ChemE.info – Chemical Engineering Consulting

One of my strangest (and scariest) experiences was my visit to the SASOL plant at Secunda. This is the largest synthetic fuels plant in the world and is situated on an open grassy plain a few hours drive east of Johannesburg.

My recollection was that my plane arrived in Johannesburg around dawn after an overnight flight from London so I was pretty exhausted and jet-lagged. I had to get a rental car and then drive for hours across the Veldte (on the wrong side of the road… South Africa is an ex-British colony). The scenery was pretty spectacular… Definitely “big sky” country although it was strikingly different from anything I have seen in the US or Australia.

Eventually I arrived at Secunda and that’s where things got surreal. I had been checked into Graceland Hotel Casino & Country Club (see photo). I drove up the sweeping drive surrounded on both sides by a treeless, windswept, and totally empty golf course to the hotel casino itself which was a fantastic thing like something out of a Disney theme park.

Under the entrance portico I was met by a tall, thin black man dressed up as Uncle Sam (all in red, white, and blue satin with a top hat, tail coat, and brightly colored suspenders… Braces if you are from the UK.).

Inside, the theme was 1800′s New Orleans and the Mississippi… The staff were dressed up like saloon keepers with straw boater hats and sleeve garters. The hotel was quite luxurious and new but it seemed very odd to find all this pseudo-Americana in the depths of Africa.

The next morning I drove out the SASOL plant and that also was more than a bit surreal. The synthetic fuel plant had been developed to circumvent the oil embargo imposed by the world community during the apartheid era and, inevitably, was a target for the anti-apartheid guerrillas. On the morning I drove out, a huge thunder storm was drifting in from the west, looming over the plant… And the plant was heavily fortified with armored watch towers. The overall impression that morning was like something out of a high-tech, industrial Lord of the Rings.

This visit took place a few years after the collapse of apartheid. Unfortunately, the end of apartheid has made South Africa, if anything, even more dangerous than when there was an ongoing civil war. All the South African engineers I worked with routinely carried guns when they drove to and from work… There were lockers at the gatehouse where you left you gun while at the plant. As a visitor and not being familiar with the local modus vivendi, I was pretty worried. Rather like a Japanese tourist who got parachuted into the South Bronx or Watts.

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.