Reboiler inlet valve

In wide-boiling mixtures, insufficient circulation can be recognized by a high temperature difference between rehoiler outlet and reboiler inlet on the process side. Alternatively, if heat transfer coefficient improves when reboiler sump level is raised or when the reboiler inlet valve is opened, insufficient circulation is implied. Very low heat transfer coefficients, excessive fouling, and low pressure drop across some sections may also suggest insufficient circulation (360). 

Figure 15.7 Thermosiphon reboiler oscillations and their dampening by throttling reboiler inlet valve ("Experimental Study of a Vertical Thermosiphon Reboiler, K. R. Shellene, C. V. Sternling, DM. Church, and N. H. Snyder, Chemical Engineering Progress Symposium ries, vol. 64, no. 82, p. 102, 1968. Reproduced by permission of the American Institute of Chemical Engineers.)...

Used for small duties, clean process, and only infrequent cleaning required. Vaporization is usually less than 30%, but less than 15% if the fractionator pressure is below 50psig. The viscosity of the reboiler feed should be less than 0.5 cp. Put a butterfly valve in the reboiler inlet piping. This type is used in nearly 100% of chemical plant thermosyphon applications (70% of petrochemical). 

Installing a throttling valve in the reboiler inlet line (Fig. 15.4a). Throttling this valve reduces the length of the preheat zone. This technique is mainly effective in vacuum reboilers. 

Installing a valved dump line connecting the column bottom outlet line with the reboiler inlet line (Fig. 15.4a). This technique is only needed when the column reboiler sump is separated from the column bottom sump by a baffle or when the reboiler liquid comes from a trapout pan. The valve remains shut during normal operation, but is opened during startup to lower the level and inspire thermosiphon action during startup. One case where this technique was successfully used has been described (237) the author has had several similar experiences. 

Typical examples are steam reboilers and refrigeration vapor reboilers. The control valve may be located either in the reboiler inlet line (Fig. 17.1c) or in the reboiler condensate outlet line (Fig. 17.16). 

The dynamic response of the vapor inlet control scheme is far superior to that of the condensate outlet scheme. Manipulating the inlet valve immediately changes the vapor flow. Reboiler pressure and heat transfer rate only lag a few seconds behind. On the other hand, the condensate outlet valve has no direct effect on vapor flow. Condensate flow determines condensate level, and this level changes slowly. Reboiler vapor rate and heat transfer tread on the heels of the level change. Because of this slow response, manipulating vapor flow is a far better means of control than manipulating condensate flow. 

The cause of this near disaster is shown in Figure 17-2. Tower pressure was held by adjusting steam flow to the reboiler. A single pressure transmitter signaled both the high-pressure alarm and the pressure control valve (i.e., the reboiler steam inlet valve). 

Dry inlet gas that has been dehydrated by molecular sieves (qv) or alumina beds to less than 0.1 ppm water is spHt into two streams by a three-way control valve. Approximately 60% of the inlet gas is cooled by heat exchange with the low pressure residue gas from the demethanizer and by external refrigeration. The remainder of the inlet gas is cooled by heat exchange with the demethanized bottoms product, the reboiler, and the side heater. A significant amount of low level refrigera tion from the demethanizer Hquids and the cold residue gas stream is recovered in the inlet gas stream. 

The thermosiphon reboiler has inherent instabilities. A valve or other flow restriction in the inlet line helps overcome these instabilities. Adjustment possibilities of a valve also compensate for variations in reboiler duty as imposed by changes in operation of the fractionator. 

Thermosyphon reboilers can suffer from flow instabilities if too high a heat flux is used. The liquid and vapour flow in the tubes is not smooth but tends to pulsate, and at high heat fluxes the pulsations can become large enough to cause vapour locking. A good practice is to install a flow restriction in the inlet line, a valve or orifice plate, so that the flow resistance can be adjusted should vapour locking occur in operation. 

The flow of steam to a reboiler can be controlled by using a control valve on either (1) the steam inlet line or (2) the condensate outlet line. 

Figure 8.4 shows a control valve on the steam inlet line. The rate of steam flow to the reboiler is not really controlled directly, however, by this control valve. The actual rate of steam flow to the reboiler is controlled by the rate of condensation of the steam inside the tubes. The faster the steam condenses, the faster it flows into the channel head. The function of the control valve is to reduce the steam pressure in the channel head of the reboiler. For example, in case 1 ... 

It is better not to use a steam inlet control valve when using low-pressure steam. The channel head pressure will then always equal the steam header supply pressure. The flow of steam to the reboiler can then be controlled only by raising or lowering the water level in the channel head, as shown in Fig. 8.5. This sort of control scheme will work perfectly well until the water level drops to the bottom of the channel head. If the condensate drain control valve then opens further, in an attempt to increase steam flow into the reboiler, the condensate seal is blown, and the reboiler heat duty drops. 

Condensate pumps are sometimes used to overcome such backpressure problems. However, these pumps are often not kept in good repair, and condensate is still lost to the sewer. Eliminating the steam inlet control valve of the type shown in Fig. 8.4 has helped recover condensate from many reboilers, supplied with low-pressure steam. 

For control valves and other areas of high-turbulence (velocity 8 ft/s [2.5 m/s]) (I e, downstream of control valves, rich carbonate inlet of carbonate regenerator, reboiler tube sheet, and baffles), use type 304 SS plus i/32 in, (1 mm) CA Do not use miters long radius elbows are preferred. See piping specifications for other limitations on miters. 

The heating mediums of steam or a hot process stream always connect to the tube side of horizontal reboilers. The inlet piping should have a control valve — with block valves and a bypass globe valve, if required. This should be arranged near the reboiler s tube side inlet. 

I cut the debutanizer pressure back to 100 psig and immediately the flooding in the main fractionator subsided. The operators then closed the inlet block valve to the hot-oil side of the reboiler and opened up a drain. Naphtha poured out instead of gas oil. This showed that the debutanizer reboiler had a tube leak. 

Failure of steam controller. A common practice with steam reboilers (293) is to assume the steam valve is wide open, the steam pressure in the reboiler is the same as in the steam supply line, and that reboiler area remains constant (condensate is removed as soon as it is formed). The process side is usually assumed to have the same temperature rise (outlet minus inlet) as in usual operation. 

In an existing installation, or in a service where oscillations are anticipated, an oscillation problem can be overcome by installing a restriction orifice or a throttling valve (see earlier in this section Fig. 15.4a) in the inlet line to the reboiler (150c, 313,358,360). Figure 15.7 (360) demonstrates the effectiveness of a throttling valve in dampening reboiler oscillations. 

It has been recommended to design a forced-circulation reboiler for a high pressure drop (134). In some cases (68), a restriction is placed in the vapor line downstream of the reboiler and sized to prevent vaporization in the reboiler. This restriction is often placed at the column inlet nozzle (68), but this may generate an undesirable high-velocity jet at the column inlet (see Sec. 4.1, guideline 5). Further, a restriction downstream of the reboiler may interfere with the action of a control valve located in the liquid line to the reboiler. The author is also familiar with a case where such a restriction experienced erosion at an intolerable rate. 

The condensate outlet scheme permits reboiler (condensate-side) operation at a higher pressure because it eliminates the pressure drop in the inlet control valve. 

Tuning can be troublesome with the vapor inlet scheme if flow across the valve changes from noncritical to critical upon reboiler turndown (67, 68, 362). As boilup falls, so does the absolute pressure downstream of the valve. When the ratio of upstream to downstream pressure exceeds a critical value, critical flow is established through the valve, and the downstream pressure ceases to affect the vapor flow rate. The controller dynamics differ under critical and noncritical flow. A loop tuned for noncritical flow tends to be unstable when flow becomes critical, while a loop tuned for critical flow tends to be sluggish when flow becomes noncritical (67, 68). 

Steam reboilers are subject to a wide variety of serious capacity problems. The most common is steam condensate backup into the channel head. This is checked by manually draining the steam condensate to a nearby sewer and observing if the reboiler duty increases or the steam inlet control valve closes. 

The temperature recorder controller (TRC) holding the reboiler outlet temperature signaled the steam-inlet control valve to pinch back. 

It is better not to use a steam inlet control valve when using low-pressure steam. The channel head pressure will then always equal the steam header supply pressure. The flow of steam to the reboiler can... 

If a column is shut down by closing foe steam and drawoff valves, foe entire column contents accumulate in foe lower section of foe column. Typically foe liquid level rises well above foe vapor inlet from foe reboiler and up over some of foe lower trays. (Some column designers prefer to provide enough space below foe first tray that it can never be flooded.) Such a shutdown may be made to accommodate production scheduling or may have been forced by process interruptions elsewhere. The column may be started up again simply by turning on foe steam. 

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