Hydrocarbon Processing - March 2021 - 38

Petrochemical Technology
plants and the autoclave process offers up
to 150,000-tpy capacity for EVA. The tubular process can also provide up to 20%
EVA copolymers, while the autoclave
process can produce up to 40% EVA. No
further advancement in single-line capacity, either by tubular or by autoclave processes, is expected in near future.

DRIVING REACTOR
CAPACITIES
Two primary areas of continuous innovation and advancement have driven
the capacity of the single-reactor line. One
area is specific to process technology and
the other is common for all technologies.
Reactor technology. The specific area

pertinent to each technology or each licensor is reactor technology. Regardless of
technology, the common area is extruder
or pelleting that converts resin powder to
product pellet through melting, which is
more rheological scale-up and machinery
advancement in the pelleting equipment.
Ethylene and propylene polymerization reactions are highly exothermic. The
reaction demands heat removal as fast as
possible to control the reactor temperature and avoid " hot spots " or localized
reactions. Reactor pressure is usually held
constant and temperature is controlled,
depending on the type of polymer being
produced, within +/-1°C. The heat is removed either by jacket cooling or by external coolers, or a combination of both,
and the mechanism varies from technology to technology.
The role of the catalyst system is
unique in polymerization in that catalyst enables the reaction and dictates the

polymer characteristics or polymer product properties. This mandates certain
minimum residence time in the reactor,
which again varies for each technology.
Therefore, reactor size or volume is primarily determined considering the aspects of heat transfer, mass transfer and
reaction kinetics.
Catalyst yield or catalyst productivity-as defined by tons of polymer
produced per kg of catalyst (metric t/
kg)-has been the single most important factor driving PE/PP technology advancement. Though this is not the theme
of this article, it should be noted that catalyst productivity through continuous
research and innovation has advanced
multi-fold to a sufficiently high level to
enable the large single-reactor line capacities that are now prevalent. Key factors
contributing to reactor capacity differ for
each type of technology.
CSTR technologies for PE. Two main
factors ae involved: reactor volume and
reactor agitator. The reactor is a pressure
vessel where pressures are moderate, so
design for a given volume is not a controlling factor. It is the agitator system design
for large vessels that is critical.
Ethylene polymerization reaction is
heterogeneous, involving a combination
of gas (ethylene, hydrogen and co-monomer), liquid (co-catalyst and hexane) and
solid (catalyst particles and resin). The
agitator design must meet the following
minimum criteria:
* Fast or almost instantaneous
dispersion of gas into the
reaction medium
* Fast reaction velocities
* Solid-liquid suspension consistency

FIG. 6. PP: Stirred-bed gas phase process. Source: Lummus Novolen.

MARCH 2021 | HydrocarbonProcessing.com

* Heat transfer or heat removal
as fast as possible for close
temperature control.
Intelligent balance of radial and axial
agitation pattern is the key to agitation
design. The agitator scale-up has become
more predictable and accurate with the
advent of computational fluid dynamic
(CFD) analysis; nevertheless, experience-based empirical scale-up along with
CFD analysis dominates the design. The
established vendors have lived up to the
challenge to scale up to large capacities,
including designing robust agitator sealing
systems. A minimum of two reactors are
in operation either in parallel or cascade
mode. Hostalen ACP technology offered
by LyondellBasell employs three reactors.
Multiple reactors are intended for
manipulation of product properties, but
also to assist in reducing the throughput
per reactor. Individual jacketed reactors
have reached the size of approximately
350 m3-400 m3. Most of the heat is removed by external coolers by circulating
slurry pumps.
Loop slurry reactor technologies for
PE. Again, reactor volume and a loop cir-

culation pump are the two main factors.
The reactor is in the form of long vertical vessels called reactor loops or legs.
The heterogeneous reaction includes gas
(ethylene, co-monomer and chain terminating agent), liquid (iso-butane) and
solid (resin and catalyst). The residence
time or volume and heat transfer aspects
are determined by L/D ratio. This has
given rise to multiple legs or loops operated in series or parallel with multiple
circulation pumps. Therefore, increasing
the diameter and L/D ratio per loop combined with multiple loops has enabled
the reactor scale-up to a higher capacity.
More than the reactor volume, the scaleup of the slurry circulation pump has
been the challenge for speciality pump
manufacturers. The circulation pump
must have a large flowrate operating with
low ΔP-but at high suction pressure-
to maintain high velocity to sustain high
slurry consistency or high solid-to-liquid
ratio. A mechanically challenging task
has been to provide a sealing system to
handle ethylene and light hydrocarbons
like isobutane, in addition to scale-up of
pump impeller design.
The established vendors have overcome this problem gradually to supply the

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