Antonio Marzocchella

Fluidization regimes and transitions from fixed bed to dilute transport flow

Characterization by means of Kolmogorov entropy shows that the dynamics of the bottom bed in small size circulating fluidized bed risers are significantly different from the dynamics of the dense bottom bed in large size risers and, as a consequence, two types of circulating regimes are introduced: the exploding bubble bed for large risers and the circulating ‘slugging’ bed for small risers, the latter at high superficial gas velocities. In a pictorial fluidization diagram ten gas—solid fluidization regimes are given, seven of which are experimentally identified with the Kolmogorov entropy by varying the superficial gas velocity, riser solids holdup and diameter (or width) of the riser: bubbling bed, slugging bed, exploding bubble bed, intermediate turbulent bed, circulating ‘slugging’ bed, intermediate dilute flow, and dilute transport flow. No transition could be identified between the exploding bubble bed at captive conditions and the exploding bubble bed at circulating conditions in the dense bottom bed of the two largest facilities in this study. This suggests that the dense bottom bed in large size risers can be considered as a bubbling bed. A turbulent bed was found in none of the facilities of this study with the Geldart B solids used. As well as by the Kolmogorov entropy (chaos analysis), the hydrodynamics have been characterized by amplitude of pressure fluctuations, while a solids distribution analysis has also been carried out. The study has been made in four (circulating) fluidized beds of different size and design, all operated with 0.30 mm silica sand. The dimensions of the fluidized bed risers are 1.47 × 1.42 × 13.5 m, 0.70 × 0.12 × 8.5 m, 0.12 m i.d. × 5.8 m, and 0.083 m i.d. × 4.0 m.

Self-segregation of high-volatile fuel particles during devolatilization in a fluidized bed reactor

Abstract The interaction between fuel particles and incipiently bubbling gas fluidized beds during devolatilization has been investigated by X-ray imaging. The fuel consisted of a ligneous biomass ( Robinia pseudoacacia ) reduced into millimeter-sized particles and doped with lead nitrate in order to make particles visible upon X-ray irradiation. A purposely designed single-particle-injector was used to impulsively introduce fuel particles one at a time at a given depth into the fluidized bed. Experiments highlighted three main features of the phenomenology, namely: (a) the formation of ( endogenous ) volatile matter bubbles around devolatilizing fuel particles; (b) the uprise of endogenous bubbles; and (c) the uprise of fuel particles closely associated to endogenous bubble motion. Bubble and particle trajectories and bubble cross sections as functions of time were worked out in order to assess fuel particle segregation times and endogenou s bubble growth rate. The choice of operating under incipient bubbling conditions enabled thorough assessment of interactive processes establishing between gas-emitting particles and the fluidized suspension. The formation, growth and motion of endogenous volatile bubbles and the associated motion of the fuel particle could be characterized without the perturbation caused by exogenous gas bubbles (i.e. bubbles formed under freely bubbling conditions). This represents a first step towards the characterization of the interaction between gas-emitting particles and freely bubbling beds.

Deep eutectic solvent pretreatment and subsequent saccharification of corncob

Ionic liquid (ILs) pretreatment of lignocellulosic biomass has attracted broad scientific interest, despite high costs, possible toxicity and energy intensive recycling. An alternative group of ionic solvents with similar physicochemical properties are deep eutectic solvents (DESs). Corncob residues were pretreated with three different DES systems: choline chloride and glycerol, choline chloride and imidazole, choline chloride and urea. The pretreated biomass was characterised in terms of lignin content, sugars concentration, enzymatic digestibility and crystallinity index. A reduction of lignin and hemicellulose content resulted in increased crystallinity of the pretreated biomass while the crystallinity of the cellulose fraction could be reduced, depending on DES system and operating conditions. The subsequent enzymatic saccharification was enhanced in terms of rate and extent. A total of 41 g fermentable sugars (27 g glucose and 14 g xylose) could be recovered from 100g corncob, representing 76% (86% and 63%) of the initially available carbohydrates.

Segregation of fuel particles and volatile matter during devolatilization in a fluidized bed reactor—II. Experimental

An experimental study on the segregation of fuel particles and of volatile matter during devolatilization in a fluidized-bed reactor is presented. The study was purposely carried out with beds of solids kept at incipient fluidization in order to focus attention on the segregation phenomena associated with the generation of endogenous bubbles. This term is used to denote bubbles of volatile matter formed around each fuel particle during devolatilization, as opposed to exogenous bubbles of fluidizing gas always present in aggregative fluidized beds. Experimental evidence of volatile bubble formation and of fuel particle as well as of volatile matter segregation is provided. Qualitative characterization of segregation patterns and quantitative assessment of the effect on segregation of variables like fuel particle size, density and volatile matter content, of bed solids size, of bed temperature are presented and discussed. Experimental data are further analyzed in the light of the theory developed in Part I (Fiorentino et al., 1996) in order to validate the model.

Butanol production by bioconversion of cheese whey in a continuous packed bed reactor

Butanol production by Clostridium acetobutylicum DSM 792 fermentation was investigated. Unsupplemented cheese whey was adopted as renewable feedstock. The conversion was successfully carried out in a biofilm packed bed reactor (PBR) for more than 3 months. The PBR was a 4 cm ID, 16 cm high glass tube with a 8 cm bed of 3mm Tygon rings, as carriers. It was operated at the dilution rate between 0.4h(-1) and 0.94 h(-1). The cheese whey conversion process was characterized in terms of metabolites production (butanol included), lactose conversion and biofilm mass. Under optimized conditions, the performances were: butanol productivity 2.66 g/Lh, butanol concentration 4.93 g/L, butanol yield 0.26 g/g, butanol selectivity of the overall solvents production 82 wt%.

Segregation of fuel particles and volatile matter during devolatilization in a fluidized bed reactor—I. Model development

The phenomenology of fuel particles segregation at the surface of fluidized beds during devolatilization is modeled. The beds are assumed to be at incipient fluidization in order to focus the attention on the segregation mechanism caused by the formation of endogenous bubbles of volatiles. These are generated during fuel particle devolatilization immediately after its underbed feeding to a fluidized-bed reactor, and are distinct from exogenous bubbles, which are always present in the bed under aggregative fluidization conditions. Submodels describing the volatile flow pattern, the volatile bubble formation, the vertical motion of the volatile bubble and of the fuel particle are developed and combined in a coherent framework. Results of extensive model computations are presented and discussed. Qualitative and quantitative features of the fuel particle and of the volatile matter segregation processes are assessed by looking at the time required for the fuel particle to become segregated, at the frequency of volatile bubble formation, at the amount of volatiles emitted in the bed prior to particle segregation on the top of it. The proposed framework should complement previous studies on the segregation of fuel particles in freely bubbling fluidized beds based on the effects of exogenous bubbles only.

Cellulosic butanol production from alkali-pretreated switchgrass (Panicum virgatum) and phragmites (Phragmites australis).

A potential dedicated energy crop (switchgrass) and an invasive (North America) plant species (phragmites) were compared as potential substrates for acetone butanol ethanol (ABE) fermentation. Both biomass were pretreated with 1% (w/v) NaOH and subjected to enzymatic hydrolysis. Total reducing sugar yields were 365 and 385gkg(-1) raw biomass for switchgrass and phragmites. Fermentation of the hydrolysates resulted in overall ABE yields of 146 and 150gkg(-1) (per kg dry plant material), with a theoretical maximum of 189 and 208gkg(-1), respectively. Though similar overall solvent yields were obtained from both crops, the largest carbon loss in the case of switchgrass occurred during pretreatment, while the largest loss in the case of phragmites occurred to enzymatic hydrolysis. These findings suggest that higher overall yields are achievable and that both crops are suitable feedstocks for butanol fermentation.

Development of simple protocols to solve the problems of enzyme coimmobilization. Application to coimmobilize a lipase and a β-galactosidase

This paper shows the coimmobilization of β-galactosidase from Aspergillus oryzae (β-gal) and lipase B from Candida antarctica (CALB). The combi-biocatalyst was designed in a way that permits an optimal immobilization of CALB on octyl-agarose (OC) and the reuse of this enzyme after β-gal (an enzyme with lower stability and altogether not very stabilized by multipoint covalent attachment) inactivation, both of them serious problems in enzyme co-immobilization. With this aim, OC-CALB was coated with polyethylenimine (PEI) (this treatment did not affect the enzyme activity and even improved enzyme stability, mainly in organic medium). Then, β-gal was immobilized by ion exchange on the PEI coated support. We found that PEI can become weakly adsorbed on an OC support, but the adsorption of PEI to CALB was quite strong. The immobilized β-gal can be desorbed by incubation in 300 mM NaCl. Fresh β-gal could be adsorbed afterwards, and this could be repeated for several cycles, but the amount of PEI showed a small decrease that made reincubation of the OC-CALB–PEI composite in PEI preferable in order to retain the amount of polymer. CALB activity remained unaltered under all these treatments. The combi-catalyst was submitted to inactivation at 60 °C and pH 7, conditions where β-gal was rapidly inactivated while CALB maintained its activity unaltered. All β-gal activity could be removed by incubation in 300 mM NaCl, however, SDS analysis showed that part of the enzyme β-gal molecules remained immobilized on the OC-CALC–PEI composite, as the inactivated enzyme may become more strongly adsorbed on the ion exchanger. Full release of the β-gal after inactivation was achieved using 1 M NaCl and 40 °C, conditions where CALB remained fully stable. This way, the proposed protocol permitted the reuse of the most stable enzyme after inactivation of the least stable one. It is compatible with any immobilization protocol of the first enzyme that does not involve ion exchange as only reason for enzyme immobilization.

Continuous lactose fermentation by Clostridium acetobutylicum – Assessment of acidogenesis kinetics

An assessment of the growth kinetics of acidogenic cells of Clostridium acetobutylicum DSM 792 is reported in the paper. Tests were carried out in a continuous stirred tank reactor under controlled conditions adopting a complex medium supplemented with lactose as carbon source to mimic cheese whey. The effects of acids (acetic and butyric), solvents (acetone, ethanol and butanol) and pH on the growth rate of acidogenic cells were assessed. The conversion process was characterized under steady-state conditions in terms of concentration of lactose, cells, acids, total organic carbon and pH. The growth kinetics was expressed by means of a multiple product inhibition and interacting model including a novel formulation to account for the role of pH. The model has the potential to predict microorganism growth rate under a broad interval of operating conditions, even those typical of solvents production.

Hydrodynamic interaction between a coarse gas-emitting particle and a gas fluidized bed of finer solids

Abstract The paper addresses phenomena associated with the hydrodynamic interaction between a gas-emitting coarse particle and a gas-fluidized bed of finer solids. The study is relevant to segregation of particles undergoing fast decomposition (e.g. solid fuel pyrolysis) in fluidized beds. Hydrodynamic patterns associated with gas-emission from a coarse particle into a gas-fluidized bed have been characterized by means of 2D experiments by simulating the gas-emitting particle as a porous cylinder. Formation of endogenous bubbles around the gas source was highlighted. The momentum exchange between bed solids and the gas-emitting particle was characterized by means of experiments carried out in a cylindrical fluidized bed. The gas-emitting particle was simulated by a porous sphere connected to a gas source and continuously weighed. Gas-emission from the sphere induced momentum transfer from the fluidized suspension to the sphere resulting into a lift force opposed to gravity. Quantitative assessment of the lift force was based on analysis of data obtained with different gas-emitting sphere sizes, gas superficial velocities and flow rates of emitted gas.