The Columns with Side Strippings

We now turn to the columns with side strippings (for the colunms with side rectifiers, the calculation of minimum reflux mode is carried out the same way). 

We shortly examine the general algorithm for multicomponent nonideal mixtures. 

In the example under consideration, in the second column as in the first one there is indirect split 1,2 3, but if product of section S2 contains more than one product component, then there is an intermediate split. 

The last two-section column containing sections and 53 is calculated in the same way as the second one. The described algorithm is a general one it embraces columns with any number of side strippings and with any number of product components in each product. 

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We propose a method of design calculation based on decomposition of the column with side strippings into a system of two-section columns, on rigorous design calculation of each two-section column and on simultaneous design of distillation columns and of system of heat exchange. 

The choice between the absence and availability of liquid flow from one two-section column into the other and the determination of optimal flow rate are performed using the same method. Such optimization simultaneously concerns the distillation column and the system of heat exchangers. As a result of such optimization in different two-section columns entering into the column with side strippings, it turns out to be profitable to use the different modification (reboilers or steam stripping, availability or absence of liquid flow between columns) (Fig. 7.16). 

If a sequence of simple colunms has minimum number of columns (n - 1) and a summary number of reboilers and condensers 2(n - 1), then the column with side strippings or rectifiers (partially coupled sequence) has a summary number of reboilers and condensers n. 

We understand by distillation complex a countercurrent cascade with branching of flows, with recycles or bypasses of flows. Columns with side stripping or side rectifier and columns with completely connected thermal flows (the so-called Petlyuk columns ) are examples of distillation complexes with branching of flows. A column of extractive distillation, together with a column of entrainer regeneration, make an example of a complex with recycle of flows. Columns of this complex work independently of each other therefore, we do not examine it in this chapter, and the questions of its usage in separation of azeotropic mixtures and questions of determination of entrainer optimal flow rate are discussed in the following chapters. 

Columns with side stripping (Fig. 6.12b) are used at refinery beginning with the first decades of the twentieth century (see Watkins, 1979). 

Figure 6.15 shows the simple example of separation of a four-component mixture into four pure components in a column with side strippings. As for the columns with side withdrawals of the products, the calculation of minimum reflux mode should be started with determination of the conditions of joining of trajectories of two sections adjacent to feed cross-section. For section ri, the pseudoproduct equals the sum of top and two side products. The minimum reflux mode for the first two-section column is calculated the same way it is done for the corresponding simple column with split 1,2,3 4 (indirect split). In a more general case, when the bottom product contains more than one product component, the intermediate split will be in this column. 

The main task of designing columns with side strippings is the determination of necessary number of plates in each section, of optimal thermal duties on pumparounds and of the rates of steam for stripping or thermal duties on reboilers. 

Design calculation of columns with side strippings is carried out on the basis of the algorithm of design calculation of two-section columns and on the basis... 

What is the pseudoproduct of the second section above the feed cross-section of the main column with side strippings if its products in the direction downward are... 

The comparison of various distillation complexes and of ordinary flowsheets is given in Tedder and Rudd (1978). Columns with side strippings and side rectifiers, Petl50ik columns, flowsheet with prefracionator, and also some other feasible configurations of two columns were examined. It was shown, in particular, that Petlyuk columns are preferable at big content of average volatile component. 

Besides sequences of simple columns, some types of distillation complexes, each of which can replace two adjacent simple columns, were examined in work (Ghnos Malone, 1988). The following complex columns and distillation complexes were examined column with side output above the feed cross-section, column with side rectifier, column with side stripping flowsheet with prefractionator, Petlyuk column top and side flows from the first column into the second one (Fig. 8.3a),... 

Columns with side stripping sections were used for petroleum separation already in the first decades of twentieth century. This choice is quite grounded by the main purposes of designing increase of separability, and decrease of energy and capital expenditures on separation. 

Because petroleum is a mixture with a wide interval of bubble temperatures of components and the required purity of products is not very great, reflux and vapor numbers in the sections are not large. Therefore, the heat brought in is used up mostly not for creation of vapor reflux, but for evaporation of those products that are withdrawn above the feed cross-section. Therefore, the split (direct, indirect, intermediate) has but a weak influence over energy expenditures, but it is very important to exclude multiple evaporation and condensation like in multicolumn sequences of simple columns. Columns with side strippings exclude multiple evaporation and condensation. 

Among multisection distillation complexes, only columns with side strippings bring practically the whole heat into the feeding and bring hve steam into the bottom. Application of pumparounds decreases energy expenditures and recuperates withdrawn heat for heating of petroleum before separation. 

Therefore, column with side strippings, with live steam into the bottom, and with pumparounds is the best distillation complex for petroleum refining. An optimum way of designing such column is discussed in Section 7.5.2. 

Only part of these products is obtained in each unit. Nevertheless, it is not possible to obtain all the products in one column with side strippings because of... 

Therefore, the main succession of petroleum refining includes two main units that of atmospheric distillation and that of vacuum distillation. The lighter products are obtained in the first unit right up to mazut (bottom product of atmospheric column with side strippings) and, in the second unit, the heavier products are obtained right up to tar (bottom product of vacuum column with side strippings). 

The first unit usually consists of one column with side strippings (one-column flowsheet) (Fig. 8.32a) or of superatmospheric column of partial recovery of benzine and of main column with side strippings (two-column flowsheet) (Fig. 8.32b). 

For petroleum mixtures, their peculiarities predetermine the choice of the main distillation complex (column with side strippings) and of main sequence (atmospheric and vacuum columns), but different modifications are possible for different sets of products and compositions of crude are possible. These modifications may significantly improve the economy of the equipment. 

In the basic a/c-path there exist two sections with twofold separations, see Fig. 11.2-9 (Brusis 2003). These twofold separations can be avoided by arranging the rectifying section of colitrrm C-2 at the top of colitrrm C-1 and by arranging the stripping section of C-2 at the bottom of colurrm C-1. Two heat exchangers of the basic process (Fig. 11.2-5) can be discarded. The internal concentration profiles of the process with side colurrm are depicted in Fig. 11.2-10. No double fractionation exists in the modified process. The feeds into the side column are taken at the concentration maxima of the intermediate boiling compound b. 

Figure 6.14. Knch zones in column with side product and intermediate and stripping section trajectories for the ideal ternary mixture (a) side product composition equal to tear-off point composition and (b) side product composition unequal to tear-off point composition. Attraction region of point (Reg ft) is shaded.

Regardless of the procedure used, certain initial steps must be taken for the determination or specification of certain product properties and yields based on the TBP distillation curve of the column feed, method of providing column reflux, column-operating pressure, type of condenser, and type of side-cut strippers ana stripping requirements. These steps are developed and ilhistrated with several detailed examples by Watkins (op. cit.). Only one example, modified from one given by Watkins, is considered briefly here to indicate the approach taken during the initial steps. 

In the section below the feed, the more volatile components are stripped from the liquid and this is known as the stripping section. Above the feed, the concentration of the more volatile components is increased and this is called the enrichment, or more commonly, the rectifying section. Figure 11.1a shows a column producing two product streams, referred to as tops and bottoms, from a single feed. Columns are occasionally used with more than one feed, and with side streams withdrawn at points up the column, Figure ll.lt . This does not alter the basic operation, but complicates the analysis of the process, to some extent. 

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... 

The simulation shows that acroleine is the most difficult to isolate. If not removed in C-1A it will be found in the top of C-2 and further in the end product The column C-1A is designed with a ratio stripping/rectificahon 3 1 to ensure over 99.9% HCN recovery. Despite a Rvalue of 1.7 the acroleine concentrates in the middle of the stripping zone, from which a quantitative removal by a large side stream or secondary recovery column is not efficient. The best solution is chemical conversion in heavies. 

A simultaneous countercurrent movement of solid and gaseous phases makes it possible to enhance the efficiency of an equilibrium limited reaction with only one product (Fig. 4(a)) [34]. A positive effect can be obtained for the reaction A B if the catalyst has a higher adsorption capacity for B than for A. In this case, the product B will be collected mainly in the upper part of the reactor, while some fraction of the reactant A will move down with the catalyst. Better performance is achieved when the reactants are fed at some side port of the column inert carrier gas comes to the bottom and the component B is stripped off the catalyst leaving the column (Fig 4(a)). The technique was verified experimentally for the hydrogenation of 1,3,5-trimethylbenzene to 1,3,5-trimethylcyclohexane over a supported platinum catalyst [34]. High purity product can be extracted after the catalytic reactor, and overequilibrium conversion can be obtained at certain operating conditions. 

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