Contact: David Brenchley
Pagemaster: Amanda Kissire
Updated: December 18, 2002
Anna Lee Tonkovich, Yong Wang & Dave VanderWiel
This project aims to develop and demonstrate integrated chemical process systems—reactors and separators—built upon Pacific Northwest National Laboratory's micro-channel technology. A key objective is to demonstrate the capability to fabricate and operate catalytic micro-reactors and integrate the reactor into a complete chemical process system, including separation, recycle, and control. The primary process target is the production of synthesis gas from methane through either partial oxidation or steam reforming. Secondary conversion of carbon dioxide is also of interest because it is a byproduct in many commercial processing systems, and emission requirements are becoming more stringent. In both cases, thermal integration in a microchemical reactor is paramount to obtaining high efficiency, which ultimately shrinks the size of the process.
Microchemical reactors hold great promise for enabling technology breakthroughs for fast exothermic reactions, including partial oxidation reactions. The work in milli-second reactions by Schmidt has shown that extremely high synthesis gas yields (>90%) can be obtained from methane. Safety considerations, however, have limited the deployment of the large-inventory tubular reactor design pursued by Schmidt. The microreactor has inherent safety, and thus scalability, features. Rapid heat production from the exothermic reaction is quickly removed through a microchannel heat exchanger to quench the desired reaction products. In addition, these processes are run under isothermal conditions to prevent thermal runaway and possible explosions. Thermal excursions are an extremely serious consideration for conventional reactors, where low heat-removal rates require high reactant dilution with an inert gas or permit only a few percent conversion per-pass. Further, the most striking inherent safety feature of a microchemical reactor is the low reactant inventory, which eliminates the accumulation of flammable gases that are well into the explosion regime. Microchannels behave much like a flame arrester placed in conventional systems because the channel dimensions are designed to be equal to or smaller than the explosion quench diameter.
Methane is selected as the initial target for demonstrating potential applications for microchemical reactors. Con-ventional methane conversion technologies such as partial oxidation (not yet commercial) and steam reforming (economic only at extremely large scales) are not well suited for distributed processing. The primary limitation with partial oxidation processes, as previously articulated, is the necessity to remove rapidly the copious quantities of heat generated from the exothermic reactions. Steam reforming, on the other hand, is highly endothermic and requires careful thermal integration to add enough heat to maintain fast reaction rates. Microchemical reactors can address each limitation. The intimate coupling of reaction and heat transport (either removal or addition) enables fundamental technology breakthroughs for methane con-version at point sources, including homes, wellheads, and centralized power plants. The other driving force for targeting methane is the interest by several DOE agencies in carbon management for mitigation of global warming and for hydrogen production at stationary power plants.
Carbon dioxide is often formed as a byproduct during methane steam reforming, and many other commercial reactions. As federal emissions regulations for carbon dioxide are becoming increasingly stringent, options for the conversion of carbon dioxide to useful products are being investigated.
Because commercial catalysts used for methane production typically use nickel as an active component (for both carbon monoxide and carbon dioxide hydrogenation) supported on alumina, this was the first type of catalyst tested. Other Group VIII metals are highly active for methanation, and several options have been explored for using them as catalysts. A variety of oxide supports have been examined.
To examine the effects of residence time and temperature on both the carbon dioxide conversion and methane selectivity, the reaction was studied over a wide range of feed and reactor conditions. Initial testing was conducted using powder catalyst beds in quartz microreactors. Such systems allow residence times from about 1 to over 1000 milliseconds and temperatures up to 1200°C to be examined. The Sabatier reaction typically operates at temperatures around 300°C, so the experimental temperature range was centered on this value. Longer reaction times are required for complete conversion, so residence times as high as 1 second were tested.
Initial testing of this system suggests that methane formation from carbon dioxide occurs with intrinsically rapid kinetics and so may be successfully used in microreactor systems. Hence, the advantages of novel catalysts can be fully realized for this reaction.
The reverse water-gas shift reaction (RWGS) reaction is catalyzed on a variety of surfaces and, again, the Group VIII transition metals are good candidates. Typical RWGS catalysts contain copper and/or zinc dispersed on an oxide support. Unfortunately, the forward reaction is also catalyzed by these reactions to a high degree, and operating conditions dictate carbon dioxide conversion and product selectivity. The higher temperatures of RWGS require more stable supports, so only those known to be stable at elevated temperatures were tested.
Initial testing used powder catalysts in quartz micro-reactors. The short residence times achievable in these screening reactors and in full-scale microchemical reactors should prove advantageous in suppressing the forward WGS reaction. Therefore, shorter contact times of about 1 millisecond were studied. Reaction tempera-tures for RWGS are typically higher than Sabatier, often over 500°C.
This system has been tested using several different catalysts, both conventional and novel. In addition, simulated mixed feed streams have been tested to examine the potential effects of multiple competing reactions. Initial results are quite promising, with order-of-magnitude improvements in residence time over highly active, novel catalysts. The effects of side reactions remain an inherent hurdle, however.
Tonkovich AY, JL Zilka, MJ La Mont, Y Wang, , and RS Wegeng. 1999. "Microchannel Reactors for Fuel Processing Applications. I. Water Gas Shift Reactor." Chemical Engineering Science, 54(13-14):2947-2951.
Tonkovich AY, JL Zilka, Y Wang, M LaMont, S Fitzgerald, D VanderWiel, and RS Wegeng. April 1999. "Microchannel Reactors for Automotive Fuel Processors. II. Compact Gasoline Vaporization." Proceedings of the Third International Conference of Microreaction Technology, Frankfurt, Germany.
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