Investigations on the Liquid‐Phase Decomposition of AdBlue Urea for the Selective Catalytic Reduction Process

Difficulties in decomposing AdBlue to ammonia limit the applicability of selective catalytic reduction systems at low exhaust temperatures. Investigations on the decomposition of AdBlue in the liquid phase under elevated pressure at temperatures up to 165 °C were carried out. Besides effects of inorganic catalysts, the impact of pH on urea decomposition was examined. After dissolution in aqueous phase, the compounds ZnO, WO₃, and MoO₃ were found to be effective in liquid‐phase AdBlue decomposition. However, the efficiency was dropping significantly over few hours. Decomposition of AdBlue urea was also found to be favored for alkaline and acidic conditions.     

Monitoring the Ammonia Loading of Zeolite‐Based Ammonia SCR Catalysts by a Microwave Method

Exhaust gas aftertreatment systems, which reduce nitrogen oxide emissions of heavy‐duty diesel engines, commonly use a selective catalytic reduction (SCR) catalyst. Currently, emissions are controlled by evaluating NO_x or NH₃ in the gas phase downstream the catalyst and calculating the NH₃ loading via a chemical storage model. Here, a microwave‐cavity perturbation method is proposed in which electromagnetic waves are excited by probe feeds and the reflected signals are measured. At distinct resonance frequencies, the reflection coefficient shows a pronounced minimum. These resonance frequencies depend almost linearly on the NH₃ loading of a zeolite‐based SCR catalyst. Since the NH₃ loading‐dependent electrical properties of the catalyst material itself are measured, the amount of stored ammonia can be determined directly and in situ. The cross‐sensitivity towards water can be reduced almost completely by selecting an appropriate frequency range.     

Detailed modeling of the evaporation and thermal decomposition of urea‐water solution in SCR systems

This work is aimed to develop a multicomponent evaporation model for droplets of urea‐water solution (UWS) and a thermal decomposition model of urea for automotive exhausts by using the selective catalytic reduction systems. In the multicomponent evaporation model, the influence of urea on the UWS evaporation is taken into account using a nonrandom two‐liquid activity model. The thermal decomposition model is based on a semidetailed kinetic scheme accounting not only for the production of ammonia (NH₃) and isocyanic acid but also for the formation of heavier solid by‐products (biuret, cyanuric acid, and ammelide). This kinetics model has been validated against gaseous data as well as solid‐phase concentration profiles obtained by Lundstroem et al. (2009) and Schaber et al. (2004). Both models have been implemented in IFP‐C3D industrial software to simulate UWS droplet evaporation and decomposition as well as the formation of solid by‐products. It has been shown that the presence of the urea solute has a small influence on the water evaporation rate, but its effect on the UWS temperature is significant. In addition, the contributions of hydrolysis and thermolysis to urea decomposition have been assessed. Finally, the impacts of the heating rate as well as gas‐phase chemistry on urea decomposition pathways have been studied in detail. It has been shown that reducing the heating rate of the UWS causes the extent of the polymerization to decrease because of the higher activation energy. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Oxygen Transfer in a Baffled Rectangular Air‐Lift Loop Reactor

The objective of this study is to characterize mass transfer in a rectangular air‐lift loop reactor in two‐phase flow. In a previous work, it has been shown that the reactor presents a complex gas flow pattern. Therefore, first, the global mass transfer volumetric coefficient k_La was measured in two‐phase flow, by three methods (two based on the liquid phase mass balance, one based on the gas phase mass balance). Then, second, a localized analysis was implemented in order to obtain more information about the phenomena governing the gas phase flow.     

Study of Hydrodynamics,Mixing and Gas‐Liquid Mass Transfer in a Split‐Rectangular Airlift Reactor

Global hydrodynamic characteristics, liquid mixing and gas‐liquid mass transfer for a 63 L split‐rectangular airlift reactor were studied. Correlations for gas holdup and overall liquid circulation velocity were derived for the air‐water system as a function of the specific power input; these were compared to data and correlations for reactor volumes between 4.7 L and 4600 L. A partial recirculation of small bubbles in the riser was observed when U_gr > 0.03 m/s, which was attributed to the use of a single‐orifice nozzle as the gas phase distributor. The dimensionless mixing time and the overall axial dispersion coefficient were nearly constant for the range of gas flow rates studied. However, values of K_L/d_B were greater than those reported in previous studies and this is caused by the partial recirculation of the gas phase in the riser. While scale effects remain slight, the use of a gas distributor favouring this partial recirculation seems adequate for mass transfer in split‐rectangular airlift reactors.     

Urea thermolysis studied under flow reactor conditions using DSC and FT-IR

The thermal decomposition of urea has been studied under flow reactor conditions using a differential scanning calorimeter (DSC) and Fourier transformed infrared spectroscopy (FT-IR). Samples of urea were administered using either a cordierite monolith impregnated with a urea/water solution or a silica cup with a dry urea sample. The samples were heated between 25 and 700 °C with heating rates of 10 and 20 K/min and the thermal response of the sample and off gas concentrations of ammonia and isocyanic acid were recorded. Biuret and cyanuric acid were decomposed in a separate set of experiments to verify some of the features observed in gas phase data during urea decomposition.Results show that depending on the way the sample is administered, i.e. cup or monolith different behavior in the evolved gases are observed. This is due to different reactions taking place in the vessel induced by the different conditions under which the pyrolysis of urea is preformed.     

An empirical thermodynamic model for the ammonia—water—carbon dioxide system at urea synthesis conditions

An empirical thermodynamic model for the NH₃–H₂O? CO₂ system at equilibrium at urea synthesis conditions (140 ≤ T ≤ 200 °C, 50 ≤ P ≤ 250 atm, 2.4 ≤ NH₃/CO₂ ≤ 6, H₂O/CO₂ ≤ 0.5) is presented. This model can quantitatively or semi-quantitatively describe a number of important phenomena in the urea synthesis, such as the bubble-point and gas—liquid equilibrium conditions and the effect of excess ammonia and water on the conversion to urea in the liquid phase. The model also sheds light on the problem of the conversion to urea at high temperatures (T > 190–200 °C).     

Catalytic Ammonia Decomposition Over Fe/Fe4N

The catalytic ammonia decomposition over iron and iron nitride, Fe₄N, under the atmosphere of ammonia–hydrogen mixtures of different amounts of ammonia in the temperature range of 400–550 °C by means of thermogravimetry has been studied. A differential tubular reactor with mixing has been used. The ammonia concentration in the gas phase during all the process was analysed. The balance between the inlet and outlet ammonia quantity has been used to determine a degree of ammonia conversion and the values of decomposition reaction rate. The activation energy of ammonia decomposition reaction over Fe and Fe₄N was found to be 68 and 143 kJ/mol, respectively.     

Liquid-Phase Catalytic Decomposition of Novel Ammonia Precursor Solutions for the Selective Catalytic Reduction of NOx

Aqueous solutions of NH₃-precursor compounds (i.e. urea and methanamide) were catalytically hydrolyzed in the liquid phase by applying a pressure of 50 bar during contact with the catalyst in a heated tube. Methanamide could be hydrolyzed on an Au/TiO₂ catalyst to yield not only NH₃, but also H₂. Decomposition of urea under high pressure in the liquid phase could even be achieved without the addition of a catalyst. Due to the large excess of water present during decomposition, side products could be avoided. As the reactor tube is electrically heated, the presented method provides a possibility to reliably transform NH₃-precursor compounds into gaseous NH₃ independent of the engine exhaust gas temperature. The NH₃-flow can be added to the main exhaust duct to enable the selective catalytic reduction of NO_x at the light-off of the SCR catalyst.     

Hydrogen production from urea wastewater using a combination of urea thermal hydrolyser-desorber loop and a hydrogen-permselective membrane reactor

This work presents novel application of palladium-based membrane in a wastewater treatment loop of urea plant for hydrogen production. Urea wastewater treatment loop is based on combined thermal hydrolysis-desorption operations. The wastewater of urea plant includes ammonia and urea which in the current treatment loop; urea decomposes to ammonia and carbon dioxide. The catalytic hydrogen-permselective membrane reactor is proposed for hydrogen production from desorbed ammonia of urea wastewater which much of it discharges to air and causes environmental pollution. Therefore hydrogen is produced from decomposition of ammonia on nickel-alumina catalyst bed simultaneously and permeates from reaction side to shell side through thin layer of palladium-silver membrane. Also a sweep gas is used in the shell side for increasing driving force. In this way, 4588 tons/yr hydrogen is produced and environmental problem of urea plant is solved. The membrane reactor and urea wastewater treatment loop are modeled mathematically and the predicted data of the model are consistent with the experimental and plant data that show validity of the model. Also the effects of key parameters on the performance of catalytic hydrogen-permselective membrane reactor such as the temperature, pressure, thickness of Pd-Ag layer, configuration of flow and sweep gas flow ratio were examined.     

Preparation of sorbitol from D‐glucose hydrogenation in gas–liquid–solid three‐phase flow airlift loop reactor

A new process for D ‐glucose hydrogenation in 50 wt% aqueous solution, into sorbitol in a 1.5 m³ gas–liquid–solid three‐phase flow airlift loop reactor (ALR) over Raney Nickel catalysts has been developed. Five main factors affecting the reaction time and molar yield to sorbitol, including reaction temperature (T_R), reaction pressure (P_R), pH, hydrogen gas flowrate (Q_g) and content of active hydrogen, were investigated and optimized. The average reaction time and molar yield were 70 min and 98.6% under the optimum operating conditions, respectively. The efficiencies of preparation of sorbitol between the gas–liquid–solid three‐phase flow ALR and stirred tank reactor (STR) under the same operating conditions were compared. Copyright © 2004 Society of Chemical Industry     

Selective catalytic reduction by urea in a fluidized‐bed reactor

The selective catalytic reduction (SCR) of NO_x by urea as a reducing agent was carried out over fresh and sulfated CuO/γ‐Al₂O₃ catalysts in a fluidized‐bed reactor. The optimum temperature ranges for NO reduction on the fresh and sulfated CuO/γ‐Al₂O₃ catalysts were 300–350 °C and 400–450 °C, respectively. NO reduction with the sulfated CuO/γ‐Al₂O₃ catalyst was somewhat higher than that with the fresh CuO/γ‐Al₂O₃ catalyst. N₂O formation increased with increasing reaction temperature. Ammonia (NH₃) slip increased with increasing gas velocity and decreased with increasing reaction temperature. Copyright © 2003 Society of Chemical Industry     

Synthesis Gas Production Configurations for Gas‐to‐Liquid Applications

Different syngas configurations in a gas‐to‐liquid plant are studied including autothermal reformer (ATR), combined reformer, and series arrangement of gas‐heated reformer and ATR. The Fischer‐Tropsch (FT) reactor is based on a cobalt catalyst and the degrees of freedom are steam‐to‐carbon ratio, purge ratio of light ends, amount of tail gas recycled to synthesis gas (syngas) and FT synthesis units, and reactor volume. The production rate of liquid hydrocarbons is maximized for each syngas configuration. Installing a steam methane reformer in front of an ATR will reduce the total oxygen consumption per barrel of product by 40 % compared to the process with only an ATR. The production rate of liquid hydrocarbons is increased by 25.3 % since the flow rate of the purge stream for the ATR is the highest one compared to other configurations and contains mainly CO₂.     

Effect of Ethylene Glycol on the CO2/Water Gas‐Liquid Mass Transfer Process

The effect caused by the presence of ethylene glycol on the gas‐liquid mass transfer velocity of CO₂ in a aqueous phase has been studied. In this study two different gas‐liquid contactors have been used, a bubbling stirred reactor and a flat surface stirred vessel. The first contactor, gas phase, was introduced using a porous bubbling plate. The influence of operational variables, stirring rate, gas flow rate and ethylene glycol concentration were studied. The experiments were carried out at 298.15 K using a semicontinuous regime. The final aim was to obtain empirical equations that allow the calculation of the mass transfer velocity for this system a priori.     

Liquid Flow Visualization in Packed‐Bed Multiphase Reactors: Wire‐Mesh Sensor Design and Data Analysis for Rotating Fixed Beds

Wire‐mesh sensors are increasingly used for flow imaging in packed beds. In this study, a capacitance wire‐mesh sensor is applied to measure the cross‐sectional liquid phase distribution in a rotating fixed‐bed reactor. The liquid filling level is derived as a crucial parameter defining the operational window of the reactor concept. Contrary to the standard sensor configuration, wireless data transfer and autonomous power supply is integrated. Furthermore, appropriate data processing is required to visualize the liquid flow of the three‐phase system (nitrogen, cumene and γ‐Al₂O₃ particles).     

Formation of NOx and SOx precursors during the pyrolysis of coal and biomass. Part IV. Pyrolysis of a set of Australian and Chinese coals

The formation of HCN and NH₃ from the pyrolysis of a small set of Chinese and Australian coals were studied using a novel fluidised-bed/fixed-bed reactor and a fluidised-bed/tubular reactor. The fluidised-bed/fixed-bed reactor has some features of a fluidised-bed reactor and of a fixed-bed reactor, allowing the evaluation of the effects of coal properties on the formation of HCN and NH₃ to be carried out on a similar basis for a wide range of coals. The thermal cracking of volatiles was investigated in a tubular reactor in tandem with the fluidised-bed/fixed-bed reactor where the nascent volatiles were generated in situ from the pyrolysis of coal. Our experimental results indicate that, in addition to coal rank, the petrographic composition and/or geographic origin of the coal are important factors influencing the formation of HCN and NH₃ during pyrolysis. Among the few Chinese and Australian coals studied, the inertinite-rich Chinese coals tend to give more NH₃ during pyrolysis than the Australian coals of similar carbon contents. It is believed that the structure of inertinites of less caking properties favours the formation of H radicals in the pyrolysing solid over a ‘correct’ temperature range to overlap with the activation and subsequent hydrogenation of the N-containing ring systems for the formation of NH₃ in the solid. If the coal properties favour the release of coal-N as volatiles, the formation of HCN in the gas phase is more likely. Under the current experimental conditions, where volatiles may be deposited on the reactor wall, the formation and destruction of the sooty materials on the reactor wall play an important role in the formation of HCN from the cracking of volatiles.     

Gas‐Liquid Mass Transfer in Fibrous Bed Reactor with Counter‐Current Liquid Recycle

In this work, the gas‐liquid mass transfer in a lab‐scale fibrous bed reactor with liquid recycle was studied. The volumetric gas‐liquid mass transfer coefficient, k_La, is determined over a range of the superficial liquid velocity (0.0042–0.0126 m.s^–1), gas velocity (0.006–0.021 m.s^–1), surface tension (35–72 mN/m), and viscosity (1–6 mPa.s). Increasing fluid velocities and viscosity, and decreasing interfacial tension, the volumetric oxygen transfer coefficient increased. In contrast to the case of co‐current flow, the effect of gas superficial velocity was found to be more significant than the liquid superficial velocity. This behavior is explained by variation of the coalescing gas fraction and the reduction in bubble size. A correlation for k_La is proposed. The predicted values deviate within ± 15 % from the experimental values, thus, implying that the equation can be used to predict gas‐liquid mass transfer rates in fibrous bed recycle bioreactors.     

Optimal Design of a Gas‐to‐Liquids Process with a Staged Fischer‐Tropsch Reactor

The optimal design of a natural gas‐to‐liquid hydrocarbons (GTL) process with a multistage cobalt‐based Fischer‐Tropsch reactor and interstage product separation is considered. The objective function is to maximize the wax (C₂₁₊) production rate at the end of the reactor path. Sectioning of the Fischer‐Tropsch reactor increases the chain growth probability inside the reactor which results in a higher production of wax. The carbon efficiency of the two‐stage reactor is distinctly higher than that of the single‐stage reactor.     

Experimental Study on a Co-current Gas–Liquid Down Flow Contactor with Gas Entrainment by a Liquid Jet

Two-phase flow co-current vertical downflow reactor with gas entrainment by a liquid jet is investigated in an air–water system. Experiments are carried out in order to clarify the flow behavior of the reactor under various conditions. Gas entrainment flow rates and gas holdup are quantified experimentally and their dependency on the liquid jet flow rates are shown. The experimental program also included determination of liquid phase residence time distribution (RTD) characteristics for different liquid jet flow rates. The result of the analysis of the liquid phase RTD curves justified the tank-in-series model flow for the liquid phase. On the basis of these analyses, the reactor hydrodynamics are modeled by the tank-in-series model including dead zones. Good agreement is obtained between theoretical and experimental results assuming the reactor is operating as perfectly mixed. The volumetric mass transfer coefficient k_La_Lis determined experimentally by a “gasing out” method. The interfacial area is deduced from the bubble diameter measurements which are determined by visualization experiments.     

Conceptual feasibility studies of a CO<Subscript><Emphasis Type="Italic">X</Emphasis></Subscript>-free hydrogen production from ammonia decomposition in a membrane reactor for PEM fuel cells

CO_X-free hydrogen production from ammonia decomposition in a membrane reactor (MR) for PEM fuel cells was studied using a commercial chemical process simulator, Aspen HYSYS®. With process simulation models validated by previously reported kinetics and experimental data, the effect of key operating parameters such as H₂ permeance, He sweep gas flow, and operating temperature was investigated to compare the performance of an MR and a conventional packed-bed reactor (PBR). Higher ammonia conversions and H₂ yields were obtained in an MR than ones in a PBR. It was also found that He sweep gas flow was favorable for X_NH3 enhancement in an MR with a critical value (5 kmol h^-1), above which no further effect was observed. A higher H₂ permeance led to an increased H₂ yield and H₂ yield enhancement in an MR with the reverse effect of operating temperature on the enhancement. In addition, lower operating temperature resulted in higher X_NH3 enhancement and H₂ yield enhancement as well as NG cost savings in a MR compared to a conventional PBR.