TRAY DESIGN FOR TRAY DESIGN FOR DISTILLATION COLUMNDISTILLATION COLUMN

Sanjeev kumarSanjeev kumar

05516114060551611406

B.Tech. VIIth semB.Tech. VIIth sem

TRAY DESIGN FOR TRAY DESIGN FOR DISTILLATION COLUMNDISTILLATION COLUMN

Cross-flow plates are the most common type of plate contactor used in Cross-flow plates are the most common type of plate contactor used in distillation and absorption columns.distillation and absorption columns.Three principal types of cross-flow tray are used, classified according to the Three principal types of cross-flow tray are used, classified according to the method used to contact the vapour and liquid.method used to contact the vapour and liquid.

1. 1. Sieve plate (perforated plate)Sieve plate (perforated plate)

This is the simplest type of cross-flow plate. The vapour passes up through This is the simplest type of cross-flow plate. The vapour passes up through perforations in the plate; and the liquid is retained on the plate by the vapour flow. perforations in the plate; and the liquid is retained on the plate by the vapour flow. There is no positive vapour liquid seal, and at low flow-rates liquid will “weep” There is no positive vapour liquid seal, and at low flow-rates liquid will “weep” through the holes, reducing the plate efficiency. The perforations are usually small through the holes, reducing the plate efficiency. The perforations are usually small holes, but larger holes and slots are useD.holes, but larger holes and slots are useD.

2. 2. Bubble-cap platesBubble-cap plates

In which the vapour passes up through short pipes, called risers, covered by In which the vapour passes up through short pipes, called risers, covered by a cap with aserrated edge, or slots. The bubble-cap plate is the traditional, a cap with aserrated edge, or slots. The bubble-cap plate is the traditional, oldest, type of cross-flow plate, and many different designs have been oldest, type of cross-flow plate, and many different designs have been developed. Standard cap designs would now be specified for most developed. Standard cap designs would now be specified for most applications.applications.

The most significant feature of the bubble-cap plate is that the use of risers The most significant feature of the bubble-cap plate is that the use of risers ensures that a level of liquid is maintained on the tray at all vapour flow-ensures that a level of liquid is maintained on the tray at all vapour flow-rates.rates.

3. Valve plates (floating cap plates) 3. Valve plates (floating cap plates) Valve plates are proprietary designs. They are essentially sieve plates with Valve plates are proprietary designs. They are essentially sieve plates with

large-diameterlarge-diameter holes covered by movable flaps, which lift as the vapour flow increases.holes covered by movable flaps, which lift as the vapour flow increases. As the area for vapour flow varies with the flow-rate, valve plates can As the area for vapour flow varies with the flow-rate, valve plates can

operate efficientlyoperate efficiently at lower flow-rates than sieve plates: the valves closing at low vapour at lower flow-rates than sieve plates: the valves closing at low vapour

rates.rates.

Bubble CapBubble Cap Vapor rises up through “risers” or “up-takes” into bubble cap, out through Vapor rises up through “risers” or “up-takes” into bubble cap, out through

slots as slots as bubbles into surrounding liquid on tray. Bubbling action effects bubbles into surrounding liquid on tray. Bubbling action effects contact. Liquid flows over caps, outlet weir and down comer to tray below, contact. Liquid flows over caps, outlet weir and down comer to tray below,

CAPACITY: CAPACITY: moderately , high, maintains efficiency.moderately , high, maintains efficiency. Efficiency: Efficiency: most data are for most data are for this this type, type, as as high high as as other tray designs.other tray designs. Entrainment: about three times that of perforated type plate or sieve tray. Entrainment: about three times that of perforated type plate or sieve tray.

Jet-action accompanies bubbling.Jet-action accompanies bubbling. Flexibility: most flexible of tray designs for high and low vapor and liquid Flexibility: most flexible of tray designs for high and low vapor and liquid

rates. Allows positive drain of liquid from tray. Liquid heads maintained rates. Allows positive drain of liquid from tray. Liquid heads maintained by weirs.by weirs.

Application: all services except extremely coking, polymer formation or Application: all services except extremely coking, polymer formation or other high fouling conditions. Use for extremely low flow conditions other high fouling conditions. Use for extremely low flow conditions where tray must remain wet and maintain a vapor seal.where tray must remain wet and maintain a vapor seal.

Tray Spacing :18-in. average, 24 to 36in. for vacuum conditions.Tray Spacing :18-in. average, 24 to 36in. for vacuum conditions.

Bubble Cap Tray DesignBubble Cap Tray Design

Tray DesignTray DesignTower application or service: Product FinishingTower application or service: Product FinishingTower Inside Diameter: 6 ft, 0 in.Tower Inside Diameter: 6 ft, 0 in.Tray Type: Cross FlowTray Type: Cross FlowTray Spacing, 24 in.; Type outlet weir: EndTray Spacing, 24 in.; Type outlet weir: EndNo. downcomers /tray 1; Located: EndNo. downcomers /tray 1; Located: EndCap Data:Cap Data:(1) Cap I.D., ID, 3% in., Spacing: 5% in. A 60" centers(1) Cap I.D., ID, 3% in., Spacing: 5% in. A 60" centers(2) Total height, 3% in.(2) Total height, 3% in.(3) No./tray, Nc, 129(3) No./tray, Nc, 129(4) Slots: No., N,, 50(4) Slots: No., N,, 50(5) Height, H,, 1%in .(5) Height, H,, 1%in .(6) Width, w,, % in.(6) Width, w,, % in.(7) Skirt Height, s, 54 in.(7) Skirt Height, s, 54 in.(8) Shroud Ring height, hSr, % in.(8) Shroud Ring height, hSr, % in.(9) Height of inside surface of cap above tray, 3.94 in.(9) Height of inside surface of cap above tray, 3.94 in.(10) Riser I.D., 2.68 in.(10) Riser I.D., 2.68 in.(1 1) Riser height above tray floor, 3 in.(1 1) Riser height above tray floor, 3 in.

Areas:Areas:

(12) Riser inside cross-sectional, a,, 5.43 in.2 per riser(12) Riser inside cross-sectional, a,, 5.43 in.2 per riser

(13) Total riser inside cross-sect. area/tray, A,, 4.95 ft2(13) Total riser inside cross-sect. area/tray, A,, 4.95 ft2

(14) Riser outside cross-sectional area aro, 5.94 in2 perriser. Riser is (14) Riser outside cross-sectional area aro, 5.94 in2 perriser. Riser is 22⅓⅓O.D.; O.D.; пп**2.752.75ⁿn/4 = 5.94.(n=2)./4 = 5.94.(n=2).

(15) Cap inside cross-sectional area a,, 11.79 in2 per(15) Cap inside cross-sectional area a,, 11.79 in2 per

(16) Total cap inside cross-sectional area, 4, 10.53 ft'(16) Total cap inside cross-sectional area, 4, 10.53 ft'

(17) Annular area per cap, aa, in2, (1 1.79 - 3.94) = 5.85(17) Annular area per cap, aa, in2, (1 1.79 - 3.94) = 5.85

(18) Total annular area per tray, &, 5.24 ft2(18) Total annular area per tray, &, 5.24 ft2

(19) Reversal area per cap, ar', in.2 = n(2.69) (3.94 - 3.0)(19) Reversal area per cap, ar', in.2 = n(2.69) (3.94 - 3.0)

(20) Total reversal area, per tray, Afr, ft2 (129/144)(20) Total reversal area, per tray, Afr, ft2 (129/144)

(21) Slot area per cap, a,, (50) ( X ) (1.5) = 9.39 in.2(21) Slot area per cap, a,, (50) ( X ) (1.5) = 9.39 in.2

(22) Total slot area per tray, &, 8.40 €t2(22) Total slot area per tray, &, 8.40 €t2

Tray Detail:Tray Detail:

(23) Length of outlet overflow weir, l,, 4.0 ft(23) Length of outlet overflow weir, l,, 4.0 ft

(24) Height of weir (weir setting) above tray floor, h,,(24) Height of weir (weir setting) above tray floor, h,,

(25) Inlet weir (downcomer side) length (if used), 4.0 ft(25) Inlet weir (downcomer side) length (if used), 4.0 ft

(26) Inlet weir height above tray floor, 3 in.(26) Inlet weir height above tray floor, 3 in. (27) Height of top of cap slots above tray floor, 2 in.(27) Height of top of cap slots above tray floor, 2 in. (28) Static slot submergence or static slot seal (2.5-2.0),(28) Static slot submergence or static slot seal (2.5-2.0), (29) Height of bottom of downcomer above tray floor,(29) Height of bottom of downcomer above tray floor, (30) Downcomer flow areas: (a) Between downcomer(30) Downcomer flow areas: (a) Between downcomer (31) (b) Between bottom downcomer and tray floor,(31) (b) Between bottom downcomer and tray floor, (32) (c) Between downcomer and inlet weir, 0.740 ft2(32) (c) Between downcomer and inlet weir, 0.740 ft2 (33) Riser slot seal, (3.0 - 2), 1.0 in.(33) Riser slot seal, (3.0 - 2), 1.0 in.

Tray Operations summaryTray Operations summary and Pressure Dropand Pressure Drop

A. A. Tray number 20 Tray number 20 B.B. Operating pressure, mm. Hg 75 Operating pressure, mm. Hg 75 C.C. Operating temperature, Operating temperature, "F "F 6060 DD. Vapor flow, lbs/hr 6,565. Vapor flow, lbs/hr 6,565 EE. Vapor volume, ft3/sec @. Vapor volume, ft3/sec @ operating conditions, V 132.2operating conditions, V 132.2 F.F. Vapor density, lbs/ft3 operating Vapor density, lbs/ft3 operating conditions 0.0138conditions 0.0138 G.G. Liquid flow, gallons/minute, L, 3.74 Liquid flow, gallons/minute, L, 3.74 H.H. Liquid flow, Ibs/hr., L‘ 1,515 Liquid flow, Ibs/hr., L‘ 1,515 I.I. Liquid flow, ft3/sec @ operating Liquid flow, ft3/sec @ operating Conditions 0.00834Conditions 0.00834 J.J. Liquid density, lbs/ft3 @ operating Liquid density, lbs/ft3 @ operating conditions 50.5conditions 50.5 K. K. Superficial vapor velocity, basedSuperficial vapor velocity, based on Tower I.D., ft/sec, 132.2/28.28 4.67on Tower I.D., ft/sec, 132.2/28.28 4.67 L.L. Vapor velocity based on cap area Vapor velocity based on cap area between inlet and outlet weirs,between inlet and outlet weirs, ft/sec 132/[28.28 - 2(2.12) 1 5.49ft/sec 132/[28.28 - 2(2.12) 1 5.49

11001006,565

105

0.017353.741,515

0.00776

54.2

3.7

4.37

Top bottom

M.M. Volume of downcomer: Area top Volume of downcomer: Area top segment, segment, Perry's Perry's Hdbk. 3rd Ed.Hdbk. 3rd Ed. pg. 32. h/D = 9% in./72 =pg. 32. h/D = 9% in./72 =

0.1276, A = 0.03799(6)2 = 2.08 ft20.1276, A = 0.03799(6)2 = 2.08 ft2Lower taper, use h @ % of vert.Lower taper, use h @ % of vert.taper for estimate. 8/72 = 0.111,taper for estimate. 8/72 = 0.111,A = 0.04763(6)2 = 1.71 ft2 VolumeA = 0.04763(6)2 = 1.71 ft2 Volume= (2.08) (0.5) + (1.71) (21/12) = 4.04 ft3= (2.08) (0.5) + (1.71) (21/12) = 4.04 ft3

N.N. Liquid residence time in downcomer, Liquid residence time in downcomer,seconds, (4.04)/0.00834 = 485 485seconds, (4.04)/0.00834 = 485 485

0.0. Throw over downcomer weir Throw over downcomer weir(sideflow), inches 1.17(sideflow), inches 1.17

PP. Throw over downcomer weir. Throw over downcomer weir (center flow), min. = -(center flow), min. = -Q. Tray layout, actual downcomer 9.3125Q. Tray layout, actual downcomer 9.3125width, in. 5.5width, in. 5.5Taper downcomer has 6 in. verticalTaper downcomer has 6 in. verticaldimension at 9% in. wide. Tapers todimension at 9% in. wide. Tapers to5% in., 24 in. below tray.5% in., 24 in. below tray.

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4.04

520

1.17

-9.31255.5

RR. Slot velocity: minimum 3.4 /(pc)1/2. Slot velocity: minimum 3.4 /(pc)1/2

ft/sec 29ft/sec 29

S.S. Slot velocity: maximum = 12.1/ Slot velocity: maximum = 12.1/

(pc)'l2 = 12.1/(.0138)1/2 and 12.1/(pc)'l2 = 12.1/(.0138)1/2 and 12.1/

(0.01735)1/2, ft/sec 103.1(0.01735)1/2, ft/sec 103.1

T.T. Slot velocity: Superficial, u, = Slot velocity: Superficial, u, =

V/As = 132.2/8.40 and 105/8.4V/As = 132.2/8.40 and 105/8.4

ft/secft/sec 15.715.7

Pressure Drop, Inches Liquid on Tray:Pressure Drop, Inches Liquid on Tray:a.a. Height of liquid over weir (straight weir) Height of liquid over weir (straight weir)

Lg/ (lw)2.5= 3.74/ ( 4 ~ 1 2 )= ~0.1. 1~68Lg/ (lw)2.5= 3.74/ ( 4 ~ 1 2 )= ~0.1. 1~68

IW/D = 4/6 = 0.667IW/D = 4/6 = 0.667

Read Fw = 1.018 from Figure 8-105Read Fw = 1.018 from Figure 8-105

how = 0.092 (1.018) (3.74/4)2/3 0.0989how = 0.092 (1.018) (3.74/4)2/3 0.0989

25.9

92

12.5

0.0989

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TopTop BottomBottom

Use W in.-V-notched weir, 2.5 in. fromUse W in.-V-notched weir, 2.5 in. from

tray floor to bottom of notch. Thistray floor to bottom of notch. This

is necessary because of low liquid flow.is necessary because of low liquid flow.

b.b. Static submergence, h,, in. Static submergence, h,, in. 0.50.5 0.5 0.5

c.c. Caps Caps

Modified Dauphine and Cicalese,Modified Dauphine and Cicalese,

[l l, 13] dry cap basis.[l l, 13] dry cap basis.

1. Riser pressure drop, reversal area1. Riser pressure drop, reversal area

greater than riser area.greater than riser area. 0.0633 0.04620.0633 0.0462

2. Reversal and annulus pressure drop2. Reversal and annulus pressure drop

Riser height > 2.5 in.Riser height > 2.5 in. 0.0450.045 0.03220.0322

3. Rectangular slot dry pressure drop3. Rectangular slot dry pressure drop 0.0308 0.0308 0.02310.0231

4. Total dry cap pressure drop4. Total dry cap pressure drop

h', = h, + h,, + h,' = 0.0633h', = h, + h,, + h,' = 0.0633

+ 0.045 + 0.0308 = 0.139+ 0.045 + 0.0308 = 0.139 0.1391 0.1391 0.10150.1015

5. Wet cap pressure drop5. Wet cap pressure drop

From Figure 8-115, C, = 0.16From Figure 8-115, C, = 0.16

h, = h,'/CI, = 0.1391/0.16 = 0.87 h, = h,'/CI, = 0.1391/0.16 = 0.87 0.87 0.87 0.8470.847

TopTop BottomBottom6.Check maximum pressure drop6.Check maximum pressure dropthrough wet caps:through wet caps:h, max. = 0.0633 + 0.045 +h, max. = 0.0633 + 0.045 +Since h, is less than h, ma., cap isSince h, is less than h, ma., cap is0.K and not blowing under shroud0.K and not blowing under shroud(1.5 + 0.25), in. (1.5 + 0.25), in. 1.81.8 1.3 1.3Ring .Ring .Bolles' recommendationBolles' recommendation7. Riser, reversal, annulus pressure drop7. Riser, reversal, annulus pressure drop+/ar = 5.85/5.43 = 1.073+/ar = 5.85/5.43 = 1.073From Figure 8-114, K, = 0.598 From Figure 8-114, K, = 0.598 0.118 0.118 0.08610.08618. Slot pressure drop, Rectangular slots8. Slot pressure drop, Rectangular slots 0.6260.626 0.5660.566

d. Liquid Gradientd. Liquid GradientMean tray width = (4 + 6)/2 = 5 ftMean tray width = (4 + 6)/2 = 5 ftGPM/ft mean tray width = 3.74/5 = 0.75GPM/ft mean tray width = 3.74/5 = 0.75Assumed mean liquid depth, hl =Assumed mean liquid depth, hl =2.5 + 0.0989 + 0.12.5 + 0.0989 + 0.1Uncorrected A'/row caps = approx.Uncorrected A'/row caps = approx.0.02 in.0.02 in.Cv, from Figure 8-113 = estimated 0.55(off chart)Cv, from Figure 8-113 = estimated 0.55(off chart)

TopTop BottomBottomNo. cap rows = 11No. cap rows = 11

Corrected Corrected ∆∆ = (0.02) (0.548) (11) = (0.02) (0.548) (11)

= 0.1206 inches= 0.1206 inches 0.12 0.12 0.12 0.12

∆∆ /2, inches (essentially negligible/2, inches (essentially negligible

in this case) in this case) 0.06 0.06 0.060.06

1. Modified Dauphine1. Modified Dauphine

ht = 0.87 + 0.5 + 0.0989 + 0.06ht = 0.87 + 0.5 + 0.0989 + 0.06 1.528 1.528 1.502 1.502

2. Bolles2. Bolles

ht = h,, + h, + h,, +how + ht = h,, + h, + h,, +how + ∆∆ /2 /2

ht = 0.118 + 0.626 + 0.5 + 0.0989 +0.06 ht = 0.118 + 0.626 + 0.5 + 0.0989 +0.06 1.502 1.502 1.3101.310

f. Pressure drop for 15 trays inf. Pressure drop for 15 trays in

1.1. Modified Dauphine, 15 (1.528)Modified Dauphine, 15 (1.528)

= 22.9 in. liquid = 34.2 mmHg = 22.9 in. liquid = 34.2 mmHg 34.2 mm34.2 mm

2. Bolles, 15 (1.502) = 22.4 inches2. Bolles, 15 (1.502) = 22.4 inches

liquid = 33.4 mm Hgliquid = 33.4 mm Hg 33.4 mm33.4 mm

TopTop bottom bottom

Pressure drop for 5 trays inPressure drop for 5 trays instripping sectionstripping section1.1. Modified Dauphine, 5 (1.505)Modified Dauphine, 5 (1.505)= 7.52 in. liquid == 7.52 in. liquid = 11.1 mm11.1 mm2. Bolles, 5 (1.42) = 6.56 in. liquid = 2. Bolles, 5 (1.42) = 6.56 in. liquid = 9.7mm9.7mmTotal pressure drop for 20 traysTotal pressure drop for 20 trays1. Modified Dauphine 1. Modified Dauphine 45.3 mm45.3 mm2. Bolles2. Bolles 43.1 mm 43.1 mmgg. Height liquid in downcomer. Height liquid in downcomer1. Segmental, underflow plus1. Segmental, underflow plusfriction friction 0.000077 0.000077 0.0000770.0000772. Segmental, upflow when inlet2. Segmental, upflow when inletweir used weir used Neg.Neg. Neg. Neg.hd' = 0.3 Vdu2hd' = 0.3 Vdu23. Total segmental loss, hd 3. Total segmental loss, hd 0.000077 0.000077 0.00007'70.00007'74. Circular downspout4. Circular downspout5. Liquid height in downcomer5. Liquid height in downcomerHd = hw + how + hd + h, + AHd = hw + how + hd + h, + A= 2.5 + 0.0989 + 0.000077 +1.638 + 0.35= 2.5 + 0.0989 + 0.000077 +1.638 + 0.35 4.584.58 4.564.56

TopTop BootomBootom6. Free height in downcomer6. Free height in downcomer

F = St + hlv - Hd = 24 +2.5 - 4.58 F = St + hlv - Hd = 24 +2.5 - 4.58 21.6921.69 21.71 21.71

7. Throw over weir7. Throw over weir

f = 0.8 [how (F)J112f = 0.8 [how (F)J112

= 0.8 [0.0989 (21.69)]'1/2 = 0.8 [0.0989 (21.69)]'1/2 1.17 1.17 1.171.17

h. Vapor distribution ratioh. Vapor distribution ratio

Rv = A/h, = 0.12/0.87 Rv = A/h, = 0.12/0.87 0.1380.138 0.141 0.141

i. Slot seali. Slot seal

Dynamic, hd, = h,, + how +Dynamic, hd, = h,, + how +

A/2 = 0.5 + 0.0989 + 0.06 A/2 = 0.5 + 0.0989 + 0.06 0.65 0.65 0.650.65

Liquid Velocity in Downcomer:Liquid Velocity in Downcomer:

Minimum cross-section area of downcomer = 0.886 ft2Minimum cross-section area of downcomer = 0.886 ft2

Liquid rate = 0.00834 ft3/secLiquid rate = 0.00834 ft3/sec

Velocity = 0.00834/0.886 = 0.00942 ft/secVelocity = 0.00834/0.886 = 0.00942 ft/sec

This is very low and confirms that there should beThis is very low and confirms that there should be

ample disengaging capacity in the downcomers. Theample disengaging capacity in the downcomers. The

downcomers are too large for good design.downcomers are too large for good design.

Slot velocitySlot velocity The results of lines R, S, and T indicate that the vapor velocity through the cap The results of lines R, S, and T indicate that the vapor velocity through the cap

slots is lower than desirable for good bubblingslots is lower than desirable for good bubbling ..

Slot OpeningSlot OpeningThe slot opening, h,, given in line c8 is only slightly lower than the normal design of The slot opening, h,, given in line c8 is only slightly lower than the normal design of

50-60% of Hs, or 0.75 in. to 0.90 in.50-60% of Hs, or 0.75 in. to 0.90 in.Vapor Distribution RatioVapor Distribution RatioThe values of line (h) are quite in line with good vapor flow through all the caps. This The values of line (h) are quite in line with good vapor flow through all the caps. This

is as would be expected since the hydraulic gradient is low; too low to require any is as would be expected since the hydraulic gradient is low; too low to require any compensationcompensation..

Liquid Entrainment Liquid Entrainment vf= vf= 132.2/[28.28 - 2(2.12)] = 5.5 ft/sec132.2/[28.28 - 2(2.12)] = 5.5 ft/secReading Figure 8-1 16Reading Figure 8-1 16For 27 dynes/cm surface tensionFor 27 dynes/cm surface tensionW,/h, + h,, + h, = 0.026W,/h, + h,, + h, = 0.026We = (.026) (0.098 + 0.5 + 0.626) = 0.0317 lbs/min ft2)We = (.026) (0.098 + 0.5 + 0.626) = 0.0317 lbs/min ft2)Entrainment = (0.0317) [28.28 - 2(2.12)] = 0.764 lbs/minEntrainment = (0.0317) [28.28 - 2(2.12)] = 0.764 lbs/minEntrainment ratio = 0.764/ (6565/60) = .00698Entrainment ratio = 0.764/ (6565/60) = .00698This value of entrainment is negligible. For a new columnThis value of entrainment is negligible. For a new columndesign, this would indicate that the tower was toodesign, this would indicate that the tower was toolarge, and a smaller shell should be consideredlarge, and a smaller shell should be considered..

ConclusionConclusion This is not a good tray design, but it This is not a good tray design, but it

should operate.should operate. However, a reduced efficiency is to be However, a reduced efficiency is to be

expected due to low vapor velocities.expected due to low vapor velocities. Because the liquid flow is low also, %in. v-Because the liquid flow is low also, %in. v-

notched weirs should be used to ensure notched weirs should be used to ensure uniform flow and level across the tray. The uniform flow and level across the tray. The bottom of the notches should be bottom of the notches should be 2.5 2.5 in. in. above the tray floor.above the tray floor.