David W. Savage

Promotion of CO2 mass transfer in carbonate solutions

A general, physico-chemical analysis of mass transfer rate promotion in the system CO2-potassium carbonate-water-promoter is presented. Different possible mechanisms of promoter action including homogeneous catalysis, “shuttle” mechanism and surface reactions are discussed and classified. A unified picture of promoter chemistry is presented, showing that differences between inorganic and organic promoters are predominently quantitative, not qualitative. The “shuttle” mechanism is analyzed for absorption. The analysis leads to the prediction that the CO2 mass transfer rate may be influenced by the liquid hold up. This is related to the fact that, although the reaction in the interface region may be fast enough to enhance the mass transfer rate, the different reaction in the bulk may not be fast enough to maintain chemical equlibrium. This complex type of chemical absorption process has not been considered previously in the literature.

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.

Sterically-hindered amines for acid-gas absorption

Abstract This paper reviews sterlcally-hindered amines for removal of add gases such as CO2 and H2S from gaseous streams. Steric hindrance of amines reduces carbamate stability. Moderately hindered amines are characterized by high rates of CO2 absorption and high capacities for CO2. The moderately hindered amine in use with organic solvent has considerably higher capacity than the conventional amine-solvent system for simultaneous removal of CO2and H2S from synthesis gas and natural gas. A severely-hindered-amine absorbent, characterized by a very low rate of CO2absorption, has much higher capacity and selectivity than the current industry standard, methyldiethanolamine, for selective removal of H2S from CO2-containing streams. Use of hindered amines represents new advances in gas treating. Hindered amines save energy and capital in gas treating significantly. In addition, hindered amines used commercially have much better stability than conventional amines. As of today, fourteen commercial plants use hin...

Chemical absorption and desorption of carbon dioxide from hot carbonate solutions

Abstract Absorption and desorption rate data for the system CO 2 -hot carbonate solutions are presented. The data are interpreted on the basis of a film-theory model developed following the procedure recently presented by Astarita and Savage [1]. The agreement is very satisfactory. Values of the kinetic constant of the rate-determining step, previously known only up to a temperature of 40°C, have been obtained up to 110°C.

Zeolite composition for use in olefinic separations

Basically, the present invention is predicated on the discovery that zeolites that have a high silica to alumina ratio, e.g., a ratio of 20 and above, pore diameters of greater than about 5.0Å, and which have substantially no active acid sites, i.e., zeolites which are non-reactive toward olefin isomerization and oligomerization, are especially useful in substantially separating linear olefins and paraffins from hydrocarbon mixtures containing at least linear and branched aliphatic hydrocarbons and optionally containing aromatic and other hydrocarbons.

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.

Gas absorption and desorption with reversible instantaneous chemical reaction

Abstract The general problem of gas absorption and desorption accompanied by an instantaneous, reversible chemical reaction is approached from the viewpoint that chemical equilibrium prevails everywhere in the liquid phase. The approach allows absorption and desorption to be analysed along the same lines. A complete analytical solution is given for the film theory model and the penetration theory equations are set up in general form. The equations for the enhancement factor reduce to the classical “irreversible” result for absoption when the driving force, φ, becomes very large, rather than in the limit of very large equilibrium constant as the word irreversible seems to suggest. The corresponding asymptote for desorption is obtained by setting φ = 0. A third asymptotic region, in which φ is in the neighborhood of unity, is established for both absorption and desorption. Physically this is the region where driving forces are very small, which from an industrial viewpoint is what happens in absorption and desorption units near pinches. The asymptotic analysis for the pinch region is applied to hydrogen sulfide absorption and desorption from aqueous diisopropanolamine solution. For this system use of the “irreversible” equations grossly overestimates the enhancement factor. Gas-phase controls mass transfer at the lean end of the absorber but at the other terminal locations of the towers the liquid-phase resistance cannot be neglected.

Simultaneous absorption with reversible instantaneous chemical reaction

The approach presented recently [1] for analyzing absorption and desorption mass transfer problems with instantaneous chemical reaction is extended to the case of simultaneous absorption of two gases, A and A′. The analysis is developed for arbitrary stoichiometry. The following simple case is discussed in detail: A + B1⇌B2 A′ + B1⇌B3 where B1 is the liquid phase reactant and B2, B3 the reaction products. The analysis takes into account the “shift” reaction, which for the simple case above is: A + B3⇌A′ + B2 This reaction takes place in the region near the interface. The analysis differs from previous work which, with one exception, ignored the “shift” reaction and restricted attention to zero values of the concentrations of B2 and B3 in the bulk liquid. The analysis shows that the conditions where the physical driving forces (ai-ao) for absorption of both gases are large and positive does not imply that the chemical driving forces (αi-αo) are both positive. In fact, it is shown that cases arise where one component may desorb even though its physical driving force is positive. A simplified thermodynamic model useful for extrapolation of mixed CO2 and H2S equilibrium data in amine solutions to very low values of acid gas loading in solution is developed. Tower profiles for simultaneous absorption of CO2 and H2S in monoethanolmine solution are considered in light of the new analysis. The good kinetic selectivity measured for H2S at the absorber lean end is due to the fact that carbon dioxide is not absorbed in the instantaneous reaction regime. At the absorber rich end, where a temperature bulge develops, CO2 is absorbed in the instantaneous regime, causing H2S to be desorbed even though its physical driving force favors absorption.

Carbon-sulfur surface compounds—novel regenerable adsorbents for the removal of aromatics from aqueous solutions

Abstract Carbon-sulfur surface compounds, CxS, have been studied as novel adsorbents for the removal of aromatics from aqueous solutions using naphthalene, phenol, and resorcinol as model adsorbates. Equilibrium isotherms obtained at room temperatures show that CxS materials have adsorption capacities for the aromatics which are similar to those of Filtrasorb 300, a conventional activated carbon. The rate of adsorption for CxS is also similar to that for the activated carbon. However, spent CxS adsorbents can be regenerated thermally at milder temperatures. They can also be regenerated more completely than spent Filtrasorb 300 with organic solvents at room temperatures. These results suggest that energies of adsorption for the novel CxS material are lower than those for the conventional activated carbon. The difference in binding energy for aromatic compounds on these two adsorbents can be attributed to differences in surface groups through which the adsorbate is complexed. Spectroscopic characterizations on spent adsorbents support the donor-acceptor complex mechanism postulated by earlier investigators for the activated carbon.