Gas Separation & Purification最新文献

Chemical solvent degradation during bulk removal of CO₂ has been studied for aqueous solutions of methyldiethanolamine (MDEA) and triethanolamine (TEA) promoted by piperazine (PIP) and morpholine (MOR), and for 40% aqueous solutions of diethanolamine (DEA). These solvents are either used commercially or have good commercial potential. Physical properties of the degraded solutions were measured. The study has been extended to determine the foaming characteristics of the solutions. The effectiveness of granular and powdered activated carbon in suppressing foaming was studied. The data generated by using accelerated degradation tests are of considerable interest to process engineers in the gas treating industry.

Both desorption and displacement kinetic experimental data of gases in activated carbon are presented in this article for ternary systems of ethane, propane and n-butane. Experiments are carried out under various temperatures, particle sizes and shapes. The collected data are compared with the predictions obtained from a multicomponent heterogeneous diffusion model proposed by Hu and Do (AIChE J (1993) 39 1628) using single-component equilibrium and mass transfer parameters.

Subject to the basic assumption of uniradial flow, a model has been established which allows the prediction of gas flow distribution in annular packed beds. Whilst of general application, the model has been used to simulate the flow distribution characteristics of a specific type of air filter incorporating an annular bed of activated carbon. All four possible flow arrangements have been investigated and it has been shown that U-type flow gives better flow distribution than Z-type flow.

A rigorous rate-based steady-state model of an absorber/desorber unit for the removal of CO₂ by amine solutions was developed. The model, which essentially contains no empirically assigned parameters, is based on a ‘mixing cell’ approach and considers both interphase heat and mass transfer along with simultaneous chemical reaction rates. This deterministic model, which simulates tray and packed columns at steady state, has been validated using the data of two commercial monoethanolamine (MEA) plants. For tray columns, each tray was considered to be a mixing cell, while for packed columns, the required dispersion was provided by choosing a finite number of mixing cells. For a specified CO₂ slippage rate, the effects of certain operating parameters, such as type and total concentration of amine employed, flow rate and composition of entering gas, on solution circulation rate and reboiler duty can be conveniently predicted.

The computer program, written in basic language, is designed in a user-friendly form and is a menu-driven interactive simulator. The mathematical model consists of non-linear algebraic equations and can be operated on a PC. A typical running time on an IBM PS/2 with 25 MHz was 4.39 s for the absorber and 1.1 s for the desorber.

Carbon membranes with 0.2 and 1.0 μm pore sizes are commercially available for liquid microfiltration applications. These membranes may be modified for gas separation applications by providing a gas separation layer with pores in the 1 to 10 nm range. With such pores, gases are separated by Knudsen diffusion with an individual gas species permeation rate inversely proportional to the ratio of the square root of the molecular weight of the permeating species. This paper describes some of the techniques used for depositing a suitable layer starting with various organic polymeric precursors. The in situ polymerization technique was found to be the most promising, and pure component tests with membrane samples prepared with this technique indicated Knudsen diffusion behaviour. The gas separation factors obtained by mixed-gas permeation tests were found to depend strongly on gas temperature and pressure indicating significant viscous flow at high-pressure conditions.

A ternary feed mixture ABC can be separated into individual components through the use of a main distillation column with a thermally linked side rectifier. To enhance such a separation, a heat pump can be implemented to transfer heat from the condenser at the top of the side rectifier to the reboiler at the bottom of the main column. In this paper, one such heat pump is described and applied to an air distillation system separating the ternary mixture containing nitrogen, oxygen and argon. The separation is performed by a conventional double column with a crude argon side column. When this system is operated at an elevated pressure to obtain higher product pressures, the separation of oxygen and argon becomes very difficult and leads to reduced argon recovery. The proposed heat pump enhances the separation by providing a supplementary crude argon condensing duty through the vaporization of a liquid oxygen stream from the bottom of the low pressure (LP) column. This scheme improves the liquid/vapour ratio (L/V) in the bottom section of the LP column and, more importantly, increases the vapour feed to the crude argon column. This increased feed rate leads to a substantial increase in argon recovery for the elevated pressure air distillation process.

The separation characteristics of silicone rubber membranes are determined for CO₂N₂ gas mixtures. The analysis is performed as a function of composition, flow rate and pressure of the feed gas. Results are presented in terms of the variation in component permeability and separation factor as a function of the above parameters. Component permeabilities are calculated using the complete mixing model. Data analysis over the studied pressure range shows that the permeability coefficient of pure CO₂ gas in silicone rubber is 15 times higher than that of pure N₂ gas. This behaviour is completely altered for a mixture of the gases, where the calculated separation factors at low feed pressures and low CO₂ mole fractions in the feed stream are two- to three-fold lower than the separation factors for the pure gases. At higher feed pressures and high CO₂ mole fractions in the feed stream, the above behaviour is reversed; the separation factors for the gas mixture are now higher than those for the pure gases. Comparison of the permeation characteristics of silicone rubber and cellulose acetate membranes for CO₂N₂ gas mixtures shows similar ranges and values for the gas permeabilities and separation factors. However, much higher separation factors are obtained for the cellulose acetate membrane in the case of pure gas permeation.

Studies on the preparation of carbonaceous sorbents from coal reject, a waste material generated in coal preparation processes, by pyrolysis and activation, and their applications to acidic gases removal are reviewed. The kinetics of coal reject pyrolysis and structural changes of coal reject as well as activation of coal reject chars are studied by both experiment and modelling. It is demonstrated that coal reject can be converted to effective adsorbents for NO_x and SO₂ removal. A two-consecutive-reaction model developed in this work can well describe the pyrolysis kinetics and confirm the softening effect on pore structure development during carbonization of coal reject. Structure-based kinetic models for the activation reactions are developed and validated by experimental data. Experimental and modelling studies of adsorption and reaction of sulfur dioxide on activated coal reject char are summarized. A macropore and surface diffusion model is successfully used to describe and predict the dynamic adsorption and desorption behaviour of SO₂ on coal reject-derived adsorbent. It is found that sorption of SO₂ in the presence of water vapour and oxygen enhances the capacity owing to sulfuric acid formation.

Studies were made on the membrane absorption of CO₂ and/or SO₂ using hydrophobic microporous hollow-fibre (HF) membrane modules. The absorbent liquids used were aqueous solutions of NaOH, K₂CO₃, alkanolamines and Na₂SO₃, flowing on the lumen side of the HF in laminar flow. A semi-empirical correlation was derived for the gas-phase mass-transfer coefficient on the shell side, by including geometrical factors of the HFs and the shell tube in the general correlation for mass transfer. It was found that the CO₂ absorption rate in various aqueous solutions of alkalis and alkanolamines is successfully described by a model based on gas diffusion through the membrane pores subsequent to gas absorption accompanied by chemical reaction. The simultaneous membrane absorption of SO₂ and CO₂ was also studied using aqueous Na₂SO₃ solution, the selective removal of SO₂ to CO₂ being successfully achieved when both the liquid flow rate and solute concentration are low. This suggests that this membrane absorption method provides an energy saving process for SO₂ removal from flue gases.