Separations Technology最新文献

The performance of three hybrid membrane and distillation processes is compared theoretically, using the separation of propylene and propane as a representative case. For each system configuration the effect of the main design variables is illustrated through parametric studies. Results dealing with both design of new processes and retrofit of existing distillation-based separation processes are presented. The results show qualitative trends which may serve as guidelines for design of hybrid membrane and distillation processes. If the membrane is placed parallel to the column, the optimal position for the membrane feed stream is close to the column feed plate, which represents a potential pinch point in the column. The optimal membrane cut rate for this configuration is generally close to the molefraction of propylene in the membrane feed stream. The comparison of the systems performances indicates that placing the membrane in parallel or on the bottom stream of the column gives the best performance of the hybrid process, both in terms of compressor duty and in terms of installed membrane area.

Different types of large-pore zeolites with 12-oxygen-member-ring structure were synthesized by a hydrothermal method and used for adsorbing and separating mixtures of β,β-DMN isomers. Among all the zeolites tested, zeolites Y and BEA which possess three-dimensional pore channels were found to adsorb β,β-DMN isomers effectively. On the other hand, SAPO-5, Mordenite and ZSM-12 scarcely adsorbed β,β-DMN molecules because of the large diffusional resistance offered by their one-dimensional pore channels. The adsorptive capacity of zeolite BEA was found to decrease while the adsorption selectivity increase by transition metals isomorphous substitution. Adsorption simulations by the Monte Carlo method indicate that the differences in the adsorption of β,β-DMN molecules on metal-incorporated zeolite BEAs may be attributed to their different adsorption energies.

The tailing effect in the displacement washing operation of lime mud in the pulp and paper industry was investigated using two different model approaches. Both approaches utilise the dispersion model, the first approach adding micropore diffusion and particle size distribution, the second adding a disturbance in the inlet boundary condition. To investigate the models, washing tests were performed in an apparatus in which a filter cake was formed and subsequently washed. The parameters in the washing experiments were type of lime mud, type of salt as solute, flow velocity and bed length. It was found that a good fit could be obtained with the inlet disturbance model, while the micropore diffusion model could not explain the tailing in a satisfactory way. Furthermore, it was found that the salt used had some influence on the dispersion, and that both the salt and the type of lime mud influenced the adsorption constant.

The growth kinetics of thiourea adducts formed with the guests of cyclohexane and methylcyclopentane (MCP) mixtures by adductive crystallization were studied in a batch cooling crystallizer. The effects of the guest concentration on the growth rate and the dissolution rate of adducts were investigated. The growth rate of true crystal (thiourea) and the inclusion rate of guests were also investigated and compared. The method used to evaluate the crystal growth kinetics from the integral mode batch experiments performed in a cooling crystallizer required the determination of the first two initial derivatives of supersaturation and temperature according to time. The orders of the growth for thiourea adducts with respect to supersaturation were found to be 0.87, 0.86 and 0.84, and the activation energies of thiourea adducts were determined as about 75.3, 68.3 and 60.7 kJ/mol for cyclohexane-thiourea adduct, cyclohexane-MCP-thiourea adduct, and MCP-thiourea adduct, respectively. Furthermore, studies on the composition profile for guest and host in the solution suggest that thiourea adduction is controlled by the integration of thiourea molecules and not by the diffusion of the guest molecules.

This work reports a new approach that can effectively separate and recover Tc (as pertechnetate, TcO₄⁻) from contaminated groundwater. Activated carbon was used in both batch adsorption and column leaching studies. The adsorption experiments indicated that activated carbon adsorbs TcO₄⁻ selectively and effectivele over a wide range of pH values and from various dilute electrolyte solutions (< 0.01 M). The partitioning coefficient (K_d) of TcO₄⁻ exceeded 27 000 ml/g when actual groundwater was used, and exceeded 12000 ml/g when background solutions of 0.01 M CaCl₂ and Na₂SO₄ were used. TcO₄⁻ removal efficiency was > 99% under these conditions, except in a 0.01 M NaNO₃ background solution. Column studies confirmed a high adsorption capacity and selectivity of activated carbon for TcO₄⁻. Within the detection limit, no Tc breakthrough was observed when more than 14000 pore volumes of contaminated groundwater (containing ∼3000 pCi Tc/1) were passed through a small column (6.6 × 30 mm) with 0.5 g activated carbon. Recovery of TcO₄⁻ from activated carbon was studied using various chemical reagents such as salicylate, phthalate, NaNO₃, NaCl, and Na₂SO₄. Salicylate was found to be the most effective in desorbing and recovering the adsorbed TcO₄⁻ (as high as 100%). Therefore, the spent carbon can be disposed as low-level radioactive wastes or may be regenerated. Results of this work suggest that the use of activated carbon to remediate Tc-contaminated groundwater can be a promising technology — it is cost-effective and requires minimal installation and maintenance during the pump-and-treat processes.

Copper adsorption by granular activated carbon is reported in this paper. The experimental section includes titrations of activated carbon, as well as equilibrium and kinetic studies of copper adsorption. The potentiometric titration results show that the point of zero charge is 9.5, and that the surface charge increases with decreasing pH. The adsorption of copper strongly depends on solution pH and increases from 10 to 95% at pH ranging from 2.3 to 8. A dramatic increase in pH and emission of small gas bubbles are observed during the experiments, which may result from adsorption of hydrogen ion and/or reduction-oxidation reactions. The two-pK triple-layer model is employed to describe copper adsorption. KINEQL, an adsorption kinetics algorithm, is used to represent the experimental data, and it is found that the model can describe reasonably well the experimental measurements of surface charge, adsorption equilibrium, and adsorption kinetics. Calculations show that formation of the surface-metal complexes SO⁻Cu²⁺ and SO⁻CuOH⁺ (a hydrolysis product of SO⁻Cu ²⁺) in the outer layer around the surface of carbon results in removal of copper ion. It is also found that mass transfer controls the adsorption rate, and that adsorption occurs in the micropore region where both external mass transfer and diffusion are important.

Biodegradation enhanced by aeration through soil venting, a technique commonly referred to as bioventing, is gaining in popularity as a means for in situ remediation of soils contaminated with organic compounds. The effectiveness of this technique at a particular site is dependent upon achieving and maintaining sufficient oxygen levels in the contaminated soil zones to support aerobic biodegradation. This paper uses analytic and numerical models of the gas flow in soil surrounding vents with simplified biodegradation relationships to predict the oxygen profiles in the soil under given site properties and operating conditions. The results may be used to decide whether bioventing is a feasible remediation technique at a particular site and to investigate the effects of vent placement and flow rate upon performance.

Pertechnetate oxyanion (TcO₄⁻), which is highly soluble in water and readily mobile in the environment, can be immobilized through an ion exchange /adsorption process and chemical reduction followed by adsorption and/or precipitation. Previous studies have focused on the separation and removal of ⁹⁹TcO₄⁻ from high-level waste streams; however, little information is available for ⁹⁹TcO₄⁻ removal from only slightly contaminated groundwater. This paper describes treatment of ⁹⁹TcO₄⁻-contaminated groundwater with both batch and column flowthrough experiments. Synthetic resins and sponges, and zero-valence iron filings were used to evaluate their capacities and the rates of ⁹⁹TcO₄⁻ removal. The toxicity characteristic leaching procedure (TCLP) was applied to evaluate the leachability of ⁹⁹Tc adsorbed or co-precipitated on iron. Results suggest that both iron and synthetic resins remove ⁹⁹TcO₄⁻ from groundwater and that at a high flow rate (with residence time of less than 1 min), ⁹⁹TcO₄⁻ removal capacity is greater for iron filings than for the synthetic resins on a volume basis. Additionally, the rate of ⁹⁹TcO₄⁻ sorption on the sponge is slow (approximately 3 days), and the capacity is relatively low. No appreciable amount of ⁹⁹Tc can be leached out from the spent iron filings by the TCLP test. Overall, zero-valence iron filings provide fast reaction and high removal capacity for ⁹⁹TcO₄⁻ in groundwater. The high removal efficiency, low cost, and the small waste production of zero-valence iron are attractive for remediation of ⁹⁹TcO₄⁻-contaminated groundwater.

Biological activated carbon treatment is applied to a type of wastewater collected from plating industries. The water contains small amounts of refractory organic pollutants, such as anionic surfactants, small amounts of heavy metals, such as cupric and chromic ions, and large amounts of sodium salts. It is found that the thickness of biofilm formed around activated carbon particles increases with time, even though the existence of heavy metals is unfavorable to the growth of microorganisms. As a result, about 50% of organic substances are removed from the water. Present removals for the ionic species of copper and chromium are about 80% and 30%, respectively. Heavy metals are removed from the wastewater by uptake in the bodies of microorganisms, while organic substances are removed by biological decomposition and partly by adsorption.