Study On The Multisacle Transport And Hydrodesulfurization Performances Of Multiphase Monolithic Reactors

Monolithic reactors (MRs) present advantages such as low pressuredrop, uniform flow distribution, enhanced mass transfer, low liquid axialmixing, and high catalyst effectiveness. Therefore, MRs are viewed asnew "process intensification" three-phase reactors to replace theconventionally used trickle beds and slurry bubble columns.This work aims to intensification of the hydrodesulfurization (HDS)process by using MRs. The transport and HDS performance of MRs werestudied experimentally and theoretically at a single scale and monolithbed scale.Firstly, the characteristics of liquid side mass transfer and liquidaxial mixing under Taylor flow in capillaries were studied withComputational Fluid Dynamics (CFD) method. Three peak values oflocal mass transfer coefficient were found on bubble surface. Masstransfer coefficient in liquid film decreases with an increase in filmcontact time, whereas that in the caps changes little. Moreover, theprinciple of field synergy was used to explain the characteristics andenhancement of local mass transfer. The results also show that short filmcontact time for the same contact area could enhance the gas-liquid masstransfer under Taylor flow. Empirical correlations were proposed topredict the liquid side volumetric mass transfer coefficients for short andlong film contact time respectively, and the predicted values in capillarieswith diameters of0.25-3mm have a relative error within±20%. To study the liquid axial mixing, the vessel dispersion number (VDN) wasevaluated by using particle tracking method at high Bodenstein numbers.The VDN increases with increasing liquid film length and bubble velocity.An empirical correlation was proposed to predict the vessel dispersionnumber for Bo>10⁵.Secondly, total pressure drop, liquid holdup and gas-liquid masstransfer coefficients in three different monolith packings wereinvestigated experimentally in the Taylor flow regime. The effect ofdistributor design on the flow distribution was investigated using twodifferent types of distributor (nozzle distributor and packed beddistributor with1mm glass beads). A close relation of the distributordesign to the hydrodynamics in monolith beds was observed to exist,evidenced by the larger pressure drop and liquid hold-up values for themonolith packings with the use of the packed bed distributor, whencompared with the nozzle distributor. An analysis of the unstable flowphenomenon characterized by negative pressure drops within bed showsthat the unstable region is not only dependent on the operating conditionsand properties of monolith but also on the distributor design. Empiricalcorrelations of friction factor and liquid hold-up for the three monolithswere proposed. The mass transfer coefficient increases with increasingsuperficial gas or liquid velocities. Moreover, the mass transfercoefficients for these three types of monolith packing show a linearrelation to bed pressure drop. An empirical correlation of mass transfercoefficients based on Jepsen's correlation was proposed, which has arelative error within±30%.Thirdly, flow distribution experiments were carried out in a columnwith monolith packings of cell density of50cpsi with two differentdistributors. Liquid saturation in individual channels was measured byusing a self-made micro-conductivity probe. A mal-distribution factor was used to evaluate uniform degree of phase distribution in monoliths.For liquid flow distribution, it is found that the superficial liquid velocityis a crucial factor and the packed bed distributor is better than the nozzledistributor. A semi-theoretical analysis using single channel modelsshows that the packed bed distributor always yields shorter and uniformlydistributed liquid slugs compared to the nozzle distributor, which in turnensures a better mass transfer performance. A bed scale mass transfermodel is proposed by employing the single channel models in individualchannels and incorporating effects of non-uniform liquid distributionalong the bed cross-section. The model predicts the overall gas-liquidmass transfer coefficient with a relative error within±30%.Finally,reaction kinetics of thiophene and dibenzothiophene (DBT)hydrodesulfurization over Ni₂P/SBA-15/cordierite catalyst wasinvestigated at temperatures of300-380°C and total pressures of3.0-5.0MPa. Comparison studies of the performance of the two types of reactors(the trickle bed and monolithic reactor) were performed by using theLangmuir-Hinshelwood model and the first-order model for fitting thethiophene and DBT experimental data in a batch recycle operation,respectively. It was found that, both the activation energy and the rateconstant over Ni-P monolithic catalyst in the stated operating conditionsare close to those over conventionally used HDS catalysts. The resultsindicat that, the productivity of the monolithic reactor is3times higherthan that of the trickle bed reactor on the catalyst weight basis due to aneffective utilization of the catalyst in the monolithic reactor, but thevolumetric productivity of the monolithic reactor is lower for HDS ofDBT. Here, we proposed two-stage reactors, i.e., a monolithic reactorfollowed by a tickled bed reactor and it was found that, the two-stagereactor outperforms the trickle bed reactor in both reactor volume andcatalyst weight basis. The effect of the flow distribution on HDS performance of the monoliths was discussed. The results show the HDSperformance of the monoliths is improved with the increasing liquidvelocity and decreasing gas velocity and the packed bed distributor isbetter than the nozzle distributor. At high liquid velocity, thoughmal-distribution was observed, the HDS simulation results of themonoliths with packed bed distributor are close to those of the singlechannel with10%decrease.