ETHYLENE TECHNOLOGIES-Conclusion

Marcello Picciotti
Technipetrol SpA
Rome

While these processes may play certain niche roles in ethylene production, they are likely to find little room in industry.

The first article in this series presented new ethylene processes that use conventional feedstocks. This final article details novel routes that use alternative feeds.

A review of recent advances in conventional steam cracking reveals that the major process licensors all offer advanced ethylene technologies. These companies must rely much more on their selling ability and market conditions than on superior process features.

Future and ongoing developments in ethylene technology will take their momentum from both market forces and technological advances. The preferred route to ethylene will be the one based on the cheapest raw material, minimum energy consumption, and lowest investment.

Methane oxydimerization

P.S. Chernykh and V. Menschikov of the All-Union Research Institute for Organic Synthesis (Vniios), Moscow, developed a process for catalytic oxydimerization of methane. The process uses a magnesium and lanthanum oxide catalyst.

In 1991, methane conversion of 31-33% and a per-pass yield of 18-19% were achieved at the following conditions:

• Temperature-860-870° C.

• Space velocity-3,000-5,000 hr-1

• Feed-a fresh mixture of methane and oxygen with a molar methane-to-oxygen ratio of 3.0 or 3.5 to 1.

Under optimal operating conditions and catalyst formulation, C2 yields of 18-22% were achieved. Kinetic limitations are barriers to further yield increases. Only with diluted methane (inerts-to-methane ratios of 20-40) are the maximum yields of 30% per-pass achievable. Such diluted conditions make the process commercially unacceptable.

Catalyst stability and the tendency to form by-products have been investigated. Lithium-bearing catalysts lose their key component during operation, while halogen-bearing catalysts tend to form chloro-organics.

Based on the test conditions, a complete feasibility study was developed, including material and energy balances, process flow diagrams, main equipment and machinery characteristics, capital cost, etc.

With methane priced 2-3 times less than ethane, ethylene production cost is 25-35% cheaper than for conventional ethane pyrolysis, according to Menschikov. Further information on this process has been published.^{1 2}

Oxydehydrogenation

Also in 1991, the University of California's Lawrence Berkeley Laboratory (LBL) signed an agreement with a research company for the complete development of a new process to produce ethylene and propylene from methane^.3 The process, known as catalytic oxydehydrogenation, was invented and patented by the research team of Professor Heinz Heinemann.

The process involves passing a mixture of methane, oxygen, and steam over a mixed calcium/nickel/potassium oxide catalyst at about 600° C. and at atmospheric pressure. Conversions of 10-15% per pass and almost 100% selectivity to C2/C3/C4 cuts are reported. The C2-4 cut comprises about 88% C2s, 10% C3s, and 2-3% C4s. Typical performance figures are shown in Table 2 [15469 bytes].

Compared to other catalytic processes, this process has the advantage of reduced carbon dioxide formation. (Carbon dioxide formation prevents higher selectivities.)

The research program was aimed at determining whether the technology could be economically scaled up to the commercial level. A semicommercial plant planned for Italy was not built because the process was found economically unfeasible.

Heinemann believes it is unlikely that oxidative dehydrogenation or oxidative methane dimerization will be successful commercially. In fact, it seems impossible to avoid the production of appreciable amounts of carbon oxides.

Among the modern ethylene-production techniques being studied, says Heinemann, many put their hopes in proton-conducting membranes. These systems involve methane dehydrogenation and coupling on one side of a catalytic membrane, and water formation from protons and oxygen on the other, thus avoiding hydrocarbon conversion to carbon oxides.

The flexibility of this scheme has been shown, but, in Heinemann's view, the proton transfer rate is too low for industrial application. Heinemann's remarks indicate that the outer limits of basic research into ethylene production have been reached.

Other methane processes

In the U.S., a number of companies and academics have done much work on oxidative methane coupling. The work of Arco Chemical Co. and Professor Lunsford at Texas A&M University must be mentioned, because some of it precedes Heinemann's work.

Other companies that have studied this process include: Standard Oil Co. (Ohio), Phillips Petroleum Co., Amoco Chemical Co., Broken Hill Proprietary Co. Ltd., Commonwealth Scientific & Industrial Research Organisation, and Exxon Research & Engineering Co.

Chem Systems Inc., Tarrytown, N.Y., published a set of reports dedicated to methane coupling.^4-6 These reports detail a wide range of recently patented membrane processes.

Mobil methanol process

Mobil Chemical Co.'s Methanol-to-Olefins conversion process was demonstrated in a 2,200 ton/day fluid-bed facility in Germany. The catalyst is essentially a small-pore, modified synthetic borosilicate zeolite with a high silica-to-alumina ratio.⁷

Modified zeolites provoked intense interest in the ethylene industry after their large-scale utilization for crude oil processing.

UOP/Hydro process

Among the technologies that convert methanol to olefins, a process codeveloped by UOP and Norsk Hydro ASA is certainly the newest. The two companies are running a field-test unit in a Norsk Hydro facility at Porsgrum, Norway. Unit capacity is 20 kg/hr.

Using a two-step sequence, low-cost natural gas can be converted to ethylene and propylene. The process is not intended to compete with steam cracking but, in areas where methane is abundant and cheap, this technology may provide a cost-effective way to convert natural gas to olefins.

Overall material balance and estimated investment for a methanol-to-olefins unit are shown in Table 3 [14684 bytes].

The economics of methanol-to-olefins processes were demonstrated by comparing a conventional 500,000 mt/y naphtha cracker with a natural-gas-integrated complex to produce olefins.

For a U.S. Gulf Coast plant including off sites as 25% of the building life-cycle cost, a return on investment (ROI) of about 30% is achievable for a natural-gas-based, methanol-to-olefins plant. This compares to about 26% for a steam cracker. For a methanol-to-olefins unit alone, using a methanol cost of $100/ton, the relevant ROI jumps to more than 36.5%.

The conclusion is that, although the capital cost of a natural-gas-based olefins complex may be significantly higher than that of a conventional naphtha cracker, the feedstock cost advantage can overturn the resulting ethylene production cost.⁸

Currently, UOP is focusing its near-term energies on the application of the UOP/Hydro methanol-to-olefins process to very specific revamp projects.

State-of-the-art

No one should miss reading Peter H. Spitz's history of the petrochemical industry.⁹ This book is a provoking mosaic of the rise of the modern petrochemical industry. The "apotheosis," in Spitz's representation, is the introduction of world-size steam crackers.

It is true that U.S. engineering companies have made an essential contribution to the development of the steam-cracker designs. During the 1970s, Stone & Webster Engineering Corp. was the leading U.S. ethylene contractor. It introduced the concept of feedstock flexibility.

But, in Spitz's book, little credit is given to European contractors, some of whom have contributed heavily to theoretical and technological research, throwing light on basic notions of the fundamental cracking mechanism, its laws, and inherent problems in naphtha and heavy hydrocarbon cracking. This also applies to cracked-gas handling for recovery and purification of valuable components.

The European industry, in fact, is based on liquid hydrocarbon cracking. Since the early '70s, the refining industry trade journals have given extensive coverage to developments and improvements in steam cracking and associated cold-section separation. This has encouraged European researchers and scientists to share with U.S. readers their findings in the field of liquid and heavy-liquid cracking.

The state-of-the-art European ethylene technology is still little known in the U.S. Using this technology, Technipetrol has built a 720,000 mt/y plant in Priolo, Sicily, and a 400,000 mt/y ethylene plant in Brindisi, Italy.

Both plants are unique. The Brindisi plant is rated second in Europe for optimal feed and energy consumption. And the Priolo plant, based on 50/50 naphtha and gas oil feed, has great feed and products flexibility. At the time it was built, it was the world's largest.

The size of ethylene plants continues to increase. Many of the half million ton/year plants operated today were built in the late '70s. Even though beyond a certain size no incentive exists, the trend is toward megacapacity.

In the author's view, average plant size in the near future is likely to remain around 450,000-550,000 mt/y. Although ethylene production by megacapacity plants is commonplace, producers are increasingly aware of the disadvantages of these "megaunits."

This new generation of ethylene plants is the culmination of a golden era in the petrochemical industry. But, apart from this megacapacity trend, what other technological advancements are being made?

After the apotheosis

In truth, during the last 10-20 years, no real breakthroughs have occurred in ethylene technology. All of the processes currently available are fairly stable and similar. This may be because of the maturity of the technology.

The long development programs that engineering contractors went through in the last 10-15 years to survive in an extremely competitive ethylene-contracting business have progressively exhausted possibilities of further improvement.

In terms of overall process performance, it would be difficult to cite the best process, and even more difficult to find significant process differences between the schemes. However, perhaps one of the major breakthroughs is the so-called millisecond furnace developed in the late '70s by M.W. Kellogg Co.

The first prototype for this process was built in Japan. The furnace development program also led to design and construction of a double-pipe heat exchanger to replace the so-called transfer line exchangers (TLX), also called quench boilers. This is probably the most recent step change in steam cracking technology.

Kellogg experienced some setbacks because of a phenomenon involving overly fast coking with light feeds. But today, millisecond furnaces operating on ethane and other gases are providing high ethylene yields, according to Ray Orriss.

In pyrolysis, much progress is being made on inhibiting coke formation. This work may soon produce a commercial breakthrough.

The economics and design of ethane crackers will be affected by the use of coke inhibitors. A number of papers on coke inhibition have been published in the last few years by Amoco, Kellogg, and Phillips.¹⁰

A second approach to coke inhibition may be to modify the radiant coil surface. The concept is not new, but many practical and effective innovations may soon emerge.

In the area of furnace design, the use of new materials containing 35% chromium has been successful. These materials show much higher resistance to carburization and deterioration, both of which can cause coking.

In the separation area, attention is being focused on progressive distillation. The contribution of V. Kaiser to this area has been huge.^11-13

The major ethylene technology suppliers have incorporated distributed distillation into their technologies. This scheme is targeted to low energy usage, with a low-to-medium-pressure demethanization system, and a predemethanizer for larger plants.

Highly technological equipment has certainly contributed to further improvement in the performance of conventional ethylene technology, with additional capital cost sometimes surpassing direct cost benefits.^14-16 These applications include: dephlegmators, high-flux heat exchangers, high-efficiency distillation trays, turboexpanders, inter-reboilers, and advanced controls.

For the purpose of designing low-capital-cost plants, a new process replaces modern cryogenics with a solvent application for ethylene recovery.¹⁷ This concept has seen perhaps a previous application.¹⁸

Finally, a new membrane-based olefin recovery process has been developed by BP Chemicals and Stone & Webster. The process, called Selective Olefin Recovery (SOR), involves chemical absorption by silver nitrate solution. It would replace cryogenic distillation.

In the author's view, this process will not fit commercial applications in large-capacity ethylene plants. However, its long-term economic viability has yet to be determined.

Although there is much work under way on improving ethylene technology, the success of current processes must rely much more on contractors' selling ability and on market conditions than on superior process features.

The future of ethylene

Despite research and development efforts during the last 2 decades, the announced new technologies for ethylene are unlikely to find a place in industry. Yet future and ongoing developments will, as always, take their momentum from both market forces and technological advances.

The preferred route to ethylene will be the one based on the cheapest raw material, minimum energy consumption, and lowest investment. So far, the responses to these requirements have been:

• Upgrading lower-value raw materials for use as cracker feedstock

• Increasing unit capacities and feedstock flexibilities

• Devising new routes, such as methane and methanol conversion

• Developing high-temperature catalytic pyrolysis systems.

Although the possibility of a change in raw material for olefin production must not be neglected, the conventional steam cracking of gas and naphtha feedstocks will remain the dominant technology.

Cracking furnaces will remain, for many years, the sole industrial apparatus for large-scale ethylene production. Potential alternative routes will play no more than a niche role.

Acknowledgment

The author wishes to thank: Heinz Heinemann of Lawrence Berkeley Laboratory and V. Menschikov of Vniios for revising the manuscript, and for their suggestions; Ray Orriss of M.W. Kellogg; Lionel Chambers of Stone & Webster Engineering Co.; N. Curtis of UOP; Mark Scharre of Phillips Petroleum Co.; Wilfred Lam of Brown & Root Braun; and Dr. Victor Kaiser.

References

1. Catalysis Today, Vol. 13, 1992, pp. 571-74.

2. Thirteenth World Petroleum Congress, Forum 11 Poster.

3. European Chemical News, Dec. 2, 1991.

4. Chem Systems Inc. PERP Report 8803, "Methane Conversion to Olefins and Liquids," August 1989.

5. Chem Systems Inc. PERP Report 89S14, "Ethylene from Methane," March 1991.

6. Chem Systems Inc. PERP Report 92S3, "Methane to Ethylene-BHP Process," March 1994.

7. H"lderich, W., and Gallei, E., "Industrielle Anwendung Zeolithisher Katalyzatoren bei Processen," Chem. Ing. Tech., Vol 56, No. 12, 1984, pp. 908-15.

8. Vera, B.V., et al., "UOP/Hydro MTO Process: the Critical Link in Upgrading Natural Gas to Olefins," 1996 Chemical Marketing Associates Inc., World Petrochemical Conference, Mar. 20-21, 1996, Houston.

9. Spitz, P.H., Petrochemicals: The Rise of an Industry, John Wiley & Sons Inc., New York, 1988, pp. 385, 437, 453, 459.

10. Reed, L.E., et al., "The Effect of Sulfur Compounds and Phillips Antifoulants in Ethane Pyrolysis," 208th national meeting, American Chemical Society, Aug. 20-25, 1995, Chicago.

11. Kaiser, V., and Orion, J.P., "Modern Gas Separation Technology Applied to Ethylene Plants," AIChE spring national meeting, March 1988, New Orleans.

12. Kaiser, V., and Picciotti, M., "Better Ethylene Separation Scheme," Hydrocarbon Processing, Vol. 67, No. 11, November 1980, pp. 57-61.

13. U.S. Patent No. 5,253, 479, R. Di Cintio, et al., Oct. 19, 1993.

14. Lacadamo, G.A., et al., "Dephlegmator Technology for Improved Hydrocarbon Recovery from Gas Streams," AIChE spring national meeting, Apr. 3, 1989, Houston.

15. O'Neil, P.S., and Ragi, E.G., "Recent Application Trends for Enhanced Boiling Surface Tubing," AIChE winter meeting, November 1986, Miami.

16. O'Neil, P.S., et al., "Experience with High-Performance Process Equipment in Large Ethylene Plants," 7th Large Chemical Plant Symposium, Oct. 7, 1988, Brugge, Belgium.

17. ECN Chemscope, June 1966, pp. 8-9.

18. Foust, A.S., Wenzel, L.A., et al., Principles of Unit Operations, John Wiley & Sons Inc., New York, 1960, pp. 52-53.

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