Liquid Levels in Columns

Ellingsen (107) reported a survey of 18 incidents of column damage that occurred in DuPont s columns. Table 13.1 summarizes the results. Over half (55 percent) of the incidents were caused by excessive liquid level in the bottom of the column. In another 17 percent of the incidents, trays ruptured due to a vacuum that was present in some section of the column. Poor installation accounts for another 14 percent of the incidents. Other culprits—including corrosion, high vapor rates, and fatigue failure accounted for the remaining 14 percent of the incidents. 

Assuming the sample of incidents is representative, one out of two tray damage incidents can be prevented by avoiding excessive liquid level in the column (Sec. 13.2). Six out of seven incidents can be prevented by also preventing vacuum formation in sections of the column (Sec. 11.2, 11.7, 11.8, 12.6, 12.9, 12.11) and by adequate installation and inspection (Chap. 10). 

If the liquid level in the bottom sump of the column rises above the reboiler return nozzle (or, alternatively, the bottom vapor inlet nozzle), vapor from the reboiler has to travel upward through a layer of liquid. If this layer is shallow, the vapor can bubble through it or atomize the liquid and carry over liquid as a mist into the first tray from the bottom. This may lead to premature flooding (71, 145, 192, 207, 237, 238) and possibly some wave action that would interfere with level control. 

If the liquid layer is deep (i.e., several of the lower trays may be flooded), vapor may travel through the liquid in the form of slugs. These can loosen bolting and other fasteners, damage trays, or lift trays off their supports (61,150a, 192, 207, 295), collapse packing supports (237), and cause column vibration and structural damage. 

Case no. No. of trays Diameter of col. in Operating pressure vacuum High liquid in bottom High vapor rate Fatigue failure Poor tray installation Corrosion 

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Several incidents have been reported where trays were lifted off their supports due to such slug (49, 107, 231, 296). The author is familiar with some additional similar experiences. Ellingsen s survey (107) suggests that over half the tray damage incidents in the chemical industry are caused by deep liquid levels in columns (Sec. 13.1). 

It is just as important to maintain a good control of the bottom liquid level in columns where the bottom feed, stripping steam, or reboiler return nozzle enters through a submerged sparger (Fig. 4.3a). Here the liquid level serves as a desuperheater, and its loss may overheat the column or its internals. One incident was reported (440) where plastic packings melted because this level was lost. 

Fractionating columns usually operate with a normal liquid level in the bottom of the column and a level of liquid on each tray. It is reasonable to assume that the wetted surface be based on the total liquid within the height limitation—both on the trays and in the bottom. 

This represents the fraction of the total available head between points A and B, which represents the sensible heating zone. This neglects liquid friction in the sensible zone and assumes the liquid level in the distillation column is maintained even with the top of the tubesheet. Equation 10-170 then gives the fractional tube length devoted to sensible heating. Refer to Figure 10-110 and note that ... 

The mechanical design of thermosiphon reboiler piping must be carefully examined for (a) system pressures and (b) elevadon reladonship between the liquid level in the disdl-ladon column and the verdcal or horizontal reboiler. Kem provides an excellent presentadon on this topic, including the important hydraulics. AbboT also presents a computer program for this topic. 

A centrifugal pump is required to circulate a liquid of density 800 kg/m2 and viscosity 0.5 x 10 3 Ns/m" from the reboiler of a distillation column through a vaporisor at the rate of 0.004 m3/s, and to introduce the superheated vapour above the vapour space in the reboiler which contains a 0.07 m depth of liquid. If smooth-bore 25 mm diameter pipe is to be used, the pressure of vapour in the reboiler is 1 kN/m2 and the Net Positive Suction Head required by the pump is 2 m of liquid, what is the minimum height required between the liquid level in the reboiler and the pump ... 

If all losses in the operation of the pump were neglected, the pressure at the point of introduction of the compressed air would be equal to atmospheric pressure together with the pressure due to the column of liquid of height hs, the vertical distance between the liquid level in the suction tank, and the air inlet point. Therefore ... 

The mass balance relationships for the feed plate, the plates in the stripping section, of the column and for the reboiler must, however, be modified, owing to the continuous feed to the column and the continuous withdrawal of bottom product from the reboiler. The feed is defined by its mass flow rate, F, its composition xp and the thermal quality or q-factor, q. The column bottom product is defined by its mass flow rate, W, and composition, xw and is controlled to maintain constant liquid level in the reboiler. 

A process liquid is pumped from a storage tank to a distillation column, using a centrifugal pump. The pipeline is 80 mm internal diameter commercial steel pipe, 100 m long. Miscellaneous losses are equivalent to 600 pipe diameters. The storage tank operates at atmospheric pressure and the column at 1.7 bara. The lowest liquid level in the tank will be 1.5 m above the pump inlet, and the feed point to the column is 3 m above the pump inlet. 

The top tube sheet is normally aligned with the liquid level in the base of the column Figure 12.58. The outlet pipe should be as short as possible, and have a cross-sectional area at least equal to the total cross-sectional area of the tubes. 

Gas Recycle is a relatively simple operation. Rather than being circulated through a variety of pipes, pumps and columns, the catalyst remains in one place. A key control variable is maintaining a constant liquid level in the reactor. This is not as simple as it might first seem because in addition to butanal isomers forming, butanal condensation products including dimers and trimers also form to give what are collectively termed heavies . 

In addition to the basic control loops, all processes have instrumentation that (1) sounds alarms to alert the operator to any abnormal or unsafe condition, and (2) shuts down the process if unsafe conditions are detected or equipment fails. For example, if a compressor motor overloads and the electrical control system on the motor shuts down the motor, the rest of the process will usually have to be shut down immediately. This type of instrumentation is called an interlock. It either shuts a control valve completely or drives the control valve wide open. Other examples of conditions that can interlock a process down include failure of a feed or reflux pump, detection of high pressure or temperature in a vessel, and indication of high or low liquid level in a tank or column base. Interlocks are usually achieved by pressure, mechanical, or electrical switches. They can be included in the computer software in a computer control system, but they are usually hard-wired for reliability and redundancy. 

An early indication of flooding in a distillation column is loss of liquid level in the bottom of the column. 

In the 300-gallon-per-day plant the mean ice particle sizes have been calculated from measurements of ice bed permeability and porosity made on the ice harvested at the top of the column. From these results the important design parameters can be calculated, such as particle diameters, linear ice velocities, residence time of ice in the column, frictional losses in the wash water flowing down the column and in brine flowing toward the screens in the bottom of the coltam, and the fraction of voids occupied by air above the liquid level in the column. Typical ranges for some of these measured or calculated quantities are shown in Table III from measurements in the 12-inch diameter column. 

The product-acid solution is withdrawn from the column using a level control valve on this line. The liquid level in the base of the... 

Figure 8-54 shows a depropanizer controlled by reflux and boil-up ratios. The actual mechanism through which these ratios are manipulated is as D/(L + D) and B/(V + B), where L is reflux flow and V is vapor boil-up, which decouples the temperature loops from the liquid-level loops. Column pressure here is controlled by flooding both the condenser and accumulator however, there is no level controller on the accumulator, so this arrangement will not function with an overloaded condenser. Temperatures are used as indications of composition in this column because of the substantial difference in boiling points between propane and butanes. However, off-key components such as ethane do affect the accuracy of the relationship, so that an analyzer controller is used to set the top temperature controller (TC) in cascade. 

Subsequently, we used Aspen Dynamics for time-domain simulations. A basic control system was implemented with the sole purpose of stabilizing the (open-loop unstable) column dynamics. Specifically, the liquid levels in the reboiler and condenser are controlled using, respectively, the bottoms product flow rate and the distillate flow rate and two proportional controllers, while the total pressure in the column is controlled with the condenser heat duty and a PI controller (Figure 7.4). A controller for product purity was not implemented. 

The final pH adjustment (Figure 6B) takes place in the feed system to the extraction (WE-1) column. Preneutralized solution in Tank WM-1 is recirculated to provide mixing. Part of the recirculated solution is diverted to the WM-2 static mixer tank where it is mixed with NaOH the volume of NaOH added is controlled by the pH of the WM-2 tank outlet stream. Most of the solution leaving the WM-2 tank is returned to the WM-1 tank the remaining portion feeds the WE-1 Column. The flowrate of this latter stream is adjusted to maintain the liquid level in the WM-1 tank approximately constant. When functioning satisfactorily, the two-stage acid adjustment procedure provides an aqueous E1F stream at pH 0.75 flowing at a constant rate to the extraction column. 

In the second control structure (Fig. 2.11b), which does work, the fresh feed makeup of the limiting reactant (.F0B) is flow-controlled. The other fresh feed makeup stream (FCvl) is brought into the system to control the liquid level in the reflux drum of the distillation column. The inventory in this drum reflects the amount of A inside the system. If more A is being consumed by reaction than is being fed into the process, the level in the reflux drum will go down. Thus this control structure employs knowledge about the amount of component A in the system to regulate this fresh reactant feed makeup to balance exactly the amount of B fed into the process. 

Both of the control structures discussed in Sec. 2.7.2 (CSl and CS4j work because they detect the inventories of the reactant components A and B in the system and bring in fresh feed streams to balance the consumption of the two components. Structure CSl does this by using the liquid level in the reflux drum of the second column as an indicator of the amount of A in the system and the liquid level in the base of the first column as an indicator of the amount of B in the system. Structure CS4 uses a composition analyzer to measure directly the concentration of one of the reactants in the reactor. But both of these structures lack a direct handle on production rate. 

Two of the control degrees of freedom must be consumed to control the two liquid levels in the process reflux drum level and base level. Reflux drum level can be held by changing the flowrate of the distillate, the reflux, the vapor boilup, the condenser cooling, or the feed (if the feed is partially vapor). Each of these flows has a direct impact on reflux drum level. The most common selection is to use distillate to control reflux drum level, except in high reflux-ratio columns (RR > 4) where Richardson s rule7 suggests that reflux should be used. 

To avoid the high-pressure safety constraint, we must control reactor pressure. We can use the distillate valve from the purge column, the flooded condenser cooling water valve, or the DIB column feed valve. The most logical variable to use for control of the flooded condenser (.reactor) pressure is the DIB column feed valve (as shown in Fig. 5.5). Based upon the discussion in Step 3, we would then use the flooded condenser cooling water valve to keep the liquid level in a good control range. 

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