DRECTiFiNg Section Interrb01lR With Trmseht Pinch

(v) Compared to a conventional column, the liquid and vapor traffic below the interreboiler is lower. This may permit a smaller column diameter, or a lower tray spacing, tc be used below the interreboiler. This acts to offset seme cf the increase in capital cost caused by the greater stages requirement (fii) above). i

An extreme case, in which interreboilers were applied for !

the sole purpose of reducing column diameter was reported ir. j water-D20 distillation (45). In this very close separation, j

8 towers in series were used for stripping H20 from D20. j

Interreboiler application permitted the diameters to be ! progressively lowered from 15 feet near the feed to 10 inches near the D20-rich end.

(vi) Adding an interreboiler adds stages and an extra pinch point ' into the column. For these reasons, there is often an incentive to use a slightly higher ratio of reflux to minimum reflux compared to a conventional column. This may slightly increase reboiler, interreboiler, and condenser duties, but the increase is generally small compared to the ; savings achievable by the interreboiler.

(vii) At the bottom of the column, the temperature is sensitive |

only to bottom composition, and is unaffected by feed j composition. Further up the column, the temperature is also sensitive to feed composition. An increase in the concentration of heavy components in the feed may result in an increase in interreboiler tray temperature, thus reducing j the AT available for boiling. Where feed fluctuations are | expected, either a sufficiently large AT must be provided ! for the interreboiler, or overdesign must be incorporated in j the bottom reboiler, and some of the benefits mentioned in ! (v) above may not be achievable. j i

(viii) Application of interreboilers can be particularly attractive j for columns that are restricted by a pinch in the rectifying section (eg, a tangent pinch, Figure 2.22, 2.33; alternatively, some multi-feed columns). For such columns j the shape of the equilibrium curve creates a high reflux requirement near the top, which is not needed elsewhere. Below the pinched region, the operating lines can be made to move a lot closer to the equilibrium curve. This can be achieved with an interreboiler, and such an interreboiler can be located relatively close to the feed. An even better technique is to locate an interreboiler in the rectifying section (Figure 4.2d). This technique increases the total column heating load, but it enables j heating at an even lower a temperature than an interreboiler j below the feed. Note, however, that the rectifying section interreboiler technique can only be applied when the shape j of the equilibrium curve resembles that shown in Figure 4.2d. Columns whose overhead products are azeotropes are a typical example.

(ix) A unique and novel application of interreboilers and intercondensers to upgrade product purities in a column operating at maximum throughput was reported by Niedzwiecki (12). In this column, reflux decreased from the top of the tower towards the feed because of variations in the latent heat of vaporization. This caused trays above the feed to dry up, while the top trays were at maximum liquid loading. An intercondenser and an interreboiler were installed to increase the liquid traffic through the middle of the column, without affecting the loading on the top trays.

(x) Another novel application of interreboilers and inter-condensers for upgrading product purities was described by King et al (43). The column described was a demethanizer, which utilized a very high-grade refrigerant in the top condenser. The high-grade refrigerant was in short supply, so a lower-grade (warmer) refrigerant was used just upstream of the high-grade refrigeration condenser to make up the balance cold required to condense sufficient reflux. With this system, losses of the heavy component in the column overheads were relatively large.

King et al (43) proposed a modified system, in which the lower-grade (ie, warmer) refrigeration was introduced in an intercondenser, located a few stages below the top of the column. This reduced the top reflux requirement sufficiently to enable the high-grade refrigerant to fulfill all the condensation requirements alone. This lowered the top product temperature, and enabled better top-product purity to be achieved.

A similar philosophy can also be applied with an interreboiler, when bottom purity is limited by the availability of sufficient high-grade heating medium.

A shortcut procedure for designing columns with interreboilers using an x-y diagram is presented in the articles by the author in Section 2. The following example indicates how a preliminary analysis can be carried out to determine the potential impact of installing an interreboiler.

EXAMPLE 4.1 The temperature at the bottom of the benzene-toluene column in Example 2.1 is 223°F, and the column is reboiled using 50 psig steam. Flash steam is available at 8 psig in large quantities. 50 psig st-eam is in short supply, while flash steam is vented to atmosphere. The column has 10 trays in the bottom section, which operate at 50 percent efficiency. The column has excess capacity, and tray spacing can be reduced to fit a total of 14 trays in the bottom section.

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(i) Can interreboiling with flash steam be applied? To which I tray? (assume a 20°F approach between boiling medium j temperature and interreboiling temperature).

J (ii) By how much would the present usage of 50 psig steam be

! reduced by interreboiling?

| (iii) By how much would the present 50 psig steam usage be reduced

| if the interreboiler temperature approach is raised to 25°F?

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