A three-dimensional direct simulation model for studying pressure-driven dead-end microfiltration was developed. The simulation model employs the Euler–Lagrange hybrid method to analyze concentrated suspensions, and it is useful for understanding the fouling process in dead-end microfiltration. We performed numerical simulations using a membrane model with a straight pore for particle concentrations of 2% and 5% and used the permeate flux and particle rejection to evaluate the filtration performance. The simulations reproduced the well-known experimental fact that the decrease in the permeate flux increases with increasing particle concentration. Snapshots of particles dynamics obtained from the simulation are presented, and the difference between the fouling processes corresponding to the two particle concentrations is discussed. The starting point of fouling approached closer to the feed side of the membrane as the particle concentration increased. In addition, we demonstrated the simulation of backwash operation, determined the transient properties of the permeate flux during three backwash cycles, and observed that the backwash led to the removal of the deposited particles.
Type: Short Communication
Subject area: Transport Phenomena and Fluid Engineering
2010 Volume 43 Issue 10 Pages
Published: October 20, 2010
Released: October 20, 2010 [Advance publication] Released: July 09, 2010
The effect of elevated pressures on the gas holdup and flow regime transition in a gas–liquid bubble column is examined. Experiments are carried out in a bubble column of 0.3 m diameter at system pressures between 0.1 and 0.6 MPa, using different liquids sparged with nitrogen. The following results are obtained. (1) The superficial gas velocity at the flow regime transition point (Utrans) increases with the system pressure. (2) In the bubbly flow regime, the gas holdup shows no notable change when the superficial gas velocity is maintained at a low value. However, in the churn-turbulent flow regime, the gas holdup increases with the system pressure. These phenomena are in agreement with those predicted on the basis of the equations for Utrans and gas holdup, proposed previously.
A comprehensive computational method based on the Eulerian–Eulerian approach is presented for gas–liquid flows in a mechanically stirred vessel. Separate submodels were developed to investigate the influence of the drag coefficient on bubbles due to the interaction between bubbles and turbulence. A new model is used for the definition of drag coefficient and a detailed simulation method is used to estimate the interphase drag force. A standard k−ε turbulent model is used and the impeller rotation is modelled by the multiple reference frame approach. Simulation results are compared with previous drag models and the experimental measurements. The results indicate that the new model is able to predict the influence of turbulent intensity on drag coefficient and the distribution of gas holdup. The computational model and the proposed correlation for drag in turbulent flow will be useful for simulation gas holdup distribution and flow regimes in stirred vessels.
High-precision control of the dispersion of aggregates in a stirred media mill requires a thorough understanding of the motion of multiphase flows. Here, we present numerical methods to study the dispersion mechanism and estimate the impact and fluid shear powers arising in such a mill. Volume-averaged four-way coupling equations are used to simulate the bead motion and turbulent flow: a distinct element method (DEM) is employed for simulating bead motion, while large eddy simulation (LES) is used for simulating turbulence. The simulated particle and fluid velocities are in good agreement with the experimental findings at various stirring rates, showing the reliability of the DEM–LES method. Next, methods to calculate the powers of fluid shear, bead collision, and friction are presented. These methods are used to study the effect of each type of power on the dispersion process at various bead filling ratios. An increase in the filling ratio from 0 to 50 vol% causes a dramatic increase in the power of the fluid shear of pore fluid flow within the beads and a similar increase in the collision and friction powers. Further increase in the filling ratio to 83 vol% results in only a slight decrease in the collision and friction powers. However, the associated increase in the total bead–bead contact area leads to an increase in the effect of these powers on dispersion. The fluid shear power dominates the initial stage of the dispersion, whereas the collision and friction powers are dominant during the subsequent gradual dispersion process.
The effect of surface deprotonation of colloidal silica particles on settling mode transition has been studied quantitatively. We observed the settling mode transitions of silica aqueous suspensions by adding a pH controlled CaCl2 electrolyte solution as a coagulant. Depending on the solution pH, three types of settling modes were observed, i.e. “no settling mode”, “free settling mode” and “hindered settling mode”. In these observations, a large pH drop occurred immediately after mixing of the coagulant. To describe the pH drop, we developed a simple mathematical model consisting of a 1-pK Stern model and a mass balance equation of protons on silica particles. The model shows excellent agreement with experiments in the number of dissociated protons from a silica surface (np). Furthermore, the boundaries of the three setting modes in the pH-φ phase diagram are also well explained by the value of np. Our results clarify that the extent of colloidal instability is quantitatively described by np.
Heat integrated distillation columns (HIDiCs) may have multiple steady-states, which may cause abrupt changes in process states. The present work presents two possible approaches to avoiding such multiplicity. The first approach is related to compressor operation strategies in the LV control structure; the strategy of constant compressor rotation speed can realize stable operation without multiple steady-states. The second approach is to adopt the DV or LB control structure. By employing one of these two structures, no multiplicity appears and stable operation of HIDiC can be realized. Herein, the usefulness of the proposed approaches is demonstrated though rigorous dynamic simulations.
Exchange between the SO42− ions in ettringite and borate is believed to occur during borate removal from wastewater by the Ca(OH)2–Al2(SO4)3 addition method. To investigate borate uptake by ettringite, we synthesized ettringite in the presence of H3BO3 and prepared suspensions of ettringite in H3BO3 solution. At the theoretical Ca(OH)2 / Al2(SO4)3 ratio for ettringite, with an increase in the H3BO3 / Al2(SO4)3 ratio, the ettringite concentration decreased but the concentration of amorphous compounds increased; further, the amount of borate uptake by the precipitate formed also increased. Upon alkalization of the H3BO3–Al2(SO4)3 solution to pH = 10 with NaOH solution, borate-containing amorphous Al(OH)3 was precipitated. It was proposed that the borate coprecipitated along with amorphous Al(OH)3 (which showed boron uptake) and did not undergo exchange with SO42− ions in ettringite; this was strongly supported by the results of IR and TG analyses performed on the boron-containing precipitates. At a high H3BO3 / Al2(SO4)3 ratio, ettringite decomposed to afford amorphous Al(OH)3, and borate coprecipitation was promoted. Upon suspension in H3BO3 solution, the ettringite precipitate showed low boron uptake, which was presumably due to the substitution of SO42− in ettringite with B(OH)4−.
A thermal spraying technique was adopted to change the surface properties of heat exchanger tubes in pulverized coal fired boilers in order to reduce ash deposition. Initially, four types of coal ash with different melting points were tested as samples for ash deposition experiments. As a result, ash particles with higher fouling factor selectively adhered on the tube surface. Even for the ash particles with lower melting point, Ni alloy as a thermal spraying material played an effective role in reducing deposition. Ni alloy can reduce the ash deposition by limiting formation of molten slag at high temperature. In addition, long-term ash adhesion experiments were also carried out using a precise tension tester at high temperature. The effectiveness of the Ni alloy was also proved after long term contact between an ash pellet and a rod thermally sprayed Ni alloy at high temperature. This is because the Ni alloy can control the diffusion of Fe compounds from the tube rod to the ash deposition layer.
This paper addresses the polyethylene quality transition in gas phase fluidized bed reactors using feedback control. Specifically, a nonlinear model predictive controller is utilized to steer the polymer density and the entire molecular weight distribution to follow predefined targets. The hydrogen and co-monomer inflows are used as the manipulated variables. In this work, the molecular weight distribution is modeled as a function of the reaction kinetics and hydrogen to monomer ratio. The simulation results indicated successful implementation of the control algorithm to attain the desired molecular weight distribution and density, even with the existence of modeling errors. Economically undesirable operations such as excessive purge and/or reduced production rate are avoided by the limited use of monomer and catalyst inflows.
Various thin films are produced by coating and drying of slurries. Cracks and fractures in the film, which frequently are encountered under a large drying rate condition, can be avoided by a reduction in drying rate. In this work, we propose a novel drying technique (pressure controlled drying), where the coated slurry is dried in a container filled with vapor of the drying solvent. If the inner pressure of the container is held reasonably higher than the saturation vapor pressure of the solvent, the drying rate can be successfully depressed. It was also turned out that the residual amount of the solvent in the coated slurry before and after holding the pressure has to be carefully controlled in order to work this drying technique effectively. The residual solvent is affected mainly not only by the time of holding the pressure but also by the container volume, decompressing capacity, and co-drying solvent. When the drying rate could be suppressed till the end of the constant drying rate period, the resultant film had a tightly packed cellular pattern and a smooth surface. This is probably due to the enhancement of particle arrangement with sufficient amount of the solvent and long drying time.
Enthalpy wheels have been widely used for achieving comfortable air-conditioning with low energy consumption. Desiccant materials such as silica gel and molecular sieves are used so as to give it a latent heat exchanging function. However, this important function has become the cause of cross-contamination because substances with offensive odors are also adsorbed onto desiccants. In this study, cross-contamination of an enthalpy wheel consisting of ion-exchange resin particles as a desiccant material is examined and compared with a commercialized 3A zeolite wheel. It is found that the ion-exchange resin type wheel shows a smaller cross-contamination ratio than the other for all contaminants tested in this study, i.e. ammonia, carbon dioxide, formaldehyde and propane, although water-soluble and smaller contaminants such as ammonia still indicated a higher contamination ratio.
A new fine aggregate recycling process from waste concrete using high-pressure carbon dioxide (CO2) solution is proposed. The basic mechanism of the recycling process is extraction of calcium from a hydrated cement component, which reacts readily with acid. Calcium extraction would be effective in promoting the separation of hydrated cement from aggregates. The aggregate component, mainly composed of SiO2, does not react with carbonic acid and would be recovered as the solid phase on filtration. Dissolved calcium can be recovered as calcium carbonate under low CO2 pressure conditions. The feasibility of the recycling process through laboratory experiments was examined using real concrete waste samples, which cannot be recycled effectively by current processes. A combination of CO2 solution treatment and ball-mill crushing was examined. High-quality recycled fine aggregate that can be used for the production of new concrete could be obtained using the proposed process.
Phosphate behavior is an important factor that controls the eutrophication of inland lakes. Dianchi Lake is the largest lake in southwest China, and it is divided into Caohai Lake (CL) and Waihai Lake (WL). Although the problem of eutrophication is more serious in CL than in WL, algal blooms are more frequent in the latter. In this study, phosphate–sediment interactions were studied with an emphasis on the differences between CL and WL sediments. A pseudo-second-order kinetic model could describe phosphate release kinetics well. The phosphate adsorption capacity of the CL sediments was thrice that of the WL sediments. Although phosphate release in these two lakes was comparable, the total phosphate concentration in the CL sediments was seven times that in WL sediments. Because phosphates and dissolved organic matter (DOM) compete for adsorption sites on the sediment particles, the presence of DOM results in a significant increase in phosphate release in both sediments. The results suggest that both endogenous and exogenous phosphates should be controlled in order to reduce algal bloom frequency in Dianchi Lake.