Once-through reboiler

Figure 5.3 A once-through reboiler, with vertical baffle plate. 

We said before that it was wrong to return the effluent, from a once-through reboiler, with a vertical baffle, to the cold side of the tower s bottom. Doing so would actually make the once-through thermosyphon reboiler work more like a circulating reboiler. But if this is bad, then the once-through reboiler, must be better than the circulating reboiler. But why ... 

The once-through reboiler functions as the bottom theoretical separation stage of the tower. The circulating reboiler does not, because a portion of its effluent backmixes to its feed inlet. This hackmixing ruins the separation that can otherwise be achieved in reboilers. 

Regardless of the type of reboiler used, the tower bottom product temperature has to be the same, so as to make product specifications. This is shown in Fig. 5.5. However, the reboiler outlet temperature must always be higher in the circulating reboiler than in the once-through reboiler. This means that it is more difficult to transfer heat in the former than in the latter. 

Figure 5.6 Forced-circulation once-through reboiler.

Once-through reboiler arrangement Here reboiler liquid is withdrawn from the bottom downcomer or from a chimney tray located above the bottom sump. Typical arrangements are shown in Fig. 4.7. 

Figure 4.7 Reboiler trapout-pan arrangements, (a) Single-pass trays (once-through reboiler) (6) two-peiss trays, single reboiler (once-through reboiler) (c) two-pass trays, two reboilers (once-through reboiler) (d) single-pass trays (recirculating reboiler).

In tray columns, liquid to once-through reboilers (Fig. 4.7a to c), as well as liquid to many recirculating reboilers (Fig. 4.7cf), is normally withdrawn from the bottom tray via a trapout pan. In packed columns, a chimney tray is usually preferred for this purpose. This trapout pan can often be troublesome. Successful operation requires close attention to the following guidelines ... 

The trapout pan is most suitable for once-through reboilers (Fig. 4.7o to c). With a recirculating reboiler, the trapout pan can still be used, but its arrangement is more complex. Here the reboiler return nozzle needs to be located above the trapout pan, with liquid returning from the reboiler routed into the trapout pan and away from the seal pan. This is usually accomplished by a baffle (Fig. 4.7d) and/or by using a wide pan. Failure to properly route the reboiler return liquid into the trapout pan may starve the reboiler of liquid, and/or lower reboiler AT, and/or reduce the reboiler mass transfer efficiency. 

In packed columns, chimney trays are the most effective liquid collection devices for high liquid flow rate services, for interreboilers, and for once-through reboilers. They are also sometimes used for collecting liquid from an upper bed for redistribution (when the redistributor is not self-collecting), and as vapor distributors. 

A single stage is allowed for a once-through reboiler, a kettle reboiler, or when the bottoms draw compartment is separated by a preferential baffle from the reboiler compartment (Sec. 4.5). Half or zero theoretical stages are allowed for an unbaffled recirculating reboiler arrangement. 

We said before that it was wrong to return the effluent from a once-through reboiler with a vertical baffle to the cold side of the... 

Super-fractionation Effect of partial condenser or once-through reboiler on fractionation. 

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