Gianni Astarita

CO2 absorption in aqueous solutions of hindered amines

Sterically hindered amines constitute a new class of chemicals which have recently come into industrial use in a variety of gas-treating processes: chemical solvents in both aqueous and non-aqueous solutions, rate promotion additives for the hot carbonate process, and chemical solvents for the selective removal of hydrogen sulfide. The scant published data on the behavior of hindered amines do not allow one to estimate the actual values of the apparent kinetic constants or equilibrium constants, and even less to establish the chemical steps involved. n nIn this paper, the results of an investigation of the behavior of one particular amine, 2-amino-2-methyl-1-propanol (AMP), as a chemical solvent for CO2 in aqueous solutions are reported. The equilibrium behavior of a hindered monoamine like AMP in aqueous solutions is dominated by the values of two equilibrium constants: the protonation constant Kp and the carbamate stability constant Kc. The value of Kp at infinite dilution has been determined experimentally, and is large enough to neglect formation of the carbonate ion. The value of Kc has been found experimentally to be significantly less than 10−1 1/gmol, as is expected for a hindered amine. n nA thermodynamic model has been developed and tested against experimental equilibrium data. Preliminary kinetic data seem to indicate that the reaction with CO2 is first order with respect to both CO2 and AMP. A first step in the elucidation of the chemical steps involved is presented.

Kinetics of carbon dioxide absorption in solutions of methyldiethanolamine

The kinetics of the reaction of carbon dioxide in MDEA solutions were studied both experimentally and theoretically. It is concluded that MDEA acts as a homogeneous catalyst for CO2 hydrolysis, and as a result the rate of absorption in aqueous MDEA solutions is significantly larger than one would calculate by simply taking into account the alkalinity of the reaction. A possible zwitterion mechanism is proposed for this reaction. The minor effect of ionic strength were also studied with the presence of other ions.

Diffusion-relaxation coupling in polymers which show two-stage sorption phenomena

Abstract The diffusion behaviour of swollen polymer films in interval sorption experiments cannot be explained satisfactorily by the available models for diffusion into polymeric materials, and compels us to accept that the state of the swollen polymer should be characterized not only by the state variables such as temperature and pressure but also by a ‘degree of swelling’. The concept of activity parameter as an internal state variable is introduced as an yardstick for the degree of swelling and the rate of evolution of this has been related directly to the concept of relaxation time. Assuming a simple empirical constitutive rate equation for the physical process involved, asymptotic as well as the complete solution of the proposed mathematical model are developed and the results agree well with the available experimental data on two-stage sorption.

Sample-dimension effects in the sorption of solvents in polymers — a mathematical model

Abstract It has recently been reported in the literature that, given a polymer-solvent pair, different sorption behaviors are observed according to the size of the sample. A mathematical model is developed for the coupled swelling-diffusion phenomenon in the polymer sample, which allows the prediction of such sample-dimension effects. Five different kinds of behavior are predicted for a given polymer-solvent pair, and the corresponding size bounds are related to the relevant physicochemical properties of the system. Comparison with a large body of literature data is satisfactory.

Analysis of entry flow to determine elongation flow properties revisited

The minimisation technique proposed by Binding (J. Non-Newtonian Fluid Mech., 27 (1988) 173) was used in our Generalised Engineering Bernoulli Equation framework to relate the entry pressure and stress power. We arrived at a final result similar to Bindings using assumed kinematics. Through subsequent assumptions to the kinematics we finally arrive at a result exactly equivalent to Cogswells technique (Trans. Soc. Rheol., 16 (1972) 383). Thus, these two techniques are related in this general framework. The techniques were used to predict elongation flow properties of a polymer melt and polymer solution. The results for the polymer melt clearly show Cogswells technique is adequate at high elongation rates. All these techniques require minimisation of the stress power with respect to the flow volume and discussion is given as to the validity of this minimisation technique. In addition, the approximate variational technique we propose gives clears limits as to when a technique, such as Cogswells, can be applied.

Theory of chemical desorption

Abstract The general problem of desorption of a volatile component accompanied by chemical reaction is considered. Basic points of difference between chemical absorption and desorption are emphasized. The conditions required for chemical absorption theory to apply to chemical desorption are established. A general procedure for solving chemical desorption problems when the rates of reaction are very high is developed; the procedure applies up to and including the limit where the reactions are instantaneous.

Coupled diffusion and morphological change in solid polymers

Abstract Sorption of low molecular weight solutes in glassy polymers is known to often result in swelling of the polymer phase. The swollen polymer may on occasion crystallize. One thus has a phenomenon where three different rate processes take place simultaneously and are coupled with each other: diffusion, swelling and crystallization. Correspondingly, there are as many as four time scales: the one for diffusion, the swelling time, the crystallization time, and the relaxation time for the mass flux; the last is shown to be crucial to the possible development of discontinuities and quasi-discontinuities in the state of the polymer. A model has been developed to take into account all these phenomena, and experimental data are successfully compared with the model predictions. Specific numerical techniques have been developed to deal with the resulting quasi-linear hyperbolic equations.

Variational principles and entropy production in creeping flow of non-newtonian fluids

Abstract The extension of Helmholtz—Kortewegs theorem to purely viscous non-Newtonian fluids yields a variational principle for steady creeping flow for a rather wide class of fluids. Only for a restricted subclass (Newtonian and power-law fluids) does the quantity to be minimized coincide with the total rate of entropy production. It is argued that the Helmholtz—Korteweg theorem is a purely mechanical result which has no significant thermodynamic content.

Selective absorption of hydrogen sulphide in tertiary amine solutions

Abstract An approximate model for the rate of simultaneous absorption of H2S and CO2 into aqueous solutions of a tertiary amine is presented, and the model is successfully compared with experimental results of simultaneous absorption. An effect which is not taken into account in the model is shown to become possibly important when the liquid phase is almost a solution of only free amine, and an approximate method of taking such an effect into account is presented. The procedure discussed will be used in a forthcoming work for developing a methodology of designing the required height for a packed absorber for selective removal of H2S.

Design of packed towers for selective chemical absorption

Abstract The selectivity obtained in industrial processes of gas treating with chemical solvents is of kinetic nature, and accurate procedures for designing the required packed height are therefore needed. A procedure for packed towers is presented here, which is based on the model of simultaneous absorption presented in a previous paper. The procedure takes into account the absorption of two gases, heat transfer and water evaporation.