According to Jason Akey, energy technologist for Marathon Petroleum Corp., crude units are "the biggest lever" for facility energy consumption.
Jason Akey, energy technologist for Marathon Petroleum Corp.
"It stands to reason that the most efficient refineries are usually the most profitable," he said.
Focusing on sustainability metrics for crude unit designs, Akey said that even though a complex refinery has many more downstream units, its overall energy demand based on crude-unit charge rate is not much higher than a simple refinery's rates.
"Larger integrated refineries generally have a lower intensity than small facilities, and the relative size of the steam system does affect the sustainability metrics," he said. "More steam demand generally means higher energy intensity and emissions. It is interesting to see that the crude and vacuum units consume a large portion of a site's fuel gas -- between 25 and 30 percent."
Crude charge heaters, in this case, can be the single largest fired unit at the site, occasionally rivaled by reformer unit combined-charge meters, Akey said.
"The point to be made is that crudeunit design and operation present the greatest impact, driving the refinery's energy consumption," Akey explained. "This isn't surprising, considering that all feedstock goes through this one unit."
According to Akey, energy utilities in the form of natural gas, electricity and purchased steam can be 30 to 45 percent of a site's operating expenses.
"With energy costs going up recently and a focus on reducing fixed costs through COVID-19 demand destruction, it's a fair statement that the most efficient and sustainable refineries should also be the most profitable," he added.
Over-flash optimization, steam and sustainability
Akey shared that there are several methods for managing hot-wash and over-flash rates to a level that will minimize energy use while maintaining wash-bed health throughout the turnaround cycle.
He defined "over-flash" as the amount of hydrocarbon liquid that exits the bottom of the wash-bed above the flash zone in a crude or vacuum tower. "Excessive wash rates will increase the over-flash rate, which will normally recycle back to the front end of the unit," he said. "This results in more fired heater duty and hydraulic load."
There are a number of crude and vacuum unit optimization strategies that can impact both profit and sustainability.
"Maximizing the recovery of light products from crude and vacuum towers can be assisted by changing partial pressures using live steam injection or light gas stripping," he said.
Atmospheric crude towers can use between four and 10 pounds of stripping steam per barrel of feed, Akey said, while side strippers generally use less to assist with achieving a product's minimum flashpoint target.
"Most sites now have reliable power and equipment fed from different substations, meaning there is less reliability incentive to have the steam turbine driver," Akey said. "Consider electrifying by retrofitting turbines over to high-efficiency motors, especially if they are condensing turbines, or when operation results in steam imbalances."
As more and more facilities transition from carbon-based thermal fuel to renewables, and power grids evolve toward cleaner energy sources, "the electrification of refinery equipment becomes an important sustainability strategy," Akey said.
Noting that new and innovative products are emerging from the process industries, Akey said medium-voltage electrical heaters can have reasonably low capital costs for small and mid-sized installations.
"Energy efficiency and profitability of crude units are influenced by design and utilization, as well as by the operation and maintenance of the preheat train and fired heaters," Akey concluded. "Consider how you should plan for the transition to clean energy."
