Decreasing Gas Flow Rate Research Articles

Numerical simulation for mass transfer characteristics of CO2 capture in a rotating packed bed

Rotating packed bed (RPB) has owned the significant advantages for gas-liquid mass transfer process intensification. This study presented a mathematical model for mass transfer characteristics of CO2 capture in a RPB, which involved diffusion and reaction process. In current setup, main focus was given on analysis of local liquid-side mass transfer coefficient (kL), which was applied to obtain overall gas-side mass transfer coefficient (KGa) and CO2 absorption efficiency (E). The effects of operating parameters, such as Higee factor, temperature, pressure, gas and liquid flow rate, on KGa and E were investigated. The results indicate that KGa increases with increasing Higee factor, temperature and liquid flow rate and decreasing gas flow rate but is independent on pressure; E is positively dependent on Higee factor, temperature, liquid flow rate and pressure but negatively dependent on gas flow rate. Meanwhile the numerical simulation exhibits that transient droplet lifetime is the essence of process intensification for RPBs. Furthermore, the simulated values of E have satisfactory agreement with experimental and published data with maximum deviations within ±20%, which recommends this model suitable for providing the theoretical basis for future design and application of RPBs.

Oxidative destruction of monocyclic and polycyclic aromatic hydrocarbon (PAH) contaminants in sulfur recovery units

The requirement of low sulfur content in fuels and the stringent environmental regulations has led to an increased production of acid gas (H2S and CO2) as byproducts in oil and gas industry. Consequently, the need for economic and efficient treatment of acid gas has surfaced. Sulfur recovery units (SRU), consisting of a furnace and catalytic reactors, are widely deployed to recover sulfur and thermal energy from acid gas. The contaminants in acid gas such as benzene, toluene, ethylbenzene, and xylenes (BTEX) and frequent changes in its composition cause flame instability and produce unwanted byproducts such as polycyclic aromatic hydrocarbons (PAHs), CO, COS, and CS2, which reduce process efficiency and increase operational cost through frequent catalyst deactivation. In this paper, a detailed reaction mechanism is presented for SRU that includes reactions for the combustion of acid gas and its contaminants, and the formation and oxidation of large PAHs by several oxidants (O, O2, OH, and SO2). The mechanism is validated with different sets of experimental data, and is used to investigate the process conditions that triggers the oxidative destruction of aromatics (BTEX and PAHs) in the furnace. With decreasing acid gas flow rate, and increasing feed preheating temperatures and oxygen concentration in air, the aromatics concentration was found to decrease substantially due to their enhanced oxidation by SO2 and O2, which indicates that the destruction of harmful aromatics in the high-temperature zone of the furnace is possible with optimized SRU process conditions. The developed reaction mechanism provide viable means of optimizing SRU to achieve aromatics destruction, enhanced catalyst life, and reduced sulfur production cost.

Gas induced semi-solid process effects on microstructure and mechanical properties of 319 aluminum alloy

Abstract The exclusive effects of gas induced semi-solid parameters such as rheocasting temperature, agitation intensity and flow rate of gas injection into the melt on the average size and shape factor of primary α-Al particles and subsequently tensile properties of 319 aluminum alloy have been studied. Applying vigorous agitation and rapid heat extraction simultaneously in the early stages of solidification in order to achieve a slurry with fine and dispersed primary α-Al particles is essential. Hence, in order to investigate these two factors separately, three different types of graphite diffusion in three proportional gas flow rates have been used. It was found that a reduction in rheocasting temperature leads to a decrease in the average size of the primary α-Al particles and an increase in their sphericity and improvement of tensile properties as well. The average size of primary particles slightly decreases as the agitation intensity increases as expected; while a decrease in agitation intensity in a given gas flow rate leads to the formation of gas porosity. Also the gas was trapped due to an increase in gas flow rate at constant agitation intensity whereas the decrease in gas flow rate has no significant effect on morphology of primary α-Al particles.

Emission characteristics of kerosene-air spray combustion with plasma assistance

A plasma assisted combustion system for combustion of kerosene-air mixtures was developed to study emission levels of O2, CO2, CO, and NOx. The emission measurement was conducted by Testo 350-Pro Flue Gas Analyzer. The effect of duty ratio, feedstock gas flow rate and applied voltage on emission performance has been analyzed. The results show that O2 and CO emissions reduce with an increase of applied voltage, while CO2 and NOx emissions increase. Besides, when duty ratio or feedstock gas flow rate decreases, the same emission results would appear. The emission spectrum of the air plasma of plasma assisted combustion actuator was also registered to analyze the kinetic enhancement effect of plasma, and the generation of ozone was believed to be the main factor that plasma makes a difference in our experiment. These results are valuable for the future optimization of kerosene-fueled aircraft engine when using plasma assisted combustion devices to exert emission control.

Drastic increase in catalyst productivity of nanoclay-supported CVD-grown carbon nanotubes by organo-modification

The organo-modification of montmorillonite (Mt) prior to catalyst insertion was proposed as a promising approach to increase carbon nanotube (CNT) yield on nanoclay (NC) supports in catalytic chemical deposition (CVD) reaction. The behavior of organo-modified montmorillonite (OMt) as the support in CVD reaction was compared with that of Mt. According to morphological images and Raman spectra, the carbon atoms form CNT in case of both Fe-loaded pristine Mt and OMt. Decreasing gas flow-rate and diminishing hydrogen fraction in growth atmosphere were found to favor improvement of CNT yield, whereas generally tended to deteriorate its quality. TGA results showed that the CNT content of OMt-supported hybrids were drastically increased compared to that of non-modified Mt. There was about two- to fifteen-fold increase in CNT yield depending on the CVD conditions. This observation was found correlated to the larger specific surface area (SSA) and d001-value of Fe-loaded OC. The evident advantages of OMt as catalyst support include efficient and scalable synthesis of high thermally stable NC/CNT hybrid at low cost.

Photo-catalytic destruction of ethylene using microwave discharge electrodeless lamp

A double tube type microwave discharge electrodeless lamp was employed to investigate the photo-catalytic decomposition of ethylene gas, an important VOC species. The anatase TiO2 film photo-catalyst balls prepared by a low pressure metal organic chemical vapor deposition method were used. In addition, the advantages of microwave/UV/TiO2 photo-catalysts hybrid process were analyzed. The removal performance was examined under different conditions with different initial ethylene concentrations, gas residence times and oxygen concentrations. At all microwave powers tested, UV-C exhibited much larger irradiance than UV-A and UV-B. The degradation efficiency of ethylene increased with increasing microwave intensity, with decreasing inlet concentration, and with decreasing reaction gas flow rate. Taking the energy cost into account, residence time should be determined considering inlet concentration, volume of degradation, capacity of devices, and admitted costs. Microwave intensity was shown to be a critical operation variable for the photo-catalytic degradation of ethylene, required to be determined depending on initial ethylene concentration.

Effect of H2 Flow Rate on High-Rate Etching of Si by Narrow-Gap Microwave Hydrogen Plasma

For the purpose of realizing a low-cost production process of silane (SiH4) gas, we have proposed the high-rate etching of metallurgical-grade Si by narrow-gap microwave hydrogen plasma. In this paper, effect of hydrogen gas flow rate (0–10 L/min) on the etch rate has been investigated and correlated with the relative variation of hydrogen-atom density estimated by actinometry. By decreasing hydrogen gas flow rate, the etch rate gradually increases up to the maximum value of 11 μm/min at 2 L/min. This increase is well correlated with the increase of hydrogen-atom density due to the longer residence time of hydrogen molecules in the plasma. On the other hand, when the gas flow rate is lower than 2 L/min, the etch rate abruptly decreases with decreasing gas flow rate in spite of the increase of hydrogen-atom density. From the surface observations and Raman measurements, it is found that the decrease in etch rate in the lower flow rate range is attributed to the formation of microcrystalline Si particles due to the decomposition of generated-SiH4 molecules in the plasma.

Experimental studies on simultaneous removal of CO2 and SO2 in a polypropylene hollow fiber membrane contactor

Membrane gas absorption technology is a promising alternative to conventional technologies for the mitigation of acid gases. In this study, simultaneous removal of SO2 and CO2 from coal-fired flue gas was carried out in a polypropylene hollow fiber membrane contactor using aqueous monoethanolamine as the absorbent. The influences of liquid and gas flow rates on the simultaneous absorption performance of CO2 and SO2 were investigated. The experimental results indicated that the membrane contactor could eliminate these two sour gases simultaneously and effectively. Absorption of SO2 and CO2 was enhanced by the increase in liquid flow rate and decrease in gas flow rate. It was observed that a small amount of SO2 in the flue gas had a slight influence on the absorption of CO2. In addition, the membrane contactor was operated continuously for two weeks to evaluate its duration performance. The results showed that the CO2 mass transfer rate was decreased significantly with the operating time due to partial wetting of membrane pores. After 14days of continuous operation, the CO2 mass transfer rate of the wetted membrane contactor was decreased by 41% but could be retrieved to 86% of the fresh one by increasing the gas phase pressure.

Numerical simulation using CFD software of countercurrent gas–liquid flow in a PWR hot leg under reflux condensation

In order to improve the countercurrent flow model of a transient analysis code, countercurrent air–water tests were previously conducted using a 1/15 scale model of the PWR hot leg and numerical simulations of the tests were carried out using the two-fluid model implemented in the CFD software FLUENT 6.3.26. The predicted flow patterns and CCFL characteristics agreed well with the experimental data. However, the validation of the interfacial drag correlation used in the two-fluid model was still insufficient, especially regarding the applicability to actual PWR conditions. In this study, we measured water levels and wave heights in the 1/15 scale setup to understand the characteristics of the interfacial drag, and we considered a relationship between the wave height and the interfacial drag coefficient. Numerical simulations to examine the effects of cell size and interfacial drag correlations on numerical predictions were conducted under PWR plant conditions. Wave heights strongly related with the water level and interfacial drag coefficient, which indicates that the interfacial drag force mainly consists of form drag. The cell size affected the gas velocity at the onset of flooding in the process of increasing gas flow rate. The gas volumetric fluxes at CCFL predicted using fine cells were higher than those using normal cells. On the other hand, the cell size did not have a significant influence on the process of decreasing gas flow rate. The predictions for the PWR condition using a reference set of interfacial drag correlations agreed well with the Upper Plenum Test Facility data of the PWR scale experiment in the region of medium gas volumetric fluxes. The reference interfacial drag correlations employed in this study can be applied to the PWR conditions.

Formation mechanism and size prediction of bubble in opposite-flowing T-shaped microchannel

Bubble formation in an opposite-flowing T-shaped microchannel with 40 μm in depth and 120 μm in width was real-time visualized and investigated experimentally by means of a high speed camera. N2 bubbles were generated in glycerol-water mixtures with different concentrations of surfactant sodium dodecyl sulfate (SDS). And the images were captured by the high speed camera linked to a computer. Results indicated that the bubble formation process can be distinguished into three consecutive stages, i.e., expansion, collapse and pinching off. The bubble size decreases with the increase of liquid flow rate and viscosity of liquid phase as well as the decrease of gas flow rate. The surface tension of the liquid phase has no measurable influence on the bubble size. Moreover, a new approach to predicting the size of bubbles formed in the T-shaped microchannel is proposed. And the predicted values agree well with the experimental data.

Enhanced toluene removal using granular activated carbon and a yeast strain Candida tropicalis in bubble-column bioreactors

The yeast strain Candida tropicalis was used for the biodegradation of gaseous toluene. Toluene was effectively treated by a liquid culture of C. tropicalis in a bubble-column bioreactor, and the toluene removal efficiency increased with decreasing gas flow rate. However, toluene mass transfer from the gas-to-liquid phase was a major limitation for the uptake of toluene by C. tropicalis. The toluene removal efficiency was enhanced when granular activated carbon (GAC) was added as a fluidized material. The GAC fluidized bioreactor demonstrated toluene removal efficiencies ranging from 50 to 82% when the inlet toluene loading was varied between 13.1 and 26.9 g/m 3/h. The yield value of C. tropicalis ranged from 0.11 to 0.21 g-biomass/g-toluene, which was substantially lower than yield values for bacteria reported in the literature. The maximum elimination capacity determined in the GAC fluidized bioreactor was 172 g/m 3/h at a toluene loading of 291 g/m 3/h. Transient loading experiments revealed that approximately 50% of the toluene introduced was initially adsorbed onto the GAC during an increased loading period, and then slowly desorbed and became available to the yeast culture. Hence, the fluidized GAC mediated in improving the gas-to-liquid mass transfer of toluene, resulting in a high toluene removal capacity. Consequently, the GAC bubble-column bioreactor using the culture of C. tropicalis can be successfully applied for the removal of gaseous toluene.

Visualization of Liquid Water Distribution in an Operational PEMFC by Soft X-Ray Radiography

Liquid water distribution in the operational Proton Exchange Membrane Fuel Cell (PEMFC) was visualized by soft X-ray radiography. To achieve a maximum sensitivity towards liquid water, low electron beam energy in soft X-ray range was chosen. As a result, the spatial resolution of 0.8 um and temporal resolution of 2 sec were obtained. In addition, in-plane and through-plane observation of operational cell was carried out and correlation between the accumulation of liquid water and the gas flow rate was observed. At high gas flow rate condition, liquid water accumulation was only observed in the interface between the catalyst layer and the gas diffusion layer (GDL). With decrease of gas flow rate, the liquid water content within the GDL increased and erupt of the liquid water to the channel was observed.

CO2 Removal from CO2−CH4 Gas Mixture Using Different Solvents and Hollow Fiber Membranes

Commercial hollow fiber membrane modules have been investigated for the selective removal of CO2 from the CO2−CH4 gas mixture. Experiments were carried out to evaluate the effectiveness of different membrane modules and different solvents for the removal of CO2 from a gas mixture containing 10% CO2 and 90% CH4. Three different membrane modules with fibers made from polypropylene, super phobic polypropylene, and silicone rubber were used in this study. The effects of gas and liquid flow rates, temperature, solvent type and concentration, and membrane module type were investigated. The experimental results indicated that CO2 removal increased with increasing the liquid flow rate, decreasing gas flow rate, and increasing solvent concentration. For physical absorption, the CO2 removal decreased with increasing temperature, whereas temperature had negligible effect using chemical absorption. The polypropylene module showed the best performance in terms of CO2 flux.

Study of gas cavity size hysteresis in a packed bed using DEM

When a high velocity gas jet is introduced into a packed bed a cavity is formed. The size of the cavity shows hysteresis on increasing and decreasing gas flow rates. This hysteresis leads to different cavity sizes at same gas flow rate depending on the bed history. The size of cavity affects the gas flow profiles in the packed bed. In this study the cavity size hysteresis phenomenon has been modeled using discrete element method along with turbulent gas flow. A reasonable agreement has been found between computed and experimental results on cavity size hysteresis. The effect of various parameters, such as nozzle height from the bed bottom and packing height, on the cavity size hysteresis has been studied. It is found that inter-particle interaction forces along with gas drag and bed porosity play an important role in describing the cavity size hysteresis. The injection of gas flow allows the particles to go to an unconstrained state than they were previously in, and their ability to remain in that state, even under decreased gas drag force, leads to the phenomenon of cavity size hysteresis.

Low refraction properties of F-doped SiOC:H thin films prepared by PECVD

Low refractive index materials which F-doped SiOC:H films were deposited on Si wafer and glass substrate by low temperature plasma enhanced chemical vapor deposition (PECVD) method as a function of rf powers, substrate temperatures, gas flow ratios (SiH4, CF4 and N2O). The refractive index of the F-doped SiOC:H film continuously decreased with increasing deposition temperature and rf power. As the N2O gas flow rate decreases, the refractive index of the deposited films decreased down to 1.378, reaching a minimum value at an rf power of 180 W and 100 °C without flowing N2O gas. The fluorine content of F-doped SiOC:H film increased from 1.9 at.% to 2.4 at.% as the rf power was increased from 60 W to 180 W, which is consistent with the decreasing trend of refractive index. The rms (root-mean-square) surface roughness significantly decreased to 0.6 nm with the optimized process condition without flowing N2O gas.

Selective desorption of carbon dioxide from sewage sludge for in situ methane enrichment—part I: Pilot‐plant experiments

In situ methane enrichment in anaerobic digestion of sewage sludge has been investigated by experiments and by modeling. In this first part, the experimental work on the desorption of carbon dioxide and methane from sewage sludge is reported. The bubble column, had a diameter of 0.3 m and a variable height up to 1.8 m. At operation the dispersion height in the column was between 1 and 1.3 m. Outdoor air was used. The column was placed close to a full-scale sewage sludge digester, at a municipal wastewater treatment plant. The digester was operated at mesophilic conditions with a hydraulic retention time of about 20 days. The bubble column was operated to steady-state, at which carbon dioxide concentration and alkalinity were determined on the liquid side, and the concentration of carbon dioxide and methane on the gas side. Thirty-eight experiments were performed at various liquid and gas flow rates. The experimental results show that the desorption rates achieved for carbon dioxide ranges from 0.07 to 0.25 m(3) CO(2)/m(3) sludge per day, which is comparable to the rate of generation by the anaerobic digestion. With increasing liquid flow rate and decreasing gas flow rate the amount of methane desorbed per amount of carbon dioxide desorbed increases. The lowest methane loss achieved is approximately 2% of the estimated methane production in the digestion process.

Effects of process parameters on the refractive index of F-doped SiOC:H films grown by PECVD

F-doped SiOC:H films were deposited on P-type (100) Si wafers by plasma enhanced chemical vapor deposition (PECVD) method using an SiH 4, CF 4 and N 2O gas mixture. The refractive index of the F-doped SiOC:H film continuously decreased with increasing deposition temperature and rf power. As the N 2O gas flow rate decreases, the refractive index of the deposited films decreased down to 1.3778, reaching a minimum value at an rf power of 180 W and 100 °C without flowing N 2O gas. The fluorine content of F-doped SiOC:H film increased from 1.9 at.% to 2.4 at.% as the rf power was increased from 60 W to 180 W, which is consistent with the decreasing trend of refractive index. Also, the maximum fluorine content of the deposited film was 5.5 at.% when it was deposited under the identical process condition exhibiting the lowest refractive index. The surface roughness (root-mean-square) significantly decreased to 0.6 nm with the optimized process condition without flowing N 2O gas.

Fractionation of Alfalfa into Leaves and Stems using a Three Pass Rotary Drum Dryer

Fractionation/separation of chopped alfalfa into leaf and stem components was investigated using a small industrial three-pass rotary drum dryer. Both drying and fractionation of chopped alfalfa were successfully achieved in a single operational process. Generally, fractionation is carried out only as a separate process and, therefore, more costly overall. The success of fractionation was determined by the total amount of the desired component (leaf or stem) recovered, relative to the amount entering the process (i.e. separation efficiency) and by the purity (leaf or stem proportion) of the component in a product stream collected at the cyclone or drum exit. A series of experiments under different operating conditions were performed to establish the optimum process conditions leading to maximum separation efficiency and leaf purity. The operating variables used during experimentation were the outer drum end gate opening area, material feed rate, gas flow rate and gas temperatures at the drum inlet. The experimental results indicated that leaf purity decreased and separation efficiency increased with a decrease in dryer drum end gate opening area. A decrease in separation efficiency was observed with an increase in material feed rate. On the other hand, leaf purity increased and separation efficiency decreased with a decrease in gas flow rate. The decrease in separation efficiency was most significant at low gas flow rates. Separation efficiency decreased with a decrease in gas inlet temperature. Stem moisture content was observed to be consistently higher than the leaves. Lowering stem moisture levels to an acceptable level of 8% moisture content results in over drying of the leaves, leading to a loss in product quality. It was determined that the relative difference in moisture levels of stems and leaves at the drum and cyclone exit, respectively, can be averted by re-circulating in the order of 20% of the wet material (stems) from the drum exit back through the dryer inlet.

The Influence of Periodic Operation on the Effective Radial Thermal Conductivity in Trickle Bed Reactors

AbstractThe effect of a periodic liquid feed on the effective radial heat conductivity, Λr, in a trickle bed was investigated. The mean Λr under periodic operation was found to be well estimated by time‐weighed averaging of steady state values. The actual effect of periodic operation can therefore be predicted by the shape of the Λr, uL — curve in steady state operation. The effect of periodic operation is attenuated with decreasing gas flow rates and with decreasing liquid flow rate amplitudes in a bed of spherical particles. Practically no effect was observed with cylindrical particles.

Ammonia absorption from a bubble expanding at a submerged orifice into water

To investigate the mechanism of gas absorption from a bubble containing soluble and insoluble components, a gaseous mixture of ammonia and nitrogen was bubbled into water. The growth curve, volume, surface area and shape of the growing bubbles were measured with parameters such as inlet gas composition, gas flow rate and gas chamber volume. The bubble volume decreased with the increasing composition of ammonia in a bubble, decreasing gas chamber volume and decreasing gas flow rate. To reasonably express the mass transfer from the bulk of a gas in a bubble to the bulk of a liquid, the overall mass transfer resistance was evaluated by the mass transfers in the gas phase, interface and liquid phase. The non-spherical bubble formation model combined with the overall mass transfer resistance estimated well experimental bubble shape, bubble volume at its detachment from an orifice, growth rate and mass transfer rate. Moreover, the change of concentration with bubble growth time and the fractional absorption during bubble formation were simulated.