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CENG 221
Lecture 2. Simple Distillation Processes (4.5 h)
Learning Objectives:
(1) Simple batch distillation
(2) Binary flash distillation
(3) Sizing of flash distillation drum
Learning Guides:
(1) Lecture handouts
(2) Chapters 11.1-11.2, 3.1-3.6 and 2 of Textbook:
Equilibrium-Staged Separations
(3) Please review the notes on VLE, bubble and dew point
calculations
CENG 221
Lecture 2. Simple Distillation Processes (4.5 h)
Learning Objectives:
(1) Simple batch distillation
(2) Binary flash distillation
(3) Sizing of flash distillation drum
Learning Guides:
(1) Lecture handouts
(2) Chapters 11.1-11.2, 3.1-3.6 and 2 of Textbook:
Equilibrium-Staged Separations
(3) Please review the notes on VLE, bubble and dew point
calculations
Simple Batch Distillation
Used only for small scale processes,especially in pharmaceuticals andspecialty products.
Simple Batch Distillation
Water in
Water out
F = W + DF Xf = W Xw + D Xd
Rayleigh Equationd(W Xw) = Yd dW = Xw dW + W dXw
dW/W = dXw /(Yd - Xw)
F = W + DF Xf = W Xw + D Xd
Rayleigh Equationd(W Xw) = Yd dW = Xw dW + W dXw
dW/W = dXw /(Yd - Xw)
Simple Batch Distillation
Yd
Xw
Equilibrium
W - dW
dW
Material Balance:(W-dW) Xw + Yd dW = W (Xw + dXw)dW/W = dXw/(Yd-Xw)…………….Rayleigh Equation
X(ethanol) X(water) Y(ethanol) Y(water) T(C)0 1 0 1 100
0.019 0.981 0.17 0.83 95.50.0721 0.9279 0.3891 0.6109 890.0966 0.9034 0.4375 0.5625 86.70.1238 0.8762 0.4704 0.5296 85.30.1661 0.8339 0.5089 0.4911 84.10.2377 0.7623 0.5445 0.4555 82.70.2608 0.7392 0.558 0.442 82.30.3273 0.6727 0.5826 0.4174 81.50.3965 0.6035 0.6122 0.3878 80.70.5079 0.4921 0.6564 0.3436 79.80.5198 0.4802 0.6599 0.3401 79.70.5732 0.4268 0.6841 0.3159 79.30.6763 0.3237 0.7385 0.2615 78.740.7472 0.2528 0.7815 0.2185 78.410.8943 0.1057 0.8943 0.1057 78.15
1 0 1 0 78.3
Simple Batch Distillation
Example 9: An alcohol soln. contains 20 mole % ethanol was distilledin a batch distillation unit. The distillation was stopped when thetemperature of the still reaches 90 C. Please determine the following:(1) What is the bubble point temperature of the starting and end
mixtures?(2) What is the concentration of first bubble of vapor formed from
the solution and the last bubble before the distillation was stopped?(3) Determine the amount of liquid remaining in the still if the
starting solution is 1 Kmole.(4) How much distillate was obtain and its average concentration?(5) What is the expected bubble point temperature of the distillate?
75
80
85
90
95
100
105
0 0.2 0.4 0.6 0.8 1
X,Y (ethanol)
T(C
)
(1) Plot the T-x-y data for ethanol-water solution
T(C) X Y Y-X 1/(Y-X)100 0 0
95.5 0.019 0.17 0.151 6.62251789 0.0721 0.3891 0.317 3.154574
86.7 0.0966 0.4375 0.3409 2.93341285.3 0.1238 0.4704 0.3466 2.8851784.1 0.1661 0.5089 0.3428 2.91715382.7 0.2377 0.5445 0.3068 3.25945282.3 0.2608 0.558 0.2972 3.36473881.5 0.3273 0.5826 0.2553 3.9169680.7 0.3965 0.6122 0.2157 4.63606979.8 0.5079 0.6564 0.1485 6.73400779.7 0.5198 0.6599 0.1401 7.13775979.3 0.5732 0.6841 0.1109 9.017133
78.74 0.6763 0.7385 0.0622 16.0771778.41 0.7472 0.7815 0.0343 29.1545278.15 0.8943 0.8943
78.3 1 1
(2) Calculate the parameters in the Rayleigh Equation:
F(x) = 1/(Y-X) vs X
(3) Plot F(x) vs X
0
10
20
30
0 0.2 0.4 0.6 0.8
X
F(x
)
(3) Graphical Integration
0
2
4
6
8
10
0 0.1 0.2 0.3
X
F(x
)
(3) Graphical Integration
0
2
4
6
8
10
0 0.1 0.2 0.3
X
F(x
)
Numerical IntegrationSimpson Rules:
Area = (Xf - Xi) 1/6 [F(Xi) + 4F(Xm) + F(Xf)]where Xf is the final point
Xi is the initial pointXm is the midpoint given by 0.5Xf + 0.5Xi
Simple Batch Distillation
Water in
Water out
Example 10: A 100 g solution of isoamyl alcohol (65 wt.%) and ethanol (35 wt.%) was to be separated using a laboratory still (see figure). Thebinary solution has a relative volatility of 2.25. At the end of thedistillation, 30 g of solution was left in the still.(1) What is the concentration of the first drop of distillate collected
from the distillation unit?(2) What is the concentration of isoamyl alcohol in the remaining
solution in the still?(3) What is the concentration of ethanol in the last drop of distillate
collected?(4) How much distillate is produced and what is its ethanol content?(5) If an isoamyl concentration of 95 % is needed, what is maximum
yield one can obtain?
(1) Plot y vs x
0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1
x (mass fraction of EtOH)
y (m
ass
frac
tion
of E
tOH
)
Isoamyl alcohol-ethanol solution
x y y-x 1/(y-x)0 0
0.04 0.085714 0.045714 21.8750.08 0.163636 0.083636 11.956520.12 0.234783 0.114783 8.7121210.16 0.3 0.14 7.142857
0.2 0.36 0.16 6.250.24 0.415385 0.175385 5.7017540.28 0.466667 0.186667 5.3571430.32 0.514286 0.194286 5.1470590.36 0.558621 0.198621 5.034722
0.4 0.6 0.2 50.44 0.63871 0.19871 5.0324680.48 0.675 0.195 5.1282050.52 0.709091 0.189091 5.2884620.56 0.741176 0.181176 5.519481
(2) Calculate and plot 1/y-x vs x (Rayleigh equation)
0
5
10
15
20
25
0 0.2 0.4 0.6
x (mass fraction of EtOH)
F(x
) =
1/y
-x
0
5
10
15
20
25
0 0.2 0.4 0.6
x (mass fraction of EtOH)
F(x
) =
1/y
-x
Isoamyl alcohol-ethanol solution
Ethanol + Water
LIQUID
VAPOR
Ethanol
Water
Flash DistillationSingle stage distillation process for continuous separation.
Continuous FEEDContinuous
DISTILLATE
Flash DistillationUsed mainly for simple separation. Separate component with largedifference in volatility.Example: distilled water
Pb Pc PdPa <<> >
pump
heater
throttlevalve
Tb Tc TdTa ≥~ <
Liquid productL, x, hL
Vapor productV, y, Hv
TdrumPdrum
Q
TF, PF, hF
FeedF, z,T1, P1
Degree of Freedom (F)
F = {total # of independent variables} - {total # of equations}
Variables No Relation NoFEEDF, feed rate 1 Enthalpy (H) 3zi, feed composition C-1 VLE data (y = Kx) CT1, temperature 1 Mass Balance CP1, pressure 1 Energy Balance 1
LIQUIDL, liquid flow rate 1xi, liquid composition C-1hL, liquid enthalpy 1
VAPORV, liquid flow rate 1yi, liquid composition C-1HL, liquid enthalpy 1
STILLTdrum, temperature 1Pdrum, pressure 1Q, heating rate 1
F = {total # of independent variables} - {total # of equations}
Variables No Relation NoFEEDF, feed rate 1 Enthalpy (H) 3zi, feed composition C-1 VLE data (y = Kx) CT1, temperature 1 Mass Balance CP1, pressure 1 Energy Balance 1
LIQUIDL, liquid flow rate 1xi, liquid composition C-1hL, liquid enthalpy 1
VAPORV, liquid flow rate 1yi, liquid composition C-1HL, liquid enthalpy 1
STILLTdrum, temperature 1Pdrum, pressure 1Q, heating rate 1
Variables = 3C + 8Equations = 2C + 4degree of freedomF = C + 4
Variables = 3C + 8Equations = 2C + 4degree of freedomF = C + 4
Degree of Freedom (F)
Flash Distillation: F = C + 4
Binary flash distillation# components: C = 2
F: 2 + 4 = 6 = number of parameters that needed to bespecified in order to solve a flash distillation problem.
= number of design parameters in order to build flash distillation unit.
Parameters usually specified:(1) Feed conditions:
F (feed rate), z (feed, composition)T1, P1
(2) Still condition: Pdrum
The last parameters will dictate the solution procedure:
Binary flash distillation# components: C = 2
F: 2 + 4 = 6 = number of parameters that needed to bespecified in order to solve a flash distillation problem.
= number of design parameters in order to build flash distillation unit.
Parameters usually specified:(1) Feed conditions:
F (feed rate), z (feed, composition)T1, P1
(2) Still condition: Pdrum
The last parameters will dictate the solution procedure:
Flash Distillation (Sequential Solution -1)
Parameters usually specified:(1) Feed conditions:
F (feed rate), z (feed, composition)T1, P1
(2) Still condition: Pdrum
The last parameters will dictate the solution procedure:condition of the drum-1: Tdrum, y or x
Parameters usually specified:(1) Feed conditions:
F (feed rate), z (feed, composition)T1, P1
(2) Still condition: Pdrum
The last parameters will dictate the solution procedure:condition of the drum-1: Tdrum, y or x
Example 11: 1000 mole/h of 0.4 mole fraction ethanol-water solution(T = 24 C, P = 101.3 kPa) enters a flash drum operated at 101.3 kPa.If the concentration of ethanol in the vapor leaving the flash drum is0.6 mole fraction, what is the Tdrum, x, L, V and Qheater?
Flash Distillation (Sequential Solution -1)
75
80
85
90
95
100
105
0 0.2 0.4 0.6 0.8 1
X,Y (ethanol)
T(C
)
Flash Distillation (Sequential Solution -2)
Parameters usually specified:(1) Feed conditions:
F (feed rate), z (feed, composition)T1, P1
(2) Still condition: Pdrum
The last parameters will dictate the solution procedure:condition of the drum-2: V, L, f = V/F, q = L/F or L/V
Parameters usually specified:(1) Feed conditions:
F (feed rate), z (feed, composition)T1, P1
(2) Still condition: Pdrum
The last parameters will dictate the solution procedure:condition of the drum-2: V, L, f = V/F, q = L/F or L/V
Example 10: 1000 mole/h of 0.4 mole fraction ethanol-water solution(T = 24 C, P = 101.3 kPa) enters a flash drum operated at 101.3 kPa.Determine the flowrate and composition of the products leaving the flash drum and the Tdrum given:(a) amount of vapor produced is 500 mole/h,(b) the fraction of liquid remaining is 0.7,(c) for (b) also calculate the heat needed to achieve the distillation.
(1) Plot VLE data in form of McCabe-Thiele diagram:
Operating Equation:OMB: F = V + LCMB: Fz = Vy + Lx solve the 2 eqns and set in form of y = f(x)
y = -(L/V)x + (F/V)z = mx + b (equation for a line)y = -(1-f/f)x + z/fy = -(q/1-q)x + z/(1-q)
slope, m = -L/V = -(1-f/f) = -(q/1-q)y-intercept, b = (F/V)z = z/f = z/(1-q)
Operating Equation:OMB: F = V + LCMB: Fz = Vy + Lx solve the 2 eqns and set in form of y = f(x)
y = -(L/V)x + (F/V)z = mx + b (equation for a line)y = -(1-f/f)x + z/fy = -(q/1-q)x + z/(1-q)
slope, m = -L/V = -(1-f/f) = -(q/1-q)y-intercept, b = (F/V)z = z/f = z/(1-q)
0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1
X (ethanol)
Y (
eth
an
ol)
Flash Distillation (Sequential Solution -2)
Flash Distillation (Sequential Solution -2)
75
80
85
90
95
100
105
0 0.2 0.4 0.6 0.8 1
X,Y (ethanol)
T(C
)
Flash Distillation (Simultaneous Solution)
Parameters usually specified:(1) Feed conditions:
F (feed rate), z (feed, composition)T1, P1
(2) Still condition: Pdrum
The last parameters will dictate the solution procedure:condition of the feed entering the drum: TF or hF
Parameters usually specified:(1) Feed conditions:
F (feed rate), z (feed, composition)T1, P1
(2) Still condition: Pdrum
The last parameters will dictate the solution procedure:condition of the feed entering the drum: TF or hF
Example 11: 1000 mole/h of 0.4 mole fraction ethanol-water solution(T = 24 C, P = 101.3 kPa) enters a flash drum operated at 101.3 kPa.Determine the flowrate, composition and enthalpy of the productsleaving the flash drum and the Tdrum given:(a) TF is 83 C(b) enthalpy of the feed entering the drum, hF is 150 kcal/kg
Different Representation of VLE Data
(6) Ponchon-Savarit Diagram:
H-x-y diagram
Sizing of Flash Drum
D
D = (4Ac/π)0.5
Ac (ft2) = W(lbmol/h)MWvapor(lbm/lbmol)uperm(ft/s)(3600)ρv(lbm/ft3)
uperm = Kdrum(ρL - ρv/ρv)0.5
Kdrum = exp(A + BlnFlv + C(lnFlv)2
+ D (lnFlv)3 + E (lnFlv)4)where: A = -1.87748
B = -0.81458C = -0.18707D = -0.01452E = -0.00101Flv = WL/Wv (ρv/ρL)0.5
Sizing of Flash Drum
D
hv = 0.5D + 36” > 48”
hf = 0.5D + 12”
hL = 4Vs/πD2
Note: L = hv + hf + hL3 < L/D < 5
Example 12: Determine the size (i.e., diameter and height) of theflash drum needed to separate 1000 mole/h of 0.4 mole fraction ethanol-water solution (T = 24 C, P = 101.3 kPa) enters a flash drum operated at 101.3 kPa. If the the fraction of liquid remaining is 0.7.
Sizing of Flash Drum
0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1
X (ethanol)
Y (
eth
an
ol)
Sizing of Flash Drum
75
80
85
90
95
100
105
0 0.2 0.4 0.6 0.8 1
X,Y (ethanol)
T(C
)
Flash Drum in Series (Cascade)
1 2 3
V1V2 =(F3)
V3
L1 (=F2)L2 L3
F1
Example 13: Determine the composition and flowrate of each product streams leaving the flash drums 1, 2 and 3. A 0.3 mass fractionethanol-water solution was feed to the first drum in the cascade.The temperature of the first drum was 89°C. The feed to the seconddrum was preheated so that its enthalpy (hf2) is 200kcal/kg. The vaporfraction in the last drum is 0.2.
Please use the Ponchon-Savarit Diagram for your solution.
Flash Drum in Series (Cascade)
1
V1
L1 (=F2)
F1
Flash Drum in Series (Cascade)
2
V2 =(F3)
L1 (=F2)L2
Flash Drum in Series (Cascade)
3
V2 =(F3)V3
L3