Advanced
separation process
Lec2
Instructor:
Eng: Abdullah Ali Alsharafi
Email :abdoamer.2553@gmail.com
Thermodynamics of Separation Operations
Thermodynamic properties play a major role in separation operations with
respect to energy requirements, phase equilibrium ,biological activity, and
equipment sizing.
Experimental thermodynamic property data should be used, when available,
to design and analyze the operation of separation equipment.
THERMODYNAMICS OF SEPARATION OPERATIONS
Which
n is molar flow rate
z mole fraction
h molar enthalpy
s molar entropy
b molar availability
Q flow lf the heat
Ws shaft work
LW lost work
Ts absolute Temp.
THERMODYNAMICS OF SEPARATION OPERATIONS
From the previous figure we summary this equations :
The energy balance equation for system:
(nh Q w ) (nh Q w ) 0
out
s
in
s
The entropy balance equation for system:
Q Q
out
(ns ) ( ns ) Sirr
Ts in Ts
Availability balance equation for system by combination first and second low
To To
in
(nb Q (1 ) Ws ) (nb Q(1 ) Ws ) LW
Ts out Ts
THERMODYNAMICS OF SEPARATION OPERATIONS
The Minimum work of separation:
Wmin nb nb
out in
Second-law efficiency:
Wmin
Lw Wmin
where
b h To s =availability function
Lw To Sirr =lost work
THERMODYNAMICS OF SEPARATION OPERATIONS
In the entropy balance, the heat sources and sinks in at absolute
temperatures,
Unlike the energy balance, which states that energy is conserved, the
entropy balance predicts the production of entropy, Ds,irr, which is the
irreversible increase in the entropy of the universe. This term, which must
be positive, is a measure of the thermodynamic inefficiency. In the limit, as
a reversible process is approached, Ds,irr tends to zero. Unfortunately,
Ds,irr is difficult to apply because it does not have the units of energy/unit
time (power).
EXAMPLE
EXAMPLE
SOLUTION
SOLUTION
SOLUTION
SOLUTION
SINGLE EQUILIBRIUM STAGES AND FLASH CALCULATIONS
The simplest separation process is one in which two phases in
contact are brought to physical equilibrium, followed by phase
separation.
If the separation factor, between two species in the two phases
is very large, a single contacting stage may be sufficient to
achieve a desired separation between them; if not, multiple
stages are required:
if a vapor phase is brought to equilibrium with a liquid phase,
the separation factor is the relative volatility, a, of a volatile
component called the light key, LK, with respect to a less-
volatile component called the heavy key, HK
SINGLE EQUILIBRIUM STAGES AND FLASH CALCULATIONS
If the separation factor is 10,000, a near perfect separation is
achieved in a single equilibrium stage. If the separation factor
is only 1.10, an almost perfect separation requires hundreds of
equilibrium stages.
BINARY VAPOR–LIQUID SYSTEMS
BINARY VAPOR–LIQUID SYSTEMS
BINARY VAPOR–LIQUID SYSTEMS
BINARY VAPOR–LIQUID SYSTEMS
BINARY VAPOR–LIQUID SYSTEMS
For the water–glycerol system, the difference in boiling points is 190C.
Therefore, relative volatility values are very high, making it possible to
achieve a good separation in a single equilibrium stage. Industrially, the
separation is often conducted in an evaporator, which produces a nearly
pure water vapor and a glycerol-rich liquid. For example, as seen in the
Table 4.1a, at 207C, a vapor of 98 mol% water is in equilibrium with a liquid
containing more than 90 mol% glycerol.
For the methanol–water system, in Table 4.1b, the difference in boiling
points is 35.5C and the relative volatility is an order of magnitude lower
than for the water–glycerol system so its difficult separation with a single
stage and a 30 tray distillation column is required to obtain a 99 mol%
methanol distillate and a 98 mol% water bottoms.
For the paraxylene–metaxylene isomer system in Table 4.1c, the boiling-point
difference is only 0.8C and the relative volatility is very close to 1.0, making separation
by distillation impractical because about 1,000 trays are required to produce nearly
pure products. Instead, crystallization and adsorption, which have much higher
separation factors, are used commercially.
BINARY VAPOR–LIQUID SYSTEMS
BINARY VAPOR–LIQUID SYSTEMS
BINARY VAPOR–LIQUID SYSTEMS
BINARY VAPOR–LIQUID SYSTEMS
Vapor–liquid equilibrium data for methanol–water in Table 4.2 are in the
form of P–YA–XA for temperatures of 50C, 150C, and 250C. The data cover
a wide pressure range of 1.789 to 1,234 Psia, with temperatures increasing
with pressure. At 50C, AB averages 4.94. At 150C, the average AB is only
3.22; and at 250C, it is 1.75. Thus, as temperature and pressure increase,
AB decreases.
For the data set at 250C, it is seen that as compositions become richer in
methanol, a point is reached near 1,219 psia, at a methanol mole fraction
of 0.772, where the relative volatility is 1.0 and distillation is impossible
because the vapor and liquid compositions are identical and the two phases
become one. This is the critical point for the mixture. It is intermediate
between the critical points of methanol and water:
BINARY VAPOR–LIQUID SYSTEMS
Critical conditions exist for each binary composition. In industry,
distillation columns operate at pressures well below the critical
pressure of the mixture to avoid relative volatilities that
approach 1.
Thermodynamic Properties and Phase Equilibrium
EQUATIONS OF STATE
The relationship between pressure, volume and temperature for fluids
is described by equations of state.
For example:
if a gas is initially at a specified pressure, volume and
temperature and two of the three variables are changed,
the third variable can be calculated from an equation of
state.
Gases at low pressure tend towards ideal gas behavior.
For a gas to be ideal.
PV = NRT (1)
EQUATIONS OF STATE
The ideal gas law describes the actual behavior
of most gases reasonably well at pressures
below 5 bar.
For gas mixtures, the partial pressure is defined as the
pressure that would be exerted if that component alone
occupied the volume of the mixture. Thus, for an ideal gas,
PV
i Ni RT (2)
where
pi = partial pressure (N·m−2)
Ni = moles of Component i (kmol)
EQUATIONS OF STATE
