Steam stripped side columns

Side columns are used, for instance, in the most important distillation processes worldwide, the fractionation of air (see Fig. 11.2-18) and the distillation of cmde oil (Meyers 1996). The atmospheric tower of oil refineries consists of a main column and four stripping side columns (Fig. 11.2-12). In this tower the crude oil is split into six fractions which are processed further in several subsequent columns. Oil refineries also have some other interesting features. Steam is fed into the bottom of the main column and most of the side columns. This causes a stripping effect and reduces the temperatures in the columns (steam distillation). The overhead fractions of all side columns are fed into the main colunrn thus increasing the vapor flow there. So-called pump arounds effect a partial condensation of the vapor in the main column and, in turn, a reduction of the vapor flow rates in the upper sections. 

Bottoms and three side-cut strippers remove light ends from products and may utilize steam or reboilers. In Fig. 13-92 a reboiled stripper is utilized on the light distillate, which is the largest side cut withdrawn. Steam-stripping rates in side-cut strippers and at the bottom of the atmospheric column may vary from 0.45 to 4.5 kg (1 to 10 lb) of steam per barrel of stripped liquid, depending on the fraction of stripper feea hquid that is vaporizea. 

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. 

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). 

There are a number of additional modes of operating a distillation column. These include partial condenser open steam introduced at the column bottom without a reboiler enriching distillation column stripping distillation column operation with a side stream, etc. We will briefly describe each one of these in the context of equilibrium stages/ plates and constant molal overflow. 

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... 

An inefficiency in the original configuration of the case study is shown where the product of the AGO side stripper is spht into two streams one of is cooled and introduced again in the main column. This sub-cooled returned liquid performs the function of a pump around. This separation and remixing is clearly inefficient because of the higher amount of stripping steam required. 

One way to remove this inefficiency is done where the side draw from the main column is split first and returned as sub-cooled stream while the other stream is stripped in the side stripper. Therefore, the required stripping steam is reduced tremendously as the feed is reduced. The total savings in operating cost are 40,000/ for this simple modification in the piping connections. Another example of inefficiency is mixing two vapour streams with a big difference in their composition causes this inefficiency. The returned vapour from the KE side stripper is mixed with the vapour inside the main column which resulting in mixing losses. 

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. 

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. 

Pumparound duties/ Side stripping steam rates These values are not routinely measured during true column operation and may vary significantly. It is generally inadvisable to rely on these values to make a simulation converge. 

---