Lp

column

Distillate figure 4.10(a)

SlfíPLE SPLIT FEED

Bottom distillate

FIGURE 4.10 hi/iTi-EFFECT £i¿TllLftTION (continued)
Distillate Stream

¡In the forward-feed arrangement (Figure 4.10c), all the feed enters j the HP column. When the feed, is rich in the less volatile component, : this arrangement produces a pure bottom stream and an impure Icistillate stream as shown in Figure 4.10c. The impure overhead i stream is further separated into a distillate stream ar.c a second bottom stream. when the feed is rich in the more volatile component, \a variation of this arrangement is used (39) in which the HP column produces a pure distillate stream and an impure bottom stream, the latter being processed in the LP column. Another variation of the |scheme shown in Figure 4.10c is one in which the HP column is not ;refluxed (39), or used as a stripper, but this variation suffers from ¡problems of heat balancing (39).

J The backward-feed arrangement (Figure 4.10d) is the reverse scheme, j in which all the feed enters the LP column. It was described by ;O'Brien (33) for a case in which the feed was rich in the mere I volatile component. In the arrangement shown in Figure 4.10d, the LP column produces a pure distillate stream and an impure bottom stream. The impure bottom stream is further separated in the HP tower to produce a bottom stream and a second distillate stream in the HP I column. A case study in which a column was revamped using this |technique in conjunction with feed preheaters was reported by 0'3rier. j (33). Although not previously proposed, there is'no reason why a !variation of this backward-feed scheme cannot be applied to mixtures ¡rich in the less volatile component. In this case, the LP column will produce a pure bottom stream and an impure distillate stream, with the latter undergoing further separation in the HP column. This variation may have a distinct advantage for processing thermally ¡unstable materials.

¡In the presplitter arrangement (Figure 4.10e) the HP column separates ! the feed into two crude cuts which are further refined into final ;products in the LP column. In general, this system gives slightly j lower heat requirements than the forward-feed system (39).

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