Emilio Molina

Tubular photobioreactor design for algal cultures

Principles of fluid mechanics, gas-liquid mass transfer, and irradiance controlled algal growth are integrated into a method for designing tubular photobioreactors in which the culture is circulated by an airlift pump. A 0.2 m(3) photobioreactor designed using the proposed approach was proved in continuous outdoor culture of the microalga Phaeodactylum tricornutum. The culture performance was assessed under various conditions of irradiance, dilution rates and liquid velocities through the tubular solar collector. A biomass productivity of 1.90 g l(-1) d(-1) (or 32 g m(-2) d(-1)) could be obtained at a dilution rate of 0.04 h(-1). Photoinhibition was observed during hours of peak irradiance; the photosynthetic activity of the cells recovered a few hours later. Linear liquid velocities of 0.50 and 0.35 m s(-1) in the solar collector gave similar biomass productivities, but the culture collapsed at lower velocities. The effect of dissolved oxygen concentration on productivity was quantified in indoor conditions; dissolved oxygen levels higher or lower than air saturation values reduced productivity. Under outdoor conditions, for given levels of oxygen supersaturation, the productivity decline was greater outdoors than indoors, suggesting that under intense outdoor illumination photooxidation contributed to loss of productivity in comparison with productivity loss due to oxygen inhibition alone. Dissolved oxygen values at the outlet of solar collector tube were up to 400% of air saturation.

A process for high yield and scaleable recovery of high purity eicosapentaenoic acid esters from microalgae and fish oil

A low expense process is developed for recovering esterified eicosapentaenoic acid (EPA) from microalgae and fish oil. Over 70% of the EPA content in the esterified crude extract of microalgae were recovered at purities exceeding 90%. The recovery scheme utilizes either wet or freeze-dried algal biomass. The process consists of only three main steps: 1) simultaneous extraction and transesterification of the algal biomass; 2) argentated silica gel column chromatography of the crude extract; and 3) removal of pigments by a second column chromatographic step. Argentated silica gel chromatography recovered about 70% of the EPA ester present in the crude fatty ester mixture of fish oil, but at a reduced purity ( approximately 83% pure) compared to the microalgal derived EPA. The optimal loading of the fatty ester mixture on the chromatographic support was about 3% (w/w) but loadings up to 4% did not affect the resolution significantly. The process was scaled up by a factor of nearly 320 by increasing the diameter of the chromatography columns. The elution velocity remained constant. Compared to the green alga Monodus subterraneus, the diatom Phaeodactylum tricornutum had important advantages as a potential commercial producer of EPA. For a microalgal EPA process to be competitive with fish oil derived EPA, P. tricornutum biomass (2.5% w/w EPA) needs to be obtained at less than

Production cost of a real microalgae production plant and strategies to reduce it.

4/kg. If the EPA content in the alga are increased to 3.5%, the biomass may command a somewhat higher price. The quality of microalgal EPA compares favorably with that of the fish oil product. Compared to free fatty acid, EPA ester is more stable in storage. Shelf-life is extended by storing in hexane. The silver contamination in the final purified EPA was negligibly small (<210 ppb).

Interaction between CO2-mass transfer, light availability, and hydrodynamic stress in the growth of phaeodactylum tricornutum in a concentric tube airlift photobioreactor

The cost analysis of a real facility for the production of high value microalgae biomass is presented. The facility is based on ten 3 m3 tubular photobioreactors operated in continuous mode for 2 years, data of Scenedesmus almeriensis productivity but also of nutrients and power consumption from this facility being used. The yield of the facility was close to maximum expected for the location of Almería, the annual production capacity being 3.8 t/year (90 t/ha·year) and the photosynthetic efficiency being 3.6%. The production cost was 69 €/kg. Economic analysis shows that labor and depreciation are the major factors contributing to this cost. Simplification of the technology and scale-up to a production capacity of 200 t/year allows to reduce the production cost up to 12.6 €/kg. Moreover, to reduce the microalgae production cost to approaches the energy or commodities markets it is necessary to reduce the photobioreactor cost (by simplifying its design or materials used), use waste water and flue gases, and reduce the power consumption and labor required for the production step. It can be concluded that although it has been reported that production of biofuels from microalgae is relatively close to being economically feasible, data here reported demonstrated that to achieve it by using the current production technologies, it is necessary to substantially reduce their costs and to operate them near their optimum values.

Studies of mixing in a concentric tube airlift bioreactor with different spargers

The microalga Phaeodactylum tricornutum was grown in a concentric tube airlift photobioreactor. A maximum specific growth rate of 0. 023 h-1 was obtained using a superficial gas velocity around 0.055 m/s. Lower or higher gas flow rates limited the culture performance. To establish if the observed limitation was due to CO2 or to the photosynthetically active irradiance, characteristic times for mixing, mass transfer and CO2 consumption, and the photon flux absorbed by the culture were analyzed. The CO2-gradients in the culture were shown to be responsible for the limitation during the exponential growth phase, and both CO2 and light irradiance were limiting in the linear growth phase. The decrease in specific growth rate relative to the maximum was found to be related to the specific gas-liquid interfacial area, the length scale of the microeddies and the shear rate. Copyright 1998 John Wiley & Sons, Inc.

RETRACTED: A process for high yield and scaleable recovery of high purity eicosapentaenoic acid esters from microalgae and fish oil

Abstract Gas hold-up, e, liquid velocity, JLr, axial dispersion coefficient, Ez, and mixing time, tm, have been measured in a concentric tube airlift bioreactor (12 × 10−3 m3 in volume) using sea water, as a function of the superficial gas velocity, JGr, (up to 0.21 m/s). Seven different spargers were tested. Four of them were cylindrical (pore size from 60 μm to 1 × 10−3 m) and three were porous plates (pore size from 30 to 120 μm). Three different flow regimes are observed in hold-up and liquid velocity over the range of JGr covered: uniform bubbly flow at low gas velocities, heterogeneous flow at high gas velocities and a transition flow between bubbly and heterogeneous. The spargers with smaller pore sizes produce more hold-up and slow down velocity, because more gas recirculates into the downcomer. The change from uniform bubbly flow to transition flow appears because of the start of coalescence. In heterogeneous flow, where the bubble size is set by the degree of coalescence, no influence of the difference between the spargers used could be detected. Gas hold-up in the riser, er, could be represented, over the whole range of JGr used, by E r =α J Gr J Lr β When the Bodenstein number for the gas-liquid mixture, BoLG, and the Froude number, Fr, are used to represent axial dispersion, three flow regimes appear which correspond to those observed when hold-up and liquid velocity are plotted vs JGr. The axial dispersion coefficient, Ez, could be represented by E z =K 5 d e J Gr E n 4 where de is the equivalent diameter of the reactor. This equation suggests that Ez is dominated by bubble slip, and fits 78% of the measured data with less than 20% error. It has also been applied satisfactorily to the results obtained by other authors who used different systems and liquid phases. Mixing time depends on sparger geometry and pore size only at low gas velocities. At high gas velocities, mixing time is practically independent of sparger and gas velocity.

Steady-state axial profiles of dissolved oxygen in tall bubble column bioreactors

This article has been retracted at the request of the Editor. Please see Elsevier Policy on Article Withdrawal ( http://www.elsevier.com/locate/withdrawalpolicy ). Reason: It duplicates significant parts of a paper that has already been published by the same authors in Enzyme Microb. Technol., volume 26 (2000) 516–529, doi: 10.1016/S0141-0229(99)00191-X . One of the conditions of submission of a paper for publication is that authors declare explicitly that the paper is not under consideration for publication elsewhere. The scientific community takes a very strong view on this matter and we apologize to readers of the journal that this was not detected during the submission process.

Axial inhomogeneities in steady-state dissolved oxygen in airlift bioreactors: predictive models

A model is developed for prediction and interpretation of the observed steady-state axial dissolved oxygen concentration profiles in tall bubble columns. The observed concentration profiles are non-linear, unlike what would be expected if the hydrostatic pressure alone influenced the profiles. The non-linear profiles result from the axial mixing of liquid in the column. Several other factors influence the profiles, including the overall gas holdup, the volumetric overall gas—liquid mass transfer coeƒcient, and the static height of liquid in the column. The e⁄ect of mixing can be adequately accounted for using an axial dispersion coeƒcient. Because the axial dispersion coeƒcient is sensitive to the diameter of the column and to gas flow rate, the overall behavior of the profile is a⁄ected by the aspect ratio of the column and the superficial gas velocity in it. The mass transfer coeƒcient and the axial dispersion coeƒcient have mutually opposing e⁄ects on the shape of the profile. Because both those variables increase with increasing gas flow rate, the shape of the profile is a⁄ected less than would be the case if only mixing influenced the profile. The non-linearity of concentration profiles increases with increasing overall height of the column especially when the height exceeds about 2 m in a 0.24 m diameter column. The model-predicted axial concentration profiles agree closely — within

A reassesment of relationship between riser and downcomer gas holdups in airlift reactors

3% — with the measured data. Using the measured profile, the model allows for calculation of the liquid-phase axial dispersion coeƒcients. This method does not require the use of tracers. Being a steady-state method, the operation of the bioreactor does not need to the interrupted in any way for the determination of the axial dispersion coeƒcient or the state of mixing. Consequently, the proposed method is particularly suited to characterizing the axial dispersion coeƒcient in an operating bioreactor without disturbing the operation. If the axial dispersion coeƒcient is known, the model allows for quantifying the spatial inhomogeneities in oxygen concentration in a bioreactor vessel. ( 1999 Elsevier Science Ltd. All rights reserved.

GAS HOLDUP, LIQUID CIRCULATION AND MIXING BEHAVIOUR OF VISCOUS NEWTONIAN MEDIA IN A SPLIT-CYLINDER AIRLIFT BIOREACTOR

Models were developed for prediction and interpretation of the observed steady-state axial dissolved oxygen concentration profiles in tall airlift bioreactors. The observed concentration profiles were non-linear because of a combination of hydrodynamic and mass transport factors. The profiles were influenced mainly by the liquid-phase axial dispersion coefficient, the volumetric overall gas–liquid mass transfer coefficient, the gas velocity, the induced liquid circulation velocity. The model-predicted concentration profiles agreed within ±2% with the measured data in a tall (working aspect ratio ∼ 15) airlift vessel operated under aeration regimens that are typically used during wastewater treatment. Axial inhomogeneities in dissolved oxygen increased with increasing aeration rate. This phenomenon may influence activated sludge processes in airlift and deep-shaft reactors. The maximum attainable concentration of dissolved oxygen at the bottom of a typically aerated airlift reactor, ≥ 3.5 m deep, always remained at 10% axially up the reactor.