Single stage unit operations: flash

Maurizio FermegliaMaurizio.fermeglia@units.it

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

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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