Flash Distillation

Question 1

what is flash distillation?

Answer 1

flash distillation is a single-stage phase-creation process

Question 2

how are the phases created in flash distillation?

Answer 2

the phases are changed in the process by means of heat and/or work

the two phases mixture is either achieved by the liquid feed being partially vaporised or the vapour feed being partially condensed

Question 3

list the components of a single stage flash process

Answer 3

feed stream (F, z, Tf, Pf)
heat exchanger (Q)
expansion valve
flash drum (T,P)
vapour stream (V,y)
liquid stream (L,x)

Question 4

what two intensive variales specify an isothermal flash process?

Answer 4

temperature and pressure

Question 5

how are the intensive variables x and y determmined?

Answer 5

from independent phase equilibria equations
k1=y/x
k2= 1-y / 1-x

or they can be read for the T,x,y diagram

Question 6

how are the extensive variables of an isothermal flash process computed

Answer 6

L,V,Q can be determined from independent balance equations

F=L+V TOTAL MASS BALANCE
zF= xL+yV MVC MASS BALANCE
hfF+ Q = hlL +hvV ENERGY BALANCE

Question 7

what four other variables need to be specified in an isothermal flash process

Answer 7

F, Z Tf, Pf

Question 8

what are the intensive variables concerning an isothermal flash process

Answer 8

x,y,z,T,P,Tf,Pf

Question 9

what are all the extensive variables concerning an isothermal flash process

Answer 9

F,L,V,Q

Question 10

how do you determine phase compositions in equilibrium at given T and P

Answer 10

use the isobaric T,x,y diagram
read across from the given temperature
x is read from the bubble curve
y is read from the dew curve

Question 11

how would you solve the full flash process inclusing determination of heat duty

Answer 11

stream enthalpies need to be calculated

Question 12

how do you calculate enthalpy of pure liquid?

Answer 12

from the ideal liquid model:
hliq(T) = ∑i xi- hliqi(T)
where hliq,i is the enthalpy of pure liquid i (assume independent of of pressure)

Question 13

how do you calculate enthalpy of pure vapour

Answer 13

from the ideal gas model:
hvap(T) = ∑i -yi- hvap,i(T)
where hvap,i is the enthalpy of pure vapour i (assume independent of of pressure)

Question 14

how do you calculate enthalpy for a specific temperature?

Answer 14

enthalpies are defined relative to an assigned value at a reference temperature and state
therefore, enthalpies at other temperatures are calculated by integration

for liquids,
hliq,i(T)= hliq,i(reference temperature) + Cp,liq,i (T- ref.T0)

for vapours,
hlvap,i(T)= hliq,i(reference temperature)+ Δh,vap,i(ref.T) + Cp,vap,i (T- ref.T0)

Question 15

how do you convert from kJ/h to MJ/h?

Answer 15

1 kJ/h is 0.001 MJ/h

so, to convert we need to multiply the number by 0.001

Question 16

how do you convert from MJ/h to kW?

Answer 16

The energy in kilowatt-hours is equal to the megajoules multiplied by 0.277778.

Question 17

what is required for an isenthalpic flash process? ( adiabatic flash process)

Answer 17

specified pressure and zero heat duty

hmix and Pf must be specified for the two phase system entering the flash drum
solve for x,y and q using a VLE model (Raoult’s law)

Question 18

how do you solve flash problems with specified P and q?

Answer 18

isobaric x,y diagram

Question 19

describe the isobaric x,y diagram

Answer 19

different to isobaric T,x,y diagram
temperature varies along the curve]there is a 45∘ line on the diagram as a plotting aid

y= αx/ 1+x(α-1) for constant relative volatilites
x=y/α-y(α-1)

y(x) is known as the equilibrium curve
the isobaric x,y diagram gives y as a function of x for the equilibrium liquid and vapour phases in a binary mixture at specified pressure

Question 20

what does the isobaric x,y diagram show?

Answer 20

the isobaric x,y diagram gives y as a function of x for vapour iquid equilibrium vapour and liquid phases ina binary mixture at specified pressure

Question 21

what is y(x)

Answer 21

equilibrium curve

Question 22

what is the feed line/ q-line

Answer 22

y= z/1-q - qx/1-q

this is obtained by making y the subject of the material balance

Question 23

does the q-line satisfy the material balance and equilibrium criterion?

Answer 23

all pointes on the q-line sattisfy the material balance criterion
the point at which the q-line crosses the equilibrium curve also satisifies the equilibrium criterion

Question 24

where does the q-line cross the 45∘ line?

Answer 24

the q-line passes through the point x=y=z on the 45∘ line

Question 25

what are some properties of the q-line?

Answer 25

equation of the line: y = z/1-q -qx/1-q
slope: -q/1-q
passes through x=y=z

Question 26

what is the general two phase split?

Answer 26

the general two phase split is and equilibrium phase composition where
the liquid fraction can be equal to any value between 0 and 1 [0,1]
L/V can be equal to any value between 0 and infinity [0,inf]

Question 27

what is the bubble point phase split?

Answer 27

q=1

q line is vertical on the diagram

Question 28

what is the dew point split phase?

Answer 28

q=0

q line is horizontal on the diagram

Question 29

what process equipment is needed for flash distillation?

Answer 29

upstream, heat exchanger,
variable throttle valve,
flash vessel,
liquid+ vapour outlets,
pressure, liquid- level and temperature sensors
process controller to manipulate heat exchanger duty and valve positions to maintain steady state throughout at the specified flash conditions