Troubleshooting reboilers

Shah, G. C., Troubleshooting Reboiler Systems, Chemical Engineering Progress, July 1979, p. 53. 

For column analysis and troubleshooting it is important to have pressure drop measured with a DP cell. The differential pressure can also be used to control column traffic. A good way to do this would be to let the differential pressure control the heating medium to the reboiler. The largest application for differential pressure control is with packed columns where it is desirable to run at 80 to 100% of flood for best efficiency. 

Safety, environmental protection, and equipment protection (see Section 16.7 this could include redundant temperature sensing and alarms on reactors and reboilers handling corrosive chemicals such as HF), smooth operation, product quality, profit, monitoring, diagnosis, and troubleshooting. Identify the objective and select a pertinent variable. Direct measurement of the variable is preferred. If direct measurement is impractical, select an inferential or calculated variable. For example, temperature can infer conversion and composition. 

Reboilers, condensers, and operation difficulties amount to about half of the remaining problems. Thus three out of four incidents are caused by either these or the factors previously mentioned. Familiarity with these problems, therefore, is of great importance to persons involved in distillation and absorption operation and troubleshooting. For this reason, these topics are also highlighted in this book. 

Figure 1.1 Column troubleshooting case history, (a) Hot oil from the fractionator supplies heat to gas-plant reboilers.

Figure 1.1 (Continued) (b) Leaking debutanizer reboiler upsets gas plant. Parts a and b from Norman P. Lieberman Troubleshooting Process Operations 2nd ed., PennWell Books, Tulsa, 1985. Reprinted courtesy of PennWell Publishing Co.)...

Obtain a clear, factual definition of the symptoms. A poor definition of symptoms is one of the most common troubleshooting pit-falls. In the debutanizer case history above, the following definitions were used by different people to describe the symptoms of a reboiler tube leak problem ... 

Nevertheless, reboilers and condensers are an integral part of a distillation system. As shown in Chap. 1, problems with reboilers and condensers account for a sizable fraction of distillation malfunctions. Distillation supervisors or troubleshooters cannot perform their duties effectively unless acquainted with the pitfalls in the operation of the column heat exchangers. There have been many instances in which the cause of an apparent poor column performance problem was traced back to the reboiler or condenser. 

Fired heaters. When the process temperature and the heat duty are high, a fired heater is occasionally used as a forced-circulation reboiler. Fired heater technology is outside the scope of this distillation book, and the reader is referred to review articles (30, 319) as well as some troubleshooting guidelines (237, 239) on this subject. A few of the process-side-related considerations pertaining to fired reboilers are... 

Trim gas oil, 284, 293, 299 Trim steam reboiler, 88 Trimming burner operation (combustion air supply), 318 Trip elimination, 359—361 Troubleshooting problems. See specific process. 

Troubleshooting malfunctioning reboilers can be the toughest part of determining capacity limitations on a distillation tower. 

By supplying heat to reboilers from a hot pumparound stream, waste heat from the main fractionator is gainfully utilized. On the other hand, heat-integrating two process units creates the opportunity for some interesting troubleshooting assignments. The following story is not a myth it really happened. 

Oantrol of the condensate valve is sometimes achieved by a steam-flow measurement, sometimes by a condensate-flow measurement (more accurate and cheaper), and sometimes by another process variable. Response is more sluggish than when steam flow is throttled. No trap is required. The theory and design equations are presented in Chapter 15. For troubleshooting, performance tests, startup, and, in some cases, overrides, a liquid-level transmitter should be installed on the reboiler shell side. 

---