Phase transfer catalysis: chemistry and engineering

Phase transfer catalysis (PTC) uses catalytic amounts of phase transfer agents which facilitate interphase transfer of species, making reactions between reagents in two immiscible phases possible. PTC is used widely in the synthesis of various organic chemicals in both liquid-liquid and solid-liquid systems. Existing literature on PTC is chemistry-intensive and a mere handful of recent articles constitute the entire information on engineering analysis. This article reviews the field comprehensively by combining the existing knowledge from chemistry with insights into mechanistic and kinetic analysis and mathematical modeling of soluble and insoluble PTC. By its very nature, PTC involves a series of equilibrium and mass-transfer steps, beside the two main reactions. Neglect of mass-transfer effects can grossly overpredict the conversion of a PTC mediated reaction. A practical way of using PTC, which enables easy separation, is to immobilize the catalyst on a solid support. Mass-transfer limitations and higher costs, however, have precluded its commercial use so far, requiring further analysis of mass-transfer limitations in these complex three-phase systems. The use of PTC, combined with other rate enhancement techniques like sonochemistry, microwaves, electroorganic synthesis, and photochemistry, is being increasingly explored. Applications in this area in the manufacture of organic intermediates and fine chemicals seem almost unlimited

Effect of interaction and mobility on fixed-bed reactor performance

The roles of interaction and mobility in determining surface rates and hence reactor performance, as reflected in the space time values required for achieving a desired conversion or selectivity, are demonstrated. It is shown that localized adsorption models predict larger space times in comparison to the mobile models. Further, repulsive forces lead to higher space times in comparison to attractive forces, and the divergence between models is most marked for localized adsorption

Influence of catalyst deactivation on the nature of the steady state solutions for reactions on catalytic surfaces

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Effectiveness factors in bidispersed catalysts under conditions of catalyst fouling

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Studies on the dissolution rate of sparingly soluble calcium citrate in water

The mass transfer coefficient of calcium citrate-water system was determined by the conductivity measurement. The range of the measured coefficient is 2.65 × 10-6m/s ± 35% at 25.5°C, independent of particle size and rpm under a full solid suspension condition. The prediction of the effects of particle size and agitation on the coefficient based on the classic mass trafer theories is also discussed

Preliminary evaluation of spectral, normal and meteorological crop stage estimation approaches

Several of the projects in the AgRISTARS program require crop phenology information, including classification, acreage and yield estimation, and detection of episodal events. This study evaluates several crop calendar estimation techniques for their potential use in the program. The techniques, although generic in approach, were developed and tested on spring wheat data collected in 1978. There are three basic approaches to crop stage estimation: historical averages for an area (normal crop calendars), agrometeorological modeling of known crop-weather relationships agrometeorological (agromet) crop calendars, and interpretation of spectral signatures (spectral crop calendars). In all, 10 combinations of planting and biostage estimation models were evaluated. Dates of stage occurrence are estimated with biases between -4 and +4 days while root mean square errors range from 10 to 15 days. Results are inconclusive as to the superiority of any of the models and further evaluation of the models with the 1979 data set is recommended

Simulação da dinâmica do carbono orgânico em latossolo sob plantio direto e convencional: uma análise dos modelos CENTURY e CQESTR.

O objetivo deste trabalho foi avaliar a performance dos modelos Century e CQESTR em simular a dinâmica do carbono orgânico em latossolo vermelho-amarelo sob diferentes sistemas de preparo em àreas do cerrado do Piauí.Disponível também em papel na biblioteca do CPAMN

CQESTR: a simple model to estimate carbon sequestration in tropical soils.

CQESTR simulates the effect of management practices on soil organic carbon (SOC) stocks. The beta version of the model had been calibrated and validated for temperate regions. Our objective was to evaluate the CQESTR model performance for simulating carbon dynamics as affected by tillage practices in two tropical soils (Ultisol and Oxisol) in southeastern and northeastern Brazil. In the southeast (20.75 S 42.81 W), tillage systems consisted of no tillage (NT); reduced tillage (RT) (one disc plough and one harrow leveling [RT1] or one heavy disc harrow and one harrow leveling [RT2]); and conventional tillage (CT) (two heavy disc harrows followed by one disc plough and two harrow levelings). In the northeast (7.55 S 45.23 W), tillage systems consisted of NT, RT (one chisel plow and one harrow leveling), and CT (one disk plow, two heavy disk harrowings, and two harrow levelings). CQESTR underestimated SOC at both sites, especially under NTsystems, indicating that adjustments (e.g., the inclusion of clay mineralogy factor) are necessary for more accurate simulation of SOC in the tropics. In spite of this, measured and simulated values of SOC in the 0?20 cm depth were well correlated (southeast, R2 =0.94, p<0.01; northeast, R2=0.88, p<0.05). The model estimated carbon emissions varying from 0.36 (NT) to 1.05 Mg ha-1 year-1 (CT) in the southeast and from 0.30 (NT) to 0.82 (CT) Mg ha-1 year-1 in the northeast. CQESTR showed acceptable performance to predict SOC dynamics in two tropical soils of Brazil.1 CD-ROM