I (ii) The number of stages available in the bottom section is 7 (=14 x 50 percent). By trial and error, an operating line that will give 7 stages in the bottom section can be found. This operating line is shown on Figure 4.2(a). Slope of this operating line is 2.0. Above the interreboiler, the L/V ratio is the same.as that in Example 2.1, ie L'/V* « 363/234 » 1.55 (Figure 2.15). Let the quantity of liquid vaporized in the interreboiler be AV, then
Solving gives ¿V/V' » 0.45, or 45 percent of the vapor generated by the reboiler in Example 2.1 can be supplied by the interreboiler.
(iii) With a 25°F temperature approach, a point in the column with a temperature of 210°F («235-25) is sought. This corresponds to a benzene mole fraction of 0.30 in the liquid.
Line 1 in Figure 4.2(c) shows the corresponding operating line that gives 7 theoretical stages in the bottom section. Its slope is 1.85.
Solving gives &V/V* - 0.35, or 35 percent of the vapor generated by the reboiler in Example 2.1 can be supplied by the interreboiler. This illustrates that the further up the f column an interreboiler is located, the smaller is the fraction of reboiler heat it can supply.
INTERCONDENSERS The application of an intercondenser (Figure 4.1b) to a column is fully analogous to the application of an interreboiler. While in a conventional column, all the heat is removed by the top condenser, the application of an interconder.ser enables heat removal at two different locations in the rectifying section. The total heat removed usually remains the same whether an intercondenser is used or not.
The application of an intercondenser enables some heat removal using a warmer (and therefore cheaper) cooling medium than can be used at the top of the column. Typically, between 1/3 and 2/3 of the column condensing duty is carried out by the intercondenser.
Application of intercondensers is limited to cases in which a lower grade (warmer) condensing medium is available and is of satisfactory temperature and quantity to carry out condensing at a point further | down the column. Since cooling water and air are cheap and | plentiful, intercondensers are seldom used when these cooling media I are used in the top condensers. The main application of intercondensers is in refrigerated columns, where their application enables shifting significant condensing loads from one refrigerant level to a warmer (and therefore cheaper) level, or to cooling water.
i Comments made on the application of interreboilers extend to the application of intercondensers. These are summarized below. The reasoning and discussion follow the same logic as that described for interreboilers.
(i) The point of application of an intercondenser depends mainly on the temperature of the available low-grade condensing medium.
(ii) Application of an intercondenser increases the column stage requirement.
(iii) The quantity of heat that can be removed by an intercondenser is limited by the approach to a pinched condition.
(iv) When a sufficient quantity of low-grade condensing medium is available, there is incentive to locate the intercondenser at the coldest possible tray that will permit satisfactory heat transfer.
(v) Compared to a conventional column, the liquid and vapor traffic above the intercondenser is lower. This may permit reduction of diameter or tray spacing above the intercondenser.
(vi) Adding an intercondenser adds stages and an extra pinch point into the column.
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(vii) At the location of the intercondenser, the temperature is sensitive to variations in feed composition as well as variations in product composition. This must be allowed for in the design.
(viii) The application of intercondensers can be particularly attractive for columns that are restricted by a pinch in the stripping section (eg tangent pinch in the stripping section; alternatively, some multi-feed columns restricted by minimum stripping). Here the slope of the rectifying section operating line can be substantially changed without altering the approach to pinching.
A shortcut procedure for designing columns with intercondensers using an x-y diagram is presented in the articles by the author in Section 2.
FURTHER READINGS References 7 and 8 contain valuable contributions on the application of interreboilers and intercondensers in industrial practice.
In most applications, the thermal condition of a column feed is set at the upstream unit which feeds the column. Preheaters and precoolers can alter this thermal condition to reduce the column energy consumption. A feed preheater can save energy when it uses a lower grade (and therefore, less expensive) heating medium than the jheating medium used in the reboiler. Similarly, a feed precooler can J j save energy when it uses a lower grade cooling medium than the j cooling medium used in the condenser. Preheaters and precoolers can i often be regarded to compete against interreboilers and interconaensers respectively.
PREHEATERS Some typical energy-saving preheater arrangements are j shown in Figure 4.3. When the reboiler is heated by MP or HP steam, ! I preheating can be carried out by LP steam or a hot process stream | (water, oil etc; Figure 4.3a), or any other lower-grade heating :medium. One common arrangement is a feed-bottom interchanger ! (Figure 4.3b). The bottom stream is conveniently located and is always a lower-grade heat source than the reboiler heating medium (otherwise heat would not flow from the reboiler to the bottom of the column). When the column overhead or top product temperature is j sufficiently high to preheat the feed, a feed-product interchanger or ! a feed preheater/overhead condenser can be used instead of the j feed-bottom interchanger. When the column feed comes from the J
overheads of another column (Figure 4.3c), taking the overheads !
product as a vapor or a vapor-liquid mixture instead of liquid ;
essentially preheats the feed without a heat exchanger, while j reducing the previous column condenser load.
jPatterson and Wells (7) analyzed the effect of feed preheating with a I low-grade heating medium on exchanger duties and column economics. They considered two cases: one in which 80 percent of the feed was made up of lighter components (those that leave in the tower top product) and the other in which 80 percent of the feed was made up of i heavier components. When the feed mainly consists of the lighter |
components (Figure 4.4a), reboiler duty sharply drops, while !
condenser duty only slowly rises as the vapor fraction in the feed is j raised. On the other hand, when the feed mainly consists of the ;
heavier components (Figure 4.4b), reboiler duty only slowly drops, , while condenser duty sharply rises as the vapor fraction in the feed is raised.
The reason for this behavior is that with the lighter feed, the preheat mainly vaporizes the light component and sends it up the column, a function that would otherwise be carried out by the reboiler. With the heavier feed, the preheat largely vaporizes the heavy component and sends it up the column. This component cannot leave in the top product, so it is condensed and returned downwards, thus increasing the condenser duty without having a marked effect on the reboiler duty.
LP STEflfl OR HOT Oil/WATCR ETC.
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