Assessment of CO2 adsorption capacity on activated carbons by a combination of batch and dynamic tests

In this work, batch and dynamic adsorption tests are coupled for an accurate evaluation of CO2 adsorption performance for three different activated carbons obtained from olives stones by chemical activation followed by physical activation with CO2 at varying times, i.e. 20, 40 and 60 h. Kinetic and thermodynamic CO2 adsorption tests from simulated flue-gas at different temperature and CO2 pressure are carried out both in batch (a manometric equipment operating with pure CO2) and dynamic (a lab-scale fixed-bed column operating with CO2/N2 mixture) conditions. The textural characterization of the activated carbon samples shows a direct dependence of both micropore and ultramicropore volume on the activation time, hence AC60 has the higher contribution. The adsorption tests conducted at 273 and 293 K showed that, when CO2 pressure is lower than 0.3 bar, the lower the activation time the higher CO2 adsorption capacity and a ranking ωeq(AC20)>ωeq(AC40)>ωeq(AC60) can be exactly defined when T= 293 K. This result can be likely ascribed to a narrower pore size distribution of the AC20 sample, whose smaller pores are more effective for CO2 capture at higher temperature and lower CO2 pressure, the latter representing operating conditions of major interest for decarbonation of a flue-gas effluent. Moreover, the experimental results obtained from dynamic tests confirm the results derived from the batch tests in terms of CO2 adsorption capacity. It is important to highlight that the adsorption of N2 on the synthesized AC samples can be considered negligible. Finally, the importance of a proper analysis of characterization data and adsorption experimental results is highlighted for a correct assessment of CO2 removal performances of activated carbons at different CO2 pressure and operating temperature

Discontinuous permeable adsorptive barrier design and cost analysis: a methodological approach to optimisation

The following paper presents a method to optimise a discontinuous permeable adsorptive barrier (PAB-D). This method is based on the comparison of different PAB-D configurations obtained by changing some of the main PAB-D design parameters. In particular, the well diameters, the distance between two consecutive passive wells and the distance between two consecutive well lines were varied, and a cost analysis for each configuration was carried out in order to define the best performing and most cost-effective PAB-D configuration. As a case study, a benzene-contaminated aquifer located in an urban area in the north of Naples (Italy) was considered. The PAB-D configuration with a well diameter of 0.8 m resulted the best optimised layout in terms of performance and cost-effectiveness. Moreover, in order to identify the best configuration for the remediation of the aquifer studied, a comparison with a continuous permeable adsorptive barrier (PAB-C) was added. In particular, this showed a 40% reduction of the total remediation costs by using the optimised PAB-D

Pump-and-treat configurations with vertical and horizontal wells to remediate an aquifer contaminated by hexavalent chromium

Pump-and-treat technology is among the most used technologies for groundwater remediation. While conventional, vertical wells (VRWs) are well-known and used from long time, horizontal wells (HRWs) have been explored for remediation technologies only in last few decades. HRWs have shown to outperform vertical wells in terms of versatility, productivity and clean-up times under certain conditions. In this paper, the efficacy of an innovative pump-and-treat (P&T) configuration for groundwater remediation obtained by adopting either VRWs or HRWs technology is comparatively tested. A 3D transient finite element model of an unconfined aquifer containing a hexavalent chromium (Cr(VI)) contamination plume is considered to compare a single horizontal well configuration vs a range of spatially-optimised arrays containing vertical wells. A sensitivity analysis aimed at finding the best configuration to minimise the remediation time and the related cost is carried out by comparing different well diameters, D, pumping rates, Q, and position of wells. A comparative cost analysis demonstrates that, for the examined case-study, a single HRW achieves the clean-up goals in the same time span as for a greater number of vertical wells, but at higher price due to the excavation costs

An optimized configuration of adsorptive wells for the remediation of an aquifer contaminated by multiple aromatic hydrocarbon pollutants

Adsorptive wells arrays are an innovative outline of Permeable Reactive Barrier (PRBs) made of a definite number of passive deep wells opportunely distributed in the aquifer, known as PAB-D (Discontinuous Permeable Adsorptive Barrier). They are generally located downstream the contaminated groundwater flow and perpendicularly to the groundwater flow direction. Being PAB-D wells filled with adsorbing media, whose hydraulic permeability is higher than the surrounding media, the array will create a targeted capture zone, which will force the contaminated water to pass through the whole PAB-D, allowing for both the interception of the contaminant plume and its treatment. In this work, an optimized configuration of PAB-D is presented, for the in situ-remediation of an aquifer simultaneously contaminated by benzene and toluene. The design optimization of the PAB-D was performed by using COMSOL Multiphysics®, in which numerical simulations reproduced the transport and the adsorptive phenomena occurring inside the aquifer and the barrier itself. The proposed technique was applied to the remediation of an aquifer located in an urban area in the north of Naples (Italy), in proximity of numerous landfills, where the contamination was spread over an area of 0.10 km2. Simulation results confirm the effectiveness of the PAB-D, being both pollutant plumes intercepted and their concentrations reduced below their correspondent Italian regulatory threshold values. The best array configuration of PAB-D resulted made of 741 wells, each having a diameter of 0.6 m, which was also compared with a continuous barrier (PAB-C) showing a reduction of about 49% of the volume and 35% of the overall remediation cost

Solubility of benzene in copolymer aqueous solutions for the design of gas absorption unit operations

In this work, an amphiphilic block copolymer (ABC) aqueous solution is proposed as innovative green liquid for benzene recovery from polluted gas. This ABC has been selected among four commercially available copolymers, two thermosensitive ABC containing poly-ethylene oxide and poly-propylene oxide blocks-Pluronic (R) L62 and Pluronic (R) P123 - one non-thermosensitive ABC - poly(vinyl pyrrolidone)-polystyrene (PVP-PS) and one non-ionic surfactant - polyoxyethylene sorbitol monooleate (Tween 80 (R)). For each copolymer aqueous solution, benzene Henry's constant (H) has been experimentally calculated and the effects of temperature (4-90 degrees C) and copolymer concentration (4-20%) have been investigated. Experimental results at 20 degrees C and 4% copolymer concentration showed that the H values are in the range 50-280 atm, with a ranking PVP-PS 1100 atm) has been measured in P123 aqueous solution at 90 degrees C, corresponding to its critical point (i.e. phase separation and no micelles presence). As a result of its benzene H values, the P123 aqueous solution was found to be the most promising liquid for benzene recovery among those investigated, being effective for both operations of gas absorption (at low temperature) and solvent regeneration by thermal stripping (at high temperature)

Sustainability Indicators for Materials and Processes

The concept of sustainability is nowadays employed to compare manufacturing processes or to define the correct path for material selection. Sometimes, this concept is only partially defined, including just low costs, profit maximization and/or CO2 emission reduction. Actually, a process or material can be defined as sustainable only if an objective function related to the economic, environmental and social impacts is simultaneously maximized. To this aim, it is necessary to define appropriate and specific sustainability indicators (i.e., values related to the economic, social and environmental aspects of a process or material under analysis). These indicators come about from simple calculations, and they are defined in terms of percentages and represented and compared using radar diagrams. Then, a process or specific material is identified by an objective function (i.e., the area included by the polygon that links the scores reported on the diagram). The scope of this representation of data is to individuate the major weaknesses of the process/material, proposing methods of optimization and trying to maximize the objective function in the retrieved diagram. This work aims to propose a general and simple method to calculate sustainability indicators on the basis of specific definitions related to a given process/material. To highlight the potential of this calculation and comparison instrument, two case studies are proposed: the first aims at comparing processes for the production of energy, while the second aims at driving the choice of manufacturing material. The selected indicators and adopted algorithm allowed for the identification of hydroelectric and eolic as the most sustainable processes for energy production; for materials, the results strictly depended on the assumptions made regarding favorable mechanical properties

Fixed-bed adsorption of trichloroethylene onto activated carbon

The present work deals with the study of the adsorption of Trichloroethylene (TCE) onto a commercial granular activated carbon (GAC). Thermodynamic (batch) and dynamic (fixed-bed column) tests were carried out, under a wide range of operating conditions. In particular, adsorption isotherms were preliminary determined to support the investigation on a fixed-bed column at lab-scale. The main fluid dynamic and physical parameters such as flow rate, TCE concentration and GAC particle size were investigated. Experimental data on fixed-bed column show that an increase in TCE initial concentration and, more markedly, an increase in flow rate lead to a shorter breakpoint time. In fact, the breakthrough curves become steeper as a consequence of higher velocity that enhances the external mass transport. Finally, a thorough modeling analysis of the fixed-bed column was carried out using a model that includes axial dispersion and external-film diffusion followed by internal diffusion. A good accordance between experimental data and model results was observed

Cd(II) removal from aqueous solutions by adsorption onto activated carbon

This paper documents the results of an experimental study on cadmium removal from model aqueous solutions by adsorption onto a granular activated carbon (GAC). Adsorption isotherms have been carried out at different pH, salinity and temperature levels. Cadmium adsorption is the highest at neutral pH levels while it strongly decreases by decreasing pH. Due to Cd(OH)2 precipitation, the adsorption becomes irrelevant in alkaline systems. On the contrary temperature and salinity have more limited effects on cadmium capture by the GAC