Reboiler problems

There are dozens of other types of reboilers, but these four represent the majority of applications. Regardless of the type of reboiler used, the following statement is correct—almost as many towers flood because of reboiler problems as because of tray problems. 

Rgura 15.4 Techniques for overcoming thermosiphon reboiler problems. (a) The dump line technique and the throttling valve technique (6) residue removal from reboiler base (c) water accumulation to solve a water-induced reboiler surging problem. 

Blown condensate seal (reboiler problems), 273-275 Blue fire, 133 Boiler leaks, 112, 120 Bottlenecks (process), 482 Bottom txrol tem 5eralure, 283-284 Bottom-head removal (coking cycle), 59-60... 

Plugged blowdown lines, 42 Plugged reboilers (reboiler problems), 272-273... 

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... 

For most distillation columns, the bottom level is controlled by the bottom draw-off. Sometimes, the bottom draw-off is used for quality control. In that case, the bottom level can be controlled, by adjusting the load of the reboiler. Problems can originate when there is an inverse response. This happens when X > 0.5. 

Up until 1986 the major use for 2-j -butylphenol was in the production of the herbicide, 2-j -butyl-4,6-dinitrophenol [88-85-7] which was used as a pre- and postemergent herbicide and as a defoHant for potatoes (30). The EPA banned its use in October 1986 based on a European study which showed that workers who came in contact with 2-j -butyl-4,6-dinitrophenol experienced an abnormally high rate of reproduction problems. Erance and the Netherlands followed with a ban in 1991. A significant volume of 2-j -butyl-4,6-dinitrophenol is used worldwide as a polymerization inhibitor in the production of styrene where it is added to the reboiler of the styrene distillation tower to prevent the formation of polystyrene (31). OSBP is used in the Par East as the carbamate derivative, 2-j -butylphenyl-Ai-methylcarbamate [3766-81-2] (BPMC) (32). BPMC is an insecticide used against leaf hoppers which affect the rice fields. 

Since the changes usually occur close to the source of the problem, the source can usually be quickly found with proper instrumentation. Too little reflux or too much side-stream withdrawal are two examples of insufficient liquid entering a section. Too hot a feed or too much reboiling are examples of excessive liquid boiloff. 

High Pressures. Thermosyphon reboilers present design problems at the two extremes of the pressure scale. Near the critical pressure, the maximum allowable flux drops. 

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. 

Pattinson. Scott, Changes in Demethanizer Reboiler Solve Efficiency Problems, Oil and Gas Journal, May I, 1989, p. 102. 

Problem A fired reboiler outlet temperature would not rise above 350°F no matter how hard the heater w as fired. 

The vertical thermosiphon reboiler is a popular unit for heating distillation column bottoms. However, it is indeed surprising how so many units have been installed with so little data available. This indicates that a lot of guessing, usually on the very conservative side, has created many uneconomical units. No well-defined understanding of the performance of these units exists. Kern s recommended procedure has been found to be quite conservative on plant scale units yet it has undoubtedly been the basis for more designs than any other single approach. For some systems at and below atmospheric pressure operation, Kern s procedure gives inconsistent results. The problem is in the evaluation of the two-phase gas-liquid pressure drop under these conditions. 

A common case where intense general corrosion is experienced in a very restricted section of plant is where an acidic vapour is condensing. As a vapour the acid is usually non-corrosive, but when condensed it can only be handled in expensive materials. Another variation on this theme is that only at the region of initial condensation is there a corrosion problem, either the condense/reboil condition being particularly corrosive or else corrosion only takes place at or near the boiling point. Several variations in design are possible to cope with these situations ... 

Where the corrosion problem is limited to the condense/reboil situation, i.e. where, due to variations in vapour temperature (or temperatures of the surfaces with which the condensate can come into contact), the condensate reboils, the answer may be to use resistant material at... 

To produce a high purity product two distillation columns are operated in series. The overhead stream from the first column is the feed to the second column. The overhead from the second column is the purified product. Both columns are conventional distillation columns fitted with reboilers and total condensers. The bottom products are passed to other processing units, which do not form part of this problem. The feed to the first column passes through a preheater. The condensate from the second column is passed through a product cooler. The duty for each stream is summarised below ... 

The problem of distillation sequencing was discussed in Chapter 11, where the distillation columns in the sequence were operated on a stand-alone basis using utilities for the reboilers and condensers. Following the approach in Chapter 11, the best few nonintegrated distillation sequences would be found. These sequences would then be heat integrated as discussed above. Figure 21.8 shows how heat integration can be applied within a two-column direct... 

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