Experimental kinetics and thermodynamics investigation: Chemically activated carbon-enriched monolithic reduced graphene oxide for efficient CO2 capture.

In this research, we have developed solid MGOs by self-assembled reduction process of GO at 90 °C with different weight ratios of oxalic acid (1:1, 1:0.500, and 1:0.250). The as-synthesized monoliths were carbonized (at 600 °C) and chemically activated with varying proportions of NaOH (1:1, 1:2, and 1:3). This materials offer the CO2 adsorption effect under dynamic conditions, fast mass transfer, easy handling, and outstanding stability throughout the adsorption-desorption cycle. FE-SEM, and HR-TEM analyses confirmed the porous nature and shape of the adsorbents, while XPS examination revealed the presence of distinct functional groups on the surface of the monolith. By increasing the mass ratios (MGO:NaOH) from 1:1 to 1:2, the surface areas increased by approximately 2.6 times, ranging from 520.8 to 753.9 m2 g⁻1 (surface area of the untreated MGO was 289.2 m2 g⁻1). Consequently, this resulted in a notable enhancement of 2.10 mmol g⁻1 in dynamic CO2 capture capacity. The assessment encompassed the evaluation of production yield, selectivity, regenerability, kinetics, equilibrium isotherm, and isosteric temperatures of adsorption (Qst). The decrease in CO2 capture effectiveness with rising adsorption temperature indicated an exothermic and physisorption process. The regenerability of 99.1 % at 100 °C and excellent cyclic stability with efficient CO2 adsorption make this monolithic adsorbent appropriate for post-combustion CO2 capture. The significant Qst lend support to the heterogeneity of the adsorbent's surface, and the pseudo-second-order kinetic model along with the Freundlich isotherm model emerged as the most fitting. Therefore, the current investigation shows that the carbon-enriched adsorbents enhance the CO2 adsorption capacity. It may be used as a low-cost pretreatment method on an industrial scale before carbon capture.

Enhancing CO2 capture through innovating monolithic graphene oxide frameworks.

The advancement and engineering of novel crystalline materials is facilitated through the utilization of innovative porous crystalline structures, established via KOH-treated monolithic graphene oxide frameworks. These materials exhibit remarkable and versatile characteristics for both functional exploration and applications within the realm of CO2 capture. In this comprehensive study, we have synthesized monolithic reduced graphene oxide-based adsorbents through a meticulous self-assembly process involving different mass ratios of GO/malic acid (MaA) (1:0.250, 1:0.500, and 1:1 by weight). Building upon this foundation, we further modified MGO 0.250 through KOH-treatment by chloroacetic acid method, leading to the creation of MGO 0.250_KOH, which was subjected to CO2 capture assessments. The comprehensive investigation encompassed an array of parameters including morphology, specific surface area, crystal defects, functional group identification, and CO2 capture efficiency. Employing a combination of FT-IR, XRD, Raman, BET, SEM, HR-TEM, and XPS techniques, the study revealed profound insights. Particularly notable was the observation that the MGO 0.250_KOH adsorbent exhibited an exceptional CO2 capture performance, leading to a significant enhancement of the CO2 capture capacity from 1.69 mmol g-1 to 2.35 mmol g-1 at standard conditions of 25 °C and 1 bar pressure. This performance enhancement was concomitant with an augmentation in surface area, elevating from 287.93 to 419.75 m2 g-1 (a nearly 1.5-fold increase compared to MGO 1.000 with a surface area of 287.93 m2 g-1). The monolithic adsorbent demonstrated a commendable production yield of 82.92%, along with an impressive regenerability of 98.80% at 100 °C. Additionally, adsorbent's proficiency in CO2 adsorption, rendering it a promising candidate for post-combustion CO2 capture applications. These findings collectively underscore the capacity adsorbents to significantly amplify CO2 capture capabilities. The viability of employing this strategy as an uncomplicated pre-treatment technique in various industrial sectors is a plausible prospect, given the study's outcomes.

Biodegradation kinetic modeling of pro-oxidant filled polypropylene composites under thermophilic composting conditions after abiotic treatment.

This work aims at modeling and characterizing the kinetics of biodegradation of polypropylene loaded with cobalt stearate as pro-oxidant after abiotic treatment. Eight films of these composites were prepared using different pro-oxidant loadings. These films were treated abiotically using accelerated weathering for 40 h, and biotically using aerobic composting as per ASTM D 5338. The experimental data were analyzed using an eight-parameter Komilis model containing a flat lag phase. The model formulations involved hydrolysis of primary solid carbon and its subsequent mineralization. The first step was rate controlling and it included hydrolysis of slowly (Cs), moderately (Cm), and readily (Cr) hydrolyzable carbon fractions in parallel. The model parameters were evaluated by means of nonlinear regression technique. The surface morphology of the films before and after the biodegradability test supported the biodegradation results. The model parameters and undegraded/hydrolyzable/mineralizable carbon evolutions involved moderately and readily hydrolyzable carbons but with the absence of slowly hydrolyzable carbon. These exhibit degradability in the range of 11.20-36.42% in 45 days. Biodegradability increases with progressive increase in pro-oxidant loading. The rate of degradation reaches maximum (0.322-0.897% per day) at around the 39th-12th day. For all the films, readily hydrolyzable carbon fractions and their hydrolysis rate constants (kr) are appreciably increased with increasing pro-oxidant loading. All the films show the presence of growth phase because of their high initial readily hydrolyzable carbon fractions. The SEM images after the abiotic and subsequently biotic treatments were progressively rougher. The methods presented here can be used for the design and control of other similar systems.

Adsorption of CO2 on KOH activated carbon adsorbents: Effect of different mass ratios.

Nitrogen and oxygen enriched carbons were prepared by the cost-effective synthesis route of carbonization of polyacrylonitrile (PAN) and subsequent KOH activation for CO2 capture. The effect of four impregnation mass ratios (KOH: PAN = 1-4) and activation temperatures (600-900 °C) on the synthesized carbon adsorbent properties was explored by different analyses. The X-ray photoelectron spectroscopy (XPS) revealed the existence of basic nitrogen and oxygen functionalities on the adsorbent's surface which increases the adsorption rate for CO2 by providing its basic sites. By increasing mass ratio (KOH:PAN) from 1:1 to 3:1, the surface area increased from 1152.4 to 1884.2 m2 g-1 and the dynamic CO2 adsorption capacity also increased from 2.1 to 2.5 mmol g-1 respectively, at 30 °C (approximately ten times the adsorption capacity of untreated PAN, 0.22 mmol g-1). Physisorption and exothermic nature of the process were confirmed by the decrease in the adsorption capacity of the adsorbents with the increase in adsorption temperature. Moreover, good cyclic stability and regenerability over 5 adsorption-desorption cycles were obtained for the adsorbents. The fractional order kinetic and Temkin isotherm models fitted best with the adsorption data. A heterogeneous interaction between CO2 and the surface of adsorbents was suggested by the isosteric heat of adsorption values. Combined with the simple method for the preparation of activated carbon adsorbents, efficient CO2 adsorption and excellent regeneration make it appropriate adsorbents for post-combustion CO2 capture.

Biodegradation kinetic modeling of oxo-biodegradable polypropylene/polylactide/nanoclay blends and composites under controlled composting conditions.

Polypropylene/polylactide/nanoclay blend/composite films with/without pro-oxidants/compatibilizer were prepared and aerobically degraded to measure the CO2 evolution under controlled composting conditions as per ASTM D 5338. A first-order Komilis model in series with a flat lag phase was postulated involving two stages; hydrolysis of solid carbon followed by its rapid mineralization. The first, rate-limiting stage further comprised of three possible parallel paths: the solid hydrolysis of readily, moderately, and slowly hydrolyzable carbon fractions. The model parameters were computed after correlating with the experimental data using nonlinear regression analysis. The results of the model characteristic parameters, un-degraded/hydrolyzable/mineralisable-intermediate carbon kinetics, and degradation curves exhibit two distinct kinetic regimes. The first regime comprising of slowly and moderately hydrolyzable carbon is shown by the first four films without pro-oxidants. This causes low degradability and degradation rate. The second regime comprising of the readily and moderately hydrolyzable carbon is shown by another four films containing pro-oxidants. They exhibit relatively high degradability and degradation rate, which peaks at around 11-14th day in the range of 0.219-0.268% per day. The values of their moderately hydrolyzable carbon fractions and the corresponding hydrolysis rates are significantly higher than that of the first regime. For the first regime, the degradability and degradation rate decreases with increase in the slowly hydrolyzable carbon impervious to microbial attack. Their degradation rate profiles show an absence of growth phase due to the absence of readily hydrolyzable carbon. The rate decreases monotonously starting from the maximum value ranging from 0.043 to 0.180% per day. The approach presented can also be implemented to model and design equipment for other waste biodegradation systems.

Porous carbons derived from polyethylene terephthalate (PET) waste for CO2 capture studies.

Oxygen augmented carbon adsorbent has been developed using polyethylene terephthalate (PET) waste by first carbonizing at different temperatures (500-800 °C) and then chemically activating using different ratios of KOH: PET (mass ratio 1 to 4). The textural characterization divulges the effect of activation in terms of the development of the high surface area and micropore volume of 1690 m2 g-1 and 0.78 cm3 g-1 respectively, for the optimum sample (PET-3-700). Elemental analysis of PET-3-700 illustrates the presence of 34.33% oxygen and XPS results confirmed the occurrence of oxygen moieties which enhance the basicity of the adsorbent and promote CO2 capture. The CO2 adsorption capacity of prepared carbons was determined thermogravimetrically under dynamic conditions, at different concentrations of CO2 (6-100%) and temperatures. The maximum CO2 uptake capacity of 2.31 mmol g-1 was exhibited by PET-3-700 at an adsorption temperature of 30 °C under 100% pure CO2 flow. Four adsorption-desorption cycles corroborate almost complete regenerability of the prepared adsorbent. Adsorption kinetics at all adsorption conditions was described best by fractional order kinetic model. Freundlich isotherm fit indicates the surface of adsorbent being heterogeneous and low values of isosteric heat shows physisorption behavior of the process. Negative values of thermodynamic parameters indicate exothermic and feasible nature of adsorption process.

Development of chemically activated N-enriched carbon adsorbents from urea-formaldehyde resin for CO2 adsorption: Kinetics, isotherm, and thermodynamics.

Nitrogen enriched carbon adsorbents with high surface areas were successfully prepared by carbonizing the low-cost urea formaldehyde resin, followed by KOH activation. Different characterization techniques were used to determine the structure and surface functional groups. Maximum surface area and total pore volume of 4547 m2 g-1 and 4.50 cm3 g-1 were found by controlling activation conditions. The optimized sample denoted as UFA-3-973 possesses a remarkable surface area, which is found to be one of the best surface areas achieved so far. Nitrogen content of this sample was found to be 22.32%. Dynamic CO2 uptake capacity of the carbon adsorbents were determined thermogravimetrically at different CO2 concentrations (6-100%) and adsorption temperatures (303-373 K) which have a much more relevance for the flue gas application. Highest adsorption capacity of 2.43 mmol g-1 for this sample was obtained at 303 K under pure CO2 flow. Complete regenerability of the adsorbent over four adsorption-desorption cycles was obtained. Fractional order kinetic model provided best description of adsorption over all adsorption temperatures and CO2 concentrations. Heterogeneity of the adsorbent surface was confirmed from the Langmuir and Freundlich isotherms fits and isosteric heat of adsorption values. Exothermic, spontaneous and feasible nature of adsorption process was confirmed from thermodynamic parameter values. The combination of high surface area and large pore volume makes the adsorbent a new promising carbon material for CO2 capture from power plant flue gas and for other relevant applications.

Radiotracer investigation on the measurement of residence time distribution in an ethyl acetate reactor system with a large recycle ratio.

A radiotracer investigation was carried out on the measurement of residence time distribution (RTD) of process fluid in an industrial-scale ethyl acetate reactor system, which consists of two independent reactors with recirculation and connected in series with each other. Bromine-82 as ammonium bromide was used as the radiotracer for the RTD experiments at different operating conditions. The individual reactors and the overall reactor system were modelled using physically representative phenomenological models comprising of continuously stirred tank reactors (CSTRs). The results showed that the recirculation rate considerably affected the flow mixing behaviour and mean residence time of the process fluid in the reactor system. The results also showed that there was bypassing of the fluid in the first reactor that ranged from 12% to 22% and 40% dead volume at different operating conditions, whereas the second reactor behaved closely as an ideal CSTR. The results of the investigation can be used to optimise the process parameters and design new improved reactor systems for the production of ethyl acetate.

Radiotracer investigation and modeling of an activated sludge system in a pulp and paper industry.

A radiotracer investigation was carried out in an activated sludge process (ASP) system of an effluent treatment plant in a pulp and paper industry. The system consists of an aeration tank and a secondary clarifier connected in series. The primary objective of the investigation was to measure mean hydraulic retention times (MHRTs) of wastewater and investigate the hydraulic performance of the ASP. Residence time distributions (RTD) of the wastewater were measured in an aeration tank and a secondary clarifier of the system using Iodine-131 as a radiotracer. The measured RTD data was treated and MHRTs were estimated. No bypassing was found to exist in the aeration tank and the secondary clarifier. However, the dead volume in the aeration tank and the secondary clarifier was found and estimated to be 2.34% and 4.6%, respectively. The treated curves were further simulated using suitable hydraulically representative mathematical models and detailed flow patterns in the aeration tank and the secondary clarifier were deciphered.

Melamine-formaldehyde derived porous carbons for adsorption of CO2 capture.

In this work, we report carbon adsorbents obtained from high nitrogen content melamine-formaldehyde resin as starting material and mesoporous zeolite MCM-41 as template through nanocasting technique. To synthesize different carbon structure adsorbents with improved textural and surface properties, the material undergo carbonization followed by physical activation under CO2 atmosphere at different temperatures. Characterizations of the adsorbents using SEM, TEM, XPS, nitrogen sorption, CHN, TKN, and TPD have been carried out. Characterization results reveal the development of nanostructured carbon adsorbents with better texture and surface properties as compared to the sample prepared by direct carbonization. Sample prepared at carbonization-activation temperature of 700 °C shows highest basicity, surface area (193.28 m2 g-1) and pore volume (0.32 cm3 g-1). Performance evaluation of adsorbent was performed thermo gravimetrically at different temperatures and concentrations and was found that the adsorbent synthesized at 700 °C exhibit highest CO2 uptake of 0.93 mmol g-1 with nitrogen content of 22.73%. It was found that both surface area and nitrogen functional group have a major impact on adsorption capacity. Physiosorption process was confirmed by a decrease in adsorption capacity with increase in temperature. Three kinetic models and isotherms were used in this study and found that fractional order kinetic model and Freundlich isotherm best fitted with the experimental data. Isotherm study depicts the heterogeneous nature of adsorbent surface. Adsorbent exhibited complete regenerability and was stable over four adsorption-desorption cycles. Low value of isosteric heat of adsorption of 15.75 kJ mol-1, indicates physiosorption process. Negative value of ΔG0 and ΔH0 confirms spontaneous, feasible and exothermic nature of adsorption process.

Measurement of residence time distribution of liquid phase in an industrial-scale continuous pulp digester using radiotracer technique.

A series of radiotracer experiments was carried out to measure residence time distribution (RTD) of liquid phase (alkali) in an industrial-scale continuous pulp digester in a paper industry in India. Bromine-82 as ammonium bromide was used as a radiotracer. Experiments were carried out at different biomass and white liquor flow rates. The measured RTD data were treated and mean residence times in individual digester tubes as well in the whole digester were determined. The RTD was also analyzed to identify flow abnormalities and investigate flow dynamics of the liquid phase in the pulp digester. Flow channeling was observed in the first section (tube 1) of the digester. Both axial dispersion and tanks-in-series with backmixing models preceded with a plug flow component were used to simulate the measured RTD and quantify the degree of axial mixing. Based on the study, optimum conditions for operating the digester were proposed.

Development of nitrogen enriched nanostructured carbon adsorbents for CO2 capture.

Nanostructured carbon adsorbents containing high nitrogen content were developed by templating melamine-formaldehyde resin in the pores of mesoporous silica by nanocasting technique. A series of adsorbents were prepared by altering the carbonization temperature from 400 to 700 °C and characterized in terms of their textural and morphological properties. CO2 adsorption performance was investigated at various temperatures from 30 to 100 °C by using a thermogravimetric analyzer under varying CO2 concentrations. Multiple adsorption-desorption experiments were also carried out to investigate the adsorbent regenerability. X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed the development of nanostructured materials. Fourier transform infrared spectroscopy (FTIR) and elemental analysis indicated the development of carbon adsorbents having high nitrogen content. The surface area and pore volume of the adsorbent carbonized at 700 °C were found to be 266 m(2) g(-1) and 0.25 cm(3) g(-1) respectively. CO2 uptake profile for the developed adsorbents showed that the maximum CO2 adsorption occurred within ca. 100 s. CO2 uptake of 0.792 mmol g(-1) at 30 °C was exhibited by carbon obtained at 700 °C with complete regenerability in three adsorption-desorption cycles. Furthermore, kinetics of CO2 adsorption on the developed adsorbents was studied by fitting the experimental data of CO2 uptake to three kinetic models with best fit being obtained by fractional order kinetic model with error% within range of 5%. Adsorbent surface was found to be energetically heterogeneous as suggested by Temkin isotherm model. Also the isosteric heat of adsorption for CO2 was observed to increase from ca. 30-44 kJ mol(-1) with increase in surface coverage.

Mesoporous carbon adsorbents from melamine-formaldehyde resin using nanocasting technique for CO2 adsorption.

Mesoporous carbon adsorbents, having high nitrogen content, were synthesized via nanocasting technique with melamine-formaldehyde resin as precursor and mesoporous silica as template. A series of adsorbents were prepared by varying the carbonization temperature from 400 to 700°C. Adsorbents were characterized thoroughly by nitrogen sorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), elemental (CHN) analysis, Fourier transform infrared (FTIR) spectroscopy and Boehm titration. Carbonization temperature controlled the properties of the synthesized adsorbents ranging from surface area to their nitrogen content, which play major role in their application as adsorbents for CO2 capture. The nanostructure of these materials was confirmed by XRD and TEM. Their nitrogen content decreased with an increase in carbonization temperature while other properties like surface area, pore volume, thermal stability and surface basicity increased with the carbonization temperature. These materials were evaluated for CO2 adsorption by fixed-bed column adsorption experiments. Adsorbent synthesized at 700°C was found to have the highest surface area and surface basicity along with maximum CO2 adsorption capacity among the synthesized adsorbents. Breakthrough time and CO2 equilibrium adsorption capacity were investigated from the breakthrough curves and were found to decrease with increase in adsorption temperature. Adsorption process for carbon adsorbent-CO2 system was found to be reversible with stable adsorption capacity over four consecutive adsorption-desorption cycles. From three isotherm models used to analyze the equilibrium data, Temkin isotherm model presented a nearly perfect fit implying the heterogeneous adsorbent surface.