Thermosyphon reboiler steam

Two-phase flows oscillations induced by the flow boiling process are of interest to designers of macroscale thermal systems and equipment, such as thermosyphon reboilers, steam generators, evaporators in the refrigeration industry and chemical... 

A vertical thermosyphon reboiler is required for a column. The liquid at the base of the column is essentially pure n-butane. A vapour rate of 5 kg/s is required. The pressure at the base of the column is 20.9 bar. Saturated steam at 5 bar will be used for heating. 

Make a preliminary mechanical design for the vertical thermosyphon reboiler for which the thermal design was done as Example 12.9 in Chapter 12. The inlet liquid nozzle and the steam connections will be 50 mm inside diameter. Flat plate end closures will be used on both headers. The reboiler will be hung from four bracket supports, positioned 0.5 m down from the top tube plate. The shell and tubes will be of semi-killed carbon steel. 

A distillation operation separating a low-viscosity hydrocarbon mixture requires three shell-and-tube heat exchangers. The liquid feed is to be preheated to saturated liquid by heat recovery from another low-viscosity hydrocarbon stream. The reboiler is to be a vertical thermosyphon using steam heating. 

The total condensation of a vapor to a liquid is best illustrated by the condensation of steam to water. Figure 13.1 is a rather accurate reproduction of the radiator that heated my apartment in Brooklyn. Steam flowed from the boiler in the basement. The steam condensed inside the radiator, and flowed back into the boiler, through the condensate drain line. This is a form of thermosyphon circulation. The driving force for the circulation is the differential density between the water in the condensate drain line and the steam supply line to the radiator (see Chap. 5, discussion of thermosyphon reboilers). 

Also such factors as allowable tensile strengths, weld efficiencies, and possible inaccuracies of formulas used to calculate shell and head thicknesses may be pertinent—that is, the relative uncertainty or error in the function is related hnearly to the fractional uncertainties of the independent variables. For example, take the case of a steam-heated thermosyphon reboiler on a distillation column for which the heat transfer equation is... 

Referring to Fig. 47.3, for many refinery operations, the circulating thermosyphon reboiler shown can be eliminated and replaced with the same quantity of stripping steam, thus further simplifying the design of your waste water stripper installation. 

In this case th should be 20 minutes or more. In addition, the level controller should be cascaded to a steam-flow controller with a linear flow transmitter (or orifice AP transmitter, and square-root extractor). Furtho", for a thermosyphon reboiler, one should make the volume Ag x AHt- at least ten times the volume inside the reboiler tubes. 

Much difficulty has been encoimtered when trying to control base level by adjusting steam flow to a thermosyphon reboiler. It Aerefore is recommended that all such applications be subjected to a dynamic analysis such as presented in Chapter 16. 

Perhaps the most common system that has given trouble is base level control via steam. This is particularly true if a thermosyphon reboiler is employed or if the column has valve trays, or both. It is the result of inverse response (see Chapter 13). At low boilup rates, sieve trays give the same trouble. To rninirnize difficulties the design recommendations of Chapter 16, Section 7, should be followed. One of the authors has shown, in an unpublished study, that an inverse response compensator can be designed and implemented on a computer or with some microprocessor controls. 

To shut down a column or train, it is only necessary to switch the master switch to shutdown this shuts oflF the steam, feed, and drawoflf valves. Liquid then drains down into each column base. This procedure has worked well for. small columns, say, 6 feet or less in diameter. For larger columns there have been some cases where the lower trays were damaged. It is probably safer, therefore, before startup to pump column-base contents down to a level well below that of the bottom tray. In doing this one should also allow for displacement of liquid from a thermosyphon reboiler once boilup has been established, most of its mbe volume will be vapor. 

There are diree factors that limit the accuracy of the preceding analysis. The first of these relates to the phenomenon of inverse response discussed in Chapter 13. It is characteristic of valve tray columns and some sieve tray columns operating at low boilup rates. It exercises its most serious effect in those columns where base level is controlled via steam flow. If the level becomes too high, the level controller increases the steam flow. But this causes a momentary increase in base level due to the extra liquid coming down the column (also due to thermosyphon reboiler swell ). Without proper design the level controller can become very confused. This is discussed in detail in Chapter 16. 

A similar line of reasoning may be followed at the base of the column, and leads to the conclusicHi that we would ncnmally prefer to ccaitrol base compositicm by manipulating boilup. Controlling base level by steam has another disadvantage if a thermosyphon reboiler is use interchange of inventory between column base and reboiler sometimes leads to severe dynamic problems. This is discussed in Chapters 4, 15, and 16. 

The partially flocxled reboilcr is similar in many ways to the partially flooded condenser. Usually, although not always, it is a verticad thermosyphon reboiler. As discussed in Chapter 4, Section 2, it is controlled by throttling the stsam condensate, which in turn varies the condensate level in the shell and thereby the heat-transfer area for condensation. That area covered by liquid permits only sensible heat transfer fixMn the condensate this is a small heat load compared with that of the condensing steam and is treated as negligible. 

When considering the steam side of steam heated reboilers, it is best to think about the reboiler as a steam condenser. The steam, at least for a conventional horizontal reboiler, is usually on the tube side of the exchanger, as shown in Fig. 8.1. The steam is on the tube side, because the shell side was selected for the process fluid. If the reboiler is a thermosyphon, or natural-circulation reboiler, then low-process-side pressure drop is important. For a horizontal reboiler, it is easiest to obtain a low pressure drop for the fluid being vaporized by placing it on the shell side. 

Kettle reboilers have five connections, two on the tube side and three on the shell side. Steam or hot oil flows through the tube side and provides the heat source. Flow rate is carefully controlled and frequently linked to the bottom temperature control system. The higher the flow rate, the hotter the bottom product. The shell side has three nozzles one liquid-product feed line, one vapor-return line to the column, and one heavy-liquid-out product line. A kettle reboiler can be used to (1) control the liquid level on the bottom of the column, (2) control the temperature of the column, and (3) help control product purity in the bottom of the column. Figure 6-5 shows what a kettle reboiler looks like, and Figure 6-6 shows two thermosyphon and one kettle reboiler arrangements on a distillation column. 

The consequences of vapor lock on the performance of a vertical thermosyphon steam reboiler, with steam on the shell side, is shown in Fig. 13.3. 

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