Anne K. Rhodes
Refining/Petrochemical Editor
Refiners and merchant manufacturers in the U.S. are gearing up to produce the large volumes of methyl tertiary butyl ether (MTBE) needed to comply with oxygenated gasoline requirements.
The 1990 U. S. Clean Air Act Amendments specify that, as of the first of this coming November, gasoline containing a minimum of 2.7 wt % oxygen must be sold in 39 CO-nonattainment cities. Refiners and others are scurrying to bring MTBE capacity on line in time to meet this requirement.
Many U.S. refiners already have some operating MTBE capacity, but this will not be nearly enough to meet the looming increase in demand. As a result, additional capacity is being constructed worldwide.
Edward J. Swain, process planning engineer for Bechtel Corp., recently completed a study of operating and planned capacity for oxygenate units. According to the study, U.S. MTBE capacity at the end of 1991 was 128,970 b/d. If planned projects remain on schedule, this number is expected to reach 209,170 b/d by the end of this year.
This rapid growth rate should continue in 1993, with 84,900 b/d coming on stream, bringing the U.S. total to 294,070 b/d. Looking a little further out, projections for yearend 1995, according to the study, are 389,270 b/d. This total includes projects in the engineering phase, and two projects, totaling 37,500 b/d, under study.
Refiners are choosing a variety of routes and technologies to produce MTBE (Table 1). Projects of several U.S. refiners will be outlined to illustrate this point and to reveal the resulting changes in refinery operations.
VALERO
Valero Refining & Marketing Co. currently has 2,000 b/d of MTBE capacity at its Corpus Christi, Tex., refinery. Its process removes butadiene from the alkylation unit feedstock. This improves alkylate quality and reduces acid consumption, while producing MTBE.
A butane/butylene mixture from the heavy oil cracker (HOC) vapor recovery unit is mixed with hydrogen and charged to the hydrogenation reactor, where the butadiene is converted to butylene (Fig. 1).
The butane/butylene is then mixed with methanol in an MTBE synthesis unit, which converts the contained isobutylene and methanol to MTBE. The remaining butane/butylene stream is charged to the alkylation unit.
Valero broke ground on a $230 million butane upgrade/MTBE project in January of this year (Fig. 2). The 13,000 b/d facility is expected to begin operating by second quarter 1993.
All of the normal butane needed to produce isobutylene for the new MTBE plant will be supplied by Valero-related operations--either Valero Natural Gas Partners L.P. or the refinery itself. The feed to the new MTBE unit will be equal in purity to the stream from the fractionator--probably 95-96% butane, the rest being isobutane.
Methanol feedstock will be purchased from Gulf Coast manufacturers. Valero has arranged for most of its supply under a term contract.
PROCESS DESCRIPTION
Valero will use UOP's Butamer, Oleflex and Huls processes for, respectively, isomerization, dehydrogenation, and MTBE synthesis (Fig. 3). Another MTBE unit utilizing this same route is Enron Corp.'s LaPorte, Tex., plant, purchased from Tenneco Inc. (OGJ, May 20, 1991, p. 25).
The C4 stream from storage is fed to a caustic treating unit to remove mercaptan sulfur. It then goes to the deisobutanizer (DIB), which separates isobutane from n-butane. A vapor sidedraw off the DIB yields normal butane recycle.
Normal butane from the DIB is dried in molecular sieves, then combined with hydrogen at high pressure and vaporized by heat exchange.
Isomerization takes place over a platinum catalyst in two series-flow reactors. The reactor effluent is cooled and recycle hydrogen separated from it in a high-pressure separator. It is then compressed and combined with dry makeup hydrogen.
Separator liquid is preheated with reactor effluent and fed to the stabilizer, which removes propane and lighter materials. The stabilizer overhead vapor flows to the plant fuel gas system after caustic scrubbing.
Stabilizer bottoms flow to the DIB, along with butanes from the alkylation unit and from the Complete Saturation Process (CSP) unit, and are used to convert olefins to paraffins during the MTBE production process.
CSP is a Huls technology, licensed by UOP, that uses a palladium catalyst to convert leftover butylenes to butanes. The process saturates the butylenes in the recycle loop to produce butanes, which are then fed to the Butamer unit for reprocessing.
Isobutane from the DIB is fed to the Oleflex unit. (The DIB bottoms stream consists of isopentane and heavier material which is routed to gasoline blending.)
The isobutane passes through a nitrogen guard bed, then molecular-sieve driers. The feed is then chilled and mixed with hydrogen recycle. The cold combined feed stream is preheated with reactor effluent and charged to the first reactor heater.
The feed is contacted with a platinum catalyst at high temperature and low pressure, thus producing isobutylene in three radial-flow reactors in series configuration. These reactors are designed for continuous downward flow of catalyst.
As the reaction proceeds, the mixture cools and is reheated by interheaters between the reactors. Catalyst withdrawn from the first reactor is fed to the top of the second, and then third, reactor, concurrently with the feed, by means of lift gas systems.
Spent catalyst from the third reactor is lifted to the top of the continuous catalyst regeneration unit, where it is continually regenerated.
Cooled effluent from the third reactor is compressed in a two-stage compressor system with intermediate cooling, then passed through an alumina treater to trap residual chlorine from the regeneration process. Molecular sieve beds remove residual water and hydrogen sulfide.
A cryogenic separation plant separates liquid product from the hydrogen recycle and product gases. The net gas is compressed and processed in a pressure swing adsorption (PSA) unit. The PSA unit produces high-purity hydrogen, for use within the refinery, and a fuel gas stream.
Liquid product from the Oleflex unit is received in the feed surge drum, then combined with fresh methanol from storage and recovered unreacted methanol. The mixture passes through an in-line static mixer and a cooler before proceeding to the reactor.
The effluent from the first reactor is cooled and a portion recycled for temperature control. The net flow passes through a trim cooler to adjust the inlet temperature to the second reactor. A butene column then separates MTBE from the C4 raffinate.
The MTBE product is drawn from the column bottoms, cooled, and sent to storage. The net overhead liquid (containing the unreacted methanol, isobutane, and light ends) flows to the raffinate water wash column for methanol recovery via extraction. The raffinate leaves at the column top and is pumped to the depropanizer, then to the oxygenate removal unit (ORU).
The ORU removes residual oxygenates, such as methanol, dimethyl ether, and tertiary butyl alcohol (TBA), from the MTBE raffinate using molecular sieve driers. Feed passes through two of three beds in series. The third bed is regenerated while the other two are online.
The ORU product is sent to the CSP unit. The CSP reactor effluent is fed to the CSP stripper, which Strips dissolved hydrogen from the effluent. The stripper bottoms liquid is cooled, then sent to the Butamer unit, where it is fed to the DIB as reflux.
BUTAMER UNIT
Valero's Butamer unit was built when the plant was originally constructed, but was not run because of the unfavorable iso/normal spread. The refinery has been using parts of that unit for MTBE synthesis in its 2,000 b/d plant. It will now convert the unit back to Butamer service.
The isobutylene stream to the operational MTBE unit will go back to the alkylation unit when the Butamer unit conversion begins. The refinery will make alkylate out of that stream until the new MTBE project is finished.
The new unit is expected to have no negative influence on the refinery's HF alkylation unit, which currently has excess capacity. This excess capacity will enable Valero to alkylate these isobutylenes while alkylating all the butylenes from the HOC unit.
The company also anticipates no changes in the operation of the cracker to supply feed for the MTBE unit.
Valero Refining & Marketing President Martin Zanotti said the presence of the Butamer unit did not influence the company's subsequent choice of UOP dehydrogenation technology -all available technologies were investigated.
The company also considered using a catalytic distillation process, but was well into the engineering phase and decided there were not enough advantages, in this particular case, to redo the engineering. Valero will have the capacity in the synthesis tower to retrofit this process later, if desired.
AFTER START-UP
The butane upgrade project will involve a workforce of about 800 during construction. When completed, the facility is expected to create 34 permanent jobs. Annual expenditures for routine maintenance work are estimated to be about $1.75 million. The contractor on the project is Fluor Daniel Inc.
Most of the produced MTBE will be blended into gasoline as time goes on, but Valero also has the option of selling it. The refinery has a sophisticated computer inline blending system with on-line octane and volatility measurement. A large portion of gasoline is blended directly into ships.
Valero has no retail outlets; it sells all of its gasoline wholesale. The marketplace has yet to determine exactly what grades of reformulated gasoline Valero will be required to blend, although 100% of its gasoline output ultimately will be reformulated.
Zanotti says the company expects to undergo a period where it is making both conventional and reformulated gasolines for its customers, in addition to selling blending components. The refinery will, of course, produce the most profitable product slate possible.
Valero believes that somewhere between 1995 and 2000, virtually all gasoline will be reformulated.
CATALYST LIFE
Catalyst life in the Oleflex unit is expected to be greater than 3 years. Valero attains about 1-1/2 years of catalyst life on its operating MTBE unit, and expects similar results with the new plant.
In terms of product storage capacity, the refinery is adding about 120,000 bbl of MTBE storage to the 50,000 bbl it already has. Valero also has a tertiary amyl methyl ether (TAME) unit in the permitting process.
EXXON, BATON ROUGE
Exxon Co. U.S.A. broke ground in late third quarter 1991 on a 7,000 b/d MTBE unit at its refinery at Baton Rouge, La. Start-up is projected for late 1992 (Fig. 4).
The unit is somewhat oversized to provide for future isobutylene yield increases. Feed sources will include isobutylene from the refinery's fluid catalytic cracking unit (FCCU) and purchased methanol.
Exxon is utilizing a catalytic distillation process for MTBE synthesis. Exxon Chemical America's Baton Rouge chemical plant produces about 3,000 b/d MTBE via the same method.
The refinery MTBE plant site was chosen based on land availability and proximity to utilities and feed and product lines. MTBE product will exceed accepted commercial specifications (i.e., 95% MTBE, < or equal to 0.7% methanol).
Because the MTBE plant feedstock will be an FCC-generated stream that currently goes to alkylation, the size of the alkylation stream will be correspondingly reduced.
Although Exxon is not currently planning any changes in the operation of its FCCU, it is keeping an eye on catalysts some vendors are developing to expand FCC isobutylene production.
In terms of air controls, Exxon is constructing its MTBE unit to meet all applicable Louisiana air quality regulations and federal new source performance standards.
This winter, for CO-nonattainment areas, Exxon will need to deliver gasoline with about 15 vol % MTBE. The company is currently blending an undisclosed, but small, amount of MTBE in its gasoline grades (OGJ, Sept. 16, 1991, p. 34). For the future, Exxon is exploring other oxygenates options.
CATALYTIC DISTILLATION
CDTech's catalytic distillation process, as designed for use at Exxon's Baton Rouge refinery, is shown in Fig. 5.
The FCCU-generated stream is fractionated to isolate the C4 portion. This stream usually contains nitrogen compounds that shorten the MTBE catalyst's life. Therefore, a water wash step is included to extract these compounds. (Exxon has determined other uses in the refinery for this water.)
The mixed-C4 stream then proceeds, with the addition of methanol, to a boiling-point (BP) reactor, which is a patented feature of the CDTech process. This reactor prevents the formation of hot spots in the catalyst. These hot spots can destroy the synthesis catalyst.
About 90% of this equilibrium reaction takes place in the BP reactor.
When isobutylene conversion in the BP reactor falls below 87%, the catalyst should be changed. When this occurs--typically in 1-2 years, according to the licensor--the catalytic distillation (CD) tower will compensate for that lost conversion, still achieving an overall conversion rate of 97%.
The catalyst in this reactor loses activity faster than that in the CD tower. Catalyst life for the CD tower is expected to be about 3 years.
The effluent from the BP reactor is two-phase. The heat contained in the vapor phase is not removed by cooling water; it is utilized in the CD tower, thus improving energy efficiency.
The catalyst in the reactor is a conventional ion-exchange resin available from several suppliers. Although the MTBE synthesis catalyst is regenerable, the process uses a strong mineral acid, making in situ regeneration cost-prohibitive.
In the BP reactor, the catalyst is loosely filled to form a packed bed. In the CD tower, it is packed in bales, sometimes called "Texas tea bags," manufactured by CDTech.
Catalytic distillation is the simultaneous reaction and fractionation of MTBE reactants and products. The process removes the MTBE from the mixture, thus overcoming the equilibrium limitation to high conversion to MTBE.
The product stream may be sent directly to gasoline blending, or it may be fractionated further to remove C5s. Exxon will send the C5 stream to gasoline blending.
The raffinate from the CD tower is washed with water, then fractionated to recover the methanol. The unrecovered C4s go to downstream processing.
TRANSAMERICAN REFINING
TransAmerican Refining Corp. in Norco, La., is instituting a three-phase MTBE program, designed to reach an ultimate 20,000-22,500 b/d MTBE capacity some time in 1994.
TransAmerican is conveniently situated on the Mississippi River to receive materials by barge, ocean-going vessel, road, and rail.
This will essentially be a merchant MTBE plant until the refinery comes on stream. TransAmerican hopes to accomplish this in the near future.
PHASE 1
Phase 1 of the project, close to completion, is expected to bring 7,200 b/d capacity on-line in early 1993.
The principal feedstock to produce isobutylene for this MTBE plant will be purchased mixed field butanes from South Louisiana or Mont Belvieu, Tex. Alternatively, prefractionated n-butane or isobutane can be used. Approximately 1 bbl MTBE is produced from each 0.95 bbl field butanes and 0.33 bbl methanol.
Phase 1 is centered around a Catofin dehydrogenation unit previously used to manufacture butadiene at Firestone Synthetic Rubber & Latex Co.'s Orange, Tex., plant. The unit, with its five large reactors, was moved to Norco about 2 years ago (Fig. 6).
The Catofin technology was purchased from Air Products & Chemicals Inc. by United Catalysts Inc. ABB Lummus Crest licenses the process.
TransAmerican is also converting an old middle distillate hydrodesulfurization (HDS) unit to ABB Lummus Crest's butane isomerization technology. When Phase 1 starts up, this will be the first operating unit utilizing Lummus isomerization technology.
This unit, run at reasonably high pressure, will be utilized below capacity in Phase 1. A fractionation tower will separate n-butane from isobutane and recycle the n-butane to isomerization.
MTBE synthesis will be performed in an existing Huls unit, which will be expanded from 4,000 to 7,200 b/d. All utilities and ancillaries--including cooling towers, electric substation, and air compressors--will be provided by the existing refinery.
A simplified process flow diagram is shown in Fig. 7.
About 7,000 b/sd of field-grade butanes will be required in Phase 1. Fractionation will remove light and heavy ends from this feedstream. The separated n-butane will be routed to the isomerization unit for conversion to isobutane.
The product stream from the isomerization unit, which contains some unconverted n-butane, will be piped to storage, and from there, back to the DlB towers for separation and n-butane recycling.
The DIB towers are presently part of TransAmerican's alkylation unit and are being modified for the new service.
The isobutane stream from the DIB towers will be routed to the Catofin unit for dehydrogenation to isobutylene. The main product stream from this unit, containing both isobutylene and unreacted isobutane, will be routed to MTBE synthesis.
Catofin, a low-pressure catalytic process, produces a by-product gas stream containing hydrogen. This hydrogen will be recovered and purified using a PSA unit. The purified hydrogen will then be used to meet the isomerization unit requirements.
The MTBE unit will synthesize MTBE by reacting isobutylene with purchased methanol. The product is purified by distillation, with the unreacted isobutane returning to the Catofin unit. The finished MTBE is sent to storage.
TransAmerican says it is saving money relative to grassroots MTBE plants by using the two existing refinery units and purchasing the Firestone unit. In addition, plant start-up will occur much sooner than would have been possible for a grassroots facility.
Two gasoline tanks at the refinery will be used to store methanol. MTBE will be held in existing internal floating roof tanks. An adjacent oil terminal owned by General American Transportation Corp. (GATX) offers additional storage capacity if needed.
Pressure storage for LPG will come from both the refinery and leased GATX Storage.
PHASES 2 AND 3
TransAmerican plans to bring in two additional Catofin reactors for Phase 2 of the project.
MTBE capacity will be expanded through the use of another idle reactor, making a total of two synthesis reactors. The butane isomerization unit will be operated at full capacity during this phase.
The company will construct Phase 2 equipment while Phase 1 is operating. Long lead time items are currently being manufactured. Phase 2 is expected to be completed in late 1993.
The core of the Phase 3 expansion is a shutdown Catofin unit from Brazil, which was acquired through a process equipment broker in Louisiana. This phase will require a new MTBE unit and expansion of the isomerization capacity.
Negotiations for these technologies are ongoing. Catalytic distillation is being considered for MTBE synthesis.
DIAMOND SHAMROCK
Diamond Shamrock Refining & Marketing Co. currently operates an MTBE plant feeding FCC-produced isobutylene at its McKee refinery in the Texas panhandle. Nameplate capacity is 1,830 b/d of 94% MTBE. The technology from ARCO employs a Rohm & Haas catalyst.
In ARCO's process, isobutane and oxygen are reacted to produce an intermediate peroxide and tertiary butyl alcohol (TBA). (The peroxide can be reacted with propylene to produce propylene oxide and TBA.) The produced TBA is then reacted with methanol to synthesize MTBE.
The plant flow scheme and process conditions are typical of FCC-feed designs. The unit was built in 1985-86 to provide additional octane and to unload the refinery's alkylation unit. Today Diamond Shamrock uses the MTBE to supply its retail outlets in the Denver and Albuquerque regions, which require oxygenated fuels.
The unit is located adjacent to a retired C4 isomerization unit. Salvage equipment was used in the construction.
There are no special features of the process or control scheme. Product is stored in dedicated as well as swing tankage as Diamond Shamrock accumulates MTBE for winter demand.
The company purchases methanol on the open market, primarily along the Gulf Coast, and transports it by rail to the McKee refinery.
Diamond Shamrock is currently engineering a TAME plant, also feeding FCC-produced olefins, to parallel the MTBE unit. The TAME plant will utilize CDTech technology, using CDTech's proprietary catalyst system in the CD column. The primary reactor catalyst has not been chosen.
Methanol feed source will also be Gulf Coast open market, although Diamond Shamrock is investigating building a methanol plant at the site. The methanol plant project is on hold because other refining projects demonstrate higher rates of return for the company.
The schedule for engineering and construction of the new TAME plant is:
- Complete engineering by end of 3rd Quarter 1992
- Complete construction by end of 3rd Quarter 1993.
The environmental permit is not yet issued, so construction timing is questionable.
Feed preparation includes the addition of a depentanizer to the FCC gas plant train, additional mercaptan removal, water wash, and selective hydrogenation of diolefins by Engelhard's process. The technologies chosen were believed to be the best-developed in the field.
The unit will be located adjacent to the existing MTBE unit in order to share methanol facilities and operating staff.
In justifying the project, Diamond Shamrock compared the economics of oxygenate value to olefin blending and oxygenate blending values. The project was attractive in both respects, but the company feels there is substantial uncertainty in the value of oxygenates in the market.
Diamond Shamrock has not made any projections of changes in FCC operations particular to supplying feed to the oxygenates units. However, it is working on modification of its FCC cyclone system to allow higher reactor temperatures and shorter contact times. This will directionally produce more oxygenate feed.
The company does not currently use a selective FCC additive like ZSM. It is looking ahead at alkylating the other amylenes, but will need an expansion to handle a substantial increase in feed quantity. The alkylation unit expansion is a future item without a firm schedule.
Diamond Shamrock believes that both diolefin removal from the TAME feed and oxygenate removal from the raffinate are required for acceptable operation of the alkylation unit on amylenes.
MTBE is currently blended into all grades of gasoline for the Denver area during the winter control months. Typical formulas include 14-15 vol % MTBE, depending on gasoline density, as needed to comply with 2.7 wt % oxygen.
Gasoline blending is normally performed in-line, using MTBE as a component. Octanes are monitored as usual, using both on-line octane engines and laboratory check samples. Similar techniques will likely be used when TAME becomes available.
In the past, and now occasionally, Diamond Shamrock has blended MTBE strictly for octane enhancement. For octane, typically the percentage of MTBE in a blend is considerably lower gen content regulations.
Diamond Shamrock has no current oxygenate projects under way at its Three Rivers refinery in South Texas, but it does expect to build facilities there in the future.
COASTAL
Two Coastal Corp. subsidiaries are building 4,000 b/d of MTBE capacity at a site in Cheyenne, Wyo. The plant's feed sources will be field-grade butanes and natural gas.
The butanes will be isomerized via Engelhard's process, and the isobutane will be dehydrogenated using Phillips 66 Co.'s STAR process. Coastal will also use ICI's technology to produce methanol from natural gas, and Phillips' MTBE route to synthesize MTBE.
The project is scheduled to take 12 months, start to finish, with start-up expected by early September.
Coastal is also producing 12,000 b/d MTBE at its Corpus Christi, Tex., refinery. This MTBE is produced for ARCO Chemical Co. via ARCO's MTBE technology.
FUTURE ROUTES
One of the more-promising routes to the production of MTBE likely to be in commercial operation in the relatively near term involves the isomerization of n-butylene to produce isobutylene. At least three licensors are hoping to make such a process commercially available within the next few years (OGJ, Mar. 23, p. 42).
Although this will cut costs by reducing the number of steps required to produce MTBE, its effect on n-butylene supply and markets are unknown.
With this new technology and other advances, producing MTBE will become cheaper, and the number of options available to manufacturers will grow. Whether this will enable supply to keep up with demand and refiners to achieve comfortable profit margins remains to be seen.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
