Ammonia/water phase equilibrium

A. System NH3 H S-H20. The dissociation of water (re-action 9) and the second dissociation of H2S (reaction 6) are neglected at given temperature and total molalities of NHo and H2S there remain four unknown molalities in the liquid phase (e.g. NH3, NH4+, H2S and HS ), the composition of the vapor phase and the total pressure, which are calculated from 8 equations The dissociation constants of ammonia and hydrogen sulfide (eqs.I and III) together with the phase equilibrium for hydrogen sulfide (eq. XII) are combined resulting in a equilibrium constant K 2... 

Gas and Liquid Phases. Equilibrium data (P-V-T) and thermodynamic properties for the single-component systems water (steam) and ammonia are complete and apparently of the best accuracy because of the extensive use of these substances in cyclic systems 14,20). 

Construct the equilibrium line. The equilibrium line relates the mole fraction of ammonia in the gas phase to that in the liquid phase when the two phases are at equilibrium. Equilibrium is assumed to exist between the two phases only at the gas-hquid interface. For dilute systems, Henry s law will apply. It applies for liquid mole fractions less than 0.01 in systems in general, and, as can be seen in Example 11.5, for the ammonia-water system it applies to liquid mole fractions as high as... 

Enthalpy-concentration charts are particularly useful for two-component systems in which vapor and liquid phases are in equilibrium. The Gibbs phase rule (Equation 6.2-1) specifies that such a system has (2 -I- 2 - 2) = 2 degrees of freedom. If as before we fix the system pressure, then specifying only one more intensive variable—the system temperature, or the mass or mole fraction of either component in either phase—fixes the values of all other intensive variables in both phases. An H-x diagram for the ammonia-water system at 1 atm is shown in Figure 8.5-2. 

We conclude this section by noting that, in some cases states involving both chemical and phase equilibrium can be considered to be chemical equilibrium problems only, but with different states of aggregation for the standard states of the various species present. Consider, for example, the dissolution of gaseous ammonia in water, and its subsequent reaction to form ammonium hydroxide. This process can be considered either to occur in two steps, the first involving phase equilibrium,... 

Carbon Dioxide/Water Equilibrium 345 Sulfur Dioxide 348 Ammonia/Water Equilibrium 353 Nitric Acid/Water Equilibrium 355 Equilibrium of Other Important Atmospheric Gases Aqueous-Phase Reaction Rates 361 S(IV) to S(VI) Transformation and Sulfur Chemistry 363... 

If the column is to be operated isothermally, then the typical problem is to use the energy balance (12.4.23), together with material balances and phase-equilibrium relations, to compute Q and the composition x< for the liquid leaving the column. If the column is to be operated adiabatically, then the typical problem is to determine both the temperature T and the composition x< at the liquid outlet. We illustrate both problems using absorption of ammonia from air into water the following problem was originally analyzed by Sherwood and Pigford [13]. 

Check whether we have a closed problem. We have Np = 4 streams and C = 3 components (ammonia, water, and "air"). Since the phases do not reach equilibrium in the unit, Fgx is given by (12.3.9)... 

Assuming that 1000 compounds are of technical interest, phase equilibrium information for about 500000 binary systems are required to fit the required binary parameters to describe aU possible binary and multicomponent systems. Although more than 64500 VLE data sets for nonelectrolyte systems have been published up lo now, VLE data are available for only 10300 binary systems, since for a few systems a large number of data sets were published, for example, for the systems ethanol-water, ammonia-water, water-carbon dioxide, methanol-water, methane-nitrogen more than 150 data sets are available. This means that only for 2% of the required systems at least one VLE data set is available. If only... 

As in the gas-liquid systems, the equilibrium in vapor-liquid systems is restricted by the phase rule, Eq. (10,2-1). As an example we shall use the ammonia-water, vapor-liquid system. For two components and two phases, F from Eq. (10.2-1) is 2 degrees of freedom. The four variables are temperature, pressure, and the composition of NH3 in the vapor phase and in the liquid phase. The composition of water (B) is fixed ify or. 4 is specified, since + ys = TO and, x + Xg = 1.0. If the pressure is fixed, only one more variable can be set. If we set the liquid composition, the temperature and vapor composition are automatically set. 

The basic requirement for the separation of the components by distillation is that the composition of the vapor be different from the composition of the liquid with which it is in equilibrium at the boiling point of the liquid. Distillation is concerned with solutions where all components are appreciably volatile, such as in ammonia-water or ethanol-water solutions, where both components will be in the vapor phase. In evaporation, however, of a solution of salt and water, the water is vaporized but the salt is not. The process of absorption differs from distillation in that one of the components in absorption is essentially insoluble in the liquid phase. An example is absorption of ammonia from air by water, where air is insoluble in the water-ammonia solution. 

We have selected as base cases those of dry air and of humid air with ammonia water interactions, both with the liquid fraction of = 0.85. These were evaluated in more detail using the model AERCLOUD. Figures 21.1a to d show the computed numbers of moles of ammonia (vapor and liquid phases). The headlines of figures show also the selected initial droplet sizes (1,000 /u,m and 100 /um). The curves marked with HE-limit show the model predictions in the homogeneous equilibrium limit. 

Coefficients of the equadon of state and of the equation for transport properties are stored for each substance. Parameters of the critical point and coefficients of equations for calculadon of the ideal-gas functions, the saturated vapor pressure and the melting pressure are kept also. The thermal properties in the single-phase region and on the phase-equilibrium lines can be calculated on the basis of well-known relations with use of these coefficients. The system contains data for 30 reference substances monatomic and diatomic gases, air, water and steam, carbon dioxide, ammonia, paraffin hydrocarbons (up to octane), ethylene (ethene), propylene (propene), benzene and toluene. The system can calculate the thermophysical properties of poorly investigated gases and liquids and of multicomponent mixtures also on the basis of data for reference substances. 

Pb(CH3)4 is soluble In absolute ethanol [1, 2], but not in 96% ethanol [3]. It is soluble in ether [1, 2], hydrocarbons, benzene, toluene, and other usual organic solvents [3], but Insoluble in liquid ammonia at —78 C [4]. Solutions of Pb(CH3)4 in C7Hi4-n (components are about equally volatile) are used as standards in atomic absorption spectrometry. Such solutions are stable for more than 6 months In contrast to Pb(C2H5)4 solutions [27]. For a phase equilibrium study of the systems Pb(CH3)4-toluene and Pb(CH3)4-benzene, see [38]. Estimated values of free energies of transfer of Pb(CH3)4 from methanol to water, alcohols, and several other solvents are given in [12, 21]. A solution of 17 to 90% methanol in 0.1 molar acetate buffer is used as a mobile phase for the separation of (CH3)4 nPb(C2H5)n by HPLC [39, 40]. Pb(CH3)4 and acetonitrile form an azeotrope [6]. Pb(CH3)4 is quantitatively extracted from dust samples into cold ammoniacal methanol [24]. The llpophllicity of Pb(CH3)4 is lower than that of Pb(C2H5)4 [41]. 

Various amines find application for pH control. The most commonly used are ammonia, morpholine, cyclohexylamine, and, more recently AMP (2-amino-2-methyl-l-propanol). The amount of each needed to produce a given pH depends upon the basicity constant, and values of this are given in Table 17.4. The volatility also influences their utility and their selection for any particular application. Like other substances, amines tend towards equilibrium concentrations in each phase of the steam/water mixture, the equilibrium being temperature dependent. Values of the distribution coefficient, Kp, are also given in Table 17.4. These factors need to be taken into account when estimating the pH attainable at any given point in a circuit so as to provide appropriate protection for each location. 

In a packed column, operating at approximately atmospheric pressure and 295 K, a 10% ammonia-air mixture is scrubbed with water and the concentration of ammonia is reduced to 0.1%. If the whole of the resistance to mass transfer may be regarded as lying within a thin laminar film on the gas side of the gas-liquid interface, derive from first principles an expression for the rate of absorption at any position in the column. At some intermediate point where the ammonia concentration in the gas phase has been reduced to 5%. the partial pressure of ammonia in equilibrium with the aqueous solution is 660 N/nr and the transfer rate is ]0 3 kmol/m2s. What is the thickness of the hypothetical gas film if the diffusivity of ammonia in air is 0.24 cm2/s ... 

Acetylene is sufficiently acidic to allow application of the gas-phase proton transfer equilibrium method described in equation l7. For ethylene, the equilibrium constant was determined from the kinetics of reaction in both directions with NH2-8. Since the acidity of ammonia is known accurately, that of ethylene can be determined. This method actually gives A f/ acid at the temperature of the measurement. Use of known entropies allows the calculation of A//ac d from AG = AH — TAS. The value of A//acij found for ethylene is 409.4 0.6 kcal mol 1. But hydrocarbons in general, and ethylene in particular, are so weakly acidic that such equilibria are generally not observable. From net proton transfers that are observed it is possible sometimes to put limits on the acidity range. Thus, ethylene is not deprotonated by hydroxide ion whereas allene and propene are9 consequently, ethylene is less acidic than water and allene and propene (undoubtedly the allylic proton) are more acidic. Unfortunately, the acidity of no other alkene is known as precisely as that of ethylene. 

A mixture of ammonia and air is scrubbed in a plate column with fresh water. If the ammonia concentration is reduced from 5 per cent to 0.01 per cent, and the water and air rates are 0.65 and 0.40 kg/m2s, respectively, how many theoretical plates are required The equilibrium relationship may be written as Y = X, where X is the mole ratio in the liquid phase. 

The solubility of gaseous weak electrolytes in aqueous solutions is encountered in many chemical and petrochemical processes. In comparison to vapory-liquid equilibria in non reacting systems the solubility of gaseous weak electrolytes like ammonia, carbondioxide, hydrogen sulfide and sulfur dioxide in water results not only from physical (vapor-liquid) equilibrium but also from chemical equilibrium in the liquid phase. 

Mass transfer controlled by diffusion in the gas phase (ammonia in water) has been studied by Anderson et al. (A5) for horizontal annular flow. In spite of the obvious analogy of this case with countercurrent wetted-wall towers, gas velocities in the cocurrent case exceed these used in any reported wetted-wall-tower investigations. In cocurrent annular flow, smooth liquid films free of ripples are not attainable, and entrainment and deposition of liquid droplets presents an additional transfer mechanism. By measuring solute concentrations of liquid in the film and in entrained drops, as well as flow rates, and by assuming absorption equilibrium between droplets and gas, Anderson et al. were able to separate the two contributing mechanisms of transfer. The agreement of their entrainment values (based on the assumption of transfer equilibrium in the droplets) with those of Wicks and Dukler (W2) was taken as supporting evidence for this supposition. 

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