single stage unit operations: flash
TRANSCRIPT
Single stage unit operations: flash
Maurizio [email protected]
Department of Engineering & Architecture
University of Trieste
Process Simulation – Maurizio Fermeglia Trieste, 15 November, 2021 - slide 2
Agenda
Vapor-liquid equilibria
Relative volatility
Bubble point and dew point calculations
Flash calculation
Flash unit operation in Aspen+
Process Simulation – Maurizio Fermeglia Trieste, 15 November, 2021 - slide 3
Equilibrium ratio and relative volatility
3
IMPIANTI CHIMICI – A.A. 2019-2020
VLE in summary
0 < P < 2 bar
2 < P < 40 bar
P > 40 bar
1. Equilibrium ratio (capacity factor):
2. Relative volatility (selectivity):
3. For a binary system:
𝐾𝑖 =𝑦𝑖𝑥𝑖
𝛼𝑖,𝑗 = 𝑆𝑖,𝑗 =𝐾𝑖𝐾𝑗
𝛼 = 𝛼1,2 =Τ𝑦 𝑥
1 − 𝑦 Τ) ( 1 − 𝑥
Process Simulation – Maurizio Fermeglia Trieste, 15 November, 2021 - slide 4
Equilibrium ratio and relative volatility
At low pressure
Relative volatility is NOT constant, depends on composition ◼ *For ideal solution is nearly constant
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IMPIANTI CHIMICI – A.A. 2019-2020
At the azeotrope: x = y α = 1α >1
α < 1
Process Simulation – Maurizio Fermeglia Trieste, 15 November, 2021 - slide 5
Binary systems: Brown’s equation
Rearranging
Limit for thermal separation processes
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IMPIANTI CHIMICI – A.A. 2019-2020
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1
0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1
y
x
100
10
5
2
1,1
0,8
= 0,4
x = y
→ 0
→
𝑦 =𝛼𝑥
1 + (𝛼 − 1)𝑥
|𝛼𝑖,𝑗 − 1| ≥ 0.05
𝛼 = 𝛼1,2 =Τ𝑦 𝑥
1 − 𝑦 Τ) ( 1 − 𝑥
Process Simulation – Maurizio Fermeglia Trieste, 15 November, 2021 - slide 6
Vapor liquid equilibrium calculation types
Bubble point calculation, ◼ Given: the liquid composition (x) and equilibrium T or P
◼ Calculate: the vapor composition (y) and equilibrium P or T
Dew point calculations◼ Given: the vapor composition (y) and equilibrium T or P
◼ Calculate: the liquid composition (x) and equilibrium P or T
Flash calculation◼ Given: the global composition (z) and equilibrium T and P
◼ Calculate: the liquid (x) and vapor (y) compositions
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Process Simulation – Maurizio Fermeglia Trieste, 15 November, 2021 - slide 7
Vapor-Liquid Equilibrium Binary Systems
0 1x, y
PVAPOR
LIQUID
→
P
B
PA
AB
T = cost
p
x
y
b
r
bubbledew
P
→
= cost
LIQUID
VAPOR
x, y
T
T
T
B A0 1
B
A
dew
bubble
x
y
b
r
T
→
LIQUID
VAPOR
T
p
z = cost
T
T
b
b
r
r
= costj
p
p
C
bubble
dew
Process Simulation – Maurizio Fermeglia Trieste, 15 November, 2021 - slide 8
Bubble point and dew point calculations
For a binary system:◼ Unknowns: T, P, x1, x2, y1, y2
◼ Equilibrium equations: 2 isofugacity criteria
◼ Summation of mole fractions: 2 equations
◼ If two indipendent variables are fixed (P,x) two dependent variables are calculated (T,y)
◼ The equations to be solved are the isofugacity criterion for two components in two phases
For multicomponent systems:◼ Unknowns: T, P, xi, yi
◼ Equlibrium equations: n isofugacity criterion
◼ Summation of mole fractions: 2 equations
◼ If n+1 (composition and P) is fixed, n+1 variables may be calculated (T and yi)
◼ The equations to be solved are the isofugacity criterion for n components in 2 phases
Process Simulation – Maurizio Fermeglia Trieste, 15 November, 2021 - slide 9
Single stage operation (Flash, mixer-settler,..)
→ →
→
P , T
jF
j(1- )FL =
V =
FF
1
01
(1- j)j
j = 0
j = 1
y
xx z
y
F
E
r
b
→x
y
Process Simulation – Maurizio Fermeglia Trieste, 15 November, 2021 - slide 10
FLASH calculation
For a binary system and isothermal flash:◼ Unknowns: T, P, F, z1, z2, V, L, x1, x2, y1, y2 (11)
◼ Material balance equations: 2 chemical species
◼ Equilibrium equations: 2 isofugacity criteria
◼ Summation of mole fractions: 3 equations
◼ If z1, F, T and P is specified, the problem is solved for z2, x1, x2, y1, y2, L, V
For a multicomponent system and isothermal flash:◼ Unknowns: T, P, F, zi, V, L, xi, yi (3(n-1) +5) = 3n+2
◼ Material balance equations (n)
◼ Equilibrium equations (n)
◼ TOTAL Equations= 2n
◼ IF zi, F, T and P is specified (n-1+3)= n+2 → the problem is solved
Process Simulation – Maurizio Fermeglia Trieste, 15 November, 2021 - slide 11
Flash in Aspen+
Input stream 1:◼ water – methanol
◼ T=85°
◼ P=1.5 atm
◼ Mole flow 50 – 50 kmol/hr
Block Flash input◼ Pressure = 1 atm
◼ Duty = 0
Process Simulation – Maurizio Fermeglia Trieste, 15 November, 2021 - slide 12
Flash in Aspen+: results