Liquid-Phase Concentrations

The freeze concentration process is based on the partial solidification of water into ice in a fluid food product followed by the removal of the solid ice phase from the concentrated liquid phase. This process has some inherent advantages over evaporation and reverse osmosis for concentrating fluid foods as well as other process streams (1). One advantage is that essentially none... 

Frozen Aqueous Solutions Concentration Effects. As solute is rejected by the growing ice and as its concentration increases in the shrinking liquid phase, the temperature drops toward the eutectic point, where the entire system approaches complete solidification. Of major importance is the fact that a highly concentrated liquid phase can persist indefinitely at any point above the eutectic temperature. 

Coacervation, a fourth type of aggregation, in which the silica particles are surrounded by an adsorbed layer of material that makes the particles less hydrophilic, but does not form bridges between particles. The particles aggregate as a concentrated liquid phase immiscible with the aqueous phase. 

This chapter treats reactions within a singie phase. Muitiphase CSTRs are treated in Chapter 11. For the singie-phase case, there is no essentiai difference between iiquid and gas reactors except for the equation of state. Density changes in iiquid systems tend to be smaii, and the density is usuaiiy assumed to be a iinear function of concentration. Liquid phase CSTRs can be hydrauiicaiiy fuii but frequentiy operate with a fixed ievei and have a free surface in contact with a vapor phase. Occasionaiiy they are mounted on ioad ceiis or use radiation-ievei detectors and operate with a fixed mass. 

At low sulfur dioxide concentrations (< 1000 ppm) gas diffusion is the controlling reaction. There is negligible liquid-phase resistance because sufficient absorbent is being dissolved to react with the sulfur dioxide. However, at increased sulfur dioxide concentrations liquid-phase resistance may become controlling. In this case the rate of absorbent dissolution may not increase fast enough to react with the sulfur dioxide. 

An important feature of transient supersaturation is its duration or, in other words, the rates of solute nucleation and crystal growth. Some of the factors that influence these rates have already been discussed for the case of pure water. For crystallisation from solutions, two additional factors are the viscosity of the residual freeze-concentrated liquid phase and probably also the conflgurational complexity of the crystal structures of the solutes involved. Thus, for NaCl, a degree of supersaturation of up to 6 M, reached at ca. —25°C, seems reasonable, at which temperature the salt is likely to crystallise spontaneously. For a typical cooling rate of 10°C min the duration of supersaturation would then be ca. 5 min, with the NaCl concentration transient reaching 6 M ... 

The concentrated liquid phase was always clear at the highest temperature... 

L dilute liquid phase L = concentrated liquid phase L = liquid phase near the consolute solution composition (where composition 1. equals composition L2). 

The separation operation of drying converts a solid, semisolid, or liquid feedstock into a solid product by evaporation of the liquid into a vapor phase through application of heat. In the special case of freeze drying, which takes place below the triple point of the liquid that is removed, drying occurs by sublimation of the solid phase directly into the vapor phase. This definition thus excludes conversion of a liquid phase into a concentrated liquid phase... 

Figure 7.39 Saccharose-water state diagram t = temperature (°C), c = concentration, T , = equiiibrium formation-meiting of ice (equilibrium melting temperature of ice), 7, = equilibrium saturated solutlon-oversaturated solution, = glass transition temperature, = glass transition temperature of maximum concentrated liquid phase with saccharose content...

It was shown some time ago that in ionic surfactants (such as SDS), the trajectory of CMCs versus temperature intersects the Krafft phase boundary at the CMC Krafft point [7,44,45]. Just as the CMC itself is not a thermodynamic discontinuity [46], there is no kink or cusp in the Krafft boundary at this intersection. Nevertheless, this behavior is important because below the temperature of the CMC Krafft point micellar structure does not exist in equilibrium surfactant solutions. Metastable micellar solutions may, however, easily be formed below the Krafft boundary by cooling concentrated liquid phases [47]. Cooling liquid-crystal phases below the Krafft eutectic typically yields metastable liquid-crystal (not liquid) phases. 

A particularly interesting compound in this context is C12E3, which displays a lamellar liquid-crystal solubility boundary at room temperature, an L3 solubility boundary between 39°C and 58°C, and a concentrated liquid-phase solubility boundary >58 C. The change in the qualitative nature of the solubility boundary from the lamellar liquid crystal to the L3 liquid profoundly alters the swelling behavior of this compound [71]. 

There is much evidence to suggest that the metastable liquid-liquid region of a protein solution is responsible for cataract formation. As the eye ages, its protein concentration changes and one (or more) proteins may achieve a concentration that exceeds the saturation concentration as indicated in the figiue. Following phase separation, the highly concentrated liquid phase does not set-tie but remains in the eye as a fine dispersion whose optical properties seriously interfere with vision. The experimental data show that when the concentration of native y-crystaUin exceeds about 100 mg cm at body temperature (310 K, 37 °C), a second liquid phase is formed whose concentration is about 700 mg cm. It is this... 

Figure 1 gives an enthalpy-concentration diagram for ethanol(1)-water(2) at 1 atm. (The reference enthalpy is defined as that of the pure liquid at 0°C and 1 atm.) In this case both components are condensables. The liquid-phase enthalpy of mixing... 

The equilibrium conversion can be increased by employing one reactant in excess (or removing the water formed, or both). b. Inerts concentration. Sometimes, an inert material is present in the reactor. This might be a solvent in a liquid-phase reaction or an inert gas in a gas-phase reaction. Consider the reaction system... 

Clearly, in the liquid phase much higher concentrations of Cfeed (kmol m ) can be maintained than in the gas phase. This makes liquid-phase reactions in general more rapid and hence leads to smaller reactor volumes for liquid-phase reactors. 

Note that in liquid phase chromatography there are no detectors that are both sensitive and universal, that is, which respond linearly to solute concentration regardless of its chemical nature. In fact, the refractometer detects all solutes but it is not very sensitive its response depends evidently on the difference in refractive indices between solvent and solute whereas absorption and UV fluorescence methods respond only to aromatics, an advantage in numerous applications. Unfortunately, their coefficient of response (in ultraviolet, absorptivity is the term used) is highly variable among individual components. 

This is an analysis frequently conducted on oil lubricants. Generally, the additive is known and its concentration can be followed by direct comparison of the oil with additive and the base stock. For example, concentrations of a few ppm of dithiophosphates or phenols are obtained with an interferometer. However, additive oils today contain a large number of products their identification or their analysis by IR spectrometry most often requires preliminary separation, either by dialysis or by liquid phase chromatography. 

It was pointed out that a bimolecular reaction can be accelerated by a catalyst just from a concentration effect. As an illustrative calculation, assume that A and B react in the gas phase with 1 1 stoichiometry and according to a bimolecular rate law, with the second-order rate constant k equal to 10 1 mol" see" at 0°C. Now, assuming that an equimolar mixture of the gases is condensed to a liquid film on a catalyst surface and the rate constant in the condensed liquid solution is taken to be the same as for the gas phase reaction, calculate the ratio of half times for reaction in the gas phase and on the catalyst surface at 0°C. Assume further that the density of the liquid phase is 1000 times that of the gas phase. 

In order to describe any electrochemical cell a convention is required for writing down the cells, such as the concentration cell described above. This convention should establish clearly where the boundaries between the different phases exist and, also, what the overall cell reaction is. It is now standard to use vertical lines to delineate phase boundaries, such as those between a solid and a liquid or between two innniscible liquids. The junction between two miscible liquids, which might be maintained by the use of a porous glass frit, is represented by a single vertical dashed line, j, and two dashed lines, jj, are used to indicate two liquid phases... 

Instead of concentrating on the diffiisioii limit of reaction rates in liquid solution, it can be histnictive to consider die dependence of bimolecular rate coefficients of elementary chemical reactions on pressure over a wide solvent density range covering gas and liquid phase alike. Particularly amenable to such studies are atom recombination reactions whose rate coefficients can be easily hivestigated over a wide range of physical conditions from the dilute-gas phase to compressed liquid solution [3, 4]. 

Zaikin A N and Zhabotinsky A M 1970 Concentration wave propagation in two-dimensional liquid-phase self-oscillating system Nature 225 535-7... 

At equilibrium, in order to achieve equality of chemical potentials, not only tire colloid but also tire polymer concentrations in tire different phases are different. We focus here on a theory tliat allows for tliis polymer partitioning [99]. Predictions for two polymer/colloid size ratios are shown in figure C2.6.10. A liquid phase is predicted to occur only when tire range of attractions is not too small compared to tire particle size, 5/a > 0.3. Under tliese conditions a phase behaviour is obtained tliat is similar to tliat of simple liquids, such as argon. Because of tire polymer partitioning, however, tliere is a tliree-phase triangle (ratlier tlian a triple point). For smaller polymer (narrower attractions), tire gas-liquid transition becomes metastable witli respect to tire fluid-crystal transition. These predictions were confinned experimentally [100]. The phase boundaries were predicted semi-quantitatively. 

The relative concentrations of either constituent, say B, in the vapour and liquid phases will be ... 

Thus a solution containing mol fractions of 0-25 and 0-75 of A and B respectively is in equilibrium with a vapour containing 16-7 and 83 -3 mol per cent, of A and B respectively. The component B with the higher vapour pressure is relatively more concentrated in the vapour phase than in the liquid phase. 

In practice, it is more convenient to predict the behavior of an ion, for any chosen set of conditions, by employing a much simpler distribution coefficient, which is defined as the concentration of a solute in the resin phase divided by its concentration in the liquid phase, or ... 

Equation 28 and its liquid-phase equivalent are very general and valid in all situations. Similarly, the overall mass transfer coefficients may be made independent of the effect of bulk fiux through the films and thus nearly concentration independent for straight equilibrium lines ... 

Nearly every chemical manufacturiag operation requites the use of separation processes to recover and purify the desired product. In most circumstances, the efficiency of the separation process has a significant impact on both the quality and the cost of the product (1). Liquid-phase adsorption has long been used for the removal of contaminants present at low concentrations in process streams. In most cases, the objective is to remove a specific feed component alternatively, the contaminants are not well defined, and the objective is the improvement of feed quality defined by color, taste, odor, and storage stability (2-5) (see Wastes, industrial Water, industrial watertreati nt). 

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