Fractionator Reflux Drums

Commonly their orientation is horizontal. When a small amount of a second hquid phase (for example, water in an immiscible organic) is present, it is collected in and drawn off a pot at the bottom of the drum. The diameter of the pot is sized on a hnear velocity of 0.5 ft/ sec is a minimum of 16 in dia in drums of 4-8 ft dia, and 24 in. dia. in larger sizes. The minimum vapor space above the high level is 20% of the drum diameter or 10 in (Sigales, 1975). 

A method of sizing reflux drums proposed by Watkins (1967) is based on several factors itemized in Table 18.1. A factor Fj is applied to the net overhead product going downstream, then instrument factors F and labor factors Fi which are added together and applied to the weighted overhead stream, and finally a factor Ft is applied, which depends on the kind and location of level indicators. When L is the reflux flow rate and D the overhead net product rate, both in gpm, the volume of the drum (gal) is given by 

Operation w/Alarm w/o Alarm Good Fair Poor 

Although this method seems to take into account a number of pertinent factors, it is not rigorous. Some practitioners may size drums on the basis of 5 minutes holdup half-full. 

Since the rise or fall of liquid droplets is interfered with by lateral flow of the hquid, the diameter of the drum should be made large enough to minimize this adverse effect. A rule based on the Reynolds number of the phase through which the movement of the liquid drops occurs is proposed by Flooper and Jacobs (1979). The Reynolds number is Df,up/fi, where Dh is the hydrauhc diameter and u is the linear velocity of the continuous phase. The rules are  

TABLE 18.1 Factors for Sizing Reflux Accumulators a. Factors F, and F2 on the Reflux Flow Rate 

Under good control Under fair control Under poor control Feed to or from storage 2.0 3.0 4.0 1.25  

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FIGURE S-11 This type of reboiled absorber can lose efficiency as absorption is improved in the fractionator reflux drum. 

Clean steam condensate is the best source of wash make-up water. Condensate collected from fractionator reflux drums is sometimes acceptable. River, lake, or well water with high total dissolved solids (TDS) is bad. These solids can precipitate out on the mix valve and cause excessive pressure drop. In one refinery, using untreated river water in a crude-unit desalter for a few months reduced desalting efficiency from 90% to 65% salt removal. 

Fig. 21.5 shows two waste-heat boilers that are generating superheated, 150-psig steam from a fractionator hot oil pumparound. After years of trouble-free operations, water was observed in the gas-oil pumparound return stream. The symptoms of the water infiltration into the fractionator were an increase in the fractionator pressure and an increase in the water production from the fractionator reflux drum s draw-off boot. 

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-... 

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. 

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-... 

Figure 5.220. Liquid phase mole fractions of components A, B, C and D in the reflux drum.

Petroleum distillation units generate considerable wastewater. The process water used in distillation often comes in direct contact with oil and can be highly contaminated. Both atmospheric distillation and vacuum distillation produce an oily, sour wastewater (condensed steam containing hydrogen sulfide and ammonia) from side-stripping fractionators and reflux drums. 

A single feed stream is fed as saturated liquid (at its bubblepoint) onto the feed tray N,. See Fig. 3.12. Feed flow rate is F (mol/min) and composition is z (mole fraction more volatile component). The overhead vapor is totally condensed in a condenser and flows into the reflux drum, whose holdup of hquid is Mj) (moles). The contents of the drum is assumed to be perfectly mixed with composition Xo The liquid in the drum is at its bubblepoint Reflux is pumped back to the top tray (iVj-) of the column at a rate R. Overhead distillate product is removed at a rate D. 

The vast majority of fractionators have top reflux. Cold liquid from the reflux drum is pumped onto the top tray of the tower. The cold liquid flashes to a hotter vapor. For example, let s say 1500 lb/h of liquid butane, at 100°F, flashes to 1500 lb/h of vapor at 260°F. 

Overhead of the fractionator T-2 is partially condensed in E-1A (on crude) and E-IB (on cooling water). A gas product is withdrawn overhead of the reflux drum which operates at 15 psig. The light gasoline is pumped with J-5 to storage and as reflux. 

In order for a process to be controllable by machine, it must represented by a mathematical model. Ideally, each element of a dynamic process, for example, a reflux drum or an individual tray of a fractionator, is represented by differential equations based on material and energy balances, transfer rates, stage efficiencies, phase equilibrium relations, etc., as well as the parameters of sensing devices, control valves, and control instruments. The process as a whole then is equivalent to a system of ordinary and partial differential equations involving certain independent and dependent variables. When the values of the independent variables are specified or measured, corresponding values of the others are found by computation, and the information is transmitted to the control instruments. For example, if the temperature, composition, and flow rate of the feed to a fractionator are perturbed, the computer will determine the other flows and the heat balance required to maintain constant overhead purity. Economic factors also can be incorporated in process models then the computer can be made to optimize the operation continually. 

Feed split means the fraction of the feed that leaves in one product stream, e.g., the DIF ratio. Feed split can be set directly and explicitly by using either D or B to control one composition. Or it can be set indirectly and implicitly by using reflux or vapor boilup to control one composition and removing D or B to hold reflux drum or base level. 

Figure 6.9 shows typical control structures for two special types of columns. Figure 6.9a is for a column whose feed contains a small amount of a component that is much more volatile than the main component. The distillate product is a small fraction of the feed stream. It is removed from the reflux drum as a vapor to hold column pressure. Reflux flow is fixed, and reflux drum level is controlled by manipulating condenser coolant. In the petroleum industry, this type of column is called a stabilizer. The first column in the HDA process is this type. 

Column 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 temperature at flash-zone pressure must be sufficient to vaporize the total distillates plus the overflash, which is necessary to provide reflux between the lowest sidestream-product drawoff tray and the flash zone. Calculations are made by using the crude oil EFV curve corrected for pressure. For the example being considered, percent vaporized at the flash zone must be 53.1 percent of the feed. 

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. 

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