Ultrasound in gas–liquid systems: Effects on solubility and mass transfer

The effect of ultrasound on the pseudo-solubility of nitrogen in water and on gas–liquid mass transfer kinetics has been investigated in an autoclave reactor equipped with a gas induced impeller. In order to use organic liquids and to investigate the effect of pressure, gas–liquid mass transfer coefficient was calculated from the evolution of autoclave pressure during gas absorption to avoid any side-effects of ultrasound on the concentrations measurements. Ultrasound effect on the apparent solubility is very low (below 12%). Conversely ultrasound greatly improves gas–liquid mass transfer, especially below gas induction speed, this improvement being boosted by pressure. In typical conditions of organic synthesis: 323 K, 1100 rpm, 10 bar, kL a is multiplied by 11 with ultrasound (20 kHz/62.6 W). The impact of sonication is much higher on gassing out than on gassing in. In the same conditions, this enhancement is at least five times higher for degassing

Adsorption of aqueous organic mixtures on activated carbon: experiments and modelling of competitive adsorption

Experiments and modelling of competitive adsorptio

Refractory compounds abatement by adsorption and oxidative catalytic regeneration on activated carbon Experimental and modelling of competitive adsorption

Experimental and modelling of competitive adsorptio

Selective hydrogenation in trickle-bed reactor. Experimental and modelling including partial wetting.

A steady state model of a trickle bed reactor is developed for the consecutive hydrogenation of 1,5,9-cyclododecatriene on a Pd/Al2O3 catalyst. Various experiments have shown that the selectivity of this reaction towards the product of interest is much lower in co-current down-flow (trickle-bed) than in up-flow. This is due to uneven liquid distribution and to partial wetting of the catalyst surface at low liquid flow rates. The non-isothermal heterogeneous model proposed here takes into account the partial wetting of the catalyst, as well as the resistances to heat and mass transfer at the gas-liquid, liquid-solid and solid-gas interfaces. It assumes that the catalyst particles can be divided into two distinct concentration zones corresponding to the wetted and dry catalyst surfaces; mass transfer between these two zones is described by a simplified diffusion mechanism. Compared to previous models assuming a uniform concentration of liquid-phase components inside the catalyst particles, this model improves the prediction of the outlet concentrations of hydrogenation products

Theoretical analysis of tracer method for the measurement of wetting efficiency

This work investigates the tracer technique for the measurement of catalyst wetting efficiency, f, in trickle-bed reactor. The model of Ramachandran et al. (1986), based on a 2D description of the tracer diffusion, is applied for the full range of wetting efficiency. It is also extended to account for the effects of axial dispersion, liquidsolid mass transfer, pattern of the wetted zone on the pellet, and distribution of the partial wetting along the reactor. The numerical method for parameter optimisation implies a frequency domain least-squares procedure. A sensitivity analysis has been performed proving the wetting efficiency may be derived with convenient accuracy in usual trickle-bed conditions: high Peclet and Biot numbers. For f>0.3, it is shown that wetting efficiency can be accurately calculated from apparent particle diffusivities derived in liquid-full conditions (Deapp,LF) and in partial wetting regime (Deapp,TB), using the following relation: f=sqrt(Deapp,TB)/(Deapp,LF)

AD–OX: A sequential oxidative process for water treatment— Adsorption and batch CWAO regeneration of activated carbon

A sequential process for water treatment involving usual adsorption on activated carbon (AC) followed by wet air catalytic oxidation of the adsorbed pollutants has been carried out in a fixed bed reactor with a mixture of two model pollutants. The first step achieves water purification while the second one reduces the organic pollution but also, more importantly, performs some AC in situ regeneration. The experimental work has been done with AC yet extensively used and stabilized by long range continuous oxidation. The two steps have been analyzed successively showing very important drop of adsorption capacity with respect to fresh AC but efficient oxidative partial regeneration. As expected with used AC no more evolution occurs in between two consecutive runs. The first step of competitive adsorption has been simulated by a model leading to higher diffusivities than estimations based on correlations. The main features of the complex second step, involving simultaneous non-isothermal desorption and three phase catalytic reaction, are qualitatively explained

Dynamics of internal diffusion during the hydrogenation of 1,5,9-cyclododecatriene on Pd/Al2O3

Dynamic and pseudo-steady state diffusion–reaction models were simulated for the three-phase consecutive hydrogenation of 1,5,9-cyclododecatriene on a shell Pd/Al2O3 catalyst in order to examine the time evolution of concentration profiles inside the catalyst pellet: a model accounting only for the dynamics of the active layer and a model taking also into account the inert part of the catalyst were compared. In the conditions of the semibatch experiments (T=433 K, pH2=1.2 MPa), all the models lead to the same bulk concentration–time curves, but the hydrocarbon concentration profiles in the pores are dependent on the model. The influence of the diffusion in the inert part of the catalyst on the bulk concentrations becomes nonnegligible only when the external liquid volume (out of the catalyst) is reduced. The transient evolution of the concentration profiles in the pores show that hydrogen concentration reaches its steady state within a few seconds, while the evolution of the organic concentration profiles is slower. Furthermore, the reaction rate has been found to be only affected by the hydrogen diffusion. The diffusion of organics can control the reaction rate only for low values of organic concentration and higher pressure in hydrogen

Effect of partial wetting on liquid/solid mass transfer in trickle bed reactors

The wetting efficiency of liquid trickle flow over a fixed bed reactor has been measured for a wide range of parameters including operating conditions, bed structure and physico-chemistry of liquid/solid phases. This data bank has been used to develop a new correlation for averaged wetting efficiency based on five different non-dimensional numbers. Finally liquid/solid mass transfer has been determined in partial wetting conditions to analyse what are the respective effects of wetting and liquid/gas flow turbulence. These effects appear to be separated: wetting being acting on liquid/solid interfacial area while the liquid/solid mass transfer coefficient is mainly connected to flow turbulence through the interstitial liquid velocity. A correlation has been proposed for liquid/solid mass transfer coefficient at very low liquid flow rate

Selective hydrogenation of 1,5,9-cyclododecatriene in up- and down-flow fixed-bed reactors: experimental observations and modeling

The performance of trickle and flooded-bed reactors has been investigated and compared for an exothermic multi-step catalytic reaction. Selective hydrogenation of cyclododecatriene over Pd/Al2O3 has been studied in both up- and down-flow modes of operation in the same pilot reactor. In the down-flow mode, hot spots and runaway could not be avoided without diluting both catalyst bed and liquid reactant. With this diluted system, the up-flow reactor leads to a higher productivity and a much better selectivity. A non-isothermal plug-flow reactor model predicts the performances of the up-flow reactor satisfactorily, but is found to be unsuitable to the case of a trickle-bed reactor. In the latter case, the productivity was underestimated, when complete wetting of catalyst particles was assumed. On the other hand, when partial wetting effect was incorporated, the calculated selectivity was always much higher than that observed actually in a trickle bed, due to heterogeneities of liquid velocity and partial wetting (poorly irrigated zones

Catalytic Wet Air Oxidation of Aqueous Organic Mixtures

Catalytic Wet Air Oxidation (CWAO) has been investigated for the treatment of water contaminated by 4-hydroxybenzoic acid (4HBA) and equimolar mixture of phenol-4HBA. Both batch measurements for kinetics determination and continuous fixed bed operation have been performed on the same Activated Carbon (AC). After a fast initial deactivation AC was proved stable and efficient at moderate temperature and oxygen pressure, like for phenol degradation. The kinetic study in the case of highly adsorbing material as AC may require complex approach to account for the variation of adsorbed reactants during batch oxidation. Adsorption isotherms at reaction temperature and with aged AC have been obtained according to Langmuir equation and used in 4HBA mass balance to derive more significant kinetic parameters. At high catalyst loading and relatively low pollutant concentration, the variation of 4HBA during the batch may be even higher on the solid than in the aqueous phase