Binary Flash Flash Distill ation
Concept of phase equilibrium
Equilibrium relationships Flash distillation Design issues Binary flash distillation design Problem to test your understanding
Compul sor y reading : Chapter 2 (section s 2.1 to 2.3, 2.3, 2.4 2.4,, 2.4. 2.4.1, 1, and and 2.5) 2.5) fro m th e text bo ok
Farooq/CN3132/Flash Farooq/CN3132/Flash Distillation-1
Flash Distill ation – simpl est separation separation proc ess in chemical indus try V, y A, y B, TV Vapor pressure of A: p A Vapor pressure of B: pB
Demister
Heat
Pdrum Tdrum Feed A+B
Expand
Demister prevents liquid droplet entrained in vapor
• Feed is a liquid mixture Methanol (A) – water (B) Propane (A) – butane (B) Water (A) – salt (B) • p A>pB (A is lighter component) • y A>z A (lighter component enriched in V) • xB>zB (heavier component enriched in L)
Pressurize • Degree of separation high if p A>pB Mole
Mole
fraction
fraction
of A
of B
Feed, F
zA
zB
Vapor, V
yA
yB
Liquid, L
xA
xB
L , X A , XB, TL
• Very sui table table fo r desalination
Farooq/CN3132/Flash Farooq/CN3132/Flash Distillation-2
What can we say from the equilibrium design method? V and L are in equilibrium TL = TV = Tdrum Liquid and vapor stream pressures = P drum y= f(x) yi / xi= Ki (distribution coefficient) = f(P, T and all xi) x A + xB = 1 = y A + 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 t he flash process depends on goo d equilibrium data Experimental data
DePriester chart for hydrocarbons
Estimated K values
Simple thermodynamic model for ideal systems
Farooq/CN3132/Flash Distillation-3
Experimental vapor-liquid equilibrium data (including non-ideal systems) T-x-y plo t
Tabular (complete with temperature and pressure) XEt 0 0.019 0.0966 0.1661 0.2608 0.3965 0.5198 0.6763 0.7472 1.0
Xw 1.0 0.981 0.9034 0.8339 0.7382 0.6035 0.4802 0.3237 0.2528 0
YEt 0 0.17 0.4375 0.5089 0.5580 0.6122 0.6599 0.7385 0.7815 1.0
Yw 1.0 0.83 0.5625 0.4911 0.4420 0.3878 0.3401 0.2615 0.2185 0
T, C 100 95.5 86.7 84.1 82.3 80.7 79.7 78.74 78.41 78.3
P, atm
1
x-y diagram
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
Farooq/CN3132/Flash Distillation-4
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 Farooq/CN3132/Flash Distillation-5
Vapor-liquid equilibrium for an ideal sy stem In general:
yi xi
K i , where K i f P , T , all xi yi
For ideal systems :
xi
K i , where K i f P , T
The following identities apply for both ideal and nonideal systems n n n n y yi 1; xi 1; K i xi 1; i 1 i 1 i 1 i 1 i 1 K i Raoult’ s L aw:
pi
Dalton's Law: yi
pio xi
i. ii. iii. iv. v. vi.
Set system pressure (say, P =1 atm) 0.1 (depends on how many data points) x A = 0.1 x B = 1- x A Guess T Calculate K A and K B from Eqs (1) and (2) or from DePriester Chart Check if K x K B xB 1 Δ x =
pi
vii.
P
viii. ix.
y
x. xi. xii.
Store x A , y A and T Check if x A = 1 If yes, stop. If no, x A = x A + Δ x and go to (iv)
For an ideal syst em : K i
An toi ne eq uat io n:
How to generate x-y-T data from Ki values Illustration for a binary system
ln pio
pio
(1)
Ai
Bi
T C i
(2)
If no, go back to (iv). If yes, continue
K x ; y B K B x B
Repeat from (iii) for another pressure
Ai, Bi, Ci are constants for pure components. These are tabulated in various data sources. Tables of pure component vapor pressures are also available. Farooq/CN3132/Flash Distillation-6
Ap proximate equ ilibrium eq uation for a binary sy stem
Relative volatility,
AB
K A K B
o p A o p B
for ideal system Short-cut to vapor-liquid equilibrium data
B means no separation >1 or <1 necessary for vapor-liquid separation
K
or from DePriester Chart at the desired P and a representative T
f T ; K B f T ;
A good assumption for many systems is AB
AB
y A K x A A K B y B x B
y A x A 1 y A 1 x A
Upon rearrangement :
y A
Obtain K A and KB from Eq (1) and (2)
AB x A
1
B 1 x
(3)
f T
Use AB
K A K B
in Eq (3) to generate
y A vs x A data
Useful for preliminary design Cannot give the operating T Multicomponent systems 2D plots cannot be used Polynomial equations fitted to DePriester chart are used See eq (2-30) and Table 2-3 in text book
Farooq/CN3132/Flash Distillation-7
Degrees of fr eedom in a bin ary two-phase system Gibbs phase rule: F = C – P + 2
T, P, x A and y A are the 4 independent variables
For a binary vapor-liquid system: F = 2
You can choose to fix o nly 2 variables
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.
Farooq/CN3132/Flash Distillation-8
What do we mean by design ing a flash sy stem? Remember: x and y are in equilib rium Loop 1
Typicall y F, z, T1 and P1 will be known
Loop 2
Given Tdrum, Pdrum x, y,
F, z T1, P1
V L or F F
Calculate V L x, y, , F F Tdrum, Pdrum
Overall and component mass balances around Loop 1
TF and PF
TF from energy balance
TF, PF, hF
Pumping & Heating
QH V, y, HV F, z TF, PF, hF
How?
IDEAL STAGE
QH
around Loop 1 PF is chosen to prevent boiling at TF
QH from energy balance around Loop 2
L, x, hL
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
Drum height and diameter
Empirical method depends only on densities and mass flow rates of L and V
Farooq/CN3132/Flash Distillation-9
Simultaneous solution of (6a) and (7)
Binary Flash Distil lation
Numerical
Loop 1
V, y, HV F, z
General method including multi-component Analytical Equation for equilibrium data, e.g., eq (4) Graphical – simple and insightful
IDEAL STAGE
TF, PF, hF
L, x, hL
Overall mass balance
F V L
(5)
1 f f Eq (6a) intersects diagonal at x y z Slope of eq (6a)
1
Component balance (more volatile species)
f=0
0.8
Fz Vy Lx
e n e z n e B
1
y 0.4
or, z
(6)
1 f
0.2
Z = 0.5
0 0
1 f f
x
z f
(Operating line)
Equilibrium relationship: y
0.2
0.4 0.6 x Benzene
0.8
1
Benzene-toluene equilibrium
Rearranging eq (6a):
y
Diagonal (x = y)
0.5
0.6
V L y x fy (1 f )x F F V L FV f and therefore where F F F
L v
f x,T,P
(6a) (7)
Test
y our u nderstanding
Effect of vapor rate on ybenzene? Range over which y benzene can be varied? Farooq/CN3132/Flash Distillation-10
Condition to prevent boiling at TF yi K i TF,PF xi <1
Feed Temperature, TF
Loop 1
V, y, HV F, z TF, PF, hF
L x hL
(8)
(9a)
xiCLi (Tdrum Tref )
(9b)
ziCLi (TF Tref )
(9c)
i
Hv
F, z T1, P1
Pumping & Heating
b
QH
i
hF
Loo p 2
a
Qflash VHv LhL
where Qflash 0 (adiabatic) hL
i
Pump size to pressurize from P 1 to PF P P g V 2 a v a2 Wp F 1 b a b b gc 2gc
IDEAL STAGE
Energy balance around Loop: 1 FhF
i
yi i Cvi (Tdrum Tref )
(9d)
i
In (a) – (d) CL is liquid heat capacity is latent heat of vaporization Cv is vapour heat capacity When F,zi ,xi ,yi,Tdrum and Pdrum are known, after substituting (a) – (d) and the physical properties in eq (8), the only unknown is TF
hf
TF, PF, hF
Heat Load, QH Energy balance around Loop: 2 QH Fh1 FhF or, QH
F hF h1
(10)
After determining TF, hF is calculated from eq (9c) h1
zC i
Li (T1
Tref )
(11)
i
When F,zi,TF and PF are known, QH is obtained after substituting eqs (9c) and (11), and the physical properties in eq (10) Farooq/CN3132/Flash Distillation-11
Test
you r underst anding: Probl em 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
o
C, find x, y,
V L , . F F
(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)
Farooq/CN3132/Flash Distillation-12
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
V L and . F F
(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
V =0.5. This line will give you the composition of the vapor and liquid products. F
Use this information in the T vs. x or y plot to find required flash drum temperature.
Farooq/CN3132/Flash Distillation-13
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.
180
Vapor
160 ) C ( 140 T
o
120
Liquid 100
0
0.2
0.4
0.6
x T or y T
0.8
1
Figure: 2
Farooq/CN3132/Flash Distillation-14
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
V 0.7 and the liquid product composition is 25 mole % methanol. Equilibrium data at 1 F 2 Table 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 C
0
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 Farooq/CN3132/Flash Distillation-15
Farooq/CN3132/Flash Distillation-16
Test y our 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. Farooq/CN3132/Flash Distillation-17
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.
Fresh air
Hexane saturated outlet air 10 moles/hr
Paint formulation chamber
Maximum 10% of hexane in
Nearly pure
K values of hexane. Pressure Temp K o 1 atm 25 C 0.2 200 kPa 0 oC 0.035
(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.
Farooq/CN3132/Flash Distillation-18
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 73.6 oC 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) Pressure (kPa) Ethylene Ethane Propylene Benzene Ethylbenzene 1,4Diethylbenzene
15
16
17
18
19
20
73.6
73.6
81.4
145.4
139.0
191.1
110.0
110.0
105.0
120.0
110.0
140.0
0.54 7.0 2.0 8.38 0.71 0.013
0.0 0.0 0.0 169.46 91.54 10.35
0.0 0.0 0.0 0.17 89.72 0.0001
0.0 0.0 0.0 0.0 0.91 10.35
Flow rate (kmol/hr) 0.0 0.0 0.0 0.0 0.0 0.0 169.23 0.17 0.92 90.63 0.0 10.35
Farooq/CN3132/Flash Distillation-19
Block 1
Figure Q1: Process flow diagram for an ethyl benzene manufacturing process.
Farooq/CN3132/Flash Distillation-20
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 H2S CO2 CH4 C2H6 C3H8 C4H10
Mole % 5 10 75 5 3 2
Farooq/CN3132/Flash Distillation-21
