Volumetric Oxygen Transfer Coefficient Research Articles

Bioconversion of hemicellulosic fraction of wheat straw biomass to bioethanol by Scheffersomyces stipitis: A kLa-based scale-up study

The potent utilization of hemicellulosic fraction, which comprises one-third of carbohydrates in lignocellulosic biomass (LCB), is vital to strengthen the bioethanol production from it. Most studies on bioethanol production from LCB focused on cellulosic fraction only, or very low ethanol productivity is reported from hemicellulosic fraction. The present study investigated a process to convert the hemicellulosic fraction of wheat straw biomass to bioethanol effectively using Scheffersomyces stipitis. This work aimed to develop and demonstrate a systematic approach for scaling up the fermentation process, keeping the volumetric oxygen transfer coefficient (kLa) nearly constant in different scale bioreactors. It was found that 99.11±0.13% of synthetic xylose and 99.04±0.10% of lignocellulosic xylose were converted to bioethanol with a yield and productivity of 0.4706±0.0022 g/g and 1.5325±0.0020 g/Lh; 0.4290±0.0030 g/g and 0.7399±0.0043 g/Lh, respectively at an optimum kLa of 0.5776 (0.5735, 0.5817) min−1 and 0.6924 (0.6887, 0.6961) min−1, respectively in a 0.5 L bioreactor. After that, these optimum kLa were validated in 1 L and 5 L bioreactors. The successful kLa validation revealed that the hemicellulosic fraction of lignocellulosic biomass could also be well utilized for bioethanol production.

Validation of a CFD model for cell culture bioreactors at large scale and its application in scale-up

Among all the operating parameters that control the cell culture environment inside bioreactors, appropriate mixing and aeration are crucial to ensure sufficient oxygen supply, homogeneous mixing, and CO2 stripping. A model-based manufacturing facility fit approach was applied to define agitation and bottom air flow rates during the process scale-up from laboratory to manufacturing, of which computational fluid dynamics (CFD) was the core modeling tool. The realizable k-ε turbulent dispersed Eulerian gas-liquid flow model was established and validated using experimental values for the volumetric oxygen transfer coefficient (kLa). Model validation defined the process operating parameter ranges for application of the model, identified mixing issues (e.g., impeller flooding, dissolved oxygen gradients, etc.) and the impact of antifoam on kLa. Using the CFD simulation results as inputs to the models for oxygen demand, gas entrance velocity, and CO2 stripping aided in the design of the agitation and bottom air flow rates needed to meet cellular oxygen demand, control CO2 levels, mitigate risks for cell damage due to shear, foaming, as well as fire hazards due to high O2 levels in the bioreactor gas outlet. The recommended operating conditions led to the completion of five manufacturing runs with a 100% success rate. This model-based approach achieved a seamless scale-up and reduced the required number of at-scale development batches, resulting in cost and time savings of a cell culture commercialization process.

Exploring the use of hexadecane by Yarrowia lipolytica: Effect of dissolved oxygen and medium supplementation

This work aimed to evaluate the effect of medium composition and volumetric oxygen transfer coefficient (kLa) on Y. lipolytica growth and production of microbial lipids and enzymes from hexadecane. In the stirred tank bioreactor, increasing kLa from 11 h−1 to 132 h−1 improved the hexadecane assimilation rate, biomass concentration, and lipids synthesis (0.90 g·L−1). A cost-effective hexadecane-based medium supplemented with corn steep liquor and a low amount of ammonium sulfate boosted lipids production up to 2.1 g·L−1, composed of palmitic, palmitoleic, oleic, and linoleic acids. The unsaturated/saturated fraction was dependent on the C/N ratio. Lipids of Y. lipolytica CBS 2075 are promising feedstock for animal feed, food additives, or the biodiesel industry. Simultaneous synthesis of extracellular lipase and protease from hexadecane was observed, which is a new feature that was not previously reported. The highest enzyme activity was obtained at the highest C/N ratio conditions. These results open new perspectives on the application of Y. lipolytica-based cultures for the biotransformation of hexadecane-polluted streams into valuable compounds, fulfilling an interesting strategy towards the circular economy concept.

Oxygen and hydrocarbon volumetric transfer coefficients in the production of an oil-degrading bacterial consortium: emulsifying activity and surface tension in a bioreactor.

Our work shows that in multiphase systems, it is more important to take into account the mass transfer of oil rather than that of just oxygen. The oxygen volumetric transfer coefficient is important in aerobic bioreactor design. However, in multiphase systems with non-soluble substrates, oil transfer can impose larger restrictions but is usually not considered. Emulsification and surface tension could play an important role due to effects on oil droplet size and interfacial transfer area. Petroleum oil and is derivates such as diesel can negatively affect living organisms. This study evaluated the effects of the volumetric transfer coefficients (kLa) of hydrocarbons and oxygen on the production of an oil-degrading consortium in an airlift bioreactor relative to emulsifying activity and surface tension, which play important roles in the biodegradation of non-soluble substrates such as diesel due to a combined mass transfer constraint. Our results showed a clear difference in kLa values, which ranged from 15 to 91h-1 for oxygen and from 0 to 0.0014h-1 for diesel. Most aerobic biodegradation studies focus on the oxygen volumetric transfer coefficient (kLaoxygen), but our results indicated that non-soluble constraints, such as the volumetric transfer coefficient of diesel (kLadiesel), could be more important. Additionally, d32diesel decreased as superficial gas velocity (Ug) increased. Lower Ug rates (0.15cms-1) resulted in higher values of d32diesel (0.38cm-1), whereas higher Ug rates (2.7cms-1) resulted in lower values of d32diesel (0.21cm-1) at the beginning of the cultivation.

Fermentation development using fruit waste derived mixed sugars for poly(3-hydroxybutyrate) production and property evaluation

Free sugars from fruit wastes were evaluated for the production of poly(3-hydroxybutyrate) (PHB) in Paraburkholderia sacchari fed-batch bioreactor fermentations. Different initial sugar concentration, carbon to inorganic phosphorus (C/IP) ratio, IP addition during feeding and volumetric oxygen transfer coefficient (kLa) were evaluated to promote PHB production. The highest intracellular PHB accumulation (66.6%), PHB concentration (108.3 g/L), productivity (3.28 g/(L·h)) and yield (0.33 g/g) were achieved at 40 g/L initial sugars, C/IP 26.5, 202.6 h−1kLa value and 20% IP supplementation in the feeding solution. The effect of different microbial mass harvesting time on PHB properties showed no influence in weight average molecular weight and thermal properties. The harvest time influenced the tensile strength that was reduced from 28.7 MPa at 22 h to 13.3 MPa at 36 h. The elongation at break and Young’s modulus were in the range 3.6–14.8% and 830–2000 MPa, respectively.

Performance of rotating cage biological contactors based on the partial nitrification of immobilized bioactive fillers

The rotating cage biological contactors (RCBC) is based on the partial nitrification immobilization of bioactive fillers, incorporating the advantages of traditional the rotating biological contactor (RBC). The RCBC is more energy-efficient, with higher ammonia oxidation rate (AOR) and nitrite accumulation rate (NAR) levels compared to the RBC Under the conditions of different rotational speeds and immersion rates of an RCBC, the volumetric oxygen transfer coefficient (KLa) was measured. The experimental results show that these two factors significantly affected the KLa of the reactor, which ranged from 1.08 to 42.02 h−1. Regarding long-term ammonia oxidation performance, the RCBC can successfully start partial nitrification in only 5 days. After stable operation, the AOR was 23.34 ± 0.55 mg (L·h)−1, with an NAR of 93.42 ± 0.84 %. This indicates a good adaptability to variations in influent ammonia nitrogen (NH4+-N) in the range of 95.92–403.14 mg·L−1 and good resistance to free nitrous acids (FNA). In addition, high-throughput sequencing analysis showed that the operation mode of the RCBC could effectively inhibit the growth of nitrite-oxidizing bacteria (NOB) in immobilized bioactive fillers, and the internal oxygen-limited environment provided favorable conditions for reducing the relative abundance and activity of NOB. In this study, a partial nitrification device was established by immobilized bioactive fillers in the form of an RCBC for the first time, providing a partial nitrification technology without aeration.

Strategy to reduce acetic acid in sugarcane bagasse hemicellulose hydrolysate concomitantly with xylitol production by the promising yeast Cyberlindnera xylosilytica in a bioreactor.

The yeast Cyberlindnera xylosilytica UFMG-CM-Y309 has been identified as a promising new xylitol producer from sugarcane bagasse hemicellulosic hydrolysate (SCHH). However, SCHH pretreatment process generates byproducts, which are toxic to cell metabolism, including furans, phenolic compounds, and carboxylic acids, such as acetic acid, typically released at high concentrations. This research aims to reduce acetic acid in sugarcane hemicellulose hydrolysate concomitantly with xylitol production by yeast strain Cy. xylosilytica UFMG-CM-Y309 in a bioreactor by strategically evaluating the influence of volumetric oxygen transfer coefficient (kLa) (21 and 35h-1). Experiments were conducted on a bench bioreactor (2L volumetric capacity) at different initial kLa values (21 and 35h-1). SCHH medium was supplemented with rice bran extract (10gL-1) and yeast extract (1gL-1). Cy. xylosilytica showed high xylitol production performance (19.56gL-1), xylitol yield (0.56gg-1) and, maximum xylitol-specific production rate (μpmáx 0.20 gxylitol·g-1h-1) at kLa value of 21h-1, concomitantly slowing the rate of acetic acid consumption. A faster acetic acid consumption (100%) by Cy. xylosilytica was observed at kLa of 35h-1, concomitantly with an increase in maximum cellular growth (14.60gL-1) and reduction in maximum xylitol production (14.56gL-1 and Yp/s 0.34gg-1). This study contributes to pioneering research regarding this yeast performance in bioreactors, emphasizing culture medium detoxification and xylitol production.

Microaerophilic activated sludge system for ammonia retention from high-strength nitrogenous wastewater: Biokinetics and mathematical modeling

It is crucial to shift from conventional ammonia removal to recovery to solve environmental problems and resource scarcity. Recovering ammonia from high-strength nitrogenous wastewater can simultaneously save the energy and cost involved in ammonia oxidation, produce ammonia without CO2 emissions, and prevent N2O emissions. A technology for concentrating and recovering ammonia by retrofitting a conventional activated sludge (CAS) system is promising but faces significant challenges in oxidizing organic carbon but not ammonia in wastewater. This study employed biokinetic analysis and mathematical modeling to comprehensively assess the operation and effectiveness of a microaerophilic activated sludge (MAS) system for simultaneously removing chemical oxygen demand (COD) and retaining ammonia. We show that heterotrophic bacteria (HB) in the MAS system had higher maximum specific growth rate (µHB), biomass yield coefficient (YHB), and decay coefficient (bHB) but lower affinities for oxygen and COD than HB in CAS. Simulations using the developed model demonstrated that COD removal efficiencies of > 90%, ammonia retention efficiencies of > 90%, and low N2O emission factors (N2OEF) of < 0.01% can be achieved at solids retention times (SRTs) of 3–50 d, hydraulic retention times (HRTs) of 0.1–1 d, and a volumetric oxygen transfer coefficient (KLa) of (24.58 ± 0.30)HRT−1. The combination of the biokinetics and mathematical modeling revealed the favorable MAS operating conditions allowing efficient ammonia retention and COD removal, which paves the way for turning a CAS system into a MAS system by retrofitting the configuration without high capital expenditures, requiring future intensive study.

Oxygen uptake and transfer rates throughout production of recombinant baculovirus and rabies virus-like particles

The technologies used in rabies vaccines manufacturing for human use are based on the inactivated virus platform. An alternative to traditional vaccines is virus-like particles (VLPs). This work aimed to characterize the oxygen uptake and transfer rate parameters throughout recombinant baculovirus (rBV) and rabies VLPs production using Sf9 cells in stirred tank bioreactor (STB) for a better bioprocess understanding and scalability. Four runs in a bench STB were performed: cell culture without infection; cells infected singly with rBV bearing rabies virus glycoprotein (rBVG, multiplicity of infection, MOI=0.1 pfu/cell) and matrix protein (rBVM, MOI=0.1 pfu/cell), and coinfected with BVG and BVM at MOI of 3 and 2 pfu/cell, respectively. The specific oxygen uptake rate (qO2) and volumetric oxygen transfer coefficient (kLa) were monitored throughout the reactions, as well as viable cell concentration, viability, rBV titers, and protein concentration. According to the results herein, the aeration and agitation systems in a bioreactor at a higher scale could be designed using the criterium for scale-up of constant kLa, without oxygen facilities. Besides, rabies VLPs volumetric yield of 2.8 mg/L with a typical size (55–68 nm) was obtained. These findings suggest a promising bioprocess for rabies VLPs at a commercial scale.

New insight into the role of oxygen supply for surfactin production in bench-scale bioreactors using induced surface aeration.

Surfactin biosurfactant produced by Bacillus sp. has been studied, because it has enormous potential in several applications in the oil and cosmetics industry. The cultivation conditions for obtaining this bioproduct, however, still require attention, as, for example, parameters related to oxygen supply and consumption. In this study, different volumetric oxygen transfer coefficient (KLa) levels (0-11.56h-1) were tested in bench-scale bioreactor for surfactin biosurfactant production by Bacillus velezensis H2O-1, using induced surface aeration. While conditions close to anaerobiosis showed insignificant production of surfactin, an intermediated KLa condition (4.24h-1) generated the best surfactin concentration (579.6mg L-1), with a volumetric productivity of 11.9mg L-1h-1. These results showed that the oxygen demand to produce surfactin is not high, being possible to use induced surface aeration strategy in bioreactors, minimizing foam formation. In addition, in all KLa conditions tested, surfactin homologues C14 and C15 had higher relative abundance. Nevertheless, the KLa parameter seems to have had minimal influence on affecting the relative abundances of surfactin homologues produced. Particularly noteworthy in this study is the possibility of producing surfactin using a low-cost and scale-up feasible aeration strategy, unlike the foam collection strategies developed in other studies to obtain this bioproduct.

An experimental study of venturi aeration system

Aeration with a venturi aerator is efficient method to increase the dissolved oxygen (DO) concentration in water sources like rivers, waterways, ponds and water treatment plants. It is employed in aquatic animals' respiration and other biological processes as well as the removal of dissolved gases, dissolved metals, and volatile organic chemicals (VOC) in water. Aeration system efficiency is a crucial design factor, as aeration systems often account for the most energy draw and consumption. The study aims to evaluate the standard oxygen transfer rate (SOTR), standard aeration efficiency (SAE), volumetric oxygen transfer coefficient (KLa20) of venturi aeration system, air velocity and airflow rates at different water levels of the tank. Venturi aerator designed by Green Pumps and Equipments was used for experimentation. Air entrainment hole was fixed to 25 mm and 66 mm water inlet diameter. It was discovered that the rate of air flow rises as the tank's water level falls. The KLa20 was achieved at a rate of 2.58 h−1, SOTR at a rate of 0.323191 kgO2/hr, and SAE at a rate of 0.293594 kgO2/KWh, respectively. The findings suggest that venturi aeration may have a considerable impact on air entrainment and aeration efficiency. As a result, the venturi aeration system can be employed as an efficient aerator in aeration procedures.

Oxygen transfer of microbubble clouds in aqueous solutions – Application to wastewater

This study aims at improving the knowledge on the effects of gas injection, bubbles sizes and contaminants on oxygen transfer in microbubble clouds. First the effects of gas injection on oxygen transfer are studied and linked to several parameters that change together with changes in flow rate, namely bubble sizes and rise velocities. Oxygen transfer is then studied in the presence of contaminants that are shown to affect bubble size distribution, modify bubble dynamics and interfacial mass transfer. Oxygen transfer efficiencies are also measured in wastewater and compared with those obtained in aqueous solutions. The agreement between contaminated water in the lab (Triton X100) and wastewater experiments is emphasised as this offers the possibility to develop fundamental understanding relevant to wastewater under laboratory conditions. The role of the surfactants on the volumetric oxygen transfer coefficient is further analysed in terms of specific interfacial area and transfer coefficients, respectively. Interestingly, this shows that the increase in oxygen transfer efficiency as the concentration in Pentanol increases is due to the increase in interfacial area while the transfer coefficients decrease.

A comparative study of removal efficiency of organic contaminant in landfill leachate-contaminated groundwater under micro-nano-bubble and common bubble aeration.

Landfill leachate-contaminated groundwater is widespread all over the world. In order to study the organic contaminant removal efficiency of landfill leachate-contaminated groundwater under oxygen micro-nano-bubble (MNB) aeration, a series of lab-scale experiments of oxygen MNB aeration as well as common bubble (CB) aeration were conducted. Firstly, the difference in mass transfer, microbial activity enhancement, and contaminant removal efficiency between MNB and CB aeration was estimated. Then, the composition variations of dissolved organic matter (DOM) in groundwater treated by MNB or CB aeration were characterized by ultraviolet-visible (UV-VIS) absorption spectrum and fluorescence excitation-emission matrix (EEM). The test results showed that the oxygen utilization efficiency and volumetric oxygen transfer coefficient of MNB aeration were 10 and 50 times that of oxygen CB aeration, respectively. On the 30th day after MNB aeration, the dehydrogenase activity (DHA) of groundwater increased by 101.25%. Compared with CB aeration, the chemical oxygen demand (COD), 5-day biochemical oxygen demand (BOD5), and ammonia nitrogen removal efficiency under MNB aeration increased by 29.72%, 13.43%, and 138.59%, respectively. With the biodegradation effect of MNB aeration, a large number of protein-like and soluble microbial by-product substances were degraded, and humic and fulvic acid-like substances were degraded to a certain level. Oxygen MNB aeration played a chemical oxidation effect while enhancing the biodegradation of groundwater, and it was an energy-efficient landfill leachate-contaminated groundwater treatment method.

Evaluación hidrodinámica de un reactor airlift de recirculación interna con fluidos newtonianos y adelgazantes: Experimentación vs Simulación CFD

The hydrodynamics of an internal-loop airlift reactor was numerically and experimentally characterized. The gas holdup, liquid velocity, shear rate, flow pattern and volumetric oxygen transfer coefficient (kLa) were evaluated as a function of the air velocity and medium rheology. Tap water and CMC solutions were used as Newtonian and non-Newtonian fluids, respectively. The standard  model was employed for modeling turbulence, and unsteady three-dimensional simulations with the Euler–Euler model were performed. Gas holdup, liquid velocity and kLa measurements were performed for validating simulations. An increase in bubble coalescence and a decrease in kLa was observed with CMC solutions. The presence of recirculation loops inside the riser for CMC solutions is reported, which was not observed with tap water. The higher the CMC concentration, the larger the recirculation region seems to be. Results show that recirculation loops play a substantial role in the reactor’s hydrodynamic performance, and it turns out that the gas holdup in the riser increases with increasing the CMC concentration.

Modelo matemático para estimar el coeficiente volumétrico de transferencia de oxígeno en biorreactores usando cálculo conformal

This work proposes a novel mathematical model based on time conformable derivative convective mass transfer equation to calculate the volumetric oxygen transfer coefficient (kLa) in a bioreactor. To validate the novel model, a full mixed-level experimental design was proposed with two factors: agitation speed and dispersed phase. The model employs the conformable derivate order operator (α) and the electrode constant (kp), which changes with electrode use and the operating conditions of the bioreactor. The results show that when the viscosity increases and the agitation decreases, the value α increases, and vice versa. Therefore, α is a parameter that has a physical meaning in the process. The correlation coefficient of the proposed model with the experimental data (R2 &gt; 0.985) is higher than the one obtained with conventional models. The Akaike information criterion determined that the proposed conformal model describes the experimental data by 59%, while the conventional models describe the experimental data by 25% and 15%. There are no reports of similar mathematical models that determine mass transfer coefficients in bioprocesses.

Relation of shear stress and KLa on bikaverin production by Fusarium oxysporum CCT7620 in a bioreactor.

This study involved evaluating the effects of rotational impeller speed agitation (N) and specific air flow rate (Фair) on bikaverin production and on the growth of Fusarium oxysporum employing 11 bench-scale bioreactor assays. The results showed that the maximum bikaverin production (close to 300mgL-1) was achieved after 48h of fermentation in rice medium (20gL-1 milled rice in water) at 28°C with a volumetric oxygen transfer coefficient (KLa) and shear stress values of approximately 20h-1 and 17Nm-2, respectively. We reached this combination of parameters using an N of 340rpm and Фair of 0.935vvm. These KLa and shear stress values can be used as references when upscaling this process. Thus, this study was important to demonstrate how the main parameters in bioreactors affect bikaverin production and it presented important indications for upscaling this bioprocess.

Volumetric Oxygen Transfer Coefficient Effect on Biomass, Bioactive Compounds Production, and Kinetic Behavior of G. lucidum in Submerged Culture Using a Complex Medium.

HIGHLIGHTS G. lucidum is able to grow in a low-cost complex medium in a stirred tank bioreactor. KLa values between 17.3 - 32.8 h-1 are suitable to scale up bioprocess using complex media. Oxygen availability in the bioreactor affects the biomass and biomolecules production.

Effect of process parameters and surfactant additives on the obtained activity of recombinant tryptophan hydroxylase (TPH1) for enzymatic synthesis of 5-hydroxytryptophan (5-HTP)

5-hydroxytryptophan (5-HTP) is an intermediate molecule in the biosynthesis of serotonin, an important neurotransmitter, regulating a series of metabolic and psychological functions in humans. In this work, we studied the heterologous production of Human tryptophan hydroxylase (TPH1) in Escherichia coli, for the synthesis of 5-hydroxytryptophan (5-HTP) from Tryptophan (Trp). To quantify TPH1 activity, a simple fluorescence-based microtiter plate assay was established, based on the changes in fluorescence emission at 340 nm between substrate and product when excited at 310 nm, allowing quick and reliable quantification of released 5-HTP. To increase enzyme production, heterologous TPH1 production was studied in stirred tank bioreactor scale. The effect of rate of aeration (0.25, 0.50 and 0.75 vvm) and agitation (150, 250 and 500 rpm) was evaluated for biomass production, pH, volumetric oxygen transfer coefficient (kLa) and volumetric TPH1 activity. We determined that high agitation and low aeration allowed reaching the maximum measured enzyme activity. Under such conditions, we observed a 90% substrate conversion, obtaining 90 µM (~0.02 g/L) 5-HTP from a 100 µM Tryptophan substrate solution. Finally, we observed that the addition of Tween 20 (0.1%) in the culture broth under production conditions expanded the pH operation range of TPH1. Our results establish a base for a biocatalytic approach as a potential alternative process for the synthesis of 5-HTP using recombinant TPH1.

The dilution effect of media culture on mixing time, Kla O2, and hyaluronic acid production in S. zooepidemicus fed-batch culture.

Microbial production of biopolymers is typically associated with high viscosity and suitable mixing plays an important role in their production. Due to the nature of Streptococcus strains in high production of lactic acid and consequently high consumption of NaOH, which is associated with increased viscosity and reduced mixing caused by hyaluronic acid production, the injected NaOH accumulates and causes cells loss, and decreases in quantity and quality of the produced hyaluronic acid. In this study, the effect of increasing dilution of media culture of Streptococcus zooepidemicus fed-batch culture during pH control by NaOH on mixing time, volumetric oxygen transfer coefficient, and increasing hyaluronic acid production in a 2-L fermenter were studied. The results showed that significant increasing dilution causes reduction mixing time, remarkable improvement volumetric oxygen transfer coefficient, hyaluronic acid production enhancement from 6.6 to 8.4g/L, and diminution the consumption of NaOH. Dilution of media culture of S. zooepidemicus fed-batch culture by the pH controlling agent achieved one of the highest amounts of hyaluronic acid that was reported recently. This method does not require any automatic control and can be used at a low cost to produce other soluble extracellular biopolymers.

Evaluating the performance of fine bubble diffused aeration systems in cylindrical aeration tanks by fuzzy c-means algorithm.

In order to make a comprehensive evaluation of the performance of fine bubble diffused aeration systems in cylindrical aeration tanks, the following parameters are considered: distribution of DO concentration in the horizontal direction of the different water depth of an aeration tank, distribution of DO concentration in the vertical direction of the aeration tank, distribution of DO concentration in all the points of the aeration tank and ratio of total mass increment of DO in the aeration tank to total mass of oxygen in aeration air. The aeration characteristic criterion (ACC) is proposed to synthesize these parameters, and weighted sums of the similarity degrees derived from the extensions of fuzzy c-means (FCM) are used to construct ACC. The results show that compared with total volumetric oxygen transfer coefficient (KLα) and specific standard oxygen transfer efficiency (SSOTE), ACC is demonstrated to be more marked and sensitive for the performance evaluation of the fine bubble diffused aeration systems equipped with fine-pore aeration tubes. Moreover the performance of aeration systems equipped with different layouts of fine-pore aeration tubes is comparatively studied, and their performance from best to worst is ring-type diffuser > square-type diffuser > parallel-lines-type diffuser > cross-type diffuser.