Reflux drums

Cohimn pressure at the reflux drum is established so as to condense totally the overhead vapor or some fraction thereof. Flash-zone pressure is approximately 69 kPa (10 psia) higher. Crude-oil feed temper-... 

Component mole balances for total-coudeuser-reflux drum, trays, and reboiler, respectively ... 

Molar holdups in condenser-reflux drum, on trays, and in reboiler ... 

Derivatives or rates of change of tray and condenser-reflux drum hquid holdup with respecl to time are sufficiently small compared with total flow rates that these derivatives can be approximated by incremental changes over the previous time step. Derivatives of liquid enthalpy with respect to time eveiywhere can oe approximated in the same way. The derivative of the liquid holdup in the reboiler can likewise be approximated in the same way except when reflux ratios are low. 

Calculate liquid densities, molar tray and condenser-reflux drum holdups, ana hquor and vapor enthalpies. Determine holdup and enthalpy derivatives with respect to time by forward difference approximations. 

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... 

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... 

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. 

Tower reflux drum See Table 2 5-based on reflux flow 5 to 10-based on total flow —... 

This article deals enly with reflux drums. Use only the larger vessel volume determined. Do not add two volumes such as reflux plus product. If a second liquid phase is to be settled, additional time is needed. For water in hydrocarbons, an additional 5 minutes is recommended. 

Liquid Residence Rules of Thumb for Reflux Drums... 

Sigales, B., How to Design Reflux Drums, Chemical Engineering, March 3, 1975. 

Sigales has provided an optimizing method for reflux drums. Here are some useful comments from that article. 

Reflux drums are usually horizontal because the liquid load is important. 

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... 

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. 

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. 

Take-off rate from reflux drum (high)... 

Finally, at 12 noon, the reflux drum overflowed, and there was a spillage of flammable oil. From 7 a.m. onward, a trainee served as operator, but a lead operator was present, and the foreman visited the control room from time to time. 

Reflux drum Mixer or stirrer Breathing apparatus (BA) Slip-plate Refrigeration plant Knock-on (or domino effects) Nonreturn valve Blinded... 

TR14, TR15 = Column Temperature LIC3 = Level in Reflux Drum FIC8 = Reflux Flow FIl, FR15 = Crude Flow at Entry Point TRC8 = Crude Temperature at Entry Point. 

Reflux drums usually are horizontal, with a liquid holdup of 5 min half full. A takeoff pot for a second liquid phase, such as water in hydrocarbon systems, is sized for a linear velocity of that phase of 0.5 ft/sec, minimum diameter of 16 in. 

Holdup time is 5 min half full for reflux drums, 5-10 min for a product feeding another tower. 

In the inverted configuration the feed is charged to the reflux drum and then continuously added to the top of the column. The inverted column seems to be better suited for cases in which the products are to be recovered at high purity from a feed low in light components, whereas the regular column appears to be best for cases in which the products are to be recovered at high purity from a feed rich in light components. 

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. 

Consider the binary batch distillation column, represented in Fig. 3.58, and based on that of Luyben (1973, 1990). The still contains Mb moles with liquid mole fraction composition xg. The liquid holdup on each plate n of the column is M with liquid composition x and a corresponding vapour phase composition y,. The liquid flow from plate to plate varies along the column with consequent variations in M . Overhead vapours are condensed in a total condenser and the condensate collected in a reflux drum with a liquid holdup volume Mg and liquid composition xq. From here part of the condensate is returned to the top plate of the column as reflux at the rate Lq and composition xq. Product is removed from the reflux drum at a composition xd and rate D which is controlled by a simple proportional controller acting on the reflux drum level and is proportional to Md-... 

The liquid volumes in the still, reflux drum and on the column plates are well-mixed regions of uniform composition. 

Total mass and component mass balance equations are written for all the plates of the column, for the still and for the top reflux drum. 

The maintenance of constant liquid level in the reflux drum can be expressed by the following proportional control equation... 

---