Never too late to simulate: modeling existing plants
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.