José A. Caballero

Pyrolysis kinetics of almond shells and olive stones considering their organic fractions

The kinetics of thermal decomposition of two lignocellulosic materials (olive stones and almond shells) were studied using dynamic TG at heating rates between 2 and 25 K min−1 at atmospheric pressure. Different kinetic models were tested, that consider that the thermal decomposition of lignocellulosic materials can be grouped in two or three overall processes (each one being the result of a great number of reactions) corresponding to the hemicellulose, cellulose and lignin decompositions. The best results were obtained with a model that considers that the biomass decomposes via three independent reactions. There is a qualitative agreement between the decomposition of hemicellulose, cellulose and lignin with the peaks observed in the TG-DTG diagram, but the simple addition of the kinetics of isolated compounds cannot satisfactorily reproduce the kinetic behaviour of any raw material, and consequently some interactions must exist between the fractions considered.

Comments on the validity and utility of the different methods for kinetic analysis of thermogravimetric data

This work deals with the kinetic analysis of data obtained in thermobalance. It consists of a review of kinetic models used for material decomposition, and also of the methods used for the analysis. The topics presented comprise numerical problems related with kinetic analysis, best conditions for a kinetic run, discussion about correlation vs. actual models, nth order models, more complex models (models using a great number of reactions, models using various fractions), calibration of the temperature, position of the thermocouple, sample mass and particle size. Other topics treated are the validation of kinetic models using MS data and the different models available for kinetic studies in thermobalance.

Characterization of sewage sludges by primary and secondary pyrolysis

An apparatus formed by the combination of a primary pyrolyzer (Pyroprobe 1000) and a secondary reactor was used to study the thermal decomposition of three different chemical sewage sludges, using temperatures in the secondary reactor between 290 and 650°C and 700°C in the primary pyrolzer. Yields of 12 pyrolysis products were determined (methane, ethylene, ethane, propylene, propane, methanol, acetic acid, acetaldehyde, C4-hydrocarbons, CO, CO2 and water). The pyrolysis of chemical sewage sludge is an alternative to incineration because it presents the advantage of concentrating the heavy metals in the final residue, and of avoiding the formation of toxic organic compounds. This technique allows the liquids formed in primary pyrolysis to be transformed almost completely into light gases that do not present significant environmental problems. Only about 1–2% of liquids remains without cracking. The gases formed are mostly CO, CO2 and water. Hydrocarbons yields are only around 1–2% wt%, on a dry basis of the sludge. The paper presents the characterization of three sewage sludges by pyrolysis using simple equipment that can be useful for studying any type of wastes.

Mathematical programming approaches to the synthesis of chemical process systems

This paper presents a review of advances that have taken place in the mathematical programming approach to process design and synthesis. A review is first presented on the algorithms that are available for solving MINLP problems, and its most recent variant, Generalized Disjunctive Programming models. The formulation of superstructures, models and solution strategies is also discussed for the effective solution of the corresponding optimization problems. The rest of the paper is devoted to reviewing recent mathematical programming models for the synthesis of reactor networks, distillation sequences, heat exchanger networks, mass exchanger networks, utility plants, and total flowsheets. As will be seen from this review, the progress that has been achieved in this area over the last decade is very significant.

Thermogravimetric studies on the thermal decomposition of polyethylene

Abstract The thermal decomposition of polyethylene was analyzed by dynamic and isothermal experiments. Dynamic runs with different heating rates, exposed surface and initial mass were carried out. Isothermal runs of different initial mass were also carried out. Several kinetic models were tested and the best fit to the experimental thermogravimetry curves was obtained with a model involving a surface zero-order reaction.

Design of distillation sequences: from conventional to fully thermally coupled distillation systems

Abstract This work presents a novel superstructure optimization approach for the design of distillation sequences of zeotropic mixtures. The approach considers as alternatives from conventional sequences in which each final distillation column has a condenser and a reboiler, to fully thermally coupled distillation sequences going through all possible intermediate combinations. A two-stage procedure is proposed. In the first one, a sequence of tasks is selected and then in a second stage the best configuration of actual columns is extracted among all the thermodynamically equivalent configurations. The model uses the Underwood–Fenske–Gilliland approximation and is formulated as a generalized disjunctive programming problem. The model has proven to be very robust and reliable. This formulation also makes the model very flexible and allows the easy modification of the equations and even migration to other model types including rigorous tray-by-tray models. Rigorous simulations in HYSYS [AEA Technology, Hyprotech Ltd., Calgary, Canada, 2001] have shown good agreement with the shortcut model.

Kinetic model for the combustion of tyre wastes

Abstract The decomposition of tyre wastes at different heating rates in an oxidizing atmosphere is explained by means of a kinetic model including a first step of pyrolysis, which assumes three organic fractions not forming residues, and a step of combustion. The atmosphere used varied from 10 of oxygen to 20% (v/v). The activation energy for the step of combustion is in the range 221–235 kJ/mol, and there exists a dependence of the rate of decomposition on the partial pressure of oxygen.

Thermogravimetric analysis of olive stones with sulphuric acid treatment

Abstract The thermal decomposition of olive stones, with different time-lengths of 72% sulfuric acid treatment (from olive stone to klason lignin), was studied by thermogravimetry (TG-DTG). The TG curves indicate that the isolation of lignin begins with the easy elimination of hemicellulose and some of the cellulose, while the lignin fractions remain practically unaltered. More intensive treatment produced the progressive removal of the cellulose, but the formation of volatiles at relatively low temperatures indicated that thermal resistance had decreased due probably to the breakdown of the three-dimensional structure of lignin. However, even with prolonged treatment, it is difficult to remove all the cellulose and even when this is removed, the lignin structure is greatly altered. A method based on the kinetic analysis of the thermal decomposition is proposed to characterize the isolation degree of klason lignin and to monitor alterations in its structure.

Study of the primary pyrolysis of Kraft lignin at high heating rates: yields and kinetics

The primary pyrolysis of Kraft lignin was studied in a Pyroprobe 1000 apparatus. Experiments were carried out at a nominal heating rate of 20 °C ms−1 over a range of temperatures between 450 and 900 °C and for several pyrolysis times (1–30 s). The evolution with temperature of the main gaseous and volatile products was analysed. A mathematical model that takes into account physical phenomena (heat transfer limitations in the heating and cooling stages), the kinetics of thermal decomposition and the endothermic character of the reaction was developed to explain the experimental data. In the kinetic analysis, the C function model for primary decomposition was used. This model assumes that lignin is formed by a large number of fractions, each one of which begins to decompose at a characteristic temperature.

Comparative study of the pyrolysis of almond shells and their fractions, holocellulose and lignin. Product yields and kinetics

A comparison between the thermal decomposition of almond shells and their components (holocellulose and lignin) was carried out, considering the yields of the most important products, under flash conditions, and the decomposition kinetics. The yields of the main gaseous products obtained in the fast pyrolysis of almond shells can be reproduced from the yields obtained with holocellulose and lignin. The best results were obtained with CO, water and CO2. The differences were greater with the minor hydrocarbons, CH4, C2H6, C2H4, etc. The kinetics of the slow thermal decomposition (TG-DTG) of almond shells cannot be reproduced by the sum of lignin and holocellulose. The cellulose from almond shells decomposes at lower temperatures than almond shells, and the behavior of isolated lignin is very different from that found when it forms part of the raw material, proving the importance of the interactions between its components.