Katbeh, Tala (2017-12). An Energy Integrated Approach to Design Supercritical Fischer-Tropsch Synthesis Products Separation and Solvent Recovery System. Master's Thesis.
Thesis
Extensive research has been done in the advancement of gas-to-liquid (GTL) technology for producing a cleaner source of energy through the conversion of natural gas into ultra-clean fuels and value-added chemicals. The Fischer-Tropsch (FT) synthesis, which is a catalytic process that converts synthesis gas (or syngas, which is a mixture of CO and H2) into longer chain hydrocarbons is considered to be the heart of the GTL process. Conventional FT processes are currently utilizing two most common types of reactors: the multi-tubular fixed bed reactor (in which the reaction takes place in a gas phase medium) and the slurry bubble column reactor (where the reaction takes place in a liquid phase medium). However, they possess heat transfer and mass transfer limitations, respectively.
In order to avoid the challenges, the application of a supercritical fluid (SCF) solvent in the Fischer-Tropsch synthesis was introduced. The SCF-FT process, in essence, combines the benefits of the two major reactor technologies used in conventional GTL processes due to the SCF's gas-like diffusivity, liquid-like solubility and heat transfer.
The SCF-FT synthesis involves co-feeding the SCF solvent along with the syngas into the reactor at a specific solvent to syngas ratio (set as 3:1 in this work). Introducing the supercritical solvent (which was selected to be n-hexane in this work) requires adjustments in the SCF-FT products' separation sequence due to the significantly large amount of solvent available in the process. The major additional costs associated with the SCF-FT synthesis is in the product separation and solvent recovery. For SCF-FT to be adopted on a large-scale, the economics from operation under high pressure supercritical conditions must exceed the additional cost required for the separation of the solvent.
The aim of this work is to construct an optimum separation design to target the separation of synthetic crude oil (or syncrude) obtained from SCF-FT synthesis while recovering the supercritical solvent. Aspen Plus(R) was used as the process simulator to determine the energy consumption and quantify the sensitivity of the various parameters on the solvent recoverability, purity, product yield, and operation feasibility while comparing it to the typical FT process.
Three separation sequences were developed using existing GTL plants as references. The three scenarios were compared with regards to their energy requirements. The simulation results showed that despite the addition of a large amount of solvent, the separation of the products, water, and the recovery of the solvent was achieved.
