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Prospect of Success in Scaling-up Fischer-Tropsch Synthesis Reactor Operates in near-Critical and Supercritical Phase Media

Nimir O. Elbashir, Chemical Engineering, Texas A&M at Qatar, Doha, Qatar

The first generation of commercial scale reactors for Fischer-Tropsch synthesis (FTS) was fixed-bed reactors of different design configurations including; multitubular reactor with sets of double concentric tubes, adiabatic fixed-bed reactor with either single or multiple adiabatic beds, and adiabatic fixed-bed reactor with large recycle of heavy condensate passing in upflow through the bed. The latest commercial version of fixed-bed reactors is the mulitubular reactor of Shell's Middle Distillate Synthesis (SMDS) technology, which is part of their Bintulu plant in Malaysia (an upgraded version of this reactor will be used in Shell PearlGTL plant in Qatar). FTS reactor technology was then moved to the fixed fluidized beds and the slurry bubble reactors (liquid-phase) to overcome some of the limitations of fixed-bed-reactors in FTS, such as local overheating of catalyst surface, high degrees of methane selectivity, and pore diffusion limitation due to condensation of heavy hydrocarbons inside catalyst pores. Sasol took the leadership in the development of most of the aforementioned reactor technologies, which are claimed to have several advantages over the fixed-bed reactors, such as being more compact than fixed-bed reactor (in particular reduced height), requiring less energy for gas circulation, and easier operation and maintenance, all of which result in substantial reductions in capital and operating costs. Nevertheless, slurry phase FTS processes suffer from slow diffusion of syngas into the catalyst pores resulting in lower rates of reaction, the difficulty of separation in product wax or catalyst from slurry as well as the capability of the catalyst to resist attrition and abrasion, and finally the complexity of the design in the gas distribution devices and the other reactor internals.

Conducting FTS reactions in supercritical fluid (SCF) media has been demonstrated to have certain advantages over the traditional routes because of the unique characteristics of the supercritical phase (combination of liquid-like heat capacity and solubility for optimum temperature distribution and in situ extraction of heavy hydrocarbons from the catalyst pores and gas-like diffusivity for higher conversion). The pioneer study in this field was conducted by Karu Fujimoto and his team at the University of Tokyo (published in a short communication in Fuel volume 68, 1989). Series of studies then followed led by Bala Subramanian team at the University of Kansas, Drago Bukur team at Texas A&M, Burt Davis at the University of Kentucky, and Christopher Roberts team at Auburn University. The aforementioned studies concluded several advantages of operating FTS in SCF media versus operation in conventional media due to the followings: (1) in-situ extraction of heavy hydrocarbons from the catalyst pores resulting from high solubility in the supercritical phase, (2) elimination of interphase transport limitations thus promoting reaction pathways toward the desired products, (3) enhancement of -olefins desorption that promote the chain growth process prior to secondary reactions, and (4) excellent heat transfer compared to gas-phase reaction that results in more long chain products. Most of these studies have been funded by governmental agencies and a few of them funded by industry. Nevertheless, the implementation of SCF in FTS has not yet moved beyond the lab-scale reactors. The scale-up of SCF-FTS to a pilot plant is still a challenge because product yield and selectivity of a large scale reactor requires a deep understanding of phase behavior of reaction mixture, reaction kinetics, and catalytic system and its FTS chemistry. In this communication an overview of studies conducted in this field will be summarized. Methods to overcome the challenges facing upgrading SCF-FTS reactors to a pilot scale will be discussed. In addition a design of a high-pressure fixed-bed reactor setup that facilitates FTS operation under near-critical and supercritical solvent condition will be described. The design of the reactor considers utilizing the high pressure operation under SCF-FTS in separation of hydrocarbon products from supercritical solvent as well as in hydrocarbon fractionation.