What is the leanest stream to sustain a nonadiabatic loop reactor: Analysis and methane combustion experiments

While previous studies experimentally demonstrated that loop reactor (LR) can be sustained with a lean feed (using ethylene combustion) and have analyzed the single‐reaction adiabatic case, this work analyzes the effects of heat loss and of reactor size to determine the leanest stream (expressed in terms of adiabatic temperature rise ΔT_lim) that will sustain the operation. For an adiabatic infinitely long reactor ΔT_lim→0 while for a finite reactor ΔT_lim scales as (1 + Pe/4)^?1 where Pe = Luρc_pf/k, and heat loss increases this limit by (β/Pe)^1/2. Thus, a good design of a LR will aim to decrease conductivity (k) and radial heat‐transfer coefficient (β) while increasing throughput (u) and reactor length. This article is also the first experimental demonstration of auto‐thermal operation in a LR for catalytic abatement of low‐concentration of methane, showing the leanest stream to be of 8000 ppm vs. 33,000 ppm in a once‐through reactor. Experimental combustion results of methane and of ethylene are compared with model predictions. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2030–2042, 2017     

Experimental study and simulation of the polymerization of methyl methacrylate at high temperature in a continuous reactor

The bulk polymerization of MMA at high temperature (120–180°C) in a continuous pilot‐plant reactor has been studied. The polymerization is initiated by diterbutyle peroxide and the chain transfer agent is 1‐butanethiol. A simulation program has been developed to predict the steady state behavior of the reactor. The particular features of the kinetic at above‐T_g temperature are included in the model, especially the thermal initiation of the reation and the attenuation of the autoacceleration effect. For the flow and mixing model, the actual vessel cannot be approximated to a single ideal reactor because of its design and of the moderate agitation imposed by the high viscosity of the reacting fluid. A tanks in series model with a recycle stream between tanks is proposed to evaluate the backmixing caused by the special design of the agitator. The parameters of the model are determined with the help of the experimental residence time distribution measured on the reactor. The data collected on the actual reactor, i.e., operation, conversion, molecular weight, temperature, are compared to the calculated one. The agreement is satisfactory but the tendencies are slightly underestimated. The program is a tool to evaluate the effect of modifications of the design of the reactor or changes on the operation parameters like input rate, temperature, and agitation on its behavior. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2038–2051, 2001     

Study of unsteady-state catalytic oxidation of sulfur dioxide by periodic flow reversal

A systematic study of catalytic oxidation of sulfur dioxide in a fixed bed reactor operated in flow reversal mode was made. A heterogeneous transient model of the reactor was developed. The global rate equations and the heat transfer parameter correlation were obtained, based on a series of previous experiments. The experiments of unsteady-state oxidation of low concentration SO₂ were conducted in a bench-scale fixed bed reactor, packed with a domestic commercial catalyst. The model can successfully predict the transient concentration and temperature profiles when a correction factor is introduced to the global rate equations.     

Are 3‐D models necessary to simulate packed bed reactors? analysis and 3‐D simulations of adiabatic and cooled reactors

Simulations and analysis of transversal patterns in a homogeneous three‐dimensional (3‐D) model of adiabatic or cooled packed bed reactors (PBRs) catalyzing a first‐order exothermic reaction were presented. In the adiabatic case the simulation verify previous criteria, claiming the emergence of such patterns when (ΔT_ad/ΔT_m)/(Pe_C/Pe_T) surpasses a critical value larger than unity, where ΔT_ad and ΔT_m are adiabatic and maximal temperature rise, respectively. The reactor radius required for such patterns should be larger than a bifurcation value, calculated here from the linear analysis. With increasing radius new patterned branches, corresponding to eigenfunction of the problem emerge, whereas other branches become unstable. The maximal temperature of the 3‐D simulations may exceed the 1‐D prediction, which may affect design procedures. Cooled reactor may exhibit patterns, usually axisymmetric ones that can be characterized by two anomalies: the peak temperature may exceed the corresponding value of an adiabatic reactor and may increase with wall heat‐transfer coefficient, and the peak temperature in a sufficiently wide reactor need not lie at the center but rather on a ring away from it. In conclusions, we argue that transversal patterns are highly unlikely to emerge in practical adiabatic PBRs with a single exothermic reaction, as in practice Pe_C/Pe_T > 1. That eliminates patterns in stationary and downstream‐moving fronts, whereas patterns may emerge in upstream‐moving fronts, as shown here. This conclusion may not hold for microkinetic models, for which stationary modes may be established over a domain of parameters. This suggests that a 1‐D model may be sufficient to analyze a single reaction in an adiabatic reactor and a 2‐D axisymmetric model is sufficient for a cooled reactor. The predictions of a 2‐D cylindrical thin reactor with those of a 3‐D reactor were compared, to show many similarities but some notable differences. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

Comparison of the performance of a direct-contact bubble reactor and an indirectly heated tubular reactor for solar-aided methane dry reforming employing molten salt

A theoretical approach is presented for the comparison of two different atmospheric pressure reactors—a direct-contact bubble reactor (DCBR) and an indirectly heated tubular reactor (IHTR)—to evaluate the reactor performance in terms of heat transfer and available catalytic active surface area. The model considers the catalytic endothermic reactions of methane dry reforming that proceeds in both reactors by employing molten salts at elevated temperatures (700–900 °C) in the absence of catalyst deactivation effects. The methane conversion process is simulated for a single reactor using both a reaction kinetics model and a heat transfer model. A well-tested reaction kinetics model, which showed an acceptable agreement with the empirical observations, was implemented to describe the methane dry reforming. In DCBR, the heat is internally transferred by direct contact with the three phases of the system: the reactant gas bubbles, the heat carrier molten salts and the solid catalyst (Ni-Al₂O₃). In contrast, the supplied heat in the conventional shell-and-tube heat exchanger of the IHTR is transferred across an intervening wall. The results suggest a combination system of DCBR and IHTR would be a suitable configuration for process intensification associated with higher thermal efficiency and cost reduction.     

Performance optimization of an electromembrane reactor for recycling and resource recovery of desulfurization residuals

An environmentally friendly method for electrochemically regenerating alkali‐sorbent (NaOH) and recovering sulfur in the flue gas as H₂SO₄, while producing H₂ as a clean energy source from flue gas desulfurization (FGD) residuals in an electromembrane reactor, was proposed in this article. To optimize and improve the performance, the optimal operating conditions were deduced from the numerical simulation and validated using experimental data. Under the optimized conditions, the current efficiencies of alkali‐sorbent regeneration and H₂SO₄ reached 84 and 87%, respectively, which is comparable to those obtained in the chlor‐alkali industry. Therefore, this method has the potential to be scaled up. If this technology is integrated into an existing FGD facility, the money‐consuming chemical process could be transferred into a renewable resource and clean energy conversion process. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2613–2624, 2014     

Mathematical modeling,steady state simulation,parametric analysis and optimization of SO2 capture in a countercurrent moving bed (CaO and C) reactor

The use of absorbents based on calcium for the reduction of SO₂ emissions from power plants has been studied for the last 30 years. The present work is part of a research project aimed at the development of a moving bed limestone filter. It is placed after burning in order to capture the SO₂ from the flue gases of pulverized lignite combustors. A heterogeneous mathematical model was developed for the steady‐state simulation of a gas–solid countercurrent moving bed reactor. The mathematical model was solved using the method of finite elements and gave results for the temperature and conversion profile along the reactor. Also, parametric analysis (for T_g,in, T_s,in, u_s, q_o, , , d_o, z_o) was performed and useful conclusions concerning the behavior of a moving bed reactor in different conditions were drawn. Finally, optimization of conditions was performed to give an SO₂ conversion higher than 0.95. The results are of important technological interest as a dry process in applications of SO₂ capture. © 2018 Society of Chemical Industry     

Methylcyclohexane dehydrogenation for hydrogen production via a bimodal catalytic membrane reactor

The dehydrogenation of methylcyclohexane (MCH) to toluene (TOL) for hydrogen production was theoretically and experimentally investigated in a bimodal catalytic membrane reactor (CMR), that combined Pt/Al₂O₃ catalysts with a hydrogen‐selective organosilica membrane prepared via sol‐gel processing using bis(triethoxysilyl) ethane (BTESE). Effects of operating conditions on the membrane reactor performance were systematically investigated, and the experimental results were in good agreement with those calculated by a simulation model with a fitted catalyst loading. With H₂ extraction from the reaction stream to the permeate stream, MCH conversion at 250°C was significantly increased beyond the equilibrium conversion of 0.44–0.86. Because of the high H₂ selectivity and permeance of BTESE‐derived membranes, a H₂ flow with purity higher than 99.8% was obtained in the permeate stream, and the H₂ recovery ratio reached 0.99 in a pressurized reactor. A system that combined the CMR with a fixed‐bed prereactor was proposed for MCH dehydrogenation. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1628–1638, 2015     

SMCA—the real optimal operation of a multistage adiabatic fixed-bed reactor

A general rule and a straightforward approach of the real optimal operation of a multistage adiabatic fixed-bed reactor (MAFBR) for a single reaction system, namely, the stagewise maximum conversion approach (SMCA), were derived based on the analysis of the operation of a Fauser—Montecatini type, five-stage ammonia synthesis reactor. The SMCA can be implemented in reactors which have ageing catalysts and maldistributed gas. The SMCA has been applied efficiently to industrial SO₂ oxidation and NH₃ synthesis plants. Suggestions on the design of a new MAFBR are made.     

Effects of enzyme microcapsule shape on the performance of a nonisothermal packed-bed tubular reactor

The effects of enzyme microcapsule shape (spherical, cylindrical and flat plate) on the performance of a nonisothermal, packed-bed reactor have been modeled as a function of Biot number and Peclet number for mass and heat transfer (Bi_m, Bi_h, Pe_m and Pe_h), and dimensionless heat of reaction α. Under the given simulation conditions, only higher values of Bi_m and Bi_h (>2·5) confirm the influence of microcapsule shape on the reactor performance such that the axial and overall conversion and bulk temperature decrease as follows: spherical > cylindrical > flat plate. In terms of the shape-independent modified Biot number, Bi* = Bi/{(n + 1)/3}, this order is retained for 2 < Bi* < 8. The influence of increasing Pe_m, Pe_h, and α on conversion and bulk temperature also follows the above order. For the flat plate, the exit conversion and temperature are not influenced by Pe_m and Pe_h, that is, mass transfer and thermal backmixing effects, respectively. On the other hand, for the spherical and cylindrical microcapsules, overall backmixing effects are negligible only beyond a critical value of Pe_m (∼7) and Pe_h (∼1·75). The conversion and bulk temperature increase with the increase in α, independent of the microcapsule shape. The spherical and cylindrical microcapsules, unlike the flat plate, cannot be considered isothermal.     

Rapid measurement of gas diffusivity in liquids using a tube-in-tube reactor with an unsteady-state strategy

In this work, an unsteady-state strategy for rapid measurement of gas diffusivity in liquid is proposed, which has a quick perturbation of the liquid flow rate in inner tube for obtaining the change of gas flow rate across the membrane with time. The strategy has taken full advantages of the tube-in-tube reactor that possesses a high permeability Teflon AF-2400 membrane to accelerate the diffusion rate of gas to liquid without direct contact between the two phases. With a developed mathematical model fitting the recorded variation of gas flow rate with time, the gas diffusivity in liquid can be determined within 0.5–3 min compared with the conventional methods of 4–14 hr. In addition, the strategy is demonstrated with several gas–liquid systems (O₂-DMSO, CO₂-[Emim] [NTf₂] and CO₂-[Bmim] [BF₄]) with varied viscosities and temperatures, showing a good agreement with literature values with less than 10% deviation.     

Performance of an internally circulating fluidized-bed reactor for the catalytic oxidative coupling of methane

The oxidative coupling of methane to C₂-hydrocarbons (OCM) over a La₂O₃/CaO catalyst (27 at.%) was investigated in an internally circulating fluidized-bed (ICFB) reactor (ID_eff = 1.9 cm, H_riser = 20.5 cm). The experiments were performed in the following range of conditions: T = 800?900°C, p_CH4:p_O2p_N2 = 57.1–64:16–22.9:20 kPa. The obtained C₂ selectivities and C₂ yields were compared with the corresponding data from a spouted-fluid-bed reactor (ID = 5 cm) and a bubbling fluidized-bed (FIB) reactor (ID = 5 cm). The maximum C₂ yield in the internally circulating fluidized-bed (ICFB) reactor amounted to 12.2% (T = 860°C, 38.7% C₂ selectivity, 31.5% methane conversion), whereas in the FIB reactor a maximum C₂ yield of 13.8% (T = 840°C, 40.4% C₂ selectivity, 34.2% methane conversion) was obtained. The lowest C₂ yield was achieved in the spouted-bed (SFB) reactor (Y = 11.6%, T = 840°C, 36.2% C₂ selectivity, 32.0% methane conversion). The highest space-time yield of 24.0 mol/kg_cat.h was obtained in the ICFB reactor, whereas in a FIB reactor only a space-time yield of 9.6 mol/kg_catcould be obtained. The performance of the ICFB reactor was strongly influenced by gas-phase reactions. Furthermore, stable reactor operation was possible only over a narrow range of gas velocities.     

A priori modelling of an adiabatic spouted bed catalytic reactor

An a priori reactor model for an adiabatic spouted bed reactor has been developed. This model uses first-principles mass and energy balances to predict the concentration and temperature profiles in the spout, annulus and fountain regions of the reactor. The particle circulation and voidage profiles in the spout are calculated using previously developed analytical techniques. Particle circulation patterns in the annulus are determined by a minimum path-length analysis. The spout and fountain are shown to contribute significantly to the overall conversion in the bed. Predicted and experimental conversions at flowrates up to 1.2U_ms show that extension of the fountain reaction zone and increased particle circulation with increasing inlet flow makes up for the higher average voidage in the spout and fountain. Experimental data confirm the calculated results for a stably spouting bed with CO oxidation over a Co₃O₄/αAl₂O₃ catalyst. The effects of flowrate and inlet reactant concentration are confirmed.     

A mathematical model for the dehydrogenation of methylcyclohexane in a packed bed reactor

A model for the dehydrogenation of methylcyclohexane in a tubular reactor over an industrial catalyst Pt-Sn/Al₂O₃ has been established. This model takes into account the axial dispersion at the inlet of the catalytic bed reactor as well as the heat transfer at the wall of the reactor. The heat transfer at the wall is satisfactorily represented by using a heat transfer coefficient correlation for which the parameters are obtained by fitting to the experimental data. The model provides a good representation of the radial and axial temperature profiles in the packed bed and can be also used to calculate the conversion.     

Semicontinuous thermal bulk copolymerization of styrene and maleic anhydride: Experiments and reactor model

Thermal bulk copolymerization of styrene (St) and maleic anhydride (MAH) has been carried out at 110–130°C and up to around 55 wt % conversion in a stirred tank reactor with an anchor impeller to prepare the random copolymer of St–MAH (R-SMA). A series of experiments in semicontinuous monomer adding process were done to investigate the effects of operating condition on monomer conversion, copolymer composition, and its uniformity. It has been shown that a random copolymer with constant composition can be obtained by semicontinuous copolymerization. A reactor model was developed to simulate the copolymerization processes. The numerical method in which the gel effect on the copolymerization is incorporated has exhibited excellent agreement between the model calculation and the experimental data. However, when using the assumption that (1) k₂₂ = 0; (2) k₂₁[M₁] ≫ k₁₂[M₂]; and (3) (R₁/2k_t)^1/2 is a constant, an analytical solution to the model was found to be available also. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1905–1912, 1998     

NOx removal by selective noncatalytic reduction with urea solution in a fluidized bed reactor

A fluidized bed reactor has been developed to overcome the plugging problem of urea injection by employing a sparger rather than nozzles in the SNCR process for simultaneous removal of SO₂ and NO_x. In a developed fluidized bed reactor, the optimum temperature to remove NO_x is shifted to lower values, the reaction temperature window is widened with the presence of CO in flue gas, and NO conversion is higher than that in a flow reactor. The optimum amount of urea injection in the reactor is found to be above 1.2 based on the normalized stoichiometric molar ratio (NSR) with respect to NO conversion. In the simultaneous removal of SO₂/NO, conversions of SO₂ and NO reach 80–90%, nearly the same values for the individual removal of SO₂ and NO above 850 ‡C.     

Scale-up analysis of autothermal operation of methane oxidative coupling with La2O3/CaO catalyst

The exothermicity of oxidative coupling of methane (OCM) renders a cooled packed-bed reactor impractical or impossible. Recently, we proposed an adiabatic autothermal reactor as a solution to this problem and reported the first results for stable autothermal operation (AO) with feed at ambient temperature. AO on the ignited branch is possible only in the region of steady-state multiplicity. High per-pass conversion and productivity requirements demand a stable ignited branch at the lowest possible feed temperature and high flow rate. To achieve OCM scale-up, many conditions must be satisfied simultaneously. Using a kinetic model for La₂O₃/CaO catalyst, we examine the impact of space time, feed methane to oxygen ratio, feed temperature, particle size, inter-phase heat and mass transfer gradients, pore-diffusion, bed scale heat/mass dispersion on the region of AO for large scale adiabatic packed-bed reactors. We show that while it is possible to achieve CH₄ conversion of about 20% and C₂ selectivity of about 80% in scaled-up reactors, these values are sensitive to the design and operating parameters.     

A packed‐bed solar reactor for the carbothermal zinc production – dynamic modelling and experimental validation

Integration of concentrated solar energy into the pyrometallurgical Zn production process as clean source of high‐temperature process heat could significantly reduce fossil fuels consumption and its concomitant CO₂ emissions. The solar‐driven carbothermal reduction of ZnO is investigated using a 10‐kW_th solar reactor featuring two cavities, the upper one serving as the solar absorber and the lower one containing a packed‐bed of ZnO and beech charcoal as the biogenic reducing agent. Experimentation in a high‐flux solar simulator is carried out under radiative fluxes of 2300–2890 suns, yielding a peak solar‐to‐chemical energy conversion efficiency of 18.4%. The reactor performance under variable operating conditions is analyzed via a dynamic numerical model coupling heat transfer with chemical kinetics. The model is validated by comparison to the experimental data obtained with the 10‐kW_th packed‐bed solar reactor and further applied to predict the effect of incorporating semi‐continuous feeding of reactants on the process efficiency. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4586–4594, 2016     

Auto-thermal chemical looping combustion of sulphur with Fe2O3 oxygen carriers for sulphuric acid production: Thermodynamics

Chemical looping is a novel fuel conversion and material separation technology. It can be applied to obtain sulphur through selective oxidation of H₂S. Further, chemical looping combustion (CLC) of sulphur can generate SO₂ with a high concentration without NO_x formation. The high SO₂ concentration is adjustable and facilitates large-scale H₂SO₄ production. In this study, we examined the thermodynamics of the CLC of sulphur for H₂SO₄ production, which has not been reported previously. We analyzed the effects of reactor temperature and sulphur to Fe₂O₃ oxygen carrier (OC) ratios on sulphur allotrope transformations and on the distributions of reaction products. Moreover, the reactors were operated auto-thermally. Based on this design, we examined the effects of fuel reactor (FR) and air reactor temperatures on the minimum recirculation of the OC, as well as the gas and solid products and heat released from the air reactor. Our results showed that the CLC of sulphur with Fe₂O₃ OC could occur through an auto-thermal process. The FR in a sulphur CLC system should be operated over a temperature range of 800–950°C, with an Fe₂O₃ OC recirculation between 45 and 143 kg/kg_S(s). Furthermore, when the FR was operated in the auto-thermal mode, we achieved 100% SO₂ conversion. The findings of this study may be applied to reactor design for large-scale H₂SO₄ production through CLC of sulphur.