Chemical Engineering June 2023 - 40

Applied Technologies
PVDF Bubble Caps for Distillation Columns
Polymer components in column trays can avoid issues associated with corrosion of
metal equipment
Sachin Upadhye, Arkema
Himanshu Mistry, Sangir Plastics
G
ujarat Alkalies & Chemicals
Ltd. (GACL) - a
major chemical producer
in Gujarat state in western
India - was facing corrosion challenges
with its distillation columns.
In chemical processing applications,
distillation columns are often used
to separate and recover solvents
and acids. Though these chemicals
vary, many of the distillation systems
experience corrosion issues if not
designed properly for long-term applications.
These columns typically
utilize metallic components, but the
harsh acidic conditions required for
this application were not suitable
for metals. And while metals can
handle high temperatures and have
good physical strength, they can be
sensitive to rapid attack from acidic
conditions and rapid acid concentration
changes. In cases where
strong acids are present, and when
acid concentrations can change
within the system, polymer components
can be a viable option to
prevent corrosion and avoid rusting.
This case study describes a project
where metal bubble caps in distillation
column trays were replaced with
bubble caps made from polyvinylidene
fluoride (PVDF) resin (Figure 1)
in an effort to address corrosion issues
with the metal caps in a phosphoric
acid plant.
Arkema
PVDF versus metal
FIGURE 1. Distillation tray bubble caps made
from polyvinylidene fluoride (PVDF) resin can be
a viable option in cases where metallic components
are subject to acid attack
40
The major purpose of the distillation
column is to separate, recover and
remove volatile organic compounds
(VOCs), solvents and other lighterthan-water
substances, which may
be present in the flow. For many
years, distillation columns have used
SS-304 and SS-316 stainless-steel
bubble caps, but stainless-steel
bubble caps corrode in the presence
of some chemicals, such as amines,
chlorides and ketones, because the
heat-affected zones tend to suffer
amine-stress-corrosion cracking or
chloride-stress-corrosion cracking,
even at lower temperatures. Additionally,
stainless-steel bubble caps
are more bulky (in weight). Because
of this, the distillation column needs
to be over-designed and capital
costs are high. Another option to
avoid corrosion is to use fiber-reinforced
polymer (FRP) bubble caps.
These are an inexpensive solution,
but they need to be frequently replaced
due to delamination, which
leads to embrittlement and failure.
The decision to switch from a metallic
construction to PVDF was driven
by the use of phosphoric acid in the
plants. Previous distillation systems
using metal components had experienced
corrosion, and resolving the
issue became essential for GACL to
keep the plant running. When GACL
sought to resolve corrosion-related
issues in the distillation columns of
their food-grade phosphoric acid
plant, project consultant Worley India
recommended they replace conventional
metal bubble caps in column
trays with bubble caps made from
Kynar PVDF. Sangir Plastics Pvt. Ltd.
is one of the major users of Kynar
PVDF raw material, and is also a
leading producer of customized thermoplastic
products. They were able
to successfully manufacture bubble
caps using Kynar PVDF resins. The
fabricator to which the products were
supplied was Dolf Industries Pvt. Ltd.
Vadodara Gujarat.
Arkema
FIGURE 2. PVDF components can withstand a
wide range of pH conditions
Kynar PVDF is known to have resistance
to a wide range of solvents
and acids. Kynar PVDF homopolymers
are known to withstand pH
values of <1 to 12 while Kynar Flex
PVDF copolymers handle a wider
pH range, from <<1 to 13.5. Operating
parameters dictate the use
of engineered thermoplastics, such
as PVDF, and based on the service
conditions and the chemicals characteristics,
Kynar PVDF was selected
for this project as the most
suitable and compatible material of
construction. GACL acheived cost
savings due to the lighter weight of
the PVDF materials, and also was
able to optimize the critical process
conditions by using lower temperatures
and pressures.
Producing PVDF bubble caps
The Kynar PVDF bubble caps were
manufactured from Kynar raw material
supplied by Arkema. The manufacturing
process involved extrusion,
injection molding and fabrication.
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