Feed injection, riser-termination systems replaced in fast-track revamp

B.F. Dahlstrom, Kevin Ham, Myron E. Becker, Tom P. Hum
Consumers' Co-operative Refineries Ltd.
Regina, Sask.

Larry Lacijan, Tom Lorsbach
UOP
Des Plaines, Ill.

In 1995, Consumers' Co-operative Refineries Ltd. (CCRL) successfully revamped the 19,500 b/sd UOP fluid catalytic cracking (FCC) unit at its Regina, Sask., refinery.

The low-cost revamp included a new, elevated Optimix feed-injection system and the first commercial implementation of vortex separation system (VSS) riser termination. VSS is a novel application of high-containment, vortex separation technology for the rapid separation of catalyst and hydrocarbon products.

The project has markedly improved FCC performance and profitability.

This first of two articles outlines the objectives of the revamp and describes the technologies incorporated. The second article will discuss the project execution schedule and detail the economic and operational benefits realized as a result of the revamp.

History

CCRL is the refining division of Federated Co-operatives Ltd., a large, member-owned Canadian cooperative. CCRL operates a 50,000 b/d heavy oil refinery. The refinery-the world's first cooperative petroleum refinery-started up in 1935.

The original refinery capacity was 500 b/d. Over the years, the refinery has undergone numerous expansions and changes to meet the evolving needs of the cooperative retailing system.

The most recent major change involved converting the light oil refinery to a fully integrated, heavy oil refinery. This transformation comprised the construction of an adjacent, heavy oil upgrading facility.

The principal process units in the heavy oil upgrader include hydrogen production, atmospheric resid desulfurization, hydrocracking, distillate hydrotreating, and sulfur recovery. In 1996, the combined refinery and upgrader complex produces a variety of finished gasoline and diesel products, and synthetic crude.

Fig. 1 [20670 bytes] is a schematic of CCRL's heavy oil processing scheme.

Catalytic cracking was incorporated into the refinery in 1954, when a UOP stacked, fluid catalytic cracking unit (FCCU) was brought on-stream. The FCCU has remained an integral part of CCRL's refining operation. Today, the FCCU processes a mixture of hydrotreated vacuum gas oil and coker gas oil.

Modifications to the FCCU over the years have increased capacity from 4,500 to 19,500 b/d. The unit evolution has taken advantage of advancements in FCCU process and catalyst technology.

Changes included shifting from bed cracking to riser cracking, with the installation of a T-type disengager in 1976. CCRL upgraded the feed injection system from the simple bayonet feed pipe to the shower head nozzle in 1977, and to the wye Premix distributor in 1993.

Catalyst regeneration was changed from partial to complete-combustion mode in 1975. Regenerator air distribution was changed from a perforated plate distributor to a pipe grid in 1960, and then to a mushroom grid in 1990.

The regional economy, which is primarily agricultural, generates high demand for diesel fuel. The gasoline-to-diesel sales ratio is less than 1-to-1 when farming activity is at its highest in the spring and fall. In recent years, the growth rate of diesel sales has exceeded that of gasoline sales, and this trend is expected to continue.

This somewhat unusual market environment places unique demands on the refinery operation. The operating objectives of the FCCU have expanded to include maximizing not only gasoline but also distillate product.

The FCC distillate products-the sum of light and heavy cycle oil (LCO and HCO)-are hydroprocessed and become part of the diesel blending pool. The value of the distillate product is roughly equivalent to that of gasoline.

The proposed benefits of UOP's Optimix feed injectors and VSS riser termination device-namely, increased liquid volume yield of gasoline and distillate-were compatible with CCRL's objectives.

Revamp objectives

As is the case with most refinery revamp projects, the primary objective of CCRL's FCC revamp was to improve overall refinery profitability.

Typically, FCCUs are revamped either to eliminate bottlenecks that limit fresh feed throughput or to update the unit to incorporate technological features that improve selectivity to desired products. CCRL's FCCU was not capacity limited. The route CCRL chose to improve performance, therefore, was through technology enhancements that improve selectivity.

Specifically, the objectives of the revamp were to:

• Increase the combined gasoline and distillate product yields

• Improve coke selectivity (higher conversion per unit of coke burned)

• Reduce dry gas production.

Technology description

Two strong trends in FCC development in the last 10 years are improved feed distribution and more highly contained riser-termination systems. Both of these technology trends continue to move the FCC reaction zone in the direction of a plug-flow scheme.

Improved feed distribution and a novel, improved riser-termination system are the main features of the CCRL FCC revamp.

Both the elevated feed injection and the highly contained riser-termination technologies represent significant steps toward the ideal of a plug-flow system for catalyst and hydrocarbon contacting and reaction. These improvements accomplish rapid, uniform, initial contacting of catalyst and feed, followed by rapid, uniform, separation of catalyst and products at the end of the riser.

Feed Distribution

The FCC revamp includes four Optimix feed distributors oriented radially on the riser and elevated above the wye section. The lower riser between the wye piece and the feed distributors serves as a catalyst preacceleration zone. In this zone, the catalyst is accelerated to produce a stable, uniform, flowing phase of moderate density prior to feed injection.

Fig. 2 [20416 bytes] illustrates the catalyst preacceleration zone and the arrangement of the elevated radial Optimix feed distributors on the riser.

Development work and commercial testing by UOP have shown that uniform radial injection of well-dispersed oil into a stable, moderate-density phase of flowing catalyst increases the uniformity of oil/catalyst contacting, vaporization, and initiation of catalytic cracking reactions. The details of the development of the Optimix feed distributors and the utility of catalyst preacceleration were described in a paper by Schnaith, et al.¹

Fig. 3 [20136 bytes] shows the basic internal arrangement of the Optimix feed distributor. Fig. 4 [22447 bytes] shows the orientation of the distributor mounted radially on the riser.

The Optimix distributor comprises three stages of atomization.

Oil enters through an internal, axial oil pipe, and steam enters through an annular steam chamber surrounding the oil pipe. The first stage of atomization is the generation of an internal conical oil spray from the tip of the internal oil pipe.

The second stage of atomization is the shearing and internal mixing of the oil spray and steam exiting the steam annulus. The third stage of atomization is the dispersion of oil droplets in a continuous steam phase as the mixture is discharged through machined holes in the distributor cover plate.

The machined holes in the cover plate are oriented to generate finely dispersed oil droplets in a flat, fan-shaped spray pattern. This spray pattern of small, well-dispersed oil droplets penetrates the moderate-density phase of preaccelerated catalyst to provide uniform mixing, vaporization, and initiation of vapor-phase cracking.

Riser termination

Before the 1995 revamp, the FCCU was equipped with a T-style riser disengager. In the years preceding the revamp, CCRL had become interested in installing a more highly contained riser-termination system.

Options at that time were direct-connected cyclones (DCC), vented riser, and the vortex disengaging system (VDS), a design variant of the UOP vortex separation technology. None of these disengaging systems, however, would fit into the CCRL reactor without an extensive and costly vessel enlargement.

In 1993, UOP began developing a new concept for the highly contained separation of catalyst and hydrocarbon vapors. This novel, riser-termination device provided the additional benefit of being quite compact, thus allowing it to be retrofitted in most existing FCC reactors. This development yielded what is known as the VSS riser termination system.

An important feature of the VDS and the newer VSS riser-termination systems is that stripped hydrocarbon vapors and steam rising from the stripper can be directed up through the vortex chamber countercurrent to the flow of spent catalyst. This countercurrent contacting with stripper vapor provides rapid prestripping of the spent catalyst immediately following the initial disengagement of the catalyst and hydrocarbon vapors exiting the riser.

With spent catalyst prestripping, much of the entrained hydrocarbon product is displaced. The vapor entrained with the catalyst flow into the stripper, along with vapor from the cyclone dipleg, consist primarily of steam. In fact, the hydrocarbon containment within the VDS and VSS riser termination systems is 98 wt % or more. This high level of containment contributes to improved product selectivity.

By contrast, in DCC riser termination systems, the fluidization vapor entrained with the full flow of catalyst down the cyclone diplegs is almost entirely hydrocarbon product. The entrained hydrocarbon is typically on the order of 6 wt % of feed. This entrained product remains in the stripper and reactor for an extended period and there undergoes further nonselective reaction and concomitant degradation of product quality.

CCRL became the first commercial FCCU to use the VSS system. The operating principle of this riser-termination device is illustrated in Fig. 5 [20650 bytes].

Catalyst is discharged centrifugally in the horizontal plane, swirls downward along the wall of the enclosure and contacts the prestripping vapors before entering the stripper vessel.

Fig. 6 [26330 bytes] shows the configuration of the CCRL FCC reactor before and after the revamp. The existing reactor cyclones were modified and reused in the revamped VSS configuration.

Fig. 7 [25426 bytes] shows the plan view of the revamped FCC reactor containing the VSS riser disengager.

Reference

1. Schnaith, Mark W., Gilbert, Allyn T., Lomas, David A., and Myers, Daniel N., "Advances in FCC Reactor Technology," Paper AM-95-36, NPRA Annual Meeting, San Francisco, Mar. 19-21, 1995.

The Authors