Energy Saving In Distillation

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j 4.1.1 Classification of Energy Saving Techniques j Energy saving techniques can be classified into two categories:

! 1 Techniques that involve a considerable amount of capital i expenditure. These techniques are generally capable of achieving large energy savings, and are mainly suitable for new i plants or major revamps.

j 2 Techniques that involve a relatively small amount of capital i expenditure. These techniques generally (but not always) are

! capable of achieving smaller savings than those under item 1

above, and are mainly suitable for in-house improvements.

; The techniques generally involving considerable amount of capital j expenditure are discussed in Section 4.2, while those involving a : smaller amount of capital expenditure are outlined in Section 4.3 I Finally, Section 4.4 surveys some of the literature which I specifically describes energy saving techniques, pointing out the strengths and major topics dealt with in each literature source.

■4.2 ENERGY SAVING TECHNIQUES FOR NEW PLANT AND REVAMPS

i4.2.1 Interreboilers and Intercondensers i

¡In a conventional distillation column, all the heat input is applied | in the bettor? reboiler, and removed in the top condenser. Commonly, I heat is supplied by steam in the reboiler, and rejected to cooling |water or air in the condenser. This practice is thermodynamicallv i inefficient because heat is added at the highest (most valuable) ¡level and removed at the lowest (most difficult to recover) level, iInterreboilers and intercondensers permit some of the duty of heat addition or heat removal to be carried out at intermediate heat levels.

INTERREBOILERS In most cases, the application of an interreboiler !to a column (Figure 4.1a) does not significantly alter the total iquantity of heat input to the column. The main difference the iinterreboiler makes is that it enables heat input at two different !locations in the stripping section, instead of just at the bottom. !Since the column is hottest at the bottom, heating at that location I must be provided by a relatively high grade (ie, high temperature) !heating medium. Further up in the column, the column is cooler, and ¡heating can be provided by a lower-grade (ie lower temperature), anc |therefore cheaper, heating medium. Application of an interreboiler j thus shifts a portion of the column heating duty from the reboiler heating medium to the lower grade heating medium used in the interreboiler. Typically, between 1/3 and 2/3 of the column heat duty is carried out by the interreboiler.

|Application cf interreboilers is limited to cases in which a jlower-grade heating medium is available and is of satisfactory |temperature and quantity to carry out boiling at a point further up : in the column. An ideal application is when the column is reboiled |by steam, while a waste-heat stream can be used for interreboiling. ¡Other common applications are in multi-level steam systems, where 1 lower-pressure steam can be used for interreboiling than that needed ¡at the bottom reboiler, and in refrigerated columns, where an ' interreboiler can be used to condense refrigerant vapor at a lower temperature and pressure than at the bottom reboiler. In cold services, where refrigerant vapor condensing pressure is a primary factor in determining compressor power consumption, there is often incentive to install two or even more interreboilers.

It has been argued (8) that interreboiler application is generally most attractive when the difference between column top and bottom temperature is large. This is generally a good guideline when the reboiler duty is relatively low. However, when the reboiler duty is high, interreboilers can be very attractive also in applications where temperature differences between column top and bottom are relatively low, such as in ethylene-ethane separations (9-11) .

Interreboiler

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FIGURE <Uot INTERREßOlLER ARRANGEMENT

APPLICATION When applying interreboilers to a column, the following iconsiderations are imnortant:

i i j (i) The point of application of an interreboiler depends mainly j on the temperature of the available low-grade heating i medium. The highest tray temperature at which an

' ir.terreboiler can be applied must be lower than the heating

| medium by a margin large enough to enable satisfactory heat j transfer. For instance, if the heating medium is available at 180°F, and a 20°F margin is considered satisfactory, the highest tray temperature at which the interreboiler can be applied is 160°F. See item (d) for further discussion of interreboiler location.

(ii) Application of an interreboiler increases the column stages j requirement. This is illustrated in Figure 4.2(a). Above the interreboiler, the liquid and vapor flows are the same as in the case in which no interreboiler is applied. Below the interreboiler, both the liquid flow and vapor flow are diminished by an equal amount, the reduction being equal to the quantity of liquid boiled (or vapor generated) at the I interreboiler. Since in the stripping section there is more

| liquid than vapor flow (L/V ratio greater, than unity; see

Sections 2.1 and 2.2), an equal reduction of both L and V increases the L/V ratio. The greater L/V ratio causes the operating line to steepen (Figure 4.2(a)), and therefore, to move closer to the equilibrium curve. This in turn causes the stages to become smaller, and therefore, a greater number of stages is required.

| (iii) The quantity of heat that can be supplied by an j interreboiler is limited by the approach to a pinched condition (Figure 4.2(b)). As described above, the slope of j the operating line below the interreboiler increases with the quantity of heat supplied by the interreboiler. The highest quantity of heat that theoretically can be supplied by an interreboiler corresponds to an L/V ratio where the operating line for the section below the interreboiler just touches the equilibrium curve. At this point, an infinite number of stages is required to achieve the separation, and the column is pinched. This is not an operable condition. The quantity of heat supplied by the interreboiler must be low enough to give an L/V ratio below the interreboiler which provides a sufficiently large margin away from the pinch point.

(iv) When a sufficient quantity of low-grade heating medium is available, there is incentive to locate the interreboiler at the hottest possible tray that will permit satisfactory heat transfer. Figure 4.2(c) shows that the further down the column the interreboiler is located, the greater is the slope of the operating line in the section below the interreboiler at the same margin from pinching. It follows that the lower the interreboiler is located, the more heat it can supply.

Interreboiler

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