The Reboiler

For the reboiler, we have to include the energy balance to compute the vapor flow rate and overall material balance. 

All machines have drivers. A distillation column is also a machine, driven by a reboiler. It is the heat duty of the reboiler, supplemented by the heat content (enthalpy) of the feed, that provides the energy to make a split between light and heavy components. A useful example of the importance of the reboiler in distillation comes from the venerable use of sugar cane, in my home state of Louisiana. 

If the cut cane is left in the fields for a few months, its sugar content ferments to alcohol. Squeezing the cane then produces a rather low-proof alcoholic drink. Of course, one would naturally wish to concentrate the alcohol content by distillation, in the still shown in Fig. 4.1. 

The alcohol is called the light component, because it boils at a lower temperature then water the water is called the heavy component, because it boils at a higher temperature then alcohol. Raising the top reflux rate will lower the tower-top temperature, and reduce the amount of the heavier component, water, in the overhead alcohol product. But what happens to the weight of vapor flowing up through the trays Does the flow go up, go down, or remain the same  

There are two ways to answer this question. Let s first look at the reboiler. As the tower-top temperature shown in Fig. 4.1 goes down, more of the lighter, lower-boiling-point alcohol is refluxed down the tower. The tower-bottom temperature begins to drop, and the steam flow to the reboiler is automatically increased by the action of the temperature recorder controller (TRC). As the steam flow to the reboiler increases, so does the reboiler duty (or energy injected into the tower in the form of heat). Almost all the reboiler heat or duty is converted to vaporization. We will prove this statement mathematically later in this chapter. The increased vapor leaving the reboiler then bubbles up through the trays, and hence the flow of vapor is seen to increase, as the reflux rate is raised. 

Now let s look at the reflux drum. The incremental reflux flow comes from this drum. But the liquid in this drum comes from the condenser. The feed to the condenser is vapor from the top of the tower. Hence, as 

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Requires more heat in the reboiler but less cooling in the condenser... 

If decomposition in the reboilers contaminates the product, then this dictates that no finished products be taken from the bottoms of columns. 

A widening of the temperature differences across columns, since light nonkey components cause a decrease in condenser temperature and heavy nonkey components cause an increase in the reboiler temperature. 

First consider thermal coupling of the simple sequences from Fig. 5.1. Figure 5.14a shows a thermally coupled direct sequence. The reboiler of the first column is replaced by a thermal coupling. Liquid from the bottom of the first column is transferred to the second as before, but now the vapor required by the first column is supplied by the second column instead of by a reboiler on the first column. The four column sections are marked as 1, 2, 3, and 4 in Fig. 5.14a. In... 

Consider again the simple process shown in Fig. 4.4d in which FEED is reacted to PRODUCT. If the process usbs a distillation column as separator, there is a tradeofi" between refiux ratio and the number of plates if the feed and products to the distillation column are fixed, as discussed in Chap. 3 (Fig. 3.7). This, of course, assumes that the reboiler and/or condenser are not heat integrated. If the reboiler and/or condenser are heat integrated, the, tradeoff is quite different from that shown in Fig. 3.7, but we shall return to this point later in Chap. 14. The important thing to note for now is that if the reboiler and condenser are using external utilities, then the tradeoff between reflux ratio and the number of plates does not affect other operations in the flowsheet. It is a local tradeoff. 

The dominant heating and cooling duties associated with a distillation column are the reboiler and condenser duties. In general, however, there will be other duties associated with heating and cooling of feed and product streams. These sensible heat duties usually will be small in comparison with the latent heat changes in reboilers and condensers. 

Both the reboiling and condensing processes normally take place over a range of temperature. Practical considerations, however, usually dictate that the heat to the reboiler must be supplied at a temperature above the dew point of the vapor leaving the reboiler and that the heat removed in the condenser must be removed at a temperature lower than the bubble point of the liquid. Hence, in preliminary design at least, both reboiling and condensing can be assumed to take place at constant temperatures. ... 

The consequences of placing distillation columns in different locations relative to the pinch will now be explored. There are two possible ways in which the distillation column can be integrated. The reboiler and condenser can be integrated either across the pinch or not across the pinch. 

Fig. 14.1a. The background process (which does not include the reboiler and condenser) is represented simply as a heat sink and heat source divided by the pinch. Heat Qreb is taken into the reboiler above pinch temperature and rejected from the condenser at a lower temperature, which is in this case below pinch temperature. Because the process sink above the pinch requires at least Q min to satisfy its...

If botb reboiler and condenser are integrated with the process, this can make the column difficult to start up and control. However, when the integration is considered more closely, it becomes clear that both the reboiler and condenser do not need td be integrated. Above the pinch the reboiler can be serviced directly from hot utility with the condenser integrated above the pinch. In this case the overall utility consumption will be the same as that shown in Fig. 14.16. Below the pinch the condenser can be serviced directly by cold utility with the reboiler integrated below the pinch. Now tlje overall utility consumption will be the same as that shown in Fig. 14.1c. 

Let us now consider a few examples for the use of this simple representation. A grand composite curve is shown in Fig. 14.2. The distillation column reboiler and condenser duties are shown separately and are matched against it. Neither of the distillation columns in Fig. 14.2 fits. The column in Fig. 14.2a is clearly across the pinch. The distillation column in Fig. 14.26 does not fit, despite the fact that both reboiler and condenser temperatures are above the pinch. Strictly speaking, it is not appropriately placed, and yet some energy can be saved. By contrast, the distillation shown in Fig. 14.3a fits. The reboiler duty can be supplied by the hot utility. The condenser duty must be integrated with the rest of the process. Another example is shown in Fig. 14.36. This distillation also fits. The reboiler duty must be supplied by integration with the process. Part of the condenser duty must be integrated, but the remainder of the condenser duty can be rejected to the cold utility. 

The composite curves for this flowsheet are shown in Fig. 14.86. The composite curves are dominated by the reboilers and condensers of the two distillation columns and the feed vaporizer for the acetone feed. It is immediately apparent that the two distillation columns are both inappropriately placed across the pinch. Linnhoflf and Parker ... 

Example A.4.1 The purchased cost of a distillation column is 1 million, and the reboiler and condenser are 100,000. Calculate the annual cost of installed capital if the capital is to be annualized over a 5-year period at a fixed rate of interest of 5 percent. 

Beyond 340°C in the reboiler, the residue begins to crack thermally. If the distillation is stopped at this point, the residue is called the atmospheric residue. In order to continue, the distillation is conducted under a low pressure, vacuum", so as to reduce the temperature in the reboiler. 

Dry inlet gas that has been dehydrated by molecular sieves (qv) or alumina beds to less than 0.1 ppm water is spHt into two streams by a three-way control valve. Approximately 60% of the inlet gas is cooled by heat exchange with the low pressure residue gas from the demethanizer and by external refrigeration. The remainder of the inlet gas is cooled by heat exchange with the demethanized bottoms product, the reboiler, and the side heater. A significant amount of low level refrigera tion from the demethanizer Hquids and the cold residue gas stream is recovered in the inlet gas stream. 

Fig. 3. Rough layout sketch (/) the two fined heaters F-1 and F-2 are located together but are separated from the other equipment with a subpipeway connecting the process area to the heater area (2) the reboiler E-2 is located adjacent to its column, T-1. The preheat exchanger E-4 is located adjacent to tower T-3 (J) the elevated overhead condenser E-3 is located next to the overhead accumulator V-1. Also, the ain condenser EE-3 is located adjacent to its overhead accumulator V-2 (4) the rest of the ain coolers (EE-1—3, -5) are grouped together ia a common fan stmcture (5) all equipment and related piping is routed to and from the existing piperack saving the addition of a new piperack (6) all pumps (P-1—P-6) are located ia a row under the piperack, and each...

Up until 1986 the major use for 2-j -butylphenol was in the production of the herbicide, 2-j -butyl-4,6-dinitrophenol [88-85-7] which was used as a pre- and postemergent herbicide and as a defoHant for potatoes (30). The EPA banned its use in October 1986 based on a European study which showed that workers who came in contact with 2-j -butyl-4,6-dinitrophenol experienced an abnormally high rate of reproduction problems. Erance and the Netherlands followed with a ban in 1991. A significant volume of 2-j -butyl-4,6-dinitrophenol is used worldwide as a polymerization inhibitor in the production of styrene where it is added to the reboiler of the styrene distillation tower to prevent the formation of polystyrene (31). OSBP is used in the Par East as the carbamate derivative, 2-j -butylphenyl-Ai-methylcarbamate [3766-81-2] (BPMC) (32). BPMC is an insecticide used against leaf hoppers which affect the rice fields. 

Reboiler. The case shown in Figure 8 is common for reboilers and condensers on distillation towers. Typically, this AThas a greater impact on excess energy use in distillation than does reflux beyond the minimum. The capital cost of the reboiler and condenser is often equivalent to the cost of the column they serve. 

Fig. 8. ATin the reboiler (a) schematic (b) corresponding graphic representation.

The steam balance in the plant shown in Figure 2 enables all pumps and blowers to be turbine-driven by high pressure steam from the boiler. The low pressure exhaust system is used in the reboiler of the recovery system and the condensate returns to the boiler. Although there is generally some excess power capacity in the high pressure steam for driving other equipment, eg, compressors in the carbon dioxide Hquefaction plant, all the steam produced by the boiler is condensed in the recovery system. This provides a weU-balanced plant ia which few external utiUties are required and combustion conditions can be controlled to maintain efficient operation. 

Chlorobenzene mixtures behave in distillation as ideal solutions. In a continuous distillation train, heat may be conserved by using the condensers from some units as the reboilers for others thereby, saving process energy. 

A good approximation for the vapor rate leaving the reboiler, 1% for any type of distillation is... 

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