Reduction of NOx Stored at Low Temperatures on a NOx Adsorbing Catalyst

Isothermal storage and reduction of NO₂ with CO, C₃H₆ and H₂ as reducing agents on a lean NO _x adsorber was investigated by temperature programmed desorption (TPD) and temperature programmed reduction (TPR) studies. The reduction of NO _x was clearly favoured with H₂ as reducing agent. Carbon monoxide and C₃H₆ showed fairly low reduction of NO _x . The NO _x reduction at low temperatures with H₂ as reducing agent was found to be effective, clearly much more effective than for CO.     

Integrated design of diesel hydrotreating processes

This work presents an integrated approach for the design of diesel hydrotreating processes employing a simulated annealing optimisation algorithm. The modelling of reactor, separation and heat recovery system for diesel hydrotreating processes is discussed, and a novel optimisation framework is developed for the design of complex refinery processes. A comparison with conventional approach to process design, i.e. sequential evolution of design, is given to illustrate the ability of proposed approach to obtain overall hydrotreating process designs with minimum total annualised costs. The proposed integrated approach takes into account the trade-offs between capital and operating costs, as well as interactions between the hydrotreater, distillation column, and the associated heat exchanger network.     

Predicting River Travel Time from Hydraulic Characteristics

Predicting the effect of a pollutant spill on downstream water quality is primarily dependent on the water velocity, longitudinal mixing, and chemical∕physical reactions. Of these, velocity is the most important and difficult to predict. This paper provides guidance on extrapolating travel-time information from one within bank discharge to another. In many cases, a time series of discharge (such as provided by a U.S. Geological Survey stream gauge) will provide an excellent basis for this extrapolation. Otherwise, the accuracy of a travel-time extrapolation based on a resistance equation can be greatly improved by assuming the total flow area is composed of two parts, an active and an inactive area. For 60 reaches of 12 rivers with slopes greater than about 0.0002, travel times could be predicted to within about 10% by computing the active flow area using the Manning equation with n = 0.035 and assuming a constant inactive area for each reach. The predicted travel times were not very sensitive to the assumed values of bed slope or channel width.     

Induced low temperature catalytic ignition by transient changes in the gas composition

The effect of gas composition changes on the low temperature activity for supported platinum model catalysts has been studied. By introducing well-controlled periodic O₂ pulses to a simple diluted gas mixture of CO and O₂, a substantial improvement of the low temperature oxidation activity was observed. The reason for low activity on noble metals at low temperatures is often attributed to self-poisoning by CO. The improved catalytic performance observed is proposed to origin from the transients causing a surface reactant composition that is favourable for the reaction rate, i.e. lower degree of self-poisoning. This was also confirmed by in situ Fourier transform infrared (FT-IR) spectroscopy in combination with mass spectrometry (MS) measurements, which gave evidence for the existence of a strong interplay between the gas phase concentration and the adsorbate composition for these catalysts.     

Estimation and reduction of CO2 emissions from crude oil distillation units

Distillation systems are energy-intensive processes, and consequently contribute significantly to the greenhouse gases emissions (e.g. carbon dioxide (CO₂). A simple model for the estimation of CO₂ emissions associated with operation of heat-integrated distillation systems as encountered in refineries is introduced. In conjunction with a shortcut distillation model, this model has been used to optimize the process conditions of an existing crude oil atmospheric tower unit aiming at minimization of CO₂ emissions. Simulation results indicate that the total CO₂ emissions of the existing crude oil unit can be cut down by 22%, just by changing the process conditions accordingly, and that the gain in this respect can be doubled by integrating a gas turbine. In addition, emissions reduction is accompanied by substantial profit increase due to utility saving and/or export.     

Multi-period design of heat exchanger networks

Heat exchanger networks are an integral part of chemical processes as they recover available heat and reduce utility consumption, thereby improving the overall economics of an industrial plant. This paper focuses on heat exchanger network design for multi-period operation wherein the operating conditions of a process may vary with time. A typical example is the hydrotreating process in petroleum refineries where the operators increase reactor temperature to compensate for catalyst deactivation. Superstructure based multi-period models for heat exchanger network design have been proposed previously employing deterministic optimisation algorithms, e.g. (0005 and 0180). Stochastic optimisation algorithms have also been applied for the design of flexible heat exchanger networks recently (0110 and 0115). The present work develops an optimisation approach using simulated annealing for design of heat exchanger networks for multi-period operation. A comparison of the new optimisation approach with previous deterministic optimisation based design approaches is presented to illustrate the utilisation of simulated annealing in design of optimal heat exchanger network configurations for multi-period operation.     

An investigation of the thermal stability and sulphur tolerance of Ag/γ-Al2O3 catalysts for the SCR of NOx with hydrocarbons and hydrogen

The sulphur tolerance and thermal stability of a 2 wt% Ag/γ-Al₂O₃ catalyst was investigated for the H₂-promoted SCR of NO_x with octane and toluene. The aged catalyst was characterised by XRD and EXAFS analysis. It was found that the effect of ageing was a function of the gas mix and temperature of ageing. At high temperatures (800 °C) the catalyst deactivated regardless of the reaction mix. EXAFS analysis showed that this was associated with the Ag particles on the surface of the catalyst becoming more ordered. At 600 and 700 °C, the deactivating effect of ageing was much less pronounced for the catalyst in the H₂-promoted octane-SCR reaction and ageing at 600 °C resulted in an enhancement in activity for the reaction in the absence of H₂. For the toluene + H₂-SCR reaction the catalyst deactivated at each ageing temperature. The effect of addition of low levels of sulphur (1 ppm SO₂) to the feed was very much dependent on the reaction temperature. There was little deactivation of the catalyst at low temperatures (≤235 °C), severe deactivation at intermediate temperatures (305 and 400 °C) and activation of the catalyst at high temperatures (>500 °C). The results can be explained by the activity of the catalyst for the oxidation of SO₂ to SO₃ and the relative stability of silver and aluminium sulphates. The catalyst could be almost fully regenerated by a combination of heating and the presence of hydrogen in the regeneration mix. The catalyst could not be regenerated in the absence of hydrogen.     

Fundamental studies of NOx storage at low temperatures

A commercial NO_x storage catalyst (Pt, BaO and alumina containing) was investigated by temperature programmed desorption (TPD) experiments in the temperature range 100–400 °C. The catalyst stored a substantial amount of NO_x at 100 °C using NO + O₂. Nitrites or loosely bound NO species are suggested for this storage, since no NO was oxidised at this low temperature. In addition, the released NO_x during the temperature ramp consisted of mainly NO and at lower temperatures the NO₂ dissociation is limited. Water and CO₂ was found to decrease the storage substantially, 92% for the NO + O₂ adsorption at 100 °C. The total storage for 60 min using NO₂ + O₂ at 200 °C was similar when introducing CO₂ and H₂O. However, the initial total uptake of NO_x was decreased. Initially we probably formed loosely bound NO_x species, which likely are strongly influenced by water and CO₂. After longer time periods are barium nitrates probably formed and they can remove the carbonates by forming stable nitrates, thus resulting in the same total uptake of NO_x.     

A highly active Ag/alumina catalytic converter for continuous HC-SCR during lean-burn conditions: from laboratory to full-scale vehicle tests

Topics in Catalysis - A common-rail diesel vehicle was equipped with a full-scale Ag/alumina catalytic converter. The converter consisted of several Ag/alumina bricks, with free space in between...