Process Wednesday: electrostatics and your glass-lined reactors

We all love a good non-polar solvent, right? Well, we might, but our glass-lined steel reactors may not. Non-polar solvents (hexane, heptane, xylene, toluene) all have quite low conductivities (less than 2 picoSiemens per meter, where 'low conductivity' is less than 50 pS/m). What's wrong with that? Well, according to a 2003 article in Organic Process Research and Development [1], typical operations like mixing and stirring generate static charge:

Mixing and stirring. The presence of two phases in a reactor/vessel can lead to the generation of charge on stirring. The higher the agitation speed the greater the charge generation. In certain systems where a solid is being dissolved into a liquid, very high electrostatic charges can be generated. In this case special precautions may be required.

What happens when static charge builds up in a reactor? Well, there's always the issue of a spark or (in the long-term) pitting of your reactor walls:

If a large-enough electrostatic charge is present in the solvent and there is an earthed object in close proximity, then a discharge or spark can occur. If there is a flammable atmosphere present and the spark has enough energy to ignite the solvent, then a fire can occur. There are many case histories where fires have been started due to static discharges from liquids involving a range of low-conductivity solvents.  

A nitrogen blanket can be used to prevent a flammable atmosphere, but this can still lead to problems with reactor pitting. An example of this is the damage caused to enamellined reaction vessels due to discharges of static electricity. Enamel itself cannot become dangerously charged, unlike some plastics, and is therefore used extensively in the chemical and pharmaceutical industries. However, experience in the use of enamel vessels has shown that under certain conditions high electrostatic charging can occur and the sparks generated can cause pitting of the reactor wall. If this remains unchecked, it can lead to corrosion, extensive damage, reactor downtime, and even reactor replacement. Similar experiences have been seen for glass-lined reactors.

 What to do? Well, there's a chemical solution, of course, with anti-static additives:

One example is Octastat, which can improve the conductivity of solvents at ppm levels. A typical graph is shown in Figure 3 for the dosing of toluene with Octastat 3000. A dose level of 1-2 ppm is required to raise the conductivity above 1000 pS/m. The exact dosage for many of these additives depends on the particular solvent and the manufacturing process in which it is being used. It is therefore advised to measure the conductivity of the solvent directly using a conductivity meter for each application.  

In pipes another way of limiting the buildup of static is to reduce the flow velocity. The recommended maximum flow for a low conductivity solvent is 1 m/s where a solid or second liquid could be present. Otherwise a maximum limit of 7 m/s is suggested. With conducting liquids in metal pipes, flow control is rarely required. 

A careful selection of inlet and outlet points can help minimise the problems of static generation due to splashing, as does the use of wide bore valves.

I'm sure there's a scaling reason as to why we tend not to worry about these sorts of things in lab-scale reactions. Probably has something to do with charge density (?).

[1] Giles, M.R. "Electrostatic Hazards in Liquids and Relevance to Process Chemistry." Org. Process. Res. Dev. 2003, 7, 1048−1050.