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Shock-wave cracking technology comes back to the future.

In 1965, a patent was awarded for manufacturing acetylene using a shock-wave tube. Although the process was successful, the idea fell into hibernation, laying dormant for the next several decades. Since then, shock waves have played important roles in laboratory kinetic research, but have not been seriously applied to hydrocarbons.

Like a lost key to an old lock, shock waves are now being mixed with modern technology in an attempt to find the ideal natural-gas cracking technique - and researchers are finding that their predecessors may have been on the right track.

At the Univ. of Washington, Seattle, researchers led by Tom Mattick, Abraham Hertzberg, and David Russell are using a shock-wave reactor to break up to 70% ethylene from ethane-gas feedstock. This surpasses current techniques which are able to crack only 55% of the available ethylene in ethane, Mattick says.

"There are really interesting elements to the technology," says Ray Orriss, director of ethylene technology for MW Kellogg, an engineering firm that has specialized in ethylene-plant design for more than 40 years. "The shock-wave idea could have slipped through the cracks for all of these years. They still only have it at the laboratory level, however. It'll take four to five years to find the rest of the answers."

Shock waves are disturbances in gas flow that form when gas speeds change suddenly from supersonic to subsonic levels. The shock-wave reactor uses energy released from these waves to heat natural gas to 1,250 [degrees] C. This surpasses the heating capabilities of existing furnaces.

Because the feedstock can be heated in microsecond bursts, users can precisely moderate temperatures to specify the type of gas desired from the separation. In addition, the apparatus remains free of coking throughout the process.

"Supersonic mixing and shock waves are not standard processes in chemical engineering, and their use probably would not occur to a plant designer," Mattick says. "This rediscovery might open people's eyes to the possibility of applying gas dynamics to petro-chemicals."

In the shock-wave reactor, an inert carrier gas is heated in the boiler, forming steam. After being injected into the reactor channel at supersonic speeds, the steam is mixed with a supersonic flow of preheated feedstock. The mixture does not react prematurely because its speed drops the temperature.

As the mixture slows to subsonic speeds, a shock wave is created, initiating pyrolysis. After reacting for a few moments, the gas mixture passes through a quencher, which cools it to preserve its chemical properties. The system can heat ethane in microseconds to the ideal cracking temperature.

"Use of the shock-wave reactor in place of existing tube-in-furnace cracking technology could reduce the cost of the production of ethylene, enable longer run durations between decoking, and be less costly to build," Mattick says. "System studies of the entire plant must be done" to determine the exact benefits, he says.

Even small savings on the cracking process could add up to huge savings for the industry. Ethylene is the foundation for the plastic industry, while other cracking byproducts are used to make synthetic cloth fiber and antifreeze, as well as many other products. World production of ethylene increases 4% to 5% each year, Orriss says. This year, more than 2 million kilograms were produced.

Ethylene cracking technology has not changed in many years because cracking furnaces are mature technologies, according to Bob Seravskas, principal technical manager, combustion, for the Gas Research Institute, Chicago.

"It's an expensive 'game' with a conservative industry," agrees Peter Barone, principal project manager with the Gas Research Institute. "That's why there haven't really been new developments. Companies have invested a lot of capital in their current equipment."

Begun with a grant from DOE's Advanced Energy Research Div., the shock-wave reactor may provide several other boons to the petrochemical industry. The reactor is much more compact than existing furnaces. If it is adopted in a cracking assembly, retrofitting costs should be low because only the furnace must be replaced and not the entire cracking apparatus.

Companies will also save expensive decoking costs. In current cracking assemblies, gases are exposed to short bursts of high temperatures, then cooled to freeze their chemical compositions. To do this, the feedsource is sent through tubes heated by furnaces. This heating coats the equipment with carbon, forcing plants to shut down once a month for decoking.

Because the shock wave reactor generates microsecond bursts of heat, it doesn't build up carbon. Mattick estimates that up to $1 million is lost each day a furnace is decoked in a plant with no back-up furnaces.

"I think the next step to make furnaces more efficient is not to make materials more efficient, but to eliminate coke production," Orriss says. "The shock-wave reactor may be a tool for this."

Kellogg's millisecond furnace, first used in a full-scale factory in 1985, is one of the most effective technologies currently available for cracking natural gas. Equipped with small tube diameters and high flow velocities, the furnace is made of steel and produces about 15% to 17% ethylene from liquid feedstock, Orriss says.

"The ability of furnaces to crack natural gas is limited by metallurgical properties," Mattick says. "Current research approaches have more or less reached their practical limits due to the temperature limits of furnace tube materials. Our process is not fundamentally limited by tube wall materials."

Orriss agrees, suggesting that strengthened ceramic materials may eventually be used to make furnaces with higher heat tolerances. Otherwise, he says, a radical shift in technology may replace furnace technology with more effective processes.

"We have efficiency on a number of levels, and two major ethylene companies have expressed interest," says Greg Hauth, Univ. of Washington licensing officer for the Office of Technology Transfer.

Because the shock wave reactor uses a higher steam dilution than existing furnaces, the system must be adapted to efficiently recover energy and maintain costs comparable to current processes. The university is working with interested companies to investigate the potential of using turbines at the back end of the process to recover this energy, Hauth says.

"With effective energy recovery, the energy consumption is comparable to or less than that used in existing furnaces," Mattick says. "However, since the product stream is enriched in olefins, the cost to separate chemical components and the cost of feedstock is expected to be lower."

The shock-wave reactor may be able to handle liquids as well as gases if the liquids are evaporated or injected as miniature droplets into the stream, Mattick says. It also may find applications in other fields like hazardous-waste incineration because it creates a controllable high-temperature furnace.

"Exothermic reactions might actually realize a more immediate benefit than the endothermic olefin-producing reactions since the steam dilution could be kept small," Mattick says.

While the shock-wave reactor is efficient at cracking ethane, which requires around 86 kcal/mole, other technologies are under development to crack methane. Methane requires 103 kcal to crack, but it makes up a much larger percentage of natural gas and exists in enough quantity to satisfy the world's energy needs for 500 years, according to S.H. Bauer, chemistry professor at Cornell Univ., Ithaca, N.Y.

Under Bauer's direction, researchers are using shock waves to crack methane. Instead of a continuous flow process, however, Bauer uses a shock tube that gives intermittent bursts of heat. By introducing 2% iso-octane into the methane, enough free radicals are introduced to induce fragmentation. Although the process is not commercially viable because shock-tube reactions are not sustainable, the research may lay the groundwork for techniques to crack methane in the future.

"The wave reactor looks like it may be a practical solution" for using shock waves on a commercial scale today, Bauer says.

"An idea like (using shock waves) could really catch on," Barone agrees. "Unfortunately, you must look at the costs of selling an idea in a conservative industry, and not the merits of the project. If someone were to stay diligently behind this, it could have a huge impact."
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Author:Vandendorpe, Laura
Publication:R & D
Date:Dec 1, 1996
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