Wednesday, September 16, 2015

Process Wednesday: more on static electricity

From a new-to-me book "Chemical Process Safety" [1], a nice summary of the different places that charge accumulation could result in electrostatic discharge in the plant: 
1. Contact and frictional charging: When two materials, with one being an insulator, are brought into contact, a charge separation occurs at the interface. If the two objects are then separated, some of the charges remain separated, giving the two materials opposite but equal charges.  
2. Double-layer charging: Charge separation occurs on microscopic scale in a liquid at any interface (solid-liquid, gas-liquid or liquid-liquid). As the liquid flows, it carries a charge and it leaves a charge of opposite sign on the other surface, for example, a pipe wall.  
3. Induction charging: This phenomenon is applicable only to materials that are electrically conductive. A person with insulated shoes, for example, may approach an overhead vessel that is positive charged (previously filled with positively charged solids). Electrons in the person's body (head, shoulders and arms) migrate toward the positive charge of the vessel, thus accumulating an equal quantity of positive charges on the opposite side of the body. This leaves the lower part of the body positively charged by induction. When a metal object is touched, there is transfer of the electrons, creating a spark.  
4. Charging by transport: When charged liquid droplets or solid particles settle on an isolated object, the object is charged. The transferred charge is a function of the object's capacitance and of the conductivities of the droplet, particle and interface. 
The myriad of places that static electricity can build up in the plant is not something that I've considered before; I tend to think of it as a phenomenon that only happens inside the reactor with certain solvents (hexane, for example) or during the transfer of various solvents from drums/totes to the reactor (or vice versa.) Lots to learn, I suppose.

1. Crowl, D.A.; Louvar, J.F. "Chemical Process Safety: Fundamentals with Applications." 3rd Edition, Pearson Education, 2013.

1 comment:

  1. I had the unfortunate privilege of observing the ignition of two solvent fires caused by static discharge. In one MTBE was being pumped (air-driven PTFE diaphragm pump, PTFE corrugated tubing) to hose down the reactor walls. The operator suffered second degree burns to 20% of his body. In another incident hexane was drained from a 30L glass reactor body and free-falling about 2 ft into a PE bucket (a "homer"). The operator suffered minor burns and severe lacerations from flying glass shards.

    It is very hard to communicate the hazards of black and grey swans - once in a lifetime incidents that don't cause massive casualties. It is even harder to make the lessons learned stick. Habits change for a short time after they happen and slowly slide back to the old "normal".