There were news reports (here, here, and here) of an explosion at a chemical facility in Mississippi involving a herbicide being heated in a hot box. As is typical for such news reports, there were few details involved and the authorities (public and private) only provided the vaguest of information. So, let’s take a general look at ‘hot box’ operations.
Hot Box Basics
Chemicals are frequently stored in portable containers in solidified form or in a very viscous state that are difficult to remove from containers. Thus, there is a need for facilities to heat those containers to empty them. Typically, a ‘hot box’ is used.
Now I have used ‘hot boxes’ that were nothing more than a heavy tarp and a steam hose. If there is a frequent need for such container heating, there are more sophisticated options available. They will typically include a closeable metal box (with or without insulation), heat source (steam or electric), and some sort of temperature control system.
Allow for container venting, place in the hot box, apply heat and wait. When the container is heated through, it is removed from the hot box, and the contents can be poured or pumped out. More sophisticated systems will include temperature probes to put into the container to monitor the heating.
Safety Issues
The most obvious safety issue is pressure buildup. Heating any gas will increase the pressure in a closed system. A properly designed hot box will have some sort of safety venting, frequently just an opening to the atmosphere. Portable containers do not have such venting designed in. Pressure relief valves are, however, available that can be screwed into bung holes on the container. Or the top can be simply loosened.
Another potential safety issue is the rise of a flammable atmosphere. While most people have a tendency to ignore flashpoints over 100°F, if the temperature in the hot box is above the flashpoint of any of the materials being heated in the hot box, there may be a flammable atmosphere in the box. The question that needs to be answered in that case is if the hot box has been designed for operation in a classified atmosphere so that the heating system or the box electronic controls cannot ignite that atmosphere.
Another issue that must be considered is chemical compatibility when multiple products are put into the hot box at the same time. Potential gas phase interactions need to be considered at the operating temperature to be found in the box when the containers are vented inside the box.
The final issue has only recently become a more public concern; the security of the electronic control systems (if present) on the box. Recent attacks on controllers for commercial freezers have shown that temperature controllers are potentially subject to electronic attacks. Since it is becoming obvious that anything can be hacked, safety considerations need to take into account the maximum temperature that can be reached within the hot box.
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