Rectifying trays

Reducing ethylene glycol in the water product to a significantly lower value requires only the addition of one or more rectifying trays in the recovery column. An increase in the reflux-feed ratio will do as an alternate method, but to change the ethanol in the water product, the extractive distillation column would have to be operated for higher or lower ethanol recovery than the 99.99% m value. This can be done without difficulty. 

The effect of liquid leakage from the tray is more important for stripping trays than for rectifying trays. For stripping trays, liquid leakage carries more volatile components toward the draw point for the less volatile product, thus reducing the effectiveness of the stripper. Fortunately, for most bubble cap trays, there is no tray leakage of consequence. 

In a multi-component system, the optimum feed location depends on the light and heavy key components and their desired concentrations in the products. A feed location that is optimum for one set of specifications may be a poor selection for another. The number of rectifying trays must be sufficient to remove from the overhead as much of the components heavier than the light key as is needed to meet the required overhead composition. Similarly, the number of stripping trays must be sufficient to strip from the bottoms as much of the components lighter than the heavy key as is needed to meet the required bottoms composition. 

If the feed is placed on the fourth tray from the top, there would be only four rectifying trays and, therefore, a higher reflux ratio, calculated at 2.0, would be required to obtain the same rectification. The stripping section becomes over-trayed, with several trays showing little change in the separation parameter. If the feed tray is too low, the rectifying section becomes over-trayed. Figure 7.11 is a schematic plot (not to scale) of the separation parameters for the different feed tray locations. 

A reboiled stripper is the opposite of a rectifier in that it has no rectifying section. The feed is sent either directly to the condenser or, if sufficiently cold, to the top tray, requiring no condenser. The liquid portion of the feed acts as reflux. The main purpose of this type of column is to produce a bottoms product that is enriched in heavier components and stripped of the lighter ones. With no rectifying trays, a reboiled stripper could result in some heavies being lost in the overhead. 

Since no condenser is used, the colder feed, serving as external reflux, must be fed to the top tray. With no rectifying trays available above this feed, some of the heavy key component in this feed is "lost" in the overhead. In this example ethane is the heavy key (methane is the light key). Table 9.1 shows that, of the 250 kmol ethane in the upper feed, over 230 mol go overhead. 

In selecting the feed tray for the main feed, it is desirable to maximize the number of stripping trays in order to minimize the reboiler duty required to achieve the specified methane stripping. However, the feed tray should not be too high. If it is, an excessive amount of ethane is lost in the overhead because of an insufficient number of rectifying trays. Table 9.2 shows reboiler duty requirements and ethane recoveries in the bottoms for different feed tray locations. The methane mole fraction in the bottoms is fixed in all cases at 0.001. The results indicate that the optimum feed tray is the third tray from the top. 

Measurement of internals is particularly important for those dimensions that change from one section of the column to another. Assembly errors can often result in internals being interchanged. If these are not detected, severe hydraulic problems can arise. One experience has been described (295) in which the rectifying trays were interchanged with the stripping trays. 

II. Exploiting Equipment Interactions The designer needs to consider total system interactions before finalizing scope for any equipment. Consider a revamp project with the main objective of increasing the duty of a column s feed pre-heater it will require assessment of the condenser s maximum capacity and top rectifying tray section. Simulation of the whole process is needed to understand system interactions. Lee et al. (2007) and Zhu (2014) described ethylene plant retrofit cases to emphasize the importance of system interactions in revamps. 

Reactive distillation is also different from conventional distillation in that there are both product compositions and reaction conversion specifications. The many design degrees of freedom in a reactive distillation column must be adjusted to achieve these specifications while optimizing some objective function such as total annual cost (TAC). These design degrees of freedom include pressure, reactive tray holdup, number of reactive trays, location of reactant feedstreams, number of stripping trays, number of rectifying trays, reflux ratio, and reboiler heat input. 

The number of stripping trays is assumed to be equal to the number of rectifying trays because the relative volatilities between the key components are the same in each... 

The numbers of stripping and rectifying trays are assumed to be equal (Ns = Nr) because the relative volatilities between the components being separated in the two sections are the same. In the rectifying section the separation is between C and A, where the relative volatility is ac/a = 2 in the stripping section the separation is between B and D, where the relative volatility is as/ctD = 2. 

Then the number of the stripping and rectifying trays is varied over a range, and steps 3-15 are repeated. 

Note that the optimum number of stripping trays is larger than the optimum number of rectifying trays. This is caused by the higher temperatures in the lower part of the column, which means lower relative volatilities. 

There are five stripping trays, five rectifying trays, and nine reactive trays. The column operating pressure is 8 bar, and the holdup on the reactive trays is 1000 mol. The vapor boilup is 28.91 mol/s, and the column diameter is 0.805 m. Figures 4.2 and 4.3 give composition and temperature profiles in the column, respectively. 

The final parameter explored in this chapter is the number of trays used in the two separation sections. In Chapter 2 we found that increasing the number of stripping and rectifying trays decreases energy consumption in the quaternary system. In Section 5.1.7 in this chapter we found that there is an optimum number of stripping trays in the ternary system without inerts. What are the effects for the ternary system with inerts ... 

Note the increase in the composition of A in the rectifying section for the Nr = Ns = 4 case. A large amount of A is escaping from the top of the reactive zone, but the rectifying trays are only able to keep heavier components B and C from leaving in the distillate. 

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