Process Simulation 1 – Coursework
Question 9 Multi-Component Distillation – C2/C3 Fractionator
Tom Hart
Student ID: 4249967
Submitted: 11/05/2018
PS1 Coursework Q9 Thomas Hart 42499667
Q9 Multi-Component Distillation – C2/C3 Fractionator
The purpose of this report is to undertake the design of a C2/C3 fractionator, and to evaluate the effect
of design parameters on economics, particularly reflux ratio.
Fluid package: Peng Robinson
Specifications: 99.99% ethane recovered in overheads, 95% propene recovered in bottoms. Full reflux
condenser (top product is vapour only).
From the above, the heavy key is identified as propene and the light as ethane.
An initial mass balance is undertaken for use in the shortcut column. The assumptions of this mass
balance are that all components lighter than the light key leave in the distillate and all heavier than the
heavy key leave in the bottoms. The results of the initial mass balance can be seen in Table 1.
Table 1 - Initial mass balance based on the specifications
Feed Distillate Bottoms
Component
Mol frac lbmol/hr Mol frac lbmol/hr Mol frac lbmol/hr
Hydrogen 0.2885 2400 0.2997 2400 0.0000 0
Methane 0.1202 1000 0.1249 1000 0.0000 0
Ethene 0.3246 2700 0.3371 2700 0.0000 0
Ethane 0.2284 1900 0.2372 1899.81 0.0006 0.19
Propene 0.0216 180 0.0011 9 0.5531 171
Propane 0.0102 85 0.0000 0 0.2749 85
n-Butane 0.0048 40 0.0000 0 0.1294 40
n-Hexane 0.0016 13 0.0000 0 0.0420 13
Total 1 8318 1 8008.81 1 309.19
A shortcut column is then used to estimate the number of trays needed. The shortcut column uses the
Fenske-Underwood equations to estimate the number of trays.
Assumptions of the shortcut column: No pressure drop through trays, reboiler or condenser, ideal tray
R
behaviour. Initially, a reflux ratio of =1.5 is used.
R min
Results of the shortcut column are as follows:
Minimum reflux ratio: 0.267
R
Reflux ratio (based on assumption): 0.4
R min
Minimum number of trays: 12.390
Actual number of trays: 29.332
Optimum feed tray: 5.775
Distillate rate: 8009 lbmol/hr
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PS1 Coursework Q9 Thomas Hart 42499667
Inputs/assumptions of the rigorous distillation column: 30 trays, feed in at tray 6 (from top), initial reflux
ratio 0.4, initial distillate rate 8009 lbmol/hr, pressure drop 2kPa per tray, negligible pressure drop
across reboiler and condenser, full reflux condenser (vapour top product). When the column converges
on the two component recovery specifications, the reflux ratio becomes 0.288.
The mass balance resulting from the converged column can be seen in Table 2.
Table 2 - Mass balance around the converged column
Feed Distillate Bottoms
Component
Mol frac lbmol/hr Mol frac lbmol/hr Mol frac lbmol/hr
Hydrogen 0.2885 2400 0.2996 2400 0.0000 0
Methane 0.1202 1000 0.1248 1000 0.0000 0
Ethene 0.3246 2700 0.3371 2699.9996 0.0000 0.0004
Ethane 0.2284 1900 0.2372 1899.8099 0.0006 0.1901
Propene 0.0216 180 0.0011 9.0001 0.5580 170.9999
Propane 0.0102 85 0.0002 1.7266 0.2685 82.2734
n-Butane 0.0048 40 0.0000 0.0022 0.1305 39.9978
n-Hexane 0.0016 13 0.0000 0 0.0424 13
Total 1 8318 1 8010.5384 1 306.4616
The effect of reflux ratio on parameters influencing cost will now be investigated. Effects on the number
of trays and condenser and reboiler duties can be seen in Figure 1 and Figure 2 respectively.
Figure 1 - Variation in number of trays required with reflux ratio
The number of trays required declines with increasing reflux ratio, however the rate of decline
decreases as R increases. Although not shown, Rmin is calculated at total reflux, and therefore would
R
require an infinite number of trays. The number of trays becomes manageable at around = 1.1. A
R min
greater number of trays leads to a reduced capital cost when constructing the column, so a reflux ratio
as high as possible would be ideal from an economic perspective. However, this has the effect of
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PS1 Coursework Q9 Thomas Hart 42499667
increasing operating cost, as will be seen. The optimal feed tray, a critical design factor, decreases in
parallel with the number of trays required.
Figure 2 - Variation in condenser and reboiler duties with reflux ratio
Both condenser and reboiler duties increase linearly with reflux ratio. This leads to an increase in
operating cost as reflux ratio is increased. Therefore a balance must be struck between the reduced
number of trays and the increased utility demand when evaluating economics.
In these studies the column pressure has remained constant at 445 psia. Column pressure is significant
as it affects the shape of the overall x-y diagram in binary distillation, and the relationships between
each of the components in multicomponent distillation. A higher pressure increases both bubble and
dew points of the mixture, therefore increasing the temperatures of the reboiler and condenser,
meaning different heat and cooling sources are may be required. Duties of these also increase with
pressure, leading to a greater operating cost. Increasing pressure also increases the minimum reflux
ratio and number of theoretical stages, therefore increasing overall capital cost. In addition, the relative
volatility difference between the key components will decrease with pressure, leading to a more
difficult, costly separation. All these phenomena hold true for the example at hand.
For the reasons described above, it is logical for distillation to be carried out at low pressures, unlike the
445 psia used here. However, there are a number of reasons to use a high pressure, including avoiding
the need for refrigeration of the condenser, the availability of different utilities for the reboiler and
avoiding an azeotrope.
In conclusion, the economics of distillation are affected by the reflux ratio such that an increasing ratio
decreases capital cost but increases operating cost. An increased column pressure often has a negative
effect on both separation and economics, although there are several reasons that a high column
pressure may be used, such as those described above.
Reference: Górak, A. and Sørensen, E. (2014). Distillation - Fundamentals and Principles. Elsevier Inc.
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