Arthur G. Shaffer Jr.
Ashland Oil Inc.
Ashland, Ky.
Charles L. Hemier
UOP
Des Plaines, Ill.
The reduced crude conversion (RCC) process at Ashland Oil Inc.'s Catlettsburg, Ky., refinery has demonstrated throughout the past 7 years that it is a reliable and efficient means of converting residual fractions into transportation fuels.
The 40,000 b/sd capacity RCC unit has shown good feedstock flexibility, which results from the ability to control the overall unit heat balance.
As a result of continuing improvement, the unit today is able to process an even more contaminated feedstock than the feedstock for which it was originally designed.
When the RCC unit was first commissioned in 1983, it set a new standard for cracking facilities designed to process contaminated feedstocks. This unit, which was designed by a cooperative technical effort between Ashland Oil Inc. and UOP, produces higher-value transportation fuels from poorer-quality atmospheric residues and heavy oils.
Such a unit was a natural fit to the Ashland operation because the facilities produce a product slate emphasizing transportation fuels to satisfy a local demand.
The performance and proven history of the RCC operation has led to the installation and start-up recently of an RCC unit at Statoil's Mongstaad, Norway, refinery. A third unit for a refinery in Southeast Asia has entered the engineering phase.
ASHLAND'S CATLETTSBURG REFINERY
Ashland's refinery at Catlettsburg is a large, complex, inland refinery with atmospheric distillation capacity of 213,400 b/cd and vacuum distillation capacity of 92,000 b/sd. The primary crude supply for the refinery comes by pipeline from the Gulf Coast, and much of the crude is of offshore origin.
In some years, as many as 25 different crude oils have been processed. These crude oils are processed by three different crude distillation units, one of which is operated to optimize feed to a lube oil complex.
Additional conversion is provided by 2,600 b/sd of thermal cracking and a 55,000 b/sd atmospheric residual treating (ART) unit. The ART unit has not been operating for the past 2 years.
The refinery is also equipped with a 12,000 b/sd CS-C6 isomerization unit, 52,000 b/sd of reforming capacity that includes a 27,000 b/sd unit with continuous catalyst regeneration (CCR), a 60,000 b/sd conventional fluid catalytic cracking unit (FCCU), the 40,000 b/sd RCC unit, an HF alkylation unit, several feed and product hydrotreating units, and an extensive petrochemicals complex.
RCC UNIT DESIGN
The, need for flexibility in handling a frequently changing slate of crude oils with wide-ranging qualities while optimizing transportation fuel yields led to the construction of the RCC unit. Ashland management envisioned the unit as a means of expanding the yields of higher valued transportation products and cutting the costs of raw material by running less expensive grades of crude oil.
The RCC process catalytically cracks high-boiling feedstocks, such as atmospheric reduced crude oil, directly into more valuable transportation fuel components and light products. The contaminants contained in the crude oil present additional challenges when compared with more conventional gas oil cracking because a residual fraction is being processed.
The metals in the crude oil that deposit on the circulating catalyst lead to more difficult maintenance of catalyst activity, loss of selectivity, and increased gas production. Also, the high boiling, more aromatic nature of the feed results in a higher level of carbon deposition (delta coke) on the circulating catalyst.
This higher delta coke can lead to problems with the unit heat balance and unacceptably higher regenerator temperatures. Several new process features were required to cope with high coke make, to manage a higher heat release in the regenerator, and to minimize the impact of metals contamination of the catalyst.
A schematic diagram of the reactor-regenerator section of the RCC unit is shown in Fig. 1. This unit configuration was established to provide several unique zones that help contribute to the overall improved unit performance.
The first of these zones is a combined lift-gas and feed-distribution section at the base of the riser. In this zone, a light hydrocarbon gas is used to lift and accelerate the catalyst before the residual feed is injected into the dilute-flowing catalyst stream through a specially designed feed atomizer.
This patented section provides rapid, intimate catalyst and oil contact, which leads to an improved yield pattern and a partial passivation of the metallic contaminants that have deposited on the circulating catalyst. By helping to control the undesirable metal-catalyzed side reactions, the unit can be operated with high metals levels on the equilibrium catalyst and still provide a highly selective yield pattern.
Another patented feature, the vented riser, provides a reaction zone in which hydrocarbon and catalyst are efficiently contacted and then quickly separated at the termination of the riser. The outlet configuration also helps prevent coke formation within the reactor because no stagnant zones are present.
The Catlettsburg RCC unit design incorporated a two-stage regenerator. A catalyst cooler is located between the regeneration stages.
The arrangement achieved a low level of carbon on regenerated catalyst (CRC) and permitted a high level of carbon monoxide in the single flue gas stream leaving the unit. With the arrangement, catalyst is also protected from harmful high temperatures, thereby permitting the unit to operate at peak catalyst performance with lower levels of fresh catalyst consumption.
The combination of high-activity catalyst and the variable-duty catalyst cooler achieves flexible heat-balance control which results in optimum performance over a broad range of feed contaminant levels.
FEEDSTOCK FLEXIBILITY
Perhaps the most significant benefit of an RCC installation, particularly at an isolated location, or to a refiner with no captive crude oil supply, is the flexibility available for the processing of various feedstocks.
The options available for the processing of gas oils, vacuum bottoms, and reduced crudes enable the refiner to produce an optimum product mix from a more varied range of crude oil inputs. The advantages allow a refiner to produce a desired volume of light products from fewer barrels of crude oil, produce a given product slate from a lower-quality crude oil, or produce a product slate of higher value from the present crude oil input.
A refiner located in an area with fixed product demands is primarily interested in the first two categories. A refiner with an outlet for incremental product can benefit from all three categories, depending on market conditions.
Fig. 2 graphically illustrates how the addition of an RCC unit can improve the overall distribution of the refinery products.
During the past 7 years, the RCC unit at Catlettsburg has processed feedstocks with a wide range of qualities. Although these feedstocks have been predominantly atmospheric residues, the RCC unit has occasionally handled supplemental feed components, such as vacuum residue and lube oil extract.
For several years during the winter months, some of the more heavily metal-contaminated crude residues were treated in the ART unit and then introduced as one component of the RCC feed. This pretreatment for metals removal has not been practiced at all for more than 2 years.
As an indication of the range of feedstock characteristics that the RCC unit routinely processes, Table 1 shows the range of contaminants in the feedstock to the unit for the last year. These data reflect the fact that much of the Catlettsburg refinery, including the FCC unit but excluding the RCC unit, underwent a major scheduled turnaround during the period.
For that reason, the lower range of values listed for the feed metals and Ramsbottom carbon content include data for the period when the RCC unit was running gas oil in addition to the normal reduced crude charge. The average characteristics of the RCC unit charge stock during 1989 are similar to those of the full-range, untreated Arabian Light reduced crude for which the unit was designed.
Table 1 also shows that during 1989, the high level of nickel plus vanadium content of the feedstock had exceeded 65 ppm. As a result, the equilibrium metals level on catalyst had occasionally approached 10,000 ppm, with a range of 7,000 to 8,000 ppm being more typical.
During the year, because of the high metal content in the feed, catalyst makeup averaged 27.6 tons/day.
Equilibrium FCC catalyst makes up 10 tons/day of the catalyst addition. This catalyst is cascaded through the unit to act as a metals adsorbent.
One of the primary factors that determines the addition of catalyst to the unit is a target catalyst surface area, with the target value dependent on the particular catalyst type.
Another point of great interest to refiners is that the variations in feed quality cause very little change in product quality. With the ability to control feed temperature, coke burn, and catalyst-to-oil (C/O) ratios, the unit allows the optimization of operating conditions for a wide range of feedstocks.
Table 2 shows representative operations from two 1989 plant surveys.
Test A was obtained for a feedstock of relatively low API gravity and high Ramsbottom carbon and metals contents. Test B was produced from a lighter feed that had lower carbon residue but higher metals content.
Even for these contaminated feeds, high yields of gasoline and light products have been obtained.
RCC UNIT IMPROVEMENTS
Several significant changes have been made to the unit operation since its start-up in 1983. These changes have been due to an ongoing program between Ashland and UOP to further improve the RCC process at the commercial level.
One significant change to the unit involved the move from a combination steam-and-gas lift section at the bottom of the riser to a modified gas-only lift system.
This change reduced the quantity of sour water from the RCC unit that has to be treated, and also improved the unit product-yield distribution.
More significantly, the change had an additional passivating effect on the metals carried on the equilibrium catalyst. This passivating effect leads to lower gas production and lower catalyst addition rates.
A second change of significance was a second catalyst cooler added to the unit. This additional cooler was added to further control the C/O ratio at optimum levels when running higher carbon feedstocks without the ART unit on-stream.
The new cooler is of a backmix design, which was developed following simulation work by UOP, and plant testing using the existing flow-through cooler on the Catlettsburg RCC unit. Although the new design achieves a slightly lower overall heat-transfer coefficient, it was less expensive to install because it has no standpipes and slide valves.
The operational flexibility of the two coolers is evident from Fig. 3. This graph represents the total cooler duty for the period from May to September of 1989.
During that period, the catalyst cooler duty varied between 50 and 200 MMBTU/hr. This was in response to variations in feedstock quality caused by asphalt demand in the product slate.
The catalyst cooler was used during these periods to keep the unit running at optimum catalyst-to-oil ratios. Because of the success of the second cooler on the RCC unit at Catlettsburg, Ashland added a catalyst cooler to the FCCU at its Canton, Ohio, refinery to take advantage of the heat balance adjustment.
Several other refiners around the world have also installed catalyst coolers on a revamp basis.
Another significant operational change, that is related in part to the addition of the backmix catalyst cooler, is that all of the RCC charge, whether atmospheric or vacuum residue, is now routed directly to the unit without any pretreatment, even during the winter, nonasphalt season.
The ART unit has not been operated for 2 years, confirming that pretreatment is not required for successful RCC operation.
The decision not to operate the ART unit has a further effect on the RCC unit because these units share a common gas-recovery facility and flue-gas treating section.
Still another significant change that was implemented involves not hardware but software. When the unit was originally constructed, a distributed control system was installed for process control.
Since that time, Ashland has embarked on a major program called RCU (refinery control upgrade). One of the first areas of work was the RCC unit because the presence of an up-to-date distributed control system allowed the quick implementation of a computer-based advanced control system.
The fact that a comprehensive data-reporting system was already in place allowed a realistic analysis of unit operation before and after implementation of advanced controls. The system has been an unqualified success, resulting in smoother and more-efficient operation.
These changes have led to a unit with even more flexibility in operation and the ability to process a more-varied range of feedstocks.
RCC UNIT IS RELIABLE
The key to the successful operation of a refinery unit, and the result of the operating flexibility previously discussed, is unit reliability and on-stream efficiency. Without these characteristics, no process will be successful.
For the period from original unit start up in March 1983 to Jan. 1, 1990, the on-stream efficiency of the RCC unit has been greater than 94%.
This period included 73 days of downtime as a result of three scheduled turnarounds. Other than the normal turnarounds, the unit experienced unscheduled outages less than 3% of the time in the 7-year operating history of the unit, the first of its kind.
On-stream reliability has been the result of a good initial unit design, an efficient reactor-regenerator system, and improvements made during the operation of the unit. Fig. 4 presents a graphical look at the on-stream efficiency history for the RCC unit.
This figure shows that, as expected, the on-stream factor has been improved as operational experience with the unit has grown. One area in which the RCC has excelled is the low-coking tendency experienced in the unit.
Inspections during all the major turnarounds have revealed essentially no coke deposition in the reactor system or reactor vapor line. This freedom from coke deposition has been even better than that experienced on Ashland's conventional FCC units.
The lack of coking is a large contributor to the fine on-stream record of the unit because nuisance shutdowns attributable to coke pluggage and resulting pressure drop have been avoided. A side benefit of the RCC unit at Catlettsburg has been the improvement in feed quality to the previously existing FCC unit in the refinery.
Because the poorer quality feed is routed to the RCC unit, the FCC unit has benefitted from a higher-quality charge stock.
OTHER RCC OPERATIONS
In the fall of 1989, another RCC unit was commissioned at Statoil's complex at Mongstad, Norway. Like its Ashland counterpart, this 41,000 b/sd unit has a two-stage regenerator and a catalyst cooler to provide operating flexibility.
In the several months that it has been operating, the Statoil RCC unit has not used as wide a variety of feedstocks because the crudes processed have been predominantly from the North Sea.
With a carbon residue of about 4 wt % and a feed metal content of less than 10 ppm, the atmospheric residues from North Sea crudes are less contaminated than those charged at Ashland.
Because the North Sea residues are hydrogen rich, obtaining high gasoline and liquid yields from these stocks at moderate operating conditions is possible. A typical yield pattern from such operations is shown in Fig. 5.
The high liquid yield demonstrates that an RCC unit produces a selective yield pattern for good-quality feedstocks. When market conditions shifted, the unit adjusted to provide a more distillate-oriented yield pattern.
In addition, the unit has the capability of adjusting operating severity to provide an effective conversion of much more contaminated feeds.
A third RCC unit is in the engineering design phase. It will be installed in a large complex in Southeast Asia designed to process local crudes.
As for the existing RCC operations, the focus for the third unit will be to provide a high-severity, gasoline-oriented yield structure.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.
