Advanced process control (APC) can be a very powerful tool to solve problems not amenable to more basic techniques such as PID control. In the right situation, APC can deliver benefits that more than compensate for the time and expense required to implement and maintain these often complex solutions.
For example, Saudi Aramco's six gas plants use AspenTech's DMCplus APC software to improve unit stability, balance and optimize feed rate, and reduce steam and power usage. In the NGL Fracionator liquid recovery unit, improved quality, increased recovery rate and minimized feed disturbances resulted in a fuel gas savings of about $200,000 per year. Similar improvements were made in the propane recovery units and depropanizer units.
Results were even more impressive at a GALP Energia hydrodesulphurization plant in Portugal, where APC generates savings of about €1.2M ($1.54 million) per year. The AspenTech APC software in use there controls sulphur in the final product to below the spec limit, reduces hydrogen in the hydrotreater and hydrodesulphurisation units, and saves fuel gas in pre-heating furnaces.
"We offer core MPC technology for both linear and non-linear applications and products for developing inferential measurements," explains Robert Golightly, APC product marketing manager at AspenTech. "On top of the APC technology, we offer a range of optimization products, including composite technology supporting very large applications and full real-time optimization based on our modeling products, Aspen Hysys and Aspen Plus."
He adds that AspenTech can provide services in the form of augmentation to existing staff or turnkey services. Sometimes such help is needed, because APC requires specialized skills not always available within process plants or companies. Because of this lack of familiarity, many plants that could benefit from APC haven't done so, despite many success stories.
LG Chem's Daesan Ethylene plant in South Korea was plagued by disturbances, making quality and throughput control difficult. The plant had to deal with feed quality variations, de-coking activities, naptha tank switching every three days, furnace switching every day, and changing ambient conditions.
To solve the problem, LG Chem's engineers implemented Aspen APC software across all sections of the plant to stabilize plant operation, reduce losses, maximize throughput and minimize energy consumption. Aspen controller software was installed on the furnaces and the C2 splitter; another controller coordinated all the other controllers; and a simulator was used to provide inferential measures of composition and severity, and to determine the gains for the controllers.
The effort reduced losses by about 35% across all disciplines; throughput went up 2%; and energy consumption went down 1.5%.
AspenTech was one of the originators of APC software for general purposes in the process industries, so their vision of the future is worth noting. "Based on the changes we have seen over the last five years, my expectation is APC will become more accessible to non-experts," predicts Golightly. "The inclusion of knowledge, best practices and workflow automation within the products will enable APC deployments in organizations where it was previously cost- and/or resource-prohibitive to do so. In the near future, the traditional maintenance issues will be eliminated with built-in technology that automates much of the maintenance cycle and keeps controller models matched with plant performance, all without the need for the level of expertise required today."
For a process plant with a number of control loops, or just a few high value ones, that can't be closely held to setpoint using conventional control techniques, APC is worth investigating. Recent advances have reduced both the cost and complexity of implementation and maintenance, making APC a viable option in a wider range of applications.