This article was included in one of Hydrocarbon Processing’s three daily show newspapers at the NPRA Q&A and Technology Forum in San Antonio, Texas.
Hydrocracker high temperature excursions and runaway reactions can be avoided with the help of new advanced regulatory control strategies such as quench overrides, two officials said at the NPRA Q&A and Technology Forum on Tuesday.
Paul Robinson of Criterion Catalysts & Technologies said that preventing excursions requires proper maintenance and operations, process monitoring, process training, turnaround planning and quality hardware and software.
“Early detection and preemptive action can stop excursions before they become serious enough to require depressurizing,” Mr. Robinson said to an audience on plant automation. “Quench and heater overrides provide both.”
CITGO’s Dennis Zelmanski spoke about a September 2009 incident at his company’s Lake Charles facility. In that case, a loss of preheat caused cycling of temperatures.
From there, quench controls looked at bed inlets only. Cycling caused a low inlet temperature. In response, the quench flow decreased and heater firing increased.
In the end, it led to a flaring event, production disruption of three to four days, and a unit shutdown.
From that event, however, CITGO learned several lessons. Those included never adding heat in a hurry, slowing down the tuning on heater controls, limiting how quickly heater fuel gas can increase and limiting how quickly hydrogen quench can increase.
The company also said it learned to limit the allowed size of set-point and output changes. In addition, it wanted to protect against any bed temperature rate-of-change that exceeds 1 degree Fahrenheit per minute.
Now, quench overrides significantly help the company to meet those goals.
The quench override takes over by increasing the hydrogen quench when a bed outlet temperature or a bad temperature rate of change is too high, the company said. The system executes once per second.
On the whole, the quench overrides help the company with early detection of a temperature excursion, as well as initiate preemptive action and return to normal control - all without operator intervention.
They have been proven very effective, and operators seem to approve of their use, Mr. Zelmanski said.