Process design

Further Analysis of the T2 Incident & Recommended Reading

After the incident, the US Chemical Safety Board examined the T2 chemistry using reaction calorimetry – at technique for looking at heat and gas evolution from reaction mixtures.[1]  To study this in more depth, see Reaction Calorimetry

Both mixtures of solvent (diglyme) and metallic sodium and solvent, sodium and Methylcyclopentadine dimer (MCPD) were tested.  In both cases, at just under 200oC, extremely exothermic events initiated, with associated large increases in pressure.  The exothermic decomposition was so violent that the test cells were damaged beyond repair.

A large exothermic event due to decomposition of the solvent is seen both in the presence of MCPD and with just sodium and diglyme alone. In the presence of MCPD, a smaller exotherm due to reaction between Na and methylcyclopentadiene is seen before solvent decomposition. Diglyme is known to decompose in the presence of strong bases at ~200°C.

If an exothermic incident is known, then (at least in a batch reactor) a general rule is to stay at last 100oC away from the onset temperature.  Looking at the T2 process, this would give a safe range up to 100oC, with the safety window of 100-200oC. However, T2 regularly ran the reaction up to ca. 190oC during routine operation of the plant. With a cooling failure and inadequate pressure venting, this allowed a thermal runaway situation to develop.  The decomposition of diglyme most likely involves initial elimination to give methanol and vinyl ethers. This would generate more gas and reactive compounds in the reactor.

More information on thermal runaways can be found in a report from the Health and Safety Executive.[2]

Before scaling up a reaction, many factors need to be considered, and backed up by appropriate safety testing.

  • Is the reaction exothermic and can you safely remove heat under all scenarios?
  • Cooling capacity of reactor.
  • Multi-phase reactions.
  • Evolution of gas/flammables/pressure.
  • Reagent accumulation.
  • Materials of construction – potential corrosion.
  • Use or generation of potentially explosive materials.
  • Safe waste treatment and disposal.

To study this in more depth, see Process Safety.

Recommended reading:

General articles on process safety:

T. A. Kletz, How We Changed the Safety Culture, Org. Process Res. Dev., 2007, 11, 1091-1095.

D. J. Frurip, Selection of the Proper Calorimetric Test Strategy in Reactive Chemicals Hazard Evaluation, Org. Process Res. Dev., 2008, 12, 1287-1292.

How to Minimise Scale-up Difficulties (Last accessed: ).

More specific examples of process safety investigations:

E. Bassan, R. T. Ruck, E. Dienemann, K. M. Emerson, G. R. Humphrey, I. T. Raheem, D. M. Tschaen, T. P. Vickery, H. B. Wood and N. Yasuda, Merck’s Reaction Review Policy: An Exercise in Process Safety, Org. Process Res. Dev., 2013, 17, 1611-1616.

Z. Wang, S. M. Richter, B. D. Gates and T. A. Grieme, Safety Concerns in a Pharmaceutical Manufacturing Process Using Dimethyl Sulfoxide (DMSO) as a Solvent, Org. Process Res. Dev., 2012, 16, 1994-2000.

S. Shilcrat, Process Safety Evaluation of a Tungsten-Catalyzed Hydrogen Peroxide Epoxidation Resulting In a Runaway Laboratory Reaction, Org. Process Res. Dev., 2011, 15, 1464-1469.

 

 

 

  1. Investigation report T2 laboratories (Last accessed: ).
  2. HSE Chemical reaction hazards and the risk of thermal runaway (Last accessed: ).