Gravity return reflux

GRAVITY-RETURN REFLUX VERSUS PUMPED-BACK REFLUX... 

Many arguments have taken place as to whether it is better to locate the condenser overhead and provide gravity return reflux, or to locate the condenser at ground level (or nearly so) and pump the reflux back to the column. With the condenser ovo head, the vapor and reflux lines can be short, which favors good overtiead composition ccHitroL On the other hand, a ground level condenser... 

For those columns with gravity return reflux, a severe oscillation in overhead vapor flow to the condenser is sometimes encountered. This is commonly called reflux cycle and has a typical period of several minutes. It has been observed primarily in columns where reflux flow is the difference between rate of condensation and distillate flow rate that is, where distillate is on automatic flow control or column-composition control. Reversing the controls—that is, employing automatic reflux flow or flow-ratio control and allowing distillate to be the difference flow—provides a positive cure. 

Gravity-Return Reflux Versus Pumfed-Back Rflux 93... 

Gravity-Return Reflux Versus Pumped-Back Riflux... 

Another important point for gravity return reflux is the method of connecting the reflux piping to the column. Each of the two piping arrangements of Figure 3.24 has an undesirable upward loop just before entry into the column. Inerts sometimes accumulate in this pocket, thereby causing a reflux flow oscillation as a result of an intermittent siphon action. There have been cases where hot vapor was sucked back into this pocket and caused such severe hammer that the reflux line and column nozzle were ruptured. 

Pressure drop is only likely to be a major consideration in the design of vacuum condensers and where reflux is returned to a column by gravity flow from the condenser. 

FIGURE 8.3 Columns for water removal used for enzymatic reactions conducted in refluxing solvent. Portions of this figure were adapted from Reference 239. Key to letters (a) exit from reaction vessel (e.g., round-bottom flask) (b) arm for vapor transport (c) entrance to condenser (d) packed bed of molecular sieves (or other drying agent) (e) arm to return dehydrated solvent to reactor and (f) gravity-settling or phase separation column. 

The plant shown in Fig. 18.7 is often simplified for small installations. In place of the reboiler, a heating coil may be placed in the bottom of the column and generate vapor from the pool of liquid there. The condenser is sometimes placed above the top of the column and the reflux pump and accumulator are omitted. Reflux then returns to the top plate by gravity. A special valve, called a reflux splitter, may be used to control the rate of reflux return. The remainder of the condensate forms the overhead product. 

Condensers are usually conventional tubular heat exchangers, generally arranged horizontally with the coolant inside the tubes, but also vertically with the coolant on either side of the tubes [27]. The condenser may be placed above the tower for gravity flow of the condensed reflux to the top tray, but it is usually more convenient for purposes of construction and cleaning to place the condenser nearer the ground and to return the reflux from an accumulator drum [55] to the top tray by pumping. This procedure also provides more pressure drop for operation of control valves on the reflux line. 

Horizontal tube bundle with coolant in tubes (Figure 3.3). The vapor comes up from below and condensate drops into an annular space around the vapor nozzle. The latter has a haf over it to prevent condensate from dropping back down the column. Reflux may return internally via an overflow weir, or externally through a gravity flow line with a control valve. 

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