THERMODYNAMICS OF ALKANOLAMINE + WATER SYSTEM

Design of gas treating processes requires knowledge of the vapor-liquid equilibrium behavior of the (acid gas + aqueous alkanolamine) system. The present study is focused on thermodynamics and associated nonideal behavior of binary MEA + H₂O, DEA + H₂O, and MDEA + H₂O systems, which is required to predict the vapor-liquid equilibrium of acid gases such as CO₂ and H₂S over aqueous alkanolamine solutions. Determination of binary interaction parameters and analytical prediction of infinite dilution activity coefficient, heats of solution at infinite dilution, the excess Gibbs free energy, and excess enthalpy for nonideal alkanolamine-water systems are the objectives of this study.     

Detoxication of nitrose gases formed in the production of adipic acid: The two-stage catalytic cleaning process

A two-stage process was developed for the catalytic cleaning of the nitrose gases formed in the production of adipic acid. The process involves the decomposition of N₂O over a Co-ZSM-5-type catalyst and the reduction of NO _x with ammonia over a V₂O₅/Al₂O₃ catalyst. The proposed process provides for the cleaning of the effluent gases from N₂O and NO _x at an efficiency of over 99% and has the advantage of natural gas economy over the thermal process. The Co-ZSM-5-type zeolite was used to make an N₂O decomposition catalyst characterized by high moisture resistance. The kinetic characteristics of the process were determined, and hydrothermal endurance tests conducted with the catalysts (Co-ZSM-5 and V₂O₅/Al₂O₃) were recommended for the process. Technical and design parameters were established for the catalytic reactors forming part of the system for the cleaning of the effluent gases in the production of adipic acid.     

Simultaneous Absorption of H2S and Co2 Into Aqueous Methyldiethanolamine

Abstract

The tertiary araine methyldiethanolamine (MDEA) is finding increasing application as a chemical solvent for selective absorption of hydrogen sulfide from gases containing hydrogen sulfide and carbon dioxide. Gas streams of this type include some natural gases, synthetic gases from coal and heavy oil gasification and tail gases from sulfur plants. Selectivity for H2S is needed either to enrich Glaus sulfur plant feed in H2S or to remove only H₂S when CO₂ removal is not necessary or economic. For the absorption of hydrogen sulfide into MDEA, the reaction which occurs can be considered to be instantaneous while carbon dioxide undergoes a second-order reaction with MDEA.

In this work, the simultaneous absorption of two gases into a liquid containing a reactant with which both gases react is modelled using the film theory. Physical properties and kinetic rate parameters used in the model have been measured in our laboratory. The model is used to study the effect of process variables on the selectivity of MDEA for H₂S over C0₂. The simultaneous absorption of H₂S and CO₂ gases into aqueous MDEA is studied experimentally using a continuous stirred tank absorber. Experimental absorption rates are compared to predictions based on a multicomponent mass transfer model. The average deviations of the theoretical calculations from the experimental results are 10.2% and 12.9% for C0₂ and H₂S, respectively.     

Absorption of CO2 and H2S in aqueous alkanolamine solutions using a fixed-bed reactor with cocurrent downflow operation in the pulsing flow regime: Absorption von CO2 und H2S in wäßrigen alkanolamin-lösungen in einem Festbettreaktor bei abwärtsfließendem Gleichstrombetrieb und pulsierendem Arbeiten

Absorption rates of H₂S and CO₂ in several aqueous alkanolamines in a cocurrent downflow fixed-bed reactor operated in the pulse flow regime have been measured in order to obtain information on the potential selectivity and on the mass transfer parameters. From these experiments it can be concluded that this type of reactor seems to be very suitable for the selective removal of H₂S from acid gases.It was not possible to derive correlations which are always valid for the calculation of the mass transfer parameters in laboratory-scale contactors. The results of the present study in combination with the data published in the literature were correlated with only one unknown parameter in which the influence of the physical properties of the system and the shape and material of the packing were combined. This parameter has to be determined empirically.Small amounts of acid gases in aqueous alkanolamine solutions have a pronounced effect on the properties, such as the foaming behaviour, of the liquid phase. The pressure drops measured in the cocurrently operated fixed-bed reactor were affected substantially by small amounts of acid gases present in the aqueous alkanolamine solutions and up to now this effect could not be explained satisfactorily.For the design of contactors for industrial purposes, detailed scale-up rules are required. In the present study this aspect is not investigated.     

Verfahrenstechnische und wirtschaftliche Aspekte der Verarbeitung verdünnter SO2-Gase zu Schwefelsäure

Chemical engineering and economic aspects of converting dilute SO₂ gases into sulphuric acid. In the contact process for production of concentrated sulphuric acid the lowest limit for the autothermal processing of dilute SO₂ gases lies at around 2 vol.-% SO₂ provided that a pre-drying concentrating system is in use. Yet more dilute SO₂ gases can be processed by Ciba Geigy's nitrogen oxide/sulphuric acid process – a modern version of the well-known lead chamber process – which is already producing 70 to 75% H₂SO₄ from molybdenum disulfide roasting waste gases of ca. 1 vol.-% SO₂ on an industrial scale. The paper considers not only the specific boundary conditions, and chemical engineering characteristics of the two fundamentally different processes but considers above all the economics of processing low-concentration SO₂ gases for sulphuric acid.     

Intensification of nitrous acid oxidation

An alternative solution to the reduction of a discharge of residual nitric oxide and nitrogen dioxide into atmosphere has been proposed. Instead of using methane or ammonia for SCR or gas absorption into alkali solutions, which are the most popular treatment methods of tail gases, now the use of powerful oxidant—ozone capable of transforming nitrous acid and nitric oxides into nitrogen of the highest oxidation level—could be employed for this purpose. As the intensive oxidation and ozonation of nitrous acid is the heterogeneous gas-liquid process, the solubility of oxygen and ozone in HNO₂/HNO₃ aqueous solution was necessary to be determined. Variations of reaction rates depending on temperature, ozone dose and nitrous and nitric acid concentrations were studied experimentally. The kinetic model of the reactions, 2HNO₂+O₂→2HNO₃ and HNO₂+O₃→O₂+HNO₃, were proposed and the kinetic parameters (rate constants and activation energies) were estimated on the basis of experimental data in semi-batch laboratory gas-liquid contactor with the liquid phase drawn from an absorption column in the nitric acid plant. The determined kinetic parameters were then used in designing and modeling of the oxidation of nitrous acid using ozone-oxygen mixture in a continuous bubble column. The model consists of mass transfer kinetic equations and material balance equations for the gas and liquid phases. The co-current flow of gas and liquid phases and the complex kinetics of chemical reaction in the liquid phase were taken into account. The variation of the following process conditions, flow rate, compositions of the gas and liquid phases, temperature, and pressure in the bubble column of different diameters and heights, were studied in numerical solutions of the proposed model.     

Reduction of CO2 emissions by a membrane contacting process☆

A membrane-based gas–liquid contacting process was evaluated in this work for CO₂ removal from flue gases. The absorption of CO₂ from a CO₂–N₂ mixture was investigated using a commercial hollow fiber membrane contactor and water or diethanolamine as absorbing solvents. Significant CO₂ removal (up to 75%) was achieved even with the use of pure water as absorbent. By using aqueous amine solutions and chemical absorption, mass transfer improved, and CO₂ removal was nearly complete (∼99%). A mathematical model was developed to simulate the process and it was validated with experimental data. Results show that membrane contactors are significantly more efficient and compact than conventional absorption towers for acid gas removal.     

New absorption liquids for the removal of CO2 from dilute gas streams using membrane contactors

A new absorption liquid based on amino acid salts has been studied for CO₂ removal in membrane gas-liquid contactors. Unlike conventional gas treating solvents like aqueous alkanolamines solutions, the new absorption liquid does not wet polyolefin microporous membranes. The wetting characteristics of aqueous alkanolamines and amino acid salt solutions for a hydrophobic membrane was studied by measuring the surface tension of the liquid and the breakthrough pressure of the liquid into the pores of the membrane. The dependence of the breakthrough pressure on surface tension follows the Laplace-Young equation. The performance of the new absorption liquid in the removal of CO₂ was studied in a single fiber membrane contactor over a wide range of partial pressures of CO₂ in the gas phase and amino acid salt concentrations in the liquid. A numerical model to describe the mass transfer accompanied by multiple chemical reactions occurring during the absorption of CO₂ in the liquid flowing through the hollow fiber was developed. The numerical model gives a good prediction of the CO₂ absorption flux across the membrane for the absorption of CO₂ in the aqueous amino acid salt solutions flowing through the hollow fiber.     

Simulation of CO<Subscript>2</Subscript> removal in a split-flow gas sweetening process

Split-flow gas sweetening is known to consume less energy than a conventional gas sweetening process when the inlet sour gas contains a high concentration of acid gases. In this work, a computer simulation of a split-flow natural gas sweetening process based on absorption/stripping process with alkanoamine (MEA and DGA) solutions, using Aspen plus, was performed. The input of parameters such as the concentration of sour gases (CO₂, H₂S) in the feed gas has been examined. Simulation results show that the split-flow gas sweetening process can reduce the reboiler duty of a stripping tower better than the conventional gas sweetening process according to the concentration of CO₂ in the feed gas.     

Activation energy variation for catalytic oxidation of aqueous SO2

SO₂ oxidation in aqueous solutions catalysed with manganous sulphate was studied to determine temperature dependencies of the reaction rate. The process was carried out at relatively high sulphuric acid (reaction product) concentrations with regard to its application for SO₂ removal from waste gases. Variation of the apparent activation energy has been linked with alteration of reaction rate determining steps.     

Zum Stand der Schwefelsäure-Produktion

Sulphuric acid production – state of the art. Apart from pyrites and other metal sulfides, sulphur is mainly used today as raw material for the production of sulphuric acid. The oxidation of roaster gases or sulphur combustion gases is performed almost exclusively in multistage reactors over catalysts containing vanadium pentoxide. SO₂ conversions of more than 99.5% are achieved by intermediate absorption of SO₃ (“double catalysis”) and by using efficient catalysts. To further improve the economics of the manufacturing process, new technologies are being tried out, and attempts are being made to develop even more active catalysts. Although there is still some uncertainty regarding the kinetics of SO₂ oxidation, empirical findings can be explained quite satisfactorily with existing models.     

Computational simulation of gas separation using nonporous polymeric membranes: Experimental and theoretical studies

In this study, preparation and simulation of polydimethylsiloxane (PDMS) membranes for gas separation is carried out. The membranes are synthesized by solution‐casting method via silicon oil as precursor. Gas permeation experiments for single gases of CH₄ and N₂ were conducted at different feed pressures (2–10 bars). PDMS membrane as a rubbery polymer showed that are more permeable toward more condensable gases, i.e., CH₄ compared to N₂. It was indicated that increasing feed pressure enhances permeability of CH₄ through the membrane slightly, but the permeability of nitrogen was almost constant over enhancement of feed pressure. Moreover, a mathematical model was developed to predict the permeation of gases across PDMS membrane. The model is based on solving conservation equations for gases in the membrane phase. Finite element analysis was utilized for numerical simulation of the governing equations. The simulation results were used to predict the concentration of gases inside the membrane. POLYM. ENG. SCI., 55:54–59, 2015. © 2014 Society of Plastics Engineers     

Equilibria of H2S and CO2 in triethanolamine solutions

The equilibrium solubility of H₂S and CO₂ has been measured in 2.0, 3.5 and 5.0 mol/L triethanolamine solutions. Data were obtained at temperatures of 25°, 50°, 75°, 100° and 125°C at partial pressures of the acid gases ranging from 0.01 to 6360 kPa. Enthalpies of solution were calculated from the solubility data.     

Simulation of NOx gas absorption under adiabatic condition and comparison with plant data

A mathematical model has been developed for the absorption of NO_x gases in aqueous solutions of NaOH. The present model is for adiabatic operation, which is an extension of the previous isothermal model of Pradhan and Joshi (Chem. Eng. Sci. 55(7) (2000) 1269). Heat effects have been considered for gas phase reactions such as the oxidation of NO, dimerisation of NO₂, and formation of N₂O₃, HNO₂ and HNO₃. Absorption of NO₂, N₂O₃, HNO₂(g), HNO₃(g), into NaOH solutions is accompanied by large heat effects which have been incorporated in the model. Evaporation/condensation of water has also been considered. A comparison between isothermal and adiabatic operation has been presented. The model was validated using data from an operating small-scale plant which uses the process of absorption of NO_x in NaOH and a good match between plant and simulation results was obtained. Minor modifications were suggested in the existing plant which would improve the selectivity of the product NaNO₂.     

Modeling the Simultaneous Transport of Two Acid Gases in Tertiary Amines with Reversible Reactions

Abstract

The objective of this work is to develop a model for the simultaneous mass transfer of two acid gases in tertiary amines accompanied by reversible chemical reactions. The model has been applied to the industrially important system of simultaneous. absorption or desorption of CO ₂ and H ₂ S in aqueous methyldiethanolamine (MDEA). In most applications the treated gas must be virtually free of H ₂ S; however, it is often not necessary or economical to remove substantial amounts of CO ₂. Hence, selective removal of H ₂ S from gas streams such as natural or synthetic gases which contain CO ₂ is desirable.     

Solvent selection for CO<Subscript>2</Subscript> capture from gases with high carbon dioxide concentration

Amine absorption processes are widely used to purify both refinery and process gases and natural gas. Recently, amine absorption has also been considered for application to CO₂ removal from flue gases. It has a number of advantages, but there is one major disadvantage-high energy consumption. This can be solved by using an appropriate solvent. From a group of several dozen solutions, seven amine solvents based on primary amine, tertiary amine and sterically hindered amine were selected. For the selected solutions research was conducted on CO₂ absorption capacity, an absorption rate and finally a solvent vapor pressure. Furthermore, tests on an absorber-desorber system were also performed. In this study the most appropriate solvent for capturing CO₂ from flue gases with higher carbon dioxide concentrations was selected.     

SOLUBILITY OF CO2AND H2S IN A MIXED SOLVENT

Mixed solvents are a combination of chemical and physical solvents and have some advantages over traditional treating solvents for the removal of acid gases from gas streams. The solubility of H₂S and CO₂in a mixed solvent consisting of AMP (2-amino-2-methyl-l-propanol), sulfolane, and water has been measured at 40 and 100°C at partial pressures of the acid gas to 6000 kPa. The solubility in the mixed solvent was compared with the solubility in an aqueous solution of equivalent amine concentration. At solution loadings less than 1 mol acid gas/mol amine, the solubility of the acid gas is lower in the mixed solvent than in the corresponding amine solvent. At higher loadings, the trend is reversed and the solubility is greater in the mixed solvent. The results are rationalized in terms of the effect of the physical solvent component on the chemical reaction and physical vapor-liquid equilibria. The solubility model of Deshmukh and Mather was used to correlate the data.     

Ozone Solubility in Aqueous Solutions of NaCl,Na2SO4, and K2SO4: Application of the Weisenberger-Schumpe Model for Description of Regularities and Calculation of the Ozone Molar Absorption Coefficient

The ozone solubility in aqueous solutions of salts, NaCl (up to 5 М) and Na₂SO₄ (up to 2 М) and their mixtures, was studied. The results are adequately described by the semi-empirical model proposed by Weisenberger and Schumpe for calculation of the solubilities of atmospheric gases in concentrated solutions of electrolytes. By comparing the data obtained experimentally and calculated in terms of the Weisenberger and Schumpe model, the ozone molar absorption coefficient was found to be ε = 2860 ± 200 M^?1 cm^?1 at 260 nm.     

Determination of the combustion properties of natural gases by pseudo-constituents

This paper presents a methodology for a rapid determination of important natural gas combustion properties (lower heating value, Wobbe index and the stoichiometric air-fuel ratio) using easily detectable physical properties. It is possible to determine natural gas composition by measuring two physical properties and using specific ternary diagrams (CH₄-C₂H₆-C₃H₈ and CH₄-C₂H₆-N₂). The first part of the work deals with the selection of two physical properties from a group comprising thermal conductivity, refraction index, and speed of sound. Then, in the second part, a sensor using the best couple of physical property is used to determine the ternary pseudo-constituents of the gas mixture. The model and the sensor are applied to specific situations such as the online determination of LHV. The error on the combustion properties of natural gas is less than 1% over the gases considered in the present study and over about 20 typical gases supplied over Europe. The effect of small errors in the measurement of physical properties has also been highlighted.     

An oxygen sensor based on the semiconducting properties of (U0.3Y0.7)O2−x

Urania-yttria fluorite solid solutions with a U/Y ratio below 0.33/0.67 and having n-type conductivity are suggested as oxygen sensing materials. The electrical properties of one composition, (U_0.3Y_0.7)O_2–x , have been studied in detail. The resistance of this composition follows a (pO₂) ⁿ relationship over an oxygen concentration range of 0.1 to 100% and shows long-term stability. The value ofn decreases with increase in temperature (0.23 at 400° to 0.15 at 800°C), but is reproducible for a large number of specimens. A method for determining oxygen partial pressure in hot gases using these urania-yttria solid solutions as the sensing material is described.