Reboiler surging

With kettle reboilers, because bottom product is withdrawn from the kettle reboiler surge compartment and not from the column. 

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. 

To avoid reboiler surging, the following have been recommended (358) ... 

Surging may also occur when the column bottom contains water-insoluble components along with a small quantity of water. Because of steam distillation, the water acts as a light component. A case where severe tray damage was caused by this mechanism has been reported (358). The author is familiar with another experience where smdl quantities of water caused reboiler surging. The problem was eliminated by elevating the reboiler liquid offtake about a foot (Fig. 15.4c), and converting the section below the offtake into a reservoir which constantly supplied a small amount of water to the reboiler. 

The reboiler may also experience inverse response, often referred to as reboiler surge or reboiler swell. An increase in heat input may increase the volume of vapor in the reboiler or the pressure drop in the reboiler and its outlet piping. This will temporarily back up liquid into the column bottom, causing liquid level to rise. 

The upward flow of gas and Hquid in a pipe is subject to an interesting and potentially important instabiHty. As gas flow increases, Hquid holdup decreases and frictional losses rise. At low gas velocity the decrease in Hquid holdup and gravity head more than compensates for the increase in frictional losses. Thus an increase in gas velocity is accompanied by a decrease in pressure drop along the pipe, a potentially unstable situation if the flows of gas and Hquid are sensitive to the pressure drop in the pipe. Such a situation can arise in a thermosyphon reboiler, which depends on the difference in density between the Hquid and a Hquid—vapor mixture to produce circulation. The instabiHty is manifested as cycHc surging of the Hquid flow entering the boiler and of the vapor flow leaving it. 

The kettle reboiler is shown in Fig. ll-3.5ishell-side, this common design provides adequate dome space for separation of vapor and hquid above the tube bundle and surge capacity beyond the weir near the shell cover. 

Scale blocking pinched downcomer. Very poor separation with limited ability to change reflux. At low reboiler duty, feed rates greater than design could be sent through the column. Any increase in reboiler duty resulted in surges of liquid overhead. This was an installation error. 

Effects. Trays can become damaged several ways. A pressure surge can cause damage. A slug of water entering a heavy hydrocarbon fractionator will produce copious amounts of vapor. The author is aware of one example where all the trays were blown out of a crude distillation column. If the bottom liquid level is allowed to reach the reboiler outlet line, the wave action can damage some bottom trays. 

When the reboiler was brought back on line, the water was swept into the heat transfer oil lines and immediately vaporized. This set up a liquid hammer, which burst the surge tank. It was estimated that this required a gauge pressure of 450 psi (30 bar). The top of the vessel was blown off in one piece, and the rest of the vessel was split into 20 pieces. The hot oil formed a cloud of fine mist, which ignited immediately, forming a fireball 35 m in diameter. (Mists can explode at temperatures below the flash point of the bulk liquid see Section 19.5.)... 

The bottom tray of a tower must have its downcomer sealed to prevent upflow of reboiled vapors. The downcomer of this tray is usually equal to or 6 in. longer than the other downcomers to ensure against bottom vapor surges or pulses in pressure breaking the seal. The seal pan is designed to avoid liquid back pressure and minimum restrictions to liquid flow. 

CONSTANT MOLAR OVERFLOW AND CONSTANT PLATE HOLDUP CONSTANT HOLDUP IN REBOILER AND SURGE DRUM... 

For example, it is important to have large enough holdups in surge vessels, reflux drums, column bases, etc., to provide effective damping of disturbances (a much-used rule of thumb is 5 to 10 minutes). A sufficient excess of heat transfer area must be available in reboilers, condensers, cooling Jackets, etc., to be able to handle the dynamic changes and upsets during operation. The same is true of flow rates of manipulated variables. Measurements and sensors should be located so that they can be used for eflcctive control. 

The selected system comprises three direct-fired hot oil heaters, surge vessels ond circulation pumps. The hot oil is circulated in a closed loop through the tube bundles in the glycol reboilers. The system allows accurate control of the glycol tempercture ond by designing the fired heaters os a redundant utility system cvailoble to the three process troins the reliability of the system is thereby improved. 

It is rare to encounter a satisfactory long-term operation at more than about 92% of vapor capacity. Above about 95%o of maximum load, small surges in feed rate or reboil rate may cause temporary local flooding, which interferes with good separation. 

Over the years some heuristics have been developed for estimating liquid holdups in most systems. Holdup times (based on total flow in and out of the surge volume) of about 5 to 10 minutes work well. If a distillation column has a fired reboiler, the base holdup should be made larger. If a downstream unit is particularly sensitive to rate or composition changes, then holdup volumes in upstream equipment should be increased. Such matters should be considered when the vessels are designed. 

When the bottom sump does not supply liquid to a reboiler (e.g., when a reboiler trapout pan is used, or when the column has a bottom feed and no reboiler), the design in Fig. 4.86 can be used with thermally unstable materials. This arrangement eliminates the need for a surge drum and self-venting lines, and immediately quenches liquid reaching the column base. 

Figure 9.5a shows another control system that can increase heat input to the reboiler if the bottom pump fails. Pump failure will interrupt column vapor flow, the column will dump, and the temperature controller will increase the furnace fuel. Unless a reliable trip system (discussed below) is installed, the furnace will overheat. In one incident (239), resumption of circulation caused rapid vaporization, which resulted in a pressure surge that dislodged... 

Effects of heat exchanger tube leaks into the column include off-spec products and/or undesirable chemical reactions. In some cases, this reaction may lead to rapid corrosion or plugging. It is important to realize that material leaking at the condenser or at an intermediate exchanger may travel to the column base and decompose there. In one vacuum column, water leaking at the condenser reached the reboiler and caused a pressure surge and tray damage (358). 

Controller cycling This is usually caused by inadequate tuning. Switching the controller to manual should stop the cycling if the controller is the culprit. Special caution is required with cycling of reboiler controllers these can cause surging (98). 

Surging may also occur when reboiler AT is small and either col-... 

Bottom product surge. The liquid draw compartment of kettle reboilers is much smaller than most column bottom sumps, and usually provides less liquid residence time and product surge. It is often impractical to incorporate the desired residence time (Sec. 4.4) in this draw compartment, and one needs to either live with the lower residence time or add a surge drum downstream of the reboiler. 

Arrangements 17.1c, d, and f require a large surge volume in the condensate pot in order to prevent reboiler level variations from flooding or draining the pot. In arrangement 17.le (with or with-... 

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