The economic curves on Figure 4.4 show that with the lighter feed, an |all vapor feed was optimum; with the heavier feed, 0-40 percent ¡vaporization vas the optimum. Perhaps a general guideline that can ¡be inferred from Patterson and Wells' analysis is that the optimum fraction of feed vaporized increases with the concentration of lights iin the feed.
Application of feed preheating (or increasing the feed vapor fraction) affect column design and performance as follows:
(i) It affects the ideal feed stage location, and often, the number of stages required for the separation. These effects are complex, and must be analyzed for each situation separately.
(ii) It tends to increase vapor and liquid traffic in the ; rectifying section, while decreasing vapor and liquid
! traffic in the stripping section.
(iii) It tends to increase the reflux ratio and the loads on the condenser and reflux system, while unloading the reboiler.
(iv) The quantity of vapor in the feed has a major effect on the design of the feed piping and feed inlet to the column.
All the above aspects must be studied in detail when installing a feed preheater, especially in revamps. Special attention needs to be focused on feed point location, feed piping design, and potential bottlenecks in the rectifying section and the overhead system.
Some additional application guidelines are:
(i) A feed preheater competes with an interreboiler for the use of lower-grade heat media to reduce reboiler duty. Often, an interreboiler has the potential for saving more energy, but the capital cost for adding a preheater is usually lower, and the feed preheater has the advantages of simplicity and ease of operation compared to an interreboiler. Therefore, a feed preheater is often preferred even in situations when an interreboiler can save more energy. In addition, the feed preheater is generally a far more suitable revamp device compared to an interreboiler.
(ii) When a feed-bottom interchanger is planned (Figure 4.3b), some consideration must be given to downstream units. A cooler column bottom stream may increase energy consumption in the unit processing the bottom stream.
(iii) Fluctuations in feed composition may affect the performance of the feed preheater. Heavier feed may reduce its heat input. Some overdesign may need to be incorporated in the reboiler and stripping section in order to accommodate such fluctuations.
(iv) The amount of preheat that can be supplied is often limited by the temperature approach or the quantity of low grade heat available. This is commonly the case in feed-bottom (or feed-product) interchangers. In such situations, correct location of the feed control valve is important; it should be located upstream of the preheater to achieve maximum vaporization.
(v) Excessive vaporization may occur if a preheater is overdesigned, when the exchanger tubes are clean, or when the feed contains an excessive quantity of lights. This may cause bottlenecks in the column rectifying section, reflux system, or affect column separation. Attention must be paid to the control system in order to ensure that preheater vaporization can be sufficiently reduced under the circumstances.
(vi) The control of column feed is often part of the control system of an upstream unit, eg another column. Introduction of a feed preheater often has an effect on this system, and the control system may need to be modified to suit.
PRECOOLERS Precoolers are analogous to preheaters, and the above discussion for preheaters is extensible to precoolers. Their main application for energy saving is in refrigerated columns, where cooling the feed by a wanner (and therefore, less expensive) refrigerant or by cooling water can reduce the consumption of the colder refrigerant in the condenser. Arrangements analogous to those in 4.3a and b are usually used (with a feed-product interchanger being most commonly used in the arrangement analogous to Figure 4.3b). The analysis of Patterson and Wells (7) discussed for preheaters is also extensible to precoolers, with the analogous guideline being that the optimum fraction of feed condensed increases with the concentration of heavies in the feed. In an analogous manner to preheaters, precoolers affect the feed point location and feed piping design. Their introduction increases the loads on the reboiler and stripping section, while lowering the reflux ratio and the vapor and liquid loads in the rectifying section and in the column overhead system.
In an analogous manner to preheaters, precoolers compete with intercondensers, with precoolers usually being cheaper, simpler and more suitable for revamps. An increase in the concentration of lights in the feed may reduce the condensation rate at the precooler, while an increase in heavies concentration, clean exchanger, or excessive heat exchange area may cause excessive condensation; both need to be catered for in the design. When a precooler is installed, the feed control valve should be located downstream of the precooler to achieve maximum condensation. The control system of the upstream unit should be revised to cater for the addition of a precooler.
Energy savings from interreboilers and preheaters depend on utilizing a heating medium which is lower grade than that used in the reboiler. Cften, such a heating medium is not available in sufficient quantity,
!or its supply is not sufficiently reliable, to justify installing the j
Ipreheater or interreboiler. !
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