Total Vapor Distillate

The control of partial condenser columns is more complex than total condenser columns because of the interaction among the pressure, reflux-drum level, and tray-temperature control loops. Both pressure and level in the reflux dmm need to be controlled, and there are several manipulated variables available. The obvious are reflux flow, distillate flow, and condenser heat removal, but even reboiler heat input can be used. In this section, we explore three alternative control structures for this type of system, under two different design conditions (1) a large vapor distillate flow rate (moderate RR) and (2) a very small vapor distillate flow rate (high RR). 

As the dynamic simulation results will show, the preferred control structure depends on the control objectives of the entire process. For example, when the distillate goes to a downstream unit and large variability in its flow rate is undesirable, the control structure should control pressure with condenser heat removal, control level with reflux, and maintain a constant RR. 

Process Studied. The numerical example used in this section with an aU-vapor distillate stream is a depropanizer with a feed that contains a small amount of ethane, but is mostly propane, isobutane, and -butane. Two cases are considered. The first has a feed composition that is 2mol% ethane and 40mol% propane, so the distillate flow rate is large and the RR is moderate (RR = 2.6). In the second case, the propane in the feed is only 4 mol% (with 0.02 mol% ethane), which gives a small vapor distillate flow rate and a large reflux ratio (RR = 20). Table 8.1 gives design parameters for the two cases. The Chao-Seader physical properties are used. 

Design specifications are 1 mol% isobutane impurity in the distillate and 0.5 mol% propane impurity in the bottoms. The column contains 30 trays (32 stages) and is fed in the 

If the column is designed with a vapor distillate product, the column operates with a reflux drum pressure of 210psia, which gives a reflux dram temperature of 110°F and permits the use of cooling water in the condenser. If a total condenser were used, the column pressure would have to be 230 psia to give a reflux dram temperature of 110 °F. Of course, higher ethane concentrations in the feed would increase the difference between the operating pressures of total and partial condenser columns. 

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EXAMPLE 113-2. Sirttple Differential Distillation A mixture of 100 mol containing 50 mol % n-pentane and 50 mol % -heptane is distilled under differential conditions at 101.3 kPa until 40 mol is distilled. What is the average composition of the total vapor distilled and-the composition of the liquid left The equilibrium data are as follows, where x and y are mole fractions of n-pentane. 

A third fundamental type of laboratory distillation, which is the most tedious to perform of the three types of laboratory distillations, is equilibrium-flash distillation (EFV), for which no standard test exists. The sample is heated in such a manner that the total vapor produced remains in contact with the total remaining liquid until the desired temperature is reached at a set pressure. The volume percent vaporized at these conditions is recorded. To determine the complete flash curve, a series of runs at a fixed pressure is conducted over a range of temperature sufficient to cover the range of vaporization from 0 to 100 percent. As seen in Fig. 13-84, the component separation achieved by an EFV distillation is much less than by the ASTM or TBP distillation tests. The initial and final EFN- points are the bubble point and the dew point respectively of the sample. If desired, EFN- curves can be established at a series of pressures. 

Example 11.2 Using the Underwood Equations, determine the best distillation sequence, in terms of overall vapor load, to separate the mixture of alkanes in Table 11.2 into relatively pure products. The recoveries are to be assumed to be 100%. Assume the ratio of actual to minimum reflux ratio to be 1.1 and all columns are fed with a saturated liquid. Neglect pressure drop across each column. Relative volatilities can be calculated from the Peng-Robinson Equation of State with interaction parameters assumed to be zero (see Chapter 4). Determine the rank order of the distillation sequences on the basis of total vapor load for ... 

The errors associated with the Underwood Equations were discussed in Chapter 9, which tend to underpredict the minimum reflux ratio. This introduces uncertainty in the way that the calculations were carried out in Examples 11.2 and 11.3. The differences in the total vapor load between different sequences are small and these differences are smaller than the errors associated with the prediction of minimum reflux ratio and minimum vapor load using the Underwood Equations. However, as long as the errors are consistently low for all of the distillation calculations, the vapor load from the Underwood Equations can still be used to screen between options. Nevertheless, the predictions should be used with caution and options not ruled out because of some small difference in the total vapor load. 

Determine the best distillation sequence for minimum total vapor flowrate. 

Distillation sequencing. The separation of homogeneous nonazeotropic mixtures using distillation usually offers the degree of freedom to choose the distillation sequence. The choice between different sequences can be made on the basis of total vapor load, energy consumption, refrigeration shaft power for low-temperature systems, or total cost. However, there is often little to choose between the best few sequences in terms of such measures of system performance if simple distillation columns are used. 

Because the boil-up rate associated with a specific distillation system should be known from past experience with the system, this calculation of total vapor load immediately produces a time required for the new separation. 

Flashing is a distillation process in which the total vapor removed approaches phase equihbrium with the residue hquid. 

The composition of the distillate is calculated in the following way. For two components, A and B, the total vapor pressure, P, is the sum of the vapor pressures of the components. Pa and P - Since the partial pressure of a component in a gaseous mixture is proportional to its molar concentration, the composition of the vapor is given by Eq. (6),... 

Steam distillation. If two liquids are nearly insoluble in each other, neither one lowers the vapor pressure of the other therefore the total vapor pressure of a mixture of these two liquids will be the sum of their vapor pressures. If the mixture is heated, boiling begins when the combined vapor pressure of the two immiscible components equals the pressure of the atmosphere. The vapor and hence the distillate contain both components in the ratio of their vapor pressures. If, for example, at the temperature of the distillation 95 per cent of the vapor pressure is due to component A (of a mixture A and B), then the composition of the distillate will be 95 mole per cent A and 5 mole per cent B. This principle is applied in the separation of organic compounds from a mixture, at temperatures which are far below their boiling point, by distillation with steam. Consider, for example, a mixture of aniline, which boils at 184°, and water. At 100° the vapor pressure of aniline is 45 mm and that of water 760 mm at 98° the vapor pressure of water is 727 mm and that of aniline 40 mm. Therefore the combined vapor pressure of a mixture of water and aniline at 100° is 805 mm, and at 98°, 767 mm. It is evident that near 98° the total vapor pressure will be one atmosphere and the mixture will boil., The distillate will contain water and aniline in the mole ratio of their partial pressure. The process is called steam distillation and is further discussed in Experiment 27 (page 163). 

If the vapor pressures of the two pure constituents are close together, then any appreciable positive deviation from Raoult s law will lead to a maximum in the total vapor pressure curve similarly, a negative deviation will, in the same circumstances, be associated with a minimum in the curve. In any event, even if the vapor pressures of the pure constituents are appreciably different, marked positive or negative deviations can lead to a maximum or a minimum, respectively, in the total vapor pressure curve. Such maxima and minima are the cause of the formation of the familiar constant boiling mixtures or azeotropic mixtures. A liquid mixture having the composition represented by a maximum or a minimum will distil without change of composition, for the proportions of the two constituents are then the same in the liquid and vapor phases. That this must be the case will be shown in the next section. 

The condenser is the stage where overhead vapors are condensed and liquid is returned to the top of the column as reflux. The condenser is partial if only part of the vapor is condensed and refluxed and the remainder leaves the condenser as vapor distillate. This type of condenser adds one equilibrium stage to the column trays because it holds a vapor phase and a liquid phase at equilibrium with each other. A total condenser is one where the entire overhead vapor is condensed (cooled to the bubble point temperature or subcooled to a lower temperature), part of the condensate is returned as reflux, and the remaining part is taken as liquid distillate. This type of condenser does not count as an equilibrium stage because no vapor-liquid separation takes place in it. The liquid distillate composition is identical to the composition of the vapor leaving the column top tray. 

The stream defined below is sent to a column at a fixed pressure. The column has a partial condenser with a vapor distillate product and a partial reboiler with a liquid bottoms product. Assuming total reflux and constant relative volatilities, calculate by the Fenske equations the minimum number of trays and the product rates and compositions to meet the following specifications ... 

Rigoroas stage calculations can be mode for steam distillation columns, using methods described in Section 5.3. It is also possible to make approximate calculations on a sieam-frea basis, as if the tom] pressure in die columa were equal to the total vapor pressure of the material being distilled. 

Seehofer, E. and H. Borowski Eine Wasserdampf-DestUlationsapparatus zur Nikotinbestimmung in Tabak und Rauchkondensaten sowie zur Gesamtsstickstoffbestimmung [A vapor-distillation apparatus for analyses of nicotine in tobacco and tobacco smoke condensates as well as for the determination of total nitrogen] Beitr. Tabakforsch. 2 (1963) 37-38. 

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