The use of the chemical method for the determination of interfacial areas in gas-liquid contactors

The interfacial area achem in a gas-liquid contactor as determined by the chemical method deviates from the true geometrical interfacial area ageo, because the overall conversion of the gas phase reactant represents an incorrect average if bubble sizes and residence times are not uniform. The deviations of achem fromageo become larger the broader the distribution τb/db and the higher the overall conversion ΩA of the reactant in the gas phase. Model calculations, which take into account both the effect of gas phase backmixing as well as the effect of bubble coalescence on the deviation of achem from ageo, are performed for a mechanically agitated gas—liquid reactor and a bubble column at practical micro- and macromixing conditions. For a gas-liquid model reaction, which is first-order in the gas phase reactant, it is found that: (1) for a mechanically agitated reactor the error in achem will always be smaller than 10% if ΩA is lower than 0.99, and (2) for a bubble column the error in achem will be smaller than 20% for most practical applications if ΩA is lower than 0.99. Gas-liquid model reaction systems with absorption of CO2 in alkanolamine solutions are recommended for the determination of interfacial area in gas—liquid contactors

Mass transfer phenomena and hydrodynamics in agitated gas—liquid reactors and bubble columns at elevated pressures: State of the art = Stoffübergangserscheinungen und hydrodynamik in gerührten gas-flüssigkeitsreaktoren und blasensäulen bei hohen drucken: Stand der technik

All important studies on the influence of pressure on mass transfer phenomena in gas—liquid systems and reactors are reviewed critically. Points of agreement and conflict are indicated and discussed.\ud It is concluded that: (1) the initial bubble size at a single orifice decreases with increasing pressure; (2) the gas-phase mass transfer coefficient kG is inversely proportional to the pressure to the power n, where n depends on the mass transfer mechanism; (3) the liquid-phase mass transfer coefficient kL is not influenced by pressure; (4) the gas hold-up G in bubble columns increases with increasing pressure.\ud However, insufficient data on the influence of the operating pressure on the interfacial areas in gas—liquid contactors are available

The kinetics of the reaction between CO2 and diethanolamine in aqueous ethyleneglycol at 298 K: a viscous gas—liquid reaction system for the determination of interfacial areas in gas—liquid contactors

The reaction between CO2 and diethanolamine (DEA) in aqueous ethyleneglycol (ETG) at 298 K has been studied over the complete composition range. The application of this reaction as a viscous gas—liquid system for the determination of interfacial areas in gas—liquid contactors by the chemical method is discussed. The reaction kinetics have been determined by mass transfer experiments of CO2 into solutions of DEA in aqueous ETG. To this end laboratory-scale stirred cell reactors with a flat surface have been used. In accordance with the same reaction in water at 298 K the reaction between CO2 and DEA in aqueous ETG at 298 K can be described by the zwitterion mechanism of Caplow. Special attention has been paid to the reversibility of the reaction between CO2 and DEA. Calculation show that the influence of the reversibility on the mass transfer rate can be neglected for partial pressures of CO2 below 3 kPa. It is demonstrated that the reaction between CO2 and DEA in aqueous ETG can be used for the determination of interfacial areas in gas—liquid contactors at higher viscosities

Development of catalytic hydrogenation reactors for the fine chemicals industry

A survey is given of the problems to be solved before catalytic hydrogenation reactors can be applied in a multiproduct plant in which selectivity problems are experienced. Some results are reported on work done on the reaction kinetics of two multistep model reactions and on mathematical modelling and experimental verification of the models. Since hydrogenation reactions are often very exothermic, cooling by solvent evaporation has been applied where appropriate. Sufficient information has been collected and correlated to enable operation of multiproduct catalytic reactors of the slurry or packed bubble column type; interdependence of operating variables is so complex that a mathematical model is indispensable

Methanol synthesis in a countercurrent gas-solid-solid trickle flow reactor. An experimental study

The synthesis of methanol from CO and H2 was executed in a gas-solid-solid trickle flow reactor. The reactor consisted of three tubular reactor sections with cooling sections in between. The catalyst was Cu on alumina, the adsorbent was a silica-alumina powder and the experimental range 498–523 K, 5.0–6.3 MPa and 0.2–0.33 molar fraction of CO. Complete conversion in one pass was achieved for stoichiometric feed rates, so that the gas outlet could be closed. The experimental results are compared with the model presented in the previous paper [Westerterp, K.R. and Kuczynski, M. (1987) Chem. Engng Sci.42,]; agreement is close over the entire conversion range from 15% to 100%

Interfacial areas and gas hold-ups in gas-liquid contactors at elevated pressures from 0.1 to 8.0 MPa

Interfacial areas and gas hold-ups have been determined at pressures up to 8.0 MPa in a mechanically agitated gas—liquid reactor and a bubble column with a diameter of 81 mm for superficial gas velocites between 1 and 5 and 1 and 10 cm/s, respectively. The interfacial areas have been determined by the chemical method using the model reaction between CO2 and aqueous diethanolamine (DEA). Contrary to earlier reported results on interfacial areas in a mechanically agitated reactor at pressures up to 1.7 MPa, a positive influence of pressure on the interfacial areas has been observed for higher pressures and higher superficial gas velocities. The product of the gas density G and the superficial gas velocity at the orifice υG, or was found to be an important parameter for the manifestation of the pressure effect. For values of GυG, or larger than 200 kg/m2 s the interfacial areas increase with increasing reactor pressure. Below this value of 200 kg/m2 s no influence of pressure could be observed. The gas hold-ups in the bubble column in water as well as in an aqueous solution of DEA with antifoam increase with increasing pressure. This pressure effect on the gas hold-up in bubble columns originates from the formation of smaller bubbles at the gas distributor. The relative increase in the gas hold-ups is smaller in water and also if a porous plate instead of a perforated plate is used as gas distributor. The differences in the magnitude of the pressure effect are caused by differences in the coalescence behaviour of the gas bubbles in both liquids and by differences in the bubble formation process at the two types of gas distributors, respectively. The interfacial areas in the bubble column also increase with increasing pressure. The relative increase in the interfacial areas aP/aatm with increasing pressure may be as large as 200% for a pressure increase from P = 0.15 to 8.0 MPa, depending on the type of gas distribution and the superficial gas velocity used