Sealing PHOSGENE 4

[israelkk]

Can Teflon (PTFE) chemically compatible and be used for sealing PHOSGENE (I hope the spelling is correct)?

Can anyone recommend a suitable rubber compound that can do the job too?

 

[moltenmetal]

Phosgene services are almost always considered "lethal service", and have very special design rules for all components handling this gas or its liquid. High integrity sealing methods are required. Lethal service valves, for instance, generally have bellows seals or a double packing arrangement with the weep connection between these connected to a continuously operating scrubber.

Viton fluoroelastomer is listed as incompatible with phosgene at 20 C or higher in the only reference I could find. That leaves virtually all elastomers out of consideration, with the possible exception of Kalrez and its analogs.

I would suspect that PTFE would not be chemically incompatible with phosgene, but the worry would be its significant permeability to halogen gases. I'd infer that it is probably significantly permeable to phosgene also, necessitating special seal design to make it safe for use.

 

[Montemayor]

All:

I would remind everyone interested in this subject about what we "pioneers" learned about the miracle of Teflon when we started to use it in the 1960's: it's a panacea that all engineers should be warned about.

Like other, so-called engineered miracles such as Stainless Steel, the silicones, Freons, etc. there are inherent trade-offs that must be considered. Once again, the Universal Engineering Law prevails: There are no free lunches or rides.

Back in 1965 we found out that, yes, Teflon is totally inert to some of the Cosmos' worst bad actors (such as Phosgene); however, this comes at a price of a mechanical Achille's Heel: it has a rotten, stinking memory! Without proven purity and proper re-inforcement (such as with glass fibers), Teflon will draw-out under sustained stress and not return to its original shape. It has no memory; as a potential gasket material that we originally envisioned, it is a total failure. It's sad, but true. But for experienced and scarred old engineers, it's a story that has been repeated many times in engineering history: all that shines is not gold. Beware of any substance or gadget that is claimed to solve all your problems at a cheap price, without any brain cell activity, faster than anything you imagined, and available on the spot. It is not common sense to believe that this is possible.

moltenmetal is right on target with his inference that Phosgene should be treated on its own terms of a bad reputation as a lethal substance. Nothing should be spared in considering its containment and control. There exist specific and detailed instructions and directions on how to deal with this extremely bad actor. One should not generalize nor experiment with this application.

My main point is: don't concentrate on the chemical properties of the containment material simply because of the lethal nature of the substance and the perceived "inertness" of some materials. The total mechanical design is just as (or more) important than the process design. This is a made-to-order situation where Mechanical and Chemical Engineers should be both deeply involved in the design of any containment or transport system for handling Phosgene. Mechanical integrity should not be compromised because there is no allowance for mistakes.

Art Montemayor
Spring, TX

 

[rmw]

My only contribution to this topic is that I was involved with a chemical plant some years ago that made phosgene, among a lot of other nasty things, but when it came to this specific bad actor, we could not use Plate and Frame Hx's, and had to use sealless pumps in this process.

I can't remember if it was specifically the gasketing and or sealing materials for the Hx's and pumps, or their (the P&F's) propensity to leak in general. Once I knew the parameters, I simply moved on to other acceptable devices to use.

I do remember that phosgene was one serious bad actor, so be careful in your evaluation of teflon in this situation.

rmw

 

[saxon]

I hope everyone's aware of of the isolation distances to protect health and safety. Small leaks, 95 m, downwind-day 0.8 km, downwind-night 2.7 km. For large leaks, 765 m, downwind-day, 6.6 km, downwind-night 11.0 km.

When dealing with such toxic substances, it's best not to trust to seals; seals/gaskets leak. Everthing needs to be welded and when this isn't practicable, such as at valve stems and the like, use specially designed seals with excess inert positve pressure purging.

Hope this helps.
saxon

 

[EGT01]

Israelkk,

If you are just a phosgene user, your best bet is to discuss your needs with your phosgene supplier who will certainly have the expertise you seek.

As a matter of better educating yourself about phosgene, get a copy of Handbook of Compressed Gases from the Compressed Gas Association, Inc, published by Van Nostrand Reinhold. It covers a wide range of information about different gases (including phosgene) relating to physical constants, physiological effects, materials of construction and safe storage and handling.

You may also be interested in the following

http://www.phosgenepanel.org/

 

[hacksaw]

you might check with the phosgene institue. they have a lot of design installation practices that you may find helpful

 

[moltenmetal]

Art's absolutely right, as he usually is! PTFE creeps like a bastard. It is NOT an elastomer, and can't be designed to behave as one. Reinforcement with glass just makes it stiffer, not more elastomeric.

Those things taken as true, I'm as glad as hell that it was invented- used properly with the correct mechanical design, PTFE and its analogs truly ARE a wonder material. Ditto for viton, only moreso- it IS an elastomer, and viton o-rings have been of tremendous benefit to reliable sealing at modest temperatures. But don't confuse it with PTFE- viton has very different chemical resistance properties.

Lethal service implies the absolute minimization of seals and other potential leakage points in the design, as well as a high percentage of radiography for welds etc. Agree that the manufacturers of phosgene would be a great source of info on proper design codes for this material.