The UOP* RCD Unionfining* reduced-crude desulfurization process represents the merg-er of three of the world’s leaders in residual oil processing and catalyst technology. UOP’sacquisition of the Unocal PTL Division in January 1995 resulted in the merging of UOP andUnocal’s catalyst technology, commercial know-how, and design experience to create anew, improved residual hydrotreating process. Prior to this acquisition in 1993, UOPentered into an alliance with Catalyst & Chemicals Ind. Co. Ltd. (CCIC) in Japan thatenabled UOP to offer CCIC’s commercially proven portfolio of residual hydrotreating cat-alysts. In addition UOP has catalysts available from other leading catalyst manufacturers.
The RCD Unionfining process provides desulfurization, denitrification, and demetalliza-tion of reduced crude, vacuum-tower bottoms, or deasphalted oil (DAO). Contaminantremoval is accompanied by partial conversion of nondistillables. The process employs a fixedbed of catalyst, operates at moderately high pressure, consumes hydrogen, and is capable ofgreater than 90 percent removal of sulfur and metals. In addition to its role of providing low-sulfur fuel oil, the process is frequently used to improve feedstocks for downstream conver-sion units, such as cokers, fluid catalytic crackers (FCCs), and hydrocrackers.
MARKET DRIVERS FOR RCD UNIONFINING
The first commercial reduced-crude desulfurization unit, which came on-stream in 1967,was a licensed design from UOP. The residual hydrotreating units produced a low-sulfurfuel oil product that was in increasing demand as a result of the stringent laws relating toair pollution that were being enacted in the industrialized countries. Units were designedin the 1970s to produce fuel oil with a sulfur level as low as 0.3 wt %. The trend towardlow-sulfur fuel oil has now extended around the world.
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Other drivers have added to the need for the RCD Unionfining process. Fuel oildemand has been declining at a rate of about 0.2 percent per year since the 1980s, and thisdecline has been coupled with a growth of about 1.4 percent in the demand for refinedproducts. As the demand for heavy fuel oil has fallen, the price differential between lightand heavy crude oil has increased. This price differential has given the refiner an econom-ic incentive to process heavy crude. However, heavy crude not only produces a dispropor-tionate share of residual fuel, but also is usually high in sulfur content. Because heavy,high-sulfur crude is a growing portion of the worldwide crude oil reserves, refiners look-ing for future flexibility have an incentive to install substantial conversion and desulfur-ization capacity to produce the required product slate. In addition to providing low-sulfurfuel oil, the RCD Unionfining process provides excellent feedstock for downstream con-version processes producing more valuable transportation fuels.
CATALYST
Catalysts having special surface properties are required to provide the necessary activityand stability to cope with reduced-crude components. The cycle life of the catalyst used inthe RCD Unionfining process is generally set by one of three mechanisms:
Excessive buildup of impurities, such as scale or coke, that leads to unacceptable pres-sure drop in the reactor
Coke formation from the decomposition and condensation of heavy asphaltic mole-cules
Metal deposition in catalyst pores from the hydrocracking of organometallic com-pounds in the feed
UOP provides a complete portfolio of catalysts to handle each of these three mechanisms(Table 8.4.1).
Feed filtration for removing scale particulates is a standard part of the RCDUnionfining design. In most cases, this filtration satisfactorily prevents buildup of scale onthe catalyst bed, and the resulting pressure drop does not limit cycle life. However, somefeeds contain unfilterable components. Under these circumstances, the catalyst bed itselfacts as a filter, and the impurities build up in the top section of catalyst to create unac-ceptable pressure drop. The CDS-NP series of catalyst helps prevent this problem byincreasing the amount of void space in the top of the reactor for the impurities to collect.The CDS-NP catalysts are designed as a macaroni shape in 1/4-in and 1/6-in sizes. Themacaroni shape maximizes the amount of void space and catalyst surface area available fordeposition of the impurities. The catalysts are also loaded from the larger to smaller size.This kind of loading, which is called grading the catalyst bed, helps to maximize the voidspace available for deposition.
Coke formation reduces catalyst effectiveness by decreasing the activity of the reactivesurface and decreasing the catalyst-pore volume needed for metals accumulation. For agiven catalyst and chargestock operating under steady-state conditions, the amount of cokeon the catalyst is a function of temperature and pressure. Successful hydrodesulfurizationof reduced crudes requires that temperatures and pressures be selected to limit coke for-mation. When coke formation is limited, ultimate catalyst life is determined by the rate ofmetals removal from organometallic compounds in reduced crude and by the catalyst-porevolume available for the accumulation of metals.
The deposition rate of metals from organometallic compounds correlates with the lev-el of desulfurization for a given catalyst. Thus, the rate of catalyst deactivation by metals
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UOP RCD UNIONFINING PROCESS 8.45
deposition increases with the increasing level of desulfurization. Metals deposition resultsfrom the conversion of sulfur-bearing asphaltenes. Conversion exposes catalyst pores tothe portion of the feed that is highest in organometallics.
The catalyst deactivation rate is also a function of feedstock properties. Heavierreduced crudes with high viscosities, molecular weights, and asphaltene contents tend tobe more susceptible to coke formation. Hence, higher pressures and lower space velocitiesare required for processing these materials. Correlations developed on the basis of com-mercial data and pilot-plant evaluation of many different reduced crudes can predict therelationship between the hydrodesulfurization reaction rate and the deactivation rate andreduced-crude properties.
The UOP-CCIC catalyst portfolio has been developed to maximize both the removal ofsulfur, metals, and other impurities such as nitrogen and Conradson carbon and the life ofthe catalyst. The CDS-DM series catalysts are typically loaded downstream of the CDS-NP catalysts and are designed for maximum metal-holding capacity. Although their met-als-removal activity is high, they maximize the removal of the resin-phase metals andminimize the removal of asphaltene-phase metals. (For an explanation of the terms resinphase and asphaltene phase, see the following “Process Chemistry” section.) The removalof asphaltene-phase metals can lead to excessive formation of coke precursors, whichultimately reduce the life of downstream catalysts. The CDS-R9 series catalysts are typi-cally located downstream of the CDS-DM catalysts. These transition catalysts have inter-mediate activity for demetallization and desulfurization and are used to gradually movefrom maximum demetallization to maximum desulfurization. Once again, the gradual tran-sition helps to minimize the formation of coke precursors, which could lead to shortenedcatalyst life. The final catalysts are the CDS-R25/R55 series, which have maximum desul-furization activity. By the time the residual oil reaches this series of catalysts, the metalslevel is sufficiently low to prevent metals deactivation.
In addition to this portfolio of conventional desulfurization and demetallization catalysts,several custom catalysts are available for the RCD Unionfining process. The R-HAC1 cata-lyst is a residual, mild-hydrocracking catalyst intended for use with lighter feedstocks.Although it has the same hydrodesulfurization activity as conventional HDS catalysts, it pro-duces 3 to 4 vol % more diesel fuel without an increase in naphtha or gas yields. The CAT-X catalyst is designed as an FCC feed pretreatment catalyst. The FCC microactivity testing(MAT) of feeds processed over the CAT-X catalyst has shown an increase in the gasolineyield of as much as 5 percent. Typical catalyst loadings are shown in Fig. 8.4.1.
TABLE 8.4.1 CCIC Catalyst Portfolio
Catalyst name Application Size and shape
CDS-NP1 �P relaxation and HDM 1/4 in shapedCDS-NP5
CDS-NPS1 �P relaxation and HDM 1/6 in shapedCDS-NPS5CDS-DM1 HDMCDS-DM5 HDMCDS-R95 HDM/HDS 1/8, 1/12, 1/16, 1/22 in cylindrical or shapedCDS-R25H HDSCDS-R55 HDS
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PROCESS CHEMISTRY
A residual (or “resid”) is a complex mixture of heavy petroleum compounds that are richin aromatic structures, heavy paraffins, sulfur, nitrogen, and metals. An atmospheric resid-ual (AR) is a material that has been produced in an atmospheric-pressure fractionation col-umn as a bottoms product (ATB) when the boiling endpoint of the heaviest distilledproduct is at or near 343°C (650°F). The bottoms is then said to be a 343°C� (650°F�)atmospheric residual. A vacuum residual (VR) is produced as bottoms product from a col-umn running under a vacuum when the boiling endpoint of the heaviest distilled productis at or near 566°C (1050°F). The bottoms is then said to be a 566°C� (1050°F�) vacu-um residual.
Residual components can be characterized in terms of their solubility:
Saturates. Fully soluble in pentane; this fraction contains all the saturates.
Aromatics. Soluble in pentane and separated by chromatography; this fraction con-tains neutral aromatics.
Resins. Soluble in pentane and absorb on clay; this fraction contains polar aromatics,acids, and bases.
Asphaltenes. Those that are insoluble in pentane (pentane insolubles) and those thatare insoluble in heptane (heptane insolubles); the weight percent of pentane insolublesis always greater than the weight percent of heptane insolubles.
Typically, the conversion reaction path in the RCD Unionfining process is fromasphaltenes to resins, resins to aromatics, and aromatics to saturates.
With the exception of the lightest fractions of crude oil, impurities can be foundthroughout the petroleum boiling range. Impurity concentrations of each fraction increasewith the boiling point of the fraction. Examples of this situation for sulfur and nitrogen areshown in Figs. 8.4.2 and 8.4.3.
Of all the components in the residual, the asphaltene components are the most difficultto work with. Asphaltene molecules are large and are rich in sulfur, nitrogen, metals (Fe,Ni, V), and polynuclear aromatic compounds. These components are primarily the onesthat deactivate the catalyst through metals contamination or coke production.Characterization of some typical atmospheric residuals along with their respective asphal-
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tene components is shown in Table 8.4.2. An example of an asphaltene structure can beseen in Fig. 8.4.4. Typically, the impurities are buried deep inside the asphaltene molecule,and so severe operating conditions are required to remove them.
PROCESS DESCRIPTION
Operating Variables
For a specific feedstock and catalyst package, the degree of demetallization, desulfurization,and conversion increases with the increasing severity of the RCD Unionfining operation. Theoperating variables are pressure, recycle-gas rate, space velocity, and temperature.
UOP RCD UNIONFINING PROCESS 8.47
FIGURE 8.4.2 Sulfur distribution.
FIGURE 8.4.3 Nitrogen distribution in Hondo California crude.
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Pressure. Increasing hydrogen partial pressure decreases the catalyst deactivationrate at constant reactor temperature because the formation of carbonaceous deposits,which deactivate the catalyst, is thereby retarded. The increased pressure alsoincreases the activity for desulfurization, demetallization, and denitrification.Increased hydrogen partial pressure can be obtained by increasing total pressure or byincreasing the hydrogen purity of the makeup gas, which together with a recycle-gasscrubber to remove H2S maximizes hydrogen partial pressure.
Recycle-Gas Rate. Increasing the recycle-gas rate increases the hydrogen/hydrocarbon ratio in the reactor. This increased ratio acts in much the same manner asincreased hydrogen partial pressure.
Space Velocity. Increasing the space velocity (higher feed rate for a given amount ofcatalyst) requires a higher reactor temperature to maintain the same impurity removallevel and results in an increase in the deactivation rate.
Temperature. Increasing temperature increases the degree of impurity removal at aconstant feed rate. operating at an increased temperature level increases the catalystdeactivation rate. as the catalyst operating cycle proceeds, reactor temperature isusually increased because of the disappearance of active catalyst sites.
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Impact of Feedstock Quality and Processing Objectives
The ease of processing a feedstock depends on the nature of the asphaltenic molecule andthe distribution of contaminants throughout the resin and asphaltene fractions. Relativeprocessing severity is dependent on feedstock type and processing objectives (Fig. 8.4.5).Consequently, the process operates over a large range of operating conditions: 1500 to3000 lb/in2 and 0.10 to 1.0 LHSV. Feedstocks with high contaminants, such as vacuumresidues, typically have higher pressures and lower space velocities.
Process Flow
A simplified flow diagram of the UOP RCD Unionfining process is presented in Fig. 8.4.6.The filtered liquid feed is combined with makeup hydrogen and recycled separator offgasand sent first to a feed-effluent exchanger and then to a direct-fired heater. In this flowscheme, the direct-fired heater is shown as a two-phase heater, but the alternative of separatefeed and gas heaters is also an option. The mixed-phase heater effluent is charged to a guardbed and then to the reactor or reactors. As indicated earlier, the guard bed is loaded with agraded bed of catalyst to guard against unacceptable pressure drop, but this catalyst also per-forms some impurities removal. Removal of the remaining impurity occurs in the reactor.
The RCD Unionfining reactors use a simple downflow design, which precludes prob-lems of catalyst carryover and consequent plugging and erosion of downstream equipment.Because this reactor system has three phases, uniform flow distribution is crucial. UOPprovides special reactor internals to ensure proper flow distribution. The reactor-effluentstream flows to a hot separator to allow a rough separation of heavy liquid products, recy-cle gas, and lighter liquid products. The hot separator overhead is cooled and separatedagain to produce cold separator liquid and recycle gas, which is scrubbed to remove H2Sbefore being recycled. A portion of the scrubbed recycle gas is sent to membrane separa-tion to reject light components, mainly methane, that are formed in the reactor. If thesecomponents are not removed, they could adversely affect the hydrogen partial pressure inthe reactor. Hot separator liquid is fed to a hot flash drum, where the overhead is cooledand mixed with cold separator liquid, and the mixture is charged to the cold flash drum.Bottoms from both the hot and cold flash drums are charged to the unit’s fractionation sys-tem, which can be set up to either yield low-sulfur fuel oil or match feed specifications fordownstream processing.
UOP RCD UNIONFINING PROCESS 8.49
Desulfurization
Cat
alys
t V
olum
eP
ress
ure
DA
O∆A
R
VacuumResid
AtmosphericResid
DAO
VGO
FIGURE 8.4.5 Required processing severity versus feedstock type.
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Process Applications
As trends toward heavier crudes and lower fuel oil demand have become evident, UOP hasdevoted increased attention to bottom-of-the-barrel processing. As a result, several flowschemes that offer a variety of advantages have been developed. The most common ofthese flow schemes is shown in Fig. 8.4.7. Atmospheric residual oil is directly hydrotreat-ed to provide FCC feed. Hydrotreating allows a high percentage of the crude to be cat-alytically cracked to gasoline while maintaining reasonable FCC catalyst consumptionrates and regenerator SOx emissions. Hydrotreating can also help refiners meet some of thenewly emerging gasoline sulfur specifications in most parts of the world.
For upgrading residual oils high in metals, the best processing route may be a combi-nation of solvent extraction (UOP/FWUSA solvent deasphalting process) and the RCDUnionfining process. The SDA process separates vacuum residual oil with a high metalcontent into a deasphalted oil (DAO) of relatively low metal content and a pitch of highmetal content. The pitch has several uses, including fuel oil blending, solid-fuel produc-tion, and feed to a partial-oxidation unit for hydrogen production. If the metal andConradson carbon content of the DAO are sufficiently low, it may be used directly as anFCC or hydrocracker feed component. In some cases, however, hydrotreating the DAO pri-or to cracking is desirable, as shown in Fig. 8.4.8. This combination of processes showsbetter economics than either process alone. The arrangement provides an extremely flexi-ble processing route, because a change in feedstock can be compensated for by adjustingthe ratio of DAO to pitch in the SDA unit to maintain DAO quality. In some cases, thetreated material can be blended with virgin vacuum gas oil (VGO) and fed directly to theconversion unit.
When the RCD Unionfining process is used to pretreat coker feed (Fig 8.4.9), it reducesthe yield of coke and increases its quality and produces a higher-quality cracking feed-stock.
Of course, these examples are just a few of the bottom-of-the-barrel upgrading flowschemes involving the RCD Unionfining process. The correct selection of flow scheme istypically specific to a given refiner’s needs and crude type.
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OPERATING DATA
The yield and product properties for processing a blended Middle Eastern reduced crudein an RCD Unionfining unit are shown in Table 8.4.3. Utilities required to operate a 132.5-m3/h [20,000 barrels per stream-day (BPSD)] RCD Unionfining unit are shown in Table8.4.4. The estimated erected cost for this unit is $70 million.
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COMMERCIAL INSTALLATIONS
The first commercial direct reduced-crude desulfurization unit was a UOP unit that wenton-stream in 1967 at the Chiba, Japan, refinery of Idemitsu Kosan. The first commercialdirect vacuum-resid conversion unit was a UOP unit that went on-stream in 1972 at theNatref, South Africa, refinery. A total of 27 RCD Unionfining units have been licensed. Asof early 2002, more than 143,000 m3/h (900,000 BPSD) of RCD Unionfining capacity hasbeen licensed. These units process a variety of feeds, including DMOs and vacuum residsand atmospheric resids. Applications for this process include conventional desulfurization,downstream conversion unit pretreatment, and resid nondistillable conversion.
8.52 HYDROTREATING
TABLE 8.4.3 Yields and Product Properties of a Middle East Blend Reduced Crude
Yields on reduced crude
Specific Sulfur, Nitrogen, Viscosity, V � N,Wt % Vol % gravity wt % vol % cSt at 50°C ppm
