percent holdup

Figure 3.18b. Minimum Batch Time vs Column Holdup at different q'

percent holdup

Figure 3.19a. Optimum Reflux Ratio vs Column Holdup at different qe percent holdup

Figure 3.19a. Optimum Reflux Ratio vs Column Holdup at different qe

Note that q measures how difficult the given separation is in a given column for a particular mixture. However, the batch times are not identical for different mixtures with the same value of q. For the same value of q the batch times can be different for different binary mixtures although the optimum holdup for the binary mixtures are about 10% (Mujtaba and Macchietto, 1998).

The distillate accumulator, plate 1 (top) and the reboiler liquid composition (for benzene) profiles for case 3 are presented in Figures 3.20-3.22. Figure 3.23 presents the plate 1 liquid composition profile for case 4. Using these composition profiles Mujtaba and Macchietto (1998) made the following observations for better understanding of the role of column holdup.

percent holdup

Figure 3.19b. Optimum Reflux Ratio vs Column Holdup at different qf percent holdup

Figure 3.19b. Optimum Reflux Ratio vs Column Holdup at different qf

As already mentioned, a q value of 0.3-0.4 may be interpreted to mean that 3040% of the plates available in the column are the required minimum for the given separation and therefore 60-70% of the plates are in excess.

As the holdup on the plates always acts as an accumulator, larger holdup gives slower dynamic responses in the concentration profiles. With very low holdup on the plates, most of the liquid is located in the reboiler. The light component initially moves very quickly up the column, giving an early and high peak in the instant distillate and top plates composition profiles, followed by a fairly quick drop off (Figure 3.20, Holdup=2%). With slightly larger holdup (Figure 3.20, Holdup=10%) the composition profiles become flatter, with a smaller initial overshoot of the distillate composition than before and also a slower change in the composition profiles past the peak. Overall, a lower reflux ratio would be sufficient, resulting in slightly shorter batch times (for the same separation task) than in the previous case (as seen more clearly in Figure 3.21).

Figure 3.20. Plate 1 Liquid Composition Profiles for Benzene6

Figure 3.20. Plate 1 Liquid Composition Profiles for Benzene6

Figure 3.21. Accumulated Distillate Composition Profiles for Benzene8

Figure 3.22. Reboiler Liquid Composition Profiles for Benzene1"

Figure 3.22. Reboiler Liquid Composition Profiles for Benzene1"

Figure 3.23. Plate 1 Liquid Composition Profiles for Benzene11

For a given separation task, further decrease in batch time (due to lower reflux ratio operation) can be obtained by increasing plate holdup until such a stage where any further holdup increases result in the reboiler being quickly depleted of the light component as the distillation progresses at which point the heavy component then begin to rise up the column. At this value of holdup and beyond, low reflux operation is no longer possible. So a high reflux is needed to maintain the product purity which consequently means longer batch time. Figure 3.22 clearly explains the above fact. For the 20% holdup case the column has to operate at a quite high reflux ratio to maintain the product quality because of the significant drop in light component in the reboiler.

With increasing q (more difficult separation) there will be fewer plates in excess of the minimum required and the column must be operated at high reflux ratio to achieve the given separation. The effects of holdup on batch time will be more significant. In case 4 (Table 3.3), for example, the minimum batch time increases from approximately 4.5 hours to about 7 hours as the holdup distributed on the plates increases from 2% of the initial charge to 20% (Figure 3.23). The effects observed at low q with high holdup only are now dominant for any value of the holdup. For the cases studied it has been found that beyond q = 0.67 increasing column holdup above zero always increase the minimum batch time (Figure 3.23). Effect of Condenser Holdup on the Column Performance

This study is carried out only for binary mixture 1 with a q value of 0.332. The role of the condenser holdup is examined for three condenser holdup values, 0%, 2% and 5% of the total initial charge. The plate holdup is varied in all cases as a percentage of the total initial charge to the column. There are a total of 8 internal plates with a separation requirement of 90% purity (x D) of benzene.

Figure 3.24 shows the effect of the condenser holdup on the performance of the column. Since, for a total condenser holdup only plays as an accumulator of material but not as a separation stage, larger condenser holdup means longer batch time is always required to achieve a given separation. This is quite clear from Figure 3.24. In practice there must always be a certain amount of condenser holdup to ensure a neat reflux operation, however this should be kept to a minimum. Luyben (1971) also arrived at similar conclusions.

Although the case study presented here identifies an optimum amount of holdup (as a percentage of the total initial charge), in practice the size of the reboiler and the amount of holdup on the plates and in the condenser are fixed for an existing column and are usually dictated by design geometry and pressure drop requirements. Since it is always desirable to charge the reboiler to its full capacity, the holdup to charge ratio will, in general, not be an optimal one for a particular distillation task on hand. In such circumstances it is always possible to trade-off between batch time and recovery of the product to achieve a profitable operation (discussed in detail in Mujtaba and Macchietto, 1993; 1996 and in Chapter 5).

Nevertheless the investigations presented here show that if the holdup effects are understood for the range of separations and mixtures to be handled by a column, it may also be possible to use this information (optimum holdup) at the design stage, and balance the design requirements against additional dynamic effects and column performance variations due to holdup. For further details see Mujtaba and Macchietto (1998).

percent total column holdup

Figure 3.24. Effect of Condenser Holdup1

percent total column holdup

Figure 3.24. Effect of Condenser Holdup1

3.6. Campaign Operation

More than one batch is considered in a long-term production campaign if the total amount of mixture to be processed is more than the capacity of the column. During processing a particular batch, as the overhead composition varies (Figure 3.25, Mujtaba, 1989), a number of main-cuts and off-cuts are made at the end of various distillation tasks or periods (section 3.1). In campaign mode, each intermediate offcuts may be collected and stored separately and fed to the reboiler sequentially and reprocessed in subsequent batches (Quintero-Marmol and Luyben, 1990; Mujtaba and Macchietto, 1992a). The other alternative is to collect and store each off-cut separately for sometime and reprocessed when the amount of material of each offcut reaches to the level of one full batch charge (Mujtaba and Macchietto, 1994).

Figure 3.25. Typical Instant Distillate Composition Profile. [Mujtaba, 1989]

Main-cut 1 (PI) Off-cut 1 Main-cut 2 (P2) Off-cut 2

Charge Product

Figure 3.26. Operational Sequence for Fresh Feed Processing (Product cuts in parenthesis)

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