S Block Heroes – The Innovative Operator
I started work at Monsanto's West Footscray plant as a new graduate chemical engineer in 1974. There I was assigned to the plant technical services group (PTS) responsible for supporting the styrene polymerisation plants. We were known as the S Block heroes due to an atmosphere of frenetic endeavour to reduce batch cycle times, improve yields and troubleshoot the many inevitable variabilities of batch processes.
Dept 81 where we spent most of our time, consisted of a closed, agitated, jacketed, glass lined reactor (described as a kettle). A batch involved Styrene monomer droplets suspended in hot water with a peroxide catalyst to initiate & sustain the polymerisation.
The key controls were: batch temperature profile, (which controlled the molecular weight distribution of the polymer and hence the polymer's physical properties); and, the timing of the addition of suspending agent, (which controlled the finished polystyrene bead size).
Once the batch cycle time was complete the kettle was cooled and the contents transferred into a stainless-steel agitated slurry tank, which could hold 3 kettle batches.
The suspended polystyrene beads were dewatered in a continuous horizontal decanter centrifuge. Then the beads were dried in a horizontal rotary drier with direct contact hot air heated to about 60°C. Finally, they were screened into the required size range for production of expandable polystyrene (EPS) beads.
Our main tasks were to increase bead size and reduce batch cycle times to increase capacity for the growing EPS business. In undertaking these tasks, we worked closely with plant operators explaining the issues to them and seeking their support in conducting trials aimed at meeting our capacity objectives.
The polystyrene plant operators were a diverse lot of guys mostly in their fifties, many migrants from the UK and Europe. Most had only a basic education, but a good understanding of their job even though they didn’t always really understand how the processes they operated actually worked. There was one particularly sharp process operator, a Pom, who was probably in his mid-thirties.
The batch process comprised of coincident charging of hot water and styrene monomer to the closed reactor under an inert gas blanket of CO2 to avoid oxygen in the kettle, which resulted in a kettle pressure in excess of 100kPag at the end of charging. A vented catalyst charge bomb mounted above the kettle was valved to facilitate catalyst addition to the pressurised kettle.
One Saturday afternoon shift our intrepid operator (I'll call him Fred) forgot to shut the vent on the catalyst bomb before opening the valve into the reactor. This resulted in the pressure from the kettle blowing the catalyst out of the bomb vent to atmosphere above the building.
Not knowing how much catalyst (if any) had been charged to the kettle, Fred then elected to proceed and to follow the standard batch temperature-time cycle. He observed that the jacket cooling temperature seemed to be significantly higher than normal during the reaction cycle. Our man understood that jacket temperature profile could provide an indication of the stability of the monomer suspension in water, however it also indicated a lower than the normal cooling load due to reduced rates of polymerisation. He figured more stable batches were good, possibly even providing scope for larger batches. When Fred handed over to the night shift there was no mention of his ‘novel’ process and when finished, the batch was transferred to the slurry tank.
On the Sunday afternoon shift our sharp operator arrived with a spring in his step. While there had been no mention of his charging error of the previous afternoon’s shift, he was determined to repeat his charging ‘modification’ to confirm the benefits this procedure offered. By the end of his shift someone mentioned that the rotary bead drier below the control room was making a bit of a banging sound. Fred, however was keen to get home to watch the Big Match on TV, leaving batch transfer to the night shift.
Early on the Monday morning things took a decided turn for the worse in Dept 81. The next batch was transferred to the slurry tank however, the drier was sounding increasingly noisy “like there was someone trapped inside wanting to get out” explained the foreman as the S Block Heroes raced to the plant to pore over the batch charts and sort out this latest crisis.
The drier was thumping and was shut down and opened, revealing the extent of the problem. All the internal lifting flights were full of a honeycomb like polystyrene, while polystyrene boulders of varying diameter banged into the drier walls. The control laboratory confirmed very high levels of residual styrene in the polymer from the dryer.
By lunch time it was clear that catalyst charging had not been carried out to the standard procedure not just once but for two batches. Our afternoon shift operator was summoned to the supervisor’s office and was sacked on the spot for failing to follow standard operating procedures, a cardinal sin for any process plant operator. In retrospect our Innovative Operator was probably a little unlucky in that his error occurred on the weekend, when supervision was significantly reduced. During the week the S Block Heroes were always about, trying some process change or other in an effort to increase polymer production, which may have also contributed to Fred’s downfall.