Reflux drum liquid separation

The horizontal reflux drum also separates hydrocarbon liquid from water. 

Saturated isomerate from the separator is heated and fed to the stabilizer. The stabilizer s overhead vapors are cooled and fed to a reflux drum. Liquid hydrocarbons from the reflux drum are returned to the stabilizer as reflux while uncondensed light hydrocarbons are separated and sent to the offgas system. The bottom product or isomerate is cooled and sent to gasoline blending. 

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... 

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. 

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. 

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... 

Distillation with vapor product. When a partial condenser is used, the flash drum plays the role of a vapor/liquid separator. In the setup known as a stabilizer there is only vapor distillate, while the liquid is returned as reflux. The column has a pasteurization section when a gaseous stream leaves at the top, while the... 

Step 2. There are 26 control degrees of freedom in this process. They include three feed valves for oxygen, ethylene, and acetic acid vaporizer and heater steam valves reactor steam drum liquid makeup and exit vapor valves vaporizer overhead valve two coolers and absorber cooling water valves separator base and overhead valves absorber overhead, base, wash acid, and liquid recirculation valves gas valve to CO removal system gas purge valve distillation column steam and cooling water valves column base, reflux, and vent valves and decanter organic and aqueous product valves. 

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. 

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. 

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. 

Separation of two liquid phases. If two liquid phases are to be separated, the reflux drum must provide sufficient settling time. The rate of settling can usually be calculated from Stoke s law. A more detailed discussion on settling and decanting is available in Ref. 319 and its cited literature. 

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

There are three streams in the table shown in Figure 3.35. Stream 1 is the feed inlet to the column. Stream D is the liquid distillate leaving the reflux drum. Stream B is the liquid bottoms leaving the base of the column. We can see that there is about 12 mol% iCA in the distillate and 8 mol% C3 in the bottoms. The purities are too low, so we need to increase the reflux ratio or add more stages to get a better separation. 

Both a gas stream and a liquid distillate are removed from the reflux drum. In addition, since live steam is fed into the bottom of the column and separates into an aqueous phase in the reflux dmm, a water stream is removed from a small boot at the bottom of this drum that serves as a decanter. The stripping steam flow rate is 5000 Ib/h and its temperature is 400 °F. Both the gas and the liquid products contain some water. The water decanted is 2441b/h. 

The process uses a distillation column to separate methanol and water. During the PHA, the team identified that failure of the level control loop in the reflux drum could cause the drum to overfill, resulting in vessel overpressure, opening of a pressure relief valve to the atmosphere, and the liquid release of methanol with possible fire. An SIL 1 SIF is installed to prevent the level in the reflux drum from exceeding 95%. The SIF consists of an SIF level transmitter, a trip amp, a block valve on the steam to the column, and a block valve on the feed to the column. The capacity of the column is such that the column could be shut down for a short time period without affecting the production operation. 

We must also calculate the size of the reflux drum and the column base. These provide liquid surge capacity, which helps to filter disturbances in both flow and composition to downstream units. They also permit the column to ride through large disturbances without upsetting the column to the point where liquid or vapor hydraulic limitations are encountered (flooding or weeping), which can result in the loss of separation and the production of off-specification products. 

Other equipment sizing followed the recommendations in Chapter 4. The volumes of the column base for the Cl column and also for the C2 column, and the reflux drum of the C2 column, are all sized to give 10 min holdup with 50% liquid level. The decanter is sized to be bigger to allow for two hquid phases to separate. The holdup time of 20 min for the decanter is used in the dynamic simulation. 

Figure 13.5 Liquid compositions in the reflux drum and in the colunm bottoms for the acetone methanol separation.

Now we consider the other extreme case, where the feed has a composition identical to the distillate product specification. Again no separation is required we need to provide sufficient reboil duty to fully vaporise die feed, and sufficient condenser duty to return it to liquid in the reflux drum, but no reflux is required. 

Analysis of Partial Condenser Data. Gunness (Ref. 1) reports experimental data obtained on the partial condenser oi a rectifying column stabilizing absorption naphtha. The vapor from the column was passed downward through the partial condenser, and the uncondensed vapor and condensate from the bottom of the condenser passed together to the reflux drum where they were separated. Owing to the concurrent flow of the vapor and the liquid, it would be expected that this system might approximate equilibrium partial condensation. The data for a test... 

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