A. Gianetto

Trickle-bed reactors: state of art and perspectives

Abstract Theoretical and phenomenological aspects of trickle-bed reactors as reported in a wide number of papers of the last 35 years have been reviewed. The following engineering topics have been focused by trying a critical interpretation of the attained results: hydrodynamics, fluid-solid contacting, theoretical and semiempirical models, mass transfer phenomena, heat and pressure effects. The more recent themes considered in the literature and some important future trends of scientific and applied research have been enlightened.

Powdered activated carbon in an activated sludge treatment plant

Abstract Addition of powdered activated carbon (PAC) to the aeration basin of an activated sludge treatment plant fed with dye-works waste waters increases the purifying capacity of the plant: removal efficiency rises from 55.8 to 75.6% (COD) and from 78 to 98.5% (BOD 5 ) and the nitrification-denitrification capacity of the system also increases. The sludge growth parameters and the kinetic constant of biological oxidation were determined on the plant with and without PAC. The addition of PAC decreased overall sludge growth rate and the auto-oxidation factor, but increased the biological removal rate of the substrate by about one order of magnitude.

CONTINUOUS EXTRACTION LOOP REACTOR (CELR): ALCOHOLIC FERMENTATION BY FLUIDIZED ENTRAPPED BIOMASS

Abstract For the study of the continuous fermentation in a loop bioreactor, the entrapment and the activity of cells into a Ca-alginate matrix and the solvent extractive fermentation were considered. These proposals had already been presented as single ideas and sometimes supported by theoretical approaches. The initial tests, performed for more then a month of work, seem indicate that the extractive fermentation increases the specific production rate, but the alginate matrix shows many serious problems: a low mechanical resistance, the leakage of viable cells, a not negligible internal mass transfer resistance and the decrease in viability of the entrapped microorganisms. The reported results concern only the initial part of the research and need to be integrated with data of further research.

Theoretical and practical aspects of circulating fluidized bed reactors (CFBRs) for complex chemical systems

Abstract Selectivity and yield in complex reactions were calculated for CFBRs with reference to industrial processes; the effect of longitudinal dispersion, catalyst effectivness, heat dissipation and co-reactant axial distribution were also considered. Two mathematical models with and without longitudinal dispersion provided the theoretical results to be compared with experiments.

Pseudo-homogeneous approach to CFB reactor design

Hydrodynamic parameters were evaluated in a laboratory scale apparatus in order to provide data for CFBR (Circulating Fluidized Bed Reactor) design. Pressure drop and hold-up axial profiles were measured in a wide range of conditions with particular emphasis paid to dilute phase conveying where a pseudo-homogeneous flow is established in most of the upper part of the reactor. A safe correlation between pressure drop and solids hold-up is the link which permits a prompt evaluation of the reactor performance after exponential fitting of the pressure gradient along the axial direction. Ozone decomposition was chosen a as tracing reaction; comparison between measured and predicted conversion values was the ultimate goal of the present study aimed at demonstrating a possible approach for reactor design from gross parameter measurements.

Diffusion phenomena in the spherical matrices used for cell immobilization

Abstract A transient method was used to determine the effective diffusion coefficient Deff of glucose and ethanol in a 3.5% calcium alginate matrix unde

MECHANISMS OF ACTIVATED CARBON BIOREMOVAL

Abstract The removal of an organic azo dye-stuff particularly resistant to biodegradation was studied in an activated sludge pilot plant on the basis of the PACT process. Different runs were carried out by adding to the oxidation basin both adsorbent solids (granular activated carbon (GAC), and powdered activated carbon (PAC)) or practically non-adsorbent solid (semisilica fire brick crushed particles (SFB).) Biological removal of the dye-stuff was observed with GAC or PAC addition, whereas no removal was observed with the fire brick particles. Comparison of the results obtained with either GAC or PAC with those from the SFB non-adsorbent solid showed carbon bioregeneration, existing only in the presence of adsorbent solids, to be probably the fundamental mechanism governing biological dye removal. The specific removal rate obtained with carbons, probably dependent on many physical and biological phenomena, was interpreted with saturation type equations typical of biological phenomena. The apparent adsorp...

Citric acid production by A. niger with banana extract

Abstract Experimental runs were carried out on citric acid bioproduction using banana extract as carbon source. The micro-organism used was a strain of Aspergillus niger. Different types of batch bioreactor were tested: bubble column (BCR) and stirred tank reactor (STR). With the STR the effects of two different impeller types were studied: Rushton six-bladed turbine and Lightnin A310 turbine, both at various rotational speeds. During the tests various parameters were measured: biochemical (biomass as w/v and mean mould pellet diameter), chemical (total sugar and citric acid concentration) and physical (dissolved oxygen tension, DOT; oxygen uptake rate, OUR; viscosity; gas/liquid volumetric mass transfer coefficient, KLa). The results showed that citric acid production is strictly connected to the morphology of the microorganism; the mould must grow in pellet form, as occurs when a gentle agitation is provided such as in the BCR; otherwise, with mechanical agitation the pellets are broken, the biomass morphology becomes filamentous and the citric acid production markedly decreases. The fermentation results are discussed to achieve a better comprehension of the main influential parameters for process scale-up.

Conversion rates in a laboratory trickle-bed reactor during the oxidation of ethanol

Abstract We examined the conversion rate during the oxidation of ethanol in a trickle-bed reactor in a wide range of hydrodynamic conditions, temperatures and concentrations. The results have confirmed the findings of other authors that mass transfer of the gaseous reactant to the catalyst surface also takes place through zones of liquid which are not effective during a physical process. However, they have also shown an influence of the temperature and concentration of the volatile liquid reactant never reported before. This effect has been qualitatively explained by assuming that the supply of the ethanol to liquid zones with a poor renewal occurs mainly by an evaporation—condensation process. Accordingly, the data were well correlated by the ratio between the partial pressures of ethanol and oxygen in the gas phase.

Production of ethanol with Saccharomyces cerevisiae in a continuous reactor. Note I: kinetics of immobilized yeast

Continuous production of ethanol by fermentation with immobilized yeasts gives a better yield and capacity and can be better controlled than conventional processes with batch reactors and a suspended biomass. This paper (the first of three) examines the operative conditions for immobilizing Saccharomyces Cerevisiae on beech wood cubes. After preliminary evaluation of the non-influence of mass transfer phenomena and the extent of alcoholic inhibition, an experimental investigation of the kinetics for a metabolic reaction in a continuous recycle reactor was carried out. The kinetic equation was found to be a Michaelis-Menten type with uncompetitive inhibition by substrate, and linear inhibition by product.