Phase leaning

Model Dense phase Lean phase Homogeneous reactions Heterogeneous reactions Reference... 

Steam can be considered a cor-eactant of oxidation during rich-phases (lean in O2) [3-5], In oxy-steam conversion of propane, we showed (fig.l) that propane oxidation was catalyzed by platinum (between 200 and 350°C) while rhodium was the key-component in the catalysis of steam reforming (between 350 and 600°C). Ceria was an excellent promotor of steam reactions [3, 6], particularly when this reaction was carried out in the presence of oxygen. Therefore, the steam reforming activity is an excellent indicator of the rhodium surface state since the activity systematically decreases when the metallic rhodium area decreases [7]. On the other hand, oxidation activity is a more complex indicator of platinum surface state because there exists an optimum dispersion [8,9]. 

The overhead vapor of compositionj/gj is totaHy condensed into two equiHbrium Hquid phases, an entrainer-rich phase of composition x and an entrainer-lean phase of composition The relative proportion of these two Hquid phases in the condenser, ( ), is given by the lever rule, where ( ) represents the molar ratio of the entrainer-rich phase to the entrainer-lean phase in the condensate. 

The two condensate Hquids must be used to provide reflux and distiUate streams. NormaHy, the reflux ratio, r, is chosen so that r = L jD > (j). This requires that the reflux rate be greater than the condensation rate of entrainer-rich phase and that the distiUate rate be correspondingly less than the condensation rate of entrainer-lean phase. This means that the distiUate stream consists of pure entrainer-lean phase, ie, Xj = x, and the reflux stream consists of aU the entrainer-rich phase plus the balance of the entrainer-lean phase. Thus, the overall composition of the reflux stream, Hes on the... 

In eatalytie eombustion of a fuel/air mixture the fuel reaets on the surfaee of the eatalyst by a heterogeneous meehanism. The eatalyst ean stabilize the eombustion of ultra-lean fuel/air mixtures with adiabatie eombustion temperatures below 1500°C. Thus, the gas temperature will remain below 1500 °C and very little thermal NO will be formed, as ean be seen in Figure 10-21. However, the observed reduetion in NOx in eatalytie eombustors is mueh greater than that expeeted from the lower eombustion temperature. The reaetion on the eatalytie surfaee apparently produees no NOx direetly, although some NOx may be produeed by homogeneous reaetions in the gas phase initiated by the eatalyst. 

Figure 1 shows water content of lean, sweet natural gas. It can be used also for gases that have as much as 10% CO2 and/or HiS if the pressure is below 500psia. Above 500psia, acid gases must be accounted for by rigorous three-phase flash calculations or approximation methods. ... 

To extract a desired component A from a homogeneous liquid solution, one can introduce another liquid phase which is insoluble with the one containing A. In theory, component A is present in low concentrations, and hence, we have a system consisting of two mutually insoluble carrier solutions between which the solute A is distributed. The solution rich in A is referred to as the extract phase, E (usually the solvent layer) the treated solution, lean in A, is called the raffinate, R. In practice, there will be some mutual solubility between the two solvents. Following the definitions provided by Henley and Staffin (1963) (see reference Section C), designating two solvents as B and S, the thermodynamic variables for the system are T, P, x g, x r, Xrr (where P is system pressure, T is temperature, and the a s denote mole fractions).. The concentration of solvent S is not considered to be a variable at any given temperature, T, and pressure, P. As such, we note the following ... 

Consider a lean phase, j, which is in intimate contact with a rich phase, i, in a closed vessel in order to transfer a certain solute. The solute diffuses from the rich phase to the lean phase. Meanwhile, a fraction of the diffused solute back-transfers to the rich phase. Initially, die rate of rich-to-lean solute transfer surpasses that of lean to rich leading to a net transfer of the solute from the rich phase to the lean phase. However, as the concentration of the solute in the rich phase increases. 

Throughout this bocdt, several mass-exchange operations will be considered simultaneously. It is therefore necessary to use a unified terminology such that y is always the composition in die rich phase and x is the composition in the lean phase. The reader is cautioned here that tiiis terminology may be different ftom other literature, in which y is used for gas-phase composition and x is used for liquid-phase composition. 

Whenever die rich and the lean phases are not in equilibrium, an interphase concentration gradient and a mass-transfer driving force develop leading to a net transfer of the solute from the rich phase to the lean phase. A common method of describing the rates of interphase mass transfer involves the use of overall mass-transfer coefficients which are based on the difference between the bulk concentration of the solute in one phase and its equilibrium concentration in the other phase. Suppose that the bulk concentradons of a pollutant in the rich and the lean phases are yi and Xj, respectively. For die case of linear equilibrium, the pollutant concnetration in the lean phase which is in equilibrium with y is given by... 

Let us define two overall mass transfer coefficients one for the rich phase, Ky, and one for the lean phase, Kj,. Hence, the rate of interphase mass transfer for... 

The stage efficiency may be defined based on the rich phase or the lean phase. For instance, when the stage efficiency is defined for the rich phase, r)y, Eq. (2.11) becomes... 

This value is in good agreement with the experimental conslant reported by Machay and Shiu (1981) to be 0.673 kPa - m /gm mol. It is instructive to demonstrate the convetsion between dilTerent ways of reporting Hemy s coefficient. First, the repotted value is inverted to be in the units of composition in the rich phase divided by composition in the lean phase, i.e., 1.486 gm mol/(kPa - m ) which can be converted into units of mole fraction as follows ... 

Cost estimation and screening external MSAs To determine which external MSA should be used to remove this load, it is necessary to determine the supply and target compositions as well as unit cost data for each MSA. Towards this end, one ought to consider the various processes undergone by each MSA. For instance, activated carbon, S3, has an equilibrium relation (adsorption isotherm) for adsorbing phenol that is linear up to a lean-phase mass fraction of 0.11, after which activated carbon is quickly saturated and the adsorption isotherm levels off. Hence, JC3 is taken as 0.11. It is also necessary to check the thermodynamic feasibility of this composition. Equation (3.5a) can be used to calculate the corresponding... 

Chapters Three, Five and Six have covered the synthesis of physical mass-exchange networks. In these systems, the targeted species were transferred from the rich phase to the lean phase in an intact molecular form. In some cases, it may be advantageous to convert the transferred species into other compounds using reactive MSAs. Typically, reactive MSAs have a greater capacity and selectivity to remove an undesirable component than physical MSAs. Furthermore, since they react with the undesirable species, it may be possible to convert pollutants into other species that may either be reused within the plant itself or sold. 

In order to establish the conditions for thermodynamic feasibility of reactive mass exchange, it is necessary to invoke the basic principles of mass transfer with chemical reactions. Consider a lean phase j that contains a set Bj = z —... 

It is now useful to recall the concepts of molarity and fractional saturation (Astarita etal., 1983). The molarity, mj, of areactive MSA is the total equivalent concentration of species that may react with component A. On the other hand, the fractional saturation, uj, is a variable that represents the degree of saturation of chemically combined A in the jth lean phase. Therefore, ujmj is the total concentration of chemically combined A in the yth MSA. Hence, the total concentration of A in MSA j can be expressed as... 

As discussed earlier, the admissible compositions may be selected as the lean-phase composition at some particular instant of time, or any other situation which is compatible with stoichiometry and mass-balance bounds such as Eqs. (8.20) and (8.21). Let us aihitrarily select the admissible composition of to be zero. 

The new pressure loss equation presented here is based on determining two parameters the velocity difference between gas and conveyed material and the falling velocity of the material. The advantage of this method is that no additional pressure loss coefficient is needed. The two parameters are physically clear and they are quite easily modeled for different cases by theoretical considerations, which makes the method reliable and applicable to various ap>-plications. The new calculation method presented here can be applied to cases where solids are conveyed in an apparently uniform suspension in a so-called lean or dilute-phase flow. 

EXj = Total mols of all liquid phase components absorbed per mol of lean oil (omitting lean oil present in liquid phase, considered = 1.0)... 

Xm + 1 = Number liquid phase mols of component entering stripper per mol of lean oil... 

Xoi = Number liquid phase mols of component entering absorber with lean oil per mol of lean oil Yi = Number vapor phase mols of component leaving top plate of absorber per mol rich gas entering absorber Y = Mols component in vapor phase from tray i /mol rich gas entering absorber... 

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