Escobar-Vásquez, Zalleris O.
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Publication Performance of landfills with coal combustion byproducts aggregates as an alternative reactive daily cover(2012-05) Escobar-Vásquez, Zalleris O.; Hwang, Sangchul; College of Engineering; Rivera Santos, Jorge; Padilla, Ingrid Y.; Perales-Pérez, Oscar; Department of Civil Engineering; Arroyo, JoséA landfill is an engineered facility for the disposal of wastes. It is designed and operated to minimize public health and environmental impacts. However, land availability is the limiting factor for the operation and development of landfills. Landfill daily cover is a standard practice where inorganic soil is placed over the waste to: keep waste from blowing away; minimize disease vectors; restrict access to rodents, birds, and insects, to control leachate and erosion, reduce fire hazard potential and noxious odors, and provide an aesthetic appearance. The use of alternative materials for daily covers could conserve landfill space and soil resources while also meeting environmental and operational requirements. Energy wastes, such as coal combustion byproducts aggregates (CCAs), can meet dual purposes simultaneously. They can achieve resource recovery and reclamation as a reactive daily cover (ADC) by being reutilized in landfills as a key design and operating component for daily cover. The CCAs are an agglomerate of fly ash and bottom ash that are produced during the coal combustion process. The purpose of this research was to evaluate the potential of the CCAs as ADC materials to achieve resource recovery, enhance biological decomposition, and induce early settlement of landfills. Biochemical decomposition and settlement were simulated using physical landfill models (PLMs) in an environmental chamber (one PLM used sandy soil as a daily cover, whereas another PLM used CCAs). The environmental chamber was equipped with a thermal circulator to support the rate of waste decomposition in lab-scale PLMs. The PLMs were equipped with gas extraction ports, water spraying systems on the top, and leachate drain ports on the bottom. Settlement was monitored through a side-wall window on the PLMs. Synthetic solid wastes were formulated in accordance to the average characteristics of Puerto Rican solid wastes. Rate of leachate production through the CCAs cover, as a measure of hydraulic performance, was quantified by comparing infiltration rates through the soil cover under identical hydrological conditions. Leachate volume and the concentrations of organic and inorganic substances in leachate were monitored and compared between the CCAs PLM and control PLM. Results showed that physical conditions in the control and CCAs PLMs produced similar hydraulic characteristics (leachate quantity) are attributed to similar void fractions controlling the flow through the sand and CCA. The concentrations of organic and inorganic components were found reduced more in the CCA PLM than in the control PLM. Higher microbial activity resulting from more optimal pH conditions for methanogens and higher contribution of nutrients for microbial growth. Higher active biodecomposition of solid waste is also supported by higher settlement and biogas production in the CCAs PLM, compared to the control PLM. To understand potential influence of the CCAs packing density on leachate characteristics, three smaller physical landfill models (SPLMs) were constructed. One had the same packing density of the CCAs PLM, another had the same packing density of the control PLM, and the other had a higher packing density than the CCAs PLM (1.23). Results of this study indicated that packing density of the CCA did not significantly alter the production of leachate substances. The concentrations of organic and the inorganic compounds were reduced and the production of biogas increased regardless of the packing density validating the results of the CCA PLM. An important aspect in the use of CCAs as an ADC is the possibility of using it as a reactive daily cover for heavy metals removal. Removal of heavy metals by CCAs was confirmed in SPLM tests. Quantification of the CCA’s point of zero charge, which was found at 8.7 ± 0.2, suggests that removal is mainly caused by precipitation processes, not by adsorption. Microbial activity enhancement due to CCAs was confirmed with a separate experiment where four order of magnitude greater colony forming unit (CFU) was observed from the system having CCAs than the control system without them (30~150 x 1010 CFU/100 mL vs. 140 x 106 CFU/100 mL). Therefore, the greater microbial populations, activity, and enhanced biological waste decomposition are expected to occur in landfills with CCAs as daily cover, as was observed in the CCAs PLM. This also provides extra space for more wastes to be disposed of in the landfill and leads to significant conservation of natural soils which otherwise would have been excavated for use as daily cover.Publication A new mixing rule in the modeling of the solubility of solids in supercritical fluids(2006) Escobar-Vásquez, Zalleris O.; Estévez-De Vidts, L. Antonio; College of Engineering; Suleiman Rosado, David; Ramírez Beltrán, Nazario D.; Department of Chemical Engineering; Carrasquillo, ArnaldoCurrently, supercritical-fluid technology in the pharmaceutical and microelectronics industries is increasingly applied to solve difficult processing problems. The solubility of a solute in the supercritical fluid is the most important thermophysical property that needs to be determined and modeled as a first step to develop any supercritical fluids application. This research was undertaken to develop a mathematical model to compute the solubility of solids in supercritical fluids. As a result, a new combination rule is proposed along with a novel approach to obtain general correlations for its parameters. The new combination rule is a modification of the classical van der Waals mixing rules where the binary cohesive parameter a12 is correlated in terms of the reduced pressure. A database containing experimental solubility data for 126 isotherms was used in this study. Half of the isotherms were judiciously selected to develop the correlations in the new combination rule. The rest of the isotherms were then used to validate the results. Detailed error calculations were carried out for different thermodynamic models that included the Peng-Robinson and Patel-Teja equations of state and van der Waals, cubic, and Rao mixing rules. The conclusion, after comparing the calculated errors for various models, was that the best results were obtained for the Patel-Teja EoS and the new mixing rule proposed here. This work is a significant contribution in the field in two ways. First, it provides a specific correlation that gives excellent values of solubility. Second, it proposes a novel approach that can be extended to other mixing rules and may result in a fully predictive method.