Milan Popovic

Mixing characteristics of external-loop airlifts : non-newtonian systems

Abstract Mixing time (θ) was measured in a 45 or 60 1 working liquid volume external-circulation-loop airlift (ECL-AL) fermentor, and in a geometrically similar 6 1 prototype, both equipped with perforated-plate spargers (1 mm dia. holes). Carboxymethylcellulose solutions were used to simulate the rheological behaviour non-Newtonian fermentation broths. The specific mixing time (θ/ V ) is shown to be highly dependent on the dispersion height and gas superficial velocity in the riser section, as well as on the riser diameter and the effective viscosity of the liquid phase. In the range studied (0.111⩽ A D / A R ⩽0.46), the downcomer-to-riser cross-sectional-area ratio has a relatively minor effect on θ/ V . The superficial velocity of the circulating liquid in the riser ( u LR ) was also measured. The results have been combined with those of a previous investigation, by Popovic and Robinson, to better elucidiate the effect of dispersion height on the rate of liquid circulation. A modified correlation of u LR is presented and discussed.

Dual feeding strategy for the production of α-amylase by Bacillus caldolyticus using complex media

In this study, the objective was to investigate an exponential feeding strategy for fed-batch production of thermostable alpha-amylase (E.C. 3.2.1.1.) from the Bacillus caldolyticus (DSM405). The parameters for establishing compositions of feed media and feeding rate were obtained by statistical analysis of batch and continuous shake flask experiments. These parameters were casitone to starch ratio of 2.67g(casitone)g(starch)(-1), maintenance coefficient 0.174g(casitone)g(DW)(-1)h(-1), cell yield 0.62g(DW)g(casitone)(-1) and mu(opt)=0.2h(-1). The exponentially fed fermentation resulted in yield of 120Uml(-1) alpha-amylase that was thermostable up to 105 degrees C. Results of the exponentially fed fermentation have been discussed in the light of a feed-back controlled fed-batch fermentation reported earlier by the authors. A comparison of the temperature and pH effects on amylase produced by B. caldolyticus and on several other commercially available amylases has also been presented.

Absorption and reaction of isobutene in sulfuric acid—I: The effect of tert-butanol on the absorption rate in a bubble column

Abstract The absorption and reaction of isobutene in sulfuric acid yielding tert-butanol was studied in a bubble column of 4·6 cm diameter and 105 cm length. The inlet concentration of acid was varied from 28·5 to 56·9 wt%. A simple absorption model was used which shows that the absorption rate depends on the parameter group ( K A a / H ). From steady state and transient experiments the influence of tert-butanol on this group was determined. At low concentrations of tert-butanol the increase of ( K A a / H ) is mainly due to the increase of the specific interfacial area a , which was obtained by taking photographs. Whereas at high concentrations of tert-butanol (>1·8 g-mole/l) ( K A a / H ) is increased probably since the solubility of isobutene increases nonlinearly. The highest absorption rate was obtained with an inlet concentration of acid of 46·6 wt%. This agrees well with the feed concentration applied in the industrial process.

Alkali hydrolysis of trinitrotoluene.

Data for alkali hydrolysis of 2,4,6-trinitrotoluene (TNT) in aqueous solution at pH 12.0 under static (pH-controlled) as well as dynamic (pH-uncontrolled) conditions are reported. The experiments were conducted at two different molar ratios of TNT to hydroxyl ions at room temperature. The TNT disappeared rapidly from the solution as a first-order reaction. The complete disappearance of aromatic structure from the aqueous solution within 24 h was confirmed by the ultraviolet-visible (UV-VIS) spectra of the samples. Cuvet experiments in a UV-VIS spectrophotometer demonstrated the formation of Meisenheimer complex, which slowly disappeared via formation of aromatic compounds with fewer nitro groups. The known metabolites of TNT were found to accumulate only in very small quantities in the liquid phase.

Gas residence time distribution in stirred tank bioreactors

Abstract Residence time distributions of gas phase have been measured in fermentation broths and in simulated media of low as well as high viscosity (1 × 10 −3 − 0.5 Pas) using a step-function-Helium-tracer technique. On the basis of these, simple mathematical models have been developed and their parameters are qualitatively correlated with operating conditions. It was shown that even in low viscosity Newtonian fluids, gas phase does not behave as well mixed. Differences between gas flow patterns in low and high viscosity fluids, already reported in literature, have been verified.

A Mathematical Model for Vegetative Growth of Bacillus Thuringiensis

A structured but unsegregated model was developed for the vegetative growth of Bacillus thuringiensis (Bt) cells. This model included cell growth, limiting‐substrate consumption, production and consumption of acetic acid, and production of poly‐β‐hydroxy butyric acid. The parameter estimation and model validation were conducted using data from carbon‐limited batch experiments. The model successfully predicted batch behavior of biomass and glucose concentrations.

Chemical Interactions Between Aqueous and Organic Phases in a Reactive Extraction Process

Partitioning of lactic acid between aqueous phase and kerosene containing trin-octyl phosphine oxide (TOPO) as a reactive agent was measured and analyzed in terms of an association model between lactic acid and TOPO. Equilibrium association was calculated to involve 1.1 mol of lactic acid /mol of TOPO. 31P-NMR and FT-IR analyses found no evidence of covalent bond formation between lactic acid and TOPO in solutions in kerosene, and pointed to hydrogen bond formation between the two chemicals. In a hydrophobic, microporous, hollow-fiber-membrane module, the simultaneous presence of lactic acid, TOPO, and the back-extractant NaOH resulted in drastic membrane fouling as evidenced by the loss of kerosene flux through the membrane. The membrane could be restored back to the original kerosene flux after extensive washing.