F. Camacho Rubio

Prediction of dissolved oxygen and carbon dioxide concentration profiles in tubular photobioreactors for microalgal culture

A model is developed for prediction of axial concentration profiles of dissolved oxygen and carbon dioxide in tubular photobioreactors used for culturing microalgae. Experimental data are used to verify the model for continuous outdoor culture of Porphyridium cruentum grown in a 200-L reactor with 100-m long tubular solar receiver. The culture was carried out at a dilution rate of 0.05 h-1 applied only during a 10-h daylight period. The quasi-steady state biomass concentration achieved was 3.0 g. L-1, corresponding to a biomass productivity of 1.5 g. L-1. d-1. The model could predict the dissolved oxygen level in both gas disengagement zone of the reactor and at the end of the loop, the exhaust gas composition, the amount of carbon dioxide injected, and the pH of the culture at each hour. In predicting the various parameters, the model took into account the length of the solar receiver tube, the rate of photosynthesis, the velocity of flow, the degree of mixing, and gas-liquid mass transfer. Because the model simulated the system behavior as a function of tube length and operational variables (superficial gas velocity in the riser, composition of carbon dioxide in the gas injected in the solar receiver and its injection rate), it could potentially be applied to rational design and scale-up of photobioreactors. Copyright 1999 John Wiley & Sons, Inc.

Carbon dioxide uptake efficiency by outdoor microalgal cultures in tubular airlift photobioreactors.

The influence of solar irradiance and carbon dioxide molar fraction of injected CO2-air mixtures on the behavior of outdoor continuous cultures of the mi- croalga Phaeodactylum tricornutum in tubular airlift pho- tobioreactors was analyzed. Instantaneous solar irradi- ance, pH, dissolved oxygen, temperature, biomass con- centration, and the mass flow rates of both the inlet and outlet oxygen and carbon with both the liquid and gas phases were measured. In addition, elemental analysis of the biomass and the cell-free culture medium was per- formed. The oxygen production rate and carbon dioxide consumption rate increased hyperbolically with the inci- dent solar irradiance on the reactor surface. Carbon losses showed a negative correlation with the daily varia- tion of the carbon dioxide consumption rate. The maxi- mum CO2 uptake efficiency was 63% of the CO2 supplied when the CO2 concentration in the gas supplied was 60% v/v. Carbon losses were >100% during the night, due to CO2 production by respiration, and hyperbolically de- creased to values of 10% to 20% in the midday hours. An increase in the carbon fixed in the biomass with the solar cycle was observed. A slight daily decrease of carbon content of the cell-free culture medium indicated the ex- istence of carbon accumulation in the culture. A decrease in CO2 molar fraction in the injected gas had a double benefit: first, the biomass productivity of the system was enhanced from 2.05 to 2.47 g L ˛1 day ˛1 by reduction of CO2 inhibition and/or pH gradients; and second, the car- bon losses during the daylight period were reduced by 60%. The fluid dynamics in the reactor also influenced the carbon losses: the higher the liquid flow rate the higher the carbon losses. By using a previous mass transfer model the experimental results were simulated and the usefulness of this method in the evaluation and scale-up of tubular photobioreactors was estab- lished.

Modeling the effect of free water on enzyme activity in immobilized lipase-catalyzed reactions in organic solvents

The influence of water content on the lipase-catalyzed acidolysis of triolein (glycerol-trioleate, TO) and caprylic acid (CA) in hexane, using an immobilized enzyme was studied. An adequate water content (RW) ranged from near zero to 0.1 g of water/g of dry enzyme. Over these values there was a decrease in the rate of incorporation of CA into triglyceride. This decrease was attributed to the progressive flooding of the carrier’s pores, in which the enzyme was immobilized. The flooding reduced the number of the enzyme molecules at the water–hexane interface and therefore, hindered the accessibility of the hydrophobic substrates (TO and CA) to the enzyme. A simple physical model based on a characterization of the immobilized enzyme particle by mercury porosimetry was developed. The model agreed well with both the experimental data and the prior published data. The model may partly explain the observed inhibition when using low molecular weight alcohols and carboxylic acids in immobilized lipase-catalyzed processes.

Production of structured triglycerides rich in n-3 polyunsaturated fatty acids by the acidolysis of cod liver oil and caprylic acid in a packed-bed reactor: equilibrium and kinetics

Structured triglycerides (ST) enriched in n-3 polyunsaturated fatty acids (PUFAs) (eicosapentaenoic acid, EPA, and docosahexaenoic acid, DHA) in position 2 of the triglyceride backbone were synthesised by acidolysis of cod liver oil (CLO) and caprylic acid (CA) catalysed by the 1; 3-speci=c immobilised lipase Lipozyme IM. The reaction was carried out in three ways: (1) in a batch reactor (where the in?uence of temperature on the incorporation of CA into the CLO triglyceride was studied); (2) in an immobilised lipase packed-bed reactor (PBR) by recirculating the reaction mixture from the exit of the bed to the substrate reservoir (product recirculation) to determine the equilibrium composition; and (3) in a PBR without recirculation. A “lag” period of duration inversely proportional to the initial water amount of the lipase, was observed when new lipase was used. Apparently, during this “lag” period the hydro-enzymatic layer that surrounds the lipase surface reaches its water equilibrium content. A reaction scheme, where only the fatty acid in the positions 1 and 3 of the glycerol backbone were exchanged by CA, was proposed. The exchange equilibrium constants between CA and the native fatty acids of CLO were determined. The n-3 PUFAs (EPA and DHA) were the most resistant native fatty acids to exchange with exchange equilibrium constants of 1.32 and 0.28, respectively. Also, average reaction rates and kinetic constants of exchange of CA and native fatty acid of CLO were calculated. Low kinetic constants were observed for EPA, DHA and palmitic acid. For acidolysis reaction in the continuous mode PBR, the lipase amount=(?ow rate × substrate concentration) ratio (mL=q[TG]0) could be considered as the intensive variable of the process for use in scale up of the PBR. A simple equation was proposed for the prediction of the fatty acid composition of the ST at the exit of the PBR as a function of the intensive variable mL=q[TG]0. At equilibrium, the ST produced had the following composition: CA 57%, EPA 5.1%, DHA 10.0% and palmitic acid 6.3% (only considering the major fatty acids). In addition, the proportion of EPA and DHA that esteri=ed the position 2 of the ST was 13.5%, which represented 44% of the total fatty acids in the position 2 of the resultant ST. ? 2002 Elsevier Science Ltd. All rights reserved.

Kinetics of aerobic treatment of olive-mill wastewater (OMW) with Aspergillus terreus

A kinetic study was carried out on the aerobic treatment of olive-mill wastewater by fermentation with Aspergillus terreus. The bioreactor used was batch fed at several concentrations. The aerobic treatment process followed a Quiroga and Sales model, which is represented by a second-grade polynomial from which the kinetic constants; maximum substrate concentration (h), maximum specific growth rate (P) and organic matter present in the medium (q) were calculated by using a non-linear regression. The mean values for chemical oxygen demand (COD) elimination velocity were 126.3 mg l − 1 h − 1 in the first 24 h and 77.3 mg l − 1 h −1 at 72 h. An increase in airflow allowed higher degradation percentages in less time (COD: 65.77% and BOD: 85.41%). The kinetic parameters ‘q=S’, corresponds to the organic matter present in the medium, which cannot be metabolized by the microorganisms under operating conditions. The value of P =max was influenced by the transfer of pure oxygen, the value of c M (minimum cellular retention time) decreased from 22.42 h for air with a porous plate to 8.31 h for pure oxygen.

THE BIOLOGICAL PURIFICATION OF WASTE PRODUCTS FROM OLIVE OIL EXTRACTION

Aspergillus terreus gave the overall best results in alpechin at approximately 80% concentration, degrading organic material by 53%, expressed as COD, and 67%, expressed as BOD. Degradation of the total phenol content, which included the great majority of phenolic compounds, reached 69%.

Production of structured lipids by acidolysis of an EPA-enriched fish oil and caprylic acid in a packed bed reactor: analysis of three different operation modes.

Structured triacylglycerols (ST) enriched in eicosapentaenoic acid (EPA) in position 2 of the triacylglycerol (TAG) backbone were synthesized by acidolysis of a commercially available EPA‐rich oil (EPAX4510, 40% EPA) and caprylic acid (CA), catalyzed by the 1,3‐specific immobilized lipase Lipozyme IM. The reaction was carried out in a packed bed reactor (PBR) operating in two ways: (1) by recirculating the reaction mixture from the exit of the bed to the substrate reservoir (discontinuous mode) and (2) in continuous mode, directing the product mixture leaving the PBR to a product reservoir. By operating in these two ways and using a simple kinetic model, representative values for the apparent kinetic constants (kX) for each fatty acid (native, Li or odd, M) were obtained. The kinetic model assumes that the rate of incorporation of a fatty acid into TAG per amount of enzyme, rX (mole/(h g lipase)) is proportional to the extent of the deviation from the equilibrium for each fatty acid (i.e., the difference of concentration between the fatty acid in the triacylglycerol and the concentration of the same fatty acid in the triacylglycerol once the equilibrium of the acidolysis reaction is reached). The model allows comparing the two operating modes through the processing intensity, defined as mLt/(V[TG]0) and mL/(q[TG]0), for the discontinuous and continuous operation modes, respectively. In discontinuous mode, ST with 59.5% CA and 9.6% EPA were obtained. In contrast, a ST with 51% CA and 19.6% EPA were obtained when using the continuous operation mode. To enhance the CA incorporation when operating in continuous mode, a two‐step acidolysis reaction was performed (third operation mode). This continuous two‐step process yields a ST with a 64% CA and a 15% EPA. Finally, after purifying the above ST in a preparative silica gel column, impregnated with boric acid, a ST with 66.9% CA and 19.6% EPA was obtained. The analysis by reverse phase and Ag+ liquid chromatography of the EPA‐enriched ST demonstrated that the CA was placed in positions 1 and 3 and the EPA was occupying position 2 of the final ST.

Oxygen absorption in alkaline sodium dithionite solutions

Abstract The absorption rate of oxygen in alkaline sodium dithionite solutions, in a stirred tank with a flat interfacial surface, has been studied experimentally. Under the conditions studied (12.0 ⩽ pH ⩽ 13.7, 20–45°C, partial oxygen pressure 0.20 at 1 atm and a dithionite concentration of 0.005–0.20 M), absorption occurs by means of a fast reaction of zero order in oxygen and 1.5 order in dithionite. A chain mechanism of free radicals is proposed to explain these results and the apparent kinetic constant is adjusted to the following equation as a function of temperature: