Wire-mesh honeycomb catalysts for selective catalytic reduction of NO with NH3

Both flat and corrugated wire mesh sheets were coated with aluminum powder by using electrophoretic deposition (EPD) method. Controlled thermal sintering of coated samples yielded uniform porous aluminum layer with a thickness of 100 μm that was attached firmly on the wire meshes. Subsequent controlled calcination formed a finite thickness of Al₂O₃ layer on the outer surface of each deposited aluminum particles, which resulted in the formation of Al₂O₃/Al double-layered composite particles that were attached firmly on the wire surface to form a certain thickness of porous layer. A rectangular-shaped wire-mesh honeycomb (WMH) module with triangular-shaped channels was manufactured by packing alternately the flat sheet and corrugated sheet of the Al₂O₃/Al-coated wire meshes. This WMH was further coated with V₂O₅-MoO₃-WO₃ catalyst by wash-coating method to be applied for the selective catalytic reduction (SCR) of NO with NH₃. With an optimized catalyst loading of 16 wt%, WMH catalyst module shows more than 90% NO conversion at 240 °C and almost complete NO conversion at temperatures higher than 300 °C at GHSV 5,000 h⁻¹. When compared with conventional ceramic honeycomb catalyst, WMH catalyst gives NO conversion higher by 20% due to reduced mass transfer resistance by the existence of three dimensional opening holes in WMH.     

Counter-current absorption using wire gauze packings

The dynamic liquid hold-up, ?_LD, effective interfacial area, a, and the liquid side mass transfer coefficient k_La were determined for 0.1 m and 0.2 m multifilament wire gauze packings, 0.0125 m double walled wire gauze partition rings and 0.025 m wire gauze saddle packings in columns operated countercurrently. The theory of gas absorption accompanied by fast pseudo mth order reaction was used to determine the effective interfacial area. The values of liquid side mass transfer coefficient for the multifilament wire gauze packings were obtained by absorbing lean carbon dioxide in a buffer solution of sodium carbonate and sodium bicarbonate. K_La values for the other packings were obtained by absorbing pure carbon dioxide in tap water. The values of a and k_La for multifilament wire gauze packings were found to be two to four times higher as compared to the conventional ring or saddle packings. Further, the superficial liquid velocity was found to have marginal effect on a. The double walled wire gauze partition rings offered a values which were 1.5–2.0 times higher than that offered by 0.016 m s.s. Pall rings at low values of superficial liquid velocity (<3 × 10^{?3 m/s}.     

On the use of internal mass transfer coefficients in modeling of diffusion and reaction in catalytic monoliths

We utilize the recently developed concept of internal or intraphase mass transfer coefficient to simplify the problem of diffusion and reaction in more than one spatial dimension for a washcoated monolith of arbitrary shape. We determine the dependence of the dimensionless internal mass transfer coefficient (Sh_i) on washcoat and channel geometric shapes, reaction kinetics, catalyst loading and activity profile. It is also reasoned that the concept of intraphase transfer coefficient is more useful and fundamental than the classical effectiveness factor concept. The intraphase transfer coefficient can be combined with the traditional external mass transfer coefficient (Sh_e) to obtain an overall mass transfer coefficient (Sh_app) which is an experimentally measurable quantity depending on various geometric and transport properties as well as kinetics. We present examples demonstrating the use of Sh_app in obtaining accurate macro-scale low-dimensional models of catalytic reactors by solving the full 3-D convection-diffusion-reaction problem for a washcoated monolith and comparing the solution with that of the simplified model using the internal mass transfer coefficient concept.     

On the relationship between Aris and Sherwood numbers and friction and effectiveness factors

We present a general solution of the diffusion-reaction problem for linear kinetics and an expression for the effectiveness factor (η) in terms of the shape normalized Thiele modulus (Φ) for a catalyst particle of arbitrary shape and with an arbitrary activity profile. We also show that the Sherwood number (Sh_Ω) or the dimensionless mass transfer coefficient between the interior of a particle or region (denoted by Ω) and its boundary (∂Ω) is related to the effectiveness factor and Thiele modulus by η=1/(1+Φ²/Sh_Ω). Further, the coefficients in the expansion of η in terms of Φ (the Aris numbers, Ar_i) are related to the asymptotic Sherwood number (Sh_Ω∞) obtained in the limit of slow reaction. We show that the curve Sh_Ω versus Φ is universal for most common particle or channel geometric shapes and for the case of uniform activity is described by the two asymptotes Sh_Ω=Sh_Ω∞=1/Ar₁ for Φ?1 and Sh_Ω≈Φ for Φ?1. For two-dimensional ducts we show that the friction factor times Reynolds number (fRe) is equal to 8Sh_Ω∞ and provide a physical interpretation of this result. In the second part of this work, we derive low-dimensional models for solving multicomponent nonlinear diffusion-reaction problems using the concept of an internal mass transfer coefficient. We also present low-dimensional models for catalytic reactors using external and internal mass transfer coefficients. Finally, the Aris/Sherwood numbers are presented for some commonly used catalyst particles in packed-bed reactors and washcoat shapes in catalytic monoliths.     

Automated measurements of gas‐liquid mass transfer in micropacked bed reactors

Gas‐liquid mass transfer in micropacked bed reactors is characterized with an automated platform integrated with in‐line Fourier transform infrared spectroscopy. This setup enables screening of a multidimensional parameter space underlying absorption with chemical reaction. Volumetric gas‐liquid mass‐transfer coefficients (k_La) are determined for the model reaction of CO₂ absorption in a methyl diethanolamine/water solution. Parametric studies are conducted varying gas and liquid superficial velocities, packed bed dimensions and packing particle sizes. The results show that k_La values are in the range of 0.12～0.39 s^?1, which is about one‐to‐two orders of magnitude larger than those of conventional trickle beds. An empirical correlation predicts k_La in micropacked bed reactors in good agreement with experimental data. © 2017 American Institute of Chemical Engineers AIChE J, 64: 564–570, 2018     

Liquid-solid mass transfer in a rotating packed bed reactor with structured foam packing

A rotating packed bed (RPB) reactor has substantially potential for the process intensification of heterogeneous catalytic reactions. However, the scarce knowledge of the liquid–solid mass transfer in the RPB reactor is a barrier for its design and scale-up. In this work, the liquid–solid mass transfer in a RPB reactor installed with structured foam packing was experimentally studied using copper dissolution by potassium dichromate. Effects of rotational speed, liquid and gas volumetric flow rate on the liquid–solid mass transfer coefficient (k_LS) have been investigated. The correlation for predicting k_LS was proposed, and the deviation between the experimental and predicted values was within ± 12%. The liquid–solid volumetric mass transfer coefficient (k_LSa_LS) ranged from 0.04–0.14 1⁻¹, which was approximately 5 times larger than that in the packed bed reactor. This work lays the foundation for modeling of the RPB reactor packed with structured foam packing for heterogeneous catalytic reaction.     

Lovastatin production by Aspergillus terreus using lignocellulose biomass in large scale packed bed reactor

The effect of superficial air velocity on lovastatin production by Aspergillus terreus PL10 using wheat bran and wheat straw was investigated in a 7 L and a 1200 L packed bed reactor. Mass transfer and reaction limitations on bioconversion in the 1200 L reactor was studied based on a central composite design of experiments constructed using the superficial air velocity and solid substrate composition as variables and lovastatin production as response. The surface response prediction showed a maximum lovastatin production of 1.86 mg g⁻¹ dry substrate on day 5 of the bioconversion process when the reactor was operated using 0.19 vvm airflow rate (23.37 cm min⁻¹ superficial air velocity) and 54% substrate composition (w_C). Lovastatin production did not increase significantly with superficial air velocity in the 7 L reactor. Variation in temperature and exit CO₂ composition was recorded, and the Damköhler number was calculated for lovastatin production at these two scales. The results showed that in larger reactors mass transfer limitation controlled bioconversion while in smaller reactors bioconversion was controlled by reaction rate limitations. In addition, mass transfer limitations in larger reactors reduced the rate of metabolic heat removal, resulting in hot spots within the substrate bed.     

Exact universal uniqueness criteria for the adiabatic tubular packed bed reactor

Exact, universal a priori bounds and regions of multiplicity for the entire tubular packed bed reactor are developed by application of a technique reported in Chang and Calo, Chem. Engng Sci. 1979 34 285 to a cascade two-phase cell model for an nth order chemical reaction with interphase resistance to mass and heat transport, Le ≠ 1 (or Le = 1) and either lumped parameter catalyst particles or with intraparticle concentration gradients with uniform temperature. Both the more common case of interphase heat transfer greater than interphase mass transfer rate and the inverse case of particle over-temperature have been considered. In all cases it has been shown that the reactor conservation equations can be decoupled at certain points along the bed determined by the Lewis number, and that questions of multiplicity and uniqueness reduce to consideration of a single algebraic equation which is actually a form of the two-phase adiabatic CSTR. Also as for the CSTR, the topology of the adiabatic packed bed reactor is shown to be the simple cusp catastrophe. The application of the resultant criteria is quite simple and represents a practical step in the design procedure for highly exothermic reactions in packed beds. A flow chart of a suggested procedure is included.     

Process intensification of catalytic hydrogenation of ethylanthraquinone with gas‐liquid microdispersion

In this article, to miniaturize the hydrogenation reactor and make the H₂O₂ production with more safety a gas‐liquid microdispersion system was generated to intensify the process of catalytic hydrogenation of ethylanthraquinone by passing the gas‐liquid microdispersion system through a generally packed bed reactor. A microdispersion device with a 5 μm pore size microfiltration membrane as the dispersion medium has been developed and microbubbles in the size of 10–100 μm were successfully generated. The reaction and mass transfer performance was evaluated. The conversion of ethylanthraquinone as much as 35% was realized in less than 3.5 s. The overall volume mass transfer coefficient in the microdispersion reaction system reached in the range of 1–21 s^?1, more than two orders of magnitude larger than the values in normal gas‐liquid trickle‐bed reactors. A mathematical model in the form of Sh = 2.0 + 54.7Sc^1/3We^1/2?^1/10 has been firstly suggested, which can well predict the overall mass transfer coefficient. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Gas film coefficient of mass transfer in low Péclet number region for sphere packed beds

The limiting Sherwood number between particles and fluid in the low Péclet number region has been determined experimentally for gas phase mass transfer in packed beds. A value of Sh_ho = 8·33 (or Sh_po = 12·5 at ?_f = 0·50), was found where Sh_po and Sh_ho are the Sherwood numbers based on the particle diameter and the hydraulic diameter respectively and ?_f is the void fraction of the beds. A high accuracy of measurement was obtained by applying a pulse response of CO₂ tracer gas to determine the film coefficients.     

Wall heat transfer to chemical reactors

The effects of chemical reactions on wall heat transfer coefficients in packed bed reactors operating at equilibrium or near equilibrium conditions are explored by means of one-dimensional and two-dimensional models. Both approaches suggest using Damköhler number or ratios of reacting to frozen heat capacity as parameters in heat transfer correlations. Available data for the methane-steam reforming reactors were used to establish a suitable correlation for use in thermal design of reacting beds of solids, i. e. N_Nu = 0.195 N_Re^0.8 exp. (0.21 σ) This correlation can be applied with reasonable accuracy for beds which are 3 to 5-in. in diameter and bed to particle diameter ratios from 5 to 10. A review of available correlations for wall coefficients obtained under non-reacting packed bed conditions is also presented.     

Mass transfer with chemical reaction in liquid foam reactors

Mass transfer studies were conducted in a stable liquid foam reactor under various operating conditions to evaluate gas holdup, effective interfacial area, liquid-phase mass transfer coefficient and a modified interfacial mass transfer coefficient to include the surface-active agents employed. Gas holdup and effective interfacial area were evaluated experimentally. The interfacial mass transfer coefficient was evaluated semitheoretically, by considering the interfacial region as a separate phase and using the experimental data developed for mass transfer accompanied by a fast first-order chemical reaction. The liquid-phase mass transfer coefficient was also evaluated semitheoretically, using Danckwert's theory for the liquid phase and the experimental data on mass transfer accompanied by a slow pseudofirst-order chemical reaction. An experimental unit was set up to provide a stable flowing foam column, simulating the foam reactor. Mass transfer rates were studied for superfacial gas velocities in the range from 1.5 × 10⁻² m/s to 5 × 10⁻² m/s, giving gas residence times in the range from 20 to 55 seconds. A cationic and nonionic surface-active agent and three different wire mesh sizes, giving bubble size distributions in the range from 2.2 to 5.4 mm Sauter mean diameters, were employed. It is observed that gas holdup is insensitive to the type of surface-active agent; it is however, dependent on wire mesh size and gas velocity. The bubble diameter and, hence, the interfacial area are found to be insensitive to gas velocity in the range studied; they are, however, strong functions of wire mesh size. The liquid-phase mass transfer coefficient increases with increase in gas velocity. The surface-active agent introduces additional resistance to mass transfer in both reaction cases, this being the controlling one in the case of the fast reaction. A comparison with conventional packed bed contactors indicates the mass transfer rates to be about 8 times lower for the foam reactor, for the fast reaction case; for slow reactions, the foam reactor has mass transfer rates approximately 2-4 times higher than those for conventional packed bed contactors.     

Lyapunov stability analysis of porous catalyst particle systems: extension to interphase transfer and time-varying surface conditions

A weighted Lyanunov functional is used to derive local and global stability conditions for the porous catalyst particle system. Results are presented for finite interphase transfer, Nu and Sh ≠ _∞, and for general time-varying surface conditions. Improvements on previous results for the case of unit Lewis number and negligible interphase transfer are also developed.     

Nitrification by immobilized cells in a micro‐ecological life support system using packed‐bed bioreactors: an engineering study

The nitrifying component of a micro‐ecological life support system alternative (MELISSA) based on microorganisms and higher plants was studied. The MELISSA system consists of an interconnected loop of bioreactors to allow the recycling of the organic wastes generated in a closed environment. Conversion of ammonia into nitrates in such a system was improved by selection of microorganisms, immobilization techniques, reactor type and operation conditions. An axenic mixed culture of Nitrosomonas europaea and Nitrobacter winogradskyi, immobilized by surface attachment on polystyrene beads, was used for nitrification in packed‐bed reactors at both bench and pilot scale. Hydrodynamics, mass transfer and nitrification capacity of the reactors were analysed. Mixing and mass transfer rate were enhanced by recirculation of the liquid phase and aeration flow‐rate, achieving a liquid flow distribution close to a well‐mixed tank and without oxygen limitation for standard operational conditions of the nitrifying unit. Ammonium conversion ranged from 95 to 100% when the oxygen concentration was maintained above 80% of saturation. The maximum surface removal rates were measured as 1.91 gN‐NH₄⁺ m^?2 day^?1 at pilot scale and 1.77 gN‐NH₄⁺ m^?2 day^?1 at bench scale. Successful scale‐up of a packed‐bed bioreactor has been carried out. Good stability and reproducibility were observed for more than 400 days. Copyright © 2004 Society of Chemical Industry     

Mass transfer in packed beds at low Reynolds numbers

Mass transfer coefficients for packed beds of Raschig rings were measured by the electrochemical technique in the low Reynolds number range, where both, free and forced convection are important.From the analysis of the governing basic equations the parameters controlling the combined free and forced convection mass transfer were found to be Sh/(ScGr)^{$\text{1}\text{4}$} and Re/Gr^{$\text{1}\text{2}$}. The experimental results were correlated in terms of these variables for both, aiding and opposing flows.Satisfactory agreement was found when comparing previous mass transfer data for widely varying systems with the proposed expressions.     

An experimental study of a single layer packed bed cathode in an electrochemical flow reactor

Experimental measurements are reported for a packed bed electrode consisting of a single planar layer of uniform copper plated spheres located between platinum anodes and restrained by two plane porous PVC diaphragms. Two mass transfer controlled reactions, namely the reductions ofm-nitrobenzene sulphonic acid and copper sulphate, were investigated and the electrochemical mass transfer data in the range 23 <Re < 520 correlated by the equationShSc ^?1/3 = 0-83Re ^0.56, the Reynolds and Sherwood numbers being defined in terms of a particle diameter. Variations of electrode potential throughout the bed were found to be small enough to ensure reaction selectivity in the system.     

Elongated gas bubble dissolution under a turbulent liquid flow

Mass transfer between an elongated homogeneous gas bubble under a turbulent liquid flow in a duct is investigated experimentally. Elongated gas bubble dissolution is encountered in bioengineering tubular photobioreactors. Such reactors are interesting because they are compact, they have a low contamination risk and a low mechanical stress for a liquid phase containing fragile microalgae cells. It is demonstrated from experimental mass transfer measurements, that the interface of an immobilised elongated bubble can be approximated to a flat plane. Measured mass transfer experimental data, estimated using this simplification, appear to be well fitted by Sh_L = 1.76 × 10⁻⁵ × Re^1.506 × Sc^0.5, a correlation from Lamourelle and Sandall [8], given for a turbulent liquid flow in wetted-wall columns. A formula drawn from this hypothesis is proposed for mass transfer prediction in photobioreactors. For different applications, it is suggested that the results obtained for the studied geometry could be used to build mass transfer feedback control systems.     

Design of immobilised glucoamylase reactors using a simple kinetic model for the hydrolysis of starch

A comparative study is reported of the three main types of continuous reactors, continuous feed stirred tank reactor (CFSTR), packed bed reactor (PBR) and fluidised bed reactor (FBR), for the conversion at 50°C of a 30% (w/w) corn syrup 40 DE using glucoamylase (exo-1,4-D-glucosidase, EC 3.2.1.3) immobilised on carbonyl derivatives of titanium(IV)-activated porous silica by a method developed previously.^1–3 The hydrolysis of starch in these reactors is described by a simple kinetic model⁴ which involves the intrinsic kinetic constants as well as mass transfer and dispersion effects, and allows the computation of enzyme activity values under continuous operation. FBR appears to be more effective for the hydrolysis of starch. This reactor also confers a better operational stability (t_1/2=775h) on the immobilised enzyme than the other immobilised glucoamylase reactors (PBR, t_1/2=629h; CFSTR, t_1/2=239h).     

Characteristics of integrated micro packed bed reactor-heat exchanger configurations in the direct synthesis of dimethyl ether

The performance of three integrated micro packed bed reactor-heat exchangers (IMPBRHEs) for direct DME synthesis over physical mixtures of CuO–ZnO–Al₂O₃ and γ-Al₂O₃ catalysts was experimentally investigated. Systematic variations in reactor and slit dimensions and configuration were analyzed in terms of thermal behaviour, mass transfer, pressure drop and residence time distribution (RTD). The pressure drop was always small (<0.12 bar) relative to the total pressure (50 bar), and linear dependence with GHSV confirms the predicted laminar flow for Re = 0.1–2. A narrow RTD was estimated by the dispersion analysis. Careful temperature measurements confirmed that the reaction temperature is mainly controlled by the oil heat exchange to give a practically uniform temperature profile for set inlet oil temperatures of 220–320 °C. The micro packed beds were found free of the internal as well as external mass transfer limitations, as showed by no significant change in the CO conversion and DME yield for different catalyst particle sizes, no effect of varying the linear gas velocity, and no effect of manipulating reactant diffusion coefficient. Packed bed microstructured reactors hence provide an isobaric and isothermal environment free from transport limitations for the direct DME synthesis, in the kinetic regime as well as at equilibrium conversion.