Effects of free ammonia and dissolved oxygen on nitrification and nitrite accumulation in a biofilm airlift reactor

The purpose of this research is to find out the effects of free ammonia concentration and dissolved oxygen on nitrification and nitrite accumulation in a biofilm airlift reactor. Free ammonia seriously inhibited the activity of nitrite oxidizers at the concentration higher than 0.1 mg NH₃-N/L and it was very effective for nitrite accumulation. Dissolved oxygen limitation in the biofilm also caused nitrite accumulation. Long term inhibition decreased the growth rate for nitrite oxidizers, and ammonia oxidizers were the dominant nitrifiers in the wastewater nitrification. Selective accumulation of ammonia oxidizers in the biofilm could be another reason for nitrite accumulation. Free ammonia inhibited nitrite oxidizers immediately, and adaptation to free ammonia was not observed. Therefore, the optimum control of free ammonia and dissolved oxygen concentration is critical for nitrite accumulation and the strategy can be used for selective accumulation of ammonia oxidizers in a bioreactor system.     

Ammonia oxidation in Ba0.5Sr0.5Co0.8Fe0.2O3−δ membrane reactor

Selective oxidation of ammonia to NO was studied in a dense mixed ion electron conducting Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ membrane reactor, which integrates the separation and catalytic reaction process in a single reactive separation unit. The influence of the temperature and feed concentration on the membrane reaction performance were investigated in detail. Under reaction conditions, the oxygen permeation flux through the dense membrane increases with increasing temperature and ammonia flow rate. The lower temperature and ammonia concentration can favor the formation of NO, in which higher catalytic performance is obtained, suggesting that the membrane reactor operation is much beneficial for selective oxidation of ammonia.     

Harnstoffhydrolyse für die selektive katalytische Reduktion von NOx: Vergleich der Flüssig‐ und Gasphasenzersetzung

In order to reduce the NO_x concentration in car exhausts usually the selective catalytic reduction with ammonia is used. However, to avoid the transport of ammonia in vehicles urea is applied as NH₃ precursor. Controlled urea decomposition before the injection into the exhaust gas system itself may be accomplished by the use of a separate reactor. Urea decomposition to ammonia in the liquid phase under pressure in a heated reactor was compared to its decomposition in the gas phase. In the liquid phase, higher conversion rates relative to the reactor volume were realized than in the gas phase. Catalysts which showed high activity in the gas phase influenced the decomposition in the liquid phase only slightly.     

Design and operational considerations of catalytic membrane reactors for ammonia synthesis

Production of ammonia using hydrogen derived from renewable electricity instead of hydrocarbon reforming would dramatically reduce the carbon footprint of this commodity chemical. Novel technologies such as catalytic membrane reactors (CMRs) may potentially be more compatible with distributed ammonia production than the conventional Haber–Bosch process. A reactor model is developed based on integrating a standard industrial iron catalyst into a CMR equipped with an inorganic membrane that is selective to NH₃ over N₂/H₂. CMR performance is studied as functions of wide ranges of membrane properties and operating conditions. Conversion and ammonia recovery are dictated principally by the ammonia permeance, and the benefits by using membranes become significant above 100 GPU = 3.4 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹. To be effective, the CMR requires a minimum selectivity for ammonia of 10 over both nitrogen and hydrogen and purity scales with the effective selectivity. Increasing the pressure of operation significantly improves all metrics, and at P = 30 bar with a quality membrane, ammonia is almost completely recovered, enabling direct recycle of unreacted hydrogen and nitrogen without need for recompression. Temperature drives conversion and scales monotonically without thermodynamic limitations in a CMR. Alternatively, the temperature may be reduced as low as 300°C while achieving conversion levels surpassing equilibrium limits at T = 400°C in a conventional reactor.     

The mechanism of the poisoning of ammonia synthesis catalysts by oxygenates O2, CO and H2O: an in situ method for active surface determination

The effect of adding an oxygenated poison (O₂, CO or H₂O) to a hydrogen/nitrogen stream producing ammonia over a triply promoted (K₂O, CaO, Al₂O₃) commercial catalyst is not unsurprisingly rapidly to poison the catalyst. However, immediately the oxygenated poison reacts with the catalyst and before total poisoning has occurred, which in these experiments took 10 min, there was an explosive release of ammonia producing concentrations in the gas phase in excess of the equilibrium value. This is thought to be due to a convulsive reorganisation of the surface of the catalyst in forming regions of an oxide overlayer, resulting in the expulsion of the standing surface nitrogen atom coverage as ammonia. However, in contradistinction to the observation of complete poisoning of the triply promoted catalyst shortly after switching the water (2.9%) into the hydrogen/nitrogen stream, when polycrystalline iron was used as the catalyst, after the initial pulse of ammonia was observed, the small quantity of water (2.9%) in the hydrogen/nitrogen stream resulted in an increased rate ( ×3) of ammonia synthesis which declined only slightly over the twenty minute duration of the experiment. The difference in behaviour between the triply promoted catalyst and the polycrystalline iron is thought to be due to the relative ease of reduction of the latter, so that submonolayer quantities of oxide can be stabilised on the surface of the polycrystalline iron. The promoting effect of this oxide overlayer is either structural or electronic; no distinction can be made from these experiments. The technique of injecting either O₂ or CO into a hydrogen/nitrogen stream which is producing ammonia over promoted catalysts in quantities insufficient to cause complete poisoning and measuring the oxygen coverage of the catalyst to a measured decrease in the ammonia synthesis rate, appears to be a ready, in situ method for the determination of the active catalyst area.     

Application of solar light for degradation of ammonia in petrochemical wastewater by a floating TiO2/LECA photocatalyst

In this study, photocatalytic degradation of ammonia in petrochemical wastewater was investigated by solar light/TiO₂ photocatalysis. The TiO₂ nanoparticles were used as photocatalysts which were immobilized on light expanded clay aggregate (LECA) granules as a new porous and light weight support. Maximum concentration of ammonia (975 mg/L) in petrochemical wastewater was selected, and to optimize the photocatalytic reaction, the effect of pH and catalyst dosage was investigated in two aerated and agitated reactor systems. Also, the morphology and chemical structure properties of the prepared catalysts were characterized by SEM and XRF analyses. The experimental results were shown that the performance of two types of aeration reactor systems was almost the same. Also, the ammonia removal efficiency was increased by increasing the pH value, and after solar light irradiation in three days, the solar reactor system lead degradation of ammonia in pH = 11–96.5%. The floating of photocatalyst can be reused at least three consecutive times with 14% decreases on the ammonia removal efficiency. The results suggest that the photocatalytic purification followed by solar photocatalytic reactor would be a promising method for the purification of chemical wastewater.     

Study of the ammonia decomposition over iron catalysts

The decomposition of ammonia is a reaction associated with the process of the nitriding of metals. The kinetics of the ammonia decomposition on iron catalysts has been studied using a differential reactor with internal mixing. The balance between the inlet and outlet ammonia quantity has been used to determine the degree of conversion. The rate of ammonia decomposition could be described by the following expression: r = k₀ exp (Ea/RT)pNH₃. The activation energy of the ammonia decomposition process has been found for samples with potassium as E _a= 96 kJ/mol, for samples without potassium as E _a= 87 kJ/mol. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

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.     

Nitrite accumulation characteristics of high strength ammonia wastewater in an autotrophic nitrifying biofilm reactor

Selective nitrification was carried out to accumulate nitrite from high strength ammonia wastewater in an autotrophic nitrifying biofilm reactor. Nitrification efficiencies and nitrite accumulation characteristics were investigated at various operating conditions such as ammonium load, oxygen supply and free ammonia concentration. The biofilm reactor showed very stable nitrification efficiencies of more than 90% at up to 2 kg NH₄‐N m^?3 d^?1 and the nitrite content was maintained at around 95%. Inhibition by free ammonia on nitrite oxidizers seems to be the major factor for nitrite accumulation. Batch kinetic analyses of ammonium and nitrite oxidation showed that nitrite oxidation activity was selectively inhibited in the presence of free ammonia. However, the activity recovered quickly as the free ammonia concentration decreased below the threshold inhibition concentration. Examination of specific ammonia and nitrite oxidation activities and the most probable number indicated that the number of nitrite‐oxidizing microorganisms in the nitrite‐accumulating system was less than that in the normal nitrification system due to long‐term free ammonia inhibition of the nitrite oxidizers. The reduced population of nitrite oxidizers in the biofilm system was also responsible for the accumulation of nitrite in the biofilm reactor. © 2003 Society of Chemical Industry     

Selective Catalytic Reduction of NOx over H-ZSM-5 Under Lean Conditions Using Transient NH3 Supply

The selective catalytic reduction (SCR) of NO _x over zeolite H-ZSM-5 with ammonia was investigated using in situ FTIR spectroscopy and flow reactor measurements. The adsorption of ammonia and the reaction between NO _x , O₂ and either pre-adsorbed ammonia or transiently supplied ammonia were investigated for either NO or equimolar amounts of NO and NO₂. With transient ammonia supply the total NO reduction increased and the selectivity to N₂O formation decreased compared to continuous supply. The FTIR experiments revealed that NO _x reacts with ammonia adsorbed on Brønsted acid sites as NH₄ ⁺ ions. These experiments further indicated that adsorbed -NO₂ ^– is formed during the SCR reaction over H-ZSM-5.     

High throughput multiscale modeling for design of experiments,catalysts, and reactors: Application to hydrogen production from ammonia

A novel approach for design of experiments (DOE) is outlined that combines high throughput multiscale modeling, sensitivity analysis, and information extraction from massive computational data using informatics tools. This approach is implemented by conducting experiments of ammonia decomposition on a Ru/γ-Al₂O₃ catalyst in a fixed bed reactor. It is shown that a relatively small number of experiments chosen from this new DOE approach can enable refinement of microkinetic models and render them predictive over the (large) experimentally important parameter space. Microkinetic models are subsequently used for process and product design. Specifically, a membrane fixed bed reactor is simulated and is shown to outperform the conventional fixed bed reactor at intermediate temperatures for hydrogen production. Also, the attributes of the best catalyst for ammonia decomposition are identified as a function of processing conditions. It is shown that for NH₃ decomposition, processing conditions do not significantly affect the best catalyst choice. In contrast, fundamental physicochemical phenomena, such as adsorbate–adsorbate interactions, can have a profound effect on catalyst discovery.     

Ammonia removal from effluent streams of wet oxidation under high pressure

The formation of ammonia is inevitable during industrial-scale wet oxidation of wastewater if nitrogen-containing compounds are present. This undesired side-reaction requires additional measures for disposal. Common routes are either the use of noble metal-containing catalysts in the first oxidation step or end-of-pipe treatment. Problems rise for example from the insufficient stability of solid catalysts against hydrothermal impact. As most of the wet oxidation processes run at elevated pressure and temperature, running the heterogeneously catalysed oxidation of ammonia in the gas phase in a downstream reactor could protect the catalysts mainly from leaching and offers an economic alternative by avoiding loss of unused oxygen after depressurisation. This work reports on the oxidation of ammonia with air in steam atmosphere using Cu,Cr-containing supported and bulk catalysts at 235–305 °C and 30–60 bar. A copper chromite catalyst gave best performance (86% conversion at 305 °C, 45 bar, contact time 1 s). The spinel-type phase CuCr₂O₄ seems to be the active phase and shows superior stability. The results indicate that phase behaviour of water strongly influences activity and lifetime of catalysts. Characterisation of the solids (BET, XRD, XPS, ICP) proved that deactivation is mainly caused by leaching of Cr(VI) species from catalysts when the reaction runs near to dew point of water and by loss of BET surface area of supported catalysts due to hydrothermal impact.A member of the EU-funded Coordination Action of Nanostructured Catalytic Oxide Research and Development in Europe (CONCORDE).     

Electrolytic removal of ammonia from brine wastewater: scale‐up,operation and pilot‐scale evaluation

Brine wastewater with a high ammonia content from an iodine processing plant (commonly called kansui in Japan) was treated by electrolysis. The system, which can be considered as an indirect electrolytic treatment process, generates chlorine at the anodes and initiates the formation of mixed oxidants like hypochlorous acid. The oxidants then act as agents for ammonia destruction. Laboratory‐scale experiments showed that high ammonia concentrations (as much as 200 mg dm^?3) could be completely removed within a few minutes, and could be considered a good alternative for efficient ammonia removal from saline wastewaters. From laboratory‐scale experiments in the batch and continuous modes, the charge dose was analyzed and used as the operating and scale‐up factor. The value of the charge dose was not severely affected by changes in operating conditions such as electrode spacing and temperature. The charge dose from batch and continuous runs was found to be in the range of 23 C (mg NH₄‐N removed)^?1 to 29 C (mg NH₄‐N removed)^?1. Using the charge dose obtained from laboratory‐scale continuous electrolysis experiments as the scale‐up factor, a pilot‐scale reactor was designed, and the operating conditions were calculated. In the pilot‐scale reactor tests at different flow rates, the effluent ammonia concentrations were reasonably close to the calculated values predicted from the charge dose equation. Copyright © 2004 Society of Chemical Industry     

Ammonia synthesis enhanced by magnesium chloride absorption

Conversion to ammonia with Haber–Bosch catalysts can be increased above 95% by selective absorption of ammonia by MgCl₂. The maximum conversion depends on reaction and absorption equilibria. At very short times, the measured conversion rate is the same with and without absorption by the MgCl₂ salt; the overall rate constants are comparable to those in the literature. At larger times, conversion to ammonia can be over seven times greater with MgCl₂ than without. However, the overall rate constants can be over 10 times slower because they are controlled by ammonia diffusion in the solid salt. An approximate, pseudosteady state theory consistent with these results provides a strategy for improving the overall rate while keeping the conversion over 90%. For example, the absorption rates might be increased using smaller particles of absorbent on a porous inert absorbent support. The results provide part of the basis for designing small scale ammonia plants. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1364–1371, 2015     

Apparatus for steady-state kinetic measurements of the catalytic reduction of nitric oxide by ammonia on iron oxide

The reduction of nitric oxide with ammonia on an unsupported iron oxide catalyst has been studied in a continuous-flow recycle reactor using simulated flue gas. The responses of the employed reactor system to step and pulse inputs of tracer indicate that the system could be regarded as a continuous stirred tank reactor (CSTR). Preliminary tests were carried out to determine the effect of temperature and particle size on the measured reaction rates. Additional experiments were performed in order to study the influence of oxygen and water concentration on these rates. A gas chromatographic system has been developed to analyze the gas components NO, N₂O, NO₂, NH₃, H₂O, O₂, CO₂ and N₂. In addition, the concentrations of NO and NO₂ were measured with a nondisperse infrared (NDUV/NDIR) analyzer.     

Segregation effects on consecutive competing reaction in a CSTR

The steady state behaviour of a continuous stirred tank reactor at a constant temperature of 30°C was studied using the saponification of ethylene glycol diacetate by sodium hydroxyde. The reactor studied was respectively fed by premixed and unmixed reactants. Experimental values of the level of segregation lay within the range of 0.15 to 0.5. The level of segregation has been correlated with the physicochemical dimensionless group k₁C_Ao/β and we may conclude that the reactor would be in a state of maximum mixedness when the magnitude of this group is smaller than 10^?4.     

Improvement of the settling properties of Anammox sludge in an SBR

Biological systems for the treatment of wastewater have to provide optimum sludge retention to achieve high removal efficiencies. In the case of slow‐growing micro‐organisms, such as anaerobic ammonia‐oxidizing (Anammox) bacteria, episodes of flotation involving biomass wash‐out are especially critical. In this study a strategy based on the introduction of a mix period in the operational cycle of the Anammox Sequencing Batch Reactor (SBR) was tested for its effects on biomass retention and nitrite removal. Using this new cycle distribution the biomass retention inside the reactor improved as the solids concentration in the effluent of the SBR decreased from 20–45 to 5–10 mg VSS dm^?3 and the biomass concentration inside the reactor increased from 1.30 to 2.53 g VSS dm^?3 in a period of 25 days. A decrease of the sludge volume index (SVI) from 108 to 60 cm³ g VSS^?1 was also observed. Complete depletion of nitrite was achieved in the reactor only with the new cycle distribution treating nitrogen loading rates (g N‐NO₂^? + g N‐NH₄⁺ dm^?3 d^?1) up to 0.60 g N dm^?3 d^?1. Copyright © 2004 Society of Chemical Industry     

Dynamic modelling of a three-phase catalytic slurry reactor

Two dynamic models for a three-phase catalytic slurry reactor with appropriate solution procedures were developed in this work. The models consist of mass and heat balance equations for the catalyst particles, for the gas and liquid bulk phases as well as for the heat exchange through the jacket of the reactor. The models of the tubular reactor were applied to describe the dynamic behaviour of the reactor during the hydrogenation of o-cresol on Ni/SiO₂ catalyst. These models differ in solid phase modelling, which allows to evaluate the reactor dynamic behaviour prediction capacity. The models successfully reproduce the main characteristics of the reactor dynamic behaviour.