The effect of surface properties on CaSO4 scale formation during convective heat transfer and subcooled flow boiling

In this paper the effects of surface energy and surface roughness on the deposition of calcium sulphate during convective and subcooled flow boiling heat transfer to aqueous CaSO₄ solutions are studied. The surfaces of several test heaters have been treated by Ion Beam Implantation, Unbalanced Magnetron Sputtering, Mixed Sputtering and Plasma Arc Deposition to reduce surface energy. One heater was electropolished to reduce surface roughness and one heater was etched by an electrochemical method to increase surface roughness. Fouling runs with these heaters, and with an untreated surface as control, were carried out at different heat fluxes, flow velocities and salt concentrations. The results show that heat transfer surfaces with low surface energy experienced significantly reduced fouling, while electropolishing did not have a notable beneficial effect. The combined effect of reduced surface energy and flow velocity on fouling reduction is considerably stronger than previously reported for pool boiling.     

Water soluble copolymers for calcium carbonate and calcium sulphate scale control in cooling water systems

Mineral scales are formed in cooling water systems and they cause heat transfer problems. Much research has been carried out to reduce the carbonate and sulphate scales of calcium. To inhibit the scale formation in cooling water systems, vinyl acetate–acrylic acid (VA–AA) and vinyl acetate– methacrylic acid (VA–MAA) copolymers were synthesized, characterized, and the ability of the polymers to mitigate the calcium carbonate and calcium sulphate scale formation was tested through chemical screening, constant potential electrolysis, and electrochemical impedance techniques. XRD (X‐ray diffraction) and SEM (scanning electron microscope) studies were performed to understand the morphological changes of the scales in the presence of the polymers. The biocidal, gelation, and iron dispersion ability of the polymers were also noted. Among the two threshold inhibitors, VA–AA shows slightly better antiscaling properties even at higher temperatures and pH for both CaCO₃ and CaSO₄ scales compared to the methacrylic copolymer. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1451–1459, 2005     

Chemical descaling of acid dosed desalination plants

The heat transfer efficiency of sea water desalters is very often impaired by the deposition of scale of various compositions on the heat transfer surfaces. Four major classifications of such scales can be recognised—corrosion products such as iron and copper oxides, alkaline scale Ca(OH)₂ and MgO, additive scales consisting of calcium and magnesium combined with additive materials that are usually soft and easily removed, and calcium sulphate.Acid cleaning techniques have been developed and successfully applied to the problem of removing in the first 3 categories of scale and these are reviewed. However for calcium sulphate scale there is as yet no widely accepted chemical cleaning process and mechanical methods are usually adopted. These also are briefly considered and shown to be inadequate to deal effectively with every instance of calcium sulphate scaling. The need therefore for chemical descaling techniques able to adequately remove calcium sulphate is shown to be clear.A number of chelating agents which can dissolve calcium sulphate and therefore are potential candidates for the formulation of a suitable reagent have been reviewed and two such reagents taken to pilot scale trials. These trials included corrosion tests on a typical range of desalter construction materials and in later work used actual scale removed from operational plants.From the data obtained an economic assessment of the use of these reagents for the descaling of a typical desalter heat input vessel was made, from which it was concluded that the once through application of either reagent could only be considered feasible for limited treatment. To overcome this economic problem, processes for the regeneration of the spent reagents have been developed to the pilot plant stage. From this data one reagent has been chosen and following continuing development, a design for a mobile chemical descaling unit has been evolved. This unit, which also has the capability of carrying out a wide range of descaling operations is described.     

Calcium carbonate inhibition by a phosphonate-terminated poly(maleic-co-sulfonate) polymeric inhibitor

The precipitation of calcium carbonate scale on heat transfer surfaces widely occurs in numerous industrial processes. For the control of calcium carbonate scale and in response to environmental guidelines, the new low phosphonic copolymer was prepared through reaction of maleic anhydride with sodium p-styrene sulfonate in water with redox system of hypophosphorous and hydrogen peroxide as initiator. The anti-scale property of the low phosphonic copolymer towards CaCO₃ in the artificial cooling water was studied through static scale inhibition tests, and the effect on formation of CaCO₃ was investigated with combination of scanning electronic microscopy (SEM), X-ray powder diffraction (XRD) analysis and Fourier transform infrared spectrometer, respectively. The results showed that the low phosphonic copolymer was excellent calcium carbonate scale inhibitor in artificial cooling water. The crystallization of CaCO₃ in the absence of inhibitor was rhombohedral calcite crystal, whereas a mixture of calcite with vaterite crystals was found in the presence of the low phosphonic copolymer. For actions of carboxyl and phosphonic acid groups, the calcite was inhibited and the metastable vaterite was stabilized in the presence of the low phosphonic copolymer during the CaCO₃ formation process.     

Ionic mass transfer by free convection with simultaneous heat transfer

Rates of electrochemical mass transfer by free convection under the influence of simultaneous thermal free convection have been measured for upward facing horizontal disc electrodes. The electrode reaction was the cathodic deposition of copper from cupric sulphate solutions containing H₂SO₄ as swamping electrolyte. Results have been correlated by the equations.

and

where GR_m is a combined Grashof number for concentration and thermal buoyancy effects.

.Mass transfer has also been measured for the novel situation of nucleate boiling at the electrode surface and diffusion layer thicknesses as small as 1·7 × 10^?3mm have been obtained.     

Improvement of the electrochemical performance of an NCA positive-electrode material of lithium ion battery by forming an Al-rich surface layer

A positive-electrode material of LiNi_0.80Co_0.15Al_0.05O₂ (NCA) having an Al-rich surface layer was synthesized, and its characteristics and electrochemical performance were investigated and compared with those of pristine NCA prepared by a conventional solid-state reaction. The Al-rich surface layer with its compositional gradient was formed by a wet-coating method using a fluidized bed coating technique and a subsequent heat treatment: fluidized NCA powder and a mist of 2-propanol solution containing an aluminium oxide sol were mixed in a chamber, followed by heat treatment at 600 °C in an oxygen atmosphere. For the electrochemical measurement, the positive electrode fabricated using the positive-electrode material was assembled into a 2032-type coin cell with lithium metal as an anode and LiClO₄ in ethylene carbonate-diethyl carbonate as an electrolyte. The measurement revealed that the Al-rich surface layer effectively contributed to the improvement of charge-discharge cycle life performance; NCA with the Al-rich surface layer retained 96% of the initial capacity after 100 cycles, compared to the 90% retained by pristine NCA. For a better understanding of the improvement, the structure of the positive-electrode materials after 100 cycles was examined by scanning transmission electron microscopy (STEM) and scanning electron microscopy (SEM). High-angle annular dark-field signal images obtained from STEM showed no clear difference between these materials in terms of the growth of inactive rock-salt-like structure layer near the surface of the primary particles. On the other hand, low-magnification SEM observation revealed that the pristine NCA had widened microcracks among the primary particles in the secondary particles, while the NCA with the Al-rich surface layer did not. Therefore, the Al-rich surface layer improved the fracture strength of the secondary particles during the charge-discharge cycling, thus preventing the formation of isolated primary particles, which do not contribute to the charge-discharge behaviour.     

Mathematical analysis of nonisothermal mass transfer process-application to oxidation of carbon with CO2-CO mixtures

A mathematical analysis is made of the nonisothermal mass transfer occurring in reactions of gases with porous media involving appreciable enthalpy changes. The mathematical analysis of a nonisothermal reaction controlled by heat and mass transfer and by chemical reaction involves simultaneous solution of two second-order differential equations derived from the laws of conservation of mass and energy. For the purpose of demonstrating the mathematical solution, the oxidation of porous carbon in CO₂-CO mixtures is considered, on the assumption that no structural changes occur during oxidation, as would be the case in the early stages of oxidation. A similar mathematical treatment can be made for reactions of gases with other porous media. Although the thermal conductivity of coke is relatively low, it is found that the oxidation in CO₂ occurs under essentially isothermal conditions and the heat transfer to the sample is by radiation from the surface of the reaction tube. The results of the mathematical analysis were tested experimentally. During the early stages of burning in 1 atm CO₂ at 1200°C, the temperature at the center of a 22 mm dia. cylinder was found to be about 7°C below that at the surface, which was a few degrees below the furnace temperature. These findings are in accord with the results of the mathematical analysis.     

Experimental investigation of flow resistance and convection heat transfer of CO2 at supercritical pressures in a vertical porous tube

Experiments were conducted to measure the convection heat transfer in a vertical porous tube with particle diameters of 0.2–0.28 mm at supercritical pressures. The local heat transfer coefficients, fluid bulk temperatures and wall temperatures were measured to investigate the influence of the inlet fluid temperature, pressure, heat flux and flow direction on the convection heat transfer in the porous tube. The measured friction factors in a heated tube are much larger than those predicted using the Aerov-Tojec correlation for both upward and downward flows. Therefore, two new correlations are presented for upward and for downward flows to predict the friction factors of supercritical pressure CO₂ in heated porous tubes. The experimental results also show that the inlet temperature, pressure and heat flux all significantly influence the convection heat transfer. When the inlet temperature (T₀) is higher than the pseudocritical temperature (T_pc), the local heat transfer coefficients are much less than those when the inlet temperature is lower than the pseudocritical temperature. The convection heat transfer coefficients are found to vary nonlinearly with heat flux. For T₀ < T_pc, the local heat transfer coefficients along the porous tube have a maximum for upward flow and have a peak value for downward flow when the local fluid bulk temperatures are near T_pc and the wall temperatures are slightly higher than T_pc. The different variations of the local heat transfer coefficients along the porous tube for upward and downward flows are attributed to the effect of buoyancy. However, when the wall temperatures are much higher than T_pc, the local heat transfer coefficients along the porous tube decrease continuously for both upward and downward flows.     

Characteristics of the surface heat transfer coefficient for Al2O3 ceramic in water quench

In this paper, we determined the surface heat transfer coefficient of Al₂O₃ ceramics quenched from different initial temperatures into a water bath at room temperature. By using the multipoint temperature measurement technique and the inverse heat conduction method, this coefficient was measured as function of surface temperature of the ceramics during the water quench. The obtained results indicate that the surface heat transfer coefficient largely depends not only on the initial quenching temperature and their evolution in quenching media but also on the sizes of tested specimens. In addition, brief discussion was completed on the rationality of the traditionally used approach, which considers the surface heat transfer coefficient as a joint constant of materials and quenching media, in previous studies on heat transfer and thermal stresses.     

Heat treatment of 6H-SiC under different gaseous environments

Silicon carbide is a useful material for the reactors in chemical processes. In recent years, microreactors have gained significant attentions due to the high demand for process miniaturization. As heat and mass-transfer are highly improved inside the gas flow channels in microreactors, any change on the surface of inner channels under heating becomes critical to the performance of microreactors. To investigate the surface changes of silicon carbide during the heat treatment, 6H-SiC coupons were processed in five different gases—Ar, N₂, air, 0.9% O₂ in Ar and 50% H₂O in air—that are commonly encountered in high temperature chemical processes. While the formation of oxide film was found to be dependent on the partial pressure of oxidizing gas, surface decomposition was found from the treatment in nitrogen environment. Characterization of the SiC surface by Raman spectroscopy and SEM–EDX revealed that a graphitic layer has formed at the oxide film/SiC interface. Crystallinity of graphitic layer at the interface seemed to be dependent on the partial pressure of oxidizing gas, which was revealed by the relationship between G peak position and R(I_D/I_G). The intensity ratio of FTO(0)/FTO(2/6) bands showed that stacking faults on the surface of SiC coupons were reduced after heat treatment.     

The cathode process in sodium chloride melts containing sulphate

The effect of addition of sulphate to sodium chloride and sodium fluoride melts was investigated by chronopotentiometry in the temperature range 820-1000 °C. It was found that the numerical value of the chronopotentiometric term jτ^1/2/c⁰ increased with increasing cathodic current density. This behaviour was explained by chemical decomposition of sodium sulphate into SO₃ and Na₂O, followed by electrochemical reduction of SO₃. The decomposition of sodium sulphate is enhanced with increasing temperature, resulting in an increase in the chronopotentiometric transition time. Numerical simulation of this electrode process supports this explanation of the electrode process. It is not a classical so-called CE mechanism when a chemical reaction precedes the electrochemical reduction as described in the literature, because the formation of SO₃ is suppressed by the presence of Na₂O in the melt.     

On the theory of the decomposition of a metastable gas hydrate

Methane hydrate decomposition at atmospheric pressure in the overheated state relative to the equilibrium temperature (T _S = 193 K) at positive (T ₀ > 273 K) and negative (T ₀ < 273 K) temperatures is discussed with reference to available experimental data. Two temperature ranges (193 K < T ₀ < 240 K and 240 K ≤ T ₀ < 273 K) arte distinguished at negative temperatures, and one temperature range (T ₀ > 273 K) at positive temperatures. For the lower range of negative temperatures, it is accepted in the construction of the theoretical model that the major factors determining the intensity of gas hydrate decomposition into ice and gas are Arrhenius-type kinetics and conductive heat transfer. Two schemes, namely, frontal and bulk ones are considered. For the upper range of negative temperatures, where an anomalous preservation effect is observed in experiments, it is assumed in the theoretical model that the release of the gas from the hydrate is controlled by the diffusion mechanism of gas transport through the solid phase or through the surface ice crust. For the positive temperatures, it is accepted that the decomposition rate is determined by the heat flux through the draining water film that has resulted from hydrate decomposition. Calculations have been carried out for different initial and boundary temperatures, and the results of the calculations have been analyzed and have been compared to available experimental data.     

Mathematical modelling of mixed salt precipitation during convective heat transfer and sub-cooled flow boiling

The main purpose of this investigation was to study the mechanisms of mixed salt crystallisation fouling on heat transfer surfaces during convective heat transfer and sub-cooled flow boiling conditions. In the present investigation, the effects of various operating parameters such as solution composition and hydrodynamics of the system, on crystallisation fouling of mixtures of calcium sulphate and calcium carbonate have been studied experimentally. After clarification of the effects of operating parameters on the deposition process, the results of the experiments were used to develop a mechanistic model for prediction of fouling resistances, caused by crystallisation of mixed salts, under convective heat transfer and sub-cooled flow boiling conditions. Model predictions were compared with the measured experimental data when calcium sulphate and calcium carbonate form deposits on the heat transfer surface simultaneously. Deviations ranging from 6% to 25% were observed which confirm the suitability of the model.     

Fiber-modified scaling in heat transfer fouling mitigation

The study of heat exchanger fouling using supersaturated calcium sulphate solutions has been widely reported. In this study fouling was investigated in a larger-scale heat exchange apparatus using stainless-steel pipe, and data were obtained at different flow rates, concentrations and temperature differences. The deposits were examined using a scanning electron microscope, X-ray diffraction and conventional photography. In a novel approach, wood pulp fibers were added to the fouling solution at various concentrations to mitigate fouling. Heat transfer enhancement above the solution-alone was observed initially and the onset of fouling delayed. When fouling eventually developed the final asymptotic level was lower than the fiber-free case for the experimental conditions specified. At a fiber concentration of 0.15% heat transfer augmentation occurred for 11 days. However, at 0.25% fiber concentration, heat transfer augmentation (no fouling) was sustained over the experimental duration of 45 days. It can be concluded that the service-life cycle of a heat exchanger can be prolonged with the addition of asymmetric, flexible, natural fibers. In this work it is argued that fibers modify the onset of deposition by boundary layer scavenging, and interact with turbulent eddies to reduce the rate of mass transfer of the foulant to the heated surface. When scale forms, the crystalline structure of the scale is interrupted by the fibers, which appear to roughen the heat transfer surface initially and increase the heat transfer coefficient. However, the scale deposit continues to build up very slowly, causing the thermal resistance to eventually override the turbulence augmented heat transfer effect of the fibers.     

FIBER-MODIFIED SCALING IN HEAT TRANSFER FOULING MITIGATION

The study of heat exchanger fouling using supersaturated calcium sulphate solutions has been widely reported. In this study fouling was investigated in a larger-scale heat exchange apparatus using stainless-steel pipe, and data were obtained at different flow rates, concentrations and temperature differences. The deposits were examined using a scanning electron microscope, X-ray diffraction and conventional photography. In a novel approach, wood pulp fibers were added to the fouling solution at various concentrations to mitigate fouling. Heat transfer enhancement above the solution-alone was observed initially and the onset of fouling delayed. When fouling eventually developed the final asymptotic level was lower than the fiber-free case for the experimental conditions specified. At a fiber concentration of 0.15% heat transfer augmentation occurred for 11 days. However, at 0.25% fiber concentration, heat transfer augmentation (no fouling) was sustained over the experimental duration of 45 days. It can be concluded that the service-life cycle of a heat exchanger can be prolonged with the addition of asymmetric, flexible, natural fibers. In this work it is argued that fibers modify the onset of deposition by boundary layer scavenging, and interact with turbulent eddies to reduce the rate of mass transfer of the foulant to the heated surface. When scale forms, the crystalline structure of the scale is interrupted by the fibers, which appear to roughen the heat transfer surface initially and increase the heat transfer coefficient. However, the scale deposit continues to build up very slowly, causing the thermal resistance to eventually override the turbulence augmented heat transfer effect of the fibers.     

Early Events in the Precipitation Fouling of Calcium Sulphate Dihydrate under Sensible Heating Conditions

Supersaturated aqueous solutions of calcium sulphate, an inverse solubility salt, were circulated through a 9 mm i.d. stainless steel tube in which they were subjected to a constant and uniform heat flux at Reynolds numbers ranging from 2100 to 36 000. Precipitation fouling of the tube surface, which was monitored and measured thermally, resulted in calcium sulphate dihydrate (gypsum) scales. Measured delay times decreased with increasing bulk solute concentration, C_b, in accord with classical nucleation kinetics, decreased with increasing surface temperature T_s, its reciprocal correlating with T_s in the Arrhenius manner, and decreased with increasing fluid velocity V up to V &#8776; 0.5 m/s before flattening out. Initial linear fouling rates,R_fo, at C_b = 3400 ppm, also yielded Arrhenius plots, with fouling activation energies increasing almost eight‐fold over the 0.1–1.6 m/s velocity range. This result implies a shift in the mechanism of fouling rate control with increasing velocity from mass transfer, which is weakly temperature dependent, to surface integration, which is strongly temperature dependent. The “Initial Fouling Rate Model,” which simulates this mechanistic shift with a unique dependence of the surface‐integration step on the residence time of the fluid at the tube wall, was used to represent, by least‐squares fitting, 84 fouling rate data points at different values of V and T_s, with three adjustable parameters. Observed trends of R_fo with V and clean wall T_s were qualitatively captured by the model, but quantitative agreement was imperfect. Substantial quantitative improvement was effected by introducing into the model an additional temperature dependence of the surface‐integration term—resulting in one additional adjustable parameter—rationalized as a method of accounting for the previously neglected temperature‐dependent nucleation that accompanies crystal growth. The fouling investigation was supplemented by an experimental study of crystal growth kinetics.     

池式沸腾条件下CaSO4污垢生成的实验研究

在池沸腾装置中,100℃下CaSO4饱和溶液以及热通量55—300 kW/m2,研究了传热表面性质对硫酸钙污垢沉积的影响。通过现代表面处理技术,例如动态混合离子注入和动态混合磁控溅射技术,对若干加热器表面进行处理,以减小金属表面的表面能。研究表明,表面能低的表面,即经离子注入或磁控溅射的表面具有良好的抗垢性能。     

Spatial Scale Effects on Rayleigh Convection and Interfacial Mass Transfer Characteristics in CO2 Absorption

Concentration gradient‐induced Rayleigh convections in the CO₂ absorption process were investigated by the hybrid Lattice‐Boltzmann/finite‐difference method (LBM‐FDM). The spatial scale effects on Rayleigh convection were studied by simulating Rayleigh convections with different liquid layers. The scale of convection increased with the liquid layer height but the average mass transfer coefficient showed the adverse tendency. The Rayleigh convection had a pronounced effect on the surface renewal. For better assessment of the renewal intensity, two statistical quantities were proposed. The transient variations of these quantities provided a good following feature with the mass transfer coefficient which confirms their accuracy and precision in characterizing the mass transfer process.     

Composite fibers of La0.5Sr0.5CoO3-δ-LaSrCoO4±δ with high catalytic activity toward oxygen reduction reaction

To promote catalytic activity toward oxygen-reduction reaction in the intermediate temperature solid-oxide fuel cells, we prepared a dual-phase composite cathode material 75 mol%La0.5Sr0.5CoO_3-δ-LaSrCoO_4±δ with a fibrous structure via one-pot electrospinning of a polymer containing solution. We then investigated the micro-structure and electrochemical performance of this fibrous composite. The results confirm that the fibrous composite is composed of intimately mixed nano-crystalline grains and that the compositional phases are compatible with each other. Compared with the corresponding single-phase fibers, the nano-crystalline size of the composite is more stable, and the fine nano-crystalline grains are more resistant to growth and coarsening, indicating the mutual dispersion and suppression between the constituent phases. Furthermore, compared with the corresponding single-phase fibers, the electrochemical performance of this fibrous composite cathode is more favorable. At 800 °C, its area specific resistance (ASR) is as low as 0.03 Ω cm², and maximum output power density is as high as 960 mW cm⁻², which is achieved from an electrolyte-supported single cell that was developed using this cathode. After aging this cathode for a long time, ASR is less worsened and the output power density decreases only slightly, indicating that the prolonged electrochemical performance in the running cells is more stable.     

Electrochemical regeneration of ceric sulphate in an undivided cell

Ceric sulphate (0–0.5 m) was generated electrochemically from cerous sulphate slurries (0.5–0.8 m total cerium) in 1.61 m sulphuric acid, at 50 °C, using a bench scale differential area undivided electrochemical cell with an anode to cathode ratio of eleven. A cell current efficiency for Ce(IV) of 90% was obtained at an anode current density of 0.25 A cm^–2. An empirical model illustrates an increase in overall current efficiency for Ce(IV) with an increase in electrolyte velocity, an increase in total cerium concentration, and a decrease in the cell current. From separate kinetic studies on rotating electrodes, both, anode and cathode kinetics were found to be affected by cerium sulphate adsorption processes. Anode adsorption of cerous sulphate species leads to inhibited mass transfer and negatively affected current efficiencies for Ce(IV). Cathode adsorption of cerium sulphate is thought to be responsible for high cathode current efficiencies for hydrogen (93–100%). The dissolved cerous sulphate concentration increased with increasing ceric sulphate and total cerium sulphate concentrations resulting in slurries with a stable dissolved cerous sulphate concentration of as high as 0.851 m in 1.6 m H₂SO₄ at room temperature.