ChemE.info – Chemical Engineering Consulting

I spent about a year at Kennecott Development working on a project for Kennecott’s Carborundum subsidiary (I think Carborundum is now owned by Saint Gobain). We were developing two processes for making boron nitride ceramic fibers. One process was intended to make tensile BN fibers for use in woven composite materials. The second process made a loose BN fiber mat that looked very much like the pink fiberglass mat used in home construction (the BN wasn’t pink of course ;).

What I want to focus on here is the second fiber mat process. One cannot spin Boron Nitride fibers directly so the process involved spinning boron oxide glass fibers first and then nitriding the BO with ammonia to convert it to BN.

As I mentioned, the fiber mat we wanted to end up with looked a lot like fiberglass wall insulation so the BO glass spinning apparatus was modeled on a commercial fiberglass spinning system. This was comprised of an electrically heated metal tank that stored the molten boron oxide glass, a metering valve, a perforated spinning cup, and a torch system.

The molten boron oxide glass was metered into the spinning cup where it was thrown against the side by the centripetal force. It then extruded through the holes in the cup a formed a cloud of glass fibers surrounding the cup. The torch was positioned so that it melted through the fibers once they reached a certain radius from the center of the cup so that one would get a consistent fiber length in the mat.

In the commercial fiberglass system, the torch would have been supplied with either natural gas or propane. But boron oxide glass is very hygroscopic; in other words it soaks up moisture from the air. When exposed to humidity it ends up looking like sticky cotton candy. And, of course, a major combustion product of both natural gas and propane is H2O.

So what to do? We needed a torch fuel that didn’t produce water vapor as a product of combustion. I don’t think it was me that thought of it but somebody on the team came up with a very elegant solution. We used carbon monoxide as the fuel and that worked quite well. It burned hot enough to cut the fibers as needed and the only combustion product was CO2 so we didn’t end up with cotton candy. Neat, eh?

A lot of process design is cut and dried but the fun part is finding creative ways to change or improve the process.

I worked for a couple of years on the CUPROSUL process. This process used copper sulfate to scrub H2S out of various gas streams (the principal application was scrubbing geothermal steam). In the original process concept, the sulfur contain in the H2S ended up as ammonium sulfate. It was originally hoped that the ammonium sulfate could be sold as fertilizer. Unfortunately, ammonium sulfate is not widely used as fertilizer in the developed world…

And even worse, the ammonium sulfate that resulted from using the process to scrub geothermal steam contained contaminants that were unacceptable in a fertilizer. This meant that there was no market for the ammonium sulfate and that one would probably have to pay for its disposal.

So, what to do? I had done a literature survey on the reactions used to regenerate the copper sulfate from the copper sulfide produced in the scrubber. From reading the articles it was evident that scrubbed sulfur was briefly present as elemental sulfur in the stirred tank regen reactors but that it was quickly oxidized to the sulfate given the rather severe temperature and oxygen levels.

It occurred to me that, if we could somehow protect the elemental sulfur from further oxidation, we might be able keep it in its elemental form rather than end up with the sulfate form. So I went looking for good sulfur solvents that were immiscible in water and were poor solvents for oxygen. It turns out there are quite a number (including olive oil) but I decided to run an experiment with a chlorinated hydrocarbon which we had in the lab and that seemed to have the desired solvent characteristics.

I took a quantity of the aqueous copper sulfide slurry produced by the scrubber and I oxidized it in an agitated beaker in the presence of the chlorinated solvent. The slurry eventually disappeared and I stopped the agitation; allowing the two solvents to separate. I then decanted the chlorinated solvent into a pan and left it to evaporate in a fume hood. The next morning the pan was dry and covered in sulfur crystals.

That was very satisfying but, of course, we still had to look at the economics of an elemental sulfur by-product. A first pass analysis showed that elemental sulfur by-product did look more promising than sulfate but that the impact would vary by region. The cost of sulfur varies quite widely around the world. Some areas have vast amounts of mineral sulfur that is cheap to mine. Other areas have widely used processes that produce elemental sulfur as a by-product.

So the process modification was not a complete homerun but it did offer the prospect of changing the by-product produced to suit the local market.