Volume 22, Issue 3

Anaerobic digestion is a prevailing technology for the treatment and utilization of municipal organic solid waste (MOSW). In this technology, the macromolecular organic matter in waste degrades into small molecular substances through the anaerobic decomposition of microorganisms, producing biogas that can provide enormous energy. This paper focuses on the research progress of anaerobic digestion of various organic wastes for biogas production. The principle and process of anaerobic digestion for biogas production are introduced, along with the key factors affecting anaerobic digestion efficiency, such as temperature, pH, and sealing conditions. At the same time, the current cycle treatment technology and comprehensive treatment system of MOSW are also summarized. Furthermore, the paper explores biogas purification technologies, including desulfurization, deoxidation, drying, and decarbonization. Finally, the state-of-the-art of the utilization of MOSW for biogas production in the world and the problems faced by the utilization of MOSW for biogas production in China are reviewed. By summarizing the anaerobic digestion technology of MOSW, this review hopes to provide some reasonable solutions for the high-value utilization of MOSW.

Heavy metals such as Lead(II), Nickel(II), Manganese(II), Cadmium(II), Chromium(VI), etc., are leached from the coal ash of thermal power plants. These metals contaminate ash pond water and subsequently contaminate groundwater. Phycoremediation using microalgae/cyanobacteria is an emerging technology for removal of heavy metals. The present study aims at phycoremediation of the said heavy metals from ash pond water using cyanobacterial consortium of Limnococcus limneticus and Leptolyngbya subtilis followed by the development of an accurate data-driven Multigene Genetic Programing (MGGP) approach for modeling and optimization of the process. The developed model was used to obtain a correlation between the average removal of metals and biomass production with all input factors such as the initial metal concentrations, pH, and days of incubation. To maximize metal removal and biomass production, the Water Evaporation Optimization (WEO) technique was applied to determine optimal values of input parameters. The application of WEO for the optimization of the phycoremediation process is the first of its kind and here lies the novelty of the study.

Conventional dimethyl maleate (DMM) synthesis relies on the use of sulfuric acid as a catalyst which requires water washing and produces a large amount of wastewater that harms the environment. The use of this method is expensive since it involves numerous processes such as neutralization of sulfuric acid, washing with water, distillation etc. and the yield of dimethyl maleate is not very high. Reactive distillation integrates the two important industrial processes such as reaction and distillation into a single unit which shortens the overall process, reduces capital and operating costs, and maintains the reaction in a forward direction to enhance the conversion of maleic anhydride by continuously removing of the products. In this work, simulation study based on experimental investigation of reactive distillation process for the synthesis of dimethyl maleate was carried out using Aspen Plus V11. DZH strong cation exchange resin was used as catalyst for esterification reaction of maleic anhydride with methanol. A reactive section of column was packed with Katapak SP type packing loaded with DZH catalyst while non-reactive sections consist of wire gauze packing. In order to describe a reactive distillation process for synthesis of dimethyl maleate, RAD-FRAC equilibrium stage model was employed. The NRTL activity model and RK equation of state model were selected to describe vapor-liquid equilibrium of the system. The reliability of the developed simulated model was verified by validating the simulation results with the experimental ones. The effect of various design and operating parameters on the conversion of maleic anhydride and purity of dimethyl maleate has been studied. It was found that the optimal condition for RD, were as follow: total number of theoretical stages 17, rectifying stages 3, reactive stages 7, stripping stages 5, reflux ratio 0.25, operating pressure 0.1 MPa, reboiler duty 250 Cal/Sec, feed mole ratio 1:5. Under optimized condition the water formed as a results of esterification reaction was constantly removed from the reactive section of the column to maintain reaction balance, the conversion of maleic anhydride was 99.95 %, purity of dimethyl maleate achieved was 0.997 and the reactive distillation process was feasible to produce dimethyl maleate without any wastewater.

The implementation of a scraping rotor has enhanced the distribution of the liquid film in wiped film molecular distillation while also preventing groove flow and drywall phenomena. To investigate the structure of the rotor and its surrounding cylinder, a three-dimensional mathematical model of the interleaved rotor has been established using computational fluid dynamics (CFD), and the accuracy of the simulation model has been validated through experiments. Additionally, a three-dimensional staggered cylindrical rotor has been developed and used to explore the impact of different structural parameters on the distribution of the head wave and liquid film thickness. These findings have provided a theoretical basis for the creation of new rotor devices and furnished valuable empirical parameters for practical engineering applications. Furthermore, this research has been instrumental in elevating the efficiency and effectiveness of wiped film molecular distillation, and at the same time, it provides important support for improving the structure of the scraping film molecular distillation equipment in the industry.

Conversion of corn straw into biochar by pyrolysis was considered as an environmentally friendly and efficient method for biomass sources because porous biochar has excellent properties and wide applications. Porous biochar production from corn straw in a microwave heated reactor was the main purpose of this study. The yields and microstructures of biochar produced at different pyrolysis temperatures (500, 600, 700, 800, and 900 °C), microwave powers (600, 650, 700, 750, and 800 W), and pyrolysis times (30, 40, 50, 60, and 70 min) were studied. The results showed that biochar yield ranged from 44.69 to 59.90 wt% with the changes of reaction conditions. Biochar with the nanoscale pores was first obtained at pyrolysis temperature of 800 °C, microwave power of 600 W, and pyrolysis time of 30 min, indicating that corn straw biochar had the potential to produce nanoscale pores. This study also quantitatively described the relationship between microstructure and productivity of corn straw biochar at different experimental conditions (pyrolysis temperature, microwave power, pyrolysis time), which can provide guidance for the reuse of corn straw and the production of porous biochar.

The operational efficiency of the airlift reactors relies significantly on the aeration and mixing provided by the inlet system. The diffused aeration system is the most energy-intensive component affecting the operation of the bioreactor, accounting for 45–75 % of the energy costs. This study presents a coupled CFD-PBM to investigate the collective impacts of multiple bubble diameters, variations in inlet gas distribution types, and flow rates on the hydrodynamic characteristics of bubble columns. The simulation results were validated through comprehensive comparisons with experimental data. The experimental data and simulations of the single bubble size model (SBSM) and multi-bubble size model (MBSM) were compared, proposing an enhanced inlet gas distribution type. The results indicate a close resemblance between the MBSM data and the experimental results, with an error margin not exceeding 5 %. Moreover, different flow rates were found to cause varying sensitivities in the bubble size distribution (BSD) within the column. Furthermore, the simulation results validate the similarity between lift coefficients and critical diameters to experiments and shed light on favorable conditions for reactor design. The key findings of this study encompass: (1) the use of MBSM can accurately predict the tower system characteristics; (2) the column circulation is intensified with small inlet bubble size and high gas velocity, which is favorable for chemical reactions and microbial aggregation to proceed; and (3) the BSD is not sensitive to the inlet gas distribution type at high flow rates.

The effect of temperature and silicon concentration on iron precipitation and morphology of hematite particles was discussed. The results showed that increase of initial silicon centration will lead to the decrease of iron removal efficiency, and the Fe content decreases and S content increases in hematite. Increasing temperature is beneficial to increase iron removal rate and Fe content in hematite, but increasing temperature can not decrease silicon content in hematite. The content of Si in precipitate is proportional to the concentration of silicon in solution. Silicon is adsorbed on the surface of hematite in the form of silicic acid. Hematite particle was agglomerated microsphere. Increasing concentration of silicon will increase the disorder degree of hematite crystallization. With the increase of silicon concentration, the agglomeration of hematite particle was more obvious, and the average particle size of hematite particles increased. At 175 °C, the morphology of iron precipitation particles are composed of flake particles and rhomboid massive particles, but when the temperature rises to 195 °C, the agglomeration phenomenon is obvious, small flake particles agglomerate into microspherical aggregates.

In order to promote the yield and calorific value of combustible gas products in the pyrolysis, a CaO-based biochar (CSC) derived from waste crab shell was used as a low-cost sorption-enhanced catalyst to enhance oil-fried sludge (OS) pyrolysis. The effects of CSC addition and pyrolysis temperature on OS pyrolysis characteristics were analyzed in detail. The results indicated that adding CSC significantly enhanced OS pyrolysis and produced more combustible gas products with CO 2 removal. At 700 °C, adding CSC promoted the combustible gas yields and greatly reduced CO 2 by 96.9 %. And the corresponding LHV of pyrolysis gas products increased by 26.8 % and reached up to 33.7 MJ/N m 3 . Additionally, TG-FTIR analysis revealed that adding CSC reduced the formation temperature of CH 4 . Importantly, although there was only 35.5 wt% of Ca in CSC, CSC exhibited almost the same sorption-catalysis-enhanced effects compared to pure CaO. These findings suggest that biochar derived from crab shell has the potential to replace CaO for enhancing sludge pyrolysis into value-added fuel products.

In the present work, furfuryl alcohol (FAL) alcoholysis towards ethyl levulinate (EL) was studied over a mesoporous SO 3 H-SBA-15 catalyst. The effect of various operating parameters i.e., temperature, catalyst dose, furfuryl alcohol amount, and time was studied and optimized via robust Response Surface Methodology through central composite rotatable designs (CCRD) method on the conversion of FAL to EL. According to Response Surface Methodology, under optimum reaction conditions viz. temperature 110 °C, catalyst dose 0.42 g, time 3 h, and FAL amount of 1.46 g, maximum EL yield (95 %) was recorded. Further, the effect of reaction parameters on the kinetics of the said reaction was also examined, suggesting the second-order kinetic concerning all operating parameters. Eventually, the reusability of the catalyst is evident in a decrease of almost 40 % yield towards EL in the fourth cycle.

This work concerns the application of the homotopy method to solve the mathematical model of a non-adiabatic chemical continuous stirred tank reactor (CSTR) and tubular reactor with mass recycle (TRR) (Berezowski 2000. Spatio-temporal chaos in tubular chemical reactors with the recycle of mass, Chaos, Solitons & Fractals , vol. 11, no. 8, pp. 1197–1204). This method was associated with the parametric multivariable continuation algorithm. Thanks to this, this method can automatically find all the multiple steady states of the reactor model without the need to use any iteration. The parametric continuation method is used to determine a curve whose each point is a solution of the tested model. Therefore, the starting point must be very precisely designated so that it lies on this curve. Otherwise, the result is a graph that deviates from the correct graph. However, this condition is not required when the homotopy method is also introduced into the calculations. The starting point can then be a point with any coordinates. Different curves are also obtained, but the homotopy method ensures that each of them passes through the point where the parameter p  = 1. The solution we are looking for in the model is just such a point. This is undoubtedly a big advantage resulting from the combination of both above-mentioned methods.