Computational flow modelling and design of bubble column reactors

Circulating bubble columns or loop reactors form one of the important classes of modified bubble columns. The present paper analyses the performance of external loop air lift reactors (EL-ALR). The EL-ALR has many advantageous features especially at large scale. These arise from its feature of having controlled liquid circulation which is the key parameter for the design and operation of EL-ALR contactors. Therefore, a reliable and generalised circulation model has been presented. The reliability of the model depends on the accuracy of the predictive methods for the gas hold-up and the two-phase fractional pressure drop. Detailed analysis of these fundamental parameters of gas-liquid flows has been presented. The effects of design (area ratio of riser to downcomer, height to diameter ratio and volume of reactor) and operating (gas flow rates and sparging locations) parameters on the performance of the EL-ALR have been analysed in detail. A rational basis has been developed for the estimation of pressure drop, mixing time and mass transfer coefficient. An optimum combination of design and operating parameters has been suggested and a criterion has been developed for the optimum location of spargers. An attempt has been made to provide critical analysis of the published information and to construct a coherent picture of EL-ALR.

Sparged loop reactors

Publisher Summary This chapter reviews that there are four types of fischer-tropsch (FT) reactor in commercial use at present. Three broad categories of catalyst are used in these reactors. The four types of reactor are: circulating fluidized bed reactor, fluidized bed reactor, tubular fixed bed reactor, and slurry phase reactor. It discusses that the fluidized bed reactors operate in the temperature range 320oC to 350oC. This temperature range is 100oC higher than the typical operating temperature range used with the reactors of around 220oC to 250oC. Hence, the term high temperature fischer-tropsch (HTFT) is used to describe the reactors on the left hand side and the term low temperature FT is used to describe the reactors on the right hand side. The key distinguishing feature between the HTFT and LTFT reactors is the fact that there is no liquid phase present outside the catalyst particles in the HTFT reactors. The chapter also discusses that as compared to many industrial operations the FT reaction is highly exothermic. This is an order of magnitude higher than the typical catalytic reactions in the oil refining industry. Any increase in the operating temperature of the FT synthesis will result in an undesirable increase in the production of methane and may result in catalyst damage.

Fischer-Tropsch Reactors

A review of existing literature for the purpose of comparing correlations which allow the explicit predictiosn of gas holdup in two- or three-phase bubble columns shows wide disagreement concerning the effect of various variables. Further, various correlations, apparently of equal validity, show equally wide disagreement in the predicted absolute values of gas holdup among three test systems; water-air, hydrocarbon solvent—air, and trichloroethytene air. Some possible reasons for these disagreements are discussed.



Extensive gas holdup data obtained in a 0.30 m diameter column for the three test systems were correlated statistically for the transition and the turbulent region. The resulting correlation gives agreement with experiment to within ±20%, with most data showing ± 10% agreement. The proposed correlation was also tested against thirteen data sets (from eight researchers) which corresponded to its region of validity and gave good agreement. Three additional relevant data sets were not satisfactorily predicted.



On a precede a une etude bibliographique dans le but de comparer des correlations permettant de predire explicitement la retention des gaz dans des colonnes a bulles a deux ou trois phases et on a montre que ces correlations ne s'accordent pas sur l'effet des differentes variables. De plus, diverses correlations, apparemment de měme validite, montrent egalement un desaccord dans les valeurs absolues predites pour la retention des gaz dans le cas de trois systemes-tests: eau-air, solvant a base d'hydrocarbures–air et trichlorethylene-air. On analyse quelques raisons pouvant expliquer ces desaccords.



On a correle de maniere statistique un grand nombre de donnees de retention des gaz obtenues dans une colonne de 0,30 m de diametre pour les trois systemes-tests, dans la region de transition et la region turbulente. L'ecart entre la correlation obtenue et l'experience est de ±20%, la majorite des donnees ne presente pas d'ecart superieur a ± 10%. La correlation proposee a ete egalement testee avec treize series de donnees (produites par huit chercheurs differents) compatibles avec sa region de validite et montre un bon accord. Trois autres series de donnees pertinentes ont ete predites de maniere non satisfaisante.

A correlation for gas holdup in turbulent coalescing bubble columns

Liquid phase mixing in bubble columns with Newtonian and non-Newtonian fluids

Liquid Telocity distribution was measured in a bubble column 5.5 m in diameter and approximately 7 m in liquid height. Liquid flow was upward in the column center and downward near the wall. The upward velocities were from 52 to 115 cm/sec in the superficial gas velocity range from 2.4 to 8.5 cm/sec. The liquid velocity distribution is analyzed by a circulating flow model. Values of the turbulent viscosity-the parameter of the model-are in the range from 3000 to 4000 g/cm • sec. These values agree well with those estimated by an empirical formula for turbulent viscosity proposed by Miyauchi and Shyu for smaller-diameter bubble columns.

Hajime Unno

Papers

Liquid phase velocity in A 5.5M diameter bubble column.