Removal of Pollutants from Solid Matrices Using Supercritical Fluids

Abstract

Several supercritical fluid extraction (SCFE) processes have been proposed for removing toxic and intractable organic compounds from a range of contaminated solids. These include soil remediation and the regeneration of adsorbents used to treat wastewater streams such as granular activated carbon (GAC). As a separation technique for environmental control, SCFE has several distinct advantages over conventional liquid extraction methods and incineration. Most notably, the contaminant is removed from the solvent in a concentrated form via a change in pressure or temperature and can be completely separated upon expansion to atmospheric pressure.

The viability of SCFE hinges on process conditions such as solvent-feed ratio and solvent recycle ratio. The necessity of recycling solvent complicates the contaminant separation step since a complete reduction to atmospheric pressure would create large recompression costs. Because of this, the pressure and temperature dependence of contaminant solubility must be understood so that operating conditions for the separation step can be defined. Fortunately, this is the most developed aspect of SCF technology. However. the mass transfer limitations to removing contaminants from solids change with solvent flow rate.

This paper discusses the use of SCFE for environmental control and presents results for the removal of DDT and 2-chlorophenol from GAC. 2-chlorophenol is almost completely removed with pure CO₂ at 40°C and 101 bar while only 55% of the DDT is removed at 40°C and 200 bar. These differences in regeneration efficiency cannot be understood solely in terms of solubility but point to a need for detailed studies of adsorption equilibrium and mass transfer resistances in supercritical fluid systems.     

Separation of Fine Particles from Nonaqueous Media: Free Energy Analysis and Oil Loss Estimation

Abstract

In industrial processes for the separation of fine particles from oil to produce clean liquid fuels, a significant portion of oil is entrained by the solids and is lost, A semi-empirical model is proposed to explain the mechanism of oil loss and floe carryunder. The favorability of the transfer of fine particles from oil to water was determined by conducting a free energy analysis of the process. The ratio of the water drop to particle radius (size ratio) and the contact angle were found to be important parameters affecting the free energy change. The oil loss was found to increase with an increase in size ratio. To verify the model, experiments to determine oil loss in a practical system were conducted for two different surfactants. The model was found to fit experimental data for oil loss very well. The results show that by choosing a suitable surfactant and demulsifier, it is possible to decrease the oil loss considerably. Hence, the proposed model is useful for evaluating the efficiency of the separation process.     

Drying in the Context of the Overall Process

Abstract

Although drying has traditionally been considered as a unit operation, it is strongly affected by upstream operations such as crystallization and solid–liquid separation, and in turn can affect downstream processes such as gas cleaning and micronization. Process design needs to consider the complete flowsheet and the interactions between the different steps as early as possible. Key points are the particle formation method and the final product specification. The mean particle diameter and particle size distribution are vital parameters throughout the process, as smaller particles and fines make solids handling, dewatering, and washing more difficult. This in turn affects the inlet moisture content to the dryer, and hence the heat duty and performance. Intermediate size enlargement or reduction may be used to give a more easily dried particle or agglomerate. There are important links to the new subject of product engineering, for example in the choice of processing route to achieve a given product quality and specification. The interactions between the different process steps can affect process design, equipment selection and troubleshooting, and this is illustrated by industrial case studies. A holistic approach is proposed to allow the whole solids processing flowsheet to be optimized as an entity, rather than optimising each unit operation in isolation and then finding a conflict between them.     

Dielectrophoretic Gold Particle Separation

Abstract

We present a novel process for gold particle separation from aqueous setup with a high separation efficiency and without any environmental risk. Dielectrophoresis (DEP), as the main mechanism of this separation process, is applied for the first time to separate gold even continuously from a raw mineral mixture. Electrothermal and high-pass-filter effects, occurring in DEP with water as liquid phase, were investigated and considered during the design of the separation process. The experimental results demonstrate that even ultra thin gold particles can be separated from a raw mineral mixture with an efficiency of up to 88% at an electric field of 32 kV/m and 200 kHz in continuous operation with specific fluid flow of about 400 m³/(m h).     

Influence of Polymeric Suspension Characteristics on the Particle Coating in a Spouted Bed

Abstract

The influence of the coating suspensions and particle properties on the coating process in a conventional spouted bed is presented. Glass beads were coated at fixed operating conditions with different formulations of aqueous polymeric coating suspensions in a spouted bed of laboratory scale. The wettability of the solids by the liquid was quantified by the contact angle and surface tension of the coating suspensions. The coating efficiency and particle growth were correlated with the adhesion of the coating suspension to the solid particle, which is a function of the solids and liquid characteristics. The physical properties of the coated particles—particle mean diameter, sphericity, bulk, absolute and apparent densities, porosity and flow velocity—were determined and compared to the properties of uncoated particles.     

Evaluation of Inertial Filter for Dissolver Solution Clarification

Abstract

The performance of some types of solvent extraction contactors is adversely affected by a high concentration of entrained solids in the process feed streams. Therefore, during reprocessing of spent reactor fuels it is desirable to separate undissolved solids and insoluble fission product residues from dissolver solutions.

A series of statistically designed filtration tests were conducted to evaluate the separation efficiency of a Mott inertial filter for the removal of sub-micron particulate matter from dissolver slurries following centrifugation. Slurries used for testing consisted of 0.1 weight percent solids (needle-shaped yellow Fe₂O₃, rhombehedral-shaped red Fe₂O₃, or SiC hulls from HTGR fuel spheres) dispersed in water. Particle diameters ranged from less than 0.1 micron to 10 microns. The operating conditions varied temperature, inlet and outlet pressures, backwash intervals and particle shape. Measured separation efficiencies were greater than 90% indicating that all particles larger than 0.2 microns were removed.     

Separation of Anthracene from Crude Anthracene Using Gas Antisolvent Recrystallization

Abstract

Pure anthracene is mostly used for conversion to anthraquinone, an intermediate for the synthesis of very powerful vat dyestuffs. A coal tar distillate, crude anthracene, which contains 30% anthracene, 25% phenanthrane, 15% carbazole, and other impurities, was used as the model mixture. In this study, 90% by weight purity anthracene was obtained using gas antisolvent (GAS) recrystallization. The GAS process induces the separation of solids by introducing an antisolvent, carbon dioxide (or the supercritical fluid), into acetone which was used as the liquid solvent. The dissolution of the compressed gas into the solute-laden solution selectively lowers the solubilities of solid solutes and salts them out. The results showed that high purity anthracene was obtained at a high feed concentration and high pressure conditions. The separation factor of anthracene versus phenanthrene is close to 30.07.     

Separation of Ethylbenzene from Mixed Xylenes by Continuous Adsorptive Processing

Abstract

Separation of ethylbenzene at high purity from its mixtures with xylenes has been accomplished commercially by superfractionation. Because of the low relative volatility (1.05) between ethylbenzene and para-xylene, this operation is difficult and energy-intensive, requiring the use of several hundred fractionation trays at a reflux/feed ratio of about 18.

To improve economics and reduce energy usage, a process has been developed to accomplish this separation by selective adsorption from the liquid phase onto a solid adsorbent. In this operation, ethylbenzene is separated from the mixed xylenes within the adsorbent bed, and the two products are recovered from the adsorbent by displacement with a liquid of different boiling point, referred to as desorbent. Desorbent is recovered from the net products by distillation and is recycled.

Results of liquid chromatographic tests used to screen various adsorbents are presented. Adsorbents have been developed that show a higher separation factor than the 1.05 existing in distillation, with ethylbenzene being the least strongly adsorbed component.

The process has been demonstrated by operation of a pilot plant in which a simulated moving-bed technique is used to obtain the process characteristics of continuous countercurrent flow of adsorbent and process fluid, without actual movement of solids. Results of these operations are presented.

The adsorptive process requires less than half of the energy input required by superfractionation. An economic comparison of the two operations is presented.     

Separation of Solids at Liquid-Liquid Interface

Abstract

The possibility of separation of solids at the interface of two liquid phases by utilizing surface tension to overcome gravity was studied. The proposed theoretical model was verified in several experimental examples.     

The Improvement of Separation in Zone Refining Processes with Zone Length Varied along the Ingot

Abstract

Adjusting the zone length properly and continuously along the ingot during zone melting substantially increases the separation efficiency by controlling the solute concentration in the liquid zone. Considerable improvement in separation was obtained by employing the process with a partially optimized zone length versus distance.     

Electrochemical Treatment of Black Liquor from Straw Pulping

Abstract

The conventional black liquor regeneration process is not always suitable for pulping plants of nonwood fibers due to the unfavorable ratio of organic to inorganic solids. This paper presents an alternative treatment based on an electrolysis process of the soda black liquor from straw pulping. This alternative method minimizes the environmental impact by recovering the caustic at the same time that the liquor is acidified, which favors the later separation of the lignin.     

Aerosol Precipitation

Abstract

The efficiency of a single-stage wet electroprecipitator was investigated. This separation process is based on ionization of the particles in the first step. The precipitation electrode is formed by the scrubbing liquid. The operating voltage is limited to 20 kV.     

Electrostatic Splitting of the Emulsion Used in Liquid Surfactant Membranes Process for Metals Separation

Abstract

A study has been carried out to investigate the effect of different parameters on the splitting process of the emulsion used in the liquid surfactant membranes process for metals separation. The influence of electric field strength, frequency of the applied ac voltage, and membrane recycling on emulsion stability have been studied. The investigations have revealed that there exists a unique optimal frequency range in which the best separation of the emulsion phases is obtained. The effect of the membrane composition and the variation of tri-n-butyl phosphate (TBP) concentration has been shown to vary linearly with the splitting efficiency. This finding correlates quite well with Sadek and Hendrick's equation on the force causing coalescence in an electric field.     

Electro-Acoustic Dewatering (EAD) a Novel Approach for Food Processing,and Recovery

Abstract

Separation of liquids from fine particle suspensions plays an important role in many industrial processes. In the past few years a number of technologies have been developed for the separation of slurries with coarse particle suspensions and intermediate particle sizes. However, separation of fine particles from their suspensions can be difficult and prohibitively costly.

Battelle has developed a solid/liquid separation technology that utilizes differences in electro-kinetic and acoustic properties to enhance the efficiency of conventional solid/liquid separation techniques such as vacuum filters or presses. This method can dewater colloidal stable suspensions better than conventional techniques. Typical applications of this technology to food processing will be presented. Mechanisms involved during separation will also be discussed.     

Production of Motor Fuel Grade Alcohol by Concentration Swing Adsorption

Abstract

A new cyclic process concept called “concentration swing adsorption” for separation of bulk binary liquid mixtures is described. The process carries out the primary separation by selective liquid-phase adsorption of one of the components of the feed mixture on an adsorbent. The adsorbed component is then desorbed by a desorbent liquid which is equally or more strongly adsorbed than the slectively adsorbed component of the feed mixture. The desorbent liquid is removed from the adsorbent by displacing it with the less strongly adsorbed component of the feed mixture so that the adsorbent can be resused. The process also includes a complementary step where the adsorbent is rinsed with the more strongly adsorbed component of the feed mixture so that two essentially pure products are produced from the feed mixture with high recoveries of both components. At least one simple distillation is also required by the process which separates the desorbent liquid from the less strongly adsorbed component of the feed mixture. The process can be used to separate liquid mixtures with close boiling components or azeotropic mixtures which require energy intensive distillation. A very efficient separation can be achieved in these cases by spending only a fraction of the distillation energy. An example of such an application, viz., separation of a bulk ethanol–water mixture for motor fuel grade alcohol production, is described. A local equilibrium model of the process is used to evaluate the performance of the process for that separation using an activated carbon as the adsorbent and acetone as the desorbent liquid. Experimentally measured equilibrium adsorption characteristics for ethanol–water, acetone–water, and acetone–ethanol binary liquid mixtures on the carbon as well as adsorption column dynamics for the steps of the process are reported.     

A Study of Kinetics on Induced-Air Flotation for Oil-Water Separation

Abstract

Induced-air flotation makes use of the centrifugal force of a high-speed backspin impeller through which gas is introduced at the top and liquid at the bottom. The gas and liquid become fully intermingled and, after passing through a disperser outside the impeller, form a multitude of gas bubbles, thus completing the flotation separation process of a liquid-solid or liquid-liquid heterogeneous separation process of a liquid-solid or liquid-liquid heterogeneous system. The flotation machine used for treating oily sewage generally belongs to the four-cell series. Its good points are that the machine has a high removal efficiency for oil separation, shortens retention time, and has great treatment capacity. The device has been widely used in treating oily sewage in recent years (1, 2). In order to provide a basis for establishing a method of design computation and for screening appropriate chemicals, this study was initiated to gain an understanding of the variables controlling the induced-air flotation process for oil-water separation. An experimental device for simulating tests has been developed. In order to analyze its mechanism, a kinetic model of oil-water separation has been put forward. By the use of the experimental result, correlative equations, which can estimate the parameters of the model, have been obtained.     

Concentration-Thermal Swing Adsorption Process for Separation of Bulk Liquid Mixtures

Abstract

A novel concentration-thermal swing adsorption process is described for separation of bulk binary liquid mixtures. The process is designed to produce essentially two pure products with high recoveries of both components. It is particularly suited for separation of azeotropic or close-boiling liquid mixtures which are difficult to separate by distillation. An example of the performance of the new process for separation of an azeotropic water-methyl acetate mixture is given. Experimental binary surface excess equilibrium isotherms, adsorptive mass transfer coefficients, and column dynamics for adsorption of water-methyl acetate mixtures on NaX zeolite are reported.     

Compressibility of Paniculate Structures in Relation to Thickening,Filtration, and Expression—A Review

Abstract

Compressibility of cakes is basic to separation of solids from slurries. Cake compression is caused by collapsing, particulate structures in which particles are forced into existing voids. Stresses causing the collapse arise from unbuoyed weight of solids in thickeners, radial acceleration in centrifuges, frictional pressure drop in filters, and mechanical forces in press belts and membrane actuated filters. Compressibility is a function of particle size and shape and the degree of aggregation.

Flow through very compressible cakes produces a highly non-uniform structure with a tight skin of low porosity next to the supporting medium. This skin leads to adverse effects in which increasing filter pressure has little effect on flow rate or average porosity. Similarly increasing the squeezing pressure indefinitely in expression has little effect upon the rate of removal of liquid.     

DISTILLATION UNDER ELECTRIC FIELDS

ABSTRACT

Distillation is the most common separation process used in the chemical and petroleum industry. Major limitations in the applicability and efficiency of distillation come from thermodynamic equilibria, that is, vapor-liquid equilibria (VLE), and heat and mass transfer rates. In this work, electric fields are used to manipulate the VLE of mixtures. VLE experiments are performed for various binary mixtures in the presence of electric fields on the order of a few kilovolts per centimeter. The results show that the VLE of some binary mixtures is changed by electric fields, with changes in the separation factor as high as 10% being observed. Batch distillation experiments are also carried out for binary mixtures of 2-propanol and water with and without an applied electric field. Results show enhanced distillation rates and separation efficiency in the presence of an electric field but decreased separation enhancement when the electric current is increased. The latter phenomenon is caused by the formation at the surface of the liquid mixture of microdroplets that are entrained by the vapor. These observations suggest that there should be an electric field strength for each system for which the separation enhancement is maximum.

     

Mixed Waste Separation Technologies

Abstract

The United States Department of Energy policy for management of mixed (radioactive and hazardous) waste is in the development stage. Regardless of the approach to treatment of mixed waste, significant preprocessing will be necessary. The separation requirements for preprocessing of mixed waste will vary greatly depending on the downstream treatment requirements, the contents of the mixed waste stream, and the regulatory requirements at the waste treatment site. During the past year, the Department of Energy's Mixed Waste Integrated Program (MWIP) has begun to identify separation requirements, applicable commercial technologies, and emerging technologies that may meet specific requirements.

The current emphasis on developing emerging separation technologies for the MWIP include: freeze crystallization for gross separation of dissolved solids and organics from an aqueous waste stream; a bio-catalytic process for nitrate destruction; the General Electric KI/I₂ process for separating mercury from noncombustible solids and aqueous sludges; and the 3M/IBC membrane technology for separating mercury, cesium and strontium from aqueous streams.