Title Flash and Decanter Aspen Plus Phase (Matter) Vapor Mechanics Branches Of Thermodynamics Nature 842.4 KB 11
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CHAPTER SEVEN

FLASHES AND DECANTER

Aspen Plus’s model library contains two different rigorous flash blocks that solve the
appropriate material, energy balance, and equilibrium equations. The Flash2 block is
designed to produce a single vapor phase and a single liquid phase that are in equi-
librium when the flash conditions are specified such that a multicomponent mixture’s
state is in the two-phase region. Similarly, the Flash3 block is designed to produce one
vapor phase and two liquid phases in equilibrium for suitably specified process condi-
tions. The Flash3 block is also capable of solving a liquid–liquid equilibrium problem
under conditions where no vapor is produced. A similar block is Decanter, which is
designed to produce two liquid phases in equilibrium in the absence of a vapor phase.
The Flash2, Flash3, and Decanter blocks can be found in the model library under the
Separators tab.

7.1 Flash2 BLOCK

Figure 7.1 is a graphical depiction of a Flash2 block. The nomenclature used is as
follows: F represents the total feed flow, in moles/time; fi the flow of component
i in the feed, in moles/time; V the total vapor flow, in moles/time; vi the flow of
component i in the vapor, in moles/time; L the total liquid flow, in moles/time; li the
flow of component i in the liquid, in moles/time; and n the number of components.
Including the flash temperature Tf and pressure Pf results in 2n + 2 independent
variables given the feed state. These are equilibrium temperature; flash pressure; two
total flows, L and V ; and 2(n − 1) component flows; or alternatively, 2n component
flows, excluding the total flows. Mole fractions are calculated from the independent

Teach Yourself the Basics of Aspen Plus™ By Ralph Schefflan

93

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94 FLASHES AND DECANTER

Flash2
PFeed, F, f

Heat, Q

Liquid, L, l

Vapor, V, v

Figure 7.1 Flash2 model.

variables by an equation such as

yi =
vi∑n
j=1 vj

(7.1)

where yi is the mole fraction of component i in the vapor.
The applicable material balances are n componential equations such as

fi − vi − li = 0 (7.2)

or alternatively, n − 1 equations such as equation (7.2) and one overall material balance
given by

F − V − L = 0 (7.3)

Additionally, n equilibrium equations which describe the equality of the fugacities
of components in each phase are required. When the liquid fugacity is represented by
an equation of state, where φL

i
and φV

i
are the fugacity coefficients of component i in

the liquid and vapor phases, respectively, the result is

yiφ
V
i − xiφLi = 0 (7.4a)

When the liquid fugacity is represented by an activity coefficient equation, where
γi is the activity coefficient of component i and the vapor phase is represented by an
equation of state

yiφ
V
i P − γixipvi = 0 (7.4b)

where pv
i

is the vapor pressure of component i , results. For the sake of simplicity, the
Poynting correction (see Prausnitz et al., 1999), which has a contribution only for very
light components, has been omitted.

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102 FLASHES AND DECANTER

TABLE 7.1 Feeds for Workshop 7.1

Component Feed 1 (lbmol/hr) Feed 2 (lbmol/hr)

Ethanol 40
Water 50
Acetone 10

Workshop 7.2 Based on the solution to Workshop 7.1, using 760 mmHg as a base
case, create a sensitivity study for pressure varying between 10 and 760 mmHg. Show
the effect of varying flash pressure on fraction vaporized, mole fraction acetone in the
vapor and liquid, and flash temperature using Aspen Plus’s stored Uniquac parameters.

Workshop 7.3 The feeds in Table 7.2 are to be fed to a Flash3 block at a pressure
of 760 mmHg and a temperature of 80◦C. Using Aspen’s stored Uniquac parameters,
use a sensitivity study to find the temperature at which the vapor phase disappears
when operating at 760 mmHg. The temperature and pressure of both feeds are 25◦C
and 760 mmHg.

TABLE 7.2 Feeds for Workshop 7.3

Component Feed 1 (lbmol/hr) Feed 2 (lbmol/hr)

Ethanol 10
Water 40
Toluene 50

Workshop 7.4a Using the feeds from Workshop 7.3, employ the Decant block
to calculate the composition and quantity of the resulting phases at 25◦C using the
parameters provided in Table 7.3. Be sure to set up appropriate units.

TABLE 7.3 Uniquac Parameters for a Toluene–Water–Ethanol System

Parameter/ Parameter Estimate
Element No. Component Pair (SI units)

UNIQ/2 Toluene–H2O −926.6623
UNIQ/2 H2O–Toluene −258.8106
UNIQ/2 EtOH–H2O 301.8589
UNIQ/2 H2O–EtOH −119.5998
UNIQ/2 EtOH–Toluene −49.37053
UNIQ/2 Toluene–EtOH −100.2406

Workshop 7.4b Develop a sensitivity study to determine the effect of temperature
on this operation. Vary the temperature from 30 to 100◦C, displaying the amount of
ethanol extracted into the water-rich phase and the ethanol composition of the toluene-
rich phase. Be sure that the base case is executed last. Note all observations concerning
this sensitivity study. Are the results reasonable?

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REFERENCES 103

Workshop Notes

Workshop 7.1 A duplicator block will permit all three simulations to be done at once,
and the results will facilitate easy comparison. Two of the results are comparable. The
third displays a similar trend, but which are correct?

• Do the activity coefficient models work for this system?
• Were the parameters in the database fit properly?
• Should new data be located and custom-fit?
• Would an “in the field” design be acceptable for controlling the product compo-

sition (e.g., a secondary sensor loop feeding the set point of the primary control
loop, to make up for model deficiencies)?

Workshop 7.3 A sensitivity study with a plot should be a good guide for a design
specification. See the solutions to Workshops 7.4a and 7.4b.

Workshop 7.4 All Uniquac parameters except Uniq/2 equal must be set to zero.

• Where did the list of Uniquac parameters come from? How can their quality be
assessed?

• Results show that extraction improves as the temperature gets colder.

REFERENCES

Poling, B. E., Prausnitz, J. M., and O’Connell, J. P., The Properties of Gases and Liquids , 5th ed., McGraw-
Hill, New York, 2000.

Prausnitz, J. M., Lichtenthaler, R. N., and de Avezedo, E. G., Molecular Thermodynamics of Fluid-Phase
Equilibria , 3rd ed., Prentice Hall, Upper Saddle River, NJ, 1999, p. 41.

Walas, S. M., Phase Equilibria in Chemical Engineering , Butterworth, Woburn, MA, 1985, p. 388.