Chemicals I Have Known (and Made) – Acrylonitrile

acrylonitrile-500x500

 

The last chemical I wrote about, hydrogen peroxide, I described as a cute, cuddly chemical. The next chemical I was involved with was anything but cuddly. Acrylonitrile, or Acrylo as we called it, is an organic chemical that is used to make acrylic plastics and fibers. By itself, it has toxicity as it will release cyanide within the body. But the process to make the chemical is also very nasty, and especially so where we made it in Memphis. A little background first, though.

I received a promotion and gained the title of Production Supervisor in the Acrylo process. This was a big enough process that it had two production supervisors. I was placed in charge of planning the annual shutdown, which required intense logistical planning. I served as a backup to the real process supervisor. He was a Memphis native who had come up from the hourly wage roll to his exempt role position. He actually was a classmate of Elvis Presley when they were both in junior high school, but he did not have any good stories about their shared time. So I had the advantage of being able to learn about supervision while only occasionally really taking charge.

chemical plant

The Acrylo process is a huge process, more like an oil refinery than a standard chemical plant. It had six huge reactors where the chemicals propylene, ammonia, and oxygen (from air) are mixed with a catalyst in fluidized bed reactors. These reactors were about 12′ in diameter, and some 40′ tall. The reaction itself creates significant heat, so the reactors are full of tubes containing water which turn into steam that helps to drive the later separation processes. Once the chemicals have reacted, the off-gases are sent into an absorbing tower. This tower was over 100′ tall, and about 15′ in diameter. After the gases are absorbed in water, it is necessary to separate out the other reaction products. The primary one is hydrogen cyanide, which I wrote about earlier. There were two distillation towers used to separate and purify the hydrogen cyanide, which was then sent by pipeline to the other part of the plant that produced cyanide as its primary product. I remember that one of the pumps that transferred the cyanide developed a leaky seal, and since it was several months before the scheduled shutdown, the solution was to barricade off a section of the process with good old yellow and black warning rope, guaranteed to be a barrier against all chemicals. NOT! In fact, even beyond the tape, you could taste the cyanide, and this is how I became sensitive to cyanide and was able to easily pass the sniff test during my annual physical at the plant. It does not smell like bitter almonds, rather, it is an unpleasant sensation that grabs at the back of the throat.

Once the cyanide was removed, the crude acrylonitrile had to be separated out of the ammonia-laden water. There were a total of five distillation towers, each with a different purpose, until finally the refined acrylonitrile was pure enough to go into the storage tanks. One of the distillation towers actually concentrated another byproduct, acetonitrile, which is used as a solvent. Eventually though, the ammonia-laden water was neutralized with sulfuric acid, and had to be disposed of. Now every other commercial acrylo plant in the US was in a location where the waste stream could be injected into the earth in a deep well. In Memphis with its extensive aquifer system near the Mississippi River, this was not a viable option. So when the plant was built in the 1960’s, and energy was extremely cheap, the solution implemented was to incinerate this stream. We had three huge stacks that could be used to “thermally oxidize” the solution, and release nitrogen, water, and sulfur dioxide to the atmosphere. We were the 2nd largest sulfur dioxide emitter in western Tennessee. Only the Tennessee Valley Authority’s coal-fired power plant was a larger source. As you can imagine, when the dual energy shocks of the 1970’s came, burning a water waste stream put a larger and larger burden on profitability. So much so that when our plant suffered a major freezing incident one winter, that proved to be the final straw that led to the plant’s closure and eventual dismantlement. Chemical plants really, really do not like cold, freezing weather. And seeing 12″ diameter burst water pipes start to leak when they finally thaw is not something I ever want to witness again.

But before the process was closed, there were some really wild times I had. One in particular involved a one ton cylinder of sulfur dioxide. Now pure sulfur dioxide was used as a polymerization inhibitor in the vapor space in the columns where cyanide was purified. So we had tubing running from the cylinder up to the tops of the distillation towers. Even though sulfur dioxide boiled at 14ºF, it took a little extra push to ensure that enough gas flowed up to where it was needed. So we had a simple plywood enclosure where we kept the cylinder, and we had steam coils underneath the cylinder. Such a complex system couldn’t ever go wrong, could it? Well, it did go wrong, and the fusible plug in the cylinder that kept it from over-pressurizing, that plug melted and began to release the content of the cylinder to the atmosphere. That was one of the days where the other Process Supervisor wasn’t there, and I was in charge. I had to direct the evacuation of the adjacent laboratory and technical building, but what saved us was one operator who was able to get onto a forklift with breathing air, and pulled the cylinder out, where it could be sealed by hammering a wooden plug into the hole where the fusible plug had been. We prevented releasing the entire cylinder contents, which could have affected a large area, including US 51 highway which ran parallel to the plant.

To this day I don’t remember what we did to get another cylinder in and fix the tubing that had torn away when the cylinder was pulled out, but I do remember that we didn’t create a huge environmental incident.

When we finally did get into our planned shutdown, the biggest job was the replacement of our 100+’ tall absorbing tower. We got cranes in that were able to lift the entire tower – the big crane to lift from the top, a smaller crane to guide the bottom section. Then the process was reversed so that the new column was installed. We did this on a weekend when most of the lab people and other technical engineers weren’t around. My job? To run the video camera that captured the move. Somewhere there was a VHS tape that documents the replacement of this absorbing tower, which was used for about one year before the entire process was shut down.

Propylene is the main reactant to make acrylo. It has properties very similar to its chemical cousin, propane. So you know those long cylindrical tanks that hold propane? We had four big tanks that held the propylene. One thing that most folks don’t know about chemical piping is that there is almost always a little bit of leakage that comes out of valves and flanges. And for whatever reason, propylene attracts wasps. So going up on the storage tanks was a bit of an adventure. It was necessary to keep watch in order to knock down wasp nests before they got too big.

One other similarity to a refinery was that the residual gases from all of the columns was released through a flare stack. This stack was 175′ tall at its tip, and one of the tasks for the shutdown was to inspect the flare. I, being a novice supervisor, didn’t always think about my decisions. We had an intern who had his own pilot’s license, and was clearly unafraid of heights. So he asked, and I gave permission, for him to do the inspection on his own, and trusted him to do it safely. If he had an accident, my career would have been over at that time. But he completed the inspection, and came down safely. It was only years later after I gained more experience that I realized what a risk I took with his life and with my own career in my company.

The equipment for this process was huge. We used air as an ingredient. So it was a 2500 horsepower air compressor that fed the reactors. That was one impressive motor that ran that compressor.

Of all of the chemical processes I worked with, this one was by far the “dirtiest”. We emitted tons per day of sulfur dioxide. We sometimes had cyanide leakage. We had another wastewater stream that did go to the sewer system, that we had to monitor for compounds that had the nitrile (or cyanide) functional group – the CN on the end of the molecule. Before I worked in the process, they had tried to see if they could use the ammonium sulfate waste stream as a fertilizer for soils that needed acidification. They had a section of ground near the plant set up to receive the waste, and monitored the soil to see how it worked. Spoiler alert – it didn’t work.

One thing that I appreciated in my time in this process was that we had a Superintendent who believed that his supervisors should know what we were expecting workers to do. So all of us had to put on self-breathing air packs (like scuba tanks), put on chemical-proof suits, and disassemble and reassemble a flange with its bolts. It did show me how exhausting working in that type of environment was. When I took off the suit, I was drenched in sweat. But in my mind, I thank my old supervisor’s supervisor for giving me a taste of what it really is like to work in such an environment.

In the Memphis plant where I worked, there were four large chemical processes. I’ve shared the stories of three of them. One more to go.

 

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