Accumulators reflux drum

Example 10 Calculation of Multicomponent Batch Distillation A charge of 45.4 kg mol (100 Ih-mol) of 25 mole percent heuzeue, 50 mole percent monochlorohenzene (MCB), and 25 mole percent orthodichloro-henzene (DCB) is to he distilled in a hatch still consisting of a rehoiler, a column containing 10 theoretical stages, a total condenser, a reflux drum, and a distillate accumulator. Condenser-reflux drum and tray holdups are 0.0056 and... 

Some of the stripping steam condenses in the overhead condenser, shown in Fig. 10.3. The condensed steam, which accumulates in the reflux drum, is totally refluxed back to the top tray of the stripper tower. 

Let s assume that the liquid draining from the condenser is not quite cold enough to absorb the entire latent heat of condensation of the vapors flowing through the hot-vapor bypass line. The vapors will then be only partially condensed. Vapor will start to accumulate in the reflux drum. This accumulation of vapor will increase the reflux drum pressure by a small amount. The higher drum pressure will back up the liquid level in the condenser by a few inches. The higher height of liq-... 

The molar holdup in the base of the column is MP and in the overhead accumulator is Mn. These holdups are not constant but vary with time. The total molar and component balances for the base and reflux drum are given below ... 

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. 

Low accumulator liquid levels are likely to damage pumps and/or induce flow of vapor to downstream units. One case history of pump damage due to loss of reflux drum level has been described (237). 

Slug flow. When partial condensers are located below the reflux drum (Fig. 15.15e), and the velocity in the riser is too slow, vapor and liquid segregate in the riser. A head of liquid builds up and exerts back pressure against the column. Periodically, a slug of liquid breaks through and releases the back pressure. The riser then gradually fills up with liquid, and the cycle repeats itself. This causes fluctuations in column pressure and accumulator level. One troublesome case history of slug flow has been reported (70). 

In some systems, the reflux drum is omitted or run flooded (e.g.. Fig. 17.56). This eliminates one variable (accumulator level). This also eliminates a manipulated stream by making the condensation rate a slave to the top product rate. Similarly, when an internal condenser is employed, the accumulator level is eliminated, together with one manipulated stream (reflux becomes a slave to the condensation rate). In either situation, the rest of the variables are paired in the normal manner. The discussions below therefore apply to that situation too. 

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. 

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. 

The reflux scheme can be difficult to start up because initially there may not be enough light components accumulated in the reflux drum, and the column temperature may be too hot so, the controller may want more reflux flow than is available. This can pump the reflux drum level down until the distillate flow stops and then proceed to pump the reflux drum empty. However, this situation can be handled with computer control by using a low-level constraint control that will constrain the reflux flow rate to maintain a low-level constraint setpoint until the column temperature is low enough, so the temperature controller calls for less reflux. This reflux scheme is recommended when the reflux/distillate ratio is less than... 

The manipulated boilup scheme is quite easy to start up after some liquid is accumulated in the reflux drum. However, this scheme does not shed environmental disturbances very well. When a sudden rain storm hits the distillation tower, the temperature disturbance is shed by the temperature controller increasing the steam flow rate to the reboiler. This can be a problem if the distillation tower needs to be running at its maximum capacity near to flooding. 

Methods to identify and correct these problems are discussed in Chapter 12. Remember that a refrigerant accumulator drum is similar to the tower reflux drum. 

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. 

If naphtha is accumulating in the regenerator reflux drum, excessive concentrations of propane and butane will occur in sulfur plant feed. A commercially proven method to eliminate this problem is detailed in Figure 4-7. The rich amine surge drum is retrofitted with baffles. Figure 4-7 is roughly drawn to scale. This baffle arrangement will automatically skim off the hydrocarbons. 

Are light hydrocarbons accumulating in the amine regenerator reflux drum ... 

Occasionally, towers in this service must be washed to remove sulfate or acid coke deposits on the tray decks. The wash liquid is accumulated in the reflux drum and then pumped to the top tray with the reflux pump. The wash liquid can then be pumped from the bottom of the tower and recirculated back to the reflux drum. 

---