|
GOALS
Replace the existing chlorine dioxide (ClO2)
generation and
feed system with a safer, simpler, and more
reliable system.
SITE
A rendering plant in the western United
States.
Summer 2003
HISTORY
In the mid-1990s, a rendering plant found
itself located in an area experiencing a
population boom. Neighboring citizens complained
regularly about the odor emitted from the
plant. While the plant used a masking agent
for designed for odor control, the mixture
of odors proved even more offensive. In
addition, two oil refineries, two feed mills,
a food processing plant, and an animal food
plant were also located in the same vicinity—all
of which contributed to the nuisance of
foul odors. In 1997, the county and state
health departments began to take action
to address the odor issues. About that same
time, the rendering plant made a transition
from the masking agent to chlorine dioxide
(ClO2) as an oxidizing agent for odor control.
A traditional three-precursor ClO2 generation
system using sodium chlorite, hydrochloric
acid, and sodium hypochlorite was installed
and provided an immediate reduction in the
odors being emitted from the plant.
PROBLEM
The three-precursor ClO2 generation system
did not have the capacity to maintain the
oxidant level at the necessary ORP set point—500
mV—especially during the high volume
times that occurred in conjunction with
summer’s warmer temperatures.
The generation system required constant
monitoring to ensure it was operating effectively.
Operators needed to check and adjust precursor
rotometer settings every 15-30 minutes,
and had to check for air bubbles in lines.
Eduction was used to deliver precursors
into the generator and ClO2 solution was
educted from the batch tank into the scrubber
system. Multiple eduction processes resulted
in very poor dosage control. Most importantly,
if the system failed, it was not easy for
on-site personnel to fix the problem.
The generation system relied on the use
of a ClO2 batch tank in order to provide
the necessary conversion of precursors to
ClO2. In order to accelerate the rate of
ClO2 generation, excess acid and sodium
hypochlorite were used to speed production.
The presence of the batch tank, containing
~1000 ppm ClO2 with excess HCl and NaOCl
was a serious safety concern to plant operators.
Despite the use of excess acid and bleach,
the ClO2 conversion only achieved 80% to
85%_far below performance expectations,
especially when managers factored in the
amount of time and effort operators invested
in keeping the system operational.
Because of chlorine dioxide’s excellent
ability to eliminate odor—and because
citizen complaints had dropped off dramatically
following the implementation ClO2 for odor
control—the plant wanted to continue
using the molecule. However, they needed
to upgrade to a larger, safer, simpler,
and more reliable ClO2 generation system.
SOLUTION
 The rendering plant installed an SVP-Pure® Chlorate-Based Chlorine Dioxide Generator. The
two-chemical ClO2 generation
system
uses Purate® (40% sodium chlorate/10% hydrogen
peroxide) and 78% sulfuric acid solution.
Advantages of the SVP Pure® system includes:
» Improved safety, simplicity, and
reliability
» Savings on cost of chemical precursors
» Savings from improved ClO2
conversion efficiency
» Savings on operator service time
(automated feed control)
» Reduced citizen odor complaints
RESULTS
The plant staff were very pleased with the
performance and simplicity. During the first
couple of months of the trial, PureLine
service technicians visited often to perform
standard changes to the ClO2
dosage settings in order to optimize the
feed rate with the system flow rate, and
to train on-site personnel to troubleshoot
problems. Once the feed rate was optimized,
the generator automatically provided the
appropriate ClO2 dose via flow-pacing
software built into the SVP-Pure® control
system. Multiple safety interlocks and alarm
features ensured safe and simple operation,
and the PureLine generator was much easier
for on-site personnel to troubleshoot that
the previous three-precursor generator.
Service visits decreased and were replaced
with monthly system check-ups and quarterly
preventative maintenance—all performed
by PureLine field technicians as part of
the contract.
Daily operator time for generator maintenance
was reduced from constant monitoring to
a routine, once-a-day, five-minute system
check, as well as periodic level check in
the Purate and sulfuric acid storage tanks.
This new simplicity and improved reliability
allowed plant personnel to focus on other
projects instead of monitoring previous
old generator every 15-30 minutes. The plant
manager was pleased that “The generator
just ran, and it ran well.”
Equally important, the SVP-Pure® system did
not require a batch tank since SVP-Pure® technology rapidly converts two precursors
into a high purity ClO2 solution
at >95% efficiency. This ClO2
product solution is then educted into the
rendering scrubber water system using the
generator’s automated feed control
system directed by ORP. Chlorine dioxide
generation without the batch tank greatly
improved the safety and simplicity of the
feed system while the SVP Pure automation
system ensured optimal feed control with
minimal operator involvement.
In order to produce the same pounds per
day of ClO2, the SVP-Pure® generator
used 25% less Purate than the amount of
sodium chlorite required by the plant’s
outdated system. Even if the dollar-per-pound
cost of Purate were the same as sodium chlorite,
there would be a 25% reduction in the cost
for replacing sodium chlorite with Purate.
In addition, replacement of hydrochloric
acid and sodium hypochlorite with sulfuric
acid generated even greater cost savings.
Additional savings were realized due to
improved ClO2 conversion efficiency.
While the old, three-chemical generator
operated at 80% to 85% efficiency, the SVP-Pure® system achieved 95% to 99% efficiency.
The following table compares statistics
that reflect the plant’s switch first
from a “masking agent” to the
three-precursor ClO2 generator, and finally
to the SVP-Pure® generator during
the summer months.
| Odor Control
Agent |
|
Frequency of Complaints |
| “Masking agent”
|
|
1-5 per day |
| 3-Precursor ClO2 Generator |
|
1-2 per week |
| Pureline SVP-Pure ClO2
generator (unoptimized) |
|
1 every two weeks |
| Pureline SVP-Pure ClO2
generator (optimised) |
|
1-4 a year |
CONCLUSION
While the plant’s initial transition
from a “masking agent” to chlorine
dioxide did result in better odor control
and a drop off in citizen complaints to from
daily to weekly, complaints did persist because
of the three-precursor ClO2 generator’s
poor reliability and insufficient capacity.
By upgrading to the SVP-Pure® ClO2
generator, odor complaints were dramatically
reduced from weekly to quarterly.
The SVP-Pure® ClO2 generator was
able to provided a dramatic improvement in
safety and simplicity over the old generator
for several reasons—two versus three
chemical precursors; elimination of the ClO2
batch tank; a more reliable automated ClO2
feed system; and low-maintenance operation.
The SVP-Pure® ClO2 generator is
more cost effective than the plant’s
old three-precursor generation system for
several reasons—only two, not three,
chemical precursors are needed; the
system uses 25% less Purate® than sodium chlorite;
and a 10% to 15% improvement in ClO2 conversion
efficiency. |
