TRAY DESIGN FOR DISTILLATION COLUMN
Sanjeev kumar 0551611406 B.Tech. VIIth sem
TRAY DESIGN TRAY DESIGN FOR DISTILLATION COLUMN
Cross-flow
plates are the most common type of plate contactor used in distillation and absorption columns. Three
principal types of cross-flow tray are used, classified according to the method used to contact the vapour and liquid. 1. Sieve plate (perforated plate) This is the simplest type of cross-flow cr oss-flow plate. The vapour passes up through perforations in the plate; and the liquid is retained on the plate by the vapour flow. flow. There is no positive vapour liquid seal, and at low flow-rates liquid will “weep” through the holes, h oles, reducing the plate efficiency. efficiency. The perforations are usually u sually small holes, but larger holes and slots are useD.
2. Bubble-cap plates 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, oldest, type of cross-flow plate, and many different designs have been developed. Standard cap designs would now be specified for most applications. The most significant feature of the bubble-cap plate p late is that the use of risers ensures that a level of liquid is maintained on the tray at all vapour flowrates.
3. Valve plates (floating cap plates) Valve plates are proprietary designs. They are essentially sieve plates with large-diameter holes covered by movable flaps, which lift lift as the vapour flow increases.
As the area for vapour flow varies with the flow-rate, valve plates can operate efficiently
at lower flow-rates than sieve plates: the valves closing at low low vapour rates.
Bubble Cap
Vapor rises up through “risers” or “up-takes” into bubble cap, out through slots as bubbles into surrounding liquid on tray. Bubbling action effects contact. Liquid flows over caps, outlet weir and down comer to tray below,
CAPACITY: moderately , high, maintains efficiency.
Efficiency: most data are for this type, as high as other tray designs.
Entrainment: about three times that of perforated type plate or sieve tray. Jet-action accompanies bubbling. Flexibility: most flexible of tray designs for high and low vapor and liquid rates. Allows positive drain of liquid from tray. Liquid heads maintained by weirs.
Application: all services except extremely coking, polymer formation or other high fouling conditions. Use for extremely low flow conditions where tray must remain wet and maintain a vapor seal.
Tray Spacing :18-in. average, 24 to 36in. for vacuum conditions.
Bubble Cap Tray Design Tray Design Tower application or service: Product Finishing Tower Inside Diameter: 6 ft, 0 in. Tray Type: Cross Flow Tray Spacing, 24 in.; Type outlet weir: End No. downcomers /tray 1; Located: End Cap Data: (1) Cap I.D., ID, 3% in., Spacing: 5% in. A 60" centers (2) Total height, 3% in. (3) No./tray, Nc, 129 (4) Slots: No., N,, 50 (5) Height, H,, 1%in . (6) Width, w,, % in. (7) Skirt Height, s, 54 in. (8) Shroud Ring height, hSr, % in. (9) Height of inside surface of cap above tray, 3.94 in. (10) Riser I.D., 2.68 in.
Areas: (12) Riser inside cross-sectional, a,, 5.43 in.2 per riser (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 2⅓O.D.; п*2.75ⁿ/4 = 5.94.(n=2). (15) Cap inside cross-sectional area a,, 11.79 in2 per (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 (18) Total annular area per tray, &, 5.24 ft2 (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) (21) Slot area per cap, a,, (50) ( X ) (1.5) = 9.39 in.2 (22) Total slot area per tray, &, 8.40 €t2 Tray Detail: (23) Length of outlet overflow weir, l,, 4.0 ft (24) Height of weir (weir setting) above tray floor, h,, (25) Inlet weir (downcomer side) length (if used), 4.0 ft
(26) Inlet weir height above tray floor, 3 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),
(29) Height of bottom of downcomer above tray floor,
(30) Downcomer flow areas: (a) Between Between downcomer
(31) (b) Between bottom downcomer and tray floor,
(32) (c) Between downcomer and inlet weir, 0.740 ft2
(33) Riser slot seal, (3.0 - 2), 1.0 in.
Tray Operations summary and Pressure Drop Top
A. Tray number B. Operating pressure, mm. Hg C. Operating temperature, temperature, "F D. Vapor flow, lbs/hr E. Vapor volume, ft3/sec @ operating conditions, V F. Vapor density, lbs/ft3 operating conditions G. Liquid flow, gallons/minute, L, H. Liquid flow, Ibs/hr., L‘ I. Liquid flow, ft3/sec @ operating Conditions J. Liquid density, lbs/ft3 @ operating conditions K. Superficial vapor velocity, based on Tower I.D., ft/sec, 132.2/28.28 L. Vapor velocity based on cap area between inlet and outlet weirs, ft/sec 132/[28.28 - 2(2.12)
bottom
20 75 60 6,565
1 100 100 6,565
132.2
105
0.0138 3.74 1,515
0.01735 3.74 1,515
0.00834
0.00776
50.5
54.2
4.67
3.7
1 5.49
4.37
Top
bottom
M. Volume of downcomer: Area top segment, Perry's Hdbk. 3rd Ed. pg. 32. h/D = 9% in./72 = 0.1276, A = 0.03799(6)2 = 2.08 ft2 Lower taper, use h @ % of vert. taper for estimate. 8/72 = 0.111, A = 0.04763(6)2 = 1.71 ft2 Volume = (2.08) (0.5) + (1.71) (21/12) (21/12) = N. Liquid residence time in downcomer, seconds, (4.04)/0.00834 = 485 0. Throw over downcomer weir (sideflow), inches P. Throw over downcomer weir (center flow), min. = Q. Tray layout, actual downcomer width, in. Taper downcomer has 6 in. vertical dimension at 9% in. wide. Tapers to 5% in., 24 in. below tray.
4.04 ft3 485
4.04 520
1.17
1.17
9.3125 5.5
9.3125 5.5
Top R . Slot velocity: minimum 3.4 /(pc)1/2
ft/sec
29
bottom 25.9
S. Slot velocity: maximum = 12.1/
(pc)'l2 = 12.1/(.0138)1/2 and 12.1/ (0.01735)1/2, ft/sec
103.1
92
15.7
12.5
T. Slot velocity: Superficial, u, =
V/As = 132.2/8.40 and 105/8.4 ft/sec
Pressure Drop, Inches Liquid on Tray: a. Height of liquid over weir (straight weir)
Lg/ (lw)2.5= 3.74/ ( 4 ~ 1 2 )= ~0.1. 1~68 IW/D = 4/6 = 0.667 Read Fw = 1.018 from Figure 8-105 how = 0.092 (1.018) (3.74/4)2/3
0.0989
0.0989
Top
Bottom
0.5
0.5
0.0633
0.0462
Riser height > 2.5 in.
0.045
0.0322
3. Rectangular slot dry pressure drop
0.0308
0.0231
0.1391
0.1015
0.87
0.847
Use W in.-V-notched weir, 2.5 in. from tray floor to bottom of notch. This is necessary because of low liquid flow. b. Static submergence, h,, in. c. Caps
Modified Dauphine and Cicalese, [l l, 13] dry cap basis. 1. Riser pressure drop, reversal reversal area greater than riser area. 2. Reversal and annulus pressure drop
4. Total dry cap pressure drop h', = h, + h,, + h,' = 0.0633 0.063 3 0.0633 + 0.045 + 0.0308 = 0.139 5. Wet cap pressure drop From Figure 8-115, C, = 0.16 h, = h,'/CI, = 0.1391/0.16 = 0.87
Top Bottom 6.Check maximum pressure drop through wet caps: h, max. = 0.0633 + 0.045 + Since h, is less than h, ma., cap is 0.K and not blowing under shroud (1.5 + 0.25), in. 1.8 1.3 Ring . Bolles' recommendation 7. Riser, reversal, annulus pressure drop +/ar = 5.85/5.43 = 1.073 From Figure 8-114, K, = 0.598 0.118 8. Slot pressure drop, Rectangular slots
0.0861 0.626
d. Liquid Gradient Mean tray width = (4 + 6)/2 = 5 ft GPM/ft mean tray width = 3.74/5 = 0.75 Assumed mean liquid depth, hl = 2.5 + 0.0989 + 0.1 Uncorrected A'/row caps = approx. 0.02 in. Cv, from Figure 8-113 = estimated 0.55(off chart)
0.566
Top
Bottom
0.12
0.12
0.06
0.06
1.528
1.502
1.502
1.310
No. cap rows = 11 Corrected ∆ = (0.02) (0.548) (11) = 0.1206 inches ∆ /2, inches (essentially negligible in this case) 1. Modified Dauphine ht = 0.87 + 0.5 + 0.0989 + 0.06 2. Bolles ht = h,, + h, + h,, +how + ∆ /2 ht = 0.118 + 0.626 + 0.5 + 0.0989 +0.06 f. Pressure drop for 15 trays in 1.
Modified Dauphine, 15 (1.528)
= 22.9 in. liquid = 34.2 mmHg
34.2 mm
2. Bolles, 15 (1.502) = 22.4 inches liquid = 33.4 mm Hg
33.4 mm
Top
Pressure drop for 5 trays in stripping section 1. Modified Dauphine, 5 (1.505) = 7.52 in. liquid = 2. Bolles, 5 (1.42) = 6.56 in. liquid == Total pressure drop for 20 trays 1. Modified Dauphine 2. Bolles g. Height liquid in downcomer 1. Segmental, underflow plus friction 2. Segmental, upflow when inlet weir used hd' = 0.3 Vdu2 3. Total segmental loss, hd 4. Circular downspout 5. Liquid height in downcomer Hd = hw + how + hd + h, + A = 2.5 + 0.0989 + 0.000077 +1.638 + 0.35
bottom
11.1 mm 9.7mm 45.3 mm 43.1 mm
0.000077
0.000077
Neg.
Neg. Neg.
0.000077
0.00007'7
4.58
4.56
Top Bootom 6. Free height in downcomer F = St + hlv - Hd = 24 +2.5 - 4.58
21.69
7. Throw over weir f = 0.8 [how (F)J112 = 0.8 [0.0989 (21.69)]'1/2
1.17
1.17
0.138
0.141
0.65
0.65
h. Vapor distribution ratio Rv = A/h, = 0.12/0.87 i. Slot seal Dynamic, hd, = h,, + how + A/2 = 0.5 + 0.0989 + 0.06
Liquid Velocity in Downcomer Downcomer: Downcomer::
Minimum cross-section area of downcomer = 0.886 ft2 Liquid rate = 0.00834 ft3/sec Velocity = 0.00834/0.886 = 0.00942 ft/sec This is very low and confirms that there should be ample disengaging capacity in the th e downcomers. The downcomers are too large for good design.
21.71
Slot velocity The results of lines R, S, and T indicate that the vapor velocity through the cap slots is lower than desirable for good bubbling.
Slot Opening 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. i n.
Vapor Distribution Ratio 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 compensation.
Liquid Entrainment vf= 132.2/[28.28 - 2(2.12)] = 5.5 ft/sec Reading Figure 8-1 16 For 27 dynes/cm surface tension W,/h, + h,, + h, = 0.026 We = (.026) (0.098 + 0.5 + 0.626) = 0.0317 lbs/min ft2) Entrainment = (0.0317) [28.28 - 2(2.12)] = 0.764 lbs/min Entrainment ratio = 0.764/ (6565/60) = .00698 This value of entrainment is negligible. For a new column design, this would indicate that the tower was too large, and a smaller shell should be considered .
Conclusion
This is not a good tray design, but it should operate. However, a reduced efficiency is to be expected due to low vapor velocities. Because the liquid flow is low also, %in. vnotched weirs should be used to ensure uniform flow and level across the tray. tray . The bottom of the notches should be 2.5 in. above the tray floor.
