1 Install a vent above the bottom liquid level in the heating side of the reboiler. This was difficult to implement in the field and was not done.
2 Install a vent in the horizontal leg of the condensate line upstream of the vertical leg.
CASE STUDY No. 5 Reboiler Condensate Seal (Reference 6)
INSTALLATION Vertical thermosyphon reboiler on a demethanizer, using refrigerant vapor as a heating medium. Condensate flow out of the reboiler was controlled by the control tray temperature (Figure 8.1.5A).
PROBLEM At the time, the column operated at low rates. The bottom purity was critical, while the overhead purity was less important, so the column was being overreboiled. Despite this, bottom product went off-spec.
Column control tray temperature slightly dropped, and the controller called for more reboil (Figure 8.1.5B). As reboil rate was raised, the control temperature continued dropping. This continued until the operator placed the controller on manual. The temperature continued to drop, but at a slower rate. The low control temperature was accompanied by a large increase in methane in the bottom stream.
ANALYSIS The problem was caused by losing the condensate seal. When a control valve is located at the outlet of the reboiler (Figure 8.1.5A), a liquid level is held in the reboiler shell, which covers a portion of the tubes. This level also ensures the reboiler is liquid-sealed, so that no vapor escapes with the condensate. When the rate of flow through the outlet valve exceeds the rate at which vapor condenses in the reboiler, the liquid level, and therefore the liquid seal, may be lost. When this occurs, a further increase in vapor flow rate increases pressure drop in the reboiler inlet lines pressure drop, reducing the reboiler condensation pressure. This in turn reduces reboiler AT, and in low AT services (such as the demethanizer reboiler) significantly lowers heat transfer.
SOLUTION The condensate valve was heavily throttled to reestablish the condensate seal. When the seal was reestablished, column operation returned to normal.
Figure a Dtnwthanizar and FteboJter Arrangement
Refrig. Vapor Row
Figura a Dtnwthanizsr Rstooilsr Loss of SmI
CASE STUDY No. 6 Establishing Thermosyphon Action in a Demethanizer
Reboiler (Reference 6)
INSTALLATION Vertical thermosyphon reboiler on a gas plant demethanizer, using column feed as the heating medium (Figure 8.1.6). The column feed leaving the reboiler flowed on to be used as the heating medium in a side reboiler. The column was at its initial operating period following plant commissioning.
PROBLEM The start-up of the bottom reboiler tended to be erratic. At times, the reboiler would start thermosvphoning immediately without any problems. At other times, it was necessary to inject gas downstream of the reboiler, which would create a gas-lifting effect, and this would get the thermosyphon started. Yet at other times, even gas lifting would not work, and the thermosyphon could not be established. In almost all cases, once the thermosyphon was !
established, the column operation was normal, until the next time the \ column would shut down. The erratic behavior would then be repeated in the next start-up.
CAUSE In one of the subsequent shutdowns, the reboiler piping was pulled apart. A piece of masking tape, used during construction as a gasket cover, was found in the reboiler outlet flange. The gasket cover was split in the middle. The split was such that the gasket cover could either stick together or separate, causing the erratic behavior. It was believed that separation sometimes was effected by the rise in pressure following some vaporization at the reboiler, and at other times by the relative vacuum pulled by the gas injection downstream of the flange.
CURE The masking tape was removed from the flange. Further use of masking tape as flange covers was banned. Only plastic flange covers were allowed from then on - these have to be removed during construction, or no bolts can be installed at the flange.
Figure 111 Demethanizer Experiencing Start-up Problems
8.2 Condensers That Did Not Work
CASE 1 Poor Venting (Reference 2)
INSTALLATION A horizontal 16-foot long cross-flow condenser (Figure 8.2.1) .
PROBLEM Exchanger only achieved 30 percent of its design capacity.
TROUBLESHOOTING It was observed that although the vapor temperature was 220°F, the surface temperature at points from zero to six feet from the tube sheets was low - in fact, one could place his hand over the shell along all but the center part. The ends were full of non-condensable gas, and only the middle part was active.
SOLUTION Add vents at both ends of the condenser.
VATM PATHS tanftaur VI**
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