Saturation carrying capacity at high archimedes number of vertical concurrent gas-particle flow

The existing literature data on the saturation carrying capacity for vertical concurrent gas-particle flow are limited to Archimedes numbers Ar below 1000. By experimental measurement and dimensionless analysis the present article extended Ar to 2500 to demonstrate how varies with gas velocity under these high Ar values. The obtained result revealed that is subject to the same kind of correlation with gas velocity irrespective of Ar. The degree that depends on gas velocity, however, generally decreases with raising Ar and it tends to be a constant when Ar becomes higher than 530. This shows in fact that the flows with heavy/large particles or dense/viscous gas possess a low increase in particle-carrying capability with increasing gas velocity until a steady specific capability per m/s is reached at Ar of 530. The article also reanalyzed the influence of column diameter on , demonstrating that the column diameter influence exists only in small columns and for flows with heavy/large particles. The influence is negligible in commercial conveying columns and differs according to Ar in those small columns employed in laboratory studies. All of these findings resulted in an upgraded correlation for , which manifested not only wide applicability to flows in differently sized columns and with Ar of up to 2500 but also better accuracy in the mentioned flow conditions when comparing with several other literature correlations.     

Dynamic Monte Carlo study on the polymer chain in random media filled with nanoparticles

The effect of nanoparticles on the dynamic and configurational properties of a single polymer chain has been studied by using dynamic Monte Carlo simulation. An attractive potential, ε, is considered between fixed nanoparticles and polymer beads. The diffusion coefficient of the polymer D remains constant for weak interactions, i.e. small β=−ε/k_BT, and then gradually drops to zero at a critical value . However, the mean square end-to-end distance 〈R²〉 shows a different behavior with an increase in the attractive interaction. 〈R²〉 decreases at small β and reaches a minimum at , then goes up slowly and at last saturates at strong interaction. The values and scale with the concentration of nanoparticles. Interesting findings are: (1) a chain has its minimum size before the chain stops its diffusion, and (2) a chain continues its configuration adjustment even for .     

Desulfurization in the gas-continuous impinging stream gas-liquid reactor

An investigation is made to evaluate the flue gas desulfurization (FGD) by absorption in a gas-continuous impinging stream gas-liquid reactor recently developed for systems involving fast reaction(s) in liquid. The mixture of air and SO₂ was used as the pseudo-flue gas and Ca(OH)₂-water suspension as the absorbent. By employing horizontal two-impinging streams, the reactor is simple in structure with few internal parts, while exhibits satisfied overall performance for FGD. Under moderate conditions, the content of SO₂ in the cleaned gas can achieve a level much lower than that permitted, while the pressure drop across the device is about 400 Pa only. The influences of some operating and structural parameters, such as V_L/V_G, Ca/S mole ratio, SO₂ concentration in flue gas, impinging distance S, and nozzle location, etc., are examined. The gas-film mass transfer coefficient, k_G, is determined based on Sauter mean diameter of spray droplets. The results show that k_G is essentially independent of concentration of SO₂ in flue gas, implying the process can be considered to be controlled by diffusion through gas film. The relationship between k_G and impinging velocity, u₀, is fitted to be with the standard deviation of , suggesting u₀ is a strong effecting variable on mass transfer, and, consequentially, important operating variable. In the range of u₀ from 5.53 to , the values determined for the volumetric mass transfer coefficient, k_Ga, are 0.577-, and those for k_G are ranged from 0.00641 to .     

Droplet size and velocity profiles in liquid-liquid horizontal flows

The size and vertical distribution of drops were studied experimentally in dispersed liquid-liquid pipeline flows. Under most conditions the pattern was dual continuous where both phases retain their continuity and there is entrainment in the form of drops of one phase into the other. The investigations were carried out in a stainless steel test section with 38 mm ID with water and oil (density and viscosity ) as test fluids. Mixture velocities from 1.5 to and input oil volume fractions from 20% to 80% were used. A dual sensor impedance probe allowed drop chord length and drop velocity measurements at different locations in a pipe cross section.It was found that in dual continuous flows drop concentration and size decreased with increasing distance from the interface. There were only small differences in size between oil drops in the lower water continuous layer of the flow and water drops in the upper oil continuous layer. Mixture velocity did not affect significantly the drop size of either phase since higher velocities that would result in smaller drops were accompanied by increased entrainment of one phase dispersed into the other that favoured larger drops. The Rosin-Rammler function was found to fit satisfactorily the experimental drop size distributions, while literature correlations on entrained and maximum drop sizes in a turbulent field underpredicted the values found experimentally.     

Macro- and micromixing studies in an unbaffled vessel agitated by a Rushton turbine

Macromixing characteristics, power number and visual observation of the vortex behaviour and micromixing in an unbaffled tank agitated with a Rushton turbine are reported. The latter has also been compared in detail with earlier results from an identical tank containing baffles. The maximum mean specific energy dissipation rate, , in the unbaffled tank that can be utilised without severe air incorporation is compared to with baffles. However, at this lower , the micromixing efficiency is always significantly greater without baffles except when addition is made onto the top of the liquid or into the trailing vortex very close to the impeller. In these latter cases, they are approximately the same but even a small submergence of the feed tube below the liquid surface greatly enhances micromixing in the unbaffled case whilst it is still very poor with baffles. This good micromixing performance of the unbaffled vessel was very unexpected. Furthermore, an established method of estimating the local ε_T gave values of at every feed position where measurement was undertaken. Since the spatially averaged value of φ=1, this result suggests the possibility that the accepted concept of micromixing being totally controlled by the local ε_T at the feed point may not be valid for such swirling flows.     

Transition between stratified and non-stratified horizontal oil-water flows. Part II: Mechanism of drop formation

The conditions and mechanism of drop formation at the interface of oil-water wavy stratified flows that lead to the onset of drop entrainment and the transition to dual continuous flow pattern were investigated both experimentally and theoretically. Experimentally, high-speed video imaging was used to capture the mechanism of drop detachment from waves during oil and water stratified flow in a diameter horizontal acrylic pipe. The visual observations revealed that the faster phase undercuts the other one while the waves present in both phases deform until drops are detached. The wave deformation was attributed to the drag force, that originates from the relative movement between the two phases, exceeding the stabilising surface tension force. Based on this force balance an equation was developed that related the wavelength to the amplitude that can lead to drop detachment. This drop entrainment equation and the wave stability equation suggested in part I of the paper [Al-Wahaibi, T., Angeli, P., 2007. Transition between stratified and non-stratified horizontal oil-water flows. Part I: Stability analysis. Chemical Engineering Science, in press, doi:10.1016/j.ces.2007.01.024 ], defined three regions in a wave amplitude versus length graph, namely the stable waves, the unstable waves and the drop entrainment region. The intersection of the lines produced by these two equations gives the critical minimum wave characteristics for drop formation. These agreed well with experimental data when a new correlation for the drag coefficient on the waves was used, suitable for liquid-liquid flows. Also the characteristics of waves that were experimentally found to form drops fell within the predicted entrainment region.     

Synthesis, characterization and thermodynamic properties of poly(3-mesityl-2-hydroxypropyl methacrylate-co-N-vinyl-2-pyrrolidone)

Poly(3-mesityl-2-hydroxypropyl methacrylate-co-N-vinyl-2-pyrrolidone) P(MHPMA-co-VP) was synthesized in 1, 4-dioxane solution using benzoyl peroxide (BPO) as initiator at 60 °C. The copolymer was characterized by ¹H ¹³C NMR, FT-IR, DSC, TGA, size exclusion chromatography analysis (SEC) and elemental analysis techniques. According to SEC, the number-average molecular weight (M_n), weight-average molecular weight (M_w) and polydispersity index (PDI) values of PMHPMA-co-VP were found to be 58,000, 481,000 g/mol and 8.26, respectively. According to TGA, carbonaceous residue value of PMHPMA-co-VP was found to be 6% at 500 °C. Also, some thermodynamic properties of PMHPMA-co-VP such as the adsorption enthalpy, ΔH_a, molar evaporation enthalpy, ΔH_v, the sorption enthalpy, , sorption free energy, , sorption entropy, , the partial molar free energy, , the partial molar heat of mixing, , at infinite dilution was determined for the interactions of PMHPMA-co-VP with selected alcohols and alkanes by inverse gas chromatography (IGC) method in the temperature range of 323-463 K. According to the specific retention volumes, , the weight fraction activity coefficients of solute probes at infinite dilution, , and Flory-Huggins interaction parameters, between PMHPMA-co-VP-solvents were determined in 413-453 K. According to and , selected alcohols and alkanes were found to be non-solvent for PMHPMA-co-VP at 413-453 K. The glass transition temperature, T_g, of the PMHPMA-co-VP found to be 370 and 363 K, respectively, by IGC and DSC techniques, respectively.     

Analytical and numerical evaluation of two-fluid model solutions for laminar fully developed bubbly two-phase flows

An analysis of the two-fluid model in the case of vertical fully developed laminar bubbly flows is conducted. Firstly the phase distribution in the central region of the pipe (where wall effects vanish) is considered. From the model equations an intrinsic length scale L is deduced such that the scaled system reduces to a single equation without parameters. With the aid of this equation some generic properties of the solutions of the model for pipes with diameter greater than about 20L (the usual case, since L is of the order of the bubble radius) are found. We prove that in all physically meaningful solutions an (almost) exact compensation of the applied pressure gradient with the hydrostatic force occurs (with ρ_eff the effective density and the gravity). This compensation implies flat void fraction and velocity profiles in the central region not affected by the wall, even when no turbulence effects are accounted for.We then turn to consider the complete problem with a numerical approach, with the effect of the wall dealt via wall forces. The previous mathematical results are confirmed and the near-wall phase distributions and velocity profiles are found. With the numerical code it is also possible to investigate the regime in which the pressure gradient is greater than the weight of the pure liquid, in which case a region of strictly zero void fraction develops surrounding the axis of the pipe (in upward flow of bubbles).Finally, the same code is used to study the effect of reducing the gravity. As decreases, so does the relative velocity between the phases, making the lift force increasingly dominant. This produces, in upward bubbly flows, narrower and sharper void fraction peaks that also appear closer to the wall.     

Flow behavior and regime transition in a high-density circulating fluidized bed riser

Flow behavior and flow regime transitions were determined in a circulating fluidized bed riser (0.203 m i.d. × 5.9 m high) of FCC particles (, ). A momentum probe was used to measure radial profiles of solids momentum flux at several heights and to distinguish between local net upward and downward flow. In the experimental range covered (; ), the fast fluidization flow regime was observed to coexist with dense suspension upflow (DSU). At a constant gas velocity, net downflow of solids near the wall disappeared towards the bottom of the riser with increasing solids mass flux, with dense suspension upflow achieved where there was no refluxing of solids near the riser wall on a time-average basis. The transition to DSU conditions could be distinguished by means of variations of net solids flow direction at the wall, annulus thickness approaching zero and flattening of the solids holdup versus G_s trend. A new flow regime map is proposed distinguishing the fast fluidization, DSU and dilute pneumatic transport flow regimes.     

The effect of magnetic fields on the electrodeposition of cobalt

The effect of a magnetic field on the electrodeposition of Co has been investigated with respect to the strength and the orientation of the magnetic field (B). Two different effects of the magnetic field B on the electrodeposition of cobalt have been observed. The first is the magnetohydrodynamic (MHD) effect caused by the Lorentz force (). The second is an effect due to the paramagnetic force (), caused by the concentration gradient () and therefore the gradient of the molar susceptibility (). The limiting current density and the deposition rate of Co increases if the B-field is oriented parallel to the electrode surface. This is mainly due to the expected convection induced by . Both, the Co deposition and the reduction of hydrogen ions, are affected by this. At high cathodic potentials the contribution of the hydrogen reduction to the process changed, which led to homogeneous deposits. A decreasing deposition rate was measured for B-fields oriented parallel or antiparallel to the flow of ions. These results are attributed to the effect of on the electrochemical processes close to the surface.     

Solids mixing in the riser of a circulating fluidized bed

Gas/solid and catalytic gas phase reactions in CFBs use different operating conditions, with a strict control of the solids residence time and limited back-mixing only essential in the latter applications. Since conversion proceeds with residence time, this residence time is an essential parameter in reactor modelling. To determine the residence time and its distribution (RTD), previous studies used either stimulus response or single tracer particle studies.The experiments of the present research were conducted at ambient conditions and combine both stimulus response and particle tracking measurements. Positron emission particle tracking (PEPT) continuously tracks individual radioactive tracer particles, thus yielding data on particle movement in “real time”, defining particle velocities and population density plots.Pulse tracer injection measurements of the RTD were performed in a 0.1 m I.D. riser. PEPT experiments were performed in a small ( I.D.) riser, using ¹⁸F-labelled sand and radish seed. The operating conditions varied from 1 to 10 m/s as superficial velocity, and 25- as solids circulation rate.Experimental results were compared with fittings from several models. Although the model evaluation shows that the residence time distribution (RTD) of the experiments shifts from near plug flow to perfect mixing (when the solids circulation rate decreases), none of the models fits the experimental results over the broad (U,G)-range.The particle slip velocity was found to be considerably below the theoretical value in core/annulus flow (due to cluster formation), but to be equal at high values of the solids circulation rate and superficial gas velocity.The transition from mixed to plug flow was further examined. At velocities near U_tr the CFB-regime is either not fully developed and/or mixing occurs even at high solids circulation rates. This indicates the necessity of working at U> approx. ( to have a stable solids circulation, irrespective of the need to operate in either mixed or plug flow mode. At velocities above this limit, plug flow is achieved when the solids circulation rate . Solids back-mixing occurs at lower G and the operating mode can be described by the core/annulus approach. The relative sizes of core and annulus, as well as the downward particle velocity in the annulus (∼U_t) are defined from PEPT measurements.Own and literature data were finally combined in a core/annulus vs. plug flow diagram. These limits of working conditions were developed from experiments at ambient conditions. Since commercial CFB reactors normally operate at a higher temperature and/or pressure, gas properties such as density and viscosity will be different and possibly influence the gas-solid flow and mixing. Further tests at higher temperatures and pressures are needed or scaling laws must be considered. At ambient conditions, reactors requiring pure plug flow must operate at and . If back-mixing is required, as in gas/solid reactors, operation at and is recommended.     

Gas-solid adsorption of selected volatile organic compounds on titanium dioxide Degussa P25

This paper focuses on the adsorption of gaseous trichloroethylene, toluene and chlorobenzene on the photocatalyst TiO₂ Degussa P25. An optimized EPICS (Equilibrium Partitioning In Closed Systems) methodology was used to study equilibrium partitioning. For the three compounds investigated, equilibrium adsorption was reached within of incubation. Adsorption isotherms, determined at a temperature (T) of and relative humidities (RH) of 0.0% and 57.8% were found to be linear (R²>0.993,n=5), indicating that no monolayer surface coverage was reached in the concentration interval studied ). Within the linear part of the isotherm, the influence of both relative humidity and temperature was investigated in a systematic way and discussed from a thermodynamic point of view. Data analysis resulted in a double linear regression for 22% ?RH?90% and . The equilibrium adsorption coefficient represents the equilibrium concentration ratio and ΔU_ads is the internal energy of adsorption . At RH=0.0%, experimental K values were a factor 5-10 higher than those expected from the regression equation, indicating that another adsorption mechanism becomes important below monolayer surface coverage of TiO₂ by water vapour molecules. Since surface interactions are of primary importance in photocatalytic reactions, this paper contributes to a better understanding of the basic mechanisms of TiO₂ mediated heterogeneous photocatalysis and is an interesting tool for developing optimized mathematical models.     

Role of hydrodynamic flow parameters in lipase deactivation in bubble column reactor

The dynamic environment within the bioreactor and in the purification equipment is known to affect the activity and yield of enzyme production. In the present work, the effect of hydrodynamic flow parameters and τ_N,max) and interfacial flow parameters ( and ) on the activity of lipase has been comprehensively investigated in bubble column reactors. Lipase solution was subjected to hydrodynamic flow parameters in 0.15 and 0.385 m i.d. bubble column reactors over a wide range of superficial gas velocity (0.01<V_G<0.4-). The flow parameters were estimated using an in-house CFD simulation code based on k-ε approach. The extent of lipase deactivation in both the columns was found to increase with an increase in hydrodynamic and interfacial flow parameters. However, at equal value of any of these parameters, the extent of deactivation was different in the two columns. The rate of deactivation was found to follow first order kinetics. An attempt has been made to develop rational correlations for the extent of deactivation as well as for the deactivation constant. The rate of deactivation was found to be depending on the average turbulent normal stress and interfacial flow parameters such as bubble diameter and bubble rise velocity.