Vapor return nozzle

Tower-bottom liquid level covering the reboiler vapor return nozzle. 

Should the liquid level in the bottom of the tower rise to the reboiler vapor return nozzle, the tower will certainly flood, but the reboiler heat duty will continue. Unfortunately, reboiler shell-side fouling may also lead to tray flooding. This happens because the fouling can cause a pressure-drop buildup on the shell side of the reboiler. 

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. 

For most once-through and many circulating thermosyphon reboilers (see Figs. 18-1 and 18-2), a high liquid level covering the reboiler vapor return nozzle will retard thermosyphon circulation and cause a precipitous loss in reboiler duty, instead of tower flooding. 

In summary, whenever a distillation tower s performance is improved by lowering the bottom s liquid level, foam formation should be suspected. If the apparent liquid level is well below the reboiler vapor-return nozzle before the level is reduced, the existence of a thick froth layer in the bottom of the column is most probable. Reduction in reboiler circulation is one proven method to diminish the foam height in the bottom of a tower. Injection of a silicon defoaming chemical is also effective in fighting foam. 

However, the tower flooding was being induced by a high tower bottoms liquid level. As the reboiler duty was increased, the reboiler pressure also increased. This raised the level in the tower bottoms. When this level reached the reboiler vapor return nozzle, the tower flooded due to entrainment of liquid from the bottom of the tower up to the bottom tray. The operators had noticed that the liquid level rose when the reboiler duty was increased, but they thought that the bottoms liquid level would not cause tray flooding until this level reached the bottom tray. This point deserves emphasis ... 

Foam-induced flooding has been considered a problem occurring on tray decks or inside packed beds. Certainly this is correct. For many columns, however, it is a high foam level formed in the bottom of the tower which causes premature flooding. When this foam level rises to cover the reboiler vapor return nozzle, flooding results. 

I saw hundreds of such reboilers in Amoco s many refineries. I never stopped to think what caused the liquid to circulate through the reboilers. I never thought about it, even though the reboiler feed nozzle on the tower was below the vapor return nozzle. Now, with my fish tank experience as a guide, I was able to understand ... 

I have seen towers equipped with kettle reboilers flood due to high liquid levels a dozen times in my career. The story is always the same. The elevation difference between the reboiler vapor return nozzle and the overflow baffle inside the kettle is only 2 or 3 ft. The designer has forgotten about shell-side fouling. 

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. 

Bottom feed and reboiler return nozzles should not be too close to the maximum liquid level. The space between the bottom of the reboiler return (or bottom feed) nozzle and the maximum liquid level (Fig. 4.16) should be at least 12 in (143, 192, 207, 208, 237, 375). Failure to do this will promote turbulence on the liquid surface, erratic level control, and liquid entrainment into the rising vapor. 

In large-diameter packed colxunns where I-beams support the packing support plate, a larger space between the reboiler return nozzle and the packing support plate may be required. The prime consideration here is allowing sufficient open area between the bottom of the beam and the high liquid level for adequate vapor distribution. Detailed discussion is in Sec. 8.2. 

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. 

Premature tower flooding occurred when level in the bottom of the column rose above rdxnler return nozzle and badqnessured a uniquely designed bottom surge drum. Level rose either because v xrr was present in liquid line to beater or liquid was entrained in drum vapor. Provisions which lowered surge dr level solved the problem. 

Liquid level in the vapor body is an important variable affecting operation of natural circulation calandrias. Normally units are operated with the evaporator liquid level at the top tubesheet of the calandria. For non-fouling fluids, the liquid level can be lowered to the optimum value in order to minimize heat transfer surface or maximize performance. The optimum value is approximately half the distance between the top and bottom tubesheets of the calandria, and will vary with each system. The liquid level should not be appreciably above the top tubesheet and certainly should not be maintained above the caiandria outlet nozzle. Liquid levels above the vapor return will limit the performance of the calandria and may result in damage to the evaporator. Flow instabilities may also be experienced. 

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. 

When the liquid level in the bottom of a column rises to the reboiler return nozzle, the liquid in the bottom of the column is forcibly lifted by the reboiler vapors. The entrained liquid is blown against the underside of the bottom tray. Since flooding will progress up a lower, a liquid level covering the reboiler return nozzle will cause the entire fractionator to flood. 

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. 

The great advantage of forced circulation is that careful calculation of the pressure drop through the reboiler and associated piping is not critical. But as we can see in Fig. 8.6, the operator now has two tower-bottom levels to control. Further, if the hot-side liquid level rises above the reboiler return nozzle, the force of the vapor and liquid rushing back into the column will cause the trays to flood, but the reboiler heat input will not be affected. 

The best designs provide for the percentage vaporization per pass to have been completed by the time the fluid mixture reaches the upper end of the tube and the mixture is leaving to enter the bottom chamber of the distillation column. In order to assist in accomplishing this, the initial reboiler elevation should be set to have the top tubesheet at the same level as the liquid in the column bottom section. A liquid-level control adjustment capability to raise or lower this bottoms level must exist to optimize the recirculation. Sometimes, the level in the bottom of the column may need to be 25-30% of the reboiler tube length above the elevation of the tubesheet. Therefore, the vapor nozzle return from the reboiler must enter at sufficient elevation to allow for this possibility. 

RECIRCULATION ROM PUMP DISCHARGE THROUGH A aOW CONTROL ORIRCE (WHEN NECESSARY) TO THE SEAL AND BACK TO THE SUCTION NOZZLE. THE ORIRCE IS TO BE SIZED IN ACCORDANCE WITH THE THROAT BUSHING AND THE RETURN UNE. SIMILAR TO PLAN 11 BUT FLOW BACK TO SUCTION SIDE WILL EVACUATE VAPORS THAT MAY COLLECT IN SEAL CHAMBR. RECOMMENDED IN UGHT HYDROCARBON SERVICE. 

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