01 lecture note - binary flash distillation

Farooq/CN3132/Flash Distillation-1

Binary Flash Distillation

Concept of phase equilibrium Equilibrium relationships

Flash distillation Design issues Binary flash distillation design Problem to test your understanding

Compulsory reading: Chapter 2 (sections 2.1 to 2.3, 2.4, 2.4.1, and 2.5) from the text book


Flash Distillation simplest separation process in chemical industry

MolefractionofA

MolefractionofB

Feed,F zA zB

Vapor,V yA yB

Liquid,L xA xB

Feed is a liquid mixture Methanol (A) water (B) Propane (A) butane (B) Water (A) salt (B)

pA>pB (A is lighter component)

yA>zA (lighter component enriched in V)

xB>zB (heavier component enriched in L)

Degree of separation high if pA>pB

Very suitable for desalination

Pdrum Tdrum

Pressurize

Heat

Expand Feed A+B

Vapor pressure of A: pA Vapor pressure of B: pB

V, yA, yB, TV

L, XA, XB, TL

DemisterDemister prevents liquid droplet entrained in vapor


What can we say from the equilibrium design method?

V and L are in equilibrium TL = TV = Tdrum Liquid and vapor stream pressures = Pdrum y= f(x) yi / xi= Ki (distribution coefficient) = f(P, T and all xi) xA + xB = 1 = yA + yB

Models relating y to x at equilibrium were covered in CN2121 Simple equations for ideal systems Complex equations for non-deal systems

Design of the flash process depends on good equilibrium data

Experimental data Estimated K values

DePriester chart for hydrocarbons Simple thermodynamic model for

ideal systems


Experimental vapor-liquid equilibrium data (including non-ideal systems) Tabular (complete with temperature and pressure)

XEt Xw YEt Yw T, C P, atm 0 1.0 0 1.0 100

0.019 0.981 0.17 0.83 95.5 0.0966 0.9034 0.4375 0.5625 86.7 0.1661 0.8339 0.5089 0.4911 84.1 0.2608 0.7382 0.5580 0.4420 82.3 1 0.3965 0.6035 0.6122 0.3878 80.7 0.5198 0.4802 0.6599 0.3401 79.7 0.6763 0.3237 0.7385 0.2615 78.74 0.7472 0.2528 0.7815 0.2185 78.41

1.0 0 1.0 0 78.3

x-y diagram is widely used in binary vapor-liquid

separation

x-y diagram usually scales from 0-1 on both axes

The diagonal joins (0, 0) and (1,1) points and is an important reference line

T-x-y plot

x-y diagram


DePriester chart: K values for light hydrocarbons This is Figure 2-12 from the text book. For lower temperature see Figure 2-11

Read K values methane, ethylene, iso-propane and n-octane at 200 kPa and 50 oC


Vapor-liquid equilibrium for an ideal system

In general: iiii

i xallTPfKKxy

where ,,,

For ideal systems : TPfKKxy

iii

i ,, where The following identities apply for both ideal and non-ideal systems

1;1;1;11111

n

i i

in

iii

n

ii

n

ii K

yxKxy

Raoults Law: ioii xpp

Dalton's Law: Ppy ii

For an ideal system : PpK

oi

i (1)

Antoine equation: i

ii

oi CT

BAp ln (2) Ai, Bi, Ci are constants for pure components. These are tabulated in various data sources. Tables of pure component vapor pressures are also available.

How to generate x-y-T data from Ki values Illustration for a binary system

i. Set system pressure (say, P=1 atm)

ii. x = 0.1 (depends on how many data points) iii. xA = 0.1 iv. xB = 1-xA v. Guess T

vi. Calculate KA and KB from Eqs (1) and (2) or from DePriester Chart

vii. Check if 1 BBAA xKxK viii. If no, go back to (iv). If yes, continue

ix. BBBAAA xKyxKy ; x. Store xA, yA and T

xi. Check if xA = 1 xii. If yes, stop. If no, xA = xA + x and go to (iv)

Repeat from (iii) for another pressure


Approximate equilibrium equation for a binary system

Relative volatility, systemidealforoB

oA

B

AAB p

pKK

;; TfKTfK BA TfABissystemsmanyforassumptiongoodA

(3):entrearrangemUpon

AAB

AABA

AA

AA

BB

AA

B

AAB

xxy

xy

xy

xy

xy

KK

11

11

AB means no separation >1 or


Degrees of freedom in a binary two-phase system

F: degrees of freedom C: number of components P: number of phase Test your understanding You are asked to determine the system pressure that is required to bring a 50:50 liquid mixture of A:B to come

to equilibrium with a 70:30 vapor mixture of A:B at 70oC. Is this possible? Yes No

Which of the following is/are possible?

1 / 2 / 3

1. A binary vapor-liquid system is at 50oC and 1 atm pressure. Find the composition of the two phases. 2. From a given liquid mixture, obtain a specific vapor mixture at specified temperature and pressure. 3. Condense a vapor mixture to obtain a desired liquid mixture.

Gibbs phase rule: F = C P + 2

For a binary vapor-liquid system: F = 2

T, P, xA and yA are the 4 independent variables

You can choose to fix only 2 variables


What do we mean by designing a flash system? Remember: x and y are in equilibrium

Typically F, z, T1 and P1 will be known

Given Calculate How? Tdrum, Pdrum x, y, V

F, L

F

Overall and component mass balances around Loop 1 x, y, V

F or L

F Tdrum, Pdrum

TF and PF TF from energy balance around Loop 1

PF is chosen to prevent boiling at TF

QH QH from energy balance

around Loop 2

Drum height and diameter

Empirical method depends only on densities and mass flow rates of L and V

Loop 1

Loop 2

IDEAL STAGE

F, z TF, PF, hF

V, y, HV

L, x, hL

Pumping & Heating

F, z T1, P1

TF, PF, hF

QH

Adiabatic process. Pressure reduction from PF to Pdrum across the

valve causes part of the feed to flash from the liquid to the vapor state. Internal enthalpy supply causes Tdrum < TF


Loop 1

IDEAL STAGE

F, z TF, PF, hF

V, y, HV

L, x, hL

Binary Flash Distillation

Overall mass balance F V L (5) Component balance (more volatile species)

Fz Vy Lx

or, V Lz y x fy (1 f )xF F

(6)

where V fF

and therefore L F V 1 fF F

Rearranging eq (6a):

1 f zy xf f

(Operating line) (6a)

Equilibrium relationship: y f x,T,P (7)

Simultaneous solution of (6a) and (7)

Numerical General method including multi-component

Analytical Equation for equilibrium data, e.g., eq (4)

Graphical simple and insightful Slope of eq (6a) L 1 f

v f

Eq (6a) intersects diagonal at x y z

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1xBenzene

y

B

e

n

z

e

n

e

Diagonal (x = y)

Z = 0.5

f = 0

0.5 1

Benzene-toluene equilibrium

Test your understanding

Effect of vapor rate on ybenzene?

Range over which ybenzene can be varied?


Condition to prevent boiling at TF i i F F ii i

y K T ,P x


Test your understanding: Problem 1 A flash distillation chamber operating at 1 atm (101.3 kPa) is separating ethanol-water mixture containing 60 mole % ethanol. (a) Is

it possible to get a vapor product containing 80 mole% ethanol from this unit? (b) If the drum temperature is 85 oC, find x, y,VF

,LF

.

(c) What should be the drum temperature if we want 50% of the feed to evaporate in the flash drum? Find the corresponding changes in x and y.

(b) (c)

(a)


Hints

(a) Draw the operating quadrant of a flash unit (see page 2-10, if in doubt) Now locate the vapor product leaving the unit. Remember, Vapor and liquid products leaving a flash unit are assumed to be in equilibrium. Hence, vapor/liquid product composition is a point on the vapor-liquid equilibrium line. Knowing any one composition is sufficient to locate the point.

(b) You can read x and y directly from the T vs. x or y plot. This (x, y) is a point on the vapor-liquid equilibrium line. You know the feed composition and therefore you have another point on the diagonal. The line joining these

two points is the operating line. From the slope of this line you should be able to find VF

andLF

.

(c) In part (b), you went from T vs. x or y plot to x vs. y plot. Now you do the reverse. Since the feed composition is known, the point you had identified on the diagonal remains unchanged. You now find the slope of the

operating line corresponding to VF

=0.5. This line will give you the composition of the vapor and liquid products.

Use this information in the T vs. x or y plot to find required flash drum temperature.


100

120

140

160

180

0 0.2 0.4 0.6 0.8 1xT or yT

T

(

o

C

)

Liquid

Vapor

Figure: 2

To be solved in class

1. The T-x-y data for toluene-phenol system at 1 atm pressure is given in Figure 2. It has been decided that a 35:65 mixture of toluene and phenol will be flash distilled at a feed rate of 1000 kmol/hr in a flash drum at 1 atm pressure and 140 oC.

a. Find toluene mole fractions in the vapor and liquid

streams leaving the flash unit. Show your drawings in Figure 2.

b. Calculate the vapor flow rate leaving the flash unit c. Calculate the diameter of the drum if the permissible

vapor velocity is 2 m/s. Average molecular weight of the vapor is 92.5. Molar density of the vapor is 0.0295 kmol/m3.


2. Two flash distillation chambers are hooked together as shown in the diagram. Both are at 1 atm pressure. The feed to the

first drum is a binary mixture of methanol and water that is 55 mole % methanol. Feed flowrate is 10,000 kg moles/hr. The

second drum operates with 2

V 0.7F

and the liquid product composition is 25 mole % methanol. Equilibrium data at 1 atm are given in Table 1.

a. What is the fraction vaporized in the first flash drum?

b. What are y1, y2, x1, Tdrum1 and Tdrum2?

Mole % Methanol

Liquid Vapour Temp C0 0 100

2.0 13.4 96.4

4.0 23.0 93.5

6.0 30.4 91.2

8.0 36.5 89.3

10.0 41.8 87.7

15.0 51.7 84.4

20.0 57.9 81.7

30.0 66.5 78.0

40.0 72.9 75.3

50.0 77.9 73.1

60.0 82.5 71.2

70.0 87.0 69.3

80.0 91.5 67.6

90.0 95.8 66.0

95.0 97.9 65.0

100.0 100.0 64.5

Table 1


Test your understanding

Separation process design is an integration of knowledge acquired in various CN modules. Identify the roles of the following modules in the context of a flash system design: CN1111 Chemical Engineering Principles CN2121 Chemical Engineering Thermodynamics CN2122 Fluid Mechanics CN 2125 Heat and Mass Transfer How would you describe integrating role of the present module? What is the role of CN3121 Process Dynamics and Control in separation process design? Answer this question after you have developed some idea about the content of CN3121.


Homework 1 (CN 3132)

1. The outlet air from the paint formulation chamber of a paint industry shown in the figure is saturated with solvent hexane. The outlet air stream is at atmospheric pressure and 25 oC. A consultant has been hired to design a flash separator that will remove at least 90% of hexane from the outlet air before discharging the air steam to atmosphere. Available in the store is a cooling system that can cool the outlet air stream up to -5 oC. Also available is a compressor that can compress the air stream up to 600 kPa. The consultant has recommended that the flash separator be operated at 0 oC and 200 kPa. K values of hexane at 2 different conditions are also included in the figure below.

(i) Find the flow rate of the hexane stream leaving the bottom of the flash unit assuming 90% removal. (ii) Do you agree with the recommendation of the consultant? Show the calculations in support of your answer.

Paint formulation chamber

Hexane saturated outlet air

10 moles/hr

Nearly pure

Maximum 10% of hexane in

Fresh air

K values of hexane. Pressure Temp K

1 atm 25 oC 0.2 200 kPa 0 oC 0.035


2. The process flow diagram for an ethyl benzene manufacturing process is shown in Figure Q1. The separation processes, consisting of a flash unit (V-302) and two multi-stage distillation units (T-301 and T-302), are shown in Block 1 of the figure. The temperatures, pressures and compositions of the streams entering and exiting these units are given in Table Q1.

i) What is the composition (in mole fractions) of the stream entering the flash unit (V-302)? ii) According to Table Q1, the flash unit (V-302) operates at 73.6oC and 110kPa. Calculate the partition coefficients (K values)

of all the components in this unit.

Table Q1: Information on the streams entering and exiting the separation units.

Stream Number

Temperature (oC)

73.6 73.6 81.4 145.4 139.0 191.1

Pressure (kPa)

110.0 110.0 105.0 120.0 110.0 140.0

Flow rate (kmol/hr) Ethylene 0.54 0.0 0.0 0.0 0.0 0.0 Ethane 7.0 0.0 0.0 0.0 0.0 0.0

Propylene 2.0 0.0 0.0 0.0 0.0 0.0 Benzene 8.38 169.46 169.23 0.17 0.17 0.0

Ethylbenzene 0.71 91.54 0.92 90.63 89.72 0.91 1,4-

Diethylbenzene0.013 10.35 0.0 10.35 0.0001 10.35

15 16 17 18 19 20


Figure Q1: Process flow diagram for an ethyl benzene manufacturing process.

Block 1


3. Natural gas is considered as the cleanest among the fossil fuels. In many natural gas fields, the gas contains N2, CO2,

C2H6, C3H8 and C4H10 in addition to CH4. A representative composition of natural gas found in the Middle East is given in Table Q3 below. Design a flash unit to recover 99% of C2H6.Clearly state your assumptions and verify them after completing the design.

Table Q3: Composition of natural gas from a gas field in the Middle East.

Component Mole % H2S 5 CO2 10 CH4 75C2H6 5C3H8 3C4H10 2

01 lecture note - binary flash distillation

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