Wednesday, August 30, 2017

Harvey Related Chemplant Explosion Predicted

It is not often that we get to watch a major chemical safety event as it progresses to its catastrophic end-point. As unusual as such an event is, it is hardly surprising that it is Harvey that is the proximate cause of the incident. News stories (here, here and here) provide the background to the unfolding event.

Organic Peroxides

Organic peroxides are a class of chemicals that contain an oxygen-to-oxygen bond in the central portion of the molecule (commonly represented as RCOOR; where R is any of a variety of organic compounds). These molecules are important in chemical manufacturing because they decompose to produce free-radicals (commonly represented as •OR). Free radicals are necessary to start many industrial chemical reactions; many polymerization reactions, for example, require the use of free radical technology.

From a process chemical point of view, organic peroxides are very useful because each organic peroxide starts its decomposition process at a characteristic temperature and has a characteristic rate of decomposition. This makes it relatively easy to control the resulting chemical reactions by controlling the temperature of the mixture containing the organic peroxide.

The bad thing about organic peroxides from a process safety point of view is that the decomposition process produces excess energy that heats the mixture containing the peroxide (most are sold in a solution with an organic solvent to ease handling). The rising temperature increases the rate at which the free radicals are formed, which raises the amount of energy produced. The cycle proceeds quite quickly once it passes a critical characteristic temperature for that particular compound.

Unless controlled by external cooling, the temperature can easily exceed the boiling point of the solvent, causing pressure to rise in the container until it reaches the point where the container catastrophically fails. While not technically an explosion (no burning has taken place at this point in the process) most observers would characterize the failure of the container as an ‘explosion’. This is especially true since the resulting solvent cloud can ignite when it comes in contact with the atmosphere and a source of ignition.

Safety Measures

I have worked in a couple of different facilities that used organic peroxides in industrial chemical manufacturing operations. Whenever organic peroxides are introduced to a facility a safety review typically comes up with the same safety procedures to try to prevent decomposition incidents and to limit the damage if such an incident does occur. First, an industrial cooler/freezer is obtained to store the material at a temperature well below the critical decomposition temperature. That storage temperature is monitored and alternative cooling methods are identified for when the storage temperature starts to approach the critical temperature. At a facility near Baton Rouge in 2012 when Hurricane Issac approached, we filled the freezer with dry ice before we closed-up the facility and kept it topped with dry ice after the storm passed until power was restored.

The problem is quite different at a facility that manufactures organic peroxide, like this one in Crosby, TX. While I’m sure that they would manage their inventories quite closely, they are going to have a lot more of the material on hand at any given time than a facility that just uses the organic peroxide in a manufacturing process. Thus, they can be expected to have more formal backup measures in place. According to at least one of the articles that I have seen, the plant lost both their primary and two separate backup power supplies to their cooling systems.

The Problem

The problem here is, of course, that Harvey presented a situation beyond the design basis for the facility. I do not know what the facility safety management team used for their flooding risk basis, but I am almost positive that it was not the 40+ inches of rain that the facility received. Flooding is not unexpected in that part of Texas (flat does not begin to describe the topography), but six-foot of standing water was certainly not expected by anyone.

What is interesting here is that the temperature monitoring systems are still working in the storage area. Another of the news reports mentions that the company is still able to watch the temperatures rise. We will ignore for the moment that this situation could be used as a text book example of why facility management wants to see remote access to industrial control systems, but this will almost certainly provide the company the ability to provide emergency response personnel with quite good predictions of when to expect the onset of catastrophic consequences.

There are going to be other chemical safety events associated with the aftermath of Harvey. The Chemical Safety Board has published a brief safety reminder about the special challenges in starting up chemical manufacturing process after a catastrophe like Harvey. While the information there is very valuable, it fails to address the type issue being seen in this unfolding event. There are a large number of other industrial chemicals (monomers come quickly to mind) that have decomposition issues related to lack of cooling. Generally, they are not quite as severe as organic peroxides, but they do provide their own safety issues that will have to be dealt with during the recovery phase from this unusual storm.

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