Mass change rate

A slope change of a TG curve is the main feature used to analyze a sample. Sometimes, the slope change is uncertain in this case, a derivative TG curve can be used. The derivative TG (DTG) curve is a plot of versus temperature. Figure 10.28 shows comparison of TG and corresponding DTG curves. A peak in a DTG curve represents a maximum of mass change rate. DTG does not contain any new information other than the original TG curve however, it clearly identifies the temperature at which mass loss is at a maximum. 

The growth terms need some further attention. Considering that for fluid particles the mass change rate (or an imaginary velocity in the internal coordinates) of the particle, is negative for the case of condensation or particle dissolution, and is positive in the case of evaporation or mass diffusion into the particle the distribution function was discretized using an upwinding approach. 

The histories of mass change rate of Specimen B and C are shown in Figure 4. The air flow of the arc wind tunnel was ejq)ected to be chemically active. However, the mass reduction rate for the arc-heated wind tunnel test was unexpectedly smaller than that for the hot gas flow test. This can possibly be attributed to the ambient pressure, because the ambient pressure for the hot gas flow test was... 

Data Recording Unit 4.3 Temperature Calibration 4.4 Sample 4.5 Atmosphere 4.6 Heating Rate 4.7 Classification of TG Curves 4.8 Calculation of Mass Change Rates 4.9 Derivative Thermogravimetry (DTG) 4.10 Intercomparison of TG and DTA 4.11 TG Reports 4.12 References 5... 

Figure 3.9. Adsorption equilibria of helium (5.0) on activated carbon (AC) NORIT R1 EXTRA at T = 298.15 K and T = 323.15 K. Data of the apparent weight of the sorbent sample are sketched as function of the density of the helium gas (pHe) within the region (0 < pne < 2 kg / m ) corresponding to the pressure range (0 < p < 2.5 MPa). Data were always taken 15 minutes after increase of hehum gas pressure at a relative mass change rate of (Am / m At = 5.10 g / (gh)).

As also noted in the preceding chapter, it is customary to divide adsorption into two broad classes, namely, physical adsorption and chemisorption. Physical adsorption equilibrium is very rapid in attainment (except when limited by mass transport rates in the gas phase or within a porous adsorbent) and is reversible, the adsorbate being removable without change by lowering the pressure (there may be hysteresis in the case of a porous solid). It is supposed that this type of adsorption occurs as a result of the same type of relatively nonspecific intermolecular forces that are responsible for the condensation of a vapor to a liquid, and in physical adsorption the heat of adsorption should be in the range of heats of condensation. Physical adsorption is usually important only for gases below their critical temperature, that is, for vapors. 

The equations of combiaed diffusion and reaction, and their solutions, are analogous to those for gas absorption (qv) (47). It has been shown how the concentration profiles and rate-controlling steps change as the rate constant iacreases (48). When the reaction is very slow and the B-rich phase is essentially saturated with C, the mass-transfer rate is governed by the kinetics within the bulk of the B-rich phase. This is defined as regime 1. 

Momentum Flow Meters. Momentum flow meters operate by superimposing on a normal fluid motion a perpendicular velocity vector of known magnitude thus changing the fluid momentum. The force required to balance this change in momentum can be shown to be proportional to the fluid density and velocity, the mass-flow rate. 

For many laboratoiy studies, a suitable reactor is a cell with independent agitation of each phase and an undisturbed interface of known area, like the item shown in Fig. 23-29d, Whether a rate process is controlled by a mass-transfer rate or a chemical reaction rate sometimes can be identified by simple parameters. When agitation is sufficient to produce a homogeneous dispersion and the rate varies with further increases of agitation, mass-transfer rates are likely to be significant. The effect of change in temperature is a major criterion-, a rise of 10°C (18°F) normally raises the rate of a chemical reaction by a factor of 2 to 3, but the mass-transfer rate by much less. There may be instances, however, where the combined effect on chemical equilibrium, diffusivity, viscosity, and surface tension also may give a comparable enhancement. 

It is seen that the viscosity of the gas will change significantly during a temperature program and, thus, at a constant gas mass flow rate, the inlet pressure will rise proportionally. This increase in inlet pressure will result in an increase in the inlet/outlet pressure ratio and, as a consequence, will extend the retention time and oppose the effect of any increase in temperature. It also follows that the effect of... 

Technical calculations dealing with humid air are reasonable to solve with dry air mass flow rates, because these remain constant in spite of changes in the amount of water vapor in the air. For that reason a definition for enthalpy,... 

Calculate the shell-side dry-gas film coefficient, hg or h, for outside tube conditions. Assume a baffle spacing or about equal to one shell diameter. Use the shell-side method described in Equation 10-48 and Figure 10-54. This is necessary for inlet conditions and then must be checked and recalculated if sufficient change occurs in the mass flow rate, G, to yield a change in hg. 

In addition, it was concluded that the liquid-phase diffusion coefficient is the major factor influencing the value of the mass-transfer coefficient per unit area. Inasmuch as agitators operate poorly in gas-liquid dispersions, it is impractical to induce turbulence by mechanical means that exceeds gravitational forces. They conclude, therefore, that heat- and mass-transfer coefficients per unit area in gas dispersions are almost completely unaffected by the mechanical power dissipated in the system. Consequently, the total mass-transfer rate in agitated gas-liquid contacting is changed almost entirely in accordance with the interfacial area—a function of the power input. 

A change in the agitation intensity or gas flow rate will change 0. As a result, b, c0, and the total mass-transfer rate are affected. Thus, the effect of mixing in the vessel is considered by indirect mechanisms. 

Whatever the physical constraints placed on the system, the diffusional process causes the two components to be transferred at equal and opposite rates and the values of the diffusional velocities uDA and uDB given in Section 10.2.5 are always applicable. It is the bulk How velocity uF which changes with imposed conditions and which gives rise to differences in overall mass transfer rates. In equimolecular counterdiffusion. uF is zero. In the absorption of a soluble gas A from a mixture the bulk velocity must be equal and opposite to the diffusional velocity of B as this latter component undergoes no net transfer. 

At radius r within sphere, mass transfer rate — — (4nr2)D The change in mass transfer rate over a distance dr... 

A pure gas is absorbed into a liquid with which it reacts. The concentration in the liquid is sufficiently low for the mass transfer to be covered by Fick s Law and the reaction is first-order with respect to the solute gas. It may be assumed that the film theory may be applied to the liquid and that the concentration of solute gas falls from the saturation value to zero across the film. The reaction is initially carried out at 293 K. By what factor will the mass transfer rate across the interface change, if the temperature is raised to 313 K ... 

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