Study On Coking Behavior Of Medium-low Temperature Coal Tar During Catalytic Hydroprocessing

Catalytic hydrogenation for medium-low temperature coal tar produce fuel oil which can relieve the shortage of oil supply in China.However,coking on catalyst surface during catalyic hydroprocessing of tar is an important issue.Understanding the formation and evolution of coke deposited on the catalyst surface can provide a theoretical basis for solving the problem of coking.In this work,we used Yulin middle-low temperature coal tar as feed source and investigated the effects of heating temperature and a hydrogen donor solvent,tetrahydronaphthalene,on the coking(Tetrahydrofuran insoluble)of the reactor wall and the catalyst surface.By measuring the concentration of radical concentration in the system and on the surface of the catalyst after reaction,we associated the amount of coke on the catalyst with its the radical concentration.The aged sample were analyzed by a variety of techniques,which include elemental analysis,temperature-programmed oxidation(TPO),solid-state carbon-13 nuclear magnetic resonance spectroscopy(13C NMR),fourier transform infrared spectroscopy techniques(FT-IR).The changes in composition and properties of coke deposited on the catalyst surface with reaction temperature and time were analyesd and preliminary understanded the coking mechanism.The main conclusions are described in the followings:1.Tetrahydrofuran insoluble of the tar is 0.5%.When the tar is heated at 380-440 ? for 1 min,the coking yields on reactor wall reache 2-3%.At 380?,the coking yields have a little changes with increasing heating time.But when the temperature is higher than 400 ?,after heating for 10 minutes,the coking yields on reactor wall increase linearly with increasing heating time and increase exponentially with increasing heating temperature.The coking yields at 420 ? and 440 ? for heating 1 h are about 8%and 15%,respectively.The addition of hydrogen donor solvent and catalyst during hydroprocessing can significantly reduce the coking yields on reactor wall.When the temperature is 420 ?,the coking yield on reactor wall decreases to 2-3%.2.At the same temperature,the coking yields of tar on catalyst surface was lower than reactor wall.At 420 ? and 440 ? for heatig 1 h,the coking yields on catalyst surface are about 4.0%and 5.4%,respectively.The addition of hydrogen donor solvent during the hydroprocessing significantly reduced the coking yields on catalyst surface,but the coke cannot be completely suppressed,indicating that there may be high-concentration asphaltenes in the tar which are difficult to hydrogenate.3.A positive correlation between the amount of coke on the catalyst surface and its radical concentration.The greater the amount of coke,the higher the radical concentration,indicating that the coking behavior on catalyst surface during the tar catalytic hydrprocessing reaction also follows the free radical mechanism.4.The properties of coke deposited on the catalyst at 440 ? change with heating time,including the decrease of aliphatic carbon content,the increase of aromatic carbon content,and the decrease of H/C ratio.When THN was added,the H/C ratio of coke decreases and the degree condensation of the coke has little diffenence with the average ring number maintaining at 4-6.The heating time has little effect on the reactivity of coke on the catalyst surface and the oxidation reaction starts at 220 ?.