Vapour fraction

The vapour fraction of the solute is moved down the column by the carrier gas and the equilibrium between the two phases is destroyed. However, in an attempt to re-establish an equilibrium, solute molecules leave the liquid phase restoring the partial pressure above the solution. The solute vapour which has been moved down the column encounters fresh solvent and a new equilibrium is established. 

Alternatively, the mean of G (calculated at the relief pressure) and G (calculated at the maximum accumulated pressure) can be used. Since Leung s method assumes that the reactor contents are homogeneous during relief, the consistent assumption for the vapour fraction at inlet to the relief system (needed to calculate G) is that it is the same as the average for the reactor, i.e. at the inlet to the relief system the void fraction is given by ... 

The fraction of gas or vapour in the two-phase mixture entering the pressure relief system is an important parameter if the detailed pressure/ time history for the vented runaway reaction is to be predicted. This type of calculation is performed by a number of computer codes (see Annex 4). In order to predict the gas or vapour fraction in the two-phase flow entering the relief system, the "coupling equation", a material balance for the vessel and relief system, needs to be solved iteratively. This is discussed in detail by DIERS[5J and is beyond the scope of this Workbook. 

A material balance of the flow from the reactor and jithe flow into the relief system, taking account of the vapour fraction at inlet to the relief system as calculated using level swell methodology. See. A3.4. 

It is also possible to distil fractions I, II and III below 185 °C in vapour (fraction la) or fractions I and II below 150 °C in vapour (fraction lb). Fractions I-IV self-flow or are sent by vacuum from receptacles 3 into collector 7, and fractions la and lb into collector 8. After the distillation is completed, the electric heating of the tank is switched off, the vacuum pump is stopped and tank 1 is filled with nitrogen (0.07 MPa). When the pressure in the system is atmospheric, the tank residue which boils above 250 °C (at 1.3-6.6 GPa) is sent by vacuum through cooler 4 into batch box... 

Distillation The process of separating a liquid from a solid, or other liquids, by vaporizing it and then condensing the vapour. Fractional distillation separates... 

As an exercise, the reader is invited to demonstrate that both for condenser and reboiler, the degrees of freedom are (A +4), identical with a flash. Typically, the specifications are input stream N. + 2) variables plus two others. Outlet pressure is usually imposed. The remaining variable may be liquid or vapour fraction, including bubble-point liquid (1=1), dew-point vapour (1=0), or sub-cooled liquid or superheated vapour (unusual). The above specifications enable to compute the duty Q, but this may be given also as specification. Note also that in steady state flowsheeting the reflux drum is included in the simulation of condenser. The type of condenser (partial, total, or sub-cooled liquid), as well as the type of reboiler (kettle or thermosyphon) does not change the analysis. 

A phase envelope maps the equilibrium behaviour of a complex mixture in a P-T space at constant composition. Figure 6.1 displays the plot obtained with ASPEN Plus with curves of constant vapour fraction, from bubble (V=0) to dew points (V=l). 

We also define porosity ri= - liquid saturation and vapour fraction... 

The final state variables are saturation j , temperature T, and vapour fraction f Inertial terms and gravitation are neglected and the total pressure is assumed to be constant. The model consists of the conservation of the mass of the water species, and the thermal energy conservation, i.e.. 

For the majority of reactions performed in organic solvents with a low viscosity, it may be assumed that the vapoiu mass fraction of the fluid will be very small. This can be explained by the dependence of the volumetric vapour fraction s on the vapour mass fraction. Figure 7-4 shows this relationship for four solvents at 6 bars absolute pres-sme. 

Fig. 7-4. Dependence of volumetric vapour fraction on vapour mass fraction for four different organic solvents at 6 bar...

From this diagram it can be seen that 5% vapour mass fraction already correspond to 65% of volumetric vapour fraction on average. Actually this value of 5 % may be regarded as a good first estimate if a rigorous design is acceptable. For such cases, with a low vapour mass fractions the critical mass flow density may be approximated with the help of the following equation. 

The hot effluent is cooled to -1°C (in practice this very large temperature difference can only achieved by direct contact heat exchange, ie a quench system). The pressure drop across the large condenser is 50 kPa. Under these conditions, the product stream has a vapour fraction of about 0.8 and the task of recovering condensable liquid ethylene oxide begins. The cool product stream is fed into a 3-phase separator and the light liquid phase is separated from the heavy liquid phase and vapour residual. HYSYS normally puts water in the heavy phase when there is a non-zero water stream. 

Fig. 10.12 Mass flow rate and vapour quality at the beginning of release as functions of the volumetric vapour fraction a...

The volumetric vapour fraction at the surface of the liquid/vapour mixture is given by... 

At a leak elevation of z we finally have the vapour fraction... 

In the first place the mean vapour fraction is calculated according to Sect. 10.2.5.1. If the leak lies above the liquid, respectively liquid/vapour surface, the flow regime and the corresponding mass flow rate are determined according to Sect. 10.2.3. If the leak is below the surface the volumetric vapour firaction is calculated according to Eq. (10.25). The methods of Sect. 7.4 for dealing with two-phase flow are then used. 

This is added to the initial vapour fraction of 1.558 x 10 " so that 0.293 results. In view of the vaporization by heat transfer due to mixing with the surrounding air (20 °C), which has not been accounted for so far, one may assume that the vapour fraction is even higher. Since propylene is heavier than air the vapour will be dispersed as a dense gas (vid. Sect 10.5.2). ... 

Thus the total vapour fraction after flash vaporization amounts to 0.23 + 0.511 + 0.2555 = 0.9965, so that it is assumed for the subsequent dispersion calculation that there is nothing but propylene vapour. ... 

The vapour fraction after depressurization is calculated according to Eq. (10.172) using the data of Table 10.13. This gives... 

An expansion energy of 2 MJ/m is read from the corresponding graph in [37]. This refers to the vapour fraction in the vessel, since flash vaporization of the liquid fraction is improbable at this temperature. The uncertainty in assessing the expansion energy is accounted for by using a constant pdf according to Eq. (C.33). The interval [1.8, 2.2] MJ/m is used. 

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