Distillation feed condition

No attempt should be made to optimize pressure, reflux ratio, or feed condition of distillation in the early stages of design. The optimal values almost certainly will change later once heat integration with the overall process is considered. 

Though the total degrees of freedom is seen to be (C + 4) some of the variables will normally be fixed by general process considerations, and will not be free for the designer to select as design variables . The flash distillation unit will normally be one unit in a process system and the feed composition and feed conditions will be fixed by the upstream processes the feed will arise as an outlet stream from some other unit. Defining the feed fixes (C + 2) variables, so the designer is left with ... 

To solve Equation 9.50, start by assuming a feed condition such that q can be fixed. Saturated liquid feed (i.e. q = 1) is normally assumed in an initial design as it tends to decrease the minimum reflux ratio relative to a vaporized feed. Liquid feeds are also preferred because the pressure at which the column operates can easily be increased if required by pumping the liquid to a higher pressure. Increasing the pressure of a vapor feed is much more expensive as it requires a compressor rather than a pump. Feeding a subcooled liquid or a superheated vapor brings inefficiency to the separation as the feed material must first return to saturated conditions before it can participate in the distillation process. 

The Underwood Equation is based on the assumption that the relative volatilities and molar overflow are constant between the pinches. Given that the relative volatilities change throughout the column, which are the most appropriate values to use in the Underwood Equations The relative volatilities could be averaged according to Equations 9.47 or 9.49. However, it is generally better to use the ones based on the feed conditions rather than the average values based on the distillate and bottoms compositions. This is because the location of the pinches is often close to the feed. 

For a simple distillation column separating a ternary system, once the feed composition has been fixed, three-product component compositions can be specified, with at least one for each product. The remaining compositions will be determined by colinearity in the ternary diagram. For a binary distillation only two product compositions can be specified independently, one in each product. Once the mass balance has been specified, the column pressure, reflux (or reboil ratio) and feed condition must also be specified. 

A direct sequence of two distillation columns produces three products A, B and C. The feed condition and operating pressures are to be chosen to maximize heat recovery opportunities. To simplify the calculations, assume that condenser duties do not change when changing from saturated liquid to saturated vapor feed. This will not be true in practice, but simplifies the exercise. Assume also that the reboiler duty for saturated liquid feed is the sum of the reboiler duty for saturated vapor feed plus the heat duty to vaporize the feed. Data for the two columns are given in Tables 21.7 and 21.8. 

The height of a distillation column depends on the feed conditions, the product purity specifications and the extent of separation through the vapour-liquid equilibrium relationship, but also on the type of tray or packing used in the column as this affects the rate of separation. Column vendors will normally provide information on tray or packing efficiencies.9,10... 

The height of an absorption column depends on the feed conditions, the product purity specifications, the solvent used and the extent of separation through the absorption equilibrium relationship, but also on the rate of separation. If the rate of mass transfer of the gaseous component from the gas phase into the liquid phase is slow, then the column needs to be longer to ensure that the required amount is removed. The rate of mass transfer depends on the mass-transfer coefficient, normally denoted kG or k. The value of the mass-transfer coefficient depends on the components in the gas feed and on the solvent used and is often determined experimentally. The type of packing used in the column will also have an impact on the column height as for distillation. 

Robinson (1970) considered an industrial 10-component batch distillation operation. The feed condition is shown in Table 5.3. The distillation column was currently producing the desired product using constant reflux ratio scheme. Table 5.4 summarises the results of the application of minimum time problem using simple model with and without column holdup. 

The examination of heat duties in Figure 5.21 suggests heat-integration opportunities. A major impact has used the energy released in reactor for driving the distillation column (C-2), the largest consumer. In addition, the reactor effluent can preheat the fresh and recycled phenol, as well as ensure a suitable feed condition for (C-l). 

The simulator packages such as Aspen Plus and Hysys may be useful in analyzing distillation column systems to improve recovery and separation capacity, and to decrease the rate of entropy production. For example, for the optimization of feed conditions and reflux, exergy analysis can be helpful. A complete exergy analysis, however, should include both an examination of the exergy losses related to economic and environmental costs and suggestions for modifications to reduce these costs. Otherwise, the analysis is only theoretical and less effective. 

Most distillation columns have two control degrees of freedom, once pressure and feed conditions are set. The typical control structure holds the composition profile in the column by controlling a tray temperature somewhere in the column. The other degree of freedom is then normally consumed by fixing some other variable such as the flowrate of reflux, the reflux ratio, or the heat input. 

For binary distillation without solvent, the feed condition was taken as saturated liquid at the tower pressure. 

The BRS includes a feed conditioning system, two brine concentrators, two evaporator/crystallizers, two distillate liquid-phase carbon filters, three solids dewatering units, and an offgas treatment system (BRS OTS) (not shown in Figure 2-7). The BRS feed conditioning system converts carbonate salts to carbon dioxide, which is then removed... 

Step 1 The flowsheet is Figure 6.1, and you enter the components and feed conditions in the usual manner. (See other examples in Chapters 2-5 and Appendix C.) The thermodynamics model chosen is the Refinery, Chao-Seader option. The parts specific to the distillation column are the block parameters. Shown in Figure 6.2 is the screen where you select the block parameters. You choose the light and heavy keys (propane and Z-butane), the splits desired (99 and 1 percent), the pressure of the column (138 psia), a total condenser, and the desired number of stages (10). 

The separation is specified by fixing the distillate and bottoms compositions, and Xg. The product compositions determine the product rates, which are calculated by material balance. The product compositions are used to locate one end point of each operating line on the Y-X diagram. The feed conditions determine the ty-line (Figure 6.4). 

Pressure drop per plaie, 0.1 psi Feed condition, liquid at bubble point Distillate condition, liquid at bubble point Reflux subconiing. none Murphsee plate efficiency, 75%... 

Use the Underwood equation to estimate the minimum external reflux ratio for the separation by distillation of 30 mole% propane in propylene to obtain 99 mole% propylene and 98mole% propane, if the feed condition at a column operating pressure of 300psia is ... 

Through Equations 7.11 7.14 we now have formal conditions which dictate when changes in the global FP will occur. Notice that each of these equations is a linear function in terms of v and d> and depend only on the reflux in the topmost (reference) CS (Fai), the Distillate, Feed, and Side-draw flowrates (D, F, and S, respectively). Recall, however, that for a fixed feed flowrate, the quantities D and S are dependent on the specified product compositions one wishes to achieve. Interestingly, these net flow boundaries are independent of the feed quality or whether the side-draw is removed as a vapor or liquid product. Thus, for an arbitrary choice of product compositions and Fai> we can represent the transition from one FP to the next in a d>y versus 4> diagram, as shown in Figure 7.14. 

Flash calculations and the application of the lever rule (overall mass balance relating the feed, distillate and bottoms product streams) to predict feasible sharp splits for a given feed condition. 

The normal situation in distillation design is that the feed conditions are given flow rate F (mol/h), composition Zj (mole fraction component j, temperature and pressure Pp. The... 

In Chapter 3, we studied how to design a distillation column, given the feed conditions, the desired product specifications, and the total number of stages. The calculated design parameters included the operating pressure of the column, the reboiler and condenser heat duties, and the length and diameter of the column vessel. 

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