Alkaline scale formation restriction in desalination plants by means of antiscalant additives

Rotating foam reactors: Mass transfer and reaction rate

The work is devoted to the study of heat and mass transfer in a liquid film flowing down on a heated surface under conditions of evaporation into a crossflow of a gas neutral with respect to the liquid. The work aimed to experimentally determine the average heat transfer coefficients from a heated surface to the film, heat transfer and mass transfer from the film to the gas flow and to establish their dependence on the input parameters of the heat and mass transfer process. To achieve this goal, an experimental setup was created, and a research technique was developed based on the proposed mathematical model of the heat and mass transfer process. The results of the study showed that the dependences of the average heat and mass transfer coefficients on the initial liquid flow rate are extreme with the minimum values of these coefficients at the liquid flow rate, which corresponds to the critical value of the Reynolds criterion Re l cr ≈ 500, which indicates a transition from the laminar falling films to turbulent mode under the considered conditions. The dependences of the heat and mass transfer coefficients on other process parameters for both modes of film falling are established. A generalization of the experimental data made it possible to obtain empirical equations for calculating these coefficients. Keywords: heat and mass transfer, cross flow, film apparatus, heat and mass return coefficient, neutral gas.

Solar desalination based on humidification process—I. Evaluating the heat and mass transfer coefficients

This study evaluated the effect of the size and layout of adiabatic patch surfaces on heat exchange surfaces to improve the efficiency of heat and mass transfer and their exchange within a certain duct length, considering three factors: (1) the length of the heat exchange surfaces, (2) the location and layout of the adiabatic surfaces, and (3) the number of local adiabatic surfaces. Based on computational fluid dynamics (CFD), a simple numerical simulation model was developed to analyze the heat and mass transfer coefficients. The results provide dimensionless parameters at the heat exchange surface with the partitioning (patch) effect for various conditions in terms of the size, location/layout, and number of local adiabatic surfaces. Finally, the heat exchange efficiency, determined according to the locally installed adiabatic patch surface conditions, was examined, with a focus on the design of the joint component for a compact heat exchange element.

Numerical simulation of ultrasonic enhancement by acoustic streaming and thermal effect on mass transfer through a new computation model

Water and electrolytes cause significant changes in brittle fracture strength and subcritical fracture propagation velocities in quartz and quartz rocks. The changes may be caused, in part, by changes in surface free energy. Experimental fracture surface energies of quartz range from about 400 mJ m−2 to about 11.5 J m−2. Thermodynamic surface free energies are likely to be lower than fracture surface energies owing to dissipative energy losses and failure to achieve equilibrium surface structure. Thermodynamic surface free energies are sensitive to environmental composition. Reaction of water vapor with pristine fracture surfaces reduces surface energy by hydroxylation, but the extent of reduction is not known. Adsorption of water vapor on the hydroxylated surface and immersion in liquid water reduce surface energy by 75–230 and 72 mJ m−2 respectively. In an electrolyte, the surface free energy is maximum at the point of zero charge, where adsorption of ionic solutes is least. Adsorption of hydrogen ion, hydroxide ion, and electrolytes reduce surface energy as concentration increases, by tens of mJ m−2. All of these surface energy changes are qualitatively consistent with changes in fracture behavior caused by the same variables.

Intensification of heat and mass transfer processes in industrial drop cooling towers is one of the main areas of research in modern heating engineering. Increasing the efficiency of transfer processes during the direct interaction of water and air in cooling tower sprinklers is possible when moving from creating a contact surface in the form of a flowing liquid film to sprinklers creating drop-film surfaces. One of the possible devices that create such surfaces are devices with vertical contact grids, which make it possible to obtain significant drop surfaces at low energy costs. A mathematical description of mass and heat transfer processes in devices with vertical grids is impossible without the presence of functional dependencies of mass and heat transfer coefficients depending on the hydrodynamic parameters of the process. Obtaining analytical expressions convenient for engineering modeling is difficult due to the complexity of the mathematical description, therefore in some cases it is more convenient to use experimental dependences of the coefficients on the process parameters. The experiments carried out on mass and heat transfer in devices with vertical grids made it possible to confirm the possibility of using the Lewis relation, based on the similarity between the processes of heat and mass transfer under conditions of water cooling in cooling towers equipped with devices with vertical grids. This made it possible to use only one experimental dependence in the mathematical description of transfer processes. An experimental dependence of the volumetric heat transfer coefficient on the hydrodynamic parameters of the interaction of water and air on the contact grids was also obtained.

An important advantage of microreactors is the significant intensification of heat and mass transfer processes. As components of microsystems, they facilitate controlled chemical reactions. To date, there is no definitive answer regarding the predominant role of convection and diffusion in mass transfer processes within two-phase flows in microchannel devices. The aim of this study was to determine the influence of circulation frequency on the mass transfer coefficient in a liquid-liquid microchannel system to assess the role of convection. To achieve this goal, an experimental investigation of mass transfer was conducted through the extraction of succinic acid in a two-phase slug (Taylor) flow regime within a model system consisting of water, succinic acid, and n-butanol. For this purpose, a microreactor installation was developed, assembled and tested, which is equipped with a glass microchannel with a coaxial-spherical mixer at the very beginning, and the internal diameter of the microchannel dc = 1.3 mm. The methodology for processing experimental data to calculate the mass transfer coefficient was based on the two-film model of Lewis and Whitman. In the course of experimental studies, the conditions for a stable slug flow at various phase flow rates were found, approximations were obtained for calculating the specific surface area of phase contact, surface and volumetric mass transfer coefficients and the dependences of these coefficients on the circulation frequency in a drop of a continuous phase (slug) were determined. The study confirmed the linear dependence in the Taylor flow regime in a two-phase liquid-liquid flow of the mass transfer coefficient on the circulation frequency inside droplets of the continuous phase, discovered earlier. Thus, it was determined that convection has a significant effect on the overall mass transfer in the liquid-liquid system.

Simulation of the coal pyrolysis process in downer reactor with a mass transfer model based on the spatial superposition assumption

The often observed tailing of tracer breakthrough curves is caused by a multitude of mass transfer processes taking place over multiple scales. Yet, in some cases, it is convenient to fit a transport model with a single‐rate mass transfer coefficient that lumps all the non‐Fickian observed behavior. Since mass transfer processes take place at all characteristic times, the single‐rate mass transfer coefficient derived from measurements in the laboratory or in the field vary with time . The literature review and tracer experiments compiled by Haggerty et al. (2004) from a number of sites worldwide suggest that the characteristic mass transfer time, which is proportional to , scales as a power law of the advective and experiment duration. This paper studies the mathematical equivalence between the multirate mass transfer model (MRMT) and a time‐dependent single‐rate mass transfer model (t‐SRMT). In doing this, we provide new insights into the previously observed scale‐dependence of mass transfer coefficients. The memory function, g(t), which is the most salient feature of the MRMT model, determines the influence of the past values of concentrations on its present state. We found that the t‐SRMT model can also be expressed by means of a memory function . In this case, though the memory function is nonstationary, meaning that in general it cannot be written as . Nevertheless, the full behavior of the concentrations using a single time‐dependent rate is approximately analogous to that of the MRMT model provided that the equality holds and the field capacity is properly chosen. This relationship suggests that when the memory function is a power law, , the equivalent mass transfer coefficient scales as , nicely fitting without calibration the estimated mass transfer coefficients compiled by Haggerty et al. (2004).

Rotor-stator spinning disc reactor

This paper has reported the effect of oxygen and argon plasma treatments of CIIR rubber using Attenuated Total Reflectance (ATR) and surface energy measurements. Plasma treatment led to changes in the surface energy from 31 to 45.7 mN/m. Plasma treatment conditions influenced both the changes in surface energy and stability, and they also became more difficult to obtain good contact angle measurements. However, plasma treatments made the interfacial properties to be stabilized. ATR measurements revealed that changes in surface energy with treatment time are due mostly to increased oxygen functionality.

Heat/mass transfer analogy in the case of convective fluid flow through minichannels

This work investigates the mass transfer process with and without first order chemical reaction by direct numerical simulation of two-fluid flows within mini-channels. The large potential of two-fluid flows for mass and heat transfer processes, operated in mini- and micro-systems such as micro bubble columns and monolithic catalyst reactors, motivated the present research. The study is based on the implementation of the species conservation equation in computer code TURBIT-VoF. The implementation of the equation is validated against different solutions of simplified mass transfer problems. The demanding treatment of the interfacial concentration jump described by Henry's law is examined with great concern. The diffusive term is successfully compared against one- and two-dimensional theoretical solutions of diffusion problems in two-phase systems. The numerical simulation of mass transfer during the rise of a 4mm air bubble in aqueous glycerol is performed and compared against another numerical simulation in order to test the convective term. The implementation of the source term for homogeneous and heterogeneous chemical reaction is successfully validated against theoretical solutions of mass transfer with chemical reaction in single-phase flows. The numerical simulations are focused on bubble train-flows flowing co-currently in mini-channels. Taking advantage of the periodic flow conditions exhibited in axial direction, the analysis is restricted to a flow unit cell, which consists of one bubble and one liquid slug. As concerns the hydrodynamics of all simulations performed, good agreement is obtained for the non-dimensional bubble diameter, the ratio of bubble velocity to the total superficial velocity and for the relative velocity in comparison with experimental data. The influence of the unit cell length on mass transfer from the bubble into the liquid phase of an arbitrary species is investigated in square channels having the hydraulic diameter D* h = 2mm. Short unit cells are found more effective than long unit cells for mass transfer, in agreement with published investigations performed for circular channels. This is related to the length of the liquid film between bubble and wall which becomes rapidly saturated due to short diffusion lengths and long contact time and leads to a decrease of the local concentration gradient. The major contribution to mass transfer occurs through the cap and the bottom of the bubbles, as reported also in experimental investigations. For mass transfer with heterogeneous chemical reaction more mass is consumed at the wall for systems having long unit cells, as a consequence of the increased lateral surface and more vigorous recirculation in the liquid slug. For species having a large solubility in the continuous phase, diffusion dominates over reaction allowing short unit cells to be more effective for mass transfer with heterogeneous reaction. A formulation of the mass transfer coefficient based on averaged concentrations is proposed for mass transfer processes and successfully compared against another approach based on the mass balance at interface. In complete agreement with experimental and theoretical studies, the study reveals that long liquid slugs and short bubbles are more efficient than short liquid slugs and long bubbles, respectively.

Changes in surface energy and electrical conductivity of polyimide (PI)-based nanocomposite films filled with carbon nanotubes (CNTs) induced by UV exposure are gaining considerable interest in microelectronic, aeronautical, and aerospace applications. However, the underlying mechanism of PI photochemistry and oxidation reactions induced by UV irradiation upon the surface in the presence of CNTs is still not clear. Here, we probed the interplay between CNTs and PIs under UV exposure in the surface properties of CNT/PI nanocomposite films. Changes in contact angles and surface electrical conductivity at the surface of CNT/PI nanocomposite films after UV exposure were measured. The unpaired electron intensity of free radicals generated by UV exposure was monitored by electron paramagnetic resonance. Our study indicates that the covalent interactions between CNTs and radicals generated by UV irradiation on the PI surfaces tailor the surface energy and surface conductivity through anchoring radicals on CNTs. Surprisingly, adding CNTs into PI films exposed to UV leads to antagonistic contributions of dispersion and polar components to the surface energy. The surface electrical conductivity of the CNT/PI nanocomposite films has been improved due to an enhanced hopping behavior with dense π-conjugated CNT sites. To explain the observed changes in surface energy and surface conductivity of CNT/PI nanocomposite films induced by UV exposure, a qualitative model was put forward describing the covalent interactions between UV-induced PI free radicals and CNTs, which govern the chemical nature of surface components. This study is helpful for characterizing and optimizing nanocomposite surface properties by tuning the covalent interactions between components at the nanoscale.