J.R.F. Guedes de Carvalho

An experimental study of the wake of gas slugs rising in liquids

A photographic study of the wakes of slugs rising in tubes of 19 mm and 52 mm internal diameter is presented. The dependence of the flow pattern in the wake upon the Reynolds number of the rising slug, R , is established for different slug lengths. Values of R covered in this study are in the range 25 to 1.3 × 10 4 . For low values of R the flow pattern in the wake is laminar and axisymmetric and values of wake length and wake volume could be determined from the photographs: these values were correlated with the other variables in the system by means of dimensional analysis.

Measurement of the coefficient of transverse dispersion in flow through packed beds for a wide range of values of the schmidt number

Experimental values of the coefficient of transverse dispersion (DT) were measured with the system 2-naphthol/water, over a range of temperatures between 293K and 373K, which corresponds to a range of values of viscosity (μ) between 2.83×10−4 Ns/m2 and 1.01×10−3 Ns/m2 and of molecular diffusion coefficient (Dm) between 1.03×10−9 m2/s and 5.49×10−9 m2/s. Since the density (ρ) of water is close to 103 kg/m3, the corresponding variation of the Schmidt number (Sc=μ/ρDm) was in the range 1000 – 50. More than 200 experimental values of the transverse dispersion coefficient were obtained using beds of silica sand with average particle sizes (d) of 0.297 and 0.496mm, operated over a range of interstitial liquid velocities (u) between 0.1mm/s and 14mm/s and this gave a variation of the Reynolds number (Re=ρduε/μ) between 0.01 and 3.5.Plots of the dimensionless coefficient of transverse dispersion (DT/Dm) vs. the Peclet number (Pem=ud/Dm) based on molecular diffusion bring into evidence the influence of Sc on transverse dispersion. As the temperature is increased, the value of Sc decreases and the values of DT/Dm gradually approach the line corresponding to ‘gas behaviour’ (i.e. Sc ≈ 1), which is known to be well approximated by the equation DT/Dm=1/τ+ud/12Dm, where τ is the tortuosity with regard to diffusion.

Dense phase expansion in fluidised beds of fine particles: The effect of pressure on bubble stability

Abstract An experimental study is described of the behaviour of Synclyst particles ( d av = 48μm) fluidised by air in a 0.05 m i.d. column; pressures in the range 1–22 bar were used. The expansion of the dense phase during bubbling is compared with maximum bed expansion before bubbling. The data are used to estimate the possible effect of Taylor instability on bubble.break-up at high pressure.

Mixing induced by air slugs rising in narrow columns of water

Abstract A series of equal-sized slugs of air was injected at the bottom of water columns of internal diameters 19 and 32 mm, respectively. The dispersion of tracer initially contained in the lower half of the columns was measured. A simple physical model was developed based on the idea that dispersion is mainly due to the action of the wakes of slugs, considered to be fully mixed; this model accounts remarkably well for the experimental results observed. An alternative dispersion model, based on the analogy with one-dimensional molecular diffusion, is also presented and the relationship between the two models is discussed. Experiments were performed for a range of values of slug volume, the number of slugs injected and the frequency of slug injection.

Liquid-side mass transfer coefficient for gas slugs rising in liquids

Abstract The present work reports a series of experimental measurements of liquid-side mass transfer coefficient for slugs rising in liquids with viscosities in the range 0.9 × 10 −3 −0.14 Pa s. Three column diameters were used (19, 32 and 52 mm) and slugs with values of L/D up to 38 were studied. The data obtained compare well with the theoretical predictions of van Heuven and Beek and they show the limited range of applicability of the empirical equations of Filla and Niranjan. The data reported for slugs rising in water in the 52 mm i.d. column clearly suggest that transition to turbulent film flow occurs at a value of z/D of about 11, the mass transfer in the turbulent portion being well predicted by correlations developed for wetted wall columns. A simple asymptotic expression for the evaluation of mass transfer coefficients in long slugs (i.e. slugs alongside which fully developed film flow is observed over a large portion of the total length) in viscous liquids was also developed and it is seen to compare very well with the experimental data.

Mass transfer during bubbling in single and multi-orifice absorbers

Abstract An experimental study is described of the process of mass transfer during continuous bubbling in single and multi-orifice plates. The regions of bubble formation and bubble rise are considered separately and the values of the corresponding interfacial areas, gas side and liquid side transfer coefficients are determined. For the multi-orifice absorbers the values of these parameters per orifice are compared with the corresponding values for single orifice absorbers. This provides a reasonable test of the idea that a multi-orifice absorber may be regarded as a set of single orifice absorbers operating in parallel.

Interfacial shear stress as a criterion for flooding in counter current film flow along vertical surfaces

Abstract This paper deals with the prediction of flooding in wetted-wall flow along vertical surfaces. A critical review of some published data, on flooding in wetted-wall tubes, is initially presented, and the data are compared with the predictions of two well-known empirical correlations. An alternative method is then developed for the prediction of flooding, based on physical reasoning coupled with dimensional analysis. The main conclusion is that, for any given set of properties of the liquid [in fact, for a constant value of the parameter Z=σ( ρ gμ 4 ) 1 3 ], the dimensionless interfacial shear stress between gas and liquid, at the point where flooding is about to start, should correlate with the Reynolds number for flow in the liquid film, Re= 4Q ν . Published data are then used to establish the form of that correlation, for systems where the liquid is water at room temperature (i.e. Z ≅3360). The proposed correlation is given as Eqs. (14) and (15) in the paper and these equations are then manipulated to yield a simple approximate criterion for the prediction of flooding, in vertical round tubes; according to this new criterion, flooding may be expected to occur when U g +u i >0.353 (ρgσ) 2 D μ g ρ 3 g 1 7 F where u i and F are known functions of Re . This criterion is then shown to accurately predict the recent finding that gas slugs will be subject to flooding at high pressure.

Drag force on the particles at the upstream end of a packed bed and the stability of the roof of bubbles in fluidised beds

Abstract A study is reported on the “bed support experiment” in which measurements of the minimum supporting velocity are performed for a wide range of conditions. Values of the drag coefficient for the particles at the upstream end of a packed bed are then calculated, and the data are seen to be well represented by The analysis of drag forces is applied to a study of stability of the roof of bubbles in fluidised beds. It is concluded that gas throughflow is sufficient to hold up the particles on the roof of spherical bubbles in 3D- beds, but that it is insufficient to prevent particles from raining down across cylindrical bubbles in 2D-beds.

Stripping in a bubbling pool under vacuum

Abstract A study is presented of the stripping of pentane from both a mixture of heavier n -paraffins (C 10 –C 13 ) and of sunflower seed oil. In the experiments a stream of pure nitrogen was bubbled through a small pool of liquid, at room temperature, for the three values of the absolute pressure (10 3 , 10 and 3 mbar) measured above the liquid. A theoretical model of the physical situation is presented and its predictions are compared with the experimental data. Two important findings related to operation at (sufficiently) low pressures are that (i) the efficiency of mass transfer may be lower for the lower pressures and (ii) equilibrium between gas and liquid may be unattainable irrespective of the depth of bubbling pool.

A new experimental technique to study backmixing in packed bubble columns

Abstract A new experimental technique is presented for the measurement of axial dispersion of a liquid in semibatch packed bubble columns. Values of the dispersion coefficient were obtained in a 0.10 m diameter column with five different packings. The liquid used were water and mixtures of glycerine and water with viscosities up to 1.0 Pa s. The data reported show the dispersion coefficient to decrease markedly with decrease in the size of the packing elements and with increase in the liquid viscosity.