Gas Separation & Purification最新文献

The performance of zinc ferrite promoted with V₂O₅ was investigated as a hot gas desulfurization sorbent. The sorbents were prepared by the incipient impregnation technique, and the vanadium loading and the calcination temperature were varied. The sorbents were tested in a packed-bed microreactor set-up for five sulfidation-regeneration cycles to investigate their breakthrough behaviour. Sulfidation was performed at 873–973 K with H₂S-H₂-H₂O-N₂ mixtures, while regeneration was carried out at 923–973 K with O₂-N₂ mixtures. The fresh, sulfided, and regenerated sorbents were characterized by atomic absorption spectroscopy, X-ray diffraction, surface area and porosity measurements. The results of the investigation indicated that the sorbents exhibited a more stable cyclic performance with higher vanadium loading, all vanadium-promoted sorbents reduced the H₂S content of the gas to levels below the equilibrium level for zinc oxide, some H₂S was chemisorbed by vanadium, and two-step calcination imparted structural stability to the sorbents.

This paper compares two methods for the determination of periodic states of adsorption cycles based on mathematical modelling. Emphasis is on pressure swing adsorption, although the methods are valid for cycles of any type. The methods are (1) the traditional, successive substitution approach in which the final condition of a cycle is used as the initial condition of the next cycle, and (2) a new approach for directly determining the periodic states. Rates of convergence of the methods are illustrated using a literature example for air separation using carbon molecular sieve to produce N₂.

In air separation units, aluminium structured packing may be used instead of aluminium sieve trays for the cryogenic distillation of air. In previously published experiments regarding the flammability of packing in oxygen-enriched atmospheres, strong energy releases have been reported under conditions which did not correspond to realistic operating conditions.

In additional tests with packing under simulated operating conditions, no propagation of combustion was observed even with strong promoters. Therefore, the authors confirmed the compatibility of aluminium packing for use in air separation units under operating conditions.

In this paper, experiments are described to compare the flammability of aluminium sieve trays, which have been used for more than 30 years in distillation columns, with aluminium packing under similar conditions.

It was found that under similar test conditions, sieve trays as well as packing are flammable. In flammability tests, violent energy releases were observed with the strong promoter thermite; however, no differences in flammability between sieve trays and packing were noticed. Comparing the flammability of the sieve trays, in use for more than 30 years in distillation columns, with packing, it was concluded that sieve trays can be substituted by packing without any additional risks.

Models are developed to simulate on-site membrane enrichment of natural gas wells. The models consider transient behaviour of the enrichment process, the effects of permeability functions, flow patterns and separation of multicomponent systems. The gas mixture considered in the analysis includes CH₄, CO₂, N₂ and H₂S. Comparison of models shows that for either flow pattern, the presence of low permeating species reduces the final CH₄ mole fraction in the enriched mixture. In addition, higher CH₄ recoveries are predicted by the four-component model than by the binary model. This is a result of the lower driving force for permeation in the four-component model, caused by lower mole fractions. Similar behaviour is also predicted for the removal of CO₂ and its final mole fraction in the gas mixture. The presence of other fast permeating species (H₂S) resulted in lower permeation rates of CO₂. As a result, higher CO₂ mole fractions in the gas mixture and lower CO₂ removal are predicted by the four-component model. The use of variable permeabilities versus constant permeability ratio showed similar predictions for some of the system variables with partial matching between model predictions at either low or high pressure.

Experimental work has shown that trays and packing fabricated from aluminium are flammable under certain conditions normally encountered in oxygen distillation columns. Therefore, the safety of trays and packing depends entirely on the exclusion of ignition mechanisms.

This paper presents an evaluation of ignition mechanisms with respect to their relevance to the adiabatic section of the column. The combustion of hydrocarbons has been identified as the only realistic ignition mechanism. Therefore, most of the experiments described in the paper refer to this mechanism. It is shown that the combustion of an oil film on packing (approximately 5 g m⁻²) is not able to ignite aluminium packing, but that accumulated oil (a few grams) is able to ignite aluminium trays and packing as well as copper packing. This result stresses the importance of controlling any hydrocarbon accumulation.

A short review of accumulation mechanisms for lubrication oil from fabrication and hydrocarbons from air shows that accumulation within the adiabatic section of the distillation column is practically impossible.

Finally, it is concluded that no ignition sources for aluminium trays and packing are present in the low-pressure column of oxygen distillation columns. This explains the very good safety record of aluminium trays for more than 30 years of industrial experience characterized by no single case of tray combustion. Aluminium packing, now in operation for more than five years, can be expected to be as safe as trays.

A previously reported approach (E. I. Bozhenko and S. V. Bozhenko Gas Sep Purif (1993) 7 123) and software for simulation of multicomponent membrane gas separation processes are further developed for two-stage processes with permeate recycling. The described method was successfully applied to the design and optimization of a membrane device manufactured by the CLIMBI Company, Moscow, Russia, for storage of museum relics in controlled gas media at the Hermitage in St. Petersburg, Russia.

A commercial-scale, single-stage, spiral-wound membrane system has been operated for approximately 20 months to upgrade low-quality natural gas from a well in East Texas. Throughout the test period the retentate product (“sales” gas) met pipeline specifications. Data were obtained on two membrane modules containing two types of asymmetric cellulose acetate membranes, one “standard” and the other one of higher density. A summary of the field test data shows the effects of the operating variables of pressure, feed flow rate, and CO₂ concentration in the feed (from 3 to 25 mole percent). Concentrations greater than 6 mole percent were obtained by adding pure CO₂ to the feed gas. In addition, computer models for the separation of gases under “perfect mixing” and cross-flow conditions were applied to the analysis of the field data. In general, the field test data were consistent with a flow regime which was intermediate between perfect mixing and cross-flow.

There are three processes for adsorptive generation of oxygen from air using molecular sieve zeolites, namely pressure swing adsorption (PSA), vacuum swing adsorption (VSA) and pressure vacuum swing adsorption (PVSA). Two new variations have recently come onto the market: low-temperature VSA (LT VSA) and two-bed VSA (PVSA). Because of their reliability and long service life, swing adsorption systems have been producing good results for more than ten years, which is why the market share of these systems in the overall oxygen market is constantly rising. Capacity limits with adsorption processes stem at present from the design of the valves and vacuum pumps.

Total, static and dynamic holdups have been measured for 0.012 m ceramic Raschig rings with air rates from 0 to 700 kg m⁻² h⁻¹ and sodium citrate solution rates from 2000 to 60 000 kg m⁻² h⁻¹ at 40°C using the draining method. Equations presented for estimating holdups are discussed.

In this paper we describe a composite-metal membrane that shows promise for practical large-scale applications including the manufacture of ethylene, recovery of hydrogen from refinery streams, and the environmentally clean decomposition of hydrogen sulfide in sour natural gas and refinery off-gases. Specifically, this composite membrane exhibits improvements over previously developed metal membranes that include (1) stable hydrogen flux at 700°C, (2) low cost and (3) chemical compatibility with many common feed-stream impurities, including hydrogen sulfide.