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Flash distillation
1. Theoretical background
Flash distillation is a special operation within distillation, where a liquid mixture is heated up and fed – with constant flowrate – into a distillation equipment. The resulting vapor and liquid phases enter a phase separator – an equilibrium chamber – and are drained separately. During the operation, the total pressure and temperature of the system, as well as the compositions of the two phases in equilibrium remain constant over time. One implementation of this operation is shown in Figure 1.
Figure 1 Flash distillation
The mixture to be separated is fed from the feed tank (1) by a pump (2) through a heat exchanger (3) at pressure P 3. Here, it is heated above the boiling point it would have at P 5 , the pressure inside the phase separator (5). The pressure of the mixture is then decreased by flowing it through a valve (4), so that it partially evaporates and yields a vapor and liquid phase with equilibrium compositions. Based on Figure 1, the following balance equations can be obtained: mass balance
F = L + V (Eq.1)
component balance:
Fx (^) F = Lx + Vy (Eq.2)
All necessary calculation can be carried out by using balance equations (Eq.1) and (Eq.2). The balance equations are valid whether using molar flows or mass flows.
F
xF
V; y
L; x
P 3 > P 5
Another implementation of flash distillation is feeding a mixture of quantity F and composition x F into a heated tank, yielding a liquid phase of quantity L and composition x , and a vapor phase of quantity V and composition y (which is the equilibrium composition for liquid phase composition x ). (Eq.1) and (Eq.2) apply in this case as well, thus all results are calculated with the same methods. This second implementation requires a startup period, which will be explained in detail later.
1.1. Calculation of flash distillation
The vapor phase composition y expressed from (Eq.1) and (Eq.2) yields the following equation:
y
L
V
x
F
V
=− + xF (Eq.3)
Figure 2 shows (Eq.3) and the equilibrium curve.
Figure 2 Equilibrium diagram for flash distillation
To calculate this operation, feed quantity and composition ( F , x F) and either the liquid and vapor quantities ( L , V ) or the liquid composition x (and potentially, vapor composition y ) is always given. Based on (Eq.3), the intersection point
1.2.1. Calculating startup time
For case b), startup time is calculated with condition F = V. This can be easily implemented by maintaining a constant fluid level in the tank using an overflow drain. The entire operation can be thus considered a batch distillation with refill. In the startup period, the component balance for the operation is:
Fx Vy ( (^) ) L
dx F (^) x dt − = 0 (Eq.4)
where F feed flowrate, kg/h V distillate flowrate, kg/h xF = x 0 feed concentration, mass fraction y ( x ) distillate concentration changing over time, mass fraction
Considering the startup period condition F = V rearranging and integrating the differential equation (Eq.4) yields:
( )
dt t
L
F
dx y x
t
x x^ F
x
F
0 0
(Eq.5)
(Eq.5) can be integrated graphically using a ( )
y x
x x −^ F
− diagram. (Eq.5) shows
that given a constant feed stream, startup time is shorter for smaller amounts of initial mixture L 0 ; and for a given amount of initial mixture L 0 , startup time can be reduced by increasing the feed stream flowrate ( F ).
2. Description of the experimental apparatus
The schematic of the apparatus is shown in Figure 3. The apparatus consists of a feed tank (1), a flow control tap (2), a capillary flow meter (3), an internally heated flask (4), the residue cooler (5) and distillate cooler (6). The residue and distillate are collected in flasks. The distillate forms two phases, these will be separated in a separation funnel.
3. Description of the measurement
The aim of this experiment is the recovery of ethyl acetate from an aqueous solution saturated with ethyl acetate. The solubility and vapor–liquid equilibrium data for the ethyl acetate–water system are shown in Figure 4. Ethyl acetate and water have limited miscibility, thus the condensed vapors cooled to room temperature typically form two phases. The enrichment due to
distillation is further amplified by the separation of the distillate. In this case, the industrial approach is to drain the ethyl acetate phase as the final product. Since the composition of the water phase composition usually equals the feed composition, it is recycled into the distillation tank. Due to the phase separation, (Eq.1) and (Eq.2) must be augmented with the balance equations of the phase separator (Florence flask). Mass balance of the phase separator:
V = L 1 (^) + L 2 (Eq.6)
Component balance of the phase separator:
Vy = L x 1 (^) 1 + L x 2 2 (Eq.7)
In (Eq.6) and (Eq.7) L 1 denotes the amount of the ethyl acetate phase, L 2 denotes the amount of the water phase (both in kg/h), x 1 denotes the composition of the etyl acetate phase, x 2 denotes the composition of the water phase (both in mass fraction). Typically, x 2 = x 0 = xF.
Figure 3
L; x
V ; y
Figure 4 Equilibrium diagram of the water–ethyl acetate system
3.2. Measurement in steady state
When the boiling point reaches 90 °C, increase the feed by 8–10 times so that the boiling point stabilizes between 87 and 93 °C. A constant boiling temperature indicates that steady state has been reached. At this point, take clean collector flasks and measure in steady state for 20 minutes. Decrease the feed if the temperature dereases, increase it if the temperature increases. After 20 minutes, measure the mass of the residue and determine its concentration by refractometry. Measure the mass of the two phases of the distillate at room temperature and determine their concentration from the equilibrium diagram. The exact mass of the feed is given by the sum of the residue and distillate masses. The capillary flow meter is only used to maintain a constant feed rate.
Laboratory report:
a.) Startup period initial mixture L 0 = g heating voltage U = V
feed temperature ϑ = °C
(room temperature) feed composition based on temperature xF = tömeg %
temperature at the end of startup ϑ = °C
measured startup time t = min
b.) Steady state measurement time t = min heating voltage U = V feed according to flow meter F = g/min
boiling point ϑ = °C
distillate temperature ϑ = °C
feed refractive index nD^20 =
component balance
Distillate Residue upper phase lower phase L 1 g
x 1 mass%
L 2
g
x 2 mass%
L
g
x mass%
