Reverse flow reactor operationfor control of catalyst surface coverage

Simulation of reverse flow operation for manipulation of catalyst surface coverage in the selective oxidation of ammonia

In chemical engineering design, process plants are often capable of turndown and flexible operation to maximise profitability depending on demand and price signals, and the optimisation of these processes needs to take into account the variability in the plant operation. By performing the optimisation with flexible operating conditions the overall plant performance can be improved. The optimisation process makes use of two computational modelling techniques: multi-objective optimisation (MOO) and surrogate modelling. Multi-objective optimisation allows for the optimisation of two or more competing variables simultaneously. In the cases studied here, the long individual simulation time coupled with the large number of individual simulations required meant that the total simulation time was prohibitive for dynamic MOO. In order to reduce computation time surrogate models of the processes are developed using artificial neural networks. A statistical dynamic modelling framework within a multi-objective optimisation methodology is developed to optimise a process plant’s performance. Here the dynamic modelling is used to optimise the economic and environmental performance of natural gas combined cycle power plant systems fitted with carbon capture and storage (CCS) and solar thermal energy (STE) in a time-integrated MOO framework. This framework allows for variable process inputs and flexible operation based on external signals, such as product price. By introducing a probabilistic approach to the modelling of flexible operation allows for more data to be included in the analysis. The methodology for dynamic modelling of flexible operation could be applied to other industrial processes that face similar dynamic economic and/or environmental conditions. From a CCS standpoint dynamic MOO was used to show that variable capture can be used to make the CCS plant more profitable over steady state operation. At a capture rate of 80 % the NPV of a gas turbine combined cycle with CCS improved by 38 m$AU by utilising flexible operation of the capture process. This also allows for various economic scenarios to incentivise the reduction of CO₂ emissions from natural gas based power plants, with the use of CCS and STE used to reduce the greenhouse emissions. The economic strategies of a carbon price and governmental grants are investigated to determine the levels of policy help to incentivise implementing CO₂ reduction technologies. In order to achieve a financial incentive for limiting CO₂ emissions from a combined cycle power plants equipped with CCS a government co-investment grant of 50 % of the capital costs and a carbon price of 83 $AU/tCO₂ is required.

The variable refrigerant flow (VRF) air conditioning systems have been widely used to home air conditioner. The VRF system can provide variable refrigerant flow by changing the speed of compressor for energy saving and more comfortable air conditions. Recent years, the energy saving technique for central air condition system becomes important, and the inverter applied as variable frequency drive control the speed of compressor motor technologies have been applied to central air conditioning systems. The central air conditioning system with the inverter compressor (ICCACS) providing variable refrigerant flow becomes an important solution for energy saving. This study built the central air conditioning systems dynamic model with operation data. The appropriate refrigerant flow is calculated by refrigerating capacity. A PI controller is designed to control the speed of compressor for keeping the temperature of chilled water in command temperature. The energy consumption is investigated under transient and steady processes.

This paper deals with reactor design, modeling, and simulation of a pilot-scale reverse flow operation used for catalytic oxidation of lean benzene-air mixtures. The reactor was designed for conducting lean benzene with adiabatic temperature rise less than 54°C. According to the rule of thumb, this kind of condition requires the reaction to be accomplished under reverse flow operation. In the reactor design, some criteria were calculated such as Mears, Weisz-Prater and Ergun’s criteria The reactor model was developed for 1D, pseudo-homogeneous, involving mass and energy balances. The fixed bed consisted of the inert section, placed at the outer part of the catalyst section. The results of the simulation indicated that the switching time, inlet concentration and heat extraction imposed an important influence on the performance of the reactor. Under the suitable operating conditions, the system can achieve a favorable running status, without reactor extinction, and can be operated auto-thermal, in fact, more thermal energy from the catalyst section was feasible to extract. The temperature profile remained smooth and continuous in the case of desired energy extracting from the heat exchanger.

The extension of the Dutch 380 kV high voltage grid is necessary in order to guarantee security of electricity supply to the consumers. To achieve this extension, there are two new 380 kV connections under construction in the Randstad area, a densely populated area in the western part of the Netherlands. In these 380 kV connections, underground cables are applied. The work described in this thesis forms a part of a monitoring program that is managed by the Dutch transmission grid operator TenneT. In this program, the behavior of the two new underground cable connections in the Dutch 380 kV grid is being investigated and the work described in this thesis contributes to this program. Unlike the common overhead transmission line, which has an inductive behavior, a cable acts as a capacitance when it is in operation. This difference in electrical behavior makes research on the grid behavior necessary. One of the research goals is to evaluate whether the power flows and voltage levels in the 380 kV grid stay within the prescribed limits. Another goal of this work is investigating whether the peak of the transient voltages at the cable terminals after lightning currents will stay below the Basic Insulation Level. The research is divided into two parts: steady state behavior and transient behavior. For the steady state part, PI sections are used to model the underground cable and overhead line to perform load flow studies in order to investigate the power flows and voltage levels in the 380 kV grid. For the transient part, the widely applied Frequency Dependent Phase Model (FDPM), which is a transmission line model that takes into account the frequency dependency of cable and line parameters, is used to model the cable and overhead line sections. For the transient part, the focus is on the modeling work of the complete underground cable system. Field measurements are performed for both the 380 kV cross-bonded cable and the overhead line sections and the results are used for transient cable model verification. The scientific contribution of this work is the determination of the parameters for the 380 kV cross bonding system by using reflection measurements and the validation of the FDPM transmission line model by using field measurements on the actual cross bonded 380 kV cable system. Finally, based on the studies described in this thesis, one can conclude that no remarkable overvoltages occur during the studied steady state and transient situations for the given dimensions of the observed 380 kV system.

Reverse-flow operation of catalytic reactors is known to result in very efficient regenerative heat integration. While reverse-flow operation is in particular ideally suited for high-temperature short-contact-time reactions, the application of this reactor concept under such extreme reaction conditions also requires particular care in the design and operation of the reactor. We report results from an experimental study on some of the main design and operating parameters for high-temperature catalytic reverse-flow reactors. Results for reverse-flow operation are compared to conventional stationary reactor operation. Temporal and spatial temperature profiles at periodic steady state are presented, and the influence of the structure of the catalyst and inert zones, the role of homogeneous reactions, and the frequency of flow switching are investigated and discussed.

The world is steadily moving toward greener alternatives of established chemical production processes. Process modeling and simulation play a vital role in conceiving, designing and optimizing these greener processes. In many cases, these simulation tools are limited to a few project design stages, thus limiting their true potential. This thesis presents the benefits of process modeling and simulation at different levels of process development. The work focuses on ammonia production, which sustains global agricultural and mining activities. Ammonia production has traditionally been based on large-scale plants. The thrust toward large-scale production in order to gain economic advantages has overshadowed the benefits that could be derived from small-scale production plants. Additionally, the ammonia industry is a major consumer of global fossil fuels, and in the process of providing ammonia, also burdens the planet with greenhouse gases. In order to ensure sustainable ammonia production, this thesis highlights the techno-economic advantages that result from small-scale ammonia plants based on biomass feedstock. The use of biomass as a potential feedstock can help neutralize the carbon footprint of ammonia production. To establish the feasibility of such a process, this thesis presents the application of process simulations at different modeling levels.<br> <br> A Computational Fluid Dynamics (CFD) model was used to understand the flow patterns inside a dual fluidized bed gasifier. This elevated the understanding of the hydrodynamics of the gasifier freeboard, which is neglected by the conventional two-phase methodology. The CFD simulation was utilized to perform a Residence Time Distribution (RTD) analysis of the reactor. Four different tracer approaches namely the frozen velocity approach, the snapshot approach, the data sampling approach and the transient approach, were compared. The RTD analysis formed the basis of a steady-state compartment model that was developed in ASPEN Plus simulation software. The ASPEN Plus gasifier model decoupled the pyrolysis, gasification, and combustion sections of the gasifier to affect a better comprehension of the process and the results. The model predicated satisfactory results upon validation. Additionally, the model could also be used to predict the output for different biomass feedstocks.<br> <br> ASPEN Plus is used to simulate the biomass gasification process as well as downstream gas conditioning that leads to ammonia production. Apart from major constituent gases, the model accounted for tar production in the gasifier process as well as for its subsequent removal. Three different configurations of the biomass-to-ammonia process, namely, Autothermal Reforming (ATR), Steam Methane Reforming (SMR), and CO₂ Reforming (CR) have been modeled and compared. The configurations differ on the basis of the reforming of the methane and tars, which are present in the syngas. The output of the ASPEN Plus simulation is fed into an MS Excel-based framework, which performs Life Cycle Costing (LCC) and Life Cycle Assessment (LCA) for the simulated flowsheet. Finally, these outputs are utilized by an MS Excel-/VB-based Multi-Objective Optimization (MOO) framework in order to optimize both LCC and LCA for changing process parameters.<br> <br> Three biomass feedstocks, namely, wood, straw pellets, and bagasse, which are available in Australia, India, and Brazil, respectively, have been compared in order to understand the effect of biomass composition, the supply chain, and the national economic and environmental scenario on the viability of the biomass-to-ammonia process. The MOO results that were derived for changing flowsheet configurations as well as for different biomass feedstocks that were grown at different locations worldwide are expected to highlight a more rigorous analysis of the biomass-to-ammonia process. Such an analysis will provide decision makers with consistent and comparable data to objectively judge the viability of the proposed process. This analysis distinguishes this thesis from past studies that focused on ammonia production from biomass, and presented their LCC and LCA results for a single process configuration and for a single feedstock. The results predicted that different biomass feedstocks, process configurations, and geographical regions have their own advantages. The results provide a benchmark for comparison of other biomass-to-ammonia processes on a techno-enviro-economic scale. The methodology can be employed as an example in the development of different sustainable chemical production processes.

We propose a new circuit analysis method, namely Semi-Implicit Linear-Centric Analysis (SILCA), for efficient SPICE-accurate transient simulation of deep-submicron VLSI circuits with strong parasitic coupling effects introduced by interconnect lines, common substrate, power/ground networks, etc. SILCA is based on two linear-centric techniques. First, a new semi-implicit iterative numerical integration scheme is developed, which applies dynamic time step control accounting for stiff systems and meanwhile keeps constant equivalent conductance for capacitor/inductor companion models. Its convergence and stability properties are characterized. Second, to achieve constant linearized conductance for nonlinear devices during nonlinear iteration process, a successive variable chord method is introduced as an alternative of the Newton-Raphson method and the rank-one update technique is implemented for fast LU factorization. With these techniques, SILCA reduces the number and cost of required LU factorizations dramatically. Experimental results on substrate and power/ground networks have demonstrated that SILCA yields SPICE-like accuracy with an over 80X reduction in LU factorization cost, and an about 20X overall CPU time speedup over SPICE3 for circuits with tens of thousands elements, and the efficiency increases further with the size of a circuit.

The catalytic oxidation of methane for abating the emission vented from coal mine or natural gas transportation has been known as most reliable method. A reverse flow reactor operation has been widely used to oxidize this methane emission due to its capability for autothermal operation and heat production. The design of the reverse flow reactor requires a proper kinetic rate expression, which should be developed based on the operating condition. The kinetic rate obtained in the steady state condition cannot be applied for designing the reactor operated under unsteady state condition. Therefore, new approach to develop the dynamic kinetic rate expression becomes indispensable, particularly for periodic operation such as reverse flow reactor. This paper presents a novel method to develop the kinetic rate expression applied for unsteady state operation. The model reaction of the catalytic methane oxidation over Pt/-Al2O3 catalyst was used with kinetic parameter determined from laboratory experiments. The reactor used was a fixed bed, once-through operation, with a composition modulation in the feed gas. The switching time was set at 3 min by varying the feed concentration, feed flow rate, and reaction temperature. The concentrations of methane in the feed and product were measured and analysed using gas chromatography. The steady state condition for obtaining the kinetic rate expression was taken as a base case and as a way to judge its appropriateness to be applied for dynamic system. A Langmuir-Hinshelwood reaction rate model was developed. The time period during one cycle was divided into some segments, depending on the ratio of CH4/O2. The experimental result shows that there were kinetic regimes occur during one cycle: kinetic regime controlled by intrinsic surface reaction and kinetic regime controlled by external diffusion. The kinetic rate obtained in the steady state operation was not appropriate when applied for unsteady state operation. On the other hand, the kinetic rate expression obtained in the unsteady state operation fitted quite well. It was proven that in one cycle period the kinetic rate would always shift according to the ratio of CH4/O2.

This report presents a system thermal-hydraulic analysis code ATHLET-MF --- a new version of ATHLET. This code has been developed on the basis of ATHLET for the thermal-hydraulic analysis of multi-fluid systems, includingthe liquid LBE-cooled systems. A fluid index was introduced in ATHLET-MF so that the user can easily adapt the code for various fluids. The current version of ATHLET-MF has the fluid options of water, liquid LBE and Diphyl THT. Empirical equations of physical properties of liquid LBE and of Diphyl THT, and the heat transfer correlation of liquid LBE were implemented in ATHLET-MF for its application to the LBE-cooled ADS systems. The physical properties and heat transfer correlation of liquid LBE were used based on the comprehensive review and assessment of the thermophysical properties of liquid LBE and of the heat transfer correlations for heat transfer in heavy liquid metals in the open literature. The ATHLET-MF code was applied to the analyses of the target cooling systems of XADS, MEGAPIE and MITS under various transient conditions. The code was assessed against two home-made system analysis codes HERETA and HETRAF by performing the simulation of the dynamic behavior of the target cooling system of XADS under beam power switch-on conditions. Results from the three different codes show a good agreement, indicating the applicability of the ATHLET-MF code to LBE cooled systems. Simulation of the beam power interruption of XADS shows that transient with longer beam interrupt time undergoes a deeper drop of the fluid temperature and of the mass flow rate. However, the drop of fluid temperature is limited by the heat transferred from the reactor pool and the reactor core after the switch-off of the beam power. It is shown that the beam interrupts with duration shorter than 0.1 s are less critical than those with duration longer than 0.1 s. In the case of loss of heat sink, the proton beam should be switched off in 200 s after the occurrence of the transient in order to avoid the failure of the window. For the beam trips of MEGAPIE and MITS, a proper regulation of the 3-way valve in the intermediate cooling loop can effectively limit the LBE temperature fluctuation at the exit of the target heat exchanger. The peaks of LBE temperatures at the inlet and exit of the target heat exchanger after the beam power recovery can be reduced or even eliminated by opening the 3-way valve in the intermediate cooling loop at an early time. Comparison shows that the drop of fluid temperature under the transient of beam trip of MITS is much smaller than that of the MEGAPIE. The steady state natural circulation of LBE is established after the loss of pump power supply for both MEGAPIE and MITS. The natural circulation flow rate is about 46% of its initial value for MEGAPIE and about 37% of its initial value for MITS, which is about 24% less than that calculated based on the proposed scaling principles of a scaled experiment.

This paper addresses the steady-state and transient responses of un-controlled faulty 6-pulse diode rectifier bridge circuits. The faulty bridges can include several simultaneous diode valves’ open- or short-circuits. An efficient representation of the diodes is adopted in order to describe their performance in both the forward and reverse directions. A set of simultaneous differential and algebraic equations simulating the faulty rectifier bridge is presented. Its numerical solution is obtained through the use of a Mathematica code. The paper includes the results of a detailed investigation involving four case studies. They demonstrate the applicability of the suggested approach and the reduced computation burden in comparison with other currently available techniques.

The residence time distribution (RTD) is very reliable, providing valuable information about the performance of process equipment such as reactors and columns. The aim of this work is to study the effect of agitation in the hydrodynamic column of flow-rig. In this case, residence time distribution (RTD) is determined using the radiotracer approach. The radiotracer is the Technetium Tc-99m with half decay 6 h. It also emits gamma rays with an energy of 0.140 MeV. 2 mL of approximately 9 mCi activity Tc-99m was injected inside the column. The results show that in the conditions studied the column presents the anomaly, dead volume. The Intensity function A(t) confirms this result. Agitation has a beneficial effect in the reduction of dead volume. To revise the conception of the column or study other parameters like the variation flow rate can reduce or eliminate the dead volume. The RTD model indicated the column behaved as a Plug flow reactor (PFR) with mixing cells in serie (J = 4) model and the RTD experiment verified the model well.

Grid-forming inverters are the essential components in the effort to integrate renewable energy resources into stand-alone power systems and microgrids. Performance of these inverters directly depends on their control parameters embodied in the controller. Even the most conscientiously designed controller will exhibit suboptimal performance upon implementation due to the presence of parasitic elements in the existing hardware. Hence, the controller has to be tuned and optimized. In the present article, the process of implementation, laboratory verification, and tuning of a matching-controlled grid-forming inverter is presented. In order to assess the efficiency of the grid-forming controller, its operation has been tested and analyzed in blackstart, steady state, and transient operation. For this purpose, a systematic sensitivity analysis has been conducted and the control parameters have been tuned in laboratory tests. The laboratory results verify proper operation of a 7 kW grid-forming inverter in all three test scenarios. After applying the proposed method on the tested grid-forming inverter in steady state operation, total harmonic distortion (THD) of the output voltage is less than 0.5% for its practical loading range (maximum THD is less than 1% in no-load condition). The system is able to blackstart and supply the loads. Finally, the studied grid-forming inverter is stable in the presence of severe step load changes and disturbances, i.e., voltage overshoot is managed well and compensated for with a low settling time using this approach.

The performance of a reverse flow reactor (RFR) is strongly influenced by the switching time used to alternate the flow direction. This research aimed to study the effect of the switching time on reactor dynamics including the heat propagation along the bed and reaction rate in the oxidation methane for low concentration using catalytic reverse flow reactor. The experimental results show that the reverse flow operating mode can influence heat propagation along the reactor and reaction conversion. Based on the three switching times was tested, the temperature dynamics formed were in the sliding regime. The effect of switching time on RFR on conversion is very significant. When compared to steady operation, RFR operation provides the highest conversion at smaller switching times. At large switching times, the effect of reversal of flow direction becomes less dominant and reactor behavior approaches steady state.

Electron Beam Induced Deposition (EBID) allows deposition of three-dimensional micro- and nano-structures of conductive and insulating materials on a wide range of substrates. The process is based on the decomposition of molecules of a pre-selected precursor by a focused electron beam. In recent decades EBID of several metals, namely Au, W, Cu and Pt, from different families of precursors, has been achieved and the technique has found some application for small-scale production of laboratory devices and for repair of masks and micro-optoelectronic devices. The weak point of the technique is at present the low purity of the deposited material, caused by metal-organic precursors and by the lack of selectivity of the electron-induced decomposition process. This work is dedicated to EBID of Rh nanostructures from the precursor [RhCl(PF3)2]2. High metal content deposits are expected because the precursor does not contain C atoms and because Rh is one of the less reactive metals. [RhCl(PF3)2]2 as EBID precursor has been characterized by vapor pressure, mass spectrometry and surface residence time measurements. The vapor pressure of 7.5 Pa at room temperature reveals that the precursor is sufficiently volatile for room temperature EBID. The knowledge of the vapor pressure allows also to estimate the number of precursor molecules delivered to the reaction area per unit time. Mass spectrometry measurements allow to know the decomposition path of the precursor under electron impact in the gas phase. The measured spectrum indicates that the molecule decomposes by successive loss of PF3 groups, as confirmed by density functional theory calculations. This is compatible with a high metal content deposit. Residence time measurements show that [RhCl(PF3)2]2 does not decompose on stainless steel surfaces. The measured residence time of 2 ms allows to estimate that the activation energy for desorption of [RhCl(PF3)2]2 on stainless steel is about 0.6 eV and that precursor molecules can travel distances in the micrometer range before being desorbed. EBID structures obtained from [RhCl(PF3)2]2 have been characterized with a wide range of techniques for a better knowledge of the material properties and the deposition process. The deposit morphology has been studied by Transmisssion Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) to characterize the different steps of the deposition process. Crystallographic analysis is carried out by TEM in diffraction mode. Chemical analysis is carried out by Auger Electron Spectroscopy (AES) and Electron Energy Loss Spectroscopy (EELS). Morphological analysis of deposits carried out at different exposure times reveals that the first phase of the growth process, in close proximity to the substrate, is characterized by an increase of the deposit height and the deposit diameter. On the other hand the second phase of the growth process is characterized by increasing height and constant diameter. TEM contrast profiles of dots and Atomic Force Microscopy (AFM) sections of lines have clearly shown that the EBID rate is highest in the center of the beam and decreases in the peripheral regions. Deposition at variable distances from the precursor source allowed to obtain hollow structures, whose morphology reveals that the precursor reaches the reaction area mainly by direct gas phase transport. Structural analysis and TEM revealed that, independently of the deposition conditions, the deposited material is made up of Rh nanocrystals immersed in a lighter amorphous matrix. Chemical analysis by Auger Electron Spectroscopy revealed that, after removal of the C rich contamination layer by Ar ion sputtering, the average composition of the deposits is about: 60 at.% Rh, 20 at.% P, 5 at.% Cl, 7 at.% N, 8 at.% O. The absence of C and the presence of N and O in the deposit bulk have been confirmed by Electron Energy Loss Spectroscopy. This technique allowed also to prove that Rh is dominant also in deposits of sub-micrometer size (not analyzable with AES) and to determine the elemental distribution in the deposit with nanometer resolution. Comparison of the deposit composition and the positive ions detected by mass spectrometry revealed that EBID, compared to low pressure gas phase ionization, involves a higher number of events, i.e. multi-electron decomposition and rearrangements of partially decomposed species.