ETHYLENE TECHNOLOGIES-1: Novel ethylene technologies developing, but steam cracking remains king

Marcello Picciotti
Technipetrol SpA
Rome

Despite hundreds of patents filed in recent years and myriad new processes touted as revolutionary ethylene technologies, a breakthrough has yet to be found that economically replaces conventional steam cracking technology.

A review of the progress made in developing novel ethylene technologies will reveal barriers to commercialization.

Aside from attempts to produce olefins via different reactor types, in which reaction heat is supplied by fixed or circulating inert solid beds, research today focuses on finding the right catalyst for oxidative dehydrogenation of light hydrocarbons, or for oxidative dimerization of methane.

In addition, two recent commercial offerings are based on dehydration of methanol, with reported total olefin yields of more than 80%. The economics of methanol-based olefins production will rely primarily on attractively priced feeds or other special situations.

Here is a review of the latest announced ethylene technology developments, followed by details on the state of the art of conventional steam cracking.

The evolution of ethylene processes over the years has been slow but progressive, even if some backward steps have occurred here and there. After more than 50 years, thermal homogeneous pyrolysis, in its shining maturity, is the sole process on which the most modern and competitive petrochemical production is based.

This technology will likely stay with us for years to come, and will economically satisfy mushrooming ethylene demand, while alternative technologies will serve only limited market niches.

This first of two articles discusses ethylene processes that use conventional feedstocks. The second article will detail novel processes that convert feeds such as methane and methanol to olefins.

Conventional feeds

Developments can be separated into those that convert conventional feeds through the use of catalysts, oxygen, or both, and those that process alternative feeds such as methane or methanol. These are further subdivided, depending on reactor type, contact means, heat-method, process developer, and so forth.

Catalytic pyrolysis

The search for catalysts that perform hydrocarbon pyrolysis began many years ago. Research into applications of heterogeneous catalytic systems, mainly based on metals, started in the late 1960s, with initial papers published in the early '70s. Despite accumulated data confirming the effectiveness of metal-based catalytic processes, many in the industry refute high-temperature catalytic processes.

Heterogeneous catalytic systems, which are applied to highly endothermic reactions, feature high reaction rates and consequent temperature decreases. Both features are in contrast to the basic requirements for high ethylene yields: high temperatures and low residence times.

In addition to these outstanding thermodynamic and kinetic constraints, there are doubts about catalyst stability and regenerability to initial activity levels.

The following sections detail two important catalytic pyrolysis processes: one developed by S.P. Chernykh of the All-Union Research Institute for Organic Synthesis (Vniios), Moscow, and the other by Asahi Chemical Industry Co. Ltd., Tokyo.

Vniios process

The Vniios laboratories have performed a great deal of work on the catalytic pyrolysis of hydrocarbons.¹Table 1 [22339 bytes] shows results obtained with several different catalysts.

As can be seen in Table 1, potassium vanadate on corundum and indium oxide on pumice (Catalyst Nos. 1 and 2) have the highest activities, in terms of ethylene yields. Carbon oxide yields are highest with calcium and aluminum oxides (Catalyst No. 4). With indium and iron-chromium catalysts (Catalyst Nos. 2 and 3), coke formation is significantly higher than with vanadium.

Ethylene yields are, in all cases, substantially higher than with homogeneous pyrolysis, although the high dilution-steam ratios used (twice as high as in conventional pyrolysis) favor conversion. Propylene yields, in contrast, are somewhat lower. It is important to note that the sum of ethylene plus propylene is in the range of results seen with homogeneous pyrolysis (i.e., 45-46%).

Finally, ethane yield is substantially lower, except when indium catalyst is used.

The results of catalytic pyrolysis in a laboratory unit processing different hydrocarbon feedstocks are shown in Table 2 [43941 bytes].² With reaction temperatures about 60-70° C. lower than those needed in homogeneous pyrolysis, the following changes occur:

• Ethylene yields are substantially higher than those obtained with conventional pyrolysis (about 5-10% absolute).

• Propylene yields are lower, but the sum of ethylene plus propylene is 45-46%.

• Recently, the Vniios-design catalyst was charged in a commercial-scale demonstration furnace with a capacity of 7.5 tons/hr on a naphtha basis. The catalyst has shown a cumulative on-stream service of more than 10,000 hr.³ This process could be used to revamp existing units or to build new ones, but no commercial installations have been announced.

Asahi process

Asahi has recently announced two catalysts for light hydrocarbon cracking:

• Aluminum silicate zeolite with a medium pore size and an organic peroxide additive

• Loaded iron (100 ppm-1 wt %) on aluminum silicate zeolite with a medium pore size (5-6.5 Å).

Typical experimental results for these two systems are shown in, respectively, Table 3 [17208 bytes] and Table 4 [16956 bytes].

Operating conditions for both processes are:

• Temperature, 600-800° C.

• Pressure, 1-30 kg/sq cm g

• Space velocity, 1-800 hr^-1 (wt/wt).

Features of these catalytic systems are:

• Propylene yields 10% higher than with conventional pyrolysis

• Aromatics more than 20% higher

• Ethylene about 10% lower

• Ethane and propane higher.

For the moment, Asahi is not planning to utilize this process in its Mizushima plant.

For both of these catalytic systems, total olefins are in the range obtained with a conventional process. Additionally, a high propylene-to-ethylene ratio (0.80-0.95) and high aromatics yield are indicative of overall reaction conditions equivalent to or close to those of very-low-severity, conventional cracking.

The author believes that, in this last example, substantial overcracking produces higher propylene and aromatics production at the expense of ethylene. According to Dr. V. Menschikov, the fast aromatization rate is due to the action of the zeolites.

For these reasons, the enthusiasm for catalytic pyrolysis should he mitigated. The reaction endothermicity decreases temperatures at the catalyst's active centers and within the catalyst pores, where the reaction demand for heat is highest. Feed moves into the pores, while the catalyst remains enveloped in a film of olefins.

So the global rate of reaction is controlled by feed diffusion through the olefin film while olefins counterdiffuse from the catalyst surface into the feed. Coking on the catalyst surface, where olefin concentration is at its highest, appears to be unavoidable.

Hydropyrolysis

In the early 1970s, a new process was announced for ethylene production by thermal cracking of light to heavy hydrocarbons in the presence of hydrogen at elevated pressures and very short contact times.⁴ The principle itself-to replace steam by hydrogen or hydrogen-donating compounds-was certainly not new.

Sufficiently high temperatures and short contact times are needed to prevent excessive olefin hydrogenation. On the other hand, hydrogenation is an exothermic process, and may relax the heat requirements for the endothermic cracking reactions.

Features of this technique were:

• High ultimate combined yields of ethylene plus propylene (40-70 wt %, including recycle ethane cracking)

• The possibility of processing gas oil and other steam cracker by-products, such as butylene, pyrolysis fuels, propane, and propylene

• Reduction of coking.⁵

The mild hydrogenation conditions prevent diolefin formation (mainly butadiene and acetylenes) and condensation reactions; therefore, no fouling occurs.

Preliminary results obtained with a 1,000 ton/year pilot plant seemed very promising. More than the simple dilution role that steam plays in the conventional ethylene process, hydrogen takes part in the reactions, thanks to high pressure. This greatly reduces the tendency toward coking.

Very high temperatures (up to 900° C.), very short residence time (generally far less than 0.1 sec), high pressures, and high hydrogen partial pressures were the features of this system, which, at the time, received great interest. But, despite early enthusiasm, no further commercial or industrial developments were announced.

Circulating beds

The history of ethylene technology has seen the periodic appearance of new processes in which a hydrocarbon/steam mixture is heated by direct contact with an inert, or sometimes active, solid heat-carrier, instead of being heated through a wall.

The major limitations to conventional ethylene technology are:

• Fouling of coils, caused by coke formation

• Development of a high heat flux through cracking coils and relevant metallurgy

• Processing of feed with a high hydrogen-to-hydrocarbon ratio to facilitate olefins formation.

Conventional fluidized beds are not easily adaptable to highly endothermic, high-temperature reaction systems that require low residence times. The simplest thermodynamic model of this system, in fact, may involve a completely mixed, batch-reactor mode in the solid phase, and a piston-flow mode in the gas phase. So, again, high temperatures and very low residence times are, in principle, difficult to achieve.

M.A. Bergougnon has performed extensive research on the myriad aspects of these gas/solid systems, including: mixing, separation, and the application of ultra-rapid fluidized (URF) reactors for both heavy hydrocarbon cracking and catalytic system application.⁶ Typically, these fast-fluidization reactors are called circulated fluidized bed (CFB) reactors, or more generically, circulating bed reactors (CBRs).

A study of the hydrodynamics of these systems by J.R. Grace revealed that, as has often been the case in fluidization, applications have tended to precede understanding.⁷ This creates the risk of misapplication and underutilization of the technology.

In the author's experience with a CBR designed for ethylene production, reaction heat was supplied to the steam/hydrocarbon mixture by injecting into the gaseous stream, at 600-650° C., silica sand at 1,000-1,050° C. Higher sand temperatures would have caused localized melting of the sand.

The use of sand as a heat carrier accomplishes three major goals:

• Removing pyrolysis limitations caused by coil metallurgy and heat flux

• Allowing the use of heavy oil feeds

• Abating coking phenomena.

The research has been planned in three stages:

• A transparent model to study sand rheology and general flow dynamics⁸

• A field-test unit to check performance (commissioned in fall 1996)

• Complete basic design of a commercial unit.

Several other companies offer processes in which the reaction heat is supplied by a solid carrier. These companies include Lurgi AG, Phillips Petroleum Co., Kureha Chemical Industry Co. Ltd., Union Carbide Corp.,⁹ Chevron Chemical Co., and Stone & Webster Engineering Corp.

Stone & Webster has two versions: Thermal Regeneration Cracking (TRC) and Quick Contact (QC). TRC uses an inert solid carrier, and QC uses a solid with catalytic properties.^{10 11}

The QC technology is a circulating fluid-solid system employing a novel reactor that accomplishes hydrocarbon/solid contacting and separation in less than 250 ms. The QC technology can process a broad range of feedstocks.

The TRC process was demonstrated in a 500 b/d unit at the former Gulf Oil Co. plant at Cedar Bayou, Tex., in the early 1980s. The principal thrust of this project was to crack heavy feeds without pyrolysis coils.

In addition to these processes, recent catalyst developments in China allow cracking of a vacuum-gas-oil-type feedstock.

So, for the Stone & Webster processes, replacing the inert heat carrier in the TRC systems with an active solid produces the QC process. Lower temperatures can be used with active solids, compared to thermal process.

This catalytic system produces a yield shift to propylene and heavier components. QC system peculiarities have been described.¹²

The inherent difficulties in designing and operating a gas/solid system are so numerous that the main benefits of its use-heavy feed cracking, no coil, no coke-seem to lose their importance. The truth is, the cracking process demands extremely high energy (4 million Kcal/ton), very high temperatures (840-860° C.), and the shortest possible time (0.1-0.2 sec).

These basic requirements cannot be met easily by a CBR-type system because the solid carrier has the following drawbacks:

• The reaction specific heat is low because the quantity of solid to be circulated is huge (270-300 ton/ton of ethylene).

• Mechanical inertia makes it impossible to avoid local acceleration or deceleration and the resulting large pressure drops and discontinuous pressure profiles for gases.

• Quick solid/gas separation is not easy.

• Pipe erosion at extremely high rates is unacceptable.

• Fines production is unavoidable (3-5% of sand is entrained in cracked effluent).

• There is a risk of contaminating the oxidative atmosphere of sand heater with the reducing atmosphere of the reactor.

• Instability phenomena exist in the sand standpipe and across the valves.

• Huge and expensive solid-dosing valves are not commercially available.

• The weight of sand requiring hold-up poses a difficult engineering problem.

These difficulties make the author skeptical about the possibility of successful utilization of a CBR reactor or, even worse, the spouted bed reactor-especially for large ethylene plants.

Shock wave reactor

A new-concept technology called a shock wave reactor (SWR) eliminates high energy supply through the wall and avoids coke formation, according to a team of researchers at the University of Washington, Seattle.¹³ A gas dynamic effect is used to crack steam/ethane feed at high temperatures.

Shock waves, produced by the deceleration of a supersonic stream at subsonic velocity, result in an instantaneous increase in the temperature of the gas mixture. This temperature jump initiates pyrolysis.

Ethylene yields of 58 wt %, with a 70% ethane conversion, are obtained with residence times less than 0.2 sec. The supersonic feed stream at about Mach 2, 900° K., and 2.2 bar is brought down to 1.4 bar and subsonic velocity of Mach 0.77. Kinetic energy content is thus suddenly turned into thermal energy.

The resulting temperature increase calls for the formation of high-energy-content hydrocarbons, such as double or triple-bond-bearing compounds. Thanks to the speed of the process, no coke formation can take place.

Tests are planned to measure yields from gas pyrolysis under operating conditions suitable for commercial applications.

Deep catalytic cracking

China's Sinopec International and Research Institute of Petroleum Processing (RIPP) have developed a new process and proprietary zeolite catalyst to convert heavy feedstock to olefinic products at high yields. The process, called deep catalytic cracking (DCC), is colicensed by Stone & Webster.

Two distinct modes of DCC operations are reported-maximum propylene and maximum iso-olefins. Each mode uses a unique catalyst and specific operating conditions.

In China, five commercial units are in operation, a large one is under construction, and a very large one is under engineering. Another very large one is under construction in Thailand.^14-17

References

1. Chernykh, S.P., "New Organic Synthesis Process," Mir Publisher, Moscow, English translation, G. Leib, 1991, pp. 10 ff.

2. Chernykh, S.P., Adelson, S.V., and Mukhina, T.N., "Novel Production Methods for Ethylene, Light Hydrocarbons, and Aromatics," A Non-Traditional Method of Producing Lower Olefins by Pyrolysis of Hydrocarbon Feed in the Presence of Heterogeneous Catalyst, ed. by L.F. Albright, L. Crynes, and S. Novak, Marcel Dekker Inc., 1992.

3. Chernykh, S.P., Russian-American Conference, "Chemistry-XXI Century," Las Vegas, Dec. 1-6, 1996.

4. Gu?ry, C., "Heurtney: de l'?thyl?ne sans vapoeraquer," Le Figaro, Feb. 16, 1997.

5. Barre, C., Chahvekilian, E., and Dumon, R., "New Route to Ethylene," Hydrocarbon Processing, November 1976, pp. 176-78.

6. Vogiatzis, A.L., et al., "Selected Applications of Ultrarapid Fluidized (URF) Reactors: Ultra Pyrolysis of Heavy Oils and Ultrarapid Catalytic Cracking," AIChE Symp. Series: Fluidization and Fluid-Particle Systems-Fundamental and Application, No. 270, Vol. 85, 1989, pp. 69-76.

7. Grace, J.R., "High-velocity Fluidized Bed Reactors," Chemical Engineering Science, Vol. 45, No. 8, 1990, pp. 1953-66.

8. Picciotti, M., "Specify Standpipes and Feeder Valves for Packed Beds," Chemical Engineering Progress, January 1995, Vol. 91, No. 1, pp. 54-63.

9. Wilkinson, L.A., and Gomi, S., Hydrocarbon Processing, May 1974.

10. Gartside, R.J., and Elhis, A.F., "TRC: A Development Update," AIChE national meeting, Anaheim, Calif., June 8, 1982.

11. Gartside, R.J., "TRC: Flexibility for the Furnace," AIChE meeting, New Orleans, Apr. 6, 1986.

12. U.S. Patent No. 4,663,019, "Olefin Production from Heavy Hydrocarbon Feed," Gartside, et al., May 5, 1987.

13. Mattick, A.T., Russell, D.A., Knowlen, C., and Christiansen, W.H., "A Shock Wave Reactor for Ethylene Manufacture," AIChE spring meeting, New Orleans, 1996.

14. Johnson, A.R., et al., "Maximize Chemicals from Crude," 21st annual DeWitt Petrochemical Review, Mar. 19-21, 1996, Houston.

15. Chambers, L.E., "Future Developments in Petrochemical Conversion Processes," Middle East Petrotech, 1996, June 11, 1996, Bahrain.

16. Hutching, D.J., and Hood, R.S., "Catalytic Cracking to Maximize Light Olefins," P?trole et Techniques, No. 400, March-April 1996, pp. 29-40.

17. Marcilly, C., "La production d'olefins legers au FCC, l'existent et le futur," P?trole et Techniques, No. 400, March-April 1996, pp. 41-49.