November 1, 2021 [Bicmagazine] – If you ask Joshua Santrock, crude/vacuum technologist for Marathon Petroleum Corp., whether crude oil tanks are still considered a viable location to dewater crude charging to a crude unit, expect a direct answer..
“The answer is definitely ‘yes,’ in my opinion,” Santrock said. “The water, in and of itself, is not harmful to the crude unit. In fact, 1-2 percent of clean water is typically injected on the front end in a well-designed unit to limit the dehydration of salts in the crude train.”
However, that water takes up capacity and tends to drag surfactants, brine, sediments, sludge and transients with it, Santrock said.
“Over time, getting flooded with water or getting too much water can also overwhelm systems on your desalters, depending on your hydraulics, so that’s why it’s still viable,” he said. “This is going to be driven based largely on the refinery and configuration.”
Crude capacity, in this case, is the “No. 1 driver,” Santrock said.
“If you have a lot of tankage and excess crude capacity that many refiners don’t have, the best system would be to take on a receipt, block it in and still the tank for 8-12 hours. Then you can drain the water to actually give it some settling time.”
Santrock said he believed the best system he’s ever experienced was one in which the storage was able to be still for eight hours.
“Then you turn on paddle mixers and actually stir the tank, and get it well stirred prior to charging the unit,” he said. “It is not typical that a refinery would have that much capacity, but I have seen it. Some folks probably have that capability now.”
Other systems, Santrock explained, entail offloading or discharging while crews are taking receipt “so you don’t even have enough storage to still the tank.”
In those particular situations, he said, “Many times you’ll still be getting enough water to form an aqueous base so you can drain the tank.”
Santrock said that while he encourages that method, the best thing he’s found to utilize valve mixtures for a system that’s on a running gauge while discharging and taking on receipts is to keep the tank continuously stirred in order to eliminate layering.
“Get it as uniform as you can in the unit,” he said. “Also, employ flooding suctions and things like that so if you do start to build some water up, you have a little bit of time to gauge and drain the water.”
Another good thing to consider, especially if there is sufficient time to still the tank, Santrock said, is pre-treating with emulsifier.
“That can help quite a bit, as well,” he said. “If you’re able to get that in the tank upstream while there are receipts coming in, that could help quite a bit when it comes to getting excess water down to the crude unit feed.”
Coke and shock coat formation
Santrock also suggested that technicians monitor basic sediments and water in the feed “so you can turn it every time and figure out which receipts coming in are high in brine and sediment.”
Jason Nigg, director of crude, fractionation and heat transfer for Phillips 66, said mixing in the crude tank helps to manage the peak of solids.
“It makes it more uniform and there’s less layering, but if you’re getting hit with a lot of solvents, there’s not a lot you can do except figure out where it’s coming from,” Nigg said. “If you’re more uniform and getting hit with a certain crude coming in that’s loaded with solvents and it’s impacting coalescence in the desalter such that you’re building an emulsion, it’s going to help because you’re getting a uniform feed.”
Speaking about the increasing occurrence of rapid fouling in the coker furnace after starting to process high percentages of light tight oil, Tracie Gurtler, process engineer for HollyFrontier Corp., said higher concentrations can lead to the coker feed being inherently unstable, which will produce shock coat formation.
“We’ve had quite a lot of experience with this, and we’ve set limits on the ratios of our crude plants to be able to manage it, as well as other issues it can cause in the crude unit,” she said.
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