Distillation side-stripper

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

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. 

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. 

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. 

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

Complex columns are distillation devices that can handle a mixture of minimum three components and deliver more than two products. A complex column consists of a main tower surrounded by additional columns, as prefractionator, side strippers and side rectifiers. As illustration, Figure 3.7 presents five alternatives for separating a ternary mixture ABC ... 

The SR method is suitable for modeling absorbers and strippers. For some extremely wide boiling systems, especially those with noncondensables, it is the best method. It has been found to work very well for the side strippers of a refinery fractionator. Absorbers typically have a rich gas bottom stage feed and a lean oil top stage feed. The equations of the SR method do not allow its direct use for reboiled absorbers, absorbers with condensers, or distillation columns. For these columns, other methods like that of Tomich (Sec. 4.2.8) or Russell Sec. 4.2.10) can be used. 

However we choose to look at it, a basic distillation column has two control degrees of freedom. When we turn to more complex column configurations with sidestreams, side strippers, side rectifiers, intermediate reboilers and condensers, and the like, we add additional control degrees of freedom. These more complex systems are discussed in Sec. 6.8. 

The transition split divides direct-type sphts from indirect-type splits as discussed by Doherty and Malone (Conceptual Desisn of Distillation Systems, 2001, chaps. 4 andS) also see Fidkowski, Doherty, and Malone [AlChE J., 39,1301(1993)]. The upper line in Fig. 13-70 is the minimum vapor flow leaving the reboiler of the main column, which also corresponds to the minimum vapor flow for the entire system since all the vapor for the total wstem is generated by this reboiler. For P = 0 the minimum vapor flow for the entire thermally coupled system (i.e., main column) becomes equal to the minimum vapor flow for the side rectifier system (i.e., main column of the side-rectifier system see Fig. 13-65b or c) (Vsr) for P = 1 it is equal to the minimum vapor flow of the entire side stripper system (Vss) (which is the sum of the vapor flows from both the reboilers in this system see Fig. 13-66h or c). Coincidentally, the values of these two minimum vapor flows are always the same (Vsr), = (Vss)mm- For P = Pr the main column is pinched at both feed locations i.e., the minimum vapor flows for separations A/B and B/C are equal. 

Several of the commercial simulation programs offer preconfigured complex column rigorous models for petroleum fractionation. These models include charge heaters, several side strippers, and one or two pump-around loops. These fractionation column models can be used to model refinery distillation operations such as crude oil distillation, vacuum distillation of atmospheric residue oil, fluidized catalytic cracking (FCC) process main columns, and hydrocracker or coker main columns. Aspen Plus also has a shortcut fractionation model, SCFrac, which can be used to configure fractionation columns in the same way that shortcut distillation models are used to initialize multicomponent rigorous distillation models. 

The atmospheric distillation column in a refinery is highly complex system because of the interactions between the main column with different side strippers and draw streams where the study of this complex system will be more difficult. However, the decomposition of the complex column into a series of simple columns ease and simplify its study. There are a number of advantages of decomposing a complex tower, namely ... 

The internal modifications to retrofit the distillation system consider the distillation column and its internals. In this level of retrofit, some changes are made in the internals or nozzles of the main column and the side strippers. Moreover, the reboilers and pump around loops are examined and adjusted as required. Therefore, more complex modifications are required at this level that lead to higher investments. The objective of these modifications is to find out the ideal distribution of stages for each section in the main column [4]. This ideal design is applied to the existing column by modilying its internal to the niinimrim extent to reduce the costs. 

A distillation column with an attached side stripper is used to separate a feed stream F into products A, B, and C, as shown in the diagram. The main column has a total condenser and a partial reboiler, and the side stripper has a partial reboiler. The columns are existing units with fixed number of trays and feed and draw locations. The external feed, a product of an upstream unit, is of fixed flow rate, composition, and thermal conditions. The pressure in the columns is determined independently and is not available as a variable. Using basic modules representation, determine the degrees of freedom of this system. What variables would you specify to define the performance of these columns ... 

Figure 12.8 shows a model of a main column and a side-stripper. Vapor feed F goes to the bottom of the main column, which is steam-stripped with stream 51, using no reboiler. The column has a partial condenser with vapor distillate D. Side draw SD is taken from around the middle of the main column and steam-stripped in the side-stripper, with the overhead vapor, OH, returned to the draw tray in the main column. The side-stripper is steam-stripped with stream 52. 

Earlier chapters use simplified and binary models to analyze in a very informative manner some fundamentals such as the effect of reflux ratio and feed tray location, and to delineate the differences between absorption/stripping and distillation. Following chapters concentrate on specific areas such as complex distillation, with detailed analyses of various features such as pumparounds and side-strippers, and when they should be used. Also discussed are azeotropic, extractive, and three-phase distillation operations, multi-component liquid-liquid and supercritical extraction, and reactive multistage separation. The applications are clearly explained with many practical examples. 

Specialised units are used to simulate complex fractionation processes in petroleum refining. Typical configuration consists of a main column with pump-around and side strippers (Fig. 3.14). Among applications, we may cite pre-flash tower, crude atmospheric distillation, or Fluid Catalytic Cracking (FCC) main fractionator. 

Simple Distillation. In this category, we include the separation of ideal or slightly non-ideal mixtures that do not form azeotropes, based on the differences in the relative volatilities of components. A simple column designates a device that separates one or several feeds in only two products top distillate and bottoms. Complex columns are can deliver more than two products. In this category we include columns with side-streams, columns equipped with auxiliary devices, as prefractionators, side-strippers and side-rectifiers, as well as thermally integrated columns. 

Determine the number of independent variables and suggest a reasonable set for (a) a new column, and (b) an existing column, for the crude oil distillation column with side stripper shown below. Assume that water does not condense. 

FKiURE 1.3 Examples of complex columns (a) distributed feed, (b) side draw columns, (c) thermally coupled side stripper, (d) reactive distillation column with reaction zone (R). 

Both configurations can be divided into four CSs for the case of a ternary system where we wish to obtain relatively pure distillate, bottoms, and sidestream products. For the sake of consistency, the side unithas been numbered as CS4 while the internal CS is labeled as CS2 in both configurations. In the side-stripper unit, the liquid coming from CSi is divided into two streams one that is directed to the main column body and another one that is directed toward the side-stripping unit. The vapor flow in CSi is also the sum of the vapor flows from the side stripping unit and the main column. Similarly, the vapor flow from CS3 in the side rectifier is directed toward the main column and the rectifying unit, while the liquid flowrate in CS3 is the sum of the liquid flowrates in the main column and the rectifier. The location of the sidestream withdrawal and addition at the thermally coupled junction are assumed to take place at the same location. This assumption can be relaxed however, an additional CS would be created and this case will not be further discussed in this text... 

Notice that in both configurations that they are very closely related to simple columns. For the side stripper, for instance, the simple column has a feed of flowrate F and quality q, a bottoms product of flowrate B and composition x. One can show by mass balance that CS2 produces a distillate product, or a pseudo distillate, of flowrate A2 = V2 — L2 and composition Xa2. Thus, for all practical purposes the CS above and below the feed is just a simple column with one feed and two products. One can in a similar way deduce that the CS2 in the side rectifier acts as a conventional product producing stripping section with a flowrate of — A2 =L2 — V2 (since product flowrates have to be positive) and bottoms composition of X. ... 

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