(21a) Highly Conductive Catalyst Support Structure for Use in Large Diameter Highly Exothermic Gas-to-Liquid Reactors

Highly Conductive Catalyst Support Structure for Use in Large Diameter Highly Exothermic Gas-to-Liquid Reactors

William R. Yantz, Jr.¹*, Carlos F. Gonzalez¹, Min Sheng¹, Hongyun Yang², Paul S. Dimick², Donald R. Cahela¹, Bruce J. Tatarchuk^1,2

1         Center for Microfibrous Materials Manufacturing (CM3), Department of Chemical Engineering, Auburn University, Auburn, AL 36849 (USA)

2         IntraMicron Inc., 368 Industry Dr., Auburn, AL 36832 (USA)

*wry0001@auburn.edu

Introduction

From a thermal management perspective, many highly exothermic gas-to-liquid reactions are problematic. Due to the high heat of reaction, many strict limitations are placed on reactor design to ensure safe and productive operation. Such limitations can include the reactor diameter, reactant recycle, heat exchanger design, catalyst loading, and the catalyst support structure in order to mitigate heat transfer problems. To overcome these limitations, a novel catalyst support structure that is highly thermally conductive has been utilized. The catalyst support known as microfibrous entrapped catalyst (MFEC) composed of sintered micron-diameter metallic fibers entrapped with catalyst particulates was studied for use in highly exothermic gas-to-liquid reactions.

Three reactions have been studied while utilizing the MFEC support: Fischer-Tropsch synthesis (FTS), single-step dimetheyl ether (DME) synthesis, and methanol synthesis. These reactions were carried out in both a 15 mm ID reactor and a 41 mm ID reactor. Of these reactions, the FTS reaction was studied in depth due to its temperature sensitive product selectivity [1,2] as well as its extremely high adiabatic temperature rise [3] that limits the reactor diameter and correspondingly, the volumetric throughput.

Materials and Methods

Two downflow, stainless steel (SS) tubular reactors (15mm and 41mm ID) were utilized to compare the MFEC reactors to traditional packed bed reactors (diluted to the same catalyst loading). A thin sheath multipoint thermocouple with low axial thermal conductivity (Omega, 316SS probe, 1.5mm wall thickness) was used to measure temperatures in the catalyst beds. For FTS (225-255°C, 20bar), a Co/Al2O3 catalyst was prepared through incipient wetness impregnation, whereas for DME (275°C, 20-40bar), the catalyst used was commercially available CuZnOAl₂O₃/γ-Al₂O₃. The methanol (MeOH) synthesis reaction was performed under the same conditions as DME, but with only CuZnOAl₂O₃. Product composition was determined though the use of an online Agilent 6890 GC.

Results and Discussion

The use of MFEC provided a favorable path for the heat generated on catalyst particles to be transported out of the reactor. The poor thermal conductivity of alumina catalyst particles was compared to the thermal conductivities MFEC composed of various metallic fibers. Under stagnant conditions, the thermal conductivities were 0.16 W/mK for alumina, 1.09 W/mK for SS, 3.77 W/mK for Ni, and 9.05 W/mK for Cu [4]. The Cu fibers showed an increase of 56 fold that of the alumina particles.

This increase in thermal conductivity of the Cu MFEC explains the difference in temperature profiles observed in the packed bed and copper MFEC bed during reactor startup for the FTS reaction. Using MFEC, a uniform radial temperature profile was observed in the 15mm ID reactor, while the packed bed showed a radial temperature deviation of 5.7°C. At the increased diameter of 41mm, the MFEC produced a 6.4°C temperature deviation; the packed bed underwent a 460°C temperature deviation that signaled thermal runaway in the reactor [5].  The results from these studies showed that packed beds must be run in small diameter tubes due to the observed thermal gradients. This result was mirrored in the 15 mm ID DME reaction with the MFEC observing a 15°C radial temperature deviation and the packed bed showing a 63°C temperature deviation.

The MFEC bed allowed for a nearly constant α-value (FTS) and low methane selectivity as temperature increased, whereas the packed bed displayed the opposite trend. Furthermore, the activity of the catalysts was constant throughout the run for the MFEC bed, whereas the catalyst activity was significantly reduced in the packed bed reactor.

Significance

The enhanced heat transfer characteristics of MFEC structures compared to those of a packed bed were demonstrated by comparing temperature profiles established within FTS, DME, and MeOH reactors operated at equivalent particle size, catalyst loading, and volumetric reactivity. The use of copper MFEC resulted in uniform temperature profiles, higher selectivity to desired hydrocarbons, higher conversion, and less catalyst deactivation when compared to their packed bed counterparts. The enhanced thermal management properties of the MFEC allows for larger diameter tubes than those typical in industry while at the same time maintaining product selectivity and volumetric throughput.

References

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3. S.T. Sie, and R. Krishna, Applied Catalysis A, 186 (1999), 55-70.
4. Sheng, Min et al., Journal of Catalysis, 281 (2011), 254-262.
5. Sheng, Min et al., Applied Catalysis A, 445-446 (2012), 143-153.