| dc.description.abstract | The increasing demand for liquid fuels draws attention to synthetic fuels. Currently, two well-known Gas-to-Liquids technologies are used to convert syngas to liquid fuels. One is Fischer-Tropsch Synthesis (FTS); the other one is the methanol-based approach. It can be Syngas-to-Gasoline (STG), which is based on methanol synthesis and Methanol-to-Gasoline (MTG). The MTG process composed of methanol dehydration to form Dimethyl Ether (DME) and DME converts on zeolites to synthesize gasoline. If we combine methanol synthesis with methanol dehydration, the Syngas-to-DME (STD) process could be formed, which is also an effective method to generate synthetic fuel. Although DME is not in liquid phase under atmosphere pressure, it can be liquefied with minor pressure (0.5 MPa). Also, DME is an excellent diesel fuel alternation with high cetane number. Hence, people treat DME as a liquid fuel, and DME synthesis is also discussed in this dissertation. In short, the two major approaches of gas-to-liquids are FTS and methanol-based approach including STD and STG.
These two approaches have their advantages and disadvantages. FTS process produces a broad spectrum of high value-added products including liquefied petroleum gas, gasoline, jet fuels, diesel, and waxes. Typically, it requires complex and expensive subsequent processes to upgrade the initial products to meet liquid fuel specifications. For instance, the heavy wax products need to be hydrocracked or thermally cracked into middle distillates to maximize liquid fuels production. The FTS and hydrocracking process can be integrated into a single reactor by physically mixing FTS catalysts and hydrocracking catalysts. The problem is that both FTS (H0 = -165.0 kJ/mol) and hydrocracking processes are highly exothermic processes, and the product distribution of FTS is strongly dependent on the reaction temperature. High temperatures favor the formation of undesirable light products like methane. Therefore, a reactor system with an excellent heat transfer property is necessary for process intensifying these two highly exothermic processes together.
Both the STD and STG approaches need to go through multiple reaction steps to synthesize the final liquid fuels. It typically consists of methanol production from syngas and dimethyl ether production via methanol dehydration. For the STD process, the reaction is stopped at methanol dehydration and DME as the final product. If for the STG process, an additional reaction step needs to be added to convert DME to gasoline. The STD process can produce high quality DME that used as diesel fuel alternation. The STG process can synthesize high quality gasoline with high selectivity, but it cannot produce jet fuels or diesel. Also, methanol synthesis is a reversible pressure dependent reaction and is subject to an equilibrium limitation. Moreover, every reaction step requires independent reactor systems, relevant product separation systems, and recycle loops. Hence, the conventional STD or STG process is not an efficient process to convert syngas to liquid fuels. There is a great opportunity to integrate methanol synthesis and methanol dehydration reaction, which overcomes the pressure dependent equilibrium yield of methanol and push the reaction forward. It must be noted that both methanol synthesis (H0 = -90.8 kJ/mol) and methanol dehydration (H0 = -23.6 kJ/mol) are highly exothermic reactions. In addition, Water-Gas-Shift (WGS) reaction is another highly exothermic reaction (H0 = -41.1 kJ/mol), which is unavoidable occur with methanol synthesis reaction. The net reaction of STD is an extremely exothermic process (H0 = -246.3 kJ/mol). Therefore, a reactor system must have an efficient heat transfer mechanism to accommodate the added thermal load from integrating these highly exothermic reactions into a single reactor.
As we mentioned before, the STG process is operated in three consecutive reaction steps: methanol synthesis, methanol dehydration to form DME, DME converts to gasoline. Every reaction step needs their own reactor system, heating system, product separation system, and recycle loops, etc. Therefore, the STG process is complex and expensive if operating in the traditional method with multiple reaction steps. However, there are two novel process intensified approaches. Since the first two reaction steps could be integrated together, then the STG process can be reduced to a two-step process. Furthermore, if we can continue to process intensification of the two-step STG process into a single-step STG process, we could greatly reduce the CAPEX & OPEX. The key issue is like the one-step STD process, all these reactions in the STG process are highly exothermic reactions. When compared with the one-step STD process, the single-step STG process is even more exothermic because one more exothermic reaction has been added. All in all, new means of enhancing intra-bed heat transfer are required to intensify these highly exothermic reactions.
Microfibrous Entrapped Catalyst (MFEC) structure is a novel catalyst structure developed by our group, which is a micron-sized metal fibers network with high thermal conductivity. Previous efforts on this unique catalyst structure have shown significant enhancement in intra-bed heat transfer and mass transfer. Especially for highly exothermic reactions like FTS, Cu MFEC structure can efficiently transfer the reaction heat out of the reactor, maintain a stable reaction temperature, and improved FTS product selectivity and process stability.
This dissertation is focusing on process intensification of FTS and hydrocracking process together, process intensification of the STD into a one-step process, and process intensification of the conventional STG process into a single vessel. The Cu MFEC structure has been used to handle the added thermal load, to maintain a stable reaction temperature, and to scale up the tubular reactor to a larger scale. In the first part, a highly active and stable iron-based FTS catalyst (Fe-FTS) has been developed. Based on that, a hybrid catalyst was formed by physically mixing the Fe-FTS with an equal mass of mesoporous aluminosilicate. This hybrid catalyst demonstrated FTS and hydrocracking activity simultaneously. Furthermore, the reaction has been scaled up to a large tubular reactor (34.0 mm I.D.) packed with Cu MFEC. It demonstrated a radial temperature gradient of less than 5 ºC, while the comparative packed bed (34.0 mm I.D.) reached a radial temperature gradient around 54 ºC. In the second part, the direct STD process was carried out in a 34.0 mm I.D. tubular reactor with the assistance of Cu MFEC structure. As a result, the one-step STD process demonstrated a stale temperature profile throughout the entire reaction process, and the radial temperature deviation is around 7.6 ºC. In contrast, the comparable packed bed showed a temperature deviation of 37.5 ºC. The direct STD process has been carried out under different reaction conditions (H2:CO ratio, GHSV or WHSV, T, reactor size and type). The results showed that it offers a high per pass conversion (68.8 %) and high carbon productivity (0.51 g of C/gcatalyst/h) when a reactor packed (34.0 mm I.D.) with Cu MFEC structure and operated at H2:CO = 1:1, WHSV = 4.18 L/gcatalyst/h, and T = 275 ºC. In the third part, a single vessel STG (SV-STG) with multiple reaction zones has been developed. The SV-STG demonstrated a high gasoline selectivity of 74.8 wt% under a high CO conversion of 78.3 % in a small tubular reactor (9.0 mm I.D. Moreover, Cu MFEC structure has been successfully used to scale up SV-STG to a large tubular reactor (41.mm I.D.) without compromising the reaction activity or gasoline selectivity. The SV-STG packed with Cu MFEC structure exhibited a maximum radial temperature gradient of less than 5 ºC, while the comparable packed bed showed a maximum radial temperature gradient around 40 ºC. This difference has only been demonstrated in a bench scale reactor (41.0 mm I.D. tubular reactor), and the difference can be much greater in a larger diameter tube. Furthermore, Cu MFEC structure has high voidage (60 – 80 vol%) which prevents the bed from experiencing severe pressure drop even when using small catalyst particles (80 – 170 mesh). Therefore, the Cu MFEC structure is greatly heat transfer medium, which enables process intensification of these highly exothermic reactions, significantly reduces CAPEX and OPEX and enhances scalability and modularity. | en_US |
