Analysis of fouling characteristic in enhanced tubes using multiple heat and mass transfer analogies☆

This paper provides a comprehensive analysis on cooling tower fouling data taken from seven 15.54 mm I.D. helically ribbed, copper tubes and a plain tube at Re=16000. There are two key processes during fouling formation:fouling deposition and fouling removal, which can be determined by mass transfer and fluid friction respectively. The mass transfer coefficient can be calculated through three analogies:Prandtl analogy, Von–Karman analogy, and Chilton–Colburn analogy. Based on our analyses, Von–Karman analogy is the optimized analogy, which can well predict the formation of cooling tower fouling. Series of semi-theoretical fouling correlations as a function of the product of area indexes and efficiency indexes were developed, which can be applicable to different internally ribbed geometries. The correlations can be directly used to assess the fouling potential of enhanced tubes in actual cooling tower water situations.     

Investigation of extraction fraction in confined impinging jet reactors for tri-butyl-phosphate extracting butyric acid process☆

The extraction fraction E and overall volumetric mass transfer coefficient kLa of TBP extracting butyric acid pro-cess in confined impinging jet reactors (CIJR) with two jets were investigated. The main variables tested were the concentration of tri-butyl-phosphate (TBP) and butyric acid, the impinging velocity V, the impinging velocity ratio of two phases Vorg/Vaq, the nozzle inner diameter di and the distance L between the jet axes and the top wall of the impinging chamber. The results showed that E and kLa increase with an increase of the impinging ve-locity V, the concentration of TBP Corg, and the impinging velocity ratio Vorg/Vaq. However, E and kLa decrease with an increase of the inner diameter di from 1 to 2 mm, the concentration of butyric acid Caq from 0.5%(v/v) to 2%(v/v). The factor L ranging from 3 to 11 mm has a negligible effect on E and kLa. A correlation on these variables and kLa was proposed based on the experimental data. These results indicated good mass transfer performance of CIJR in the extraction operation.     

PIV Measurement for Rayleigh Convection and Its Effect on Mass Transfer☆

The velocity distribution in Rayleigh convection caused by acetone volatilization in acetone-ethyl acetate binary system was observed in a vertical cross section of an initially quiescent liquid layer ...     

Numerical Studies of Convective Mass Transfer Enhancement in a Membrane Channel by Rectangular Winglets☆

Numerical calculations were conducted to simulate the flow and mass transfer in narrow membrane channels with and without flow disturbers. The channel consists of an impermeable solid wall and a membra...     

A Reynolds mass flux model for gas separation process simulation:II. Application to adsorption on activated carbon in a packed column☆

Simulations of adsorption process using the Reynolds mass flux model described in Part I of these serial articles are presented. The object of the simulation is the methylene chloride adsorption in a packed column (0.041 m id, packed with spherical activated carbon up to a length of 0.2 m). With the Reynolds mass flux model, breakthrough/regeneration curves, concentration and temperature as well as the velocity distributions can be obtained. The simulated results are compared with the experimental data reported in the literature and satisfactory agreement is found both in breakthrough/regeneration curves and temperature curves. Moreover, the anisotropic turbulent mass diffusion is characterized and discussed.     

Convective mass transfer enhancement in a membrane channel by delta winglets and their comparison with rectangular winglets☆

Numerical calculations were conducted to simulate the flow and mass transfer in narrow membrane channels equipped with delta winglets, which are often used as longitudinal vortex generators to enhance heat transfer in heat exchanger applications. The channel consists of an impermeable solid wall and a membrane. The delta winglets are attached to the solid wal surface to enhance the mass transfer near the membrane surface and sup-press the concentration polarization. The winglet performance was evaluated in terms of concentration polariza-tion factor versus consumed pumping power. Calculations were implemented for NaCl solution flow in a membrane channel having a height of 2.0 mm for Reynolds numbers ranging from 400 to 1000. The delta wing-lets were optimized under equal pumping power condition, and the results of optimization suggest winglet height of 5/6 of the channel height, aspect ratio of 2.0, attack angle of 30°, and a winglet interval equal to the chan-nel height. The optimal delta winglets were compared with the optimal rectangular winglets we found previous-ly, and it is shown that the rectangular winglets yield a somewhat better performance than the delta winglets. ? 2015 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. Al rights reserved.     

Modeling and experimental studies of methyl methacrylate polymerization in a tubular reactor☆

In this study, rheological examination of the mixture of a tubular reactor in which methyl methacrylate was po-lymerized has been studied. The n (flow behavior index) value of Power Law Model of mixture contained in the reactor has been determined within the span of 0.3492 to 0.9889 by curve fitting. Employing these numerical data for velocity profile, the reactor has been modeled. Moreover, the functions of the reactor have been com-pared in the three modes of plug, mixed and laminar flow. The results obtained in this research indicate that the polymethyl methacrylate mixture contained in the reactor is pseudo-plastic. Moreover, as the conversion grows, the velocity profile starts as a parabolic profile and approaches the plug mode;although it never reaches the plug. The other conclusions borne in this study indicate that when the reactor's radius is decreased, the conversion rate grows. However, as decreasing the radius would also reduce the productions rate, this procedure is not economical. Finally, in this modeling, the amount of conversion is equal to 56.47%at the end and according to its laboratory proportion which is 55.88%, it has reached the conclusion that the modeling duly undertaken is applicable and valid.     

CFD modeling of a headbox with injecting dilution water in a central step diffusion tube☆

For engineering applications of water dilution controlling system,the fluid dynamics of a mixed flow was studied with computational fluid dynamics(CFD) simulations and self-designed experimental set-up.In order to examine the predictability of CFD model for the headbox in industrial scale,two pulp suspensions before mixing were treated as homogeneous flows separately.Standard k-ε turbulence models with the mass diffusion in turbulent flows-species transport approach were applied in the simulations.A numerical simulation of this headbox model was analyzed with semi-implicit method for pressure linked equations scheme with pressure–velocity coupling.Results show that the model can predict hydrodynamic characteristics of headbox with injecting dilution water in a central diffusion tube,and the distribution of water content at the outlet of the slice lip is ideally normal at different speeds.     

Influence of impeller diameter on overall gas dispersion properties in a sparged multi-impeller stirred tank☆

The impeller configuration with a six parabolic blade disk turbine below two down-pumping hydrofoil propellers, identified as PDT + 2CBY, was used in this study. The effect of the impeller diameter D, ranging from 0.30T to 0.40T (T as the tank diameter), on gas dispersion in a stirred tank of 0.48 m diameter was investigated by experimental and CFD simulation methods. Power consumption and total gas holdup were measured for the same impeller configuration PDT + 2CBY with four different D/T. Results show that with D/T increases from 0.30 to 0.40, the relative power demand (RPD) in a gas–liquid system decreases slightly. At low superficial gas velocity V_S of 0.0078 m·s^-1, the gas holdup increases evidently with the increase of D/T. However, at high superficial gas velocity, the systemwith D/T=0.33 gets a good balance between the gas recirculation and liquid shearing rate, which resulted in the highest gas holdup among four different D/T. CFD simulation based on the two-fluid model along with the Population Balance Model (PBM) was used to investigate the effect of impeller diameter on the gas dispersion. The power consumption and total gas holdup predicted by CFD simulation were in reasonable agreement with the experimental data.     

HYDRODYNAMIC CHARACTERISTICS AND GAS-LIQUID MASS TRANSFER IN A DRAFT TUBE SLURRY REACTOR

Fundamental characteristics of hydrodynamics and mass transfer have been measured in an air lift slurry reactor with a draft tube. The solid suspension capacity, i.e., the critical solid holdup, the gas holdup and the volumetric gas-liquid mass transfer coefficient were measured in the two draft tube columns of 0.1485 and 0.10?m in diameter. Four activated carbon beads ranging in size from 0.25 to 2.19?mm in average diameter were utilized as suspended solids in the experiments.

The critical solid holdup in the draft tube slurry column is found to be much greater than that in the conventional bubble column. An empirical correlation is developed to account for the critical solid holdup behavior in the draft tube column. The gas holdup in the draft tube column agrees well with that in the bubble column. The overall gas-liquid mass transfer coefficient, k₁awas measured by the oxygen probe method. The effect of solid holdup on k₁a is found to be negligible in the present system. The empirical equation is developed to correlate k₁a in the draft tube slurry reactor.     

LIQUID-SOLID MASS TRANSFER IN A THREE PHASE DRAFT TUBE FLUIDIZED BED REACTOR

Liquid-solid mass transfer coefficients in a three phase draft tube fluidized bed reactor have been measured using spherical ion exchange particles. The particle diameters ranged from 655 to 1119μm and solids volume fractions of approximately 5 and 10% were employed in water at 28°C. The experimental data can be successfully correlated using a Reynolds number derived using Kolmogoroffs theory of isotropic turbulence, although it is doubtful whether isotropic turbulence actually prevails in the fluidized bed over the range of conditions employed. Comparison with correlations determined for bubble columns and gas-liquid fluidized beds is performed. A model which considers the draft tube reactor as comprising two distinct fluid mechanical regions is developed to explain the apparently lower values of mass transfer coefficients obtained in a draft tube as opposed to conventional fluidized bed reactor.     

MASS TRANSFER AND REACTION IN BUBBLE COLUMN SLURRY REACTOR WITH DRAFT TUBE

Gas-liquid and liquid-solid mass transfer coefficients were obtained in a draft tube bubble column slurry reactor (abbreviated as DTBCSR) from the measurements of gas absorption and ion-exchange, respectively. Oxidation of sulfur dioxide on activated carbon was carried out in the same reactor. The effects of gas flowrates, diameters of draft tube and solid concentrations were investigated. These results were compared with those in a bubble column slurry reactor without draft tube (abbreviated as BCSR). Minimum gas velocities for complete suspension of solid particles in DTBCSR were lower than those in BCSR. Gas-liquid and liquid-solid mass transfer coefficients in DTBCSR were higher than those in BCSR. Both coefficients were almost independent of solid loadings. Reaction rates in DTBCSR were higher than those in BCSR and a stirred slurry reactor (abbreviated as SSR) except for lower gas flow rates and smaller particles.     

H2O2吸收SO2的降膜传质系数模型的建立

为中小型燃煤锅炉烟气脱硫提出一种以列管式并流降膜塔为脱硫反应器,H2O2溶液为脱硫剂的简便高效、无二次污染的脱硫技术.建立了气、液相传质系数模型,通过实验数据回归了降膜管传质关联式.对气、液相传质系数模型进行验证,结果显示,得出的计算结果与实验数据基本相符,这表明建立脱硫模型对基本设计参数的确定具有一定的指导意义.     

Impact of Feed Spacer Filament Spacing on Mass Transport and Fouling Propensities of RO Membrane Surfaces

Material build-up on membrane surfaces is one of the vital challenges faced by Reverse Osmosis operations, leading to many operational and maintenance issues. To date, several modeling studies are dealt with flow behavior and concentration patterns for cross-flow membrane operations. However, the relative fouling propensities of top and bottom membrane surfaces are never addressed in any study for narrow channels filled with ladder-type spacers. In the present work, fluid flow patterns through different spacer configurations are visualized using ANSYS FLUENT by varying the dimensionless filament spacing, L (ratio of top or bottom filament spacing and channel height). Results clearly indicate that the average shear stress values at the top membrane surface are always higher (3–8 times) than that at the bottom membrane surface, but the mass transfer coefficient for the two walls yielded approximately similar average values having low to moderate filament spacings of L ≤ 3 (SP22 and SP33), indicating similar fouling propensities of membrane surfaces. Further increase in filament spacing with L ≥ 4 (SP44 and SP66), the average mass transfer coefficient for the top membrane indicated a sharp decline, suggesting high fouling propensity compared to bottom membrane which is not a desirable feature. Among the four spacer arrangements studied, SP44 (with L = 4) was found to be the optimal arrangement, yielding moderate pressure drop with nearly equal/higher area weighted values of mass transfer coefficient for the two walls and would lead to lower and equal fouling tendencies for top and bottom membrane surfaces, respectively.     

Region-dependent mass transfer behavior in a forced circulation airlift loop reactor

The contribution of local regions to global mass transfer holds the key to optimization and scale-up of a reactor. Extensive study has been conducted to investigate gas-liquid mass transfer occurring in the internal airlift loop reactor, but mostly restricted to global mass transfer performance. A cold model forced circulation internal airlift loop reactor was employed and divided into six regions in which dissolved oxygen concentration in slurry and mass transfer interfacial area were measured respectively. Different models were utilized to calculate volumetric mass transfer coefficient. Contributions of individual region to global mass transfer performance were calculated and compared. It was found that mass transfer coefficient and mass transfer interfacial area of individual region increases with increasing superficial gas velocity and slurry feed flowrate. The feed affected region has the greatest mass transfer coefficient and volumetric mass transfer coefficient, contributing more than 30% to global mass transfer in most operating condition. Mass transfer interfacial area is close in the gas distributor region, feed affected region and the gas-slurry separator region. In the present work, circulating bubbles are rare and contribute negligibly to the global mass transfer. Global volumetric mass transfer coefficient is close to that of the gas-slurry separator region, ranging from 0.02 to 0.1 1/s. Comparison of k_La is made between this work and literatures, suggesting a great improvement of mass transfer due to external liquid circulation.     

Hydrodynamics and mass transfer study of aliphatic alcohols in airlift reactors

Gas holdup and gas–liquid mass transfer coefficient were considered in an external airlift reactor. Air was sparged through some aliphatic alcohols (methanol, ethanol, n-propanol, and n-butanol) with different concentrations (0–1%, v/v). It was observed that gas holdup and mass transfer coefficient increased with increasing the number of carbons in alcohols. Furthermore, an increment in alcohols concentration increased gas holdup and mass transfer coefficient. The same behavior was observed in external and internal loop airlift reactors although gas holdup and mass transfer coefficient values were less than those of internal airlift reactor. According to the experiments, two correlations for gas holdup and mass transfer were developed.     

Ozone Mass Transfer in Stirred Vessel

A method is proposed and a model is developed which are capable of providing a correlation of the mass transfer coefficient k_La, with stirrer speed and gas superficial velocity. The method can be adopted for deriving a correlation which can be profitably used for ozone gas–liquid reaction both for assessing the absorption regime and for the simulation of oxidation processes which evolve according to slow reaction regime.     

Solid–liquid mass transfer in a gas sparging contactor equipped with a tube of circular fins

The solid–liquid mass transfer rate at a stack of circular fin surfaces in a gas sparging contactor was investigated. A diffusion-controlled dissolution technique of copper in an acidified chromate solution was employed. Variables studied included the number of actively exposed fins ranging from 5 to 20, pertinent physical properties of the solution, and air superficial velocity. Experimental data showed that the rate of the diffusion-controlled mass transfer increases with increasing superficial air velocity and decreases with increasing chromate solution acid concentration. Moreover, at relatively low superficial air velocity, increasing the number of actively exposed fins results into a continuous increase in the mass transfer coefficient. At relatively higher superficial air velocity, however, the mass transfer coefficient decreases in the 5–10 range of actively exposed fins and then reverts to increase in the 15–20 range. An empirical correlation relating the mass transfer j factor to Re_g, Fr, and a dimensionless height defined as the ratio of the height of actively exposed fins to the column equivalent diameter was developed based on the data generated in this study, with ±6.45% average deviation.