Horizontal reboilers

Yilmaz comments that several unexpected results have developed from the current horizontal reboiler research studies. 

Ketde horizontal reboilers consist of either a U-bundle or a shell and tube bundle inserted into an enlarged shell. The enlarged shell provides disengaging space for the vapor outside and above the liquid, which is usually held by level control at the top level of the tube bundle or possibly a few in. below the top of the tubes. The heating medium is inside the tubes. 

Palen recommendation corrects single tube boiling data (outside) to the bundle effect in a horizontal reboiler by ... 

The mechanical design of thermosiphon reboiler piping must be carefully examined for (a) system pressures and (b) elevadon reladonship between the liquid level in the disdl-ladon column and the verdcal or horizontal reboiler. Kem provides an excellent presentadon on this topic, including the important hydraulics. AbboT also presents a computer program for this topic. 

Thermosyphon reboilers are the most economical type for most applications, but are not suitable for high viscosity fluids or high vacuum operation. They would not normally be specified for pressures below 0.3 bar. A disadvantage of this type is that the column base must be elevated to provide the hydrostatic head required for the thermosyphon effect. This will increase the cost of the column supporting-structure. Horizontal reboilers require less headroom than vertical, but have more complex pipework. Horizontal exchangers are more easily maintained than vertical, as tube bundle can be more easily withdrawn. 

Divide the exchanger tube into sections and calculate the pressure drop section-by-section up the tube. Use suitable methods for the sections in which the flow is two-phase. Include the pressure loss due to the fluid acceleration as the vapour rate increases. For a horizontal reboiler, calculate the pressure drop in the shell, using a method suitable for two-phase flow. 

Estimate the heat transfer area, using the maximum allowable heat flux. Take as 39,700 W/m2 for vertical and 47,300 W/m2 for horizontal reboilers. 

When considering the steam side of steam heated reboilers, it is best to think about the reboiler as a steam condenser. The steam, at least for a conventional horizontal reboiler, is usually on the tube side of the exchanger, as shown in Fig. 8.1. The steam is on the tube side, because the shell side was selected for the process fluid. If the reboiler is a thermosyphon, or natural-circulation reboiler, then low-process-side pressure drop is important. For a horizontal reboiler, it is easiest to obtain a low pressure drop for the fluid being vaporized by placing it on the shell side. 

Fair, J. R. and Klip, A. (1983) Chem Eng Prog. 79 (3) 86. Thermal design of horizontal reboilers. 

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. 

Bottom Draw-off. At horizontal reboilers, whether the drawoff nozzle is elevated or located in the bottom of the tower, the hydraulic conditions are the same. For available energy, the AP equation and Figure 7-74 can be used. 

Figure 7-74. Relative elevation between the tower bottom and a horizontal reboiler.

In vertical reboiler circuits, reboiler losses are greater and pipe losses are smaller than in horizontal circuits. This often results in a difference of two pipe sizes between the downcomer and riser — also, in larger pipe sizes than in horizontal reboilers (assuming the same liquid, flow and evaporation rate). 

Piping to horizontal reboilers is designed to be as simple and direct as possible within the limitations of thermal expansion forces (see Figure 7-72). 

The heating mediums of steam or a hot process stream always connect to the tube side of horizontal reboilers. The inlet piping should have a control valve — with block valves and a bypass globe valve, if required. This should be arranged near the reboiler s tube side inlet. 

Venting inerts from the floating head end of a horizontal reboiler can be difficult. A novel method which successfully accomplished this (232) was installing a 1-in internal pipe that extended a top tube from the channelhead tubesheet to a vent in the channel head (Fig. 15.10c). The top tube was thus converted into a "vent tube. The internal pipe was coupled to the vent from inside the channel head, to permit removal. This technique cured a CO2 corrosion problem attributed to poor venting at the floating head end. 

Refinery Inability to vent accumulated CO2 from the steam (tube) side of a horizontal reboiler caused corrosion and tube leakage near the floating head. Problem was solved by extending an upper tube to make up a vent tube finm the floating head to a vent valve located at the channel head. A novel technique was developed to solve problem. 

When considering the steam side of steam-heated reboilers, it is best to think about the reboiler as a steam condenser. The steam, at least for a conventional horizontal reboiler, is usually on the tube side of the exchanger, as shown in Fig. 12.1. The steam is on the tube side. 

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