How Reboilers Work

Four types of reboilers are discussed in this chapter  

There are dozens of other types of reboilers, but these four represent the majority of applications. Regardless of the t) e of reboiler used, the following statement is correct Almost as many towers flood because of reboiler problems as because of tray problems. 

DRW = specific gravity of fluid in the riser in this case 1.0 HRT = height of the aerated water in the riser tube, ft 

In a thermosyphon or natural irculation reboiler, there is, of course, no source of air. The aerated liquid is a froth or foam produced by the vaporization of the reboiler feed. Without a source of heat, there can be no vaporization. And without vaporization, there will be no circulation. So we can say that the source of energy that drives the circulation in a thermosyphon reboiler is the heating medium to the reboiler. 

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To answer this, let us see how such a gravity-fed or kettle reboiler works ... 

Compare a kettle reboiler with a thermosyphon reboiler. Explain how each works and the primary differences between them. 

There are three balances for the binary system considered here the total mole balance, the component mole balance, and the heat balance. It is customary to apply these in imison and in turn to three separate regions of the fractionation column. One set each is used to describe conditions above and below the feed tray, i.e., the rectifying and stripping sections. These balances are taken over the entire upper and lower portions and include the reboiler and condenser as well as the product stream. A third set of balances is applied to an isolated stage, the feed tray, and includes the flow rates and thermal condition of the incoming feed. Let us see how this works out in practice. For the rectifying section, we have... 

The analysis presented in this chapter is an example of how the principles of thermodynamics can be applied to establish efficiencies in separation units. We have shown how exergy analysis or, equivalently, lost work or availability analysis can be used to pinpoint inefficiencies in a distillation column, which in this case were the temperature-driving forces in the condenser and the reboiler. The data necessary for this analysis can easily be obtained from commonly used flow sheeters, and minimal extra effort is required to compute thermodynamic (exergetic) efficiencies of various process steps. The use of hybrid distillation has the potential to reduce column inefficiencies and reduce the number of trays. We note that for smaller propane-propene separation facilities (less than 5000bbl/day [10]), novel technologies such as adsorption and reactive distillation can be used. 

The final concern we have about the control structure in Fig. 5.16 is how to start up and turn down the plant. For example, how would we start up the columns without running the furnace and the reactor Also, how could we turn off the heat to any of the reboilers when the reactor and the furnace are running The bypass valves may not be designed to take the full gas stream when fully opened. This implies that we need two control valves working in tandem around each reboiler or a three-way valve. Neither of those options is particularly attractive. See Jones and Wilson (1997) for further discussions on process flexibility related to heat integrated designs. 

As in the case of reboilers and condensers, distillation control is too wide a topic to be adequately covered in a handful of chapters. Entire texts (68, 89, 301, 332, 362) deal exclusively with distillation control. Most of these strike a balance between theory, practice, controls design, and controls optimization. In contrast, the coverage here emphasizes operational aspects what various control schemes can and cannot do, how to put together a control system (not necessarily optimum, but one that works), how to recognize and avoid a troublesome system, what are the ill effects of various poor control schemes, and what corrective action can restore trouble-free operation. 

The stripper tower separates the acid gases from the amine systems, and so its efficiency dictates how effective the system works. Increased pressure drop caused by fouling means higher firing on the reboiler. This condition may lead to corrosion and more amine lost through carryover. 

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