Bubble-point liquid

Underwood minimum reflux constant XjF = Mol fraction of component i in the feed XjD = Mol fraction of component i in the distillate q = Thermal condition of the feed Bubble point liquid q =1.0 Dew point vapor q =0 General feed q = (Ls - Lr)/F... 

Bubble point liquid feed, q = 1.0 (q-line vertical)... 

For a saturated (bubble point) liquid, pipe vertically downward from the drawoff nozzle as close to the nozzle as possible. This gives maximum static head above any horizontal sections or piping networks ahead of the pump. 

In a total condenser all of the overhead vapor is condensed to the liquid state. When the heat load or duty on the condenser is exactly equal to the latent heat of the saturated or dew point of the overhead vapor from the distillation column, the condensed liquid will be a saturated bubble point liquid. The condenser and accumulator... 

Figure 7.8 Three-dimensional curved envelope of the binary fluid P-T-xB surface (left), showing the upper bubble-point (liquid) surface, the lower dew-point (vapor) surface, and the hatched inside of the envelope, together with the three 2D projections (right) that result from slicing the envelope through the plane of constant temperature (upper), pressure (middle), or composition (lower).

Note that the propane vapor is still condensing to propane liquid at 120°F. The condensed liquid is in intimate contact with the propane vapor, as it drips off the outside surface of the colder condenser tubes. The saturated propane vapor condenses directly to saturated propane liquid at 120°F. The saturated, or bubble-point, liquid then drips from the condensation zone of the condenser into the subcooling zone of the condenser. This is the zone where the tubes are submerged in liquid. 

But the liquid in the reflux drum is in equilibrium with a vapor space. This liquid is then at its bubble, or boiling, point. If the liquid draining from the condenser is colder than this bubble point liquid, then it must be subcooled. But how can a vapor condense directly into a subcooled liquid Well, it cannot. 

Answer—yes But why Well, the liquid is cooled by 5°F after it leaves the drum. The cooled liquid is not in equilibrium with the vapor in the drum. It has been subcooled by 5°F. This means that the bubble-point liquid has been cooled, without altering its composition. The vapor pressure of the liquid has been reduced. As can be seen in Fig. 25.3, subcooling this particular liquid by 5°F reduces its vapor pressure by about 2 psi. As the specific gravity of the liquid is 0.58, this is equivalent to an increase in the NPSH by 8 ft. Once again, our objective is to increase the flow from 250 to 300 GPM. Figure 25.2 tells us that the required NPSH increases from 20 to 26 ft. However, when we subcool the liquid by 5°F, the available NPSH increases from 20 to 28 ft. As the available NPSH now exceeds the required NPSH by 2 ft, the flow can be increased without risk of pump cavitation. 

The pressure at which the slope changes is the bubble-point pressure of the mixture. The volume at this point is the volume of the bubble-point liquid. Often it is given the symbol Vsat. The volume of the bubble-point liquid can be divided by the mass of reservoir fluid in the cell to obtain a value of specific volume at the bubble point. Specific volume at the bubble point also is measured during other tests and is used as a check on the quality of the data. 

The circulating pumps should be specified on the basis of the bubble point liquid at the net positive suction head (NPSH). 

When the liquid starts to boil at temperature 7 (point B), the first vapor formed has a composition yx and is therefore at its dew point, At thia point, the vapor is as rich in the light component as it will ever be. As temperature is further raised, more of the heavier component is boiled off. The quantity of vapor formed increases, but the mole fraction of the light component in both vapor and liquid drops. At temperature T2, the liquid composition is x2 and the vapor composition is y2. Some of the initial charge is now vapor and some is liquid. A further increase in temperature to Ta will vaporize the rest of the liquid. The vapor composition will now be xlt and the last drop of liquid vaporized has a composition x3, The liquid always travels along its bubble-point curve (BEH) while the vapor always travels along the dew-point curve iDFG), Therefore, in distillation, bubble-point liquid is always in equilibrium with dew-point vapor. 

Graphical Underwood method. To eliminate the trial and error involved, Van Winkle and Todd (37) developed a graphical solution technique for obtaining 8. This technique is only applicable to bubble-point liquid feeds. 

Applicability Saturated (at the bubble point) liquid mixtures. Input data Tc, V , and Xj. 

A.2.2 Liquid/gas methods (bubble point, liquid expulsion permporometry)... 

When the appropriate values for lFi and vFi are employed, Eq. (2-48) may be used to calculate b-Jdi for a feed of any thermal condition. For bubble-point liquid and subcooled feeds, = FXt and vFi = 0. For feeds that enter the column as dew-point and superheated vapors, vFi = FX and lFi = 0. 

The column has a partial condenser which is to be operated at 300 lb/in2 abs. The distillate is removed as dewpoint vapor. The bottoms and the sidestream products are removed as bubble-point liquids. Feeds enter as liquids at their bubble point at the column pressure. Vapor-liquid equilibrium data and enthalpy data are given in Tables B-l and B-2. 

Unlike the binary case, the choice of two keys does not give determinate mass balances, because not all other mole fractions are calculable by mass balances alone and equilibrium calculations are required to calculate the concentrations of the dew-point vapor from the top plate and the bubble-point liquid leaving the reboiler. 

Flow rate tmol/sl Feeds (both bubble-point liquids at 17 atm) ... 

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. 

These tables summarize the thermophysical properties of air in the liquid and gaseous states as calculated from the pseudo-pure fluid equation of state of Lemmon et al. (2000). The first table refers to liquid and gaseous air at equilibrium as a function of temperature. The tabulated properties are the bubble-point pressure (i.e., pressure at which boiling begins as the pressure of the liquid is lowered) the dew-point pressure (i.e., pressure at which condensation begins as the pressure of the gas is raised) density (/ ) enthalpy (H) entropy (S) isochoric heat capacity (CJ isobaric heat capacity (C ) speed of sound (u) viscosity (rj) and thermal conductivity (A). The first line of identical temperatures is the bubble-point (liquid) and the second line is the dewpoint (vapor). The normal boiling point of air, i.e., the temperature at which the bubble-point pressure reaches 1 standard atmosphere (1.01325 bar), is 78.90 K (-194.25 °C). 

For the conditions of Problem 12.7, compute the minimum external reflux rate and the distribution of the nonkey components at minimum reflux by the Underwood equation if the feed is a bubble-point liquid at column pressure. 

Calculate and plot the minimum external reflux ratio and the minimum number of equilibrium stages against percent product purity for the separation by distillation of an equimolal bubble-point liquid feed of isobutane/n-butane at lOOpsia. The distillate is to have the same 1C4 purity as the bottoms is to have nC4 purity. Consider percent purities from 90 to 99.99%. Discuss the significance of the results. 

As an example, consider Fig. 7.10a where n-hexane (H) is separated from n-octane by a series of three flashes at 1 atm (pressure drop and pump needs are ignored). The feed to the first flash stage is an equimolal bubble-point liquid at a flow rate of 100 Ibmole/hr. A bubble-point temperature calculation yields 192.3°F. Using Case 5 of Table 7.4, where the vapor rate leaving stage 1 is set... 

Example 8.5. One hundred kilogram-moles per hour of a feed containing 30mole% n-hexane and 70% n-octane is to be distilled in a column consisting of a partial reboiler, a theoretical plate, and a partial condenser, all operating at I atm (101.3 kPa). The feed, a bubble-point liquid, is fed to the reboiler, from which a liquid bottoms product is continuously withdrawn. Bubble-point reflux is returned to the plate. The vapor distillate contains 80 mole% hexane, and the reflux ratio (LrID) is 2. Assume the partial reboiler, plate, and partial condenser each function as an equilibrium stage. 

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