The SAIChE 2018 Innovation Award was celebrated with a luncheon at the Wanderers Club in Johannesburg and alcohol free champagne, in keeping with Sasol’s strict safety protocol.
The winning team from Sasol’s Research and Technology division won the award for an automated control system they developed for the bench-scale evaluation of new naphtha reforming catalysts for their continuous catalytic regeneration (CCR) plants.
“We at Sasol have an ongoing need to model the performance of commercial catalyst’s on a bench scale, to allow us to continuously improve the performance of our Fischer-Tropsch (FT) syngas-to-liquid process,” Prinsloo says.
“Specifically, we need to evaluate new commercial catalysts at a constant research octane number (iso-RON) – and this needs to happen on the benches of our laboratories,” he explains, adding that this is has long been a significant challenge because the parameter changes that occur while reforming a bench-scale raw naphtha sample happen too fast for manual control, especially when dealing with naphtha’s derived from Sasol’s FT-based coal-to-liquids (CTL) process.
“So we needed to automate the manipulation of process conditions and this is what we have managed to achieved in this work,” he says.
Catalytic naphtha reforming is used at all refineries to convert naphthas – the low-octane raw liquid hydrocarbons usually distilled from crude oil – into high-octane liquid products called reformates. “These reformates are then used to blend the high-octane fuels we use in our motor vehicles,” he continues.
“The process is a true industry workhorse and almost every refinery in the world has such a processing unit to produce blend products with the octane numbers required,” Prinsloo informs MechChem Africa.
Naphtha reforming at Sasol
In essence the naphtha reforming process used at Sasol’s coal to liquids (CTL) complex, involves the isomerisation and aromatisation of feeds containing paraffins such as heptane, octane and nonane into highly branched and aromatic liquid hydrocarbons. “We typically look for molecules with six or more carbon atoms, which we refer to as C5+ naphthas.
“The critical number for a fuel blending product, however, is its research octane number (RON), which governs the fuel’s suitability for use in different modern engines,” says Prinsloo, adding that Sasol’s commercial CTL processes strive to produce fuel products with specific target RONs.
“The catalysts are critical for achieving this. They are critical for speeding up the reaction and making it more efficient. But they operate at high temperatures and they coke up, so they need to be routinely regenerated to keep them working effectively. Typically, the catalyst will deactivate within a week under the process conditions in a laboratory reactor,” he says.
“The naphtha reforming technology has evolved from a simple fixed bed reactor system, where the catalyst has to be removed to be regenerated externally, to a highly sophisticated system where the catalyst is continuously moved through the reactor into the regenerator and back,” Prinsloo explains.
In the regenerator, the catalyst is purged to remove hydrocarbons and hydrogen, the coke is burned off the catalyst before it is re-chlorinated and reduced. “The whole process is known as continuous catalytic regeneration (CCR) and it is currently used in Sasol’s modern plants in the manufacture of our high-octane fuel blending products,” he says.
He adds that the paraffinic feed used in the Sasol process is particularly lean, which requires a much higher temperature to produce the specific octane number required. This lowers product yield and causes catalyst to degenerate far more quickly.
“Pushing catalyst and processing boundaries has always been imperative at Sasol. This has become especially true when targeting reformate with a RON above 95 while retaining an acceptable yield and keeping within commercial fuel specifications, particularly with regard to benzene.
The bench-scale challenge
A key role of Sasol’s Research and Technology team is to seek out new and better catalysts for use in its CTL plants. “We routinely perform bench-scale tests of catalysts for use in on our FT refinery processes using conditions that are as close as possible to those in our production units,” Prinsloo continues.
But doing this manually presents challenges. “First, we are looking to see the effect of the catalyst on the yield of the targeted fuel and its RON number. Ideally, the whole sample collected needs to have the same RON, but this can be difficult to achieve because the catalyst is slowly deactivating as the test proceeds, sometimes faster than the sample can be analysed,” he says.
“We also need to collect a large enough sample of the reformate to do engine analyses, which can take a week for each sample,” he adds.
The process involves several endothermic and exothermic reactions and temperature swings introduce significant variation in the product composition. “The temperature of the catalytic bed directly affects the RON number and, as well as keeping the temperature as uniform as possible, a gradual increase in bed temperature can be used to counteract the deactivation process,” Prinsloo tells MechChem Africa.
Sasol’s new bench scale solution
“The control system we developed can compare very small differences in the reformate yield when using different commercial catalysts,” he says. “By using multidisciplinary analytical and advanced process control techniques, we have achieved substantial improvement in bench-scale naphtha reforming.”
Summarising Sasol’s new bench process, he says that the RON of the product is continuously measured while the temperature and the heat flow over the catalyst bed is accurately controlled. “We slowly ramp up the catalyst bed temperature based on real-time RON measurements of the reformate. In addition, the control loop maintains an exceptionally uniform bed temperature with far less heat flux drift across the reactor zone,” he says.
The core innovation involved in achieving this was developing a way of measuring the RON in real time so that it could be used in the control loop to automate the whole process.
“Octane numbers of reformate samples are measured in a number of ways. The most accurate of these is to use a calibrated CFR engine to determine a motor octane number (MON). But 500 to 1 000 ml samples are required before the test can begin, which takes at least a week to reform at bench scale, so using this test routinely is impractical for research.
“An alternative is to do detailed compositional analyses and spectroscopy, but special chromatographic systems have to be used and, although used in many large laboratories, the equipment requires specialised knowledge, is very expensive and the tests take time.
“We have developed a cost-effective alternative that can approximate the RON number using simple gas chromatography (GC) for component separation, combined with more sophisticated calculations and modelling to arrive at an estimated RON and MON. These values are then occasionally calibrated against more accurate CFR engine tests.
“None of these methods were suitable for real time use in a feedback-based control system, however,” Prinsloo says.
“We therefore pursued the use of near-infrared (NIR) spectrometry. We used our historical GC RON estimates to calibrate a real time on-line NIR analyser and set it up to continuously monitor the reformate stream from the bench-test system. By adopting error-based feedback control between the output reformate octane number and the preset required RON, we are able to slowly adjust the bed temperature upwards to keep the activity level of the catalyst at the point where it produces a reformate with the exact RON required,” Prinsloo explains.
“This system has enabled us to generate 1 900 equivalent octane number analyses that were corroborated via only 80 CFR engine tests, which translates into a more than 20-fold reduction in the testing load for laboratory octane number determination.
“And this has already benefited our production units. By identifying catalysts better suited to our processes, we are achieving significantly better yields while lowering catalytic reforming demands and associated costs,” he concludes.
A project worthy of winning any innovation award.
Acknowledgements: This work has been shared with the international community via a keynote lecture by Nico Prinsloo at the 2016 CATSA conference and a paper published in the American Chemical Society’s Industrial Engineering and Chemistry Research journal of May 2017.