This document examines common practices of introducing reflux and intermediate feed into tray columns, outlines the preferred practices, highlights the consequences of poor practices, and supplies guidelines for troubleshooting and for reviewing designs of reflux and intermediate feed inlets.
The main consideration for introducing reflux or intermediate feed into a tray tower is to achieve adequate hydraulics in the inlet area. Failure to achieve this may result in premature flooding, excessive entrainment, and mechanical damage. When the tower contains multipass trays, it is also important to split the feed or reflux adequately among the passes. With single-pass trays, and sometimes with two-pass trays, achieving good distribution of the feed to the tray is of secondary, but not negligible, importance.
1. Top-Tray Feed and Reflux Inlet Arrangements
Figure 1 shows methods for introducing top-tray feed or reflux into a column. Table 1 lists the dimensions restricting each design. All the arrangements shown in Fig. 1 are suitable for liquid reflux or top feed. If the feed contains some vapor, only arrangements b, d, e, and h of Fig. 1 are suitable. Arrangements a, b, c, e, and f of Fig. 1 are usually preferred for cost reasons. Arrangements d and h are usually used when there is a distinct advantage for orienting the inlet nozzle at an angle other than about 0° to the liquid flow. Arrangements a, d, h, and to a lesser extent g have the disadvantage of inducing weeping through the tray inlet rows of perforations or valves because of hydraulic jump over the inlet weir.
Figure 1 - Top-tray feeds and reflux-inlet arrangements. Dimensions for x, y and z are listed in Table 1.
Table 1 - Dimensions for Top Feed/Reflux Inlet Arrangements
dn = Inlet pipe dia., in
hcl = Clearance under downcomer, in
Wd = Downcomer width, in
NS = Not Suitable.
*Dimensions as recommended by the author. All other dimensions recommended by the cited reference.
Note 1: Drill a 1/4-in vent hole on top.
Note 2: Wear plate may be required.
Note 3: Ensure nozzle enters behind the baffle. If it does not, hydraulic jump could be a problem
Internal inlet pipes should be removable for maintenance.
Arrangement b (often referred to as the false downcomer) is popular. It offers better liquid distribution than the others, does not suffer from hydraulic jump, and provides some flexibility in inlet nozzle orientation. The width of the false downcomer should be the same as the width at the bottom of a downcomer. If entrainment due to liquid splashing in the false downcomer is a concern, a horizontal baffle, with dimensions of about 2dn by Wd can be installed directly above the nozzle entry into the false downcomer. This baffle is installed at some clearance above the false downcomer. In most cases, there is no need for this baffle.
Arrangement e is also popular. It is one of the least expensive arrangements, does not suffer from hydraulic jump, and minimizes inlet splashing. The baffle is open both at its sides and bottom.
2. Intermediate Feed Inlet Arrangements
Figure 2 shows methods for introducing an intermediate feed to the the column. Table 2.2 summarizes the applications for which each arrangement is suitable.
Figure 2 - Intermediate-feed arrangements for columns
Table 2 - Intermediate Feed Inlet Arrangements
*Assuming insulation plate is provided.
Arrangement a is suitable only for subcooled low-velocity liquids such as circulating reflux streams. If the liquid contains some vapor or is hotter than the downcomer liquid, flashing will occur and downcomer capacity will be reduced. Even with subcooled liquid feeds, this arrangement introduces turbulence in an area where phase separation is important. It should therefore be avoided where downcomer capacity is critical, such as in high-pressure systems and systems in which fouling tends to progressively limit downcomer capacity. One designer recommends avoiding this arrangement altogether, because vapor may find its way into the feed (e.g., by tube leakage in an upstream heat exchanger).
Arrangement b is similar to arrangement a and is also only suitable for subcooled, low-velocity liquids where downcomer capacity is not critical. Compared to arrangement a, it may reduce the turbulence generated inside the downcomer; otherwise it suffers from the same disadvantages.
Arrangement c is also suitable only for low-velocity liquid feeds. If the feed contains vapor, impingement of the feed against liquid in the vapor space can cause premature entrainment. One designer, however, recommends this arrangement for general use for liquid as well as liquid-vapor feeds. This designer implies that significant interference with tray action can be avoided by locating the bottom of the feed nozzle 6 in above the tray floor for liquid feeds, and half a tray spacing above the tray floor for liquid-vapor feeds. Arrangement c has the advantage of being the least expensive.
Arrangement c can be troublesome with feeds whose temperature substantially exceeds the tray liquid. If the feed nozzle is positioned too close to the outlet weir, the joint liquid path may be too short to permit proper mixing between feed and tray liquids. Hot liquid will overflow the weir and induce vaporization, and, therefore, a capacity restriction, in the downcomer. In one column, this resulted in premature flooding and loss of efficiency.
Arrangement d is similar to arrangement c, but a channel baffle is added to avoid the impingement problem. This arrangement is suitable for vapor-containing feeds. The baffle may be straight or round and is open at its sides, top, and bottom.
Arrangement e is similar to arrangement d, but a plate is added below the nozzle so that the liquid initially flows sideways instead of downward. This reduces the feed velocity at the inlet and is particularly suitable for high-velocity feeds, whether liquid or vapor.
Arrangement f is similar to e, but the feed is introduced above the downcomer instead of above the tray to minimize interference with tray action. Minimizing this interference can be a distinct advantage with high-velocity feeds.
Arrangement g is often considered optimum for columns whose outlet-weir length is less than 5 ft; in larger columns, a distributor is better. This arrangement has the advantage of introducing feed at the tray inlet, thus improving separation, minimizing interference with tray action, and providing ample mixing distance for hot liquid feeds.
An insulation plate (Fig. 2g) is required on the outside wall of the downcomer if the feed enters at a temperature higher than the liquid in the downcomer. Failure to provide such a plate will cause flashing in the downcomer with a reduction in downcomer capacity.
Arrangement h is similar to g, but a wear plate is added on the outside wall of the downcomer and a horizontal impingement baffle is added below the nozzle to prevent entrainment. This arrangement is recommended for high-velocity feeds. Other advantages and recommended dimensions for this arrangement are similar to those for arrangement g above.
Arrangement i is a typical feed distributor arrangement. The recommended clearance between the distributor and the downcomer is 3 to 4 in, with the distributor openings oriented 45° from the vertical toward the downcomer. Additional guidelines are in following Sec. 4. This arrangement provides similar advantages to those of arrangement g, and in addition, provides superior liquid distribution, which is important in large columns.
Arrangement j is unique for high-velocity feeds in which vapor is the continuous phase and liquid is present in the form of a spray. This arrangement is common when the feed makes up the bulk of the vapor traffic in the column section above the feed. It is also used when the feed flashes upon entry to a low-pressure column. Typical examples are feeds to refinery crude and vacuum columns and rich solution feeds to hot carbonate regenerators. A tangential helical baffle or vapor horn, covered at the top, open at the bottom, and spiraling downward, is used at the feed entry. This baffle forces the vapor to follow the contour of the vessel as it expands and decreases in velocity. Liquid droplets, due to their higher mass, tend to collide with the tower wall, which deflects them downward, thus reducing entrainment to the tray above. Large forces, generated by vapor flashing, are absorbed by the entire column wall rather than by a small area. A wear plate is required at the tower wall.
It is important to ensure that the helical baffle spirals downward and is covered. There are example with situations where failure to do this caused excessive entrainment to the trays above the feed.
Pilot-scale experiments showed that compared to a radial vapor inlet, the tangential inlet gives better vapor distribution above the feed inlet zone. The experiments also showed that with the tangential arrangement, vapor velocity around the periphery of the zone above the feed was higher than in its center. Addition of an annular deflection ring in the tower, above the feed zone, further improved vapor distribution. An additional improvement resulted when two tangential inlets were used instead of one.
3. Dos and Don'ts for Reflux, Top-Tray, and Intermediate Feed Inlets
Below are guidelines for avoiding operating problems with reflux, top feed, and intermediate feed inlets.
- The inlet arrangements must be suitable for the service as described above. This should be checked not only during the initial design but also in any revamp and whenever feed conditions change. An example is an aromatics plant where equipment upstream of a column was revamped for energy savings. The revamp replaced the all-liquid column feed by a partially vaporized feed. The column feed distributor (similar to Fig. 2i) was not modified. The mixture issued at excessive velocities, and premature flooding resulted.
- When the feed can contain vapor, the tray sections and baffles that contact the entering feed can be subjected to abnormally high forces. To avoid structural damage, these sections and baffles should be strengthened. Also, the feed pipe should be anchored to the tower shell.
- Inlet lines containing two-phase feeds should be designed so that the flow is outside the slug-flow regime. When a horizontal pipe run precedes a vertical rise, a lift orifice or a trap is often advocated in order to prevent liquid from accumulating in the horizontal run. Slugging at the column inlet can lead to severe hydraulic pounding and tray damage, as well as column instability.
- When the feed is liquid, nozzle velocity should not exceed 3 ft/s. This ensures that the entering jet is broken up immediately on entering the column. For vapor or mixed feeds, it has been recommended that the velocity head at the tower inlet not exceed 10 percent of the pressure drop across one tray or across the packed bed above. Industry experienced many tray columns working well with feed velocity heads between 10 and 100 percent of a single tray pressure drop. Perhaps it is appropriate to follow the above conservative practice in general, but to somewhat relax it if it incurs a substantial cost penalty (e.g., if it significantly increases column height). An alternative practice preferred is to size the inlet nozzle to have a pressure drop lower than that of the tray above. Feed velocity heads largely exceeding a single tray pressure drop should be avoided. If a large-velocity head cannot be avoided at the inlet to a packed section, a vapor-distributing device should be used.
- Tray spacing should be increased, usually by 6 to 12 in, if vapor is present at the feed, or if large-diameter internal feed pipes are used. This is particularly important if the feed tray is heavily loaded.
- High-velocity two-phase and vapor feeds should be avoided. Although wear plates can protect column internals from damage, there have been cases when high-velocity vapor feeds cut holes through wear plates as thick as 1/4 in.
- All internal feed pipes should be removable.
- It is preferable to locate large internal liquid feed pipes below the trusses of the next higher tray.
- Components which are present in the column overhead vapor above their dew points may condense locally upon contacting highly subcooled reflux or internal reflux pipes. If the condensing component is corrosive, the reflux piping or tray areas contacting the condensate may experience severe local corrosion. A typical example is where column overheads contain hydrocarbons, steam (above its dew point), and chlorides. Water may condense on the cold surfaces, dissolve and hydrolyze chlorides, thus forming acid.
- Pipe supports should be located near the feed nozzle so that the pipe is not supported by the nozzle. Pipe guides should be used to prevent pipes from swaying in the wind.
- Alternative feed nozzles are often provided to allow for uncertainties and add flexibility to the design. The location of these nozzles should be carefully reviewed, particularly if column, tray, downcomer, or packing dimensions change from the section above the feed to the section below. Feed should be piped to an alternative nozzle only if the column section between the main feed nozzle and the alternative feed nozzle is suitable for processing the liquid and vapor loads which prevail both above and below the feed point. Failure to observe this can cause premature flooding, downcomer unsealing, or packing wettability problems when the feed is inserted into the alternative feed point or if the valve at the alternative feed point leaks.
4. Guidelines for Distributors and Multi-Pass-Tray Inlets
Feed inlet distributors are recommended for large-diameter single-pass trays. In multi-pass trays, feed and reflux distributors are essential to ensure uniform distribution. The only exceptions are:
- Pure liquid feeds into two-pass trays, which can be introduced into the central downcomer by arrangements similar to those shown in Fig. 1 (arrangements a, b, d, g, h) and Fig. 2 (arrangement a).
- When feed to multi-pass trays is to be split unevenly, such as in a three-pass tray. In such cases, a feed trough (Fig. 3) is often preferred to a distributor.
Figure 3 - Feed trough for three-pass trays
The guidelines listed earlier for reflux, top-tray, and intermediate feed inlets apply whether a distributor is used or not. In addition, guidelines for avoiding operating problems unique to distributors are listed below.
- The distributor must distribute the incoming stream evenly among the passes. If a feed is not split equally, liquid maldistribution between the passes will be established that may persist throughout the trays below the feed, with a resulting reduction in capacity and efficiency. Therefore, correct sizing of the distributor pipe and distributor perforations as well as proper orientation are important. This guideline is most important when more than two tray passes are used. With two-pass trays, a disturbance generated by an unequal liquid split is unlikely to persist beyond the few neighboring trays below. However, the disturbance to the neighboring trays can be substantial.
- When good distribution is important (Fig. 4a), fluid velocity through the distributor perforations should be considerably higher than through the distributor pipe. It was recommended to make the pressure drop through the distributor perforations at least 5 or 10 times greater than the pressure drop through the distributor pipe. Alternatively, another recommends making the hole velocity three times the distributor pipe velocity. It also recommends a distributor pipe velocity of 5 ft/s for liquid feeds. This guideline need not be adhered to when the quality of inlet stream distribution is not of major importance, e.g., when distributing feed to a single-pass tray or to the center area of a two-pass tray. With vapor and two-phase feeds, this guideline may be difficult to adhere to. Unless very large hole velocities are acceptable, this guideline calls for low pipe velocities. This, in turn, leads to impractically large pipes. A common compromise is to use a hole area of the same order as the pipe area, at the expense of the inferior distribution profile described in item 3 below (Fig. 4b, c). This, in turn, may lead to a tendency of vapor to flow toward one wall (Fig. 5a). When vapor maldistribution can be troublesome (e.g., beneath a packed bed), flow-straightening tubes (Fig. 5b) can alleviate the problem. Tube length is usually two to three times the perforation diameter.
Figure 4 - Distribution profiles of perforated pipe distributors - (a) Ideal distribution; (b) excessive fluid velocity through pipe; (c) same as for b, but with column vapor sucked in; (d) insufficient perforation pressure drop; (e) severe hydraulic disturbance near pipe inlet.
Figure 5 - Application of flow-straightening tubes - (a) Feed tends to channel; (b) flow-straightening tubes alleviate channeling
- Excessive fluid velocity through the distributor pipe may cause excessive flow through perforations near its closed end. (Fig. 4b). In the extreme case, vapor from the column may even be sucked into the distributor pipe through perforations near the feed end of the pipe (Fig. 4c). If the feed is a subcooled liquid, sucking in vapor may cause hammering. It has been recommended to make the pressure drop through the distributor holes at least 10 times the kinetic energy of the inlet stream.
- Pressure drop through the distributor perforations should range between 1 to 2 psi and 15 to 20 psi. A lower perforation pressure drop may cause maldistribution (Fig. 4d), while a higher perforation pressure drop may cause mist formation. At turned-down conditions, perforation pressure drop is low and the pattern shown in Fig. 4d sets in. Perforations near the closed end of the pipe may dry, allowing vapor into the pipe. If the feed is a subcooled liquid, vapor sucked in may collapse onto the liquid, causing instability and hammering. In one case, a column operating at 30 percent of its design rate was shaken by water hammer induced by this mechanism. The hammering was eliminated by orienting the perforations upward so that the pipe was kept full of liquid. A deflection bar was added above the orifices to arrest impingement into the tray above (see item 12 below). An alternative solution to the problem would have been to make distributor modifications.
- Severe hydraulic disturbances near the distributor inlet may cause excessive (Fig. 4e) or insufficient flow. It is best to avoid sharp bends and high-pressure-drop fittings close to the distributor inlet.
- Long, continuous slots that are parallel to the length of the distributor pipe are not recommended because they may partially block or partially corrode and then cause maldistribution. Circular perforations or short rectangular slots are preferred.
- The distributor perforations are a high-velocity area and are likely to deteriorate in service, particularly if the service is corrosive or erosive. Correct material selection is therefore important.
- Impingement of high-velocity jets issuing from distributor perforations on column walls and other internals should be minimized. Cases have been reported in which such impingement caused severe corrosion; in other situations, it may also cause mechanical damage and erosion.
- Distributor design should be reviewed for simplicity. The simpler the distributor, the less expensive it is, and the less likely it is to cause trouble.
- Feed distributors should be located at least 8 in above the tray floor for liquid feeds, and at least 12 in above the tray floor for flashing feeds. A distributor of the type shown in Fig. 2, arrangement i, is best located so that its centerline is two-thirds of a tray spacing above the tray floor.
- With long distributor pipes (>10 ft), a good practice is to "tee" the feed pipe into the distributor at the center, so that the feed flows from the center toward both ends of the distributor (as shown in Fig. 2i). With shorter distributor pipes, one end of the distributor is usually connected to the feed nozzle, with feed flow from the inlet end of the distributor pipe toward the other end.
- At times, with vapor feeds, it may appear attractive to orient the distributor (sparger) perforations upward. In one case, this was successful in overcoming a hammering problem (item 4 above). This practice is troublesome and should only be used as
a last resort. The following considerations apply to spargers with upward-oriented openings:
- A dead pocket conducive to accumulation of debris, deposits, and undesirable components forms at the sparger floor. The debris may originate in the feed or drop in through the perforations.
- Adequately located drain holes must be provided for shutdown drainage. With vapor-containing feeds, these drain holes must also prevent liquid accumulation in the sparger pipe (e.g., due to liquid pockets in the feed, or liquid weeping through the perforations, or vapor condensation at low rates). In one incident, poor drainage of such a vapor-feed sparger caused liquid buildup, which in turn led to maldistribution and corrosion by impingement of feed on the tower wall.
- Impingement of feed on the tray above (and often also on the tower wall) must be avoided using properly placed baffles. Corrosion, maldistribution, and excessive entrainment can result from poorly baffled spargers.
- The upward orientation of sparger openings makes feed distribution patterns difficult to predict, more rate dependent, and often inferior compared to those of standard spargers.