Feed thermal condition

Aid optimization. The graphics can show the effects of parameter changes such as compositions, feed thermal condition, and multiple feed or draws. 

Q FEED THERMAL CONDITION (RATIO OP LIQUID TO VAPOR... 

FIGURE 5.5 Plots of (/-lines for different feed thermal conditions. 

Equation 5.19 is known as the -line equation. The (/-line represents the feed thermal conditions and is the locus of intersection points of the operating lines above and below the feed tray. It is a straight line with a slope of qHq - 1) and an intersection point with the diagonal at Y = X = Xp. This may be shown by setting Y = X in Equation 5.19 and solving forX. 

The values of q for different feed thermal conditions at feed tray temperature and pressure, 7 and were discussed in Section 5.2.1. The corresponding values of the (/-line slope are summarized in the table below. Example of (/-lines are shown in Figure 5.5. 

The complete definition of column performance when the number of stages and feed location are fixed requires two specifications (Section 5.2.2). In studying the effect of reflux ratio on separation, other parameters, including product rates, are assumed constant. In Y-X graphical representation, a fixed product rate is implied by fixing the 7-line slope. This slope depends on the feed thermal conditions at feed tray temperature and pressure (Section 5.2.2). The column pressure is fixed, but its temperature depends on the product rates. Therefore, in order to maintain a fixed product rate, the (/-line slope must be held constant. The actual product rates corresponding to a given (/-line slope are calculated by material balance once the product compositions have been determined. 

The next step is to plot the q-line, which intersects the diagonal at the feed composition. The slope of the q-line is computed on the basis of the feed thermal conditions at the feed tray temperature, which is dependent on the product rates. It is assumed that the product rate (distillate or bottoms) is such that the feed is almost a saturated liquid at feed tray conditions so that the q-line is close to vertical. 

To complete the construction of the Y-X diagram from simulation results, the feed line must be drawn. The intersection with the diagonal of a straight line drawn through the feed composition determines one point on the q-line. One other point is determined by the feed equilibrium vapor and liquid compositions at the feed tray conditions. If the feed is a saturated liquid, the equilibrium liquid composition is the same as the feed composition, and the equilibrium vapor composition is the bubble point composition on the equilibrium curve. In this case the q-line is vertical. For a saturated vapor feed, the equilibrium vapor composition is the same as the feed composition, the equilibrium liquid composition is the dew point composition, and the q-line is horizontal. For a mixed-phase feed, the c/ line slope is determined by the feed thermal condition (Section 5.2.2). Note that, for a multi-component mixture, the feed equilibrium vapor and liquid compositions from the simulation output may not lie exactly on the equilibrium curve because of the discrepancies resulting from lumping the light components in one pseudocomponent. 

A binary stream at the rate of 1000 kmol/h containing 35% mole of component 1 (the lighter component) is to be separated in a distillation column to produce 95% component 1 in the distillate and 90% component 2 in the bottoms. The column will have a partial reboiler and a partial condenser, and will operate at 1 atm. It is proposed to utilize an existing hot process stream in the plant as a heat source for the reboiler, which limits the reboiler duty to 58 x 10 kJ/h. Using either the Y-X diagram or the H-X diagram, determine the number of theoretical trays required, the optimum feed location, the product rates, and the condenser duty. Assume feed thermal conditions that result in a saturated liquid at the feed tray pressure. Use thermodynamic data from Problem 6.1. 

The feed thermal conditions are such that it is a saturated vapor at the... 

The fourth column specification is the reflux ratio, which controls the quality of separation between the products. The reflux ratio sets the L/V ratios in the column. These ratios tend to be fairly constant within each section, depending on the distillation characteristics of the mixture. They are different, though, in each section because the side draws alter the fluid flow in the column. If, for instance, a side draw is a liquid, the liquid flow on the trays below the draw tray is smaller than that on the trays above it. In the feed section, the L/V ratio is generally different above and below the feed tray, depending on the feed thermal conditions. 

The definition of minimum reflux is linked to a specification of separation between two components. In multi-component mixtures, the specified components are the light key and the heavy key components. Minimum reflux is meaningless if the column is specified in a manner that does not define a particular separation between two components, such as a specification of the number of trays and a product rate. Also, minimum reflux in this context is associated with singlefeed, two-product columns. For a given feed composition, the minimum reflux depends on the key components, their separation specification, and the feed thermal conditions. 

The feed composition Zj (mole fractions j= 1, NC), the desired distillate composition Xpj (/ = 1, NC), and the feed thermal condition q are specified. The relative volatilities ot, (/ = 1, NC) of the multicomponent mixture are known. 

The upper feed is represented graphically by its g-line drawn with a slope corresponding to the feed thermal conditions. The g-line starts at the intersection with the diagonal of a vertical line drawn through the feed composition. The intersection of the g-line with the previous operating line determines the upper end of the next operating line. With a known slope, or LIV ratio, this line is now drawn, and the stages between the upper feed and the next discontinuity, the liquid draw, are stepped off. 

With higher bottoms flow rates, lower reboiler duties are required at minimum reflux because less liquid must be vaporized. For any given bottoms rate, a minimum amount of reflux is required to maintain both liquid and vapor phases on all trays. Curve AB in Figure 6.5 is a plot of the minimum reflux ratio. This minimum is not to be confused with the minimum reflux ratio required to bring about a specified separation with a given number of stages. The curve characteristics depend mainly on the feed thermal conditions. Below this curve either the liquid or the vapor dries up on some of the trays. The upper limit of the reflux ratio is determined by practical considerations such as duty limitations and column diameter. 

Feed flow rate —Feed composition —Feed thermal condition —Steam supply pressure —Cooling-water suf ly temperature —Cooling-water header pressure... 

Two different feed thermal conditions dew-point vapor and bubble-point liquid. 

Values Calculated for Each System. The following quantities were calculated for each feed composition at each feed thermal condition,... 

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