Experimental and Numerical Study of Gas-Liquid Flow in a Sectionalized External-Loop Airlift Reactor

Process engineering strategy for improved methanol production in Methylosinus trichosporium through enhanced mass transfer and solubility of methane and carbon dioxide

Methanol was produced in a two-stage integrated process using Methylosinus trichosporium NCIMB 11131. The first stage involved sequestration of methane to produce methanotrophic biomass, which was utilized as biocatalyst in the second stage to convert CO₂ into methanol. A combinatorial process engineering approach of design of micro-sparger, engagement of draft tube, addition of mass transfer vector and elevation of reactor operating pressure was employed to enhance production of biomass and methanol. Maximum biomass titer of 7.68 g/L and productivity of 1.46 g/L d^-1 were achieved in an airlift reactor equipped with a micro-sparger of 5 µm pore size, in the presence of draft tube and 10 % v/v silicone oil, as mass transfer vector. Maximum methane fixation rate was estimated to be 0.80 g/L d^-1. Maximum methanol titer of 1.98 g/L was achieved under an elevated operating pressure of 4 bar in a high-pressure stirred tank reactor.

A Turbulent Mass Diffusivity Model for Predicting Species Concentration Distribution in the Biodegradation of Phenol Wastewater in an Airlift Reactor

In this study, a three-dimensional CFD transient model is established for predicting species concentration distribution in the biodegradation of phenol in an airlift reactor (ALR). The gas–liquid flow in the ALR is determined by the Euler–Euler method coupled with the standard k-ε model, and the bubble size is predicted by the population balance model (PBM). A turbulent mass diffusivity model is developed to simulate the turbulent mass transfer process and to predict the species concentration distribution. No empirical methods are needed as the turbulent mass diffusivity can be expressed by the concentration variance c2¯ and its dissipation rate εc. A good agreement is found between simulated and experimental results in the literature. It is not reasonable to assume a constant turbulent Schmidt number because the calculated distribution of turbulent mass diffusivity is not identical to that of turbulent viscosity. Finally, the hydrodynamic characteristics and biodegradation performance of the proposed model in a novel ALR are compared with that in the original ALR.

Influence of Sparger Type on Mass Transfer in a Pilot-Scale Internal Loop Airlift Reactor

In a pilot-scale internal loop airlift reactor with a height of 5.5 m and a main column diameter of 0.484 m, the influence of three gas sparger structures (ladder distributor, tri-nozzle sparger and perforated plate) on the volumetric mass transfer coefficient kLa was investigated. It was found that the perforated plate produces the highest gas holdup difference and circulating liquid velocity between the riser and the downcomer. The perforated plate provides the most efficient mass transfer due to the more uniform gas distribution and higher circulating liquid velocity, followed by the ladder distributor and tri-nozzle spargers. Compared with the tri-nozzle sparger, the perforated plate increases the value of kLa by up to 16% at a superficial velocity of 0.15 m/s. Interestingly, the analysis of the liquid-phase mass transfer coefficient kL and specific area a with respect to gas velocity shows that the mass transfer rate is primarily controlled by a. By comparing the predictions of different mass transfer models, the slip velocity model based on penetration theory yields a satisfactory agreement with the experimental results within ±15% error. Meanwhile, empirical correlations regarding gas holdup and kLa were developed and were found to have good consistency with experimental values.

Process optimization and CFD simulation in external loop airlift reactor and sectionalized external loop airlift for application of wastewater treatment

The industrial wastewater treatment is carried out in an external loop airlift reactor (EL-ALR) and sectionalized EL-ALR. The airflow rate is optimized in EL-ALR and sectionalized EL-ALR for better degradation of wastewater. The 74% degradation is obtained in sectionalized EL-ALR as compared to EL-ALR for continuous aeration up to 32 h. For higher superficial gas velocity (U G ) percentage degradation is decreased due to shear stress on microorganisms. The extent of degradation is obtained 77 and 80% for the effect of hydrogen peroxide (H2O2) and combination of H2O2 and Fenton reagent in an EL-ALR. The computational fluid dynamic (CFD) simulation validated with experimental results of gas hold-up and liquid circulation velocity compared in EL-ALR sectionalized EL-ALR over a broad range of superficial gas velocity 0.0024 ≤ U G  ≤ 0.016 m/s. The comparison of CFD and experimental values of gas hold-up and liquid circulation velocity are in good agreement.

Degradation of sulfadiazine in aqueous media by peroxymonosulfate activated with biochar-supported ZnFe2O4 in combination with visible light in an internal loop-lift reactor

Solid waste resource utilization and the treatment of wastewater are two important aspects in environmental protection. Here, biochar (BC) derived from municipal sewage sludge has been combined with ZnFe₂O₄ to form the photocatalyst ZnFe₂O₄/biochar (ZnFe/BC), and it was used to degrade sulfadiazine (SDZ) in the presence of peroxymonosulfate (PMS) under visible (Vis) light irradiation in an internal loop-airlift reactor (ALR). The surface morphology and structure of ZnFe/BC have been characterized by X-ray diffraction (XRD), scanning electron microscopy equipped with an attachment for energy-dispersive spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS), and UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS). ZnFe/BC displays outstanding photocatalytic performance and reusability. After four reuse cycles of ZnFe/BC in the Vis/ZnFe/BC/PMS system, the SDZ degradation rate and efficiency still reached 0.082 min⁻¹ and 99.05%, respectively. Reactive species in this system included free radicals SO₄˙⁻, ˙OH, and ˙O₂⁻, as well as non-radicals ¹O₂, e⁻, and h⁺, as established from the results of chemical quenching experiments and electron paramagnetic resonance (EPR) analyses. Moreover, a mechanism of action of the Vis/ZnFe/BC/PMS system for SDZ degradation was proposed. The acute toxicity of the reaction solution towards Photobacterium phosphoreum T3 spp. in the Vis/ZnFe/BC/PMS process increased during the first 40 min and then decreased, illustrating that Vis/ZnFe/BC/PMS provided an effective and safe method for the removal of SDZ.

Solid waste resource utilization and the treatment of wastewater are two important aspects in environmental protection.