Dewpoint calculation

The calculation of single-stage equilibrium separations in multicomponent systems is implemented by a series of FORTRAN IV subroutines described in Chapter 7. These treat bubble and dewpoint calculations, isothermal and adiabatic equilibrium flash vaporizations, and liquid-liquid equilibrium "flash" separations. The treatment of multistage separation operations, which involves many additional considerations, is not considered in this monograph. 

Dewpoint calculations must be made when we know the composition of the vapor yj and P (or T) and want to find the liquid composition Xj and T (or P). Flash calculations must be made when we know neither Xj nor yj and must combine phase equilibrium relationships, component balance equations, and an energy balance to solve for all the unknowns. 

If FLFEES is not zero, the value of jretyp is examined to find which of either the reboiler vapor or bottom liquid has been set in the problem description. If reboiler vapor has been set, the program examines flfees to see if it is less than the fixed amount of reboiler vapor, fixrv. If so, all of FLFEES is taken as reboiler vapor, the bottom liquid is set at zero, KALTER is set to 1, the temperature is determined by a dewpoint calculation on the reboiler vapor, and the calculation returns to the basic program. On the other hand, if flfees is more than large enough to supply FIXRV, the remainder is taken as bottom liquid and the temperature and compositions are obtained by calculation of a flash for which the total vapor and total liquid are known (isovfl). If the bottom liquid is specified, exactly the same procedure is followed except that in the case of a mass input less than the specified bottom liquid, fixbl, all of the input... 

The factor yt is called an activity coefficient. Bnbblepointand dewpoint calculations made witli tliis equation are only a bit more complex than the same calculations made with Raoult s law. Activity coefficients are functions of temperature and liquid-phase composition, and ultimately are based on experiment (Sec. 12.1). For present purposes, the necessary values are assnmed... 

Thus for dewpoint calculations, where the y, are known, die problem is to find die set of K-values diat satisfies Eq. (10.14). 

For these assumed products, the distillate temperature is obtained from a dewpoint calculation as 11.5°F (-11.4°C) and the bottoms temperature is obtained from a bubble-point calculation as 164.9°F (73.8 C). The dew-point calculation also gives values of (jCi)o, from which values of (y,)i for the top stage of the enricher are obtained from a... 

To obtain the mean values for Ps and Pd per year, the yearly mean values for temperatures and dewpoints calculated as described above have been used, respectively. 

For me, it s easy to remember how to do bubble-point and dewpoint calculations. I carry a pocket-size vapor pressure chart in my wallet. When I m out in the plant troubleshooting, that s all I need to check temperatures versus pressures for light hydrocarbon systems. 

The control system requires the values of T and AT obsei-ved during the first minutes of operation to be stored as the basis for the above calculation of end point. When the exhaust temperature then reaches the value calculated, diying is terminated. Coefficient K can be estimated from models but requires adjustment on-hne to reach product specifications repeatedly. Products having different moisture specifications or particle size will require different settings of K, but the system does compensate for variations in feed moisture, batch size, air moisture, and inlet temperature. Some exhaust air may be recirculated to control the dewpoint of the inlet air, thereby consei-v-ing energy toward the end of the batch and when the ambient air is especially diy. 

Even for this simplest CCGT plant, iterations on such a calculation are required, with various values of p, in order to meet the requirements set on T, the steam turbine entry temperature, and 7s (the calculated value of 7s has to be such that the dewpoint temperature of the gas (7jp) is below the economiser water entry temperature (7b) and that may not be achievable). But with the ratio /i satisfactorily determined, the work output from the lower cycle Wl can be estimated and the combined plant efficiency obtained from... 

A mixture with initial dewpoint 139.9°C and final bubblepoint 48.4°C is to be condensed with coolant at a constant temperature of 27°C. The gas film heat transfer coefficient is 40 W/m2 K and the overall coefficient is 450. Results of the calculation of the condensing curve are... 

Figure 10 shows the relationship between yx and xx for different values of an calculated from Eq. (8). When two components have close boiling points, by implication they have similar vapor pressures, so that an is close to unity. Separation of mixtures by distillation becomes more difficult as an approaches unity. Figure 11 indicates some of the x, y diagrams that can be obtained for distillation systems. Also shown are corresponding temperature-composition diagrams. The saturated vapor or dewpoint curve is determined by finding the temperature at which liquid starts to condense from a vapor mixture. Similarly, the saturated liquid or bubble-point curve corresponds to the temperature at which a liquid mixture starts to boil. For ideal mixtures, the dewpoint and bubble-point curves can be calculated as follows. From Eq. (3), at the dew point, since... 

If p is determined separately by a static method. Mi can be calculated. A dewpoint method, including an adaptation to the transport procedure, has been described. 6... 

Diaphragm pump, 142, 143 Deentrainment commercial equipment, 619 cyclone dimensions, 620 cyclones, 616,620,621 empty drums, 615,617 wire mesh nads. 615.616.618.620 Densities, hulk, data, 677-679 Desalination, 632 performance, 640 Design basis, 12 questionnaire 13, 14 Detergent manufacture, 35 Dewpoint conditions, 376 calculation, 378... 

Calculate the temperature and composition of a liquid in equilibrium with a gas mixture containing lO.O mole% benzene, 10.0 mole% toluene, and the balance nitrogen (which may be considered noncondensable) at 1 atm. Is the calculated temperature a bubble-point or dewpoint temperature ... 

Although VLE problems with other combinations of variablesare possible, engineering interest centers on dewpoint and bubblepoint calculations there are four classes ... 

Dewpoint and bubblepoint calculations are readily made witlr software packages such as Mathcad and Maple , in wliich iteration is an integral part of an equation-solving routine. Mathcad programs for solution of Ex. 10.3, parts ( aitlirough(d), are given in App. D,2. 

The nature of dewpoint and bubblepointcalculatioiis is evidentfrom the examples of Secs. 10.4 and 10.5 for Raoult s law and modified Raoult s law. All such calculations made by the gammdplii fommlation require iteration because of its complex functionality ... 

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