Reboiler overheating

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. 

Overheating of glycol in reboiler due to excessive temperature or hot spots on firetube... 

Abnormal Heat Input From Reboiler - Reboilers are designed with a specified heat input. When they are new or recently cleaned, additional heat input above the normal design can occur. In the event of temperature control failure, vapor generation may exceed the process system s ability to condense or otherwise absorb the buildup of pressure, which may include noncondensibles due to overheating. 

Figure 3.14. The lower ends of fractionators, (a) Kettle reboiler. The heat source may be on TC of either of the two locations shown or on flow control, or on difference of pressure between key locations in the tower. Because of the built-in weir, no LC is needed. Less head room is needed than with the thermosiphon reboiler, (b) Thermosiphon reboiler. Compared with the kettle, the heat transfer coefficient is greater, the shorter residence time may prevent overheating of thermally sensitive materials, surface fouling will be less, and the smaller holdup of hot liquid is a safety precaution, (c) Forced circulation reboiler. High rate of heat transfer and a short residence time which is desirable with thermally sensitive materials are achieved, (d) Rate of supply of heat transfer medium is controlled by the difference in pressure between two key locations in the tower, (e) With the control valve in the condensate line, the rate of heat transfer is controlled by the amount of unflooded heat transfer surface present at any time, (f) Withdrawal on TC ensures that the product has the correct boiling point and presumably the correct composition. The LC on the steam supply ensures that the specified heat input is being maintained, (g) Cascade control The set point of the FC on the steam supply is adjusted by the TC to ensure constant temperature in the column, (h) Steam flow rate is controlled to ensure specified composition of the PF effluent. The composition may be measured directly or indirectly by measurement of some physical property such as vapor pressure, (i) The three-way valve in the hot oil heating supply prevents buildup of excessive pressure in case the flow to the reboiier is throttled substantially, (j) The three-way valve of case (i) is replaced by a two-way valve and a differential pressure controller. This method is more expensive but avoids use of the possibly troublesome three-way valve.

Incorrect bottom (or reboiler) temperature indication This can be due to a faulty thermocouple, but a more likely cause is fouling of the thermowell. Ironically, bottom or reboiler thermocouple fouling tends to occur in services that are most vulnerable to a malfunction of this instrument, i.e., when heat-sensitive materials are distilled. A fouled thermowell will read low this in turn will enhance heat input into the column bottom, either automatically or by operator action. The greater heat input will accelerate thermal degradation in one peroxide service incident (97), it caused overheating and an explosion. 

Insufficient circulation. Insufficient circulation forms a mist flow zone in the upper portions of the reboiler tubes. This gives rise to poor heat transfer, accelerated fouling rates, and possible tube overheating as described earlier. 

To obtain good heat transfer coefficients, it has been recommended to maintain liquid velocities of 10 to 15 ft/s in the reboiler (253, 254, 358). Low velocities promote fouling and overheating, while high velocities erode tubes and add little to the heat transfer rate (253, 358). Film boiling should be avoided, in a similar manner to thermosiphon reboilers. 

If tube metal can be overheated, the tubes must always be flooded. This is usually achieved by an overflow weir (Fig. 15.1d). The overflow liquid constitutes the tower bottoms stream. Even with an overflow weir, it has been recommended (68) to monitor the liquid level in the tube chamber in order to protect the tubes in the event that boilup temporarily exceeds reboiler feed. A low level can be alarmed or used to cut back on heat input until the level is reestablished. 

If overheating is not a concern (e.g., in refrigerated services), a reboiler level controller can replace the overflow weir, and used to control the boiling rate by flooding or unflooding tubes. In some cases, bottom product is drawn from the bottom of the column, and... 

Solvent contaminated] carryover from upstream equipment example oil from compressor brines, corrosion inhibitors, sand, [corrosion products, see Section 1.3] /oxygen leaks into storage tank/inadequate corrosion control, example low pH causing corrosion/degradation via overheating, ex hot spots in reboiler tubes or fire tubes/ineffective filters/ineffective cleaning before startup/for amine absorbers corrosion products/FeS/chemicals used to treat well. 

Dehydration capacity vs. temperature (gas drying), 449-455 glycol circulation vs. tray capacity, 450-451 tray design, 451-452 reboiler temperature variable, 453 overheating glycol, 453—454... 

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