Chemical Engineering December 2013 - 41

Butene via
Ethylene Dimerization
By Intratec Solutions
C4+
O
lefin dimerization is a reaction where two
molecules of a monomer combine to form
a dimer. It can be utilized to produce
normal butenes from ethylene, which can be
provided by a steam cracker.
Dimerization of ethylene is generally applied
when butene is required in another part
of a petrochemical complex. For instance,
the dimerization unit may be integrated with
a metathesis unit, to produce polymer-grade
propylene, a commodity chemical with a high
added value. In this integration, n-butene from
dimerization is reacted over a metathesis
catalyst with ethylene to form two molecules
of propylene, as shown in the block flow
diagram, in Figure 1.
The process
A dimerization process for butenes production,
similar to CB&I Lummus (The Woodlands, Tex.;
www.cbi.com) Ethylene Dimerization Technology,
is analyzed and depicted in the flowsheet
(Figure 2). The following is a brief description
of the dimerization processing scheme:
Catalyst preparation and treatment section.
Catalyst and co-catalyst are properly unloaded,
stored and pumped into the catalyst preparation
section. Ethylene is treated in the purification
unit, prior to mixing with the catalyst, to remove
potential dimerization-catalyst poisons.
Reaction section. The reaction system produces
normal butenes, mainly 2-butene, as the major
product, with higher olefins (such as hexenes
and octenes), as byproducts. The reaction
occurs in a liquid-phase loop system, with a
high circulation flowrate. The reaction medium
- containing the catalysts, n-butenes and recirculated
unreacted ethylene - is fed to the surge
drum, which provides liquid surge to the loop. A
heat exchanger, which removes the exothermic
heat from the circulating liquid, acts as the dimerization
reactor. After the reactor stage, part
of the reaction medium is sent to the quench
Purge to
cracker
3
2
Ethylene
Co-catalyst
Catalyst
4
1
1
CW
5
8
To water
treatment
Fresh caustic
FIGURE 2. n-butenes production process similar to Lummus Ethylene Dimerization Technology
ST
Fuel oil
CW Cooling water
SC Steam
condensate
ST Steam
RF Refrigeration fluid
To flare
drum
ST
6
SC
5
7
ST
n-butenes
CW
Gasoline
Ethylene
from cracker
Treatment
unit
Treated
ethylene
Dimerization
Gasoline
Catalyst and
chemicals
Fuel oil
FIGURE 1. Integration between dimerization and metathesis
section while the remainder is recirculated.
Catalyst quench section. In the quench section,
the catalysts are deactivated with caustic
solution. Fresh caustic is added to the reaction
product, after which it goes to the liquid-liquid
separator drum, where the caustic is withdrawn
from the bottom and purged to the flash drum
prior to being recycled. The same occurs with
the steam condensate, which is used to remove
traces of caustic. In the flash drum, hydrocarbons
in the purge stream are sent to flare,
while the liquid is sent to water treatment.
Purification section. In the debutenizer tower,
the overhead butenes and any unconverted
ethylene are separated from heavier reaction
byproducts. The bottom is sent to the heavyends
tower, which produces gasoline as distillate
and fuel oil as bottoms, while the overhead
is sent to the butenes purification tower.
In the butenes tower, unreacted ethylene is
separated from the product and recycled to the
reaction section. In case of metathesis integration,
the purification tower is not utilized, since
both olefins will be used as raw materials.
Total installed cost
An economic evaluation of the ISBL (inside
battery limits) total installed cost, which acCW
9
counts
for the ethylene treatment unit and the
dimerization unit, was conducted based on
data from the fourth quarter of 2012, for a
plant with a nominal capacity of 200,000
ton/yr of butenes erected on the U.S. Gulf
Coast (the process equipment is represented in
the simplified flowsheet below).
The ISBL total installed cost is about $30
million. The cost includes: the process equipment
and equipment spares; housing for process
units; others direct material; and indirect
costs (field supervision, payroll burdens and
so on).
Recently, the shift to lighter feedstocks from
low-cost natural shale gas is reducing the
propylene production in steam cracker units.
Thus, the construction of a dimerization unit integrated
to a metathesis unit can be a solution
when there is an excess of ethylene production
and a demand for propylene. ■
Editor's note: The content for this column is supplied
by Intratec Solutions LLC (Houston; www.intratec.us)
and edited by Chemical Engineering. The analyses
and models presented herein are prepared on the
basis of publicly available and non-confidential information.
The information and analysis are the opinions
of Intratec and do not represent the point of view of
any third parties. More information about the methodology
for preparing this type of analysis can be found,
along with terms of use, at www.intratec.us/che.
RF
1) Catalysts vessels
2) Fixed bed dryer
3) Surge drum
4) Dimerization reactor
5) Liquid-liquid
separator drum
6) Flash drum
7) Debutenizer tower
8) Heavy ends tower
9) Butene purification
tower
Catalyst and
chemicals
Metathesis
Propylene
purge
http://www.cbi.com http://www.intratec.us http://www.intratec.us/che
Chemical Engineering December 2013

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