Zhen-Min Cheng

Kinetics of Hydrogenolysis of Glycerol to Propylene Glycol over Cu-ZnO-Al2O3 Catalysts

Abstract A series of Cu-ZnO-Al 2 O 3 catalysts with various metal compositions of Cu/Zn/Al were prepared by the co-precipitation method, and screened for glycerol hydrogenolysis to propylene glycol. The catalyst with a Cu/Zn/Al molar ratio of 1 : 1 : 0.5 exhibited the best performance for glycerol hydrogenolysis, and thus selected for kinetic investigation. Under elimination of external and internal diffusion limitation, kinetic experiments were performed in an isothermal fixed-bed reactor at a hydrogen pressure range of 3.0-5.0 MPa and a temperature range of 493-513 K. Based on a dehydration-hydrogenation two-step hydrogenolysis mechanism, a two-site Langmuir-Hinshelwood kinetic model taking into account competitive adsorption of glycerol, acetol and propylene glycol was proposed and successfully fitted to the experimental data. The average relative errors between observed and predicted outlet concentrations of glycerol and propylene glycol were 6.3% and 7.6%, respectively. The kinetic and adsorption parameters were estimated by using the fourth-order Runge-Kutta method together with the Rosenbrock algorithm. The activation energies for dehydration and hydrogenation reactions were 86.56 and 57.80 kJ·mol −1 , respectively.

Hydrodynamic behavior of a trickle bed reactor under “forced” pulsing flow

The hydrodynamic properties in a trickle bed reactor under forced pulsing flow are studied. It has been demonstrated by the experiments that the axial and radial liquid distributions become more uniform than those of the natural pulsing flow regime. The thickness of the liquid film over the particles is sharply thinned by the induced liquid flow. Effects of the operating conditions on liquid holdup are studied in detail. The frequency of the forced pulsing flow is found to be the most significant parameter affecting the liquid holdup.

Initial estimation of heat transfer and kinetic parameters of a wall-cooled fixed-bed reactor

Abstract A numerical method is developed for the initial guesses of heat transfer and kinetic parameters in a wall-cooled fixed-bed reactor accompanied with side-stream analysis in both the axial and radial directions. The differential method is found to be the only way to decouple the heat and mass transport equations, and thus enables the estimation of the heat transfer and kinetic parameters by separate steps. Compared with simultaneous estimation by the integral methods, the differential method possesses a unique advantage arising from the separate estimation of the parameters, because the parametric correlation can be reduced since the parameter number in the separate estimating process is less than that in the integral process. Numerical analysis and experimental studies reveal that the most appropriate method for presenting the axial derivative is by the Pad≐ approximation, and that for the radial derivatives is by the orthogonal collocation with one point.

Influence of hydrodynamic parameters on performance of a multiphase fixed-bed reactor under phase transition

Abstract Reactor performance of a trickle-bed reactor (TBR) concurrently under gas–liquid downward flow and that of a flooded-bed reactor (FBR) concurrently under gas–liquid upward flow was experimentally investigated under an elevated temperature and pressure (150°C and 1.0 MPa ) for benzene hydrogenation. It was shown that the different hydrodynamics of TBR and FBR could result in quite different reactor behaviors, as typically observed from the temperature profiles along the reactor. The reason for this is because the present reaction is controlled by the supply of hydrogen in the liquid phase, thus external partial wetting of the catalyst pellets could increase the reaction rate. Moreover, the pronounced vapor pressure of benzene under the prescribed temperature would make the reaction remarkable over the non-wetted catalysts. Operation in the FBR is superior to the TBR, considering the operational safety. However, TBR should be considered when the catalyst partial wetting is negligible under liquid flow rates higher than 0.58 cm / s as shown in this work.

Support Effects on Thiophene Hydrodesulfurization over Co-Mo-Ni/Al2O3 and Co-Mo-Ni/TiO2-Al2O3 Catalysts

Abstract Hierarchically macro-/mesoporous structured Al2O3 and TiO2-Al2O3 materials were used as supports to prepare novel Co-Mo-Ni hydrodesulfurization (HDS) catalysts. A commercial Co-Mo-Ni/Al2O3 catalyst without macroporous channels was taken as a reference. The catalysts were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), energy-dispersive spectrometry (EDS), N2 adsorption-desorption, X-ray diffraction (XRD), and temperature programmed reduction (TPR). The apparent activities of the hierarchically porous catalysts for thiophene HDS were superior to those of the commercial catalyst, which was mainly ascribed to the diffusion-enhanced effect of the hierarchically bimodal pore structure. The addition of titania to alumina in the support helped to weaken the interaction between the active phase and the support, and as a result, the novel Co-Mo-Ni/TiO2-Al2O3 catalyst with a low titania loading (28%, by mass) in the support exhibited high HDS activities, even without presulfiding treatment. However, the catalyst with a high titania loading (61%, by mass) showed much lower activities, which was mostly caused by its low surface area and pore volume as well as the non-uniform distribution of titania and alumina. The kinetic analysis further demonstrated the support effects on HDS activities of the catalysts.

An innovative reaction heat offset operation for a multiphase fixed bed reactor dealing with volatile compounds

Hydrogenation of benzene to cyclohexane was conducted in an adiabatic fixed-bed reactor under a phase transitional condition. In which process, a mixture of benzene and cyclohexane was fed into the reactor at the bottom under the liquid condition, however, the liquid phase disappeared and turned into gas phase at the reactor outlet through vaporization. It follows from this novel idea that the problems associated with reaction heat removal and low catalyst efficiency of the liquid-filled catalyst could be solved simultaneously with emptying of the liquid from the catalyst. However, the benzene concentration in the mixture should not exceed 14% if simple phase transition is employed, since the reaction heat is seven times the liquid vaporization heat. To optimize this primitive operation, a side stream quenching operation was proposed and attempted experimentally, and it was found that the side stream quenching with double injection points could be an acceptable operating strategy.

Catalytic oxidation of dilute SO2 over activated carbon coupled with partial liquid phase vaporization

Abstract The paper presents theoretical analysis and experimental results of dilute SO2 oxidation over activated carbon coupled with partial liquid phase evaporation. A dynamic model has been first developed for this combination of reaction with vaporization of liquid in the packed bed. The model reveals the interactions among the exothermic reaction, endothermic vaporization and partial internal wetting of activated carbon pellets. Reliability of the model is verified by the experiments. Secondly, the effects of the two different operation modes and the states of the wetted catalysts on SO2 oxidation are compared. The results have demonstrated that this method has an increased SO2 removal efficiency and a higher sulfuric acid concentration.

Correlations for dynamic liquid holdup under pulsing flow in a trickle-bed reactor

Based on theoretical analysis and experimental results, a new attempt has been made to characterize the dynamics of the fluid flowing under conditions of pulsing flow in a trickle-bed reactor (TBR). Two kinds of correlation are proposed for the dynamic liquid holdup under pulsing flow, which can predict the dynamic liquid holdup for a given packing type and given operating conditions.

Simultaneous estimation of kinetic and heat transfer parameters of a wall-cooled fixed-bed reactor

Abstract Heat transfer and kinetic parameters of wall-cooled fixed-bed reactors are traditionally obtained through separate heat transfer and kinetic experiments. In spite of its simplicity, the uncertainties resulting from the low parametric sensitivity and high correlationship among the parameters inevitably make the estimation unsuitable for safe extension to actual reacting conditions. In this paper, an effort was made to estimate simultaneously both heat transfer and kinetic parameters under reacting conditions in a single tube wall-cooled fixed-bed reactor, and a two-stage parameter estimation procedure was developed. The correlation and confidence region analysis performed in this paper verifies both theoretical and experimental feasibility of simultaneous estimation of heat transfer and kinetic parameters under real reacting conditions.

Ultrafast and Stable CO2 Capture Using Alkali Metal Salt-Promoted MgO–CaCO3 Sorbents

As a potential candidate for precombustion CO2 capture at intermediate temperatures (200-400 °C), MgO-based sorbents usually suffer from low kinetics and poor cyclic stability. Herein, a general and facile approach is proposed for the fabrication of high-performance MgO-based sorbents via incorporation of CaCO3 into MgO followed by deposition of a mixed alkali metal salt (AMS). The AMS-promoted MgO-CaCO3 sorbents are capable of adsorbing CO2 at an ultrafast rate, high capacity, and good stability. The CO2 uptake of sorbent can reach as high as above 0.5 gCO2 gsorbent-1 after only 5 min of sorption at 350 °C, accounting for vast majority of the total uptake. In addition, the sorbents are very stable even under severe but more realistic conditions (desorption in CO2 at 500 °C), where the CO2 uptake of the best sorbent is stabilized at 0.58 gCO2 gsorbent-1 in 20 consecutive cycles. The excellent CO2 capture performance of the sorbent is mainly due to the promoting effect of molten AMS, the rapid formation of CaMg(CO3)2, and the plate-like structure of sorbent. The exceptional ultrafast rate and the good stability of the AMS-promoted MgO-CaCO3 sorbents promise high potential for practical applications, such as precombustion CO2 capture from integrated gasification combined cycle plants and sorption-enhanced water gas shift process.