Search Results (6)

Biorefinery systems play a critical role in the transition towards a sustainable bioeconomy, and bioreactors are a key component in these systems. While mechanically stirred reactors have been extensively studied, there is a lack of research on pneumatically driven systems like air-lift reactors (ALRs). This study aims to address this gap by examining the hydrodynamic behavior of a double draft tube airlift bioreactor using Computational fluid dynamics simulations. Ten different geometric configurations were investigated, with variations in draft tube placement, liquid height, distance between draft tubes and draft tube diameters. Results showed that the placement of the draft tubes heavily influenced hydrodynamic behavior, with smaller distances between draft tubes and a funnel configuration leading to higher velocities. Stable downcomer velocities were achieved by maintaining a consistent distance between the bottom clearance and the sum of the distance between draft tubes and the bottom clearance on the top clearance. The model was validated against literature experimental data. This study provides insight into the optimal design of ALRs, which can contribute to the development of more efficient and effective bioreactor systems. The findings can be used to forecast the most optimal configurations of airlift bioreactors and have significant value for the development of more efficient biorefining concepts in light of the increasing importance of studying biorefineries and their components in the shift towards a biomass-based economy. Full article

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 $\overline{c^2}$ and its dissipation rate ε_c. 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. Full article

This article focuses on the study of the mass transfer involved in the application of a bacterial antifouling technique for membrane bioreactors (MBR), via the addition of solid media. These alginate objects can contain a biological system capable of producing an enzyme that degrades the signal molecules responsible for membrane fouling. The objective of this article is to quantify the mass transfer by distinguishing two main types: the transfer from the liquid to the solid media and the transfer from solid media to the liquid phase. For this purpose, a model molecule was chosen, and experiments were specifically developed with an optical device to track the concentration of the dye in the liquid phase, considering three different shapes for the particles (beads, hollow cylinders, and flat sheets). The experiments were first performed in jar tests and then in a lab-scale reactor. The results of this study revealed that the total amount of dye transferred into the sheets was greater than that transferred into the cylinders or the beads, which was attributed to the sheets having a larger exchange area for the same volume. When the dyed media were implemented in the MBR (loading rate of solid media: 0.45% v/v—no biomass), the global transfer coefficient from the sheets to the liquid was found to be greater than for the other shapes, indicating a faster transfer phenomenon. The effect of aeration in the MBR was investigated and an optimal air flowrate for fostering the transfer was found, based on the highest transfer coefficient that was obtained. This study provided key information about mass transfer in MBRs and how it is affected by the particle shapes and the MBR operating conditions. Full article

Even bubble column reactors (BCR) and airlift reactors (ALR) have been developed in terms of various related aspects towards mass transfer enhancement, the effective analysis of gas diffuser types on mass transfer and gas–liquid hydrodynamic characteristics is still limited. Therefore, the present study aims to analyze the relative effect of different types of air diffusers on bubble hydrodynamics and mass transfer performance to understand their behaviors and define the best type. The experiments were conducted by varying different diffuser types, reactor types (BCR and ALR), and superficial gas velocity (Vg) (0.12 to 1.00 cm/s). Five air diffusers including commercial fine sand (F-sand) and coarse sand (C-sand) diffusers, and acrylic perforated diffusers with orifice sizes of 0.3 mm (H-0.3), 0.6 mm (H-0.6), and 1.2 mm (H-1.2), were used in this study. For every condition, it was analyzed in terms of bubble hydrodynamics and oxygen mass transfer coefficient (K_La). Lastly, the selected diffusers that provided the highest K_La coefficient were evaluated with a solid media addition case. The results of both reactor classes showed that F-sand, the smallest orifice diffuser, showed the smallest air bubbles (3.14–4.90 mm) compared to other diffusers, followed by C-sand, which larger about 22–28% on average than F-sand. ALR exhibited a better ability to maintain smaller bubbles than BCR. Moreover, F-sand and C-sand diffusers showed a slower rising velocity through their smaller bubbles and the tiny bubble recirculation in ALR. Using F-sand in ALR, the rising velocity is about 1.60–2.58 dm/s, which is slower than that in BCR about 39–54%. F-sand and C-sand were also found as the significant diffusers in terms of interfacial area and gas hold-up. Then, the K_La coefficient was estimated in every diffuser and reactor under the varying of Vg. Up to 270% higher K_La value was achieved from the use of F-sand and C-sand compared to other types due to their smaller bubbles generated/maintained and longer bubble retention time through slower rising velocity. After adding 10% ring shape plastic media into the reactors with F-sand and C-sand diffusers, a better performance was achieved in terms of K_La coefficient (up to 39%) as well as gas hold-up and liquid mixing. Lastly, ALR also had a larger portion of mixed flow pattern than BCR. This eventually promoted mass transfer by enhancing the mixed flow regime. Full article

Many researchers have focused on multi-phase reactor development for improving mass transfer performance. However, solid particle addition in gas–liquid contactor for better oxygen mass transfer performance is still limited. Hence, this study aims to analyze the relative effect of different types of local solid media on the bubble hydrodynamic characteristics towards mass transfer enhancement in bubble columns (BCR) and airlift reactors (ALR). This was investigated by varying solid media types (ring, sphere, cylinder, and square), solid loadings (0%–15%), and superficial gas velocities (Vg) (2.6–15.3 × 10⁻³ m/s) in terms of the bubble hydrodynamic and oxygen mass transfer parameters. The result showed that bubble size distribution in BCR and ALR with additional plastic media was smaller than that without media addition, approximately 22%–27% and 5%–29%, respectively, due to the increase of the bubble breaking rate and the decrease of the bubble rising velocity (U_B). Further, adding media in both reactors significantly decreased the U_B value. Since media increased flow resistance, resulting in decreased liquid velocity, it can also be the moving bed to capture or block the bubbles from free rising. Therefore, oxygen mass transfer performance was investigated. The oxygen transfer coefficient (K_La) in BCR with solid media addition was enhanced up to 31%–56% compared to a non-addition case, while this enhancement was greater at higher solid loading due to its higher effective surface, resulting in a higher bubble break-up rate compared to the lower loading. In ALR, up to 38.5% enhanced K_La coefficient was archived after adding plastic media over the non-addition case. In conclusion, ring and cylinder media were found to be the most significant for improving K_La value in BCR and ALR, respectively, without extra energy. Full article

Barbotage reactors such as airlift reactors (ALR) and bubble column reactors (BCR), due to their two-phase flow systems, were investigated in many research papers. In their basic design variants, they are typically used to lift, mix, and aerate liquids, while, when equipped with additional elements in hybrid variants, their individual properties, i.e., lifting, mixing, and aeration of liquids, can significantly change with the same reactor geometry. The object of this study was to develop a hybrid barbotage reactor in various structural design variants. The structure consisted of a barbotage column of 50 mm in diameter, used to transport a water–air mixture outside the reactor (so-called external loop). The installation was additionally equipped with a nozzle in order to improve mixture aeration and circulation efficiency. The nozzle was mounted at various heights of the column pump segment. Additionally, the reactor was equipped with s moving bed in two variants (20% and 40% reactor capacity) in order to determine its effect on the mixture aeration and circulation conditions. Based on the measurement results, aeration curves were prepared for various structural design and column packing variants of the reactor. Properties of the two-phase mixture were determined for both parts—ALR and BCR. Technological and energy parameters of the aeration process were calculated, and the results obtained for the individual structural design variants were compared. It was found that, for the most advantageous design, in terms of aeration efficiency, the aeration nozzle should be placed in the mid-length of the pump segment of the barbotage column, irrespective of the hybrid reactor packing rate with the moving bed. The reactor packing with the moving bed resulted in a decreased mean water velocity in the reactor. For most analyzed structural design variants, the respective packing with the moving bed had no significant effect on aeration efficiency. Only for one structural design variant did the lack of packing significantly improve oxygen levels by as much as approximately 41%. Full article