Physical water treatment using RF electric fields for the mitigation of CaCO3 fouling in cooling water

The present study investigated the feasibility of RF electrical fields in mitigating CaCO₃ fouling in cooling water. Three different voltages and two frequencies were used for the RF electric fields produced directly in water with varying cold-side velocity. Artificial hard water was used. Fouling resistances for the PWT-treated cases decreased by 34–88% from the values for no-treatment cases, depending on the cold-side flow velocity. The results showed blunt crystal structures for the PWT-treated cases, while no-treatment cases had sharp and pointed crystal structures. The new PWT method using RF electric fields presents a valid tool to mitigate CaCO₃ fouling in cooling water.     

Calcium Sulfate Fouling-Precipitation or Particulate: A Proposed Composite Model

Though it is of great importance, the majority of predictive models tend not to incorporate water chemistry in their formulations. The ionic diffusion model which was developed for CaCO₃, is based purely on crystallization, and is one of the few models that incorporates water chemistry. This model does not provide satisfactory predictions for CaSO₄ fouling. In this article, a new model is proposed for CaSO₄ fouling which takes into account the effect of both crystallization and particulate fouling and is capable of predicting the fouling resistance during the cleaning cycle as well as the fouling cycle. A removal term is incorporated into the model, as the occurrence of particulate fouling for CaSO₄ tends to weaken its crystalline structure and makes it more prone than CaCO₃ to removal. Properties of the electrolyte were evaluated using MINTEQA2 computer code, which is approved by the U.S. Environmental Protection Agency. In this model, particulate fouling is estimated using the physical mechanism for particle transport and adherence, crystallization is estimated by ionic diffusion, and the removal term is approximated using hydrodynamics of flow and deposit properties. The inclusion of both crystallization and removal terms incorporates the effects of both water chemistry and hydrodynamics of the flow and provides a relationship which not only can predict fouling but also can predict dissolution, by change of water quality and/or stopping the operation, or removal by shear stress. The proposed model was assessed using published experimental data. The results indicate that this model provides good predictions: the slope of predicted rates as a function of the experimental rates is 1.05. The experimental results, though limited in number, suggest that crystallization is not the main or only mechanism contributing to CaSO₄ fouling. Particulate fouling seems to be a major contributor. Further experimentation is in process to confirm the degree of particulate fouling and to substantiate or to modify the model accordingly.     

Impact of Tube Surface Properties on Crystallization Fouling in Falling Film Evaporators for Seawater Desalination

ABSTRACT

Crystallization fouling on heat transfer surfaces is a severe problem and a complex phenomenon in multiple-effect distillation plants with horizontal tube falling film evaporators for seawater desalination. The choice of tube material affects the wettability, the adhesion forces between surface and deposit, and the induction time of crystallization fouling. The effects of surface properties on crystallization fouling from seawater have been investigated in a horizontal tube falling film evaporator in pilot plant scale. Experiments were performed with artificial seawater and various tube materials. The tube surfaces were characterized by measuring surface roughness and contact angles and by determining surface free energies. The tube materials show qualitative and quantitative differences with respect to scale formation. The interfacial defect model was applied to the system. Spreading coefficients of CaCO₃ scale on the aluminum alloys 5052 and 6060 and stainless steel grade 1.4565 were calculated to be higher than those on copper–nickel 90/10 and aluminum brass, but the quantities of CaCO₃ scale measured on the tube surfaces were much lower compared to CuNi 90/10 and aluminum brass. The application of advanced approaches such as the interfacial defect model depends on the precise knowledge of interfacial free energies, which are very difficult to find. However, results suggest that more similar values of the interfacial free energies of heat transfer surface and deposit lead to increased scale formation.     

DFT investigation of high temperature water gas shift reaction on chromium–iron mixed oxide catalyst

As part of high temperature water gas shift reaction mechanism, CO adsorption and H₂O adsorption on Fe₃O₄ (111) and chromium atom substituted Fe₃O₄ (111) slab surfaces are investigated by means of periodic DFT approach using VASP. Fe₃O₄ bulk structure has been computed including the Hubbard (U) parameter. One oxygen site (Ooct1) is studied as a probable site among the six Fe₃O₄ (111) terminations. Cr atom substitution on this surface is also examined. Cr atoms prefer being on the surface rather than in the bulk structure and Cr atoms substitute on the octahedral iron atom layer (Ooct2Cr). Adsorption energies of CO on Ooct1 and Ooct2Cr are found as −96 kcal/mol and −47 kcal/mol. Water adsorption on Ooct1 surface is molecular with −54.88 kcal/mol adsorption energy. On the other hand, water adsorption on Ooct2Cr surface is dissociative with nearly same adsorption energy, −55.12 kcal/mol, indicating the catalytic effect of chromium atom.     

The mechanism of the water dissociation and dehydrogenation of glycerol on Au (111) and PdAu alloy catalyst surfaces

Hydrogen is an energy carrier found from renewable sources such as biomass, geothermal, solar, or wind. Water splitting and dehydrogenation of glycerol is a sustainable process of H₂ production from renewables because water is abundant, and the glycerol is formed from the biomass-derived compounds. However, finding a suitable and best catalyst for these processes is challenging. Thus, this paper proposed a theoretical study to find the mechanism of the dissociation of water and dehydrogenation of glycerol using Au metal and PdAu alloy catalysts using the density functional theory (DFT) method. Four PdAu alloys have been constructed with different atomic compositions ranging from 1 to 3 of Pd metal to Au metal. The result showed strong adsorption on the Pd₁Au₃ catalyst surface, and the water splitting is best on the Pd₃Au₁ catalyst surface. Simultaneously, the glycerol adsorption on catalyst surfaces is tested before proceeding for the complete dehydrogenation mechanism of glycerol. Strong adsorption was found at the Pd₁Au₃ catalyst compared to other catalyst surfaces on the glycerol adsorption. The dehydrogenation mechanism was found toward a downhill reaction and removed eight hydrogens from the glycerol compared to Au metal, referring to easy dehydrogenation of glycerol using the alloy PdAu. The final species that adsorbed on the Pd₁Au₃ surface is the carbon monoxide will be turned later into carbon dioxide.     

Influence of Scaling on the Performance of Shell-and-Tube Heat Exchangers

Fouling in shell-and-tube heat exchangers was modeled by combining Hasson's ionic diffusion model for scaling from CaCO₃ solutions with a model for predicting the temperature distribution developed by Gaddis and Schlünder. Using the computed results, clean heat exchanger design rules were tested for fouling conditions. The effects of fouling on the efficiency of heat exchanger configurations were determined.     

Study on oil fouling in a double pipe heat exchanger with mitigation by a surfactant

Fouling of oils on heat exchanger surfaces and pipelines is a common problem in a variety of industrial applications. This is because the oil deposits on the heat transfer surface causes an increase in pressure drop and a decrease in heat exchanger efficiency. In the current work, oil fouling in double pipe heat exchanger was investigated and mitigated using a surface‐active agent for the flow of a dispersion fluid containing different dispersed oil fractions in water. The effect of the dispersed oil fraction (5%vol and 10%vol) and temperature (35°C‐55°C) on the oil fouling rate was studied and discussed under turbulent flow conditions for both hot and cold fluids. Different amounts of alkylbenzene sulfonate as a surfactant were added to reduce the fouling rate under turbulent flow. It was found that the fouling thermal resistance (R_f) increases when the fluid temperature decreases. The higher the dispersed oil fraction, the higher the R_f for all temperatures due to higher oil deposition. Addition of 0.2%vol to 0.5%vol of alkylbenzene sulfonate caused an appreciable reduction in R_f depending on oil fraction and Reynolds number. The mitigation percent was higher for a lower Reynolds number, reaching up to 96%.     

Steam reforming of hexadecane over a Rh/CeO2 catalyst in microchannels: Experimental and numerical investigation

Hexadecane is chosen as diesel surrogate to experimentally and numerically investigate diesel reforming in microchannels coated with Rh/CeO₂. A detailed kinetic model is presented and discussed using experimental data on steam reforming of not only hexadecane but also of methane and propane providing a more detailed understanding also of conversion of hexadecane fragments. The turnover frequencies of these linear alkanes were found to be inversely proportional to the number of carbon atoms per hydrocarbon molecule. Based on these results, a kinetic model was developed that links a global reaction equation for the dissociative adsorption of long-chain hydrocarbons with an elementary surface reaction mechanism of steam reforming of methane over Rh/Al₂O₃ catalysts. The model adequately describes the observed correlation between turnover frequency and the number of carbon atoms the hydrocarbon contains. Furthermore, a significant impact of the ceria support on the reformate composition was observed.     

New insights into the interfacial interactions of O2 and H2O molecules with PuH2 (110) and (111) surfaces from first-principles calculations

The interfacial reaction of highly active plutonium hydride in humid circumstance is of great interest in nuclear safe handling and storage, but it is poorly understood so far. In this paper, we first studied the O₂ adsorption on (110) and (111) surfaces of PuH₂ by first-principles DFT + U method. The results show that there are dissociative and non-dissociative adsorption of oxygen on the surfaces. We analyze the vibrational frequencies of non-dissociative oxygen adsorbed on the surfaces. It is found that the corresponding frequency of oxygen with bond length of 1.330–1.340 Å is 1094.8–1098.2 cm^?1. The corresponding frequency of oxygen with bond length of 1.448–1.500 Å is 726.3–905.2 cm^?1. It shows that non-dissociative oxygen could be considered as superoxide (O₂^?) or peroxide (O₂^2?) species. In order to expound the atomistic evolution process of oxidized surface exposed to moist air or corrosive solution, the interactions between H₂O molecules and the strongest oxygen adsorption structures were further explored. The results indicate that H₂O molecules could dissociate into OH groups and H atoms, then they were captured to create Pu–O and H–O bonds. This work could provide new insights into the adsorption morphology of oxygen on hydride surface and the interaction between oxide/hydride interface and water.     

A study of CaCO3 fouling with a microscopic imaging technique

The present study introduces a new experimental method to visualize the fouling process of CaCO₃. A mini-channel heat exchanger system with a microscopic imaging technique was developed for real-time visualization of the fouling process. The present study discussed how scale started initially, how scale formed thick layers, and how a small crystal grew into a large one, touching the adjacent one. Detail microscopic images of scale crystals and corresponding fouling resistances were obtained over the entire fouling process. The microscopic observation indicated that the fouling process could be divided into three stages: an induction period, a period of uniform generation of nuclei, and a period of uniform growth of scale. Sudden appearance of numerous small nuclei indicated the end of the induction period, a key event before the rapid increase in the fouling resistance. The present experimental method using microscopic images of the wet fouling process provides a valuable insight on the fouling mechanism.     

Hydrogen adsorption on palladium dimer decorated graphene: A bonding study

We report a density-functional theory study of dihydrogen adsorption on a graphene sheet functionalized with palladium dimers considering different adsorption sites on the carbon surface and both molecular and dissociative Pd₂H₂ coordination structures. Our results show that a (PdH)₂ ring without an H–H bond and not dissociative Pd₂(H₂) complexes are stable adsorbed systems with more elongated Pd−Pd and Pd–H bonds compared to the unsupported configurations caused by C–Pd interactions. In contrast, individual Pd atoms supported on graphene react with H₂ to form only a Pd(H₂) complex with a relaxed but not dissociated H–H bond. We also performed the Mulliken analysis to study the bonding mechanism during the adsorption process. In most cases, we found donor-acceptor C−Pd and Pd−H interactions in which C 2p, Pd 5s, and H 1s orbitals played an important role. We also found that the adsorption of a second Pd atom close to a PdH₂ system destabilizes the H−H bond. In this work we contribute to shed more light on the relation between Pd clustering and the possibility of hydrogen storage in graphene-based materials.     

Density functional theory study on adsorption of Pt nanoparticle on graphene

The mechanism of CO oxidation was catalyzed by Pt nanoparticles on graphene through first-principle density functional theory (DFT) calculations. The simulation results show that the lowest-energy Pt₇ nanoparticle carries slightly negative charges which enhance the O₂ binding energy compared to the corresponding graphene surfaces. We placed the Pt nanoparticle on different adsorption sites, and the Pt₇ nanoparticle was found to preferentially absorb on Bond (B) site. To gain insight into the high-catalytic activity of the Pt nanoparticles, the interaction between the adsorbate and substrate was also analyzed by detailed electronic analysis such as activation barrier, adsorption energy and Mulliken charge analysis.     

Atomistic study of LaNbO4; surface properties and hydrogen adsorption

We have calculated fundamental properties of pure and hydrogen-covered (010), (101), (100) and (001) surfaces of the low temperature monoclinic phase of LaNbO₄ (LN). The (010) surface was the most stable one, exhibiting electronic structure and local geometric configurations similar to bulk. As the first stage of proton migration into the electrolyte, the ability of LN surfaces to split H₂ molecules was probed indirectly by calculating the adsorption energy of H atoms on two of the LN surfaces. H adsorption on the (010) surface was found to be strongly endothermic, and thus cannot contribute much in splitting H₂. The adsorption energy on the relatively unstable (101) surface was on the other hand approximately −0.6 eV, in the right range for surface H₂ to be catalyzed beneficially. H adsorption on this surface was induced by surface states in the band gap of the clean surface. Since the unstable (101) surface is not abundant, the rate of dissociative adsorption of H₂ on the LN surface can be anticipated to be very low. Application of the energies to simple adsorption isotherm calculations for typical proton conducting fuel cells (PCFCs) operating temperatures correspondingly showed very low H coverage, and it is not expected that LaNbO₄ surfaces can contribute much to the H₂ activation reaction of a PCFC anode.     

Crystallization fouling of CaCO3 – Analysis of experimental thermal resistance and its uncertainty

Crystallization fouling occurs when dissolved salts precipitate from an aqueous solution. In the case of inversely soluble salts, like calcium carbonate (CaCO₃), this may lead to crystal growth on heated walls. Crystallization may also take place in the bulk solution either via homogeneous nucleation or heterogeneous nucleation on suspended material.In this paper, surface crystallization of CaCO₃ and crystallization in the bulk fluid and its effect on the fouling rate on a heated wall are studied. The fouling experiments are done in a laboratory scale set-up of a flat plate heat exchanger. Accuracy of the results is analyzed by uncertainty analysis. SEM and XRD are used to determine the morphology and the composition of the deposited material.The uncertainty analysis shows that the bias and precision uncertainties in the measured wall temperature are the largest source of uncertainty in the experiments. The total uncertainty in the fouling resistance in the studied case was found to be ±13.5% at the 95% confidence level, which is considered to be acceptable.Surface crystallization rate is found to be controlled by the wall temperature indicating that the surface integration dominates the fouling process. The flow velocity affects the fouling rate especially at high wall temperature by decreasing the fouling rate with increasing flow velocity. Crystallization to the bulk fluid is found to enhance significantly the fouling rate on the surface when compared to a case in which fouling is due to crystal growth on the surface.     

Effect of CaO hydration and carbonation on the hydrogen production from sorption enhanced water gas shift reaction

Sorption enhanced water gas shift reaction (SEWGS) based on calcium looping is an emerging technology for hydrogen production and CO₂ capture. SEWGS involves mainly two reactions, the catalytic WGS reaction and the bulk carbonation of CaO with CO₂, and the solid product is CaCO₃, and the Ca(OH)₂ may be formed from the reaction of CaO with H₂O with the presence of steam in gas phase. The effect of Ca(OH)₂ and CaCO₃ on the catalytic WGS reaction and carbonation reaction was studied in a fluidized bed reactor. It was found that the hydrated sorbent and CaCO₃ did not show any catalytic reactivity toward WGS reaction at 400 °C. When the temperature was increased to 500 °C and 600 °C, the catalytic reactivity of hydrated sorbent was recovered partially, but this will depend on the steam fraction in gas phase, the recovery of fresh CaO surface from dehydration of Ca(OH)₂ may be the reason of catalytic reactivity recovery. CaCO₃ can catalyze the WGS reaction at the high-temperature (>600 °C), this may due to the CaCO₃ decomposition and recarbonation processes in which the CaO is transiently formed. The possible mechanism was discussed.     

Experimental Investigation of Crystallization Fouling on Grooved Stainless Steel Surfaces During Convective Heat Transfer

The beneficial aspects of enhanced or extended heat transfer surfaces may be offset if operated under fouling conditions. In this article, preliminary experimental results for crystallization fouling of CaSO₄ solutions onto surfaces with different structures are reported. Flat stainless steel plates (50 mm × 59 mm) with “V”-shaped grooves on the side of fluid flow were used as heat transfer surfaces. Experiments were carried out under both clean and fouling conditions to discern how the same surface structures perform under such circumstances. In addition, the impact of both the direction of grooves with respect to fluid flow (crossed, longitudinal, and mixed flow grooves) and the groove dimensions has also been investigated. Fouling trends are discussed in terms of induction time and fouling rate. Significant differences have been found for the various flow conditions.     

Dissociative adsorption of hydrogen and methane molecules at high-angle grain boundaries of pipeline steel studied by density functional theory modeling

Pipelines provide an economic and efficient means for hydrogen transport, contributing to accelerated realization of a full-scale hydrogen economy. Dissociative adsorption of hydrogen molecules (H₂) occurring on pipe steels generates hydrogen atoms (H), potentially resulting in hydrogen embrittlement of the pipelines. This is particularly important for existing pipelines transporting hydrogen in blended form with methane (CH₄). In this work, a density functional theory model was developed to investigate the dissociative adsorption of H₂ and CH₄ at high-angle grain boundaries (HAGB), a typical type of hydrogen traps contained in steels, and the stable adsorption configurations. Results demonstrate that the dissociative adsorption of both H₂ and CH₄ at the HAGB is thermodynamically feasible under pipeline operating conditions. Compared with crystalline lattice sites, the HAGB possesses the most negative free energy change, a lower energy barrier and the lowest H-adsorption energy, making the HAGB, especially the quasi three-fold site, become the most stable site for hydrogen adsorption. The saturation coverage of hydrogen at HAGB is calculated to be 1.33. The iron-H bonds are formed at the HAGB by charge consumption at Fe atoms and electron accumulation at H atoms, following a so-called electron hybridization mechanism. The CH₄ adsorption at HAGB affects the H₂ adsorption. Without pre-adsorption of CH₄, the hydrogen adsorption at the HAGB is more stable. Although an elevated CH₄ partial pressure decreases the thermodynamic tendency for H₂ adsorption, it cannot hinder occurrence of the H₂ dissociative adsorption.     

Enhanced catalytic activities of Fe anchored on graphene substrates for water splitting and hydrogen evolution

Water splitting on single Fe atom catalyst anchored on defective graphene surfaces by using first-principles density functional theory. The structure and electronic features of isolated Fe atom anchored on three graphene surfaces with single vacancy (SV), double vacancy (DV) and Stone-Wales structure (SW) defect were systematically explored. The three structures prove to be high activity and high stability on catalytic. The adsorption and the energy barrier of water splitting as well as hydrogen adsorption free energy ΔG_H1 on single-atom Fe were also studied. The sequence of promoted splitting activity is found to be Fe@SW > Fe@DV > Fe@SV. Furthermore, by hydrogen adsorption free energy ΔG_H1 analysis, we predict that the HER catalytic activity of graphene nanosheet can be improved by anchoring Fe atom on SV and DV structures, which are comparable to or even better than noble metals. It is found that the catalytic activity of water splitting and HER can be changed with the shift in d-band center with respect to Fermi-level. Detailed investigations on electronic structure of Fe@graphene catalytic systems disclose an obvious orbital hybridization coupling and charge transfer between atom Fe on carbon surfaces and water molecule. These results provide us with new insight into design of high performer and low-cost catalysts and may inspire potential applications in the fields of clean and renewable energy.     

Effects of ash fouling on heat transfer during combustion of cattle biomass in a small‐scale boiler burner facility under unsteady transition conditions

Combustion of cattle biomass (CB) as a supplementary fuel has been proposed for reducing emissions of NO_x, Hg, SO₂, and nonrenewable CO₂ in large coal‐fired power plants; however, its high ash content resulted in fouling and slagging problems when the CB was co‐fired with coals during small‐ and pilot‐scale tests. Ash depositions during combustion of the CB as a reburn fuel were investigated using a 30 kW_t (100 000 Btu h^?1) boiler burner facility with water‐cooled heat exchangers (HEXs) under unsteady transition conditions and short‐term operations. Two parameters were used to characterize the effects of the ash deposition: (1) Overall heat transfer coefficient (U) and (2) Burnt fraction (BF). A methodology was presented and empirically demonstrated for the effects of ash deposition on heat transfer under unsteady transition conditions. Experiments involving ash deposition during reburning the CB with coals were compared with experiments involving only ash‐less natural gas. It was found that the growth of the ash layer during reburning the CB and coals lowered the heat transfer rate to water in the HEXs. In low‐temperature regions, the thin layer of the ash deposition promoted radiation effects, while the thick layer of the ash deposition promoted the thermal resistance of the ash layer. A chemical analysis of the heavy ash indicated that the BF increased when a larger fraction of the CB was used in the reburn fuels, indicating better performance compared with coal combustion alone. However, the results of ash fusion temperature indicated the ash deposited during combustion of the CB and coals was more difficult to remove than the ash deposited during coal combustion alone. Copyright © 2010 John Wiley & Sons, Ltd.     

Adsorbed water and CO on Pt electrode modified with Ru

Highly sensitive ATR-SEIRA spectroscopy was exploited to elucidate water, CO and electrolyte anions adsorbed on the Ru modified Pt film electrode. CO on Ru domains was oxidized below ca. +0.3 V, followed by pronounced water adsorption. Since the oxidation potential of CO on Pt domain was significantly reduced compared to bare Pt, these water molecules on Ru obviously prompt CO oxidation on adjacent Pt surface as consistent with the bifunctional mechanism. Diffusion of adsorbate from Ru to Pt surfaces was indicated in dilute CH₃OH solution by spectral changes with potential.