Extraction equilibria determination Distribution

There are several processes for the separation of liquid mixtures using porous membranes or asymmetric polymer membranes. With porous membranes, separation may depend just on differences in diffusivity, as is the case with dialysis, where aqueous solutions at atmospheric pressure are on both sides of the membrane. For liquid-liquid extraction using porous membranes, the immiscible raffinate and extract phases are separated by the membrane, and differences in the equilibrium solute distribution as well as differences in diffusivity determine the extract composition. 

The derivation of Dt from coherent QENS is similar to a computation of Dt from the fluctuations in an equilibrium density distribution. This was accomplished by Tepper and co-workers for Ar in AIPO4-5 [6]. Using the Green-Kubo formahsm, they were able to extract this non-equilibrium quantity from just one equihbrium simulation. Moreover, the calculations being performed in reciprocal space, the variation of the diffusivity upon the wave vector was used to check when the system was in the linear regime [6]. The first application of non-equihbrium molecular dynamics (NEMD) to zeolites was performed by Maginn et al. on methane in sihcalite [7]. Standard equi-libriiun MD techniques were later used by Sholl and co-workers to determine the concentration dependence of diffusivities [8]. 

If the PT catalyst in both the phases is in extractive equilibrium (i.e., there is equilibrium distribution of catalyst) and mass transfer resistances are absent or neglected completely, then ipQY and ipqx are each equal to 1. In other words, the reaction is in regime 1, and no further modeling besides determining the intrinsic kinetics is required. 

Metal picrates (2.5 x lO" M) were prepared in situ by dissolving the metal hydroxide (0.01 mol) in 2.5 x 10 M picric acid (100 mL) triply distilled water was used for all aqueous solutions. Two phase solvent extraction was carried out between water (5 mL, [alkali picrate] = 2.5 x 10 M) and H2 l2 (5 mL, [ionophore] = 2.5 x 10 M). The two-phase mixture was shaken in a stoppered flask for 2 h at 25° . We confirmed that this period is sufficient to attain the distribution equilibrium. This was repeated three times, and the solutions were left standing until phase separation was complete. The extractability was determined spectrophotometrically from the decrease in the absorbance of the picrate ion in the aqueous phase as described by Pedersen [31]. 

Essentially, extraction of an analyte from one phase into a second phase is dependent upon two main factors solubility and equilibrium. The principle by which solvent extraction is successful is that like dissolves like . To identify which solvent performs best in which system, a number of chemical properties must be considered to determine the efficiency and success of an extraction [77]. Separation of a solute from solid, liquid or gaseous sample by using a suitable solvent is reliant upon the relationship described by Nemst s distribution or partition law. The traditional distribution or partition coefficient is defined as Kn = Cs/C, where Cs is the concentration of the solute in the solid and Ci is the species concentration in the liquid. A small Kd value stands for a more powerful solvent which is more likely to accumulate the target analyte. The shape of the partition isotherm can be used to deduce the behaviour of the solute in the extracting solvent. In theory, partitioning of the analyte between polymer and solvent prevents complete extraction. However, as the quantity of extracting solvent is much larger than that of the polymeric material, and the partition coefficients usually favour the solvent, in practice at equilibrium very low levels in the polymer will result. 

The actual SFE extraction rate is determined by the slowest of these three steps. Identification of the ratedetermining step is an important aspect in method development for SFE. The extraction kinetics in SFE may be understood by changing the extraction flow-rate. Such experiments provide valuable information about the nature of the limiting step in extraction, namely thermodynamics (i.e. the distribution of the analytes between the SCF and the sample matrix at equilibrium), or kinetics (i.e. the time required to approach that equilibrium). A general strategy for optimising experimental parameters in SFE of polymeric materials is shown in Figure 3.10. 

For determination of phenol distribution coefficients the extraction proceeded for 15 minutes in order to reach equilibrium. The time required to reach equilibrium was determined by making five replicate injections of the headspace onto the SFC system. The first injection was after the extraction had proceeded for 15 minutes at 50°C and 100 atm. Following the equilibration time, four further injections at ten minute intervals were made, after which the pressure inside the extraction apparatus was increased and the system was again allowed to equilibrate (i.e. 15 minutes). The five replicate injection process was then repeated. The amount of phenol in each injection was then noted by referring to an external phenol standard calibration curve. As the total volume of the system was known, the amount of phenol in the SF could be calculated. The amount of phenol in the aqueous phase could then be calculated by mass balance. 

In some systems a series of adducts ML(n )A aB, ML(n 2)A2 bB,..., MA yB are involved in the extraction. An attempt was recently made24,251 to determine their composition and the corresponding equilibrium constants by obtaining the metal ion distribution ratio data using a fixed [B] and varying [HA]. 

Because of interference from the radioactive decay of other nuclides (which are typically formed with much higher yields), extraction systems with relatively high decontamination factors from actinides, Bi, and Po must be chosen, and the transactinide activity can only be measured in the selectively extracting organic phase. For this reason, measurement of distribution coefficients is somewhat difficult. By comparing the Rf or Db detection rate under a certain set of chemical conditions to the rate observed under chemical control conditions known to give near 100% yield, distribution coefficients between about 0.2 and 5 can be determined. If the control experiments are performed nearly concurrently, many systematic errors, such as gas-jet efficiency and experimenter technique, are cancelled out. Additionally, extraction systems which come to equilibrium in the 5-10 second phase contact time must be chosen. 

Vp and VL are the volumes of the extraction agent and the liquid sample, respectively, and Kle - Ce/Ce is the distribution coefficient. In practice, an extraction yield higher than 99% is usually considered to be quantitative. With the use of the same volumes of the extraction agent and the sample, this result can be obtained even in a single extraction step if Kle < 0.01. Sometimes the entire procedure can be complicated by a chemical reaction taking place, e.g., in the extractive alkylation (see p.59) or in the preparation of volatile metal chelates (see p.194), and the total yield of the extraction then involves, in addition to the interphase distribution of the initial compounds and products, also the chemical equilibrium which is attained by the reaction. If the quantitative yield of the extraction cannot be predicted on the basis of the character of the system, the extraction efficiency must be determined, otherwise the quantitative evaluation is questionable. 

For practical heterogeneous catalyst kinetics this principle has the following consequence. Usually, the assumption of a homogeneous surface is not valid. It would be more realistic to assume the existence of a certain distribution in the activity of the sites. From the above, certain sites will, however, contribute most to the reaction, since these sites activate the reactants most optimally. This might result in an apparently uniform reaction behaviour, and can explain why Langmuir adsorption often provides a good basis for the reaction rate description. This also implies that adsorption equilibrium constants determined from adsorption experiments can only be used in kinetic expressions when coverage dependence is explicitly included otherwise they have to be extracted from the rate data. 

The model I is very simple, and it is not very sensitive to the physical properties of the bed, but the values of the overall mass transfer coefficients optimised are strongly dependent from the equilibrium relation assumed and it only is able to describe the initial part of the extraction. Ke was determined by mass balance assuming a uniform distribution in solid bed. Ke values of 0.5, 0.2, and 0.6 were obtained for 7, 10 and 15 MPa. Assuming a = 3000 nAn", the mass transfer coefficients calculated with model I are of some orders of magnitude lower than those for external mass transfer coefficients. This type of models have being applied with success to the extraction of edible oils from seeds were the solute is in a... 

Several chemical reactions, including calcium carbonate and hydroxyapatite precipitation, have been studied to determine their relationship to observed water column and sediment phosphorus contents in hard water regions of New York State. Three separate techniques have been used to Identify reactions important in the distribution of phosphorus between the water column and sediments 1) sediment sample analysis employing a variety of selective extraction procedures 2) chemical equilibrium calculations to determine ion activity products for mineral phases involved in phosphorus transport and 3) seeded calcium carbonate crystallization measurements in the presence and absence of phosphate ion. 

Two phases in contact with each other are said to be in equilibrium when the temperature, pressure, composition, and all other variables that characterize each phase do not change with time. Many chemical process operations—particularly separation processes such as distillation, absorption, crystallization, liquid extraction, and adsorption—work by distributing mixture components between two phases and then separating the phases. An essential step in analyzing such processes is determining how the feed mixture components distribute themselves between the two phases at equilibrium. This chapter summarizes common procedures for making this determination,... 

Liquid extraction is a separation process in which a liquid feed solution is combined with a second solvent that is immiscible or nearly immiscible with the feed solvent, causing some (and ideally most) of the solute to transfer to the phase containing the second solvent. The distribution coefficient is the ratio of the solute mass fractions in the two phases at equilibrium. Its value determines how much solvent must be added to the feed solution to achieve a specified solute transfer. When the two solvents are partially miscible, a triangular phase diagram like that in Figure 6.6-1 simplifies balance calculations on extraction processes,... 

Phenol, a common priority pollutant, was extracted from two environmental matrices, soil and water, using near critical and supercritical carbon dioxide. The primary objective of this study was to determine the distribution of the contaminant between the soil or water and the supercritical phase, and the effect of soil moisture and co-solvents on the distribution coefficients. Static equilibrium extractions were performed on dry and wetted soil contaminated with 1 wt.% phenol and on water containing 6.8 wt.% phenol. Supercritical carbon dioxide (with and without en-trainers) was chosen as the solvent for the study. An appropriate entrainer for dry soil extractions (methanol) ffiffered from that found for aqueous extractions (benzene). However, soil moisture was found to have a significant impact on the effectiveness of en-trainers for soil extractions of phenol. Entrainers appropriate for extracting wetted soil were found to be the same as those advantageous for aqueous extractions. Benzene was also extracted from dry and wetted soil to investigate the extractability of a hydrophobic compound. 

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