Production of Transportation Fuel Range Middle Distillates via Fischer-Tropsch Synthesis with Integrated Product Upgrading under Supercritical Phase Conditions

CATEGORY: FISCHER-TROPSCH
THESIS
Production of Transportation Fuel Range Middle Distillates via Fischer-Tropsch Synthesis with Integrated Product Upgrading under Supercritical Phase Conditions
Sihe Zhang
Auburn University (2013)
Abstract:
There has been a great deal of contemporary interest in the utilization of a variety of carbonaceous feedstocks to produce readily usable transportation fuels via synthesis gas (syngas, a mixture of H2 and CO). Specifically, Fischer-Tropsch synthesis (FTS) can be used to convert synthesis gas into hydrocarbon products and oxygenates. In Fischer-Tropsch synthesis, a set of surface-catalyzed polymerization reactions take place which convert syngas into hydrocarbons and oxygenates with a broad range of carbon chain lengths and type, typically over an iron or cobalt based catalyst. With appropriate product separation and upgrading procedures, these FTS products can be further processed and converted into high quality fuels and value-added chemicals. Utilization of supercritical fluid (SC) media in FTS (SC-FT) has been demonstrated to provide certain benefits including a reduction in the selectivity towards CH4 and CO2 as a result of the enhanced heat transfer that the supercritical solvent offers compared to gas phase FTS (GP-FT). In addition, the improved hydrocarbon solubilities in the SC medium can further result in prolonged catalyst life and activity maintenance.
The objective of this work is to explore and demonstrate the effect of integration of product upgrading reactions (such as oligomerization and hydrocracking) subsequent to FTS in a single pass operation and the benefits of introducing supercritical fluid media into these heterogeneous reactions.
In chapter 1, a background introduction is given with respect to Gas-To-Liquid (GTL) technology, FTS, FTS product upgrading reactions, and the utilization of supercritical fluid media as reaction solvent. In chapter 2, the catalytic performance results are presented for each individual reaction, namely the FTS reaction, oligomerization reaction and hydrocracking reaction, separately. A traditional precipitated iron-based low temperature FTS catalyst (since iron-based FTS catalysts are more feasible for a wider range of inlet syngas H2/CO ratio than cobalt-based FTS catalysts) was prepared and evaluated under gas phase conditions (GP-FT). The catalytic oligomerization activity of amorphous silica alumina (ASA) has been examined to convert the light olefin FTS products into middle distillate range hydrocarbons. A Pd/ASA (1.0 wt.%) hydrocracking/isomerization catalyst has been made using wetness impregnation method, to alter the long-chain FTS hydrocarbons into shorter fuel range products. In chapter 3, the performance of FTS with direct subsequent product upgrading has been evaluated using a newly designed reactor system. A vertical fixed bed reactor system with three catalyst beds arranged sequentially has been designed and used to incorporate Fischer-Tropsch synthesis in the first bed, oligomerization in the second bed and hydrocracking/isomerization in the third bed (FTOC). In addition, the reactor system performance has been examined under both gas phase and supercritical phase conditions. The gas phase FTOC (GP-FTOC) results of this study have shown a reduction of olefin selectivity and a marked enhancement of branched-paraffins. Furthermore, wax in C26+ range was decreased in GP-FTOC operation compared to GP-FT operation. Also, a considerable amount of branched paraffins and aromatics were generated in the gasoline/kerosene range in GP-FTOC. The work in this chapter also has examined the utilization of supercritical hexane as the reaction medium in SC-FT and supercritical phase FTOC (SC-FTOC) where the use of this supercritical solvent medium resulted in a significant reduction in both methane selectivity and carbon dioxide selectivity as well as a well maintained catalyst activity compared to the analogous gas phase operations. Significant quantities of aldehydes and cyclo-parafins were collected as reaction intermediates in SC-FT and in SC-FTOC, respectively, though these species were not observed in appreciable amounts in the traditional gas phase operation. To improve our understanding of the reactions that take place in each of these upgrading beds, detailed studies were performed and reported in chapter 4. FTS plus oligomerization (FTO) and FTS plus hydrocracking/isomerization (FTC) have been investigated using a dual reaction bed experimental apparatus. This particular study provided a detailed evaluation of effect that each of the product upgrading reactions had on the FTS products that were produced in the first reactor bed. Significantly improved CO conversion has been observed in supercritical phase FTO (SC-FTO) compared to the CO conversion that was obtained from gas phase FTO (GP-FTO). Similarly, greatly enhanced CO conversion has been shown to occur in supercritical phase (SC FTC) compared to the value of CO conversion that was obtained from the gas phase FTC operation (GP-FTC). Moreover, the selectivity toward CO2 and CH4 was greatly reduced under supercritical phase conditions compared to gas phase operation which is consistent with previously documented observations for the use of a supercritical solvent in FTS. The liquid product distribution obtained from GP-FTC exhibited a substantial enhancement in the amount of branched hydrocarbon products generated as well as a markedly decreased heavy wax selectivity. This result indicates that the isomerization and cracking activity is significant in the hydrocracking/isomerization stage in GP-FTC. Analysis of the liquid products that were obtained from SC-FTC also reveals that a high degree of activity towards the hydrogenation reaction occurred in the hydrocracking/isomerization stage. Characterization of each of the catalysts employed in each of these catalytic stages (FTS, oliomerization, and hydrocracking/isomerization) were performed using BET surface area and pore volume analysis as well as SEM microscopy. Moreover, the phase behavior of the FT reaction mixtures under these supercritical phase conditions has also been studied and described within this dissertation, presented in chapter 5. Experiments to determine the critical point loci of different model SC-FTS reaction mixtures have been performed using a high pressure, variable-volume view cell system. Specifically, the critical point loci of the mixtures of syngas + hexane (with different syngas/hexane ratios), syngas + hexane + tetradecane (which serves as a typical FTS paraffin product), and syngas + hexane + tetradecane + H2O (which is an important FTS side product that can significantly affect the phase behavior of the SC-FTS reaction mixture) have been carefully measured in order to understand the effects of these FTS reaction and product species on the phase behavior of these highly nonideal mixtures. Finally, in chapter 6, a series of future investigations have been proposed that will further improve the feasibility of this multi-bed reactor system. These proposed studies include determination of the optimal operational parameters of each of the catalytic reaction bed of this multi-bed system, optimization of the multi-bed catalytic system by further modification of the catalyst system employed catalyst system as well as more elaborate evaluation of the FTS fuels derived.
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