Earlier this week the Chemical Safety Board (CSB) announced
that it was sending an investigation team to the site of a major chemical
release in Atchison, KS that occurred on October 21st. The release
was caused by the inadvertent mixing of two common industrial chemicals and
resulted in a large chemical cloud sending hundreds to local hospitals with
complaints of difficulty breathing.
The Incident
According to news reports (for example see here,
here
and here)
the incident started at 8:00 am when a bulk chemical delivery was put into the
wrong storage tank. The two chemicals involved were industrial strength bleach
and sulfuric acid, both apparently being used in the facilities waste treatment
plant. The chemical reaction between the two produced a large cloud of steam
that also included chlorine gas, a byproduct of the reaction between the two
chemicals.
There is no publicly available information about which
chemical was being unloaded, but due to the odor of chlorine bleach being
involved, I would guess that the delivery was sulfuric acid. Adding sulfuric
acid to a bleach tank actually produces two separate reactions that would have
contributed to the cloud.
First, since bleach is mainly water (only 6 to 12% sodium
hypochlorite) the addition of sulfuric acid (which is typically shipped and
stored at concentrations above 95% for safety reasons) produced a large amount
of heat due to the ‘heat
of dilution’. That heat and the lack of mixing would quickly raise the
surface temperature of the bleach above the boiling point of water producing a
large steam cloud. That steam cloud would be expected to contain trace amounts
of unreacted bleach and sulfuric acid.
The chemical reaction between sodium hypochlorite and
sulfuric acid produces chlorine gas and even more heat. The reaction is
virtually instantaneous and consumes essentially all of which ever chemical is
least available (typically the chemical being added to the tank because
addition is usually stopped as soon as the steam cloud is observed). That is
why I suspect that the sulfuric acid was being added to the bleach tank.
How Could This Happen?
This type of accident is way too common, especially at waste
water treatment facilities. Such facilities typically rely on delivery drivers
to unload bulk chemical shipments instead of facility personnel who would be
more familiar with which tank contains which chemical. Hose connections are
made from the delivery truck to piping that leads to the chemical storage tank.
A single bulk truck unloading station typically has separate connections for
each of the storage tanks at these facilities. Inadequate marking of the pipe
connections, and/or inexperienced (for that facility) drivers results in the
truck being hooked up to the wrong piping connection.
Larger chemical facilities avoid these types of incidents
through a combination of personnel and design activities vetted through a
chemical safety program under either the EPA’s Risk Management Program (RMP)
and/or OSHA’s Process Safety Management (PSM) program. Typically, there are
only a limited number of personnel on-site who are authorized to unload bulk
deliveries of chemicals. They are specifically trained on the hazards
associated with the bulk chemicals they will be handling, including the risks
associated with mixing of chemicals in storage tanks, bulk unloading lines or
hoses. Non-facility delivery drivers are never allowed to unload bulk chemicals
without specific facility supervision.
Where there is a specific hazard from the mixing of
chemicals being stored at that site (for example bleach and sulfuric acid)
engineering measures are taken to prevent that mixing. The tanks may be located
in separate tank farms, the bulk unloading lines may be physically separated at
different unloading stations, or different types of hose connections are used
with the unloading lines to make it more difficult to inadvertently mix those
chemicals. Depending on the potential consequences involved (and this
particular incident was nowhere near a worst-case incident) combinations of
these and other engineering controls could be used.
The CSB Investigation
The CSB usually limits its investigations to larger more
severe events that kill people or result in large scale damage. This is mainly
due to their Congressional mandate, limited funds and limited personnel. Taking
up this incident is almost certainly due to the amount of level of publicity
related to the large cloud and how common this type of incident is.
Compared to other investigations this one should consume
much less in the way of CSB resources. That does not mean that the report will
be completed and published any sooner; the CSB will likely place a low priority
on the completion of this investigation.