Miguel C. Mussati

Life cycle assessment of corn-based ethanol production in Argentina

The promotion of biofuels as energy for transportation in the world is mainly driven by the perspective of oil depletion, the concerns about energy security and global warming. In Argentina, the legislation has imposed the use of biofuels in blend with fossil fuels (5 to 10%) in the transport sector. The aim of this paper is to assess the environmental impact of corn-based ethanol production in the province of Santa Fe in Argentina based on the life cycle assessment methodology. The studied system includes from raw materials production to anhydrous ethanol production using dry milling technology. The system is divided into two subsystems: agricultural system and refinery system. The treatment of stillage is considered as well as the use of co-products (distillers dried grains with solubles), but the use and/or application of the produced biofuel is not analyzed: a cradle-to-gate analysis is presented. As functional unit, 1MJ of anhydrous ethanol at biorefinery is chosen. Two life cycle impact assessment methods are selected to perform the study: Eco-indicator 99 and ReCiPe. SimaPro is the life cycle assessment software used. The influence of the perspectives on the model is analyzed by sensitivity analysis for both methods. The two selected methods identify the same relevant processes. The use of fertilizers and resources, seeds production, harvesting process, corn drying, and phosphorus fertilizers and acetamide-anillide-compounds production are the most relevant processes in agricultural system. For refinery system, corn production, supplied heat and burned natural gas result in the higher contributions. The use of distillers dried grains with solubles has an important positive environmental impact.

CONTROL STRATEGY EVALUATION FOR COMBINED N AND P REMOVAL USING A BENCHMARK WASTEWATER TREATMENT PLANT

The COST wastewater treatment plant simulation benchmark is a useful example for evaluation of control strategies aiming at improving biological nitrogen removal. In the SMAC project, the phosphorus removal process, based on the ASM2d model, was added as an extension to the existing benchmark wastewater treatment plant. This results in an example for evaluation of control strategies for combined nitrogen and phosphorus removal from wastewater, a necessity in the frame of the SMAC project. The phosphorus removal treatment plant is illustrated via a case study, where the addition of extra carbon source and metal salts for improving phosphorus removal are considered.

ANAEROBIC BIOFILM REACTOR MODELING FOCUSED ON HYDRODYNAMICS

This work deals with an experimental and theoretical investigation of anaerobic biofilm reactors for treating wastewaters. Bioreactors are modeled as dynamic (gas-solid–liquid) three-phase systems. The anaerobic digestion model proposed by Angelidaki et al. (1999) was selected to describe the substrate degradation scheme and was applied to a biofilm system. The experimental setup consists of two mesophilic (36°±1°C) lab-scale anaerobic fluidized bed reactors (AFBRs) with sand as inert support for biofilm development. The experimental protocol is based on step-type disturbances applied on the inlet substrate concentration (glucose and acetate-based feeding) and on the feed flow rate considering the criterion of maximum efficiency. The predicted and measured responses of biological and hydrodynamic variables are investigated. Experimental data were used to estimate empirical values of biofilm detachment coefficients. Under the evaluated operating conditions, the proposed model for biofilm detachment rate, assumed as a first-order function of the energy dissipation parameter, is appropriate to represent the interaction between biofilm systems and fluidization characteristics in non-highly disturbed flow conditions. Model validation was carried out using the experimental data reported by Mussati et al. (2006). The results do not differ from those above. This seems to indicate that the proposed AFBR model is able to reproduce the main biological and hydrodynamic successes in the bioreactor.

Modeling of real biological reactors for the treatment of complex substrates. Dynamic simulation

A mathematical model for the dynamic simulation of anaerobic bioreactors containing biofilm and degrading complex substrates is presented. The model allows to simulate different reactor configurations: single CSTR, multiple CSTRs and non-ideal reactors. Our simulator was developed on the basis of gPROMS, which allows to study a wide variety of problems such as optimal start-up, design under forced regimen and parameter estimations of the biofilm kinetic from experimental data. The digestion of cattle manure in different reactor configurations was selected as example to show the model and the simulator performances.

Simultaneous Flowsheet Optimization and Heat Integration of a Bioethanol Processor for PEM Fuel Cell System

Abstract The conceptual design of a bioethanol processor for fuel cell applications with heat integration was analyzed via model-based optimization formulation. Basically, the system consists of steam reforming of bioethanol and hydrogen purification process that are coupled to a proton electrolyte membrane fuel cell (PEMFC). The system was implemented within General Algebraic Modeling System (GAMS). The model simultaneously handles the problem of optimal heat integration while performing the optimization of process flowsheet. A 50 kW power generation system is presented as case study; the objective was to obtain the operative conditions for the process units that maximize the system efficiency. The operating variables of the system considered as decision variables were: system pressure, water-ethanol molar ratio and reforming temperature, input temperatures to hydrogen purification reactors, fuel cell temperature, and fuel cell hydrogen and oxygen utilizations. The results obtained predict a global efficiency of 43%, about 5% higher than the reference case. These results demonstrate the importance of simultaneous optimal heat integration for fuel cell-based processes.

Modeling and optimization of biological sequential batch reactors

Abstract This work deals with modeling and operation optimization of biological sequential batch reactors (SBR). The SBR is a fill-and-draw biological sludge system for wastewater treatment. In this system, wastewater is added to a single batch reactor, treated to remove undesirable components, and then, discharged. In this paper, a global model of a gas-solid-liquid SBR is presented to investigate and optimize operational strategies. The model can address the differences between aerated and anaerobic systems by assigning adequate parameter values related to the aeration and reaction systems. Fluctuating operation conditions during cycles such as disturbances in the organic loading rate, stirring rate and cycle time, result in strong numerical discontinuities that can be included in the simulation schedules. An existing set of experimental data is used to show a model application based on an anaerobic SBR. A good agreement was obtained between experimental and predicted values. Optimization results are based on minimizing the reaction time/total cycle time ratio subjected to path pH constraints and interior- and end-point constraints related to the pollutant removal efficiency and settling conditions. A decrease of 22% in the total cycle time, i.e. an increase in the organic loading rate from 787 to 985 mg dm−3 d−1 is reached without modifying the quality of effluent.

Optimal Synthesis of Heat Exchanger Networks Using Enthalpy-Temperature Functions to Describe Streams

Abstract This work presents a mixed integer non linear programming (MINLP) model for heat exchanger network synthesis including a detailed description of the process streams resorting to a mathematical function that matches temperature with enthalpy. This allows the model to account for non linear behavior of streams by taking into consideration their composition. To achieve the objective, the energy balance equation proposed in the “synheat” model by Yee and Grossmann (1990) is modified but equations involving temperature are kept the same. A new equation is added to that model to describe the relationship between temperature and enthalpy by correlation. As the original model, this modified MINLP model provides the network structure that minimizes the total annual cost. The mathematical program has non-convex equations, and only locally optimal solutions can be guaranteed. The approach between cold and hot streams has more detailed information because describes the real behavior of streams, and avoids matches that a linear functionality or a classical model may allow. Correlations between temperature and enthalpy are a function of the components present in a particular stream. The required information can be collected from bibliography or public databases as those provided by NIST (National Institute of Standards and Technology). The proposed methodology is a useful tool when the minimum temperature approach between streams is small. Examples are presented and discussed to compare results when streams are described by linear and non linear functionality between temperature and enthalpy.

Synthesis and Design of Combined Biological Nitrogen and Phosphorus Removal WWT Plants

Abstract In the present work, a previous superstructure model developed for simultaneous optimization of the process configuration and equipment dimensions- i.e., process synthesis and design- and the operation conditions of activated sludge wastewater treatment plants (Alasino et al., 2007) will be extended to account for phosphorus as well as nitrogen removal. Continuous operation is supposed, and the influent wastewater flowrate and composition are assumed known. The performance criterion selected is to minimize the net present value including investment and operating costs while verifying compliance with the effluent permitted limits. The Activated Sludge Model No. 3 extended with the Bio-P module for computing biological phosphorus removal is used to model the reaction compartments, and the Takacs model for representing the secondary settler. In the present work, new variables and equations for components accounting for phosphorus removal processes are incorporated and the superstructure is also enlarged to embed the most widely used combined N and P removal process configurations. Therefore, the complexity of the problem has been increased. The problem is posed as a NLP problem, specifically a nonlinear programming problem with discontinuous derivatives -DNLP-, as it results in a highly nonlinear system with non-smooth functions. The model is implemented and solved using General Algebraic Modelling System (GAMS), and proved to be robust and flexible. Results for case studies are presented and discussed.

Combined nitrogen and phosphorus removal. Model-based process optimization

Abstract An optimization model based on a superstructure embedding several activated sludge process configurations for nutrient removal is formulated and solved. Simultaneous optimization of the process configuration (process synthesis) and operation conditions for given wastewater specifications and influent flow rate in steady state operation are investigated. The performance criteria selected is the total annual operation cost minimization while predicting compliance with the effluent permitted limits. As the piece of equipment is supposed given, investment costs are not considered. The Activated Sludge Model No. 3 extended with the Bio-P module for computing biological phosphorus removal are used to model the reaction compartments, and the Takacs model for representing the secondary settler. The resulting mathematical model is a highly non-linear system, formulated as a Non-Linear-Programming Problem, specifically as a DNLP. The model is implemented and solved using GAMS and CONOPT, respectively. The optimal solution computed from the superstructure model provides cost improvements of around 10% with respect to conventional processes.

A three-phase fluidized bed anaerobic biofilm reactor model for treating complex substrates

Abstract The main purpose of this paper is to present a model of a three-phase solid-liquid-gas system to investigate the hydrodynamic and biological behavior and performance of fluidized bed anaerobic biofilm reactors (FBABRs). A general one-dimensional axial dispersive dynamic model is proposed for computing the variation of the properties such as hold-ups and superficial velocities of all phases, biofilm thickness and biological and chemical specie concentrations. Biochemical transformations are assumed occurring only in the fluidized bed zone but not in the free-support material zone. The biofilm process model is coupled to the hydrodynamic model of the system through the biofilm detachment rate, which is assumed as a first-order function of the energy dissipation parameter. Non-active biomass is considered as particulate material subject to hydrolysis. A scheme of carbohydrate degradation, kinetic parameters accepted in the literature and design characteristics of a hypothetical FBABR are taken into account to show the model predictions. The performance of the FBABR is analyzed for different flow patterns through different dispersion coefficients for the phases.