Reboiler circulation

With forced-circulation reboilers. Here the large reboiler circulation rate (compared to the bottom product rate) makes it difflcult to achieve a steady liquid overflow across the baffle. 

When reboiler circulation is much larger than bottom flow rate, baflles can cause unstable bottom flow. Small variations in the reboiler heat input or column flow rates show up as large changes in reboiler sump overflow, and therefore in bottom sump level. Bottom level fluctuations cause bottom flow fluctuations, which may be troublesome if the bottom flow is the feed to another column or is used to preheat column feed. This problem is illustrated elsewhere (258) and is most severe when the bottom sump compartment is relatively small. Providing additional residence time in the bottom sump can minimize this problem. 

If neither of the above methods helps increase reboiler circulation, the shell-side flow areas are likely restricted. This is cross-checked by slowly... 

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. 

Reactor vapor, 184, 186 Reboiler circulation, 272-273 Reboiler corrosion, 109 Reboiler leaks, 92 Reboiler problems (distillation lowers), 271-275, 280, 388 Irapoul pans, 271-272 plugged reboilers, 272-273 trapout pan repair, 273 steam-side problems, 273 blown condensate seal, 273-275 distillation, 388... 

The temperature difference obtained should be compared to the design AT (temperature difference). If the measured AT is much greater (more than 50%) than the design AT, circulation through the reboiler is insufficient. The two usual causes of decreased reboiler circulation are plugged bundles and leaking trapout pans. 

The process engineer can perform a series of experiments to determine whether the reboiler bundle needs to be cleaned to remove plugging or whether the distillation tower must be opened to repair a malfunctioning trapout pan. Following are details in diagnosing and correcting insufficient reboiler circulation. 

The purpose of the circulation system is to provide the necessary boil-up at the bottom and a portion of the reflux at the top. This system consists of a reboiler, circulation pump, heat exchanger, and an expansion valve. 

The upward flow of gas and Hquid in a pipe is subject to an interesting and potentially important instabiHty. As gas flow increases, Hquid holdup decreases and frictional losses rise. At low gas velocity the decrease in Hquid holdup and gravity head more than compensates for the increase in frictional losses. Thus an increase in gas velocity is accompanied by a decrease in pressure drop along the pipe, a potentially unstable situation if the flows of gas and Hquid are sensitive to the pressure drop in the pipe. Such a situation can arise in a thermosyphon reboiler, which depends on the difference in density between the Hquid and a Hquid—vapor mixture to produce circulation. The instabiHty is manifested as cycHc surging of the Hquid flow entering the boiler and of the vapor flow leaving it. 

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. Ketde-type reboders 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 reboders. However, ketde reboders require a larger-diameter shed that is more cosdy, and the reboder must be supported at a sufficient elevation to get the product to the bottoms pump with adequate NPSH. 

Primary alkanolamine solutions require a relatively high heat of regeneration. Also excessive temperatures or localized overheating in reboilers cause the MEA to decompose and form corrosive compounds. An inhibitor system, such as the Amine Guard system developed by Union Carbide, is an effective method of corrosion control (52). Inhibitors permit the use of higher (25—35%) concentration MEA solutions, thus allowing lower circulation rates and subsequendy lower regeneration duty. 

Heat transfer by nucleate boiling is an important mechanism in the vaporization of liqmds. It occurs in the vaporization of liquids in kettle-type and natural-circulation reboilers commonly usea in the process industries. High rates of heat transfer per unit of area (heat flux) are obtained as a result of bubble formation at the liquid-solid interface rather than from mechanical devices external to the heat exchanger. There are available several expressions from which reasonable values of the film coefficients may be obtained. 

The reboiler tube bundle should be placed on a slide about six inches above the bottom of the shell to provide good circulation. 

Inlet Line. Unstable circulation can result if the inlet line to a vertical theimosyphon reboiler is too large. The tubes of a vertical thermosyphon reboiler fire individually. The tubes can backfire excessively if the liquid inlet line is too large. They don t have to backfire all the way into the tower to cause problems, just to the inlet tubesheet. It is common to put flanges in the inlet liquid line so an orifice can be added later, if required, to provide proper dampening effect. 

If I ever have to design an amine plant I will know, for example, that the temperature of the lean amine solution entering the absorber should be about 10°F higher than the inlet gas temperature to prevent hydrocarbon condensation and subsequent foaming, that the reboiler tube bundle should be placed on a slide about six inches above the bottom of the shell to provide good circulation, that about two percent of the total circulating flow should pass through the carbon towers, and many other necessary requirements. 

Liquid circulated in reboiler 4 vapor generated in reboiler 1... 

The previous discussion focused on the use of indirect fired heaters as line heaters to provide the necessary heat to avoid hydrate formation at wellstream chokes. Indirect fired heaters have many other potential uses in production facilities. For example, indirect fired heaters can be used to provide heat to emulsions prior to treating, as reboilers on distillation towers, and to heat liquids that are circulated to several heat users. The sizing of indirect fired heaters for these uses relies on the same principles and techniques discussed for wellstream line heaters. 

As the liquid falls, it becomes leaner and leaner in light ends, and richer and richer in heavy ends. At the bottom of the tower some of the liquid is circulated through a reboiler to add heat to the tower. As the gas goes up from tray to tray, more and more of the heavy ends get stripped out of the gas at each tray and the gas becomes richer and richer in the light... 

For most field gas units it is not necessary to specify a stripper size. Vendors have standard design amine circulation packages for a given amine circulation rate, acid-gas loading, and reboiler. These concepts can be used in a preliminary check of the vendor s design. However, lor detailed design and specification of large units, a process simulation computer model should be used. 

An excessive circulation rate may overload the reboiler and prevent good glycol regeneration. The heat required by the reboiler is directly proportional to the circulation rate. Thus, an increase in circulation rate may decrease reboiler temperature, decreasing lean glycol concentration, and actually decrease the amount of water that is removed by the glycol from the gas. Only if the reboiler temperature remains constant will an inc.rea,se in circulation rate lower the dew point of the gas. 

Design Gallons of Glycol Circulated /lb Removed Reboiler Hoot Duty Btu/Gol of Glycol Circuloied... 

Determine glycol circulation rate and estimate reboiler duty. 

Determine Glycol Circulation Rate and Reboiler Duty... 

Advantages and disadvantages will in general correspond to the type of reboiler to which forced circulation is applied. The advantages and disadvantages shown are in addition. 

Figure 10-96A. Horizontal thermosiphon reboiler, a. Recirculating feed system, b. Once-through feed system. Both are natural circulation. (Used by permission Yilmaz, S. B. Chemical Engineering Progress, V. 83, No. 11, 1987. American Institute of Chemical Engineers. All rights reserved.)...

Kem deserves a lot of credit for developing design methods for many heat transfer situations and in particular the natural circulation phenomena as used for thermosiphon reboilers and shown in part in Figures 10-96A-D. 

The horizontal natural circulation systems do not use a kettle design exchanger, but rather a 1-2 (1 shell side, 2 tube-side passes) unit, with the vaporized liquid plus liquid not vaporized circulating back to a distillation column bottoms vapor space or, for example, to a separate drum where the vapor separates and flows back to the process system and where liquid recirculates back along with make-up feed to the inlet of the horizontal shell and tube reboiler. See Figures 10-96A-C. 

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. 

For once-through natural circulation reboilers, the liquid backup height is calculated from the pressure balance equation. If this height, plus an allowance for froth, reaches the bottom tray level, flooding of the tower will occur. 

The large fraction of tube length used for sensible heating in vacuum reboilers leaves little density difference for thermal circulation. This fact, plus the frequent... 

---