Side draw stage

Similarly, one may consider the material balance across the side-draw stage too. Since the side-draw product stream is not terminated by a reboiler or a condenser, there is no exact value that the net flow in either CS3 or CS5 is required to be. Thus, just like the CSs across the feed stage, there are multiple flow directions that may exist across the side-draw stage. These are depicted in Figure 7.11a d. 

It is no surprise that the feasible flow directions across the side-draw stage are the inverse of those across the feed stage. The flow directions shown in Figure 7.1 Id are infeasible because it would imply, by mass balance, that material addition is required at the side-draw stage and not material removal. 

It is interesting to note that for all FPs other than FP3, the net flows in the CSs above and below the side-draw stage (CS3 and CS5) are in the same direction. This means that both these CSs will have the same sign for the net flow as well as reflux. This does not seem to be a problem on flkce value, but one needs to consider how the refluxes in these CSs come about and what their roles are. It can be shown that... 

TABLE 73 Geometric Description of Difference Points Across the Side Draw Stage... 

TTs of CS3 and CS5 have to overlap one another (across the side-draw stage). 

Now, let us take a closer look at the material balance across the first side-draw stage. It can be shown through a similar derivation to the one given in Figure 7.9 that... 

Similarly, the material balance across the second side-draw stage with Xas = s2 (for sharp splits only) results in... 

This modification suggests that the vapour must be returned to the stage of the liquid side draw where the vapour composition of this stage is similar to the returned vapour from the side stripper. Due to the reduction in the reboiler duty of the KE side stripper. 

The new configuration suggests increasing the bottom section by two stages which means moving the lowest side draw and pump around two stages up from the existing... 

Side Draws The 7-1 inc of a side draw passes through the intersection of the operating lines above and below the draw tray. A liquid side draw from stage j is a saturated liquid with composition Xj. The liquid draw composition is the same as the draw tray liquid composition, and therefore the liquid draw 7-line is a vertical line passing through X. A material balance on a tray with liquid draw 5 T is written as... 

The stepped stages may be altered slightly if the operating lines are redrawn based on a side draw composition of 72% instead of the original 74% mole acetone. Alternatively, the reflux ratio may be adjusted such that the liquid draw from the third stage has a composition of 74% acetone. The steps necessary for this approach are as follows ... 

Step off the stages to determine the required number of trays and the optimum feed trays below the side draw. 

It should be noted that the calculated number of trays are theoretical stages that must be converted to actual trays. Once the column is constructed with the determined number of actual trays, feed and side draw locations, the operation of the column will be controlled to satisfy the composition specifications by adjusting such operating variables as product rates, reflux ratio, and condenser and reboiler duties. 

An intermediate product such as the upper side draw, which is mostly propane, may contain impurities from components both lighter and heavier than the main component. The upper side draw is at the same time the bottom product of the top column section and the top product of the second section. The fractionation in the upper section determines to what extent ethane is stripped off from the propane product, and the fractionation in the second section determines to what extent butane is removed from the propane product. Again, in this situation since the number of stages in each section and the reflux ratio are all fixed, the fractionation is fixed. The propane recovery and purity depend mostly on its flow rate and on the flow rates of the adjacent products above and below it. If the propane product contains too much ethane, its flow rate should be cut back and the overhead rate increased. If the propane product contains too much butane, its flow rate should be cut back and the lower side draw rate increased. Table 9.14 summarizes the purities of components in different products at different flow rates. The recoveries can also be calculated from Table 9.14. The dependence of the other products compositions on their rates may be analyzed in a similar manner. 

The acetone concentration peaks at tray 19, or one stage above the reboiler. This would be a logical place to put the side draw. As the side draw rate is increased, its acetone concentration goes down since more and more water gets drawn with the acetone. The variation in the acetone concentration with the side draw rate is shown in Figure 9.8. 

Module 2 models the draw tray and consists of a mixer and an equilibrium stage. Module 3 is a splitter that takes the liquid from the draw tray and splits it into side draw SD and the remaining liquid flowing down to the bottom column section. The side-stripper and the upper column section are modeled with column sections, modules 4 and 5, and the condenser is modeled with an equilibrium stage, module 6. Using computational sequence 1,2, 3,4, 5, 6 requires initialization of streams L3, L5, OH, and R. 

The method has been applied to absorbers (Khoury, 1980), where all heat duties, side draws, and all feeds except the liquid feed at the top of the column and the vapor feed at the bottom are set to zero. The rates of change of the component molar holdup and total energy on a stage may be broken down into liquid and vapor contributions (subscripts j and i are dropped in the rates of change terms for simplicity) ... 

Since any tray (or stage) can have a heat duty and side draws, there is no need for special consideration for condensers and reboilers, except with regards to hydraulics and liquid holdups, which are calculated outside the model. 

The lower part of the column is covered by stepping off stages in a fashion similar to that in the upper part of the column, and the final conni of theoretical stages is then determined. The Ponchon-Savarit method may be used for many situations more complex lhan the simple one just described mixed vapor-liquid distillate product, side draw streams, multiple feeds, and so on. Standard unit operations textbooks should be consulted for more dentils on this methnd. As mentioned, it suffers from a need for enthalpy-concentration data, but even a crude approximation based on linear variation of enthalpy with concentration can be better than the McCabe-Thiele approach if there is a very large difference in the latent heats of vaporization of the iwo components being distillnd. 

The Thjele-Geddes approach is summarized as follows. Consider a stage n in a cascade, the stage being intermediate between the feed and either end of the cascade and not complicated by a side draw or by extraneous heal addition or removal. A mass halance for component / on the tray is... 

In a similar way to which feasible flow directions at the side-draw and feed stages were determined, a summary of the net flow directions at the bottommost thermally coupled point is given in Figure 7.12. By simple symmetry, the topmost thermally... 

The purification unit consists of three valve-tray columns. The production medium for AA in the purification stage at 130-200 °C contains up to 16% water, 26% methyl iodide, and other components, such as methyl acetate (MA), methanol (MeOH), hydrogen iodide (HI), formic acid (FA), and propionic acid (PA) (PEP Report, 1994). The fractionation column removes the light components and portions of water in the mixture, and the dehydration column treats both water and FA. The last column, which is an SSC, produces the final AA product from the side draw by cutting off the remaining light and heavy components from... 

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