Reboiler liquid head

They supply a constant liquid head to the reboiler. This is most important in vacuum systems, where a steady liquid driving head is essential for stable thermosiphon action (see Sec. 15.3). 

Where sufficient liquid head is unavailable, or additional head is expensive to provide. Alternative reboiler types generally have a lower liquid head requirement. 

Where the liquid head required either for a vertical thermosiphon reboiler or for the pump in a forced circulation reboiler is not available or is expensive to provide. 

When the reboiler sump level is very low, there is little driving head to force liquid into the reboiler, and circulation rate is low. The little liquid entering the reboiler is essentially totally vaporized. Since there is little liquid head to suppress the boiling point, the liquid preheat zone is small and nucleate boiling begins almost immediately. A mist flow zone is formed above the nucleate boiling zone, where the... 

Insufficient circulation is usually caused by plugging, a leaking reboiler preferential baffle or draw pan, or by insufficient liquid head (alternatively, excessive pressure drop in the reboiler loop). Leakage across the preferential baffle is implied when the bottom sump level influences reboiler heat transfer rate despite the presence of a baffle. 

Remedies for a leakage problem are discussed in Secs. 4.7 and 4.8. Remedies for a deficient liquid head are raising liquid level or cutting down pressure drop in the reboiler loop. If impractical, a forced-circulation reboiler should be considered. 

A major consideration with these reboilers is pump-system compatibility. Since the liquid is near its boiling point, and liquid head is costly, NPSH (net positive suction head) is critical. Oversized pumps can be detrimental to NPSH and should be avoided (134, 253, 358). One case has been reported (134) of poor reboiler performance due to pump oversizing reboiler performance was the same whether power to the pump was on or off. 

With kettle reboilers, the liquid level at the coltunn base is set by the reboiler liquid level plus the head for overcoming reboiler circuit friction. The boiling mechanism is pool boiling with some convective effects. Kettle reboilers normally operate with high ( s 80 percent) fractional vaporization and are therefore prone to fouling. 

Due to high liquid head, this vent is ineffective for venting reboiler... 

The following are several examples TEMA K-type shells, whieh allow for proper liquid disengagement for reboilers TEMA J-type shells, whieh aecommodate high vapor flows by allowing for divided flow in the shellside Two-pass TEMA F-type shells, whieh ean be used for applieations when a temperature eross exists (below) TEMA D-type front head designs, whieh are often the answer for high-pressure tubeside applieations. 

Natural circulation reboilers are effective and convenient units for process systems operating under pressure. They are usable in vacuum applications but must be applied with care, because the effect of pressure head (liquid leg) on the boiling point of the fluid must be considered. The temperature difference between the heating medium and boiling point of the fluid may be so small as to be impractical, regardless of the tube length in a vertical unit. 

This unit is usually used as the reboiler for the distillation column and, in this service, operates by the thermosiphon action of the difference in static head in the column and in the vapor-liquid phase leaving the reboiler. When tied into the bottom chamber, the liquid is usually recirculated many times, vaporizing only 10-25% of the reboiler feed per pass however, when used as a draw-off from the bottom tray seal pan, the feed to the reboiler is not recirculated flow. The basic operation is the same, however. 

A centrifugal pump is required to circulate a liquid of density 800 kg/m2 and viscosity 0.5 x 10 3 Ns/m" from the reboiler of a distillation column through a vaporisor at the rate of 0.004 m3/s, and to introduce the superheated vapour above the vapour space in the reboiler which contains a 0.07 m depth of liquid. If smooth-bore 25 mm diameter pipe is to be used, the pressure of vapour in the reboiler is 1 kN/m2 and the Net Positive Suction Head required by the pump is 2 m of liquid, what is the minimum height required between the liquid level in the reboiler and the pump ... 

The three reboilers in Figure 15.13 are shown under natural circulation. The flow of liquid from the column to the reboiler is created by the difference in hydrostatic head between the column of liquid feeding the reboiler and the vapor-liquid mixture created by the reboiler. 

Note that it is the elevation, or the static head pressure, in the tower that drives the kettle reboiler. That is why we call it a gravity-fed reboiler. Also, the pressure in the kettle will always be higher than the pressure in the tower. This means that an increase in the reboiler heat duty results in an increase of liquid level in the bottom of the tower. 

Figure 3.14. The lower ends of fractionators, (a) Kettle reboiler. The heat source may be on TC of either of the two locations shown or on flow control, or on difference of pressure between key locations in the tower. Because of the built-in weir, no LC is needed. Less head room is needed than with the thermosiphon reboiler, (b) Thermosiphon reboiler. Compared with the kettle, the heat transfer coefficient is greater, the shorter residence time may prevent overheating of thermally sensitive materials, surface fouling will be less, and the smaller holdup of hot liquid is a safety precaution, (c) Forced circulation reboiler. High rate of heat transfer and a short residence time which is desirable with thermally sensitive materials are achieved, (d) Rate of supply of heat transfer medium is controlled by the difference in pressure between two key locations in the tower, (e) With the control valve in the condensate line, the rate of heat transfer is controlled by the amount of unflooded heat transfer surface present at any time, (f) Withdrawal on TC ensures that the product has the correct boiling point and presumably the correct composition. The LC on the steam supply ensures that the specified heat input is being maintained, (g) Cascade control The set point of the FC on the steam supply is adjusted by the TC to ensure constant temperature in the column, (h) Steam flow rate is controlled to ensure specified composition of the PF effluent. The composition may be measured directly or indirectly by measurement of some physical property such as vapor pressure, (i) The three-way valve in the hot oil heating supply prevents buildup of excessive pressure in case the flow to the reboiier is throttled substantially, (j) The three-way valve of case (i) is replaced by a two-way valve and a differential pressure controller. This method is more expensive but avoids use of the possibly troublesome three-way valve.

This process differs from others in several engineering details. The severe duty placed on the reboiler of the aluminum chloride recovery tower due to fouling was a problem which was solved by a vertical thermosyphon reboiler operating ivith sufficient head to provide a high rate of liquid circulation. Another problem was suitable pumps to recycle aluminum chloride slurry to the top of the reactor. Centrifugal pumps with mechanical seals using butane flush were most satisfactory. 

Condensation of the head vapors leads to two liquid phases An organic phase consisting essentially of butyl acetate with very little water, and an aqueous phase consisting essentially of water with very little acetate. The organic phases are used as refluxes, while the combined aqueous phases are fed through a cooler into the head of a small fourth column, in which the water is stripped of its acetate by means of a reboiler energized with low-pressure steam. The fourth column is operated at the same pressure as the third column, the head vapors of the third and fourth columns being liquefied in a condenser maintained at 0.5 ATM by means of a vacuum pump. 

It is proposed to pump 10,000 kg/h of toluene at 114°C and 1.1 atm abs pressure from the reboiler of a distillation tower to a second distillation unit without cooling the toluene before it enters the pump. If the fiiction loss in the line between the reboiler and pump is 7 kN/m and the density of toluene is 866 kg/m, how far above the pump must the liquid level in the reboiler be maintained to give a net positive suction head of 2.5 m ... 

KETTLE-TYPE BOILERS. A kettle-type boiler, or reboiler as it is called when connected to a distillation column, is shown in Fig. 15.11. A horizontal shell contains a relatively small tube bundle, two-pass on the tube side, with a floating head and tube sheet. The tube bundle is submerged in a pool of boiling liquid. 

When operation is under vacuum, however, the performance of the reboiler is sensitive to changes in the liquid driving head, especially in the distillation of multicomponent mixtures. The optimum liquid level for vacuum service is midway between the tube sheets, with about 50 percent of the liquid vaporized per pass. For usual applications with saturated steam on the shell side the heat flux can be estimated from Figure 15.13, which is based on BWG 14 stainless-steel... 

Extractive Distillation of Liquid-Liquid Extract. The extract portion was fed into a continuous still, 2 inches in diameter, made up of the following sections in ascending order a 100-ml capacity reboiler, a vacuum-jacketed, silvered column 3 feet long containing helipak, an intermediate feed section, and a vacuum-jacketed silvered 15-plate Older-shaw column, another feed section and a 10-plate Oldershaw column, topped off with a solenoid operated liquid dividing reflux head. The extract entered the still through a preheater at the lower feed section. The reflux ratio and boil-up rate were adjusted to fix the bottoms compo-... 

Reboiler circuits may set tower elevations. A thermo-syphon reboiler requires enough liquid static head to provide a driving force so that the reboiler will work properly. This head determines the circulation ratio and the amount of vapor returned to the tower, thereby setting the entire tower gradient. Reboiler circuits must be considered with pump NPSH considerations as they set the tower elevation. 

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