Dew point, estimation

Figure 2.36 Dew Point Estimates by Caicuiation for Methiene Chioride...

Figure 2.40 Dew Point Estimates by Calculation for Isopropyl Lactate...

The bubble and dew-point temperature calculations have been implemented by the FORTRAN IV subroutine BUDET and the pressure calculations by subroutine BUDEP, which are described and listed in Appendix F. These subroutines calculate the unknown temperature or pressure, given feed composition and the fixed pressure or temperature. They provide for input of initial estimates of the temperature or pressure sought, but converge quickly from any estimates within the range of validity of the thermodynamic framework. Standard initial estimates are provided by the subroutines. 

The computer subroutines for calculation of vapor-liquid equilibrium separations, including determination of bubble-point and dew-point temperatures and pressures, are described and listed in this Appendix. These are source routines written in American National Standard FORTRAN (FORTRAN IV), ANSI X3.9-1978, and, as such, should be compatible with most computer systems with FORTRAN IV compilers. Approximate storage requirements for these subroutines are given in Appendix J their execution times are strongly dependent on the separations being calculated but can be estimated (CDC 6400) from the times given for the thermodynamic subroutines they call (essentially all computation effort is in these thermodynamic subroutines). 

Aluminised steel produced by hot dipping is used in the construction of parts of many exhaust systems of road vehicles. Failure of some of these exhausts does take place well within the expected two-year average life. This arises in the rear end of the exhaust where dew point corrosion occurs on the inside of the system. Acid dew of pH 2.7-3.1 is produced in the exhaust gases at temperatures below 48°C and this concentrates as the system eventually heats up towards 100°C. The aluminised coating is attacked at weak positions, e.g. where holes have been punched and the aluminium does not completely coat the steel. Eventually, the aluminium coating is undermined and the steel severely attacked. It is estimated that the use of aluminium coatings can increase the life of unprotected steel by at least 12%. 

To estimate the stage, and the condenser and reboiler temperatures, procedures are required for calculating dew and bubble points. By definition, a saturated liquid is at its bubble point (any rise in temperature will cause a bubble of vapour to form), and a saturated vapour is at its dew point (any drop in temperature will cause a drop of liquid to form). 

The average volatilities will be taken as those estimated in Example 11.5. Normally, the volatilities are estimated at the feed bubble point, which gives a rough indication of the average column temperatures. The dew point of the tops and bubble point of the bottoms can be calculated once the component distributions have been estimated, and the calculations repeated with a new estimate of the average relative volatilities, as necessary. 

As these values are close to those assumed for the calculation of the dew points and bubble points in Example 11.5, there is no need to repeat with new estimates of the relative volatilities. 

At a pressure of 10 bar, determine the bubble and dew point of a mixture of hydrocarbons, composition, mol per cent n-butane 21, n-pentane 48, n-hexane 31. The equilibrium K factors can be estimated using the De Priester charts in Chapter 8. 

If the degree of superheat is large, it will be necessary to divide the temperature profile into sections and determine the mean temperature difference and heat-transfer coefficient separately for each section. If the tube wall temperature is below the dew point of the vapour, liquid will condense directly from the vapour on to the tubes. In these circumstances it has been found that the heat-transfer coefficient in the superheating section is close to the value for condensation and can be taken as the same. So, where the amount of superheating is not too excessive, say less than 25 per cent of the latent heat load, and the outlet coolant temperature is well below the vapour dew point, the sensible heat load for desuperheating can be lumped with the latent heat load. The total heat-transfer area required can then be calculated using a mean temperature difference based on the saturation temperature (not the superheat temperature) and the estimated condensate film heat-transfer coefficient. 

The universal form is most often used to prepare isosteric charts. That is charts relating either equihbrium vapor pressure or dew point, in equiUbrium, to the loading at a given temperature. The charts have lines of constant loading depicted as functions of reciprocal temperature. Such charts are particularly useful for estimation of the Umiting dew point or partial pressure that can be achieved for a given separation problem. 

EXAMPLE 2-6 Consider a mixture of 47.6 weight percent n-pentane and 52.4 weight percent n-heptane. Estimate the specific volume of the liquid at its bubble point at 400°F. Also estimate the specific volume of the gas at its dew point at 400°F. 

A reasonably accurate procedure for estimating the dew-point pressure of a retrograde gas is given in Appendix B. 

EXAMPLE 12-7 Estimate the dew-point pressure of the mixture given in Example 12-1 at a temperature ofl50°F. Assume that the equilibrium ratio charts given in Appendix A can be used for this mixture. 

A2.1 Factors to Consider When Estimating Water Vapor Pressure A2.2 Dew-Point Method for the Determination of Water Activity A2.3 Measurement of Water Activity Using Isopiestic Method A2.4 Direct Manometric Determination of Vapor Pressure A2.5 Measurement of Water Activity by Electronic Sensors... 

Estimating the unknown but required starting values of conditions and compositions is an important and sensitive part of these calculations. The composition of the feed is always known, as is the composition of one of the two phases in bubble and dew point calculations. With the Chao-Seader, Grayson-Streed, and Lee-Erbar-Edmister methods, it is possible to assume that both phases have the composition of the feed for the first trial. This assumption leads to trouble with the Soave-Redlich-Kwong, the Peng-Robinson and the Lee-Kesler-Ploecker... 

A vapor mixture ofmethanoK 1) and water(2) containing 56-mol-% methanol enters a condenser at 101.33 kPa at its dew point of 82.85 C. It is completely condensed and leaves the condenser at its bubble point of T2.05°C. How much heat must be transferred in the condenser for each mole of mixture condensed The latent heat of vaporization of methanol at its normal boiling point of 64.7°C is 35,228 J mol-1. The heat of mixing of a liquid mixture containing 56-mol-% methanol at 72°C is estimated as -500 J mol-1. 

Estimate the amount of propane in the vapor leaving the column overhead. The vapor stream leaving the column overhead may be assumed to be at its dew point in other words, the dew point of the overhead mixture is —60°F and 295 psia. At the dew point, (Yj/Kj) = 1.0. Designate Yt,... 

Repeat the trial-and-error procedure with the aid of graphic interpolation until the dew-point temperature is found. As the next estimate of the dew point, try 110°C. At this temperature, Kt = 1.756 and K2 = 0.428, as found via the procedure outlined in the previous step. Then y /Kf) + (> 2/K2) = 0.923. Since the sum is less than 1, the assumed temperature is too high. 

The curve and the data points shown in figure 3A.2 are all dew points, incipient liquid formation. The experimental critical temperature for this mixture is -16.9°C. Therefore, the plot presents the large retrograde region for this mixture. From the PR calculations, the cricondentherm is estimated to be 29°C. In this mixture, liquid can form at a temperature almost 45 Celsius degrees higher than the critical temperature. The cricondenbar is estimated to be 12.5 MPa. It is difficult to confirm the location of either the cricondenbar or the cricondentherm with the available experimental data. However, the PR fits the data, and thus it can be concluded that the estimation of these points is quite accurate as well. 

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