New and Innovative Conceptual Designs of Gas-to-Liquid Processes

A gas-to-liquid (GTL) process is one where natural gas is converted to synthetic fuels and other products. Our main focus is on the conversion of natural gas to liquid fuels by means of Fischer-Tropsch synthesis (FTS). In addition the possibility of integrating ammonia production with FT process is investigated. FTS is a commercial technology that allows converting synthesis gas, a mixture of CO and H2, into fuels and chemicals. The objective is to improve profitability by increasing the energy and carbon efficiencies through better process design, operation and integration. There is also a great incentive to reduce the investment cost in order to make such plants profitable at higher natural gas prices. This process could be one of the players in the reduction of our dependency on oil. Conceptual design of a once-through GTL plant is performed which is suitable for placement on a Floating Production, Storage and Offloading (FPSO). Almost in all GTL plants that use autothermal reformer (ATR), pure oxygen is used as oxidant. In our design, enriched-air is proposed to be used instead. Two different membrane technologies are applied to produce enriched-air and separate H2 from CO2. FT reactor staging with product cooling and separation is performed in the design which resulted in increased production of heavy hydrocarbons. One of the distinguishing features of this design compared to other designs is that natural gas feed passes through two parallel paths: one for production of syngas and one for H2 production. The produced H2 is partly injected between stages and partly sent to product upgrading. H2 injection between stages will increase the production of heavy hydrocarbons. The proposed process is self sufficient in terms of water and power, therefore can be used in remote locations. In order to increase the profitability of the proposed GTL process, co-generation of ammonia with GTL products is investigated. In this process, without any change to the GTL process and with addition of a few extra equipment, co-generation of ammonia is achieved. This process is still self-sufficient with water and power. The estimated total capital investment of the combined GTL-ammonia process is about 900 million USD of which the ammonia process alone constitutes only 7%. Having accurate kinetic models is very important in process simulation and results will depend on the choice of kinetic model used. Therefore it is vital to have kinetic models that correctly capture major variations in reaction rate subject to process conditions. In order to find a suitable model for the FT reaction, a kinetic model structure is regressed to microchannel experimental data of Yang et al. (2016). The fitted models for reaction rate and chain growth predict the reaction rate and selectivities to CH4 and C5+ quite well. The syngas production step is one of the most costly steps in a GTL plant. It is common practice to use oxygen as oxidant in the reforming step. However, by the introduction of microchannel reactors with their remarkable heat transfer characteristics, high active catalyst site exposure to reactants and therefore high once-through conversions, the use of enriched-air maybe justified. To test this hypothesis, comparison is done in the use of enriched-air versus pure oxygen in the reforming step of a GTL plant utilizing an ATR, while microchannel reactors are used in the once-through Fischer-Tropsch step. Pure oxygen is provided by a cryogenic Air Separation Unit (ASU) and enriched-air by use of air separation membranes. By using pure oxygen less FT reactor volume is required which means lower reactor cost at the expense of having a costly cryogenic ASU. The estimated operating cost of ASU is lower than air membrane, while its installed cost is higher. Due to safety and space issues of having a cryogenic ASU offshore, the only viable option is the use of enriched-air, while in an onshore setting, the use of oxygen is more attractive. Finally, systematic staging or the path optimization method of Hillestad (Hillestad, 2010) is applied on the Fischer-Tropsch synthesis with microchannel reactors. Optimization is done on the total reactor volume of the FT synthesis and the volume distribution between stages, in addition to hydrogen distribution and coolant temperature. The productivity is maximized subject to constraints on a maximum reactor temperature and a minimum conversion. Considerable reduction of reactor volume is obtained through path optimization.