New sulfur specs will challenge US pipeline system
US oil pipelines may have difficulty adjusting operations to the new federal 15 ppm sulfur rule for ultralow-sulfur diesel, according to two experts making presentations to a workshop on the subject.
Warren R. True
Chief Technology Editor—Pipelines/Gas Processing
HOUSTON, Nov. 21 -- US oil pipelines may have difficulty adjusting operations to the new federal 15 ppm sulfur rule for ultralow-sulfur diesel, according to two experts making presentations to a workshop on the subject in Houston.
On the first day of the 2-day workshop held by the US Environmental Protection Agency and jointly sponsored by several industry associations, Wes Neff, retired from but on consultancy for Marathon Ashland Pipe Line LLC, and Buster Brown of Colonial Pipeline Co., reported on pipeline tests on their systems in 2001 and 2002.
Neff said that MAPL's distribution system has no experience handling an ultralow-sulfur specification product in a batching system. At 15 ppm sulfur, he said, ULSD would be the lowest-sulfur product in the system.
Assuming that the specification in pipelines will need to be about 10 ppm and comparing it with a heating oil specification of 5,000 ppm, he said the product specification ratio would be 500:1. "The system has only dealt with sulfur specifications in the 10:1 range," he said.
The example he gave was that a 0.1% contamination—10 bbl in 10,000 bbl—of 5,000 ppm sulfur heating oil into ULSD would raise the sulfur content in the ULSD by 5 ppm.
The MAPL tests revealed, said Neff, how easily ULSD moving through a distribution system can be contaminated, not only from residual but small amounts of higher-sulfur product isolated in a refinery's piping system but also from the higher-sulfur head end of the batch.
Once contamination occurs from the head end, however, MAPL's tests showed, it does not trail back further into the main batch of ULSD.
In the end, however, Neff said the MAPL tests suggest that pipelines should be able to transport ULSD and "maintain the sulfur specifications."
"We saw that gravity changes corresponded closely to the sulfur changes and that protective gravity cuts might be acceptable," he said. The loss to interface in MAPL's tests should be about the same as its current critical cuts for gasoline and distillates.
The tests revealed no migration of sulfur at the head or the tail ends of the batch interfaces beyond the normal gravity-interface zone, no sulfur degradation due to the pipeline and its associated facilities, and no sulfur degradation at the delivery piping of the receiving terminal.
"Keep in mind, however, that our tests were done from refinery tank to a terminal tank (at Zachary, La.) with no stops in between," he said. "We did our tests under ideal conditions."
Neff emphasized the need for inline testing equipment to find any contamination that happens during transit and to help define cut points; "there just isn't any test equipment. . .available to do this," he said.
Neff concluded that the pipeline ULSD specification might need to vary batch by batch, depending on the origin and the destination. Complex pipeline systems involving several breakout tanks and multiple pipeline segments "could result in several ppm sulfur degradation."
'It's the preseason'
Colonial Pipeline's experiences were less sanguine. "Our results were bad," said Brown, beginning his presentation.
Colonial moved two sample batches of ULSD on its 36-in. line from Baton Rouge to Greensboro, NC, with sampling at Atlanta Junction in Georgia.
Movement 1 did not show sulfur levels leveling out until long after gravity change, said Brown. "Our initial results indicated sulfur and gravity results were not tracking as well as" those from the MAPL tests, he said. "We saw sulfur changes before we saw changes in gravity," and Brown admitted Colonial could not explain why.
The other observation from Movement 1 was that sulfur levels between Louisiana and North Carolina rose significantly and inexplicably.
In short, said Brown, "We were not successful at delivering that product into our intermediate tank at an acceptable level."
But, he said, the Movement 1 tests on the Colonial system were like the preseason in US professional football: "How we do in preseason is not necessarily how we're going to perform in the regular season—at least, that's what we hope," he said.
A more recent test, however, which he called "Movement 2," suggested that sulfur levels and gravity are more closely related.
When "we reached 100% gravity, we also were very close to the correct sulfur levels, whereas with the (Movement 1) test, sulfur levels were changing well before gravity changes," said Brown.
He also said the batch in Movement 2 did not show the sharp rise in sulfur levels between lifting into the system in Louisiana and delivery at Greensboro, but sulfur levels still rose.
On both movements of ULSD test batches, Colonial saw the volume of interface—the total interface from first change to 100%—to be "quite large, twice what we would expect from a typical movement.
"We're very concerned about the amount of interface (and are) trying to understand that."
One possible explanation, said Brown, lies in the differing make ups of the two product batches: Movement 1 was wrapped completely in high-sulfur product: aviation kerosine on one side and high-sulfur fuel on the other; Movement 2, on the other hand, was completely wrapped in 500 ppm product, Colonial's current low-sulfur diesel.
"Even though they were wrapped in different products," said Brown, "we still saw roughly the same interface size."
Brown said Colonial's tests reveal that the "real issues" center on movements into and out of tankage. "The greatest risk and greatest possibility of contamination (occur) any time you go in and out of tankage," he said.
"Overdisplacement into tankage is the only way we now see to protect the product." But, he said, "it's more art than science" with several operational parameters dictating the size of displacement.
Both of Colonial's tests led to the same conclusion: The sulfur level at lifting must be lower because handling of the product along the system will, for whatever reason, elevate sulfur levels at delivery.