Stripper, side

Similarly, Fig. 5.15a shows a thermally coupled indirect sequence. The condenser of the first column is replaced by a thermal coupling. The four column sections are again marked as 1, 2, 3, and 4 in Fig. 5.15a. In Fig. 5.156, the four column sections are arranged to form a side-stripper arrangement. ... 

Both the side-rectifier and side-stripper arrangements have been shown to reduce the energy consumption compared with simple two-column arrangements. This results from reduced mixing losses in the first (main) column. As with the first column of the simple sequence, a peak in composition occurs with the middle product. Now, however, advantage of the peak is taken by transferring material to the side-rectifier or side-stripper. 

Although side-stripper arrangements are common in the petroleum industry, designers have been reluctant to use the fully thermally coupled arrangements in practical applications until recently. 

Introduce complex distillation configurations. Introduce prefractionation arrangements (with or without thermal coupling), side-rectifiers, and side-strippers to the extent that operability can be... 

Indirect sequence pairings should be replaced by side-strippers. 

Distillation appHcations can be characterized by the type of materials separated, such as petroleum appHcations, gas separations, electrolyte separations, etc. These appHcations have specific characteristics in terms of the way or the correlations by which the physical properties are deterrnined or estimated the special configurations of the process equipment such as having side strippers, multiple product withdrawals, and internal pump arounds the presence of reactions or two Hquid phases etc. Various distillation programs can model these special characteristics of the appHcations to varying degrees and with more or less accuracy and efficiency. 

In many units, the light cycle oil (LCO) is the only sidecut that leaves the unit as a product. LCO is withdrawn from the main column and routed to a side stripper for flash control. LCO is sometimes treated for sulfur removal prior to being blended into the heating oil pool. In some units, a slipstream of LCO, either stripped or unstripped, is sent to the sponge oil absorber in the gas plant. In other units, sponge oil is the cooled, unstripped LCO. 

The side-rectifier and side-stripper arrangements have some important degrees of freedom for optimization. In these arrangements, there are four column sections. For the side-rectifier, the degrees of freedom to be optimized are ... 

The side-stripper can be modeled as two columns in the thermally coupled indirect sequence, as shown in Figure 11.13. The first column is modeled as a conventional column with a partial condenser and partial reboiler. The second column is modeled as a sidestream column with a liquid sidestream on stage above the feed stage4. The vapor entering the condenser and liquid leaving can be calculated from vapor-liquid equilibrium (see Chapter 4). This allows... 

Figure 11.13 A side-stripper can be modeled as a sequence of two simple columns in the indirect sequence.

The optimization can be carried out using nonlinear optimization techniques such as SQP (see Chapter 3). The nonlinear optimization has the problems of local optima if techniques such as SQP are used for the optimization. Constraints need to be added to the optimization in order that a mass balance can be maintained and the product specifications achieved. The optimization of the side-rectifier and side-stripper in a capital-energy trade-off determines the distribution of plates, the reflux ratios in the main and sidestream columns and condition of the feed. If a partitioned side-rectifier (Figure ll.lOd) or partitioned side-stripper (Figure 11.lid) is to be used, then the ratio of the vapor flowrates on each side of the partition can be used to fix the location of the partition across the column. The partition is located such that the ratio of areas on each side of the partition is the same as the optimized ratio of vapor flowrates on each side of the partition. However, the vapor split for the side-rectifier will only follow this ratio if the pressure drop on each side of the partition is the... 

As with side-rectifiers and side-strippers, the partition wall should be insulated to avoid heat transfer across the wall as different separations are carried out on each side of the wall and the temperatures on each side will differ. Heat transfer across the wall will have an overall detrimental effect on column performance6. 

The partitioned thermally coupled prefractionator in Figure 11.14c can be simulated using the arrangement in Figure 11.14b as the basis of the simulation. However, like side-rectifiers and side-strippers, fully thermally coupled columns have some important degrees of freedom for optimization. In the fully thermally coupled column, there are six column sections (above and below the partition, above and below the feed in the prefractionator and above and below the sidestream from the main column side of the partition). The degrees of freedom to be optimized in partitioned columns are ... 

Consider now ways in which the best arrangement of a distillation sequence can be determined more systematically. Given the possibilities for changing the sequence of simple columns or the introduction of prefractionators, side-strippers, side-rectifiers and fully thermally coupled arrangements, the problem is complex with many structural options. The problem can be addressed using the optimization of a superstructure. As discussed in Chapter 1, this approach starts by setting up a grand flowsheet in which all structural features for an optimal solution are embedded. 

Calberg NA and Westerberg AW (1989) Temperature-Heat Diagrams for Complex Columns 2, Underwood s Method for Side-Strippers and Enrichers, Ind Eng Chem Res, 28(9) 1379. 

The use of complex columns (side-strippers, side-rectifiers and thermally coupled prefractionators) reduces the overall heat duties for the separation at the expense of more extreme temperatures for reboiling and condensing. Heat integration benefits from smaller duties, but more extreme temperatures make the heat integration more difficult). 

Consider the use of a side-rectifier or side-stripper for the separation of a three-product mixture. Assume that thermally coupled columns operate at the same pressure. Also, assume the feed to be saturated liquid. Data for the operation of the two arrangements are given in Tables 21.9 and 21.10. 

Whether or not a variable is independent may be more difficult to determine in other cases. For example, a distillation column with a side stripper is shown in Fig. 1. The side stripper in Fig. 1(a) has a reboiler and that in Figure 1 (b) is stripped with steam. Under the program, the liquid side stream which feeds the side stripper must be set. Also, in the column arrangement of Fig. 1(b) the amount and enthalpy of the steam fed must be set, since it constitutes an external feed. For illustration, we assume that the bottom product from the reboiler of the main column has... 

Figure 2 (c) shows a distillation column with four degrees of freedom provided by the two reboilers, the condenser, and the liquid side stream from the main column. Many possibilities of limits exist, depending on what variables have been set first. One example might be taken in which the liquid side stream was set last. The limits on the amount of this stream would then be, as a lower limit, the amount of bottom product (or reboiler vapor) in the side stripper and, as an upper limit, the total amount of liquid flow off the stage from which the stream was drawn. (The latter of these limits would not be easy to determine.)... 

Example 2. Distillation Column with Side Stripper... 

Example 2 shows a distillation column of 13 stages with a side stripper of 5 stages. The side stripper receives liquid from stage 9 of the main column and returns vapor from its top stage to stage 10 of the main column. 

The feed, introduced into stage 6 of the main column, is 1635.3 moles. The reboiler vapor of the main column is fixed at 1200 moles the reflux is also fixed at 2500 moles. The liquid side stream is set at 1210 moles, and the bottom product from the side stripper is fixed at 945 moles in order to recover component 3 in fairly pure form in the side stripper. 

EXAMPLE 2 DISTILLATION COLUMN UITH SIDE STRIPPER IS COMPONENTS -U STA6ES AND 9 STAGES) REFLUX AND STRIPPER PRODUCT SET... 

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