A Review on the Efficient Catalysts for Algae Transesterification to Biodiesel

The depletion of fossil fuel resources and increasing environmental pollution led to a trend for using alternative, clean, green, and sustainable fuel and energy resources. To attain this aim, using biomass as an alternative resource for diesel production has been a hotspot among researchers. Biodiesel has several advantages, such as being lower toxic and more renewable, and eco-friendlier than diesel from fossil fuel resources. Several edible and non-edible bio-sources were used for the production of biodiesel from the transesterification process. Algal oil as a non-edible source is considered an abundant, low cost and green substrate for biodiesel production. Various factors such as reaction conditions and the type of catalyst affect the biodiesel production process. Different catalytic systems such as basic and acidic homogeneous and heterogeneous catalysts and biocatalysts were introduced for the process in the literature, and each proposed catalyst has its own advantages and disadvantages. For instance, in spite of the lower cost and better mass transfer of base and acid homogeneous catalysts, reaction system corrosion, non-reusability, and soap formation are serious challenges of these catalysts at an industrial scale. On the other hand, acid and base heterogenous catalysts overcame the issues of corrosion and recovery, but some matters such as mass transfer limitation, high cost, and weak performance in catalyzing both esterification of FFAs and transesterification of lipids must be taken into account. In addition, bio-catalysis as a high-cost process led to a purer product formation with less side reaction. Therefore, several significant factors should be considered for transesterification catalysts such as availability, cost, reusability, stability, mass transfer, and the possibility to manage both the transesterification of triglycerides and the esterification of FFAs, selecting a catalyst with predominant pros is viable. Here, a review of the biodiesel production from algal biomass focusing on the efficient catalyst of the process is presented.

Electrochemical Reduction of CO2 to CO on Hydrophobic Zn Foam Rod in a Microchannel Electrochemical Reactor

Due to CO2 mass transfer limitation as well as the competition of hydrogen evolution reaction in electroreduction of CO2 in the aqueous electrolyte, Zn-based electrodes normally exhibit unsatisfying selectivity for CO production, especially at high potentials. In this work, we introduced a zinc myristate (Zn [CH3(CH2)12COO]2) hydrophobic layer on the surface of zinc foam electrode by an electrodeposition method. The obtained hydrophobic zinc foam electrode showed a high Faradaic efficiency (FE) of 91.8% for CO at −1.9 V (vs. saturated calomel electrode, SCE), which was a remarkable improvement over zinc foam (FECO = 81.87%) at the same potentials. The high roughness of the hydrophobic layer has greatly increased the active surface area and CO2 mass transfer performance by providing abundant gas-liquid-solid contacting area. This work shows adding a hydrophobic layer on the surface of the catalyst is an effective way to improve the electrochemical CO2 reduction performance.

Recent Status and Prospects on Thermochemical Heat Storage Processes and Applications

Recent contributions to thermochemical heat storage (TCHS) technology have been reviewed and have revealed that there are four main branches whose mastery could significantly contribute to the field. These are the control of the processes to store or release heat, a perfect understanding and designing of the materials used for each storage process, the good sizing of the reactor, and the mastery of the whole system connected to design an efficient system. The above-mentioned fields constitute a very complex area of investigation, and most of the works focus on one of the branches to deepen their research. For this purpose, significant contributions have been and continue to be made. However, the technology is still not mature, and, up to now, no definitive, efficient, autonomous, practical, and commercial TCHS device is available. This paper highlights several issues that impede the maturity of the technology. These are the limited number of research works dedicated to the topic, the simulation results that are too illusory and impossible to implement in real prototypes, the incomplete analysis of the proposed works (simulation works without experimentation or experimentations without prior simulation study), and the endless problem of heat and mass transfer limitation. This paper provides insights and recommendations to better analyze and solve the problems that still challenge the technology.

Process and Energy Intensification of Glycerol Carbonate Production from Glycerol and Dimethyl Carbonate in the Presence of Eggshell-Derived CaO Heterogeneous Catalyst

The process and energy intensifications for the synthesis of glycerol carbonate (GC) from glycerol and dimethyl carbonate (DMC) using an eggshell-derived CaO heterogeneous catalyst were investigated. The transesterification reaction between glycerol and DMC was typically limited by mass transfer because of the immiscible nature of the reactants. By varying the stirring speed, it was observed that the mass transfer limitation could be neglected at 800 rpm. The presence of the CaO solid catalyst made the mass transport-limited reaction process more prominent. Mass transfer intensification using a simple kitchen countertop blender as an alternative to overcome the external mass transfer limitation of a typical magnetic stirrer was demonstrated. A lower amount of the catalyst and a shorter reaction time were required to achieve 93% glycerol conversion or 91% GC yield, and the turnover frequency (TOF) increased almost 5 times from 1.5 to 7.2 min−1 when using a conventional magnetic stirrer and countertop blender, respectively. In addition, using a simple kitchen countertop blender with 7200 rpm, the reaction temperature of 60 °C could be reached within approximately 3 min without the need of a heating unit. This was the result of the self-frictional heat generated by the high-shear blender. This was considered to be heat transfer intensification, as heat was generated locally (in situ), offering a higher homogeneity distribution. Meanwhile, the trend toward energy intensification was promising as the yield efficiency increased from 0.064 to 2.391 g/kJ. A comparison among other process intensification techniques, e.g., microwave reactor, ultrasonic reactor, and reactive distillation was also rationalized.

Performance of Sulfide-Driven Fuel Cell Aerated by Venturi Tube Ejector

Hydrogen sulfide is frequently met in natural waters, like mineral springs, but mostly it is found in marine water with low renewal rate. The Black Sea has extremely high hydrogen sulfide content. It can be utilized in different ways, but the most promising one is direct conversion into electricity. This result can be attained by a sulfide-driven fuel cell (SDFC), converting sulfide to sulfate thus releasing electric energy up to 24 GJ/t. One of the most important problems is the mass transfer limitation on oxygen transfer in the cathode space of the fuel cell. This problem can be solved using a gas diffusion electrode or highly efficient saturation by oxygen in an ejector of the Venturi tube type. This work presents experimental data in laboratory-scale SDFC for sulfide conversion into sulfate, sulfite and polysulfide releasing different amounts of electric energy. Two types of aeration are tested: direct air blow and Venturi-tube ejector. Besides pure graphite, two catalysts, i.e., cobalt spinel and zirconia-doped graphite were tested as anodes. Experiments were carried out at initial sulfide concentrations from 50 to 300 mg/L. Sulfate, sulfite and thiosulfate ions were detected in the outlet solutions from the fuel cell. The electrochemical results show good agreement with the chemical analyses. Most of the results show attained high efficiencies of the fuel cell, i.e. up to 80%. The practical applications of this method can be extended for other purposes, like treatment of polluted water together with utilization as energy.

An algal-bacterial symbiotic system of carbon fixation using formate as a carbon source

Algae are an attractive option for CO2 sequestration due to their natural ability to simultaneously fix CO2 and accumulate algal biomass for value-added products. However, the commercial implementation of such technology for efficient capture of CO2 from fossil-derived flue gases is not a reality yet due to several major challenges, such as low gas-liquid mass transport efficiency and relatively high light irradiance demand of algal growth. This study explored an algal-bacterial symbiotic system to utilize formate, a potential intermediate liquid compound of CO2, as carbon source to support microbial growth. The algal-bacterial assemblage, after an adaptive evolution using the formate medium, demonstrated a new route to assimilate CO2 without using high pH cultivations and promote biomass production under low light irradiance condition. The formate based culture system not only resolves CO2 mass transfer limitation, but also expels algae grazers in non-sterilized cultivation conditions. Continuous cultivation of the assemblage on formate led to a carbon capture efficiency of 90% with biomass concentration of 0.92 g/L and biomass productivity of 0.31 g/L/day, which is significantly better than the control cultivation on saturated CO2. In addition, isotope tracing and microbial community analysis offer new insights into formate metabolism and algal-bacterial symbiosis under light and carbon conditions. This study demonstrates a promising route of using electrochemical-derived formate to support algal biorefining.

Mathematical Modelling of Extraction Yield, Glycosaponin and Eurycomanone Content from Eurycoma longifolia Roots

Aim: The technique of modeling was used to predict the performance of herbal extraction. Background: Eurycoma longifolia is a traditional herb which is widely used to promote overall well-being by local folks in South East Asia. Objective: The objective of this study was to model the extraction yield, glycosaponin and eurycomanone content from the roots of E. longifolia in a reflux system. Results: A quadratic model is well-fitted to the yield of extraction, while a linear model is proposed for the extraction of eurycomanone which is significantly affected by temperature. The single step of reflux process revealed that glycosaponin extraction behaves linearly with temperature and particle size, but in inverse direction. However, further reflux process using crude extract prior to gravimetric precipitation could produce higher content of glycosaponin which can be explained using a quadratic model. Conclusion: The extraction of eurycomanone was faster than glycosaponin because the mass transfer and diffusion coefficients of eurycomanone was about two times larger than glycosaponin. Other: This could be due to larger molecular size of glycosaponin, and thus higher mass transfer limitation to diffuse from plant cells into solvent during extraction.

Confined thermal transformation strategy to synthesize single atom catalysts supported on nitrogen-doped mesoporous carbon nanospheres for selective hydrogenation

Carbon-supported single-atom catalysts (SACs) have brought considerable attention in heterogeneous catalysis, which, however, often suffer from low activity due to the mass transfer limitation. Herein, we report a soft-templating method...

Ultrasonic induced mechanoacoustic effect on delignification of rice straw for cost effective biopretreatment and biomethane recovery

The mass transfer limitation of biopretreatment (BAC) of rice straw biomass is improved by a delignification process prior to BAC for energy efficient biomethane recovery. In the present study, the...

A comparative analysis on different enzyme immobilization nanomaterials: Progress, constraints, and recent trends

Abstract:: Immobilization techniques have been popularly used to preserve the operational stability of the enzymes for industrial applications. The three main components of an immobilized enzyme system are the enzyme, the matrix/support, and the technique of immobilization. So far different supports have been developed to improve the efficiency of the immobilized enzymes. But in the recent decade, nanotechnology has been considerable research interest in the field of immobilized enzyme carriers. The materials at the nano-scale due to their unique physicochemical properties including; specific surface area, mass transfer limitation, and effective enzyme loading, are considered as interesting matrices for enzyme immobilization. This review describes techniques employed to immobilize enzymes and provides an integrated focus on the most common nanoparticles for enzyme conjugation. Additionally, the pros and cons of nanoparticles as immobilization matrices are also discussed. Depending on the type of enzyme and its application, in this review, the researchers are directed to select an appropriate method and support for enzyme immobilization in terms of enzyme stability and functionality.