Gas-liquid Mass Transfer Limitations Research Articles

Determination of oxygen gas–liquid mass transfer rates in heap bioleach reactors

A detailed experimental study is described which was conducted to determine the rate of oxygen gas–liquid mass transfer within the packed bed of heap bioleach reactors at different temperatures (22–68 °C), using the Na 2SO 3 method. The raw data was analysed using a simplified film mass transfer model, making corrections for oxygen solubility in concentrated solution and for increased water vapour partial pressure at elevated temperatures. The results compared favourably against two independent experimental leach studies, indicating k La values between 33 and 46 h −1. The value varied with the particle size distribution of the packing, with k La assuming larger values for those packings that had a higher fines contents. While k La increases with temperature, the solubility of oxygen decreases simultaneously, resulting in net mass transfer rates being relatively unaffected by temperature in the range studied. This indicates that thermophile heap bioleach reactors are likely to operate under gas–liquid mass transfer limitations, especially at high altitude.

Trickle – Bed reactor simulation using a process simulator

The present study deals with a multiple reaction system in both gas and liquid phases considering the effect of gas-liquid mass transfer limitations in a trickle-bed reactor where the catalytic hydrotreating of gas oil reaction is being carried out. The hydrodesulfurization (HDS) and hydrogenation (HYD) of aromatics reactions over NiMo and CoMo catalysts are taken into account in the model. The reactor model was codified as a user-added unit operation in PROvision process simulator (V 5,0), in FORTRAN language. This model is used to predict reactant conversion and product distribution as a function of inlet condition. The differential equations are solved by the LSODE program. The PROvision process simulator provides the database for physical and thermodynamics properties of the components through appropriate liquid-vapor equilibrium models.

Simulation of Slurry Polymerization of Ethylene

A model of slurry polymerization of ethylene in a multistage continuous stirred-tank reactor (CSTR) was developed in order to find the effect of stagewise variation of parameters on polymer polydispersity and rate of polymerization. Higbie's penetration theory has been used to calculate monomer absorption rate in the presence of micron-sized single site type catalyst in slurry phase. The residence time distribution of the growing polymer particles in an ideal CSTR was modelled by a relaxation-type unsteady state approach. The effect of gas-liquid mass transfer limitations on polymer properties is predicted for various reactor configurations. An analysis of various possible configurations of two-stage CSTR with stagewise variation of the partial pressures of ethylene and/or hydrogen has been made.

Alkaline Biofiltration of H2S Odors

Hydrogen sulfide (H2S) is a very common odor nuisance which is best controlled by chemical or biological scrubbing. Under alkaline pH, the amount of H2S that can be solubilized in a scrubbing liquid increases significantly, and therefore, gas-liquid mass transfer limitations can be reduced. To date, biological scrubbing of H2S has been limited to neutral or acidic pH, despite the potential benefit of reduced mass transfer limitations at alkaline pH. In the present paper, an alkaliphilic sulfoxidizing bacterial consortium was deployed in a laboratory-scale biotrickling filter treating H2S at pH 10. The gas contact time ranged from 1 to 6 s, and H2S inlet concentrations, from 2.5 to 18 ppm(v). The results showed that under most conditions, H2S removal exceeded 98% and the degradation end-product was sulfate. At the highest H2S concentrations and shortest gas contacttimes, when the loading exceeded 30 g m(-3) h(-1), the H2S removal efficiency decreased significantly due to biological reaction limitation, and incompletely oxidized sulfides were measured in the trickling liquid. An analysis of the process demonstrated that operating the biotrickling filter at high pH results in an enhancement of the mass transfer by a factor of 1700-11 000. Overall, alkaline biotrickling filtration was shown to be very effective at low concentration of H2S and very short gas contact time. This is the first demonstration of a biotrickling filter for air pollution control operated at high pH.

Measurement and enhancement of gas–liquid mass transfer in milliliter-scale slurry reactors

Due to the limited availability of chemical reactants in the early process development of pharmaceuticals and fine chemicals, and sometimes the high-cost of catalyst, it is increasingly popular to use milliliter-scale slurry reactors with reaction volumes of 20 ml or less to screen catalyst candidates for three-phase reactions. To ensure the success of catalyst screening, it is advantageous to run reactions under kinetically controlled conditions so that the activities of different catalysts can be compared. Because catalysts with small particle sizes are used in slurry reactors, the reactions are susceptible to gas–liquid mass transfer limitations. This work presents an efficient way of enhancing gas–liquid mass transfer in milliliter-scale reactors through the use of magnetically driven agitation with complex motion. In the reactor described here, gas–liquid mass transfer coefficients can be doubled over those obtained with the agitation technique used in commercial milliliter-scale units. In addition, the reactor can achieve the top range of mass transfer coefficients obtained in a full-scale reactor. This work also presents the first measurements of gas–liquid mass transfer coefficients in milliliter-scale reactors, which are two orders-of-magnitude smaller than systems for which mass transfer coefficients have been reported earlier. Both physical and chemical absorption techniques are used.

Simultaneous measurement of in situ bubble size and reaction rates with a heterogeneous catalytic hydrogenation reaction

The effect of solvent composition (water, 2-propanol and mixtures of the two) on the three phase, heterogeneously catalysed, two-step hydrogenation of 2-butyne-1,4-diol on Pd/alumina was investigated in a stirred reactor and for the first time, bubble sizes were measured in situ simultaneously with reaction rates. The first step forming the alkene is slow and gas–liquid mass transfer limitation is unlikely whilst the second step to alkane is much faster and may be mass transfer dominated. Overall, reaction rates displayed a complex relationship with solvent composition. The smallest bubbles where observed with 5% v/v 2-propanol in water and under gas–liquid mass transfer limited conditions (energy dissipation rates < 4 W / kg ), the overall reaction closely follows the ratio of mean bubble diameter to hydrogen solubility, both of which are a function of solvent. Overall, the solvent 5% v/v 2-propanol/water gave faster reactions combined with good selectivity compared to the others and is therefore recommended.

High-rate acidophilic ferrous iron oxidation in a biofilm airlift reactor and the role of the carrier material

In this study, the feasibility and engineering aspects of acidophilic ferrous iron oxidation in a continuous biofilm airlift reactor inoculated with a mixed culture of Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans bacteria were investigated. Specific attention was paid to biofilm formation, competition between both types of bacteria, ferrous iron oxidation rate, and gas liquid mass transfer limitations. The reactor was operated at a constant temperature of 30 degrees C and at pH values of 0-1.8. Startup of the reactor was performed with basalt carrier material. During the experiments the basalt was slowly removed and the ferric iron precipitates formed served as a biofilm carrier. These precipitates have highly suitable characteristics as a carrier material for the immobilization of ferrous iron-oxidizing bacteria and dense conglomerates were observed. Lowering the pH (0.6-1) resulted in dissolution of the ferric precipitates and induced granular sludge formation. The maximum ferrous iron oxidation rate achieved in this study was about 145 molFe(2+)/m(3).h at a hydraulic residence time of 0.25 h. Optimal treatment performance was obtained at a loading rate of 100 mol/m(3).h at a conversion efficiency as high as 98%. Fluorescent in situ hybridization (FISH) studies showed that when the reactor was operated at high ferrous iron conversion (>85%) for 1 month, the desirable L. ferrooxidans species could out-compete A. ferrooxidans due to the low Fe(2+) and high Fe(3+) concentrations.

Influence of mass transfer on regio- and enantioselectivity in hydrogenation of 1-phenyl-1,2-propanedione over modified Pt catalysts

The influence of mass transfer limitations on the enantio- and regioselectivity in hydrogenation of 1-phenyl-1,2-propanedione was studied. Gas–liquid mass transfer limitation as well as liquid–solid mass transfer limitation did not influence the enantio- and regioselectivity of the reaction, compared to the performance of the system under kinetic control. Under various regimes, the enantiomeric excess of the ( R)-1-hydroxy-1-phenylpropanone was about 55% and regioselectivity remained at about 10, which makes process scale-up with respect to enantio- and regioselectivity straightforward.

Gas–liquid–liquid reaction engineering: the Koch synthesis of pivalic acid from iso- and tert-butanol; Reaction kinetics and the effect of a dispersed second-liquid phase

In gas–liquid–liquid reaction systems with fast parallel and consecutive reactions the effects of mass transfer and mixing on the product yield can be significant. The Koch synthesis of pivalic acid, using sulfuric acid as catalyst, was chosen to study these effects. Reaction kinetics and the effect of the catalyst-phase composition have been investigated by using isobutanol as reactant. For studying the effect of an immiscible liquid phase on the reaction products obtained, the more reactive tert-butanol was used. Pivalic acid can be produced from isobutanol using sulfuric acid as a catalyst solution with 2-methylbutanoic acid as main byproduct, if gas–liquid mass transfer limitations are excluded. The selectivity towards 2-methylbutanoic acid is generally less than 20% and decreases strongly with decreasing acidity. The reaction is first order in isobutanol and dehydration is likely to be rate determining. The presence of pivalic acid and isobutanol strongly reduces the apparent reaction rate constant by decreasing the solution acidity (Ho). For the industrially applied backmixed reactors in the Koch synthesis, this may imply that these operate at much lower values for Ho. On addition of an immiscible heptane phase, the reaction products are extracted to some extent and this adds to maintaining a high catalyst solution acidity. Using tert-butanol, the yield and pivalic acid selectivity was found to depend strongly on CO transport to the reaction zone through gas–liquid mass transfer and mixing. The presence of an immiscible heptane phase increased the product yield and selectivity towards pivalic acid significantly.

A Reaction Calorimetric Investigation of the Hydrogenation of a Substituted Pyrazine

Reaction calorimetry was the primary tool in investigations of the liquid-phase hydrogenation of a substituted pyrazine compound carried out over supported Pd catalysts. In addition to providing information about gas-liquid mass transfer limitations, calorimetric measurements provided a kinetic analysis of how product selectivity in this consecutive reaction network may be tuned by changing reaction conditions. Concentration profiles developed from fitting the heat flow data to a first-order model were compared with analytical measurements of concentration, demonstrating the utility of combining independent measurements of reaction progress as a means of assessing the validity of a kinetic model.

Biological sulphate reduction using gas‐lift reactors fed with hydrogen and carbon dioxide as energy and carbon source

Feasibility and engineering aspects of biological sulphate reduction in gas-lift reactors were studied. Hydrogen and carbon dioxide were used as energy and carbon source. Attention was paid to biofilm formation, sulphide toxicity, sulphate conversion rate optimization, and gas-liquid mass transfer limitations. Sulphate-reducing bacteria formed stable biofilms on pumice particles. Biofilm formation was not observed when basalt particles were used. However, use of basalt particles led to the formation of granules of sulphate-reducing biomass. The sulphate-reducing bacteria, grown on pumice, easily adapted to free H(2)S concentrations up to 450 mg/L. Biofilm growth rate then equilibrated biomass loss rate. These high free H(2)S concentrations caused reversible inhibition rather than acute toxicity. When free H(2)S concentrations were kept below 450 mg/L, a maximum sulphate conversion rate of 30 g SO(4) (2-)/L x d could be achieved after only 10 days of operation. Gas-to-liquid hydrogen mass transfer capacity of the reactor determined the maximum sulphate conversion rate.

Polymerization of olefins through heterogeneous catalysis—V. Gas-liquid mass transfer limitations in liquid slurry reactions

Journal of Applied Polymer ScienceVolume 36, Issue 6 p. 1513-1513 ErratumFree Access Polymerization of olefins through heterogeneous catalysis—V. Gas-liquid mass transfer limitations in liquid slurry reactions S. Floyd, S. Floyd Department of Chemical Engineering, University of Wisconsin, Madison, Wisconsin 53706Search for more papers by this authorR. A. Hutchinson, R. A. Hutchinson Department of Chemical Engineering, University of Wisconsin, Madison, Wisconsin 53706Search for more papers by this authorW. H. Ray, W. H. Ray Department of Chemical Engineering, University of Wisconsin, Madison, Wisconsin 53706Search for more papers by this author S. Floyd, S. Floyd Department of Chemical Engineering, University of Wisconsin, Madison, Wisconsin 53706Search for more papers by this authorR. A. Hutchinson, R. A. Hutchinson Department of Chemical Engineering, University of Wisconsin, Madison, Wisconsin 53706Search for more papers by this authorW. H. Ray, W. H. Ray Department of Chemical Engineering, University of Wisconsin, Madison, Wisconsin 53706Search for more papers by this author First published: 5 September 1988 https://doi.org/10.1002/app.1988.070360621Citations: 1AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume36, Issue65 September 1988Pages 1513-1513 RelatedInformation

Polymerization of olefins through heterogeneous catalysis—V. Gas‐liquid mass transfer limitations in liquid slurry reactors

AbstractMany processes for polymerization of olefins employ laboratory, pilot plant, or full‐scale liquid‐phase polymerization reactors with monomer introduced as a gas. Criteria for the presence of gas‐liquid mass transfer resistance in these systems are determined in terms of observed reaction rate or loading of a heterogeneous catalyst of given intrinsic activity. The effects of variables such as reactor size and configuration, temperature, and soluble polymer are also examined. The equilibrium monomer concentrations of ethylene in hexane and propylene in heptane are calculated through a modified Benedict‐Webb‐Rubin equation, and some calculations for ethylene‐propylene mixtures are tabulated. The general methodology for predicting gas‐liquid mass transfer resistance is readily extendible to copolymerization systems.

MASS TRANSFER LIMITATIONS IN COAL LIQUEFACTION

ABSTRACT In a number of contributions to coal liquefaction, gas–liquid mass transfer limitation has been referred by neglecting mass transfer limitation of solvent Into solid coal. Reported apparent activation energies of coal liquefaction reactions are In the range of 10–20 kCal/g–mole, which are too low for thermal reactions, Imply that diffusion Is the rate controlling step. In this study, the solvent diffusion effect on solvolysis of vitrinitic macerals In Alberta subbituminous coal has been studied, by performing petrographlc analyses of original coal and liquefaction residues. Quasi–steady state and transient coal–solvent diffusion-chemical reaction models are developed to Interpret the experimental data. Results indicate that solvent diffusion into the reactive maceral most likely be a rate limiting step In coal liquefaction.

Modeling the Fischer-Tropsch synthesis in the slurry phase

A model of the FT synthesis in the slurry phase is being developed. It is based on first-order rate expression for hydrogen consumption but considers all relevant hydrodynamic phenomena. The parameters involved in the model equations were estimated from literature correlations and recent experimental results. The basic set of operational conditions was that applied in the Rheinpreussen-Koppers demonstration plant. The computations show that liquid solid mass transfer resistances and catalyst settling can be neglected in larger diameter columns. Gas-liquid mass transfer limitations are also small as long as conventional catalysts and favorable gas velocities are applied. The simulated results are in general agreement with practical experience. 52 refs.