ChemE.info – Chemical Engineering Consulting

There is a tendency to move process development to the next scale prematurely. Since the cost increases substantially with each increase in scale, it makes much more sense to study the process as much as possible at the lower scale.

I have seen organizations jump the gun several times in my career. For one thing, it is never a good idea to move to the next scale if you do not understand the results you are getting at the current scale. I have seen an organization increase from pilot scale to commercial scale even though they were finding that their pilot scale experiments were not repeatable. Needless to say, they then demonstrated their lack of process understanding at the larger scale… At vastly greater expense.

Another example occurred back in the late 70’s when I worked in Polaroid’s department T-22. My role was to scale up processes that the photo and organometallic chemists developed in the lab:

One of chemists asked me to do a pilot-scale production run of a new “receiver layer” system that he had been working on. I asked him to describe how he created the system in the lab and he listed various steps, one of which was the addition of a ferric salt followed by what he called an “incubation period.”

That raised my eyebrows a bit since I didn’t recall that bit of jargon from my reaction kinetics classes and I asked him what he meant by “incubation period.” He was a bit sheepish about it but said that he was adding the ferric salt as a mild oxidizing agent and that it took several minutes before the oxidation that he was looking for was complete.

Since he was using an open beaker, I asked him if he was sure that the ferric was doing the oxidation and not oxygen from the ambient air. I suggested that the “incubation period” might simply be the time required for oxygen to transfer from the room air into the agitated beaker. He was a bit taken aback by the idea but felt that we should test it.

So we reran his laboratory experiment in a narrow mouth flask and sparged the mixture with nitrogen to exclude the room air. We found that we could leave the system indefinitely and that his oxidation would never occur. This confirmed that the ferric had no effect and that the ambient oxygen was the key to his reaction.

The point I am making is that we confirmed this by doing additional lab-scale work with quite small costs in terms of equipment, chemicals, and manpower. We would inevitably have figured out the ferric vs O2 issue at the pilot scale but it would have cost us thousands of dollars more and at least a couple of weeks of time. Once we realized the role of gaseous oxygen, I built a controlled-environment lab setup with a gas manifold that allowed us to meter in the oxygen from a gas cylinder. This allowed us to control the oxidation very precisely and was the approach we used when we finally did move to pilot-scale production.

People often think of process simulation in terms of designing new processes. But it can be provide very useful insights into an existing plant’s operation.

While working for AspenTech, I modeled an ammonia plant for a customer in Japan. The customer gave me the process flow diagrams and the current operating parameters and I built that into a steady-state model.

A key section of the process is the syngas loop where a mixture of hydrogen, nitrogen, and ammonia is circulated through a reactor where the hydrogen and nitrogen react to produce more ammonia.

And a key piece of equipment in that loop is a large multi-stage turbo-compressor. I didn’t try to model the compressor rigorously (i.e. using performance curves), I simply varied the compressor stage efficiencies until I matched the temperatures, pressures, flows, etc. that I have been given by the customer. But I was surprised to find that I kept coming up with efficiencies that seemed much lower than I expected for a turbo-compressor.

During a meeting in Japan, I discussed the issue with the customer’s engineers. The two engineers that were my main contacts (they were younger and spoke English) agreed that the efficiencies I was calculating were too low and we started discussing what might be causing the discrepancy. But there was an older customer engineer present who had been around when the plant had been built twenty years earlier. Once he figured out what we were talking about (he didn’t speak English) he told us that the low efficiencies were probably correct.

So what was going on? The ammonia process has been around for quite a while and ammonia plants can be bought “off the shelf” from a number of licensors. Most of these plants are used to feed fertilizer plants and they are typically designed to produce (if memory serves) about 1100 tons per day of ammonia. But the plant I was modeling was not being used to feed a fertilizer plant; the ammonia was being used to supply other processes in a large integrated chemical complex. And the ammonia demand was significantly less than 1100 tons per day. And the volume of gas going through the turbo-compressor was much less than it was designed for. When we manually checked the compressor performance curves with the “as operated” flows, my model’s efficiencies suddenly looked reasonable.

It was then very easy to use my model to calculate the energy savings that would result from increasing the efficiency by, say, 15%. When the customer’s engineers priced out the cost of modifying the compressor to achieve this, the payback period was about 6 months. I was later informed that the necessary modifications were made at the next plant shutdown.

A satisfactory outcome but it is sobering to think how much money was wasted over the previous years of low efficiency operation.