REFINERS HAVE SEVERAL OPTIONS FOR REDUCING GASOLINE BENZENE

Alan R. Goelzer, Agustin Hernandez-Robinson
Badger Co. Inc.
Cambridge, Mass.

Sanjeev Ram
Raytheon Engineers & Constructors Inc.
Cambridge, Mass.

Arthur A. Chin
Mobil Research & Development Corp.
Paulsboro, N.J.

Mohsen N. Harandi, C. Morris Smith
Mobil Research & Development Corp.
Princeton, N.J.

Mobil Research & Development Corp. and Badger Co. Inc. have developed several alternatives for reducing benzene levels in gasoline.

Where benzene extraction is viable and maximum catalytic reformer hydrogen is needed, the companies' cumene and ethylbenzene processes are desirable. Mobil's benzene reduction process can be an alternative to benzene hydrosaturation.

All of these processes utilize low-value offgas from the fluid catalytic cracking (FCC) unit.

BACKGROUND

Although the linkage between gasoline benzene content and evaporative, running, and tailpipe emissions is not yet defined, the U.S. 1990 Clean Air Act Amendments mandate a benzene content of less than 1.0 vol % in reformulated gasolines. Likewise, the California Air Resources Board (CARB), plans to restrict benzene to less than about 0.8 vol %.

In the U.S., 1989-90 gasoline surveys indicated that the average benzene content of gasoline consumed was about 1.6 vol %. But averages can be deceptive. More than one third of refiners produced gasoline containing 2.0-3.5 vol % benzene.

Some produced lower levels, presumably because of benzene extraction or benzene/toluene/xylene (BTX) aromatics production. A few, however, had levels as high as the 5 vol % limit.

In the U.S., the dilution effects of FCC gasoline, alkylate, and ethers favorably impact benzene levels. FCC gasoline contains just 0.5-0.8 vol % benzene, while alkylate and ethers contain none.

FCC units have reduced gasoline benzene levels in traditional U.S. refineries. As will be discussed, FCC can play an even larger role in the future of benzene reduction.

BENZENE LEVELS

The amount of benzene (in b/sd) entering the motor gasoline pool is principally a function of three factors:

The b/sd of benzene precursors in the crude slate, i.e., "straight-run" methyl cylopentane (MCP), cyclohexane (CH), and "natural" benzene (BZ).
The b/sd of benzene and benzene precursors "synthesized" in and coming from FCC units, cokers, hydrocrackers, and steam crackers.
The fraction of available MCP/CH charged to the catalytic reformer.

A catalytic reformer processing most of the available Cyclic C6 precursors often will contribute 70-80% of the benzene entering the motor gasoline pool. Reformate benzene content typically varies from about 2.5 wt % to 8 wt %, depending on the amount of cyclic C6 precursors in the reformer feed and reformer operating severity.

In the absence of low-benzene gasoline specifications, most refinery optimization models will recommend maximizing benzene by sending most of the Cyclic C6 material to the reformer. If Cyclic C6 precursors are reduced to minimum practical levels, however, reformate benzene can be as low as 1.0-1.5 wt %.

Refiners facing low-benzene gasoline specifications should first:

Review the mogas-pool benzene history at each refinery.
Assess whether there are any formula trade off benefits for benzene levels that are less than reformulated gasoline maxima.
Determine the amount of benzene and MCP/CH bypassing the catalytic reformer and entering the mogas pool. FCC gasoline, light coker gasoline, light hydro-crackate, and direct-blend light straight-run (LSR) naphtha may contain more benzene and MCP/CH than expected.

Several factors tend to exclude isomerization from more than a very limited benzene reduction role:

Large hydrogen uptake per barrel of benzene and heptanes in the feed
Associated exothermic reaction, or heat release, which reduces overall unit performance
Possible preclusion of the n-hexane distillation recycle by high levels of cyclic C6 and heptanes in the feed
Excessive upstream hydrogen cleanup costs.

The preferred feed to current commercial LSR isomerizers remains isopentane (normal boiling point = 82F.) (through n-hexane) NBP = 156 F., with minimum practical levels of cyclohexane, methyl cyclopentane, benzene and heptanes.

CONVENTIONAL METHODS

Conventional benzene reduction approaches focus on benzene and other cyclic C6 components within the naphtha processing area. This "area" includes naphtha hydrotreating, catalytic reforming, isomerization if installed, catalytic reforming H2-rich offgas compression and upgrading, and three key towers-the naphtha splitter, reformate stabilizer, and reformate splitter.

Conventional benzene reduction involves:

Prefractionation of some benzene precursors directly to the isomerizer or into a benzene hydrosaturator upstream of the isomerizer
Reformate benzene hydrosaturation (Fig. 1)
Reformate benzene hydrosaturation and integral CH-to-MCP isomerization
Prefractionation of benzene precursors into naphtha-splitter side draw heart-cut and bypassing the reformer with this stream, thus going directly into motor gasoline (Fig. 1)
Benzene extraction (Fig. 2).

Some naphtha splitter towers may have to be upgraded to achieve a much sharper separation between either n-hexane and methyl cyclopentane or benzene and methyl hexane. In many refineries, benzene reduction will involve reintroduction of the reformate splitter tower.

New reformate splitter towers will split debutanized reformate into C5-cut benzene-rich heart-cut and high-octane aromatic heavy reformate. The octane of the benzene-rich heart-cut will be less than the octane of the full-range C5 reformate because of unconverted or formed MCF, methyl pentanes, n-hexane, methyl hexane, and n-heptane.

The conventional benzene hydrosaturation approach results in reduced hydrogen availability from the catalytic reformer. This is true whether practiced downstream of the catalytic reformer, upstream of an isomerizer, or in an isomerizer.

Benzene hydrosaturation consumes 4,500 scf H2/bbl feed. Mogas pool octane is reduced by hydrosaturation because methyl cyclopentane RONC is just 91 and cyclohexane RONC is only 83. The blending octane (RONC) of benzene, on the other hand, is 106-120 or more.

The benzene extraction option, as shown in Fig. 2, has several advantages:

Over 90% of reformate benzene will disappear from mogas pool.
Total pool aromatics are reduced by 1.0-1.5 vol %.
Catalytic reformer hydrogen output can be maximized.
Chemical-grade benzene is produced.

Principal disadvantages are:

High capital cost of the benzene extraction unit
High energy input into reformate splitter, solvent regenerator, and BT tower
Possible octane replacement needs
Requirement to market benzene as a petrochemical
Benzene storage, shipping, and handling requirements.

NEW TECHNOLOGIES

Conventional benzene reduction technologies look for "solutions" strictly within the naphtha processing area and fail to recognize:

Interaction with the FCC processing complex
Catalysts that accomplish alkylation and transalkylation of light olefins and aromatics.
All alkylation reactions require acidic catalytic sites. These sites can be combined with pore structures tailored to favor the selective yield of certain molecules. One of the earliest and best known uses of the solid acid zeolite alkylation processes is the MobiL/Badger ethylbenzene (EB) process, codeveloped by Mobil R&D and Badger.
The Mobil/Badger EB process was commercialized in 1978 and is now in its third generation. The process has been used in more than 80% of the new licensed ethylbenzene capacity built in the last 15 years.
Mobil and Badger now offer three new "benzene alkylation" processes. Two of these processes are suitable in situations where the extraction of reformate benzene is, or could be, available and the production of petrochemical feedstocks is of interest:
Mobil/Badger cumene process - uses chemical-grade benzene and FCC-derived "dilute propylene" to selectively make isopropyl-benzene, or cumene.
Mobil/Badger ethylbenzene process-uses chemical-grade benzene and "dilute ethylene," as found in FCC offgas, to selectively make high-purity ethylbenzene.
The third is useful if motor gasoline is preferred over making petrochemicals, or if benzene extraction is not desired:
Mobil benzene reduction process (MBR), where a mix of "dilute light olefins," available in FCC offgas and coker gas, are used to convert "dilute benzene" into other alkyl-benzenes. MBR is an extension of the Mobil olefins-to-gasoline process (MOG), announced in 1990.

CUMENE PROCESS

Mobil has developed a new proprietary catalyst System for synthesizing cumene from benzene and propylene via the reaction shown in Fig. 3. This new catalyst is used in a fixed-bed reactor system and exhibits:

Extremely high selectivity for alkylation of benzene and propylene into isopropyl-benzene (cumene) while operating in liquid phase at moderate temperatures
Minimal activity for propylene oligomerization
Near stoichiometric conversion of both benzene and propylene to cumene.
Mobil and Badger have jointly piloted and developed the Mobil/Badger cumene process using this new catalyst system. The process can be implemented as a grassroots unit in a petrochemical plant or integrated into a refinery.
As shown in Fig. 4, integration of a cumene unit into a refinery can achieve five principal objectives:
Continued ability to operate the naphtha splitter and catalytic reformer in maximum benzene" mode, providing maximum net availability of hydrogen for distillate hydrotreating
Removal of benzene from reformate at efficiencies as high as about 95%
Higher value use of "dilute propylene" (65-80% purity propylene, as found in FCC-derived olefinic C3 LPG streams)
In-process upgrading of extracted benzene to high-purity cumene product 9 Production of a high-value petrochemical (cumene is the precursor for phenol).

Using FCC-derived dilute propylene to make cumene can add more value to the product than sending all the propylene to alkylate production. Incentives will depend, of course, on the individual refiner's costs for the last increment of isobutane (purchased or recovered) and their requirements for the incremental dilution effects of propylene alkylate.

Benzene extraction and cumene production will still lower total aromatics in the motor gasoline pool by about 1.0 vol %.

ETHYLBENZENE PROCESS

The Mobil/Badger ethylbenzene (EB) process selectively alkylates chemical-grade benzene with ethylene to make ethylbenzene. Fixed-bed reactors with ZSM-5 zeolite catalyst are used in a quenched-bed, vapor phase system (Fig. 5). Process conditions minimize formation of troublesome ethylene oligomers, polyethylbenzenes (PEBs), and residues.

Because of remarkably high alkylation selectivities, near stoichiometric yields of EB are achieved from the benzene and ethylene feeds. The latest third-generation EB technology makes this possible with either polymer-grade ethylene or FCC-derived dilute ethylene.

In mann, refineries, the number of moles of ethylene in FCC offgas is approximately equivalent to the number of moles of benzene in reformate. This led to the development and commercialization of an EB process that uses dilute ethylene instead of traditional polymer-grade ethylene (Fig. 6).

This process has been operating at full commercial scale since 1991 on resid FCC (RFCC) offgas at a European refinery and on dilute ethylene at a European petrochemical plant.

This type of unit upgrades FCC-derived ethylene from fuel value to EB petrochemical value. This can provide substantial economic incentives, offsetting the less than-world-scale capacities often present in refineries.

Because of relatively limited reformate benzene availability and ethylene production in FCC/RFCC units, the annual EB production from a given refinery will be moderate (40,000 to perhaps 180,000 metric tons/year [mty]). FCC-derived EB therefore will often be transferred to an existing styrene monomer synthesis unit in a petrochemical complex.

BENZENE REDUCTION

The Mobil benzene reduction process (MBR) is a multipurpose catalytic process that restructures gasoline components. MBR incorporates the chemistry of the Mobil olefins-to-gasoline process (MOG), with additional capabilities.

Both MBR and MOG use a dense-fluid-bed reactor containing a specially tailored ZSM-5 fluid catalyst. Typically, MBR operates "pressurized" at 170-220 psia. This is similar to the pressure of "sweet" FCC offgas exiting a modem FCC gas plant section.

In a typical MBR application, FCC offgas is a source of dilute ethylene and unrecovered propylene. Benzene-rich light reformate or heart-cut reformate is distilled out of C5+ full-range reformate. The benzene-rich stream is vaporized and co-processed over ZSM-5 with FCC offgas, along with other available olefinic LPG and cracked naphthas.

The reaction sequence in an MBR reactor is complicated:

Dilute ethylene alkylates dilute benzene to form high-octane alkylbenzenes.
Light olefins oligomerize to form C5- olefins.
C5+ olefins "crack" to form light olefins.
"Equilibrated olefins" can alkylate additional dilute benzene.
equilibrated olefins simultaneously form C5, motor-gasoline polynuclear aromatics plus some propane and butanes (key MOG reaction).
Low-octane C6- linear paraffins crack to form propane, butanes, and reactive light olefins.

This chemistry can be used to upgrade lower-value olefinic streams coming from an FCC/RFCC unit or coker while significantly reducing benzene.

MOG and MBR were demonstrated in an 8-month run in a 4 b/sd continuously circulating fluid-bed demonstration unit at Mobil's Paulsboro Research Laboratory (Fig. 7). The demonstration unit concepts have been successfully scaled up from 4 b/sd to 100 b/sd in Mobil semiworks (Fig. 8). Further, the full commercial-scale dense-flu-id-bed reactor design is similar to Badger fluid-bed reactors designed for other services.

MBR OPTION 1

A conventional scenario for benzene reduction using MBR is shown in Fig. 9 (MBR Option 1). The full-range C5+ reformate is first split into C5 cut, benzene-rich heart-cut, and high-octane C7+ heavy reformate. FCC offgas is sent through the fuel-gas amine contactor, then directly into the MBR unit without further pretreatment.

FCC offgas contains essentially all ethylene, plus some 5-10% propylene. These light olefins represent 0.8-1.5 wt % of the FCC feed. These light olefins traditionally have been valued as fuel and burned.

Trends in FCC often favor increased light olefin operations. This, plus the use of deep-cut vacuum gas oil FCC feeds and some resid feeds, increase the mass of light olefins in FCC offgas and often increase offgas contaminants.

The tailored ZSM-5 catalyst used in MBR and the dense-fluid-bed reactor with slipstream regenerator will tolerate typical contaminants while remaining highly active for light olefin recovery via chemical conversion. And because deactivation rates are low, the required daily makeup rate of the catalyst is a small fraction of the MBR unit inventory. Spent catalyst is reusable as part of an octane-enhancing ZSM-5 spike in the associated FCC/RFCC unit.

In many practical refinery situations, more than one half of incoming dilute benzene can be alkylated via MBR to form high-octane alkylbenzenes. Dilute benzene alkylation is favored by increased ethylene availability and, surprisingly, by increased C5-C7 olefins in co-feeds.

In addition, MBR will provide delta octane gains in addition to benzene reduction. These capabilities are illustrated in Table 1.

Note that light reformate feed or heart-cut reformate feed will exhibit modest octane, even though the full-range C5+ reformate is high-octane (RONC = 97-102). Most of the octane in reformates is in the C7+ portion, i.e., the 70-75% bottoms from the reformate splitter.

For the theoretical average U.S. 1990 refinery with catalytic reforming and FCC, MBR Option 1 will lower mogas benzene from 1.6 vol % to 1.0 vol %, or possibly even more, because of the dilution effects of purchased MTBE and other incremental light olefin derivatives in reformulated gasoline.

The percentage benzene reduction can be increased by selective liquid recycling. Limited recycling can be implemented with modest cost impacts, or larger external recycles can be provided via the reformate splitter tower.

MBR OPTION 2

For those refiners with considerably more benzene in the mogas pool (say 2.03.0 vol %), MBR Option 2 might be of more interest (Fig. 10). MBR Option 2 overcomes a "Catch 22" in benzene reduction.

If all or most Cyclic C6 precursors are charged to the catalytic reformer, the "synthetic" benzene formed from cyclohexane and methylcyclopentane may force an increase in the mogas benzene level, and thus force selection of potentially less attractive benzene reduction approaches. As an alternative, the naphtha splitter tower can be modified to distill out the benzene precursors into a side-draw heart-cut.

In MBR Option 2, the cyCliC C6 heart-cut bypasses the catalytic reformer and goes to the MBR unit. With Most Cyclic C6 excluded from the reformer feed, the volume of low-octane light reformate will decrease somewhat and the C5+ reformate benzene content may increase to about 1.0 vol %.

The light reformate also may be distilled out and charged to the MBR unit.

In MBR Option 2, the amount of catalytic reformer hydrogen available to distillate hydrotreaters will decline, compared to Option 1. This reduction will, however, be less than for benzene hydrosaturation options.

Bypassing cyclic C6 can reduce the b/sd of reformate benzene by one half to two thirds. The MBR unit will then alkylate about one half of the benzene remaining in its cofeeds.

Concurrently, the MBR unit will partially crack the extremely low-octane C6+ linear paraffins that are distilled out along with the cyclic C6 precursors and reformate benzene. The C6. linear paraffins crack into propane and butane LPG and light olefins. Most light olefins formed will then be converted to gasoline.

Conversions of ethylene and propylene in FCC offgas will remain high. Benzene alkylation and C6+ paraffin cracking will provide large boosts in both motor and research octanes.

MBR can alleviate the constraints of excess fuel gas from FCC offgas or low propylene recovery in the FCC gas plant. Once MBR is in place, it may be possible to increase FCCU operating severity.

PERFORMANCE COMPARISON

Table 2 compares the impact of benzene reduction options for a nominal 100,000 b/sd refinery charging a crude with high Cyclic C6 content.

As shown in Table 2, the conventional approach - benzene hydrosaturation of benzene-rich heart-cut distilled out of the full-range reformate - effectively reduces benzene.

The net catalytic reformer hydrogen available to distillate hydrotreaters, however, will decrease sharply (-20%). There is also a net loss of octane across the benzene hydrosaturator.

If benzene extraction is incorporated, about 46,800 mty of benzene (1,010 b/sd) can be produced. With benzene extraction and the Mobil/Badger cumene process, the refiner will make 70,500 mty of cumene (1,550 b/sd). This may allow the refiner to eliminate benzene storage and shipping.

With benzene extraction and the Mobil/Badger ethylbenzene process, the refiner will make more than 63,000 mty of high-purity ethylbenzene (about 1,380 b/sd). Approximately 51 mty (4,700 lb/hr) of FCC-derived ethylene is upgraded from incremental fuel value to EB value.

MBR Option 1 reduces overall pool benzene by more than 35% (from 2.5 vol % to less than 1.5 vol %).

There is a substantial gain in octane. Total aromatics increase a fraction of a volume percent from a base of about 28.0 vol %.

MBR Option 1 with recycling, or MBR Option 2 with Cyclic C6 bypassing, can achieve a pool benzene of 0.8 vol %, before further dilution from purchased MTBE and other incremental tight olefin derivatives. Octane replacement needs typically associated with cyclic C6 bypassing are offset by the cracking of associated C6- linear paraffins by MBR.

The comparison in Table 2 shows that the cumene, ethylbenzene, and MBR processes give the refiner competitive benzene reduction alternatives.

ADDITIONAL USES

The MBR process has other interesting capabilities. Refiners may elect to cofeed cracked light gasoline components to an MBR unit (Fig. 11). Light FCC gasoline, light pyrolysis gasoline, and light coker naphtha components will be restructured and upgraded by MBR. As shown in Table 3, MBR can provide:

Enhanced octane (especially MONC)
Reduced Rvp
8O% reduction in pentenes (and of residual butenes)
70 + % reduction in hexenes/heptenes
98 + % conversion of diolefins (and residual methanol)
Incremental benzene reduction
Partial desulfurization.

As butenes pentenes, hexenes, diolefins, and methanol disappear from FCC gasoline, inherent photoreactivity potential will be sharply reduced. Motor octane will increase significantly as olefinicity decreases; RONC is maintained or boosted slightly. If the full-range FCC gasoline is depentanized for TAME production, the pentene-rich TAME raffinate can be coprocessed in the MBR unit. TAME raffinate will still contain significant cyclopentene and some methanol, even after water washing. These are troublesome impurities for alkylation, but are tolerated and actually upgraded by MBR.

MBR also will partially desulfurize light FCC gasoline (C3 to 220 F.). Sulfur levels in light FCC gasoline may range from about 50 ppm with hydrotreated FCC feeds to as high as 300-1,000 ppm with higher-sulfur FCC feeds.

One half to two thirds of the organic sulfur in the C5-to-220 F. true boiling point cut will be converted to H2S and light olefins by MBR, without any hydrogen addition. MBR will "trim off" troublesome sulfur in the very-olefinic C5-to-220 F. portion of FCC gasoline, without loss of octane. Table 4 shows an example of MBR desulfurization.

Coprocessing FCC C5 Cut, TAME raffinate, intermediate FCC gasoline, or light coker gas oil in MBR should be assessed as possible elements of a comprehensive reformulated gasoline production plan. The benefits of MBR C5- restructuring can open up new blending options within reformulated gasoline formulas.