Depropanizer example

Figure 2,20 Effect of feed stage on key ratio plot. Depropanizer example, feed composition same as Example 2.4. 20 theoretical stages, R/Rmin - 1.40.

Figure 2.22 Application of x-y diagrams to analyze a computer simulation. Diagrams were prepared using computer composition printouts, Depropanizer example, 20 theoretical steges, = 1.4. (a) Correct feed...

Figure 2,23 shows dlb plots for the depropanizer example, based on... 

Figure 2,23 Application of dlb plots to examine nonkey component distribution in products, Depropanizer example. 20 theoretical stages, = 1.40.

Weir length and downeomer width. These are calculated from geometry. Bolles (2) chart expresses the geometrical relationship between downcomer area, downcomer width, and downcomer length Fig. 8.30 is a revised version. For better accuracy, the manufacturers or Perry s tables (7,9,13) or high-accuracy charts (8) should be preferred. For (he depropanizer example, using Fig. 6.30,... 

A depropanizer example is provided to illustrate the use of Aspen IPE. The depropanizer is a distillation tower to recover propane and lighter species from a normal-paraffins stream, as shown in Figure 1. The simulation flowsheet and selected results are shown in Appendix I and in the multimedia tutorial on the CD-ROM that contains these course notes ASPEN Tutorials —> Separation Principles -> Flash and Distillation). Also, a copy of the file, RADFRAC.bkp, is provided on the CD-ROM. 

Detecting pinched regions (Sec, 2.2.5) Pinching and its cause (minimum reflux, mislocated feed, tangent pinch, etc.) are readily visible on an x-y diagram. Figure 2.22 compares a well-located fe point in the depropanizer example with a mislocated feed point. The pinch is clearly seen in Fig. 2.226, while no pinch exists in Fig. 2.22a. 

TABLE 6.10 Column Loading and Physical Properties Depropanizer example 6.1 (Basis 20,000 Itemolt/h feed)... 

Figure 8-56. Examples of gamma ray scanning diagnostic diagnosis of depropanizer column to evaluate performance. Used by permission, Tru-Tec Division, Koch Engineering Co., Inc.

Enter an alpha value if you have chosen F or T for the method. Enter a K value for a light key component if you chose A. Input the factor alpha or K. Alpha is defined as simply the light key K divided by the heavy key K component. The K factor is simply the particular component s vapor phase mole fraction divided by its liquid mole fraction. The alpha value is therefore a ratio of the chosen two key components. These key components should be those that readily point to how well the fractionator is doing its job of separation. For example, for a depropanizer tower, choose propane as the light key component and butane as the heavy key, since you wish to separate the propane from the butane to make a propane product specification. For a multicomponent system, you may try several components to determine a controlling alpha and/or to factor an average tray efficiency. 

This topic is best illustrated using an example. The example used here is the depropanizer described by King (7), which has previously been analyzed by Jenny (20), Hengstebeck (21), and Edmister (22). 

Figure 2.18 Example 2A—Depropanizer (a) Equilibrium curve (b) Hengstebeck diagram (c) minimum reflux determination. (Part b. from C. J. King Separation Processes, 2d ed.. Copyright by McGrew-Hill, Inc. Reprinted by permission. Parts a and c based on C. J, King, loc. cit.)...

Figure 3.5 Stage 9-re flux relationship for the depropanizer in Example 3.4.

Figure 3.7 Feed point optimization by plotting results of computer simulation, depropanizer in Example 3.4, (a) Effect of the total number of stages F 100, D 39.9...

Figure 3.e Calculating minimum reflux and minimum stages by extrapolating the reflux stages curve obtained by computer simulation. Depropanizer In Example 3.4. D = 59.9 lb-mole/h. 

Example 3.1 Evaluate the minimum number of stages for the depropanizer in Example 2.4 using each of the methods described above. Use the following data ... 

Example 3J Calculate the minimum reflux for > depropanizer similar to that of Example 2.4 using Underwood s method, hi this case, butane is acceptable both in the top and bottom product, but it is required that 98 percent of the propane in the feed is recovered in the top product, and 9B percent of the pantane is to be recovered in the bottom product. 

Example 3.4 Calculate the number of theoretical stages for the depropanizer in Example 2.4. 

Example 3.6 A depropanizer capable of providing 20 theoretical stages, with a feed point on the 9th stage from the top, is available to separate the mixture in Example 2,4, The reflux ratio is 1.5. 

Example 3.9 A depropanizer normally operates as described in Example 2.4. The column is computer-controlled, using the Jafarey et al. algorithm. The algorithm manipulates reflux flow to control top product purity. Distillate flow rate must remain fixed, but bottom purity is allowed to vary. The top product purity spec is temporarily relaxed from 0.5 mole percant to 0.9 mole percent. What would the controller set the reflux flow at ... 

These checks will use all the parameters in the layout summary in Sec. 6.5.3 (Table 6.11, third trial). The minimum throughput calculations will be performed for stages 8 and 9 in the depropanizer, where the loads are lowest. Since the example requires column turndown to 60 percent of the expected design loads, the vapor and liquid loads shown in Table 6.10 for stages 8 and 9 are multiplied by 0.6 for the turndown checks. 

Example S.2 For the depropanizer in Examples 2.4 (Sec. 2.3.1), 3.4 (Sec. 3.2.5). and 6.1 (Sec. 6.5.2), would a packed tower be better than a tray tower Proeeas loads and physical properties are the same as those in Table 6.10. The service ia nonfouling, the streams have a negligible zolid content, the corrosive tendency is low. and preeeure surges are unlikely. 

The Table 11.1 data suggest that the top sections operate at HETPs of the order of 35 to 38 in with 1.5-in Pall rings. The bottom sections have an HETP of about 29 in with 1.5-in Pall rings, and 40 in with 2-in Pall rings. The difference (11 in) is similar to the difference predicted from Eq. (9.34), and therefore, makes sense. It follows that for design purposes, an HETP value of 38 in is a good estimate for the top section of the depropanizer in this example, while an HETP of 40 in is a good estimate for the bottom section. Note that these values are not conservative they match the available data. 

Example 9.2 asked (Sec. 9.4.2) whether a packed tower would be better than trays for the depropanizer. The design with trays is in Sec. 6.5.11. For the comparison, it is assumed that filter evaluation of the alternate design (with third-generation packing Sec. 9.4,9) it was found to be sound and not too tight. 

Vised directly. To obtain very pure propylene (99.5 per cent weight), however, it is necessary to remove the propane in a supplementary column. Table 2.10 offers a glance at the specifications that the ethylene and propylene thus separated are required to meet The heavier hydrocarbons obtained at the bottom of the depropanizer are treaty in a debutanizer, which produces a l,3>butadiene-rich cut at the top. In accordance with the severity, Table 2.11 provides a typical example of the composition of this effluent for a naphtha feedstock. The pyrolysis gasoline drawn off at the bottom may, depending on the severity, contain 50 to S5 per cent weight of aromatic hydrocarbons, of which more than half is benzene. Its composition and treatment are discussed separately in Section 2.1.5. 

A material balance for the depropanizer follows the procedure outlined in Example 15-2. The... 

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