Liquor composition, equilibrium

Importance of Design Diagrams One of the first things a designer should tiy to do is lay out a carefully constructed equilibrium curve, y° = Fix), on an xy diagram, as shown in Fig. 14-2. A horizontal line corresponding to the inlet-gas composition yi is then the locus of feasible outlet-liquor compositions, and a vertical hue corresponding to the inlet-solvent-liquor composition X9 is the locus of feasible out-... 

Correlations for Equilibrium S02 Partial Pressure and Liquor Composition... 

In order to facilitate calculations and increase the usefulness of the chemical model, the computerized model has been used to obtain simplified correlations suitable for hand calculations. These correlations predict equilibrium SO2 partial pressure and liquor composition as functions of liquor pH and the total dissolved concentrations of magnesium and chloride. 

Calculation of Liquid-to-Gas Ratio The minimum possible liquid rate is readily calculated from the composition of the entering gas and the solubility of the solute in the exit liquor, with equilibrium being assumed. It may be necessary to estimate the temperature of the exit liquid based upon the heat of solution of the solute gas. Values of latent heat and specific heat and values of heats of solution (at infinite dilution) are given in Sec. 2. 

In most cases, the process of crystallization is slow and the final mother liquor is in contact with a sufficiently large crystal surface so that the concentration of the mother liquor is substantially that of a saturated solution at the final temperature in the process. In such cases, it is normal to calculate the yield from the initial solution composition and the solubility of the material at the final temperature. Some solutions, such as sucrose and water, supersaturate easily and do not attain equilibrium with respect to the final mother liquor temperature. In such cases, an assumption must be made as to the mother liquor composition at the end temperature. 

The pressure difference between the liquid-solid equilibrium curves for pure and impure materials corresponds to the freezing temperature depression caused by the presence of an impurity. The pressure increase accompanying the increase in pure crystalline material deposited is related to the concentration of impurities in the liquid. With this information, therefore, the mother liquor composition at any stage can be estimated from the system pressure. Consequently, a highly pure solid phase can be obtained by separating the liquid phase from the solid phase while maintaining an appropriate pressure corresponding to the desired solid fraction or mother liquor composition. 

Once a particular class of unit has been decided upon, the choice of a specific unit depends on initial and operating costs, the space available, the type and size of the product, the characteristics of the feed liquor, the need for corrosion resistance and so on. Particular attention must be paid to liquor mixing zones since the circulation loop includes many regions where flow streams of different temperature and composition mix. These are all points at which temporary high supersaturations may occur causing heavy nucleation and hence encrustation, poor performance and operating instabilities. As Toussaint and Donders(72) stresses, it is essential that the compositions and enthalpies of mixer streams are always such that, at equilibrium, only one phase exists under the local conditions of temperature and pressure. 

Fig. 7-20. Equilibrium diagram showing the composition of the kraft white liquor at different pH values. Concentration, 0.2 mole/liter temp., 25°C. Based on equilibrium constants corresponding to = 10 and K> = 10 7.

The crystallization temperature depends on the composition of the mixture to be treated. The cooling diagram shows that a eutectic exists between p-xylene and each of the other components of the mixture. In the case of the m, p-xyiene binary system, the eutectic contains 13 per cent p-xylene and melts at —52 C (Fig. 4.10). It separates two iiquidus curves ME in equilibrium with solid m-xylene, PE in equilibrium with solid p-xylene. Provided that the initial mixture contains more than 13 per cent p-xyleoe. crystals of pure p-xylene are obtained by cooling to — 52 and a mother liquor, whose composition is that of the eutectic. However, it qan be noticed (bat the existence of the eutectic leads to limited recovery, and that this recovery requires beat exchanges at low temperature. 

Some properties are more clearly defined than others. For example, solubility is defined as the amount of solid in equilibrium with the solvent. Solubility can affect the capacity of the crystallization process, as well as its ability to reject undesired compounds and minimize loss in the mother liquor. In addition, solubility varies widely from compound to compound or solvent to solvent. On the other hand, there are properties that are much less well characterized or understood. For example, the mechanism and condition for the formation of oil or amorphous solid remain unclear. The composition of oil and amorphous solid can be variable, and certainly can contain a much higher level of impurities than that in the crystalline solid, which leads to a real purification challenge. In addition, oil or amorphous solid generally is less stable and can create critical issues in drug formulation and storage stability. 

A good diffraction pattern from protein crystals generally requires mounting the crystal in contact with its mother liquor inside a thin-walled glass capillary tube along with a drop or two of the mother liquor so that an equilibrium atmosphere is maintained (Fig. 19). The capillary containing the crystal is then mounted on the goniometer head. Alternatively, a flow cell, in which liquid with a controlled composition is allowed to flow over a crystal, can be used to maintain the required crystal environment. 

Let us suppose that the double salt is in equilibrium with the solution at a definite temperature, and that the composition of the solution is represented by the point e. The greater part of the solution is now separated from the solid phase, and the latter, together with the adhering mother liquor is analysed. The composition (expressed, as before, in gram-molecules of A and B to 100 gram-molecules of water) will be represented by a point e,g. f) on the line eS, where S represents the composition of the double salt. 

In the dyeing process, attention has to be paid to an even distribution of the dye in the dye liquor. The first step is a dye adsorption at the fiber surface, and the second step is the diffusion of the dye molecules into the fiber core. Finally, there is an interaction between the dye and the textile fibrous material. The individual processes are completed if a complete equilibrium has occurred between the dye liquor and the textile fibrous material. In the dyeing process the relevant process parameters are the composition of the dye liquor (textile chemicals dyeing auxiliaries, for example, leveling agents and dye), the liquor temperature, the dyeing time, the pH value of the liquor, and the transaction type and rate of the fabric. 

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