Venting reflux drum

Another pressure control method is a hot gas bypass. The author has seen a retrofitted hot gas bypass system solve an unsatisfactory pressure control scheme. In the faulty system, a pressure control valve was placed between the overhead condenser and the lower reflux drum. This produced en atic reflux drum pressures. A pressure safety valve (psv) vented the reflux drum to the flare to protect against over-pressure. A psv saver control valve, set slightly below the psv relieving... 

Problem A knockback condenser mounted on a C3 splitter reflux drum exhibited liquid carryover (as evidenced by the vent line icing-up). This indicated product loss from liquid caixying over rather than dripping back into the reflux drum. Also the vent line metallurgy would not withstand the cold temperatures produced. 

Increasing pumparound heat duty will unload the overhead condenser. This will cool off the reflux drum. A colder reflux drum will absorb more gas into the distillate product. Less gas will be vented from the reflux drum, and this is often desirable. 

There is no vapor vented in the reflux drum, but there is a vapor-liquid interface in the drum. 

We can be sure that the butane liquid did not flash after the introduction of the rat because no vapor was vented from the reflux drum. 

Hot-vapor bypass pressure control. A more modern way of controlling a tower s pressure is shown in Fig. 13.6. This is the hot-vapor bypass method. When the control valve on the vapor bypass line opens, hot vapors flow directly into the reflux drum. These vapors are now bypassing the condenser. The hot vapors must condense in the reflux drum. This is because there are no vapors vented from the reflux drum. So, at equilibrium, the hot vapors must condense to a liquid on entering the reflux drum. They have no other place to go. 

Normal vertical knockout drums are designed for a K value of about 0.20 to 0.25. If we are installing a KO drum ahead of a reciprocating compressor—and they really hate liquids in their feed—a K value of 0.14 might be selected. If we really do not care very much about entrainment, a K value of 0.4 might be selected. An example of this would be venting waste gas to the flare from a sour-water stripper reflux drum. 

Accumulators are not separators. In one application, an acciunulator placed after a total condenser provides reflux to a fractionator and prevents column fluctuations in flow rate from affecting downstream equipment. In this application the accumulator is called a reflux drum. A reflux drum is shown in Figure 6.3. Liquid from a condenser accumulates in the drum before being split into reflux and product streams. At the top of the drum is a vent to exhaust noncondensable gases that may enter the distillation column. The liquid flows out of the drum into a pump. To prevent gases from entering the pump, the drum is designed with a vortex breaker at the exit line. 

Column top pressure is controlled by a vent/bleed system i.e., inert gas is added if pressure is low, and gas from the reflux drum is vented if pressure is high. 

In the column, reflux is flow controlled, reflux drum level is controlled by distillate, base level by bottoms, pressure by vent vapor, and temperature by steam to the auxiliary reboilcr or vapor from the evaporator. 

Flooding initiated by a deep liquid layer can be particularly severe. In one incident (237), a 150-ft absorber filled with liquid. The liquid overflowed into the reflux drum, and then via a vapor vent into the fuel system, spilling out of burners and initiating several fires. 

Figure 15.14 Common types of condensers, (a) Horizontal in-shell condenser, TEMA E-type shell (6) horizontal in-shell condenser, TEMA J-type shell (c) a vent condenser mounted on top of a reflux drum (d) an internal downflow in-tube condenser (e) an internal horizontal condenser if) a direct contact condenser.

Some troublesome experiences with liquid carryover fi om these condensers have been reported (381). The author had an experience with liquid carryover from a vent condenser mounted on top of a Cg splitter reflux drum (similar to Fig. 15.14c). Carryover occurred whenever the vent control valve opened excessively, and was recognized by "watering or icing up of the line downstream of the valve (due to liquid flashing). The author is familiar with other similar experiences. Installing a valve limiter was sufficient to prevent carryover in the above case. 

Figure 17.5 Pressure control by condenser flooding, (a) Control valve in condenser outlet ib) flooded reflux drum (c) flooded reflux drum with automatic noncondensables venting [d) hot vapor bypass (c) a poorly pip hot vapor bypass if) control valve in condenser inlet. (Part c from "Unusual Operating Histories of Gas Processing and Olefins Plant Columns, H. Z. Kister and T. C. Hower, Jr., Plant/Operations Progi a, vol. 6. no. 3, p. 153 (July 1987). Reproduced by permis-...

Correct piping is mandatory for the success of the hot vapor bypass control method. Bypass vapor must enter the vapor space of the reflux drum (Fig. 17.5d). The bypass should be free of pockets where liquid can accumulate any horizontal runs should drain into the reflux drum. If noncondensables are likely, vents are required on the condenser and drum. The condenser vent can be directed to the vapor space of the drum. Most important, liquid from the condenser must enter the reflux drum well below the liquid surface. The bottom of the drum (Fig. 17.5d) is the most suitable location. In one case (164) subcooled liquid entered the drum vapor space (presumably due to unflooding of the liquid inlet). The vapor space was 100°F hotter rapid condensation sucked the liquid leg between the drum and condenser into the drum in seconds. 

Figure 17.7c shows pressure control using inerts. As column pressure falls, and when the quantity of lights in the reflux drum is insufficient to permit continuous venting, an inert gas is admitted, usually on pressure control, to raise column pressure. Sometimes the inerts are admitted on flow control, and the drum is continuously vented on pressure control. 

The quantity of inerts is small losses of vaporized liquid in the gas are of little economic consequence This case coincides with venting small quantities of inerts from the condenser and the reflux drum. In this case, the control methods shown in Figs. 17.5a-d and f 17.6 or l.la, c are suitable. The inerts as vented either manually or on flow control (except 17.7c). 

Liquid line from the condenser to the accumulator. This location is only feasible when the line is continually filled with condensate. This location gives a good dynamic response (unobstructed by the accumulator lag), and a representative sample (some vapor bubbles may be present, but because vapor density is much smaller than liquid, this has little effect on the analysis). Drawbacks of this location include an inaccurate correlation with product composition when the reflux drum is vented fairly long sample lines additional dead time in the sample line (because liquid and not vapor is sampled). This method evades the major drawbacks to the other two, and is frequently recommended (258, 301, 309, 332) whenever feasible. The author shares this view. 

Inerts accumulation in flooded reflux drum caused unflooding of the drum and poor control Manual venting could not solve problem because plant was not continuously attended. 

Sour hydrocarbons from column condense in the shell of water-cooled submerged condenser. Tube bundle severely corroded due to concentration of acidic components near outlet, and needed frequent replacement. Adding a vent line from condenser to reflux drum and modifying controls to supply a small purge stream to condenser feed doubled tube life. 

An Aspen Flash model is used for the reflux drum with pressme set at 1 bar and design specification of a vapor fraction of 10 , which makes the drum essentially adiabatic. A small vapor flow rate is necessary so that the control valve in this vent line can be sized. In the Aspen Dynamics simulation, the valve is completely closed. The liquid holdup in the drum is set to give 5 min at 50% full (diameter 3 m and length 6 m). 

Small vent stream of inert or light vapors leaving the reflux drum... 

One common strategy for controlling column pressure is to manipulate a valve in a vent line from the reflux drum. If necessary, another line and automatic valve can be added to inject inerts, for example, nitrogen, before the vent valve. If the distillate is taken as a vapor stream, then the condenser may need to be run as a partial condenser with temperature-controlled cooling liquid on the condenser. 

Vapors from the coke drum must pressure their way through to the combination tower reflux drum. Any restriction to their flow will increase the operating pressure of the coke drum. To avoid exceeding the coke drum relief valve pressure, some operators vent the reflux drum to a flare. This makes it appear as if the wet gas compressor is limiting the resid feed rate. Most often, though, the problem lies with upstream pressure drop. 

First, the reflux drum liquid level dropped as the uncondensed vapors accumulated in the drum. Next, the operators reduced the reflux rate to prevent the reflux pump from running dry. Then, the splitter pressure rose rapidly, and the relief valve popped. This effectively vented the lighter hydrocarbons from the tower and allowed the operators to regain control. 

Consider the benzene purification column, T-101, in Figure 1.5 for the toluene hydrodeallgdation process. The column feed contains noncondensables, mainly methane and a small amount of hydrogen, that must be vented from the system The vent is taken off the top of the reflux drum, V-104. Sketch a control system in which the valve on this vent line is used to control the pressure of the column. 

I have another way to explain how the condensed liquid can flow uphill from the condenser into the elevated reflux drum. That is, the liquid leaving the condenser is pushed into the reflux drum by the higher pressure in the tower. This will work okay, as long as the liquid entering the reflux drum is at its saturated bubble-point temperature and pressure. If this is not the case, then the vapors flashing out in the reflux drum will have to be vented. If you refuse to open this vent, then condensate backup must increase in the condenser, with a consequent reduction in the condenser s capacity. 

One of my friends, Steve, made such an error in designing a debutanizer. He failed to account for the methane and ethane in the existing debutanizer feed stream. The sample from the 25-psig feed vessel was taken in a bottle. The lighter components weathered off prior to lab analysis. The results were, for Steve, rather catastrophic. The debutanizer overhead product could not be fully condensed. The non-condensable vapor pressured up the overhead reflux drum. The non-condensables were vented, along with 30 percent of the butane, to the flare. Steve was... 

The preferred overhead system for atmospheric columns is shown in Figure 3.6. The condensed vapor falls into a reflux drum that should have 5—10 minutes holdup (relative to condensate rate), and inerts are vented to a flare... 

For those columns that must be protected from atmospheric oxygen or moisture, a vent system such as that shown in Figure 3.7 should be used. This is similar to the one recommended later for pressurized or vacuum columns. Note that inerts usually should be added e er the condenser, to minimize product losses. Sometimes, howler, it is necessary to add inerts ahead of the condenser, for pressure control. Figure 3.7 also shows a more commonly encountered tank arrangement where the reflux drum is common to both the top product system and the reflux system. 

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