Enhancing gas–liquid volumetric mass transfer coefficient

Oxygen mass transfer enhancement by activated carbon particles in xylose fermentation media

In this work, the effect of activated carbon particles on the production of xylonic acid from xylose by Gluconobacter oxydans in a stirred tank bioreactor was investigated. The enhancement of the oxygen transfer coefficient by activated carbon particles was experimentally evaluated under different solids volume fractions, agitation and aeration rates conditions. The experimental conditions optimized by response surface methodology (agitation speed 800 rpm, aeration rate 7 L min^-1, and activated carbon 0.002%) showed a maximum oxygen transfer coefficient of 520.7 h^-1, 40.4% higher than the control runs without activated carbon particles. Under the maximum oxygen transfer coefficient condition, the xylonic acid titer reached 108.2 g/L with a volumetric productivity of 13.53 g L^-1 h^-1 and a specific productivity of 6.52 g/g_x/h. In conclusion, the addition of activated carbon particles effectively enhanced the oxygen mass transfer rate. These results demonstrate that activated carbon particles enhanced cultivation for xylonic acid production an inexpensive and attractive alternative.

Continuous Homogeneous Catalytic Oxidation of C-H Bonds by Metal-Free Carbon Dots with a Poly(ascorbic acid) Structure

The activation of the C-H bond, a necessary step to get high-value-added compounds, is one of the most important issues in modern catalysis. Combining the advantages of both homogeneous and heterogeneous catalysis, a certain continuous homogeneous process should be one of the ideal routes for the catalytic activation of C-H bonds. Here, through machine learning (ML), we predicted and fabricated metal-free carbon dot (C-Dot) homogeneous catalysts for C-H bond oxidation. These C-Dots have an ascorbic acid unit based polymer-like structure with a polymerization degree in the range of 3-10. With C-Dots as the catalyst, three groups (aliphatic, aromatic, and cycloalkanes) of 10 hydrocarbon molecules were tested, proving its generality for the catalytic oxidation of the C-H bond. A typical example of cyclohexane that was selectively oxidized to adipic acid (AA) by using a circulation and phase-transfer process demonstrates its critical advantages, such as the continuous and large-scaled producing ability of the homogeneous catalysis process. The one-pass conversion efficiency of cyclohexane to AA reaches 77.49% with selectivity up to 84.24% in 4 h. The yield of 16.32% per hour is about 4 times over that of modern technology. Theoretical calculations suggested that the O2 activation on C-Dots plays a crucial role in determining the reaction rate of the entire catalytic oxidation process of cyclohexane.

A Critical Review on Removal of Gaseous Pollutants Using Sulfate Radical-based Advanced Oxidation Technologies

Excessive emissions of gaseous pollutants such as SO₂, NO_x, heavy metals (Hg, As, etc.), H₂S, VOCs, etc. have triggered a series of environmental pollution incidents. Sulfate radical (SO₄•^-)-based advanced oxidation technologies (AOTs) are one of the most promising gaseous pollutants removal technologies because they can not only produce active free radicals with strong oxidation ability to simultaneously degrade most of gaseous pollutants, but also their reaction processes are environmentally friendly. However, so far, the special review focusing on gaseous pollutants removal using SO₄•^--based AOTs is not reported. This review reports the latest advances in removal of gaseous pollutants (e.g., SO₂, NO_x, Hg, As, H₂S, and VOCs) using SO₄•^--based AOTs. The performance, mechanism, active species identification and advantages/disadvantages of these removal technologies using SO₄•^--based AOTs are reviewed. The existing challenges and further research suggestions are also commented. Results show that SO₄•^--based AOTs possess good development potential in gaseous pollutant control field due to simple reagent transportation and storage, low product post-treatment requirements and strong degradation ability of refractory pollutants. Each SO₄•^--based AOT possesses its own advantages and disadvantages in terms of removal performance, cost, reliability, and product post-treatment. Low free radical yield, poor removal capacity, unclear removal mechanism/contribution of active species, unreliable technology and high cost are still the main problems in this field. The combined use of multiactivation technologies is one of the promising strategies to overcome these defects since it may make up for the shortcomings of independent technology. In order to improve free radical yield and pollutant removal capacity, enhancement of mass transfer and optimization design of reactor are critical issues. Comprehensive consideration of catalytic materials, removal chemistry, mass transfer and reactor is the promising route to solve these problems. In order to clarify removal mechanism, it is essential to select suitable free radical sacrificial agents, probes and spin trapping agents, which possess high selectivity for target specie, high solubility in water, and little effect on activity of catalyst itself and mass transfer/diffusion parameters. In order to further reduce investment and operating costs, it is necessary to carry out the related studies on simultaneous removal of more gaseous pollutants.