Ethylene Decomposition - Conditions

[Cirrus9]

What are the conditions that determine an uncontrolled ethylene decomposition in a vessel, pipeline or a well?
From what I know, decomposition can occur:
1) at certain rates of compression
2) at high temperature
3) when there is water, air, rust, or other foreign matters in ethylene flow path.

Thank you for help!

 

[rbcoulter]

Do you mean hazardous polymerization instead?

 

[Cirrus9]

For sure it is hazardous. I do not know if it polymerises. I do know that a lot of heat is generated, then the system get over-pressurized, and the pipe or equipment has a great chance of rupture.

 

[25362]

Is it feasible or practicable to mix in nitrogen as an inert to avoid ignition ?

 

[linford]

I know this is from a long time ago but thought it worthwhile to comment:

Oxygen or not it will still decompose. It does not need another gas to decompose. (C2H4 = C + CH4 + 2000 btu/lbC2H4). Cirrus9 is right about the conditions (rust, water etc). The heat seriously weakens the pipe and after a few short minutes there will be a rupture. It has resulted in fatalities in the past.

What I don't know is precise ranges that it will decompose at nor do I know how best to remedy a decomp. I am told that it is best to depressurize the system, but I don't know how low it must be depressurized to stop the flame. Anyone out there know anything?

 

[25362]

The ethylene (published) uncatalysed decomposition temperature is 730^oC, quite high. And the activation energy for this decomposition is 195 kJ/mol.

There is an index known as RHI (Reaction Hazard Index)
= 10 T_d/(T_d+30*E_a)
where T_d is the decomposition temperature (K) and E_a is the activation energy in kcal/mole. Relatively low reactivities are in the range 1-3, 5-8 values are considered high reactivities. The RHI for ethylene is 4.19, an intermediate value.

I've also read that ethylene pipelines should be as short as possible, and sometimes buried 3-5 m below ground level to keep a temperature of around 20^oC, just above the critical temperature of 9.9^oC, so it can be pumped at pressures of 50-55 bar, as a very dense gas.... nearly as dense as a liquid, but still a gas.

I heard of one ethylene explosion with two fatalities, many people injured and heavy property damage as a result of a leak in a compression fitting in a high pressure (1000-2500 psi) line in a pipe trench releasing a cloud of 200 to 500 lb ethylene, that ignited resulting in an explosion equivalent to 0.12-0.30 ton of TNT.

A release of 500 lb ethylene is equivalent to 8100 moles. The combustion of ethylene releases some 314,000 cal/mole. Thus, the energy released would be 314,000*8100 = 2.54*10⁹ cal.

An explosion energy equivalent to 0.3 ton TNT is about 3*10⁸ cal, that is a 12% fraction of the combustion energy, rather high in comparison with the 2% attributed to unconfined vapour cloud explosions. This is, apparently, a result of the explosion taking place in a partially confined area.

Probable causes for such an event were attributed to:
(a) the use of nonwelded pipe; (b) the lack of instrumentation to detect/analyse vapours and provide alarms; (c) the installation of pipe in trenches where ignitable clouds can form as a result of vapour accumulation.

Other ethylene explosions I heard of were connected to breakage of a drain fitting from a high pressure compressor. Another case was the result of compressor vibration causing transverse fatigue on a 1/8-in stainless steel tubing, leading to a gage, that failed.

Trevor Kletz, Learning from Accidents in Industry Butterworth, 1988, and Walter B. Howard, "Process Safety Technology and the Resposibilities of Industry" CEP, Sept. 1988, are good reading.

 

[linford]

Where did you find those values?

Do you know what kinds of systems can arrest the decomposition once it has begun? Our system trips on temperature sensors and depressurizes the system. I can't verify that the amount that we depressurize is sufficient. Any thoughts?

 

[25362]

to linford, is that decomposition or polymerization ?

 

[linford]

decomp.

 

[25362]

to linford,

The data on thermal uncatalysed autodecomposition of ethylene is correct.

Trevor Kletz in Lessons from Disaster IChE, brings one example of that happening in a 12-in line with ethylene flowing at 1300 psig in which a sidewise double valve arrangement was closed to repair an instrument. When the nearest motor-operated valve was reopened in a lapse of about 3 seconds, the atmospheric air pocket present between the valves was compressed by the flowing ethylene acting as a piston, and heated to a temperature of about 800 deg C (on adiabatic compression, T₂ =T₁*(P₂/P₁)^(k-1)/k. At this temperature ethylene started decomposing. Kletz says that after the decomposition zone has travelled some 290 ft against the stream and its velocity equalled that of the flowing ethylene all the decomposition took place in one part of the line that heated and burst.

Kletz gives as reference McKay F.F. et al., Hydrocarbon Processing, November 1977, 56(11):487.

This example of compression heating of one gas by another isn't specific to air, it could happen to nitrogen, or hydrogen, noble gases, etc. gases having a high Cp/Cv specific heat ratio. As Cirrus9 said it: "it can happen at certain rates of compression".

The hazard could be prevented, says Kletz, by a slow pressurizing of the "dormant" pipeline sections.

 

[25362]

One source recommends that after purging a line with nitrogen, this gas should be pressurized to the same pressure of the ethylene prior to feeding C₂⁼. Another source says that ethylene by itself can become hot if pressurized too quickly, and decomposition may start as a consequence of this heating.

Although the data I collected and presented above seems to be correct, it probably applies to chemically pure ethylene.
However, it appears no commercial or "polymerization" ethylene is pure enough -from 98 to 99.99%-. Contaminants can accelerate either decomposition or polymerization, both exothermic reactions, and as such can act as initiators of decomposition reactions.

Typical trace impurities are methane, ethane, propane, acetylene, propylene, C4 hydrocarbons, CO, CO₂, hydrogen, oxygen, nitrogen, water, polymers, and sulfur.

Another source says ethylene main decomposition products are methane, hydrogen and carbon, and that decomposition may be induced by hot spots, by static electricity discharges or any other high-energy initiators, and catalysed by metals, metal oxides, molecular sieves, polymer molecules, etc.

Therefore you are absolutely right, decomposition may start unnoticed at relatively mild "normal" operating conditions. I've been told that the Ullmann Encyclopedia of Industrial Chemistry may contain useful information on ethylene decomposition. SooCS in thread124-64187 may have something useful to say. Have a look.

The following website may have a reasonably good bibliographic reference list on ethylene:

http://www.intox.org/databank/documents/chemical/ethylene/cie89.htm

Good luck.

 

[Cirrus9]

To 25362
Thank you for your input.
A lot of good information.