REV 10PROCESS PLANTS DIVISION TOWER INTERNALS DATE July 2002
FOSTER WHEELER ENERGY LIMITED 2002
1.0 GENERAL
The technical literature contains numerous references to distillation towers whichthough sized adequately have maloperated or flooded because the design oftower inlets, outlets, or internals was neglected or overlooked. This ProcessStandard presents information and sketches to assist engineers in developingsatisfactory tower designs. It is suggested that, whenever possible, appropriatelymarked-up Xerox copies of the sketches given in this Process Standard be usedas attachments to the vessel sketches which are normally issued by the ProcessDesign Department to Project. The information contained in this Standard isconcerned strictly with internals for tray-type towers. Information on internals for
packed towers is available in design manuals produced by vendors such asGlitsch.
The following are general design criteria:
a. Inlets to towers should be designed to obtain uniform fluid distributionwithin the tower without interfering with the normal flow patterns oftower traffic. Means for accomplishing this differ depending on towerlocation points, phase composition, and the number of passes per tray.
b. Outlets from towers should be designed so that tower hydraulics remainsatisfied (downcomer sealing, etc.) and vortexing in or adjacent to the
outlet is avoided. Where outlets can be plugged, e.g. by a broken floatfrom a level controller or by coke obstructions, screening means shouldbe provided.
c. Reboiler connections and tower bottom internals are described in detailand will vary according to the intentions of the designer. Whenoperating conditions tend to promote degradation of bottom product,minimum hold-up requirements will affect the designer’s choice ofequipment size and detail. Sometimes the advantage of an additionaltheoretical tray is desirable and can be obtained by providing suitablebaffling which will segregate bottoms product from the reboiler feed.
When information indicates the tendency of the reboiled fluid to foam orfroth, additional free space should be provided below the bottom tray.
REV 10PROCESS PLANTS DIVISION TOWER INTERNALS DATE July 2002
FOSTER WHEELER ENERGY LIMITED 2002
11.4
Q(SG) = A
1/2
11.4a
)Q(SG =
a
A =N=
1/2
)P (SG/38.0C
Q = A
1/2
2.0 INLETS
Presented in this section are equations and detailed sketches to be used in thedesign of reflux and feed inlets. The sketches show various arrangements,which are suitable for one, two, and four pass trays. Wherever possible,explanatory notes are given directly on the sketch rather than in a separate text.
2.1 Design of Liquid Distributors
Liquid is often distributed to a tower through one or more pipes containinga series of equally spaced holes or slots. The number and size of these
holes or slots are determined as follows:
The standard orifice equation for liquids is:
Q = 29.8 Cd2 (P/SG)1/2
Where
Q = Flow, gpm (hot)
C = Orifice Coefficient
P = Pressure drop, psi
SG = Sp. Gr. at conditions
d = Orifice Diam., inches
If A = the total area of all the holes or slots in the distributor, inch2, thenthe equation above becomes:
Since in almost all cases C = 0.6 and P = 0.25, we then get:
If a = the area of each hole or slot, inch2, then the number of holes or slots
REV 10PROCESS PLANTS DIVISION TOWER INTERNALS DATE July 2002
FOSTER WHEELER ENERGY LIMITED 2002
2.0 INLETS (Cont’d)
2.1 Design of Liquid Distributors (cont’d)
Each hole or slot should be large enough so that it won’t plug. Theminimum recommended hole diameter is ½”. Care must also be taken tomake sure that N is not so large that it is physically impossible to providethe calculated number of holes or slots on the given length of pipe. As thelimiting case, the length of cut metal should not exceed the length of uncutmetal on the pipe.
The total hole area calculated should be some reasonable multiple of thecross sectional area of the inlet nozzle. A reasonable multiple liessomewhere between 1.0 and 3.0. If the total hole area calculated does
not appear “reasonable” it may be adjusted by changing the value of P tosome value other than 0.25 psi.
Example
The flow of reflux through a 6 inch, schedule 40, line to the top tray of a10'-0" I.D. debutanizer is 640 gpm (hot). The specific gravity of the reflux,at flow conditions is 0.523. Design a hole-type distributor having an
allowable pressure drop of 0.25 psi.
Using11.4
Q(SG) A
2/1
we have,
Ratio = 40.6inch11.4
3)(640)(0.52 A
1/2
which is "reasonable".
Pipe X Section for 6 inch, schedule 40, line = 28.9 inch2
The maximum total available length for cutting holes in a distributor
located in the center of a 10'-0" tower is approximately (10.0)(12)(0.5) = 60inches.
REV 10PROCESS PLANTS DIVISION TOWER INTERNALS DATE July 2002
FOSTER WHEELER ENERGY LIMITED 2002
2.0 INLETS (Cont’d)
2.2 Design of Mixed Phase Distributors (cont’d)
If a = Area of each hole or slot, inch2, then:
N = No. of holes =a
)(V5
a
A 2/1
For other rules concerning distributors, see Section 2.1 above.
2.3 Reflux Inlets
Figures 1 through 8 given below show various reflux inlet arrangements.In most cases, the downcomer widths, downcomer clearances, andlocation of the inlet weirs are determined by the tray vendor. Thedimensions and notes given on these figures are all self-explanatory.
REV 10PROCESS PLANTS DIVISION TOWER INTERNALS DATE July 2002
FOSTER WHEELER ENERGY LIMITED 2002
FIGURE 2
REFLUX INLET TO ONE-PASS TRAY"TEE ARRANGEMENT"
NOTES: 1. "Tee" provides for somewhat better distribution than elbowarrangement and also protects tray deck from possible damage,which can result from high liquid velocity through elbow.
2. By tray vendor. Usually the same as downcomer width.
REV 10PROCESS PLANTS DIVISION TOWER INTERNALS DATE July 2002
FOSTER WHEELER ENERGY LIMITED 2002
FIGURE 3
REFLUX INLET TO ONE-PASS TRAY"FALSE DOWNCOMER ARRANGEMENT"
NOTES: 1. More expensive than either the "elbow" or the "tee" arrangement,but provides for more uniform distribution than either of those twomethods.
2. Clearance beneath false downcomer is the same as that providedfor downcomers on other trays. The specific clearance dimensionis established by the tray vendor.
PROCESS STD 103FOSTERWHEELER VESSELS & PAGE 2.0-13
REV 10PROCESS PLANTS DIVISION TOWER INTERNALS DATE July 2002
FOSTER WHEELER ENERGY LIMITED 2002
2.0. INLET (Cont’d)
2.4 Feed Inlets
Figures 9 through 14 given below show various feed inlet arrangements.For these figures, it should be noted that the dimension labeled as"minimum", between the feed inlet centerline and the downcomer from thetray above, should not be interpreted as meaning that the nozzle touch thedowncomer. Preferably a minimum clearance, say 1 inch, should beprovided. For all liquid inlets, Figures 27 and 28 provide for alternativelysuitable feed inlet arrangements.
REV 10PROCESS PLANTS DIVISION TOWER INTERNALS DATE July 2002
FOSTER WHEELER ENERGY LIMITED 2002
3.0 OUTLETS
Presented in this section are information and detailed sketches to be used inthe design of liquid drawoffs and vapor outlets.
3.1 Liquid Drawoffs
Figure 15 shows two types of drawoff pans. The dimensioning and thesizing of lines from drawoff pans presents a number of distinct problems,some of which may be overlooked by engineers. The line size from adrawoff pan is critical for the following reasons:
1. Liquid from a tower tray is aerated to some extent depending on thefoaminess of the gas-liquid mixture. The degree of aeration isseldom known nor can it be calculated. Although the gas-free liquidrate is known, it can be appreciated that a very small quantity ofgas in the liquid will increase the volumetric liquid rate substantially.
2. The depth of the drawoff pan is limited due to interference with thetray below, thus limiting the head of liquid above the drawoff nozzle.These low heads are conducive to vortex formation, whichseriously lowers the discharge rate through the nozzle.
The recommended method for sizing drawoffs employs the followingcriteria:
1. The depth of the drawoff pan to be 1½ times the nozzle diameter(IPS). The minimum allowable depth is 8".
2. The allowable velocity may vary from 2.4 ft/sec to 4.0 ft/secdepending upon the nozzle size. See Tables 1 and 2.
3. The nozzle is to be swaged down to a line size, which will notexceed 0.5 psi/100 ft pressure drop. The swage is to occur at a
point in elevation 4 ft below the nozzle drawoff. Only lines 8" IPSand larger are to be swaged down, the small lines will bemaintained at nozzle size to the pump or first exchanger. SeeTable 3.
The calculation of velocities is based upon an arbitrarily modifieddischarge coefficient, which compensates for the flow from an essentiallystationary liquid in the drawoff pan to the nozzle allowing for both a slightaeration and some vortexing. The coefficients for full vortexing may be aslow as 0.20 for heads below 8".
REV 10PROCESS PLANTS DIVISION TOWER INTERNALS DATE July 2002
FOSTER WHEELER ENERGY LIMITED 2002
3.0 OUTLETS (Cont’d)
3.1 Liquid Drawoffs (cont’d)
Discharge coefficient is defined as:
2gh
Vc V = Velocity for total nozzle area; ft/sec
H = ft of head above nozzle centerline
If vortexing does not occur, the coefficient of discharge for low heads is:
)32H
R(12gh
VC
2
2
R = Radius of nozzle in ft
The experimental value of C for a submerged sharp edged orifice withextended tube (nozzle) for water varies from 0.60 to 0.80. To account foraeration and some vortexing, we are arbitrarily dropping the value of thedischarge coefficient to 0.35. Tables 1 and 2 show values of allowablevelocities and maximum capacities for various size drawoff lines. Figure15 illustrates the elevations involved.
Figures 16 through 23 show various drawoff applications and give thedetails of their arrangement.
REV 10PROCESS PLANTS DIVISION TOWER INTERNALS DATE July 2002
FOSTER WHEELER ENERGY LIMITED 2002
TABLE 3
TYPICAL SWAGED LINES AFTER SIDE-PAN DRAWOFF NOZZLE
Assumptions:
1. Capacities of Lines in Table 1.2. Allowable Pressure Drop = 0.5 psi/100 ft.3. Assume Hot Sp. Gr. = 0.8 and Viscosity = 3 Centistokes.
1 2 3 4 5
DrawoffNozzle
AssumedFlow,BPSD
SwagedLine Size
PSI/100 Ftof Swaged Line
Velocity inSwaged Line
3 1,890 3 0.32 2.4
4 3,260 4 0.24 2.4
6 7,610 6 0.16 2.4
8 13,300 6 0.43 4.2
10 23,100 8 0.31 4.3
12 35,800 8 0.54 6.7
14 46,100 10 0.36 5.5
16 64,300 12 0.32 5.4
18 85,600 12 0.43 7.2
20 111,000 14 0.48 7.7
24 155,000 16 0.47 9.1
NOTES: Swaged line size may be slightly different depending upon physicalproperties of fluid, static head, physical layout, and position of swage inrelation to drawoff nozzle.
PROCESS STD 103FOSTERWHEELER VESSELS & PAGE 3.0-14
REV 10PROCESS PLANTS DIVISION TOWER INTERNALS DATE July 2002
FOSTER WHEELER ENERGY LIMITED 2002
FIGURE 23
WATER DRAWOFF PAN FOR LIGHT OILS COLUMN
NOTES: 1. Arrangement shown is for a one-pass tray, but the dimensionsgiven may also be used for two or four-pass trays.
2. Risers are sized for 30 ft/sec with the total number usually varyingbetween 2 and 12, symmetrically arranged.
3. The hold-up time on the pan is not critical for the design. If hold-uptime is taken into consideration, however, the calculation mayindicate a pan of very great depth. To avoid this problem, astandard 36 inch depth is recommended.
PROCESS STD 103FOSTERWHEELER VESSELS & PAGE 3.0-15
REV 10PROCESS PLANTS DIVISION TOWER INTERNALS DATE July 2002
FOSTER WHEELER ENERGY LIMITED 2002
3.0 OUTLETS (Cont’d)
3.2 Vapor Outlets
Figure 24 shows the details of two arrangements for overhead vaporoutlets. As indicated on the sketch, the Type I arrangement is preferredand the Type II arrangement is shown mainly for information purposes.
Intertray vapor outlets are rarely used and because of this, a sketch hasnot been provided. When intertray vapor outlets are required, twice thenormal tray spacing should be used, with the vapor outlet located a half
tray spacing below the upper tray. For a single vapor outlet nozzle,located among multi-pass trays, vapor tunnels through the downcomersmay be required to equalize the flow of vapor to the trays above the singlevapor outlet.
REV 10PROCESS PLANTS DIVISION TOWER INTERNALS DATE July 2002
FOSTER WHEELER ENERGY LIMITED 2002
4.0 TRANSITIONS
This section shows sketches of transitions from one- to two-pass trays, one- tofour-pass trays, and two- to four-pass trays. The transition method illustrated isone, which requires that the downcomers for the trays above the transition berotated 90 degrees with respect to the downcomers for the trays below thetransition. This method is shown in three dimensional view in Figure 25.Figures 26 through 28 give the details of the transition for each of three differentpass arrangements. Although the method of transition shown is therecommended one, it should be noted that many other methods exist which canbe used as well. Information on these other methods may be obtained from tray
vendors.
The following are notes, which apply to Figures 26 through 28:
NOTES: 1. The transition need not necessarily occur in a swagedsection of the tower, but may instead occur in a section ofconstant diameter.
2. Where a swaged section exists, the downcomer from TrayN+1 may be extended into the swaged section, if so desired.
3. The seal pan may have a “V” notch instead of a rectangular
notch, if so desired. In either case, the notch must belocated so that it discharges directly into the inlet weirs ofTray N.
4. For a rectangular notch design, some engineers prefer that atrough, parallel to the inlet weirs on Tray N, emanate fromthe notch and extend across the length of the tower. Liquidthen pours along the entire length of the trough into the inletweirs on Tray N.
5. The downcomers for all of the trays above the transitionmust be rotated 90 degrees with respect to the downcomers
for all the trays below the transition.
6. The distributors shown on Figures 27 and 28 are suitableonly for an all-liquid feed. For a mixed-phase feed, thedistributor must be similar in design to that shown in Figure26. See Section 2.0 for detailed information on distributors.
REV 10PROCESS PLANTS DIVISION TOWER INTERNALS DATE July 2002
FOSTER WHEELER ENERGY LIMITED 2002
5.0 TOWER BOTTOM DETAILS
This section contains information and sketches for various tower arrangements.These correspond to the use of different types of reboilers and a variety of traypass counts. A discussion of the various types of reboilers precedes thepresentation of the detailed sketches.
5.1 Once-Through Thermosyphon Reboiler
The horizontal once-through thermosyphon reboiler is the type of reboilernormally preferred. Its feed is trapout liquid from the bottoms tray and its
outlet consists of a two-phase mixture of bottoms product and reboiledvapor. The effective separation achieved in the once-throughthermosyphon reboiler is equivalent to one theoretical tray in the tower.This type of reboiler may be used provided the percent vaporization at thereboiler outlet does not exceed 80 percent. Above 80 percentvaporization a recirculation or kettle type reboiler is normally used. Inaddition, if the bottoms temperature is so high that a fired heater, ratherthan a heat exchanger, must be employed, a recirculation-type bottomsarrangement is then used. Figures 29 through 32 show the tower bottomdetails for a once-through thermosyphon reboiler.
5.2 Recirculation-Type Reboiler
In a recirculation-type reboiler, the reboiler feed is a mixture of trapoutliquid from the bottoms tray and Recirculation liquid from the reboileroutlet. The reboiler feed may flow to the reboiler by the thermosyphoneffect or, if required, as in the case of a fired heater, it may be pumped.Because of the presence of the Recirculation liquid, the percentvaporization at the reboiler outlet may be kept low compared to thatobtained in a once-through reboiler. For most Recirculation-type reboilers,the tower bottoms is arranged so that a single nozzle is used to withdrawthe combined bottoms product and reboiler feed. When this is done, theadvantage of having the reboiler count as one theoretical tray is lost.Usually this loss is not important. In some very low efficiency separations,however, the loss of one theoretical tray means that several more actualtrays must be installed in the tower. In such a case, a special baffle maybe installed in the bottom of the tower to allow the bottoms product andreboiler feed to be separately withdrawn and to thus maintain the effect ofone theoretical tray for the reboiler. Figures 33 through 36 show the towerbottom details for the combined withdrawal arrangement and Figure 37shows the details for the baffle arrangement.
REV 10PROCESS PLANTS DIVISION TOWER INTERNALS DATE July 2002
FOSTER WHEELER ENERGY LIMITED 2002
5.0 TOWER BOTTOM DETAILS (Cont’d)
5.3 Kettle Reboiler
A kettle reboiler is usually used for a narrow boiling range material with ahigh percent vaporization. When a kettle is used, liquid residence time isprovided in the kettle rather than in the tower bottoms section. Thebottoms product is withdrawn directly from the kettle and the reboilerreturn line to the tower contains only vapor. The tower bottoms details fora kettle reboiler are the same as those given in Figures 33 through 37 forthe recirculation-type reboiler, the only difference being that the liquidresidence time in the case of the kettle is zero.
5.4 Liquid Residence Time
The liquid residence time (from LLL to HLL) to be used in designing thebottom section of a tower is as follows:
1. Bottoms as feed to a subsequent tower on level controlis 5 minutes. In general, level control will frequentlyprove satisfactory to the second of a series of towers.
2. Bottoms as feed to a subsequent tower on flow control
is 10 to 20 minutes. This amount of residence time maybe obtained, in the case of smaller towers, by swagingout the hold-up section. Alternatively, a separate surgedrum may be required.
3. Bottoms to a heat exchanger and/or tankage is 2minutes. This may be lower in the case of a vacuumtower to prevent coking.
4. Feed to a fired coil reboiler is the sum of 5 minutes onthe vaporized portion and 2 minutes on the bottomsproduct. It is normally desirable that the 5 minutes onthe vaporized portion be employed to establish theNLL, with the subsequent 2 minutes on bottomsproduct used to establish the HLL. The minimumdistance from NLL to HLL is 1 foot.
REV 10PROCESS PLANTS DIVISION TOWER INTERNALS DATE July 2002
FOSTER WHEELER ENERGY LIMITED 2002
5.0 TOWER BOTTOM DETAILS (Cont’d)
5.5 Sketches of Various Arrangements
The sketches of the various arrangements discussed in this section aregiven below in Figures 29 through 37. The following explanatory notesapply to Figure 37:
NOTES: 1. The sketch shown is for a single-pass tray.For multiple passes, the baffle is positionedanalogously.
2. The baffle does not have to extend to thebottom shell because the flow is from thebottom product section to the reboiler feedsection. Thus, there should be no danger oftrapout liquid appearing in the bottomsproduct.
3. The baffle is located so that the HLL satisfiesthe liquid residence requirements both on thebottoms product side and on the reboilerfeed side.
4. The clearance under the baffle should be aminimum of 12" in order to provide enoughflow area for the recirculated fluid to flowtoward the reboiler feed nozzle.
Many of the sketches given below show two reboiler returnnozzles. Please note that depending upon the size of the unitand the location of the reboiler, consideration may be given toeither one or two reboiler return nozzles.
REV 10PROCESS PLANTS DIVISION TOWER INTERNALS DATE July 2002
FOSTER WHEELER ENERGY LIMITED 2002
FIGURE 31
TOWER BOTTOM ONCE-THROUGH THERMOSYPHON REBOILER TWO-PASS TRAY WITH SIDE DOWNCOMERS
NOTES: 1. The top edge of the seal pan shall be above the top of the trapoutnozzle. The seal pan height, X, shall be added to the tray spacing,to determine the height from the seal pan floor to the bottom tray.Since the exact dimension of the clearance under the downcomeris not known early in the job, one method of establishing X is:
X = O.D. of nozzle A + 6"
2. As an alternative, an arrangement similar to Figure 30 may be usedif nozzle A is large.
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