ‘a’ (xa, ya) vs Mass fraction Solvent ‘s’ (xs, ys ) Lower plot - Diluent Layer composition of ‘a’ (xa) vs Solvent Layer composition of ‘a’ (yA).

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Single Stage Extraction Calculation (calc outlet compositions and flowrates)

Raffinate

Water +

Acetic acid

L Feed

Water +

acetic acid

Extract

Isopropyl ether +

Acetic acid

V Solvent

Isopropyl ether The single stage extraction shown above can be represented this way

M, xaM, xsM

L, xa, xs

V, ya, ys Streams exiting the stage are assumed to be in equilibrium Known Quantities = L, xa, xs, V, ya, ys Unknown Quantities = exiting composition and flowrates of the raffinate and extract streams Total Material Balance to find M, xaM, and xsM: Graphical Solution:

Using Triangular phase equilibrium diagram ♦

mentioned earlier

Using the "mass fraction a vs mass fraction s" diagram 1) composition pairs (xa, xs) of streams that are added together lie on a straight line i.e. for the single stage operation, the composition pairs (xa, xs) representing stream L, and (ya, ys) representing stream V lie on a straight line

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2) Point M represents the overall composition of streams that are added (L+V = M) and its composition (xaM, xsM) also lies on the line between L and V

3) the ratio of flowrates L/V = line segment length VM/line segment length LM This is called the Inverse Lever Arm Rule

Continuous Multistage Countercurrent Extraction Design Number of Stages

McCabe-Thiele method Perform an overall mat’l balance to determine terminal compositions and flowrates…this is complicated by the fact that all 3 components exist in 2 phases requiring an iterative solution

Operating line:

Plot L/V (the operating line) on a "conc of a in the Extract vs conc of a in the raffinate” equilibrium plot this is complicated by the fact that all 3 components exist in 2 phases requiring an iterative solution Step off stages between the operating line and equilib Curve (This example is exactly like absorption)

1

L0, x0 V1, y1

L1, x1 V2, y2

2

L2, x2 Vn, yn

n

Ln, xn Vn+1, yn+1

LN-1, xN-1 VN, yN

N

LN, xN VN+1, yN+1

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Number of Stages Kresmer Equation Method (assumes constant L/V and straight equilibrium line)

If Solvent S and Diluent B are relatively immiscible in each other and if solute A is relatively dilute this procedure can be greatly simplified If the equilibrium is a straight line “ya= KDxa” you can define an extraction factor E where E = KDV/L (equivalent to stripping factor in Chap 20) and use the Kresmer Equations (MSH 20.28) to calculate N

( )[ ]E

xxxxN bbaa

ln/()ln ** −−

=

subscript a and b in the Kresmer eqn refer to compositions of the solute at the top and bottom of the column respectively. In extraction, x =mole fraction in the raffinate and y =mol fraction in the solvent

Number of Stages

Rigorous Solution Method (graphical) Overall Material Balance (to determine Terminal Compositions and Flowrates and to determine the coordinates of point M)

Derivation of Operating Point (Delta Point)

Graphical Construction to determine Number of Stages 1) Using the Mass Frac A vs Mass Frac S diagram, locate pand V1 (use the coordinates of the M point to help locate V2) Intersect the two lines LN VN+1 and L0 V1 to locate t(or use the formulas derived for the delta point to find it) 3) V1 and L1 lie on a tie line. Determine L1 using the tie l

1

L0, x0 V1, y1

L1, x1 V2, y2

2

L2, x2 Vn, yn

n

Ln, xn Vn+1, yn+1

N

LN-1, xN-1 VN, yN

LN, xN VN+1, yN+1

oints VN+1, LN, L0, 1)

he Delta Point

ine through V1

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4) Draw a straight line from L1 to delta point (this is the OPERATING LINE). The intersection of this line with "extract portion" of the mutual solubility curve gives V2. 5) V2 and L2 lie on a tie line. Determine L2. 6) Draw a straight line from L2 to delta point. The intersection of this line with the mutual solubility curve gives V3. 7) Continue in this manner until LN is reached. 8) Each tie line used denotes a single stage. Count the number of stages. 9) Once the compositions of the streams leaving and entering each stage are determined, the next step is to determine the flowrates. This can be done using material balances on each stage or by using the Inverse Lever Arm Rule

Minimum Solvent Flowrate for Extraction If solvent rate VN+1 is too low separation is not possible because operating lines for streams passing each other coincide (lie directly on top of) equilibrium tie lines for streams exiting the stage (infinite stages are required) To find the Minimum Solvent flowrate 1) Locate point L0 and the equilibrium tie line through that point 2) Draw this tie line to intersect with the "extract" layer of the mutual solubility curve 3) This point is V1min 4) Draw the lines LNV1min and L0VN+1 to find the location of Mmin (i.e. xAMmin) 5) Use a overall material balance and xAMmin to solve for VN+1

Solid/Liquid Separations (Leaching) Leaching = removal of a solute from an insoluble solid by contact with a liquid solvent.

Separation device=Countercurrent cascade

Feed Solid + Solute Underflow product

Solid + Solvent+Solute

Separating AgentLiquid Solvent

Overflow productSolvent+Solute

Separation device=Countercurrent cascade

Feed Solid + Solute Underflow product

Solid + Solvent+Solute

Separating AgentLiquid Solvent

Overflow productSolvent+Solute

Feed = Solids (L) containing unwanted solute A Separating agent = (V) liquid in which only A is soluable Products = (L) solute-free solid (underflow)

(V) liquid containing solute (overflow) Principle of separation = preferential solubility

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Applications = used in food processing industries to produce sugar from sugar beets, vegetable oil from peanuts. Also used in pharmaceuticals obtained from plants and in metal recovery Solid Preparation = prior to leaching solids are crushed to improve mass transfer. During the course of leaching the solids may change considerably Mechanisms for leaching = Multiple steps take place in transferring a solute out of a solid Leaching Equipment = goal is to bring solid and liquid phases into good contact to enhance mass transfer…phases are then separated. In leaching, the solid may be dispersed in the liquid or the liquid may be percolated through a fixed bed of solids. Leaching can be operated as a batch or continuous process

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Series of stationary solid-bed tanks operated countercurrently Moving-bed leaching

Principles of leaching = Continuous countercurrent contact using stages is most common method of leaching. Many of the same principles used in absorption & distillation apply to leaching:

Number of ideal stages (based on equilibrium and operating lines) Stage efficiency

Equilibrium Expressions for Solid/Liq System Assumptions: Notation for Solid/Liq Systems Underflow = solid-liquid stream exiting a stage (L= kg/hr liq associated w/ solid) Overflow = liquid stream exiting a stage (V=kg/hr A+S) A= kg solute B= kg solid (bone dry solid) S= kg solvent N=kg B/kg A+S XA= kg A/kg A+S in overflow YA=kg A/kg A+S in underflow

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Graphical Representation of equilibrium data (N vs x,y) Single Stage Leaching Calculation (calc outlet compositions and flowrates)

V1, X1 overflow

L=kg/hr of liquid associated with the solid Streams exiting the stage are assumed to be Known Quantities = B, Lo, yo, x2, V2, Unknown Quantities = exiting composition anstreams Total Material Balance to find M, xM, and NM Graphical Solution:

Using N vs x,y phase equilibrium diagLo, V2, M lie on straight line= use thisL1, V1, M lie on straight line (equilibrcompositions and relative flowrates

v2, x2 Solvent

Lo, No, yo,B Feed solid

r

L1, N1, y1,B underflow

in equilibrium

d flowrates of the raffinate and extract

M:

am to find location of M ium tie line)= use this to find exiting

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Example- Single stage leaching:

Continuous Multistage Countercurrent Extraction Design Number of Stages

Rigorous Solution Method (graphical) Overall Material Balance (to determine Terminal Compositions and Flowrates and to determine the coordinates of point M)

Operating Point (Delta Point)

Graphical Construction to determine Number of Stages similar to leaching (see handout)