Alvaro A. Murcia
Stone & Webster Engineering Corp.
Houston
From the start-up of the first commercial fluid catalytic cracking unit-Standard Oil Development Co.'s (SOD) Model I-to today's designs, the original upflow configuration has continuously evolved.
This evolution was initiated by the technological developments required to meet the demand for high-octane motor gasoline and alkylate feedstocks created by World War II and by U.S. Government-mandated technology sharing. Postwar, the continuance of this evolution was spurred by commercial licensing activities of the original Catalytic Research Associates (CRA) consortium members.
This article chronologically summarizes the evolution of FCC commercial designs during their 50 years of continuous productivity, beginning with SOD's original Model I, whose background and start-up is described in a related article in this section.
THE 1940S
SOD's Model I unit comprised multiple small vessels, characterized by catalyst upflow through the reactor and regenerator vessels in a riser-type flow regime (Figs. 3 and 4, p. 52 and 53). In this design, prevaporized feed was sent directly from the vaporizer furnace to the reactor.
The unavailability of centrifugal fans during World War II influenced the design of this unit. This resulted in a very low operating pressure regenerator with high-elevation external multicyclone separation systems and collecting hoppers to obtain a fluidized catalyst downflow to the reactor vessels. Within months of start-up of Model I, the basic design and configuration concepts had already evolved such that, in late 1942, SOD started Model II, also at Baton Rouge.
This new unit included larger vessels with internal multicyclones, and was characterized by catalyst downflow and greater carbon burning capacity. This allowed larger catalyst circulation rates at higher conversion levels.
With a heat balance plentiful in energy, it was possible to inject liquid feed and eliminate the feed prevaporizer. Fig. 1 shows the general configuration of this design.
The year 1945 brought improvements in the utilization of air compression equipment. This resulted in higher operating pressures in the regenerator. SOD introduced its Model III unit with lower regenerator elevation and a simplified structure. The new unit also improved spent catalyst regeneration (Fig. 2).
Also in 1945, UOP introduced its side-by-side unit, with internal cyclones in both vessels and simplified catalyst flow patterns, as shown in Fig. 3.
In 1947, UOP modified its concept of pressure utilization with the stacked configuration, where air compression was used to obtain smaller and better regenerators.
In this design, the regenerated catalyst is lifted to a bed cracking reactor by catalyst vaporized feed at the bottom of the lift line. The spent catalyst flows by gravity into the regenerator. This design introduced the catalyst stripping concept. This new configuration is shown in Fig. 4.
THE 1950S
In 1951, M.W. Kellogg Co. introduced the Orthoflow (Ortho-top, or flow upward) concept in a bed cracking design with a low elevation regenerator and a high reactor with an internal stripper. The catalyst flow was through internal vertical straight tubes, stand pipe, and lift line, controlled by their previously developed plug valves.
This configuration was called Orthoflow A, and is shown in Fig. 5.
In 1932, SOD achieved many technological advances in areas related to the FCC process, and included them in its Model IV unit. The results were smaller vessels arranged side by side, operating at higher pressure and internal velocities, all built on low elevation structures.1
Catalyst circulation was simplified by elimination of valve-controlled flow with the application of the U-bend concept. Catalyst flow control was done by changes in differential pressure between reactor and regenerator and by changes in flow rate of auxiliary air in the spent catalyst entrance to the regenerator.
A typical configuration of this unit is shown in Fig. 6. 2
As early as 1956, Shell Oil Co. had already contributed to advances in FCC development in several ways, from catalyst development to unit configuration, such as the one shown in Fig. 7. This unit included a third vessel assigned to steam stripping the spent catalyst.
This configuration dates from 1957, at which time Shell came out with the invention and early development of riser cracking.
In 1953, Kellogg had introduced its Orthoflow B configuration, in which the positions of the reactor and the regenerator were switched. This resulted in a regenerator vessel elevated above the reactor, with the spent catalyst stripper contained inside the reactor, where full bubbling bed cracking takes place.
This configuration was adopted to reduce utilities consumption. Catalyst circulation continued to be through internal vertically straight lift line and standpipes, all regulated by plug valves (Fig. 8).
In early 1960, with the improvements in catalyst technology, the need for riser cracking was definitely established. M.W. Kellogg, continuing with its Orthoflow configuration, returned to the original vessel arrangement and introduced the Orthoflow C unit. This unit had two risers, fresh feed and recycle, and one standpipe; plus all internal, straight vertical tubes, with catalyst flow regulated by plug valves. The reactor also had the flexibility of using bed cracking.
THE 1960S
In the early 1960s, one of the Orthoflow C unit; was the first FCCU to process atmospheric tower bottoms, and became the first heavy oil cracking (HOC) unit. This unit had adequate steam generating coils installed at the periphery of the regenerator bed to act as a catalyst cooler. It was the result of the combined efforts of Phillips Petroleum Co. and Kellogg, and was installed at a Borger, Tex., refinery. Fig. 9 shows the general design of this landmark development.
Also in the early 1960s, UOP introduced its riser cracking configuration, also designed to handle the new landmark advancement of this technology, the zeolitic catalyst. This design has a side-by-side vessel arrangement with a reactor-stripper vessel at a higher elevation than the regenerator, to provide room for the new riser. The spent catalyst flows by gravity from a baffled spent catalyst stripper around the upper riser. This unit arrangement is shown in Fig. 10.
In 1967, Texaco Inc. introduced its riser cracking design, which included two bent risers-one for gas oil and the other for recycle-in a side-by-side arrangement. It also had the flexibility for bed cracking, if required. This configuration is shown in Fig. 11.
THE 1970S
Later, in 1971, Gulf Oil, after a thorough review of the available FCC technology, developed its riser FCCU based on a side-by-side arrangement with straight external vertical riser and feed injection at two levels. Regeneration was countercurrent.
This development included a baffled spent catalyst stripper and riser reactor design consistent with Gulf's proprietary technology and correlations (Fig. 12).
The beginning of the 1970s also included advances in the quality of FCC catalysts. These resulted in regeneration improvements that reduced the carbon on regenerated catalyst to
The riser included two new features: a multiple-nozzle feed injection system and inertia separation at its discharge inside the disengaging vessel. This design is shown in Fig. 13.
In 1978, UOP introduced its high-efficiency regeneration unit. This design included a regeneration system designed to obtain fast regeneration in a dilute-phase environment using a small diameter vessel-combustor discharging into a disengaging chamber.
The unit has a typical side-by-side configuration, with riser cracking. Vessels are designed for the relatively high superficial velocities consistent with a small catalyst inventory. This design is shown in Fig. 14.
By 1979, Exxon had already developed its Flexicracking unit using a side-by-side configuration, with an elevated disengager-stripper vessel and a lower-elevation regenerator. This unit's reactor is a straight external vertical riser ending in a proprietary separation system (Fig. 15).
The year 1979 also brought the Ultra-Orthoflow unit, which was the result of combined technologies-Kellogg's Orthoflow design and Amoco Corp.'s UltraCat regeneration.
The resulting unit included a stacked arrangement with an external vertical riser ending in roughcut cyclones, an internal baffled stripper, and catalyst flow regulated by plug valves. The regenerator was dimensioned consistent with Ultracat technology (Fig. 16).
THE 1980S
The 1980s started with increased interest in processing residual feedstocks. By 1981, Total Petroleum USA had already developed its residue FCC unit for its refinery at Arkansas City, Kan. This unit is a two-stage regenerator in two separate stacked vessels and an adjacent disengaging vessel.
The unit includes an external straight vertical riser reactor with proprietary feed injection system and an internal exit separation system. The second-stage regenerator uses external cyclones. Fig. 17 shows this general configuration. Also in 1981, Kellogg introduced the modified configuration for its HOC unit. This unit is characterized by an external straight vertical riser with cross-over lateral entrance and roughcut cyclone ending in the disengager. The regenerator is provided with internal coils and external steam generation catalyst coolers. The unit kept the Orthoflow vessel arrangement and plug valve catalyst flow control (Fig. 18).
By the mid-1980s, UOP had already developed its residue processing unit. This unit uses a side-by-side configuration with straight vertical riser entering the baffled stripper-disengaging vessel through the bottom center.
The regenerator includes two stages of regeneration stack-arranged in a common vessel with external catalyst coolers and catalyst recycle line. The riser reactor characteristics include the application of diluents at the bottom and close-vented roughcut cyclone at the end (Fig. 19).
REFERENCES
1. Luckenback, E.C., and Worley, A.C., et al., Encyclopedia of Chemical Processing and Design, McKeeta, J.J., and Cunningham, W.A., eds., Chapter 13, Marcel Dekker Inc., 1981.
2. Saxton, A. L., Worley, A.C., Modem Catalytic Cracking Unit Design Technology, Exxon Research & Engineering Co., 1970.
3. Avidan, A.A., Edwards, M., Owen, H., "Innovative improvements highlight FCC's past and future," OGJ, Jan. 8, 1990, p. 32.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
