Reboiler return inlet

The space between the bottom tray or packing support plate and the liquid level is a prime potential troublespot in distillation and absorption columns. Numerous mishaps originating in this section have been reported (12, 49, 71, 107, 203, 231, 238, 255, 296, 375, 440). It is estimated (143, 207) that 50 percent of the problems in the lower part of the column are initiated in this space. Many of these problems are associated with the reboiler return (or bottom feed) inlet. Therefore, it is important to follow these guidelines. 

Bottom feed and reboiler return inlets should never be submerged below the liquid level (see the Sec. 4.2 for the only acceptable ex-... 

Bottom feed and reboiler return flows should not impinge on the bottom liquid surface. It is best to enter these flows in parallel to the liquid level. Impingement can occur when an internal pipe is "elled down, or if the column inlet pipe slopes downward at the entrance. Arrangements to be avoided are shown in Fig. 4.2. Impinge-... 

Reboiler return and bottom feed lines must be correctly sized. Undersized reboiler return lines can lead to liquid level backup to the reboiler outlet nozzle and may result in premature flooding. The author is familiar with an incident where a column flooded at 50 percent of its design rates because of an undersized reboiler line. Excessive inlet velocities may also induce turbulence and entrainment and have also been reported (98) to cause tray vibration, loosening of tray fasteners, and tray failure. 

A detailed sizing procedure for reboiler return inlets is discussed elsewhere (113). For bottom feeds, the criterion for sizing mixed and vapor feeds (Sec. 2.3, guideline 4) can be used (355). For reboiler return lines, it has been recommended (82) that the velocity in feet per second not exceed V4000/, where... 

The section of column wall directly opposite a bottom feed (or reboiler return) nozzle is often prone to corrosion and erosion attacks. High inlet velocities, small column diameters, and corrosive chemicals are conducive to this problem. The author has experienced a case where metal was thinned and the colximn wall was eventually punctured opposite the bottom inlet of a 2.5-ft-diameter column. Installing an impingement plate on the wall will shield it from such corrosion and erosion attacks. 

It has been recommended (67, 68, 255) to avoid orientation of level-measurement nozzles by angles greater than 90° from a vapor inlet or reboiler return nozzle, and to refrain from positioning these nozzles under the bottom downcomers. If the angle exceeds 90°, a shielding baffle should be provided in front of the measurement nozzle. 

If the liquid level in the bottom sump of the column rises above the reboiler return nozzle (or, alternatively, the bottom vapor inlet nozzle), vapor from the reboiler has to travel upward through a layer of liquid. If this layer is shallow, the vapor can bubble through it or atomize the liquid and carry over liquid as a mist into the first tray from the bottom. This may lead to premature flooding (71, 145, 192, 207, 237, 238) and possibly some wave action that would interfere with level control. 

Avoid impingement of bottom feeds (or reboiler return inlets) on instrument connections. 

Sometimes, thermosiphon reboilers seem to die at low loads. Instead of a gradual reduction in heat output, the reboiler suddenly seems to stop working. This is because the low-percent vapor in the reboiler return line does not result in very much density difference between the reboiler inlet and outlet lines. Without the density difference driving force, thermosiphon circulation stops and heat transfer is reduced. 

Because a packing support plate usually is located immediately above the gas inlet in an absorber or the reboiler return in a distillation column, this plate could be used to control vapor distribution. Obviously, vapor maldistribution can reduce column efficiency in the same way as liquid maldistribution although due to the turbulence in the vapor phase, its rate of radial cross-mixing is at least three times that of the liquid phase. The potential for vapor maldistribution increases as column diameters or operating pressures increase. Fortunately, the vapor phase tends to maintain a uniform distribution once it has been established. Thus, usually only the packing support plate immediately above the vapor inlet needs to act as a vapor distributor. This support plate should be located at least one vapor-inlet diameter plus 12-in. above the center-line of the vapor inlet nozzle. 

The use of non-radial (tangential or offset) vapor inlet nozzles often leads to vapor distribution problems. Whenever two reboiler return nozzles are required, they should be installed radially and located 180 degrees apart so as to cancel kinetic energy effects. The invert of the vapor inlet nozzle should be at least 8 in. above the maximum liquid level in the bottom of the column. Submerging the vapor inlet into the liquid can lead to premature flooding of the packed bed caused by massive liquid entrainment. 

A symmetrical piping arrangement between the draw-off and reboiler inlet nozzles and between the reboiler outlet and return connection on the tower is preferred for equal flow in the reboiler circuit. Nonsymmetrical arrangements may also be... 

Lines interconnecting exchangers with other process equipment can run just above the required headroom or about on the same level as the yard piping. Reboiler line elevations are determined by the draw-off and return line nozzles on the tower. Symmetrical reboiler piping arrangements — between the tower draw-off and reboiler inlet nozzles and between reboiler outlets and return... 

Horizontal reboilers, with natural circulation, have a simple circulation system. Liquid flow from an elevated drum, tower bottom or tower trapout boot through a downcomer pipe to the bottom of the exchanger shell. The liquid is heated and leaves the reboiler in the return piping as a vapor or vapor-liquid mixture and flows back tc the tower or drum. There is no pressure difference between the inlet and outlet nozzles. The circulation is forced by the static head difference between the two liquid columns (see Figure 7-72 Use the exchanger centerline as a reference line. 

Liquid distribution can be a severe problem and a source of instability when more than one reboiler is used, and the reboilers share common inlet and outlet lines (237). It is best to provide separate liquid draw and vapor return nozzles on the column for each reboiler. 

A column flooded prematurely after being switched to a new service. Flooding was caused by an excessive pressure drop in a kettle reboiler circuit backing up liquid to above the rdboiler return nozzla Elevating vigKn inlet above liquid level solved the problem. Ladr of level indication made diagnosing the problem difficult. 

The top of a distillation tower works like an absorber whereas the bottom of a distillation tower works like a stripper. The upper and lower parts of a tower are the rectification section, which is the upper portion above the feed inlet, and the stripping section, which is the lower portion between the feed inlet and reboiler vapor return nozzle. 

Gas iiilel/reboiler vapour return deflecior(s) Titted and secured as in drawing Wdr in base of vessel located as in plant design schematic (PDS) with relation to nozzles Liquid baffle(s) vortex breaker fitted Gas inlet pipe installed and flow area dimension and location as on PDS... 

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