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Separation Processes

Simple Equilibrium Stage Operationsp

Flash Distillation

Course Slides of Prof. Alberto A. Laurito w/ modifications by MF Benjamin

Separation Processes

Lecture Objectives To be able to define and describe the flash

distillation process;To be able to calc late b bble and de points To be able to calculate bubble and dew points of a multicomponent feed;

To be able to derive and apply the working equation for multicomponent flash distillation and be able to apply the equation in specific cases.

Separation ProcessesWhat is Flash Distillation?

It is a simple distillation process where a liquid multicomponent feed is partially vaporized and the resulting liquid-vapor p g q pmixture is separated in a flash drum at a specified pressure and temperature.

The vapor is passed through a total condenser to obtain the distillate; while theliquid portion becomes the residue.

Separation Processes

Isothermal Flash DistillationTotal Condenser

V, yVi

Heater

FlashDrumF, xFi

D, xDi

L, xLi

P, TPartial

Vapor

Separation Processes

NomenclatureF = feed rate xFi = fraction of I in F

V = overhead rate yVi = fraction of I in V

L = bottoms rate xLi = fraction of I in L

D = distillate rate xDi = fraction of I in D

P = operating pressure T = operating temperature

f = fraction of F that is vaporized (or distilled)

Ki = Equilibrium Constant = f (P,T)

Separation Processes

Basic Equations1. Over-all Material Balances:

F = V + L but V = D F = D + L

2 Component Balance:2. Component Balance:

F xFi = V yVi + L xLi = D xDi + L xLi

3. Equilibrium Relation:

yVi = Ki xLi

4. f relation:

f = V/F = D/F and L/F = 1 – f

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Separation ProcessesWorking Equations

xLi =xFi

1 + f (Ki - 1)

where f is a fraction, Ki = f (P,T)

If f = 0, start of vaporization = Bubble Point

If f = 1, complete vaporization = Dew Point

for f to be a fraction, the flash drum must operate between the bubble and dew points

Calculation Check: xLi = 1.0

Separation Processes

Bubble Point Algorithm Given Data: xFi and P or T Required: Bubble Point T or P, yFi

A T P ( b ili t id ) Assume T or P (use pure boiling pts as guide) Evaluate Ki using P and T Solve yFi = Ki xFi (1st bubble composition) Check if Sum of yFi is equal to 1.0 If Yes, assumed T or P and yFi are correct If Not, assume another T or P and repeat

Separation Processes

Dew Point Algorithm Given Data: yFi and P or T Required: Dew Point T or P, xLi

Assume T or P (use pure boiling pts as guide) Evaluate Ki using P and T Solve xLi = yFi/Ki (last bubble composition) Check if Sum of xLi is equal to 1.0 If Yes, assumed T or P and xLi are correct If Not, assume another T or P and repeat

Separation Processes

Typical Flash Distillation Problems Given xFi, f, P or T Unknown T or P, xLi,

and xDi

Given xFi, P and T Unknown xLi, and xDiand xDi

Assume T or P Evaluate Ki

Solve xLi and check if Sum will equal one

Repeat trial if needed

Evaluate Ki

Assume f (fraction) Solve xLi and check

if Sum will equal one Repeat trial if

needed

Separation Processes

Practice ProblemA liquid mixture containing 20 mole% benzene (B), 30 mole%toluene(T), and 50 mole% o-xylene (X) shall be flash distilled at 1 atm. Assuming ideal system, determine:g y ,

a. Bubble and dew points of mixtureb. Flash drum temperature and mole% compositionof the products in order that 40% of the feed will be distilledc. Mole% of the feed to be distilled and the mole% compositions of the products if flash drum temperature is 125oC.d. Percent recovery of benzene in operations (b) and (c)

Separation ProcessesSolution for Bubble Point

Bubble Point (TB)Use the individual normal boiling points (1 atm) of the components as the initial trial for the bubble point of the mixture

Component A B C Boiling PointBenzene 6.9057 1211.033 220.7900 80.100Toluene 6.9546 1344.800 219.4820 110.625O-xylene 6.9989 1474.679 213.6880 144.409

Component xfi Boiling Point Trial TbBenzene 0.20 80.100 121.412Toluene 0.30 110.625O-xylene 0.50 144.409

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Separation ProcessesSolve for Ki’s using the trial bubble point and check if theyfi = 1.0. If not assume another value for TB

Formulas: Ki = Po/PT yFi = Ki xFi

Component xfi Po Ki yfiBenzene 0.20 2326.527 3.061 0.612Toluene 0.30 1022.630 1.346 0.404O-xylene 0.50 396.458 0.522 0.261

yfi 1.277Not OK

Trial 1: 121.412oC

Separation Processes

Final Trial: 112.266oC = Bubble Point

Component xfi Po Ki yfiBenzene 0.20 1860.055 2.447 0.489Toluene 0 30 796 087 1 047 0 314Toluene 0.30 796.087 1.047 0.314O-xylene 0.50 298.343 0.393 0.196

yfi 1.000OK!

Separation Processes

Dew Point (TD)To solve for the dew point, assume a temperature higher than the bubble point for the first trial. Check if xLi = 1.0, if not, assume another TD

Solution for Dew Point

, D

Formulas: Ki = Po/PT xLi = yFi/Ki

Component yfi Po Ki xliBenzene 0.20 2249.288 2.960 0.068Toluene 0.30 984.715 1.296 0.232O-xylene 0.50 379.818 0.500 1.000

xli 1.300 Not OK

Trial 1: 120oC

Separation Processes

Final Trial: 129.128oC = Dew Point

Component yfi Po Ki xliBenzene 0.20 2784.485 3.664 0.055Toluene 0 30 1250 357 1 645 0 182Toluene 0.30 1250.357 1.645 0.182O-xylene 0.50 498.020 0.655 0.763

xli 1.000OK!

Separation ProcessesFlash Drum Temperature

Flash Drum Temperature (T)To solve for the flash drum temperature, assume atemperature between the bubble point and the dew point forthe first trial. Check if xLi = 1.0, if not, assume another TLi , ,

Formulas: Ki = Po/PT xLi = xFi/[1 + f(Ki – 1)]

Trial 1: 120oC using given f = 0.4

Component xfi Po Ki xliBenzene 0.20 2249.288 2.960 0.112Toluene 0.30 984.715 1.296 0.268O-xylene 0.50 379.818 0.500 0.625

xli 1.005 Not OK

Separation Processes

Final Trial: T = 120.553oC using given f = 0.4

Component xfi Po Ki xliBenzene 0.20 2279.304 2.999 0.111Toluene 0 30 999 431 1 315 0 266Toluene 0.30 999.431 1.315 0.266O-xylene 0.50 386.266 0.508 0.622

xli 1.000OK!

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Separation ProcessesCompositions

Formulas:xDi = xLi Ki

%Rec = DxDi/FxFi*100 = fxDi/xFi*100

Component xfi xli Ki xdiBenzene 0.20 0.111 2.999 0.333Toluene 0.30 0.266 1.315 0.350O-xylene 0.50 0.622 0.508 0.316

xdi 1.000

Component T f = D/F xfi xdi %RECBenzene 120.553 0.400 0.200 0.333 66.600

Separation ProcessesFraction of Feed Distilled

Percent of Feed that was distilled (f)Using the given flash drum temperature (125oC) assume avalue for f. Check if xLi = 1.0, if not, assume another f

Formulas: Ki = Po/PT xLi = xFi/[1 + f(Ki – 1)]

Trial 1: f = 0.6 using given T = 125oC

Component xfi Po Ki xliBenzene 0.20 2531.798 3.331 0.083Toluene 0.30 1124.107 1.479 0.233O-xylene 0.50 441.385 0.581 0.668

xli 0.984Not OK

Separation Processes

Component xfi Po Ki xliBenzene 0.20 2531.798 3.331 0.078Toluene 0.30 1124.107 1.479 0.227

Final Trial: f = 0.668 using given T = 125oC

Toluene 0.30 1124.107 1.479 0.227O-xylene 0.50 441.385 0.581 0.694

xli 1.000OK!

Separation ProcessesCompositions

Formulas:xDi = xLi Ki

%Rec = DxDi/FxFi*100 = fxDi/xFi*100

Component xfi xli Ki xdiBenzene 0.20 0.078 3.331 0.261Toluene 0.30 0.227 1.479 0.336O-xylene 0.50 0.694 0.581 0.403

xdi 1.000

Component T f = D/F xfi xdi %RECBenzene 125.000 0.668 0.200 0.261 87.018

Separation Processes

AssignmentA liquid mixture containing 35 mole% n-pentane (A), 35 mole% n-hexane(B), and 30 mole% n-heptane (C) shall be flash distilled at 2585 Torrs. Determine:

a. Bubble and Dew points of mixtureb. Flash drum temperature and mole% compositionof the products in order that 50% of the feed will be distilledc. Percent Recovery of n-pentane