HMC Polymers' propane dehydrogenation (PDH) plant is located on the eastern seaboard of Thailand in Rayong, a city 200 km southeast of Bangkok. The plant began construction in 2007 and started commercial operation early in 2011. "It produces propylene from propane through a licensed dehydrogenation technology, which enables the production of high-quality polymer-grade propylene," said Anucha Pudkhunthod, senior APC/DCS engineer, HMC Polymers, in his presentation Tuesday at the Emerson Global User Group Exchange 2014 in Orlando, Florida. The session will be presented again today at 2:15 p.m. in Osceola 2.
The PDH process is licensed from UOP, and UOP's performance guarantee requires 250-ms response times on many control loops to reduce process variation and allow the process to run closer to optimum. "They also operate some of the process at very high temperatures, close to burning the product.," said Jonas Berge, director, applied technology, Emerson Process Management and session co-presenter. "You can put the controller on 250 ms, but that doesn't include the input and output. UOP wants 250 ms end-to-end, which is hard to achieve with 4-20 mA."
To achieve the required rapid response times, the PDH plant uses FOUNDATION fieldbus with control in the field. "Control sits in the valve actuator," said Berge. "You might think you can only do that for simple loops, but this is a complex process, with 1,400 devices also being monitored for health, calibration and configuration."
With FOUNDATION fieldbus, you can do control in the controller the conventional way, but you can also put PID in the transmitter or valve positioner. DeltaV version 9 also allows control on the H1 card. "The beauty is it's time-synchronized from the transmitter to the controller to the valve," Berge said. "This is the feature used to achieve control in the field for complex loops."
The approach works for simple loops with one PID, cascade loops with two PID, and complex loops with multiple PIDs, subtraction, selectors, etc. At HMC, it is possible to execute entire loops in less than 150 ms. "It looks complicated, but don't worry about it," said Berge. "DeltaV takes care of it for you."
In a cascade loop, a secondary loop with fast response, such as flow or pressure, can be used to speed up a slow primary loop, such as level or temperature. The slow loop can be executed in a shared controller, while the fast loop uses synchronized execution in the field. Here, "It's important to put fast-loop devices on the same fieldbus segment," Berge said, "but slow, primary loop devices do not necessarily have to be on the same segment."
As an example of a complex loop, consider a fired heater controlling at 650 °C with multiple inputs, multiple PID, arithmetic, etc. "We implement this in an H1 card, single controller with all the devices on the same bus segment," Berge said. "We usually allocate 30 ms for link time, but here we used 15 ms—H1 can do that, no problem." Doing control in the H1 card minimizes the number of blocks across the bus, and they execute faster. "Same bus, same junction box for all the transmitters in the loop," he said. "It's not too complex. Just do it in the design."
It's also important to make sure all the blocks are executed in the logical order. "It's faster, and H1 won't accept it any other way," Berge said. On the fired heater, "Eleven function blocks, four transmitters and a partridge in a pear tree is executed in less than 150 ms, every time, time after time."
At HMC, "Some loops have 4-20 mA, and that's no problem. You can mix and match on DeltaV," Berge said. "But if we did the plant over again today, I think it would be all FOUNDATION fieldbus."
The resulting "digital ecosystem" provides other significant benefits. "We have diagnostics on the devices," said Pudkhunthod. "If there's some fault in a transmitter, we can inform a maintenance guy and get it fixed before the plant shuts down." There's also significant time savings in calibration. The PDH plant requires only two maintenance personnel, while the analog PP plant requires five, he said. "We estimate our savings so far at about $5 million."