Handbook of petroleum processing CCR

Chapter 5 Catalytic reforming Peter R. Pujad´o and Mark Moser* Catalytic reforming is a process whereby light petroleum distillates (naphthas) are contacted with a platinum-containing catalyst at elevated temperatures and hydrogen pressures ranging from 345 to 3,450 kPa (50–500 psig) for the purpose of raising the octane number of the hydrocarbon feed stream. The low octane, paraffin-rich naphtha feed is converted to a high-octane liquid product that is rich in aromatic compounds. Hydrogen and other light hydrocarbons are also produced as reaction by-products. In addition to the use of reformate as a blending component of motor fuels, it is also a primary source of aromatics used in the petrochemical industry (1). The need to upgrade naphthas was recognized early in the 20th century. Thermal pro- cesses were used first but catalytic processes introduced in the 1940s offered better yields and higher octanes. The first catalysts were based on supported molybdenum oxide, but were soon replaced by platinum catalysts. The first platinum-based reform- ing process, UOP’s Platforming™ process, came on-stream in 1949. Since the first Platforming unit was commercialized, innovations and advances have been made con- tinuously, including parameter optimization, catalyst formulation, equipment design, and maximization of reformate and hydrogen yields. The need to increase yields and octane led to lower pressure, higher severity operations. This also resulted in increased catalyst coking and faster deactivation rates. The first catalytic reforming units were designed as semiregenerative (SR), or fixed- bed units, using Pt/alumina catalysts. Semiregenerative reforming units are peri- odically shut down for catalyst regeneration. This involves burning off coke and reconditioning the catalyst’s active metals. To minimize catalyst deactivation, these units were operatedathigh pressures in the range of 2,760 to3,450 kPa (400–500 psig). High hydrogen pressure decreases coking and deactivation rates. * c  UOP LLC 217 218 CHAPTER 5 Catalytic reforming processes were improved by introducing bimetallic catalysts. These catalysts allowed lower pressure, higher severity operation: ∼1,380–2,070 kPa (200–300 psig), at 95–98 octane with typical cycle lengths of one year. Cyclic reforming was developed to allow operation at increased severity. Cyclic re- forming still employs fixed-bed reforming, but each reactor in a series of reactors can be removed from the process flow, regenerated, and put back into service without shutting down the unit and losing production. With cyclic reforming, reactor pressures are approximately 200 psig, producing reformates with octanes near 100. Another solution to the catalyst deactivation problem was the commercialization of the Platforming process with continuous catalyst regeneration, or the CCR Platforming process, by UOP in 1971. The Institut Fran¸cais du P´etrole announced the commer- cialization of a similar continuous regeneration reforming process a few years later. With CCR small amounts of catalyst are continuously removed from the last reac- tor, regenerated in a controlled environment, and transferred back to the first reactor. The CCR Platforming process has enabled the use of ultra low pressures at 345 kPa (50 psig) with product octane levels as high as 108. More than 95% of all new cat- alytic reformers are designed with continuous regeneration. In addition, many units that were originally built as SR reforming units have been revamped to continuously regenerable reforming units. Figure 5.1 illustrates the evolution of catalytic reforming, in terms of both process yields and octane numbers. Increase in Catalytic Reforming Performance with Catalyst and Process Innovation RON Clear 86 90 94 98 102 106 Theoretical Yield 90 C5 Yield, LV% 86 82 78 1950s 1960s 1970s 1980s 1990s Figure 5.1. Increased yields and octane with Platforming advances (reprinted with permission from UOP LLC). CATALYTIC REFORMING 219 150 200 250 300 350 400 IBP 10 30 50 70 90 EP Percent Over Temperature, °F Figure 5.2. ASTM D-86 distillation curve for naphtha (1). Feedstocks Naphtha feedstocks to reformers typically contain paraffins, naphthenes, and aromat- ics with 6–12 carbon atoms. Most feed naphthas have to be hydrotreated to remove metals, olefins, sulfur, and nitrogen, prior to being fed to a reforming unit. A typical straight run naphtha from crude distillation may have a boiling range of 150–400 ◦ F (65–200 ◦ C). In addition to naphthas from crude distillation, naphthas can be derived from a variety of other processes that crack heavier hydrocarbons to hydrocarbons in the naphtha range. Cracked feedstocks may be derived from catalytic cracking, hydrocracking, cokers, thermal cracking, as well as visbreaking, fluid catalytic cracking, and synthetic naphthas obtained, for example, from a Fischer–Tropsch process. Light paraffinic naphthas are more difficult to reform than heavier naphthenic hydro- carbons. Distillation values for the initial boiling point, the mid-point at which 50% of the naphtha is distilled over, and the end point are often used to characterize a naphtha (Figure 5.2). If available, however, it is best to have a detailed component breakdown as provided by gas chromatographic analysis (Table 5.1). Feed hydrotreating is used to reduce feedstock contaminants to acceptable levels (Figure 5.3). Common poisons for reforming catalysts that are found in naphtha are sulfur, nitrogen, and oxygen compounds (Figure 5.4). Removing these requires breaking of a carbon-sulfur, -nitrogen or -oxygen bond and formation of hydrogen sulfide, ammonia, or water, respectively. Hydrotreaters will also remove olefins and metal contaminants. Some hydrotreaters are two-stage units. The first stage operates at low temperature for the hydrogenation of diolefins and acetylenes that could polymerize and plug the second, higher severity stage. The effluent from the first stage is cooled and fed to 220 CHAPTER 5 Table 5.1. Composition of a typical naphtha Concentration (wt%) Aromatics Benzene 1.45 Toluene 4.06 Ethylbenzene 0.52 p-Xylene 0.92 m-Xylene 2.75 o-Xylene 0.87 C9+ Aromatics 3.31 Total Aromatics 13.88 Total Olefins 0.11 Paraffins and Naphthenes Propane 0.79 Isobutane 1.28 n-Butane 3.43 Isopentane 5.62 n-Pentane 6.19 Cyclopentane 0.64 C6 Isoparaffins 6 n-Hexane 5.3 Methylcyclopentane 2.58 Cyclohexane 3.26 C7 Isoparaffins 4.55 n-Heptane 4.65 C7 Cyclopentanes 2.77 Methylcyclohexane 7.57 C8 Isoparaffins 4.24 n-Octane 3.43 C8 Cyclopentanes 1.52 C8 Cyclohexanes 5.23 C9 Naphthenes 3.63 C9 Paraffins 5.93 C10 Naphthenes 1.66 C10 Paraffins 3.41 C11 Naphthenes 1.04 C11 Paraffins 0.53 C12 P + N 0 > 200 P + N 0 Total Paraffins 55.35 Total Naphthenes 30.7 CATALYTIC REFORMING 221 Heater Reactor Separator Stripper Fresh Feed Recycle Gas Compressor Light Ends Stea Desulfurized Product Hydrogen Makeup Makeup Compressor Sour Water Wash Water Steam Figure 5.3. Naphtha hydrotreater flow scheme. the second stage for the hydrogenation of olefins and the removal of sulfur and nitrogen compounds. The reformate stream from acatalytic reforming unit isinvariably used either as a high- octane gasoline blending component or as a source of aromatics—BTX (benzene, toluene, and xylenes), and C 9 + aromatics. Reforming for motor fuel applications still represents the majority of existing reforming capacity. Reformate specifications (octane, vapor pressure, end point, etc.) are set to provide an optimum blending prod- uct. The octane requirement is met through the production of high-octane aromatics, the isomerization of paraffins, and the removal of low octane components by cracking them to gaseous products. Feedstocks to these units are typically “full range” naph- thas, consisting of hydrocarbons with 6–12 carbon atoms; however, the initial boiling point may be varied to limit the presence of benzene precursors. Reforming units for the production of aromatics are often called BTX reformers. Naphthas for these units are specified to contain mostly naphthenes and paraffins of 6–8 carbons. The desired reaction is aromatization through dehydrogenation of the naphthenes, and cyclization and dehydrogenation of the paraffins to the analogous aromatic. Mercaptans Di–sulfides Thiophene R  SH R  S  R C S C  C C     Figure 5.4. Sulfur types. 222 CHAPTER 5 Table 5.2. Reformate composition mass% liq-vol% Aromatics Benzene 3.72 3.39 Toluene 13.97 12.93 Ethylbenzene 3.13 2.90 p-Xylene 3.39 3.14 m-Xylene 7.47 6.91 o-Xylene 4.83 4.47 C9+ Aromatics 36.05 33.30 Total aromatics 72.56 67.04 Total olefins 0.82 1.02 Paraffins and naphthenes Propane 0.00 0.00 Isobutane 0.14 0.20 n-Butane 0.94 1.32 Isopentane 2.52 3.29 n-Pentane 1.74 2.29 Cyclopentane 0.10 0.10 C6 Isoparaffins 3.91 4.77 n-Hexane 1.74 2.12 Methylcyclopentane 0.28 0.30 Cyclohexane 0.03 0.03 C7 Isoparaffins 7.70 9.02 n-Heptane 2.22 2.60 C7 Cyclopentanes 0.33 0.35 Methylcyclohexane 0.04 0.04 C8 Isoparaffins 2.86 3.24 n-Octane 0.62 0.70 C8 Cyclopentanes 0.14 0.14 C8 Cyclohexanes 0.06 0.06 C9 Naphthenes 0.04 0.04 C9 Paraffins 0.90 0.99 C10 Naphthenes 0.04 0.04 C10 Paraffins 0.24 0.26 C11 Naphthenes 0.00 0.00 C11 Paraffins 0.03 0.04 C12 P + N 0.00 0.00 Poly Naphthenes 0.00 0.00 > 200 P + N 0.00 0.00 Total Paraffins 25.56 30.84 Total Naphthenes 1.06 1.10 CATALYTIC REFORMING 223 Reformate properties Table 5.2 shows a typical reformate composition. For motor fuel applications, the octane number is the dominant parameter of product quality. A higher octane number reflects a lower tendency of the hydrocarbon to undergo a rapid, inefficient detonation in an internal combustion engine. This rapid detonation is heard as a knocking sound in the engine, so octane is often referred to as the antiknock quality of a gasoline. Motor fuel octanes are measured at low engine speeds (research octane number or RON) or at high engine speeds (motor octane number or MON). In the United States, the octane values posted on gasoline pumps are the arithmetic average of the MON and the RON. The acronym RONC, research octane number clear, is used to denote that there are no additives, such as lead, used to increase octane number. Table 5.3 provides a listing of the various octanes of pure hydrocarbons according to the American Petroleum Institute, API (2). Octane numbers of a hydrocarbon or hydrocarbon mixture are determined by com- paring its antiknock qualities with various blends of n-heptane (zero octane) and 2,2,4-trimethylpentane, or iso-octane (100 octane). Hydrocarbons may appear to have different octane numbers when blended with other hydrocarbons of a differ- ent composition—these are denoted as “blending octanes” and may be significantly different from the actual octane numbers of the individual hydrocarbon components (Table 5.4) (3). Other property specifications of the reformate include volatility or vapor pressure, often given in terms of the Reid vapor pressure or RVP, end point, color, etc. (3) High- end point reformates, for example, may not combust well in an internal combustion engine. Table 5.3. Examples of research and motor octanes of pure hydrocarbons RON MON Paraffins n-heptane 0 0 2-methylhexane 42.4 46.3 3-ethylpentane 65.0 69.3 2,4-dimethylpentane 83.1 83.8 Aromatics Toluene 120.1 103.2 Ethylbenzene 107.4 97.9 Isopropylbenzene 113.0 99.3 1-methyl-3-ethylbenzene 112.1 100.0 1,3,5-trimethylbenzene >120 >120 224 CHAPTER 5 Table 5.4. Octane and blending octane numbers by research method RON Blending Octane 2,2-dimethyl butane 92.8 89 2-methyl-1-butene 102 146 Cyclopentane 101 141 1,4-dimethylbenzene 117 146 Reformulated gasolines, a requirement of the 1990 Clean Air Act, are the subject of much legislation. Specifications require a lower benzene content, lower volatility, and lower end point. Other specifications may pertain to the oxygenate content and other factors that affect the burning characteristics. The gasolines available to the consumer consist of a mixture of gasoline fractions from many refinery sources, including: straight run (unprocessed fraction), isomerate, alkylate, reformate, and FCC fractions, and, on occasion, polymer gasolines. Reforming reactions In BTX production, the objective is to transform paraffins and naphthenes into ben- zene, toluene, and xylenes with minimal cracking to light gases. The yield of desired product is the percentage of feed converted to these aromatics. In motor fuel ap- plications, octane values of the feed may be raised via aromatization or through isomerization of the paraffins into higher octane branched species without sacrificing yield. Yield is typically defined as liquid product with five or more carbons. Typical catalysts that consist of platinum supported on alumina (with or without other metals or modifiers) are bifunctional in that separate and distinct reactions occur on the platinum site and on the alumina. The platinum typically performs dehydrogenation and hydrogenolysis, while the acidic alumina isomerizes, cyclizes, and cracks. The dehydrogenation of naphthenes to aromatics is probably the most important reaction. Feeds contain cyclopentanes and substituted cyclopentanes, as well as cy- clohexanes and their homologues. Six carbon ring cyclohexanes, for example, can be directly dehydrogenated to produce aromatics and hydrogen. + 3H 2 Dehydrogenation is typically catalyzed by the platinum function on the reforming catalyst. CATALYTIC REFORMING 225 Five member ring cyclopentanes must be hydroisomerized to give a cyclohexane intermediate prior to dehydrogenation to aromatics. + 3H 2 Acid-catalyzed reactions together with the Pt-catalyzed dehydrogenation function are largely responsible for hydro-isomerization reactions that lead to the formation of aromatics. Paraffin conversion is the most difficult step in reforming. For that reason, the ability to convert paraffins selectively is of paramount importance in reforming. Paraffins may be isomerized over the acidic function of the catalyst to provide higher octane branched paraffins. CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 Another acid catalyzed paraffin reaction is cracking to lighter products, thus remov- ing them from the liquid product. Octane is improved through the removal of low octane paraffinic species from the liquid product by their conversion to gaseous, lower molecular weight paraffins. H 3 C CH 3 + H 2 H 3 C CH 3 + H 3 C CH 3 Paraffins also undergo cyclization to cyclohexanes. This reaction is believed to pro- ceed through an olefin intermediate, produced by Pt-catalyzed dehydrogenation (4). The cyclization of the olefin may be catalyzed by the alumina support. + H 2 + H 2 CH 3 CH 3 CH 3 H 3 C 226 CHAPTER 5 After cyclization, cyclohexane undergoes dehydrogenation to aromatics. Cyclopen- tanes undergo hydroisomerization to cyclohexane, followed by dehydrogenation to aromatics. Aromatics are stable species and relatively inert. Reactions of substi- tuted aromatics involve isomerization, hydrodealkylation, disproportionation, and transalkylation. Small amounts of olefins are formed that also undergo a number of isomerization, alkylation, and cracking reactions. In particular, they appear to play an important role as an intermediate in cyclization reactions. The dehydrogenation of naphthenes and paraffins is rapid and equilibrium concentra- tions are established in the initial portions of a catalyst bed. Isomerization reactions are sufficiently fast that actual concentrations are near equilibrium. The observed re- action rate for dehydrocyclization is reduced by the low concentrations of the olefin intermediates that exist at equilibrium. Hydrogen partial pressure significantly affects olefin equilibrium concentrations and has a significant impact on aromatization and dehydrocyclization of paraffins. Lowering hydrogen partial pressures results in an increase in the rate of aromatization, a decrease in the rate of hydrocracking, and an increase in the rate of coke formation. Table 5.5 provides thermodynamic data for typical compounds in reforming reactions at a reference temperature of 800 ◦ K. Thermodynamic data can be obtained from Table 5.5. Thermodynamic data for reforming compounds at 800 ◦ K, ideal gas in kcal/mol Reforming reactions are typically dehydrogenations of the form A ↔ B + nH 2 with equilibrium expressed in the form K P = p B (p H 2 ) n p A such that they are a strong function of the partial pressure of hydrogen. H o f G o f Typical C 6 ’s n-hexane −48.26 73.08 2-methylpentane −49.68 72.74 3-methylpentane −49.32 73.67 Cyclohexane −37.19 75.94 Methyl cyclopentane −33.73 71.92 Benzene 15.51 52.84 Typical C 7 ’s n-heptane −54.20 87.43 2-methylhexane −55.91 87.23 3-methylhexane −55.28 87.07 Methyl cyclohexane −45.10 86.15 Toluene 6.65 61.98 [...]... temperature of the catalyst beds Platforming catalysts are capable of operating over a wide range of temperatures By adjusting the heater outlet temperatures, a refiner can change the octane of the reformate and the quantity of the aromatics produced The reactor temperature can be expressed as the weighted average inlet temperature (WAIT) The WAIT is the summation of the product of the fraction of catalyst... The amount of naphtha processed over a given amount of catalyst over a set length of time is referred to as space velocity Space velocity corresponds to the reciprocal of the residence time or time of contact between reactants and catalyst When the hourly volume charge rate of liquid naphtha is divided by the volume of catalyst in the reactors, the resulting quotient, expressed in units of h−1 , is... severities for the SR and CCR Platforming units The CCR Platforming unit operates at higher severity and lower reactor catalyst inventory In addition, the CCR unit runs continuously compared to 12-month SR Platforming cycle lengths Typical product yields for the SR and CCR Platforming units operating at the conditions presented in Table 5.6 are shown in Table 5.7 Many of the benefits of CCR Platforming are... severity of the CCR Platforming unit results in similar liquid volume for the two units However, the reformate produced by the CCR Platforming is more valuable than that produced by the SR Platforming unit Taking into account both the higher octane value and the increased on-stream efficiency of the CCR Platforming unit, 80 million more octane-barrels, or 11.4 million more Table 5.7 Yield comparison of CCR. .. year with the CCR Platforming unit than with the SR Platforming unit Octane-yield is defined as the product of the reformate yield, octane, and operating days A summary of the operating revenues and costs expected for the SR and CCR Platforming units in shown in Table 5.8 The nomenclature follows standard definitions The economics of the CCR Platforming process are superior as a direct result of the differences... flexibility of the two modes of operations The CCR Platforming unit produces more valuable reformate at 102 RONC versus the SR Platforming reformate at 97 RONC On-stream efficiency of the CCR Platforming unit is 8,640 hr per year compared to about 8,000 hr per year for the SR Platforming unit Although the CCR Platforming utility costs are higher than those for the SR Platforming unit, these costs are offset... temperature of the reactor The weighted average bed temperature (WABT) is also used to describe catalyst temperature and is the temperature of the catalyst integrated along the catalyst bed Temperatures in this chapter refer to the WAIT calculation Typically, SR Platforming units have a WAIT range of 490–525◦ C (914–977◦ F) CCR Platforming units operate at a WAIT of 525– 540◦ C (977–1,004◦ F) CCR Platforming... the unit are easily isolated to permit a shutdown of the regeneration system for normal inspection or maintenance without interrupting of the Platforming operation A few years after the introduction of the UOP CCR Platforming process, another continuously regenerable process design was offered by the Institut Fran¸ ais du P´ trole c e Though similar to CCR Platforming, the continuous reforming units... of CCR Platforming From both an economic and technical standpoint, the CCR Platforming process is superior to the SR and cyclic reforming processes The CCR Platforming unit allows for low-pressure operation, leading to higher yields At these conditions, the SR Platforming catalyst is completely deactivated after only a few days of operation Both the hydrogen and C5 + yields are maximized with the CCR. .. maximized with the CCR Platforming process Since the number of cyclic reformers is small relative to CCR Platforming process units and SR process units, the following comparison will focus on contrasting CCR Platforming units and SR units CATALYTIC REFORMING 235 Table 5.6 Relative severities of CCR versus SR Platforming units Operating mode SR CCR Charge rate, barrels/day LHSV, h−1 H2 /HC RONC Reactor . definitions. The economics of the CCR Platforming process are superior as a direct result of the differences in operating severity and flexibility of the two modes of operations. The CCR Platforming unit. ratio of mols of hydrogen in the recycle gas to mols of naphtha charged to the unit. The recycle gas is a mixture of hydrogen and light gases, typically 75–92 mol% hydrogen. The ratio of total. increased on-stream efficiency of the CCR Platforming unit, 80 million more octane-barrels, or 11.4 million more Table 5.7. Yield comparison of CCR versus SR Platforming units SR CCR Delta Hydrogen yield,