Kettle reboiler operation

When MEA is used in the presence of COS and CS2, they react to form heat-stable salts. Therefore, MEA systems usually include a reclaimer, The reclaimer is a kettle-type reboiler operating on a small side stream of lean solution. The temperature in the reclaimer is maintained such that the water and MEA boil to the overhead and are piped back to the stripper. The heat-stable salts remain in the reclaimer until the reclaimer is full. Then the reclaimer is shut-in and dumped to a waste disposal. Thus, the impurities are removed but the MEA bonded to the salts is also lost. 

Figure 10-104. Kettle reboiler—estimate of shell diameter. Example If the vapor rate is 50,000 Ib/hr and the bundle is 25 ft, then V/L is 2,000 lb/(hr)(ft). Entering the curve at this V/L ratio, with an operating pressure of 50 psia and a bundle diameter of 24 in. gives an estimate for shell l.D. of 40 in. (Used by permission Palen, J. W., and Small, W. M. Hydrocarbon Processing, V. 43, No. 11, 1964. Gulf Publishing Company, Houston, Texas. All rights reserved.)...

A kettle reboiler heat transfer area 25 m2, carbon steel shell and tubes, operating pressure 10 bar. 

Kettle reboilers have lower heat-transfer coefficients than the other types, as there is no liquid circulation. They are not suitable for fouling materials, and have a high residence time. They will generally be more expensive than an equivalent thermosyphon type as a larger shell is needed, but if the duty is such that the bundle can be installed in the column base, the cost will be competitive with the other types. They are often used as vaporisers, as a separate vapour-liquid disengagement vessel is not needed. They are suitable for vacuum operation, and for high rates of vaporisation, up to 80 per cent of the feed. 

Remember, though, that the increased tower-bottom liquid level will not be reflected on the indicated bottom level seen in the control room, which is actually the level at the end of the kettle reboiler. This is a constant source of confusion to many operators, who have towers that flood, as a result of high liquid levels, yet their indicated liquid level remains normal. 

Kettle reboilers consist of a bundle of tubes in an oversize shell. Submergence of the tubes is assured by an overflow weir, typically 5-15 cm higher than the topmost tubes. An open tube bundle is preferred, with pitch to diameter ratios in the range of 1.5-2. Temperature in the kettle is substantially uniform. Residence time is high so that kettles are not favored for thermally sensitive materials. The large shell diameters make kettles uneconomic for high pressure operation. Deentraining mesh pads often are incorporated. Tube bundles installed directly in the tower bottom are inexpensive but the amount of surface that can be installed is limited. 

Reboilers need to be located next to the tower they serve, except for the pump-through types, which can be located elsewhere. Fired heater reboilers are always located away from the associated tower and use a pump to circulate the bottoms. Kettle-type reboilers are preferred from an operational and hydraulic standpoint because they can be designed without the worry of having to ensure sufficient head for circulation required by thermosyphon reboilers. However, kettle reboilers require a larger-diameter shell that is more cosdy, and the reboiler must be supported at a sufficient elevation to get the product to the bottoms pump with adequate NPSH. 

Control of the heavies column is simpler. A fixed fraction of the feed is taken as bottom product, in a feedforward manner. The reboiler duty controls the level in the column sump. Note that this arrangement, which is required because of the small bottoms stream, cannot be implemented if a kettle reboiler is used. The column is operated at constant reflux, while the distillate rate controls the condenser level. 

Kettle Reboilers Common Operating Problems, Process Side... 

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. 

The following operating problems are commonly experienced with kettle reboilers ... 

Internal reboilers are similar in principle to kettle reboilers. The main operating difficulties experienced with internal reboilers are ... 

A kettle reboiler is used to evaporate toluene at 375°F with a heat duty of 3,000,000 Btu/hr. Steam is available at 50, 150, and 450 psig. Determine the steam pressure level to use, the steam flow rate in Ib/hr and Ib/yr, and estimate the annual steam cost if the plant-operating factor is 0.90. 

Distillation column 1 operates at 1.0 atm with constant pressure. Column 1 has 6 stages (Aspen notation) and has a kettle reboiler and a total condenser. The feed is above stage 3, and the furfural layer from the decanter is sent to above stage 2 (both Aspen notation). Use the total condenser to condense the distillate before it is sent to the decanter. Final values of the reflux ratios in both columns should be 0.01. 

Gl. [Note This problem is quite extensive.] A saturated vapor feed at 1000 kmol/h of methanol (5 mol%) and water (95 mol%) is fed to a distillation column with 18 stages plus a kettle reboiler and a total condenser (N = 20 in Aspen Plus notation). Use the NRTL VLE correlation. Operate at 80% of flooding using Fair s diameter calculation method and a tray spacing = 0.4572 m. Use an external reflux ratio of 24. Pressure is 1.0 atm... 

The inlet stream is supplied as a saturated liquid at 22.8 bar pressure the mole fraction of C3 in the top product is specified as 0.97. The column has 32 real trays with a total eondensor and a kettle reboiler. The operating specifications are reflux ratio and boilup ratio with the amount of 2.64 and 4, respectively, temperature and pressure of the main streams of the process are shown in table 1, for the case of the conventional column. Table 1. Conditions of the main streams of the conventional distillation process... 

Figure 12-4 illustrates the operation of a debutanizer equipped with a kettle reboiler. The liquid level in the bottom of the lower is determined by the pressure difference between the reboiler and the tower. (As the reboiler vapors vent back to the tower, the reboiler must be at the higher pressure.) The plant operators observed that when they increased the reboiler heat duty beyond a certain point, the lower s AP gauge reading would dramatically increase, indicating flooding. They therefore erroneously concluded that the trays were overloaded. 

A vertical steam reboiler, when operated at 20% of its design load, emitted bangs loud enough to awaken the dead. At 50% of design heat duty, it ran smoothly and quietly. A kettle reboiler produced a terrific steam hammer when it was first brought on-line. Once the contents had started boiling, it immediately quieted down. 

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