| Trickle bed reactor (TBR), which is a typical gas-liquid-solid three-phase reactor, is wildly used for hydrocreaking and hydrorefining processes including desulfurization, denitrogen and dearomatics in the refinery indurstry and for hydrogenation, oxidation and hydration process in the petroleum chemical industry. Recently, more attention has been gained on the application in the waste water treatment and biochemical engineering process. Unsteady-state operation of TBR (USTBR), a novel and developing process intensification technology, is becoming one of the most important topics in the multiphase reactor engineering. Compared with the traditional steady-operaiton, a significant enhancement of time-average performance can be obtained by the Unsteady-state operation, i.e., periodically changing some operating parameters of TBR.Regardless of the advantages of USTBR, there still exit some important issues to be solved due to its short history. (1) Most of previous research focused on the modulation of liquid flow rate on performance, but little attention has been paid on the other practicable modes for the unsteady-state operation. (2) Since most of model reactions used in the literaures were the simple reactions, such as the hydrogenation ofα-methylstyrene, up to now little information was reported about the influence of USTBR on selectivity and temperature rise, which were important issues for the indurstial process. (3) Now it is still impossible to predict the performance of a TBR under periodic operation using a mathematic model due to the lacking of deep understanding on the USTBR behaviors. Therefore, it is indispensable for the further understanding on unsteady-state operation technology and its potential application in the industry to resolve the problems described above.In the present work, the influences of several unsteady-state operation modes such as TBR under pulsing regime, periodic modulation of catalyst activity, and periodic modulations of liquid flow rate or concentration, on the reactor performances and temperature rises were systemically studied using hydrogenation of 2-ethyl-9,10-anthraquinones (EAQs) and dicyclopendidene (DCPD) as the test reactions for isothermal and nonisothermal systems. In addition, attempts were also made to develop the mathematic models for the described operating modes, and thus to analyze the unsteable behaviors of TBR under unsteady-state operations. As a result, the following results have been obtained.(1) The TBR hydrodynamics of pulsing regime for experimental system was studied to determine operating regime and properties of pulsing flow. To obtain the transition boundaries of trickling and pulsing regimes, regimes identification method was established by the statistic analysis on pressure drop signals. Pressure drop and liquid holdup of the pulsing flow were also experimentally measured. Furthermore, a theoretic method and several empirical correlations were also provided to predict regimes transition boundary, pressure drop and liquid holdup, respectively.Based on an extensive experimental database (946 measurements) set up from the literature published over past 30 years, a new correlation relying on artificial neural network (ANN) was proposed to predict basic pulsation frequency of pulsing flow in the trickle-bed reactors. Seven dimensionless groups employed in the proposed correlation were liquid and gas Reynolds (ReL, ReG), liquid Weber (WeL), gas Froude (FrG), gas Stokes (StG) and liquid Eotvos (EoL) numbers and a bed correction factor (Sb). The performance comparisons of literature and present correlations showed that ANN correlation is a significant improvement in predicting pulsation frequency with an AARE of 10% and a standard deviation less than 18%. The effects of the variables including the properties of fluid and bed, and flow rate of liquid and gas on pulsing frequency were investigated by ANN parametric simulations and the trends were compared with exiting experimental results which confirmed the coherence of the proposed method with the previous experiments.(2) EAQs hydrogenation in the TBR under pulsing and trickling regimes was studied to examine the effect of natural unsteady-state operation on the reactor performance. It is found that the hydrogenation rate under pulsing regime is 40%-100% higher than that of trickling regime. The apparent kinetics indicated that the apparent activity energy of pulsing flow is 27.86 kJ/mol, which is closer to the intrinsic activity energy of 35~37 kJ/mol and higher than trickling regime of 16.67 kJ/mol. This further confirmed that the enhancement of performance was a result of improvement of external mass transfer rate under high interaction regimes.A mathematic model for TBR operated under pulsing flow was developed with an assumption of square-wave function between mass transfer coefficients and the time. The developed model predicted the experimental results with a satisfactory accuracy. The effect of pulsing flow properties on the reactor performance was simulated with the model described above. The results showed that increase of pulsation frequency can improve the conversion up to 80%, but the effect of pulse structure is neglectable.(3) A novel unsteady-state mode was proposed to enhance TBR performance by periodic modulation of the activity of catalyst pellets. The reactor performances with different packing modes including traditional, diluted and periodic packing modes were systematically studied using EAQs hydrogenation as test reaction. It was found that periodic packing mode with higher capability packing can improve space time yields 90% and 10% as compared with traditional and diluted packing modes.A mathematic model for periodicaly packed TBR incorporating partial wetting and axial dispertion was developed, which can predict the experimental results accurately. Simulations with the developed model indicated that the higher performance packing and packing structure were essential factors for the performance enhancement.(4) In general, the periodic ON-OFF modulation of liquid flow rate was taken as a most promising technology due to its enhancement on the mass transfer rate of gaseous reactants from gas to catalyst surface, and the effective wetting of catalyst pellets. The influence of periodic operation on a consecutive reaction, the EAQs hydrogenation over Pd/Al2O3, was studied. The effects of operating parameters including cycle period, split, pressure, temperature and time-average flow rate on the reactor performance were experimentally examined in comparison with the steady state operation. The results showed that under the interested operating conditions the conversion and the selectivity were improved by 3%-21% and 4%, respectively.Based on a nonlinear relationship of local liquid holdup with superficial liquid velocity (εL, uL), a dynamic model consisting of a set of partial differential equations (PDEs) was developed for the periodic operation of TBR. The developed model was verified compared with the experimental results. The feed/drainage behavior and effect of the periodic operation on the external mass transfer rate were simulated to explain the course-reason relationship of reactor performance enhancement.(5) The intrinsic kinetics of dicyclopentadiene (DCPD) hydrogenation into endo-tetrahydrodicyclopentadiene (endo-THDCPD) over Pd/Al2O3 catalyst was investigated using stirred semibatch reactors in the absence of transport limitations over the ranges of temperature (358.15-438.15 K) and hydrogen pressure (0.5-3 MPa). A Langmuir-Hinshelwood type model was proposed to fit the experimental data and its kinetic parameters were also regressed by comparing the calculated and experimental concentrations profiles of reactants and products. The activation energy for the first and second step reaction is 7.8945 and 12.2068 kJ/mol, respectively. It was found that the developed model was accurate to predict the experimental results with the averagerelative error (ARE) less than 12.7 %, and was reliable for the reactor design and modeling.(6) Effects of the steady-state operation parameters, such as pressuer, inlet temperature, concentration and liquid flow rate, on the DCPD hydrogenation performance and temperature profiles were studied to determine the practical modulation variables for the USTBR. The results indicated that liquid flow rate and concentration were suitably used as regulation variables for the performance enhancement by the unsteady-state operations.On the basis of above results, the unsteady-state behaviors under different operating modes including ON-OFF modulations of the liquid flow rate (mode A), PEAK-BASE modulations of the liquid flow rate (mode B), ON-OFF modulations of the liquid concentrations (mode C), PEAK-BASE modulations of the liquid concentrations (mode D) and the synchronous modulations of liquid concentration and flow rate (mode E) were systemically studied. It is shown that mode A and E can improve hydrogenation rate up to 20% and 15%, and reduce MTR more than 13 K; performance enhancement were about 10% for both mode B and D, which can reduce MTR less than 10 K; mode C has an effect less than 5% on the performance, but reduces MTR up to 12 K.(7) A mathematic model incorporating enthalpy balance and phase equilibria was developed based on a three-zone pellet-scale model considering the influence of static liquid holdup on the mass transfer rate. The comparison of experimental and simulated results indicated that the developed model was reliable to predict the performance and axial temperature profiles accurately.(8) Several principles for the selections and optimizations of USTBR operation parameters for the conception design were proposed from the experimental results reported in this work and literatures.To sum up, USTBR, as a promising process intensification technology for the large-scale applications in the industries, has several significant advantages over the steady operation in enhancing mass transfer rate and utilizing reaction heat to improve the reaction rate and eliminate the local hot point and the"run away"of reactor.
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