External Mass Transfer Research Articles

Feasibility of nitrate adsorption from aqueous solution by nitrogen and oxygen-modified pine bark biochar: experimental and computational approach

Efficient and economical wastewater treatment has presented itself as a global challenge. In this context, adsorption is one of the most effective methods to remove contaminants from wastewater. The present study evaluated the feasibility of chemically modified pine bark biochar’s nitrate adsorption ability. Pine bark biochar was modified with urea and sulfuric acid to remove nitrate from an aqueous solution. The physicochemical properties of the biochar samples, such as pH, pH at point of zero charges, surface atomic composition, surface morphology, and surface area, were evaluated. The equilibrium adsorption data were fitted to the Langmuir and Freundlich isotherm models. The kinetic data were fitted to different kinetic models (pseudo-first order, pseudo-second order, intraparticle diffusion, and Elovich). The adsorption data fitted well with the Langmuir and pseudo-first order models. The maximum nitrate adsorption capacity was found to be 1.548 mg g−1. Mass transfer studies were conducted to identify the rate-limiting step, values of the external mass transfer coefficient, and diffusion coefficient in the nitrate adsorption process by the modified biochar. The external mass transfer coefficients were in the range of 2.2 × 10–11–2.86 × 10–10 m s−1. The intraparticle diffusion coefficient ranged from 6.53 × 10–10 to 1.78 × 10–9 m2 s−1. The Biot number value less than 100 indicated that the adsorption was controlled by film diffusion. Interaction energies between nitrate ions and model biochar structures were calculated DFT-based quantum chemical software (Gaussian). The positive interaction energy values (2.3485–2.485 eV) suggested nitrate adsorption on model biochar structures was thermodynamically not feasible.Graphical

Investigation of Heat and Mass Transfer in the Process of Fuel Preparation from Biomass Particles with High Moisture

Currently, in a number of countries an urgent task for development of fuel and energy complexes is to increase the share of generation by involving solid fuels in circulation. Among such projects, those that allow the disposal of waste from various industries are particularly significant. Expired food products in this context are represented as a renewable local energy resource. However, such products require serious activities to prepare them for incineration or other type of high-temperature processing in order to obtain energy. The purpose of the present work is to improve methods of preparing fuel from recycled carrot fruits (unsuitable for use in the food sector). During the fuel preparation of carrots, the drying stage is limiting for the rational organization of its processing in boilers. In addition, the drying stage is extremely energy-consuming, so reliable prediction of its kinetics largely determines the efficiency of the entire technological process. In the course of the study, the following tasks were solved: a numerical method was developed for describing the processes of internal and external heat and mass transfer problems using an explicit difference approximation of differential equations of heat and mass transfer; parametric identification of the proposed one-dimensional mathematical model was performed using empirical dependencies known from literature sources; empirical verification of the proposed mathematical model was carried out by comparing the calculated forecasts obtained with the results of their own field experiments. The fact that the proposed mathematical model and the results of the full-scale experiment are independent, while the calculated forecasts and experimental data are in good agreement, makes us possible to consider the proposed calculation method as a reliable scientific basis for a computer method for calculating of heat and mass transfer processes when organizing the preparation of fuel from carrot fruits.

Lignin-modified metal–organic framework as an effective adsorbent for the removal of methyl orange

Herein, lignin-modified metal–organic frameworks (NH2-UIO@L) are prepared using a one-step synthesis as sorbents for the removal of organic dyes from water. The introduction of lignin improved the adsorption sites. NH2-UIO@L2 adsorption of MO conforms to Langmuir model, and the adsorption capacity of NH2-UIO@L2 on MO was 214.13 mg·L−1 with an adsorption efficiency up to 99.28 %, which was significantly higher than values for other adsorbents. Due to hydrogen bonds, π–π interactions and electrostatic interactions, MO was effectively removed by NH2-UIO@L2 and its adsorption efficiency is maintained at 90.55 % after six cycles. The adsorption kinetics showed that the NH2-UIO@L2 adsorption of MO was chemical adsorption and controlled by intraparticle diffusion and external mass transfer. Further, the adsorption performance of NH2-UIO@L2 on MO and MB in mixed MO/MB solution was investigated. The adsorption capacity of NH2-UIO@L2 in mixed MO/MB solution was 207.04 mg·L−1 for MO and 243.31 mg·L−1 for MB; the adsorption of NH2-UIO@L2 on MO followed the Dubinin-Radushkevich and pseudo-second-order models, and the adsorption on MB followed the Temkin and pseudo-second-order models. Hydrogen bonds, π–π interactions, and pore filling are all implicated in the removal of MO and MB. In particular, the electrostatic attraction between MB and MO improves the adsorption efficiency of NH2-UIO@L2 on MB. NH2-UIO@L2 has good reusability, maintaining an adsorption efficiency of 97.66 % for MO and up to 99.15 % for MB after six cycles. Its simple preparation and superior adsorption suggest that NH2-UIO@L2 has considerable potential to remove organic dyes from wastewater.

Preparation and characterization of calcined Ni/Mo hydrotalcite for the effective removal of lead from wastewater

ABSTRACT In this investigation, calcined Ni/Mo hydrotalcite was prepared via co-precipitation method and used as a sorbent for efficient removal of lead from aqueous solutions. The Pb2+ removal efficiency reached 99% at pH 6 and a sorbent dose of 2 g/L. The equilibrium data were satisfactorily fitted by the Langmuir and Hill models with a maximum capacity of 196.87 mg/g, indicating the monolayer sorption for lead over calcined Ni/Mo hydrotalcite. The Pb2+ sorption kinetic follows a pseudo-second-order reaction due to high correlation coefficients (R 2 > 0.99), while the Boyd’s plots confirm the external mass transfer as the rate-limiting step in the Pb2+ sorption. The temperature effect indicated a spontaneous and exothermic Pb2+ uptake. Mechanisms involved in the removal process include surface precipitation, diffusion into the solid pores and isomorphic substitution with Ni2+ of the sheets. The results showed excellent selectivity for Pb2+ removal from multi-divalent cation solutions and good reusability of the sorbent for up to 10 consecutive sorption–regeneration cycles without significant loss of the removal efficiency. As an application, the treatment of wastewater containing Pb2+, generated from the battery industry has been undertaken. The Pb2+ concentration was reduced from 5.7 to 0.6 mg/L, corresponding to an abatement of 89.5%. Therefore, the sorption using calcined Ni/Mo hydrotalcite is an efficient and suitable method for the elimination of Pb2+.

Adsorption kinetics and equilibrium performance of Di-n-butyl phosphate on XAD-4 and XAD-7 from alkaline medium

ABSTRACT The present study investigates the feasibility of adsorption method for the removal of Di-n-butyl phosphate from sodium carbonate solution on adsorbents such as XAD-4 and XAD-7. The fundamental aspects of equilibrium and kinetics of adsorption have been studied. The experimental equilibrium data were fitted well with Sips isotherm, and maximum solid uptake capacity for monolayer coverage (qm) was found to be 0.363 × 10−3 mol/g and 0.385 × 10−3 mol/g for XAD-4 and XAD-7, respectively. The adsorption kinetics was well described by the pseudo-first-order model, and estimated first-order rate constant values were found to be 0.041 min−1 for XAD-4 and 0.027 min−1 for XAD-7. The transport mechanism of solute from the bulk solution to adsorbent was described in detail with suitable mathematical model, and experimental kinetic data were superimposed on numerically solved concentration profiles to calculate external mass transfer coefficient (kf) and effective diffusivity (De). The values of kf and De were estimated as 4.632 × 10−7 m/s and 8.849 × 10−13 m2/s, respectively, for XAD-4, whereas for XAD-7, the values were found to be 2.132 × 10−6 m/s and 1.389 × 10−12 m2/s. XAD-7 is found to have slightly better adsorption capacity and transport characteristics than XAD-4.

Mass transfer limiting step determination and a new modified empirical model for Th(ІV) biosorption in a fixed-bed column

In this paper, a new empirical model for predicting breakthrough curves of Th(ІV) biosorption using Ca-pretreated Cystoseria indica alga in a fixed bed column was presented. Also, the comparison of the proposed model with the Thomas, Yoon-Nelson, and Clark models showed that the new model had better agreement with the experimental data. Furthermore, the optimum flow rate of 2.5 mL/min with a maximum biosorption capacity of 213.20 mg/g was obtained using the proposed new model. Also, four mass transfer models, including internal mass transfer, external mass transfer, biosorption on active sites, and triple resistance, were used to determine the rate-limiting step and mass transfer mechanism. The internal mass transfer was considered the mass transfer controlling step. In addition, the sensitivity analysis of the mass transfer coefficients (kf, ks, kad) again showed that the internal resistance is the governing mass transfer resistance to the Th(ІV) biosorption process. The biosorption of Th(IV) from a real effluent by Ca-pretreated Cystoseria indica alga in a fixed-bed column was investigated to validate the new empirical model. The results demonstrated that the new empirical model could describe biosorption behavior more accurately than the Thomas model.

Synthesis and application of alginate-nanocellulose composite beads for defluoridation process in a batch and fluidized bed reactor

Electronegative Fluorine has great reactivity and it exists as organic or inorganic fluoride compounds. Biosorption feasibility of fluoride onto alginate-cellulose composites was investigated in this study. Extracted cellulose has been utilized to synthesize calcium alginate impregnated composite beads for fluoride remediation process in batch and fluidized-bed reactors. Physiochemical characteristics were analyzed by FTIR, SEM, TGA and BET. From the BET properties analysis, the surface area of prepared composite beads was 87.13 m2/g. The point zero charge (PZC) value of composite beads was attained at pH 7.32. The relationship between biosorption efficiency and independent variables have been observed to evaluate the effects on the fluoride biosorption efficiency of composites and its components. The hypothetical development of the removal technique has been explained using various nonlinear model-fitting methods to evaluate Isotherm study, bio-sorption Kinetics, Thermodynamic parameters and Mass transfer study. Maximum monolayer adsorption capacity (qm) obtained by following Langmuir model for fluoride removal was found to be 23.809 mg/g at 30 °C using adsorbent dosage of 2 g/L for an initial fluoride concentration of 6 mg/L. The optimized condition for fluoride adsorption experiment was observed by evaluating response surface methodology (RSM) was pH-5.67, dose 1.89 g/L and time 85.71 min and removal was found as 82.79%. Experimental data of fluidized-bed study were evaluated by designing mathematical modeling. Fluidization velocities was adjusted in between Umf and 2Umf for optimizing external mass transfer and adsorbent loss. Regeneration study of fluoride loaded biosorbent and cost analysis of composite production have been estimated.

Experiments and modeling of fluidized bed steam-biomass char gasification with a red-mud oxygen carrier at particle scale for chemical looping

Chemical Looping Combustion (CLC) of biomass provides a low-cost way for CO2 negative emission. It creates complex multi-component flows including solids of oxygen carrier and fuel, and gasses of steam and/or CO2 as well as other gasification products. Experiments and modeling of steam-gasification of biomass char were investigated using a low-cost red mud as oxygen carrier materials. Effects of temperature, steam concentration, H2 inhibition on chemical reaction kinetics and mass transfer were experimentally investigated and modeled. A self-designed macro-TGA was used to detect its oxidization kinetics under isothermal conditions. The red mud oxygen carrier reduced inhibitory effect of H2 on carbon reaction, thus enhancing char conversion remarkably. With the neglect of the external mass limitation on char conversion, the reaction kinetics during reduction process was calculated by an experimentally-fitted function, expressing the char conversion. Pore structure evolution during the char conversion on kinetics was considered and its function was obtained based on the experimental results. The radial gas profiles under the bed of red mud particles by the solution of interparticle and external mass transfer coupled with the kinetics of char gasification were obtained, revealing the whole process of char conversion in CLC process. A semi-empirical kinetic model, including both of chemical and diffusion processes, provided a good insight of oxidation reaction kinetics, which was proven to be better than the classic models. The experimental data and modeling results got high agreement in both reduction and oxidation reactions.

Dynamic Behavior of Pb(II) and Cr(III) Biosorption onto Dead Anaerobic Biomass in Fixed-Bed Column, Single and Binary Systems

The biosorption of lead (II) and chromium (III) onto dead anaerobic biomass (DAB) in single and binary systems has been studied using fixed bed adsorber. A general rate multi- component model (GRM) has been utilized to predict the fixed bed breakthrough curves for single and dual- component system. This model considers both external and internal mass transfer resistances as well as axial dispersion with non-liner multi-component isotherm (Langmuir model). The effects of important parameters, such as flow rate, initial concentration and bed height on the behavior of breakthrough curves have been studied. The equilibrium isotherm model parameters such as maximum uptake capacities for lead (II) and chromium (III) were found to be 35.12 and 23.84 mg g-1 respectively. While pore diffusion coefficients (Dp) were obtained to be 7.23×10-11 and 3.15×10-11 m2 s-1 for lead (II) and chromium (III) respectively from batch experiments. The results show that the general rate model was found correct for describing the biosorption process of the dynamic behavior of the DAB adsorber column.

Hindered diffusion of heavy metal cations in the adsorption rate on activated carbon fiber

Water resource pollution caused by heavy metals represents a risk to the environment because of their toxicity. Adsorption on activated carbon materials can be applied to remove heavy metals in aqueous solutions. Activated carbon fiber (ACF) is a microporous material successfully used for adsorbing heavy metals from water solutions; however, the adsorption rate on ACF has yet to be investigated in detail. The adsorption equilibrium and rate of Cd(II), Ni(II) and Zn(II) on ACF were analyzed in this study, and the experimental data were procured in a differential packed bed batch adsorber. The Radke-Prausnitz isotherm successfully interpreted the experimental adsorption equilibrium data. The adsorption rate was analyzed employing a diffusional model, supposing that the external mass transfer along with intraparticle or intrafiber diffusion affect the overall adsorption rate. Besides, the metal intrafiber diffusion was attributed exclusively to diffusion in the pores volume, excluding surface diffusion. The results revealed that intrafiber diffusion is significantly affected by hindered diffusion caused by frictional effects on pore walls (Kr), steric exclusion (Kp), and blocking provoked by metal cations adsorbed onto pore walls (Ka). The hindrance factors Kr and Kp were estimated using literature correlations, and Ka was related to the mass of metal adsorbed.

Mass transfer kinetics of chemical oxygen demand removed from palm oil mill effluent in stirred cylinder batch reactor

Understanding the mechanisms and mass transfer kinetics of chemical oxygen demand (COD) removed from palm oil mill effluent (POME) could be one of the most important steps to achieve an effective design process of engineering acetogenic bacteria. The aim of this study was to evaluate the mass transfer kinetics of COD removed from POME in stirred cylinder batch reactor (SCBR) under an anaerobic environment using the modified mass transfer factor models. The performance of SCBR for the removal of COD increased with an electromagnetic field (EMF) exposure reaches 77.6% of its maximum efficiency when the operation of SCBR was set at the hydraulic retention time of 9 days. The variation of [kLa]d value has a trend of almost similar to that of [kLa]g value and is far higher than that of [kLa]f value to conclude that the resistance of mass transfer for the removal of COD from POME by the SCBR process depends on external mass transfer. The analysis of COD removal efficiency pursuant to the [kLa]d value provides a new insight on the performance of SCBR increased with an EMF exposure contributing to advanced treatment of COD from POME for achieving an effective process of engineering acetogenic bacteria.

Enhancing Energy Efficiency and Retention of Bioactive Compounds in Apple Drying: Comparative Analysis of Combined Hot Air–Infrared Drying Strategies

The drying process is one of the oldest methods used to obtain food products that could be stored for a long time. However, drying is an energy-intensive process. Additionally, convective drying, due to the high temperature used during the process, results in loss in bioactive substances as well as nutritional value. Thus, in this research, apple slices were dried in a combined hot air–infrared air dryer with four different drying strategies and drying kinetics, internal and external mass transfer (Crank and Dincer models), and then the energy parameters were investigated. The first, second, third, and fourth strategies, respectively, include one-stage drying with a hot air (HA) or infrared energy source (IR), one stage but with two sources of hot air and infrared (HA–IR), and then there are two stages of first hot air and then infrared drying (HA+IR) and vice versa (IR+HA). According to the results, the highest effective moisture diffusion coefficient of the two Crank and Dincer models was equal to 1.49 × 10−9 and 1.55 × 10−8 m2/s, obtained in the HA70–IR750, and the lowest effective moisture diffusion coefficient was equal to 1.8 × 10−10 and 2.54 × 10−9 m2/s, obtained in IR250+HA40. The maximum (10.25%) and minimum (3.61%) energy efficiency were in the IR750 and HA55–IR250 methods, respectively. Moreover, the highest drying efficiency (12.71%) and the lowest drying efficiency (4.19%) were obtained in HA70+IR500 and HA40–IR250, respectively. The value of specific energy consumption was 15.42–51.03 (kWh/kg), the diffusion activation energy was 18.43–35.43 (kJ/mol), and the value of the specific moisture extraction rate (SMER) was in the range of 0.019–0.054 (kWh/kg). Compared to the other strategies, the second strategy (HA–IR) was better in terms of drying time and mass transfer, and the third strategy (HA+IR) was more efficient in terms of energy efficiency and drying efficiency. The infrared drying in the first strategy was better than that in the other methods in the other strategies in terms of retention of bioactive compounds.

Experimental study and modeling of a packed bed bioreactor for urea removal in wines

The study involved the development and modeling of a fixed-bed bioreactor for the removal of urea from wines. The reactor, based on the immobilization of acid urease enzyme, was studied under both stationary and non-stationary conditions. The developed model, including internal and external catalyst particle mass transfer, Michaelis-Menten kinetics, convection and dispersion in the liquid along the reactor axis, was able to produce urea concentration profiles in both the solid and liquid phases under various volumetric flow rates and inlet urea concentrations. The experimental results were in good agreement with the model predictions, the mean relative error between simulated and experimental outlet ammonia concentration ranging from 4.1 % to 16.4 %. Model simulations confirmed that in wines the reaction kinetics is of the pseudo-first order and that internal and external catalyst particle diffusion limitations are negligeable. Simulations of the decrease of urea concentration as a function of space velocity for the reactor under study operating in the continuous mode and for three different wines were obtained confirming that urea removal by immobilized urease in wines is more difficult than in sake. The results obtained form the basis for the designing and scaling up of bioreactors for the treatment of wines.

An engineered biochar for treatment of selenite contaminated water: Mass transfer characteristics in fixed bed adsorption

Biochar composites have been gaining attention in recent years as a promising and cost-effective material for wastewater treatment. In the present study, a novel procedure was developed to create a biochar (BC) composite impregnated with binary zinc (Zn) and iron (Fe) oxides (denoted as ZnFeBC). The procedure was optimized to enhance selenite, Se(IV), adsorption performance of the ZnFeBC composite. The modified biochar composite had excellent pore characteristics with a specific surface area of 854 m2/g. The maximum Se(IV) adsorption capacity of an unary Zn-loaded biochar composite was 520 µg/g, which significantly increased to 4,100 µg/g after the iron oxide impregnation. Isotherm experiments suggested that the adsorption process was likely through homogeneous monolayer adsorption. The ZnFeBC composite exhibited excellent performance over a broad range of pH values, with the highest Se(IV) adsorption capacities observed at lower pH values (maximums at pH 3). X-ray absorption near edge structure spectroscopy (XANES) indicated that the redox reaction of Se oxyanions did not occur during the adsorption process. ZnFeBC was successfully used to remove Se(IV) from a mining lake water matrix in a fixed-bed adsorption system. Additionally, a mass transfer model optimizing effective diffusion and external mass transfer coefficients was developed to comprehensively assess the adsorption system. Overall, ZnFeBC exhibited great potential as an adsorbent for treating Se(IV) contaminated waters and the developed mass transfer model is useful for scaling up fixed-bed adsorption systems.

Investigation of External Mass Transfer during Biodegradation of Congo Red Dye in a Recirculating Packed Bed Bioreactor

Investigation of External Mass Transfer during Biodegradation of Congo Red Dye in a Recirculating Packed Bed Bioreactor

Mechanism modeling and application of Salvia miltiorrhiza percolation process

Percolation is a common extraction method of food processing industry. In this work, taking the percolation extraction of salvianolic acid B from Salvia miltiorrhiza (Salviae Miltiorrhizae Radix et Rhizoma) as an example, the percolation mechanism model was derived. The volume partition coefficient was calculated according to the impregnation. experiment. The bed layer voidage was measured by single-factor percolation experiment and the internal mass transfer coefficient was calculated by the parameters obtained by fitting the impregnation kinetic model. After screening, the Wilson and Geankoplis, and Koch and Brady formulas were used to calculate the external mass transfer coefficient and the axial diffusion coefficient, respectively. After substituting each parameter into the model, the process of percolation of Salvia miltiorrhiza was predicted, and the coefficient of determination R2 was all greater than 0.94. Sensitivity analysis was used to show that all the parameters studied had a significant impact on the prediction effect. Based on the model, the design space including the range of raw material properties and process parameters was established and successfully verified. At the same time, the model was applied to the quantitative extraction and endpoint prediction of the percolation process.

Kinetic analysis of an ionic liquid-based metal extraction process using a single droplet extraction column

In this study a liquid-liquid extraction (LLX) process has been investigated based on experimental analysis and kinetic modelling. The purpose of this investigation is (1) to understand the mass transfer behaviour, (2) to determine the rate limiting step via evaluating different mass transfer models, and (3) to estimate the mass transfer and kinetic parameters. This has been discussed in the context of the extraction of Co by the ionic liquid (IL) [P8888][Oleate] as an example of LLX with chemical reaction. Mass transfer models, with and without a chemical reaction, are evaluated based on a statistical cross-validation method. The following operational parameters are included in the analysis: column lengths, droplet diameter, droplet rising velocity and continuous and dispersed phase concentrations on Co uptake. This method reveals that a single parameter representing the external mass transfer resistance can describe the forward extraction of Co (i.e., into the IL) for the whole data set sufficiently accurate (error ±30%) regardless of the studied operational conditions. Back-extraction of Co from pre-loaded IL droplets shows a different transfer mechanism. Now the mass transfer in the dispersed IL phase dominates the process which is attributed to a change of the physical properties of the pre-loaded IL.

Experimental Investigation of Removal of SO3 from Flue Gas with Modified Fly Ash Adsorbents.

The removal of nonconventional pollutants in coal-fired power plants, such as SO3, has been receiving more and more attention. However, due to its unique nature, it is difficult to remove SO3 effectively with the widely used wet flue gas desulfurization systems. Nowadays, dry-sorbent injection technology has become a promising method for SO3 emission control in coal-fired power plants. The removal characteristics of SO3 from flue gas with modified fly ash adsorbents were investigated in a fixed-bed reactor. Factors affecting the adsorption efficiency of SO3 were studied, including modification method, modified fly ash adsorbent particle size, reaction temperature, and flue gas component. Combined with adsorbent characterization analysis, the adsorption kinetics of SO3 by modified fly ash adsorbents were carried out with four different adsorption kinetics models. The results show that the SO3 adsorption efficiency of the fly ash samples increases after modification; the best SO3 removal performance of fly ash was achieved when 1.5 mol/L NaOH solution was used, with the highest SO3 adsorption efficiency of up to 98.3%. The modified fly ash adsorbent particle size, water vapor content, and the addition of NO have little effect on the adsorption efficiency of SO3. As the reaction temperature increases from 250 to 450 °C, the SO3 adsorption efficiency first increases and then decreases, with an optimal reaction temperature of 350 °C. The addition of SO2 would compete with SO3 for adsorption and inhibit the uptake of SO3 by the adsorbent. Adsorption kinetics data show that external mass transfer and chemical adsorption are the main critical mechanisms affecting the adsorption efficiency of the modified fly ash adsorbent in the SO3 removal process compared to internal diffusion.

Influence of ultrasound-pretreated convective drying of Roselle (Hibiscus sabdariffa L) leaves on its drying kinetics and nutritional quality

The drying kinetics and the nutritional qualities of Roselle (Hibiscus sabdariffa L) leaves as affected by ultrasound pretreatment during hot air drying were investigated in this work. Ultrasound frequency and power at 20 kHz and 600 W, respectively, were used for the experiment. Roselle leaves were subjected to ultrasound pretreatment in distilled water for 0 min (untreated), 5 min (UD5), 10 min (UD10) and 15 min (UD15) before hot air drying. The result showed that UD15 samples had about 47% reduction in drying time compared with the untreated samples. The effective moisture diffusivity of the treated and untreated samples ranged from 1.21 to 3.29 × 10−1m2/s. The Page and Logarithmic models best described the drying behavior for the untreated and treated samples respectively. The values of the mass transfer Biot number (ratio of internal mass transfer resistance to external mass transfer resistance) for all the samples ranged from 0.9032 to 1.2391. The samples of UD10 and UD5 had the highest amount of carotenoid and vitamin C contents, respectively, amongst the untreated and treated samples. The total color change of the treated samples were significantly lower than the untreated samples after drying. Drying of Roselle leaves is usually done using convective drying method with its attendant's effects such as long drying time, loss of nutrients and high energy usage. But this work showed that application of ultrasound pretreatment during drying of Roselle leaves can significantly reduce the drying time and at the same time retain the nutritional quality of the dried treated samples.

Mass transfer in catalytic depolymerization: External effectiveness factors and serendipitous processivity in stagnant and stirred melts

Several heterogeneous catalysts are being developed to recycle plastics. Most operate in viscous polymer melts, where external mass transfer effects could limit the supply of co-reactants to active sites. External mass transfer can also impede the diffusion of long chain products away from the catalyst after each cut. Product egress limitations could potentially confer unintentional processivity to catalyst operation, i.e. a tendency for the catalyst to repeatedly cut the same chain after an initial encounter. We formulate reaction–diffusion equations to quantify mass transfer effects on the co-reactant transport to the catalyst and the degree of serendipitous processivity. Results are developed for catalysts in stagnant or stirred melts, with simple expressions involving Damkohler, Peclet, and Sherwood numbers, i.e. dimensionless combinations of rate constants, catalyst particle size, polymer diffusivities, and shear rates (where applicable). We estimate the impact of these effects for a spherical core–shell catalyst.