Reflux drums separation

The reflux drum separates liquid and wet gas by gravity settling. A horizontal vapor-liquid separator works in much the same way as the vertical KO drum. 

With this scheme, vapor and liquid should enter the reflux drum separately. Liquid should enter below the liquid surface. 

The fix for the erratic reflux drum pressure problem was to provide for separate pressure control of the fractionator column and the reflux drum. A new pressure control valve was installed upstream of the condenser and the old condenser outlet control valve was removed. A hot gas bypass, designed for 20% vapor flow, was installed around the pressure control valve and condenser. A control valve was installed in the hot gas bypass line. The column pressure was then maintained by throttling the new control valve upstream of the condenser. The reflux drum pressure w as controlled by the hot gas bypass control valve and the psv saver working in split range. The new system is shown in the figure below. 

When a small quantity of a second liquid phase is present, a drawoff pot (commonly called a bootleg) is provided to make separation of the heavy liquid (frequently water) easier. The pot diameter is ordinarily determined for heavy phase velocities of 0.5ft/min. Minimum length is 3 ft for level controller connections. Minimum pot diameter for a 4 to 8 foot diameter reflux drum is 16 inches. For... 

Certain refinery wastewater streams are treated separately, prior to the wastewater treatment plant, to remove contaminants that would not easily be treated after mixing with other wastewater. One such waste stream is the sour water drained from distillation reflux drums. Sour water contains dissolved hydrogen sulfide and other organic sulfur compounds and ammonia which are stripped in a tower with gas or steam before being discharged to the wastewater treatment plant. 

At the lop of the tower any intermediate components going out with the gas are condensed, separated, pumped back to the tower, and sprayed down on the top tray. This liquid is called reflux. and the two-phase separator that separates it from the gas is called a reflux tank" or reflux drum. The reflux performs the same function as the cold feed in a cold-feed stabilizer. Cold liquids strip out the intermediate components from the gas as the gas rises. 

Sorensen (1999) has suggested a cyclic operation policy for batch distillation with repeated filling and dumping of the reflux drum. This configuration achieves the maximum attainable separation and requires minimal control. Furthermore, such a column can be operated very safely. 

The 23-cm-diameter distillation column under study is used to separate ethanol and water. It contains 12 sieve trays with a 30-cm spacing (Fig. 11) as well as three possible feed locations, an external reboiler, and two condensers, which are used at the bottom and the top of the column. The second condenser is also used as a reflux drum a pump sends the reflux back to the column (tray 1) and the product to the product tank. 

Develop a mathematical model for the three-column train of distillation columns sketched below. The feed to the first column is 400 kg mol/h and contains four components (1, 2, 3, and 4), each at 25 mol %. Most of the lightest component is removed in the distillate of the first column, most of the next lightest in the second column distillate and the final column separates the final two heavy components. Assume constant relative volatilities throughout the system ai, CI2, and a3. The condensers are total condensers and the reboilers are partial. Trays, column bases, and reflux drums are perfectly mixed. Distillate flow rates are set by reflux drum... 

Process designers sometimes like to use dephlegmators or partial condensers mounted directly in the top of the distillation column when the overhead product is taken off as a vapor. They arc particularly popular for corrosive, toxic, or hard-to-handle chemicals since they eliminate a. separate condenser shell, a reflux drum, and a reflux pump. Comment on the relative controllability of the two process systems sketched below. 

A distillalion column is used to separate two close-boiling components that have a relative volatility close to one. The reflux ratio is quite high (IS) and many trays are required (150). To control the compositions of both products the flow rates of the product streams (distillate D and bottoms B) an manipulated. Gas chromatographs are used to measure the product compositions. Base level is controlled by steam flow rate to the icboiler and reflux drum level is controlled by reflux flow rate. 

The horizontal reflux drum also separates hydrocarbon liquid from water. 

Liquid drums usually are placed horizontal and gas-liquid separators vertical, although reflux drums with gas as an overhead product commonly are horizontal. The length to diameter ratio is in the range 2.5-5.0, the smaller diameters at higher pressures and for liquid-liquid settling. A rough dependence on pressure is... 

Rmin and the corresponding number of trays calculated ( 2N J. The shortcut models were replaced by rigorous RADFRAC units, where the reflux and distillate feed ratio were adjusted by means of design specifications, in order to meet the desired separation. The trays were sized using Aspen s facilities. Finally, the dimensions of the reflux drum and column sump were found based on a residence time of 5 min and aspect ratio H D = 2 1. Table 9.7 presents the results of distillation column sizing. 

Figure 6.96 shows a column that is separating a mixture with a low relative volatility, so the column has a large number of trays and operates with a high reflux ratio. This type of column is called a superfractionator. Because of the high reflux ratio, reflux should be used to control reflux drum level. For the same reason, vapor boilup should be used to control base level. Therefore the two manipulators left to control composition are distillate and bottoms flowrates. Obviously these two... 

The stripped solvent is practically free from water. The water undergoes overhead azeotropic distillation and is produced as separate subphase in the reflux drum. This water is then fed to the solvent recovery stage of the extraction process. 

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. 

A fractionator separates dimethylformamide from water and acetic acid. The distillate contains a trace amount of acetic acid. Assuming that the fractionator uses a total condenser, estimate the diameter, length, and wall thickness of the reflux drum. Because the mixture contains acetic acid, use stainless steel (SS 316) for the drum. 

The reactor effluent proceeds to a cooler where dissolved ethylene is separated from the butenes stream. A reflux drum is provided for the condensation of solvent vapor and the liquid is recycled back to the reactor. The mixture of ethylene and butene-1 proceeds from the reflux drum into the outlet line. After 5 hrs of reaction time, ethylene conversion reached 25.7 percent and the product distribution was mainly butene-1 at 95.5 percent selectivity and small amounts of hexenes and other oligomers but without any polymer formation. The unit ratio for the grams of butene-1 produced per gram of triethylaluminum was about 159. 

A stream containing benzene, toluene, and biphenyl is to be separated in a distillation column to produce purified benzene in the distillate. The separation will take place in an existing column with a total condenser, a partial reboiler, and several optional feed locations. The feed stream is of fixed flow rate, composition, and thermal conditions. The entire feed may be introduced at any one of the available feed trays, but may not be split and introduced at more than one feed tray. The condenser pressure is controlled by an inert gas flowing in and out of the reflux drum. Using column modules representation, determine the degrees of freedom for this operation, and recommend a set of specifications to define the column performance. 

Consider a binary mixture of components A and B, to be separated into two product streams using conventional distillation. The mixture is fed in the column as a saturated liquid (i.e., at its bubble point), onto the feed tray / (Figure 4.10), with a molar flow rate (mol/min) F/ and a molar fraction of component A, overhead vapor stream is cooled and completely condensed, and then it flows into the reflux drum. The cooling of the overhead vapor is accomplished with cooling water. The liquid from the reflux drum is partly pumped back in the column (top tray, N) with a molar flow rate FR (reflux stream) and is partly removed as the distillate product with a molar flow rate FD. Let us call Mrd the liquid holdup in the reflux drum and xD the molar fraction of component A in the liquid of the reflux drum. It is clear that xD is the composition for both the reflux and distillate streams. 

Fig. 13.9 presents the flowsheet for the separation section of a HDA process. The first column is a stabiliser. The vapour distillate - a mixture of H2, CH4 with traces of benzene and toluene - can be used to hold constant the pressure. The quality of the bottom product is ensured by controlling a sensitive temperature in the top zone (inferential control) with the reboiler duty. Reflux and bottoms flow rates control the levels in reflux drum and sump, respectively. 

Most process and utility drums serve as separators, surge and reflux drums. These drums should be arranged in process flow sequence. 

If gravity flow is required from the condenser outlet to a collecting drum, reflux drum or separator, exchangers must be elevated. Examples are shown in Figure 7-67 and 7-68. 

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