MTG Revisited

A. Holmen et al. (Editors) ,Natural Gas Conversion 1991 Elsevier Science PublishersB.V., Amsterdam

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MTG REVISITED Clarence D. Chang Mobil Research and Development Corporation Princeton, NJ 08543-1025 USA INTRODUCTION Nearly two decades have passed since Mobil's discovery [1,2] that methanol can be transformed directly to high octane gasoline over ZSM-5 zeolite catalyst. The first commercial plant based on this unique chemistry has been onstream for five years in New Zealand, where it converts 140 million standard cubic feet per day of natural gas mainly from the offshore Maui gas field, to methanol, thence to 14,500 barrels per day of gasoline for premium unleaded specifications. The plant is designed to supply one third of New Zealand's current gasoline demand, and was key to their attainment of 50% self-sufficiency in transport fuels [3], a strategic goal of the government. In a

historical sense, the MTG (Methanol-to-Gasoline)Process was the first major advance in synfuelstechnology in half a century since the Fischer-Tropsch Process. Developed in the wake of the Arab oil embargo, MTG technology was a timely response to the subsequent "energy crisis". Much has already been written and told of MTG, from its serendipitous beginnings through its development into a viable process. That phase of its history will not be revisited here. Less has been said about the actual planning and construction of the commercial unit in New Zealand. It is a fascinating story, particularly from a perspective of the "bench chemist" who struggled with the early mysteries of the new reaction. It is in New Zealand that we pick up the story. From there we travel to West Germany, where advanced MTG concepts were tested and realized. We conclude with a brief discourse on the still unsolved reaction mechanism. NEW ZEALAND GTG New Zealand's response to the oil supply crisis of the 1970s was a bold decision to drastically curtail dependence on oil imports, which was weakening the economy and limiting growth, by utilizing a portion of their vast offshore natural gas reserves for

394

synfuels production. A target of 50% self-sufficiency in transport fuels by the mid 1980's was set. Mobil's MTG process (also called "GTG" for "Gas-to-Gasoline" in New Zealand) was selected on the basis of overall process economy and efficiency, after careful scrutiny of availabls synfuels technologies. The New Zealand Synthetic Fuels Corporation (NZSFC) was formed to build and operate the plant. Initial shareholders were the Crown (New Zealand Government) (75%) and Mobil(25%). The Crown has since sold its holding to private interests. By August 1980 project concepts were finalized, and a plant site was chosen. Prime concerns in site selection were proximity to the Maui gas field, a reliable source of water, proximity to a population center for the work force, and proximity to a suitable port for shipping and transportation access. Last, but by no means least, were stringent environmental requirements. The chosen site, encompassing some 300 acres surrounded by dairy farmland, is located on the North Island near New Plymouth on the Tasman Sea coast, with Mt. Taranaki, a dormant volcano, forming a scenic backdrop. Preparation of the Environmental Impact Report (EIR) was begun. Among the multitude of issues addressed was water use, waste management, noise levels, and the preservation of Maori archaeological sites adjacent to the plant site. To minimize intake and discharge, process water would be recycled. Sophisticated biological wastewater treatment is employed. Maximum noise at the fence line would not exceed 45 db. Housing and transportation would be provided for the estimated 1500-2000workers in the area during construction. in March 1982 the EIR was accepted, construction permits were issued, and site clearing started. The New Zealand GTG project was distinguished by some highly unusual and innovative features [4]. One of the first concerns was the lack of sufficient skilled labor in New Zealand to accomplish the task in the projected 5 year time. The decision was therefore made to design the project for modular construction, incorporating large preassemblies. Prefabricated units were to be built outside of New Zealand and shipped over for installation at the site. The Japanese firm of Hitachi Zosen was contracted for the pre-assemblyjob. The size and weight of preassemblies were dictated by their delivery route from the Port of Taranaki harbor, through the town of New Plymouth, and along existing highways to the plant site, a total distance of 25 km. The load bearing capacity of each road and bridge had to be carefully determined. As a result the maximum weight of each preassembly was limited to 600 net tons, and their

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size to 15 meters length by 33 meters width. Seventy-six preassemblies, weighing a total of 15,000 tons, were built and shipped to New Zealand over a 15 month period, in 16 sea voyages. This undertaking was rated one of the largest of its kind in the world. Situated along the juncture of the Pacific and Indo-Australiancrustal plates, New Zealand is subject to volcanic and seismic activity. The region around Mt. Taranaki is largely composed of multilayered volcanic ash. This necessitatedthe installation of piles, some 7000 in all, to support major foundations. Another concern was possible soil liquefaction during an earthquake. To stabilize the site, a series of eductor wells was drilled to lower and maintain the water table below grade. Critical plant components, for example the gas reformer furnace structures, were designed to withstand an earthquake of magnitude 7 on the Richter scale. The plant was turned over to NZSFC in phases beginning in December 1984, completed in July 1985, and started up in October 1985. Full production was achieved in April 1986. The project was brought in on schedule, 17% under budget, and can truly be regarded as an engineering tour de force. A simplified block diagram of the New Zealand GTG plant is shown in Figure 1.

D

i

e

144 U'Mr 635 GPM

Fig. 1. Simplified Block Flow Diagram. Natural gas from the Maui Platform is piped ashore and converted to crude methanol via the ICI low pressure process. Two methanol trains, each with a production capacity

396

of 2200 tons per day, are utilized. Crude methanol is fed directly to the MTG reactor section where it is converted to gasoline. The product gasoline, after treating to remove heavy components (mainly durene), is exported or sent to the Marsden Point Refinery at VJhangarei for biending. A flow diagram of the MTG reactor section appears in Figure 2.

Fig. 2. Typical MTG Fixed Bed Process Flow. Methanol feed is vaporized by heat exchange with reactor effluent and enters the dehydration reactor, where it is converted to an equilibrium mixture of dimethyl ether (DME), water and methanol. This mixture is combined with recycle gas and enters the fixed-bed conversion reactors containing ZSM-5 catalyst, where it is converted to aromatic gasoline. Light gas recycle s e w s to remove heat generated by this highly exothermic reaction. As seen in Figure 2, there are five conversion swing reactors. One of these will be on regeneration, while the others are in conversion mode. Multiple reactors are necessary to minimize pressure drop and to maintain constant product selectivity. Selectivity changes through each cycle due to the phenomenon of bandaging, peculiar to MTG in fixed bed operations. In a fixed-bed MTG occurs over a relatively small catalyst zone, or "band", which moves progressively down the length of the bed, leaving behind an ever-increasingzone of coked, deactivated catalyst and a decreasing volume of active catalyst in front. Because of the sequential nature of the reaction (vide infra), the product becomes less aromatic and more olefinic with progressive band-aging, the effect of marching back along the reaction path.

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Essentially constant overall product selectivity is therefore maintained with multiple beds "staggered" with respect to the progression of band-aging.

ADVANCEDMTGCONCEPTSANDDEVELOPMENT Band-agingeffects are eliminated by conducting MTG in a fluid-bed reactor, where constant catalyst activity (age) can be maintained by catalyst regeneration and fresh catalyst addition. A fluid bed also offers the major advantage of high efficiency for heat removal. However it was early recognized that a fluid-bed MTG process would require more extensive development than a fixed-bed process. The more conventional fixedbed process, on the other hand, could be developed within the projectedtime frame of the New Zealand venture, while the advanced fluid-bed concept would be more suitable for future larger scale operation. Therefore a "double-pronged approach" [5] was adopted to develop both concepts simultaneously to meet both short and long term goals. Two factors were of critical importance for the successful development of a fluid bed process: 1) complete methanol conversion (no bypassing) had to be assured to avoid a costly distillation step for feed recovery, which meant the need for detailed information on reactor hydrodynamics, and, 2) an abrasion-resistant ZSM-5 fluid catalyst needed to be developed. The Catalyst and Process groups in Mobil's Paulsboro Laboratory were pressed into service. Followingsuccessful catalyst development and vertical scale-up from bench reactors to a 4 B/D fluid unit [6],a full-scale cold flow model (CFM) was constructed to simulate a 100 B/D reactor, in the horizontal scale-up [A.Fabricated of carbon steel and Plexiglas, the CFM was utilizedto study the effect of numerous parameters, such as reactor baffle configurationand catalyst particle size distribution, on system hydrodynamics. The CFM studies provided confirmation of the design basis for the 100 B/D plant, Construction and operation of this semi-works plant in Wesseling, Federal Republic of Germany was a joint project of Mobil Research and Development Corporation, Union Rheinische Braunkohlen Kraftstoff AG (URBK), and UHDE GmbH, with suppon from the West German and US governments. After about one year of construction, the plant was started in December, 1982. Figure 3 is a schematic of the plant and is largely selfexplanatory.

398

Methnol Wal

Fig. 3. 100 B/D Fluid Bed MTG Demonstration Plant. It is seen that an external cooler to remove reaction heat is provided. Not shown are internal cooling coils. In operation, both external and internal cooling were evaluated.

In each case, temperature profiles were isothermal within 5°C. The 100 B/D plant logged some 8600 hours on stream, performing flawlessly and meeting all design goals

[a]. A comparison of fixed vs. fluid bed operation is given in Table 1. process gives higher ultimate gasoline yields at higher octane. TABLE 1 MTG Hydrocarbon Yields Fixed Bed

Fluidized Bed

Hydrocarbon Product, wt% Light Gas Propane Propylene iso-Butane n-Butane Butenes C5+ Gasoline C5+ Gasoline

(including Alkylate) Gasoline Octane Numbers RON

MON

1.3 4.6 0.2 8.8 2.7 1.1 81.3 100.0

4.3 4.4 4.3 11.0 2.0 5.8 68.2 100.0

-

-

83.9

91.2

93 83

95 85

The fluid bed

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Early studies at Mobil's Central Research Laboratory [ l ] elucidated the MTG reaction path:

n2

[ 2 C H 3 0 H s C H 3 0 C H 3 + H20]

nH 0

CnH2n-'

Eq. 1

n[CH2]

[CH2] = Average Paraffin-Aromatic Mixture revealing light olefins as intermediates. Subsequent kinetic and catalyst studies 191 defined conditions for decoupling olefin formation from aromatization, setting the stage for a possible methanol-to-olefins(MTO) process. Such a process could be the key to methanol conversion to distillate fuels and chemicals. For G+D production, MTO olefins can be converted utilizing the MOGD Process [lo-121 as illustrated in Figure 4. Fuel

Fractionnation Compression

Fuel

Fractionation

Garallno

Fig. 4. Schematic of Combined MTO-MOGD Process. Initial scale-up of the MTO concept was from a laboratory micro fluid-bed reactor (110 g. catalyst) to a 4 BID pilot unit (10-25 kg. catalyst) [13]. The imminent availability of

the 100 B/D MTG fluid-bed unit presented a unique opportunity to test MTO on semiworks scale at low incremental cost. This was done as the final phase of MTO development [8]. Total stream time accumulated was 3600 hours, during which 2130 tons of methanol were processed. The successful scale-up of MTO can be seen in Figure 5, where the yield of C2-C5 olefins is ptotted against the propane/propene ratio.

400 9080 -

x Yield c2-. cs'

8

a:

60 0

40

Fluid-Bed Mlno-Reactor

.01

I

.1

c; I ;c

Fig. 5. Fluid Bed MTO Scale-up. This ratio is an index of reaction severity and catalyst activity. Typical MTO selectivity is shown in Table 2. TABLE 2 Typical MTO Product Yield MTO' Product C1 c2 c3 c4 cp= c3= C4' Gasoline C5 C11 Diesel C12 - c18 Heavy Product Cg+ Water Soluble Oxygenate

-

Total Light Saturates (C1 - C3) Total Light Olefins (C2= - C4=) '482°C 102 kPa methanol partial pressure

1.4 0.3 2.3 3.9 5.0 31.8 19.6 35.7

0.3 100.0 4.0 56.3

40 1

Olefin distribution in MTO appears to be kinetically controlled 1231,therefore it seems possible, in principle, that the selectivity could be controlled by appropriate catalyst and reactor design. Indeed, ethylene selectivity is known to be enhanced in the presence of small pore zeolites and silicoaluminophosphates [14,15]. MTG MECHANISM We tum now to a brief discussion of the controversial question of MTG reaction mechanism. It will be impossible to present here a comprehensive survey. We will attempt to distill from the vast literature the essence of the question, recognizing the potential hazards in this undertaking. We also admit that this will be a somewhat personal view, for which we beg the Reader's indulgence. Actually, the running controversy is centered on the MTO portion of the reaction path (Eq. 1). The basic issue is how the first C-C bond is formed from C1 reactants. A plethora of mechanistic schemes has appeared in the open literature over the years covering virtually every possibility [16). Available experimental evidence, though provocative, has been less than definitive. Nevertheless some progress has been made, particularly in identification of reaction intermediates on the catalyst surface. It is generally agreed that surface methoxyl or methyloxonium species,

I

II

are present during the reaction. Furthermore, there is a measure of agreement that a carbene species, stabilized by interaction with the zeolite lattice, is implicated in initial C-C bond formation. Some possible structures are shown below:

\

L

/

111

/

Si

\

.+\A,/ / \ IV

402

Species 111 is an oxonium methylide (isoelectronic with surface-bound carbene), while IV represents a methylcarbene stabilized by association with an electron hole (solid-state defect) in lattice oxygen. These species may engage in C-C bond formation either by insertion into C-H bonds, for example [ l , 16b]

O'/ H+ 0 CH2 \

/

Si

\-/+\ Al

/

/

\

\

/

.

-ROH

Si

8H2CH3

\-/+\ Al

/

\

/

/

\

Si

/

\

or by undergoing methylation.

At issue is how the species 111 and IV are formed. The basic question therefore reduces to how hydrogen is abstracted from the methyl group. Every proposed mechanism inevitably meets this stumbling block. Hydrogen abstraction from C-H may either be heterolytic or homolytic. Heterolysis (deprotonation)would give a carbenoid species directly, but in the first analysis would require a strong base. Unfortunately the zeolite conjugate base itself has insufficient basicity for the task [17]. An alternative is a concerted mechanism, first considered by the Author [l], and more recently elaborated by Chuvylkin et al [18] through ab initio calculations, culminating in the following scheme:

ipH3

0 9 1

'

H&/"'

TP

la I

\ sI / O m \

/\

/ O

/\

\

-

H

i

'<'0Sp

\/$ &

H

H W C .

\sl /

/\

%

I

I

LA1 /O\

I

/ \

403

This depicts the steps of 1) adsorption of CH30H near Z-OCH3, 2) the concerted migration of 2 protons, generating an incipient methylide and CH30H2+, 3) reorientation, via rotation, of the CH30H2' bringing its carbon in close proximity to the methylide, 4) Walden inversion of the methyl group with concurrent C-C bond formation and generation of water. Homolytic mechanisms had not received serious consideration until Clarke et al [19] detected free radicals during DME conversion over HZSM-5. These workers used a spin trapping agent along with ESR. Chang et al [20] found that MTO was completely, but reversibly, inhibited by small amounts of NO, a well-known radical scavenger. However since a catalyst poison was apparently generated during NO inhibition, the Authors concluded that while the result confirms the presence of free radicals, the role of these radicals in MTO initiation is an open question. Nevertheless, since DME readily generates radicals upon gas phase pyrolysis, even at MTO temperatures [21], and since NO inhibition of DME pyrolysis is well-known [22], radical-initiatedMTO deserves further attention. A plausible mechanism might be the following [20]:

RH +OZ

___*

R.+ZOH

POSTSCRIPT

As noted in the beginning of this essay, the MTG process was developed in response to the oil supply crisis. The decision to commercialize was made at a time when the price of crude oil had rapidly risen to U S 2 8 per barrel from pre-embargo prices of around $3, and was projected by the forecasters to more than double by the end of the century. The opposite, of course, occurred and oil prices began to drop, reaching a low of $9 per barrel in 1986, rebounding to the present-day range of about

$16-$20 per barrel. At this level, MTG cannot compete with petroleum. For New Zealand GTG, however, it is estimated that when all loans are repaid in 1995, gasoline

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can be produced at a petroleum equivalent of $13 per barrel [3]. But as history clearly shows, petroleum futures are highly volatile and cannot be reliably predicted. From a technological viewpoint, the MTG introduced new C1 chemistry. The challengewill be to understand this chemistry more fully, and to explore its potentialfor novel applications. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

Chang, C. D.; Silvestri, A. J. J. Catal. 1977, 47, 249. Chang, C. D.; Silvestri, A. J. Cherntech 1987, 17,624. Maiden, C. J. Stud. Surf. Sci. Catal. 1988, 36, 1. Bem, J. Z. Stud. Surf. Sci. Catal. 1988,36,663. Penick, J. E.; Lee, W.; Maziuk, J. ACS Symp. Ser. 1983, 226, 19. Karn, A. Y.; Schreiner, M.; Yurchak, S. "Handbook of Synfuels Technology" (ed. R. A. Meyers), McGraw-Hill, NY, 1984, p.75. Avidan, A. A.; Gould, R. M.; Karn, A. Y. Circ. Fluid. Bed. Technol. Proc. Int. Conf., 1st 1985 (Pub. 1986), 287. Keim, K-H.; Krambeck, F. J.; Maziuk, J.; Toennesman, A. Erdoel Kohle, Erdgas, Petrochem. 1987, 103, 82. Chang, C. D.; Chu, C. T-W.; Socha, R. F. J. Catal. 1984,86,289. Garwood, W. E. ACS Symp. Ser. 1983,218,383. Avidan, A. A. Stud. Surf. Sci. Catal. 1988,36,307. Tabak, S.; Yurchak, S. Catal. Today 1990,6.307. Socha, R. F.; Chang, C. D.; Gould, R. M.; Kane, S. E.; Avidan, A. A. ACS Syrnp. Ser. 1987,328,34. Chang, C. D. Catal. Rev.-Sci. Eng. 1984,26,323. Kaiser, S. W. Arabian J. Sci. Eng. 1985, 10,361. For reviews see: a) Chang, C. D. ACS Symp. Ser. 1988,368,596; b) Hutchings, G. J.; Hunter, R. Catal. Today 1990,6, 279. Hellring, S. D.; Schmitt, K. D.; Chang, C. D. J. Chem. SOC.Chem. Commun. 1987,1320. Chuvylkin, N. D.; Khodakov, A. Yu.; Korsunov, V. A.; Kazanskii, V. B. Kinet. Katal. 1988,29,94. Clarke, J. K. A.; Darcy, R.; Hegarty, B. F.; ODonoghue, E.; ArnirEbrahirni, V.; Rooney, J. J. J. Chem. SOC.Chern. Comrnun. 1986, 425. Chang, C. D.; Hellring, S. D.; Pearson, J. A. J. Catal. 1989, 115, 282. Benson, S. W. J. Chem. Phys. 1956,25,27. Staveley, L. A. K.; Hinshelwood, C. N. J. Chern. SOC.1987, 1568 Chu, C. T-W.; Chang, C. D. J. Catal. 1984,86,297.