Transport phenomena, reactor design and scale-up

Mixing in biological reactors is used to improve mass transfer and provide proper micro-scale and macro-scale shear rates for effective process results. Reactors may be mixed by impellers on rotating shafts, or may be of the flow contactor type such as packed columns, bubble columns or airlift circulators.

In addition, review of kinetics can tell the process performance based on various kinds of mixing conditions. Several interesting and unique mixing studies are also included where appropriate.     

Growth and biochemical characterization of microalgal biomass produced in bubble column and airlift photobioreactors: studies in fed-batch culture

Relatively large (0.19 m column diameter, 2 m tall, 0.06 m³ working volume) outdoor bubble column and airlift bioreactors (a split-cylinder and a draft-tube airlift device) were compared for monoseptic fed-batch culture of the microalga Phaeodactylum tricornutum. The three photobioreactors produced similar biomass versus time profiles and final biomass concentration (4 kg m⁻³). The maximum specific growth rate observed within a daily illuminated period in the exponential growth phase, had a value of 0.08 h⁻¹ on the third day of culture. Because of night-time losses of biomass, the specific growth rate averaged over the 4-days of exponential phase was 0.021 h⁻¹ for the three reactors.

The biomass in the vertical column reactors did not experience photoinhibition under conditions (photosynthetically active daily averaged irradiance value of 1150±52 μE m⁻² s⁻¹) that are known to cause photoinhibition in conventional thin-tube horizontal loop reactors. Because of good gas-liquid mass transfer, the dissolved oxygen concentration in the reactors at peak photosynthesis remained <120% of air saturation; thus, oxygen inhibition of photosynthesis and photo-oxidation of the biomass did not occur. Carbohydrate accumulation (up to 13% w/w) by the biomass was favored during light-limited linear growth. A declining light intensity caused a more than five-fold increase in cellular carotenoids but the chlorophylls increased only by about 2.5-fold during the course of the culture. In the stationary phase, up to 2% of the biomass was chlorophylls and carotenoids constituted up to 0.5% of the biomass dry weight.     

Acceleration of mass transfer in methane-producing loop reactors

Gas bubbles entrapped in methanogenic granules subjected to hydrostatic pressure oscillations during recirculation in loop reactors will induce intraparticle liquid flows, and thereby enhance mass transfer in excess of diffusion. This breathing particle concept was clearly demonstrated in a well defined inorganic model system. The experimental results could be described satisfactory with a structured mathematical model, while a 30% improvement is predicted for methanogenic loop reactors as compared to constant pressure systems. It is concluded that acceleration of mass transfer in gas-producing systems offers challenging perspectives for both heterogeneous catalysis and biological fermentations.     

Survival of,and plasmid transfer between modified Enterobacter cloacae strains under long-term starvation conditions in a continuous-flow system

Continuous-flow, packed-bed column reactors, which provide an experimental model of a soil profile, were used to investigate survival of, and plasmid transfer between, strains of Enterobacter cloacae. When columns, inoculated with nutrient-sufficient donor and recipient strains, were provided with a minimal salts medium with no added carbon source, transconjugant cells appeared in their effluents. During the first few days of such experiments, the concentration of cells in the effluent declined but then the donor population stabilized, while the recipient and transconjugant populations continued to decrease. The results indicate that the amount of nutrient required to maintain and transfer plasmids is very low. No transconjugants were observed in the effluent from columns inoculated with pre-starved donor and recipient strains.     

Membrane sparger in bubble column,airlift, and combined membrane–ring sparger bioreactors

The bubble column and the two internal loop airlift reactors (riser/downcomer area ratios of 0.11 and 0.58) characterized in this study were equipped with a rubber membrane sparger, which produced small bubbles, giving high mass transfer coefficients. The low mixing intensity in the bubble column was increased by an order of magnitude in the airlift reactors. We designed a novel aeration and mixing system by adding a ring sparger to the membrane sparger in the bubble column and maintained the advantages of both airlift configuration (good mixing properties) and bubble column configuration (efficient aeration, without any internal constructions). The combined membrane–ring sparger system has unique features with respect to the efficiency of utilization of substrate gasses and energy. Model experiments showed that the small bubbles from the membrane sparger do not coalesce with the large bubbles from the ring sparger. If different gases were added through the two spargers it was possible to transfer a hazardous or expensive gas quantitatively to the liquid through the membrane sparger (dual sparging mode). In the combined membrane–ring sparger system the energy input for mixing and mass transfer is divided. Therefore, the energy consumption can be minimized if the flow distribution of air through the membrane and ring sparger is controlled by the oxygen demand and the inhomogeneity of the culture, respectively (split sparging mode). The dual sparging mode was used for mass production of the alga Rhodomonas sp. as the first step in aquatic food chains. Avoiding mechanical parts removes an important risk of malfunction, and a continuous culture could be maintained for more than 8 months. © 1999 John Wiley & Sons, Inc. Biotechnol Bioeng 64: 452–458, 1999.     

Laboratory scale bioreactor for solid state processes

Engineering aspects of solid state processes, design of fermentors, control systems for maintaining parameters are important problems for the research in this area. Laboratory scale reactors (2 kg dry-matter capacity) were constructed to allow control of the temperature and moisture level of the substrate without agitation. Forced aeration is accomplished by means of thermostated air injected at the bottom of the reactor. By bubbling in a waterbath and after heating, the thermostated air allows the regulation of temperature and moisture content of the medium during the cultivation. These reactors have been used for the study of protein enrichment of leached sugar-beet pulp by solid state processes.     

Denitrification characteristics of reject water in upflow biofiltration

Two types of bench-scale experiments using upflow biofilm reactors packed with granular floating polystyrene (GFP) or polyurethane foam cubes (PFC), were used to investigate the denitrification of reject water. A high denitrification rate was achieved in both upflow biofilm reactors since the highly concentrated volatile fatty acids in reject water served as effective hydrogen donors for denitrification. Of the two biofiltrations, the denitrification rate using GFP was 3.5 kg⁻¹ N m⁻³ per day, and higher than that using PFC. The amount of total attached biomass and solid capture capacity were also greater in GFP than in PFC. Moreover, the backwashing of the GFP packed column was optimized with air and water agitation. Of the processes investigated in this study, upflow biofiltration using GFP was the most acceptable process for reject water treatment based on treatability and operation.     

Incorporating hydrodynamics into spatiotemporal metabolic models of bubble column gas fermentation

Gas fermentation has emerged as a technologically and economically attractive option for producing renewable fuels and chemicals from carbon monoxide (CO) rich waste streams. LanzaTech has developed a proprietary strain of the gas fermentating acetogen Clostridium autoethanogenum as a microbial platform for synthesizing ethanol, 2,3-butanediol, and other chemicals. Bubble column reactor technology is being developed for the large-scale production, motivating the investigation of multiphase reactor hydrodynamics. In this study, we combined hydrodynamics with a genome-scale reconstruction of C. autoethanogenum metabolism and multiphase convection–dispersion equations to compare the performance of bubble column reactors with and without liquid recycle. For both reactor configurations, hydrodynamics was predicted to diminish bubble column performance with respect to CO conversion, biomass production, and ethanol production when compared with bubble column models in which the gas phase was modeled as ideal plug flow plus axial dispersion. Liquid recycle was predicted to be advantageous by increasing CO conversion, biomass production, and ethanol and 2,3-butanediol production compared with the non-recycle reactor configuration. Parametric studies performed for the liquid recycle configuration with two-phase hydrodynamics showed that increased CO feed flow rates (more gas supply), smaller CO gas bubbles (more gas–liquid mass transfer), and shorter column heights (more gas per volume of liquid per time) favored ethanol production over acetate production. Our computational results demonstrate the power of combining cellular metabolic models and two-phase hydrodynamics for simulating and optimizing gas fermentation reactors.     

Solubilization and concentration of carbon dioxide: novel spray reactors with immobilized carbonic anhydrase

Novel spray reactors are described that employ immobilized biocatalyst (carbonic anhydrase), enabling concentration and solubilization of emitted CO(2) by allowing catalytic contact with water spray. The reactors were fed with simulated emission gas. The performance of the reactors was investigated with respect to operation variable: emission flow rate; gas composition in the emission stream; water flow rate; area-to-volume ratio of immobilized reactor core; and the enzyme load within the core. The reactors were also investigated for pressure drop and extractability of CO(2) from the emission with single vs. multiple reactors (of combined equal volume). The biotechnological process of solubilization and concentration of CO(2) from emission exhausts or streams occurring in the spray reactors could be coupled for further biochemical/chemical conversion of the concentrated CO(2).     

Enzymatic reactors for the removal of recalcitrant compounds in wastewater

Abstract

Enzymatic treatments based on oxidative enzymes, such as peroxidases, laccases and tyrosinases, have been proposed as an alternative to conventional methods to remove a broad range of contaminants present in wastewater. The aim of this study is to discuss existing technologies for the removal of pollutants based on the use of oxidative enzymes, including a discussion on the most important factors affecting the efficiency of the proposed systems. Factors involved in the catalytic cycle of the enzyme (biocatalyst, substrates and mediators), the addition of certain components to the reaction medium (additives, surfactants or solvents) as well as operational parameters (temperature, pH or agitation) will be discussed. Finally, two types of reactors: one-stage and two-stage enzymatic membrane reactors, especially designed for the treatment of micropollutants present in secondary effluents, will be described in detail.     

<Emphasis Type="Italic">k</Emphasis><Subscript>L</Subscript><Emphasis Type="Italic">a</Emphasis> determination: comparative study for a gas mass balance method

The determination of k(L) a by a gas balance method coupled with sulphite oxidation is compared for three kinds of processes (stirred tank, bubble column and fixed-bed column reactors) with a gassing-in and with a classical chemical sulphite oxidation method. The mathematical relations required for the determination of the k(L) a value are detailed. In coalescing gas-liquid conditions, the values calculated by the three methods are shown to be comparable. The gas balance method is more rapid than either the steady-state gassing-in or the chemical sulphite reaction rate measurement methods. It is also well adapted for three-phase systems (gas-liquid-solid) in which the non-coalescing effects of sulphite solution are reduced by solid interferences.     

Oxygen transfer in liquid-impelled loop reactors using perfluorocarbon liquids

Oxygen transfer in the liquid-impelled loop reactor is described for a setup in which the perfluorochemical FC40 is aerated externally. Two sizes of reactors are investigated. The mass-transfer coefficient k appears to be lower with a factor of about 0.6 compared to gas liquid systems. the specific exchange area in the present experimental setup is found to be favorable when compared with gas liquid bioreactors at the same superficial dispersed-phase velocities. However, slow coalescence of the dispersed-phase drops in the phase separation section limits the dispersed-phase flow rate seriously. In Case this become crucial from the point of view of oxygen supply, special measures need to be found or alternatives such as combined sparging of air and solvent. (c) 1992 John Wiley & Sons, Inc.     

A modeling study of anaerobic biofilm systems: II. Reactor modeling

A rigorous steady-state model of anaerobic biofilm reactors taking into account acid-base and gas-phase equilibria in the reactor in conjunction with detailed chemical equilibria and mass transfer in acetate-utilizing methanogenic biofilms is presented. The performances of ideal completely stirred tank reactors (CSTRs) and plug-flow reactors, as well as reactors with nonideal hydraulic conditions, are simulated. Decreasing the surface loading rate increases the acetate removal efficiency, while decreasing the influent pH and increasing the buffering capacity improves the removal efficiency only if the bulk pH of the reactor shifts toward more optimal values between 6.8 to 7.0. The reactor can have negative or positive removal efficiencies depending on the start-up conditions. The respiration coefficient plays a critical role in determining the minimum influent pH required for reactor recovery after failure. Having multiple CSTRs-in-series generally increases the overall removal efficiency for the influent conditions investigated. Monitoring of the influent feed quality is critical for plug-flow reactors, becasue failure of the initial sections of the reactor may cause a cascading effect that may lead to a rapid reactor failure. (c) 1995 John Wiley & Sons, Inc.     

Anaerobic Reactor Design Concepts for the Treatment of Domestic Wastewater

Since the earlier anaerobic treatment systems, the design concepts were improved from classic reactors like septic tanks and anaerobic ponds, to modern high rate reactor configurations like anaerobic filters, UASB, EGSB, fixed film fluidized bed and expanded bed reactors, and others. In this paper, anaerobic reactors are evaluated considering the historical evolution and types of wastewaters. The emphasis is on the potential for application in domestic sewage treatment, particularly in regions with a hot climate. Proper design and operation can result in a high capacity and efficiency of organic matter removal using single anaerobic reactors. Performance comparison of anaerobic treatment systems is presented based mostly on a single but practical parameter, the hydraulic retention time. Combined anaerobic reactor systems as well as combined anaerobic and non-anaerobic systems are also presented.     

Immobilized plant cell reactors

The use of plant cells for the production of biochemicals represents a new area of biotechnological exploration. The techniques envisioned for industrial processes are related to those developed for microorganisms and a strong emphasis should be placed on immobilized cell systems. This review examines the spectrum of products that are synthesized by higher plants and the immobilization techniques that are suited to entrap plant cells from suspension culture. Different reactor configurations are described. Both packed-bed reactors with alginate-entrapped cells and hollow-fibre cartridges with sequestered cells have utility for the continuous production of biochemicals.     

Overall assessment of Monodus subterraneus cultivation and EPA production in outdoor helical and bubble column reactors

The outdoor production of Monodus subterraneus wasstudied in bubble column and helical reactors, mainly analysing the influenceofdilution rate, air flow rate and solar irradiance on growth rate andbiochemicalcomposition. Photoinhibition and photo-oxidation phenomena were also analysed.The cultures were stressed at high solar irradiance and dissolved oxygenconcentrations. A clear relationship between stress of the cultures and thefluorescence from PSII measurements was observed, the Fv/Fm ratio being lowerinthe helical reactor than in the bubble column. Growth rate and biomassproductivity were both a function of the average irradiance and the Fv/Fmratio;maximum values of 0.040 h^–1 and 0.54 gL^–1 d^–1 were measured. The influenceofphotoinhibition and average irradiance was modelled, the model also fitting theexperimental data reported by another author. The chlorophyll contenthyperbolically decreased, whereas the carotenoid content decreased linearlywiththe average irradiance. The higher the dilution rate the higher the protein andcarbohydrate content of the biomass, and the lower the lipid content. Theeicosapentaenoic acid (EPA) content ranged from 2.3 to 3.2% d.wt, the higherthe dilution rate, the lower EPA content, although the higher the EPAproportion. Maximum EPA productivity was only 9 mg L^–1d^–1, due to the stress to which the cultures wereexposed.     

On-line synthesis utilizing immobilized enzyme reactors based upon immobilized dopamine beta-hydroxylase

Immobilized enzyme reactors (IMERs) based upon dopamine beta-hydroxylase (DBH) have been developed. Immobilized artificial membrane (IAM) and glutaraldehyde-P (Glut-P) stationary phases have been used to immobilize DBH. When DBH is immobilized on the Glut-P interphase the enzyme is outside the stationary phase whereas with the IAM interphase the enzyme is embedded within the interphase surroundings. The activity of each IMER and their ability for on-line hydroxylation has been investigated. The resulting IMERs are enzymatically active and reproducible. The IMERs can be utilized through the use of coupled chromatography to characterize the cytosolic (DBH-Glut-P-IMER) and membrane-bound (DBH-IAM-IMER) forms of the enzyme. The substrate is injected onto the individual IMERs and the reactants and products are eluted onto a phenylboronic acid column for on-line extraction. The substrates and products are then transported via a switching valve to coupled analytical columns. The results demonstrate that enzyme-substrate and enzyme-inhibitor interactions can be investigated with the on-line system. These IMERs can be utilized for the discovery and characterization of new drug candidates specific for the soluble form and membrane-bound form of DBH. The effects of flow-rate, contact time, pH and temperature have also been investigated.     

Methanogenesis in thermophilic biogas reactors

Methanogenesis in thermophilic biogas reactors fed with different wastes is examined. The specific methanogenic activity with acetate or hydrogen as substrate reflected the organic loading of the specific reactor examined. Increasing the loading of thermophilic reactors stabilized the process as indicated by a lower concentration of volatile fatty acids in the effluent from the reactors. The specific methanogenic activity in a thermophilic pilot-plant biogas reactor fed with a mixture of cow and pig manure reflected the stability of the reactor. The numbers of methanogens counted by the most probable number (MPN) technique with acetate or hydrogen as substrate were further found to vary depending on the loading rate and the stability of the reactor. The numbers of methanogens counted with antibody probes in one of the reactor samples was 10 times lower for the hydrogen-utilizing methanogens compared to the counts using the MPN technique, indicating that other non-reacting methanogens were present. Methanogens that reacted with the probe againstMethanobacterium thermoautotrophicum were the most numerous in this reactor. For the acetate-utilizing methanogens, the numbers counted with the antibody probes were more than a factor of 10 higher than the numbers found by MPN. The majority of acetate utilizing methanogens in the reactor wereMethanosarcina spp. single cells, which is a difficult form of the organism to cultivatein vitro. No reactions were observed with antibody probes raised againstMethanothrix soehngenii orMethanothrix CALS-1 in any of the thermophilic biogas reactors examined. Studies using 2-¹⁴C-labeled acetate showed that at high concentrations (more than approx. 1 mM) acetate was metabolized via the aceticlastic pathway, transforming the methyl-group of acetate into methane. When the concentration of acetate was less than approx. 1 mM, most of the acetate was oxidized via a two-step mechanism (syntrophic acetate oxidation) involving one organism oxidizing acetate into hydrogen and carbon dioxide and a hydrogen-utilizing methanogen forming the products of the first microorganism into methane. In thermophilic biogas reactors, acetate oxidizing cultures occupied the niche ofMethanothrix species, aceticlastic methanogens which dominate at low acetate concentrations in mesophilic systems. Normally, thermophilic biogas reactors are operated at temperatures from 52 to 56° C. Experiments using biogas reactors fed with cow manure showed that the same biogas yield found at 55° C could be obtained at 61° C after a long adaptation period. However, propionate degradation was inhibited by increasing the temperature.     

Removal of Hexane in Biofilters Packed with Perlite and a Peat–Perlite Mixture

Removal of hexane from air–hexane mixtures in biofilters packed with different solid media under nitrogen supplementation was performed for 70 days. Two columns containing Perlite or a mixture of peat and Perlite, were used. The solid media were supplemented with nitrogen source up to 1 kg/m³ per week for high nutrient supplementation and 0.2 kg/m³ per month for low nutrient supplementation. A high rate of hexane removal: 95 g/m³ h was achieved under high nutrient supplementation, high air flow rate and high hexane concentration. However, the percentage of hexane removal decreased with increasing air flow rate and hexane inlet concentration. For high nutrient supplementation the type of solid medium did not significantly affect the biodegradation capacity. With low nutrient supplementation, the highest removal rate was achieved in the column containing the peat–perlite mixture. The column containing perlite had a significantly lower pressure drop (20 Pa/m) than the 2400–2930 Pa/m observed for the column containing the mixture. Perlite offers an opportunity of running a biofiltration process at a lower and stable pressure drop if the nutrient supplementation is managed properly.     

Oxygen transfer and scale-up in bioreactors using hydro-ejectors for gas-liquid contacting

The commonly used scale-up criteria are investigated for their applicability in the case of hydro-ejector reactors. In combination with the liquid jet momentum, which characterizes the hydro-ejector, a scale-up correlation with the oxygen transfer rate as scale-up criterion is proposed, independent of the type of hydro-ejector and the reactor configuration. The results with regard to the power input are compared with those of stirred tank and bubble column. Its competitiveness is at high power per volume input and above all in large scale reactors.