Metal organic frameworks research brings us back to the question: Just because something can be done, should we do it?

Writing last month about the use of metal-organic frameworks (MOFs) to reduce energy demand in one of the key steps in plastics processing gave rise to some additional questions about this process, which the National Institute of Standards and Technology (NIST; Gaithersburg, MD ) believes will reduce energy demand in the plastic resin manufacturing process. I began asking additional questions about the process, its cost effectiveness and the potential benefits of removing acetylene from polyethylene resin manufacturing in the manner NIST described.

MOFs are materials that can filter the acetylene from the ethylene more efficiently than conventional manufacturing processes, said NIST researchers, thus improving the resin’s purity. One point that NIST clarified for me is that the basic point of this research is not to “recover” the acetylene, but to purify the ethylene.

One of my thoughts when reading about this process is whether or not the acetylene could be recovered and reused in welding torch applications. I recalled writing for Welding magazine a number of years ago about acetylene welding, and realized that it does have applications. Wei Zhou, one of the researchers at NIST, responded to my question, saying, “As for acetylene retrieving, sure, it is doable. Whether the acetylene retrieved can offset the extra cost needed to retrieve it is another question. It has to be evaluated in an industrial environment.”

However, the researcher did say that MOFs could be easily and cheaply synthesized, reused a great many times and would enable industry to purify the ethylene without rare-earth catalysts (one potential expense) or the need to heat the ethylene (another expense).

Dr. Zhou explained that the work at NIST provides an alternative route or possibility for ethylene purification “from a fundamental scientific point of view.” He added, “This is just the initial step of a potentially promising technology. Practically, will it replace the current technology used in industry? We don’t yet know. It depends on many factors. People from industry need to get involved and do a lot of tests in an industrial environment.”

The NIST research findings, published on May 19, 2016, in the journal Science, show that MOFs can effectively remove the acetylene—considered a contaminate—from ethylene. The research suggests that filtering out acetylene using MOFs would produce ethylene at the high purity that industry demands while sidestepping the current need to convert acetylene to ethylene via a costly catalytic process.

I tracked down a person from IHS who has done “a fair amount of technical work” for some of the large chemicals companies during his career. “To look at the upside, a small amount of the acetylene in ethylene plants is recovered for things like welding gas applications,” said Tony Pavone, who wasn’t generally impressed with the idea. One of the problems Pavone pointed out in his response to me was that in his experience designing, operating and maintaining industrial-scale steam crackers, the concentration of acetylene from the pyrolysis furnaces is about 2%.

“What you want to do is to selectively hydrogenate acetylene to ethylene, which is why you are in this business in the first place,” Pavone wrote. “Why in the world would anybody want to remove acetylene? You wouldn’t. Current practice converts acetylene to your desired product ethylene using precious metal hydrogenation catalysts. There are also a couple of other key potential show stoppers in the use of metal organic frameworks.”

One of the problems Pavone pointed out is that “acetylene has a high propensity to explode, usually at the most inopportune moment.” Hence, use of the gas in acetylene welding torches—it’s very effective in that application. However, Pavone continued to note, “For an industrial-scale ethylene plant costing about $2 billion, the physical size of the MOF removing device would be about the size of a warehouse. Such an MOF plant would be treating about 10 million pounds a day of cracked gases off the furnaces at just over atmospheric pressure. This is about 150 million SCF per day. How extensively have the researchers tested their technology to make sure that the acetylene won’t explode? There is a very good safety reason why industrial practice is to intentionally avoid concentrating acetylene.”

Dr. Zhou obviously answered Pavone’s question—NIST is waiting for industrial testing to prove out its use of MOFs, and as yet does not know the practicality of using MOFs to remove acetylene from ethylene.

Another problem that Pavone addresses is the fact that “the cracked gas containing acetylene is loaded with all kinds of 'cats and dogs,' meaning that industrial practice uses different quality feedstocks than the reagent-grade material used in the lab, and that difference can have enormous practical consequences in the commercial world." This includes higher molecular weight tars and oils that would “tend to foul or plug any active surface designed to selectively remove the acetylene,” Pavone explained. “A steam cracker is designed to run three to five years between turnarounds. Have the researchers tested their MOF on an actual industrial ethylene feed stream, and can they guarantee the guy putting up the $2 billion that the device will work for three to five years without getting totally gummed up?”

Again, the answer from NIST researchers would be “no,” as this is, so far, purely a laboratory experimentation model.

“Once the acetylene is captured in the MOF, what do you do with it?” asks Pavone. “There is a very limited commercial market for acetylene, which is used primarily for acetylene polymers and welding gas. That market is satisfied,” he concluded.

Pavone believes the current technology is “clearly good enough” and refers to the Linde acetylene recovery process based on absorption using DMF (dimethylformamide) as a solvent, which “is accepted by the industry as the leading technology for acetylene recovery. So what’s the point?” That is Pavone’s question. Does the industry have a big problem that needs to be solved with a new technology? Not really.

“Ask yourself what is the unmet need to be satisfied by this research? That’s the first thing I did when I read about the MOF technology research,” said Pavone.

Dr. Zhou would probably agree with Pavone’s assessment. “In short,” said Dr. Zhou, “a lot of future works are still needed to make this technology a practical one.”

Both Dr. Zhou and Pavone offer some salient points on all of this. Science, particularly chemistry, is fascinating and the primary benefit that I can see is that trial and error, laboratory experimentation and research is the way we come up with new ideas, new materials and new (and perhaps better) solutions to old problems. But it does bring us back around to the question of the cost for something that may have unintended consequences. As my favorite chemical engineer, William Banholzer, likes to ask: “Just because we can, should we?”

Question for PlasticsToday readers: What do you think about this? We’d like to hear your opinions.