I was late in taking up my new assignment in this refinery, as it took longer than expected to obtain the work permit from the host government authorities. On arrival, I was to execute a major shutdown (called turnaround in North America) that had already been planned. Due to this delay, my predecessor stayed back to execute it. Thus I did not have an executive role for a few weeks and was able to observe the shutdown independently.

The Thermal Cracking Unit (TCU) added about US$60,000 of value per day of operation. In this unit, the long-chain heavy residue molecules were broken down by a high temperature cracking process. This yielded short-chain products like naphtha, which were valuable as feedstock to petrochemical plants. Figure 37.1 shows a schematic flow diagram of the process. The cracking of the long-chain molecules liberated free carbon which was then deposited as coke in the heaters, columns, vessels, and pipelines. In due course, the coke deposits resulted in a sharp fall in operational efficiency. The unit had to be shut down every six months to remove the fouling by coke deposits. Past records showed that these shutdowns lasted 21 days or longer, even though the number...