Unformatted text preview: Ask the Experts Distillation Dennis O'Brien is a detergents technology specialist in the UOP Engineering Dept. (25 East
Algonquin Road.; Des Plaines, IL 60017-5017; Phone: (847) 391-2802; Fax: (847) 391-2253; E-mail:
[email protected]). He has 32 years of experience with UOP and currently specializes in
detergents and petrochemical technology. O'Brien holds a BSChE from the Univ. of Tulsa and an
MBA from Roosevelt Univ. He is a member of AIChE and chair of the Chicago, IL, section.
Michael A. Schultz is a process research specialist in the UOP Engineering and Equipment Group
(Phone: (847) 375-7895; Fax: (847) 391-3491; E-mail: [email protected]). With 32 years of
experience with the firm, he now focuses on economic analysis and the development and optimization of process technologies. Schultz holds a BSChE from the Univ. of Michigan-Ann Arbor, a
PhD in chemical engineering from the Univ. of Massachusetts-Amherst and is a member of AIChE. Q. How can I design an optimized distillation column?
The ﬁrst step is to establish the optimum design procedure
for a standard distillation column. Tailor that design to optimize column operation for ﬂexibility, low capital cost, energy
efficiency or handling difficult feeds. In this article, we
assume that the feed is a mixture of hydrocarbon components
with near-ideal behavior. and a small additional capacity (~15%) in the reﬂux.
If the product speciﬁcations are variable, the tower must be
capable of achieving the most difficult speciﬁcation. The cost to
build this capability into a column is high, and should be justiﬁed. High-purity products can require a large number of trays
(e.g., 205 for a polypropylene (PP) splitter) and large reﬂux
ratios (e.g., in the PP splitter, R/D = 12:1). For a standard design, set the tower pressure such that it
will lead to the design of an economical condensing exchanger.
The cooling water or air-approach temperature usually sets the
condensing temperature. If the pressure is set too low, a large
and costly condenser will be required. However, the higher the
pressure, the lower the relative volatility between the light and
heavy keys. In other words, separation of lighter components
requires higher column pressures in order to operate at reasonable condenser temperatures. Separation of heavier components
can be accomplished with lower column pressures. UOP commonly uses a 30°F cold-end approach on air coolers. Typically,
air condenser outlets will be designed at 130–145°F. Water condensers usually cool to100°F.
The minimum number of stages and reﬂux required can be
calculated using the Fenske and Underwood equations, respectively, while the Gilliland correlation may be used to determine
the number stages vs. reﬂux rates. Set the reﬂux rate to 10%
above the minimum reﬂux value to ﬁnd the initial (or minimum)
number of stages. Run several rigorous column simulations to
establish the appropriate number of stages. Start with the minimum number of stages for the ﬁrst run and add 5% for the second run.
Stage efficiency may be determined by experimentation, correlations, consulting literature or drawing on the knowledge of
other skilled engineers. Different sections of the column may
operate at different tray efficiencies. Many towers designed at
UOP have stage efficiencies of 75–80%. Most of the vacuum
system designs have efficiencies of 15–20%. The drying section
of a benzene tower has an efficiency of 15%.
To determine the ratio of the light key to the heavy key, identify the stage in the tower where this ratio occurs in the liquid
phase. This should be the optimum feed point. Lowest capital cost vs. minimum energy design
There is a tradeoff between the amount of reﬂux and the
number of stages needed to achieve a desired separation. A
higher reﬂux means that fewer stages are required. However,
more reboiler duty is required, meaning that the column diameter is larger, and greater surface area is required in the condenser
and reboiler. There is an economic tradeoff between energy
requirements due to reﬂux changes and the capital cost. Usually,
the condenser and the reboiler are the most costly components in
a conventional fractionator, followed by the tower shell and
trays. High- efficiency trays (or packing) and close tray spacing
allow the tower diameter and tangent length to be minimized.
On the other hand, a tower with a lower energy requirement
is designed at or near minimum reﬂux. Rigorous column simulations can be used to determine the minimum reﬂux requirement. The column should then be designed with a small margin
(typically 10%) for operational variability. Reducing the operating pressure will minimize the reﬂux requirement. Flexible columns
If the feed composition and heat content are variable, such as
with naphtha or kerosene systems, stages may be added to the
column to achieve the desired separation. Additional trays
should be added to the tower section that needs more ﬂexibility.
Kerosene towers often have extra trays (2 or 3 stages) in each
column section. Columns that are designed to handle feed variations are not optimized for any one feed.
The feed-variability challenge can also be addressed with
additional reﬂux. Kerosene systems have a few additional trays 14
14 www.cepmagazine.org February 2004 CEP
January 2001 CEP Advanced options
In some cases, a better design can result by using the
advanced options, described below:
Dividing wall — UOP has commercialized several dividing
wall columns for the separation of kerosene and naphtha products (CEP, May 2002, p. 64). Energy savings of up to 30% and
capital savings of up to 25% have been obtained.
Heat-pumped — Most of UOP’s heat-pumped towers are
propylene/propane splitters, which achieve difficult propylene
purity and recovery speciﬁcations. The current design uses a
two-stage heat-pump compressor with high-ﬂux tubing, promoting better heat transfer, and a reduction in heat-exchange area.
Pressure-staged — Pressure-staged fractionation columns
improve the energy efficiency of petrochemical complexes. One
application is to use one high pressure column in a complex to
provide the reboiler heat to the rest of the columns in the complex. Another application is to split a large vacuum fractionation
tower into two small towers, one operating at 20 psia and the
other at 3 psia. All of the condenser duty in the high pressure
column is recovered in the reboiler of the second tower. The
2004 AIChE Spring National Meeting’s (New Orleans, LA;
Apr. 25–29) Seventh Topical Conference on reﬁnery processing
will contain a session on energy efficiency, one of which will
detail the advanced distillation methods. ...
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