Flash Distillation Flashcards
what is flash distillation?
flash distillation is a single-stage phase-creation process
how are the phases created in flash distillation?
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
list the components of a single stage flash process
feed stream (F, z, Tf, Pf) heat exchanger (Q) expansion valve flash drum (T,P) vapour stream (V,y) liquid stream (L,x)
what two intensive variales specify an isothermal flash process?
temperature and pressure
how are the intensive variables x and y determmined?
from independent phase equilibria equations
k1=y/x
k2= 1-y / 1-x
or they can be read for the T,x,y diagram
how are the extensive variables of an isothermal flash process computed
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
what four other variables need to be specified in an isothermal flash process
F, Z Tf, Pf
what are the intensive variables concerning an isothermal flash process
x,y,z,T,P,Tf,Pf
what are all the extensive variables concerning an isothermal flash process
F,L,V,Q
how do you determine phase compositions in equilibrium at given T and P
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
how would you solve the full flash process inclusing determination of heat duty
stream enthalpies need to be calculated
how do you calculate enthalpy of pure liquid?
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)
how do you calculate enthalpy of pure vapour
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)
how do you calculate enthalpy for a specific temperature?
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)
how do you convert from kJ/h to MJ/h?
1 kJ/h is 0.001 MJ/h
so, to convert we need to multiply the number by 0.001
how do you convert from MJ/h to kW?
The energy in kilowatt-hours is equal to the megajoules multiplied by 0.277778.
what is required for an isenthalpic flash process? ( adiabatic flash process)
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)
how do you solve flash problems with specified P and q?
isobaric x,y diagram
describe the isobaric x,y diagram
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
what does the isobaric x,y diagram show?
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
what is y(x)
equilibrium curve
what is the feed line/ q-line
y= z/1-q - qx/1-q
this is obtained by making y the subject of the material balance
does the q-line satisfy the material balance and equilibrium criterion?
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
where does the q-line cross the 45∘ line?
the q-line passes through the point x=y=z on the 45∘ line
what are some properties of the q-line?
equation of the line: y = z/1-q -qx/1-q
slope: -q/1-q
passes through x=y=z
what is the general two phase split?
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]
what is the bubble point phase split?
q=1
q line is vertical on the diagram
what is the dew point split phase?
q=0
q line is horizontal on the diagram
what process equipment is needed for flash distillation?
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
