The ammonia refrigeration and storage plant in Secunda has experienced significant losses due to flaring over the years. At the current Ammonia price these losses were calculated to be in the region of R24 million per year.
Investigations performed in 1999 indicated that an additional ammonia compressor would be required as it was believed that the flaring was a direct result of insufficient compression capacity. Installing a new compressor would have been costly and taken over a year to implement. In May 2002 Sastech and AVEC were asked to investigate the problem and to find ways of eliminating the flaring whilst minimising capital expenditure. This project fitted right into the Sastech-AVEC Alliance profile.
The first step of this project, similar to most of the other Alliance projects, was to complete a mass balance of the plant. This was followed by an Aspen model of the plant, done at the AVEC offices in Sandton. The results of the mass balance and simulation, along with discussions with plant personnel and Sastech, showed that there was no theoretical requirement for additional compression. From a process design perspective the plant seemed to have sufficient capacity.
As a result, further investigations into the control and operating philosophy of the plant were conducted, with startling results. A list of recommendations was then compiled and was followed by a test run that lasted for two months.
The first recommendation was to calibrate the control instrumentation. This was necessary because there were three control loops controlling to set points within a range of 1kPa of each other. The second recommendation was to increase the range of these three control loops to operate over a 4kPa range. Another recommendation was to keep the vapour balancing line between the main tanks open at all times to balance the load over the two sides. Other recommendations included maintaining compressors' settings and operation.
The plant ran for a week with the above mentioned recommendations implemented and this slightly reduced the flaring. It was then decided that the flare valve be tuned along with the oscillating level controller on the intermediate flash drum. This in itself dramatically reduced the flaring, but the flaring was still not completely stopped. The intermediate flash drum pressure was then reduced and the flaring ceased (in normal operation). The result is that the plant can now run with one compressor shut down for twelve hours before flaring starts. Currently, more work is underway, in phase 2 of this project, to reduce other losses on the plant.
Annual flaring costs have reduced by R17.7 million. This Alliance project was a success due to the excellent collaboration and assistance from plant and instrumentation personnel, along with the strong AVEC and Sastech relationship.