Sunday, October 30, 2016

Chemical Mixing Incident

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.

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