Reboiler outlet line

Column Bottom Temperature. The bottom temperature is often controlled on the reboiler outlet line with a control valve in the heating medium line. The control point can also be on a bottom section tray. Care must be exercised in location of the temperature control point. It is recommended, especially for large columns, that a cascade arrangement be used. The recommended scheme has a complete flow recorder/controller (FRC) in the heating medium line including orifice and control valve. The set point of this FRC is manipulated by the temperature recorder/controller (TRC). This eliminates the TRC from manipulating the control valve directly (recall that temperature is the most difficult parameter to control). This makes for smoother control for normal operations. Also, it is handy for startup to be able to uncouple the TRC and run the reboiler on FRC for a period. 

Effects. Trays can become damaged several ways. A pressure surge can cause damage. A slug of water entering a heavy hydrocarbon fractionator will produce copious amounts of vapor. The author is aware of one example where all the trays were blown out of a crude distillation column. If the bottom liquid level is allowed to reach the reboiler outlet line, the wave action can damage some bottom trays. 

Reboiler oscillations can often be dampened by increasing flow resistance in the reboiler inlet lines or reducing flow resistance in the reboiler outlet lines. This shifts flow resistance from the outlet to the inlet lines. This is analogous to dampening U-tube oscillations by increasing friction in the bottom of the tube while easing the restriction at the open end of the tube. Design practices for reboiler inlet and outlet lines were discussed in Sec. 15.2. 

The normal practice in the design of forced-convection reboilers is to calculate the heat-transfer coefficient assuming that the heat is transferred by forced convection only. This will give conservative (safe) values, as any boiling that occurs will invariably increase the rate of heat transfer. In many designs the pressure is controlled to prevent any appreciable vaporisation in the exchanger. A throttle value is installed in the exchanger outlet line, and the liquid flashes as the pressure is let down into the vapour-liquid separation vessel. 

The flow of steam to a reboiler can be controlled by using a control valve on either (1) the steam inlet line or (2) the condensate outlet line. 

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... 

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. 

Each sump must have its own drainage facilities. This is best achieved by installing an external valved dump line at a low point to interconnect the liquid outlet lines from each siunp (see Fig. 15.4a). Alternatively, a number of holes can be drilled at the bottom of the baffle, but this gives slow drainage rates and susceptibility to plugging and is therefore not recommended. The author is familiar with cases when it took over a day to drain the reboiler sump via such holes. 

Outlet line This line is usually sized so that its cross-sectional area is at least the same as the total cross-sectional area of the reboiler tubes (253, 358, 360). Excessive pressure drop in this line promotes oscillations and elongates the preheat zone. Excessive velocity in this line may also be undesirable at the column inlet (see Sec. 4.1, guideline 5). On the other hand, if the outlet line rises vertically before bending toward the column, velocities should be kept above 15 ft/s (237), or slug flow may result. 

Controlling heat input by a control valve in the reboiler condensate outlet line may adversely affect reboiler stability, especially in vacuum reboilers. This valve varies condensate level in the heating side of the reboiler. It therefore varies the point at which heating is first applied to the process side. This is equivalent to operating the process side at a fluctuating reboiler sump level. Further details of this and alternative control schemes are in Sec. 17.1.2. 

Installing a valved dump line connecting the column bottom outlet line with the reboiler inlet line (Fig. 15.4a). This technique is only needed when the column reboiler sump is separated from the column bottom sump by a baffle or when the reboiler liquid comes from a trapout pan. The valve remains shut during normal operation, but is opened during startup to lower the level and inspire thermosiphon action during startup. One case where this technique was successfully used has been described (237) the author has had several similar experiences. 

Injecting lift gas into the outlet liquid line. This reduces the density in this line and initiates flow. This technique is commonly used in pressure fractionators and when the reboiler is located well below the reboiler return nozzle (e.g., in cryogenic gas plant demethanizer side reboilers) (170, 394,421). The injection nozzle should be downstream of the reboiler outlet temperature indicator (421). 

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. 

Blown condensate seal. When this occurs, uncondensed vapor blows and channels right through the reboiler and out the condensate drain line. Heat transfer slumps and water hammer may follow. Experience shows that as much as half the reboiler duty is lost by a small amount of vapor blowing (239). Throttling the reboiler outlet reestablishes the seal. Installation of a condensate seal drum can cure this problem. Additional discussion is in Sec. 17.1.2. 

Typical examples are steam reboilers and refrigeration vapor reboilers. The control valve may be located either in the reboiler inlet line (Fig. 17.1c) or in the reboiler condensate outlet line (Fig. 17.16). 

A careful pressure drop survey indicated that there was an inexplicably high pressure drop in the liquid inlet line. When the tower was opened for inspection, the carcass of a dead rat was discovered lodged in the reboiler liquid inlet nozzle. Inadvertently, this rat caused the tower to flood as the reboiler rate was increased. If the rat had expired in the vapor outlet line, the effect would have been the same. 

If the velocity in the riser is too low, the two phases will separate. A head of liquid will build up and create back pressure against the distillation tower. Periodically, a slug of liquid will be pushed through the riser and relieve the back pressure. The riser then gradually refills with liquid. This type of slug flow causes fluctuating tower pressure. Other instances of slug flow in distillation service are thermosiphon reboiler outlets and partially vaporized feed lines. 

Fluctuating reboiler duty will have the same effect on a distillation tower as pumping the gas pedal on a car (that is, unpleasant changes in acceleration). A typical thermosiphon reboiler configuration is shown in Figure 18-12. The density difference between the liquid-filled reboiler inlet line and the vapor-liquid mixture in the outlet line drives the process-side fluid through the reboiler. 

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. 

Hot, rich glycol at reduced pressure generates flash gas that can be used as a fuel gas to the regenerator reboiler or as a stripping gas or for both. Excess flash gas is normally vented to the flare system. The flash separator pressure is maintained through a back-pressure control valve on the gas outlet line from the separator. The liquid flow is maintained through a level-control valve. 

A sufficient liquid level in a reboiler used to supply steam to the column is important from a performance and safety standpoint. The reboiler is under a slight pressure and therefore must be a pressure vessel. It usually has a gasketed and bolted lid. Welded steam outlet and stillage return lines connect the reboiler to the column. The liquid return enters the reboiler below the desired liquid level. The outflow of bottoms can be controlled by regulating the back-pressure in the bottoms line, in turn controlling the level of liquid in the pot. The pot must be designed so it can be completely drained to ease clean-out. 

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. 

Failure of steam controller. A common practice with steam reboilers (293) is to assume the steam valve is wide open, the steam pressure in the reboiler is the same as in the steam supply line, and that reboiler area remains constant (condensate is removed as soon as it is formed). The process side is usually assumed to have the same temperature rise (outlet minus inlet) as in usual operation. 

The condensate outlet scheme is often troublesome. If the condensate valve cannot handle the amount of condensate that the reboiler can generate, a m lximum vapor flow rate may be reached with condensate still covering a portion of the tubes. The remedy depends on the cause of the problem. If there is ample pressure difference between the reboiler and the condensate system downstream of the valve, it may be sufficient to resize the valve or condensate line. If the pressure difference is small, a condensate pot with a pump (Fig. 17.1c) may be needed to overcome the problem. 

The symbol used on a diagram for a plate column should indicate the type of tray used in the system bubble-cap, valve, or sieve. The first distillation column was invented in 1917. Today, a number of modifications allow modern process technicians to operate much more efficiently. The design, however, still includes the original still-on-top-of-a-still approach. The basic components of a plate distillation column are a feed line feed tray stripping section below the feed line enriching or rectifying section above the feed line overhead vapor outlet, side-stream outlet, and bottom outlet reboiler instrumentation for level, temperature, flow, pressure, and composition control outer shell and a top reflux line. 

If the tower is not equipped with a reboiler start-up line, try increasing the flow of liquid to the trapout pan. This is done by increasing the tower feed or the reflux rate. If the trapout pan is leaking, the increased liquid flow to the pan will cause the AT between the inlet and outlet to decrease. 

Forced-circulation reboilers are commonly horizontal as shown in Figure 4.5, but also occasionally are vertical. To minimize pressure drop, a restriction downstream of the reboiler is sized to prevent vaporization in the reboiler tubes. This restriction may be in the vapor line to the column or right at the vapor nozzle outlet. 

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