Gas holdup in pulp fibre suspensions: Gas voidage profiles in a batch-operated sparged tower

Gas holdup in a semi-batch operated slurry (pulp fibre suspension) bubble column was investigated for two pulp types (softwood and hardwood kraft pulps) over a range of suspension mass concentrations (C_m=0–9% by mass) and superficial gas velocities (U_g=0.0027–0.027 m/s). Three techniques were used: height difference between gassed and ungassed operation; pressure difference as a function column height; and electrical resistance tomography (ERT). Depending on the technique used the average, axial and radial holdup profiles could be determined. In the pulp suspensions, the ERT determined gas holdups correlated well with those determined using the differential height method. In water, the ERT determined gas holdups were significantly lower, but the agreement was significantly improved by increasing the background conductivity by adding 1 g/L salt to the water. This, however, reduced the overall gas holdup due to the effect of the electrolyte on bubble coalescence. Other differences between the three measurement techniques were attributed to limitations in the detection methods and the averaging procedures used to compare results.The presence of pulp fibres reduced gas-holdup at all gas flow rates and suspension concentrations studied and is attributed to increased bubble coalescence which increases bubble size and consequently bubble rise velocity through the suspension. Gas holdup (as determined by ERT) increased with column height. The radial gas profiles were non-uniform and more peaked than the corresponding water profiles. At low suspension concentrations this was attributed to asymmetric suspension recirculation within the column. As suspension concentration increased, channels formed in the suspension with the average void fraction leveling off to a plateau.     

Uncatalyzed selective oxidation of liquid cyclohexane with air in a microcapillary reactor

The uncatalyzed selective oxidation of cyclohexane with air was performed at high-p,T-conditions in a microcapillary reactor. Operation pressures were between 2.0 and 8.0 MPa and operating temperatures between 453 and 533 K. The measured space-time-yield showed only a slight dependence on the pressure, but a strong dependence on temperature. At 533 K, a space-time-yield of about 6.000 kg/(m³ h) was reached, which corresponds to a size of 2 m×2 m×2 m (8 m³) of the microstructured reactor assuming a capacity of 100.000 t/a compared to 500 m³ total reactor volume realized with a cascade of bubble columns of each about 100 m³. Unfortunately, selectivity drops at this temperature below 80% which is significantly lower than the selectivity in the conventional process. Passivating the capillary walls with silicon allowed an increase in selectivity. By fitting a simple mass transfer/reaction model for molecular oxygen, the mass transfer coefficient could be determined for T=453 K and a 0.75 mm capillary as k_L=3.04×10^?3 m/s. With the help of the Hatta number, mass transfer limitations can be excluded for the microcapillary reactor, whereas the bubble column reactor is weakly limited by the gas/liquid mass transfer of the molecular oxygen. Thus, process intensification by enhancing mass transfer using a microstructured reactor for cyclohexane oxidation with air is quite low.     

Use and recycling of Ca-alginate biocatalyst for removal of phenol from wastewater

The objective of the current work is the exhaustive study of the phenol degradation potential in both free cell and immobilized bacterium (Pseudomonas aeruginosa) in calcium alginate beads (biocatalyst) was investigated for its ability to grow and degrade phenol as its sole source of carbon and energy.The biodegradation assays were performed in liquid medium with phenol being the only substrate. It was found that P. aeruginosa is able to degrade phenol up to 500 mg L^?1 in 50 h as free cell and 900 mg L^?1 in 80 h when immobilized in the calcium alginate beads. However, for 1200 mg L^?1 concentration, the immobilized cells took much more time (290 h) for a complete degradation.The reuse of these beads in different concentrations of phenol (100–900 mg L^?1) showed that the cells keep their phenol degradation ability up to 900 mg L^?1in 78.5 h with 99% removal efficiency.Similarly, the reuse of the biocatalyst in the same initial phenol concentration (500 mg L^?1), allows us to get 9 cycles.     

Optimization of carbamazepine removal in O3/UV/H2O2 system using a response surface methodology with central composite design

This study used an ozone/ultraviolet/hydrogen peroxide (O₃/UV/H₂O₂) system to remove carbamazepine (CBZ) from water using a second-order response surface methodology (RSM) experiment with a five-level full-factorial central composite design (CCD) for optimization. The effects of both the primary and secondary interactions of the photocatalytic reaction variables, including O₃ concentration (X₁), H₂O₂ concentration (X₂), and UV intensity (X₃), were examined. The O₃ concentration significantly influenced CBZ and total organic carbon (TOC) removal as well as total inorganic nitrogen ion production (T-N) (p < 0.001). However, CBZ, TOC removal, and T-N production were enhanced with increasing O₃ and H₂O₂ concentrations up to certain levels, and further increases in O₃ and H₂O₂ resulted in adverse effects due to hydroxyl radical scavenging by higher oxidant and catalyst concentrations. UV intensity had the most significant effect on T-N production (p < 0.001). Complete removal of CBZ was achieved after 5 min. However, only 34.04% of the TOC and 36.99% of T-N were removed under optimal concentrations, indicating formation of intermediate products during CBZ degradation. The optimal ratio of O₃ (mg L^? 1): H₂O₂ (mg L^? 1): UV (mW cm^? 2) were 0.91:5.52:2.98 for CBZ removal, 0.7:18.93:12.67 for TOC removal, and 0.94: 4.85:9.03 for T-N production, respectively.     

Ammonia removal by air stripping in a semi-batch jet loop reactor

Ammonia is very toxic chemical and it can be removed by air stripping at high pH. JLRs have found applications in wastewater treatment processes due to their high mass transfer rates. In JLRs, intrinsic high turbulence result in a very large air-liquid surface area for greater mass transfer. Therefore, in this study, ammonia removal by air stripping from synthetically prepared ammonia solution at the high pH in a semi-batch JLR due to its high mass transfer capabilities have been investigated. Investigated parameters in a JLR were initial ammonia concentration (10–500 mg/L), temperature (20–50 °C), air flow rate (5–50 L/min) and liquid circulation rate (35–50 L/min). While it was demonstrated that temperature and air flow rate have a significant effect on the ammonia removal, it was determined that initial ammonia concentration and liquid circulation rate have no significant effect on the ammonia removal. The overall volumetric mass transfer coefficients (K_La) have been calculated from obtained model and it was determined that increasing temperature and air flow rate have a very significant effect on K_La. It was concluded that JLR provides higher mass transfer capabilities than other type of reactors even if less air is given.     

Chitin as biosorbent for phenol removal from aqueous solution: Equilibrium,kinetic and thermodynamic studies

Phenol removal from aqueous solution was studied employing chitin as low cost biosorbent. Initial biosorption tests carried out in the pH range 2–10 pointed out an optimum pH of 2. Temperature and initial phenol concentration were then varied in the ranges 15 ≤ T ≤ 50 °C and 10.4 ≤ C₀ ≤ 90.8 mg L⁻¹, respectively. The good applicability of Langmuir, Freundlich and Temkin models (R² = 0.990–0.993) to describe equilibrium isotherms suggested an intermediate mono-/multilayer biosorption mechanism along with a semi-homogeneous architecture of biosorbent surface. Biosorption capacity progressively increased from 3.56 to 12.7 mg g⁻¹ when starting phenol concentration was raised from 10.4 to 90.8 mg L⁻¹, and the related sorption kinetics was investigated by pseudo first-order, pseudo second-order and intraparticle diffusion models. The pseudo second-order model, which showed the best fit of experimental data (R² = 0.999), allowed estimating a second-order rate constant of 0.151 g mg⁻¹ h⁻¹ and a theoretical sorption capacity of 7.63 mg g⁻¹. Phenol biosorption capacity increased with temperature up to a maximum value, beyond which it decreased, suggesting the occurrence of a thermoinactivation equilibrium. Finally, to identify the main functional groups involved in phenol biosorption, both raw and phenol-bound materials were explored by FT-IR spectroscopy.     

Electrogeneration of hydrogen peroxide in gas diffusion electrodes modified with tert-butyl-anthraquinone on carbon black support

Hydrogen peroxide (H₂O₂) is a versatile oxidizing agent that is synthesized commercially by the reduction of oxygen in organic medium. Electrochemical technology employing a modified gas diffusion electrode (MGDE) offers a viable alternative for the industrial-scale synthesis of the oxidant. Addition of 1% (w/w) of tert-butyl-anthraquinone (TBAQ) to carbon black deposited in the form of a microporous layer onto the disk of a rotating ring-disk electrode produced an increase in the ring current, which is directly related to H₂O₂ formation, and presented an efficiency of H₂O₂ generation of 89.6% compared with 76.6% for carbon black alone. No significant changes were detected in the number of electrons transferred in the presence of the catalyst suggesting an electrochemical/chemical mechanism for H₂O₂ formation. Analogous improvements in the generation of H₂O₂ were obtained with MGDEs comprising TBAQ on carbon black. The highest concentrations of H₂O₂ (301 mg L⁻¹) were produced at the fastest rate (5.9 mg L⁻¹ min⁻¹) with the lowest energy consumption (6.0 kWh kg⁻¹) when a potential of −1.0 V vs SCE was applied to a MGDE containing 1.0% of TBAQ on carbon black. It is concluded that the application of MGDEs comprising TBAQ on carbon black support offers considerable advantages in the electrogeneration of H₂O₂.     

Kinetic studies on agrochemicals wastewater treatment by aerobic activated sludge process at high MLSS and high speed agitation

In the present work an effort has been made to study the kinetics of agrochemicals industry wastewater treatment by aerobic activated sludge process at high mixed liquor suspended solids (MLSS) and high speed agitation. MLSS concentration was varied in the range 6000–40,000 mg L^?1 and 2.5 mg L^?1 optimum dissolved oxygen (DO) was employed. Highest chemical oxygen demand (COD) reduction was found to be 76.83% at 9000 mg L^?1 MLSS at 130 rpm and DO 2.5 mg L^?1. Highest COD reduction was observed to be 80.76% at 25,000 mg L^?1 MLSS at higher agitation speed.     

Sol-gel synthesis of new TiO2/activated carbon photocatalyst and its application for degradation of tetracycline

A new efficient photocatalyst consisting of TiO₂-activated carbon composite (TiO₂/AC) was synthesized by sol-gel process and applied to decomposition of tetracycline (TC). Its properties and catalytic activity were evaluated in comparison with bare TiO₂ and P25, based on several characterization techniques and TC photodegradation kinetic studies. The results showed TiO₂/AC has better structural and electronic features for photocatalysis; S_BET of 129 m² g^–1, exclusively anatase phase, crystal size of 8.53 nm and band gap energy of 3.04 eV. The catalytic activity of the material was evaluated based on photodegradation kinetic studies of TC from aqueous solution (with initial concentration=50 mg L⁻¹ and catalyst dosage=1.0 g L⁻¹). Non-linear kinetic model of pseudo-first order were fitted to the resulting experimental data. The apparent first-order rate constant (k_app=42.9×10^–3 min^–1) and half-life time (t_1/2=16.1 min) determined for TiO₂/AC were better than those for P25 and bare TiO₂. TC degradation by-products were investigated by HPLC-MS, showing TC was completely degraded after 75 min, producing fragments with m/z smaller than 150.     

Structure and friction properties of laser-patterned amorphous carbon films

In the paper we report on laser surface modification of super hard micrometer-thick tetrahedral amorphous carbon (ta-C) films in the regime of single-shot irradiation with KrF laser pulses (wavelength 248 nm, pulse duration 20 ns), aimed at investigations of the laser-induced changes of the structure and surface properties of the ta-C films during graphitization and developing ablation processes. Based on the analysis of surface relief changes in the laser-irradiated spots, characteristics of the single-shot graphitization and ablation of the 2-μm-thick ta-C film are determined. Using Raman spectroscopy, it is found that during the graphitization regime the structure transformation and growth of graphitic clusters occur according to the relationship I(D)/I(G) ∝ L_a², but after reaching the ablation threshold the Tuinstra-Koenig relationship I(D)/I(G) ∝ 1/L_a describes further growth of the graphitic cluster size (L_a) during developing ablation of the ta-C film with nanosecond pulses. The maximal size of graphitized clusters is estimated as L_a = 4–5 nm. The studies of nanomechanical properties of laser-patterned ta-C films using the lateral force microscopy and force modulation microscopy have evidenced lower friction forces (between diamond-coated tips and film surface) and lower stiffness in the laser-graphitized areas. The laser-produced graphitic layer acts as a solid lubricant during sliding of the diamond-coated tips on the ta-C film surface in ambient air (~ 50% RH); the lubricating role of adsorbed water layers is suggested to be significant at low loads on the tips. The results of this work demonstrate that the UV laser surface texturing in the regime of graphitization is a promising technique to control the friction and surface elasticity of super hard amorphous carbon films on the micro and nanoscale.     

Comparison of the rate of CO2 absorption into aqueous ammonia and monoethanolamine

Aqueous ammonia has been proposed as an absorbent for use in CO₂ post combustion capture applications. It has a number of advantages over MEA such as high absorption capacity, low energy requirements for CO₂ regeneration and resistance to oxidative and thermal degradation. However, due to its small molecular weight and large vapour pressure absorption must be carried at low temperature to minimise ammonia loss. In this work the rate of CO₂ absorption into a falling thin film has been measured using a wetted-wall column for aqueous ammonia between 0.6 and 6 mol L^?1, 278–293 K and 0–0.8 liquid CO₂ loading. The results were compared to 5 mol L^?1 MEA at 303 and 313 K. It was found that the overall mass transfer coefficient for aqueous ammonia was at least 1.5–2 times smaller than MEA at the measured temperatures. From determination of the second-order reaction rate constant k₂ (915 L mol^?1 s^?1 at 283 K) and activation energy E_a (61 kJ mol^?1) it was shown that the difference in mass transfer rate is likely due to both the reduced temperature and differences in reactivity between ammonia and MEA with CO₂.     

Characteristics of microbubbles generated by porous mullite ceramics prepared by an extrusion method using organic fibers as the pore former

Porous mullite ceramics with unidirectionally oriented pores were prepared by an extrusion method using rayon fibers as the pore formers and the characteristics of microbubbles generated by these porous ceramics were investigated. The 1200 mm long ceramics were tubular and of thick or thin types of 20–30 mm inner diameter and 30–50 mm outer diameter, respectively. The thin and thick samples had porosities of 47 and 49% and average pore radii of 7.8 μm. The gas permeabilities of the thick and thin samples were 4.1 × 10^?14 and 5.4 × 10^?14 m², respectively. Microbubbles were generated by introducing N₂ gas through the ceramic tube by immersing it into water. The minimum pressure (bubble point pressure) for generation of microbubbles was 20 kPa, much lower than for other bubble-forming methods. The average microbubble radii ranged from about 70 to 105 μm at flow rates of 0.15–0.25 L/min in the thin sample and 0.3–0.7 L/min in the thick sample. These bubble sizes are much smaller than calculated for a Fritz-type bubble such as generally formed by bubbling from pores and/or orifices. However, the present bubble sizes agree well with the calculated value based on nanobubbles, indicating that bubble formation occurs by mixing the gas with water in small pores. Since microbubbles enhance the dissolution rate of a gas phase in water, they are potentially useful for improving water environments, especially oxygen-deficient water. The effectiveness of gas dissolution in water was confirmed by determining the dissolution behavior of CO₂ gas using these porous ceramics.     

A size-dependent thermodynamic model for coke crystallites: The carbon–hydrogen system up to 2500 K

The development is presented of a model of the thermodynamic functions of enthalpy, entropy and Gibbs energy for the elements carbon and hydrogen in coke crystallites. It is applicable to varying degrees of graphitization, described by the crystallite length L_a and the crystallite height L_c. The model parameters are derived from known properties such as bond enthalpies and entropies of formation. Good agreement has been obtained between the predicted thermal dehydrogenation of petroleum cokes and experimental data. The removal of hydrogen from idealized coke crystallites is predicted to occur mostly between 1100 and 1300 K. Agreement has also been found in the comparison of the predicted thermodynamic stability of coke relative to graphite, in a previous experimental study. This stability has been determined as at ≈900 J g⁻¹ at temperatures between 950 and 1250 K and for L_a = 10 nm. The current predictive capacity of the present model is valid for temperatures up to 2500 K.     

Chitosan modified with Reactive Blue 2 dye on adsorption equilibrium of Cu(II) and Ni(II) ions

This study aimed at immobilizing Reactive Blue 2 (RB 2) dye in chitosan microspheres through nucleophilic substitution reaction. The adsorbent chemical modification was confirmed by Raman spectroscopy and thermogravimetric analysis. This adsorption study was carried out with Cu(II) and Ni(II) ions and indicated a pH dependence, while the maximum adsorption occurred around pH 7.0 and 8.5, respectively. The pseudo second-order kinetic model resulted in the best fit with experimental data obtained from Cu(II) (R = 0.997) and Ni(II) (R = 0.995), also providing a rate constant, k₂, of 4.85 × 10⁻⁴ and 3.81 × 10⁻⁴ g (mg min)⁻¹, respectively, thus suggesting that adsorption rate of metal ions by chitosan-RB 2 depends on the concentration of ions on adsorbent surface, as well as on their concentration at equilibrium. The Langmuir and Freundlich isotherm models were employed in the analysis of the experimental data for the adsorption, in the form of linearized equations. Langmuir model resulted in the best fit for both metals and maximum adsorption was 57.0 mg g⁻¹ (0.90 mmol g⁻¹) for Cu(II) and 11.2 mg g⁻¹ (0.19 mmol g⁻¹) for Ni(II). The Cu(II) and Ni(II) ions were desorbed from chitosan-RB 2 with aqueous solutions of EDTA and H₂SO₄, respectively.     

Controllable preparation of nano-CaCO3 in a microporous tube-in-tube microchannel reactor

In this work, nano-CaCO₃ particles with tunable size have been synthesized via CO₂/Ca(OH)₂ precipitation reaction in a microporous tube-in-tube microchannel reactor (MTMCR) with a throughput capacity up to 400 L/h for CO₂ and 76.14 L/h for liquid. The overall volumetric mass-transfer coefficient (K_La) of CO₂ absorption into Ca(OH)₂ slurry in the MTMCR has been deduced and analyzed. To control the particle size, the effect of operating conditions including initial Ca(OH)₂ content, gas volumetric flow rate, liquid volumetric flow rate, micropore size, and annular channel width was investigated. The results indicated that the mass transfer in the MTMCR can be greatly enhanced in contrast with a stirred tank reactor, and the particle size can be well controlled by tuning the operating parameters. The nano-CaCO₃ particles with an average size of 28 nm and a calcite crystal structure were synthesized, indicating that this process is promising for mass production of nanoparticles.     

Surface modifications of activated carbon by gamma irradiation

Four commercial activated carbons with different chemical and textural characteristics were modified by gamma irradiation under five different conditions: irradiated in absence of water, in presence of ultrapure water, in ultrapure water at pH = 1.0 and 1000 mg L⁻¹ Cl⁻, in ultrapure water at pH = 7.5 and 1000 mg L⁻¹ Br⁻, and in ultrapure water at pH = 12.5 and 1000 mg L⁻¹ NO₃⁻. Changes in surface chemistry were studied by X-ray photoelectron spectroscopy; pH of point of zero charge, total acidic groups and total basic groups, which were determined by assessment with HCl and NaOH; and textural changes were determined by obtaining the corresponding adsorption isotherms of N₂ and CO₂. Outcomes show that the activated carbon surface chemistry can be modified by gamma irradiation and that the changes depend on the irradiation conditions. Modifications in the sp² hybridization of the surface carbons suggest that the irradiated carbons undergo graphitization. Measurements of structural parameters indicate that the irradiation treatment does not modify the textural properties of the carbons. Finally, studies of pristine and irradiated activated carbons using diffuse reflectance spectroscopy with the Kubelka–Munk function revealed a reduction in band gap energy in the irradiated carbons associated with an increase in sp² hybridization of the carbon atoms.     

Extraction of phenolic compounds from Vitex agnus-castus L.

The primary objective of this study was to valorized Vitex agnus-castus residues in terms of phenolic compounds. The effects of extraction time (30–360 min), solid to liquid ratio (0.1–0.3 g_DryBiomass/ml_Solvent), type of solvent and different tissue types (leave, roots and seeds) on total polyphenols, o-diphenols, total flavonoids and anthocyanins were evaluated. The highest total polyphenol (31.5 mg_{CaffeicAcidEquivalent}/g_DryBiomass) and o-diphenol (12.4 mg_{CaffeicAcidEquivalent}/g_DryBiomass) contents were obtained from methanolic extract of leaves after 180 min using a solid/liquid ratio of 0.1 g_DryBiomass/ml_Solvent, while total flavonoids, reached a maximum value of 19.4 mg_{CatechinEquivalent}/g_DryBiomass after 360 min under the same conditions. Roots of V. agnus-castus were found to be a good source of anthocyanins with the highest yield of 0.62 mg_{MalvidinEquivalent}/g_DryBiomass using ethanol as a solvent (180 min and 0.2 g_DryBiomass/ml_Solvent). The maximum antiradical power (178.5 μl_extract/μg_DPPH) was exhibited by the methanolic leave extract obtained after 360 min at solid/liquid ratio of 0.3 g_DryBiomass/ml_Solvent.     

Thermal inactivation of airborne viable Bacillus subtilis spores by short-term exposure in axially heated air flow

In this investigation, an experimental facility was developed for quantifying the inactivation of viable bioaerosol particles in a controlled axially heated air flow. The tests were conducted with Bacillus subtilis var. niger endospores. The thermal inactivation of aerosolized spores was measured based on the loss of their culturability that resulted from a short-term exposure to air temperatures ranging from ～150 to >1000 °C. The cross-sectional and longitudinal temperature profiles in the test chamber were determined for different heating and flow conditions. The characteristic exposure temperature (T_e) was defined using a conservative approach to assessing the spore inactivation. Experimentally determined inactivation factors (IF) were corrected to account for the temperature profiles in the axially heated air flow. The reported IF-values serve as the lower approximation of the actual inactivation. Two data sets obtained at different flow rates, Q=18 and 36 L min^?1, represent different exposure conditions. In both cases, the thermal exposure of aerosolized spores produced no effect or only a moderate inactivation when the T_e remained below ～200 °C for 18 L min^?1 and ～250^oC for 36 L min^?1. The IF-values increased exponentially by about four orders of magnitude as the temperature rose by 150 °C. Depending on the flow rate, IF exceeded ～10⁴ at T_e>320 °C (Q=18 L min^?1) or >360 °C (Q=36 L min^?1). At T_e≈375–400 °C, the spore inactivation obtained at both flow rates reached the limit of quantification established in this study protocol, which translates to approximately 99.999% viability loss. The findings were attributed primarily to the heat-induced damage of DNA and denaturation of essential proteins. Up to a certain level of the thermal exposure, these damages are repairable; however, the self-repair capability diminishes as the heat rises and then the damage becomes totally irreversible. The data generated in this study provide an important reference point for thermal inactivation of stress-resistant spores in various biodefense/counterterrorism and air quality control applications.     

Physico-chemical treatment applied to compost liquor: Feasibility study

Compost liquor was treated using a combination of physico-chemical processes: (i) lime precipitation, (ii) filtration on a rotary drum vacuum precoat filter, (iii) ultrafiltration, and (iv) reverse osmosis. Laboratory Jar tests showed the interest of using lime to precipitate compost liquor. Yields of ammonium removal up to 90% were obtained for an optimum lime concentration of 6 g L^?1. A test was run at semi-industrial scale on 400 L of highly loaded compost liquor (COD: 15,800 mg L^?1, ammoniacal pollution: 18,433 mg NH₄⁺-N L^?1, conductivity: 74,000 μS cm^?1) to demonstrate the potential of the treatment process proposed. Outstanding purification yields were obtained, especially 95% of COD removal and 93% of ammoniacal pollution removal.