Conoco Ltd. is using a flow improver to debottleneck part of the operations at its Humber refinery in Humberside, England, Alan Green, principal process engineer, and Brian Lever, division manager for the refinery's operations department report.
They and technical service representatives with Conoco Specialty Products Inc. recently completed a case study of the effects of using CDR flow improver at the Humber refinery.
Flow improver injection allows the refinery to increase overall capacity at minimal cost. The unit now generates additional revenues greater than five to ten times the cost of the CDR drag-reducing additive, depending on utilization rates, the operations department reports.
The chemical flow-improver solution to problems of this type is often overlooked, even though CDR has been successfully used in liquid hydrocarbon pipelines and piping systems since 1979.
REBOILER BOTTLENECK
The Humber refinery's extensive bottom-of-the-barrel upgrading capacity makes it one of Europe's most advanced conversion refineries. Its conversion units comprise a needle-grade coker, an anode-grade coker, a thermal cracker, and a fluid catalytic cracker. The Humber reboiler bottoms are often pumped to storage tanks located as far away as a mile.
The distillation tower separates cracked distillate from coking operations into naphtha, kerosine, and diesel. The diesel is pumped through a 6-in. pipeline from the tower bottom to storage tanks located approximately 5,000 ft from the unit.
The insufficient ability to pump diesel product away from the tower while maintaining a suitable level in the tower bottom limits the feed rate to the tower. It was thought that injecting CDR in the 6-in. rundown line would increase pumping capacity to storage, thereby allowing the tower, and ultimately the entire refinery, to operate at increased capacity.
The accompanying figure shows the arrangement of the refinery. Diesel, or column bottoms, is pumped through a fired reboiler with the net product routed to storage through the column level control valve and product coolers.
Upstream processing equipment had available capacity to handle increased distillate feed. Conoco considered three alternatives to increase the tower's capacity:
- Replace the column bottoms pump
- Install a larger rundown line
- Inject CDR flow improver.
Because the flow improver offered a quick solution with minimal capital investment, refinery management decided to proceed with an injection trial.
However, because the additive is most often used in long, relatively straight pipelines much different from those found in refineries, several technical issues had to be addressed to determine if the drag-reducing product was a viable alternative.
DRAG REDUCERS
A drag reducer lowers turbulence so that less energy is dissipated in moving liquids through a line.
Conoco claims that, when CDR flow improver is injected into a line in ppm quantities, the frictional pressure drop that occurs in moving the liquid is reduced by up to 60%, depending upon the hydraulic characteristics of the individual line and system.
By changing the liquid's hydraulic characteristics, a drag reducer either improves flow with minimal additional energy, or helps move the same amount of liquid using less energy. This results in improved system economics.
Humber engineers determined several desirable characteristics of a flow improver for use in crude oil and refined petroleum products. Among these are its solubility and shear stability.
In refinery lines, CDR solubility would be enhanced because of the typically higher-than-usual product temperature. Since higher temperatures decrease viscosity, and decreased viscosity enhances the performance of CDR, refinery conditions would thus enhance the product's drag-reducing effects.
The shearing effects of pipelines on CDR performance are difficult to determine on a general basis. It would normally be expected that the sharp bends, elbows, and line restrictions in many refinery lines would tend to cause shearing, and thus decrease the effectiveness of CDR. But the trial proved otherwise, Conoco reports.
INCREASE CONFIRMED
Injecting the flow improver gave an immediate flow increase down the diesel line.
Prior to the trial, diesel product rate was limited to about 23,000 b/sd. Injecting small quantities of CDR enabled the diesel flow rate to increase to 28,000 b/sd, or more. In other words, an increase in diesel flow of more than 20% was attained.
The product was injected into the diesel line using a compact, skid-mounted pump and a proprietary injection nozzle. Diesel product temperatures ranged from 35 to 76 C. (63 to 137 F.).
Before reaching the storage tank, the flow improver withstood the shearing effects of 75 elbows and tees, plus an air-cooled heat exchanger.
ECONOMIC BENEFITS
Removing the diesel rundown bottleneck immediately increased cracked distillate processing, allowing an increase in refinery throughput. Incremental revenues from this higher throughput are worth approximately $1 million/year, while the annual cost of the flow improver ranges from $100,000 to $200,000, depending on utilization rates.
To attain these benefits, typically CDR was used 30-60% of the time.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
