As a result, the company engaged Honeywell to provide engineering services to design and implement advanced process control (APC) on its two parallel urea units. The primary goal for installing APC in the plant was to minimize key variable variations and have constant reactor conversion, leading to energy savings and smooth process operations.
"We have two parallel urea units at our plants that had a high level of interaction, and large differences in dynamics between the front- and back-end variables in the plant," said V.K. Bhatia, unit head of TCL. "We wanted to achieve a reliable, safe and energy efficient operation that decoupled the units as needed but still optimized our plant."
Challenges for APC in the urea process included high levels of interaction between the variables and recycle streams from back end to front end; control of the CO2flow fluctuation; reduction in water cycles; and improving conversation and reduction in steam requirements downstream.
"We needed to achieve a reduction in our high pressure steam consumption by controlling the CO2header pressure and reducing the passive air in the system," Bhatia said. "We also wanted to provide safe and stable control of the compressors."
The scope of the APC project included CO2compressors; high- and medium-pressure sections; low-pressure and vacuum sections; and a process condensate treatment section. A complicated process, urea production involves the following steps:
- Urea synthesis and high-pressure recovery.
- Urea purification and recovery at medium and low pressure.
- Urea concentration in pre-vacuum and vacuum stages.
- Urea prilling.
- Wastewater treatment.
Traditional advanced process control software contains predictor, controller and optimizer modules. Though it is a proven technique, often the optimization of one controller affects the stability of the other controller, thereby affecting the overall plant optimization and stability. Disabling the optimizer module from all the APC controllers of a plant and having one unique optimizer solves the issues with a master controller meeting this purpose.
Honeywell employed several strategies to achieve this unique design. The Profit controllers were designed to see all constraints related to machine safety and to optimize the CO2header pressure. A second implemented strategy was to maintain passive air/oxygen percentage to its minimum limit by changing the air flow with changes in CO2flow, which led to a reduction in excessive passive air and vent openings in the downstream section.
"Our APC project not only met our expected benefits, but it also improved the dynamic control of both the front end and back end of the urea complex," said Bhatia.