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Energies 2020, 13, 420 47 of 96 hydrogenation is easier [596]. CO2 addition enhances the occurrence of reverse water gas shift reaction that slightly decreases both the CO conversion and the carbon deposition reactions [597]. Finally, the residence time of the gas in the reaction chamber increases the conversion of CO2 and CH4. The residence time is measured through the gas hourly space velocity (GHSV) that depends on both operating conditions and reactor design [598]. In conclusion, optimal operating conditions to obtain high methane yield, and avoid the carbon deposition, and catalyst sintering are relatively low temperature (200–500 ◦C), a high pressure (10–30 atm) and an H2/COx ratio above the stoichiometric value of 3 [599]. 4.6.1. Reactors The main issue for reactor design and operation is the control of temperature profiles due to the reaction exothermicity. Reactors operate with an adiabatic, isothermal or polytropic temperature profile [591]. Kiewidt and Thöming [600] distinguish the reactor temperature profiles introducing the Semenov number, defined as the ratio of heat production rate to cooling rate. If the Semenov number assumes values near zero, the temperature profile is isothermal (high cooling rate or low heat generation). Whereas, when the Semenov number tends to infinity, the reactor operates in adiabatic conditions. For intermediate values, the behavior is polytropic. Fixed-bed reactors with nickel or ruthenium catalyst are typically adopted, but structured and fluidized bed reactors are also developed to overcome temperature hot spots and high-pressure drops [554]. Fixed-bed reactors are typically tubular reactors packed with small catalytic pellets [601]. Packed-bed reactors are the reactor most used industrially due to high conversion efficiency, low operating cost, and continuous operation. Drawbacks are the hot spot formation, high-pressure drop and high exit temperature that could lead to catalyst cracking or sintering [602]. The temperature control on adiabatic fixed-bed reactors is obtained by a series of adiabatic reactors with intercooling, the recirculation of exit product gas or the dilution with inert gases [603,604]. Contrariwise, in cooled fixed-bed reactors, the temperature is tuned by cooling pipes or plates placed in the catalyst bed [453]. The most common industrial processes that employ fixed-bed reactors are Lurgi, TREMP and HICOM. The process developed by Lurgi employs two adiabatic fixed bed reactors with internal recirculation for heat management. Indeed, the TrempTM process recovers the reaction heat at high temperature vaporizing the water at high pressure. The process involves two or more adiabatic reactors with intercooling. Finally, HICOM process employs two or more methanation reactors fed by saturated syngas. The temperature control is obtained by cooled product gas recycling. The heat is recovered by high pressure and temperature steam generation [605,606]. Fluidized-bed reactors operate in near isothermal condition. The catalyst particles are fluidized by the inlet. The uniform mixing of catalyst and reactants guarantees a more uniform temperature profile avoiding the hot spot formation [607]. Drawbacks are the catalyst degradation due to attrition and bubbling formation [606]. Three-phase reactors as fluidized-bed reactors operate in near isothermal condition. The catalyst particles are suspended in a liquid phase. The high heat capacity of the liquid phase guarantees a uniform temperature in the reactor and the efficient removal of heat [173]. Drawbacks are the evaporation or decomposition of the liquid suspension and the mass-transfer resistance of the liquid phase [608]. Structured reactors are polytropic reactors realized with an internal metallic structure coated with the catalyst. The most employed structured reactor has a monolithic catalyst realized with several parallel channels [609]. Microchannel reactors are compact reactors with a high specific surface area that enhance the mass and heat transfer [610]. The temperature profile is uniform as the metal walls have high thermal conductivity, and the pressure drop is low [611]. Moreover, the scale-up is relatively simple since it is possible to increase easily the number of microchannels placed in the reactor [612].PDF Image | Green Synthetic Fuels
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