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Tamkang Journal of Science and Engineering, Vol. 4, No. 2, pp. 105-110 (2001)

105

Case Studies on Optimum Reflux Ratio of Distillation Towers
in Petroleum Refining Processes
Hsi-Jen Chen and Yeh-Chin Lin
Department of Chemical Engineering, Tamkang University
Tamsui, Taiwan 251, R.O.C.
E-mail:hjchen@mail.tku.edu.tw

Abstract
For an existing distillation tower such as the propylenesplitter in
this study, the number of trays is fixed and there are very few degrees
of freedom that can be manipulated to maximize operating profit; the
reflux ratio can be used to influence the steady-state operating point
and thus the daily profit. Also, in the debutanizer design, we have
discussed the trade-offs between reflux ratios (energy costs) and
annualized capital costs.
Key Words:Optimal Reflux Ratio, Propylene Splitter, Debutanizer,
Petroleum Refining

1. Introduction
Separations are “big businesses” in chemical
processing. It has been variously estimated that
the capital investment in separation equipment is
40-50% of the total for a conventional fluid
processing unit. Of the total energy consumption
of an average unit, the separation steps accounts
for about70%.
And of the separation
consumption, the distillation method accounts for
about 95% [1]. In general, initial design of a
distillation tower involves specifying the
separation of a feed of known composition and
temperature.
Constraints require a minimum
acceptable purity of the overhead and/or bottoms
product. The desired separation can be achieved
with relatively low energyrequirements by using a
large number of trays, thus incurring larger capital
costs with the reflux ratio at its minimum value.
On the other hand, by increasing the reflux ratio,
the overhead composition specification can be met
by a fewer number of trays but with higher energy
costs. In particular, the optimization of reflux
ratio is attractive for distillation columns that
operate with: 1. highreflux ratio; 2. high
differential product values between overhead and
bottom; 3. high utility costs; 4. low relative
volatility, and 5. feed light key far from 50%. In
this paper, we explore optimum reflux ratio of two
distillation columns for used in the petroleum

refining processes; one is called propylene splitter,
an existing tower in a naphtha cracking plant, the
other is a debutanizerused in a fluid catalytic
cracking plant. The optimization software we
used for the optimum reflux-ratio problem of the
propylene splitter is GAMS [2] and a flowsheet
simulator DESIGN II [3] was used for the basic
design of the debutanizer.

2. Propylene Splitter
Figure 1 shows a typical olefins plant in
which a propylene splitter is used for separating
propane and propylene. Thelighter component
(propylene) is more valuable than propane. The
overhead stream has to be at least 95% propylene.
Based on the basic principle for a two-component
system, we have to find the minimum reflux ratio,
Rm, and Nm, the minimum number of stages to
accomplish the separation at total reflux. For a
two-component distillation system, if the relative
volatility, α , is constant, then, wehave the
enriching operating line:

y=

Rm
xD
x+
(Rm + 1) (Rm + 1)

(1)

where xD is the purity of the overhead propylene.
The equilibrium line is given by
y = αx /[1 + (α − 1)x ]
(2)
The q-line relates feed quality to feed fraction

106

Hsi-Jen Chen and Yeh-Chin Lin

x F F = xD D + xB B

q
x
y=
x− F
q −1
q −1

(3)

where xF is the mole fraction of feed light key(propylene) and q is defined as the ratio of heat
needed to vaporize 1 mole of feed at entering
condition to the molar latent heat of vaporization
of feed.
Combining Eqs. (1)-(3), and eliminating x and y,
we obtain:

RmxF + qxD
α[xD (q −1) + xF (Rm −1)]
=
Rm(1− xF ) + q(1− xD ) (Rm +1)(1 − xF ) + (q −1)(1− xD )

(4)
Equation (4) can be called the Underwood
equation for a binary...
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