Gas Separation & Purification最新文献

Equilibrium and kinetic parameters for methane and nitrogen on a new, high specific area active carbon Maxsorb are reported. Volumetric and chromatographic methods are used to measure the pure component adsorption isotherm and the effective mass transfer coefficient for each gas. The adsorption isotherms at 300 K, measured up to a pressure of 550 kPa, are approximately linear for both methane and nitrogen on Maxsorb. The equilibrium separation factor is 3.0 in favor of methane. The mass transfer resistance is observed to be very low for each sorbate. The equilibrium and kinetic parameters are input in the mathematical model of binary breakthrough experiments using an axial dispersion model. The theoretical and experimental breakthrough curves are observed to be in excellent agreement.

Thermally coupled distillation is well-proven in low temperature gas processing where energy is relatively expensive. Although the thermodynamic efficiency of thermally coupled columns is similar to conventional columns, thermal coupling reduces energy flows. This can lead to attractive savings in both energy and capital cost. This paper shows that the conventional Fenske-Underwood-Gilliland method for short cut distillation design is as appropriate for thermally coupled columns as it is for conventional distillation. This enables comparisons of conventional and thermally coupled columns to be made easily, and should help to make thermal coupling a more accepted means of improving energy efficiency in distillation systems.

The status of technological developments for the separation and purification of CO from industrial gas streams is discussed. The scope and prospects for adsorptive and membrane separation processes are highlighted. An assessment has been made of the most promising technology, which is dependent on feed gas type and process conditions.

Adsorption of N₂, CO, CO₂ and NO has been studied on various molecular sieves using the gas chromatographic method to determine the potential for separation of these common atmospheric contaminants from air. The molecular sieves studied include H-Mordenite, 4A and 5A zeolite, a natural clinoptilolite and an activated carbon. Henry's law constants have been determined over a variety of temperature ranges from 243 to 473 K. Van't Hoff plots are presented for CO on all materials and for NO on all but 4A zeolite. Adsorption of CO₂ on the clinoptilolite was too strong to produce an interpretable response peak. Results of CO adsorption on 4A and 5A zeolites have been compared to and are supported by data available in the literature. Heats of adsorption for CO, NO and N₂ were determined. For CO the heats of adsorption decrease in the order of clinoptilolite > 5A zeolite > 4A zeolite > H-Mordenite > activated carbon. For adsorption of NO the heats of adsorption decrease in the order of clinoptilolite > 5A zeolite > activated carbon. Separation factors are presented for the CO/N₂ and NO/N₂ systems. The natural clinoptilolite shows most promise for the separation of CO and NO from N₂ at the temperature range 273–398 K. Diffusion coefficients for CO and N₂ on clinoptilolite between 348 and 423 K were also determined. Micropore diffusion proved to be the dominant mass transfer mechanism for both CO and N₂ in clinoptilolite under the conditions examined.

Annular packed bed configurations are used in many different systems and the radial packing region is not always composed of a single layer of material. This work is based on a commercial air filter which was found to have several different layers within the radially packed region, comprising a particulate filter and an activated carbon layer as the main flow resistances. To enable modelling of such a filter, an existing model has been extended to predict flow distributions and pressure profiles for annular packed bed systems. The enhanced model predicts the trends and orders of magnitude of experimentally obtained pressure profiles and overall pressure drops for three air flow rates through a commercial air filter.

Results obtained in a batch adsorption system with constant volume showed that preadsorption of small amounts of propane on NaA zeolite can significantly slow down the rate of carbon dioxide uptake. The observed decrease in the carbon dioxide uptake rate was related to a decrease in accessibility of the external surface of the microcrystallites due to the effect of an external film resistance created by the preadsorbed propane molecules. Small amounts of preadsorbed propane localized inside the zeolite microcrystallites were found to have no affect on the CO₂ adsorption rate. Experiments carried out using the frequency response technique confirmed the significant influence of the external film resistance on the kinetics of adsorption of carbon dioxide/propane mixtures on 4A zeolite even at low propane concentrations (2–3%).

A new process concept is outlined to utilize low and high pressure feed streams efficiently in a single pressure swing adsorption (PSA) process. The low pressure feed is processed first in the PSA unit to produce low pressure product. Following this, the high pressure stream is processed in the same unit. Low pressure generated in the first part of the process is used internally in the process to assist the production of high pressure product. Essentially 100% recovery of the high pressure product is possible. This is achieved in a single unit without the need to store gas product during the intermediate process steps. The process concept is demonstrated for pure hydrogen production. A process is simulated in which two streams, both containing 25% methane and 75% hydrogen at two different pressures, are supplied to a single PSA unit. The simulation shows that, using a five-bed process, one can produce 100% high pressure hydrogen at ultra-high purities (i.e. ppm levels of methane in the product). If the low pressure stream is not integrated, then the maximum high pressure H₂ recovery is only 85%.

In certain adsorption processes, several different adsorbents may be used, each more or less specialized for a particular task of selectively taking up certain components. The different adsorbents may be packed in different columns, or, alternatively, in the same column as superposed layers. In the latter case, they necessarily experience the same conditions in terms of throughput, pressure and temperature. The dynamic behaviour, and therefore the optimal design and operation of such beds, is different from that of single adsorbent beds. The passage of concentration waves from one layer to the other and the flow reversal induce particular refraction, dispersion and interference phenomena. We propose a method of analysis and representation of these phenomena, based on pseudo-characteristics defined as lines propagating constant values of particular concentrations. The pseudo-characteristics do not in general coincide with the classical lines generated by the method of characteristics within the framework of equilibrium models, except in some ideal situations (quasi-equilibrium without dispersion and without pressure and velocity changes). These lines can be generated by computer simulations using non-ideal complex models, and offer a convenient and compact way to analyse the start-up and convergence towards the cyclic regime, to represent the essential features of the concentration profiles at all times, to adjust the operating times and to optimize the layer thickness. This analysis is illustrated using the example of hydrogen purification from a four-component mixture using a two-bed two-adsorbent six-step PSA process.

The water adsorption capacity of desiccant materials may be affected significantly if particulates are present in the air that is to be processed. Air containing environmental tobacco smoke was used to study the effect of particulates. A smoking apparatus was designed by simulating the puffing of a cigarette by a person to generate environmental tobacco smoke. The apparatus can take a puff from a cigarette at 1 min intervals; one puff requires 2 s and entails drawing a total of 35 cm³ of air through the cigarette. The puffing process was controlled by a computer. The sidestream smoke generated during the idle burning of the cigarette in the 1 min interval between puffs was diluted with the room air to produce the environmental tobacco smoke and was used in all the experiments. Desiccants used in the present work included silica gel and molecular sieve 13X. Experiments were carried out under dynamic conditions using a packed bed adsorber at 298 K and at a relative humidity of ≈50%. The adsorption and regeneration cycles were repeated 15 times using the same adsorbent. Significant amounts of hydrocarbons and particulates were removed by both silica gel and the molecular sieve during the initial period of a run. The equilibrium water uptake capacity of silica gel at higher relative humidities decreased significantly after 15 cycles but that of the molecular sieve remained almost the same as that of a fresh sample. Also, a molecular sieve that was exposed to tobacco smoke could be regenerated at a lower temperature than the exposed silica gel.